JP2015087168A - Nondestructive inspection system and nondestructive inspection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately perform flaw inspection of an analyte even when the distance between a sensor and the analyte is long.SOLUTION: The nondestructive inspection system includes: an exciting coil 101 facing an analyte 2; a reference coil 105 electromagnetically bonded to the exciting coil 101; a detection coil 104 facing the analyte 2; and a measurement processing unit 4 that excites the exciting coil 101 by alternation voltage, and detects the amplitude and phase of a signal to be measured as the difference between the voltage occurring in the reference coil 105 and the voltage occurring in the detection coil 104, using the voltage of the exciting coil 101 or reference coil 105 as a reference signal.

Description

  The present invention relates to a nondestructive inspection apparatus and a nondestructive inspection method for performing nondestructive inspection of a subject using electromagnetic induction.

  As shown in Patent Documents 1 to 5, in an overcurrent flaw detector using electromagnetic induction, a sine wave generator, a drive circuit for driving an excitation coil, a sensor composed of an excitation coil and a detection coil, and the output of the detection coil are amplified. An apparatus composed of an analysis circuit including an amplifier circuit and a synchronous detection circuit has been proposed and used.

FIG. 23 is an example of a conventional sensor.
As shown in FIG. 23, the sensor has an exciter formed by winding an exciting coil 101 around an exciting core 102 made of a magnetic material such as a silicon steel plate or ferrite for the purpose of applying a strong magnetic field to a subject. The exciting coil 101 is excited with a constant voltage sine wave so as to face the subject 2. A detection coil 104 is arranged inside the excitation coil 101.

  For example, in Patent Document 6, in the sensor as described above, based on the output of the detection coil 104, a change in eddy current generated in the deep part of the subject is detected by means such as synchronous detection, and the detection of thinning, flaws, etc. is detected. The structure performed from the outside is disclosed.

Japanese Patent No. 3753499 Japanese Patent No. 3266128 JP 2010-48552 A Japanese Patent No. 3896489 JP 3010-54352 A Japanese Patent No. 4756409

However, a tube as an actual subject is a carbon steel tube having both magnetism and conductivity. Moreover, the tube is often covered with a heat insulating material such as glass wool, and the outside thereof is often covered with a dew-proof material such as aluminum or tin. In a carbon steel pipe or the like having both magnetism and conductivity, an excitation alternating magnetic field hardly penetrates into the subject due to the eddy current effect. Furthermore, since the tube as the subject is covered with the heat insulating material, the distance between the subject and the sensor, that is, the lift-off must be increased. As a result, since the detection signal becomes extremely small, it becomes extremely difficult to detect the thinning of the inner wall portion of the tube of the subject, the scratch, and the like.
In addition, when the detection signal is small, the sensor is more strongly influenced by the temperature characteristics, and there is a drawback that the measured value drifts due to a temperature change. That is, there is a drawback that the temperature coefficient of the electrical resistance of a copper wire such as a formal wire used for the winding of the sensor is relatively large and the measurement device becomes unstable in temperature.
Accordingly, an object of the present invention is to provide a nondestructive inspection apparatus and a non-destructive inspection apparatus that can accurately detect a test object even when the distance between the end surface of the sensor facing the test object and the test object, that is, the lift-off state is large. To provide a destructive inspection method.

The present invention employs the following means in order to solve the above problems.
That is, the nondestructive inspection apparatus of the present invention includes a first coil facing the subject, a second coil electromagnetically coupled to the first coil, and a third coil facing the subject. The first coil is excited with an alternating voltage, and the amplitude and phase of the signal under measurement, which is the difference between the voltage generated in the second coil and the voltage generated in the third coil, And a measurement processing unit that detects a voltage of the second coil as a reference signal.

  According to such a configuration, by exciting the first coil with an alternating voltage, a voltage generated in the second coil electromagnetically coupled to the first coil, and a voltage generated in the third coil There is a difference between By detecting the amplitude and phase of the signal under measurement, which is the difference between the voltage generated by the second coil and the voltage generated by the third coil, using the voltage of the first or second coil as a reference signal, The effect of lift-off, which is the distance from the subject, and temperature change can be suppressed.

  The measurement processing unit, the step of inputting the reference signal and the signal under measurement, the step of obtaining the amplitude ratio and phase difference between the input signal under measurement and the reference signal by fast Fourier transform; Establishing simultaneous equations using the amplitude ratio and the phase difference as variables, and obtaining each coefficient of the simultaneous equations from the known thicknesses of the plurality of points of the subject and the measured values of the amplitude ratio and phase difference at each point And estimating the thickness of the object at the unknown point from each of the obtained coefficients and the measured values of the amplitude ratio and the phase difference at the unknown point.

