JPS5821992B2 - ultrasonic transducer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transducer for generating ultrasonic waves used in an ultrasonic device, and particularly to a transducer for generating or detecting focused ultrasonic waves.

Even if the medium is optically opaque, as long as it is acoustically transparent, it is possible to observe a transparent image using sound waves in the same way as using X-rays.

Ultrasonic imaging of optically opaque objects can be applied to the fields of medical diagnosis, microscopy, nondestructive testing, observation of ocean floor patterns, and earthquake research.

Conventional ultrasonic transducers that use an acoustic phase plate, an annular array, an acoustic lens, a photo-acoustic transducer, and the like have been proposed.

However, the reality is that there is still room for improvement in terms of convergence of sound waves necessary for ultrasound imaging.

In order to improve the above points, the applicant and others first filed a patent application in 1983.
-31507 and others proposed an ultrasonic transducer that generates ultrasonic beams with excellent convergence.

This transducer generates an ultrasonic beam by applying an alternating current signal to interdigital electrodes (sometimes called interdigital electrodes) provided on the surface of a piezoelectric material and in contact with a liquid.

Although the conventional transducer described above can generate focused ultrasonic waves, the focused ultrasonic waves converge on a straight line.

In other words, it is one-dimensional convergence.

Therefore, an object of the present invention is to provide a transducer that further improves the conventional ultrasonic transducer and generates ultrasonic waves that converge on a point, that is, ultrasonic waves that converge on a single point in a two-dimensional plane. It is in.

The transducer according to the present invention is characterized in that a plurality of interdigital electrodes made of thin metal films are provided on one surface of a piezoelectric material in the longitudinal direction of electrode fingers to form an electrode group. The digital electrode has a structure in which a plurality of sets of comb-like electrodes each having a plurality of parallel elongated electrode fingers are arranged, and the electrode fingers of each comb-like electrode are arranged in an alternately overlapping manner, and the interval between the electrode fingers is is an ultrasonic transducer determined to focus an ultrasonic beam emitted into the liquid in contact with the piezoelectric material.

The transducer is used with interdigitated electrodes in contact with a suitable liquid, and electrical signals having the same frequency and different phases are applied to each interdigitated electrode.

Embodiments will be described below with reference to the drawings. 3 FIG. 1 shows an example of the structure of a focused ultrasonic transducer according to the present invention, in which a liquid 2 is provided in a container 1, and a transducer-shaped electrode 4 is placed on the surface of the liquid. A piezoelectric material 3 having .

Possible liquids include water, ether, acetone, glycerin, etc.

The electrodes 4 are single-phase electrodes which are interdigitally arranged with interdigital comb-shaped electrodes as shown in FIG. Connect every third digitally constructed electrode to terminal a.

A three-phase electrode that applies a three-phase AC signal from b and C, or a multiphase electrode that applies an n-phase AC signal by connecting every nth electrode (n is a natural number of 4 or more) to each other is possible. be.

In the case of a single-phase electrode, two ultrasound beams are generated, whereas in the case of a three-phase electrode, a single ultrasound beam is obtained.

As for the material of the electrode, for example, a combination of Cr and Au has strong water resistance and is good.

Piezoelectric materials include L IN b Os, crystal, Bi1
□GeO26, PZT-based porcelain (for example, 91A material manufactured by Tokyo Denki Kagaku Kogyo Co., Ltd.), etc. are possible.

Next, the spacing between the electrodes will be explained with reference to FIG.

The relationship between the wavelength λf in a liquid with a sound wave of frequency f and the direction (angle θ) of the maximum output of the beam is determined by the following equation, and this equation agrees well with the experimental result i by the inventor.

sin θ=λf/d (1) where d is the electrode period.

Therefore, in Fig. 3, in order for the sound waves generated from each electrode to converge on point P (that is, the condition that the sound waves generated at each point pass through point P and are in phase) r%= R% -L2= k 1nλfL + k2
It is necessary that n2λf2-(2) hold true.

Here, rn is the distance between the n-th electrode and point Q, Rn is the distance between the n-th electrode and point P, and 2 is a constant for kl.

The i value of (kl, 2) is (1, -!-
), there are 7 pairs of -C (winter, 1) in the 3-phase electrode.

