close

Вход

Забыли?

вход по аккаунту

?

Automation and control of the microwave processing of composite materials

код для вставкиСкачать
INFORMATION TO USERS
This manuscript has been reproduced from the microfilm master. UMI
films the text directly from the original or copy submitted. Thus, some
thesis and dissertation copies are in typewriter face, while others may be
from any type o f computer printer.
T he quality of this reproduction is dependent upon the quality of the
copy submitted. Broken or indistinct print, colored or poor quality
illustrations and photographs, print bleedthrough, substandard margins,
and improper alignment can adversely affect reproduction.
In the unlikely event that the author did not send UMI a complete
manuscript and there are missing pages, these will be noted. Also, if
unauthorized copyright material had to be removed, a note will indicate
the deletion.
Oversize materials (e.g., maps, drawings, charts) are reproduced by
sectioning the original, beginning at the upper left-hand comer and
continuing from left to right in equal sections with small overlaps. Each
original is also photographed in one exposure and is included in reduced
form at the back o f the book.
Photographs included in the original manuscript have been reproduced
xerographically in this copy. Higher quality 6” x 9” black and white
photographic prints are available for any photographs or illustrations
appearing in this copy for an additional charge. Contact UMI directly to
order.
UMI
A Bell & Howell Information Company
300 North Zed) Road, Ann Arbor MI 48106-1346 USA
313/761-4700 800/521-0600
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
AUTOMATION AND CONTROL OF THE MICROWAVE PROCESSING OF
COMPOSITE MATERIALS
By
Valerie Omega Adegbite
A DISSERTATION
Submitted to
Michigan State University
in partial fulfillment of the requirement
for the degree of
DOCTOR OF PHILOSOPHY
Department of Chemical Engineering
1995
Dr. Martin C. Hawley
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
UMI Number: 9619767
UMI Microform 9619767
Copyright 1996, by UMI Company. All rights reserved.
This microform edition is protected against unauthorized
copying under Title 17, United States Code.
UMI
300 North Zeeb Road
Ann Arbor, MI 48103
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ABSTRACT
AUTOMATION AND CONTROL OF THE MICROWAVE PROCESSING OF
COMPOSITE MATERIALS
By
Valerie Omega Adegbite
Microwave processing of composites in a single-mode resonant cavity using fixed
frequency technology, was automated to advance the current state of technology by
bridging the gap between device and process. Automatic hardware and software for
controlling the non-linear and complex electromagnetic interactions during composite
curing were developed. Efficient coupling, uniform and controlled heating were the
control objectives which were achieved through mode switching, mode tuning, and
power control.
For mode tuning a non-traditional control methodology using a 2-dimensional
simplex minimization method was used, and shown to be more efficient than manual
univariate methods. In the development of the uniform heating controller theoretical
empty cavity solutions were used to develop empirical correlations to characterize the
loaded cavity. This was necessary to overcome the computationally intensive
calculations required for solving loaded cavity equations for control purposes. Using
these empirical correlations, mode switching for achieving uniform or controlled heating
was constructed and shown to be highly effective. Power control was based upon
traditional proprotional-intergal-derivative(PID) control methodology and shown to be
more proficient than the previous on/off control.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The developed controllers were integrated into an overall closed-loop feedback
control system that was implemented in a control program called LabView. Another
control system was also built and demonstrated by interfacing with a knowledge-based
system planner. The control systems were implemented in a newly designed automated
cavity with novel mechanized drives and axial and radial mounted microwave coupling
probes.
Composites curing showed that the ease and flexibility in operating the automated
microwave process was comparable to automated thermal processes, although it contains
extensive electronics and involves complex control tasks. Compared with the manually
operated system, sample temperature gradients were reduced by 60%, mode switching
times were reduced by 67%, mode tuning reproducibility was significantly improved and
data acquisition was enhanced for processing and for diagnostics.
This marks a notable step in the advancement of the current state of technology;
from that of a manual lab-scale device to a fully automated “First generation prototype”
process. The application potential of this technology has been greatly enhanced by this
development.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
To my parents,
Victor and Jeannette Adegbite,
my siblings,
Curtis Adegbite, Alvina Adegbite-Obuobi, Gwendolyn Adegbite-Cooper,
for the amaranthine and unconditional love and support.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
ACKNOWLEDGMENTS
I wish to extend my sincere gratitude to Dr. Martin Hawley, my advisor, for his
guidance and support throughout the course of this work. His editorial and extensive
technical insights on the development of the dissertation has enhanced it to a fine and
comprehensive document. Credit is due to Dr. Asmussen, whose wealth of knowledge in
electromagnetics provided the fundamental understanding in this research. There is also
credit due to Dr. Jon Sticklen who provided the computing insight that steered the
direction of this work. I wish to also extend my thanks to Dr. Radcliffe for sharing his
knowledge in process control and for introducing the software tool that was used in this
work.
Thanks is also due to Ron Fritz, Dale Wesson, Larry Fellows, Jianghua Wei, Jim
McDowell, David Decker, Yunchang Qiu, Michael Muczynski, and Beajaye Bedell for
their various assistance through the course of this work.
This research was funded by NSFI/UCRC Polymer Processing Center at
Michigan State University.
The computing hardware and software were provided by ISL (Information System
Laboratories) at Michigan State University
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
TABLE OF CONTENTS
LIST
OF TA B LES..................................................................................................xiii
LIST
OF
C hapter
FIG U R ES................................................................................................. xiv
1: INTRODUCTION....................................................................................1
Chapter 2: REVIEW OF PERTINENT LITERATURE
2.1 Introduction............................................................................................................14
2.2 Control Philosophy............................................................................................... 16
2.2.1 Conventional / Traditional Control................................................................ 17
2.2.2 Intelligent Control........................................................................................... 17
2.3 Composite M aterials............................................................................................. 20
2.3.1 Background.................................................................................................... 20
2.3.2 Fabrication Methods...................................................................................... 20
2.4 Control in Composite Processing -Autoclave...................................................... 21
2.4.1 Composites Process M odeling.................................................................... 21
2.4.2 Conventional and Conceptual Control Methods........................................... 21
2.4.3 Intelligent Control Methods........................................................................... 21
2.5 Control in Composite Processing -Microwaves.................................................26
2.5.1 Process Modeling...........................................................................................26
2.5.2 Control in Microwave Processing................................................................ 27
2.5 Data Acquisition.................................................................................................... 27
2.6.1 Autoclave
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
2.6.2 Microwave
27
2.7 Summary............................................................................................................. 28
Chapter 3: ELECTROMAGNETIC THEORY AND MICROWAVE PROCESSING
3.1 Introduction........................................................................................................... 29
3.2 Electromagnetic Theory......................................................................................... 29
3.2.1 Maxwell’s Equations..................................................................................... 29
3.2.2 Maxwell’s Equations in a Source Free Homogeneous Region................... 31
3.3 Electromagnetic Fields in a Cylindrical Region....................................................32
3.3.1 Empty Cavity Solutions................................................................................. 32
3.3.1.1 Modes......................................................................................................35
3.3.1.1.1 Transverse Electric Modes - TE Modes.........................................36
3.3.1.1.2 Transverse Magnetic Modes-TM-modes....................................... 37
3.3.1.2 Modes Designation................................................................................. 38
3.3.1.3 Electric Field Pattern.............................................................................. 39
3.3.1.4 Cut-Off Frequency................................................................................. 39
3.3.1.5 Cavity Quality Factor.............................................................................46
3.3.2 Loaded Cavity Solutions............................................................................... 48
3.3.2.1 Small- Low loss Sam ples......................................................................48
3.3.2.2 Homogeneous and Isotropic Samples.................................................. 49
3.3.2.3 Simplified Composite Material...............................................................49
3.4 Fundamentals of Microwave Heating................................................................... 50
3.4.1 Microwave Power Absorption and Permittivity........................................... 50
3.4.2 Penetration D epth...........................................................................................51
3.4.3 Applicators...................................................................................................... 53
3.4.3.1 Multimode and Waveguide.................................................................... 53
3.4.3.2 Single-Mode Cavity................................................................................54
3.4.4 Mode Tuning..................................................................................................55
3.5 Summary................................................................................................................56
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Chapter 4: EMPTY AND LOADED CAVITY CHARACTERIZATIONS
4.1 Introduction........................................................................................................... 57
4.2 Empty Cavity Characterization...................................................
58
4.2.1 Experiments................................................................................................... 58
4.2.2 Results and Discussions - Empty cavity characterizations.......................... 58
4.3 Loaded Cavity Characterization.............................................................................61
4.3.1 Experiments................................................................................................... 61
4.3.1.1 Frequency Shift Measurement...............................................................63
4.3.1.2 Electric Field Measurement.................................................................... 64
4.3.2 Results and Discussions - Loaded cavity characterizations......................... 67
4.3.3 Loaded cavity mode estimation......................................................................89
4.3.3.1 Results and Discussions........................................................................ 91
4.4 Summary............................................................................................................... 94
Chapter 5: AUTOMATION CONTROL SOFTWARE AND HARDWARE
5.1 Introduction........................................................................................................... 96
5.2 Cavity Automation................................................................................................ 99
5.2.1 Cavity Description......................................................................................... 99
5.2.2 Automated Cavity and Mechanized Drives.................................................. 100
5.2.2.1 Stepper Motor and Driver Hardware................................................... 108
5.2.2.2 Stepper Motor Driver Software............................................................110
5.2.3 Cavity Length and Probe Depth Measurement......................................... 112
5.3 Microwave System Automation..........................................................................113
5.3.1.1 Automation of External Circuit............................................................113
5.3.1.2 Automation of Microwave Power Source........................................... 117
5.4 Data Acquisition................................................................................................... 118
5.4.1 Data Acquisition Hardware..........................................................................118
5.4.2 Data Acquisition Soft""w Prnomm............................................................118
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5.5 Process Control Software................................................................................... 121
5.5.1 Efficient Coupling-Mode Tuning................................................................121
5.5.1.1 Approach.............................................................................................. 121
5.5.1.2 Simplex Method................................................................................... 123
5.5.1.3 Simplex Logic...................................................................................... 124
5.5.2 Mode Selection and Uniform Heating.........................................................130
5.5.2.1 Approach.............................................................................................. 130
5.5.2.2 Mode Selection Logic...........................................................................131
5.5.3 Temperature Control................................................................................... 134
5.5.3.1 Approach.............................................................................................. 134
5.5.3.2 PID M ethod.......................................................................................... 134
5.6 Lab View Curing Process Control Software Program.......................................136
5.6.1 Front Panel.................................................................................................. 136
5.6.2 Program Implementation............................................................................ 138
5.7 Diagnostics System Automation......................................................................... 140
5.7.1.1 Hardware Automation......................................................................... 140
5.7.1.2 Diagnostics system software development......................................... 142
5.8 LabView Software Interface with Knowledge-Based-System Planner...........143
Chapter 6:APPLICATION TO CURING
6.1 Introduction......................................................................................................... 146
6.2 Curing Experiments.............................................................................................146
6.3 Results and Discussions...................................................................................... 148
6.3.1 24-ply Sample.............................................................................................. 148
6.3.1.1 Mode Selection.....................................................................................148
6.3.1.2 Power Control...................................................................................... 149
6.3.1.3 Tuning...................................................................................................149
6.3.2 48-ply Sample.............................................................................................. 150
6.3.2.1 Mode Selection.....................................................................................150
6.3.2.2 Tuning...................................................................................................151
6.3.2.3 Power Control
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
151
6.4 Summary and Conclusions................................................................................152
Chapter 7: SUMMARY AND CONCLUSIONS
7.1 Introduction..........................................................................................................160
7.2 Automatic Control Software............................................................................... 162
7.2.1 Mode Tuning Software Program................................................................. 162
7.2.2 Mode Selection and Uniform Heating Control Software Program
163
7.2.3 Power Control Software Program.............................................................. 164
7.2.4 Data Acquisition Interface and Control Software Platform....................... 165
7.2.5 Data Acquisiton Software program............................................................. 165
7.3 Automation Hardware......................................................................................... 166
7.3.1 Cavity and Circuit......................................................................................... 166
7.4 Verification of Control System........................................................................... 167
7.5 Diagnostic System Automation........................................................................... 168
7.6 Understanding to Enhance Utilization of Technology.......................................169
7.6.1 Dual coupling................................................................................................169
7.6.2 Coupling Probe Effects............................................................................... 170
7.6.3 Loaded Resonant Cavity Length................................................................. 171
7.6.4 Effect of Tooling.......................................................................................... 171
7.6.5 24-ply versus 48-ply curing........................................................................ 172
7.6.6 Theoretical Field Pattern Plots..................................................................... 172
7.6.7 Application of Knowledge-based System to Automation.......................... 173
7.7 Global Conclusion.............................................................................................. 174
Chapter 8: RECOMMENDATIONS AND FUTURE WORK
8.1 Automated System...............................................................................................176
8.2 Technology Advancements...................................................................
177
8.3 Variable Frequency
177
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8.4 High power source...............................................................................................178
8.5 Dual Coupling......................................................................................................179
8.6 Scale-up............................................................................................................... 180
8.7 Tooling Issues......................................................................................................180
8.8 Potential Applications.......................................................................................... 181
Appendix A: AUTOMATION HARDWARE
A.1 Stepper Motors.................................................................................................... 183
A. 1.1 Description...................................................................................................183
A. 1.2 Stepper Motor Driver.................................................................................. 183
A. 1.3 Wiring Diagram............................................................................................186
A.2 Data Acquisition Interface................................................................................... 188
Appendix B: DUAL-COUPLING
B .l Introduction..........................................................................................................191
B.2 Experimental........................................................................................................ 191
B.3 Results and Discussions.....................................................................................195
B.4 Conclusions........................................................................................................ 198
Appendix C: KNOWLEDGE-BASED SYSTEM INTERFACE
C. 1 Introduction..........................................................................................................200
C.2 Goals of the Planner............................................................................................200
C.3 Interface with KBS-planner............................................................................... 202
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix D: ELECTRIC FIELD PATTERNS
D .l Description...................................................................................................... 204
D.1.1 TM-mode..................................................................................................... 204
D.I.2 TE-mode...................................................................................................... 204
Appendix E: SOFTWARE DOCUMENTATION
E.1 Introduction......................................................................................................... 205
E.2 Compcure3-Curing program...............................................................................206
E.3 Surface.vi - implementation of tuning program................................................ 206
E.4 Replace.vi-replacement of simplex triangle vertices......................................... 206
E.5 Vertex.vi -calculation of intial vertex in the simplex triangle.............................206
E.6 CL / PD / Pwr -scaling of cavity length, probe depth and power values
207
E.7 Move Read.vi - adjustment of cavity length & probe depth and power sensing207
E.8 Limits.vi - tuning limits for a mode................................................................... 207
E.9 Contract.vi - contraction of simplex triangle in the tuning program................. 207
E.10 Pwrcontrl.vi - power controller used in the curing program
................. 208
LIST OF CITED W ORKS.................................................................................. 209
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF TABLES
Table 3-1 Penetration depth of various materials...................................................... 53
Table 4- 1 Measured and calculated resonant cavity lengths...................................... 60
Table 4- 2 Results from axial field measurements...............................................................84
Table 4- 3 Results from axial field measurements...............................................................84
Table 4- 4 Frequency shift measurement results...................................................................85
Table 4- 5 Total cavity shift for a loaded cavity for a TM(012) mode................................. 91
Table 4- 6 Graphite Epoxy Heating Results........................................................................ 93
Table 4- 7 Polyester / glass heating results..........................................................................93
Table 6-1 Loaded Cavity resonant length and estimated heating sites................................147
Table 6-2 24-ply curing results........................................................................................... 150
Table 6-2 48-ply curing results........................................................................................... 152
Table A - 1 Jumper Setting for Data Acquisiton Board....................................................... 190
Table A- 2 Data Acquisition Board Terminals....................................................................191
Table B-l Graphite/ epoxy heating d ata.......................................................................... 196
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF FIGURES
Figure 1-1 Control Loop for the Single-mode Resonant processing System..................... 11
Figure 1- 2 Overall Control Logic for Processing System....................................................12
Figure 1-3 Knowledge-based System Interface Logic..........................................................13
Figure 2- 1 Feedback control system showing traditional and non-traditional control
m ethods........................................................................................................................... 18
Figure 3- 1 Cylindrical coordinate system.............................................................................33
Figure 3- 2 Electric field pattern for TE-modes......................................................................42
Figure 3- 3 Electric field pattern for TM-modes................................................................... 43
Figure 3- 4 Mode chart for TE-modes....................................................................................44
Figure 3- 5 Mode chan for TM-modes.................................................................................45
Figure 3- 6 Q-curve calculation using half power point method...........................................48
Figure 4-1 Sample Placement................................................................................................ 62
Figure 4- 2 Frequency Shift Measurement.............................................................................64
Figure 4- 3 Electric field diagnostic holes: a) front view, b) side view, c) top view...........65
Figure 4- 4 Axial electric field pattern for empty cavity........................................................69
Figure 4- 5 Axial electric field pattern for Teflon loaded cavity (a) Elevated (b)
Lowered...........................................................................................................................70
Figure 4- 6 Axial electric field pattern for Graphite / epoxy at perpendicular fiber
direction (a) Elevated (b) Lowered................................................................................. 71
Figure 4- 7 Axial electric field pattern for Graphite / epoxy at parallel fiber direction (a)
Elevated (b) Lowered...................................................................................................... 72
Figure 4- 8 Axial electric field pattern for Nylon (a) Elevated (b) Lowered.........................73
Figure 4- 9 Axial electric field pattern for Polyester / glass (a) Elevated (b) Lowered
74
Figure 4-10 Radial electric field pattern for empty cavity..................................................... 75
Figure 4-11 Radial electric field pattern for Teflon (a) Elevated (b) Lowered.....................75
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 4-12 Radial electric field pattern for Graphite / epoxy at perpendicular fiber
direction (a) Elevated (b) Lowered................................................................................. 76
Figure 4-13 Radial electric field pattern for Graphite / epoxy at parallel direction (a)
Elevated (b) Lowered......................................................................................................77
Figure 4-14 Radial electric field pattern for Nylon (a) Elevated (b) Lowered.....................78
Figure 4-15 Axial electric field pattern for Polyester / glass (a) Elevated (b) Lowered
79
Figure 4-16 Frequency shift measurement for Graphite/ epoxy perpendicular fiber
direction (a) Elevated (b) Lowered................................................................................. 80
Figure 4-17 Frequency shift measurement for Graphite / epoxy parallel fiber direction (a)
Elevated (b) Lowered...................................................................................................... 81
Figure 4-18 Frequency shift measurement for Nylon (a) Elevated (b) Lowered............... 82
Figure 4-19 Frequency shift measurement for Polyester / glass (a) Elevated (b) Lowered 83
Figure 5-1 Components of a Typical Single-mode Resonant Cavity. From clock wise,
shorting plate and drive, cavity body, base plate with fingerstock, coupling probe 101
Figure 5- 2a Schematic of manually operated single-mode resonant Cavity......................102
Figure 5- 2b Picture of manually operated single-mode resonant Cavity............................103
Figure 5- 2c Drive of manually operated single-mode resonant Cavity.............................. 104
Figure 5- 3a Schematic of automated single-mode resonant cavity....................................105
Figure 5- 3b Picture of automated single-mode resonant cavity........................................ 106
Figure 5- 3c Drive of automated single-mode resonant cavity...........................................107
Figure 5- 4 Stepper Motor and Driver.................................................................................. 109
Figure 5- 5 Program Logic for Stepper Motor D river.........................................................I l l
Figure 5- 6 Lab View Program Version of Figure 5-6........................................... I l l
Figure 5- 7 Linear Motion Potentiometers........................................................................... 113
Figure 5- 8 Schematic of External Circuit........................................................................... 114
Figure 5- 9 Picture of Automated Microwave Processing System..................................... 115
Figure 5-10 Magnetron Power versus Voltage Calibration for Analog Control
117
Figure 5-11 Control loop from Chapter 1 ........................................................................... 120
Figure 5-12 Simplex Diagram............................................................................................. 127
Figure 5-13 Simplex Logic Flow Sheet.............................................................................. 129
Figure 5-14 Mode Selection Program Logic....................................................................... 133
Figure 5- 15 Program Flow Logic (also in Chapter 1 )........................................................139
Figure 5-16 Low Power diagnostic system.........................................................................141
Figure 5-17 Logic for Interface with planner (Figure 1-2).................................................145
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 6- 1 Curing temperature profile for 24-ply graphite epoxy............................154
Figure 6- 2 Power profile for curing 24-ply graphite epoxy composite............................. 155
Figure 6- 3 Cavity length and probe depth for curing 24-ply graphite epoxy.................... 156
Figure 6- 4 Curing temperature profile fro curing 48-ply graphite epoxy.........................157
Figure 6- 5 Cavity length and probe depth for curing 48-ply graphite epoxy.................... 158
Figure 6- 6 Power profile for curing 48-ply graphite epoxy............................................... 159
Figure A - 1 Stepper Motor and Drive Component............................................................. 184
Figure A- 2 Stepper Motor Driver TTL Circuit.................................................................... 185
Figure A- 3 Wiring Schematic for Stepper Motor Drivers..................................................187
Figure B- 1 Dual-Couplig Single mode Resonant Cavity......................................... 191
Figure B- 2 Sample placement with thermal paper.............................................................. 193
Figure B- 3 Temperature probe placement...........................................................................194
Figure B- 4 Top probe - graphite epoxy at CL=19.1 cm .................................................... 197
Figure B- 5 Side probe - graphite epoxy at CL=19.7 c m ................................................... 197
Figure B- 6 Both porbes graphite epoxy at CL=19.7 c m ................................................... 198
Figure C - 1 Global Composite Manufacturing Architecture...............................................201
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 1
INTRODUCTION
Microwave heating is a form of electroheating which spans the frequencies of
300MHz to 300GHz in the electromagnetic spectrum. The standard microwave
frequencies for industrial applications range from 400 MHz to 40 GHz in different parts
of the world(Pozar, 1991). In industrial applications in the United States the standard
frequencies are 915 MHz and 2450MHz, while other frequencies ranging from
2450MHz to 8000 MHz are used in research applications (Metaxas, 1992). Using the
2.45 GHz frequency, microwave heating of polymer composite materials has been
investigated in the waveguide, multimode oven and in the single-mode resonant
applicator (Adegbite et al. 1995; Adegbite et aL 1993; Adegbite et al. 1992; Adegbite et
al. 1992; Yunchang et al. 1995; Yunchang et al. 1995; Shidaker et al. 1995; Shidaker et
al. 1995; Shidaker et al 1995; Wei et al 1991; Lee and Springer, 1984; Lee and Springer
1984; Dhulipala et al. 1992; McNeil et al. 1992; Wei et al. 1991; Agrawal and Drzal,
1989; Jow 1988; Wei et al 1989; Wei et al 1992; Wei et al. 1992; Wei et al. 1992; U.
Hottong et al. 1991; Hawley and Wei, 1991; Wei et al. 1990; Fellows and Hawley, 1992;
Fellows et al. 1993; Fellows et al. 1994; Mijovic and Wijaya, 1990; Mijovic et al. 1992;
Gourdenne, 1982; Lewis et al. 1987; Jow et al. 1987; Jow et al. 1989; DeLong et al.
1989; Lewis et al. 1988; Vogel et al. 1989; Wilson and Salemo, 1978; Strand, 1980;
Gourdene et al. 1980; Gourdenne and Van, 1981; Karmazsin and Satre, 1985; Jow et aL
1988; Jullien and Valot, 1985; Wei et al. 1990; Chen and Lee, 1989; Mijovic and Wijaya,
1990).
Microwave heating, unlike conventional heating is dependent upon the applicator
and sample geometry and properties. Also, unlike conventional heating it involves
energy absorption and conversion to heat rather than heat transfer through conduction and
1
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2
convection. For non-magnetic materials, microwave power absorption is a function of
the microwave power excitation frequency, complex permittivity of the material, and the
magnitude of the electric field strength inside the material. The microwave excitation
frequency is typically constant and determines the depth of wave penetration into the
material. This power absorption relationship is described by Poynting’s theorem as
shown in Equation (1-1) (Pozar 1991).
The absorbed microwave energy excites the molecules of the sample through
dipole rotation and ionic and ohmic conduction which results in the conversion of the
microwave energy into heat (Lewis 1992). Thus, in order for a material to be heated by
microwaves it must have a complex permittivity which is a description of the dipolar,
ionic or ohmic properties. The complex permittivity is described mathematically as a
complex quantity. See Equation (1-2).
P = ^6)£ o£"\E\2
(1-1)
£ = £ ~ J£
_ //
£
=
_ // ,
t
£
a
U -2)
£ 0 CO
where:
P
= microwave power absorbed
£0
= permitivity of free space
co
= angular frequency
e"
= dielectric loss factor
£
- complex permittivity
E
= electric field strength
c
= electrical conductivity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3
The real part is called the dielectric constant, which is related to the energy stored
in a material, and the imaginary part is called the loss factor, which is related to the
energy dissipated in the material as heat through the motion of dipoles and charges. The
loss factor is a function of material chemical and electrical properties, frequency,
temperature and pressure. Materials with a higher loss factor are more easily heated by
microwave energy.
In general, materials undergo physical and chemical changes during heating. In
microwave heating, these sample changes can significantly alter the coupling between
the sample and the electromagnetic fields in such a way that process adjustments are
required to compensate for them. In microwave processing, the type of applicator
defines what processing adjustments must be made.
Three different types of microwave applicators are used: the waveguide, the
multimode oven and the single-mode resonant applicator. A waveguide is a rectangular
or cylindrical hollow pipe which can be used to guide electromagnetic waves. For a
given waveguide, the type and number of modes that can be excited are fixed. Thus, the
waveguide is not controllable to compensate for varying material changes such as the
size, shape and especially, changes in complex permittivity.
The commercial home microwave oven is what is known as a multimode oven
(Asmussen, 1987). In a multimode oven, several electromagnetic modes are randomly
excited simultaneously for a given applicator volume. Mode stirrers are sometimes used
to optimize the excitation of theses modes in the multimode oven (Huack, 1969). The
intent is to generate several electromagnetic heating modes such that varying sample
parameters during heating are randomly compensated for without adjusting the
multimode oven. For a given multimode applicator, the various modes that can be
excited may be known, however, the type of modes that are excited at any time are
unknown and cannot be controlled. Similar to the waveguide, this restricts the processing
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4
capability of the multimode oven to samples such as those that do not vary very much in
time and space.
A single-mode resonant applicator is designed such that different electromagnetic
modes are excited one at a time, by either fixing the frequency and varying the cavity
volume, or fixing the cavity volume and varying the frequency. The number of excitable
modes is only limited by the length of the cavity or the frequency range of the microwave
source. For a given single mode-resonant applicator operating at a fixed microwave
frequency, a finite number of electromagnetic modes can be selectively excited, but one
at a time, and controlled by the adjustment of the cavity volume. This allows for
selective and controlled heating and a potential for application to a wider range of
samples of varying chemical and electrical properties and shapes.
Using the waveguide, Springer and Lee (Lee and Springer, 1984) processed
unidirectional 32-ply, continuous graphite epoxy laminates. They reported that only
unidirectional composite materials can be heated with microwaves using linearly
polarized TEM waves with a polarization angle of 90 degrees. Using a multimode or
commercial microwave oven, Lee and Springer (Lee and Springer, 1984) investigated the
processing of graphite / epoxy and glass / epoxy laminates. They reported that curing
glass epoxy was effective, but the curing of multidirectional graphite epoxy composite
was not possible and the curing of unidirectional graphite epoxy depended upon the
polarization angle.
Chen and Lee(Chen and Lee, 1989) studied the cure of graphite / epoxy and
graphite / PEEK(polyether ether ketone) in a single-mode resonant cylindrical cavity
using a TE(112) mode at 2.45 GHz. They concluded that the coupling of interactions
between microwave energy and composites depended on the fiber orientation and sample
geometry in a complex manner.
Using a single-mode resonant cylindrical cavity, Vogel (Vogel eL al. 1989)
demonstrated that a 3- inch square, 24-ply graphite epoxy composite can be processed
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
5
with low input power, and that the heating rate and uniformity were dependent upon the
electromagnetic processing modes. Using a single-mode resonant cavity, Wei(Wei,
1989) showed that both unidirectional and cross-ply, thin and thick section graphite
epoxy composite materials could be successfully processed using hybrid modes. Also
using the single-mode resonant cavity, Fellows (Fellows, 1991) successfully processed
polyimide graphite composite panels and planar and complex shaped polyester glass
composite materials using a mode switching technique.
Using the single-mode resonant cavity, benefits of microwave processing of
polymeric composites were reported to range from enhanced mechanical properties, such
as in enhanced glass transition temperature of cured epoxy (Wei et al. 1992; Wei et aL
1990), enhanced fiber / matrix interphase properties (Agrawal and Drzal, 1989) faster
processing times, and capability to control temperature excursions(Jow, 1989; Wei,
1992). Although these benefits are significant in the composite processing industries,
microwave processing of composites is still concentrated in the research arena.
Currently in composites processing, the microwave research has been focused in
the demonstration of the microwave technology at the lab scale, using specialized
tooling(Teflon, ceramic), and through intensive and cumbersome manual operations. In
processing, the microwave cavity was operated as an open-loop system where a seasoned
operator was the necessary link to close the control-loop. A typical processing activity
included; 1) the continuous tuning of the cavity by the manual rotation of dial knobs to
adjust the cavity length and probe depth to minimize the reflected power; 2) the frequent
selection of new cavity modes by carefully adjusting the cavity length and probe depth to
new locations, while tuning the cavity and manually modulating the input power to
achieve uniform heating; 3)temperature control by the automatic on/off control of an
electronic switch to direct the microwave to or away from the cavity. Data acquisition
was done as a separate activity using both analog and digital ( RS- 232) interface which
required complicated program device drivers. Hence, the cavity was operated as an
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6
independent device from the external circuit, data acquisition unit and processing results
varied from one operator to the other and optimization was difficult at best.
In order for the microwave technology to be recognized as a viable technology, it
must be demonstrated as a process by addressing processing issues such as specialized
tooling, scalability, controllability and automation. Microwave heating in general is an
electronic process which typically contains extensive electrical components and control
requirements that are unique to the technology. In the single-mode resonant cavity
technology, while controllability is one of the attractive attributes it is also a challenge to
accomplish for the advancement of the technology. As such, automation and process
control which include, feedback systems, data logging, and instrumentation are essential
requirements in the advancement of the single-mode resonant cavity as a process.
In this work, a control system was designed and built as a process control
program was developed and implemented. Two different control software programs were
developed; one included complete control logic to meet all of the process control
objectives, and the other included only data acquisition, hardware and interface
instructions to facilitate an interface with a knowledge-based system planner. The
control logic and the overall control loop are shown in Figure 1-1 and Figure 1- 2, and
the KBS-interface control logic is shown in Figure 1-3.
Finally, the complete system was demonstrated by curing thin and thick section
graphite/epoxy composite materials, and the partial system was demonstrated by
interfacing with a Knowledge-based system(KBS) planner to control the processing of
epoxy/graphite composites.
In a single-mode resonant applicator, the microwave cavity
volume and the coupling probe depth are adjustable variables used to control the electric
fields in the cavity. Hence, the automation of this system included the design and
fabrication of an applicator with mechanized drives, instrumentation and a closed-loop
feedback control system to meet the following control objectives:
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.
Efficient coupling (Mode tuning) - Maintenance of cavity resonance in the
presence of dynamic sample and process changes by minimizing the
reflected power from the applicator.
2.
Uniform heating- Control of electromagnetic modes such that energy is
coupled optimally to all sample regions.
3.
Controlled heating - Regulation of input power such that the sample
temperature and reaction exotherm is controlled and maintained to within
5 °C of the cure setpoinL
In general, the significance of this work is realized in the advancement of the
single-mode resonant technology by proving it to be a practical process, thus enabling the
this technology to be used in the processing of polymer composites and in other
applications such as dielectric analysis, plasma and ceramics processing. Specifically,
significance would be realized in:
1.
