JPH0529200B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a piezoelectric vibrator that generates ultrasonic waves using an electrostrictive phenomenon. The present invention relates to a piezoelectric vibrator that can significantly suppress side lobes generated in connection with an ultrasonic radiation pattern by appropriately forming a pattern and applying an alternating current voltage between both electrodes of the piezoelectric vibrator.

[Background of the Invention] A piezoelectric vibrator is an electroacoustic transducer that converts an electric signal into a sonic signal, or converts a sonic signal into an electric signal, and has a so-called wave transmitting/receiving function. Among these, piezoelectric vibrators that generate ultrasonic waves have relatively low signal attenuation in liquids, especially underwater, and it is relatively easy to select the size of the sound source to be large compared to the wavelength. Acoustic sounders, because the directivity of sound waves can be sharpened, etc.
In particular, it is widely used in fish finders.

Here, the directional characteristics of sound waves express the characteristics of the range in which sound waves are transmitted from a piezoelectric vibrator that has a wave transmitting and receiving function, and the range in which sound waves are received. It can be said that this is an extremely important factor considering gender.

FIG. 1 shows an example of the directional characteristics of sound waves generated from a piezoelectric vibrator. As can be easily understood from the figure, the central axis 4 is perpendicular to the piezoelectric vibrator 2.
The upper sound pressure is the strongest, and the sound pressure decreases as the displacement angle θ from the central axis 4 increases. In this case, the central axis 4
The upper sound wave beam is called the main lobe M, and the side sound wave beams are called the side lobes S. Also, when transmitting and receiving using the same piezoelectric vibrator,
Since the directivity multiplier is squared, the angle θ _-3dB at which the sound pressure in the main lobe M decreases by 3dB from the value at the central axis 4
is also important as an index representing directional characteristics.

By the way, among the directional characteristics of sound waves shown in Figure 1,
The existence of side lobes S means that when a piezoelectric vibrator that generates the sound waves is used in a fish finder, the reflected signal related to the side lobe S, for example, the reflected signal from a school of fish, is the reflected signal from the main lobe M. Mistakes may occur. In this case, even if you trust the direction of the school of fish displayed on the display of the fish finder, that is, start casting the net with a misperception of the direction of the school of fish, there will be no school of fish in the casting area, which will reduce operational efficiency. It has been pointed out that there are drawbacks that reduce the

In order to suppress this unnecessary side lobe S, a method is known in which electrode patterns on opposing surfaces of a rectangular parallelepiped ultrasonic piezoelectric vibrator are formed into a special shape.

Before discussing the conventional technical ideas related to this method, in order to further clarify the problems of the conventional technology, we will first explain the basic electrode pattern of a piezoelectric vibrator and the side lobes that occur along with the electrode pattern. The relationship with the size of S will be described.

Usually, the amount of suppression of the sidelobe S is expressed as a sidelobe suppression ratio R; R=20log B/A.
Here, reference numeral A is the maximum sound pressure level of the main lobe M, and reference numeral B is the maximum sound pressure level of the side lobe S (see FIG. 1). Now, as shown in FIG. 2, the electrode pattern of the rectangular parallelepiped piezoelectric vibrator 6 has electrode patterns 8a,
8b, on the surface of the ultrasonic radiation surface U _s of the rectangular parallelepiped piezoelectric vibrator 6,
As shown in FIG. 3a, an amplitude characteristic curve is obtained in which the generated sound pressure level has equal amplitude with respect to the long axis l. It is known that the sidelobe suppression ratio R in this case is approximately 13.5 dB. In order to make this sidelobe suppression ratio R infinite, the amplitude characteristic curve related to the sound pressure should be made similar to the cos square characteristic curve, for example, as shown in FIG. 3b. Good things have been revealed.

