JPH06118060A - Magnetic optical defect inspecting device - Google Patents

Abstract

PURPOSE:To detect a defect with high reliability even when a steel plate to be inspected is vibrated according to its carriage. CONSTITUTION:The gap G1 between a Faraday effect element 20 and a steel plate and the gap G2 between magnetic poles 11e, 11f of a magnetizer and the steel plate 1 are detected by sensors 71, 72, respectively. According to a predetermined optimum magnetization curve, magnetic flux density is determined from G1. According to G2, the dimension of the steel plate, and kind of steel, the magnetic flux density is corrected. A magnetizing current value corresponding to the magnetic flux density after correction is determined from a predetermined characteristic curve.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for magneto-optically detecting defects in an object to be inspected by using the Faraday effect, and for detecting surface defects and surface internal defects in, for example, thick steel plates. Available.

[0002]

2. Description of the Related Art In recent years, attention has been focused on a method of magneto-optically detecting a defect in an inspection object by utilizing the Faraday effect. In this method, a test object made of a ferromagnetic material is magnetized, and linearly polarized light is incident on a Faraday effect element (or called a magneto-optical effect element) arranged close to the surface of the object and transmitted through the element. The polarization state of the reflected light reflected by the reflection film on the bottom surface and transmitted again through the Faraday effect element is observed through an analyzer. That is, the plane of polarization of the light passing through the Faraday effect element is rotated by the magnetic field that it receives, but when there is no defect in the inspection object, there is no leakage flux, so the Faraday effect element is not affected by the magnetic field. The plane does not rotate, and if the inspection object has a defect, leakage flux will occur and the Faraday effect element will be affected by the magnetic field and the polarization plane will rotate, so observe the polarization direction of the reflected light through the analyzer. Thus, the presence or absence of a defect can be detected.

The prior art of this type of magneto-optical defect inspection apparatus is disclosed in, for example, Japanese Patent Laid-Open Nos. 2-253152 and 3-276050.

[0004]

As described above, in the method of detecting the defect of the inspection object by the magneto-optical method, the Faraday method is used.
If the gap between the effect element and the surface of the inspection object is not kept constant, the S / N ratio of the detected signal tends to deteriorate. Therefore, when actually trying to detect a defect in a thick steel plate or the like, a Faraday effect element is placed on the surface of the thick steel plate or the like via a mechanical tracing device so that the gap between them is constant. The device was configured so that Similarly, an electric coil is placed on the surface of a thick steel plate through a tracing device, and the device is constructed so that the gap between the two becomes constant. Plates Steel sheets were magnetized.

However, when the defect inspection is carried out while moving the thick steel plate at a high speed, the thick steel plate vibrates, so that a predetermined gap (for example, 1 mm from the surface of the thick steel plate is followed so as to follow the vibration. It is practically impossible to accurately position the Faraday effect element and the electric coil with the tracing device at the position of), and the change of each gap causes
It is unavoidable that the signal S / N ratio deteriorates.

Therefore, the present invention prevents deterioration of the S / N ratio of the detection signal even when inspecting an inspection object whose surface position constantly changes due to vibration, such as a thick steel plate, and is high. An object of the present invention is to provide a practical magneto-optical defect inspection apparatus capable of detecting defects with reliability.

[0007]

In order to solve the above problems, a magneto-optical defect inspection apparatus according to the present invention comprises a magnetizing means (10) for magnetizing at least the surface layer portion of an inspection object; a magnetized surface of the inspection object. Faraday effect element means (20) arranged in the vicinity of and rotating the plane of polarization of light passing through it in response to a magnetic field received by the Faraday effect element means (20); Lighting means (3
0); an analyzer (40) for receiving the reflected light from the Faraday effect element means through an opening in the center of the frame; arranged in the vicinity of the analyzer and changing the intensity of light passing through the analyzer. Defect detection means (50) for detecting defects on the inspection object;
Tracing means (2) for supporting the magnetizing means and the Faraday effect element means and positioning them near the surface of the inspection object; detecting the size of the gap between the surface of the inspection object and the Faraday effect element means. First gap detecting means (7
1); second gap detecting means (72) for detecting the size of the gap between the surface of the inspection object and the magnetizing means; a target based on the size of the gap detected by the first gap detecting means A magnetic flux density determining means (73) for determining the magnetic flux density; and an optimum exciting current based on the target magnetic flux density determined by the magnetic flux density determining means and the size of the gap detected by the second gap detecting means. And an exciting current determining means (74) for activating the magnetizing means with the determined exciting current.

