JP3477270B2 - Ultrasonic transducer - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an ultrasonic transducer which is a sensor for transmitting and receiving ultrasonic waves in water and detecting an object. Things. 2. Description of the Related Art In a marine vehicle moving at high speed in water, an ultrasonic transducer is operated so that an acoustic window for transmitting and receiving ultrasonic waves is flush with the surface of the marine vehicle. Although installed in a buried form, flow noise generated during cruising or laminar flow containing cavitation small bubbles had a bad influence on the acoustic performance of the ultrasonic transducer. In order to reduce these effects, structural measures, such as the shape of the hull and the treatment of the outer surface, are taken, while a number of measures are also being considered for transducers. Is an effective means for a transducer, and practical use of an ultrasonic transducer using a frequency in the megahertz band is being promoted. Conventionally, a transducer used in a frequency band of several hundred kHz to several MHz uses the thickness direction vibration of a disc-shaped or rectangular-plate-shaped piezoelectric vibrator to position and fix such a vibrator. In general, a structure in which a waterproof structure is provided by bonding or fixing to a pedestal or the like and using a method such as molding with a rubber-like elastic body is used. By the way, in a piezoelectric ceramic vibrator utilizing the vibration in the thickness direction, a metal electrode made of silver, nickel or the like is formed on almost both surfaces opposed in the thickness direction, respectively. Polarization and electric signals are transmitted and received by these electrodes, and the vibration in the thickness direction is uniformly excited in the entire vibrator. When such a vibrator responds to higher frequencies, the vibrator thickness decreases in inverse proportion to the resonance frequency,
Also, in order to maintain the directivity width, which is the performance of the transducer, the outer diameter of the vibrator or the dimension in the width or length direction must be reduced, resulting in a reduction in the volume of the vibrator. There has been a problem in that the factors that increase the variation in the characteristics of the assembled product, such as the deterioration of the bonding and assembling workability, are increased. [0005] In a vibrator utilizing the thickness direction vibration, a phenomenon that is often observed is that other than the thickness direction vibration,
For example, there is a problem that higher-order mode resonance of vibration in the radial or width and length directions is likely to appear as parasitic vibration.
In the case of a frequency of about 200 to 300 kHz, the effect is prevented by processing the slit to a depth of about two thirds of the thickness of the vibrator and making the resonance frequency of the parasitic vibration higher than the resonance frequency of the thickness direction vibration. However, when the frequency is increased to the MHz band, the thickness of the vibrator becomes 1 to 2 mm or less, so that it is difficult to take the same measures in terms of processing and strength. Furthermore, when the dimensions such as the outer diameter or width of the vibrator determined from the requirements of the directional characteristics and the dimensions in the thickness direction of the vibrator approach a specific dimensional ratio determined by the material constant or shape of the vibrator, the vibration in the thickness direction is reduced. Coupling vibration between parasitic vibrations is likely to occur. When the coupled vibration occurs, the vibration in the thickness direction, which is the original vibration mode, cannot be effectively excited, and at the same time, the transition of the resonance frequency occurs, so that the use at the target frequency becomes difficult. In addition, the effect of the thickness of the adhesive layer for bonding and fixing the vibrator can be seen as the sound wave propagation wavelength becomes shorter due to the higher frequency. When used at a low frequency, the thickness of the adhesive layer of 0.1 to 0.2 mm is
The wavelength ratio is about several tenths, and the low mechanical sharpness Qm of the adhesive can be neglected.