The design and implementation of hardware and software for the
automation of the single-mode resonant cavity for the convenient and
practical operation of it as a process.
2.
The development and implementation of a closed-loop feedback
control system using traditional and non-traditional control
methodologies, to control the electromagnetics inside the cavity
during the microwave curing of composites by mode tuning, mode
switching and power control to achieve uniform and controlled
heating.
3.
The use of a 2-dimensional mathematical search technique called
simplex as a novel approach for automatically tuning the cavity, by
simultaneously manipulating the cavity length and probe depth, to
achieve efficient tuning as opposed to the typical univariate or manual
methods.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
8
4.
The development of empirical loaded cavity correlations to understand
and control the electric fields inside the cavity to achieve uniform
heating.
5.
The analog regulation of the input power source using proportionalintegral-derivative(PID) controller as opposed to the typical
on/off controller (where in the off position the power was wastefully
directed to a dummy load), to control the exothermic temperature
excursions to within 5° C.
6.
Development and implementation of a control system for interfacing
with a knowledge-based(KBS) planner.
7.
The design and fabrication of an automated single-mode resonant
cavity with novel mechanized drives for cavity length and probe
depth adjustments, and instrumentation for automatic position
measurements.
8.
Automatic data acquisition for fast, reliable and convenient data tracking
and maintenance.
Dissertation Layout
In chapter 2 the concepts of traditional and non-traditional process control
methodologies are discussed with emphasis on differences between the two methods and
when each method is applicable. A literature review of the current state of automated
composite materials processing in an autoclave using these methods are also presented.
In general, composite material fabrication is a manual operation which incorporates
empirical and fundamental knowledge. In the automation, emphasis is placed on the
development of cure cycles dependent upon available process models and automatic
sensing devices.
In chapter 3, fundamental theoretical background of electromagnetic and
microwave processing is discussed. Derivation of field equations and electromagnetic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
9
mode classifications, theoretical electric field plots, and their application to a resonant
cavity are presented. Empty and loaded cavity solutions for simple cases are also
presented. Additionally, the application of these fundamentals to the microwave
processing of anisotropic and inhomogenous composite materials is also discussed.
In chapter 4, experiments and results for empty and loaded cavity
characterizations are presented. These experiments include low power diagnostics for
empty and loaded cavity modes and the heating of previously cured composite materials
for mode characterization. The results in this chapter define the empirical foundation
upon which the uniform heating control strategy is developed.
In chapter 5, the process control strategies used to meet the different control
objectives are presented. The mode tuning controller, was treated as a mathematical
problem in which a 2-dimensional simplex search technique was used. This approach is
significantly unique because it uses a two dimensional tuning technique rather than a one
dimensional technique, by simultaneously adjusting the cavity length and probe depth.
Because loaded cavity solutions for non-homogenous and non-isotropic materials like
composites are computationally prohibitive, an empirical method was used to develop the
uniform heating controller. For the power controller , PID control software was used to
manipulate the input power. Each control strategy was developed independently and
then integrated to form the overall control system.
In chapter 6, results of applications using the developed automated cavity and the
developed process control software are presented for the curing of 24-ply and 48-ply
graphite epoxy composite material. During processing, the efficiency of automatic cavity
tuning, mode selection for uniform heating, and optimum temperature control are
evaluated.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
10
Finally, results and conclusions are summarized in chapter 7 and
recommendations and future work are discussed in chapter 8. Control program
documentation, detailed hardware instrumentation and wiring, dual-coupling results,
KBS-interface and other supporting results are discussed in the appendices.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
fro
3
T (sclpoint)
Pow er Controller
M agnetron
O
o
a
§
r
o
0
*T3
o’
>1
Er
ct>
00
H•
T cm pcram re (T)
Tem perature Sensor
Luxtron
AT (sclpoint)
T
M ode Selection
C ontroller
C avity Length
Stepper M otor
I’robe Depth
Stepper M otor
13
(2(T
1
3
T em perature G radient
(AT)
o
CL
ct>
P0
n>t
V
o
a
Ta3
n(/)>
00
a
f/Q
00
0>
3
3
P ref (sclpoint)
M ode Tuning
C ontroller
Tem perature Sensor
Luxtron
C avity Length
Stepper M otor
Probe Depth
Stepper M otor
R eflected Pow er
(W )
Reflected Power
M eter
12
Acquire and Scale Data
Tmax > Spt
Yes
Power Controller
No
Yes
AT > Spt
Mode Selection
Controller
No
Yes
Pref > Spt
Mode Tunin:
Controller
No
Figure 1-2 Overall Control Logic for Processing System
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
13
►
Acquire and Scale Data
y
Temperature
Power
Mode
KBS Planner
Power Level
Mode
Power Controller
Mode Tuner
Figure 1-3 Knowledge-based System Interface Logic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 2
REVIEW OF PERTINENT LITERATURE
2.1 Introduction
Microwave processing of composites in a single-mode resonant cavity has
been studied as an alternative to thermal processing methods( Fellows, 1992 ; Jow,
1988 ; Lee, 1984; Vogel, 1989; Wei, 1991; Adegbite et. al. 1995). Results have
shown benefits that include enhanced mechanical properties, increased glass
transition temperature, increased fiber matrix properties and increased reaction rates
(Jow, 1988; Wei, 1991). However, the concept of control of microwave curing of
composites in a single-mode resonant cavity is novel. The dynamics of the
microwave curing process is governed by complex non-linear, time variant
interaction between discrete electromagnetic modes and the material inside the
cavity. Hence mathematical models that describe the curing dynamics were found to
be incomplete and computationally intensive for control purposes. Process
monitoring capability of material related properties were also unavailable which
further complicated the accurate understanding of the cure dynamics for control
purposes.
This control problem however, is not unique and fits a class of control topics
that is an active area of research. Composite curing in an autoclave is an example of
a domain that shares similar control problem. Since the primary goal of this work is
to develop and implement a control system for the microwave curing process,
literature pertinent to control philosophies and applications in composites processing
in an autoclave are reviewed.
14
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
15
2.2 Control Philosophy
Control systems can be described as either closed loop or open loop
configurations which correspond to feedback and feedforward, respectively. In
feedback systems, the disturbance to the system is not measured but the output is
measured and used to regulate the input to the system. In the feedforward structure
the disturbance to the system is measured to produce a matching corrective action
and the error in the feedforward controller is not checked. Hence, the performance of
the controller may deteriorate with time because of parameter drifts(Auslander et al.
1974). However in the feedback structure there is no need to measure the
disturbance since the error of the controller is always checked against its desired
value. Hence, a feedback controller is flexible, relatively insensitive to external
disturbances, able to function in a changing environment and can even cope with
unanticipated disturbances(Auslander 1974). This marks the motivation for the use
of the closed loop, feedback control system in this work.
Historically, the first conventional feedback control device was the water
clock that was invented by the Greek Ktesibios in Alexandria Egypt around 3rd
century BC (Mayr 1970). The first mathematical model to describe plant behavior
for control purposes was J.C. Maxwell, of Maxwell’s equations in electromagnetics
(Antsaklis, 1994). In 1868 Maxwell used differential equations to explain instability
problems encountered with James Watt’s flyball governor. The governor was
introduced in 1769 to regulate the speed of steam engine vehicles.
Since the period
of the flyball governor, conventional control theory has been advanced with the use
of; frequency domain methods and Laplace transforms in the 1930s and 1940s; the
development of optimal control and state space analysis in the 1950s and 1960s;
optimal control in 1950s and 1960s; and stochastic, robust and adaptive control
methods in the 1960s to the concepts of non-traditional control methodologies today.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2.1
16
Conventional / Traditional Control
The term conventional (or traditional) control is used to refer to the theories
and methods to control dynamical systems which are described mathematically by
differential or difference equations. In order for the conventional controller to be
successfully implemented, the mathematical models must be accurate and simple
enough so that they can be solved in real-time. Today, the growing complex
structure of chemical processes due to the increasing demand for better energy and
materials management, has resulted in demanding control specifications for
increasingly complex dynamical systems.
This has introduced control problems that cannot be adequately described in a
differential of difference equation framework. The control problems mainly pertain
to the area of uncertainty due to poor or lack of knowledge or incomplete models to
avoid computational complexity. Examples of such systems include discrete event
manufacturing and communication systems(Antsaklis, 1994). To address these
control problems non-traditional control methodologies known collectively as
intelligent control systems were developed.
2.2.2 Intelligent Control
Intelligent control systems are non-traditional control methodologies that
were developed to address control problems that otherwise cannot be solved by
conventional control methods. They are designed to autonomously achieve high
level goals, while its components, control goals, plant models and control laws are
not completely defined(Antsaklis, 1994). At minimum, an intelligent control system
must have the ability to sense the environment and make decisions to control it.
Higher levels of intelligence may include the ability to recognize objects and events,
to represent knowledge in a model, and to reason about and plan for the future. In
advanced forms, intelligence provides the capacity to perceive and understand, to
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
17
choose wisely and act successfully under large variety of circumstances (Antsaklis,
1994). As such, there can be several levels of intelligence control systems.
Compared with the traditional control methods, intelligent control systems
typically use heuristics and empirical data in form of decision making units rather
than predictive models. For many intelligent control systems the controller
construction methodology is largely heuristic and based on certain principles from
artificial intelligence(Antsaklis, 1994).
Figure 2-1 shows a control loop for what
would be considered to be a traditional and non-traditional control system. The
construct of traditional feedback control is preserved while the function of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
18
controllers vary from a predictive one to a search one for the traditional and non-
Setpoint
Error
►
Controller
Controlled
Variable _
Process
Measured
variable
Traditional Method
PID
Process Models
Predictive
Non-traditional Meth
Heuristic
Empirical
v Decision Making
Figure 2- 1 Feedback control system showing traditional and nontraditional control methods
traditional approach, respectively. Hence, intelligent control can be described as an
enhancement of traditional control methodologies to solve new challenging control
problems. Currently, there are several installed intelligent control systems that can
be found in the NIST’s (National Institute for Standards and Technology) real-time
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
19
control system implementations, where most of the systems are for aerospace and
military applications(Antsaklis, 1994). Composites materials fabrication is another
domain which presents a framework for intelligent control applications.
2.3 Composite Materials
2.3.1 Background
Composites are combined materials of two or more components with
properties superior to that of the individual constituent materials. The two
constituents are typically a fiber and a matrix, where the fiber is the load carrying
material and the matrix (polymeric material) holds the fibers in place and provides
the bonding between the fibers.
Fiber reinforced polymer composites are widely
used in aerospace, commercial, military, and other engineering applications. The
most significant advantage of composites is the high strength to stiffness per unit
weight compared with conventional materials like steel(Strong, 1989; Richardson,
1987).
2.3.2 Fabrication Methods
The goals for fabricating a composite part is to achieve maximum mechanical
properties which requires uniform extent of cure, ultimate part consolidation for
uniform resin distribution and minimum void content. Fabrication of composites
requires reaction of the matrix system and pressure for the consolidation of the fiber
and matrix into a composite part. The matrix can be a thermosets which are
generally liquid resins which undergoe exothermic reaction to achieve a crosslinked
network structure. In such systems heat is required to activate the reaction and
additional heat is generated as the polymerization reaction proceeds. The heat of
reaction can cause the temperature to rise beyond the capacity of the external heating
source, whereby in uncontrolled systems runaway reactions can occur and cause
thermal degradation of the part. The matrix can also be a thermoplastic which are
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
solids which are melted, formed, and then cooled to achieve a solid structure.
Thermosets have historically been the principal matrix material for composites
although thermoplastics use is now increasing in many applications(Strong 1989).
2.4 .Control in Composite Processing -Autoclave
2.4.1 Composites Process Modeling
To overcome this problem a very slow heating rate is typically used which
leads to very long cycle times and does not always guarantee uniform heating,
especially in thick-section parts. Typically, a predetermined time dependent
processing plan is developed off-line and used to control the part temperature and
pressure during the cure cycle. The characteristics of the cure cycle depend upon
material properties of the resin, fiber, fabrication technique and mold geometry.
Hence complete mathematical models that describe the dynamics of the cure cycle
are required.
The most complete models of autoclave process curing was developed by
Loos and Springer in l983(Springer, 1987)which was later improved by (Kardos,
1983; Halpin, 1983; Gutowski, 1987; Dave, 1987). The model consist of five sub­
models for describing the thermochemical effects, flow consolidation, residual
stresses, void formation, strength and modulus. The inputs to this program are mold
geometry, material properties, applied temperature and pressure, and vacuum bag
pressure as a function of time. The outputs include temperature, pressure, degree of
cure, resin viscosity in the part as a function of time and position and the number of
compacted prepreg plies as a function time.
The strength of this model resides in its ability to predict many aspects of the
curing process ranging from heat transfer and chemical reaction during processing,
to strength and modulus of the final pan. However, because of the intensive
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
21
computations required these models were used in the simulation of the cure behavior
and not for real-time computation for control purposes. Additionally, the limited
sensing capabilities of other sample properties such as extent of cure and viscosity in
the autoclave also complicates the accurate implementation of these models.
2.4.2 Conventional and Conceptual Control Methods
A conventional real-time feedback computer control for the autoclave that
was developed in 1981 by Applied Polymer Technology Incorporated (Hinrichs,
1984). It was an automated system for controlling the curing of composite structures
through an interactive, computerized feed-back system. It included real-time sensors
for part surface point temperature and gradient (thermocouples), autoclave ambient
temperature(thermocouples), ambient pressure(pressure transducers), and material
viscosity (ultrasonic technique). The control decisions were made by comparing the
measured values to pre-determined optimal values. The pre-determined optimal
values were temperature vs. time, pressure vs. time, and viscosity vs. time data
which were generated from fundamental cure process models. Up to date this is the
only patented automated autoclave system for the curing of composite materials
using conventional feed-back control technique.
In a theoretical approach (Wu, 1990) used knowledge based systems
techniques to develop automated cure cycles for composite manufacturing in an
autoclave. In this method process models and experience gained from past
operations were used to provide an initial operation profile and on-line adjustments
during the curing. Automation of the autoclave process was simulated using the
Loos and Springer models (Springer, 1987) and the process control was simulated by
the developed expert system. There are other developed systems which have been
verified by controlling an autoclave to cure a part.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
22
In another approach to autoclave automation, (Trivisano, 1992) developed a
method for the control and optimization of the evolution of the processing variables
during the fabrication of composite materials. The method allowed for the
computation of real-time heat transfer coefficients for each selected tool, the
prediction of the temperature changes as a function of the programmed air
temperature and the optimization of the cure cycle to minimize the difference
between the actual temperature and recommended values. During real time
processing, model predictions were compared with optimum values to achieve proper
control decisions. The significant feature of this architecture is the real-time
computation of the tool thermal properties which is used optimize the cure cycle.
This may be described as the adaptation of the hardware conditions to optimize the
predetermined cure cycles.
In the other control and automation approaches, the strong dependence on
mathematical models alone was not emphasized while the use of experiential
knowledge was studied. These approaches utilized the concepts of knowledge-based
systems techniques or intelligent control methods to study the use of experiential
knowledge alone or integration of experiential knowledge with mathematical models
in the automation and control of the autoclave.
2.4.3 Intelligent Control Methods
In 1986 Servais, Lee and Browning(Servais, 1986) surveyed the limitations
and benefits of the various possible approaches of autoclave control based upon non­
fundamental methods. Experimental or trial and error methods were found to be
expensive, inefficient, time consuming, inflexible, impractical for optimization, and
not practical for on-line control. Thus, suggesting the integration of mathematical
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
23
methods, and expert systems techniques as the most efficient approach for
describing and controlling the curing process in a comprehensive manner.
Roberts(Roberts, 1987)presented a comprehensive architecture in which
experience, intuition, and mathematical models were integrated in the automation
and control of the autoclave. The control architecture included a cure model, a model
validation unit, an expert system, and a cure device control unit. Each module in the
control architecture was assigned a specific task and was developed independently.
The cure model unit was responsible for predicting the optimum cure profile to be
implemented by the expert system.
The expert system was the cure operator which was allowed to adjust the
cure cycle based on a set of rules drawn from a model. It also had the capability for
determining when there is a need for operator intervention. The device controller
was responsible for controlling the process and identifying anomalies in the sensed
data that deviated from the predicted cure profiles. It also was responsible for
interactively generating process history data files on-line. This unit used the
concepts and equipment of the automated control system developed by Applied
Polymer Technology(Hinrichs, 1984).
This is one of the most comprehensive architectures in the automation of the
autoclave. Only part of the cure model had been successfully proven, but the total
integration of all the modules had not been implemented.. One of the unmentioned
difficulties that could be experienced is software interface issues, computing power
requirements if all of the modules were implemented on one computer, and
communication issues if all the modules were implemented on multiple computers.
The approach is very robust and implementation will be challenging.
In the past ten years some of these non-traditional automation and control
concepts have been implemented for composite curing in an autoclave. One is a
prototype expen system for controlling the autoclave cure process Lee(Lee, 1987).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
24
The control knowledge base contained declarative rules that were derived
from an expert about the processing conditions and constraints. This was different
from a conventional type approach in which it would require explicitly prescribed
temperature and pressure cycles.
In another approach a rale based expert system was developed for controlling
the autoclave temperature and pressure during cure of fiber reinforced thermosetting
composite materials. Rules were established to ensure that the temperature and
pressure remained within required limit the residual stresses minimized. The inputs
to the expert system were the measured instantaneous autoclave temperature and
pressure, the composite midpoint and surface temperatures, composite thickness, and
ionic conductivity.
These inputs were examined by pre-established rules to control the autoclave
settings. The interfaces and rales were incorporated in an algorithm called SECURE
and was written in "C" language and installed on a Macintosh computer. The system
was verified through model simulations and by controlling an autoclave successfully.
In another approach a control strategy was derived from a methodology called
a "self directed" approach(LeClair, 1989). In this approach the process was divided
into a series of stages and rales were derived from knowledge and intuition of experts
to describe each stage. Included in these rales are knowledge about how to change
control parameters from the data provided by the sensors, and the conditions
required for successful completion of each cure stage. The advantage of this system
is that it can be developed without detailed knowledge of all the events that occur in
the part. The success of these non-traditional methods is depended upon the quality
and quantity of knowledge and the accuracy of data interpretation by the knowledge.
These non-traditional automation and control methodologies have classical
drawbacks in that they tend to be inefficient, inflexible, and impractical for
optimization. This may be credited to the inherently limited size of the knowledge
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
25
base and the control strategies which tend to be specific and do not allow for a wide
variety of processing opportunities.
A system in which both mathematical models and knowledge based systems
techniques are integrated to control a process was developed by Pardee (Pardee,
1987), at Rockwell International. The system was used for the pyrolysis of carboncarbon composites. It runs on two Macintosh computers in Allegro Common Lisp.
It consist of a control system workstation which provides information about real­
time process conditions, and a materials workstation which provides calculated real­
time material properties, predicted future properties, estimated current and future
index, and an on line expert description and interpretation of the current process
state. It has a capability to interpret sensed data and to solve a set of eight coupled
differential equations for reaction kinetics information.
2.5 Control in Composite Processing -Microwaves
2.5.1 Process Modeling
The key in microwave process model development is dependent upon the
ability to successfully develop a power absorption model to describe the interaction
of the electromagnetic fields and the sample(see chapter 3). One of the first
microwave curing process models was developed by Lee and Springer (Lee and
Springer, 1984) using a simplified power absorption model. The output of this model
was temperature distribution, the resin viscosity, degree of cure of the resin, resin
content of the composite and bleeder cloth during microwave curing. Another
process model was also developed using a more rigorous five-parameter estimation
power absorption model (Wei, 1991).
There are other process modeling
developments underway to completely describe the microwave curing
process(Sudaram, 1994 ). However, even with the simplified power absorption
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
26
models, these process models are computationally intensive and applicable to unique
processing conditions. The only known complete microwave power absorption
model is that developed by IBM(Intemational Business Machines). Unfortunately,
published literature is not available about this system at this time.
2.5.2 Control in Microwave Processing
The extent of literature found is a clear indication of how widely the
intelligent control problem has been studied, especially in the curing of composites in
an autoclave. As previously mentioned, the concept of control in the microwave
curing of composites is novel. The only known and published automated single­
mode resonant cavity was that developed by Alliouat(Alliouat, 1990) and his coworker in France for sintering ceramic materials. The control system was based upon
elements of intelligent control for regulating the input power and for tuning the
cavity.
A gradient search method was used for tuning the cavity where only sensed
information about the cavity length and reflected power were required. Components
of this processing include an infrared pyrometer for measuring the surface
temperature, detectors for sensing the input, reflected and absorbed power.
Controlled parameters were the microwave power supply by analog output, and
stepper motors for adjusting the cavity volume. The automatic sensing and control
were facilitated by a data acquisition board which resides on an Apple II computer.
2.6 Data Acquisition
2.6.1 Autoclave
In the autoclave process, typically sensed data are temperature and pressure of
the oven. However, unmeasurable process states such as extent of cure and changes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
27
in viscosity are the needed information. Various cure monitoring methods have been
studied, which include; ultrasonic sensors to measure viscosity and porosity to
describe the process cure states(Tittman, 1988); the use of electrodes mounted as
roven threads to sense resin flow properties in order to describe process states(Walsh,
1990); the use of acoustic ultrasonic technique to measure viscosity and relate it to
process sates (Saliba, 1992 ); the use of infrared transmitting optical fibers to
measure the disassociation and formation of chemical bonds (Young, 1988); and
dielectric analysis to measure chemical changes due to molecular motions (Ciriscioli,
1989 ; May, 1983; Day, 1986;Kranbeuhl, 1987).
Of all of these methods the dielectric technique has been shown to be most
promising for cure state monitoring. At the current state of the technology it can only
provide "event" information and not historical information during the cure cycle.
(i.e. where viscosity is minimum or when cure is completed). The limited sensing
capability combined with the computationally intensive process models provides a
framework for studying various control methods.
2.6.2 Microwave
Similar to the autoclave process, sensing technology in microwave processing
is also limited. There are power meters for sensing the power incident towards the
cavity and that reflected from the cavity. The sensed reflected power is used to tune
the cavity to resonance, and the sensed input power is used to regulate the amount of
power coupled to the cavity. The only direct sample parameter measured is the
temperature. This is done using fiber optic thermometry which is an invasive sensing
method(see Appendix B). Unlike the autoclave process, the desired process
parameter for control is the microwave curing process is the magnitude of the electric
field strength (see chapter 3). Currently, electric field measurement technology is
not commercially available and modeling techniques are highly complex and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
28
computationally extensive, especially for control purposes. For cure state
monitoring, on-line dielectric analysis has been studied at the research level using the
single-mode resonant cavity (Jow, 1989). However, this technique is not available at
this time for on-line cure state measurement
2.7 Summary
Although several control issues have been widely studied in the autoclave
composite curing, control is a very novel concept in the microwave curing process.
The control goal set forth in this dissertation is therefore, a significant challenge
which will result in a noteworthy technical contribution in the field of control and in
microwave processing, especially in the curing of composites.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 3
ELECTROMAGNETIC THEORY AND MICROWAVE PROCESSING
3.1
Introduction
In this chapter relevant fundamental and theoretical background of
electromagnetics and microwave processing are discussed. Derivation of field equations,
electromagnetic mode classifications, theoretical electric field plots and their application
to a resonant cavity are presented. Empty and loaded cavity solutions for simple cases
are also presented along with the application of these fundamentals to the microwave
processing of anisotropic and inhomogenous composites. The discussion in this chapter
provides the fundamental understanding from which the automation and control software
programs were developed. It addresses the concept of modes, mode tuning, cut-off
frequency, theoretical electric field patterns in the cavity, and empty cavity analysis
which are important in the understanding of the loaded cavity. Throughout the
derivations, vector quantities will be denoted by a boldface.
3.2 Electromagnetic Theory
3.2.1 Maxwell’s Equations
Electromagnetic theory at the macroscopic level is embodied in the mathematical
equations known as Maxwell's equations (Harrington, 1961). The differential form of the
time-varying Maxwell's equations in the general form are shown in Equation (3-1).
29
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30
V
X
*
.
1
V
E
* 1
X
II
V
V
•
D
H
B
=
P
=
—
=
0
D =
dD
at
+ J
J =
£E
OE
B = pH
where
E is the electric field strength (Volts / meter)
D is the electric flux density (Coulombs / square meter)
H is the magnetic field strength (Amperes / meter)
B is the magnetic flux density (Volts-seconds / square meter)
J is the electric current density (Amperes/ square meter)
p is the electric charge density (Coulombs / cubic meter)
e is electric permittivity of the medium (Coulombs / Volt meter)
a is electric conductivity of the medium (Coulombs - meter / Volt - seconds)
p is the magnetic permeability of the medium (Volts - seconds / Ampere - meter)
In these equations there are four conditions that must hold at any surface of
discontinuity(Young, 1952), which are:
1.
The tangential component of E is continuous
2.
The discontinuity in the normal component of D is a function of
the surface charge density
3.
The discontinuity in the tangential component of H is a function
of surface current
4.
The normal component of B is continuous
Assuming fields having sinusoidal or time-harmonic dependence, phasor notation
can be used for convenience. Thus, all fields will be assumed to be a complex vector
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
with an implied e^0* time dependence. Therefore, the time derivative in the Maxwell’s
equation can be replaced by the derivative of
as shown in Equation (3-2)
d jox
jax
— e
= jco{e
)
*
(3-2)
The Maxwell’s equations can be written in phasor form by substituting Equation
(3-2) for the time derivative term in Equation (3-1), and suppressing the e1°* term to
give Equation (3-3). So the time derivative in the Maxwell's equations is replaced by jco
as shown in Equation (3-3).
V x E = - jaiB
V•D=p
D= £E
V x H = jcoD+J
J= oE
V • B = 0
B = pH
(3-3)
3.2.2 Maxwell’s Equations in a Source Free Homogeneous Region
Assuming wave propagation along the z-axis in a source free, linear isotropic,
homogeneous region, the Maxwell's equations can be rewritten from Equation (3-3) as
shown in Equation (3-4). Note that these assumptions imply that the electric charge
density and the current charge density are both equal to zero. In writing these equations
the flux density quantities, B and D are replaced by the field strength quantities as
defined in Equation (3-3).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
32
V x E = -jcofiH
V x H =jO)£E
(3-4)
V • B=0
V *E = 0
3.3 Electromagnetic Fields in a Cylindrical Region
3.3.1 Empty Cavity Solutions
This form of Maxwell’s Equations (3-4) is the most useful form, from which
modal solutions can be derived. But, first a coordinate system must be defined since
modes are solutions to Maxwell’s equation in a defined structure. A cylindrical
coordinate system Figure (3-1) is defined since the solutions are sought for a cylindrical
cavity. The two curl Equation (3-4) can be further decomposed into the vector,
transverse, field z-components in cylindrical coordinates as shown in Equation (3-5). In
these equations the z-components on the right side of the equations are the solutions that
will be sought
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
33
Figure 3- 1 Cylindrical coordinate system
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
'i m ,
Er
£ dp
. i
i b 2e '
( . 1 1 d 2H 7
^ = -
J p .6 )£ p d p d z
pcae
l dE^
j ----------------------------t-----------a*-
PCH£ p d p d z
pco£
(3-5)
p dp
dz
The two curl equations are simplified into the form of the Helmholtz equation by
solving for either the E or H component in the two curl equations, then taking the curl of
the electric field (or the magnetic field) curl equation as shown in (3-6), and substituting
the curl identity in (3-7) to give (3-8).
V x V x E = - jco p V x H
= (-jc o p )(-jc o £ E )
=
(3-6)
oP'psE
V x V x A = V(V«A)-V A
V2E + ti>2/t£E = 0
(3-7)
(3-8)
V2H + Qp’pE R = 0
Equation (3-8) is the wave equation or the Helmholtz equation which, with the
appropriate boundary conditions yields an eigenvalue problem whose eigenfunctions are
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
35
Bessel’s functions and harmonic functions. These eigenfunctions describe the electric
and magnetic field components of a mode while the eigenvalues, index the modes and
describe the propagation characteristics of the mode. In solving these equations for a
cylindrical cavity the following assumptions are used. Additional assumptions are used
depending on the electromagnetic mode which the equation is used to describe.
Assumptions are(Pozar, 1991):
1. Tangential component of the electric field is equal to zero at
the cavity walls, i.e. E<])(r=a,<)),z)=0
2.
Fields must be finite everywhere, i.e. Bessel’sl’s function of
the second kind cannot be a solution since, when the argument
becomes zero the Bessel’s function of the second kind becomes
infinite.
3. Fields must repeat every 2 pi radians in (j)
4.
The electric field components at the top and base of the
cavity are equal to zero, i.e. Ep=0, (z=0, z=h) and E<j)=0, (z=0,
z=h)
3.3.1.1 Modes
There are two different types of modes that can propagate in a cylindrical cavity,
TE(transverse electromagnetic) or TM (transverse magnetic). TM modes are solutions to
the Maxwell's equations with the boundary condition that there are no longitudinal
magnetic field components while the TE modes are the solutions with the boundary
condition that there are no longitudinal electric field components. In other words, in TE
modes the electric field is aligned perpendicular to the direction of wave propagation and
in TM modes the electric field is aligned in the direction of wave propagation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
36
33.1.1.1 Transverse Electric Modes - TE Modes
For TE modes the electric field components are transverse and the magnetic field
components are parallel to the direction of wave propagation, which is the z-axis and thus
Ez=0. The field components from Equation (3-5) is reduced to Equations (3-9)(with the
substitution of Ez=0), where a solution to Hz is sought in the Helmholtz Equation (3-10).
E
’1 3 H A
P
e 9<p ,
yj
H
1
P
=
(
.
l d 2H.
pcoe dpdz
1 - 7 ----------------- - +
'
.
1 1 d2 H .
1--------------pcoe p 9<pdz
B
r e dp
(3-9)
'd^Hy
E=0
pcos
dt
V 2H + k2H = 0
z
z
(3-10
Using a vector potential function and separation of variables, a solution can be obtained
with the appropriate boundary conditions to give Equation (3-11) (Harrington, 1961).
Y (r,0,z) = B ^ p J J c ^ - r ) [ s m ( m P ) ] s m { ^ - z )
m=0,1,2,3....
p,n = 1,2,3....
where the TE mode field components are given in Equation (3-12).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(3-11
37
1 dyr
p dp
P
V1 17m
-p ) s in ( m p ) s m ( - ^ - z )
1 ”i1 aa
h
■p
*
=E2 Jm ( - ^ P ) cos(m p)sin(~ -z)
a
h
dp
Ez =0
=0
Wd \ } f
H.
P p p dp
X
=HlJ'm (—^ - p ) cos(mp) c o s ( ^ - z )
a
h
• 1i i d v
cope p dpdz
H _
$
Hm-
z
j
= H2 Jm (-J2m-p)sin(mp)cos(¥^-z)
a
h
1 d 2y /h d l2'y
cope dpdz
\
+P2v
=
(3-12)
H2Jm{ - MLp)cos(mp)saiir^-z)
J
J
a
h
copH^t
33.1.12 Transverse Magnetic Modes-TM-modes
For TM modes the electric field components are parallel and the magnetic field
components are transverse to the direction of wave propagation which is the z-axis, and
thus Hz=0. The field components from Equation (3-5) is reduced to equations (3-13),
where a solution to Ez is sought.
f
E
P
-2 ^
1 d2E7
J------------£
pcoe dpdz
.
( 1 1 dE.
\ p p dp
r \d E .\
(. i i < ? v
E{t)= J ------------------■
pcoe p dpdz
d E-
Hp =
Hp=~
p dp
(3-13
o2
pcoe
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
38
Using similar boundary conditions as in the TE modes solution, a final solution is derived
for the TM mode field components in the form of a vector potential as shown in (3-14).
X
P
¥ ( p ,p ,z ) = A m npJ m ( - ^ - p ) c o s ( m p ) c o s ( - ^ - z )
m = 0 ,1 ,2 ,3 ....
(3-14)
p ,n = 1,2,3.....