The distance between two electrodes is expressed by formula (1). (2) It is obtained by calculating the formula 9 using a computer.

Next, the experimental results of the beam radiation angle θ for each combination of piezoelectric material and liquid are shown in the following table.

From the above table, it can be seen that in order to reduce the value of θ, it is sufficient to combine a liquid with a low acoustic wave speed and a piezoelectric body with a high surface wave transmission speed.

Also, from the above explanation, it is clear that the speed of sound waves changes depending on the frequency of the sound waves, and the experimental results are shown in Figures 4 and 5 (the piezoelectric body is made of 91A material and the liquid is water). be).

Figure 4 shows the results of observing the directional characteristics at each frequency and determining the shape of the sound wave beam from the directional characteristic curve, where the horizontal axis shows the distance from the sound source and the vertical axis shows the beam width.

When the focal length for each frequency is determined from FIG. 4, the results are as shown in FIG. 5.

In addition, when we investigated the characteristics of the electrode pattern of the transducer on a scale of 1, we found that the focal length was 16c at a center frequency of 5MHz.
The m1 beam width showed a beam width of 3.8 degrees, and the focal length also changed as the frequency changed.

Therefore, it can be seen that the transducer according to the present invention satisfies the similarity relationship.

With the above structure, the ultrasonic beam can be focused in one direction (X direction).

Next, the convergence of the beam in the X direction will be explained.

FIG. 6 shows an example of the structure of an ultrasonic transducer according to the present invention, in which a plurality of interdigital electrodes 4a, 4b are disposed on a piezoelectric material 3.
, 4c, . . . 41 are provided, and the electrode group 10 is constituted by these interdigital electrodes.

Regarding the electrode period of each interdigital electrode, it is assumed that the conditions previously explained in FIG. 3 and the second equation are satisfied.

It is also assumed that the interdigital electrodes 4at4bj4Ct, . . . are arranged in the longitudinal direction of the electrode fingers as shown in the figure.

Note that each interdigital electrode in FIG. 6 is actually used in contact with a liquid as shown in FIG. 1, but the liquid is omitted in FIG. 6 for simplicity of illustration.

In the configuration shown in Figure 6, the electrode configuration is first applied to Figures 3 and 2.
Since the relationship explained by the equation is satisfied, the ultrasonic beam is converged in the X direction according to this relationship, and a number of X-direction convergent beams corresponding to the number of interdigital electrodes can be obtained.

On the other hand, regarding the characteristics in the y-axis direction, a plurality of interdigital electrodes 4
at4bt4Cj... functions just like a diffraction grating in optics.

Therefore, the electrical signals applied to each interdigital electrode are signals with the same frequency and phase difference (φ1.φ2.φ3.φ4...)
By doing so, the phase of the ultrasonic waves generated differs from one interdigital electrode to another, and the beam as a whole can be focused on a line parallel to the X-axis (convergence in the y-axis direction).

As a result, by combining the convergence in the X-axis direction and the convergence in the y-axis direction, it is possible to obtain a two-dimensional convergent ultrasound that converges on point F.

Now, the position in the y-axis direction of the n-th interdigital electrode in the y-axis direction is yn, the angular frequency of the signal is ω0, and the sound wave emitted when a signal with a phase shift Δφ(yn) from the reference value is applied. If the phase of the sound wave when it reaches the line located in front (y, z) of the transducer is φn(y, z), then
φn(y, z) is expressed by the following equation, where VW is the sound velocity in the liquid and t is the time.

i If the second formula holds here, the ultrasonic beams from each interdigital electrode will all be in phase, and will therefore converge on the line of in-phase.

Note that 2 is the distance from the electrode in the focal direction.

However, m is an integer, and if t is the distance between each interdigital electrode, then yn=n7.

Therefore, by applying electrical signals with a phase that satisfies the above equation to each interdigital electrode shown in FIG. 6, an ultrasonic beam converging onto a single line can be obtained.

Electrical signals with a phase difference can be obtained by a surface wave delay line with taps, or by a combination of known circuit techniques.

