JPH0635931B2 - Single-mode ultrasonic wave generation method and object thickness measurement method using it - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a single mode supersonic device used for nondestructive inspection of materials, processing / processing of materials by ultrasonic waves, and driving of motors / actuators by ultrasonic waves. The present invention relates to a method of generating a sound wave, and more specifically, for an object in which multiple vibration modes can exist and a single mode ultrasonic wave is difficult to generate,
The present invention relates to a method for generating a single mode ultrasonic wave that is easy to control and analyze.

Furthermore, the present invention relates to a method for directly measuring the dimension of an object, for example, the thickness of a plate, by directly using this method.

[Prior Art] In order to generate a single mode ultrasonic wave in an object through which ultrasonic waves of multiple modes propagate, a frequency range in which a specific mode is dominant is selected. However, this method is not effective when there are a plurality of modes having the same frequency and close propagation speeds.

In addition, there is a method in which an ultrasonic wave is obliquely incident on an object from a direction of a critical angle for exciting the ultrasonic wave in a required mode, and the ultrasonic wave in the mode is generated. In this case, it is necessary to bring a liquid or solid wedge into contact with the object, which makes it difficult to apply it to high-temperature, high-speed moving objects such as plates during hot rolling, and objects in vacuum such as thin films during vapor deposition. is there.

[Problems to be Solved by the Invention] The technical problem of the present invention is to provide a method for generating a single mode ultrasonic wave without touching an object, and further, by directly using the method, the thickness of the object can be increased. The purpose is to obtain a method for easily measuring the height.

[Means and Actions for Solving the Problems] An ultrasonic wave generation method of the present invention for solving the above problems is an object in which a plurality of ultrasonic wave modes capable of propagating exist in an object in which an energy beam of a required mode Focusing and irradiating with a width of about the wavelength of the sound wave, and scanning the irradiation position at a speed equal to the phase speed of the ultrasonic wave in the mode, thereby selectively and efficiently generating the ultrasonic wave in the mode. It is what

Further, the method for measuring the thickness of an object based on the present invention is characterized by measuring the frequency of the ultrasonic waves generated by the above method, and measuring the thickness of the object from the relationship between the frequency and the phase velocity. is there.

Hereinafter, the method of the present invention will be described more specifically.

In many cases, various structures are allowed to propagate ultrasonic waves having various wave modes. For example, a plate has a mode in which the distribution of displacement is symmetric with respect to the center line and a mode that is asymmetric with respect to the center line, and a bar has modes such as bending, twisting, and stretching. The ultrasonic waves in these modes are also used for non-destructive inspection of the shape of structures, materials and defects, signal processing by interaction with light, processing of materials, and driving. But,
Usually, many modes coexist in a place near the source of ultrasonic waves, the wave field is complicated, and measurement and control are often difficult. Therefore, there is a demand for a method of generating single-mode ultrasonic waves even near the source.

The principle of the method of the present invention for generating such a single mode ultrasonic wave will be described below with reference to the drawings.

Different modes of ultrasonic waves propagating through an object have different shapes of dispersion curves representing phase velocities as a function of frequency. Now, assume that there are two modes having a dispersion curve as shown in FIG. In the usual method of generating ultrasonic waves, a sound source that vibrates at a constant frequency F is brought into contact with an object or is incident via a liquid. That is, the frequency on the horizontal axis of FIG. 1 is defined first. As a result, when the frequency F is a value as shown in FIG. 1, and mode A phase velocity V _A, the mode B of the phase velocity V _B occur at the same time. When the ultrasonic duration generated is T, in the post-emergence _{TV 'A / (V' A} -V 'B) about time it is difficult to control the measurement overlap mode A and B. Here, V _'A and V' _B is the group velocity of the mode A and B.

However, by reversing the relationship of FIG. 1, if determining ultrasound velocity which occurs first, in the case where the speed was V _A mode A
Only mode B occurs when the speed is V _B. In this case, even if there is another mode outside the frequency range shown in the figure and the ultrasonic waves are generated at the same time,
The observed waveform can be easily removed by frequency filtering, and substantially single mode ultrasonic wave generation can be realized.

In the method of the present invention, in order to realize this velocity selection, the energy beam of the laser or the like is focused on a width of about the wavelength of the ultrasonic wave of a required mode previously calculated and is irradiated to the object. Is scanned at a speed equal to the phase velocity of the ultrasonic wave of the mode, the displacement of the ultrasonic wave of the mode is sequentially superimposed while maintaining the phase matching condition, and the ultrasonic wave of the mode is selectively amplified. That is, a single mode ultrasonic wave is generated by the acoustic velocity synchronous scanning of the energy beam. The displacements of the ultrasonic waves in other modes are superposed on each other with their phases shifted, and consequently they are eliminated.

As the energy beam used in the present invention, infrared rays, ion beams and the like can be used in addition to the above laser.