Where the TM mode field components can be simplified as shown in (3-15).
cops dpdz
. X
D7Z
= E^J (—^ p ) cos(m<p)sin(r-—:
a
h
cops p dpdz
= £ 2^ ( — p )sin (m p )sin (-^z)
a
h
Ez = - y — [
+/? V ]
cops \dz~
J
= EzJm(— p ) cos(mp) c o s ( ^ z)
a
h
H» =
= ^ i ^ ( — P )sin (m P )c o s(^ z )
a
h
j
1 d 2lIf
p p dp
H ,= - ~
/i op
a
co s(m « Co s ( ^ z )
ft
(3-15)
# ;=o
/' _—V\ r _ \\
x \ h j
-
X2
-If^ U
__ rT-? —2'(m \
.cosE,a
—
,
\P )
'
Ku,
3.3.1.2 Modes Designation
The important information embedded in these Equations (3-12) and (3-11) is the
characteristic field patterns associated with these modes. These field patterns indicate the
variations and amplitude of the field components as a function of the cavity axis. In the
cylindrical cavity the field indices (m,n,p) correspond to the components (<j>,r,z). Where
r, is the radial direction, <J>is the circumferential direction, and z is the vertical direction.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
39
Note that the order of the indices 4>,r,z is not in the cyclic order of coordinates r,<{>,z,
because it is common in circular waveguides to designate the <(>variation by the first
subscript(Ramo, 1953).
The index m is the periodicity in the radial direction, n is the number of half
wavelength in the circumferential direction, and p is the number of circular wavelengths
along the vertical axis. For a example, TMmnp or TMoiO indicates a transverse
magnetic mode with (0) wave variations along the radial direction, (1) wave variation in
the circumferential direction, and (0) wave variations in the vertical direction.
3.3.1.3 Electric Field Pattern
The field patterns for the different modes can be determined by plotting the
magnitude of the electric or magnetic field components. It is only necessary to plot these
patterns for one plane since similar patterns are repeated in all the repeating planes along
the z-axis. Several plots of the electric field patterns were generated using Mathematica
and are shown in Figure 3-2 and Figure 3-3. See Appendix D for Mathematica program
for generating these plots. These plots are shown as density plots where the white
regions represent high field strength regions and the dark areas represent low field
intensity regions. In generating this plots it was noted that for the TE modes, the <{>
component was the predominant component while for the TM modes the z component
was the predominant component. This is consistent with what would be expected for the
different modes.
3.3.1.4 Cut-Off Frequency
From the eigenvalues of the solutions to the Helmholtz Equation (3-9), the wave
propagation characteristics can be derived in a form of what is known as the cut-off
frequency equations. The cutoff frequency defines the minimum frequency for a given
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
40
cavity diameter where modes can propagate. The cut-off frequency equations for the TE
and TM modes are shown in Equations (3-16 )and (3-17). They show a relationship of
frequency as a function of cavity length, h, cavity radius, a, permittivity and permeability,
and the tabulated zeroes of the Bessel function. For a fixed cavity radius and microwave
frequency the
Waves of frequencies below the cutoff frequency of a particular mode cannot
propagate, and power and signal transmission at that mode is possible only for
frequencies higher than the cutoff frequency. At frequencies below the cutoff frequency
of a given mode the propagation constant is purely imaginary, corresponding to a rapid
exponential decay of the field and the generation of evanescent modes (Pozar, 1991).
(/)
TE
mn
rEH Y
<h
J
(3-16)
x ' = tabulated zeroes of the derivative of the Bessel’s function
mn
(/)
TM
^
k
f
f
(3-17)
= tabulated zeroes of the Bessel's function
The significance of the cut-off frequency equations is that they indicate the
frequencies and cavity lengths where a single mode can resonate in a cavity of a known
radius. Another useful form of these equations is a plot of'the frequency versus the
cavity length for a fixed cavity radius, to generate what is known as the mode chart (see
Figure 3-3 and Figure 3-4.
The mode chart shows the locations of modes with respect
to other modes as a fucntion of frequency and cavity radius. It is important to note that
these equations can be used in two forms: by fixing the frequency at a contant value and
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
41
varying the cavity length to excite different modes or b y ; fixing the cavity length at a
constant value and varying the frequency to excite different modes. The previous method
is the fixed frequency mthod which is used in this work and the latter is the variable
frequency method. In the variable frequency method more modes can be theoretical
excited than in the fixed frequency method. This is evident by the number of modal lines
that a vertical line intersects for the variable frequency method and the number of modal
lines that a horizontal line intersects for the fixed frequency method.
By inspecting the mode chart it is apparent that, as the order of modes
increase(increase in the indices) the modes become close together in frequency. Thus,
suggesting that it may be difficult to excite higher order single modes. The mode chart
also indicates that the modes that can be excited at a fixed frequency.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
42
T E (llx)
TE(Olx)
TE(21x)
TE(31x)
TE(13x)
TE(02x)
TE(22x)
TE(61x)
Figure 3- 2 Electric field pattern for TE-modes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
43
TM(Olx)
TM (31x)
TM(22x)
TM(21x)
T M (llx )
TM (12x)
TM (03x)
TM (32x)
Figure 3- 3 Electric field pattern for TM-modes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
44
w1
COw
©o
*- H-
CM
UJ UJ
CM
CVJ
o
CM
co
co
co
o ,
Ldj;
x io
e*
<0
in
•c-
CO
(z h ) Aouenbaij
Figure 3- 4 Mode chart for TE-modes - Cut-off frequency versus cavity length
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
caivly length (cm)
cm
45
C v^
CM
CM
CO
CM
CO
5
C M *—
S
H- 2hK
CO
CO)
CO
CM
2
5
5
ir>
co
CO
CM
o
CO
in
CM
o
CM
CM
CM
-
caivly length (cm)
CO
CO
CM
'C O
x 10
CM
in
^
CO
(ZH) Aouanbeii
Figure 3- S Mode chart for TM-modes- Cut-off frequency versus cavity length
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
46
In general mode has a unique resonant cavity length, resonant frequency and field
pattern associated with it. However, there are TE and TM modes where the field patterns
are different but the resonant frequency or cavity length are identical. These are known
as degenerate modes and can exist at the TM-mode index of (1 lz) and the TE-mode
index of (Olz), where the z-index can be any integer greater than 1.
Mathematically, this can be seen from the cut-off frequency Equations (3-16) and
(3-17), which shows that for a given cavity radius and resonant frequency, this can
happen when the root and derivative of the root of the Bessel function are equal. It is
also apparent from (3-16) and (3-17) that, as the indices or order of modes increase for a
given cavity radius and resonant frequency, the resonant cavity length increases. This is
logical since more half wavelengths are available with the higher order modes and thus, a
longer cavity length would be required.
3.3.1.5 Cavity Quality Factor
The Q-factor of a resonant cavity is defined as the ratio of the energy stored in
side the cavity volume to the energy lost to the cavity surface area per unit time Equation
(3-18). The Q-factor of a cylindrical cavity is a function of the resonant mode and in the
microwave frequency range is usually very high and ranges from 10,000 to 40,000 or
more(Collin, 1966). At a given frequency, as the order of modes increases the
theoretical Q increases. This is logical since for higher order modes the cavity volume
increases, and there is a greater volume-to-surface ratio, and energy is stored in the
voulme, whereas it is lost on the imperfectly conducting surface(Collin, 1966).
In practice the Q-factor can be lowered by the introduction of a feed
system(coupling probe), a large impedance, imperfections in the construction, and
imperfect conductivity at the cavity surface(Ramo, 1953 ). Equation (3-18) (Harrington,
1961) is the general mathematical definition of the quality factor. In this equation R is
the intrinsic wave resistance of the metal walls.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
47
r e\\E[ 2 d z '
V
(3-18)
R ftH p d s
The importance of the Q-factor in processing is that it provides a guideline to
how well a sample would heat at a particular mode, which is indicated by a decrease in
the cavity quality factor as it is loaded with a sample. The theoretical Q-factor equations
are only valid for ideal cavities where the internal surface of the cavity is homogeneously
smooth, and for sample loads that do not perturb the theoretical equations significantly.
However, in practice the sample loads are typically very lossy and large which
invalidates the use of these equations because they are derived from perturbation theories.
Thus, an experimental method using the bandwidth of the half-power points of a power
response curve is used to calculate the cavity Q as indicated in Equation (3-19) and
depicted in Figure 3-5. The power response curve is a trace of the frequency (x-axis)
versus absorbed power (y-axis) that is generated on an oscilloscope using the low power
cavity diagnostics technique(see appendix B).
^
^ _r energy stored _ J „
2 77
A/
Q ~ 2 r f o energy lost
r~T
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(3-19)
48
Power —Pm — —
-J- - J
frequency
Figure 3- 6 Q-curve calculation using half power point method
3.3.2 Loaded Cavity Solutions
3.3.2.1 Small- Low loss Samples
The modal solutions discussed above are for a homogeneously loaded cavity or an
empty cavity in this case. There are other solutions for a cavity loaded with a small
sample or a sample with a low dielectric constant using perturbation
techniques(Hanington, 1961). In order for the perturbation technique to be applicable it
is assumed that the modal fields in the perturbed cavity can be approximated by those in
the empty cavity. Hence, the actual perturbation due to a sample load must be less than
one percent, which is measured by the change in the resonant frequency or the change in
energy in the cavity.
For a composite material the dielectric constant of the polymer is approximately
four, which is a relatively large value that would perturb the fields beyond the
perturbation limits. Additionally, the size of composites that are processed are typically
large and would perturb the fields beyond the perturbation limits. Hence this method
cannot be applicable for a composite material load.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
49
3.3.2.2 Homogeneous and Isotropic Samples
For a cavity loaded with a homogeneous material of various dielectric properties
and load configurations, a mode matching technique was used to solve for the field
equations in the cavity(Mannering, 1992 ; Lin, 1989). The load configurations were
classified as the cavity short-type, cavity open type, and cavity image type(Mannering,
1992; Lin, 1989 ). Solutions were derived by requiring that the modal field components
at the boundaries of the loaded and unloaded regions of the cavity be continuous, or
matched, and thus the name mode matching.
Again, these solutions cannot be applied to a composite load because of the
anisotropic and inhomogeneous characteristic of composites due to the combination of
fiber and matrix. It is very important to note that the coupling of the modal fields with an
anisotropic material like a composite, is so radically different from that of isotropic and
homogeneous materials that the solutions cannot be linearly extended.
3.3.2.3 Simplified Composite Material
Wei(Wei, 1992) used a simplified five parameter model to solve for the fields in a
composite material for a TM (transverse electromagnetic) propagating wave. In this
solution it was assumed that the incident wave on each composite surface was a linearly
polarized TEM(transverse electromagnetic) wave. This means that the electric field
vector points in a direction where the magnetic field will be ninety degrees away from i t
It was also assumed that the power dissipated in the composite from the edge G f the
composite was constant through the thickness of the composite, and thus the electric field
distribution inside the composite was only due to the propagating waves from the top and
bottom of the composite.
These simplifying assumptions were made as a first approximation for solving a
very complicated problem. In the actual coupling between the composite material and
the electromagnetic fields, the power dissipated in the composite is not constant through
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
the thickness of the composite, and there is a difference in coupling between the incident
waves from the different planes of the sample due to the fiber reinforcement.
Although, these solutions are good approximations to the exact solutions they are
only valid for special cases. For composite materials the solutions are far more complex
and computationally intensive. Hence, in the microwave processing of composites
empirical relationship have been developed to aid in the understanding of the process. In
the sections that follow concepts of microwave porcessing in a single-mode resonant
cavity are discussed.
3.4 Fundamentals of Microwave Heating
3.4.1 Microwave Power Absorption and Permittivity
To this point, fundamentals of electomagnetics have been discussed in terms of
modes and modal field solution. Now the interaction of these fields with materials and
how they heat will be discussed. The material property of non-magnetic materials which
determines its ability to absorb microwave energy is complex permittivity is shown in
Equation (3-18) below.
i = e '- je "
£
= £
+ ------£ 0 0)
(3-20)
The complex permittivity is mathematically a complex quantity which has two
components; a real part which is called dielectric constant, and is related to the energy
stored in the material; and an imaginary part which is called the loss factor and is related
to the energy dissipated as heat in the material. The loss factor is due to contributions by
the dielectric loss factor or the motion of dipoles and the conductivity or the motion of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
51
charges. It is a function of material structure, composition, angular frequency,
temperature, and pressure.
As a dielectric material is exposed to an electromagnetic field, the polar segments
attempt to align up with the alternating electromagnetic field so that the normal random
orientation of the dipoles become ordered. These ordered polar segments relax and
oscillate with the fields. The energy used to hold the dipoles in place is dissipated as heat
into the material while the relaxation motion of dipoles is out of phase with the
oscillations of the field.
Materials with low dielectric loss do not absorb microwave energy as readily as
those with higher dielectric loss. The absorption of electromagnetic energy by materials
is not only dependent upon the permittivity, it is also dependent upon the angular
frequency of the microwave and the square of the magnitude of the electric field strength
inside the material as described in Equation (3-21).
i
o
P = -C O £ 0 e " |E |Z
(3-21)
Where:
P
= Microwave Power absorbed
£0 = Permitivity of free space
a>
= angular frequency
s"
= dielectric loss factor
E
= electric field
3.4.2 Penetration Depth
The angular frequency is typically constant and determines the depth of
penetration of the electromagnetic wave into the material. Typically, as the frequency
decreases the depth of penetration increases as the free space wavelength increases. A
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
52
linear dimension called skin-depth or penetration depth is described as the depth at
which the electromagnetic energy would decay or attenuate to 1/e. The parameter can be
used to select the appropriate frequency and sample size so that volumetric heating is
achieved. To achieve volumetric heating the power generated in the material over the
skin-depth must be substantial compared with the actual dimension of the sample(Binner,
1991).
Equation (3-20) can be used to calculate the penetration depth of various
polymers and composites. Using this equation the skin-depth has been calculated for
several polymers and composites at two different frequencies, 2.45 GHz and 0.915 GHz
and summarized in Table 1.
These two frequencies were chosen because they represent the two frequencies
that are widely used. They also represent current operating systems in our lab which are
used in a scale-up study of a 7 inch cavity to an 18 inch cavity to process 3 inch and 8
inch samples, respectively. Typically, in order to excite similar electromagnetic modes
in a larger radius cavity, the frequency has to be decreased. This can be better illustrated
from the cut-off frequency Equations (3-16) and (3-17). According the values in the
table, at these frequencies substantial penetration depths can be achieved for the polymer
and composite systems, and thus volumetric heating should be achieved.
(3-22)
H q H £ o£
Z =
o 2 co
1
f
\ 1
( £"^ 2 1 2
-1
1+ —
^
In Equation (3-22), Z is the penetration depth in meters, and the other variables are the
same as were previously defined. It is important to note that penetration depth is an
isotropic concept. Hence, in order to calculate it for the anisotropic composite material it
must calcualted for the principle axis of the composite.
For the AS4/3501-6 composite,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
53
the effective permittivity along the fiber direction and perpendicular to the fiber direction
are given by ( Lee and Springer 1984) to be 1-j 2500 and 14.5 - j 75.8, respectively.
Assuming the fiber direction to be the principal axis, the skin depth is calculated as
shown in the Table 3-1. This table shows that at 2.45 GHz the penetration depth for
several polymers can range between 0.4 m and 88 m; and between 0.0098m and 0.0032m
for a composite. This table also indicates that at a lower frequency, 915 MHz the
penetration depths are higher as compared with those for the 2.45 GHz values.
Material
Vinyl ester
Fully cured epoxy
Uncured epoxy
Fiberglass
Polystyrene
Polyethylene(pure)
AS4/3501-6
Graphite / Epoxy
composite(along fiber)
AS4/3501-6
Graphite / Epoxy
composite(perpendicular
to fiber)
♦effective permitivitty
£/£„
Z(m)
Z(m)
3 .4 4 -j0 .1 8
3 .5 - j 0.1
4 .2 5 - j 0.25
4.4-10.13
2.55 -j 0.00085
2.25 - i 0.0007
1-j 2500*
f=2.45
GHz
0.4
0.73
0.32
0.63
73
83
0.01
f=915
MHz
1.08
1.95
0.86
1.68
196
223
0.03
14.5 - j 75.8*
3.2
8.5
Table 3-1 Penetration Depth for Samples at various frequencies
3.4.3 Applicators
3.4.3.1 Multimode and Waveguide
There are several types of microwave applicators with benefits that range from
low cost to efficient energy coupling. Some of the different applicator types that have
been used in the processing of materials are waveguides, multimode cavities, and single­
mode cavities(Asmussen, 1987 ). The waveguide applicator is the easiest to fabricate and
is mad up of a hollow pipe. The multimode applicators comprise of over 50% of all
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
54
industrial microwave applicators and is based upon the principle of the commercial
microwave oven(Asmussen, 1987 ). In a given frequency range the multimode applicator
is designed to support a number of resonant modes. Thus, it reduces coupling
sensitivities since several modes share heating and compensate for load property
changes(Asmussen, 1987 ). To improve heating uniformity mode stirrers or the multiple
generators are used to distribute the power and provide for better exception of the
modes(Huack, 1969). The wide use of this applicator is due to the low cost, ease of
construction, and the adaptability to a wide variety of material loads of different sizes and
effective loss factors(Asmussen, 1987 ).
3.4.3.2 Single-Mode Cavity
Unlike the multi-mode applicator, the single-mode applicator is very sensitive to
load changes, but most efficient in the electromagnetic energy coupling. A single mode
resonant cavity is made up of a hollow pipe and two plates for top and bottom. The top
plate is adjustable similar to the adjustment of a piston while the bottom plate is fixed. In
the applicator studied the microwave energy is coupled to the cavity by an antenna
which is inserted horizontally through the side of the cavity w all.
At a given frequency the applicator is designed to support a single mode. As
material load properties change during heating the cavity has to be adjusted to
compensate for these changes. The adjustment to compensate for these changes is called
mode tuning. For mechanical tuning, the cavity volume is adjusted by the movement of
the cavity top and the depth of the coupling probe.
For variable frequency tuning the
cavity length and probe depth are fixed as the frequency is adjusted to compensate for
material load changes. These tuning relationships can also be observed from inspection
of the cut-off frequency Equations (3-16) and (3-17).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
55
3.4.4 Mode Tuning
The method of electromagnetic energy coupling and matching used in the single
mode resonant cavity is similar to that used in ion and plasma sources(Asmussen, 1974;
Asmussen, 1984; Asmussen, 1985; Asmussen, 1986) and electro-thermal
engines(Whitehair, 1984; Whitehair, 1987 ) The fundamental of this method can be
described from the principles of RLC circuits where the input impedance, Zin of a
microwave cavity is given in Equation (3-23)(Asmussen, 1987 ).
Pt + j 2 o ( W m - W e )
Z in =
J
----------- = * in + P i n
C «3)
2
In this equation, Pt is the total power coupled into the cavity including losses,
Wm and We are time-averaged magnetic and electric fields stored in the cavity fields,
respectively, llol is the total input current on the probe, and Rin and jXin are the cavity
input resistance and reactance or the complex load impedance as seen by the feed
transmission line. At resonance the impedance, Zin is minimum and is equal to the
resistance, Xin as shown in (3-24). Above or below the resonant frequency the
impedance is maximum and is expressed in Equation (3-25).
Z in
^in
(3-24)
Z in
^in + P i n
(3-25)
Thus, to achieve resonance two independent adjustments are required to match
the cavity load to the transmission line. One adjustment must cancel the load reactance,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
56
Xin while the other must adjust the resistance, Rin to equal to that of the characteristic
impedance of the feed transmission(Assmusen, 1987). The cavity length and probe depth
are two variables that are used to achieve this goal.
At resonance maximum energy is focused into the cavity. As the cavity is loaded,
the energy that is focused into the cavity becomes a function of the resistance of the load.
As the resistance of the load increases, more energy is lost to the load and the energy
stored in the cavity decreases. This is characterized by the flattening of the power
response curve and the decrease in the cavity quality(Q) factor.
3.5 Summary
In this chapter the fundamental concepts of electromagnetics and microwave
processing were discussed. Fundamentals of electromagnetics were presented from the
point of view of Maxwell's equations and how they relate to microwave processing. This
was done by the simplification of the Maxwell's equations to generate the wave equation
which, was then solved as an eigenvalue problem with appropriate assumptions and
boundary conditions for a cylindrical cavity..
Fundamental concepts of microwave processing was discussed from the point of
view of theoretical processing issues such as loss factor, microwave power absorption,
and skin depth. Other processing issues such as applicator challenges, and applicator
characterization as in impedance matching or mode tuning and quality factor calculations
were also discussed. In summary, this chapter should address all relevant theoretical
electromagnetic and microwave processing issues that will be used in this work.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 4
EM PTY AND LOADED CAVITY CHARACTERIZATION
4.1 Introduction
A key in the microwave processing of composites is the understanding of the
electromagnetic fields behavior inside the cavity. The magnitude of the electric field
strength determines the heating rate and the field pattern determines the heating
uniformity. These field characteristics are a function of the sample dielectric properties,
size, geometry, fiber orientation, placement, resonant frequency, and resonant cavity
length. Thus, in order to control the heating inside the cavity the magnitude and field
pattern of the electric field inside the cavity must be known.
For each electric field partem there is a corresponding electromagnetic mode. In
the single-mode resonant cavity the measurable parameter for identifying an
electromagnetic mode at a fixed microwave frequency is the cavity length. As a sample
is loaded into the cavity, the known empty cavity lengths corresponding to the
electromagnetic modes become altered(see Chapter 3) and mode identification using
cavity length becomes complex. Because the electromagnetic modes are discrete, a
fundamental mathematical approach for identifying loaded cavity modes was found to be
computationally intensive.
Using graphite/ epoxy, glass / polyester and un-reinforced nylon samples
experiments were done to understand the electric field behavior in the loaded cavity.
This was done by measuring the radial electric field along the axial and circumferential
axis, and the resonant frequency shift of the loaded cavity at a TM(012)-mode.
57
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
58
Graphite / epoxy and vinyl / ester glass composites were then heated to identify
heating characteristics of the loaded cavity modes. But first empty cavity solutions were
used to characterize the empty cavity, in order to define the actual empty resonant cavity
lengths and quality factors in the presence of circuit impedances and cavity
imperfections.
4.2 Empty Cavity Characterization
4.2.1 Experiments
With the theoretical cavity lengths available from the cut-off frequency
equations(Equations 3-16 and 3-17), the empty cavity modes were measured using the
swept frequency method. This was done by adjusting the cavity length around the
theoretical values in addition to adjusting the coupling probe depth to achieve resonance
in the empty cavity, which was measured by a power absorption trace on an oscilloscope.
Adjustment of the cavity length aligned the power absorption curve along the
excitation or center frequency and the adjustment of the coupling probe increased the
amplitude of the curve along the y-axis. The power absorption curve was then used to
calculate the cavity quality factor or Q-factor as explained in Chapter 3. This procedure
was repeated until all of the observable theoretical modes were measured.
4.2.2 Results and Discussions - Empty cavity characterizations
Table 4-1 lists the calculated cavity lengths, the corresponding measured empty
cavity length, probe depth and cavity quality factor. Of the 17 calculated modes, 15
were observed which include 10 TE-modes, 5 TM-modes and 2 degenerate modes. As
discussed in Chapter 3, TE-modes are electromagnetic modes where the electric field
component is transverse to the direction of propagation, and TM-modes are those where
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
59
the electric field component is parallel to the direction of propagation. In these
designations the vertical axis is taken as the direction of propagation. The degenerate
modes were observed at the measured cavity lengths of 11.08 cm and 22.46 cm. Also, as
mentioned in Chapter 3, degenerate modes are TE and TM modes with different field
patterns but with the same resonant frequency or resonant cavity length.
In practice cavity imperfections, circuit impedances and the presence of the
coupling probe inside the cavity could all contribute to increased energy losses in the
cavity which results in a decrease in the cavity volume required to support the
corresponding calculated mode(Mannering, 1992). Thus, measured cavity lengths are
typically lower than the calculated ones. In this work the measured cavity lengths were
approximated to the calculated ones by assuming that they correspond to the closest
values that are greater than the measured values. For example, a measured cavity length
of 11.08 cm was approximated to the calculated value of 11.283 cm and not 13.383 cm as
shown in table 4-1.
Higher order modes corresponding to TE(114) at 26.767 cm and TE(214) at
32.971 cm were not observed. Typically, as the order of modes increase (the increase in
the mode indices) the wavelength decreases, the resonant cavity length increases and the
modes become very close together in frequency(see mode chart in Chapter 3). As the
modes become closer together in frequency they become difficult to excite
independently. Hence, the un-observed modes could have existed as a hybrid of other
higher order modes that were excited in this cavity length range. It is important to note
that the un-oberservable modes were both TE-modes that were one <)) variation apart
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
60
Modes
Theoretical Measured Probe
Cavity
Cavity
Cavity
Depth
Q-Factor
Length
Length
(cm)
(cm)
(mm)
(K)
TE(211)
8.243
7.99
5.18
15
T M (lll)
11.283
11.08
4.71
4
TE(011)
11.283
11.08
4.71
4
TE(112)
13.383
13.29
5.64
18
TM(012)
14.41
14.39
12.77
7
TE(311)
15.713
15.16
4.25
20
TE(212)
16.485
16.44
4.09
8
TE(113)
20.075
20.15
7.66
8
TM(013)
21.615
21.77
13.55
3
TM(112)
22.566
22.48
20.04
9
TE(012)
22.566
22.48
20.04
9
TE(213)
24.728
24.83
4.36
4
TE(114)
26.767
TM(014)
28.82
27.01
9.21
4
TE(312)
31.427
30.71
3.47
9
TE(214)
32.971
TE(115)
33.459
33.29
4.87
23
Table 4- 1 Measured and calculated resonant cavity lengths
The corresponding cavity Q-factors are also shown in Table 4-1 which range from
3000 to 23,000. The Q-factors did not follow a specific profile by increasing as the order
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
61
of modes increased as was expected from the calculations. Again this could be due to
the characteristic impedance of the cavity and the circuit due to the imperfections in the
cavity as mentioned before. As discussed in Chapter 3 the cavity quality factor is a ratio
of the energy stored to the energy lost in the cavity, and therefore as the energy loss to
the cavity increases the Q-factor should decrease and the reverse is true. Typical Qfactor values can range from 10,000 to 40,000 or more for ideal cavities (Collin,1966),
where ideal means perfectly and homogeneously smooth and perfectly conducting cavity
walls.
The measured Q values can fall in any range and can serve as a fingerprint of the
coupling efficiency of the cavity and circuit. In processing it can be used as measure of
the coupling efficiency between sample loads and microwave energy inside the cavity. In
other words it can be used to determine how a sample would heat at a particular mode. A
decrease in empty cavity Q or low Q value is an indication that microwave energy is
being lost to the sample load, or the sample load is efficiently absorbing microwave
energy inside the cavity. In this work loaded cavity Q will be used to select the most
efficient heating modes. The lower the Q the more effective the coupling and heating
would be.
4.3 Loaded Cavity Characterization
4.3.1 Experiments
Experiments were performed using nylon (dielectric constant=2.06), polyester /
glass composite (dielectric constant=4.50) and graphite / epoxy composite(dielectric
constant = 4.23) to identify the loaded cavity characteristics of a TM(012)-mode. All
samples were cut into one inch squares, which was the size calculated from perturbation
theory to have minimal field perturbations for a material with a dielectric constant of 4.0.
Two different sample heights were used, one was l/8th of the fiee-space wavelength
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
62
away from the coupling probe which will be referred to as the lowered sample
position(free space wavelength for 2.45 GHz is 12.24 cm) and the other was a quarter of
the free space wavelength away from the coupling probe which will be referred to as the
elevated sample position. The choice of these sample heights was in accordance with
typical processing protocol such that maximum cavity volume is available for mode
excitation.
A solid Teflon block was used to elevate the samples to the appropriate heights.
For the lowered sample position a square 7.81 cm in length by a 1.88 cm thick Teflon
block was used to elevate the samples from the base of the cavity. For the elevated
position, the square block was placed on top of another solid Teflon disk that was 12.25
cm in diameter by 1.88 cm thick. The one inch square samples were placed in nine
marked positions, one at a time such that it spanned a nine inch square area which
represents the area of a typical three inch squared sample. For each sample placement the
cavity was opened and the sample was physically moved and carefully placed in the
specified location.
0 . 882 "
0 . 882"
Figure 4- 1 Sample Placement
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
63
The samples were placed one at a time where the order of sample placement
followed the numbered order in Figure 4-1, with the center position being the first
placement The coupling probe position was always close to position 7. When using
composite materials, the fiber orientation was referenced to the coupling probe
placement For example, a perpendicular fiber orientation would mean that the fiber
alignment in the unidirectional composite material was perpendicular to the coupling
probe. Positions 2 and 4 are referred to as top left and top right respectively and
positions 6 and 8 as bottom right and bottom le ft respectively.
In all of the measurements, at the resonant empty cavity length for a TM(012)
mode a sample was loaded into the cavity which caused the resonant conditions of the
empty cavity to change. The loaded cavity was then tuned to the new resonant conditions
by adjusting the cavity length and probe depth. At the new resonant cavity length and
probe depth the measurements were done to identify the characteristics of the mode.
4.3.1.1 Frequency Shift Measurement
In the frequency shift measurement a reference resonant frequency was first
determined for a TM(012) mode, by tuning the cavity to resonance using the low power
diagnostic system to generate a power absorption curve on an oscilloscope. A sample
was then loaded into the cavity and the coupling probe was adjusted to fine tune the
cavity by increasing the amplitude of the power absorption peak. By assuming that the
reference resonant frequency was 2.45 GHz, the frequency shift was measured by
graphically determining the difference between the reference resonant frequency, and that
for the loaded cavity on an oscilloscope trace as described in appendix B(see Figure 4- 2).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
64
Because a Teflon block was used to elevate the samples, it was used to set the reference
resonant frequency and the frequency shift was determined from the difference between
the resonant frequency due to the Teflon load and that due to the addition of the sample
load.
e.
frequency
^f-frequency shift
F ig u re 4- 2 F req u en cy S h ift M ea su rem en t
4 3 .1 .2 E lectr ic F ield M ea su rem en t
In the electric field measurement, a micro coax probe specifically designed for
E-field measurement was connected to a power meter and inserted into the cavity through
holes that were drilled through the cavity walls. The extension of the probe into the
cavity was limited to 1.0 mm so that it caused minimal perturbation to the resonant mode
field patterns and the resonant frequency during measurement. The power detected by the
probe and measured by the power meter is proportional to the radial electric field inside
the cavity or IEI2.
The microwave cavity was designed with radial and axial E-field diagnostic holes
specifically for accommodating the micro coax probe for the electric field measurement.
The axial E-field diagnostic holes were located at the direct opposite side of the coupling
probe and extended to the total height of the cavity(see Figure 4- 3). There were four
rows of circumferential diagnostic holes which were placed at approximately 1.2 inches
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
65
from the base of the cavity, at approximately the same height as the coupling probe(see
Figure 4- 3). These rows extend approximately 140 degrees or 40% of the cavity
circumference due to the presence of the cavity support bracket. Ideally, these
circumferential diagnostic holes should extend the full circumference of the cavity for
accurate diagnostics.
Support Bracket
Diagnostic Holes
Coupling Probe
F ig u re 4- 3 E lectric field d ia g n o stic h oles: a) fr o n t v iew , b ) sid e v iew , c) to p view
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
66
Figure 4-3b Side View of cavity showing doagnostic E-field diagnostic holes
21
22
23
24
25 O O
O
O
26 O O
O
O
27 | o O
O
O
28
OO o o
oo o o
o o o o
o o o o
o o o o
30
31
►
32
Figure 4-3c Front View of cavity showing doagnostic E-field diagnostic holes
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
67
To do the electric field measurement, the resonant empty cavity at a TM(012)
mode was loaded with a sample and then tuned again to compensate for the sample load.
Six Watts of microwave power at 2.45 GHz was then coupled into the cavity while the efield measurements were done. Electric field measurements were done from the top of
the microwave cavity and along the axial and circumferential axis.
