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Патент USA US2411162

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Nov. 19, 1946.
G. E; KING
2,411,162
CONTROL SYSTEM
Filed Deo. 5, 1944
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one/2%#
ATTORNEY
~Patented Nov. 19, 1946
UNITED `STATES `PATENT OFFÍCE
2,411,162
CONTROL SYSTEM
George E. King, Swissvale, Pa., assigner to West
inghouse Electric Corporation, East Pittsburgh,
Pa., a corporation of Pennsylvania
l
Application December 5, 1944, Serial No. 566,723
5 Claims. (Cl. 172-439)
2
My invention relates Áto electric motor con
trol systems for grinding machines, milling ma
chines and other machine tools in which a “feed
motor” is used to -move workpiece and tool rela
tive to each other while the tool performs its cut
ting or other deforming operation on the work
provided with a feed motor and a cutter motor
denoted as a whole by FM and CM, respectively.
The feed motor is energized, under control by an
electromagnetic contacter CR., from a power
transformer TR through two rectifier tubes TI
and T2 of the gaseous discharge type, for in
piece.
.
stance, as known under the name Thyratron.
For certain applications of such machine tools
These rectifier tubes are controlled by three elec
it is desirable to vary the speed of the feed motor
tronic tubes preferably of the high Vacuum type.
in such a manner as to maintain the load on the 10 These tubes are denoted by T3, T4 and T5, re
tool or cutter below a safe limit in order to pre
spectively.
vent «damaging the tool. In practice, the feed
More in detail, the milling machine MS car
ries on its stationary supporting or bed struc
ture I a horizontally displaceable bed 2 for ac
motor is usually operated at a speed lower than
the maximum permitted by the tool, because. it
is necessary to avoid overloading the tool or cut
The milling tool
or cutter 4 of the machine is connected by a
loads occur due to unevenness of surface or ir- suitable transmission with the cutter motor CM.
regularities in hardness of the workpiece. Very
This motor is energized through a switch 5 from
often, the operator, in order to avoid ruining the
an alternating current line 6. Motor CM may be
tool, will use a slower speed than necessary for 20 operated at constant speed and a control appa
a suflicient' margin of safety. The customary
ratus (not shown) may be provided for adjust
overload relays installed in the motor control
ing this speed to a desired value.
_
system ofthe machine tool for protecting the
The displaceable bed 2 of the milling machine
electric equipment are normally set for a fixed
is connected by a Suitable transmission to 4the
maximum load value and neither designed nor» 25 armature 'I of the feed motor FM. The apper
accessible to the operator for permitting a change
taining ñeld winding is denoted by 8.'
`
in adjustment to suit particular working condi
The power transformer TR has a primary wind
tions. Consequently, such machine tools are apt
ing 9 connected to the alternating current line
ter at those places _along the cut where increased
to be operated below optimum capacity.
15 commodating a workpiece 3.
6 and is equipped _with three secondary windings
It is an object of my invention to provide an 30 IIJ, II and I2. Winding l0 is connected through
electronic system for controlling the drive mo
a rectiñer I3`with ñeld winding 8 of motor FM
tors of machine tools in such a manner as to
so that this field winding is excited by 4direct ~
vary the motor speed automatically in dependence
upon the load imposed on the tool and with the
effect of permitting' an operation at or near
optimum capacity without the danger of over
loading the -tool at places of increased cross sec
tion or hardness of the material. to be worke
current of constant voltage during the operation
of the system. The armature circuit of motor
FM extends through two contacts I4 and I5 of
contacter CR whose .control coil i6 actuates also
four additional relay contacts denoted by Il, I8,
I9 and 20, respectively.
Relay coil I6 is con
nected to line 6 through a normally open control
Another object of my invention is to .provide an 40 contact 2| and a normally closed control contact
electronic control system for machine tools that
22. These contactsl are preferably designed as
upon.
-
-
.
