JPH0748068B2 - Eddy current flaw detection method and device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an eddy current flaw detection method and apparatus for nondestructively detecting scratches, cracks, defects and the like on a metal surface or in the vicinity of the surface.

[Conventional technology]

The conventional eddy current flaw detection method consists of a bridge consisting of two coils, a detection coil and a compensation coil, to balance them, output the phase difference and voltage change, observe the pattern drawn on the CRT, shake the analog meter, A method of discriminating from patterns on recording paper is used, and it requires specialized knowledge to discriminate such scratches. Further, in the conventional device, the voltage amplitude at both ends of the coil is larger when the detection coil is placed on the ferromagnetic material than when the detection coil is placed on the free space. Although the zero point adjustment was performed after placing, the microscopic projections may be present on the inspected object, the paint may be applied, or the so-called lift-off may occur, which varies the distance between the detection coil and the inspected object. This often causes the output of the detection coil to fluctuate, making it difficult to distinguish from the signal due to cracks,
Also, when inspecting from the top of the coating, it was necessary to adjust the zero point again. Furthermore, in the bridge method, the influence of temperature and power supply voltage fluctuations is large, and as the crack depth increases, the voltage change rate of the coil decreases, making it difficult to quantitatively measure crack depths deeper than 1 mm. Met.

[Problems to be Solved by the Invention]

In view of the above-mentioned situation, the present invention is to solve by detecting with high sensitivity scratches, cracks and defects on or near the surface of a metal including a ferromagnetic material, which has been relatively difficult until now. In addition to being able to quantitatively measure the depth of the scratches, the change in the distance between the metal surface and the detection coil, that is, the output change due to lift-off is minimized and only the signal due to the scratches is output. It is an object of the present invention to provide an eddy current flaw detection method and device which can be performed and which is less affected by temperature and power supply voltage fluctuations.

[Means for Solving the Problems]

In order to solve the above-mentioned problems, the present invention provides a detection coil and a compensation coil having the same resonance characteristics and having the same characteristics, the resonance frequency when the detection coil is located in free space and the detection coil located on the metal surface for flaw detection. In the intermediate frequency band of the resonance frequency, an alternating current having a frequency at which the outputs of both ends of the detection coil substantially match in both the free space and the metal surface is supplied, and the voltages generated between the ends of the detection coil and the compensation coil are compared. By establishing a eddy current flaw detection method that detects only the amplitude change caused by flaws, cracks and defects near or on the metal surface flaw-detected by the detection coil, and further reduces the influence of lift-off, Amplitude changes caused by scratches, cracks, and defects near or on the metal surface that occur at both ends of the coil, and the detection coil and compensation coil that occur when the detection coil approaches the metal surface. Than is corrected by adding a correction voltage proportional to the phase difference across the output Le.

More specifically, a detection coil and a compensation coil, which are wound around a ferrite core and are connected in parallel with capacitors having the same characteristics, have the same characteristics. An alternating current having a frequency at which the outputs of both ends of the coil are substantially the same is supplied through a resistor, and the alternating voltage generated at both ends of the detection coil and the compensation coil is rectified by the rectifier circuit to the opposite polarity, and then the opposite polarity Apply a DC voltage to both ends of the potentiometer's resistor, and use the linearization circuit to output the amplitude change signal resulting from flaws, cracks, or defects on or near the metal surface detected by the detection coil output from the slider. An eddy current flaw detection device was constructed by converting into flaw detection voltage signals proportional to the depth and length of cracks and the like.

Then, in order to further reduce the influence of lift-off, the AC voltage outputs at both ends of the detection coil and the compensation coil are branched, and one of them is input to the rectifier circuit and the other is input to comparators set to opposite polarities. Then, both outputs of the comparator are applied to both ends of the resistor of the potentiometer, and a pulse signal having a pulse width proportional to the phase difference output from the slider is supplied to the rectifier circuit with a correction voltage proportional to the pulse width. The signal is converted into a signal and the correction voltage signal is added to the amplitude change signal.