4.3.2 Results and Discussions - Loaded cavity characterizations
A key in mode identification is the differentiation between TE and TM modes,
and the identification of the mode indices. The E-field measurement from the top was
used to differentiate between TE and TM modes. Since the electric field component is
parallel along the z-axis for the TM-mode and perpendicular with the z-axis for the TEmode, the measured E-field from the top of the cavity should be a value greater than zero
for the TM-mode and equal to zero for a TE-mode. In this case a value of 12.0 was
measured which confirms the presence of a TM-mode as was expected.
The E-field measurement along the axial axis was measured to identify the
numerical value of the index z, which is the number of half wave variations along the
axial direction(see Figure 4- 3). The radial E-field along the circumferential axis was
also measured to identify the numerical value of <J), which is the number of half wave
variations along the circumferential direction(see Figure 4- 3). For the TM(012) mode
where (q=0, r= l, z=2) there should be two wave variations along the z-axis, and no wave
variations along the circumference of the cavity since the TM(012) mode is <J) symmetric.
Figures 4-4 through 4-9 show the axial electric field measurements, Figures 4-10
through 4-15 show the circumferential radial electric field measurements, and Figures 416 through 4-19 show the frequency shift measurements. For the purpose of clarity,
these results have also been summarized in tabular form in Tables 4-1 through 4-3. The
axial field measurements in Figures 4-4 through 4-9 show multiple plots for sample
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
68
placements in the different location in the fixed z-plane. The y-axis shows the electric
field diagnostic hole location along the vertical axis of the cavity, and the x-axis is the
electric field measurement The base of the cavity is represented by hole #0 and distance
between holes #0 to #21 is equal to a cavity length of approximately 13.0 cm. The
coupling probe placement height is indicated by a horizontal dashed line along the y-axis
between hole #27 and #28. The elevated and lowered sample placements were also
indicated by dashed lines between hole #27 and #28 and between #29 and #30,
respectively.
Table 4-2 summarizes the axial electric field measurements in which column 3
and 4 indicate the number of high amplitude peaks and their location along the z-axis,
respectively. The number of high amplitude peaks is equivalent to z, the number half
wavelength variations along the z-axis. From table 4-2, the empty cavity axial E-field
measurement shows two high amplitude peaks with the maximum at holes #29 and #24.
This is consistent with what was expected since the cavity length was that corresponding
to aTM(012).
Similar axial electric field measurement was also observed for the Teflon block
loaded at the elevated position as shown in Figure 4-5a. For the Teflon block loaded at
the lowered position only one fully developed high amplitude electric field was observed
at the bottom region of the cavity, and a partly developed one in the upper region of the
cavity at hole #22 as shown in Figure 4-5b. This lowered Teflon profile is similar to the
elevated Teflon profile except for that, the location of the high amplitude peaks are at
higher locations in the cavity. For the elevated sample, the peak in the lower region of
the cavity is located within the Teflon block while for the lowered sample it is located
outside the block region.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
69
R a d ia l E le c t r ic F ie ld P a t t e r n a lo n g A x ia l a x i s f o r E m p t y C a v it y
21
2223 o 24 o 2 5 -
a.
£ 26
-
27
C o u p lin g
P robe
? 28
2 9 30 —
31
12
14
16
Figure 4-4 Axial Field Electric Field Pattern for Empty cavity (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
70
A x ia l R a d ia l E - f ie ld P a tt e r n f o r T e f lo n
C a v ity L o a d a t t h e E le v a t e d P o s it io n
24
Sam ple an d
C oupling P ro b e
(a)
A x ia l R a d ia l E le c tr ic fie ld P a tte r n fo r T e flo n
P o s itio n
C a v it y L o a d a t th e L o w e r e d
22
c
o
oo
a.
o
C oupling P ro b e
e
m
M
x
0
2
4
6
10
8
12
14
16
18
IEIA2
(b)
Figure 4-5 Axial Field Electric Field Pattern for Teflon (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
20
71
A x ia l R a d ia l E - C e ld P a t t e r n f o r G r a p h it e E p o x y C a v it y L o a d
P o s itio n -p e r p e n d ic u la r
22
fib e r
a t th e E le v a te d
d ir e c tio n
—Q -—Center
—X —“Top right comer
■ A Doaom left comer
" O ' Top left co m p
■i O —Bottom right c o m a
24
Sam ple an d
C oupling P ro b e
20
(a)
A x ia l R a d ia l E - fie ld P a tte r n f o r G r a p h ite E p o x y C a v ity L o a d
P o s itio n -p e r p e n d ic u la r fib e r d ir e c tio n
O Center
• ^ 0 “ Bottom right
®
A
Top left
Bottom left
a t th e L o w ered
—X —Top right
C oupling P robe
S a m p le
IE IA2
(b)
Figure 4-6 Axial Field Electric Field Pattern for Graphite / Epoxy Perpendicular Fiber
Direction (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
72
A x ia l R a d ia l E -fie ld P a tte r n f o r G r a p h ite E p o x y C a v ity L o a d a t th e E le v a te d
P o s itio n -p a r a lle l f ib e r d ir e c tio n
21 s<
22
—O— Top left corner
••
—X —-Top righi comer
23 -•
°
" A
—E?—Bottom light comer
Bottom left corner
24 ■■
2 25 Sam ple a n d
27 ■■
C oupling P robe
0
4
2
8
6
10
14
12
16
18
20
1EIA2
(a)
A x ia l R a d ia l E -fie ld P a t te r n fo r G r a p h it e E p o x y C a v ity L o a d a t t h e L o w e r e d
P o s itio n -p a r a lle l
fib e r
d ir e c tio n
©
Top left comer
® ^C enter
■■O 'Bottom right comer
—X ^ T o p right corner
™A“ Bottoia left comer
eo
ir.
O
Q.
« 27 --
C oupling Probe
7N 28 4i
S am p le
0
2
4
6
8
10
12
14
16
18
20
IE IA2
(b)
Figure 4-7 Axial Field Electric Field Pattern for Graphite / Epoxy Parallel Fiber Direction
(a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
73
A x ia l R a d ia l E -f ie ld P a tte r n f o r N y lo n C a v it y L o a d a t t h e E le v a t e d P o s it io n
•X ,
■Center
—O— Top left comer
'Bottom right comer
- A " 'Bottom left comer
'X —Top right comer
25 -I
26 -|
Sam ple an d
27 -
C o u p lin g P robe
28 29 -
40
20
(a)
A x ia l R a d ia l E - f le ld P a tte r n f o r N v lo n L o a d a t t h e L o w e r e d P o s it io n
22
-|
23 - J
—Q “ ^ e n i e r
Bottom right corner
•Top left comer
’X '-T o p right comer
•Bottom left comer
o 24 -■
25 -•
« 26 -■
27
C oupling P robe
29 -•
S a m p le
30
(b)
Figure 4-8 Axial Field Electric Field Pattern for Nylon (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
A x ia l R a d ia l E -fle ld P a tte r n f o r P o ly e s t e r G la s s C a v ity L o a d
a t t h e E le v a te d P o s itio n
‘Top left comer
22
«■
o
Bottom right comer
■X—Top right comer
‘Bottom left corner
23
Sam ple and
27 -■
C oupling P ro b e H eight
28 -•
29 -•
30 20
(a)
A x ia l R a d ia l E -fie ld P a tte r n fo r P o I y e s te r G la s s C a v ity L o a d
a t th e L o w e r e d P o s itio n
21
—O - ^ e n ie r
" O "Bottom right comer
- O - Top left comer
—X “ *Top right comer
A " Bottom left comer
C oupling P robe
20
(b)
Figure 4-9 Axial Held Electric Field Pattern for Polyester Glass (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
R a d ia l E le c t r ic F ie ld P a t t e r n a lo n g C ir c u m f e r e n t ia l a x is f o r E m p t y C a v it y
20
18-1 6 --
■#2S
■#26
'#27
'#28
1 4 -12
- -
0 -8
- -
6
- -
4 -2
- -
Probe Depth -
Probe Depth - 2.01 mm
C ir c u m fe r e n tia l
b o le
19.28mm
lo c a t io n s (3 ,4 ,5 ,6 ,7 )
Figure 4-10 Radial Electric Field Pattern for Empty Cavity
6 - a x is R a d ia l E -F ie ld P a t t e r n f o r T e f lo n
C a v ity
L o a d a t E le v a te d a n d L o w e r e d p o s itio n s
20 T
18-
I26i
1614-.
128
12-c*
<
H
10-8
- •
6
-
4 -■
2
-•
Elevated
S a m p le
P o s itio n s
Lowered
Figure 4-11 Radial Electric Field Pattern for Teflon load at Elevated and Lowered Positions
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76
a x is R a d ia l E -F ie ld P a t te r n f o r G r a p h ite E p o x y C a v ity
L o a d a t e le v a te d p o s itio n - p e r p e n d ic u la r fib e r d ir e c tio n
20
i
18
—• —#25 —O —#26
16 ■
i
14 ■
—A —#27 ■■K~«28
12 •
«
<
a
ii
i
i
i
10'
8 ■
6 ■
X
42■
5
0■ •
C enter
Top left
T o p r ig h t
S a m p le
B o tto m
r ig h t
B o tto m
le ft
P o s itio n s
(a)
<>- a x i s R a d i a l E - F l e l d P a t t e r n f o r G r a p h i t e E p o x y C a v i t y
L o a d a t lo w e r e d p o s itio n - p e r p e n d ic u la r f ib e r d ir e c t io n
—• —#25 - 0 - ^ 2 6
A
C en ter
T o p le ft
T o p r ig h t
S a m p le
B o tto m
r ig h t
#27 —x —#28
B o tto m
le ft
P o s it io n s
(b)
Figure 4-12 Radial Electric Field Pattern for Graphite / Epoxy Perpendicular Fiber
Direction (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
77
$ - a x i s R a d ia l E -F ie ld P a t t e r n f o r G r a p h it e E p o x y C a v it y
L o a d a t e le v a te d p o s itio n - p a r a lle l f ib e r d ir e c t io n
20
18
125 —O—#26
16
—A —#27 —Jf—«21
14
12
10
8
6
4
2
0
C en ter
T op
le ft
T o p r ig h t
S a m p le
B o tto m
r ig h t
B o tto m
le f t
P o s itio n s
(a)
a x is R a d ia l E -F ie ld P a t t e r n f o r G r a p h it e E p o x y C a v it y
L o a d a t lo w e r e d p o s it io n - p a r a lle l f ib e r d ir e c t io n
C en ter
T op
R ig h t
T o p L e ft
S a m p le
B o tto m
R ig h t
B o tto m
L e ft
P o s itio n s
(b)
Figure 4-13 Radial Electric Field Pattern for Graphite / Epoxy Parallel Fiber
Direction (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
78
4>- a x i s R a d i a l E - F i e l d
P a tte r n f o r N y lo n C a v ity
L o a d a t e le v a te d p o s itio n
20
18
•#26
16
—A —#27
14
'#28
12
10
<_
Ed
8
6
4
2
0
.e n t e r
B o tto m
S a m p le
L e ft
B o tto m
R ig h t
P o s itio n s
(a)
<t>- a x i s R a d i a l E - F i e l d P a t t e r n f o r N y l o n C a v i t y L o a d a t l o w e r e d p o s i t i o n
2° j
18 -•
16 -•
14 -■
12
n
<
a
-•
108 - ■
6
-•
4 -•
2
-■
0
C en ter
T o p L eft
T o p R ig h t
S a m p le
B o tto m
R ig h t
B o tto m
L eft
P o s itio n s
(b)
Figure 4-14 Radial Electric Field Pattern for Nylon (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
79
<J>- a x i s R a d i a l E - F i e l d P a t t e r n f o r P o l y e s t e r G l a s s C a v i t y
L o a d a t e le v a t e d p o s itio n
>25 —U— #26
C en ter
_6CS_o—
D— p .
—O
T op
T o p L e ft
R ig h t
S a m p le
yrfkp.p
B o tto m
L e ft
B o tto m
r ig h t
P o s itio n s
(a)
<{>— a x i s R a d i a l E - F i e l d
P a t te r n f o r P o ly e s t e r G la s s C a v it y
L o a d a t lo w e r e d
C en ter
T op
R ig h t
p o s itio n
T o p L e ft
S a m p le
B o tto m
R ig h t
B e tto m
L e ft
P o s itio n s
(b)
Figure 4-15 Radial Electric Field Pattern for Polyester/Glass (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
80
Frequency S h ift o f C avity L oaded w ith G raphite E poxy
a t E levated Position -Perpendicular Fiber D irection
fo r T M (012) M ode
0 .0 1 4
0 .0 1 1 6 7
0 .0 0 9 3 3 3
0 .0 0 7
0 .0 0 4 6 6 7
0 .0 0 2 3 3 3
0 .0 0 2 3 3 3
0 .0 0 4 6 6 7
0 .0 0 7
0 .0 0 9 3 3 3
0 .0 1 1 6 7
0 .0 1 4
(a)
Frequency S hift o f C avity Loaded w ith G raphite E poxy
at L ow ered Position -Perpendicular Fiber D irection
fo r T M (012) M ode
0 .0 1
0 .0 0 8
0 .0 0 6
0 .0 0 4
0 .0 0 2
0 .0 0 2
0 .0 0 4
0 .0 0 6
0 .0 0 8
0.01
(b)
Figure 4-16 Frequency Shift Measurement for Graphite / Epoxy
Perpendicular Fiber Direction (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
81
freq u en cy Shift o f Cavity Loaded with Graphite Epoxy
at Elevated Position -Parallel Fiber Direction
focTM (012) Mode
0 .0 1 6
0 .0 1 3 3 3
0 .0 1 0 6 7
0 .0 0 S
0 .0 0 5 3 3 3
0 .0 0 2 6 6 7
0 .0 0 5 3 3 3
0 .0 0 S
0 .0 1 3 3 3
(a)
fre q u e n c y S hift o f C avity L eaded w ith G raphite E poxy
a t L ow ered Position -Parallel F iber D irection
f o r TM (012) M ode
0 .0 0 9
0 .0 0 7 5
0 .0 0 6
0 .0 0 4 5
0 .0 0 3
0 .0 0 1 5
0 .0 0 1 5
0 .0 0 3
0 .0 0 4 5
0 .0 0 6
0 .0 0 7 5
0 .0 0 9
(b)
Figure 4-17 Frequency Shift Measurement for Graphite / Epoxy
Parallel Fiber Direction (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
82
F requency, SI
Shift o f Cavity Loaded
w ith Nylon at Elevated Position
for 1*1(012) Mode
0 .0 4 4
0 .0 3 5 2
0 .0 2 6 4
0 .0 1 7 6
0 .0 0 S S
0 .0 0 8 8
0 .0 2 6 4
0 .0 3 5 2
0 .0 4 4
(a)
Frequency Shift o f Cavity Loaded
w ith Nylon at Lowered Position
for TM (012) Mode
0 .0 0 6
0 .0 0 4 8
0 .0 0 3 6
0 .0 0 2 4
0 .0 0 1 2
0 .0 0 1 2
0 .0 0 2 4
0 .0 0 3 6
0 .0 0 4 8
0 .0 0 6
(b)
Figure 4-18 Frequency Shift Measurement for Nylon (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
83
Frequency Shift o f Cavity loaded w ith Polyester
C lass at Elevated Position for TM (012) Mode
0 .0332
0.02846
0.02371
0.01897
0.01423
0.009486
0.004743
0.004743
0.009486
0.01423
0.01897
0.02371
0.02846
0.0332
(a)
Frequency Shift o f Cavity Loaded with Polyester G lass
a t the L o w s e d Position for TM (012) Mode
0.005 - r
0.004286
0.003571
0.002857
0.002143
0.001429
0.0007143
0.0007143
0.001429
0.002143
0.002857
0.003571
0.004286
0.005 —
(b)
Figure 4-19 Frequency Shift Measurement for Polyester / Glass (a) Elevated (b) Lowered
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
84
Sample
Sample
Height
Number of high electric field amplitude
peaks for sample placement in th e :
Center
Empty Cavity
Teflon
Graphite / Epoxy
Perp. Fiber
Graphite / Epoxy
Parallel Fiber
Polyester / Glass
Nylon
Teflon
Graphite / Epoxy
Perp. Fiber
Graphite / Epoxy
Parallel Fiber
Polyester / Glass
Nylon
_
Elevated
Elevated
Top
Left
Top
Right
Bottom
Right
Bottom
Left
__
—
2
29
30
30
24
23
23
2
2
30
23
2
2
2
2
2
-1
1
30
30
28
29
23
23
22
22
1
1
1
1
30
—
1
2
1
1
1
1
1
1
29
29
2
-2
2
2
Elevated
1
2
2
Elevated
Elevated
Lowered
Lowered
2
2
—
2
2
2
-1
2
2
Lowered
1
Lowered
Lowered
1
1
—
Peak
Locations
ho le#
—
—
—
—
—
—
Table 4- 2 Results from axial field measurements
Sample
Sample
Height
Measured axial electric field symmetry locations for
sample placements at the:
Center Top Top
Bottom
Bottom
Left Right Right
Left
Empty Cavity
—
Teflon
Graphite / Epoxy
Perp. Fiber
Graphite / Epoxy
Parallel Fiber
Polyester / Glass
Nylon
Teflon
Graphite / Epoxy
Perp. Fiber
Graphite / Epoxy
Parallel Fiber
Polyester / Glass
Nylon
—
—
Elevated
Elevated
—
27
—
27
Elevated
27,28
Elevated
Elevated
Lowered
Lowered
—
—
-27
—
none
~
none
26
26
26
none
26
26
26
26
26
26
26
26
26
26
25
none
none
none
none
Lowered
25
25
25
25
25
Lowered
Lowered
25
25
none
25
25
none
25
none
25
none
—
25
27
26
none
Table 4- 3 Results from axial field measurements
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
85
Sample
Sample
Height
Graphite / Epoxy
Perp. Fiber
Elevated
Resonant frequency shift comparison for
sample placement inside the cavity at:
0-90° 90 - 180 ° 180 - 270 ° 270 - 3600
N
S
S
S
Lowered
Elevated
S
S
Graphite / Epoxy
Parallel Fiber
Lowered S
Elevated S
Lowered S
Nylon
Elevated
S
Lowered s
S=symmetrical, N=non-symmetrical
Polyester / Glass
S
N
S
N
S
S
N
S
S
s
s
S
S
s
s
s
S
S
S
S
S
Table 4- 4 Frequency shift measurement results
m
Similar field behavior as a function of sample size has also been observed theoretically
by Mannering and was described as field confinement and field exclusion(Mannering,
1992). Reid confinement was described as the concentration of the field in or around
the sample load and conversely, field exclusion was the exclusion of the fields from the
load as the sample size is varied. Field confinement was said to be the characteristic of
lower order modes which require long coupling probe depths when microwave energy is
coupled form the side of the cavity. This is said to be due to low field magnitude outside
and away from the load and the primary axial field direction(Mannering, 1992).
The field confinement theory is consistent with what is shown in this data and
also serves as a verification of the generation of a lower order mode as was expected.
Additionally, there is another significant finding that, although the Teflon block may be
considered to be microwave transparent due to the low dielectric constant, the size of it
does have an effect on the field distribution and behavior of the fields in the cavity.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
86
Thus, considering the Teflon material to be transparent and non-interfering with
the fields based upon dielectric properties is not totally correct. This can be further
understood from perturbation theory, which tells us that field interactions can come about
from dielectric properties as well as sample placement in the cavity and
geometry(Harrington, 1961).
In general the axial field measurements for the different sample loads at the
elevated height was essentially similar to that of the Teflon loaded cavity at the elevated
position as shown in Figures 4-6 through 4-9 and in Table 4- 2. As shown inTable 4- 2,
for the graphite / epoxy sample with parallel fiber orientation, only one high amplitude
peak was measured. The reason for this difference is not understood at this time, but is
an indication of the complex nature by which the graphite / epoxy composite interacts
with the field.
For the lowered sample loads similar electric field measurements for the lowered
Teflon block load was also observed for most of the sample loads as shown in Table 4- 2.
A different measurement was observed for the graphite / epoxy load, where two high
amplitude peaks were observed for the sample placement in the center and at the top right
comer of the cavity. Similar peaks were also observed for the Nylon load with sample
placement at the top left comer of the cavity.
The difference in the axial field pattern for the center sample placement is not as
surprising as that for the edge sample placements. Because these electromagnetic fields
are theoretically symmetrical fields, it would be expected that similar symmetry is shown
in the field measurements for the sample placement at the edges. But, this was not the
case. Incidentally, it should be noted that similar results have been observed in heating
experiments where the graphite epoxy composite sample show severe localized burning
at the top left or top right comers of the sample. Possible reasons could be due to the
coupling probe allignment and depth which could have caused near-field interactions
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
87
with the sample. In summary, the axial field measurements have indicated that in
general, a mode with a z index of 2 was generated for the elevated sample loads, and that
with an index of 1 was generated for the lowered sample loads.
The circumferential field measurements are shown in Figures 4-10 through 4-15
and in Table 4- 2. In these plots the y-axis is the measured E-field and the x-axis is the
circumferential field measurements along five columns (3,4,5,6,7) and four rows(24,25,26,27). There were four circumferential rows which were numbered from 25 to 27
and each row contained five columns which were numbered from 3 to 7(see Figure 4-3).
Row 27 and 28 were at the same level as the coupling probe and column 5 is 180 degrees
away from the coupling probe.
Figure 4-10 shows the empty cavity circumferential field measurements for a
TM(012) mode at two different probe depths at a fixed cavity length where resonance
was achieved. The electric field measurements for the short probe depth at row 25 was
constant along the four columns. However, as the field measurements were taken close
to the coupling probe height at row 26 to 28, the field became more variable. When the
probe depth was increased to 19.28 mm the measured field also became variable for all
the rows. This indicates that the coupling probe location and depth has an effect on the
field pattern in the cavity.
Figures 4-12 through 4-15 and Table 4-2 show the circumferential field
measurements for the different sample loads. In these Figures the enumerated sample
placements, center corresponds to position 1, top left to position 2, top right to position 4,
bottom right to position 6, and bottom left to position 8. In Table 4-2 the third column
shows the row number where constant field measurements were achieved.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
88
This table shows that constant field measurements were obtained in the region around
row #26 for the elevated position, and around row #25 for the lowered position for most
of the sample loads. In all cases, the field measurements were more variable as the
measurements were taken closer to the coupling probe.
In summary, the circumferential field measurements suggest that interactions
from the coupling probe due to near field could have a significant effect on the fields in
the cavity. This effect could be due to the interactions between the sample and the
coupling probe or the electric field measuring micro coax probe and the coupling probe.
Ideally, the electric field diagnostic holes should be placed away from the coupling probe
to avoid such interactions. As such, these results cannot be used for conclusive
characterization of the loaded cavity mode. They can be used to map the field
characteristics of the in-plane field pattern as a function of sample placement, which
indicates that the regions of the sample that are close to the coupling probe interacts
differently than those away from it. This is shown by the similarity in the field
measurements for the top left and top right regions and the bottom right and bottom left
regions , as shown in Figures 4-12 through 4-15.
Figure 4-16 through 4-19 and table 4-3 show the frequency shift measurement for
the different sample loads. In Table 4-3, column 3 shows the frequency shift behavior
along the different quadrants of the cavity. An S was used to indicate if the shift was
similar to the other regions of the cavity and N for non-similar. Typically, as a sample is
placed in a region of high electric field, there is a large shift in the resonant frequency
shift according to perturbation theory(Harrington, 1961). As such, the frequency shift
measurements can be used to map the electric field intensity in the cavity.
For the polyester / glass and Nylon samples the shifts were essentially similar in
all four quadrants of the cavity. However, for the graphite / epoxy samples the
frequency shifts were different in the 90 0 - 180 ° and 1800- 270 0 quadrants of the
cavity, which is consistent with what was observed in the circumferential field
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
89
measurements. The measurements did not suggest a strong function of fiber orientation
for the graphite epoxy samples. The frequency shifts were always greater for the
elevated samples placements than for the lowered sample placements. This is an
indication that the field intensities are higher at the elevated sample location than at the
lowered sample location.
To this point the concept of near-fields interaction has been mentioned and not
explained. Near field interactions are known to occur in regions close to the microwave
coupling device. At regions close to an antenna, (about half of the free space wave
length), it is common to observe the effects of near-fields(Risman 1987). Near-fields
are conditions in a region in space where the time average of the non-radiating energy
density exceeds the radiating energy density. Thus, in the presence of near-fields the
electromagnetic field distribution can be significantly altered. The result of altering the
field distribution is that the electromagnetic energy distribution becomes less predictable
and heating uniformity becomes less predictable, if not impossible. It would seem logical
to design a cavity such that the samples are placed at a distance away from the antenna to
minimize the near-field effects, however it is not that simple. Not all of the near field
effects are bad. It becomes a problem when the electric field patterns are altered such
that heating uniformity is lost In the cavity used the phi-axis radial electric field
measurement holes were located at the same vertical height as the coupling probe which
could have contributed to the near-field interactions.
4.3.3 Loaded cavity mode estimation
The axial electric field measurements only provided conclusive information about
the value of the z-index of the loaded cavity mode. The circumferential field
measurements were not consistent enough to confirm the presence of a <j>-symmetric
mode. However, the field measurement from the top of the cavity did indicate the
existence of a TM. Hence, it was assumed that the loaded cavity mode used in the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
90
measurements was a TM-mode that would have been excited at an empty cavity length
of 14.39 cm versus the 12.97 and 11.97 cm in the loaded cavity. It was also assumed that
the decrease in the empty resonant cavity length was due to a summed effect from the
Teflon block and the sample load. However, since the samples used in the previous
measurements were quite small, the decrease in the empty cavity length was assumed to
be due mainly to the Teflon block.
Using the frequency shift method discussed above, experiments were done to
measure the resonant frequency shift for a 3-inch square by 0.25 inches thick Nylon,
polyester / glass, and epoxy / graphite sample loads. In these experiments the Teflon
loaded cavity resonant reference of 2.45 GHz was used as the reference. The cavity
length decrease was assumed to be proportional to the change in the resonant frequency
and was calculated from the following:
Ac/ -
total cavity length decrease due to sample load
Ecl -
empty cavity length, cm
Tcl -
Teflon loaded cavity length, cm
fx
sample loaded cavity frequency, GHz
-
fa -
Teflon loaded resonant frequency, GHz
Since it was assumed that the loaded cavity mode was a TM(012) mode, the
resonant empty cavity length was 14.4 cm. Table 4-4 summarizes the results for the
cavity length shift calculations for the three different sample loads.
The Teflon loaded
cavity lengths that were obtained from the previous measurements for the lowered and
elevated sample shown in column 2. The actual center frequency for the sample loaded
cavity is shown in column 3, and the total cavity length shift in column 4. The lowered
sample cavity length shift for all the samples was between 1.6 cm and 1.8 cm, and that
for the lowered sample placement was between 2.6 cm and 2.9 cm. Hence, it was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
91
Sample
^CL
(cm)
Nylon (elevated)
(lowered)
Polyester Glass
12.97
11.97
12.97
11.97
12.97
11.97
Graphite Epoxy
fl
(GHz)
2.381
2.431
2.379
2.364
2.423
2.413
V fo
0.972
0.992
0.971
0.965
0.988
0.985
Ad
(cm)
1.81
2.54
1.81
2.86
1.59
2.61
Table 4- 5 Total cavity shift for a loaded cavity for a TM(012) mode
assumed that the loaded cavity length shift due to the samples and Teflon block for the
lowered sample placement was approximately 3.0 cm.
To verify these results, two separateexperiments were done to heat 3 inch square,
unidirectional 24-ply graphite / epoxy and vinyl ester / glass composites. Samples were
placed on a Teflon block with four temperature probes attached to the surface of the
sample as shown in
Figure 4-1 Sample Placement The lower sample height (1.53 cm
from the base of the cavity) location was used. All tunable loaded cavity modes ranging
between 8.0 cm and 20.0 cm were used to heat the samples. Each sample was heated for
thirty minutes or until a temperature gradient of 10° or more was achieved. During
heating, the cavity was automatically tuned as needed and the power level was set at 100
% output which varied between 80 Watts and 90 Watts.
43.3.1 Results and Discussions
Table 3 summarizes the graphite / epoxy heating results and table 4 summarizes
the vinyl ester / glass heating results. Five tunable modes were located for the vinyl /
ester / glass sample and four for the graphite epoxy composite. In the empty cavity,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
92
there are six tunable modes between 8 cm and 20 cm. In these tables, column 3 lists the
measured temperatures, which are ordered from the best heating site to the w orst By
using the calculations discussed above an estimated cavity length was determined and is
shown in column 4. By inspecting the heating patterns, the corresponding theoretical
modes were determined which are listed in column 6 along with the corresponding
theoretical cavity lengths in column 5. In column 7, the actual cavity length which is the
difference between the heated cavity length and the theoretical cavity length are shown.
These results show that in general, the loaded cavity length shift estimation
method is relatively accurate for lower order modes and not for higher order modes.
This is not surprising since higher order modes tend to be closer together in frequency
and cavity length and multiple-mode overlapping are more prominent which makes mode
identification more complex.
It is important to note that these theoretical cavity modes only indicate that the
heating modes have similar characteristics as the corresponding theoretical ones. Loaded
cavity modes are not necessarily unique as the empty cavity modes. They typically
overlap other modes and exist as hybrids of other modes(Asmussen, 1987). The purpose
for identifying the loaded cavity modes is to provide an initial estimate such that mode
selection and tuning dung processing can be enhanced without the need for heating
experiments to characterize the modes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
93
Heating
CL
(cm)
Probe
Depth
(mm)
Heated sites
Estim.
empty
CL
(cm)
Calc.
empty
CL
(cm)
Calc.
mode
8.12
22.5*
T3=T4>T2>T1
11.12
11.28
TE(Oll)
Actual
A CI =
(Calc. CL) (Heat CL)
3.16
11.91
22.8*
T2>T1>T3>T4
14.9
14.40
TM(012)
2.49
12.44
6.65
T4>T2>T3>T1
15.44
15.62
TE(311)
3.17
13.47
18.2
T4>T2>T1>T3
16.47
16.47
TE(212)
3.0
19.73
20.3
T1>T3=T4=T2
22.73
21.62
TM(013)
1.88
Table 4- 6 Graphite Epoxy Heating Results
Heating
Cavity
Length
(cm)
Probe
Depth
(mm)
Heated sites
Estim.
CL
(cm)
Calc.
empty
CL
(cm)
Calc.
mode
8.67
7.67
T1>T3>T2>T4
11.67
11.283
T M (lll)
11.55
11.39
T1>T3>T2>T4
14.55
14.404 TM(012)
2.85
13.41
5.72
T3>T4>T2>T1
16.41
16.470
TE(212)
3.0
14.03
3.97
T4>T3>T2>T1
17.03
hybrid
hybrid
----
16.18
5.12
T3>T2=T4>T1
19.18
20.071
TE(113)
3.89
Actual
Ad =
(Calc CL) (Heat CL)
2.56
Table 4- 7 Polyester / glass heating results (Fellows , 1995)
From the heating results in Table 4- 6 and Table 4- 7 there are other noteworthy
results which do not necessarily relate to loaded cavity length estimation but to the
understanding of the heating characteristics of the two different composites. Longer
coupling probe depths were required to tune the cavity loaded with graphite / epoxy
sample than the vinyl ester / glass sample. With the longer coupling probes, the top right
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
94
or the top left comers of the sample always heated preferentially. This was also observed
in the frequency shift measurements from the cavity characterization section and was said
to be due to near-field interactions from the long coupling probe. For the vinyl ester /
glass sample, the edge and center heating sites were more uniquely defined than for the
epoxy / graphite sample. This could suggest that there is more mode overlapping with
the graphite / epoxy sample. This is not surprising since the graphite / epoxy sample is a
more complex system electrically than the glass vinyl ester / system.