'
permits setting the motor speed at a desired nor
pushbutton switches and serve, respectively, to
mal value and acts automatically to reduce the
start and stop the operation of motor FM. That
motor speed below this normal value if and when
is, when contact 2l is depressed by the operator,
the load onA the tool exceeds a magnitude which. 45 coil I6 is energized and closes contacts I4 and I5;
can be selected by the operator to suit the work
at the same time contact Il is closed and com
piece and tool used under particular workingv
pletes _a self-holding circuit so that coil I6 re
conditions.
mains energized after contact 2l is released. The
' 'I‘hese and other'objects will be apparent from
actuation of .contact 22 interrupts the self-hold
the following description of the control system 50 ing circuit and hence returns relay CR into the
for a milling machine shown diagrammatically
illustrated inoperative position.
in the drawing as an embodiment of my inven
The secondary winding II of transformer TR
tion.
is center tapped. »The center point is connected
The drawing illustrates schematically a mill
with contact I4 of relay CR, while the two .ter
ing machine denoted asa Whole MS which yis
minals of winding II are attached to the anodes
2,411,162
.
i
3
of rectifier tubes Tl and T2, respectively. The
cathodes of tubes Tl and T2 are interconnected
and attached to contact I5 of relay CR through
a series resistor 23. Hence when contactor CR is
energized and closes its contacts I4 and l5, the
armature 'l of -feed motor FM is energized by rec
`titled current derived from the secondary Il of
transformer TR through the plate` circuits of
tubes TI and T2. The energizing direct current
traverses the series resistor 23 and develops across
this resistor a voltage drop which is a, measure
of the >current intensity. A calibrating rheostat
4
ing if the function of tubes T4 and T5 is _at iirst
disregarded in the following discussion of the
control operation performed by the above-men
tioned grid circuit portion A--B-C-D-E-F-G
of tube T3.
.
This circuit portion includes in series the three
vresistors or rheostats 24, 2l and 4I. Each of
these three resistors is impressed by a voltage
that originates from outside voltage source and
hence represent each a separate source of a com
ponent voltage for the grid circuit. The rheo
stat 4I develops along its effective portion a volt
age drop (El) which originates from the con
stant voltage source 40 and has a magnitude se
to permit measuring an adjustable proportion of
the total voltage drop of resistor 23.
15 lected by the operator in accordance with the
~ 24 is connected in parallel to resistor 23 in order
The control grids of rectifier tubes Tl and T2 ^
are denoted `by 25 and 25, respectively. These
grids are interconnected through two lresistors
2l and~28 and the center-tapped secondary 29
desired speed of feed motor FM. The resistor 4l,l
as explained previously, ‘develops across its ter
minals a Voltage drop (E2) which Varies in ac
cordance with the rectifier termina1 voltage im
pressed on motor FM. That portion of rheo
stat 24 which is series connected in the grid
of transformer 30 is connected to the secondaryv
circuit/»imposes thereon a voltage component (E3)
i 32 of a phase shift transformer 33 whose primary
which represents an adjusted proportion ofthe
34 is supplied with constant excitation from the
voltage drop occurring across resistor 23. Since
secondary i2 of the power transformer TR. '_I‘he
connection includes a resistor 35 and a capacitor 25 resistor 23' lies in series between the rectifier
36 and functions to impose a component alter- `
cathodes and the armature l, its voltage drop
and hence the component gridvoltage across the
nating-currentvoltage on the grid circuit of the
effective portion of rheostat 24 is proportional to
rectifier tubes Ti and T2 under maintenance of
the IR drop -in the armature circuit of motor
a given phase displacement between the cycle of
~
this voltage component and that of the alternat 30 FM.
The resultant grid voltage imposed on the con
ing-current voltage to be rectified by the tubes
trol grid 45 of tube T3 is `thus dependent upon
Tl and T2.
of a transformer 30.