When the thickness is reduced to about 2 mm, the thickness of the adhesive layer cannot be ignored, and because of its low Qm, it functions as a damping factor for the vibration of the vibrator. It becomes a braking factor. Further, a lead wire for signal transmission / reception is connected to the electrode surface of the vibrator, but the mass, rigidity or elasticity of this lead wire cannot be neglected, and in many cases, the vibration of the vibrator like the adhesive does not occur. It exerts a braking effect on it. These are all obstacles to the excitation of the vibration in the thickness direction, which is the main vibration mode of the vibrator, and have caused poor characteristics such as a decrease in sensitivity as a transducer. Further, a lead wire and a solder point for connecting the lead wire exist on the electrode surface which is an acoustic surface of the vibrator, and the transducer has stable characteristics such as causing a disturbance in directional characteristics. Was very difficult to get. An object of the present invention is to solve the above-mentioned problems associated with the use of higher frequencies in underwater ultrasonic transducers and to provide a transducer having a small variation in characteristics by an easy method. At the substantially center of a circular or rectangular piezoelectric ceramic plate, a circular or rectangular counter electrode opposed in the thickness direction is provided so as not to be opposed in the thickness direction than the counter electrode. An extraction electrode is formed in each different direction, a vibrator configured to perform polarization and excitation by connecting an electric signal transmission / reception wire to the extraction electrode, and a concave portion is provided at least in a range where the counter electrode is located. It consists of a pedestal with a structure in which the opposing electrode does not directly touch. A waterproof structure is provided by a method such as molding with an elastic body. [0012] As a measure to reduce the adverse effect of the ultrasonic transducer provided on the traveling body moving underwater during high-speed cruising, it is effective means to increase the operating frequency of the transducer. Ultrasonic transducers using band frequencies are being put to practical use. However, the piezoelectric vibrator utilizing the thickness direction vibration used in the ultrasonic wave transducer has the following problems as the frequency is increased.
Not only the thickness of the vibrator but also the area of the vibrating surface must be reduced due to the requirement of the directional characteristics, and the dispersion of the characteristics is increased due to the deterioration of the workability of bonding and assembling. Parasitic vibration peculiar to the thickness direction vibration is likely to occur, and when the dimension of the vibrating surface and the dimension ratio in the thickness direction approach a specific size ratio in view of the directional characteristics, the coupled vibration between the thickness direction vibration and the parasitic vibration As a result, the vibration in the thickness direction cannot be effectively excited, and further, the transition of the target resonance frequency occurs. The thickness of the adhesive layer cannot be ignored, and becomes a factor of damping vibration of the vibrator. The mass, stiffness or elasticity of the lead wire cannot be neglected and becomes a damping factor for the vibration of the vibrator. The lead wire on the electrode surface and the solder point to connect it,
This contributes to disturbance of the directional characteristics. Therefore, in order to overcome these problems, the above-mentioned structural measures were taken and solved. Hereinafter, the present invention will be described with reference to examples. FIG. 4 (a) is a top perspective view and FIG. 4 (b) is a schematic sectional view showing an example of the configuration of a conventional transducer. A rectangular piezoelectric ceramic vibrator 1 in which electrodes are applied almost entirely on both sides in the thickness direction
Lead wires 26a and 26b are connected to both electrodes by soldering, and the rear surface of the vibrator 21 is adhered and fixed to the pedestal 22 with an adhesive. The lead wire 26a is inserted into the through hole of the case 23,
After bonding the pedestal 22 and the case 23 through 26b,
The lead wires are connected to the airtight terminals 28a and 28b, and the airtight terminals are bonded to predetermined positions of the housing 25. Case 2
3 is bonded to the housing 25, and a filler 27 such as an epoxy resin is injected into a space formed behind the case 23 to prevent deformation or breakage caused by application of water pressure, and is cured. The rubber-like elastic body 24 is molded into a watertight structure. Here, the shape and dimensions of the vibrator will be examined. The thickness of the vibrator when utilizing the thickness direction vibration is, for example, about 1.2 m when the resonance frequency is 1.5 MHz.
