JPH08122308A - Ultrasonic flaw detection method - Google Patents

Abstract

(57) [Abstract] [Purpose] In an ultrasonic flaw detection method using pulse compression technology, an optimal waveform that provides excellent distance resolution when detecting defects in the object and high measurement accuracy when measuring thickness. Ultrasonic flaw detection method for generating a transmission waveform composed of frequency-modulated waves of. In an ultrasonic flaw detection method in which a chirp wave having a predetermined waveform is transmitted to and received from a subject 5 via a probe 4, and the received signal is subjected to pulse compression processing, an electrical signal of the probe is previously stored. A frequency-amplitude characteristic in mutual conversion with an acoustic signal is obtained, and a correction component that compensates for waveform deterioration that occurs based on the frequency-amplitude characteristic of the probe in a received signal from the subject is added to a chirp wave to be transmitted in advance. What has the FM signal setting part 2 and the FM signal transmission part 3 which generate | occur | produce the added transmission waveform.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for non-destructively inspecting the inside and outside of a material using ultrasonic waves or measuring the thickness of the material.

[0002]

2. Description of the Related Art FIG. 8 is a functional block diagram of a conventional general ultrasonic flaw detector. In FIG. 8, reference numeral 21 is a synchronization unit that generates and outputs a synchronization signal necessary for each circuit, and 22 is a synchronization unit 21.
A transmitting unit that generates a transmission electric signal based on the output signal from the transmitting unit 22 generates an ultrasonic wave based on the transmission electric signal from the transmitting unit 22 and causes the ultrasonic wave to enter the inside of the subject 24. A probe 25 receives an echo from the inside of the sample and converts the echo into an electric signal, a receiving unit 25 amplifies the electric signal from the probe 23, and a display unit 26 displays an output signal from the receiving unit 25.

When the inside and outside of a material is inspected using ultrasonic waves, a device as shown in FIG. 8 is generally used. In the apparatus configured as described above, a pulse wave is generally used as the transmission signal waveform, and the pulse wave has a sharp waveform and the width of the pulse in the time axis direction is short, so that the time axis resolution is excellent. However, when detecting a material with coarse particles, a reflection echo (called a forest echo) from the boundary of the particles of the material is generated, which becomes noise and mixes with the reflection echo from the defect inside the material. In some cases, the signal-to-noise ratio (SN ratio) of the received signal deteriorates and it becomes difficult to detect flaws.

As a method for reducing this forest echo, the frequency or waveform of the ultrasonic waves entering the material is selected,
A method for adjusting the SN ratio of the received signal to be optimum is known, but when a pulse wave is used for the transmitted signal, the frequency and waveform of ultrasonic waves to be transmitted and received depend on the characteristics of the probe. That is, it depends on the frequency-amplitude characteristic (generally referred to as an amplitude spectrum) in the mutual conversion between the electric signal and the acoustic signal which the probe performs when transmitting and receiving.

FIG. 9 is a diagram showing a pulse waveform with an infinitely small pulse width ((a) in the same figure) and its amplitude spectrum ((b) in the same figure), and FIG. 10 shows a search by pulse wave excitation. It is a figure which shows the received waveform ((a) of the same figure) of a tentacle, and its amplitude spectrum ((b) of the same figure). The transmission pulse wave as an electric signal has various frequency components ((b) in the same figure) so that it becomes an amplitude spectrum of a wider frequency band as the pulse width of the pulse waveform in (a) of FIG. 9 becomes narrower, for example. On the other hand, the amplitude spectrum of the probe has a frequency-amplitude characteristic corresponding to the conversion efficiency in mutual conversion between an electric signal and an acoustic signal, as shown in (b) of FIG. Therefore, the frequency and the waveform of the ultrasonic wave when the probe is excited to transmit and receive the ultrasonic wave are different depending on the type of the probe. Therefore, in such a case, the person who performs the ultrasonic flaw detection selects a suitable one from a plurality of types of probes based on his intuition and experience, and optimally controls the waveform of the ultrasonic wave for use. There was a problem I had to do.

As a solution to the problem that the optimum probe needs to be selected from among the plurality of probes, for example, an ultrasonic flaw detector disclosed in Japanese Patent Publication No. 3-43586 has been proposed. FIG. 11 is a block diagram of the conventional ultrasonic flaw detection equipment disclosed in the above document. In FIG.
21 to 26 are the same as those shown in FIG. Two
Reference numeral 7 denotes a synchronizing unit 2 according to a control signal from the frequency setting unit 28.
1 is a frequency variable circuit for varying the frequency generated in synchronization with the output signal from 1, a frequency setting unit 28 outputs a control signal to the frequency variable circuit 27, and 29 is a frequency setting circuit according to the control signal from the wave number setting unit 30. A wave number changing circuit for changing the wave number of the output signal of the changing circuit 27, and 30 for the wave number changing circuit 2
9 is a wave number setting unit that outputs a control signal to the signal generator 9. In the device having the above configuration, the amplitude spectrum of the transmission wave, that is, the frequency range included in the transmission wave can be controlled by variably setting the transmission frequency and the transmission wave number that are optimal for flaw detection of the test material. There is no need to selectively replace the tentacles.