The primary winding 3l _
the behavior of three components. One compo
The grid circuit of the rectifier tubes are fur
nent (El) remains constant as long as the speed
ther impressed by a variable direct-current volt
age component. To this end, the.y tap point of 35 setting of rheostat 4l remains undisturbed., The
othertwo components (EZ'Yand E3) tend to change
the transformer secondary 29 is connected to the
during the operation of the system in accordance
- slider of a rheostat 3l which extends across awith the varmature voltage and the IR drop, _re
voltage source 38 in order to provide an adjust
spectively. The polarities of Áthe three compo
able constant grid bias. Rheostat 31 isÍ also con
nected with the plate circuit of tube T3. This. 40 nent voltages are so chosen that the resultant
circuit contains a resistor 39 and is energized
from a source of constant voltage, denoted by
4D, which is connected across a series arrange
ment of resistors'ßll, 42 and 43. Resistor 4I con
sists of an adjustable rheostat. and serves to se
lect a desired speed for motor FM as will be ex
voltage isr the algebraic sum of the negative value
of El, the positive value of E2 and the negative
value of E3. As a result, the condition of con
trol grid 45 is basically determined by the set
ting of rheostat 4l and changes in proportion
to the value of the motor voltage minus the IR
drop occurring in the armature circuit during
plained in a later place. A Calibrating resistor
the operation of the motor. This resultant value
44 is connected in the circuit of the control grid
is proportional to the motor speed. '
45 of tube T3. Connected across the plate cir
By virtue of the above-described connection of
cuits of the rectifier tubes TI and T2 are two 50
resistors 46 and 4l, arranged in series relation
tube T3 with the grid circuit of the rectifier tubes
to ,each other. When the motor is in operation,
TI and T2, the direct-current component of rec
the voltage drop across resistors 46 and 41 is pro
tifier grid voltage varies also in .accordance with
portional to the rectified or terminal voltage ap
« the speed of motor FM. These variations have
plied to the feed motor. Consequently, the Dro 55 the effect of displacing the firing point of _each
portion of voltage drop appearing across resistor
rectifier with respect to the cycle of the alter
4l presents a measure of the motor Voltage.
nating current to be rectified thus causing a cor
-The just-mentioned circuit connections of tube
responding change in the rectified output voltage
T3 include a composite grid circuit. A complete
and hence a change in the speed of motor FM.
loop portion of this circuit extends from control 60 Inthis manner, a decrease in motor speed below
the value ,set by rheostat 4I has> the effect of
vthrough resistor 44 to the point marked B, thence
producing a còntro1 action in the rectifier tubes
through a portion of rheostat 24.to points C and
Tl and T2 which is in proportion to the de
D, thence through resistor 4l to points E and F»A
parture of the speed from the desired value and
and through the effective portion of the speed 65 causes lan increase in the rectified output volta-ge,
adjusting rheostat 4i to point G thus ending at
thereby restoring the motor speed to thedesired
the cathode of tube T3. It will be recognized that
value. An increase inmotor speed above the
the plate circuits of tubes yT4 and T5 are tied
-desired value has a similar effect except that it
up with the just-mentioned portion of the-grid
causes a decrease in rectified Áoutput voltage in
circuit of tube T3. As will be explained later, 70 order to reduce the motor speed to the intended
however, the tubes T3 and T4 have no «effect on
value.
the operation of the above-mentioned loop por
Referring now to the „tubes T4 and T5, it will
tion of the grid circuit as long as certain operat
be seen that-the plate circuits' of these tubes
ing conditions are prevailing. `Hence it appears
contain a resistor 50 or 5i which is connected to
to be logical and conducive to easier understand 75 the positive pole of they above-mentioned current
l grid 45, through the point marked A, thence
2,411,162
source 40. Both tubes are pentodes having a
sharp cut-off characterstic. The cathodes 52
with the current source 56 so that the displace
ment cui-_rent iiowing into the capacitor upon the
and 53., respectively, of tubes T4 and T5 are in
closure of contacter CR delays the building up of
terconnected and are also in connection to a
the blocking potential across the rheostat 55.
point, marked H, of the armature circuit of mo 5 The capacitor 58 is discharged through contact
tor FM. Consequently, the source 40 which pro
I9 when the feed motor is stopped by depressing
vides the plate voltage for tube T3 serves also as
the pushbutton 22.
a plate voltage source for tubes T4 and T5. The
The control grid 52 of tube T4 is connected
plate circuit of tube T4, for instance, extends
through a resistor 60 to the slider of a rheostat
fromy the appertaining anode through resistor l0 6| which extends across a constant voltage source
50, voltage source 40, points F and E, resistor 41,
v|52. The positive pole of source 62 is connected
points D and C, resistor 23, and point H to the
cathode of the tube. The plate circuits of tubes
T4 and T5 are thus interlinked withLthe above
described grid circuit of tube T3. Consequently, 15
the appearance of a voltage drop across either
resistor 5D or 5| has the effect of imposing a
bias on the grid voltage of .tube T3 in super~
to a resistor 63 which, at point K, is in connec
tion with the cathode of tube T4. Resistor 63 is
connected across a rectifier 64. This rectiñer
is energized by a current transformer 65 which
measures the load current of the cutter motor
CM. Hence, the voltage drop developed across
resistor 63 is a measure of the load imposed on
position upon the above-mentioned resultant of
the cutter 4 of the milling machine. The circuit
the voltage components El, E2 and E3. The 20 of grid 52 extends through resistor 60 and the
occurrence and magnitude of such voltage drop
selected portion of rheostat 6|, thence -through
across resistors 50 and 5| is controlled by the
control grids 52 and 53, respectively, of tubes T4
and T5.