  In this way, by using simultaneous equations in which coefficients are set based on the known thicknesses of a plurality of points of the subject, measurement results with little variation can be obtained regardless of temperature changes.

  In the step of obtaining each coefficient of the simultaneous equations, a lift-off that is a distance between the subject and the third coil may be changed at a specific calibration point of the subject.

  Thereby, it can be set as the simultaneous equation approximated by the actual subject. Thereby, the thickness of the subject at an unknown point can be estimated with higher accuracy, and the inspection accuracy can be further increased.

  The measurement processing unit performs the measurement of the amplitude ratio between the signal under measurement and the reference signal and the measurement of the phase difference between the signal under measurement and the reference signal with the first coil at different frequencies. You may make it carry out by exciting.

  Thereby, since an output with a large amplitude and phase can be obtained, a change in the detection signal due to a change in the thickness of the subject can be detected with higher sensitivity.

  The present invention is a nondestructive inspection method in a nondestructive inspection apparatus as described above, wherein the first coil is applied with an alternating voltage in a state where the first and third coils are opposed to the subject. The alternating voltage for exciting and exciting the first coil or the voltage generated in the second coil and the voltage difference between the voltage generated in the third coil and the voltage generated in the second coil are Input to the measurement processing unit, the amplitude and phase of the alternating voltage and the difference voltage input by the measurement processing unit are obtained by fast Fourier transform, and the difference voltage using the alternating voltage as a reference signal. And an amplitude ratio and a phase difference are detected.

  As a result, the amplitude and phase of the signal under measurement, which is the difference between the generated voltage of the second coil and the generated voltage of the third coil, can be obtained using the alternating voltage of the first coil or the generated voltage of the second coil as a reference signal. By detecting it, it is possible to suppress the effects of lift-off, which is the distance between the sensor and the subject, and temperature changes.

  According to the present invention, even when the distance between the sensor and the subject is large, the subject can be detected with high accuracy.

It is a figure showing a 1st embodiment of a nondestructive inspection device concerning the present invention. It is a figure which shows the sensor which comprises the nondestructive inspection apparatus of FIG. It is a figure which shows the nondestructive inspection apparatus which concerns on 2nd Embodiment. It is a figure which shows the sensor which comprises the nondestructive inspection apparatus of FIG. It is a figure which shows the nondestructive inspection apparatus which concerns on 3rd Embodiment. It is a figure which shows the sensor which comprises the nondestructive inspection apparatus of FIG. It is a figure which shows the nondestructive inspection apparatus which concerns on 4th Embodiment. It is a figure which shows other embodiment of this invention. It is a figure which shows other embodiment of this invention. It is a figure which shows other embodiment of this invention. In the Example of this invention, it is a figure which shows the result of having measured the amplitude ratio of the difference of the voltage of a detection coil and a reference coil by the fast Fourier transform. In the Example of this invention, it is a figure which shows the result of having measured the phase difference of the difference of the voltage of a detection coil and a reference coil by the fast Fourier transform. It is a figure which shows the result of having measured the amplitude ratio of the voltage of a detection coil as a comparative example. It is a figure which shows the result of having measured the phase difference of the voltage of a detection coil as a comparative example. It is a figure which shows the result of having performed the measurement by varying a frequency by the measurement of an amplitude, and a phase, and is a figure which shows the measurement result of an amplitude ratio. It is a figure which shows the result of having measured by varying a frequency by the measurement of an amplitude, and the measurement of a phase, and is a figure which shows the measurement result of a phase difference. It is the figure which expanded the vertical axis | shaft of FIG. 16, and showed a part. It is a figure which shows the result of having estimated the tube wall thickness of the subject by simultaneous equations, and is a tube wall thickness estimation result in the case of tube wall thickness 4.2mm. It is a figure which shows the result of having estimated the tube wall thickness of a test object by simultaneous equations, and is a tube wall thickness estimation result in case the tube wall thickness is 2.4 mm. FIG. 6 is a vector diagram showing calibration points and measured values obtained by changing lift-off, which is a distance between the subject 2 and the detection coil 104, at a specific calibration point of the subject 2. FIG. 21 is an enlarged view near the B calibration point in FIG. 20. FIG. 21 is an enlarged view near the C calibration point in FIG. 20. It is a figure which shows the structure of the sensor of the conventional nondestructive inspection apparatus.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out a nondestructive inspection apparatus and a nondestructive inspection method according to the present invention will be described with reference to the accompanying drawings.