However, the transmission waveform excited by the ultrasonic flaw detector of FIG. 11 is generally called a burst wave. Since a plurality of waves are transmitted, the transmission time is longer than that of a pulse wave. FIG. 12 is a diagram showing a comparison between a burst wave and a pulse wave, which are general ultrasonic transmission waveforms. When the burst wave of FIG. 12A is used, since the transmission energy is large, a sufficient reception signal is generated. On the other hand, there is a problem in that the time-axis resolution becomes worse because the width in the time-axis direction becomes longer than that of the pulse wave shown in FIG.

As a technique for improving the time-axis resolution while keeping the transmission pulse width wide, there is a technique called pulse compression known in the field of radar (Radar hand).
book, Skolnik et. , McGraw-H
ill Inc. , 1970). FIG. 13 is an explanatory diagram of the linear frequency modulation pulse compression radar. In FIG. 13A, the frequencies are changed from f ₁ to f within the transmission time T (equal to the transmission pulse width) from time t ₁ to t _2. indicates that the frequency modulation (FM) in a straight line up to _2, shows the waveform of the (b) thus linearly frequency modulated wave (called a chirp wave). As an example of the pulse compression radar, a frequency-modulated wave as shown in (b) of FIG. 13 is transmitted as a transmission wave, and a cross-correlation calculation process is performed between the reception waveform and the waveform used for the transmission to obtain the reception wave. While compressing the pulse width in the time axis direction to obtain a waveform with sharp amplitude,
The SN ratio of the received signal is improved. FIG. 13C shows the signal waveform after the cross-correlation calculation process.

An example of a document to which the pulse compression technique is applied in the field of ultrasonic waves is an ultrasonic diagnostic pulse compression device disclosed in Japanese Patent Laid-Open No. 63-233369. In this device, in order to pulse-compress an ultrasonic echo signal, a cross-correlation process is performed between the complex number signal and the reference wave signal via the quadrature detection means. In general, the two functions f
The correlation function s (τ) as a result of performing the correlation calculation of ₁ (t) and f ₂ (t) is defined by the following equation (1).

[0010]

[Equation 1]

The above formula (1) is a correlation calculation formula when f ₁ (t) and f ₂ (t) are continuous functions, and recently, an analog received signal is digitally converted through an A / D converter. Since it is often processed as a discretized signal of the form (sampled discontinuous signal), the correlation calculation equation in this case is shown in the following equation (2). Note that N in the equation (2) is the total number of samples.

[0012]

[Equation 2]

The operation of the equation (2) can be generally realized by an FIR (finite impulse response) digital filter having a plurality of delay units and multipliers. FIG. 14 is a diagram showing a configuration example of the FIR digital filter, in which the + sign indicates an adder,
The mark X indicates a multiplier, and Z ^-1 indicates a delay device. Each delay device delays the input signal by a time corresponding to the repetition cycle of transmission and outputs the delayed signal. FIG. 15 is a waveform diagram for explaining the operation of FIG.

In the digital filter shown in FIG. 14, the received waveform x (τ) discretized into a digital signal and the reference waveform for performing the correlation calculation are sampled (discretized) at a certain sampling frequency. In the example, each discretized data value is input to one of the multipliers indicated by x as 128 c _{0 to} c ₁₂₇ . On the other hand, the discretized reception data x (τ) input from the input end in each transmission cycle
Is directly supplied to the other input of each multiplier and individually multiplied with the reference data c _{0 to} c _127. Each multiplication result except the multiplication result with c ₁₂₇ is 127 delay units and adders. And are alternately supplied to one of the inputs of the corresponding adder connected in series. Then, only the result of multiplication with c ₁₂₇ is directly supplied to the leading delay device which is alternately connected in series, and the other input of the adder which is connected in series after this delay device is multiplied with c _{12 6.} The results are provided. Then, the output of the last adder of the series combination becomes the correlation calculation output. The above calculation result is shown in Expression (3).

[0015]

(Equation 3)

The FIR digital filter having the above-mentioned structure is capable of sequentially inputting the discretized data X from the input end.
The convolution operation represented by the following equation (4) is performed on (τ) and the discretized data c (i) of the reference waveform, and y (τ) of the operation result is obtained. In the expression (4), the signal time is reversed to reverse the direction of the signal and then the time is shifted, but in the expression (3), the direction of the signal is changed to the forward direction and the time is shifted. By rearranging the arrays c _{0 to} c ₁₂₇ of the discrete data of the reference waveform in the opposite direction,
Equivalent processing can be performed.

