JP2003149208A - Nondestructive inspecting apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To shorten the time required for obtaining a three-dimensional image in a nondestructive inspection for inspecting defects through the use of a highly sensitive magnetic sensor. SOLUTION: A saw-tooth wave generating device 6 generates an electric signal of saw-tooth waveform containing modes of a large number of different frequency components from a low frequency to a high frequency and outputs the electric signal to an exciting coil 5. A magnetic field generated from an eddy current induced to a sample is measured by a highly sensitive magnetic sensor (SQUID flux meter 1). By performing FET on a time-domain signal value of the magnetic field obtained by the measurement, a frequency-domain signal value is obtained and stored in a storage part of a computer 4. The frequency-domain signal value of the magnetic field is stored as a function of a location on the sample. From the stored signal value, the three-dimensional image of a defect present in the sample is obtained by software, etc.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nondestructive inspection apparatus for inspecting defects inside a metal structure without destroying the metal structure and displaying the defects as an image, and more particularly to a superconducting quantum interference device ( SQUID: superconducting quantu
The present invention relates to a nondestructive inspection device for displaying a defect inside a structure as a three-dimensional image in the nondestructive inspection using a high-sensitivity magnetic sensor such as a magnetic interference device).

[0002]

2. Description of the Related Art In recent years, non-destructive inspection for finding defects in a metal structure using a high-sensitivity magnetic sensor such as SQUID has attracted attention. For example, SQUID can detect magnetism with high sensitivity as compared with the conventional magnetic sensors. Further, there is a feature that the detection sensitivity does not depend on the frequency in a wide frequency range from direct current to several tens of kHz.

Further, since the SQUID can be made into an extremely minute element by being composed of a thin film element, it has a feature that the spatial resolution is high in detecting magnetism.

There is a technique of displaying a defect as a three-dimensional image in order to clearly indicate to the user where the defect in the metal structure is, by using a high-sensitivity magnetic sensor such as SQUID.

In order to display a defect in a metal structure as a three-dimensional image, many different harmonic amplitude signals are required. In the prior art, sources corresponding to these different harmonics are required as many as the harmonics required to display a three-dimensional image.

[0006]

Conventionally, a plurality of sine wave generators are used, sine waves of different frequencies are generated from each sine wave generator, and magnetic responses at the frequencies are sequentially measured. It was However, by using the sawtooth wave, a response of many frequencies can be obtained at one time.

Further, after performing the nondestructive inspection with the amplitude signal of one harmonic, the nondestructive inspection is performed again with the amplitude signal of the different harmonic. It is necessary to repeat this procedure and execute this procedure for the number of harmonics required to display a three-dimensional image.

Therefore, it takes time to acquire the data necessary for displaying a three-dimensional image by the nondestructive inspection using the high-sensitivity magnetic sensor. A user who wants to perform a rapid non-destructive inspection needs to realize a non-destructive inspection device that displays a three-dimensional image of a defect by a non-destructive inspection using a high-sensitivity magnetic sensor in a short measurement time.

In view of these problems in the prior art, an object of the present invention is to provide a nondestructive inspection apparatus which takes a short time to acquire a three-dimensional image by nondestructive inspection using a high-sensitivity magnetic sensor such as SQUID. It is in.

[0010]

According to the present invention, in a nondestructive inspection apparatus for detecting a defect by inspecting the inside of a metal structure without destroying it, a generating means for generating a signal containing a plurality of frequency components. An excitation coil that excites an electromagnetic wave by inputting the signal and irradiates the electromagnetic wave to a metal structure that is a sample; a magnetic field detection unit that detects a magnetic field generated from the metal structure; and the magnetic field. From the above, a non-destructive inspection device is provided, which comprises: a calculating unit that calculates the amplitude of each frequency component of the magnetic field.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A nondestructive inspection apparatus according to an embodiment of the present invention will be described below with reference to the drawings. Figure 1
FIG. 3 is a functional block diagram of the nondestructive inspection device according to the embodiment of the present invention. The nondestructive inspection device of this embodiment is r
An f-SQUID (rf-type superconducting quantum interference device) magnetometer 1, an XY stage 2, an XY stage controller 3, a computer 4, an excitation coil 5, and a sawtooth wave generator 6 are provided. X is a metal structure that is a sample (also called a sample) in which defects are inspected without breaking.
-It is arranged on the Y stage 2. In FIG. 1, an aluminum plate is arranged as a sample on the XY stage 2.

