JPH045290B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an ultrasonic device for observing the surface or interior of an object using ultrasonic waves.

Ultrasonic imaging has recently attracted a lot of attention in fields such as medical diagnosis and non-destructive testing, such as ultrasonic observation devices for the inside of objects. Conventionally, there are various methods of generating and receiving ultrasonic waves used in devices for this purpose, including those using acoustic phase plates, annular arrays, and acoustic lenses. A so-called interdigital transducer has excellent characteristics for emitting and receiving ultrasonic waves.

A transducer has an interdigital combination of a pair of comb-shaped electrodes on a piezoelectric surface, and an alternating current signal is applied to the electrode while the electrode surface is in contact with a liquid. As a result, ultrasonic waves are emitted into the liquid, or acoustic waves propagating in the liquid are received and converted into electrical signals.

Here, we will explain the excitation of underwater ultrasound from an IDT (interdigital transducer) using a simple wave source model.
ASPW ( Angular Spectrum of P lane W
aves) method.

For simplicity, consider one period of the IDT as a line wave source with a δ function. Take the coordinate axes as shown in Figure 3,
N pairs of normal form IDTs are expressed as x _o (n=1 to N,
x _{o +1} − x _o = d). Hereinafter, a phenomenon that is uniform in the z direction will be treated.Each line wave source excites a leaky surface wave along the substrate surface, and the amplitude of the scalar component is expressed by the following equation.

u _po (x)=U(x-x _o ) ・exp{-jk _xs (x-x _o )} (1) Here, U(x) is the unit step function, and k _xs is the wave number of the surface wave. , where V _s is the surface wave velocity, k _xs = ω/
given by V _s . Surface waves are radiated in both the positive and negative directions of the x-axis, but for simplicity, only the positive direction is assumed. Longitudinal waves are radiated into the water according to equation (1). Amplitude u(x,y) at point (x,y) in water
uses the Fourier transform A _o (k _x ) of u _po (x), and n
= calculated by adding up the contributions of 1 to N wave sources. That is, u(x, y) _N 〓 ⁿ⁼¹ ∫ ^∞ _-∞ A _o (k _x ) ・exp{−j(k _x x + k _y y)}dk _x (2) A _o (k) = 1/2π・[πδ(k _x −k _xs ) +1/j(k _x −k _xs )] ・exp(jk _x x _o ) (3) However, k _x ² + ky ² = k _p ² , k _p = ω/V _L (4) V _L is the longitudinal wave velocity in water.

u in equation (2) is a scalar quantity, and considering the boundary conditions at the interface, it is appropriate to consider it as the y component of particle velocity, sound pressure, etc. However, since the relationship between the quantities excited by IDT is unknown, we did not treat the boundary conditions accurately here, and assumed that the Scully quantity in equation (1) is transmitted directly into the water.

By substituting equation (3) into equation (2) and calculating, equation (5) is obtained.

u (x, y)∫ ^∞ _-∞ A _s (k _s ) A _r (k _x ) exp {−j (k _x x + k _y y)} (5) A _s (k _x・1/2・δ(k _s −k _xs ) +1/j2π(k _x −k _xs ) (6) A _r (k _x ) =exp{j・k _x d/2・(N−1)} ・sin(k _x d/2・N)/sin(k _x d/2) (7) Equation (5) shows that u(x, y) is k _x spectrum
This shows that it is expressed as a composite of various plane waves with amplitudes A _s and A _r . A _s (k _x ) is a spectrum caused by a _surface wave excited stepwise from a single line wave source, and has _a line spectrum _{at k} ) has a size of A _r (k _x ) is a factor representing the effect of arranging N wave sources at equal intervals, and reaches a maximum at k _x =2mπ/d (m is an integer). Note that d is the electrode period of the interdigital electrode. That is, u(x,
y) has a large contribution of plane waves with k _x equal to k _xs and 2mπ/d. Now, if k _xs and 2mπ/d are sufficiently far apart, ultrasonic beams will be emitted from the IDT in the two directions of k _x .

The radiation angles of the two types of ultrasonic beams are determined from the wave number locus of equation (4) and wave number matching in the x direction. As shown in FIG. 4, the wave number locus is a semicircle with radius ω/V _L.
A perpendicular line is drawn from the point k _x = k _xs and k _x = 2π/d, and an ultrasonic beam is emitted in a direction that intersects the locus. If we define the directions as θ _d and θ _s as shown in Figure 4, we get the following (θ _d = 90°−
θ ₁ , θ _s = 90°−θ ₂ ). Note that f is the carrier frequency of the AC signal applied to the interdigital electrodes on the piezoelectric body.

θ _d = cos ^-1 (ω/V _s / ω/V _L ) = cos ^-1 (V _L /V _s ) (8) θ _s = cos ^-1 (2π/d/ω/V _L ) = cos ^{- 1} (V _L /f・d) (9) In equations (8) and (9), V _L = 1500 m/s, V _s = 2350 m/s,
The solid line in FIG. 5 plots the frequency dependence by substituting the value 87 μm for d of the normal form IDT. It can be seen that equations (8) and (9) correspond well to the two regulated radiation angles.