[0017]

[Equation 4]

FIG. 15 is a waveform diagram for explaining the operation of the digital filter shown in FIG. 14. In FIG. 15, after the frequency modulated wave is transmitted, the reception waveform obtained sequentially as time passes is divided into time τ to τ + 127. , Showing the temporal change of the correlation output which is the calculation result obtained by sequentially performing the convolution calculation (equivalent to the calculation of the similarity between two waveforms) between each of the divided reception waveforms and the reference waveform from the beginning of the reception period. There is.
Since the received echo waveform is now obtained approximately at the center of the time axis, the reference waves are sequentially shifted in time, and when the phases of the received echo wave and the reference wave match (at approximately the center of the time axis in FIG. 15). ), The correlation output of the maximum peak is obtained.

FIG. 16 is an explanatory diagram of pulse compression processing of a chirp wave cut out by a rectangular window. In the ultrasonic flaw detection method using the conventional pulse compression technique as described above, for example, as shown in (a) of FIG. 16, a predetermined transmission time width (5 μs in this example) has a uniform signal amplitude and a predetermined amplitude. The frequency-modulated (FM) signal that has undergone only the frequency transition (1 to 9 MHz in this example) is used as the transmission wave and also as the reference wave for performing the correlation calculation with the reception wave. In the case of the waveform after pulse compression by the cross-correlation calculation of the transmission wave and the reference wave, many side lobes appear on both sides of the main lobe, as shown in FIG. Then, this side lobe becomes a kind of noise and deteriorates the SN ratio. The cause of the side lobes is that when the amplitude spectrum of the transmission waveform is examined as shown in FIG. 16C, many ripples are present in the mountain-shaped flat portion of this amplitude spectrum.

17 and 18 are explanatory diagrams of Fourier transform and correlation processing of analog waveforms. Generally, an analog waveform x (t) that continues for an infinite time length ((a) in FIG. 17)
To obtain the frequency-to-amplitude characteristics (generally referred to as the amplitude spectrum) from (1), a continuous waveform is divided within a certain time, and the Fourier transform is performed by cutting out the waveform to convert from the time domain to the frequency domain. Waveform z (t) extracted by cutting (see (c) of FIG. 17)
Is represented by the product of the analog waveform x (t) and the rectangular window function y (t) (see FIG. 17B). Since the multiplication in the time domain is defined by the convolution operation in the frequency domain as shown in equation (5), the function X of x (t) in the frequency domain is calculated.
The function Y (ω) (Fig. 17) in the frequency domain of (ω) and y (t)
(See (d)) and the result of division by 2π are frequency characteristics Z (ω) of z (t) (see (e) in FIG. 17). Further, the above amplitude spectrum may take the absolute value of Z (ω) (see (f) in FIG. 17).

[0021]

(Equation 5)

On the contrary, the convolution operation in the time domain is multiplication in the frequency domain. On the other hand, the correlation function equivalent to the convolution operation in the time domain can be regarded as the product of the two functions performing the correlation operation in the frequency domain. Therefore, the amplitude spectrum of the correlation function is equal to the product of the amplitude spectra of the two functions that perform the correlation calculation. In FIG. 18, ripples appear in the amplitude spectra of the two functions before correlation. The side lobe of the correlation function, which is the product of the two, is affected by the ripple of the two functions before correlation.

To reduce the side lobes, ripples in the amplitude spectrum may be reduced. This ripple is to reduce the side lobe of the window function at the stage of cutting out the waveform. FIG. 19 is an explanatory diagram of the influence of the window function on the correlation processing. Conventionally, a Hamming window has been used instead of a rectangular window as a window function for reducing side lobes. FIG. 19 shows, as window functions, a rectangular window on the left side and a Hamming window on the right side in contrast. Left and right of FIG. 19A are waveforms of the two window functions in the time domain, left and right of FIG. 19B are waveforms of the window function of FIG. 19A in the frequency domain, and left and right of FIG. The FM signal wave cut out by the window function, the left and right of (d) are the amplitude spectrum of the FM signal waveform of (c),
The left and right sides of (e) show the autocorrelation function of the FM signal waveform cut out by each window function. It can be seen that the side lobes of the autocorrelation function are significantly reduced in the case of the Hamming window. Therefore, it can be said that in order to reduce the side lobes after correlation, it is effective to make the peak shape from the start end portion to the end portion of the FM signal waveform.

