JPH05273322A - Magneto-optical fault detecting method - Google Patents

Abstract

PURPOSE:To accurately and automatically detect a very small fault on the surface of a steel plate by storing the code of picture signals taken synchronously to the inversion of magnetization during the course of changing the positive and negative polarities of a magnetic field in an inverted state and taking the sum of the preceding and current signals. CONSTITUTION:When the light from a belt-like strobe diffusing light source 1 is passed through a magneto-optical effect element 3 composed of a rare-earth-iron-garnet (RIG) vertical film formed in a test piece 4 after passing through a polarizing plate 2 obtained by cutting a Polaroid'R' film for polarization into a belt-like piece so that only the linearly polarized component in one direction of the light can be passed through the element 3, the component is reflected on the bottom surface of the element 3 and only the component having the wavelength in the maximum detection sensitivity band of the element is transmitted and the component in the wavelength region which becomes a noise source is cut. During the two-way transmission of the light, the polarizing angle is detected 6 and the image of the angle is picked up with an image sensor 7 based on the transmitting distance, sensitivity constant of the element 3, and the vertical direction at the position where the element 3 exists. Therefore, magnetic fluxes are concentrated to the surface by the skin effect of the fluxes and a flaw in the thick test piece 4 can be detected with high sensitivity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for detecting surface defects and surface internal defects of a ferromagnetic material.

[0002]

2. Description of the Related Art A method of magnetizing a ferromagnetic material and detecting a leakage magnetic flux leaking from a defect with a Hall element or a detection coil is as follows.
Compared with other methods such as eddy current flaw detection, the surface roughness, scale, and magnetic permeability of ferromagnetic materials are higher than that of magnetic particle flaw detection. Since it is less affected by the dispersion of, it is widely used in automatic flaw detection methods for steel pipes. In addition, JP-A-3-2450
Japanese Patent Laid-Open No. 52-52 discloses a method of applying the optical magnetic field measurement method to defect detection.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to accurately and automatically detect minute defects on the surface of a steel sheet that occur during the steel sheet manufacturing process.

In the conventional method, the leakage magnetic flux is detected as a change in the magnetic field. Therefore, when the size of the defect is reduced, the magnetic field leakage area is rapidly reduced, and this can be detected at high speed. It is necessary to arrange a large number of small magnetic field detecting elements close to the surface of the material to be inspected so as to follow the partner. However, it was very difficult to fabricate a mechanism that enables a sensor head having several thousand signal wires to follow a steel strip or a thick plate that moves at high speed. Moreover, it was very difficult in terms of cost.

Further, in the rolled material, the defects generally extend in the rolling direction, and in order to detect the defects, it was necessary to scan the sensor in the width direction in the conventional method.
However, in a high-speed line, a method of mechanically scanning in the width direction is not feasible, and only short defects can be detected.

The technique disclosed in Japanese Patent Application Laid-Open No. 3-245052 is for detecting defects in the steel sheet, and although it is possible to detect the defects extending in the rolling direction described above, it is possible to detect flaws in slabs and thick plates. In such an object having a large surface area and thickness, it is difficult to generate leakage magnetic flux because the magnetic flux is diffused and attenuated inside the steel slab by direct current magnetization.

First, the background of the invention will be described.

The present inventors have studied a method of detecting a large defect at high speed in a wide range using the magneto-optical effect. As a result, there is a highly sensitive material in the rare earth / iron / garnet (RIG) perpendicular magnetization film used for optical isolators and optical switches, and it has a difficult magnetization property in in-plane directions other than perpendicular, It was found that the magnetic flux did not saturate due to the horizontal magnetic field for generating the leakage magnetic flux from the defect, and that the Faraday rotation in the vertical direction did not change, which was an excellent characteristic as a leakage magnetic flux sensor. RIG
The perpendicular magnetization film has a band-like magnetic domain structure that is connected in a labyrinth that looks black and white when viewed with a polarization microscope, and when used as a general magnetic field detection sensor, it has a large spot.
There is no problem because the Faraday rotation is detected as the average value. However, when an attempt is made to apply a high-sensitivity magneto-optical element to the method for the purpose of detecting a small defect, the high-sensitivity magneto-optical element has a wide magnetic domain pattern on the surface, and thus the magneto-optical element on the image sensor has a large width. It was found that when the field of view is zoomed, the wide black-and-white magnetic domain pattern becomes a large noise source, making accurate detection difficult.