An electric signal having a sawtooth waveform is generated by the sawtooth wave generator 6. A sawtooth electrical signal contains many different frequency component modes, from low to high frequencies. The generated electric signal is input to the excitation coil 5.

In the exciting coil 5, electromagnetic waves are induced by electric signals having different frequency components contained in the inputted electric signal. The induced electromagnetic wave induces an eddy current in the aluminum plate on the XY stage 2.

The induced eddy currents are scattered and disturbed by defects existing in the aluminum plate. Both disturbed and undisturbed eddy currents in the aluminum plate generate a magnetic field. The generated magnetic field is detected by the rf-SQUID magnetometer 1.

The amplitude value of the detected magnetic field is input to the computer 4 together with the position of the aluminum plate on the XY stage 2 which is the measurement point. The computer 4 uses a fast Fourier transform (FFT).
rmation) is executed.

The FFT converts the amplitude value of the input magnetic field for each measurement point into the amplitude value of the magnetic field for each different harmonic. That is, it becomes possible to obtain the frequency spectrum of the magnetic field for each measurement point.

Further, a designation signal for designating the position of the XY stage 2 is output from the computer 4 to the XY stage controller 3. The specified signal output is X
The drive signal for inputting to the -Y stage controller 3 and moving the XY stage 2 is output to the XY stage 2. The output drive signal is input to the XY stage 2, and the XY stage 2 is moved to the position designated by the computer 4.

The amplitude value of the amplitude signal indicating the strength of the magnetic field for each of a plurality of frequencies is stored as data at each sample position in a storage unit such as a hard disk of the computer 4. The data at all locations within the sample creates a three-dimensional image showing defects within the sample.
The software for displaying the three-dimensional image may be used to create the three-dimensional image from the data. For example, Visual Basic (registered trademark) or the like is used.

As described above, the electromagnetic wave induced from the excitation coil 5 is applied to the vicinity of the excitation coil 5 of the aluminum plate which is the sample. As a result, it becomes possible to detect whether or not there is a defect in the aluminum plate irradiated with the electromagnetic wave, and if there is, the position of the defect.

When a current is passed through a conductor such as a metal, if a defect exists inside the conductor, the defect causes a disturbance in the current and a disturbance in the magnetic field generated by the current. The disturbance of the magnetic field depends on the position, shape, and size of the defect. On the other hand, when there is no defect in the conductor, the current flowing through the conductor has a constant magnetic field distribution determined by the shape of the conductor.

Therefore, in principle, the distribution of the magnetic field generated outside the conductor is measured, and the disturbed portion of the magnetic field distribution is extracted and analyzed to detect defects existing in the conductor. It becomes possible to do.

The change in the magnetic field distribution due to the disturbance of the current is
Compared to the strength of the magnetic field generated by the current flowing through the cross section of the sample, it is very small and requires a highly sensitive magnetic sensor. For example, there are a semiconductor magnetic sensor, a fluxgate type magnetic sensor, a SQUID magnetometer, and the like. In the embodiment of the present invention, the case where the SQUID magnetometer is used is described. Of course, it is not limited to the SQUID magnetometer, and the above-mentioned high-sensitivity magnetic sensor may be used. Further, the essence of the present invention is not changed even if the rf-SQUID magnetometer 1 is changed to a DC type dc-SQUID magnetometer.

Further, based on the designation of the computer 4, the XY stage 2 on which the sample is loaded can freely operate. The XY stage 2 normally operates only in a two-dimensional direction. That is, the excitation coil 5 (or rf-SQU)
The distance between the ID magnetometer 1) and the XY stage 2 remains constant.

Furthermore, in this embodiment, the software for executing the FFT is installed in the computer 4 to execute the FFT. However, the present invention is not limited to this, and in order to increase the processing speed, it is preferable to use hardware in which a DSP (digital signal processor) for executing FFT is incorporated.

The frequency range of the sawtooth-shaped electric signal is determined by the thickness of the sample. This frequency is determined based on the so-called skin effect of the dielectric. That is, the penetration depth δ from the dielectric surface where electromagnetic waves penetrate is δ = (2 / ω · σ · μ _r ).
It is ^1/2 . Here, ω is the angular frequency of the electromagnetic wave induced in the excitation coil 5, σ is the conductivity of the aluminum plate that is the sample, and μ _r is the magnetic permeability of the sample. An electromagnetic wave is induced from the excitation coil 5 so that the penetration depth δ becomes equal to or larger than the thickness of the sample loaded on the XY stage. The penetration depth δ can be changed by the frequency of the electromagnetic wave induced from the excitation coil. That is, it is possible to irradiate the sample with electromagnetic waves having a low penetration depth of more than the thickness of the sample from a high penetration depth of almost zero. Usually, an electromagnetic wave having a frequency of about several tens Hz to about 100 Hz is induced from the excitation coil 5.