As an experimental example of convergence in the y-axis direction, piezoelectric material 91A manufactured by Tokyo Denki Kagaku Kogyo Co., Ltd. was used as the piezoelectric material, and the electrode period was 42.
When a signal with a phase satisfying the above formula was applied at a frequency of 5.0 MHz using 10 interdigital electrodes (interdigital electrodes) equally spaced at 8 μm, a plane located at a distance of 2ocIrL from the transducer was detected. We were able to obtain a single linear beam within the beam.

FIG. 7 shows another structural example of the ultrasonic transducer according to the present invention, in which a plurality of electrode groups 10 shown in FIG. 6 are provided on a single piezoelectric substrate 3, such as 10a and 10b.

The electrode group 10a includes interdigital electrodes 4a, 4b,...
and another electrode group 10b has interdigital electrodes 4a/
, 4 b/,...

It is assumed that the electrode period d and the position of the interdigital electrodes in the y-axis direction in each electrode group are different from each other.

Since the convergence position of the ultrasonic beam of each electrode group depends on the excitation frequency and the structure of the electrode group, the focus distance can be varied by using a transducer with the structure shown in Figure 7 and switching the electric signal. Ultrasonic waves can be obtained.

Of course, it is also possible to provide three or more electrode groups on a single piezoelectric material.

As explained in detail above, a transducer has a plurality of interdigital electrodes arranged at appropriate intervals on a piezoelectric material in the longitudinal direction of the electrode fingers, and the transducer is arranged so that the electrodes are in contact with the liquid. By applying alternating current signals with the same frequency and different phases to the ultrasonic waves, an ultrasonic beam that converges into a point can be obtained.

Note that the electrode can be protected from deterioration by covering the electrode surface with a protective film such as silicon rubber.

Although the above embodiments have mainly been explained using the generation of ultrasonic waves as an example, the transducer according to the present invention can also detect ultrasonic waves and convert them into electrical energy.

At this time, a strong electrical signal is generated in response to the ultrasonic waves generated from the focus.

Applications of the present invention are not limited solely to imaging, but are generally applicable to applications where a beam of sound waves needs to be focused, for example by focusing the beam onto a liquid/air interface to can be atomized.

[Brief explanation of drawings]

Figure 1 is an example of the structure of the ultrasonic transducer according to the present invention, Figures 2A and B are examples of the structure of the electrode 4, Figure 3 is an explanatory diagram of the operation of the present invention, and Figures 4 and 5 are experimental results. FIG. 6 shows another structural example of the ultrasonic transducer according to the present invention, and FIG. 7 shows still another structural example of the ultrasonic transducer according to the present invention. 1; container, 2; liquid, 3; piezoelectric material, 4°4a,
4b, 4a', 4b'; interdigital electrodes.

Claims (1)

[Claims] 1. A plurality of interdigital electrodes made of thin metal films are provided on one surface of a piezoelectric material in the longitudinal direction of the electrode fingers to form an electrode group, and the interdigital electrodes are composed of a plurality of parallel elongated electrode fingers. It has a structure in which a plurality of sets of comb tooth-like electrodes are provided, and the electrode fingers of each comb tooth-like electrode are arranged in an alternately overlapping manner, and the spacing between the electrode fingers is set so that the electrode fingers of the comb tooth-like electrodes are arranged in a liquid contacting with the piezoelectric material. An ultrasonic transducer characterized by being determined so as to converge an emitted ultrasonic beam. 2. The interdigital electrode is 3. Claim 1 characterized in that it is a phase interdiscicle electrode.
Ultrasonic transducer described in Section 1. 3. The ultrasonic transducer according to claim 1, wherein the interdigital electrode is a single-phase interdigital electrode. 4 The spacing between the electrode fingers in the interdigital electrode is r tongue = 1nλ (L + K2 n 2λ↑(rn is n
The horizontal distance between the electrode and the focal point, λf is the wavelength of the sound wave in the medium, L is the vertical distance between the electrode and the focal point, K1 and 2
is a constant).
Ultrasonic transducer described in Section 1. 5. The ultrasonic transducer according to any one of claims 1, 2, and 3, characterized in that it is covered with the interdigital electrode protective thin film. 6 Claims 1, 2, 3, or 4 in which a plurality of electrode groups are formed on the surface of a single piezoelectric material
The ultrasonic transducer described in Section.