Further, the output of the linearization circuit is detected visually and / or audibly by a display and / or a sound generator.

[Action]

According to the eddy current flaw detection method and apparatus of the present invention having the above contents, the resonance frequency of the detection coil located in the free space and the resonance frequency of the detection coil located on the metal surface to be flawed are different, but the resonance frequency between both resonance frequencies is different. Utilizing the fact that there is a specific frequency where the outputs of both ends of the detection coil match in both the free space and on the metal surface in the intermediate band, the detection coil and the compensation coil having the same resonance characteristics are supplied. By making the frequency of the alternating current substantially match the specific frequency,
By scanning the metal surface with the detection coil in a state in which the influence of lift-off of the detection coil is suppressed to a minimum, only the amplitude change of the voltage due to scratches, cracks, and defects on or near the metal surface is detected. It is detected by comparing the voltage across the coil and the voltage across the compensation coil.

Further, in order to further reduce the influence of lift-off of the detection coil, the fact that the detection coil has a phase delay that is less dependent on frequency and changes substantially in proportion to the distance between the metal surface and the detection coil is used. The phase difference between the coil and the compensation coil is detected, a correction voltage proportional to this phase difference is generated, and the correction voltage is added to the amplitude change due to cracks or the like to perform correction.

Specifically, the alternating current of the specific frequency from a variable frequency single oscillator is wound around a ferrite core, and a detection coil and a compensation coil of the same characteristic, in which capacitors of the same characteristic are respectively connected in parallel, are connected via a resistor. The voltage is supplied, and the AC voltage generated at both ends of the detection coil and the compensation coil is rectified by a rectifier circuit of opposite polarity to be positive and negative, and converted into a DC voltage. The pendulum is moved to adjust the zero point, and the detection coil is scanned over the metal surface in that state, whereby the amplitude change of the voltage due to scratches, cracks, and defects on the metal surface or near the surface generated at both ends of the detection coil. A signal is generated on the slider of the potentiometer, and the amplitude change signal is converted into a flaw detection voltage signal proportional to the depth of a crack or the like by a linearization circuit and output. . Here, the flaw detection voltage signal is displayed on a display, a tone sound is generated by a sounder in proportion to the depth and length of the crack, and both are detected at the same time.

Further, the AC voltage outputs at both ends of the detection coil and the compensation coil are respectively branched, one of which is input to the rectifier circuit as described above, and the other of which is input to comparators set to opposite polarities and converted into rectangular waves. After that, both square wave output signals are input to both ends of the resistor of the potentiometer arranged separately from the potentiometer, and the position of the slider is adjusted to be proportional to the phase difference of the AC voltage generated in the detection coil and the compensation coil. Converted into a positive / negative pulse signal of the pulse width, converted into a correction voltage signal in proportion to the pulse width in the rectification circuit, the correction voltage signal in addition to the amplitude change signal due to the crack, the detection The fluctuation of the output amplitude due to the lift-off of the coil is corrected to the true flaw detection voltage signal due to the crack.

〔Example〕

Next, the details of the present invention will be described based on the embodiments shown in the accompanying drawings.

FIG. 1 is a simplified block diagram showing a typical embodiment of the present invention, in which an alternating current is generated by a variable frequency single oscillator 1,
The current is input to two current amplifiers 2 and 3 having the same amplification factor to be amplified to a desired current value, and the respective coils are wound around a ferrite core 6 through resistors 4 and 5 as shown in FIG. 7 and a similar compensation coil 8 having the same characteristics as the detection coil 7
Supply to. Capacitors 9 and 10 having the same characteristics are connected in parallel to the detection coil 7 and the compensation coil 8, respectively, and have a resonance characteristic as shown in FIG. That is, in this embodiment, the resistor is adjusted so that the resonance frequency is about 80 to 130 kHz.
The resistance values of 4,5, the inductances of the detection coil 7 and the compensation coil 8 and the capacitances of the capacitors 9 and 10 are properly set. Of course, even if the capacitors 9 and 10 are not connected, the resonance frequency is caused by the presence of stray capacitance. However, the frequency is generally as high as several hundreds of kHz, and there are many problems in handling the oscillator 1 and transmission of AC current, which causes a problem. There is a risk that the phase delay at the point will become too large and cause inconvenience.