Further, magnetic domain growth occurs when a perpendicular magnetic field is applied to the film, but a magnetic domain structure that does not normally move even when a large in-plane magnetic field is applied is affected by a magnetic field in the direction of hard magnetization in the presence of a perpendicular magnetic field. , It was found that it was easy to align in the horizontal magnetic field direction at that time. These may cause a large measurement error depending on the case when detecting a minute magnetic field distribution due to optical scanning.

In view of the above problems, a method of improving accuracy by using a magneto-optical effect element as a magnetic flux leakage detecting element is conceivable. For example, a paper by Wakabayashi et al.
15,773−778 1991). Wakabayashi et al. Proposed a method of suppressing the influence of background light and increasing the contrast by using GPR as a magnetic effect element and inverting the contrast of the surface image.

Also, in a report by Nomura et al. (6th Annual Meeting of the Japan Society for Applied Magnetics, 1982.11), a method of increasing the contrast of a magnetic domain pattern by using a Faraday element to obtain the difference of an inverted image is proposed.

However, according to the research conducted by the present inventors,
When the magneto-optical effect element is applied to detect defects such as steel plates, increasing the contrast of magnetic domains from the following research results not only limits the size of defects that can be detected but also deteriorates the defect detection accuracy. I got the knowledge that it will lead to.

3 and 4 are examples of magneto-optical effect element images observed with a polarization microscope. However, in FIGS. 3 and 4, the polarities of the magnetic fields are reversed. Reference numeral 23 is a defect-free portion, and a maze-like black and white magnetic domain pattern is observed. 24 is the magnetic domain pattern of the artificial defect (slit with a depth of 0.1 mm),
It can be clearly observed that the magnetic domain grows according to the magnitude and direction of the leakage magnetic field.

Therefore, when the resolution of the linear array image sensor camera is high and the size of the defect to be detected is close to the thickness of the magnetic domain pattern, the defect-free magnetic domain pattern has a high frequency pulse-like noise source. Becomes It was also found that this noise can be considerably attenuated by a spatial low-pass filter, but it causes a reduction in spatial resolution and limits the size of a detectable defect.

Further comparing FIG. 3 with FIG. 4, only the defective portion is black-white inverted, and the magnetic domain pattern of the defect-free portion is unchanged because there is no vertical magnetic field. It was also found that this magnetic domain pattern is stable and returns to its original shape when the external magnetic field is removed.

The present invention is based on the above findings.

In view of such a problem, the present invention drastically solves this problem, and enables a single detection unit to inspect a wide range of leakage magnetic fields even for a large inspection target that is difficult to be magnetized by direct current, and realizes high speed. It is an object of the present invention to provide an epoch-making optical magnetic field type defect detection method that enables high-precision detection of surface and surface layer defects.

[0018]

In the present invention, a magnetic field is applied to a ferromagnetic material to be inspected, and the leakage magnetic field generated in the defective portion is
In a magneto-optical defect detection method for detecting based on the polarization plane rotation of linearly polarized light transmitted through a magneto-optical element that moves relative to a material to be inspected, the light source is a band-shaped diffuse polarization light source and an applied magnetic field is an alternating current. Also, in the process of changing the polarity of the magnetic field due to the application of alternating current, an image of the surface of the magneto-optical effect element is captured in synchronization with the reversal of magnetization. The captured image of the surface of the magneto-optical effect element is, for example, stored as a normal (positive) image in the case of positive magnetization and a black-and-white inverted (negative) image in the case of negative magnetization, and is stored as a signal of the positive image and the negative image. The feature is that the defect is detected from the addition result.

[0019]

Embodiments of the method of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the optical section used in the present invention.