(First embodiment)
FIG. 1 is a diagram showing a first embodiment of a nondestructive inspection apparatus according to the present invention. FIG. 2 is a diagram showing a sensor constituting the nondestructive inspection apparatus.
As shown in FIGS. 1 and 2, the nondestructive inspection apparatus includes a sensor 1 and a measurement device (measurement processing unit) 4.

  The sensor 1 includes an excitation core (core) 102 made of a magnetic material having an opening facing the subject 2, an excitation coil (first coil) 101 electromagnetically coupled to the excitation core 102, and the excitation core 102 similarly electromagnetically. Coupled to a reference coil (second coil) 105, a detection core 103 disposed inside the opening of the excitation core 102, and a detection that is electromagnetically coupled to the detection core 103 and faces the subject 2 A coil (third coil) 104 is provided.

  This sensor 1 excites the exciting coil 101 with an alternating signal, and electrically outputs the difference between the voltage generated in the reference coil 105 and the voltage generated in the third coil to the terminals 107b and 108b so as to output as a signal under measurement. It is connected.

  In FIG. 2, the reference coil 105 is wound around the outside of the exciting coil 101. However, when the inside of the exciting coil 101 or the first coil is a multi-layer winding, the reference coil 105 is wound in a sandwich manner between layers, and the coupling is tightly coupled. It is good also as making it.

  The measuring device 4 includes a computer 405, a display recorder 406, a digital / analog converter (DAC) 401, a power amplifier 402, a multiplexer 403, and an analog / digital converter (ADC) 404. Although the multiplexer 403 and the analog / digital converter (ADC) 404 have been described, the multiplexer 403 and the analog / digital converter (ADC) may be used.

  The measuring device 4 converts a digital signal obtained by synthesizing a plurality of sine waves generated by the computer 405 or a single frequency sine wave digital signal into an analog signal by a digital / analog converter 401. The converted analog signal is amplified by the power amplifier 402 and excites the exciting coil 101 via the terminals 106 a and 106 b of the sensor 1.

  The measuring device 4 excites the exciting coil 101 with an alternating voltage and receives a signal under measurement which is a difference between a voltage generated in the reference coil 105 and a voltage generated in the detection coil 104. Then, the measuring device 4 processes the amplitude and phase of the signal under measurement using the voltage of the excitation coil 101 as a reference signal, and estimates the thickness of the subject 2.

Hereinafter, the flow of processing for estimating the thickness of the subject 2 in the measurement apparatus 4 will be described.
First, the voltage of the output terminals 107 b and 108 b that outputs the difference between the voltages generated by the detection coil 104 of the sensor 1 and the reference coil 105 as a signal under measurement is input to the multiplexer 403.
On the other hand, the voltage between the terminals 106a and 106b of the exciting coil 101 is also input to the multiplexer 403 as a reference signal.

In the multiplexer 403, these two systems of input analog signals (signal under measurement, reference signal) are converted into digital signals by an analog / digital converter (ADC) 404 independently of each other and input to a computer 405.
The computer 405 performs signal processing on the input signal under measurement and the reference signal by fast Fourier transform (FFT), and outputs the processing result to the display recorder 406.

Here, the process in the measuring device 4 will be described in more detail.
As a result of the arithmetic processing by Fast Fourier Transform (FFT) in the measuring device 4, the amplitude and phase of each sine wave can be obtained simultaneously at the same measurement point position.
These do not change independently of each other, but change in correlation with each other. Therefore, it is more effective to measure and evaluate using an approximate expression that estimates the thickness D of the subject 2 using the amplitude and phase as variables, rather than measuring and evaluating each independently.
The approximate expression of the thickness D of the subject 2 is assumed to be linear approximation and will be described using Expression (21). Assuming that the amplitude ratio is x, the phase difference is y, and the thickness of the object is D, x and y are originally functions of D, but D is an inverse function of x and y, and the linear form shown in Equation (21) Define as

In Equation (21), the coefficients a and b and the constant term c are obtained by measuring three calibration points having different thicknesses and solving simultaneous equations such as Equation (22) based on the measured values.