As described above, the ultrasonic beam is radiated into the liquid of the transducer in a blind-like manner, and the receiving direction θ
(θ is the angle formed with the normal to the surface of the piezoelectric material) has two directions, and satisfies the following relationship.

θ ₁ = sin ^-1 V _L /V _s (10) θ ₂ = sin ^-1 V _L / fd (11) As is clear from equations (10) and (11), θ ₁ is constant regardless of frequency. and θ ₂ changes with frequency. Then, when V _s = fd, θ ₁ = θ ₂ , but otherwise there are two radiation directions of the ultrasonic beam.

An object of the present invention is to provide an effective means for observing the state of an object by using such ultrasonic beams emitted in two directions.

A feature of the present invention for achieving this object is that a plate-shaped piezoelectric body having interdigital electrodes on one surface is arranged so that the surface having the interdigital electrodes is in contact with a liquid, and f≠V _s /d is applied to the electrodes. (where f is the excitation frequency, V _s is the surface wave velocity on the piezoelectric material, and d is the electrode period of the interdigital electrode), an ultrasonic beam is generated in two directions in the liquid by applying an AC signal, and one of the two directions is The ultrasonic beam in one direction is focused on the surface of the observed object, and the ultrasonic beam in the other direction is focused on the inside of the observed object, and the first reflected wave reflected from the surface of the observed object and from the inside of the observed object are The second reflected waves are both received by the interdigital electrode, and the phase difference between the first and second reflected waves is detected using the first reflected wave as a reference. The ultrasonic device obtains an acoustic image of the object to be observed based on changes in elastic properties inside the object.

Examples will be described below with reference to the drawings.

FIG. 1 shows an embodiment of an ultrasonic device according to the present invention.
FIG. 2 shows a specific example of the electrode structure. Reference numeral 1 in the figure is a plate-shaped piezoelectric material whose thickness is the thickness at which surface waves are excited, specifically several times, preferably five times or more, the wavelength of the surface waves. On one surface of the piezoelectric body 1, interdigital electrodes 2, as shown in FIG.
3 can be made. Each electrode is constructed by interdigitally combining a pair of arc-shaped comb-tooth electrodes 2a and 2b, 3a and 3b, and one electrode 2 functions as an input, and the other electrode 3 functions as an output. do. It is clear that since the electrode configuration is arc-shaped, the focal point of the ultrasound beam will be on a vertical line passing through the center of the arc.

The transducer having the above configuration is arranged with the electrodes 2 and 3 in contact with the liquid 4. In this state, input electrode 2
If an AC signal with a frequency f ₂ other than the center frequency f ₁ (at the center frequency satisfies V _s = fd and the radiation direction of the ultrasonic beam is single) is applied to θ ₁ as shown in Figure 1, An ultrasonic beam is emitted in the _θ2 direction. The broken line propagation path in the figure indicates the above equation (10), and the solid line indicates the above equation (11). Therefore, it is clear that if the carrier frequency of the AC signal is set to _f1 , the beam will be radiated in a single direction along the broken line.

The ultrasonic beam in each direction (θ ₁ , θ ₂ ) is at point P ₁ and point P ₂
They each converge.

If the observation object 5 is arranged as shown in Fig. 1 while the ultrasonic beam is focused on points P ₁ and P ₂ , the ultrasonic beam in the radiation direction θ ₁ will be focused on the surface of the object 5 (point
P ₁ ) produces a reflected wave. On the other hand, the beam in the radiation direction θ ₂ is refracted, enters the interior of the object, and is focused at point P ₂ , producing a reflected wave. Although reflected waves from other points also exist, in this embodiment, attention is focused only on the reflected waves from point _P2 .

These reflected waves are received by the output electrode 3 as shown in FIG. 6b and extracted as electrical signals. At this time, the reflected wave received by the output electrode 3 has a delay time difference because there is a path difference due to the deviation in two directions of θ ₁ and θ ₂ . In other words, as shown in FIG. 4, two reflected waves in angular directions θ ₁ and θ ₂ reflected at points P ₁ and P ₂ shown in FIG. There is a temporal propagation time difference between the delayed output signals due to the two reflected waves.
When the signal output from the interdigital electrode 3 is monitored using an oscilloscope or the like, it is clearly seen that there is a temporal propagation time difference between the two reflected waves.

Therefore, the signal output from the interdigital electrode 3 is amplified and monitored by an oscilloscope, etc., so that two reflected signals are displayed, and the gate operation is performed according to each signal as shown in Figure 6c. For example, a sample and hold circuit is connected to the output terminal of the interdigital electrode 3. Therefore, the two reflected waves having the above-mentioned temporal propagation time difference can be extracted separately through such a sample-and-hold circuit, for example.