[0024]

However, in order to reduce the side lobes after pulse compression processing as described above, a Hamming window is used as a window function, and the (b) of FIG. 20 is used.
Transition frequency 1-9 [MHz], pulse width 5 shown on the right side of
A waveform obtained by pulse-compressing the received wave of the bottom surface reflection echo from the subject using the FM signal wave of [μs] under the actual flaw detection condition as shown in (a) of FIG. 20 is as shown in (c) of FIG. As shown on the right side, the side lobes around the main lobe are reduced, but side lobes with considerably large amplitude appear on both sides of the main lobe. The side lobes cause the following problems. In FIG. 20, the frequency of the probe used was a broadband type probe with a center frequency of 5 [MHz], and the test object used was a material with low attenuation and a thickness of 25 [mm]. (1) In ultrasonic flaw detection, an increase in side lobes on both sides of the main lobe deteriorates the distance resolution when detecting a defect inside the subject. (2) In the thickness measurement of the subject using ultrasonic waves, the measurement accuracy of the measured value is deteriorated.

As described above, the current situation is that the pulse compression in the waveform cutout using the Hamming window is inferior in effect to the pulse compression in the waveform cutout using the rectangular window.
Further, in the measurement of the thickness of the subject, it is the current situation that a rectangular window is more accurate. Therefore, in the ultrasonic flaw detection method using the pulse compression technique, the method of cutting out the frequency modulated wave by the Hamming window is not always satisfactory.

The present invention has been made to solve the above problems. In the ultrasonic flaw detection method using the pulse compression technique, the distance resolution at the time of detecting the defect of the object is excellent, and the measurement at the time of measuring the thickness is performed. An object of the present invention is to obtain an ultrasonic flaw detection method that generates a transmission waveform composed of a frequency-modulated wave having an optimum waveform with high accuracy.

[0027]

An ultrasonic flaw detection method according to a first aspect of the present invention transmits / receives a chirp wave having a predetermined waveform to / from an object through a probe, and compresses the received signal by pulse compression. In the ultrasonic flaw detection method for processing, a step of obtaining a frequency-amplitude characteristic in the mutual conversion between the electric signal and the acoustic signal of the probe in advance, and the frequency-amplitude of the probe in the received signal from the subject. And a step of generating a transmission waveform in which a correction amount for compensating for waveform deterioration caused by characteristics is added to a chirp wave to be transmitted in advance.

An ultrasonic flaw detection method according to a second aspect of the present invention is the ultrasonic flaw detection method according to the first aspect, wherein the envelope of the received signal and the reference signal for pulse compression is from the beginning to the center of the waveform. When a reference waveform that smoothly rises toward the end and smoothly falls toward the end is used, as a transmission waveform added to the chirp wave that transmits the correction component that compensates for the deterioration of the waveform of the received signal, the envelope at the beginning and end of the waveform Each of the lines has the step of generating a transmission waveform that rises and falls with a rather steep curve.

[0029]

In the invention according to claim 1, in the ultrasonic flaw detection method of transmitting and receiving the chirp wave of a predetermined waveform to and from the subject through the probe and performing the pulse compression processing on the received signal, A frequency-amplitude characteristic in the mutual conversion of the electric signal to the acoustic signal of the probe is obtained in advance, and a complementary component for compensating for waveform deterioration occurring in the received signal from the object based on the frequency-amplitude characteristic of the probe is added. , To generate a transmission waveform added to a chirp wave to be transmitted in advance. Generally, as shown in FIG. 23, the frequency characteristic S (ω) of the chirp to be transmitted is affected by the transmission characteristic of the probe during transmission and reception of the probe. This transfer characteristic is represented by a function having the frequency of the mutual conversion intensity of the electric signal of the probe to the ultrasonic signal as a parameter, represented by the frequency characteristic T (ω) of the probe, and its amplitude spectrum | T (ω ) | Is called a frequency band. The probe acts as a kind of filter on the received signal X (ω) after transmission and reception by the probe. Therefore, the reception signal X (ω) is equal to the transmission signal S (ω) and the frequency characteristic T of the probe.
It can be expressed by the product of (ω). Therefore, the amplitude spectrum | X (ω) of the frequency characteristic X (ω) of the received signal
| Has a narrower frequency range than the amplitude spectrum | S (ω) | of the transmission signal. Such a received signal X
Since the frequency range of the cross-correlation between (ω) and the reference wave S (ω) is narrower than the autocorrelation of only S (ω), the number of waves after correlation is large and the amplitude is large on both sides of the main lobe. Side lobes appear. Here, as shown in FIG. 9, a pulse wave including various frequencies with constant intensity is excited in the probe, and the frequency characteristic of the received wave is the frequency characteristic T (ω) of the transfer function of the probe. Becomes Further, as shown in FIG.
The transmission chirp wave S ′ (ω), which is obtained by correcting the transmission characteristic of the probe to the transmission signal, that is, by subtracting the transmission chirp wave S (ω) by the transmission characteristic T (ω) of the probe, If used as a transmission signal in the
The narrowing of the frequency range of (ω) | can be compensated.