m, the resonance frequency is about 0.6 mm if the resonance frequency is 3.0 MHz. Further, when the outer diameter of the vibrator is obtained from the requirement of the directional characteristics with the thickness of the vibrator as 1, the side length of the rectangular plate is obtained assuming that the rectangular plate vibrator has a -6 dB directivity width of 10 degrees. If the directional width is about 5 and -6 dB and the angle is 20 degrees, the side length is about 2.5. Therefore, if an attempt is made to obtain 20 degrees with a -6 dB directivity width at a frequency of 1.5 MHz, a rectangular plate-shaped vibrator has a thickness of about 1.2 mm and a side length of about 3.0 mm square. If you keep the frequency as it is and increase the frequency to 3.0 MHz, the thickness is about 0.6 mm, the side length is about 1.5 mm square,
That is, the vibrator is significantly reduced in size as the frequency is increased so that the volume is reduced to approximately one fourth. FIG. 5 shows the characteristics of a square plate-shaped thickness-directional vibrator designed to manufacture the transducer having the configuration shown in FIG. 4 on the assumption that it is used near 3 MHz. FIG. 5A shows the admittance frequency characteristics of the vibrator alone, and FIG. 5B shows the same characteristics before the rubber-like elastic body 24 is molded. The problem with the conventional method is that parasitic vibrations are easily generated in the vibrator. However, in the characteristics shown in FIG. 5A, in addition to the resonance f _A of the thickness direction vibration, f _B and f _C
And the resonance of the parasitic vibration is observed. If the vibration in the thickness direction is relatively low in frequency and the volume of the vibrator is large, parasitic vibrations such as higher order mode resonance in the width direction should be damped by bonding to the pedestal, etc., and the thickness direction vibration should be used effectively. Although it is not difficult, the effect of the thickness of the adhesive layer or the connection of the lead wire cannot be ignored in such a small vibrator. As shown in FIG. 5 (b), the adhesion of the vibrator 21 to the pedestal 22 and the attachment of the lead wires 26a and 26b dampen the thickness vibration which is originally required, and the admittance value of f _A Is reduced to the same level as the other parasitic vibrations f _B and f _C. Admittance is the reciprocal of the impedance, which indicates that the impedance at f _A has increased and is no longer well excitable. It can easily be inferred that it is difficult to obtain the expected performance even when assembled as a transducer in this state. If the characteristics change due to assembling is uniform, it is possible to reduce the variation in characteristics by reviewing the oscillator specifications or reviewing the assembling method, but this variation is not uniform, and similar Even in the case of showing a change tendency, the amount of change in each individual varies greatly and cannot be easily controlled. As a result, it is customary to allow a situation in which the characteristics vary widely, or to produce a large number of products and use them selectively. Was. FIGS. 1 and 2 show examples of the structure according to the present invention. FIGS. 1A and 1B show an assembly structure of a vibrator and a pedestal. FIG. 1A is a top view, FIG. 2B is a sectional view, and FIG. 1 shows a schematic sectional view of a vessel. A circular counter electrode 9 is formed substantially at the center of the rectangular plate-shaped vibrator 1, and lead electrodes 10a and 10b are formed from the counter electrode in directions different from each other by 180 degrees on the front surface and the back surface of the vibrator. A lead wire is connected to the end of 10b by soldering. The vibrator 1 is adhered and fixed to the pedestal 2 by applying an adhesive 11 only to the outer peripheral portion. The pedestal 2 is provided with a recess 12 that is larger than the area where the opposing electrode 9 is located, so that the opposing electrode on the back of the vibrator 1 does not directly touch the pedestal.
After the above configuration, the lead wires 6a and 6b are passed through the through holes of the case 3, and then the pedestal 2 and the case 3 are bonded and assembled. The lead wires 6a and 6b are connected to the hermetic terminals 8a and 8b to form a hermetic seal. The terminal is bonded to a predetermined position of the housing 5. The case 3 is adhered to the housing 5, the filler 7 is injected into the space behind the case 3, and is hardened.
Is molded on the exposed surface side of the vibrator 1 to obtain a watertight structure. The vibrator having the electrode structure as shown in FIG. 1 and exciting the thickness direction vibration is generally known as an energy trap type vibrator. In such a vibrator, the vibration in the thickness direction is strongly excited at the opposing electrode portion, and as it moves away from the opposing electrode, the vibration attenuates exponentially. As a result, the vibration is confined in the vicinity of the opposing electrode portion. Show. Utilizing such features,
By bonding and fixing only the peripheral portion of the vibrator 1 where vibration in the thickness direction is hardly excited, and connecting a lead wire to an end portion of the vibrator 1, the vicinity of the counter electrode 9 located at a substantially central portion of the vibrator 1 The structure of the transducer can be realized without damping the vibration of the transducer. However, if it is assumed that the vibrator is used in the air, a sufficient effect can be obtained only by using such a vibrator. However, in the case where the vibrator is used in water, the situation of applying a water pressure must be considered. . When water pressure is applied in such a configuration that the opposing electrode part directly contacts the pedestal, the pressure changes the contact state between the opposing electrode and the pedestal at any time, but this changes the rear impedance as viewed from the vibrator. Means to do. If this impedance changes, the acoustic energy propagating toward the pedestal changes, resulting in a fluctuation in sensitivity as a transducer, which is not preferable. Therefore, in order to use the characteristics of the vibrator 1 effectively, it is necessary to adopt a configuration in which the impedance behind the vibrator 1 hardly changes in response to a change in water pressure.
