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JPH08256395

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DESCRIPTION JPH08256395
[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the
improvement of an ultrasonic transducer which is a sensor portion for transmitting and receiving
ultrasonic waves in water and detecting an object.
[0002]
2. Description of the Related Art In a moving body moving at high speed in water, an ultrasonic
transducer is embedded in the moving body so that an acoustic window for transmitting and
receiving ultrasonic waves is flush with the surface of the moving body. However, laminar flow
including flow noise or cavitation small bubbles generated during navigation adversely affects
the acoustic performance of the ultrasonic transducer. In order to reduce these effects, structural
measures such as the shape of the vessel or the treatment of the outer surface are taken, while
many measures for the transducer are also considered, among which the high frequency of the
operating frequency is high. Is an effective means for the transducer, and the practical use of
ultrasonic transducers using megahertz frequency is in progress.
[0003]
Conventionally, a transducer used in a frequency band of several hundred kHz to several MHz
uses thickness direction vibration of a disc or rectangular plate-shaped piezoelectric ceramic
vibrator, and such vibrator is used as a pedestal for positioning and fixing, etc. The structure
which attached and fixed and provided waterproof structure by methods, such as a mold by a
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rubbery elastic body, was common.
[0004]
By the way, in a piezoelectric ceramic vibrator using vibration in the thickness direction, both
opposing surfaces in the thickness direction have metal electrodes made of silver or nickel
formed on almost the entire surface, respectively. The polarization and the exchange of electric
signals are performed, and the thickness direction vibration is uniformly excited in the whole
vibrator.
In the case where high frequency is supported by such a vibrator, the thickness of the vibrator
decreases in inverse proportion to the resonance frequency, and if it is intended to maintain the
directivity width which is the performance of the transducer, the vibrator outer diameter or
width And the size in the length direction must be reduced, which results in a reduction in the
volume of the vibrator, but this causes an increase in the factors that increase the variation in the
characteristics of the assembly, which is the deterioration in bonding and assembling workability.
was there.
[0005]
In vibrators that use thickness direction vibration, this is a common phenomenon, but other than
thickness direction vibration, for example, high-order mode resonance of vibration in the radial
direction or width or length direction tends to appear as parasitic vibration. There is. If the
frequency is about 200 to 300 kHz, prevent the effect by processing the slit to a depth of about
two thirds of the thickness of the vibrator and making the resonant frequency of parasitic
vibration higher than the resonant frequency of thickness direction vibration Although the
thickness of the vibrator becomes 1 to 2 mm or less when the frequency is increased to the MHz
band, it is difficult to take similar measures in terms of processing and strength. Furthermore,
when the dimensions such as the outer diameter or width of the vibrator determined from the
requirement of the directivity characteristics and the dimensions in the thickness direction of the
vibrator approach a specific dimensional ratio determined from the material constant or the
shape of the vibrator, Coupling vibration is likely to occur between parasitic vibrations. If the
coupled vibration is generated, the thickness direction vibration which is the original use
vibration mode can not be excited effectively, and at the same time, the transition of the
resonance frequency occurs, which makes it difficult to use at the target frequency.
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[0006]
In addition, the shortening of the sound wave propagation wavelength due to the increase in
frequency causes the influence of the thickness of the adhesive layer for bonding and fixing the
vibrator to be observed. When used at low frequencies, the thickness of the adhesive layer of 0.1
to 0.2 mm is only a few tenths of the wavelength ratio, and the low mechanical sharpness Qm of
the adhesive can be ignored, When the wavelength is shortened to about 1 to 2 mm due to the
increase in frequency, the thickness of the adhesive layer can not be ignored, and because of the
low Qm, it functions as a damping factor for the vibration of the vibrator and simultaneously
protrudes to the outer peripheral portion of the vibrator The cured adhesive also causes a
damping factor.
[0007]
Furthermore, although a lead wire for signal transmission and reception is connected to the
electrode surface of the vibrator, the mass, rigidity or elasticity of this lead wire can not be
ignored, and many of them are damped against vibration of the vibrator like the adhesive. Exert
an effect.
[0008]
These are all obstacles to the excitation of the thickness direction vibration which is the main
vibration mode of the vibrator, and have been the cause of the characteristic failure including the
decrease in the sensitivity as the transducer.