In addition, although explained as three points having different thicknesses, two points having different thicknesses of the subject are taken out of the three points, and an element having a large variation factor to be given to the measured value at one of the two points. Is preferably included.
It is effective to use lift-off as a large factor of the variation factor. For this purpose, two points B, C having different thicknesses of the subject are taken, and the thicknesses of these portions are physically measured to be D2, D3.
If the point where the lift-off is slightly changed at point B of thickness D2 is point A, then D1 = D2, and therefore, simultaneous equations such as equation (23) are established.

  Accordingly, the coefficients a and b and the constant term c are obtained by solving the simultaneous equations (23) from the measurement values (x1, y1), (x2, y2), (x3, y3) of the A, B, and C3 sets. The solutions of the simultaneous equations are shown in equations (25), (26), and (27). In order to simplify the mathematical expression, if the expression corresponding to the denominator is t, it is expressed as the mathematical expression (24).

By determining the coefficients a and b and the constant term c in this manner, the thickness D of the subject at that point can be obtained from the x and y measurement values at an arbitrary point using Equation (21). .
In this way, the thickness of the subject 2 can be detected while suppressing the influence of the lift-off change.

Furthermore, the thickness of the subject 2 can be estimated with high accuracy by the method described above. The reason will be described below.
The sum of two general trigonometric functions with different angles is expressed by the formulas (1), (2), and (3).

  Here, c and β in the formula (1) are expressed by the formulas (2) and (3). In this case, it is assumed that the output of the detection coil 104 is b and the output of the reference coil 105 is a.

  On the other hand, the tiller expansion of sinx and arctanx is expressed by equations (4) and (5).

  Since a minute angle difference is handled, the condition of equation (6) is satisfied, and if only the first term of equations (4) and (5) is taken, equations (7) and (8) are satisfied.

  In this case, since it is a difference between two general trigonometric functions, if γ is used again and the absolute value of a is slightly smaller than b and the sign is reversed, the equation (9) is established. Get.

  Here, if γ = 0.1, that is, | a | is 90% of b, for example, β = 10α, and the angle difference is enlarged 10 times. However, it should be noted that if γ is remarkably reduced, it will diverge to infinity.

  On the other hand, since cos α is approximately 1, the amplitude is as shown in equations (11) to (12).

The minute change is additive as shown in equation (12). In the case of the present embodiment, the reference coil 105 and the detection coil 104 are connected in reverse polarity. Therefore, since Δa is negative, the change is canceled out and becomes smaller.
Therefore, the change in the resistance of the winding of the exciting coil 101 due to the temperature acts almost equally on the reference coil 105 and the detection coil 104, so that they are canceled out and a more stable measurement is possible.

  On the other hand, since the changes with respect to the thickness change of the subject 2 are in opposite directions as shown in FIGS. 15 and 17 to be described later, these changes are added, and the sensitivity is increased.

  Further, when measuring the amplitude ratio and the phase difference, the measurement of the amplitude ratio and the measurement of the phase difference may be performed by exciting the exciting coil 101 at mutually different frequencies. By exciting the exciting coil 101 using different frequencies, the detection signal due to the change in the tube wall thickness of the subject 2 can be made larger, so that the accuracy of the tube wall thickness estimation using simultaneous equations can be improved.

According to the above-described nondestructive inspection apparatus, the excitation coil 101 is excited with an alternating voltage to generate a voltage between the reference coil 105 electromagnetically coupled to the excitation coil 101 and the voltage generated at the detection coil 104. There is a difference. By detecting the amplitude and phase of the signal under measurement, which is the difference between the voltage generated by the reference coil 105 and the voltage generated by the detection coil 104, using the voltage of the excitation coil 101 as a reference signal, the sensor 1 and the subject 2 It is possible to suppress the effect of lift-off, which is distance, and temperature change.
In this way, even in a state where the lift-off is large, it is possible to accurately detect a change in signal due to a change in the thickness of the subject 2, an internal flaw, and the like, and the inspection accuracy can be improved.

  Further, by changing the lift-off, which is the distance between the subject 2 and the detection coil 104, at the specific calibration point of the subject 2, simultaneous equations that are more approximate to the actual subject 2 can be obtained. Thereby, the thickness D of the subject at an unknown point can be estimated with higher accuracy, and the examination accuracy can be further increased.

  In the above embodiment, the configuration in which the reference signal and the signal under measurement are processed by the multiplexer 403 and one analog-to-digital converter 404 is shown. The digital converter may be configured to process the reference signal and the signal under measurement.