-
Grid 53 of tube T5 is connected through a re~
sistor 54 to the slider of a rheostat 55 which is
energized from a constant voltage source 56 in
series connection with a current limiting resistor
the load measuring resistor 63 to point K and
the cathode of tube T4. This grid circuit in
cludes two sources of grid voltage.
One source
is represented by the adjusted portion of rheo
stat 6I and provides a grid bias which~ is highly
negative so that tube T4 is normally prevented
from conducting plate current. The second
51 and under control by the above-mentioned
source of component grid voltage for tube T4 is
contacts I8, I9 and 20 of contactor CR. The
formed by the load measuring resistor 63. The
positive pole of source 56 is connected to point
corresponding voltage component acts in op
D of the rectifier output circuit so that the grid
position to the negative grid- bias provided by
circuit of tube T5 includes the load responsive
rheostat 6|. Consequently, when the load on
measuring resistor 23 in the following manner.
the cutter 4 increases, thereby increasing the
Starting at grid 53. the appertaining grid circuit 35 voltage drop across resistor 63, this will tend to
extends through resistor 54, a portion of rheo
make the control grid 52 of tube T4 less nega
stat 55, the point marked I, points D and C, re
tive and in this way will cause the plate current
sistor 2'3, point H, and back to the cathode of
through resistor 50 to increase. This in turn
tube T5. Consequently, the potential of grid 53
results in an increase of `voltage drop across re
depends on two voltage components. One com 40 sistor 50, and the control grid 45 of tube T3- will
ponent is represented by the voltage drop be
become less negative. The plate current of tube
tween the slider of rheostat 55 and point I. The
T3 increases accordingly. Consequently, the
other voltage component is the voltage drop
firing angle of rectifier tubes TI and T2 is de
developed across resistor 23 in accordance with
layed so that the rectified output voltage de
the rectified load current supplied to the arma
creases and causes a slow down of the feed motor.
ture 1 of motor FM. The component deter
Due to the high negative voltage bias imposed on
mined by the adjustment of the rheostat 55 is so
grid 52 by rheostat 6I, tube T4 does not conduct
chosen that the potential of grid 53_is normally
until the voltage drop across resistor 63 reaches
highly negative and thus maintains the tube T5
a definite value set by the potentiometer 6|.
normally in Iblocked condition. The voltage 50 The operating point at which the control effect of
drop across resistor 23 acts in opposition to the
tube T4 becomes operative under increasing cut
adjusted bias. Hence when the load current of
ter loads is, therefore, determined by the selected
armature 1 increases beyond a value determined
adjustment of potentiometer 6|. Hence, -by a
by the setting or rheostat 55, the grid 53 be
proper selection of this adjustment, the operator
comes suiliciently less negative to permit the 55 can limit the cutter load in order to suit the cut
passage of current through the plate circuit.
ter and workpiece for any'particular working op
Then the resistor 5| will develop a voltage drop
eration of the machine tool.
‘
which, as explained above, superimposes a con
The potentiometer rheostat 6| can be calibrated
trol bias on the grid circuit of tube T3 with the
in terms of cutter load current and a chart can -be
. result` of causing the rectifier tubes TI and T2 60 prepared to show the operating current for each
to reduce the voltage of their output current. In '
cutter. 'The operator can then' set the potenti
this manner, the speed of motor FM is reduced
ometer 4|rfor the speed best 'suited for the cutter
and 'the load correspondingly diminished. In
to enter the workpiece and then readjust it to
summary, tube T5, in conjunction with the as~
-the maximum permissible speed and let the cutter
sociated tube circuits, serves to prevent the feed 65 -load regulate the speed as set on the potentiom
motor from exceeding a maximum load of ad- .
justed value by reducing under overload condi
tions vthe motor speed below the normal value
, eter 6 l . This permits obtaining optimum produc
tion from the machine without fear of ruining the
cutters.