Next, from this point of view, an example of forming an electrode pattern according to the prior art will be described. This technical idea is disclosed, for example, in Japanese Patent Publication No. 58-32558. That is, in this example, as shown in FIG. 4, among the electrode patterns on two opposing sides of the rectangular parallelepiped piezoelectric vibrator, the electrode pattern on one side is cut diagonally to form the excitation electrode 10a and the damping electrode. 12a,1
2b are separated by insulating grooves 14a, 14b, and the damping electrodes 12a, 12b are connected to the excitation electrode 10b on the other side so that the vibration energy is suppressed at both ends, and an AC power source E _a is supplied. It is configured as follows. Therefore, the ultrasonic radiation surface
It is described that the ultrasonic waves radiated from U _s are most intensely radiated from the center of the radiation surface U _s and gradually weaken as they approach the ends due to the action of the damping electrodes 12a and 12b. .

However, in this case, as is clear from a detailed examination of the pattern shape, the arrangement shape of the excitation electrode 10a and the damping electrodes 12a, 12b is linear, so the side lobe related to the rectangular parallelepiped ultrasonic piezoelectric vibrator The effect of suppressing S becomes insufficient. That is, in this prior art, since the shape of the amplitude characteristic curve related to the sound pressure is approximately an isosceles triangular shape, the sidelobe suppression ratio R is approximately
20 dB, and the possibility that a reflected signal from a school of fish or the like related to the side lobe S may be mistaken as a reflected signal related to the main lobe M has not yet been eliminated.

Furthermore, the shape of the electrode pattern of the piezoelectric vibrator disclosed in the present invention does not satisfy the condition of vertical and horizontal symmetry with respect to the center of the vibrator, which may cause unnecessary vibrations and cause disturbances in the radiation pattern. However, it also exposes various drawbacks such as a decrease in the electrical/ultrasonic conversion efficiency.

Furthermore, as can be easily understood from the electrical wiring diagram shown in FIG.
Since a high voltage is applied to the piezoelectric vibrator 4b, it is necessary to ensure a sufficient creeping distance, and it has been pointed out that the piezoelectric vibrator has a disadvantage in that the utilization efficiency of the piezoelectric vibrator is reduced.

Furthermore, such a driving voltage application configuration has the disadvantage that electric charges are accumulated between the excitation electrode 10a and the vibration damping electrodes 12a and 12b, which reduces the electric-ultrasonic conversion efficiency. It is inherent.

[Object of the Invention] The present invention has been made in order to overcome all of the above-mentioned disadvantages, and is to provide electrode patterns on two opposing surfaces of a rectangular parallelepiped piezoelectric vibrator vertically symmetrically, and moreover, for example, to approximately follow a cos square curve. Excitation electrodes and damping electrodes are arranged in a shape. Therefore, by applying an alternating current voltage between the two electrodes, it is possible to suppress the side lobe S that occurs in the ultrasonic radiation pattern to almost zero, and in addition, it is extremely easy to form the electrodes in terms of manufacturing. The purpose of the present invention is to provide an ultrasonic piezoelectric vibrator.

[Means for Achieving the Object] In order to achieve the above object, the present invention provides a rectangular parallelepiped piezoelectric vibrator that generates ultrasonic waves, in which opposing electrode surfaces have substantially congruent shapes with each other. In addition, the electrodes are formed in an electrode shape including one excitation electrode and a pair of vibration damping electrodes that are vertically and horizontally symmetrical with respect to an axis orthogonal to the center of each surface, and the excitation electrode per unit length in the longitudinal direction of each surface is The area of the damping electrode is formed so that it gradually becomes narrower from the center to both ends, and the area of the damping electrode per unit length in the longitudinal direction becomes gradually narrower from both ends to the center. Further, a damping electrode on one surface is electrically connected to a damping electrode on the other electrode surface facing this damping electrode, and a resistor is connected to the two damping electrodes. It is characterized in that the resistor is configured to be connected to a drive voltage source connected to the excitation electrode.

[Embodiments] Next, preferred embodiments of the piezoelectric vibrator according to the present invention will be described in detail with reference to the accompanying drawings. Components that are the same as those shown in FIGS. 1 to 5 are given the same reference numerals, and detailed explanation thereof will be omitted.