The strip-shaped strobe diffusion light source 1 is constructed by disposing a milky white transmission diffusion plate on the entire surface of a rod-shaped xenon strobe lamp. The polarizing plate 2 is a polarizing polaroid film cut into strips, and transmits only the linearly polarized light component in one direction of the illumination light. Reference numeral 3 denotes a rare earth iron garnet perpendicularly magnetized film, which has a non-magnetizing property except in a direction perpendicular to the plane and which does not cause magnetic domain movement or magnetic saturation in a horizontal magnetic field of about 500 to 1000 oersteds. The top surface of the film has a non-reflective coating, and the bottom surface has a total reflection coating. The light incident on the film is reflected by the bottom surface and goes out. The polarization angle is detected by the transmission distance, the sensitivity constant of the film, and the magnetic flux in the vertical direction at the position where the film exists. Reference numeral 4 is a material to be inspected, and 5 is an interference filter, which transmits only the wavelength in the maximum detection sensitivity band of the magneto-optical effect element and cuts off the wavelength range that becomes another noise source. In this example of the method, the center wavelength is 800 nm and the half-value width is 100 nm. 6 is a polarizing plate, and the angle of light passing through the polarizing plate 2 is set to 45 degrees, and reflected on the magneto-optical effect element. The illumination light is transmitted. Reference numeral 7 denotes an image sensor, which photographs the surface of the magneto-optical effect element in accordance with the synchronization signal from the magnetizer.

FIG. 2 shows an example of the configuration of the signal processing section used in the present invention. Reference numeral 8 in FIG. 2 corresponds to the entire optical system shown in FIG.

Reference numeral 9 denotes an oscillator, the output of which is processed into a sine wave or a square wave by nonlinearizing a triangular wave synchronized with the horizontal synchronizing signal of the image of the image sensor. Transmitter 9
The output of is the signal source of the power amplifier and the polarity conversion trigger signal source of the polarity converter 16. 10 is a power amplifier,
The amplified alternating current is sent to the magnetizing coil 11. Magnetizing coil
11 generates an AC magnetic flux to magnetize the inspection object. Due to the skin effect due to the alternating-current magnetization, the magnetic flux is concentrated on the surface, and it is possible to detect a thick material to be inspected. 12 is the iron core of the magnetizing coil.
13 is a phase shifter, 14 is a strobe control power supply, and 15 is an AD converter, which AD-converts the image measured by the image sensor 7.
As an example, FIG. 3 shows a surface image of the magneto-optical effect element when the magnetization polarity is positive, and FIG. 4 shows a case where the magnetization polarity is negative after a half cycle of the AC cycle has elapsed from the point of FIG. The surface image of a magneto-optical effect element is shown. 23 indicates a normal portion and 24 indicates a defective portion.

Reference numeral 16 denotes a polarity converter, which performs polarity inversion (positive → negative) of the image converted by the AD converter 15 every half cycle. For example, the polarity converter performs the operation of inverting the polarity only when the signal of the polarity detector 17 is positive. As an example, FIG. 5 shows the inverted (negative) image of FIG. In FIG. 5, the black and white of the magnetic domain pattern is reversed, and the black and white ratio of the defective portion is the same as the image of FIG. 3 when the magnetization polarity is reversed.

The polarity detector 17 determines whether the current magnetization direction is positive or negative from the signal from the oscillator 9. Reference numeral 18 denotes a frame buffer, which simultaneously sweeps out and captures an image every half cycle. Reference numeral 19 denotes an adder that adds the image stored in the frame buffer 18 before the half cycle and the image signal immediately after being converted by the AD converter. As an example, FIG.
The defect pattern of the image obtained by adding FIG. 5 and FIG. 5 is schematically shown in FIG. In FIG. 6, the addition of the reversed magnetic domain patterns almost cancels the magnetic domain patterns, and the noise is significantly reduced. Further, there is an effect that the black and white contrast of the defective portion is doubled.

Reference numeral 20 denotes an image processing device, which performs processing such as low-pass filter, vertical averaging, and high-pass filter. As an example, FIG. 7 shows a luminance distribution signal of one line after image processing. The low-pass filter smoothes the magnetic domain pattern noise slightly left in the previous process, and further improves the S / N of the defect signal by the vertical averaging. Further, the high-pass filter removes the uneven brightness on the entire surface of the image due to the gentle vertical magnetic field distribution by the magnetizer, and extracts only the defect. 25 is the brightness distribution signal of one line before image processing,
The brightness corresponding to the magnetic domain pattern becomes noise, making it difficult to understand the signal at the defective portion.

Reference numeral 26 denotes a 1-line luminance distribution signal after image processing. By image processing, it is possible to completely cancel the magnetic domain pattern noise, which has been a problem until now and has become a large noise source, and the signal of the defective portion can be eliminated. S / N can be detected well. 22 is a defect indicator and 21 is a defect discriminator. Similarly thereafter, the inverter 16 synchronizes with the applied magnetic field frequency to obtain a normal image,
Alternatively, the reversed images are alternately sent to the frame memory 18.
In the frame memory, at the same time that one image is captured, the image recorded immediately before that is sent to the adder 19. The adder 19 adds the raw image at that time and the image from the frame memory and sends an image signal to the image processing device 20, whereby it is possible to completely cancel the noise in the magnetic domain part described above, and the defective part Can detect the signal of S / N well.