(Second Embodiment)
Next, a second embodiment of the nondestructive inspection apparatus and the nondestructive inspection method according to the present invention will be described. Note that in the second embodiment described below, the same reference numerals are given to the same components as in the first embodiment, and description thereof will be omitted.
FIG. 3 is a view showing a nondestructive inspection apparatus according to the second embodiment. FIG. 4 is a diagram showing a sensor constituting the nondestructive inspection apparatus.
As shown in FIGS. 3 and 4, the nondestructive inspection apparatus in this embodiment is reversely connected to the configuration shown in the first embodiment according to the polarities of the detection coil 104 and the reference coil 105, and the difference is obtained. In addition to the detected output terminals 107b and 108b, a terminal 108a for taking out the output of the reference coil 105 is provided. The terminals 108a and 108b are electrically connected so that the voltage between the terminals 108a and 108b is input to the multiplexer 403 as a reference signal.

By doing so, as in the first embodiment, the signal from the lift-off, which is the distance between the sensor 1 and the subject 2, and the influence of the temperature change, the change in the thickness of the subject 2, an internal flaw, etc. Can be accurately detected.
In addition, unlike the first embodiment, since the output voltage of the reference coil 105, which is the second coil, is used as a reference, it is possible to ignore changes up to that point, that is, changes due to the temperature of the winding resistance. It becomes possible. Furthermore, since the change due to the difference in the thickness of the subject 2 is added in addition to the change in the reference coil 105, a large test output with little temperature change can be obtained.

  In the second embodiment, the terminal 108a is taken from the reverse connection point of the reference coil 105 and the detection coil 104, but is not limited thereto. For example, a tap may be provided in the reference coil 105, and the reverse connection point and the extraction point of the terminal 108a may be extracted from arbitrary taps independently of each other. Further, another coil coupled to the exciting coil 101 may be provided for obtaining a reference signal.

(Third embodiment)
Next, a third embodiment of the nondestructive inspection apparatus and the nondestructive inspection method according to the present invention will be described. Note that in the third embodiment described below, components that are the same as those in the first embodiment are denoted by the same reference numerals in the drawings, and description thereof is omitted.
FIG. 5 is a view showing a nondestructive inspection apparatus according to the third embodiment. FIG. 6 is a diagram showing a sensor constituting the nondestructive inspection apparatus.

As shown in FIGS. 5 and 6, the nondestructive inspection apparatus according to this embodiment differs from the configuration shown in the first embodiment in that the sensor 1 is independent of the detection coil 104 and the reference coil 105. Terminals 107a, 107b, 108a, and 108b are provided, and output is made to the measuring device 4 through these terminals 107a, 107b, 108a, and 108b.
Further, the measuring device 4 is configured to input the voltage of the reference coil 105 to the multiplexer 403 as a reference signal. Further, the measuring device 4 is provided with an operational amplifier 407 and an adding circuit composed of resistors R1, R2, and R3. This adding circuit is electrically connected so as to take the difference between the outputs of the detection coil 104 and the reference coil 105. Although the circuit itself composed of the resistors R1, R2, and R3 is an adder circuit, since the output of the reference coil 105 is input to the resistor R2 with a reverse polarity, the output of the detection coil 104 is substantially changed from the output of the reference coil 105. It functions as a subtraction circuit that subtracts the output. The resistor R2 is semi-fixed so that the degree of subtraction can be adjusted.

By doing so, as in the first embodiment, the signal from the lift-off, which is the distance between the sensor 1 and the subject 2, and the influence of the temperature change, the change in the thickness of the subject 2, an internal flaw, etc. Can be accurately detected.
Furthermore, even when the lift-off is large, the temperature drift is small, and it is possible to extract a large amplitude and phase change output corresponding to the change in the tube wall thickness of the subject 2.

  In the above-described embodiment, the subtraction function in the adder circuit composed of the resistors R1, R2, and R3 may be implemented by, for example, processing by the computer 405 after analog / digital conversion in the analog / digital converter 404. However, as shown in the above-described embodiment, the higher signal-to-noise ratio (S / S) is obtained when the arithmetic processing is performed by providing an adder circuit closer to the preceding stage on the terminals 107a, 107b, 108a, and 108b, which are signal sources. N ratio) can be secured.

(Fourth embodiment)
Next, a fourth embodiment of the nondestructive inspection apparatus and nondestructive inspection method according to the present invention will be described. Note that in the fourth embodiment described below, components that are the same as in the third embodiment are denoted by the same reference numerals in the drawing, and description thereof is omitted.

