JP2003185426A - Method and apparatus for measuring propagation time of ultrasonic waves - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus wherein the propagation time of ultrasonic waves inside a material to be measured is measured with high accuracy when an ultrasonic-wave reflected echo is in a low S/N ratio (a noise is large), and even when the attenuation of the ultrasonic-wave reflected echo is large. <P>SOLUTION: Ultrasonic pulses are generated in the thickness direction of the material 10, a surface displacement generated at a time when the ultrasonic pulses reach the surface of the material is detected by a Sagnac interferometer 50, two ultrasonic reflected echoes appearing in an output signal of the interferometer are mutually correlation-processed, and the propagation time of the ultrasonic waves is computed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a device for measuring the propagation time of ultrasonic waves in a material with high accuracy, and particularly to the case of applying the material to a material having a large attenuation of ultrasonic waves and the ultrasonic wave. The present invention relates to an ultrasonic wave propagation time measuring method and apparatus suitable for cases where the S / N ratio of a received signal is low.

[0002]

2. Description of the Related Art It is well known that the thickness, temperature, or elastic constant of a material can be obtained by propagating an ultrasonic wave in the material and measuring the propagation time thereof. That is, as shown in FIG. 1, when an ultrasonic wave having a sound velocity V is propagated in the thickness direction of a material to be measured (hereinafter, also simply referred to as material) 10 having a thickness d, the ultrasonic wave propagates in the thickness direction of the material. The time Δt required for one round trip propagation is expressed as Δt = 2d / V (1), and therefore, when the sound velocity V in the material is known, Δt
The thickness d can be determined by measuring t. Further, since the sound velocity V of the ultrasonic wave changes depending on the temperature of the material and the elastic constant of the material, if the relational expression between the temperature or elastic constant of the material and the sound velocity V is obtained in advance, the thickness d can be calculated as By measuring Δt, it is possible to obtain it by using the temperature of the material, the elastic constant, and the sound velocity. Measuring the propagation time of ultrasonic waves in a material in this way is a powerful method for non-destructively measuring the thickness, temperature, or elastic constant of the material.

The propagation time of the ultrasonic wave in the material is determined by detecting the multiple reflection echo of the pulsed ultrasonic wave generated in the material,
The method of measuring based on the arrival time difference of two reflected echoes therein is well known. That is, as shown in FIG.
The ultrasonic wave generating means 20 propagates a pulsed ultrasonic wave in the material 10 in the thickness direction of the material, and the ultrasonic wave receiving means (also used as the ultrasonic wave generating means in FIG. 1) returns to the surface of the material. When you receive the
As shown in FIG. 2, a plurality of ultrasonic reflection echoes B1, B2,
B3, ... Appears with a time delay. Here, reflected echoes B1, B2, and B3 are reflected echoes that have reached the surface of the material after propagating in the material for one round trip, two round trips, and three round trips, respectively. From these reflected echoes, the arithmetic processing means 32, for example, B1 echo and B2 echo as shown in FIG.
The propagation time in the material can be measured by measuring the arrival time difference Δt of the echo.

Arrival time difference Δt between two ultrasonic reflection echoes
As a method of obtaining (i), other than the method of obtaining the difference between the times at which the signal intensities peak as shown in FIG. Vol. 59, No. 567, p. 286, (ii)
The zero-cross method and (iii) cross-correlation method are known. The zero-cross method is a method of determining the arrival time of each echo as the time when the echo crosses the zero point instead of the peak arrival time. In the cross-correlation method, as shown in FIG. 3, for two ultrasonic reflection echoes, data in a region having a time width tw including each echo is cut out (the cut-out waveforms are x (t) and y ( t)), and a cross-correlation function Rxy (τ) defined by the following equation is calculated.

[0005]

[Equation 1]

When τ is calculated so that this Rxy (τ) becomes maximum, and the value is Δt _f , the propagation time difference between the two echoes is _{calculated as} Δt = Δt _c + Δt _f (3). Here, Δt _c is the time difference between the cutout regions of the two echoes, as shown in FIG.

[0007]

However, in the above-mentioned (i) peak time difference method and (ii) zero-cross method, as shown in FIG.
There is a problem that a large measurement error occurs when the amplitudes and waveforms of the two ultrasonic reflection echoes are different as shown in FIG. 4 or when the S / N of the ultrasonic reception signal is low (noise is large) as shown in FIG. It was For example, in a steel material having relatively large crystal grains, the attenuation of ultrasonic waves becomes large, and the amplitude between ultrasonic reflection echoes greatly changes as shown in FIG. Further, when the scattering of ultrasonic waves inside the material is large, or when measuring a material at a high temperature, it is unavoidable that the S / N decreases as shown in FIG.