In FIG. 6, reference numeral 20 indicates a rectangular parallelepiped-shaped piezoelectric vibrator including an electric circuit driving section according to the present invention. The shape of this piezoelectric vibrator 20 is precisely a thin rectangular parallelepiped shape, and electrode patterns are formed on two opposing surfaces in a substantially congruent shape. In this case, in the figure, the front part 22 and the back part 24 have
One excitation electrode 26, 28 each extends over the entire surface from the center of the surface to both ends.
and two pairs of damping electrodes 30a, 30b and 32a, 32b extending from both ends toward the center, excitation electrodes 26, 28 and damping electrodes 30a, 3.
The circumferential surfaces of 0b, 32a, and 32b are mutually insulating groove 3.
It is formed so as to be insulated by 1a, 31b and 33a, 33b.

In this case, the electrode area per unit length in the longitudinal direction of the excitation electrodes 26, 28 is defined in an area distribution that gradually decreases corresponding to a cos square curve, with the center as the origin and directed toward both ends. At the same time, the electrode area per unit length in the longitudinal direction of the damping electrodes 30a, 30b and 32a, 32b has an area distribution that gradually decreases from both ends to the center in proportion to a curve of (1-cos squared). It is formed as. Note that each electrode can be printed and formed by a silk screen method, and therefore, the accuracy of the curved shape can be maintained and the electrode shape can be made suitable for mass production.

In such a configuration, the damping electrodes 30a, 3
0b and 32a, 32b are connected in common, and from the common connection point 34a, 34b, high resistances R ₁ and R ₂ of, for example, about 100 kΩ or more are connected to the excitation electrodes 26 and 28, respectively. resistance
Common connection point 36 between R ₁ , R ₂ and excitation electrodes 26 and 28
An AC power source E _a is connected between a and 36b.
Here, the AC power source E _a is a voltage source having a frequency that resonates with the ultrasonic waves generated from the piezoelectric vibrator 20 . Incidentally, an electric circuit wiring diagram related to the above-mentioned connection explanation is shown in FIG.

The rectangular parallelepiped piezoelectric vibrator according to the present invention is basically constructed as described above, and its operation and effects will be explained next.

By applying an alternating current power source E _a having a resonant frequency, the piezoelectric vibrator 20 emits ultrasonic waves upward from the radiation surface U _s or downward from the opposing surface of the radiation surface U _s . In this case, the directivity of the ultrasonic wave is such that the vibration damping electrodes 30a, 30b and 32a, 32b of the same potential are located opposite to each other at both ends of the piezoelectric vibrator 20 in the longitudinal direction. The vibration is damped on the side of the shaft, directed in the longitudinal direction, and the vibration is more strongly excited toward the center. In this case, as described above, the excitation electrode 26,
For example, the electrode area per unit length of 28 is approximately
Since it is formed to be proportional to the cos square characteristic,
The ultrasonic waves radiated from the radiation surface U _s can have a radiation pattern with almost no side lobes. In this case, in order to make the directivity pattern of the main lobe M have approximately the same θ _{-3 dB} angle as that of the piezoelectric vibrator with the basic electrode configuration shown in Fig. 2, the length l ₂ in the transverse direction must be Under the condition of being the same as the basic piezoelectric vibrator, it is preferable that the length l ₁ in the longitudinal direction is set such that l ₁ >l.

On the other hand, as is easily understood from the electrical circuit diagram shown in FIG.
There is a resistor at the common connection point 34a, 34b of a, 32b.
Since only a voltage of E _a /2 [V] is applied under the partial voltage action of R ₁ and R ₂ (R ₁ = R ₂ ), as a result, The dielectric strength may be formed to have half the dielectric strength of the conventional one. In other words, the creepage distance can be reduced to approximately half the value.