When there is a fixed phase difference P between the output signal of the reference coil 105 and the output signal of the detection coil 104, and there is a slight phase change ΔP due to the thickness variation of the subject 2 in this state, the reference coil Even if it is connected so as to take a difference signal between the output of 105 and the detection coil 104, the amplitude of the difference signal cannot be reduced due to the fixed phase difference P. In such a case, it is necessary to remove the influence of the fixed phase difference P.
FIG. 7 is a view showing a nondestructive inspection apparatus according to the fourth embodiment. This nondestructive inspection apparatus can remove the influence of the fixed phase difference P. Here, a case where the output signal of the detection coil 104 is delayed by the phase P from the signal of the reference coil 105 will be described.

  As shown in FIG. 7, the nondestructive inspection apparatus according to the present embodiment is an arithmetic circuit including an operational amplifier 408, resistors R4 and R6, a semi-fixed resistor R5, and a capacitor C1 in addition to the configuration shown in the third embodiment. Is provided. The output of the reference coil 105 is input to the input of such an arithmetic circuit.

  This arithmetic circuit functions as a delay phase shifter. That is, when R4 = R6, when the semi-fixed resistor R5 is changed, the output of the operational amplifier 408 functions so that only the phase changes and the amplitude does not change. By such a delay phase shifter, the output signal of the reference coil 105 is delayed by the phase P, and the signal is input to the multiplexer 403 as a reference signal, and the operational amplifier 407, resistors R1, R3 are used as signals for taking the difference. Are input to R2 of the arithmetic circuit including the semi-fixed resistor R2.

  By doing so, it becomes possible to remove and detect the influence of the constant phase difference P and to enlarge and detect a minute phase change due to a change in the thickness of the subject.

(Other embodiments)
The present invention is not limited to the above-described embodiments described with reference to the drawings, and various modifications are conceivable within the technical scope thereof.
8 to 10 show another embodiment of the present invention.
For example, as shown in FIG. 8, the exciting coil 101, the reference coil 105 electromagnetically coupled thereto, and the detection coil 104 facing the subject can all be air core coils. The excitation coil 101 and the reference coil 105 are wound around, for example, a synthetic resin bendable pipe, and are electrically connected so as to take a voltage difference between the voltage of the detection coil 104 and the voltage of the reference coil 105. .
Thus, since the excitation coil 101, the reference coil 105, and the detection coil 104 are air cores, no magnetic poles are generated unlike when a magnetic core is used. Therefore, the change with respect to lift-off becomes gradual.

Further, as shown in FIG. 9, all of the exciting coil 101, the reference coil 105, and the detecting coil 104 can be air core coils. Here, the reference coil 105 is electromagnetically coupled to the exciting coil 101 by being provided in close contact with the inner peripheral side of the exciting coil 101. In addition, the detection coil 104 was disposed at a position closer to the subject 2 than the excitation coil 101 and the reference coil 105.
According to such a configuration, there is convenience in simple inspection.

Further, as shown in FIG. 10, the excitation coil 101, the reference coil 105, and the detection coil 104 are all air-core coils, and the subject is penetrated. Further, the excitation coil 101, the reference coil 105, and the detection coil 104 are decentered without causing the center axis C1 of each coil and the center axis C2 of the subject 2 to coincide.
Such a configuration is effective for flaw detection of internal flaws close to the central axis of the subject 2.

In the above embodiment, when the subject 2 is a carbon steel pipe having magnetism and conductivity, and a large lift-off on the heat insulating material is taken, it is extremely difficult to detect the tube wall thickness, inner surface scratches, etc. However, the subject is not limited in any way. For example, the present invention can be applied to an analyte that is relatively easy to detect, has no magnetism, and has only conductivity, such as an austenitic stainless steel pipe.
In addition to this, the configuration described in the above embodiment can be selected or changed to another configuration as appropriate without departing from the gist of the present invention.

[Example]
Since the configuration shown in the first embodiment has been verified, the results are shown below.
The subject 2 was measured by the nondestructive inspection apparatus shown in FIG.
The subject 2 is covered with a heat insulating material 3.
The subject 2 was a magnetic carbon steel pipe 65ASGP having an outer diameter of about 76.3 mm and a wall thickness of 4.2 mm, and the inside was cut so that a part thereof had a wall thickness of 2.4 mm.
The heat insulating material 3 was glass wool having a thickness of 20 mm. The outer surface of the heat insulating material 3 was covered with an aluminum foil having both a thickness of 0.1 mm for heat reflection and dew prevention.
The distance between the outer surface of the subject 2 and the surface of the sensor 1 facing the subject 2, that is, the lift-off, was 23 mm including the movable space.