Therefore, by measuring the phase of the other beam using one reflected wave as a reference, the state inside the object 5 can be extracted as a change in elastic properties using the phase difference between the reflected waves of each beam in the two directions. becomes possible. That is, the propagating sound waves are affected by changes in acoustic impedance due to cracks inside the solid or differences in composition, and accordingly, a corresponding change occurs in the phase difference between the reflected waves in the θ ₁ and θ ₂ directions. Therefore, it is possible to obtain an acoustic image of the object 5 using the output signal of the electrode 3. For example, if the output signal is configured to be displayed on a CRT, the inside of the object can be observed with the naked eye.

In this embodiment, as is clear from FIG. 1, since the ultrasonic beam from the transducer is obliquely incident on the object to be observed, the proportion of the acoustic wave that can be transmitted into the object is much greater than when the ultrasonic beam is incident perpendicularly. Therefore, it is convenient to know not only the surface of the observed object but also the internal situation. Furthermore, since input and output are performed using separate electrodes, there is no need for a directional coupler for separating input and output signals, and since the transducer has a planar structure, there is a large degree of freedom in electrode design.

Further, although a transducer for surface wave excitation is used in this embodiment, it is of course possible to use a Lamb wave. In the case of Lamb waves, the thickness of the piezoelectric body should be approximately λ (λ is the wavelength of the sound wave on the piezoelectric body) or less, and if the surface wave velocity V _s mentioned above is replaced by the Lamb wave velocity, Equation (10) and Equation (11) also holds true.
Although this configuration has the advantage that the interdigital electrode can be used without coming into contact with liquid, it has a difficulty in increasing the frequency, and is therefore suitable for non-destructive testing in a relatively low frequency range.

In the configuration of this example, the interdigital electrode vibrates in direct contact with the liquid, so mechanical and chemical protection is required, but this can be easily done by creating a protective film on the electrode surface using, for example, a photoresist film. can be done. Furthermore, when using a combination of a piezoelectric thin film such as ZnO and a non-piezoelectric substrate instead of using a single piezoelectric material, it is possible to provide interdigital electrodes between the thin film and the substrate, and therefore it is possible to separately This has the advantage that there is no need to take any protective measures.

In order to confirm the effectiveness of the present invention described above,
TDK piezoelectric ceramic 91A material (length 20mm, width 20mm,
A transducer was constructed by providing arc-shaped interdigital electrodes on a plane perpendicular to the thickness of 5 mm. Here, the polarization axis is parallel to the thickness,
The electrode period is 210 μm, the number of electrode pairs is 5, the distance between the two sets of arcuate interdigital electrodes is 10 mm, and the aperture length is 70°. In this case θ ₁ is 45°, surface wave velocity
V _R is 2146 m/sec, which agrees with the value obtained by substituting the sound velocity in water of 1497 m/sec at a temperature of 25°C into equation (10).
An acrylic plate with holes near the surface was used as the observation object, and during actual observation, this observation object was
Scan in dimension. Based on the above specifications, the transducer and acrylic plate were placed underwater as shown in Figure 1, and the carrier frequency was varied. As a result, it was possible to observe changes in the output signal corresponding to the presence of holes inside the acrylic plate.

As explained above, according to the present invention, by using ultrasonic beams emitted in two directions, it is possible to observe the internal state of an optically opaque object, which is suitable for non-destructive inspection, etc. It is possible to provide an ultrasonic device. In addition, in the present invention, by changing the frequency, it is possible to adjust θ ₂ while keeping θ ₁ constant and move point P ₂ in Fig. 1, so it is not only possible to observe a single point inside an object. , it is possible to observe the inside of an object over a wide range.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of an ultrasonic device according to the present invention, FIG. 2 is a diagram showing a specific example of an interdigital electrode, and FIG. 3 is a diagram showing a surface wave leaking from a line wave source.
Figure 4 shows the underwater ultrasound excitation model, Figure 4 shows the wave number matching at the solid-liquid interface, Figure 5 shows the frequency dependence of the underwater ultrasound radiation angle, and Figure 6 shows the ultrasound according to the present invention. FIG. 2 is a diagram showing a transmitting/receiving signal waveform converted by an interdigital electrode for transmitting and receiving waves in the device and two signal waveforms via a gate circuit. 1... Piezoelectric body, 2, 3... Interdigital electrode, 4... Liquid, 5... Observation object.

Claims (1)

[Claims] 1 Place the surface of a plate-shaped piezoelectric body having interdigital electrodes on one surface in contact with a liquid, and apply f≠V _s /d to the electrodes.
(f is the excitation frequency, V _s is the surface wave velocity on the piezoelectric material,
Ultrasonic beams are generated in two directions in the liquid by applying an alternating current signal (d is the electrode period of the interdigital electrode), and the ultrasonic beam in one of the two directions is applied to the surface of the observed object, while the ultrasonic beam in the other direction is applied to the surface of the observed object. The beams are respectively focused inside the observation object, and the first reflected wave reflected from the surface of the observation object and the second reflected wave reflected from the inside of the observation object are both received by the interdigital electrode. By detecting a phase difference between the first and second reflected waves using the first reflected wave as a reference, the observation as a change in the elastic properties inside the observation object is performed based on the phase difference. An ultrasonic device characterized by obtaining an acoustic image of an object.