On the other hand, the above (iii) cross-correlation method is shown in FIG.
Although the measurement error is relatively small for the ultrasonic wave reception waveform as shown in Fig. 5, the measurement error is large for the low S / N waveform as shown in Fig. 5, and the ultrasonic wave propagation time is highly accurate. There was a problem that it could not be measured.

The present invention solves the above problems, and can measure the propagation time of ultrasonic waves with high accuracy even when the ultrasonic waveform changes or the S / N of the ultrasonic reception signal is low. It is an object of the present invention to provide a method and an apparatus therefor.

[0010]

In order to solve the above problems, in the present invention, an ultrasonic pulse is generated in the thickness direction of a material, and the surface displacement generated when the ultrasonic wave reaches the material surface is Sagnac interference. The ultrasonic wave propagation time in the material is calculated by cross-correlating the two ultrasonic wave reflection echoes appearing in the output signal of the interferometer and measuring the ultrasonic wave propagation time. .

At this time, the difference ΔL between the lengths of the two arms of the Sagnac interferometer is C / (nf) <ΔL <2dC / (nV) (where C is the speed of light in vacuum, and n is the interferometer). It is preferable to satisfy the refractive index of the arm medium, f: ultrasonic frequency, d: material thickness, V: ultrasonic sound velocity.

Further, it is desirable that the length of at least one arm of the Sagnac interferometer can be changed.

[0013]

FIG. 6 shows the configuration of a first embodiment of an ultrasonic propagation time measuring device according to the present invention. In addition, a flow chart of the ultrasonic propagation time measuring method according to the present invention is shown in FIG.
Shown in. Hereinafter, the first embodiment of the present invention will be described with reference to FIGS. 6 and 7.

First, the ultrasonic wave generating means 40 propagates an ultrasonic wave pulse in the thickness direction of the material 10 to be measured (FIG. 7).
Step 100). A piezoelectric oscillator may be used as the ultrasonic wave generating means 40, but in order to make the most of the effect of the present invention, a pulse laser oscillator capable of generating a wider range of ultrasonic wave pulses. Is preferably used. For example, a pulse Y with a pulse width of several ns
Irradiating the surface of the material with an AG laser can generate very wideband ultrasonic pulses in the material.

The ultrasonic waves generated in the material in this manner are the front surface (the surface on the left side in the figure facing the ultrasonic wave generating means 40) and the rear surface (the right side in the figure opposite to the ultrasonic wave generating means 40) of the material 10. Multiple reflections between the surfaces. Therefore, the displacement of the material surface is detected by the Sagnac interferometer 50 at substantially the same position as the ultrasonic wave generation position on the material surface (step 110 in FIG. 7). Each time the ultrasonic multiple reflection echo reaches this detection position, the material surface is displaced (vibrated) by a minute amount, so that the ultrasonic multiple reflection echo as shown in FIG. 2 can be obtained.

Thereafter, the arithmetic processing means 32 cuts out two ultrasonic reflection echoes in the same procedure as in the conventional method,
By computing the maximum of these cross-correlation functions,
The propagation time of ultrasonic waves in the material to be measured is obtained (step 120 in FIG. 7).

Next, the Sagnac interferometer 50 used in the present invention will be described based on the configuration example of the Sagnac interferometer shown in FIG. The Sagnac interferometer is a type of optical interferometer and is often used in fiber gyroscopes. In this Sagnac interferometer 50, the laser beam emitted from the continuous wave laser 52 is branched into two arms by the coupler 54. Here, each arm is an optical fiber (fiber 5
6A and fiber 56B) will be described. The lengths of the fiber 56A and the fiber 56B are La and Lb, respectively.

The branched laser beams are combined by a coupler 58, and are irradiated onto the surface of the material 10 to be measured through an optical system such as a condenser lens 60.

The laser beam reflected by the material surface is
It is guided to the coupler 58 through the optical system (60) and is branched into two fibers 56A and 56B again. After that, they are again combined by the coupler 54, input to the photodetector 62, converted into an electric signal, amplified by the amplifier 64, and output.

When a low coherent laser is used as the laser 52, the photodetector 62 has an (I) coupler 54.
⇒ Fiber 56A ⇒ Coupler 58 ⇒ Condenser lens 60 ⇒
Surface of measured material 10 ⇒ condensing lens 60 ⇒ coupler 58
⇒ Beam a traveling along the path of fiber 56B ⇒ coupler 54, and (II) coupler 54 ⇒ fiber 56B ⇒ coupler 58 ⇒ condensing lens 60 ⇒ surface of material 10 to be measured ⇒
An interference signal with the beam b traveling on the path of the condenser lens 60 ⇒ coupler 58 ⇒ fiber 56A ⇒ coupler 54 is obtained.