As explained above, the defects present in the sample affect the distribution of eddy currents induced in the sample. The magnetic field generated by this induced eddy current undergoes a change depending on whether there are any defects in the sample. In the embodiment of the present invention, an electric signal having a sawtooth wave is generated in the excitation coil 5. The sawtooth wave contains many frequency components from low frequencies to high frequencies. Therefore, a plurality of electric signal generators from low frequencies to high frequencies are not required, and only one electric signal generator that generates a sawtooth wave (that is, sawtooth wave generator 6) is sufficient.

As a result, it becomes possible to detect the defect in the sample by the sawtooth electric signal output from one electric signal generator. Therefore, the time for generating a plurality of electric signals is unnecessary, and the time required for the inspection for detecting the presence / absence of a defect in the sample and the position of the defect can be shortened.

FIG. 2A shows a sawtooth wave generated by the sawtooth wave generator 6 shown in FIG. FIG. 2B is an equation showing the amplitude of the sawtooth wave shown in FIG. 2A in one cycle. As shown in FIG. 2A, the same sawtooth waveform is periodically repeated and output from the sawtooth wave generator 6. The sawtooth waveform that appears in one cycle is calculated by the mathematical formula shown in FIG. Here, ω is an angular frequency and has a relationship of ω = 2πf with the fundamental frequency f of the sawtooth wave. That is, according to the equation of FIG. 2B, it is understood that one sawtooth waveform includes infinitely many frequencies. One sawtooth waveform contains a frequency that is an integral multiple of the fundamental frequency f.

What is important here is that the excitation signal is synthesized by many single frequencies. Therefore,
A triangular wave or a rectangular wave may be used instead of the sawtooth wave. However, since the sawtooth wave contains more high-frequency components than the rectangular wave, it is characterized in that it can generate a signal corresponding to a wide frequency range.

FIG. 3 is a flow chart for creating a three-dimensional image in the nondestructive inspection apparatus according to the embodiment of the present invention. In step ST-1, the sawtooth wave generator 6 is used as shown in FIG.
The sawtooth electric signal shown in (A) is generated, and the sawtooth electric signal is supplied to the excitation coil 5. In this step, the position where the sample is measured is set to the position (X ₀ , Y ₀ ).

At step ST-2, the position (X ₀ ,
The SQUID magnetometer 1 acquires the sample response signal at Y ₀ ). This response signal is a signal due to the magnetic field generated by the eddy current generated in the sample induced in the excitation coil 5 as described above.

The response signal at this position (X ₀ , Y ₀ ) is
It is the time domain signal S (t, X ₀ , Y ₀ ) (ST-
3). S (t, X ₀ , Y ₀ ) corresponds to the strength of the magnetic field at the position (X ₀ , Y ₀ ) at time t.

At step ST-4, S (t, X₀, Y
₀) Is transformed by complex FFT. And the spectrum
The signal F (ω, X₀, Y₀) Get. F (ω,
X₀, Y ₀) Is the position (X₀, Y₀) For each frequency
It is a function indicating the value corresponding to the strength of the magnetic field.

In step ST-5, the spectrum signal F
_{(Ω, X 0, Y 0} ) from the amplitude _{R (ω, X 0, Y} 0) is calculated. That is, R (ω, X ₀ , Y ₀ ) = [Re (F
(Ω, X ₀ , Y ₀ )) ² + Im (F (ω, X ₀ ,
Y ₀ )) ² ] ^1/2 .

At step ST-6, the harmonic angular frequency is changed to
Corresponding amplitude R (ω, X₀, Y₀) Is calculated. Sanawa
R (ω_n, X₀, Y₀) (N = 1, 2, 3 ...)
To calculate. This angular frequency ω_nIs shown in FIG. 2 (B)
Corresponding to nω. As shown in FIG. 2B, n is the original
It is a number that continues to infinity theoretically, but of course here
Set an upper limit of angular frequency, and
On the other hand, R (ω_n, X ₀, Y₀) Is not calculated. Normal,
This angular frequency ω_nUpper limit of 1kHz to 1.5kHz
It is set to a degree.