4.4 Sum m ary
All theoretical cavity modes available in the 8.0 cm radius by 40 cm long cavity
were calculated theoretically and measured experimentally. A total of 17 modes were
calculated theoretical of which 15 were located in the empty cavity using the low power
sweep oscillator. There were 12 TE-modes, 5 TM-modes, and 2-degenerate modes at
theoretical cavity lengths of 11.283 cm and 22.566 cm. The cavity quality factor was
calculated for each mode and ranged from 4,000 to 23,000 where the TE modes have the
higher values.
The loaded cavity characterization results showed th a t:
a)
Although the Teflon block used for sample placement is electromagnetically
transparent and would not preferentially heat, it does have an effect on the shape
of the field pattern in the cavity and thus the heating pattern, as was shown form
the axial electric field measurements.
b)
Sample placement height can affect the field distribution along the z-axis as was
shown in the axial field measurements.
c)
Longer coupling probe depths have an effect on the circumferential field pattern
due to near-field interactions as was shown in the circumferential field
measurements.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
95
d)
There was field symmetry when the samples were placed at a lower level, away
from the coupling probe than at the level parallel with the coupling probe as
shown from the frequency shift results.
e)
Variant field behavior was more prominent for the graphite epoxy composite
than for the polyester glass or the nylon samples in all the measurements.
f)
Variant field behavior was more pronounced at the elevated sample location than
at the lowered sample location in all the measurements.
g)
Variable field behavior was more pronounced at regions close to the coupling
probe than away from it as was shown from the circumferential field
measurements.
A simple empirical method was developed for calculating the loaded cavity
modes for using graphite / epoxy and vinyl ester / glass composite. The method was
found to be more accurate for lower order modes than for higher order modes for both
samples.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 5
AUTOMATION AND CONTROL OF THE SINGLE-MODE
RESONANT CAVITY
5.1 Introduction
Virtues of microwave processing of composites in a single-mode resonant
cavity have been well demonstrated as a viable alternative to thermal processing at
the lab-scale(Wei, 1991; Jow, 1988 ; Fellows, 1992 ; Vogel, 1989 ) where the single­
mode resonant cavity was operated as manual device that is impracticable as a
process. The hardware consisted of four basic components: 1) tunable cavity with
gear drives for manually adjusting the cavity length and probe depth, and
micrometers for measuring the cavity length and probe depth; 2) an external circuit
for processing with major components which included a microwave power source,
power meters, and a dummy load to absorb reflected power; 3) an external circuit for
diagnostics and for dielectric analysis with major components that includ sweep
oscillator, power meters an oscilloscope; 4) fiber optic thermometry for invasive
temperature measurements; 5) computer interface for automatic data acquisition of
input power, reflected power and temperature for post-analysis, and the on/off
control of a switch to direct the microwave power away or to the cavity.
In processing, the microwave cavity was operated as an open-loop system
where a seasoned operator was the necessary link to close the control-loop. A typical
processing activity included; 1) the continuous tuning of the cavity by the manual
rotation of dial knobs to adjust the cavity length and probe depth to m in im iz e the
reflected power; 2) the frequent selection of new cavity modes by carefully adjusting
96
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
97
the cavity length and probe depth to new locations, while tuning the cavity and
manually modulating the input power to achieve uniform heating; 3)temperature
control by the automatic on/off control of an electronic switch to direct the
microwave to or away from the cavity. Data acquisition was done as a separate
activity using both analog and digital ( RS- 232) interface which required
complicated program device drivers. Hence, the cavity was operated as an
independent device from the external circuit, and data acquisition unit and processing
results varied from one operator to the other and optimization was difficult at besL
In diagnostics and dielectric measurement, the cavity is also manually tuned
to generate a power absorption curve on an oscilloscope. This curve is then analysed
by inspection to calculate the cavity resonant frequency shift and cavity Q-factor(see
Chapter 3). The cavity resonant frequency shift and the Q-factor are both important
parameters in the calculation of the dielectric constant and loss factor. Although,
dielectric analysis the single-mode resonant cavity is considered to be one of the
most accurate methods(Jow , 1989) the manual measurements technique results in
inconsistent results.
Thus, in order to realize the potential of this technology as an alternative to
thermal processing it was advanced as an integrated process through automation. In
the automation, a control system was designed and built in addition to the
development and implementation of a set of sophisticated and comprehensive control
software programs for controlling the curing process in the cavity.
The single-mode resonant microwave process is governed by discrete
electromagnetic modes and a complex non-linear interactions of electromagnetics
and material. Mathematical models that completely and accurately describe the
dynamics of the microwave process were found to be complex and computationally
intensive for control purposes. Although the control of the microwave process is a
novel concept, the difficulty in modeling the dynamics of the process is a control
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
98
problem which fits a class of control topics under non-traditional control
methodologies or intelligent control system(see Chapter 2).
Using elements of traditional and non-traditional control methodologies, a
closedloop feedback control system was developed to satisfy the control objectives
of; efficient energy coupling or mode tuning, mode selection and uniform heating,
and controlled heating. Efficient energy coupling was treated as a mathematical
minimization problem in which a 2-dimensional simplex minimization search
technique was used to implement the controller. Uniform heating was based upon
heuristics and empirical data that was derived from the cavity characterization
results, and controlled heating was based upon traditional PID(proportional-integralderivative) method.
Additionally, although the integral part of this work was focused on the
automation of the processing system, elements of the diagnostics system was also
automated using GPIB(General Purpose Interface Board) and A/D interface. Thus,
allowing for the automatic and more accurate measurement of the cavity parameters
for diagnostics, and the potential for eliminating the oscilloscpe and the manual
measuring methods.
This chapter is dedicated to the presentation of the control software
developments to automate the single-mode resonant cavity for processing.
Hardware automation components and data acquisition interface are summarized in
this chapter with additional information in Appendix A. The developed LabView
control software program for automating and for controlling the microwave curing
process is also presented. Finally, elements of the hardware and software for the
automation of the diagnostics system is also discussed.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
99
5.2 Cavity Automation
5.2.1 Cavity Description
The designed control system included an automated single-mode resonant
cavity and a microwave circuit in which all the instruments were interfaced with a
data acquisition unit. The manually operated microwave circuit design was
essentially modified to accommodate automatic data acquisition, and the microwave
power source or magnetron was modified for automatic analog manipulation. The
core of the hardware automation was the design and fabrication of the cavity with
mechanized drives.
The cavity is a cylindrical brass tube with moving parts that include internal
transverse shorting plates, and a coupling probe. The transverse shorting plates are
adjustable to vary the volume of the cavity. The material load rests on the bottom
shorting plate which is fixed in place during processing and is removable for material
load. Microwave shielding material or gasket made out of silver called Finger Stock
(Varian CF-300) are soft soldered around both base plate and shorting plate. The
purpose of this is to provide good electrical contact between these transverse plates
and the cavity wall and eliminate microwave leakage.
A semi-rigid 50-ohm impedance brass coaxial probe serves as a field
excitation or coupling probe to couple microwave power into the cavity. The probe
is 0.25 inch in diameter and 2 inches long. There are a series of axial and
circumferential holes equally spaced 0.92 cm apart through the cavity wall. These
holes are used to measure the square of the electric field strength along the cavity
walls. A 2 mm copper micro coaxial probe is used as an E-field diagnostic probe.
Figure 5-1 shows a picture of a typical cavity that is disassembled into the major
components. From clockwise in Figure 5-1, there is the shorting plate which is
attached to the gear driven drives, the body of the cavity, the base plate with the
attached fingerstock, and the coupling probe.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100
5.2.2 Automated Cavity and Mechanized Drives
The overall cavity design was not novel and had been previously developed
with either an axial or a radial mounted coupling probe(Asmussen, 1988). What was
novel about this cavity design was the mechanized drives and the implementation of
dual mounted coupling probes at the axial and radial positions.
The automated new cavity was designed with mechanized drives for the shortingplate, base plate and coupling probe. The new cavity is 18.9 cm (7 inches) in
diameter and 42 cm tall which is 1.5 times taller than the manually operated
cavity(see Figures 5-2 and 5-3). This increase in cavity height allowed for the
excitation of 12 additional modes (see table 4-1). Figure 5-2 (a,b,c) show a
schematic of the manually driven cavity, a picture of an actual cavity, and the gear driven manual drive, respectively. Figure 5-3(a,b,c) show a schematic of the
automated cavity, a picture of the automated cavity and a mechanized drive,
respectively.
The mechanized drives use a belt and pulley system in which a stepper-motor
drives a ball screw(see Figure 5-3 c). By driving the ball screw rather than the knot
the frictional losses are minimized. There are two stainless steel 1/4 inch, diameter
guide rods which ride in graphite bearings. The use of the linear bearings rather than
the spur gears eliminated the binding problems that existed in the manual cavity.
Detail discussion of the drives are presented in Appendix A. Linear motion
potentiometers were installed on the cavity for measuring the cavity length and probe
depth. These potentiometers were calibrated by generating voltage and length
relationships for the cavity length and probe depth(see Appendix A) for details.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
101
Figure 5-1 Single-mode Resonant Cavity Components. From clockwise, shorting
plate and drive, cavity body, base plate with fingerstock, coupling probe
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
102
Coupling
Probe
Shorting Plate
T
Lc
Lp
Figure 5- 2a Schematic of manually operated single-mode resonant Cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
103
Figure 5-2b Picture of manually operated single-mode resonant Cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
104
Figure 5-2c Drive for manually operated single-mode resonant Cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
105
Short-plate
Drive
Jhort-plate
Inger Stock
-Top Coupling
Probe
S aijj^ e
Side Coupling
Probe
Figure 5- 3a Schematic of automated single-mode resonant cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
106
Figure 5-3b Picture of automated Single-mode resonant cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
107
Figure 5-3c Mechanized drive for automated single-mode resonant cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
108
Stepper Motors and Drivers
5.2.2.1 Stepper Motor and Driver Hardware
The stepper motor is an 8-lead motor and driver unit which was designed
specifically as a high performance positioning device(see Figure 5-4). The structure
was constructed with class B insulation material which is capable of withstanding
temperatures at the motor coil of 130°C(255 F) with no reduction in motor life. It
has a holding torque capability of 118 oz-inch and variable step angles of 0.9° and
0.45° which corresponds to 400 pulses per revolution and 800 pulse per revolution,
respectively. Table 5-1 summarizes the corresponding cavity length and probe depth
adjustment speeds.
Movement of the stepper motor was accomplished by regulating the number
and frequency of pulses sent to the driver's terminals. The motor rotates one step for
each pulse received at the pulse terminal and the direction of rotation is controlled by
the signal applied to the driver's "CW/CCW" terminal. The interface to the drivers
was by a non-standard TTL(Transducer Transducer Logic) digital logic IC(integrated
circuit). A signal at (4-5 V) was defined to be high while a signal at (0-0.5V) was
Motor Pulses
Pulses / revolution
800
400
Speed of Cavity length
adjustment
[cm/min]
[in/min]
10
25
5
12
Speed of Probe depth
adjustment
[cm/min]
[in/min]
180
70
90
35
Table 5-1 Cavity Length and Probe Depth Adjustment Rates
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
109
Stepper Motor Driver
©
Pulse
©
SW1
©
©
CW CCW
SW1
2-1
©
©
2-2
O
z
o
StripedBlack—
Red----OrangeYellow-
©
’a j
©
©
©
©
AC115
FG
Striped
Stepping
Motor
Black
Red
Orange
Yellow
Figure 5- 4 Stepper Motor and Driver
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
said to be low.
110
In a standard T IL interface a signal at (2.4 V- 5.2V) is defined as
high and that at 0.0V-0.8 V is low. Thus, a standard TTL wiring could not be used
and a 5V external power supply(Tucker Power Supply) was necessary to drive these
terminals(see Figure A-2 and Appendix A).
5.2.2.2 Stepper Motor Driver Software
The stepper motors were driven by sending pulse signals to the driver. The pulse
signals sent to the stepper motor drivers were generated as a sub-program called
“Move Read.vi” using the LabView software(Appendix E). The input to the
software driver were motor direction and number of pulses. Given this input a DOloop is iterated N times where N is the number of pulses. For every iteration a high
or low voltage signal is sent to the pulse terminal to move the motor. This pulse is
generated by comparing the remainder(R) of the iteration (i) number divided by
two, to one. If the remainder is equal to one then high value is output and a low
value is output otherwise. The output voltage is alternated between high and low to
generate a continuous pulse to the driver pulse terminal. A logic diagram is shown in
Figure 5-5 and its equivalent in LabView is shown in Figure 5-6. In the LabView
program the DIG-LINE box represents a subroutine for driving the digital line on the
data acquisition board.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ill
i <N ?
no
Exit
yes
i = i+1
R = i/2
Ouput = 0 (Low)
or 0 - 0.5 V
R=1
yes
Ouput = l(High)
or 4 - 5 V
Figure 5- 5 Program Logic for Stepper Motor Driver
Number of Pulses
Pulse Terminal
HEH
DI O
LINE
Figure 5- 6 LabView Program Version of Figure 5-6
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
112
5.2.3 Cavity Length and Probe Depth Measurement
The cavity length and probe depth are measured by a 10000 ohm linear
motion potentiometers with stroke length of (X and Y ), respectively. The
potentiometers require a 5V reference. The operation of the linear motion
potentiometers can be described by the principles of variable resistors. It consist of a
resistance elem ent, and a sliding arm that makes contact with the stationary
resistance element, see
Figure 5- 7. A Tucker variable power supply is used to
supply the 5V reference required by the potentiometers. There is a local digital
display for the potentiometer readings. This indicator requires a 9V power supply
which is also provided by the Tucker variable power supply.
The potentiometers axe calibrated by determining the relationship between the
cavity length and probe positions and output voltage. A linear relationship was
determined for the voltage and cavity length and probe depth positions as in
Equation (5-1)
Cl = a x ( V ) + kc
, o
Pd = P x ( V ) + kp
(5-i)
where
Cl = cavity length [cm]
Pd = probe depth [mm]
a = slope [Volts/cm)
P = slope [Volts/mm]
kc = y-intercept [cm]
kp = y-intercept [mm]
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
113
□ a
□ »
sv
O u t p u t V o lt a g e
□ c
S tro k e
Figure 5- 7 L inear Motion Potentiometers
5.3 Microwave System Automation
5.3.1.1 Automation of External Circuit
The processing external microwave circuit is made of the following
components, magnetron microwave power source, power meters, dummy load,
circulators, directional couplers and a crystal detector see Figure 5- 8 and Figure 5-9.
The diagnostic system consist of the same components except for that the power
source is replaced by a sweep oscillator and the power meters are replaced by the
oscilloscope. Coax cables are used for all connections and all devices are rated for a
power level range of 0-100 Watts at a frequency of 2.45 GHz. The magnetron is a
continuous wave, single frequency, multi-power microwave power source which
operates at 2.45 GHz and a power range of 0-100W.
Incident power from the magnetron is decoupled by a directional coupler,
where one end is directly sent to the cavity and the other end is attenuated and sent to
a power meter. A circulator directs the incident wave to the cavity and the reflected
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Microwave Processing System- Cavity. External Circuit & Flouroptic Thermometry
•
Cavity
Shorting Plate stepper
motor & driver
Oscilloscope
31
flQ
C
3
Ln
i
00
CO
otr
at
3
p
a.
o
Base Plate stepper
motor & driver
Sweep
Oscillator
Mechanical
Switch
Circulator
MW Source
Variable
Pwr Supply
Coupling Probe stepper
motor & driver
Shorting Plate
Circulator
S1
Directional
3
e.
n
H•
o
c
Load
28V Pwr
Supply
Coupling Probe
Potentiometer
Variable
Pwr Supply
Luxtron A
Luxtron B
Mac Ilci
Terminal
Box
A/D Board
GPIB Board
115
Figure 5- 9 Picture of Automated Microwave Processing System
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
wave to a dummy load. The temperature sensing system uses fluoroptic thermometry
technology and is manufactured by Luxtron(Luxtron, 1989). The system consist of a
sensing unit and fiber optic probes which have a capability for measuring
temperature in the range of -190 to 300 °C. It is made up of a silica fiber with a
Teflon jacket body and sensing tip made up of magnesium fluorogermanate. The
sensing unit is made up of optical heads, Xenon flash lamp, lenses and a filter. To
sense temperature the Xenon lamp emits a blue light which excites the sensor and
causes the sensor to exhibit a deep red fluorescence. The time at which it takes the
intensity of the red light to decay is exponential with time and is inversely related to
temperature. Each unit has 4-channel fiber optic temperature sensing probes with a
capability for local display, analog output and digital output
In the microwave processing circuit the power meters and temperature
measurement were interfaced using analog output (AI). This was an improvement
from the previous state of technology where the temperature measurement was
interfaced using digital input (RS-232) and the other meters were interfaced using
analog input. Although, this was a perfectly valid data acquisition system it required
very complex device level driver programming which was very difficult to maintain
or to add additional sensors.
The power meters and the temperature sensing units are analog sensing
devices with a voltage signal range of 0-5V for a zero to full scale values. These
analog devices were available with analog output connections as BNC type
connectors. A Coax cable with one end attached to a BNC type connector and the
other end stripped as the conductive wire was used to connect the analog devices to
the data acquisition board. The connection to the data acquisition board was done
using “single-ended" connection configuration, where one end of the wire is hooked
to the channel and the other to ground on the terminal box.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
117
5.3.1.2 Automation of Microwave Power Source
The magnetron, microwave power source was interfaced as an analog output
to the data acquisition board. The source was augmented using integrated circuitry to
provide for the analog output interface. This was a major contribution to the external
circuit which allowed for the automatic regulation of the microwave input power. In
the previous state of technology, the source was not directly controlled but the
output was either directed to or away from the cavity using a digital switch. The
power directed away from the cavity was directed to a dummy load which was
wasted which was an inefficient use of the microwave power. Additionally, the
on/off or band-bang control method did not provide for a wide variety of processing
flexibility. The magnetron modification allowed for both manual and automatic
regulation of the output power. See Appendix A for details of operation. The
voltage range of 0-100 W of the source corresponded to an analog output range of
0-1.112 V in an almost linear relationship Figure 5-10.
Magnetron Input Power vs. Voltage Calibration
0.8
0.6
0.4
0.2
0
20
40
60
80
100
Output Power (W)
Figure 5-10 Magnetron Power versus Voltage Calibration for Analog Control
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
5.4 Data Acquisition
5.4.1 Data Acquisition Hardware
The manual system external circuit was essentially automated by interfacing
with a data acquisition board. A National Instrument data acquisition and control
board NBMI016H was used. This board is a high performance multi-functional
analog, digital, timing input/output board for the Macintosh computer. This board
has a maximum sampling rate of 47 K samples per second. It is a 12-bit successive
approximation ADC(analog to digital converter) with 16 analog inputs, two 12-bit
DACs with voltage outputs, 8 lines of TTL-compatible digital I/O, three 16-bit
counter/timer channel for timing I/O, 2-(5 V) power supply, 1-digital ground, 1analog input ground, and 1-analog output ground(see Appendix A for details and
Table A-2).
5.4.2 Data Acquisition Software Program
Data acquisition is one of the main attributes of the LabView software
package. It is provided with several data acquisition device drivers (subroutines)
which are easily modified to meet any data acquisition needs. These drivers are
available in the data acquisition library. Typically a configuration subroutine is
required to configure all the device channels where data will be acquired, coupled
with the data acquisition subroutine. The data acquisition subroutine is used to set
the signal type and signal ranges and the number of data points to acquire per
channel. The subroutine “AI Group Config” was used to configure the channels and
“Single scan” was used to acquire the data values (see Appendix E).
In this work there were 16-temperature channels, 2-potentiometers channels
for cavity length and probe depth, and 2- power meters for input and reflected power.
The temperature, power meters and potentiometers were interfaced as analog input.
The stepper motors were interfaced as digital output, and the power source was
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
119
interfaced as analog output. To minimize noise in the acquired data, each channel
was scanned multiple times (10 times) and the values averaged. Scanning rate was
set at 1000 samples per second.
Up to this point the elements of the system hardware including the cavity,
data acquisition and its interface with the external circuit have been discussed. A
schematic of the overall microwave processing circuit is shown in Figure 5-8 and a
picture in Figure 5-9. In the sections that follow the control software for the
controllers in the controlloop are discussed in detail. These components are the mode
tuning controller, mode selection controller and the power controller as shown in the
controlloop in Figure 1-1 and in this chapter as in Figure 5-11.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
31
n>-1>
T (sctpoint)
Pow er C ontroller
M agnetron
C avity
O
T em perature (T)
o
a
g
Tem perature Sensor
Luxtron
AT (sctpoint)
M ode Selection
Controller
3
3
O
a*
a
Tem perature G radient
, (AT)
h-»
IYef (sctpoint)
M ode T uning
C ontroller
Cavity Length
Stepper M otor
Probe Depth
Stepper M otor
Tem perature Sensor
Luxtron
Cavity Length
Stepper M otor
Probe Depth
Stepper M otor
Reflected Pow er
(W )
Reflected Power
M eter
121
5.5 Process Control Software
5.5.1 Efficient Coupling- Mode Tuning
For a given cavity geometry, dimension and microwave excitation frequency,
the theoretical cavity lengths at which different modes can be excited can be
calculated(mode chart). This calculation is done from the cut-off frequency
Equations (3-16 & 3-17) which are derived from the eigenvalues to the solution of
Maxwell's equations. Once the cavity is loaded with material the empty cavity
modes become modified(Asmussen, 1974) and sometimes become in general hybrid
modes(Harrington, 1961).
Modification of the theoretical modes is a function of material placement,
volume, shape and electrical properties. The dependence on electrical properties
introduces additional material conductance and susceptance to changes in the
external circuit. As the material is heated the dielectric loss factor changes, along
with material geometiy and the required frequency of the microwave source to
maintain the mode. Since the changes that alter the resonant frequency are not
available, due to unavailable sensing techniques or prohibitive computation, it is
difficult to develop a control system for tuning from traditional control
methodologies. Hence, a radical approach is used where mode tuning is described as
a mathematical function minimization problem.
5.5.1.1 Approach
This approach is based on the justification that, conceptually mode tuning is a
minimization technique which can be described by a 3-dimensional surface.
Mathematically, mode tuning can be described as the minimization of a convex
objective function with constraints. For this type of problem an analytical search
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
122
technique could be used if the objective function is readily continuous and
differentiable and could be expressed as a mathematical function. This function
could be either as a result of mathematical modeling or curve fitting of a numerical
data(Beveridge, 1970 ). Stated differently, the analytical technique would require
extensive knowledge about the objective function, but will provide all to the function,
which is not the case with the numerical methods.
In mode tuning, extensive knowledge about the objective function cannot be
easily derived apriori, and all minimums need not be found. Thus, a numerical
search technique was chosen to be a sufficient method. The general principle of the
numerical search methods can be described as what is called "homing in"(Boas,
1963 ).
These search techniques are based on the determination of a base point,
from which a search method selects a new set of independent variables and tests the
objective function to see which set gives a better value for the objective function.
Based upon the comparison, another set of variables are chosen and the objective
function re-tested and re-evaluated. The key is to optimize the set of variables
selected such that wasteful computations and experimentation are minimized.
There are different numerical methods that can be classified as direct search
methods or gradient search methods. Direct search methods require only the
evaluation of the objective function at a particular location, while the gradient search
method requires both the evaluation of the objective function and the gradient The
direct search method may therefore be found to be efficient as far as computation
time, although it might not move in the best direction every time. Instead of seeking
directional accuracy in movement, the emphasis is placed upon speed of computation
and movement
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
123
5.5.1.2 Simplex Method
The simplest direct search method is the univariate method. In this method
where the total number of variables is n, one variable is changed at a time by
keeping n-1 of the n variables fixed. This technique has the tendency to oscillate as
the optimum is approached and may not even converge, depending on the shape of
the minimum, i.e. if the minimum is narrow as compared with the step size, or if the
minimum is not parallel to the coordinate axis(Beveridge, 1970).
Thus, the
univariate method is not robust and may be applicable to very specific cases where
the variables do not interact and each variable can be optimized independent of the
other.
However, in actual situations variables do interact, and a more robust
approach would be not to fix the direction or the step size of the search direction, but
to permit these parameters to change as a result of experimental data or evaluation of
the objective function. A good illustration of such a method would be the Sequential
Simplex Method which was first proposed by(Spendley, 1962 ). It is a highly
efficient, multi-factor, empirical feedback strategy that requires neither the large
number of experiments nor complex calculations of the evolutionary
operation(Morgan, 1974). A modification by Campey and Nichols(Campey, 1961),
Nelder and Mead(Nelder, 1963 ), and Box(Box, 1965) of the original simplex
method provides the capability for acceleration in directions that are favorable and
deceleration in directions that are unfavorable. These simplex systems are
conceptually different from the simplex methods used in linear
programming(Beveridge, 1970).
The method takes a geometric figure, known as a simplex as a basis. A
simplex is a geometric figure defined by a number of points equal to one more than
the number of dimensions of space. A simplex of two dimensions is a triangle, a
simplex in three dimensions is a tetrahedron. Experiments or evaluations are
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
124
organized such that the objective function is evaluated at the points formed by the
vertices of the geometric figure. One vertex is then rejected as being inferior in value
to the others.
The general direction of the search may then be taken in a direction away
from the center of gravity of the remaining, being chosen so that the movement
passes through the center of gravity of the remaining points. A new point is then
selected along this direction and the search proceeds by the process of vertex
rejection, regeneration and reflection, and figure expansion until the figure straddles
the optimum and is contracted to the optimum.
The simplex method adapts itself to local landscape, elongating down long
inclined planes, changing direction on encountering a valley at an angle, and
contracting in the neighborhood of the minimum. It is simple to program, and has
the advantage of needing only the smallest number of points to start and only one
new evaluation for each movement. Its main disadvantage lies in the necessity to
scale the problem beforehand so that unit changes in each variable are of equal
interest in its ability to accelerate. This is the method that was programmed as the
tuning rule in the single-mode resonant cavity.
5.5.1.3 Simplex Logic
Define
- yi for the function value at (point i) Pi,
- h(high) as the suffix such that y h = max (yi)
z
-1 (low) as the suffix such that yi = min (yi)
t
- n (in between) as the suffix such that yh < yn < yi
-Pbar as the centroid of the points with i not equal to h
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
125
An initial base simplex is defined by a set of coordinate axes with each
representing a Best, Next-to-Best, and Worst point, respectively see figure 5-2. At
each stage of the search the worst point, Ph is replaced by a new point using the
operations called, reflection, contraction, and expansion.
Reflection
The coordinate of the reflection point is defined as:
P' = ( l + a ) P - c c P h,
(5-1)
where P* is the reflection point and a is the reflection coefficient which is a
positive constant that is greater than one.
If the reflection point generates a function value y*> which is between yh and
yi, then Ph is replaced by P* and the a new simplex is formed.
Expansion
If y* < yi >i-S- if the reflection point is better than the best point, then the
current reflection point is expanded again as:
t
/>“ = yP* + ( l + r ) F
(5-2)
where gamma is the expansion coefficient, and is greater than unity.
If y**< yi, replace Ph by P** and restart the simplex operation, but
If y** > yi then the expansion has failed so replace Ph by P* and restart A
failed expansion may be thought of moving passed the minimum.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
126
Contraction
If on reflecting P to P*, y* > yi for all i not equal to h, i.e. that replacing P by
P*, y* becomes the worst point. Then a new Ph is defined to be either the old
one or P* whichever has the lower y value, and form:
P“ = p P h + ( \ ~ P ) P ,
(5-3)
where beta is the contraction coefficient and lies between 0 and 1. Ph is then
replaced by P** and then the simplex is restarted.
If the contracted point is worse than the better of P* and Ph then the
contraction has failed and all points are replaced by: (Pi+Pl)/2 and the
simplex is restarted. Failed contraction can occur when a valley is curved and
one point of the simplex is much farther from the valley bottom than the
others, and thus contraction may cause the reflected point to move away from
the valley bottom rather than towards it.
A flow diagram of this logic is shown in Figure 5-11, implementation in
LabView is in Appendix E. To further illustrate this method an example is used
below. Consider three base points identified as B for best, N for next-to-best, and W
for worse which form the vertex of the initial simplex BNW in Figure 5-10. P is the
centroid of the face remaining when the worst vertex is eliminated.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
127
Figure 5-12 Simplex Diagram
Reflection is accomplished by extending the line segment WPbar beyond
Pbar to generate the new vertex R.
If the response at R is more desirable than the best point B, then an expansion
is attempted to E.
If the response at E is not better than the best point B then expansion has
failed and BNR is the new simplex and the operation is restarted.
If the response at R is between B and N then neither expansion nor
contraction is recommended and BNR is taken as the new simplex.
If the response at R is less desirable than the response at N, then a step has
been taken in the wrong direction and the simplex is contracted by two
possible methods
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
128
1. If the response at R is worse than the response at N but not worse
than that at W, the new vertex should lie closer to R than W and the
new simplex becomes BNCr
2. If the response at R is worse than the previous worst vertex W,
then the new vertex should lie closer to W than to R and the new
simplex is BNCW.
A failed contraction is when the result at Q- is worst than the result at R or the
result at Cw is worse than the result at W. In a situation like this further contractions
are recommended(Nelder, 1963).
The simplex method described above was used in the on-line cavity tuning to
maintain the cavity at resonance. The method described is for functions without
constraints, however for the tuning problem there are constraints on both dependent
variables, which are the cavity length and probe depth. Thus, this method was
augmented to compensate for these constraints. For example, when a value was
calculated that was out of bounds, the value was set to the boundary value of that
variable. The termination was determined from the comparison of the reflected
power to a set minimum value.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
129
^ ► G en erate B ase P oin ts
C alcu late C en troid , P
P = (B + N )/2
C alcu late R eflection P oint, R
E valau te fu n ction a t R to b e y*
no,
yes
is y * < yi ?
t
is y > y i,
i n eq h?
yes
yes
:ulate y**
fo r ex p a n sio n
no
is y* > y h ?
R e p la ce Ph n y P *
no
ino
C alcu late y * *
fo r co n tra ctio n
is y** < yl ?
yes
ves
is y* * > yh ?
R e p ace P h b y P **
no
R ep la ce Ph by P*
R e p la c e all P i’s
b y (P i+ P l)/2
R ep lace P h b y P **
Jio
h a s m in im u m b een reached?
yes
•Exit
Figure 5-13 Simplex Logic Flow Sheet
The application of the simplex search method to tuning the cavity in 2dimensions is unique. This simplex method has been used in the chemical
industry(Baasel, 1965 ;Carpenter, 1965 ;Kenworthy, 1967 ;Lowe, 1964 ;Lowe,
1967 ). Emst(Emst, 1968 ) was the first to use simplex designs in analytical
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
130
chemistry by varying linear and quadratic y-axis gradient controls to rapidly optimize
NMR magnetic field homogeneity. It has also been used in designs in analytical
chemistry(Morgan, 1974).
Other methods such as the gradient method have been implemented for
tuning the cavity in 1-dimension. Alliouat(Alliouat, 1990 Successfully used it in the
automatic tuning of a single mode resonant cavity for the sintering of ceramics. In
this system, only the cavity length was tuned as function of the reflected power and
variation in the microwave source frequency. Remember that the gradient method
requires the evaluation of both the function and gradient values at any position, with
the gradient being estimated by local exploration, using several experiments.
Therefore, the successful implementation of this method in a 1-dimensional tuning
does not necessarily guarantee the same result in 2-dimensional tuning problem.
5.5.2 Mode Selection and Uniform Heating
5.5.2.1 Approach
In a single mode resonant cavity, there is a unique cavity length, probe depth
and field pattern associated with each mode. The theoretical cavity length at which a
mode can be excited for a given cavity radius, can be calculated from the cut-off
frequency equations which were discussed in Chapter 3.
The theoretical electric
field patterns associated with these modes are also derived from the solutions to
Maxwell's equations as discussed and plotted in Chapter 3, where the field patterns
indicate the electric field intensities in the cavity.
Examination of these field patterns indicate that the electric field intensities
are typically concentrated in the center or along the edge in a plane. As a result, a
single mode alone may not be sufficient to uniformly heat a sample. This is not a
unique characteristic of the applicator, but of the nature of electromagnetics. In
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
131
multi-mode ovens, i.e. home microwave ovens, this problem is addressed by the
random introduction of several modes using a mode stirrer(Huack, 1969).