In the invention according to claim 2, in the invention according to claim 1, as shown in (a) of FIG. 23, the window function of the reference wave is applied from a starting end portion such as a Hamming window to a central portion. When a window function having a mountain-like amplitude characteristic that smoothly rises and smoothly falls toward the terminal end is used (amplitude spectrum of the reference wave; see FIG. 23 (b)), the window function of the transmitted wave is As shown in (c) of FIG. 23, the amplitude characteristic of the central portion is a constant or gentle curve, and a window function having a hill-shaped amplitude that sharply rises and falls near the start end portion and the end portion is used (transmission). Wave amplitude spectrum; see FIG. 23 (d). When the bottom surface reflection echo of the subject is received under the flaw detection condition shown in FIG. 20A in this manner, the influence of the frequency characteristic of the probe on the received wave can be corrected, and as shown in FIG. Compared to the received wave before correction (see (f) in FIG. 23), the waveform becomes closer to the waveform of the reference wave, so that the frequency characteristic also has a shape equal to or close to that of the reference wave. As a result, the waveform after correlation becomes close to the waveform at the time of autocorrelation of the reference wave that does not include transmission / reception of the probe.

[0031]

1 is a functional block diagram showing an example of an ultrasonic flaw detector according to the present invention. In FIG. 1, reference numeral 1 is a synchronizing section for generating and outputting a necessary synchronizing signal for each circuit, 2 is an FM signal setting section for setting a frequency modulation signal (chirp signal wave in this example) of a predetermined waveform, and 3 is an FM signal setting. The FM signal transmitting unit 4 that generates an FM signal to be transmitted based on the FM signal set in the unit 2 and supplies the FM signal to the probe 4 generates ultrasonic waves based on the transmission signal from the FM signal transmitting unit 3 and generates an ultrasonic wave. A probe that applies an ultrasonic wave to the inside of the sample 5 and converts an echo from inside the sample into an electric signal, 5 is a sample for flaw detection, 6 is a pulse compression unit, 7 is a reference wave setting unit, 8 is a display unit.

In the apparatus shown in FIG. 1, the FM signal set by the FM signal setting unit 2 is output from the FM signal transmitting unit 3 in every predetermined cycle based on the synchronization signal output from the synchronizing unit 1 every predetermined repeated synchronization. Sent. The probe 4 generates an ultrasonic wave based on the transmitted FM signal and makes the ultrasonic wave incident on the subject 5, and at the same time, an ultrasonic wave reflection echo from a non-uniform portion of acoustic impedance existing inside and outside the subject. Is captured, converted into an electrical signal, and output. The received signal from the probe 4 is
Based on the output signal from the synchronization unit 1, pulse compression is performed in the pulse compression unit 6 by correlation calculation with the reference wave set in the reference wave setting unit 7, and the result is displayed on the display unit 8.

FIG. 2 is a diagram showing an example of a concrete hardware configuration of the apparatus shown in FIG. In FIG. 2, reference numeral 11 denotes a personal computer, which performs all functional operations of the synchronization unit 1, the FM signal setting unit 2, the FM signal transmitting unit 3, and the reference wave setting unit 7 of FIG. 12 is a D / A converter,
Reference numeral 13 is a transmission amplifier, 14 is a probe, 15 is a reception amplifier, 16 is an A / D converter, 17 is an FIR filter, and is concrete hardware of the pulse compression unit 6 in FIG. . The FIR filter 17 may have the structure shown in FIG. 16, for example. Reference numeral 18 is an oscilloscope, and 19 is a subject.

In FIG. 2, the FM waveform created by the personal computer 11 is converted into an analog signal by the D / A converter 12, amplified by the transmission amplifier 13 to a required transmission power, and then the probe. 14 is transmitted as ultrasonic waves into the subject 19. The signal received by the probe 14 is amplified by the reception amplifier 15, and the A / D converter 6
Are sequentially converted into digital signals. Then, the received digital signal is subjected to correlation calculation with the reference wave created and output by the personal computer 11 by the FIR filter 17, and pulse compression processing is performed. The waveform after the pulse compression is displayed on the oscilloscope 18.

The reason why the personal computer 11 is used here is that the waveform of the transmission wave and the waveform of the reference wave can be set to an arbitrary shape by changing the program, and the waveforms of the transmission wave and the reference wave can be changed. This is because the effect of low side lobe can be confirmed by performing Below, the waveforms of the transmitted wave and the reference wave are variously changed, and the effects of flaw detection are compared.

Example of flaw detection 1. The flaw detection example 1 is an example in which a carbon steel was used as the subject, and flaw detection inside the subject was performed.
6A and 6B are explanatory diagrams of various waveforms in this flaw detection example 1. FIG. In this flaw detection example 1, the nominal frequency of the probe 14 is 5 MH.
z, and the frequency-modulated wave was a chirp wave with a pulse width of 5 μs obtained by linearly modulating from 1 MHz to 9 MHz. 4A and 4B are diagrams showing the impulse response characteristics of the probe 14, wherein FIG. 4A shows the impulse response waveform of the probe, and FIG. 4B shows the amplitude spectrum.