[0009]
Furthermore, lead wires and solder points for connecting the lead wires are present on the
electrode surface that is the acoustic surface of the vibrator, thus providing a transducer with
stable characteristics, such as contributing to disturbances in directivity characteristics. Was very
difficult.
[0010]
SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems
associated with the increase in the frequency of an ultrasonic ultrasonic transducer, and an easy
method is to provide a transducer with less variation in characteristics.
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[0011]
A circular or rectangular counter electrode opposed in the thickness direction and a direction
different from each other in the thickness direction at substantially the center of the circular or
rectangular piezoelectric ceramic plate. And a vibrator configured to connect an electric wire for
transmitting and receiving an electric signal to the drawing electrode to perform polarization and
excitation, and a concave portion provided at least in a range where the counter electrode is
located, the counter electrode The vibrator is fixed by bonding only the outer peripheral portion
of the vibrator to the pedestal so that the counter electrode of the vibrator is positioned at the
recessed portion of the pedestal. A waterproof structure is provided by a method such as
molding.
[0012]
As a measure to reduce the adverse effect that an ultrasonic transducer with which a moving
body moves in water is equipped during high-speed voyage, increasing the operating frequency
of the transducer is an effective means, and the frequency in the megahertz band Practical use of
an ultrasonic transducer using
However, the piezoelectric ceramic vibrator using thickness direction vibration used for the
ultrasonic transducer has the following problems as the frequency increases.
Not only the thickness of the vibrator, but also the area of the vibrating surface must be reduced
due to the requirement of directivity characteristics, so the dispersion of characteristics due to
the deterioration of bonding and assembling workability increases.
Vibration in the thickness direction is likely to generate parasitic vibration, and in the light of
directional characteristics, when the dimension ratio of the vibration surface and the dimension
ratio in the thickness direction approach a specific dimension ratio, coupled vibration is
generated between thickness direction vibration and parasitic vibration. As a result, the thickness
direction vibration can not be excited effectively, and furthermore, a transition of a target
resonance frequency occurs.
The thickness of the adhesive layer can not be ignored, which is a damping factor against the
vibration of the vibrator.
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The mass, rigidity or elasticity of the lead wire can not be ignored, which is a damping factor for
the vibration of the vibrator. The lead wires on the electrode surface and the solder points for
connecting them contribute to the disturbance of the directivity. So, in order to overcome these
problems, the above-mentioned structural measures were taken and solved.
[0013]
The present invention will now be described by way of examples.
[0014]
A configuration example of a conventional transducer is shown in FIG. 4 (a) in a top perspective
view and in FIG. 4 (b) in a schematic cross-sectional view.
Lead wires 26a and 26b are connected by soldering to both electrodes of the rectangular
piezoelectric ceramic vibrator 1 in which electrodes are applied to almost the entire surface in
the thickness direction respectively, and the rear surface of the vibrator 21 is bonded and fixed
to the pedestal 22 with an adhesive. ing. After lead wires 26a and 26b are passed through the
through holes of case 23 and pedestal 22 and case 23 are adhered, the lead wires are connected
to airtight terminals 28a and 28b, and the airtight terminals are adhered to a predetermined
position of housing 25. The case 23 is bonded to the housing 25, and after filling the filler 27
such as epoxy resin and curing it in the space formed behind the case 23 in order to prevent
deformation or breakage when water pressure is applied, the vibrator 21 is exposed. A rubbery
elastic body 24 is molded on the side to form a watertight structure.
[0015]
Here, the shape and dimensions of the vibrator will be discussed. The thickness of the vibrator in
the case of utilizing the thickness direction vibration is, for example, about 1.2 mm when the
resonance frequency is 1.5 MHz, and about 0.6 mm when the resonance frequency is 3.0 MHz. In
addition, when the thickness of the vibrator is 1 and the dimension of the outer diameter of the
vibrator is determined from the requirement of directivity characteristics, in the case of a
rectangular plate-like vibrator, the side length of the rectangular plate is 10 degrees at -6 dB
directivity width The side length is about 2.5 if the directivity width is about 5 and -6 dB and 20
degrees. Accordingly, if it is intended to obtain 20 degrees at -6 dB directivity width at a
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frequency of 1.5 MHz, the thickness of the rectangular plate-like vibrator is about 1.2 mm and
the side length is about 3.0 mm square, and the directivity width is If the frequency is kept as it
is and raised to 3.0 MHz, it vibrates as the frequency increases so that the thickness is about 0.6
mm and the side length is about 1.5 mm square, ie, the volume is approximately one fourth. The
child will be downsized significantly.
