JPS6133515B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates generally to transducers, and more particularly to unidirectional transducers, such as those that generate unidirectional ultrasonic waves in a liquid.

By the way, even if an optically opaque medium is acoustically transparent, it is possible to observe a fluoroscopic image using sound waves in the same way as fluoroscopy using X-rays. It can be applied to fields such as medical diagnosis, non-destructive testing, and observation of ocean floor patterns. Conventional transducers for generating ultrasonic waves used in this type of ultrasonic device include those using an acoustic phase plate, those using an annular array, those using an acoustic lens, and those using a photo-acoustic transducer. etc. have been proposed. However, the reality is that there is still room for improvement in terms of focusing the sound waves necessary for ultrasound imaging.

Therefore, the present inventor previously proposed Japanese Patent Application No. 100929/1983 as a method for improving the above-mentioned drawbacks of the prior art. This method involves installing interdigital electrodes with a special relationship between the electrode fingers on one side of a piezoelectric substrate that is thick enough to excite Lamb waves, and placing the interdigital electrodes in contact with a liquid with the other side in contact with the liquid. By applying an alternating current voltage to the electrodes, a highly convergent ultrasonic beam is emitted into the liquid from the other side.

However, in the above technology, the ultrasonic beam is oriented 2 times in +θ direction and -θ direction with respect to the perpendicular line of the piezoelectric substrate.
Depending on the application, this may cause unwanted signals to be generated.

SUMMARY OF THE INVENTION Accordingly, the present invention aims to improve the above-mentioned drawbacks of the prior art, and its object is to provide a unidirectional transducer with good focusability. A feature of the present invention for achieving this object is to realize the unidirectionality of the ultrasonic beam by creating a special relationship between the spacing between the electrode fingers of the interdigital electrodes and the phase of the applied voltage. In order to focus the ultrasonic beam, there is also a special relationship to the spacing between the electrode fingers. Examples will be described below with reference to the drawings.

FIG. 1 shows an example of the structure of a unidirectional transducer according to the present invention, in which numeral 1 denotes a piezoelectric substrate whose thickness is approximately equal to or less than the wavelength λ of the Lamb wave in the substrate. Reference numerals 2 and 3 designate a pair of interdigital interdigital electrodes in which comb-tooth electrode fingers are arranged alternately in an interdigital manner, and are provided on one surface of the piezoelectric substrate 1. As shown in FIG. 2, the interdigital electrodes 2 and 3 are constructed so that the distance between each electrode finger and the adjacent electrode finger is asymmetrical.
4 is a planar electrode provided on the other surface of the piezoelectric substrate 1 facing the interdigital electrodes 2 and 3; and 5 is a liquid in contact with the planar electrode 4.

With the above structure, if two voltages with different phases are generated from a single-phase input using a phase shifter (not shown), and the transducer is driven by applying these to the interdigital electrodes 2 and 3, the transducer is driven.
Emission of longitudinal sound waves satisfying the following equation occurs from the surface having the plane electrode 4, and the sound waves are emitted into the liquid.

sinθ=V _W /V _L (1) Here, θ is the direction of the emitted sound wave, V _W is the speed of the sound wave in the liquid, and V _L is the speed of the Lamb wave propagating on the piezoelectric body.

Lamb waves differ from Rayleigh waves in that displacement exists on both the front and back sides of the medium in which the waves propagate, and in the case of symmetric modes, the displacement characteristics are the same.
Therefore, considering this characteristic, the displacement situation is the same on the surface with the interdigital electrode and the surface on the opposite side, and the surface containing the interdigital electrode can be moved to the opposite side without contacting the liquid. By bringing the surface (the surface having the planar electrode) into contact with the liquid, efficient radiation of sound waves into the liquid can be realized. Furthermore, it is also possible to efficiently convert the sound waves that reach this type of transducer into electrical signals, and in that case, the sensitivity is maximized for sound waves from the direction that satisfies equation (1) above. .