(1), (a) and (b) of FIG. 3 respectively show a transmission wave and a reference wave which are cut out from the chirp wave by using a window function as a square wave, and these two waveforms are the same. . In this case, the oscilloscope 18 uses the oscilloscope 18 to process the waveform obtained by transmitting the transmission signal shown in FIG. 3A and correlating the reception signal reflected from the surface of the subject with the reference wave (b). The measured values are shown in FIG. In this (c), side lobes can be confirmed around the main lobe. (2), (d) and (e) of FIG. 3 respectively show a transmission wave and a reference wave cut out from the chirp wave by using a window function as a Hamming window, and the two waveforms are the same. In this case, as in the case of the above (1), the waveform after the correlation processing is shown in (f) of the same figure. In this (f),
Side lobes are emphasized on both sides of the main lobe.

(3), (g) and (h) of FIG. 3 are examples of preferred transmission waves and reference waves in the present invention, respectively.
The two waveforms are different. That is, transmitted wave (g)
The rise and fall of the envelope at the start and end are slightly steeper than the curve in the case of a Hamming window (for example, a curve with a slightly smaller radius of curvature), and the envelope at the center is Hamming. It has a mountain-like waveform due to a gentler curve (for example, a curve with a slightly larger radius of curvature) than the curve for a window. The reference wave (h) has the same waveform as the reference wave (e) using the Hamming window. The waveform after the correlation processing when the transmission wave (g) and the reference wave (h) are used is shown in (i) of the figure. In (i), the side lobes on both sides of the main lobe are reduced, the side lobes in the peripheral portion are also reduced, and the sharpness of the main lobe is emphasized. As a result S
The N ratio is improving.

The reason why the use of the transmitted wave (g) makes the main lobe sharper than that of the transmitted wave (d) will be described below. A general probe is shown in FIG. 4 (b). Since it has such a hill-shaped wide band spectrum, the probe in the received signal is obtained by raising and lowering the start end and the end of the transmitted wave (g) with a curve that is slightly steeper than the transmitted wave (d). By compensating for the narrowing of the spectrum due to the above and widening the spectrum, it is possible to sharpen the waveform after the correlation processing. In other words, it can be said that the transmitted wave (g) is generated by adding the correction amount of the waveform deterioration to the chirp wave to be transmitted in advance, in order to compensate the waveform deterioration caused by the frequency-amplitude characteristic of the probe in the received signal.

Example of flaw detection 2. The flaw detection example 2 uses a split-type probe in which the wave-transmitting oscillator and the wave-receiving oscillator are separate.
This is an example of flaw detection in the vicinity of the bottom surface of carbon steel as the subject, and FIG. 5 is an explanatory diagram of various waveforms in this flaw detection example 2. In this flaw detection example 2, the probe 2 is a split type probe having a nominal frequency of 5 MHz which is mainly used for flaw detection of thick steel plates of steel products, and flaw detection of various artificial defects existing in the vicinity of the bottom surface having a thickness of 60 mm. The result of the above will be described with reference to FIG.

(1), the transmitted wave (a) and the reference wave (b) of FIG. 5 are the transmitted wave (d) and the reference wave (e) of FIG. 3, respectively.
Has the same waveform as, and both have a transition frequency of 1 due to the Hamming window.
It is cut out from a chirp wave having a pulse width of 5 μs and a frequency of 9 MHz to 9 MHz. 5 (c), (d), (e)
Is an artificial defect having a diameter of 5.6 mm and a depth of 1.5 mm, 1.0 mm, and 0.5 mm at the bottom surface of the subject, using the transmitted wave (a) and the reference wave (b) of FIG. Is a waveform after flaw detection is performed and the received wave is subjected to correlation processing.

(2), the transmission wave (f) and the reference wave (g) in FIG. 5 have the same waveforms as the transmission wave (g) and the reference wave (h) in FIG. 3, and preferable waveform examples in the present invention. Is. The reference wave (g) and the reference wave (b) have the same waveform, and the transmission wave (f) has a different waveform from the transmission wave (a).
5 (h), (i), and (j), the transmission wave (f) and the reference wave (g) of FIG. 5 are used to detect three artificial defects having different depths, and It is a waveform after the correlation processing. Comparing (c) and (h), (d) and (i), and (e) and (j) of FIG. 5, the transmitted wave (f) was used (h),
The SN ratios of (i) and (j) are superior, and in particular (d) and (i), which are closer to the bottom surface, and (e) and (j) are compared, (i) and (j) It is clear that in the case of 1, the bottom surface reflected wave and the defect reflected wave can be separated more easily, so that the detectability is improved. By using the transmission wave (f) and the reference wave (g) in this way, there is an effect that the detectability of defects existing on the bottom surface of the subject is improved.