[0016]
FIG. 5 shows the characteristics of a square plate-like thickness direction transducer designed to
manufacture the transducer of FIG. 4 on the premise of use near 3 MHz. FIG. 5 (a) shows the
admittance frequency characteristics of the vibrator alone, and FIG. 5 (b) shows the same
characteristics before the rubber-like elastic body 24 is molded.
[0017]
As a problem with the conventional method, there has been a tendency for generation of
parasitic vibration in the vibrator, but also in the characteristic of FIG. 5A, in addition to the
resonance fA of thickness direction vibration, the resonance of fB, fC and parasitic vibration Can
be seen. If resonance of thickness direction vibration is relatively low frequency and volume of
vibrator is large, parasitic vibration such as high-order mode resonance of width direction
vibration is damped by adhesion to pedestal etc. and thickness direction vibration is effectively
used. Although it is not difficult, the effect of the adhesive layer thickness or the lead wire
connection can not be ignored in such a small vibrator.
[0018]
As shown in FIG. 5 (b), adhesion of the vibrator 21 to the pedestal 22 and attachment of the lead
wires 26a and 26b damp the vibration in the thickness direction which is originally required, and
the admittance value of fA is another parasitic. It is reduced to the same level as the vibrations fB
and fC. The admittance is the inverse of the impedance, which indicates that the impedance at fA
is large and can not be successfully excited. Even if assembled as a transducer in this state, it can
easily be inferred that it is difficult to obtain the desired performance. If the method of
characteristic change due to assembly is uniform, it is possible to suppress the variation in
characteristics by measures such as review of the oscillator specification or review of the
assembly method, but the method of this change is not uniform but is similar Even in the case of
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showing a change tendency, individual change amounts differ greatly and can not be easily
controlled, so it is customary to allow situations where the variation in characteristics is large as
a result, or to make many and select and use them according to characteristics. The
[0019]
1 and 2 show a structural example according to the present invention, and FIG. 1 shows an
assembled structure of a vibrator and a pedestal, in which (a) shows a top view and (b) shows a
sectional view, and FIG. The cross section is shown. A circular counter electrode 9 is formed
substantially at the center of the rectangular plate-like vibrator 1, and lead electrodes 10a and
10b are formed in directions different from each other by 180 degrees on the front and back of
the vibrator from the counter electrode. Lead wires are connected to the end of 10b by soldering.
The vibrator 1 applies the adhesive 11 only to the outer peripheral portion and is bonded and
fixed to the pedestal 2. The pedestal 2 is provided with a recessed portion 12 larger than the
range in which the counter electrode 9 is located, and the counter electrode on the back surface
of the vibrator 1 does not directly touch the pedestal. After being configured in this way, lead
wires 6a and 6b are passed through the through holes of case 3 and then base 2 and case 3 are
bonded and assembled, lead wires 6a and 6b are connected to airtight terminals 8a and 8b, and
airtightness is achieved. The terminal is bonded to a predetermined position of the housing 5.
The case 3 is adhered to the housing 5, and the filler 7 is poured into the space at the rear of the
case 3 and cured, and then the rubber-like elastic body 4 is molded on the exposed surface side
of the vibrator 1 to form a watertight structure.
[0020]
A vibrator having an electrode structure as shown in FIG. 1 and exciting thickness direction
vibration is generally known as an energy confined vibrator. In such a vibrator, the vibration in
the thickness direction is strongly excited in the counter electrode portion and as it is separated
from the counter electrode, the vibration is exponentially damped, resulting in the phenomenon
that the vibration is confined in the vicinity of the counter electrode portion. Show. By bonding
and fixing only the peripheral portion of the vibrator 1 where vibration in the thickness direction
is hardly excited by using such features, and connecting a lead wire to the end of the vibrator 1,
almost all of the vibrator 1 can be The configuration of the transducer is possible without
damping the vibration in the vicinity of the counter electrode 9 located in the central portion.