By the way, unlike the present invention, when the periods of the interdigital electrodes provided on the piezoelectric substrate are equal intervals, the sound waves emitted from each section of the interdigital electrodes are as follows:
In relation to equation (1), it is emitted as a parallel sound beam in a direction that satisfies the following equation.

λ=sin ⁻¹ V _W /f·d (2) where f is the carrier frequency of the electrical signal applied to the transducer, and d is the electrode period of the interdigital electrode.

In this case, as shown in FIG. 3, two waves of ultrasonic beams are emitted in the +θ direction and in the −θ direction, so it was mentioned earlier that this is not appropriate depending on the application. On the other hand, when the applied AC voltage is three-phase, the radiation into the liquid is created by creating a special relationship between the spacing between the electrode fingers of the interdigital electrodes and the phase of the applied voltage, as described below. Unidirectionality of the ultrasound beam is achieved.

In other words, if xP is the narrower distance from the center of an electrode finger to the center of an adjacent electrode finger with respect to the electrode period P of the interdigital electrode as shown in Fig. 4, then the phase shift distance due to the electrode configuration is 2πx. Therefore, by setting the phase difference of the voltages applied to each of the interdigital electrodes to be 2πy and making the sum of these 180°, that is, satisfying equation (3), the ultrasonic beam will be in the liquid state. It is emitted as a parallel beam in one direction throughout the body.

x+y=1/2 (3) Next, the electrode configuration for focusing the above-mentioned parallel ultrasound beams will be explained based on FIG. 4.
The relationship between the wavelength λ _f of a sound wave of frequency f in a liquid and the direction of maximum beam output (angle θ) is determined by the following equation, and this equation agrees well with the experimental results by the inventor.

sinθ=λ _f /P (4) Therefore, in Fig. 3, in order for the sound waves generated from each electrode to be focused on point Q (that is, the conditions for the sound waves generated at each point to pass through point Q and be in phase) The following condition must be satisfied.

When n is a positive integer ^, X _{2o =} (R ² ₀ sin ² θ _{0 +} ^2nλ _f R _{0 +} n ² λ _f ² ₎ ^1/2 R ₀ + (2n+1) ² x ² λ _f ² ] ^1/2 When n is a negative integer, X _2o = (R ² ₀ sin ² θ ₀ +2nλ _f R ₀ +n ² λ _f ² ) ^1/2 X _{2o- 1} = [R ² ₀ sin ² θ ₀ +2(2n+1)(1-x)λ _f R ₀ +(2n+1) ² (1-x) ² λ _f ² ] ^1/2 (5) Here, R ₀ is The distance from the zeroth electrode to the beam convergence point, X _2o , X _2o+1 is the horizontal distance between the origin (0) and the electrode.

When emitting a parallel beam in only one direction,
As the interdigital interdigital electrode according to the present invention, an electrode having the structure shown in FIG. 2 is sufficient, and therefore, the electrode structure is greatly simplified in the present invention.

In the above explanation, for example, Cr
A combination of LiNbO 3 and Au has good strength and good strength, and the electrode is formed on the surface of the piezoelectric material by known techniques such as vapor deposition and sputtering. LiNbO ₃ ,
Possible materials include crystal, Bi ₁₂ GeO ₂₀ , and PZT-based porcelain (for example, 91A material manufactured by Tokyo Denki Kagaku Kogyo Co., Ltd.).

Experimental example: Piezoelectric ceramic 91A material manufactured by Tokyo Denki Kagaku Kogyo Co., Ltd. (TDK) with the polarization axis in the thickness direction as a piezoelectric substrate (length 70 mm, width 20 mm, thickness 0.2 mm, zero-order symmetric moderam wave velocity) 3500 m/sec), a transducer-shaped electrode is provided on one side of the substrate, and a flat electrode is provided on the other surface, and the phase shift distance x in the electrode configuration of the transducer-shaped electrode is
is 1/4 and 30cm from the transducer.
It was designed to converge 2.5MHz sound waves. Note that the number of electrode pairs of the interdigital electrodes is 50.