Other
approaches have been the random sweeping of frequencies to introduce different
modes at fixed cavity dimensions(Wei et. al., 1994).
Fundamentally, the basis of all of these methods is the introduction of a
variety of modes using different strategies. Some of the more pragmatic methods for
achieving uniform heating in the different applicators have been presented(Adegbite
et. al., 1995). One of the methods for achieving uniform heating is dual coupling
which is presented in Appendix B. Another method presented called mode switching
is used as the method for achieveing uniform heating in this work. Mode switching
has been used to uniformly heat polyimide panels (Fellows, 1993), heat and cure
complex shaped and planar vinyl ester glass panels(Fellows, 1994, Fellows, 1994).
A detailed discussion of this method has been presented by (Fellows, 1994)
and will be summarized here for continuity. Mode switching is a feedback mode
selection strategy which utilizes a pre-determined heating temperature profiles to
select optimum electromagnetic modes during processing.
The objective of this
method is the identification of complementary temperature heating profiles which
can be superimposed on each other to achieve uniform heating. Conceptually,
consider the combunation of the two theoretical modes TE(011) and TM(012)(see
Figure 3-2 and Figure 3-3 for electric field patterns corresponding to these modes) to
uniformly heat a surface. For the TE(011) mode alone all the electric field intensity
is concentrated at the edge of the sample in the form of a ring, and for the TM(012)
mode it is concentrated in the central region.
5.5.2.2 Mode Selection Logic
The mode switching implementation in this work differs from the rigorous
approach presented by Fellows(Fellows, 1994). In this method all the modes in the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
132
cavity were characterized from the theoretical field patterns to be either center or
edge heating(see Chapter 4). Thus, to achieve uniform heating, modes were
switched between groups of center heating modes and edge heating modes. The
success of this method hinges upon the accuracy of the loaded cavity mode
characterization into the groups of edge heating and center heating modes. As was
discussed in Chapter 4 sample placement, size, and electrical properties can affect the
field pattern in a complex manner that cannot be readily calculated or measured.
As such, theoretical modes and electric field patterns cannot be linearly
extrapolated to loaded cavity ones. An empirical method was used to characterize
the cavity by determining the relationship of different sample and sample placement
on a resonant mode. Results of these experiments were used to estimate loaded
cavity lengths and corresponding electric field patterns. Once the loaded modes were
identified, the electric field patterns were then identified and grouped into center and
edge heating modes which were used in the mode switching method. Details of this
work can be found in Chapter 4.
The program logic used for mode selection is shown in Figure 5- 14.
Implementation of this logic is depended upon the ability to group the loaded cavity
modes into center and edge heating ones. These edge and center heating modes are
required input on the LabView program front panel. The LabView representation of
the mode selection logic is shown in Level 3 of the main program in Figure 5-18
through Figure 5-20. In this mode selection logic, the temperature gradient site
must be classified into the central or edge regions of the part. This directs the
program to look for a mode that would heat the cold region. To illustrate this logic a
a cold center scenario will be used. Once the cold site is known to be the center it is
determined whether the same modes had been succcessful in heating the center
region in “Same Center Mode?”.
If it is yes, then one of the previuosly classified
edge heating modes is tried because it is assumed that the selected and estiamted
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
133
center modes are not successful. If no, then a new center mode is selected
sequentially from the center heating modes list and the list is updated by adding new
center heating modes if necessary. It is then determined if the new mode is the same
mode that is currently being used in “Mode =Previous”. If it is yes then another
mode is tried and the iteration continues.
AT > SPt
Yes
Yes
Cold Centei
No
■Same Centel
>«^ode2^
Select Center
Mode from List
Update List
Yes W
No
Yes
Yes
Same Edgi
Mode ?
[ode = Previous
No
No
Select Center
Mode from List
Update List
Yes
[ode = Previous?
No
New Mode
Figure 5-14 Mode Selection Program Logic
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
134
Note that before the mode selection logic is activated, the cold region has to
be cold for a period of time. This is a critical parmeter that must be determined from
rigorous mass transfer models of the curing composite. Currently, this is a variable
on the front panel called “Mode adjustment time”. It is typically set to 15 seconds or
5 iterations through the loop. This value was deterrmined from calculations of the
time constant of a typical curing system which was found to be 3 seconds. Thus,
theoretically this should allow for a 5°C temperature differential before selecting a
new mode.
5.5.3 Temperature Control
5.5.3.1 Approach
The approach taken in this work is the analog output regulation of the
microwave power source using the measured temperature of the sample.
This is
different from what is typically done where the power is turned off and on in a fixed
cycle. Jow (Jow, 1988) developed a temperature control scheme for processing
composite materials where the microwave power source was turned off by a switch
as the temperature setpoint was reached.
In this system a conventional
PID(proportional-integral-derivative) controller is used to regulate the input power
from measured temperature values. Since more than one temperature value was
measured, the maximum temperature value was used as the control element.
5.5.3.2 PID Method
The PID routine used was available as a package in LabView. It uses an
interacting positional PID algorithm as discussed by Shinskey(Shinskey, 1988 ). It
supports anti-reset windup and the derivative action is done on the process variable
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
135
rather than on the error. This was done to stabilize the response of the derivative
control action. The differential form of the PID equation:
m, = K,
was expressed in a preferred way for implementation in three parts as:
Derivative action
, A r(c-y)
>' = }7+ r ——
At + D I K d
(5-5)
where y is the output of the derivative filter which has a time, D/K d , and Kd is the
derivative gain limit, D is the derivative rate in minutes per repeat. The equation is
arranged such that setting the time constant to zero will produce y=c.
Proportional action
m = b ± K c[ r - c - K d( c - y ) ]
m > mh,m = mh
(5-6)
m < m;,/n = ml
where m is the manipulated variable and is limited to the high and low limits of the
manipulated variable, and b is the bias which is the output when the error is zero.
The bias could be fixed or could be the result of feedback from the output of the
controller to produce integral action.
Integral action
b = b + At f m - i )
At + I
where I is the integral reset in minutes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
136
The setpoint, process variable, and output are expressed in percent The
controller was tuned to determine the proportional gain, derivative gain, and integral
gain for the magnetron microwave source. The magnetron was also calibrated to
determine the input voltage and output power relationship in Figure 5- 10.
5.6 LabView Curing Process Control Software Program
In the previous sections, details of the specific controllers that make up the
major components of the controlloop were discussed.
In this section the integration
of these controllers into the curing control program as implemented in LabView is
discussed. In LabView the program was developed as if it is operational on a device
with a front panel where all input and output interactions occur and a back panel
where the program is stored.
5.6.1 Front Panel
The front panel for the curing program includes input and output information
for using the program. Most of the information on these panels are self explanatory
and can be readily understood at any level, which is one of the attractive attributes of
LabView. The discussions that follow will only show how some of these front panel
icons are used in the program. Due to the graphical nature of the programming code,
an acceptable print quality output cannot be generated for the dissertaion and the
program will only be discussed.
On the input panel there are options for data storage time, power control
tuning parameters, scaling values for the analog devices and data acquisition
parameters which are all hardware inputs that may not require new inputs every time
that the program is used. There are also input parameters for the “Mode Tuning”
program which were defined in the mode tuning software program section in the
previous sections. The required inputs that are sample dependent are “Glass mode
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
137
parameters” and the “Graphite mode parameters”. These are interval setting for
loaded cavity modes. They essentially represent the regions to search for a mode for
the cavity length and probe depth which are used by the mode tuning program. There
is also another input for cavity length and probe depth corresponding to each mode
which are input into “Cl Graphite Modes” and “PD Graphite Modes”. Once these
cavity length and probe depth values are input to the program, they are indexed
sequentially so that through the program they are referred by number.
Another required input is the classification of modes into “Center heating” or
“Edge heating” by the indexed mode number. These inputs are required for the
mode selection program as an initial guess values. Finally, there is the option for the
“Matrix /Fiber” which automatically sets the cure temperature setpoint. Currently,
there are inputs for epoxy/ graphite and polyester / glass. It is important to note that
all these inputs have default settings which can be used if other input values are not
known.
The output or display panel is probably one of the strengths of the LabView
software program. It is very easy to generate and yet very useful in process
monitoring. There is a spreadsheet at the top of the page for displaying the absolute
values of all the sensed parameters. Additionally, there is a temperature strip chart
and a temperature color gradient map, ‘Temp Probe Site”. This color map indicates
the local temperature at the different sites of the sample with colors ranging from
green to black, which corresponds to room temperature of 5 °C above set point.
There is also a theoretical electric field pattern indicator which shows the estimated
electric field patterns based upon the cavity length in the form 3-D and density plots.
These plots were imported from a Mathematica file as was discussed in Chapter 3.
Another important feature of the display panel is the on-line interaction with
the operator. All values on the front panel that are inputs can be modified while the
program is running. Indicators are always shown with arrows on the side of the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
138
display box, such as “Tgrad Spt” located under the ‘Temp Probe Site”. One very
important feature of this front panel is the “Shutdown” toggle switch. This switch
can be activated by using the mouse to click on i t It can stop the program at any
level of execution. It is a safety device which provides an additional level of
flexibility for operator interaction.
5.6.2 Program Implementation
The program was developed using a graphical interface programming
method(LabView,1994). In this programming method the program codes are built
into icons or virtual instruments (VI) with inputs and outputs which are accessed by
lines or wires. A flow diagram of the microwave process control program logic
shown in Figure 1-2. In LabView the overall logic is implemented using sequence
structure which provides for the sequential ordering of the program logic. In the first
level of the program sequence data acquisition, data scaling and data formatting for
storage and display are done. In this sequence the “Cl/PD Pwr Scale” VI is used
which is a subroutine written specifically for scaling the cavity
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
139
Acquire and Scale Data
Tmax > Spt
Yes
Power Controller
No
Yes
AT > Spt
Mode Selection
Controller
No
Yes
Pref > Spt
Mode Tuning
Controller
No
Figure 5-15 Program Flow Logic (also in Chapter 1)
length, probe depth, and power reading from voltage to actual values, (see Appendix
E for details of program). Other elements of the program use standard tools which
basic knowledge of LabView is required to understand.
Level 1 of the sequence structure is for implementing the power controller.
The main VI used in this sequence is the “Pwr cntrl” which contains the PID logic
discussed in the previous sections. Output of this VI is a voltage value which is sent
to the power source via analog output.
Level 2 implements the “Mode tuning controller” by calling other VT’s such
as “Move Read” and “Surface vi”. “Move Read” contains the program for the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
140
stepper motor drivers and for reading the reflected power. Surface vi contains all of
the components of the simplex method discussed in the previous section. This is a
major program which calls several Vi’s such as indicated in the program hierarchy.
Level 3 is the mode selection controller which involves several levels of
nested loops or sequence structures. This program implementation follows the logic
in Figure 5-16. It is important to note that only pertinent details of the LabView
program have been presented. The purpose is not to serve as a tutorial but to bring
forth the significant highlights of the work. To use this program does not necessarily
require knowledge about LabView since most of the interaction would occur at the
front pane which is self explanatory. However, to modify or to understand the
program code does require basic understanding of LabView.
5.7 Diagnostics System Automation
5.7.1.1 Hardware Automation
The sweep oscillator is a low power multi frequency radio frequency power
source which operates in an output power range of 10-20 mW at a variable frequency
range of 1.7-4.3 GHz. It is used for the low power swept frequency analysis of the
empty cavity to locate theoretical modes. In this analysis, the reflected power from
the cavity is rectified by a crystal detector and displayed on the Y-axis of an
oscilloscope trace the quality(Q-curve) curve of a mode.
Figure 5-16 shows the device diagram for the low power diagnostics circuit
The difference between this circuit and the processing circuit is that the microwave
power supply is a variable frequency source, sweep oscillator and not the fixed
frequency source, Magnetron. Also, since the power levels are at milliWatt levels,
the reflected power from the cavity is not attenuated but directly read by the power
meter and then sent to the computer via the A/D board, while the swept
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Cavity
Shorting Plate stepper
motor & driver
Diagnostics System
Base Plate stepper
motor & driver
ora
c►
i
in
ON
00
r
o
Mechanical
Switch
MW Source
*
o
Directional
3*
n
era
3
O
c/i
a.
Pr
Coupling Probe steppei
motor & driver
Shorting Plate
Potentiometer
Circulator
28V Pwr
Supply
Variable
Pwr Supply
Variable
Pwr Supply
Coupling Probe
Potentiometer
Load
n
<Z)
V)
J?
3
Luxtron A
Luxtron B
Mac Ilci
Terminal
Box
A/D Board
142
frequency is controlled via the GPIB interface. See Appendix A for detailed
discussion on GPIB interface. In manual operations the swept frequency was
controlled from the sweep oscillator and the reflected power was
attenuated and then sent to an oscilloscope. Thus, the oscilloscope was used to track
both the swept frequency and the reflected power.
5.7.1.2 Diagnostics system software development
The objective of the diagnostics system is to characterize the microwave
processing system, which includes the cavity and the external circuit for theoretical
cavity modes.
As was previously discussed in the hardware chapter, a low power
multi-frequency source is used to study the resonance of the system at the excitation
frequency. In the manual operation the resonance data is displayed on an
oscilloscope as a plot of frequency versus reflected power, where frequency is
displayed on the x-axis.
In the automation of this procedure, data acquisition at resonance is
automated while the decision for cavity adjustment for tuning is not automated. This
is due to the low power values (0-10 mW) that are output by the sweep oscillator and
the limited resolution of the sensing equipment and data acquisition system. To have
a fully automated system the low power circuit has to be augmented by the addition
of sensors with resolution for low power values or amplification of the signals before
being sensed and sent to the data acquisition board.
The software for the low power data acquisition system uses two different
modes of instructions, one for a GPIB interface to control the swept frequency and
analog-to-digital instructions to acquire the reflected power data. Input to the
program are the sweep oscillator address, swept frequency rate, minimum frequency
to scan and maximum frequency to scan.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
143
Given this input the program calculates the number of times to iterate the data
acquisition loop or the number of data points to acquire. A string of frequency is
then sent to the GPIB write driver at the instrument address of 19 to set the swept
frequency. Once this instruction is completed the data acquisition board is queried at
the reflected power channel to read the reflected power value at the set frequency.
This instruction is repeated until the number of iterations set by the program is
completed and then the data is displayed on the front panel of the VI. The actual
output is shown on the top and an actual oscilloscope photgraphic image is shown on
the bottom for comparison. The data can also be available for further analysis in
Excel.
The significance for the automastic generation of the Q-curve is that it
provides the ability for automatic on-line cavity diagnosis and dielectric analysis. In
further development, cavity frequency shift calculation and Q-curve calculations
were developed for a unimodal curve. Further developemnts must be made in the
developemnt of methods for handling multimodal curves. It is important to note that
the subroutines for interfacing with the GPIB where provided with the GPIB board.
Hence, the extent of the software development was the structuring of the device
drivers and other available subroutines.
5.8 LabView Software Interface with Knowledge-Based-System Planner
As previously mentioned, another software program was developed for the
purpose of interfacing with a KBS-planner. Details of this work are discussed in
Appendix C but the software development issues are presented. The program flow
logic for interfacing with the KBS-planner is shown in Figure 5-19. Note that it uses
the sub-programs for power controller, mode tuner, and data acquisition that were
developed for the processing system discussed in the previous sections. What is
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
144
different however this program is that the decision making now resides in the
planner. The acquired temperature, input power, reflected power, cavity length and
probe depth are input to the KBS planner. The output from the KBS-planner are the
power level and modes. Hence, the LabView program functioned as a low level
controller which takes the control setpoints from the planner and maintains them.
is also important to note that the interface between the planner which was
implemented in SmallTalk, and the LabView program was done using LabView
subroutines available in the file management library. Hence, this was another
demonstration of the versatility of the LabView software program and its capability
to interface with different software platforms. Details of the programs and
philosophy of this work is presented in detail in Appendix C.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
It
145
►
Acquire and Scale Data
^
Temperature
Power
Mode
KBS Planner
Power Level
Mode
Power Controller
Mode Tuner
Figure 5-19 Logic for Interface with planner (Figure 1-2)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 6
APPLICATION TO CURING
6.1 Introduction
In the previous two chapters the empty and loaded cavities were characterized in
order to understand the excitable modes using theoretical and empirical approaches. In
this chapter the empty and loaded cavity characterization results will be applied to the
automatic curing of graphite epoxy composite materials.
The goal of these experiments
is the verification of the automated system for controlling the curing process by selecting
modes, tuning, and controlling the sample temperature and temperature gradient
Using the characterization results, the heating modes are grouped into edge and
center heating modes. Thus, the uniform heating criteria is set to be the minimization of
the temperature gradient between the center and the edge temperatures first, and then
between the edges. The graphite epoxy system was used because of the complex nature
by which it interacts with the electric fields. The graphite fiber component of the
composite lends a highly conductive characteristic to the composite, making it highly
lossy and very responsive to microwave heating. Thus, it provides an interesting medium
for an extreme case by which the automated system can be verified.
6.2 Curing Experiments
In the curing experiments a graphite epoxy composite prepreg, Hercules
AS4/3501-6, was layed-up into 24-ply and 48-ply, unidirectional, 2 inch square parts.
The sample was cured at setpoint temperature of 1600 C for a total processing time of 90
minutes. The lay-up method follows that used in the autoclave processing but, unlike the
146
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
147
autoclave layed-up samples the part was not placed in a vacuum bag since pressure was
not applied in the microwave process. High temperature (200 °C) lay-up material was
used as compared with the polyester based materials which bum at temperatures less
than 160 °C.
A Teflon mold which has been specifically designed for the microwave cavity
was used to contain the sample during curing.
The Teflon mold was machined out of a
solid block to form a male and a female unit, which were fitted with pegs to lock the two
pieces in place. Sample surface temperature was measured using fiber optic
thermometry, with the temperature probes inserted through the top of the Teflon mold
and secured in place with autoclave tape. Sample placement was such that the coupling
probe was perpendicular to the fiber direction and the sample was elevated to a height of
1/8 X.
Five temperature probes were used which were placed at equal distance at the
four comers of the sample and one in the center.
The modes or cavity lengths used and
the estimated preferential heating sites are listed in Table 6-1. The probe depth was
always set to be less than 25 mm in order to minimize the near field interactions with the
sample. These modes represent the tunable modes for the sample loaded cavity(see
chapter 3). The preferential heating sites were estimated from the loaded cavity
characterization results to be those shown in Table 6-1.
Cavity Length (cm)
8.9
12.7
13.5
19.7
22.2
23.3
Heating Preference
Center
Edge
Edge
Center
Edge
Edge
Table 6-1 Loaded Cavity Resonant Cavity Length and Estimated Preferential
Heating Sites
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
148
6.3 Results and Discussions
6.3.1 24-ply Sample
6.3.1.1 Mode Selection
Figures 6-1 through 6-6 show the curing results for the 24-ply and 48-ply
samples. Results are shown in the form of temperature profile, power profile and cavity
length and probe depth as function of time. For the 24-ply sample the overall processing
results are listed in Table 6-2. The center temperature is the dashed line and the rest are
the edge temperatures. The temperature profile in Figure 6-1 shows several dips which
are indication of mode switching periods. These dips show the sensitivity of sample
heating to the electromagnetic coupling and mode switching times.
Figure 6-2 shows that during the first 10 minutes, the mode selection program
used all 5-modes , but stayed in the center heating modes the longest This is an
indication that the sample center was the coolest during the first 10 minutes of the run.
Between 20 minutes and 30-minutes only the center heating mode was used at cavity
length 19.7 cm. This was a good choice by the mode selection program which is
indicated by a uniform increase of the sample temperature profile from 60 °C to 80°C.
Between 40 minutes and 50 minutes, all the modes were used and none was
favored. This was the region where the sample temperature was in the reaction region. It
seems as though the switching times increased as the reaction temperature was reached.
This is an indication of the program keeping up with the reaction front such that the
sample is uniformly cured. After 77 minutes or 30 minutes in the reaction zone, the dips
in the temperature profile seem to be minimized although the program is selecting and
using different modes. This can be interpreted as advancement of the curing reaction.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
149
The reaction was controlled for 90-minutes, since this was a predetermined time when
complete curing can be successfully achieved(Wei, 1994).
6.3.1.2 Power Control
Power control did not start until 50 minutes into the run. During initial stages of
the run the power was kept constant at 100% output. The dips in the output power is a
characteristic of the power source and not the controller. Thus, the maximum output
power was an average of 80 W. The power controller regulated the power such that the
sample temperature never overshot the setpoint of 160°C. Note that after 70 minutes the
response of the controller became very rapid and the power level was reduced and the
sample temperature remained to be in the processing window. This could be an
indication power control to compensate for reaction exotherm
6.3.1.3 Tuning
The reflected power was considerable higher at approximately 20% of the input
power, which was due to the restricted probe depth requirement in order to minimize the
near field effects from the probe. For completely minimized reflected power a probe
depth of 30 mm or greater is typically required for this system. However, with the long
probe depth the near field effects from the probe are increased and heating patterns
become unpredictable.
Thus, the limited probe depth requirement restricted the optimum tuning
capability of the system to the best that it can do at a maximum probe depth of 25 mm.
The tuning and mode switching times were on the order of a minute or less with the
cavity adjustment time being the controlling factor and the tuning time on an average of
20 seconds or less.
Typically, the tuning algorithm can a achieve a minimum reflected power of 10%
or less of the input power. Theoretically, a minimum reflected power requirement for
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
150
tuning is zero or very close to zero. One of the consequences of partial tuning is un­
focused energy which could have also contributed to the dips in the temperature profile.
With the limitation on coupling probe depth, a more predictable heating profile was
achieved across the sample although the heating rate was slow. This condition can be
remedied by using a higher input power to achieve a higher heating rate without
sacrificing heating uniformity.
Figure 6-2 shows that the coupling probe depth remained constant during major
periods of the cure cycle, although several different modes were used. This is to note that
coupling probe adjustment were not significant in tuning the cavity during the cure cycle.
The only periods that the coupling probe adjustments were required was during the pre­
reaction period, between 30 minutes and 40 minutes which corresponds to a sample
temperature of 80 and 120°C.
Average heating rate
5 0 C / min.
Time to reach setpoint
50 minutes
Total temp, gradient
< 10 °C
20 W
Average reflected power
Actual input Power
60-80W
Adjustment time
< 30 sec
Tuning time
< 20 sec
Table 6 - 2 24-ply curing results
6.3.2 48-ply Sample
6.3.2.1 Mode Selection
Figures 6-4 through 6-6 show the curing results for the 48-ply graphite epoxy
composite material. The same modes and processing conditions set for the 24-ply
material were used and the results are summarized in Table 6-3. Figure 6-4 shows the
curing temperature profile for the 48-ply sample. The highest temperature profile is the
center temperature and the rest are the edge temperatures. Note the absence of dips that
were present in the 24-ply sample.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
151
Daring the heat-up period, the estimated two center heating modes were selected
the most since the center temperature had the lowest value. This was similar for the 24ply sample. The mode selection controller continued to use these 2-modes since the
overall temperature was maintained within 20 °C, which was the criteria for uniform
heating for this sample. By 14 minutes into the run the cure temperature of 160°C was
reached. Between 14 minutes and 30 minutes, the edge heating modes were selected in
order to push the edge temperatures into the processing window. During the whole
curing period the edge heating mode at a cavity length of 23 cm was selected between 30
minutes and 80 minutes except for at 50 minute into the run. This is a clear indication
that heat loss at the edge of the 48-ply sample was significant.
6.3.2.2 Tuning
Figure 6-4 shows the input and reflected power profile for the 48-ply material.
During the heat-up period the tuning was veiy efficient, and the reflected power of 1% of
the input power or less was achieved even with the limited coupling probe depth
requirement. As the cure period proceeded tuning became less efficient with a reflected
power of approximately 20% of the input power. This could be due to the coupling
nature of the more massive 48-ply material. During the period of 30 minutes and 81
minutes, the reflected power was approximately 20W, and between 80 minutes and 90
minutes it was 0 W.
6.3.2.3 Power Control
Power control was more interesting for the 48-ply sample than the 24-ply sample.
Because the heating response of the sample was very rapid, the response of the power
controller was very rapid. This is a very interesting result which shows how the power
controller complements the mode selection controller. Note that every time that a new
mode is selected a new region is preferentially heated while the hottest region is cooled.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
152
Hence as the measured input to the power controller is constantly changing as new modes
are selected. However, in the region between 50 minutes and 80 minutes, only one mode
was used.
Hence, the input to the power controller was fixed which caused the rapid and
instantaneous response of the controller. Similar response was also observed for the 24ply sample between 75 minutes and 90 minutes where a single mode was used.
It is also
important to note that, between 81 minutes and 90 minutes, although the power was
reduced to an average of 40 W, the sample temperature was maintained within the
processing window. This is a good illustration of the compensation for reaction exotherm
in the microwave system.
10 u C / min.
Average heating rate
14 minutes
Time to reach setpoint
Total temp, gradient
< 20 ° C
<20 W
Average reflected power
60-80W
Actual input power
Adjustment time
30 sec max.
< 20 sec
Tuning time
Table 6 - 3 48-ply curing results
6.4 Summary and Conclusions
The automated system was successfully demonstrated in the curing of 2-inch
square, 24-ply and 48-ply, unidirectional graphite epoxy composite materials.
Temperature gradients of 10 ° C or less and 2 0 0 C or less were maintained for the 24-ply
and the 48-ply samples, respectively.
Mode switching and tuning times were on the
order of a minute, which was constrained by the physical cavity adjustment times. The
48-ply sample was more forgiving and was not significantly affected by the cavity
adjustment delay times.
The actual output power was less than 100 Watts and the reflected power from
tuning was high, approximately 20% of the input power, which was due to the restriction
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
153
on probe depth and the long probe depth requirement for optimum tuning for these
materials. The estimated heating patterns and mode locations in the cavity were
sufficient in the selection of modes to achieve edge-center heating uniformity and in the
location of modes in the cavity. Input power was successfully controlled to maintain the
sample temperature at the curing setpoint of 160 °C.
In general, as energy was coupled into the sample, the sample heated well, and as
energy was not coupled into the sample temperature decreased rapidly. The center
heating mode was used most frequently than the edge heating modes. This could suggest
that heat loss from the central part of the sample was more significant for the 48-ply
graphite / epoxy sample.
The heating characteristics of the 48-ply material is veiy different from the 24-ply
sample. The time delay in mode switching did not have a significant effect on the
heating of the sample. There seemed to be enough thermal mass established in the
material such that it was more forgiving to cavity adjustment delay times and edge heat
losses. Edge heat loss was more significant and thus edge heating modes were
preferentially used.
Finally, using these two samples which characteristically heats differently, the
developed program was used to successfully cure them. These results indicate the
versatility of the automated system and its capability to adjust for different processing
requirements.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75- i n J |
Jl
in
oc
x
m
cn
vS
SO
Figure 6-1 Temperature Profile for Curing 24-ply Graphite Fiber / Epoxy
154
00
00
oo
o>o
(3 ) amjEidurex
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
30
25
15
10
5
0.87
17.67
45.33
64.5
77.83
82.5
85.17
T im c(tnin)
Figure 6-2 Cavity Length and Probe Depth for Curing 24-ply Graphite Fiber / Epoxy
J
Figure 6-3 Input and Reflected Power for Curing 24-ply Unidirectional Graphite / Epoxy
156
.
Gw) jsm o j
puB m duj
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
157
sC
oo
> v
X
c
C.
UJ
sO
oo
o
E
o
-C
oo
00
cs
'zZ
p
0
a.
1
oo
TT
00
u
3k .
3
"3
w
ca*
£
£
oo
oo
o
SC
SO
V
k.
oo
©
00
sc
04
(3) antmidmsj.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
52
'o
(mm) qjdsQ sqoij pas (aio) qiSuaq
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 6-5 Cavity Length and Probe Depth for Curing 24-ply Graphite
Fiber / E p o x y
158
5=
/» # # # # #
Figure 6-6 Input Power and Reflected Power for Curing 48-ply Graphite Fiber / Epoxy Composite
159
AM
(A \) J3 M O J
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CH A PTER 7
SUM MARY AND CONCLUSIONS
7.1 Introduction
The purpose of this work was to advance the fixed frequency single-mode resonant
technology for the processing of composites by bridging the gap between device and
process. This was important in order to realize the technical potential of the microwave
processing technology as a viable alternative to thermal processing. In the single-mode
resonant cavity, the electromagnetic modes are discrete and their interactions with
composites are complex, non-linear and time variant. In general the development of
complete mathematical models to accurately describe the dynamics of the electric field and
heating behavior inside the cavity is complex and computationally intensive. This is a major
source of difficulty which is manifested in the lack of advancement in the state of
technology of the single-mode resonant cavity.
In the previous state of technology, the single-mode resonant cavity was operated
as manual device that is impracticable as a process. The hardware consisted of four basic
components: 1) tunable cavity with gear drives for manually adjusting the cavity length and
probe depth, and micometers for measuring the cavity length and probe depth; 2) an
external circuit with major components which included a microwave power source, power
meters, and a dummy load to absorb reflected power; 3) fiber optic thermometry for
invasive temperature measurements; 4) computer interface for automatic data acquisition of
input power, reflected power and temperature for post-analysis, and the on/off control of a
switch to direct the microwave power away or to the cavity. There were two different
independent external circuits for processing and for diagnostics purposes.
160
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
161
In processing, the microwave cavity was operated as an open-loop system where a
seasoned operator was the necessary link to close the control-loop. A typical processing
activity included; 1) the continuous tuning of the cavity by the manual rotation of dial knobs
to adjust the cavity length and probe depth to minimize the reflected power, 2) the frequent
selection of new cavity modes by carefully adjusting the cavity length and probe depth to
new locations, while tuning the cavity and manually modulating the input power to achieve
uniform heating; 3)temperature control by the automatic on/off control of an electronic
switch to direct the microwave to or away from the cavity. As such, the cavity was
operated as an independent device from the external circuit, and processing results varied
from one operator to the other and optimization was difficult at best
In this work, the single-mode resonant cavity was automated in order to advance it
as a viable process. In the automation, a control system was designed and built in addition
to the development and implementation of a set of sophisticated and comprehensive control
software programs for controlling the curing process in the cavity. These control programs
combine traditional and non-traditional control methodologies. The control software
programs were developed for mode tuning, mode selection and power control which were
constructed independently and then integrated to form the overall closed-loop feedback
control system. The diagnostics system was also automated to provide for automatic
empty cavity characterizations and for automatic dielectric analysis of materials inside the
cavity.
In the sections that follow, the control system for processing is summarized under
the following headings; control software, automation hardware components and
application results. The automation of the diagnostic system is summarized in a dedicated
section, followed by a summary of discoveries and developments from the course of this
work that would lead to further understanding and utilization of this technology.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
162
7.2 Automatic Control Software
A closed-loop feedback control system was developed which included software and
hardware for physically controlling the microwave cavity and external circuit as an
integrated process for curing composites. Mode tuning, mode selection and uniform
heating and power control were the major components of the control software. Other
supporting programs were developed for the stepper motor drivers and for data acquisition.
These suporting programs are presented in appiecaible detail in appendix B. In processing,
electromagnetic modes were selected using the mode selection controller, and the modes
were tuned using the mode tuning controller, while controlling the input power to achieve
uniform and controlled heating.
The control software programs were structured in series in the following order; data
acquisition, power controller, mode selection and uniform heating controller and mode
tuning controller. This means that only one controller was active at any one time, and each
controller has to complete the control task before the successive one could be activated.
This also means that the completion of one controller activates the other controllers. Other
elements of the control software were; data acquisition and storage; real-time tracking of
process variables in the form of strip charts, color maps and spreadsheets; safety locks
such as temperature alarms, power alarms, and multiple shutdown levels.