Furthermore, the excitation electrode 26 and the damping electrodes 30a, 30
Since the electric charge stored between B and the electric charge stored between the excitation electrode 28 and the damping electrodes 32a and 32b is discharged through the resistors _R1 and _R2 every cycle of the AC power source _Ea , the electric charge It is also possible to prevent a decrease in electric/ultrasonic conversion efficiency related to storage.

FIG. 8 shows another embodiment of the piezoelectric vibrator according to the present invention. In FIG. 8, reference numeral 40 is an excitation electrode arranged on the front part 22, and reference numeral 4
2a and 42b are vibration damping electrodes. In this case, an excitation electrode 44 and damping electrodes 46a and 46b are formed on the back surface 24 in a shape substantially congruent with the front surface 22. And, electrically, vibration damping electrodes 42a, 42b
and 46a, 46b are commonly connected, and high resistances R ₁ and R ₂ are connected to excitation electrodes 40, 44, respectively, from their common connection points 48a, 48b. Therefore,
By connecting the driving power source _{E a} between the high resistances R ₁ and R ₂ and the common connection points 48 a and 48 b of the excitation electrodes 40 and 44, ultrasonic waves can be radiated from the radiation surface U _s . Also in this case, the excitation electrodes 40, 4
The electrode area per unit length in the longitudinal direction of 4 is approximately oriented toward both ends with the center point in the longitudinal direction as the origin.
The damping electrodes 42a, 42b and 46a, 46b are divided into areas proportional to the cos square curve.
An ultrasonic radiation pattern in which side lobes can be suppressed can be obtained by creating a shape with an area distribution proportional to the -cos squared) curve.

[Effects of the Invention] As described above, according to the present invention, the excitation electrodes and the damping electrodes, which are arranged in substantially congruent shapes on two opposing surfaces of a piezoelectric vibrator, are arranged in vertically and horizontally symmetrical electrode patterns. Furthermore, the electrode area is formed in a pattern shape that follows a cos square curve, and a high resistance is connected to the vibration damping electrode to drive it by a supplied power source. Therefore, various effects listed below can be obtained.

The directional characteristics of the emitted ultrasonic waves become sharper, and it becomes possible to suppress side lobes generated in the ultrasonic radiation pattern to approximately zero.

If the electrode is formed by a silk screen printing method or the like, it is possible to extremely easily form an electrode pattern with a complicated shape without increasing the manufacturing cost.

It is possible to reduce the creepage distance between the excitation electrode and the damping electrode to approximately 1/2 of that of the conventional one, and the utilization efficiency of the piezoelectric vibrator is improved.

Since a cycle in which the charge is discharged by the resistor can be ensured, deterioration of the electric/ultrasonic conversion efficiency due to charge accumulation is reduced.

Since the voltage value applied between the excitation electrode and the damping electrode is reduced to 1/2, the possibility of cracks occurring in the piezoelectric vibrator itself due to electrostrictive action is eliminated.

Although the present invention has been described above with reference to preferred embodiments, the present invention is not limited to these embodiments, and various improvements and changes in design are possible without departing from the gist of the present invention. Of course.

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram of the directional characteristics of ultrasonic waves generated from a piezoelectric vibrator, Fig. 2 is a perspective explanatory diagram of a rectangular parallelepiped piezoelectric vibrator having a conventional basic electrode shape, and Fig. 3 a
is an amplitude characteristic curve diagram related to the piezoelectric vibrator shown in FIG. 2, FIG. 3b is a cos square amplitude characteristic curve diagram, FIG.
6 is a perspective explanatory diagram of a rectangular parallelepiped piezoelectric vibrator including an electric circuit drive unit according to the present invention; FIG. 7 is an electric circuit connection diagram according to FIG. 6; FIG. 8 is a perspective explanatory view showing another embodiment of the rectangular parallelepiped piezoelectric vibrator according to the present invention. 20...Piezoelectric vibrator, 22...Front part, 24...
... Back part, 26, 28 ... Excitation electrode, 30a, 3
0b, 32a, 32b... vibration damping electrode, E _a ... AC power supply, M... main lobe, S... side lobe, U _s ... ultrasonic radiation surface.