[0028]

As described above, according to the present invention, due to the skin effect of the magnetic flux, the magnetic flux is concentrated on the surface, and it becomes possible to detect a thick material to be inspected. By subtracting, it completely cancels,
With the doubling of the defect signal and the use of a more sensitive magneto-optical effect element, it becomes possible to detect a minute defect. In the configuration of the method of the present invention, only the magneto-optical effect element and the image sensor need be made to follow the material to be measured, which is much lighter and more compact than the conventional method, and the following performance is greatly improved, and the trouble of the sensor and the wiring system is reduced. You can get the effect of reducing.

Further, the present invention has an epoch-making feature that it is possible to detect even a defect extending long in the longitudinal direction of the rolled material because it is optically scanned at high speed in the width direction.

Further, in the present invention, since scanning is performed optically, the scanning speed can be made sufficiently higher than the vibration of the target member, and by changing the magnification of the optical system, it is possible to detect even smaller defects than before. And the detection accuracy has improved.

In addition, the same signal processing as a normal image signal can be applied in the post-processing, and it becomes possible to recognize a defect pattern and determine a defect type as compared with the conventional method.

In recent years, advances in the field of optoelectronics have been remarkable, and both a high-sensitivity perpendicular magnetized film and a linear array image sensor camera having a wide width of 10,000 pixels as the image sensor 7 can be realized at low cost. The number of circuits is reduced to one hundredth of that of a large number of parallel circuits, and the economical efficiency can be improved epoch-making. Also,
Since the number of components is reduced, breakdowns are reduced and maintainability has been dramatically improved.

The present invention can be applied to a surface automatic flaw detection method for a thick plate, for example, in a region where the accurate leakage magnetic flaw detection method could not be applied due to economical difficulty, and labor-saving and productivity through automation. The improvement effect is great.

[Brief description of drawings]

FIG. 1 is a perspective view showing an optical section of an apparatus according to an embodiment.

FIG. 2 is a block diagram showing a measurement control unit of the apparatus of the embodiment.

FIG. 3 is a plan view showing a surface image of the magneto-optical effect element when the magnetization polarity is positive.

FIG. 4 is a plan view showing a surface image of the magneto-optical effect element in the case where the magnetization polarity is negative after a lapse of a half cycle of the AC cycle from FIG.

5 is a plan view showing an inverted (negative) image of FIG.

FIG. 6 is a schematic diagram showing a defect image obtained by adding FIGS. 3 and 5 together.

FIG. 7 is a waveform diagram showing a waveform of a lateral luminance distribution after image processing by the image processing device and a waveform of a lateral luminance distribution before image processing in FIG.

[Explanation of symbols]

1: Striped strobe diffused light source 2: Polarizer 3: Magneto-optical effect element 4: Inspected material 5: Interference filter 6: Analyzer 7: Image sensor 8: Optical system 9: Oscillator 10: Power amplifier 11: Magnetization coil 12 : Iron core 13: Phase shifter 14: Strobe control power supply 15: AD converter 16: Polarity converter 17: Polarity detector 18: Frame buffer 19: Adder 20: Image processing device 21: Defect indicator 22: Defect discriminator 23: Normal part 24: Defect part 25: Waveform of horizontal luminance distribution after image processing 26: Waveform of horizontal luminance distribution before image processing

Claims (1)

[Claims] 1. A magnetic field is applied to a ferromagnetic material to be inspected, and a leakage magnetic field generated in a defect portion thereof is transmitted to a magneto-optical effect element that moves relative to the material to be inspected, based on rotation of a plane of polarization of linearly polarized light. In the magneto-optical defect detection method of detecting by means of a band-diffuse polarized light source, the applied magnetic field is an alternating current, and the image of the magneto-optical effect element is synchronized with the reversal of magnetization in the process of changing the polarity of the magnetic field. Is detected, the sign of the image signal is sequentially inverted and stored for each polarity inversion, and the defect is detected based on the addition result of the current signal and the previous signal.