Here, the detection coil 7 and the compensation coil 8 have a maximum voltage at both ends at a frequency f ₀ in free space as shown in FIG. 3, while on the other hand, on the metal surface of a ferromagnetic material such as iron or steel. When positioned, it becomes maximum at a frequency f ₁ lower than the frequency f ₀ , and when positioned on a metal surface of a non-magnetic material such as aluminum, it becomes maximum at a frequency f ₂ higher than the frequency f _0. Become. Therefore, in the case of a ferromagnetic material, the frequency f ₀ and
In the intermediate band of f ₁ , there is a specific frequency f _{3 at} which the voltage across the detection coil 7 is equal in free space and on the ferromagnetic material, while in the case of a non-magnetic material, there is the frequency f ₀ .
In the intermediate band of f ₂ , there is a specific frequency f _{4 at} which the voltage across the detection coil 7 becomes equal in free space and on the ferromagnetic material. In the present invention, the specific frequency of the alternating current supplied to the detection coil 7 and the compensation coil 8 is set to f ₃ depending on whether the inspection object A is a ferromagnetic material or a non-magnetic material.
It is to be set within ± 5% in either f _4.

AC voltage outputs V ₁ and V ₂ generated at both ends of the detection coil 7 and the compensation coil 8 are amplifiers having the same amplification factor.
After amplifying to a desired voltage value at 11 and 12, it is rectified to a direct current by the rectifying circuits 13 and 14 set to opposite polarities. For example, as the rectifier circuit 13, the AC voltage output V ₁ from the detection coil 7 is rectified to a positive DC voltage by the forward diode 15 and the smoothing capacitor 16, while the rectifier circuit 13 is used.
As for 14, the AC voltage output V ₂ from the compensation coil 8 is rectified into a negative DC voltage by a diode 17 and a smoothing capacitor 18 in the reverse direction.

Then, the reverse polarity DC voltage rectified by the rectifier circuits 13 and 14 is input to both ends of the resistor 20 of the potentiometer 19, and the detection coil 7 is positioned in the free space or on the metal surface. When the detection coil 7 and the compensation coil 8 generate outputs with the same amplitude, the position is adjusted so that no voltage is generated in the slider 21, and the zero point is adjusted. The detection coil 7 with a second
As shown in the figure, the detection coil 7 is brought into contact with the surface of the inspected object A made of metal, or is scanned at a fixed minute interval, and scratches, cracks and defects a existing on the surface or in the vicinity of the surface are detected. That is, when the tip of the ferrite core 6 passes, the magnetic field created by the eddy current induced on the metal surface changes due to the fluctuating magnetic field due to the alternating current supplied to the detection coil 7,
As a result, an amplitude change occurs in the AC voltage output V ₁ of the detection coil 7, and this amplitude change is amplified by the amplifier 11 in the slider 21 of the potentiometer 19 and output as the amplitude change signal V ₃ . is there. Here, the detection coil 7 is
It is installed in a probe (not shown) connected to the device body,
On the other hand, although the compensation coil 8 is usually built in the main body of the apparatus, it may be brought into contact with a compensating material composed of the same material as the object A to be inspected so as to be in the same state as the detection coil 7. It is also possible to install the probe in a similar probe and move it along with the detection coil 7 on the surface of the object A to be inspected while compensating for the unevenness of the surface and the material change. Further, it is also practical to prepare a plurality of compensation coils 8 that are brought into contact with a plurality of types of compensation materials in advance and switch the rotor switch according to the material to be flaw-detected by the detection coil 7.