selected at rheostat 4|. When the overload
As explained in the foregoing, the tubes T4 and
conditions disappear, the correct feed motor, 70 T5 operate independently of each other. The
speed as determined by rheostat 4| is automati
speed regulation in dependence upon the cutter
cally established. In order to prevent tube T5
load is effected merely by tube T4 and the asso
from causing undesired transient phenomena
ciated circuits so that tube T5 may be omitted
during the starting and stopping periods of the
if a speed control in dependence upon the feed
machine too1 drive, a capacitor 58 is connected 75 motor load is unnecessary. The fact that the two
_
2,411,162
7
8
tubes T4 and T5 operate in parallel and inde
pendently of each other shows also that instead.
rent exceeds a value determined by the selected
adjustment of said potentiometer means.
2. A machine tool control system, comprising
of, or in addition to tube T5, one or several other
tube arrangements may be used which are re
a. cutter motor and a direct-current feed motor,
alternating current supplymeans for said feed
sponsive to cutter loads in accordance with the
motor, electronic rectifier meansof the gaseous
operating principle of tube T4. Such use Of 2»
discharge type connected to said supply means
plurality of tubes arranged similar to that of tube
for energizing said feed motor by rectified cur
T4 is of advantage for machines in which differ
rent derived from said supply means, said recti-ent types or sizes of cutters are to be used at the
same time. The different tubesyall similar to 10 fier means having a. control circuit for varying
the voltage of said rectified current and thereby
tube T4, are allotted- to the diiferentrespective
the speed of said feed motor, an electronic vacu
cutters and their grid circuits are controlled in
um tube having a plate circuit connected with
dependence upon the load current of the respec
said control circuit and a grid circuit for con
tive cutter motors. This permits the operator to
trolling said plate circuit. an adjustable potenti
ometer disposed in said »grid circuit for imposing'
adjust each potentiometer, according to potenti
ometer 6| of tube T4, to the proper load setting
for its associated cutter motor or cutter.
'I'hen
thereon a component voltage in accordance with
a speed value for said feed motor determined
l
the feed motor will operate automatically at that
`speed which is proper for the cutter motor carry
by the selected adjustment of said potentiometer,
When` a
20 circuit means also disposed in said grid circuit
fragile cutter is cutting, this cutter will determine
the maximum rate of feed, but if this cutter does
not happenl to be cutting, Isome more rugged cut
cordance with the speed of said feed motor so
as to maintain said speed normally at said se
ing the greatest proportional load.
for imposing thereon a voltage variation in ac
lected value, and'load-responsive circuit means
ter or one of the cutter motors carrying thelarg
_est percentage of its rated load will determine 25 connected with said grid circuit for superimpos
'ing thereon -another voltage variation in depend
the rate of feed. Thus, the machine will always
ence upon the l'oad current of the cutter motor
be caused to operate at the highest rate of feed
so as to reduce the speed of the feed motor below
-suitable for the conditions under which the ma
said selected value when said load current ex
chine is operating at any moment. In this man
ner the feed can be maintained at that rate which 30 ceeds a ~given magnitude.
» 3. A machine >tool control system comprising
will not overload any of the motors and will not
a tool operating motor and a direct-current feed
put too heavy a duty upon any of the cutters. In
motor, alternating current supply means for said
case the rate of feed is lower than that proper
feed motor, electronic rectifier means of the
for the work being done by the cutter or the
motor least able to withstand the load,l the feed 35 gaseous discharge type connected to said supply
means for energizing said feed motor by rectified
rate will be increased automatically until the
current derived from said supply means, said rec
optimum load for that cutter o_r motor is reached.
tifier means having a control circuit for varying
It will -be understood by those skilled in the
the voltage of said rectified current and thereby
art that other'modiñcations or alterations are
possible in systems according to the invention as 40 the speed of »said feed motor, an electronic vacu
um tube having a plate circuit connected with `
described in the foregoing without departing from
said control circuit and a grid circuit for con
the gist of the invention or from the essential
trolling said. plate circuit, an adjustable potenti
features of the invention as set forth in the claim
ometer disposed in said grid circuit for impos
attached hereto.