7.2.1 Mode Tuning Software Program
Using a non-traaitional control approach, a mathematical 2-dimensional simplex
method was used to construct the tuning control software. In this method both coupling
probe depth and cavity length were adjusted simultaneously to tune the cavity. Application
of the simplex method to mode tuning and the simultaneous adjustment of the cavity length
and probe depth to tune the cavity are both novel in this approach. This method only
required information about the cavity length, probe depth and reflected power and not the
electromagnetics inside the cavity. As such, it was simple to program and yet more
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
163
efficient in tuning the cavity than the manual and univariate methods. Using this method
tuning time was typically on the order of a minute or less which was comparable to manual
timing, but mode tuning was highly reproducible which is an improvement from the
previous state of technology. One shortcoming of the simplex method is the dependence of
the rate of convergence on the proper scaling of the cavity length and probe depth
adjustments beforehand. These scaling factors have to be determined experimentally
following similar principles as applied in controller tuning. Thus, in order to optimize the
tuning rate of convergence the scaling factors must be optimized.
7.2.2 Mode Selection and Uniform Heating Control Software Program
Mode selection and uniform heating control software was required to physically
select the appropriate cavity length and probe depth corresponding to a desirable heating
mode. The uniform heating controller was developed from a non-traditional control
methodology using empirical correlations to construct the heating functions and electric
field characteristics of the loaded cavity. Chapter 4 is dedicated to the development of these
correlations. Uniform heating was characterized as the maintenance of the spatial edge and
center sample temperatures within a 5 °C temperature band. The loaded cavity modes were
grouped into center and edge heating modes from the loaded cavity characterization results.
Uniform heating was achieved by the selective switching between these electromagnetic
modes to preferentially heat the center and edge regions of the sample, to distribute the
electromagnetic energy across the sample in an optimum manner. Ideally, the center region
of the sample was initially heated preferentially, until a gradient of >5 °C was obtained
between the center and edge temperatures and then an edge heating mode was selected and
the procedure was repeated.
The controller was found to be sluggish at times where several mode selection
iterations occurred before the correct one was selected. This was in part due to near-field
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
164
and time-variant complex electromagnetic interactions which could have caused variability
in the electric field patterns of the predicted modes. Thus, indicating that the heating
functions and the mode selection criteria can be optimized, and the loaded cavity modes
may not be neatly grouped into center and edge heating modes as was done in this
controller. Another factor that could have contributed to the sluggishness of mode
selection was the mechanical cavity adjustments involved in the mode selection. Although
the adjustment time was 10 inches / minute, which was a 67 % increase in speed from the
manual system it may not have been rapid enough.
The uniform heating method used in this controller was found to be sufficient in
the demonstration of the concept, however a better approach would be to develop more
accurate heating functions using either mathematical models or the rigorous empirical
methods proposed by Fellows(Fellows 1994) in order to optimize the mode selection
criteria. One notable point is that uniform heating in the single-mode resonant cavity is not
static. It is complex and does not only depend upon the ability to rapidly select and tune to
a mode, but highly depends upon the understanding of the time varying characteristics of
the heating field patterns.
7.2.3 Power Control Software Program
Power control software program was developed to physically modulate the input
power to maintain sample temperature within setpoint limits. Power control software was
developed using traditional proportional-integral-derivative (PID) methodology, and the
microwave power source was modified to allow for analog adjustments of the input power.
Since several temperature sites were measured, the input to the controller was the highest
temperature value. Using this controller, thermal overshoot from the reaction exothermic
was effectively controlled and temperature was maintained within 2°C of the setpoint
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
165
This demonstrated the controllability of temperature excursions in the single-mode resonant
cavity which was difficult to do in the previous state of technology.
In the previous state of technology, the concept of temperature control was the
on/off control of an electrical switch to direct the microwave power to or away from the
cavity. While the power was directed away from the cavity it was directed to a dummy
load as wasted power. Hence, in addition to the demonstrated technical advancement of the
developed controller, it was also an economic advancement in the use of the microwave
power.
7.2.4 Data Acquisition Interface and Control Software Platform
An element of the automation was the hardware control system development which
included the microwave cavity, microwave circuit and the computing and data acquisition
platform. A Macintosh computer running a LabView software development program was
used for computing and software development, respectively. The choice of the computing
platform was motivated by the capability to interface with a knowledge-based system unit,
capability to perform simulations and the ease of use. However, due to the recent
technological advancements in computing software the work developed will be portable to
other platforms with similar ease in operation. The LabView software platform was found
to be user friendly both in program development and in application. Additionally, because
LabView contains comprehensive data acquisition drivers and analysis subroutines it was
found to be flexible and proficient and easy to maintain.
7.2.5 Data Acquisiton Software program
Although the Labview software platform contains data acquisition drivers, a
program had to be developed for acquiring the process data. The program essentially
included sampling rates, scaling factors for the different instruments and data manipulations
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
166
functions for minimizing noise in the acquired data. Data acquisition components are
presented in appreciable detail in appendix B.
7.3 Automation Hardware
7.3.1 Cavity and Circuit
The manually operated microwave circuit design was essentially modified to
accommodate automatic data acquisition, and the microwave power source or magnetron
was modified for automatic analog manipulation. The core of the hardware automation was
the design and fabrication of the cavity with mechanized drives. The overall cavity design
was not novel and had been previously developed with either an axial or a radial mounted
coupling probe(Asmussen 1987). What was novel about this cavity design was the
mechanized drives and the implementation of dual mounted coupling probes at the axial and
radial positions.
The mechanized drives design was based upon a belt and pulley system in which
a stepper-motor was used to drive a ball screw which resulted in smooth adjustments.
Additionally, the use of precision graphite bearings rather than spur gears eliminated the
binding problems that existed in the manually operated cavity.
In the operation of these
drives, it was shown that one shortcoming in the design is the sensitive to alignment where
proper alignment was necessary for optimum performance of the drives. This is a problem
which can be easily remedied by the design of precision brackets to keep the drives aligned.
Although the mechanized cavity diameter was the same as the manual cavity, the length of
the cavity was 42 cm which is 1.5 times the length of the manually operated cavity. This
increase in cavity height allowed for the excitation of 12 additional modes which increased
the processing flexibility for multiple mode applications. Another element of the cavity
design that was novel, was the addition of linear motion potentiometers for electronic and
local cavity length and probe depth measurement
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
167
7.4 Verification of Control System
To verify the integrated automated system, a 2-inch square, 24-ply and 48-ply,
unidirectional graphite epoxy composite materials were cured. Chapter 6 is dedicated to the
discussion of these results. Sample specific inputs to the control program were; the matrix
type from which the cure temperature setpoint was automatically selected; cure duration
time; and heating modes grouped into center and edge heating modes. The automatic
curing results showed that benefits due to automation versus manual operation are;
1)
A 63% decrease in mode switching times due to the precision mechanized
drives and the effective mode tuning methodology. This is notable in that
sample heating interruption times and thus sample cooling times during
mode-switching were minimized which resulted in the improvement of
sample heating uniformity.
2)
A 50% decrease in sample temperature gradients within a 5 0 C bandwidth,
and thus increased sample heating uniformity. This was clearly due to
faster mode switching times, efficient power control and the mode selection
criteria for mode switching.
3)
Proficient and repeatable mode tuning and elimination of phantom modes
which minimized processing variability.
4)
Efficient control of exothermic reaction temperature excursions where
temperature was maintained within +/- 2 °C of the setpoint
5)
Efficient use of microwave power where only reflected power was directed
to a dummy load which was always less than 20% of the input power.
Other benefits that are noteworthy;
1)
User friendliness of the process control software,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2)
168
Real-time tracking of the process trends using strip-charts and other visual
tools on the computer,
3)
Integration of data acquisition and control on a single platform,
4)
Accurate documentation of the processing results for further developments
to eliminate the invasive sensing methods,
5)
Availability and easily accessed processing data in a spread sheet form, and
most importantly,
6)
Convenient turnkey processing characteristics as in the operation of thermal
ovens.
7.5 Diagnostic System Automation
As previously mentioned, the diagnostic system is an independent external circuit
which consist of similar circuit components as the processing circuit, except for that the
microwave power source is replaced by a sweep oscillator. Additionally, there is an
oscilloscope for measuring the input power and reflected power in the form a power
absorption curve. The automation of the diagnostic system was significantly different from
the processing system in that a GPIB (general purpose interface board) interface in addition
to the analog-to-digital interface used in the automation of the processing external circuit
were used. This was necessary since the major component of the low power diagnostic
system was the sweep oscillator which has only GPIB interface capability. A mechanical
switch was added between the processing and the diagnostic external circuits which would
potentially allow for automatic on-line dielectric analysis during processing by switching
between the two independent external circuits.
Essentially, automation of the diagnostic system replaced the need for the
oscilloscope and the manual calculation of the cavity quality factor, which enabled the
automation of dielectric analysis in the single-mode resonant cavity. One shortcoming of
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
169
the program was that it was only capable of calculating cavity quality factor for unimodal
power absorption curves. Hence, further development is required to complete this work
by developing a more robust algorithm for calculating the cavity quality factor for nonunimodal power absorption curves.
7.6
Understanding to Enhance Utilization of Technology
In the course of this work several high level global understanding of the microwave
processing of materials was developed or discovered which enhanced the utilization of the
technology. These areas of understanding can be realized in hardware design, processing
methodology, and the application of knowledge-based system technique to automation.
7.6.1 Dual coupling
In the design of the mechanized cavity, as an added feature which was not required
for automation, the mechanized cavity was equipped with novel axial and radial mounted
coupling probes. As was previously mentioned typically, either the axial or radial mounted
probe configuration is used and not both. Empty cavity characterization results showed
that similar modes can be generated for either probe mount configurations except for at the
cavity length corresponding to a degenerate mode.
An interesting finding was that, only at the cavity length corresponding to the
degenerate mode can simultaneous coupling from both axial and radial mounted coupling
probes be used to achieve heating. At the other cavity lengths as one coupling probe
coupled the microwave energy into the cavity the other coupled it out of the cavity and the
sample was not heated. Theoretically the side mounted probe should preferentially excite
TE-type modes while the axial mounted probe excites TM-type modes. This may suggest
that at the degenerate mode, each probe preferentially excites standing waves that are
orthogonal to one other and do not destructively interfere with one other. However, at the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
170
non-degenerate modes, both the axial and radial mounted probes excite similar modes that
may be out of phase with one other and cause destructive interference; which was indicated
by one coupling probe coupling the energy out of the cavity. This would be a hardware
related problem that can be remedied by phase locking the microwave sources attached to
the axial and radial mounted coupling probes.
The heating pattern that was observed using the degenerate mode, was uniformly
distributed across the sample surface and resemble the superposition of the degenerate
modes. What is notable about the simultaneous coupling method is that it provides for an
additional methodology for achieving uniform heating in adjuvant to the mode-switching
method using only the radial mounted coupling probe. Results of this work are discussed
in appendix. The axial mounted probe is also significant in that it can be an alternative to
the radial mounted probe in order to address the near-field interactions experienced from the
radial mounted probe. Cavity characterization results from chapter 4 indicate that the nearfiled interactions can alter field symmetry which can result in unpredictable field
characteristics.
1.6.2
Coupling Probe Effects
Even though the coupling probe is typically designed to be 40 mm long, it was
found that at greater than 20.0 mm it can interact with the electric fields in such a manner
that field behavior become highly unpredictable. This was said to be due to near-field
interactions that were more significant with longer coupling probe depths. Hence during
processing, the coupling probe depth was limited to less than 20.0 mm in order to minimize
the near-field effects.
The coupling probe alignment was also found to have an effect on the field
symmetry. The more the coupling probe was not alligned, the more non-symmetrical the
field pattern. This was found to be a hardware problem that can be resolved by properly
aligning the probe mount with the cavity axis.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
171
Sample placement height with reference to the coupling probe height was found to
affect the field pattern inside the cavity. As the sample height was close to the coupling
probe height unpredictable field behavior as was observed for the long coupling probe
depths was also observed. Sample placement was optimized to be 1/8 of the freespace
wavelength away from the coupling probe.
Another interesting finding was that during processing, once the cavity was tuned
to resonance for a unique mode, cavity length adjustments alone were enough to tune the
cavity to resonance even when new modes were selected. This was observed during both
graphite / epoxy and vinyl ester / glass composites processing.
7.6.3 Loaded Resonant Cavity Length
Typically, as a large and highly lossy material is loaded into the cavity, the electric
fields become so perturbed that the calculated empty cavity solutions become no longer
valid. As such loaded cavity modes become difficult to characterize. The empty cavity
characterization results from chapter 4 showed that, the change in the loaded resonant
cavity length from the empty cavity, for a typical 24-ply composite with a dielectric
constant ranging from 2 to 4 was approximately 3.0 cm. This approximation allowed for
the characterization of loaded cavity modes which was used in the mode selection controller
to achieve uniform heating. It was found to be a sufficient estimation for lower order
modes and invalid for higher order modes. This may be due to the more unique ordering
of lower order modes and the more random ordering of higher order modes.
7.6.4 Effect of Tooling
In the loaded and empty cavity characterization for the development of the uniform
heating controller, information about sample placement, near-fields, coupling probe
alignment and depth, resonant cavity length and Teflon material were discovered. As the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
172
size of the Teflon block was increased the fields were found to be pushed more and more to
the boundaries of the block. What is enlightening about this finding is that even though the
Teflon material may be nearly electrically transparent, it still can have an effect on the
electric field inside the cavity due to the volume. This clearly suggests that tooling effects
cannot be ignored and must be actively studied from the point of design optimization, to
other alternatives which would integrate the microwave cavity and the tooling piece as one
unit, i.e. the part shaped cavity.
7.6.5 24-ply versus 48-ply curing
In the automatic curing of the 24-ply and 48-ply graphite epoxy composites, it was
observed that the 48-ply sample was more forgiving to long mode switching times than the
24-ply sample. This was indicated by sharp dips in the temperature profile for the 24-ply
sample during mode-switching times which was not present for the 48-ply sample. These
results are discussed and depicted in chapter 6. The important finding from these results is
the implication that the mechanical tuning technology may be more applicable to the
processing of thick-section samples than to thin section ones in which the rate of heat loss
is significant. Hence, frequency switching which could potentially provide for faster
mode switching times would be more applicable to the processing of the thin-section
composites.
7.6.6 Theoretical Field Pattern Plots
In the application of the empty cavity solutions to characterize the empty and loaded
cavity, a method was developed for generating theoretical electric field patterns in the form
of density plots for the empty cavity. This method uses easy density plot functions in a
software program called Mathematica. Details of these plots are discussed in appendix A.
The generation of the electric field patterns in the form of density plots is unique in that
contour plots are typically used which are difficult to understand and the not all the field
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
173
pattern plots are available. These plots were used in the characterization of the loaded and
empty cavity modes and were found to provide more comprehensive information than the
contour plots.
7.6.7
Application of Knowledge-based System to Automation
The lack of fundamental process models and the complexity of the domain in this
technology, motivated the application of knowledge-based systems technique using a
generic task approach for the overall automation. Results from implementation of the
knowledge-based system technique for low level control showed that, it was more
effective for high level tasks such as planning than for fundamental control tasks. This
result motivated the development of the global microwave curing control task into two
parts; a low level traditional unit and a high level knowledge-based system u nit Hence, a
knowledge based system technique was applied in a collaborative research effort. The
knowledge-based system planner was developed as a core dissertation work by
Decker(Decker 1995) which was interfaced with an automated system developed as a part
of this dissertation, specifically for interfacing with the KBS system
Output from the planner to the automated system were electromagnetic modes and
power levels required to control the curing process to achieve uniform heating. Input to the
planner from the automated system were the sensed process variables. Hence, the
automated system had capabilities for acquiring data and carrying out control tasks that
were requested by the planner, where the planner was the control decision making unit.
The noteworthy element of this work is the interfacing capability between the
knowledge-based system planner that was implemented in a software language called
SmallTalk, and the automated system that was implemented in LabView.
The successful
demonstration of this system in curing experiments is a significant accomplishment in the
application of a knowledge based-system planner in the real-time control of a complex
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
174
process, and motivates new directions in process automation. It provides the potential for
capturing and representing experiential knowledge which typically resides with operators.
This can be further advanced to knowledge retention and learning with the intended goals to
eliminate the invasive sensing methods in the microwave processing technology. Chapter 7
is dedicated to the discussion of this work.
7.7 Global Conclusion
In conclusion, the fixed frequency, single-mode resonant microwave concept was
fully automated and demonstrated as an automated “1st generation prototype process”. The
benefits due to automation clearly indicate a significant step in the advancement of the
single-mode resonant technology as a viable automated process. In the course of this
work, high level understanding to enhance the utilization of the technology and potential
applications were developed. Despite the extensive amount of research that has been
expended in the development of the “ 1st generation prototype process”, there are still
several issues that must be addressed to completely advance this technology to
commercialization. Some of these issues include economics feasibility, the development of
a user friendly microwave processing unit, and most importantly specialized tooling,
elimination of evasive sensing methods and scale-up.
Whatever the commercialization goals, whether it is curing, drying, heating,
dielectric analysis, plasma applications, batch or continuous processing, practical and
convenient process characteristics are paramount. As such, the completed automation
work is a significant enabling step in providing the foundational elements to realize the
viable potential of the single-mode resonant technology, where further research can be
pursed.
Finally, although this research program was comprehensive in scope, its goals
were significant and successfully directed toward the technical advancement of a viable
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
175
process with numerous potential applications, and the motivation of new research
challenges.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CHAPTER 8
RECOMMENDATIONS AND FUTURE WORK
8.1 Automated System
The automated system developed in this work is the “ 1“ generation prototype”
which was shown to significantly advance the single-mode resonant system as a fully
automated process with a viable potential for commercialization. Although significant
amount of research has been expended to evolve the technology to this stage, there is still
work to be done to optimize the technology in order to further advance to
commercialization. In general, the automatic control system can be modified to achieve
more optimum control. This may be done by expressing the control elements in the
frequency domain rather than in the physical domain as was done. This would make the
control system more robust and easier to maintain over the years. However, achieving this
goal is contingent upon the generation of accurate and complete process functions which
can be done either from exact solutions or from “good” estimations. With the contributions
from this work and other developments such as the results from Fellows(Fellows, 1995)
this can eventually be done.
The uniform heating controller can be improved by either optimizing the heating
knowledge-base with data such as that generated by Fellows(Fellows, 1995), or by the
generation of heating functions from exact solutions. The mode tuning software can also
be improved by optimizing the cavity length and probe depth scaling parameters. The
software platform, LabView is a very comprehensive tool and most importantly transparent
to operating systems and thus, it is highly recommended as the best control software
development tool available and must be kept
176
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Ill
8.2 Technology Advancements
For further advancement, the variable frequency technology in addition to high
power processing, dual coupling, scale-up and tooling issues must be actively pursued.
These areas would provide speed and versatility to the technology which would enhance
the application potential of it
8.3 Variable Frequency
Results from the work discussed in this dissertation have indicated that long mode
switching times can be an unfavorable factor in heating certain samples. Hence, to further
enhance the application flexibility of the single-mode resonant technology it is
recommended that variable frequency processing is studied and automated. In the variable
frequency technology, the frequency of the microwave power source is manipulated to
control the electromagnetic modes rather than the cavity volume as is done in the
mechanical tuning(see Chapter 3). Technically, electronic frequency manipulations can be
done faster than mechanical cavity volume adjustments.
Furthermore, with the variable frequency technology “moving parts” associated
with the mechanical cavity adjustments are eliminated. Additionally, by the nature of the
single-mode resonant cavity technology, a wider variety of modes can be excited in the
variable frequency system than in the fixed frequency system for cavities with similar
dimensions. Compared with the fixed frequency system, the variable frequency system is
more involved electronically, so it is recommended that such a system be automated. In the
automation, the control objectives would be similar to the fixed frequency system but the
tuning would be different The frequency would be manipulated electronically to minimize
the reflected power, rather than the 2-dimensional tuning required in the fixed frequency
system.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
178
The following are recommended for the advancement of a variable frequency
system:
1.
Obtain a variable frequency microwave power source and adapt to
existing hardware. This source should be purchased as a unit that includes
all circuit components to avoid additional components for the wide
frequency band. Note that the components of the fixed frequency are
typically rated for 2-4 GHz and 100 Watts of power. It is important to note
that as the frequency increases the free-space wavelength decreases which
enforces more strict operating requirements on the system.
2.
Fundamental work to characterize the system(empty and loaded)
3.
Automation of the system
4.
Process control logic development and implementation
5.
Application to the processing of polymers and composites
With the development of the variable frequency method the application potential of
this technology would be significantly enhanced. Faster processing and more
electromagnetic modes will be available for a similar cavity dimension which would
improve the ability to process complex shaped parts and also achieve high-speed
processing capabilities. Additionally, there will be the capability for dielectric analysis at
various frequency ranges.
8.4 High power source
To be able to process real parts at true high-speeds a high power source is highly
recommended. The range of power required should be considered within the limitations of
the processing hardware, sample properties and control capabilities. For the 3 inch square
by 1 inch thick flat panel graphite / epoxy composites, preliminary heat transfer calculations
show that a 200 W source could achieve heating rates of 60 °C /min. and a 1 KW source,
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
179
340°C / m in.. Even with the electronic frequency switching system, such high heating
rates as shown for the 1 KW source could pose tremendous control challenges. As such a
1 KW source may not be recommended for this sample. On the other hand, for a
pultrusion process, preliminary technical, economic feasibility studies showed that a 1 KW
source was required to achieve desirable throughput for processing
1 inch diameter
polyester glass rods. Clearly the processing issues in these two processes would vary
since one is a batch process and the other is a continuous process. Nevertheless, total
process evaluations as well as hardware limitations must be understood in order to specify
and define the high power source required for high-speed processing.
8.5 Dual Coupling
As processing sample geometry becomes complex, the issue of near-fields could
have a significant effect on heating. Hence, vertical coupling should be explored such that
the near-field interactions can be avoided. This technology would require minimal effort
since similar modes were found to be excitable with both vertical and horizontal coupling.
Most importantly, it would provide additional processing flexibility for achieving uniform
heating. As a novel approach, it could also be used in pultrusion applications by exciting
orthogonal modes(degenerate modes) which can be used to process complex shaped parts
or to uniformly process planar parts.
To achieve this goal, further research must be done to understand this technology.
The issue of phase locking the two sources must be explored to see whether dual coupling
can be achieved at the non-degenerate cavity lengths. Also, dual coupling with various
input power levels must be explored, such as each probe with equal power levels and with
one having a higher power level than the other.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
180
8.6 Scale-up
Scale-up is another area which must be researched. In the single-mode resonant
cavity technology, as the cavity size is scaled up the applied microwave frequency has to be
scaled down in order to support similar electromagnetic modes. This means that for a
7 inch in diameter cavity which is used to process 3 inch square sample at 2.45 GHz, an
18 inch in diameter cavity would be required to process an 8 inch square sample at 0.915
GHz. Scale-up factors must be defined such that the 7 inch-diameter cavity results are used
in larger sized cavities. These parameters would include microwave power frequency,
sample placement, power density, cavity length, probe depth and tooling design.
Currently, cylindrical shaped shaped cavities are used due to the ease of
characterizing them, mathematically. However, as processing sample size increases and
geometries become complex cavity geometry may be worth understanding. It may seem
logical to process square shaped samples in square shaped cavities and discs in cylindrical
cavities.
8.7 Tooling Issues
In the current state of technology, the part to be processed is placed in a specialized
tooling that is placed inside the microwave cavity. These specialized tooling require
microwave transparent materials such as polymers and ceramics which may cannot
withstand processing pressures or handling in a manufacturing environment The
characterization results showed that although the Teflon tooling material could be
considered to be microwave transparent its size could significantly perturb the electric
fields inside the cavity. These perturbations could cause unpredictable field characteristics
which could affect the heating of the part in an adverse manner. As such, the concept of
tooling-free processing in the single-mode resonant cavity should be studied.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
181
8.8 Potential Applications
In general, economic feasibility, scale-up and tooling issues are just a few of the
fundamental studies that must be done in order to advance this technology to
commercialization. In commercialization there are several potential single-mode
applications or those that do not require several modes to achieve uniform heating where
this technology would be suitable. It is recommended that the following applications be
considered for further progression of this technology.
Pultrusion
Integration of of the single mold resonant cavity as the main heating and
shaping source (and not the pre-heater) in a pultrusion process for high
throughput and enhanced properties. This may be done using the dual­
coupling method for pultruding complex shaped parts, frequency switching
for high-speed processing of multiple rods or mats, and fixed frequency for
sheets or thin samples.
Powder prepreg sintering
Integration of the single-mode resonant system as the finishing step in the
powder pre-pregging process.
Post-curing
The post-curing of parts where the bulk of the reaction has been completed
and the part does not undergo rapid physical and chemical changes. An
example would be RTM parts which are typically post-cured in thermal
ovens and takes up to 24 hours. Potential benefit would be economics due
to the minimization of the post-cure time.
Drying
Integrated as the drying step before an RTM process or the drying of
polymers such as PET(poly ethylene terepthalate) for other applications.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
182
Potential benefit would be speed and the enhancement of part properties,
since microwave can be potentially used to dry polymers to lower moisture
levels.
Plasma support
Ideal for plasma support processes since intensive process tuning is
not required during processing. Potential benefit would be convenience of
operating the process.
Finally, these are only a few of the processing challenges that lay forth in this
technology. A strong effort should be directed towards the elimination of the invasive
temperature sensing methods to make this a true materials processing technology.
However, this can only come about from a solid fundamental understanding of the
technology. Hence, it is also very important that fundamental issues in process modeling,
electromagnetic modeling, dielectric modeling be also pursued in parallel to the process
developmental efforts in order to develop a strong foundation of the technology.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDICES
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX A
AUTOMATION-HARDWARE
A1 Stepper Motors
A 1.1 Description
The stepper motor is an 8-lead motor which was available as a motor and
driver unit (Super Vexta UMD268M-E1.5) and was designed specifically as a high
performance positioning device. It is designed with class B insulation materials and
is capable of withstanding temperatures at the motor coil of 130°C(255 F) with no
reduction in motor life. It has a holding torque capability of 118 oz-inch and
variable step angles of 0.9° and 0.45° which corresponds to 400 pulses per
revolution and 800 pulse per revolution, respectively.
A 1.2 Stepper Motor Driver
The driver for the stepper motors provide for switch selectable functions for
step angle size, current cutback, automatic thermal cutout, pulse selection and
direction selection, see Figure A -l. When SW-1 and SW-2 are switched to the off
position and SW-2 and SW-4 are switched to the on position, the motor would
183
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
184
Stepper Motor Driver
©
Pulse
©
SW1
©
CWTCCW
©
SW1
©
©
©
©
2-1
2-2
O
z
o•Si
•n
©
©
©
StripedBlack—
Red----OrangeYellow-
:□
AC115
FG
Striped
Stepping
Motor
Black
Red
Orange
Yellow
Figure A- 1 Stepper Motor and Drive Component
require a high or low signal at the CW/CCW terminal to turn the motor in the
clockwise or counterclockwise direction while pulses are sent to the pulse terminal.
When SW-1 and SW2 are switched on while SW-2 and SW-4 are switched off, the
motor would require a pulse at the CW/CCW terminal to move in the clockwise
direction and a pulse at the pulse terminal to move in the counter clockwise direction.
The latter configuration is used in this work due to the convenience in programming.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
185
The switch SW-5 is used to select the step angle of the stepper motor to be
either full step or half step, where a full step is 0.9 0 and a half step is 0.45 0 for
every pulse. In this work both different step sizes are used for the different
components depending of the travel length and speed. The switch SW-2 is used to
select the option for automatic current cutback at standstill. When SW-2 is in the off
position automatic current cutback is activated. This option allows for the reduction
of power to a fraction of running power after about 200 milliseconds of inactivity.
It also has a capability for TTL(Transducer Transducer Logic) digital logic
IC(integrated circuit) interface for automatic motor pulse and direction selection. In
a TTL type interface the voltage ranges from 0.0 V to 5.2 V, where high is defined
as a voltage value being greater than +2.4 V and low to be less than +0.8 V.
However, in the driver internal circuitry there is an optocoupler device which is used
to drive the direction and pulse terminals which require 4-5 V for high and 0-0.5 V
for low signals for TTL interface. Thus, a standard TTL wiring could not be used
and a 5V external power supply(Tucker Power Supply) was necessary to drive these
terminals(see Figure A-2).
H : 4—
5V
L: 0-0.5 V
220 £2
Fpl-MAr
i
I Pulse o r
| Direction
® Optocoupler
I
^
25 mA Max
I
V
.
Figure A- 2 Stepper Motor Driver TTL Circuit
Movement of the stepper motor is accomplished by regulating the number
and frequency of pulses sent to the driver's terminals. The motor rotates one step for
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
186
each pulse received at the pulse terminal and the direction of rotation is controlled
by the signal applied to the driver’s "CW/CCW" terminal. Note
that direction and pulse terminals are determined by the switch
settings in the driver. A signal at (4-5 V) is said to be high while a signal at (00.5V) is said to be low.
A. 1.3 Wiring Diagram
The wire connections between the stepper motor, the power supply and the
data acquisition board is as follows. The positive end of the pulse and CW/CCW
terminals are connected to the 5V power supply and the negative end of these
terminals are connected to the data acquisition terminal block. The negative end of
the power supply is connected to the ground on the terminal block. These
connections are very important in order for the proper operation of the stepper
motors. Note that the 5V power supply from the data acquisition board was not
adequate to drive these terminals because of the current requirements and the nonconventional high low voltage signals for the TTL output of the optocoupler (see
Figure A -l). Figure A-2 shows wiring schematic for stepper motor drivers with the
data acquisition interface board.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
187
Digital In p u t / O utput Term inals
+
Pulse
+
CWy CCW
5V
Tucker Variable
Power Supply
Figure A- 3 W iring Schematic for Stepper M otor Drivers
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
188
A.2 Data Acquisition Interface
A National Instrument data acquisition and control board NBMI016H was
used. This board is a high performance multi-functional analog, digital, timing
input/output board for the Macintosh computer. This board has a maximum
sampling rate of 47 K samples per second and has an external circuit breaker which
is attached to the side of the computer. It is a 12-bit successive approximation
ADC(analog to digital converter) with 16 analog inputs, two 12-bit DACs with
voltage outputs, 8 lines of TTL-compatible digital I/O, three 16-bit counter/timer
channel for timing I/O, 2-(5 V) power supply, 1-digital ground, 1-analog input
ground, and 1-analog output ground(see Table A-2).
This board supports three different input modes for the analog input channels
which can be selected by jumpers W1 and W9.
They are referenced single-ended
(RSE) input, non-referenced single ended input (NRSE) input, and differential
(DIFF) input. The referenced single-ended input configurations provides for 16
single ended channels, with the negative end of the of the input instrument
referenced to analog ground.
The differential ended input provides for eight channels with the negative end
of the input instrument wire tied to the multiplexer output of channels 8 through 15.
The non-referenced single-ended input configurations provides for 16 single ended
channels, with the negative end of the input instrument tied to AISENSE and not
connected to the ground. The RSE configuration is used for the analog input signals
in this work. This configuration corresponds to jumper settings: A-B, C-D, G-H, and
B-C. All jumper settings used in this work are summarized in Table A-l.
The board also supports different input polarity and input ranges for the
analog input signals. The two input polarities are unipolar and bipolar inputs.
Unipolar input means the input voltage range is between 0 and the (voltage
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
189
reference-1 LSB V), where the voltage reference is a positive reference voltage.
Bipolar on the hand means that the input voltage range is between the negative of the
voltage reference, and the (positive of the voltage reference - 1 LSB V).
One LSB is the voltage increment corresponding to a least significant bit
change in the digital code word. For unipolar output, 1 LSB = (Voltage
reference/4096) and for bipolar 1LSSB = (Voltage reference / 2048). The jumpers
W3 and W4 can be used to select the different ranges. The options for ranges are 0
to 10 V, -5 to +5 V and -10 to +10 V. The voltage range of -10 to +10 V is used in
this work which corresponds to the jumper setting of A-B and A-B.