At step ST-7, the XY stage 2
The position where the sample was measured (X ₀ , Y ₀ )
Is moved to the next measurement position (X ₁ , Y ₁ ). At the position (X ₁ , Y ₁ ), the above-mentioned step ST-
The operation is performed from 1 to step ST-6. That is, R (ω _n , X ₁ , Y ₁ ) (n = 1, 2, 3 ...)
To calculate. This operation is performed for all positions (X, Y) on the sample, and R (ω _n , X, Y) (n
= 1, 2, 3 ...) is calculated.

This position (X, Y) is a finite number of positions on the sample and is uniformly distributed on the sample. R (ω _n , X, Y) at these positions (X, Y) (n = 1, 2,
By calculating 3 ...), it becomes possible to display the defect in the sample on the screen.

At step ST-8, R (ω _n , X,
Y) (n = 1, 2, 3 ...) Based on this, a three-dimensional image is displayed. The frequency of the electromagnetic wave induced by the excitation coil 5 changes depending on the harmonic mode. Therefore, the modes of the harmonics change the penetration depth of the electromagnetic wave, making it possible to probe the entire area of the sample.

[0039]

EXAMPLES An example of an experiment using the nondestructive inspection device according to the embodiment of the present invention will be described below as an example with reference to the drawings. FIG. 4 is a cross-sectional view showing an aluminum plate 10 used as a sample to be inspected by the nondestructive inspection apparatus in the embodiment of the present invention, in which a central portion is artificially provided with a defect. As shown in FIG. 4, the sample is artificially provided with holes 11 and 12. These holes 11 and 12 correspond to the above-mentioned defects. That is, it is a defect to be displayed by the nondestructive inspection device of the present invention.

The aluminum plate 10 has a thickness of 10
mm, and a hole 11 having a diameter of 5 mm and a hole 12 having a diameter of 10 mm are provided in the central portion thereof. The hole 12 is provided in a cylindrical shape from the center of the lower surface to a depth of 3 mm. Further, the hole 11 has a depth of 4 mm to a depth of 7 mm from the center of the lower surface.
It is provided in a column shape.

FIG. 5 is a diagram showing a sawtooth wave generated by the sawtooth wave generator 6 shown in FIG. A sawtooth waveform similar to the sawtooth wave shown in FIG. 2 (A) is applied to the excitation coil 5. The sawtooth wave shown in FIG. 5 corresponds to the sawtooth wave generated in step ST-1 shown in FIG. The waveform shown in FIG. 5 is applied to the excitation coil 5 to induce electromagnetic waves from the excitation coil 5. The induced electromagnetic wave is applied to a specific position on the aluminum plate 10. The irradiation position is set by the XY stage controller 3.

FIG. 6 is a diagram showing the spectrum of the signal after the signal detected by the SQUID shown in FIG. 1 is subjected to the fast Fourier transform. Aluminum plate 10 induced by a sawtooth electrical signal shown in FIG.
The magnetic field generated by the eddy current generated inside is detected by SQUID, and the detection result is FFT-transformed. This operation corresponds to step ST-6 shown in FIG.

An amplitude R (ω _n , X ₀ , which indicates the strength of the magnetic field generated by the eddy current at a position (X ₀ , Y ₀ ),
Y ₀ ) (n = 1, 2, 3 ...) Is stored as data in a storage unit such as a hard disk in the computer 4.

FIG. 7 is a diagram showing the amplitude of each frequency of the signal after the fast Fourier transform corresponding to the position of the sample. Amplitude R (ω _n , X, Y) (n = 1, 2, 3 ...) (X, Y stored in the storage unit of the computer
(Corresponding to all inspected positions on the upper surface of the sample) shows the variation with position for each angular frequency ω _n .

The position of the mountain valley near the center right of FIG. 7 corresponds to the position of the artificially created defect. In FIG. 7, the curve at the bottom of the figure shows the response due to a high-frequency electromagnetic wave.
It can be seen that the curve shown at the bottom of FIG. 7 has fewer peaks and valleys.

This phenomenon indicates that a higher frequency electromagnetic wave does not penetrate into the aluminum plate 10 of the sample, in other words, a lower frequency electromagnetic wave penetrates into the aluminum plate 10. That is, it is understood that the eddy current induced by the electromagnetic wave having a lower frequency is more affected by the defect.