ing` thereon a component-Voltage in accordance
I claim as my invention:
-with a speed value, for said feed motor determined
1. A machine tool control system, comprising
a tool operating motor, a direct-current feed
by the selected adjustment of said potentiometer,
circuit means also disposed in said grid circuit
for imposing thereon a voltage variation in ac
cordance with the speed of said feed motor so as
motor, alternating current supply means for' said
` feed motor, electronic rectifier means of the gas
eous discharge type connected to’ said supply
to maintain‘said speed normally at said selected
value, and load-responsive circuit means connect
ed with said grid circuit for superimposing there-.
means for energizing said feed motor by rectiñed
current derived from said Supply means, said
rectifier means having a control circuit for vary
on another voltage variation in dependence upon A
ing the voltage of said rectified current and there- .
by the speed of said feed motor, an electronic vac
uum tube having a plate circuit connected with
the load current of said tool operating motor, said
grid circuit for Isuperimposing another voltage
is- reduced below said' selected value when said
load
current exceeds a magnitude determined byv
65
_load-responsive circuit means comprising an elec
tronic tube havingA a plate circuit connected with
said control circuit and a grid circuit for con
said control circuit and having a grid circuit
trolling said plate circuit, adjustable circuit
equipped with adjustable potentiometer means
means connected with said grid circuit for impos
ing thereon a voltage adjusted in accordance with 60 for providing a cut-'off bias of selective magnitude
and with a circuit member for providing opposite
a selected value of motor speed, and electric cir
ly acting bias variable in accordance with said
cuit means disposed for measuring the load of
load current, whereby the` speed of the feed motor
said tool operating motor and connected to said
thereon so as to vary said rectifiedvoltage in de
the selected adjustment of said potentiometer
pendence upon said load current, said circuit
means comprising an electronic tube having a
plate circuit connected with said control circuit '
means.
.
4. A machine control system, comprising a di
rect current drive motor, circuit means for sup
» and having a grid circuit equipped with adjust
able potentiometer means for providing a cut-olf 70 plying alternating current, electronic rectifier
means of the gaseous discharge type disposed be
bias of `selective magnitude and with a circuit
member for` providing oppositely acting bias vari
able inìaccordance with the load current of the
cutter motor in order to cause a reduction in volt
age of said rectified current when said load cur
tween said circuit means and said motor for en
ergizing said motor by rectified current, said rec
tiñer means having a contra; circuit for. varying
75 the voltage of said rectified current and thereby
2,411,162
10
the speed of said motor, circuit means connected '
-
-
said plate> circuit, an adjustable potentiometer
_ to said control circuit for causing it to vary said
disposed in said grid circuit for imposing thereon
rectiñed voltage in dependence upon load magni
a component voltage inpaccordance with a speed
value for said feed motor determined by the se
tudes, said circuit'means comprising a plurality
of electronic tubes all having their respective
.plate circuits connected to said control _circuit
and each having a grid circuit equipped With
adjustable potentiometer means for providing a
Íblocking bias of selective magnitude and with a
load-responsive circuit member for providing a
variable bias in_opposition tosaid blocking bias,
whereby the speed of said motor is reduced when
lected adjustment of said potentiometer, circuit
means also disposed in said grid circuit for im-u
posing thereon a voltage variation in accordance
with the speed of said feed motor so as to main
tain said speed normally at said selected value,
and load-responsive circuit means connected with
said grid circuit for superimposing thereon an
other voltage variation in dependence upon load
the load acting upon any one of said circuit mem- '
conditions of said operating motor, said load-re- .
bers exceeds a magnitude `determined by the se
sponsive circuit means comprising a plurality of
lected adjustment of said respective potenti
ometer means.
5. A machine control system, comprising an
operating motor and a direct-current feed motor,
alternating current supply means for said feed
15 electronic tubes all having their respective plate ,
circuits connected to said control circuit and
each having a grid circuit equipped with ad
justable potentiometer means for providing a
Y blocking bias of selective magnitude and with a
motor, electronic rectifier means of the gaseous 20 load-responsive circuit member for providing a
discharge type connected to said supply means
for energizing said feed motor. by rectified current
derived from said supply means, said rectiñer
_variable bias in opposition to said blocking bias,
whereby the speed of s_aid drive motor is reduced
means having a control circuit for varying the
when the load acting upon any one of said circuit
members exceeds a magnitude determined by the
voltage of said rectified current andv thereby the
selected adjustment of said respective potenti
speed of said feed motor, an electronic vacuum
ometer means.
tube havingA a plate circuit connected with said
control circuit and a grid circuit for controlling
GEORGE E. KING.
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