There is also software programmable gain settings which provides for more
accurate voltage readings for different instruments. Note that the input range
selection is based upon expected input range of incoming signals. A large input
range typically sacrifices voltage resolution. The NBMI016H board supports gain
settings of 1,2,4,8 which correspond to the actual input range and precision as
summarized in table 4-2.1. Note that the precision values are based upon the least
significant bit (LSB) of the 12-bit ADC. This means that the voltage increment
corresponds to a change of one in the ADC 12-bit count. The board also supports
different polarity and ranges for the analog output channels. The jumpers for these
settings are W-4 and W-5 for either analog output channels 1 or 0.
The direction of the digital ports can also be configured for either input or
output by software. The factory settings are that port A is for input and port B is for
output These ports support a maximum voltage rating of 5.5 V with respect to
digital ground. For TTL logic it supports high at 2 V minimum and low at 0.8 V
maximum. The current load at the high input voltage is 40 |iA and -120 p.A at low
input voltage. It also supports output voltage of 2.4 Vmaximum for high and 0.5 V
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
190
maximum for low output logic, and 2.6 mA maximum output source current for high
logic and sinks 24 mA for low output logic.
Device
Data Acquisition Board Jumper Settings
Input Mode
Analog Input (AI)-RSE
Range / Polarity
Jumper
Setting
Jumper
Setting
W1
A-B, C-D
W3
A-B
W9
G-H, B-C
W4
A-B
Table A- 1 Ju m p er Setting for D ata Acquisiton Board
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A P P E N D IX B
V E R T IC A L C O U P L IN G
B .l Introduction
In the design of the mechanized cavity, as an added feature which was not required
for automation, the mechanized cavity was equipped with novel axial and radial mounted
coupling probes. As was previously mentioned typically, either the axial or radial mounted
probe configuration is used and not both. Empty cavity characterization results showed
that similar modes can be generated for either probe mount configurations except for at the
cavity length corresponding to a degenerate mode. Using both probe mounted
configurations the cavity was characterized for heating graphite / epoxy and vinyl / ester
glass composite materials.
B.2 Experimental
The applicator used in this study was the 7 inch diameter single mode resonant
cavity, but with coupling probes mounted at the top and the side of the cavity, as shown in
Figure B -l. The drive mechanisms for the coupling probes were designed for
simultaneous or independent coupling. The vertical or top mounted probe mounted such
that it can be adjusted independent of the top shorting plate. Thus, at a fixed cavity length
the top mounted probe can be adjusted to various lengths. The dimensions and design of
the top probe was similar to the side probe except for that it is longer. The top-mounted
probe was 4" long x 0.375” in diameter and the side-coupling probe is 3" long x 0.375 "
diameter and was located at (0.25 of the operating wavelength) 1.2" from the base of the
cavity.
191
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
192
5
M
^^T
od
Probe
^
Drive
azzr^D,
T ? y p ? V '/ /; 7 7 } s a
I
Short-plate
Drive
^mz2<
i^ /W /i
AS-
ihort-plate
ringer Stock
•Top Coupling
Probe
Saijij^e
»
'
i
5
Side Coup ing
Probe
Figure B - 1 Dual-Couplig Single mode Resonant Cavity
Two microwave power sources were used with each one dedicated to each
coupling probe. The sources were both magnetrons operating at a frequency of 2.45 GHz
with a maximum power of 100W.
The sample temperature was measured using fiber
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
193
optic temperature probes and thermal paper which changed color from white to blue at
85°C. Thermal paper was placed between the sample and the Teflon block with four fiber
optic temperature probes attached to the top surface of the sample. Since the thermal paper
activated at approximately 85°C, the sample was heated until the highest temperature
measurement from the four temperature sites was 100°C. The samples were placed at 1.2"
from the base of the cavity on a Teflon block, as shown in Figure B-2. Figure B-3
show temperature probe placements and sample placemt in the cavity. Loaded cavity
modes were located by a swept frequency method.
1.2
Thermal Paper
Figure B- 2 Sample placement with thermal paper
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
194
Fiber Direction
**
►
4
2
1
3
Figure B- 3 Temperature probe placement
Two different heating experiments were done, one where only one probe was used
to couple the microwaves into the cavity, and another where both probes were used to
simultaneously couple energy into the cavity at a fixed cavity length. For the single
coupling experiments, when one coupling probe was active in coupling microwaves into
the cavity the other was retracted from the cavity. Input and reflected power were
measured from both probes at all times. In the dual coupling experiments each probe was
tuned independent of the other at a fixed cavity length.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
195
B.3 Results and Discussions
Similar empty cavity modes were located at similar cavity lengths for the top and
side mounted probes. The heating profile for these modes were also similar for both probe
configurations. The graphite epoxy composite also showed similar heating profile for both
probe configurations except for at a cavity length of 19.7 cm, where edge heating was
achieved by the top mounted probe and center heating for the side mounted probe. Figure
B-4 and Figure B-5 show the thermographs of these heating profiles.
For a single mode resonant cavity only unique modes can be generated at a given
cavity length, except for in certain regions of the cavity where two modes known as
degenerate modes can exist at the same cavity length. In the seven inch cavity, there are
two hybrid modes TM(1 lx) and TE(Olx) which can exist at a cavity length of either
11.283 cm or 22.256 cm. Which of these two modes dominates during excitation is a
function of where the coupling probe is mounted. The vertical coupling configuration
should excite the TM-mode while the side mounted configuration should excite the TEmode. The edge heating result from the top mounted probe could suggest a TE-mode
excitation and a TM-mode for the side mounted probe which is contrary to what was
expected.
Figure B-9 shows the heating profile when both probes were used to
simultaneously couple energy into the cavity. In this profile the temperature is uniform
across the sample, indicated by the darkened areas on the thermal paper. This heating
profile looks like the summation of the heating profiles from the side and top mounted
coupling probes. This result is evidence that a hybrid mode could have been excited at this
cavity length.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
196
It was only possible to do dual coupling at the regions where hybrid modes were
excited. At the other regions of the cavity, it was not possible since one probe acted as a
source and the other as a sink. In other words, as one probe coupled energy into the cavity
the other coupled it out of the cavity even when it was retracted from the cavity. The
reflected power during dual coupling was less than 10% of the input power. Table B-l
lists all of the processing parameters for the dual coupling experiment
The probe depth for the top mounted probe was always longer than that for the side
mounted probe. Since the sample was placed at 1.2” from the bottom of the cavity it was
relatively far enough away that near field effects would not be significant However, the
long metallic rod in the cavity could have a significant effect in the perturbation of the
fields and make the heating pattern unpredictable.
Side
Coupling
Top
Coupling
Both Probes
CL
(cm )
Top
Pd
(m m )
S id e
Pd
(m m )
29.4
S id e
P ro b e
P ref.
(W )
<1
T op
P ro b e
Pref.
(W )
<5
19.7
0
19.0
20.57
0
< 1
<3
19.9
31.9
40.37
<5
<8
Table B- 1 G raphite / epoxy heating d ata
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure B- 4 Top probe - graphite epoxy at CL=19.1 cm
Figure B- 5 Side probe - graphite epoxy at CL=19.7 cm
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
r
198
Figure B- 6 Both probes - graphite epoxy at CL=19.7 cm
B.4 Conclusions
An interesting finding was that, only at the cavity length corresponding to the
degenerate mode can simultaneous coupling from both axial and radial mounted coupling
probes be used to achieve heating. At the other cavity lengths as one coupling probe
coupled the microwave energy into the cavity the other coupled it out of the cavity and the
sample was not heated. Theoretically the side mounted probe should preferentially excite
TE-type modes while the axial mounted probe excites TM-type modes. This may suggest
that at the degenerate mode, each probe preferentially excites standing waves that are
orthogonal to one other and do not destructively interfere with one other. However, at the
non-degenerate modes, both the axial and radial mounted probes excite similar modes that
may be out of phase with one other and cause destructive interference; which was indicated
by one coupling probe coupling the energy out of the cavity. This would be a hardware
related problem that can be remedied by phase locking the microwave sources attached to
the axial and radial mounted coupling probes.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
199
The heating pattern that was observed using the degenerate mode, was uniformly
distributed across the sample surface and resemble the superposition of the degenerate
modes. What is notable about the simultaneous coupling method is that it provides for an
additional methodology for achieving uniform heating in adjuvant to the mode-switching
method using only the radial mounted coupling probe. Results of this work are discussed
in appendix. The axial mounted probe is also significant in that it can be an alternative to
the radial mounted probe in order to address the near-field interactions experienced from the
radial mounted probe. Cavity characterization results from chapter 4 indicate that the near­
filed interactions can alter field symmetry which can result in unpredictable field
characteristics.The side coupling configuration is typically used in polymer processing,
while vertical coupling is used in plasma generation.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX C
KNOWLEDGE BASED SYSTEM INTERFACE
C.1 Introduction
The lack of fundamental process models and the complexity of the domain in this
technology, motivated the application of knowledge-based systems technique using a
generic task approach for the overall automation. Initial implementation of the fabrication
unit was the modelling and controlling of the processing device for mode tuning(Adegbite,
1991 ; Adegbite, 1992 ; Adegbite, 1992) using FM. The knowledge-based system
technique was found to be effective for high level tasks such as planning than for
fundamental control tasks(Adegbite, 1992 ). It was found to be slow and complicated in
performing fundamental control tasks. Hence, a knowledge based system technique was
applied in a collaborative research effort. The knowledge-based system planner was
developed as a core dissertation work by Decker(Decker, 1995). which was interfaced with
an automated system developed as a part of this dissertation, specifically for interfacing
with the KBS system.
C.2 Goals of the Planner
This planner is a part of a global objective set for this project is the development of
an intelligent system for manufacturing composite materials from the design concept to
fabrication of the final part This will compose of two units with one dedicated to material
design and the other to fabrication. The output of the desig n unit will be material
properties and baseline process parameters to achieve specified application requirements.
This output will be used as input to the fabrication unit to process the composite part The
overall architecture in Figure C-l would be made up of the following components:
200
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
201
.from Designer
Material and
Properties
Sensed Parameters
Process
Conditions
Expectations
ii
Global
Planner
Process
States
Process
Conditions
Process Profiles
Reactive
Monitor
Process
States
Characterization
Set -Points
Controller
Sensed Parameters
Adjustments
Process
Sensed Parameters
Figure C- 1 Global Composite Manufacturing Architecture
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1.
202
An automated composite design unit with the capability to select
composite material components and fabrication methods, given final part
application(Kamel and Sticklen 1990; Kamel, Sticklen et al. 1990; Kamel,
Sticklen et al. 1992; Sticklen, Hawley et al. 1992),
2.
A m athem atical modeling unit that would relate mass, momentum, and
energy balances to explicitly describe the process(Sundaram, McDowell et
al. 1993)
3.
A fabrication planning u n it which would use process heuristics and
process history to specify the most optimum processing protocol(Decker,
Adegbite et al. 1993),
* 4.
A fabrication process control u n it for the real-time operation of the
fabrication process by sensing and controlling the fabrication process. This
is the focus of the work discussed in this proposal(Adegbite, Hawley et al.
1991; Adegbite, Hawley et al. 1992; Adegbite, Hawley et al. 1992;
Adegbite, Hawley et al. 1993; Adegbite, Wei et al. 1993).
C.3 Interface with KB S-planner
In this work a control system was develop to enable the implemntaion of the
planner. The planner was implementd in a computer languae called SmallTalk and process
control interface was implemented in LabView. Details of the SmallTalk software can be
found elsewhere(Decker, 1995), and details of the LabView section is presented.
Output from the planner to the automated system were electromagnetic modes and
power levels required to control the curing process to achieve uniform heating. Input to the
planner from the automated system were the sensed process variables. Hence, the
automated system had capabilities for acquiring data and carrying out control tasks that
were requested by the planner, where the planner was the control decision making unit
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
203
Several experiments were done to successfully verify this unit in the processing of
graphite composite materials. The results of this work can be found elsewhere(Decker,
1995).
The noteworthy element of this work is the interfacing capability between the
knowledge-based system planner that was implemented in a software language called
SmallTalk, and the automated system that was implemented in LabView.
The successful
demonstration of this system in curing experiments is a significant accomplishment in the
application of a knowledge based-system planner in the real-time control of a complex
process, and motivates new directions in process in automation. It provides the potential
for capturing and representing experiential knowledge which typically resides with
operators. This can be further advanced to knowledge retention and learning with the
intended goals to eliminate the invasive sensing methods in the microwave processing
technology. The LabView program for achieving this goal is called Integration Demo.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
APPENDIX D
ELECTRIC FIELD PLOTS
D.l Description
Appendix D contains the Mathematica program routines that were used in the
generation of the electric fields density plots. In this routine the spherical coordinates are
converted to cartesian coordinates before plotting the data. The equations polotted are the
magnitude of the electric field components in equations 3-12 and 3-15. For these
calculations the roots of the Bessel's function and their derivatives are input in files that are
«s
called on the first line of the program. Below is a a Mathematica program code for
generating these plots.
TM-mode
m=l
v=3.8318/3.5
TMll[x_,y_] := (BesselJ[m,v Sqrt[xA2 + y A2] ]
Sin[m AzcCos [x/Sqrt [xA2+yA2] ]])A2
DensityPlot [TMll [x,y] ,{x, -3.5,3.5 }, {y, -3.5,3.5> ,PlotPoints->90
,Mesh- >False,Axes ->Palse,Frame->False ]
TE-mode
m=l
▼si.8412/4
TE11[x_,y_] := (1/2(BesselJC(m-l),▼ Sqrt[xA2 + y A2]]
- BesselJl (m+1),v Sqrt[xA2 + yA2]])
Cos [a ArcCos [x/Sqrt [xA2+yA2] ]]) A2
DensityPlot[TE11[x,y],{x,-3.5,3.5},{y,-3.5,3.5},PlotPoints->90
,Mesh->False, Axes ->False,Fraae- >False ]
204
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
A PPENDIX E
SOFTW ARE DOCUM ENTATION
E. 1 Introduction
The LabView program code for all of the main VI and the sub-VI are not included
in the appendix because an acceptable print quality program could not be generated. This is
due to the graphical nature of the program, which does not allow formating to specify
margins or to generate a clear black and white image. Hence the progam subroutines are
desribed. Each program is printed with a hierarchy which indicates the other subroutines
or sub-Vi’s that the program calls. There is also a front panel and a diagram included in
each program. There are several subroutines used and only the ones developed to support
this work are presented. These sub-Vi’s have been previuosly described as part of the main
program in Chapter 5. Again, a basic understanding of LabView would be required to
fully comprehend the program. A title and description of each Vi is given.
205
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
206
E.2 Compcure3-Curing program
E .2 .1 Program Description
This is the main LabView program for implementing the complete control-loop for
curing. It shows in greater detail the different level of sequence structures. Parts of this
program were shown in Chapter 5, however this is a more detailed representation of i t
E. 3 Surface, vi - implementation of tuning program
E.3.1 Program Description
This is the tuning program for implementing the 2-dimensional simplex method.
It also uses several levels of sequence structure and calls several developed sub-VI’s as
shown in the program hierarchy. The algorithm and details were presented in Chapter 5.
The other sub-Vi’s are presented in detail in the sections that follow.
E.4 Replace.vi-replacement of simplex triangle vertices
E .4.1 Program Description
This a sub-VI that is only used in the Surface. Vi or tuning program. It is used to
replace the vertices of the simplex triangle when new values are determined. It specifically
replaces the cavity length and pobe depth values corresponding to the BNW vertices in the
simplex triangle(see Chapter 5).
E. 5 Vertex.vi -calculation of intial vertex in the simplex triangle
E.5.1 Program Description
This sub-VI is also used specifically in the Surface.vi program and it is used to
calcualte the intial vertices of the simplex triangle. These vertices are determined by
calculating the cavity length and probe depths corresponding to a best point, next to best,
worst point as explained in Chapter 5 in the tuning control software development
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
207
E.6 CL / PD / Pwr -scaling of cavity length, probe depth and power values
E.6.1 Program Description
This sub-VI is used specifically for scaling the cavity length, probe depth and
power sensed data. It converts the voltage readings to actual vaules. It is used in any loop
where data is acquired.
E.7 Move Read.vi - adjustment of cavity length & probe depth and power sensing
E.7.1 Program Description
This sub Vi is also used in the surface.vi program for data acquisition and cavity
length and probe depth adjustment. It contains the software for the stepper motor drivers.
It was generally used to read the reflected power once new cavity length and probe depths
were determined in the tuning program.
E. 8 Limits.vi - tuning limits for a mode
E. 8.1 Program Description
This sub-VI is used to keep the tuning program from jumping out of modes. It
keeps the tuning within the cavity length and probe depth within set limits for a desired
mode.
E.9 Contract.vi - contraction of simplex triangle in the tuning program
E .9 .1 Program Description
This sub-VI is one ot the logic ateps in the simplex algorithm It is used to contract
the triangle as described in Chapter 5 in the simplex routine.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
208
E. 10 Pwrcontrl.vi - power controller used in the curing program
E.10.1 Program Description
This is the main program where the PID controller is implemented. The
subroutines for implementing the PID algorithm were available from LabView. Thus the
program implemntation involved the constuction of the control logic using these
algorithms.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LIST OF WORKS CITED
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
LISTS OF WORKS CITED
Adegbite, V., M. Hawley, et al. (1991). Implementation of an Artificial Intelligence
Technique for Real Time Control of Composite Materials. ASM/ESD 1991 Advanced
Composites Conference and Exposition, Detroit
Adegbite, V., J. Wei, Larry Fellow, M. Hawley, (1995). Uniform Heating of Polymer
Matrix Composites. Invited paper at the ACS annual meeting. ACS Cincinnatti
Valerie Adegbite, Martin C. Hawley, Dave Decker, and Jon Sticklen, "Automation of
Microwave Processing of Graphite/Epoxy Composite Materials Using an Expert Systems
Technique", MRS Svmp. Proc.. 269. 425 (1992).
V. Adegbite, M. C. Hawley, A. Kamal, M. Pegah, and J. Sticklen, "Monitoring and
Control of Microwave Processing Utilizing Functional Modeling: Preliminary Results from
the Automation of the Empty Cavity Mode Tuning", ANTEC'92 Conf. Proc.. 2042
(1992).
Valerie Adegbite, Martin C. Hawley, and John Sticklen, "A Control Strategy for the
Microwave Processing of Composite Materials", American Institute of Chemical Engineers
1993 Summer International Conference in Seattle. Washington. (19931.
Antsaklis, P. (1994). Defining Intelligent Control. IEEE International Symposium on
Intelligent Control,
Asmussen, J., H. H. Lin, et al. (1987). “Single-Mode or Controlled Multimode
Microwave Cavity Applicators for Precision Material Processing.” Rev. Sci. Instrum .:
1477-1486.
Asmussen, J., R. Mallavarpu, et al. (1974). IEEE,
Asmussen, J. and D. Reinhard (1986). U.S. Patent No. 4 585 688 :
Asmussen, J. and J. Root (19841. Appl. Phvs. Lett. 44: 396.
Asmussen, J. and J. Root (19851. U.S. Patent No. 4 507 588:
Beveridge, G. S. G. and R. S. Schechter (1970). Optimization: Theory and Practice. New
York, McGraw-Hill.
Binner, J. G. P., Ed. (1991). Microwave Processing of Materials. Cambridge, Abington
Publishing.
209
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
210
Boas, A. H. (1963). “How Search Methods Locate Optima in Univariate Problems.”
Chemical Engineering 70: 85-88.
Bolin, B., E. T. Degens, et al. (1979). The global biogeochemical carbon cycle. The
Global Carbon Cvcle SCOPE 13. New York, John Wiley.
Box, M. J. (1965). “A New Method for Constrained Optimization and a Computation with
other Methods.” Computer Journal 8: 42-52.
Campey, I. G. and D. G. Nickols (1961). Simplex Minimization. ICI Limited- Central
Instr. Lab.
Carpenter, B. H. and H. C. Sweeney (1965). Chemical Engineering 72(14): 117.
Ciriscioli, P. R. and G. S. Springer (1989). “Dielectric Cure Monitoring- A critical
Review.” SAMPE Journal 25(3): 35-42.
Ciriscioli, P. R. and G. S. Springer (1991). “An Expert System for Autoclave Curing of
Composites.” Journal of Composite Materials 25: 1542-1587.
Collin, R. (1966). Foundations for Microwave Engineering. McGraw-Hill.
Day, D. R. (1986). “Cure Control: Strategies for Use Of Dielectric Sensors.” 31st
International SAMPE Symposium : 1095-1103.
Dhulipala Ramakrishna, Susannah Travis, and Martin C. Hawley, "Microwave Processing
of Glass Fiber/Vinyl Ester-vinyl Toluene Composites", MRS Svmp. Proc.. 269. 431
(1992).
Fellows, L. (1992). Microwave Processing of Polyimides. Michigan State University.
Fellows L. A ., M. Lin, and M. C. Hawley, "Feasibility of Microwave Processing by
Frequency Switching for Implementation of Mode-switching Techniques", Tenth Annual
Polymer Processing Society Meeting, Akron, Ohio, April 3-7,1994.
Fellows . L. A, V. O. Adegbite, and M. C. Hawley, "Determination and Control of
Microwave Processing Cycles", AIChE National Summer Meeting, Seattle, WA,
August 15-18, 1993.
Fellows, Larry , Susannah Travis, and Martin C. Hawley, "Preliminary Comparison of
Microwave heating of Complex-shaped Composites Reinforced with Conductive and
Nonconductive Fibers". MRS Svmp. Proc.. 269. 415 (1992).
Fellows, Larry , Richard Delgado, & Martin Hawley “Processing of Complex Shapes with
Single-mode Resonant Frequency Microwave Applications”, 26th International SAMPE
Technical Conf., Oct. 17-20 1994, Atlanta, GA
Gourdenne, A , 3rd International Conference on Composite Materials, Paris, 2 1514
(1980)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
211
Gutowski, T. G., T. Morigaki, et al. (1987). “The consiladtion of Laminate Composites.”
Journal of Composite Materials 21: 172-188.
Halpin, J., J. L. Kardos, et al. (1983). “Processing Science: An Approach for Prepreg
Composite Systems.” Pure and Applied Chemistry 55:
Harrington, R. F. (1961). Time Harmonic Electromagnetic Fields. New York, McGrawHill.
Hart, P. (1982). “Directions for Al in the 80s.” STGART News Letter (79):
Hart, s. and D. Kranbeuhl (1992). “Intelligent Sensor-Model Automated Control of PMR15 Autoclave Processing.” 37th International SAMPE Symposium : 224-230.
Hawley, M. C., J. Jow, et al. (1987). “Microwave Processing and Diagnosis of
Chemically Reacting Materials in a Single Mode Cavity Applicator.” IEEE Microwave
Theory and Techniques 3(12): 1435-1443.
Hinrichs, R., J. and J. Thuen M. (1984). “Control System For Processing Composite
Materials.” II S Patent-4.455.268:
Hinrichs, R. J. (1983). “Control of Composite Cure Processes.” ASTM STP 797: 29-37.
Huack, H. S. (1969). “Design Considerations for Microwave Oven Cavities.” IEEE
Transactions on Industry and General Applications (Jan/Feb):
Jow, J. (1988). Microwave Processing and Dielectric Diagnosis of Polymers and
Composites Using A Single-Mode Resonant Cavity Technique. Michigan State University.
Jullien, H. and H. Valot (1985). Polvmer (26): 506.
Kamel, A. and J. Sticklen (1990). An Artificial Intelligence-Based Design Tool for Thin
Film Composite Materials. Proceeding of the 5th Technical Conference for Composites,
Kardos, J. L., H. P. Dudukovic, et al., Ed. (1983). Void Formation and Transport during
Composite Laminate Processing, in Composite Materials. Quality Assurance and
Processing. Composite Materials.
Karmazsin, E. and P. Satre (1985). Thermochemics Acta 93: 305.
Kranbeuhl, D., D. Eichinger, et al. (1990). “On-Line In-situ Control of the Resin Transfer
Molding Process.” 35th International SAMPE Symposium : 825-834.
Kranbeuhl, D. E„ P. Haverty, et al. (1988). Monitoring the Cure Processing Properties of
Unsaturated Polyesters In-Situ During Fabrication. Society of Plastics Engineers 46th
Technical Conference Proceedings,
Kranbeuhl, D. E., P. Haverty, et al. (1987). Dynamic Dielectric Analysis for
Nondestructive Cure Monitoring and Process Control. 42nd Annual Conference,
Composite Institute, The Society of Plastics Industry, Session 22-D,
Kranbeuhl, D. E., M. Hoff, et al. (1988). “Insitu Measurement and Control of Processing
Properties of Composite Resins in a Production Tool.” 33rd International Sampe
Symposium : 1276-1284.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
212
Kranbuehl, D., M. Hoff, et al. ('1988'). International SAMPE Symposium 33: 1276.
Kranbuehl, D., P. Kingsley, et al. (1992). “Sensor-Model Prediction, Monitoring and Insitu Control of Liquid RTM Advanced Fiber Architecture Composite Processing.” 37th
International SAMPE Symposium : 907-913.
Krieger, B. (1994). Microwave Vulcanization: A lesson in Business and Technology.
M at Res. Soc. Symp. Proc.,
LeClair, S. R., F. L. Abrams, et al. (1989). “Qualitative Process Automation.” AT Edam
3(2): 125-136.
Lee, C. W. (1987). “Composite Cure Process Control by Expert Systems.” SAMPE:
Lee, I. W. and G. Springer (1984). “Microwave Curing of Composites.” Journal of
Compoaites Materials 18: 386-409.
Lee, W. I. and G. S. Springer (1984). “Interaction of Electromagnetic Radiation with
Organic Matrix Composites.” Journal of Composite Materials 18: 357-386.
Lee, Y. F. and C. Y. C. Lee (1989). Proc. of the ACS Div. of Polym. Mater. Sci. and
Eng.,
Lewis, D. (1992). Microwave Processing of Polumers- an Overview. Microwave
Vulcanization; A lesson in Business and Technology,
Lin, H.-H. (1989). Theoretical Formulation and Experimental Investigation of a
Cylindrical Cavity Loaded with Lossy Dielectric Material. Michigan State University.
McNeil D. M, M.C. Hawley, and M. DeMeuse, "Comparison-of a Microwave and
Thermally Cured Thermosetting Polycarbonate". ANTEC'92 Conf. Proc.. 2621 (1992).
Mallavarpu, R„ M. C. Hawley, et al. (1978). Plasma Sciience PS-3: 55.
Mannering, B. (1992). Electromagnetic Field Colutions for the Natural Modes of a
Cylindrical Cavity Loaded With Lossy Materials. Michigan State Univerrsity.
May, C. (1983). “The Chemical Characterization and Processing Of COmpoistes.” Pure
and Applied Chemistry 55(5V. 811-818.
Montgomery, C. G. (1947). Techniques of Microwave Measurements. New York,
McGraw Hill.
Morgan, S., L and S. Deming N (1974). “Simplex Optimization of Analytical Chemical
Methods.” Analytical Chemistry 46(91: 1170-1180.
Mijovic, J, J Wijaya, Macromocolues, 22.(15), 3677 (1990)
Mijovic, J, J Wijaya, Polymer Composites, i i 184 (1990)
Nelder, J. A. and R. Mead (1963). “A Simplex Method for Function Minimization.”
Computer Journal 7: 308-313.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
213
Pardee, W. J. and B. Hayes-Roth (1987). Intelligent Real Time Control of Material
Processing. Palo Alto Laboratory, Rockwell International.
Pozar, D. M. (1990). Microwave Engineering. Addison-Wesley Publishing Company,
Inc.
Ramo, S. and J. Whinery (1953). Fields and Waves in Modem Radio. New York, John
Wiley and Sons, Inc.
Richardson, T. (1987). Composites:A Design Guide. New York, Industrial Press Inc.
Risman, P. (1991). ‘Terminology and Notation of Microwave Power and Electromagnetic
Energy.” International Microwave Power 26(4): 243-249.
Roberts, R. W. (1987). Automated Composites Cure Control Implementation a Cure
Modeling Approach to Automation. 32nd International SAMPE Symposium,
Saliba, T., S. Saliba, et al. (1992). “In-Situ Cure Monitoring of Advanced Composite
Materials.” :
Schwartz, M. M. (1984). Composite Materials Handbook. New York, McGraw-Hill Book
Company.
Servais, R. A., C. W. Lee, et al. (1986). Intelligent Processing of COmposite Materials.
31st International SAMPE SYmposium,
Shinskey, F. G. (1988). Process Control Systems. New York, McGraw-Hill.
Springer, G. S. (1987). “Modelling the Cure Process of Composites.” 31st International
SAMPE Symposium. April 1987 : 776-787.
Stephanopoulos, G. (1984). Chemical Process Control-An Introduction to Theory and
Practice. Edgewood Cliffs, Prentice-Hall Inc.
Strong, B. A. (1989). Fundamentals of Composite
Manufacturing:Materials.Methods. Applications. Dearborn, Society of Manufacturing
Engineers.
Qiu, Yunchang, Val Adegbite, and Martin Hawley “Microwave Processing Using
Intelligent Frequency Switching”, accepted for presentation at IMPI 30th Annual
Microwave Symposiums, 1995
Qiu, Yunchang, Val Adegbite and Martin Hawley, “Intelligent Composite Processing
Using Variable Frequency Method: Preliminary Heating Resuts”,submitted to ,
Symposium of “Smart Processing of Materials” on the 27th International SAMPE
Technical Conference, 1995.
Qiu, Yunchang ,Val Adegbite and Martin Hawley “Intelligent Microwave Processing of
Composites Using Variable Frequency”,submitted to 11th Annual Advanced Composites
Conference & Exposition, 1995
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
214
Sudaram, A. (1994). Microwave Process Modeling. Michigan State University.
Sticklen, J., Kamel, A., Hawley, M., & Adegbite, V. (1992). Fabricating Composite
Materials: A Comprehensive Problem Solving Architecture Based on
a
Generic Task Viewpoint. IEEE Expert, 7(2), 43-53.
Tittman, B. (1988). “Advanced Processing of Composites.” MRS Bulletin : 21.
Trivisano, A., J. Kenny, et al. (1992). “Control and Optimization of Autoclave Processing
of High Performance Composites.” Intemations SMPE Symposium 37:1104-1116.
Vogel, G. L., J. Jow, et al. (1989). 4th Tech. Conf. Comp. Mat.,,
Wei, J. (1991). “Microwave Processing of Epoxy Resins and Graphite Fibe/Epoxy
Composite in a Cylindrical Tunable Cavity.” PhD. Dissertation. Michigan State University
Wei, J., J. Jow, et al. (1989). AIChE Annual Conference, SF, CA,
Wei, Jianghua and Martin C. Hawley, "Modeling and Controlling During Microwave and
Thermal Processing of Composites", MRS Svmp. Proc.. 269. 439 (1992).
Wei, Jianghua,Mark DeMeuse, and Martin C. Hawley, "Kinetics Modeling of Microwave
and Thermal Cure for Epoxy Resin", ACS meeting. SF, CA, April, (1992).
Wei, Jianghua, Brook Thomas, and Martin C. Hawley, "Scale-up Study of the Microwave
Heating of Polymers and Composites", 37th Intern. SAMPE. Anaheim, CA, March,
(1992).
Whitehair, S., J. Asmussen, et al. (1987). J. Propul. Power 3(136):
Whitehair, S., J. Asmussen, et al. (1984). Applied Physics Letters 55: 1014.
Wu, H.-T. and B. Joseph (1990). “Knowledge Based Control of Autoclave Curing of
Composite.” SAMPE Journal 26(6): 39-54.
Wu, H.-T. and B. Joseph (1990). “Knowledge Based Control of Autoclave Curing of
Composites.” SAMPE Journal 26(6): 39-54.
Young, P. R., M. A. Druy, et al. (1988). “In-Situ Composite Monitoring Using Infrared
Transmitting Optical Fibers.” 20th International SAMPE Technical Conference : 11-15.
Young, V., J (1952). Understanding Microwaves. New York, John F. Rider Publisher,
Inc.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Документ
Категория
Без категории
Просмотров
0
Размер файла
8 269 Кб
Теги
sdewsdweddes
1/--страниц
Пожаловаться на содержимое документа