Now, scratches generated on the slider 21 of the potentiometer 19,
The amplitude change signal V ₃ caused by the crack and the defect a increases or decreases according to the depth and the length of the flaw, the crack and the defect a, but is not completely proportional to the depth and the length thereof.
Therefore, the amplitude change signal V ₃ is amplified by the amplifier 22 by a factor of 10 to 100, and then the linearizing circuit 23 having a non-linear amplification or attenuation function, as shown in FIG. It is converted into a flaw detection voltage signal V ₄ which is proportional to the depth and length.

Here, the calibration of the flaw detection voltage signal V ₄ uses a pseudo-crack sample of which depth is known in advance, and the sample has a width of 0.2 mm with a discharge wire cut or a cutter grinder,
Use the one with a groove with a depth of 1, 3, 5 mm. It should be noted that, although the calibration curve changes when the material is different, a torch of a material that is significantly different uses a calibration sample of the same material. Thus, the calibrated flaw detection voltage signal V ₄ has a crack depth of 0 to
It corresponds to the depth with an accuracy of ± 0.2 mm in the range of 5 mm.

Then, the flaw detection voltage signal V ₄ proportional to the depths of the flaws, cracks, and defects a thus obtained is visually output to the display 24 such as an analog meter, a digital display, an oscilloscope, or the like. The flaw detection voltage signal V ₄ is output aurally by a sounding device 26 such as a speaker or an earphone via a voltage controlled oscillator 25 that modulates the current output having a frequency proportional to the voltage, or both are used in combination. In particular, when the sounding device 26 is used, for example, it is set to emit a tone of 1 kHz for a crack depth of 1 mm and a tone of 5 kHz for a crack depth of 5 mm, and the existence of scratches, cracks, and defects a and the depth thereof. It is possible to concentrate the line of sight on the detection coil 7, that is, the tip of the probe while distinguishing with the tone of sound, so that even when inspecting a complicated object A to be inspected, one person can easily perform the work. Further, the flaw detection voltage signal V ₄ so as to correspond to a plurality of crack depth range, for example 0 to 1 mm, 1 to 2 mm, when該探flaw voltage signal V ₄ is corresponding to ... range, "1 mm", "2 mm It is also practical to use a voice synthesizer so as to emit a voice such as ", ...".

As described above, by setting the frequency of the alternating current supplied from the oscillator 1 to the detection coil 7 and the compensation coil 8 in the vicinity of f ₃ or f ₄ , the detection coil 7 and the inspection object A are
It is possible to minimize the effect of lift-off that changes the distance from the surface of the, but the frequency of the frequency-variable oscillator 1 incorporated in the portable lightweight device body is manually changed to f ₃ or f ₄ . It is impossible to make a perfect match. Therefore, as shown in FIG. 6, a slight effect of lift-off remains in the area within a distance of about 0.5 mm from the metal surface. However, in the present invention, the correction circuit 27 capable of further reducing the effect is also provided. It is provided.

In the correction circuit 27 shown as an embodiment in FIG. 5, the AC voltage output V ₁ of the detection coil 7 after being amplified by the amplifier 11 is used.
Is input to the rectifier circuit 13, and the other is input to the comparator 28 in which a threshold value is set to a predetermined voltage to input a rectangular wave.
In addition to generating V ₅ , the AC voltage output V ₂ of the compensation coil 8 after being amplified by the amplifier 12 is branched, one is input to the rectifier circuit 14, and the other is input to the comparator 29 in which the threshold value is set similarly. A rectangular wave V ₆ having a polarity opposite to that of the rectangular wave V ₅ is generated by inputting both rectangular wave voltages V ₅ and V ₆ at both ends of a resistor 31 of a potentiometer 30 provided separately from the potentiometer 19. is applied to, and set the slider 32 in a position balanced for both rectangular wave voltage V _5, V _6, both the rectangular wave voltage V _5, V ₆ outputted from sliding Doko 32 Applied voltage,
That is, the pulse signal V ₇ having a pulse width proportional to the phase difference between the AC voltage outputs V ₁ and V ₂ is applied to the diode 33 and the smoothing capacitor 34.
Input to the rectifier circuit consisting of
It is designed to generate V ₀ . The correction voltage signal
As shown in FIG. 7, V ₀ sharply decreases according to the distance from the metal surface, and this correction voltage signal V ₀ is
After being set to a predetermined voltage via 35, it is added to the output of the slider 21 of the potentiometer 19, that is, the amplitude change signal V ₃ , to correct the influence of lift-off.