FIG. 8 is a view showing defects in the sample as a three-dimensional image based on the data shown in FIG. It is shown as a three-dimensional perspective view. This image is
Data on defects in the memory of the computer 4
It is displayed dimensionally by the illustrated software. For example, Visual Basic (registered trademark) or the like is used. In FIG. 8, what is displayed in black is the aluminum plate 20 which is a sample. What is displayed in white is a defect. The white part 21 is shown in FIG.
The white portion 22 corresponds to the hole 12, and the white portion 22 corresponds to the hole 12.

The position of the displayed aluminum plate 20 can be moved three-dimensionally by the scroll bars 23, 24 and 25 provided one below the other on the left side of FIG. These scroll bars 23, 2
By moving 4 and 25 it is possible to locate the defect in the aluminum plate 20.

The button "ANIMATI" at the upper right corner
When pressing "ON", the aluminum plate 20 automatically starts moving as an animation, and it becomes possible to observe the state of the defect from all directions.

Furthermore, the button "ZOOM I" at the upper right corner
"N" and "ZOOM OUT" allow zooming in and out of the displayed image, respectively. The button "STOP" is a button for stopping the animation.

The present invention is not limited to the above-mentioned embodiment, but can be carried out with various modifications within the technical scope thereof.

[0052]

According to the non-destructive inspection apparatus of the present invention, it becomes possible to obtain many amplitude signals of different harmonics at once by the sawtooth wave excitation signal and the fast Fourier transform.

Therefore, the measurement time of the nondestructive inspection is shortened. Moreover, since a plurality of signal generation sources that generate amplitude signals of different harmonics are not required, the configuration of the nondestructive inspection device is simplified.

[Brief description of drawings]

FIG. 1 is a functional block diagram of a nondestructive inspection device according to an embodiment of the present invention.

FIG. 2A is a diagram showing a sawtooth wave generated by the sawtooth wave generator shown in FIG. 2B is an equation showing the amplitude of the sawtooth wave shown in FIG. 2A in one cycle.

FIG. 3 is a flow chart for creating a three-dimensional image in the nondestructive inspection device according to the embodiment of the present invention.

FIG. 4 is a cross-sectional view showing an aluminum plate used as a sample to be inspected by a nondestructive inspection apparatus in an embodiment of the present invention, in which a central portion is artificially provided with a defect.

FIG. 5 is a diagram showing a sawtooth wave generated from the sawtooth wave generator shown in FIG. 1 in the embodiment of the present invention.

FIG. 6 shows S shown in FIG. 1 in an embodiment of the present invention.
It is the figure which displayed the spectrum of the signal after carrying out the fast Fourier transform of the signal detected by QUID.

FIG. 7 is a diagram showing the amplitude of each frequency of the signal after being subjected to the fast Fourier transform in correspondence with the position of the sample in the embodiment of the invention.

FIG. 8 is a diagram in which defects in the sample are displayed as a three-dimensional image based on the data shown in FIG. 7.

[Explanation of symbols] 1 SQUID magnetometer 2 XY stage 3 XY stage controller 4 computers 5 excitation coil 6 Sawtooth generator 10 Aluminum plate 11 holes (defects) 12 holes (defects) 20 aluminum plate 21 White part (defect) 22 White part (defect) 23, 24, 25 scroll bars

Claims (6)

[Claims] 1. A non-destructive inspection apparatus for detecting defects by inspecting the inside of a metal structure without destroying the inside of the metal structure, and generating means for generating a signal including a plurality of frequency components, and by inputting the signal. An excitation coil that excites an electromagnetic wave and irradiates the electromagnetic wave to a metal structure that is a sample, a magnetic field detection unit that detects a magnetic field generated from the metal structure, and an amplitude of each frequency component of the magnetic field from the magnetic field. A nondestructive inspection apparatus comprising: a calculating unit that calculates. 2. The non-destructive inspection apparatus according to claim 1, further comprising a display unit that displays a defect in the metal structure based on the amplitude of each frequency component of the magnetic field. 3. The non-destructive inspection apparatus according to claim 1, wherein the generation unit generates a signal having a sawtooth waveform in which a plurality of single frequencies are combined. 4. A loading means for placing the metal structure as a sample, a signal generating means for generating a signal for moving the metal structure to a position where a defect of the metal structure is to be measured, and the signal The nondestructive inspection apparatus according to claim 1, further comprising: a moving unit that moves the metal structure placed on the stacking unit. 5. The nondestructive inspection apparatus according to claim 1, wherein the magnetic field detecting means is an apparatus for detecting a magnetic field by a superconducting quantum interference device. 6. The method according to claim 1, wherein the calculation means is executed by a fast Fourier transform.
The nondestructive inspection device according to any one of 1.