In the second invention, the magnetic flux density determining means (73) determines the magnitude of the magnetic flux density when the S / N ratio of the defect detection signal becomes maximum with respect to the size of each gap. The calculation formula or constant table to be obtained is held in advance, and the magnetic flux density at which the S / N ratio of the defect detection signal becomes maximum with respect to the size of the gap detected by the first gap detecting means is obtained and the target is set. It is configured to have a magnetic flux density.

In the third aspect of the invention, the exciting current determining means (74) is further configured to input information on the size and steel type of the object to be inspected and determine the optimum exciting current in accordance with them. To do.

The symbols shown in parentheses are reference numerals of corresponding elements in the embodiments described later, but each constituent element of the present invention is a specific element in the embodiments. It is not limited to only.

[0011]

As described above, when a defect inspection of the surface of a thick steel plate is carried out while moving the thick steel plate at a high speed, the thick steel plate vibrates, so that the surface of the thick steel plate, the magnetizing means and the Faraday effect element. It is inevitable that each gap with the means will change. Therefore, in the present invention, the first aspect is to detect the size of the gap between the surface of the inspection object and the Faraday effect element means.
And a second gap detecting means for detecting the size of the gap between the surface of the inspection object and the magnetizing means, and the magnetizing means is attached according to the size of the gap detected by them. The control is performed so as to compensate the magnitude of the exciting current that is applied.

According to the investigation by the present inventor, the size of the gap between the surface of the inspection object and the Faraday effect element means, the magnetic flux density in the inspection object, and the S / N ratio of the defect detection signal. It has been found that there is a correlation between them, and when the gap is set to a specific state, the maximum S / N ratio can be obtained by setting the magnetic flux density to a certain value. Therefore, the magnetic flux density with which the highest S / N ratio is obtained can be obtained according to the above correlation and the gap detected by the first gap detecting means, and the magnetizing means is set so as to obtain the magnetic flux density obtained thereby. The highest S / N ratio can always be obtained by adjusting the exciting current to be applied. Further, since the magnetic flux density in the inspection object changes according to the size of the gap between the surface of the inspection object and the magnetizing means, the magnitude of the exciting current is
It is determined according to the size of the gap detected by the second gap detecting means and the determined target magnetic flux density. As a result, the exciting current for energizing the magnetizing means is compensated so that the highest S / N ratio can always be obtained even when each gap changes due to the vibration of the inspection object, and highly reliable defect detection is realized. To do.

[0013]

EXAMPLE FIGS. 2 and 3 show the appearances of the defect inspection apparatus of the example as seen from the front and from the side, respectively. This defect inspection device is installed on the inspection line of the manufactured thick plate steel sheet 1. The thick steel plate 1 is inspected by a defect inspection apparatus while being conveyed at a high speed in the direction of the arrow in FIG. Since the actual thick steel plate 1 is very large, the entire area cannot be inspected by a single defect inspection apparatus. Therefore, in reality, many defect inspection apparatuses are arranged on the inspection line. In the figure, only one defect inspection apparatus is shown. Although the defect inspection device does not move, it is in constant contact with the surface 1a of the thick steel plate via the four wheels 2 and is supported so as to move following the surface 1a.

This defect inspection device detects defects on the surface of a thick steel plate by utilizing the Faraday effect. That is, when the thick steel plate 1 is magnetized, if there is no defect, the leakage magnetic flux does not occur on the surface 1a, but if there is a defect, the leakage magnetic flux occurs. Therefore, if an element that produces the Faraday effect is arranged on the surface 1a, the direction of the plane of polarization of the light passing through the element changes depending on the presence or absence of the leakage magnetic flux. The presence or absence of can be detected. Therefore,
The main part of this defect inspection apparatus is a magnetizer 10, a Faraday
It is composed of an effect element 20, a light projector 30, and a light receiver 60.