In the sensor 1, a laminated directional silicon electromagnetic steel sheet having a cross-sectional area of 2 cm 2 and a magnetic path length of 17 cm was used as the exciting core 102, and a formal wire having a diameter of 1 mm was wound 210 times in total on both legs as the exciting coil 101.
Further, a formal wire having a diameter of 0.2 mm was wound 640 times as the reference coil 105.
As the detection core 103, a laminated directional silicon electromagnetic steel sheet having a cross-sectional area of 0.75 square cm and a magnetic path length of 10 cm was used. The detection coil 104 was a total of 2400 turns of a formal wire having a diameter of 0.2 mm.

FIG. 11 and FIG. 12 show the results of measuring the voltage between the terminals 106a and 106b, that is, the voltage of the detection coil 104 at a peak value of 3V and a frequency of 35Hz by fast Fourier transform (FFT) with reference to the voltage of the reference coil 105. .
Further, as a comparative example, the results of measuring the output of the detection coil 104 without taking the difference from the output of the reference coil 105 are shown in FIGS. In these graphs, the width of the vertical axis is the same for both amplitude and phase for easy comparison.

11 and 13 show the measurement results of the amplitude ratio, and the amplitude (crest value) of the detected voltage between the terminals 108b and 107b is the voltage between the terminals 106a and 106b, that is, the amplitude (crest value) of the voltage of the exciting coil 101. The amplitude ratio was obtained by dividing. This amplitude ratio is a value that is three times the relationship of the measurement range of the analog-digital converter 404, and the actual ratio is 1/3 of this.
12 and 14 show the measurement results of the phase difference (radian).
In FIGS. 11 to 14, the first half measures a site with a tube wall thickness of 4.2 mm, and the second half measures a site with a tube wall thickness of 2.4 mm.

As shown in FIGS. 11 and 13, the amplitude ratio is almost the same. However, as shown in FIGS. 12 and 14, the phase difference in this embodiment is the tube wall thickness of 4.2 mm and 2.4 mm. Deviation signal output due to differences in thickness is large.
Tables 1 and 2 are detailed data of FIGS. As can be seen from this data, the present embodiment, which uses the difference between the outputs of the reference coil 105 (described later) and the detection coil 104 (described later) as a detection signal, has a large lift-off of 23 mm on the heat insulating material. Regardless, the phase difference is about 41 times more sensitive than the comparative example.

FIGS. 12 to 13 and Tables 1 and 2 show the results of measurement at a single frequency of 35 Hz. Next, measurements were performed with different frequencies for amplitude measurement and phase measurement.
For this purpose, a computer 405 digitally generates a composite signal of sine waves of a plurality of frequencies, converts it into an analog signal by a digital / analog converter 401, and excites the exciting coil 101 via the power amplifier 402. The amplitude and phase for each frequency were measured by fast Fourier transform (FFT). At this time, the amplitude ratio was 15 Hz and the phase difference was 35 Hz, and the excitation coil 101 was excited to perform measurement.
FIG. 15 shows the amplitude ratio obtained by subtracting the value when the tube wall thickness is 4.2 mm from the value when the tube wall thickness is 2.4 mm, and FIG. 16 shows the phase difference. FIG. 17 is an enlarged view of a part of the vertical axis of FIG.

As apparent from the comparison of the respective coil outputs of the amplitude ratio in FIG. 15, the direction of the change due to the thickness of the subject is opposite between the reference coil 105 and the detection coil 104. Therefore, in the present embodiment, which takes the difference between the reference coil 105 and the detection coil 104, the amount of change due to the thickness of the subject is added, resulting in a large amplitude ratio near 15 Hz.
Further, as shown in FIG. 16, the present example shows a very large change in phase difference as compared with the comparative example, and shows that there is a peak of the change in phase difference in the vicinity of 35 Hz.
That is, it is clear that the maximum frequency of change in amplitude ratio and the maximum frequency of change in phase difference do not always coincide. Therefore, it is effective to select the best frequencies for the amplitude ratio and the phase difference change. That is, by setting the amplitude ratio and the phase difference to different frequencies, the detection signal due to the change in the tube wall thickness of the subject can be increased.
Also, in FIG. 17, it is clear that the change due to the difference in the subject tube wall thickness between the reference coil 105 and the detection coil 104 is opposite, and in the case of this embodiment that takes the difference, it is additive.