Then, using the eddy current flaw detector of the present invention, the inspection object A
In order to detect a crack or the like on the surface of and to measure the depth thereof, first, the frequency of the oscillator 1 is finely adjusted while the detection coil 7 is brought into contact with or separated from a normal metal surface having no crack or the like. Then, the effect of lift-off is minimized, that is, the indicator 24
Search for a specific frequency f ₃ or f ₄ that minimizes the deflection of the analog meter in. Next, with the detection coil 7 in contact with the surface of the display 24, the slider 21 of the potentiometer 19 is adjusted so that the analog meter points to the zero point. Finally, a pseudo-crack sample is used, as described above, to calibrate the crack depth and length to be proportional to the runout of the analog meter. Then, the detection coil 7 is thoroughly scanned along the surface of the inspection object A to detect scratches, cracks and defects a on the surface or near the surface, and at the same time, the depth of the cracks is directly read and measured. is there.

〔The invention's effect〕

According to the eddy current flaw detection method and apparatus of the present invention as described above, the resonance characteristic is retained by utilizing the fact that there is a specific frequency at which the outputs of both ends of the detection coil match in both the free space and the metal surface. Since the frequency of the alternating current supplied to the detection coil and the compensation coil having the same characteristics is supplied so as to be substantially equal to the specific frequency, the sensitivity due to lift-off of the detection coil is significantly changed, and a pseudo flaw detection signal is generated. It is possible to minimize the effect of the above, and by comparing the resonance change due to cracks etc. with the voltage across the detection coil and compensation coil of the same characteristics, it is possible to detect only the signal due to the net crack etc. 1mm, which was previously considered difficult.
Even for cracks of the above depth, it is possible to measure the depth with high sensitivity, which is unmatched by conventional methods.

Further, by utilizing the fact that a phase delay that has little frequency dependence and that changes substantially in proportion to the distance between the metal surface and the detection coil occurs in the detection coil, the phase difference between the detection coil and the compensation coil is detected. Since a correction voltage proportional to the phase difference is generated and this correction voltage is applied to the amplitude change due to cracks, etc., the above effect due to lift-off of the detection coil can be further reduced, and therefore signal level due to lift-off can be further reduced. Since there is almost no change in sensitivity and only the sensitivity changes slightly, for example, even if there is a coating of a certain thickness on the metal surface, no adjustment is required when the number of points is 0 from the top of the coating, and By raising only this, cracks on the metal surface can be easily detected.

In addition, by generating a DC voltage equal to the peak voltage of the voltage across the detection coil and the compensation coil, balancing the voltages of both with a potentiometer and comparing them, and extracting and detecting only the voltage change due to cracks, etc. There is almost no effect due to fluctuations in power supply voltage and temperature, which have been problems with the bridge method, and it is possible to use it in a stable state at all times. The sensitivity is very high, and by changing the frequency, it is possible to detect cracks in both ferromagnetic and non-magnetic materials.

In addition, it can be displayed on the display based on the flaw detection voltage signal proportional to the depth of the crack, etc., and can also generate a tone sound proportional to the depth and length of the crack by the sound generator. Therefore, it is possible to display in proportion to the depth of the crack, etc., and especially when the sound of the tone corresponding to the depth of the crack is generated by the sound generator, the line of sight can be concentrated on the tip of the detection coil, which is complicated. Even an inspected object can be easily detected by one person.