The Faraday effect element 20 is a thin plate-shaped and is formed in a strip shape (rectangular shape) long in the width direction of the thick steel plate 1, and is arranged so as to be close to and face the surface 1a of the thick steel plate 1. Has been done. The main part of the Faraday effect element 20 is a rare earth / iron / garnet (RIG) perpendicular magnetization film, which has a non-magnetization characteristic except in the direction perpendicular to the plane.
A material that does not cause magnetic domain movement or magnetic saturation in a horizontal magnetic field of about 1000 Oersted is used. The top surface (light incident surface) of the film is subjected to non-reflection coating, and the bottom surface of the film (surface facing the steel plate) is subjected to total reflection coating, which is incident on the Faraday effect element 20. The generated light passes through the perpendicular magnetic film, is reflected by the bottom surface, passes through the perpendicular magnetic film again, and exits from the Faraday effect element 20. While the light reciprocates in the perpendicular magnetization film, due to the Faraday effect,
Rotation of the plane of polarization occurs. The amount of rotation is determined by the transmission distance, the sensitivity constant of the film, and the magnitude of the magnetic flux in the vertical direction at the film position, and the magnitude of the vertical magnetic flux greatly changes depending on the presence or absence of defects on the steel plate surface or surface layer portion.

The appearance of the magnetizer 10 is shown in FIG. Referring to FIG. 1, the magnetizer 10 includes a core 11 made of a ferromagnetic material and two electric coils 12 and 13 wound around the core 11. The core 11 has four sides 11a, 11
a substantially rectangular frame body composed of b, 11c and 11d,
Electric coils 12 and 13 are respectively wound around two sides 11c and 11d facing each other. The remaining two sides 1
Protrusions 11e and 11f are formed on 1a and 11b, respectively. The two electric coils 12 and 13 have the same number of turns, and are connected so as to generate magnetic flux symmetrically with each other. That is, when one end 12a of the electric coil 12 is the N pole and the other end 12b is the S pole, one end 13a of the electric coil 13 is the N pole and the other end 13b is the S pole, and the protrusions 11e and 11f are the N pole and the N pole, respectively. A south pole is formed.

As shown in FIGS. 2 and 3, since the protrusions 11e and 11f are arranged in close proximity to the surface 1a of the thick steel plate 1, the electric coils 12 and 13 are energized and the protrusion 1
The thick steel plate 1 is magnetized by forming magnetic poles on 1e and 11f. Further, since the protrusions 11e and 11f are formed at the central portions of the respective sides 11a and 11b, the thick steel plate 1 is magnetized most strongly at the position facing the central portion of the magnetizer 10. The Faraday effect element 20 is arranged so as to face this position. Further, the core 11 is a rectangular frame, and the central portion of the core 11 forms an opening 11g. Therefore, an optical system is formed so that an optical path is formed in this portion.
That is, the Faraday effect element 20, the light projector 30, and the light receiver 60 are arranged.

The projector 30 comprises a white lamp 31 and a diffuser plate 3.
2, and a polarizing plate 33, and is arranged above the Faraday effect element 20. The white lamp 31
It is an elongated tubular white light source (xenon strobe lamp) and is arranged in parallel with the Faraday effect element 20 in the same direction. Further, the light emitted from the white lamp 31 contains various wavelength components, but particularly the wavelength of 800 nm. The wavelength of 800 nm is the wavelength at which the amount of rotation of the light of the Faraday effect element 20 is the largest and the attenuation due to absorption is small. The light emitted from the white lamp 31 passes through the milky white diffusion plate 32 and is diffused, so that the entire surface of the Faraday effect element 20 having a relatively wide area can be illuminated with a uniform illuminance. Further, in the light emitted from the diffusion plate 32, only the linearly polarized light component in one direction passes through the polarizing plate 33 and reaches the Faraday effect element 20. The polarizing plate 33 is formed by cutting a polaroid plate into a strip shape, and is arranged in parallel in the same direction as the white lamp 31.