It is also known that the shape of the dispersion curve changes depending on the typical size of the object. For example, in the case of a plate, if the material is constant, the product FD of the frequency F and the thickness D is uniquely determined by calculation as a function of the phase velocity V. Therefore, if the frequency F of the ultrasonic wave generated in synchronization with the phase velocity V is measured, the plate thickness can be easily measured as D = (FD) / F.

Example FIG. 2 shows an example of an apparatus for carrying out the method of the present invention.

In this device, a pulse beam from a laser 1 is focused into a rectangular shape by a collimator 2 and then is irradiated onto a surface of an object 5 via a beam deflector 3 and a lens 4.
The beam deflector 3 is for manipulating the irradiation beam on the surface of the object. By triggering the laser 1 with a synchronization signal from the beam deflector 3, a laser pulse moving at high speed on the object 5 can be obtained. You can

When the rotating polygon mirror as shown in the figure is used as the beam deflector 3, the rotation speed is changed so that the sweep speed of the beam on the object 5 is controlled to be a value close to the phase speed of the ultrasonic wave in the required mode. be able to. Further, the width of the beam may be controlled to a predetermined value by changing the focal length of the lens and the distance to the object.

The beam deflector 3 includes an acousto-optic deflector and 50 kHz.
It is also possible to use a microvibration mirror that can be driven at the above high frequencies. When an infrared ray or an ion beam is used as the excitation source, the beam focusing device and the beam deflector may be replaced with appropriate ones.

Figures 3 (a) and 3 (b) show the results of single-mode ultrasonic waves generated by selecting one of the symmetrical and asymmetric fundamental modes of the plate wave of a 1.5 mm thick aluminum plate. Is. The figure (a) shows the waveform of the generated signal when the laser beam is scanned at various phase velocities, and the figure (b) shows the relationship between the frequency F and the scanning rate V obtained by spectrum analysis of this waveform. Show. This relationship between velocity and frequency is almost in agreement with the calculated value (solid line) of the dispersion curve of the plate wave, demonstrating that two adjacent modes of the plate wave could be selectively generated by the method of the present invention. ing.

Although the method and apparatus for sweeping a rectangular beam in one direction have been described above, the same principle can be realized by contracting the annular beam at high speed.

FIG. 4 shows an example of the configuration of a device that realizes it.
The pulse beam from the laser 11 passes through the variable focus lens 12 and the conical lens 13 and is focused on the surface of the object 14 in an annular shape and irradiated. The varifocal lens 12 is formed of, for example, a material whose refractive index is changed by an electric field due to an electro-optical effect, and by operating the laser 11 and the varifocal lens 12 with a synchronization signal from the synchronization signal generator 15, a speed V can be obtained. An annular beam that contracts can be obtained.

With this method, a single-mode ultrasonic wave with an extremely large amplitude can be obtained at the center of the annulus, and not only measurement but also ultrasonic driving and processing can be expected.

[Effects of the Invention] According to the method of the present invention described in detail above, the effects listed below can be expected.

(1) Since the generated ultrasonic wave propagates as a single-mode ultrasonic wave immediately after the generation, it is easy to measure and control.

(2) The plate thickness can be measured from the value of the product of the frequency and the plate thickness, which is theoretically determined from the measured value of the frequency of the generated ultrasonic wave and the scanning speed.

(3) Since it generates a single-mode ultrasonic wave without contacting the object, it can be applied to high-temperature, high-speed moving objects such as plates during hot rolling and objects in vacuum such as thin films during vapor deposition. Is.

(4) Since the generated ultrasonic waves propagate in the beam scanning direction, the directivity is extremely good.

(5) Since the beam irradiation to the surface of the object is dispersed, the energy density on the surface of the object can be reduced, which has an effect of preventing damage to the object. In addition, the phase-matched waves become strong ultrasonic waves as a result of superposition, and signal detection is easy.

[Brief description of drawings]

FIG. 1 is a graph showing modes of a dispersion curve of two modes for explaining the principle of the ultrasonic wave generation method according to the present invention, FIG. 2 is a configuration diagram of an example of an apparatus for carrying out the present invention, and FIG. a) (b)
Shows the measurement results when the present invention is applied to a plate.
(a) is a waveform diagram of the generated signal when the laser beam is scanned at various phase velocities, and (b) is a graph showing the relationship between the frequency and the scanning velocity obtained by spectrum analysis of this waveform. FIG. 4 is a block diagram of another device example. 1,11 …… Laser, 3 …… Beam deflector, 5,14 …… Object, 12 …… Variable focus lens.

Claims (2)

[Claims] 1. An object in which a plurality of modes of ultrasonic waves that can propagate can exist and an energy beam is focused and irradiated to a width of a wavelength of ultrasonic waves of a required mode, and the irradiation position is ultrasonic waves of the mode. A method for generating single-mode ultrasonic waves, characterized in that the ultrasonic waves of the mode are selectively generated with high efficiency by scanning at a speed equal to the phase speed of. 2. A method for measuring the thickness of an object, which comprises measuring the frequency of the ultrasonic wave generated by the method according to claim 1 and measuring the thickness of the object from the relationship between the frequency and the phase velocity.