The most effective method is to provide a gap, which can be easily achieved by providing the pedestal 2 with the concave portion 12 by counterbore processing or the like. It is clear that the formation range of the concave portion 12 needs to be at least as large as the counter electrode 9, and it is practically desirable to form the concave portion 12 in a wider range than the counter electrode 9. Since the parts other than the vibrator 1 and the pedestal 2 have substantially the same configuration as the conventional transducer, improvement can be easily made only by replacing this part. FIG. 3 shows the characteristics of the transmitter / receiver having the configuration of FIG. 1 manufactured on the assumption that it is used near 3 MHz.
(A) is the admittance frequency characteristic of the vibrator alone, and FIG.
(B) shows the same characteristics for a plurality of rubber-like elastic bodies 4 before they are molded. In FIG. 3A, the resonances f _b , f _C, etc. of the parasitic vibration are observed as in the case of the conventional vibrator having the full-surface electrode, but are smaller than the conventional vibration f _{A in} the thickness direction. And approaches a single resonance of the thickness direction vibration.
In FIG. 3B, the parasitic oscillations of f _b and f _C have disappeared.
This indicates that the bonding effect of only the outer peripheral portion of the vibrator 1 exerts a braking effect on parasitic vibration such as higher-order mode resonance in the width direction vibration. On the other hand, in the thickness direction vibration f _a , the change in the admittance value is small, and FIG.
As shown in the characteristic example, there is no extreme change such that the admittance value is reduced to about one-fifth of the vibrator alone, indicating that the thickness direction vibration is favorably excited. The five transducers are assembled in a state in which the resonance frequency and the shape of the admittance characteristics are fairly uniform without disturbance in the characteristic change as seen in the conventional configuration. As described above, according to the present invention,
Excitation of the thickness direction vibration used as the main vibration mode of the vibrator when transmitting and receiving ultrasonic waves corresponding to the high frequency of the transducer, without damping the thickness direction vibration, and the characteristics due to assembly It is possible to easily obtain a transceiver having little variation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing an assembling structure of a pedestal portion of an underwater ultrasonic transducer according to the present invention, FIG. 1 (a) is a top view, and FIG. 1 (b).
Is a sectional view. FIG. 2 is a sectional view showing a schematic configuration of a transducer according to the present invention. 3A and 3B are diagrams showing characteristics of a piezoelectric ceramic vibrator used in the present invention. FIG. 3A shows admittance characteristics of a single unit, and FIG.
(B) Admittance characteristics of the assembly before molding. FIG. 4 is a schematic configuration diagram of a conventional underwater ultrasonic transducer, wherein FIG. 4 (a) is a top perspective view and FIG. 4 (b) is a cross-sectional view. 5A and 5B are diagrams showing characteristics of a piezoelectric ceramic vibrator used in a conventional configuration. FIG. 5A is a diagram showing admittance characteristics of a single unit, and FIG.
(B) Admittance characteristics of the assembly before molding. DESCRIPTION OF SYMBOLS 1, 21 Piezoelectric ceramic or piezoelectric ceramic oscillator 2, 22 Pedestal 3, 23 Case 4, 24 Rubbery 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 Leader electrode 11 Adhesive 12 Depressed portion

Claims (1)

(57) [Claim 1] At a substantially center of a circular or rectangular plate-shaped piezoelectric ceramic, a circular or rectangular counter electrode facing in the thickness direction is opposed in the thickness direction than the counter electrode. A pair of extraction electrodes formed in different directions so as not to be connected to each other, and a vibrator configured to perform polarization and excitation by connecting an electric wire for transmitting and receiving an electric signal to the extraction electrodes, and a range in which the counter electrode is located. And a pedestal having a structure in which the counter electrode is not directly touched by providing a concave portion, and only the outer peripheral portion of the vibrator is adhered and fixed to the pedestal so that the counter electrode is located in the concave portion of the pedestal. An underwater ultrasonic transducer characterized by having a waterproof structure provided by a mold.