Next, using the circuit shown in FIG. 1, the amplitude ratio and phase difference are obtained by fast Fourier transform (FFT) at a lift-off of 23 mm (on a heat insulating material), and the tube wall thickness of the subject 2 is estimated by simultaneous equations from the amplitude ratio and phase difference. Measured with
As a comparative example, as shown in FIG. 23, the tube wall thickness of the subject 2 was detected using the output of only the detection coil 104.
In addition, the tube wall thickness of the subject 2 was set to 4.2 mm and 2.4 mm.

The results are shown in FIGS. 18 shows the tube wall thickness estimation result when the tube wall thickness is 4.2 mm, and FIG. 19 shows the tube wall thickness estimation result when the tube wall thickness is 2.4 mm.
As shown in FIGS. 18 and 19, the conventional method using only the detection coil 104 has a large variation in detection results and a large drift due to a temperature change or the like at any tube wall thickness.
On the other hand, in the embodiment, there is little variation and there is very little drift.

  Table 3 shows detailed numerical values, and the standard deviation of the estimated value is 1/10 or less of the prior art in the embodiment of the present invention, which indicates that accurate measurement is possible.

Next, the lift-off, which is the distance between the subject 2 and the detection coil 104, was changed at a specific calibration point of the subject 2.
FIG. 20 is a vector diagram showing calibration points and measured values. FIG. 21 is an enlarged view near the B calibration point in FIG. FIG. 22 is an enlarged view of the vicinity of the C calibration point in FIG.
As shown in FIG. 20, the B calibration point is calibrated at a lift-off of 23 mm and a tube wall thickness of 4.2 mm, the C calibration point is at a lift-off of 23 mm, and the tube wall thickness is 2.4 mm. The A calibration point has the same tube wall thickness as the B calibration point, but is calibrated at 23-0.2 = 22.8 mm with slightly lower lift-off. Therefore, the tube wall thickness is distributed on the BC straight line or on its extension.
Then, as shown in FIGS. 21 and 22, due to the nature of the simultaneous equations, a pipe wall thickness estimation is made by drawing a straight line parallel to the AB straight line from the position of each measured value and estimating the intersection with the BC straight line from each measured value. Can be a value.

1 Sensor 2 Subject 4 Measuring Device (Measurement Processing Unit)
101 Excitation coil 102 Excitation core (core)
103 Detection core (first coil)
104 Detection coil (third coil)
105 Reference coil (second coil)

Claims (5)

A first coil facing the subject;
A second coil electromagnetically coupled to the first coil;
A third coil facing the subject;
The first coil is excited with an alternating voltage, and the amplitude and phase of the signal under measurement, which is the difference between the voltage generated in the second coil and the voltage generated in the third coil, A measurement processing unit for detecting the voltage of the second coil as a reference signal;
A nondestructive inspection apparatus comprising: The measurement processing unit is configured to input the reference signal and the signal under measurement;
Obtaining an amplitude ratio and a phase difference between the input signal under measurement and the reference signal by fast Fourier transform;
Establishing simultaneous equations using the amplitude ratio and the phase difference as variables, and obtaining each coefficient of the simultaneous equations from the known thicknesses of the plurality of points of the subject and the measured values of the amplitude ratio and phase difference at each point When,
Estimating the thickness of the subject at the unknown point from each of the obtained coefficients and the measured values of the amplitude ratio and phase difference at the unknown point;
The nondestructive inspection apparatus according to claim 1, wherein:   The step of obtaining each coefficient of the simultaneous equations changes a lift-off that is a distance between the subject and the third coil at a specific calibration point of the subject. Nondestructive inspection equipment.   The measurement processing unit performs the measurement of the amplitude ratio between the signal under measurement and the reference signal and the measurement of the phase difference between the signal under measurement and the reference signal with the first coil at different frequencies. The nondestructive inspection apparatus according to any one of claims 1 to 3, wherein the nondestructive inspection apparatus is performed by excitation. It is a nondestructive inspection method in the nondestructive inspection device according to any one of claims 1 to 4,
With the first and third coils facing the subject, the first coil is excited with an alternating voltage,
An alternating voltage for exciting the first coil or a voltage generated in the second coil and a voltage of a difference between a voltage generated in the third coil and a voltage generated in the second coil are measured in the measurement processing unit. Enter
In the measurement processing unit, for the input alternating voltage and the difference voltage, the amplitude and phase are obtained by fast Fourier transform,
A non-destructive inspection method, wherein an amplitude ratio and a phase difference with the difference voltage with the alternating voltage as a reference signal are detected.

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