[Brief description of drawings]

FIG. 1 is a simplified block diagram of an eddy current flaw detector according to the present invention.
The figure is an abbreviated perspective view showing a state in which the inspection object is flaw-detected by the detection coil, FIG. 3 is a graph showing the change in the resonance frequency of the detection coil, and FIG. FIG. 5 is a graph showing the state of conversion into a flaw detection voltage signal proportional to the depth, FIG. 5 is a block diagram in which a correction circuit is added to FIG. 1, and FIG. 6 is a graph showing the effect of lift-off by the set frequency. The figure is a time chart showing a state in which the voltage across the detection coil and the compensation coil is converted into a correction voltage signal by the correction circuit. A: DUT, a: Scratch, crack and defect, 1: Oscillator, 2: Current amplifier, 3: Current amplifier, 4: Resistor, 5: Resistor, 6: Ferrite core, 7: Detection coil, 8: Compensation coil, 9: capacitor,
10: Capacitor, 11: Amplifier, 12: Amplifier, 13: Rectifier circuit,
14: Rectifier circuit, 15: Diode, 16: Smoothing capacitor, 1
7: Diode, 18: Smoothing capacitor, 19: Potentiometer, 20: Resistor 20, 21: Slider, 22: Amplifier, 23: Linearization circuit, 24: Display, 25: Voltage controlled oscillator, 26: Sounder, 27:
Correction circuit, 28: comparator, 29: comparator, 30: potentiometer, 31: resistor, 32: slider, 33: diode, 34: smoothing capacitor, 35: resistor.

Claims (5)

[Claims] 1. An intermediate band between a resonance frequency when the detection coil is located in a free space and a resonance frequency when the detection coil is located on a metal surface to be flaw-detected. In the above, in the free space and on the metal surface, by supplying an alternating current with a frequency at which the outputs of both ends of the detection coil substantially match, and comparing the voltages generated at both ends of the detection coil and the compensation coil, flaw detection with the detection coil is performed. Eddy current flaw detection method, characterized in that only the amplitude change caused by scratches, cracks and defects on or near the surface of the metal is detected. 2. Amplitude changes caused by scratches, cracks and defects on or near the metal surface generated at both ends of the detection coil,
The eddy current flaw detection method according to claim 1, wherein a correction voltage proportional to the phase difference between the outputs of the detection coil and the compensation coil, which is generated when the detection coil approaches the metal surface, is applied. 3. A detecting coil and a compensating coil, which are wound around a ferrite core and are connected in parallel with capacitors having the same characteristic, respectively, and a detecting coil on a metal surface for flaw detection in a free space from a single frequency variable oscillator. An alternating current having a frequency whose outputs are substantially the same is supplied through a resistor, and the alternating voltage generated at both ends of the detection coil and the compensation coil is rectified by the rectifier circuit to have opposite polarities. A voltage is applied to both ends of the potentiometer resistor, and the amplitude change signal resulting from flaws, cracks, and defects on or near the metal surface detected by the detection coil output from the slider is cracked by the linearization circuit. An eddy current flaw detection device characterized by being converted into a flaw detection voltage signal proportional to the depth and length of the same. 4. An AC voltage output at both ends of the detection coil and the compensation coil is branched respectively, one of which is input to the rectifier circuit, and the other of which is input to comparators set to opposite polarities, and both outputs of the comparator. Is applied to both ends of the potentiometer resistor, the pulse signal output from the slider and having a pulse width proportional to the phase difference is converted into a correction voltage signal proportional to the pulse width by a rectifier circuit, and the correction is performed. The eddy current flaw detection apparatus according to claim 3, wherein a voltage signal is added to the amplitude change signal. 5. The eddy current flaw detector according to claim 3, wherein the output of the linearization circuit is detected visually and / or audibly by a display and / or a sound generator.