The transducer as described above is mounted in a water tank so that the interdigital electrodes do not come into contact with the liquid, and the distribution of sound waves emitted into the liquid from the surface with the flat electrodes is measured using point probes (piezoelectric transducer). FIG. 5 shows the measurement results, and it can be seen that the distance to the focal point of the acoustic beam is close to the designed value. The beam width here indicates the beam width at which the energy is 3 dB lower than the center. In this case, the phase difference between the two electrical signals applied to the transducer is 90° (corresponding to y=1/4), which satisfies the relationship x+y=1/2.

Figure 6 shows the measurement results of the unidirectional characteristics, and a directionality of 20 dB was confirmed. The measurement results shown in FIG. 6 are for the case where the phase distance x is 1/4, but the same thing was experimentally confirmed when x=1/3. In this case, a 60° phase shifter (y=1/6) was used.

Figure 7 shows the measurement of the relationship between the direction of the focal point and the carrier frequency in the transducer with a phase distance x of 1/4, as described above, and it is understood that the position of the focal point changes as the carrier frequency changes. Ru.

As explained above, according to the present invention, it is possible to emit a highly focused unidirectional ultrasonic beam into a liquid, and as an additional effect, the surface including the interdigital electrode comes into direct contact with the liquid. There is no need for mechanical or chemical protection of interdigital electrodes, and the planar structure eliminates the need for spherical surfaces for beam focusing, and the three-layer structure does not require the use of multilayer techniques. Phase configuration is possible, and the structure is simpler than a three-phase configuration on one surface.

Applications of the invention are not limited solely to imaging, but are generally applicable to applications where a beam of acoustic waves needs to be focused, for example to focus the beam onto a liquid-air interface to Atomization can be performed.

[Brief explanation of the drawing]

FIG. 1 shows an example of the structure of an ultrasonic transducer according to the present invention, FIG. 2 shows an example of the structure of an interdigital electrode, and FIG.
The figure is an explanatory diagram of the operation of a conventional transducer.
The figure is an explanatory diagram of the operation of the present invention, and FIGS. 5, 6, and 7 are diagrams showing examples of experimental results. 1; piezoelectric substrate, 2, 3; interdigital electrode, 4;
Planar electrode, 5; liquid material.

Claims (1)

[Claims] 1. A piezoelectric substrate having a thickness of approximately λ or less (λ is the wavelength of ultrasonic waves in the substrate), and a pair of interdigital interdigital blinds formed on one surface of the substrate. an electrode, and a planar electrode provided on a surface opposite to the surface of the substrate and facing the interdigital electrode, and applying an alternating current voltage to the interdigital electrode and the planar electrode in a state where the planar electrode is in contact with the liquid. In a transducer that generates ultrasonic waves in a liquid by applying an ultrasonic wave, the electrode fingers of the interdigital electrode are configured such that the distance between each electrode finger and the adjacent electrode finger is asymmetrical, and the electrode finger spacing is A unidirectional transformer characterized in that an ultrasonic wave is generated in one direction in a liquid by applying an electric signal having a phase difference such that the sum of the phase difference and the narrower phase difference is 180°. Duusa. _{2 The} spacing ^{between the} electrode fingers of ^the interdigital electrode _{is as} follows: When n _is ^{a positive} _{integer ,} R ² ₀ sin ² θ ₀ +2 ₍ 2n ⁺ ₁ )xλ _f R ₀ +(2n+1) ² _x ² λ _f ² ] ^1/2 If n is ^a negative integer, _{then 0} ^{+ n 2 λ} _f ² _{) 1/2 _ _ _} ^{_ _} _{_} ^_ ] ^1/2 where λ _f is the wavelength of the sound wave with frequency f in the liquid, R ₀ is the distance from the zeroth electrode to the beam focal point, θ ₀ is the direction of the beam from the zeroth electrode, and 2πx 2. The unidirectional transducer according to claim 1, wherein the unidirectional transducer is constructed so as to satisfy the phase separation of the narrower electrode finger spacing.