[0021]
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However, if the use in air is assumed, the use of such a vibrator alone can provide sufficient
effects, but when it is used in water, the situation of application of water pressure must be
considered. When water pressure is applied in such a configuration that the counter electrode
portion is in direct contact with the pedestal, the contact condition between the counter electrode
and the pedestal changes with time due to the pressure, but this changes the impedance behind
from the transducer It means to do. If this impedance changes, it is not preferable because the
acoustic energy propagating toward the pedestal changes, resulting in the sensitivity fluctuation
as a transducer. Therefore, in order to effectively use the features of the vibrator 1, it is
necessary to adopt a configuration in which the impedance behind the vibrator 1 hardly changes
with respect to changes in water pressure. As a method, providing an air gap Is most effective,
and this can be easily achieved by providing the recessed portion 12 in the pedestal 2 by spot
facing or the like. It is obvious that the formation range of the recessed portion 12 needs to be at
least as large as the counter electrode 9, and in practice, it is desirable to form the recessed
portion 12 in a wider range than the counter electrode 9. The parts other than the vibrator 1 and
the base 2 have substantially the same configuration as that of the conventional transducer, so
that the improvement can be easily achieved only by replacing this part.
[0022]
3 shows the characteristics of the transducer of the configuration of FIG. 1 manufactured on the
premise of use near 3 MHz, FIG. 3 (a) shows the admittance frequency characteristics of a single
vibrator, and FIG. 3 (b) shows rubbery elasticity. A plurality of the same characteristics before
molding the body 4 are shown.
[0023]
In FIG. 3 (a), although resonances fb and fC of parasitic vibration are seen as in the case of the
vibrator with the conventional full surface electrode, it is smaller than the conventional one in
comparison with the thickness direction vibration fA. It is approaching a single resonance of
vibration.
In FIG. 3B, the parasitic oscillations of fb and fC disappear, but by adhering and fixing only the
outer peripheral portion of the oscillator 1, the damping effect is applied to parasitic oscillations
such as high-order mode resonance of widthwise oscillation. Indicates that is appearing. On the
other hand, in the thickness direction vibration fa, the change in the admittance value is small,
and as seen in the characteristic example of FIG. 5B, the admittance value decreases to about one
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fifth of that of the single vibrator. It shows that the thickness direction vibration is well excited
without extreme change. There are no disturbances in the characteristic change as seen in the
conventional configuration, and the five transducers are assembled in a state in which the shapes
of the resonance frequency and the admittance characteristics are fairly well aligned.
[0024]
As described above, according to the present invention, when transmitting and receiving
ultrasonic waves in response to the increase in the frequency of the transducer, the thickness
direction vibration used as the main vibration mode of the vibrator is In excitation, it is possible
to easily obtain a transducer with less variation in characteristics due to assembly without
damping its thickness direction vibration.
[0025]
Brief description of the drawings
[0026]
1 is a diagram showing the assembly structure of the pedestal portion of the underwater
ultrasonic transducer according to the present invention, FIG. 1 (a) is a top view, FIG. 1 (b) is a
cross-sectional view.
[0027]
2 is a cross-sectional view showing a schematic configuration of a transducer according to the
present invention.
[0028]
3 is a diagram showing the characteristics of the piezoelectric ceramic vibrator used in the
present invention, FIG. 3 (a) is the admittance characteristics of a single body, FIG. 3 (b) is the
admittance characteristics of the assembly before molding.
[0029]
4 is a schematic configuration diagram of a conventional underwater ultrasonic transducer, FIG.
4 (a) is a top perspective view, FIG. 4 (b) is a cross-sectional view.
[0030]
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5 is a diagram showing the characteristics of the piezoelectric ceramic vibrator used in the
conventional configuration, FIG. 5 (a) is a single admittance characteristics, FIG. 5 (b) is the
admittance characteristics of the assembly before molding.
[0031]
Explanation of sign
[0032]
1, 21 Piezoelectric ceramic or piezoelectric ceramic vibrator 2, 22 pedestal 3, 23 Case 4, 24
rubber-like elastic body 5, 25 housing 6a, 6b, 26a, 26b Lead wire 7, 27 filler 8a, 8b, 28a, 28b
Airtight terminal 9 Counter electrode 10a, 10b Drawer electrode 11 Adhesive 12 Concave
portion
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