Example of flaw detection 3. The flaw detection example 3 is an example of measuring the thickness of the subject. In general, when measuring the thickness of a material by ultrasonic waves, the ultrasonic wave propagates back and forth across the cross section in the thickness direction of the material, so the time interval of the reflected wave from the bottom surface of the material is measured, Convert to thickness (distance) using the speed of sound inside. Therefore, this thickness measurement technique depends on whether the reflected wave repeatedly obtained from the bottom surface of the material can be clearly measured. FIG. 6 is an explanatory diagram of various waveforms in this flaw detection example 3.

(1), the transmission wave (a) and the reference wave (b) of FIG. 6 have the same waveforms as the transmission wave (a) and the reference wave (b) of FIG. 5, respectively, and both are cut out by the Hamming window. It is a chirp wave having a transition frequency of 1 MHz to 9 MHz and a pulse width of 5 μs. In FIG. 6C, the material thickness is 2 mm.
Is a waveform after the correlation processing of the bottom multiple reflection wave of the material when the transmission wave (a) and the reference wave (b) are used. (2), the transmitted wave (d) and the reference wave (e) of FIG. 6 have the same waveforms as the transmitted wave (f) and the reference wave (g) of FIG. 5, respectively, which are preferable waveform examples of the present invention. is there. Further, (f) of FIG. 6 is a waveform corresponding to (c) when the transmission wave (d) and the reference wave (e) are used with the same material thickness of 2 mm.
Comparing (c) and (f) of FIG. 6, S of (c) is
The N ratio is improved, the time interval of the reflected wave repeatedly obtained from the bottom surface of the material can be clearly measured, and the measurement accuracy in the thickness measurement is improved.

A summary of the above-described flaw detection examples 1, 2, and 3 is as follows. (1), the waveform after the correlation processing (convolution operation) in the case of the transmitted wave and the reference wave, which are the chirp waves cut out using the rectangular window,
There are many side lobes around the main lobe (see FIG. 3 (c)). This convolution operation is the product of the amplitude spectrum of the transmitted wave and the amplitude spectrum of the reference wave,
Since the product of the amplitude spectrum in the case of a square wave is wider than that in the case of the Hamming window in the frequency band, the waveform after the correlation processing becomes sharp. (2) The side lobes on both sides of the main lobe are emphasized in the waveform after the correlation processing in the case of the transmission wave and the reference wave in which the chirp wave is cut out using the Hamming window (see (f) in FIG. 3). , The SN ratio is poor (see (c) to (e) of FIG. 5 and (c) of FIG. 6). Further, the product of the amplitude spectrum in the case of the Hamming window is narrower than that in the case of the square wave in the frequency band, so that the waveform after the correlation process becomes dull. Further, due to the frequency-amplitude characteristic of the probe, the band of the transmitted wave is narrowed, so that the waveform of the received signal after the correlation processing is still blunted.

(3) As shown in (g) of FIG. 3, the transmitted wave has a slightly steeper curve than the curve in the case where the envelope curve at the start end and the end thereof is raised and lowered. The chirp wave is cut out by a window function that is performed so that the envelope of the central portion becomes a mountain shape (or a valley shape) with a gentler curve than the curve in the case of the Hamming window, and the reference wave is (2 If the chirp wave is cut out using the same Hamming window as in (1), it is possible to compensate for the narrowing of the spectrum in the frequency band due to the probe having the hill-shaped width spectrum. As a result, it is possible to sharpen the main lobe waveform after correlation processing, and because there are less ripples in the amplitude spectrum after correlation processing than in the case of a rectangular window, the side lobe near the main lobe and the peripheral part of the main lobe are close. The side lobes that appear can also be lowered. Therefore, in the present invention, the transmission wave and the reference wave according to (3) are adopted.

FIG. 7 is a diagram showing the preferred transmission / reception waveform of the present invention adopted in (3) above, its amplitude spectrum, and the waveform after correlation processing. 7A is a preferred transmission waveform according to the present invention, FIG. 7B is an amplitude spectrum of the transmission waveform A, FIG. 7C is a reception waveform when the transmission waveform is A, and FIG. Amplitude spectrum of waveform (c), (e)
Shows the waveforms after the correlation processing. By comparing (e) of FIG. 7 and (f) of FIG. 3, it can be seen that the former is superior to the latter as the ultrasonic flaw detection waveform.