When the surface of the material 10 to be measured vibrates due to the arrival of ultrasonic waves, for example, the document “Review of Progress in
Quantitative Nondestructive Evaluation, Vol. 1
5, Edited by D. O. Thompson and D. E. Chiment
i, Plenum Press, New York, 1996 ”, etc., since an output corresponding to ultrasonic vibration can be obtained from this photodetector 62, ultrasonic waves can be detected (received) in a non-contact manner. it can.

The present inventor has conducted extensive studies on an ultrasonic detection method using the Sagnac interferometer 50,
It has been found that when detecting pulsed ultrasonic waves, particularly wideband ultrasonic pulses, a peculiar interferometer output as shown in FIG. 9 is obtained. That is, when the ultrasonic pulse is a unipolar wideband pulse as shown in FIG. 9A, the output of the Sagnac interferometer becomes a bipolar pulse as shown in FIG. As the difference (La-Lb) is increased, a time delay t _d as shown in FIG. 9C occurs between the bipolar pulses. The value of this time delay t _d is Constituent medium (fiber 56 in this example)
Assuming that the refractive index of the core media of A and 56B is n, the speed of light in a vacuum is C, and the difference between the lengths of the two arms is ΔL (= La-Lb), then t _d = nΔL / C (4) .

The present invention is based on the above findings.
That is, since a bipolar pulse having a constant time delay t _d as shown in FIG. 9C can be easily separated from a random noise component, two echoes of the received ultrasonic multiple reflection echoes can be obtained. On the other hand, if cross-correlation calculation is performed as shown in FIG. 10, even if the received ultrasonic signal has a low S / N, or if the amplitude or waveform between echoes changes, the high It is possible to measure the propagation time with accuracy.

Next, a preferable condition of the arm length difference ΔL of the Sagnac interferometer 50 when constructing the ultrasonic wave propagation time measuring device according to the present invention will be described. When the time delay t _d as shown in FIG. 9B is small (ΔL≈0), it is not always easy to separate it from noise, so that FIG.
As described above, it is preferable that t _d is larger than the cycle of the ultrasonic pulse. On the other hand, as can be easily inferred from FIG.
The value of t _d is preferably smaller than the interval between adjacent ultrasonic reflection echoes. From the above, the preferable range of t _d is expressed by equation (5).

[0025]

[Equation 2] Where f is the frequency of ultrasonic waves, d is the thickness of the material to be measured, V
Is the ultrasonic velocity of sound in the measured material.

From the expressions (4) and (5),
A preferable difference ΔL between the arm lengths of the Sagnac interferometer 50 is as shown in equation (6).

[0027]

[Equation 3]

When the material to be measured has a large difference in thickness or sound velocity, a plurality of fibers having different lengths are prepared, and one arm of the Sagnac interferometer 50 (for example, fiber) is prepared according to the material to be measured. 56A) may be replaced by a fiber having an appropriate length. By doing so, a very versatile measurement system can be realized using a single Sagnac interferometer.

In the above description, the case where the ultrasonic wave generation position by the ultrasonic wave generation means 40 and the ultrasonic wave reception position by the Sagnac interferometer 50 are arranged on the same surface of the material 10 to be measured is described, but it is shown in FIG. As in the second embodiment, it is also possible to arrange the both on the opposite side of the measured material 10.

In measuring the ultrasonic wave propagation time,
Although the method of using the arrival time difference between the B1 echo and the B2 echo has been described, the combination of the two echos is not limited to this, and for example, the cross correlation calculation may be performed using the B1 echo and the B3 echo. Absent.

[0031]

EXAMPLES Examples of ultrasonic wave propagation time measurement according to the present invention will be described. In this example, as the ultrasonic wave generation means 40,
A pulse YAG laser with a pulse width of 5 ns and an output of 20 mJ was used. Also, Sagnac interferometer 50 for ultrasonic wave reception
Is a continuous wave semiconductor laser 52 having an output of 100 mW,
The one equipped with two arms 56A and 56B composed of optical fibers (core refractive index n = 1.48) and a photodetector 62 composed of a high-speed PIN photodiode was used. As the arithmetic processing means 32, the sampling rate 2
A high-speed A / D converter of 00 MS / s and a personal computer were used.
The material 10 to be measured was a carbon steel plate having a thickness of 1.0 to 2.6 mm, and the ultrasonic sonic velocity in this steel plate was 5930 m / s.

The ultrasonic pulse generated in the steel sheet by the above pulse laser irradiation has a center frequency of about 20 MHz.
It was a very wideband pulse.