[0048]

As described above, according to the present invention, there is provided an ultrasonic flaw detection method in which a chirp wave having a predetermined waveform is transmitted to and received from a subject through a probe, and the received signal is subjected to pulse compression processing. , The frequency-amplitude characteristic in the mutual conversion between the electric signal and the acoustic signal of the probe is obtained in advance, and the waveform deterioration that occurs based on the frequency-amplitude characteristic of the probe in the received signal from the subject is compensated. Since the correction waveform is added to the chirp wave to be transmitted in advance, the transmission waveform is generated after the pulse compression processing by compensating for the narrowing of the spectrum by the probe in the received signal and measuring the broadening of the spectrum. It is possible to sharpen the main lobe waveform of.

Further, according to the present invention, as the transmission waveform added to the chirp wave for transmitting the correction component for compensating for the waveform deterioration of the received signal, the envelopes at the start end and the end of the waveform are slightly steep curves. To generate a transmission waveform that rises and falls, and the envelope in the central portion of the transmission wave generates a transmission waveform that is a mountain shape or a valley shape due to a gentle curve. In addition, the side lobes that are close to the main lobe and the side lobes that appear around the main lobe can be reduced. As a result, it is effective in expanding the flaw detection range and improving the flaw flaw detection accuracy by improving the SN ratio of the flaw detection dead zone on the front surface and the back surface of the subject. Further, also in the case of measuring the thickness of the material, there is an effect that it becomes possible to measure the thickness of the thinner material.

[Brief description of drawings]

FIG. 1 is a functional configuration diagram showing an example of an ultrasonic flaw detector according to the present invention.

FIG. 2 is a diagram showing an example of a specific hardware configuration of the apparatus of FIG.

FIG. 3 is an explanatory view of various waveforms in flaw detection example 1 of the present invention.

FIG. 4 is a diagram showing impulse response characteristics of a probe.

FIG. 5 is an explanatory diagram of various waveforms in flaw detection example 2 of the present invention.

FIG. 6 is an explanatory diagram of various waveforms in flaw detection example 3 of the present invention.

FIG. 7 is a diagram showing a preferred transmission / reception waveform of the present invention, its amplitude spectrum, and a waveform after correlation processing.

FIG. 8 is a functional configuration diagram of a conventional general ultrasonic flaw detector.

FIG. 9 is a diagram showing a waveform of a pulse wave and its amplitude spectrum.

FIG. 10 is a diagram showing a received waveform of a probe by pulse wave excitation and its amplitude spectrum.

FIG. 11 is a block diagram of an ultrasonic flaw detector described in a conventional document.

FIG. 12 is a diagram showing a comparison between a burst wave and a pulse wave, which are ultrasonic transmission waveforms.

FIG. 13 is an explanatory diagram of a linear frequency modulation pulse compression radar.

FIG. 14 is a diagram showing a configuration example of an FIR digital filter.

15 is a waveform chart for explaining the operation of FIG.

FIG. 16 is an explanatory diagram of pulse compression processing of a chirp wave cut out by a rectangular window.

FIG. 17 is an explanatory diagram 1 of Fourier transform and correlation processing of analog waveforms.

FIG. 18 is an explanatory diagram 2 of Fourier transform and correlation processing of an analog waveform.

FIG. 19 is an explanatory diagram of the influence of the window function on the correlation processing.

FIG. 20 is a schematic diagram of an influence on a correlation process of a window function in a general ultrasonic flaw detection method.

FIG. 21 is a schematic diagram of the influence of the frequency band of the probe in the general ultrasonic flaw detection method on the correlation processing result.

FIG. 22 is a schematic diagram of a method for reducing the influence of the frequency band of the probe on the correlation processing result.

FIG. 23 is an explanatory diagram related to a transmission waveform that reduces the influence on the correlation processing result by the frequency band of the probe.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 synchronization part 2 FM signal setting part 3 FM signal transmission part 4,14 probe 5,19 subject 6 pulse compression part 7 reference wave setting part 8 display part 11 personal computer 12 D / A converter 13 transmission amplifier 15 Receiver amplifier 16 A / D converter 17 FIR filter 18 Oscilloscope

Claims (2)

[Claims] 1. An ultrasonic flaw detection method in which a chirp wave having a predetermined waveform is transmitted to and received from a subject through a probe, and the received signal is subjected to pulse compression processing. A step of obtaining a frequency-amplitude characteristic in mutual conversion with an acoustic signal, and a correction component for compensating for waveform deterioration caused based on the frequency-amplitude characteristic of the probe in a received signal from the subject, a chirp wave to be transmitted in advance. And a step of generating an added transmission waveform. 2. When a reference waveform is used in which the envelope of the received signal and a reference signal for pulse compression rises smoothly from the beginning to the center of the waveform and smoothly falls from the end of the waveform, the waveform of the received signal deteriorates. The method further comprises the step of generating a transmission waveform in which the envelopes at the start end and the end of the waveform are added to the chirp wave for transmitting the correction component to compensate for the rising and falling edges by slightly steep curves, respectively. Ultrasonic flaw detection method described.