In the present example, ΔL suitable for measuring a steel plate having a thickness of 1.0 mm is 10.1 m <ΔL <68.7 m (7) according to equation (6). ΔL suitable for measuring steel sheets
From equation (6), 10.1 m <ΔL <178.7 m (8) Therefore, as a condition applicable to all the steel plates in the thickness range of 1.0 to 2.6 mm, the difference ΔL in the arm length of the Sagnac interferometer 50 is set to 30 m.

Table 1 shows the results of measuring the ultrasonic wave propagation time of the above steel sheet by the method according to the present invention and the conventional method. The method according to the present invention is measured using the above method and apparatus. Further, the conventional method is a measurement using the above-mentioned pulse YAG laser, Michelson interferometer and cross-correlation treatment calculation.

[0035]

[Table 1]

The plate thickness in the table is a value measured with a micrometer, and the ultrasonic wave propagation time true value is a value calculated from this plate thickness and the speed of sound in the steel plate (5930 m / s). Since the output of the pulse YAG laser was relatively low in this example, the S / N ratio of the received ultrasonic waves was also low, and in the conventional method, it was observed that the ultrasonic wave propagation time deviated considerably from the true value, but according to the present invention. The method was in good agreement with the true value. From this result, the superiority of the method and apparatus according to the present invention was confirmed.

[0037]

As described above, according to the present invention, the ultrasonic wave propagation time in the material to be measured can be measured with high accuracy even when the S / N of the ultrasonic wave reception waveform is low. Further, if a pulsed laser is used as the ultrasonic wave generation means, it is possible to measure the ultrasonic wave propagation time in a non-contact manner, so that there is an effect that it can be applied to, for example, a high temperature material or a material that moves at high speed.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating the principle of ultrasonic wave propagation time measurement.

FIG. 2 is a time chart showing an example of an ultrasonic wave reception waveform in ultrasonic wave propagation time measurement.

FIG. 3 is a time chart explaining the principle of ultrasonic wave propagation time measurement by the cross-correlation method.

FIG. 4 is a time chart showing an example of an ultrasonic wave reception waveform when the amplitude and the waveform of the ultrasonic wave echo are changed.

FIG. 5 is a time chart showing an example of an ultrasonic wave reception waveform when the S / N is low.

FIG. 6 is a schematic diagram showing a configuration of a first embodiment of an ultrasonic propagation time measuring device according to the present invention.

FIG. 7 is a flowchart showing a processing procedure in the ultrasonic propagation time measuring method according to the present invention.

FIG. 8 is a schematic diagram showing a configuration example of a Sagnac interferometer used in the present invention.

FIG. 9 is a time chart showing an example of a reception waveform of an ultrasonic pulse by the Sagnac interferometer.

FIG. 10 is a time chart showing an example of an ultrasonic wave reception waveform at the time of ultrasonic wave propagation time measurement according to the present invention.

FIG. 11 is a second ultrasonic wave propagation time measuring device according to the present invention.
Schematic diagram showing the configuration of the embodiment

[Explanation of symbols]

10 ... Material to be measured 32 ... Arithmetic processing means 40 ... Ultrasonic wave generation means 50 ... Sagnac interferometer 52 ... Laser 54, 58 ... Coupler 56A, 56B ... Optical fiber 60 ... Condensing lens 62 ... Photodetector 64 ... Amplifier

Claims (4)

[Claims] 1. An ultrasonic pulse is generated in the thickness direction of a material, and a surface displacement generated when the ultrasonic wave reaches a surface of the material is detected by a Sagnac interferometer and appears in an output signal of the interferometer. An ultrasonic wave propagation time measuring method characterized in that the ultrasonic wave propagation time in a material is calculated by cross-correlating two ultrasonic wave reflection echoes. 2. An ultrasonic wave generating means for generating an ultrasonic wave pulse in a thickness direction of a material, a Sagnac interferometer for detecting a surface displacement generated when the ultrasonic wave reaches a material surface, and an output signal of the interferometer. An ultrasonic wave propagation time measuring device, comprising: an arithmetic processing unit for calculating an ultrasonic wave propagation time in a material by performing a cross-correlation process on two ultrasonic wave reflection echoes appearing in the inside. 3. The difference ΔL in length between the two arms of the Sagnac interferometer is C / (nf) <ΔL <2dC / (nV) (where C is the speed of light in a vacuum, and n is the interferometer arm). The ultrasonic propagation time measuring device according to claim 2, wherein the refractive index of the medium, f: ultrasonic frequency, d: material thickness, V: ultrasonic sound velocity) are satisfied. 4. The ultrasonic wave propagation time measuring device according to claim 2, wherein at least one arm of the Sagnac interferometer can be changed in length.