The photodetector 60 includes a bandpass filter 45,
Analyzer 40 and imaging device (two-dimensional image sensor) 5
0, and is arranged above the Faraday effect element 20 in an optically symmetrical positional relationship with the projector 30. The bandpass filter 45 is an optical filter having a characteristic of transmitting only light of a specific wavelength component and blocking other wavelength components. In this example, the center wavelength is 800.
nm, and the half value width of 100 nm is used. That is, only the wavelength component with high sensitivity of the Faraday effect element 20 is extracted, and the other components are cut as noise. The analyzer 40 is a polarizing plate, and the polarization direction of the light transmitted by the analyzer 40 is inclined by 45 degrees with respect to the polarizing plate 33. The light transmitted through the polarizing plate 33 enters the imaging device 50 and is captured as a two-dimensional image.

Further, on the member 21 supporting the Faraday effect element 20 and the projection 11e, gap sensors 71 and 72, which are commercially available laser distance meters, are installed. The gap sensor 71 is arranged with the detection surface facing downward so that the detection surface thereof coincides with the bottom surface position of the Faraday effect element 20, and the bottom surface of the Faraday effect element 20 and the surface 1a of the thick steel plate 1 are provided. To detect the gap. Further, the gap sensor 72 is arranged with the detection surface facing downward so that the detection surface thereof coincides with the tip position of the protrusion 11e, and the gap between the protrusion 11e, that is, the tip of the magnetic pole and the surface 1a of the thick steel plate 1 is formed. To detect. Since this defect inspection apparatus moves by following the surface 1a of the thick steel plate 1 by the copying mechanism including the wheels 2 and the like, there is little opportunity for the gap to change greatly, but the movement of the thick steel plate 1 may occur. Due to napping vibration,
There may be slight changes in the gap. However, when the size of these gaps changes, the degree of magnetization of the thick steel plate 1 that is the inspection object changes, the size of the leakage magnetic flux also changes, and the S / N ratio of the signal used for defect detection changes. Getting worse. Therefore, in this embodiment, the magnitudes of the currents flowing through the electric coils 12 and 13 are automatically adjusted based on the gaps detected by the gap sensors 71 and 72 so that the maximum S / N ratio is always obtained. is doing.

FIG. 4 shows the configuration of the electric circuit of this defect inspection apparatus.
Shown in. This will be described with reference to FIG. The oscillator 61 outputs a square wave signal of a constant cycle that determines the control timing of this electric circuit. Specifically, the oscillator 61 uses the imaging device 5
A triangular wave synchronized with the horizontal synchronizing signal used for 0 two-dimensional image reading scanning is generated and shaped to generate a square wave. The square wave signal output from the oscillator 61 is transmitted to the phase shifter 6
It is applied to the strobe power source 62 via 5. The strobe power source 62 supplies pulse power synchronized with the square wave signal output from the oscillator 61 to the white lamp 31, and lights the white lamp 31 at a predetermined timing.

The magnetizing coils (electric coils) 12 and 13 are constantly supplied with a current that maximizes the S / N ratio of the detection signal. This current is an alternating current and has a square waveform synchronized with the square wave signal output from the oscillator 61. The values C + and C− of the currents flowing through the magnetizing coils 12 and 13 are determined by the microcomputer CPU, and the gap G1 detected by the gap sensor 71 and the gap G detected by the gap sensor 72.
2. It is changed according to the size and steel type of the steel plate to be detected obtained from the process computer 78 that manages the production line. The positive side value C + and the negative side value C− of the current are held in the latch 75, and one of them is applied to the D / A converter 77 via the data selector 76. That is, the two signals C + and C− applied to the data selector 76 are alternately switched by the output of the polarity detector 63 for each half cycle of the signal output from the oscillator 61, and are output to the D / A converter 77. Is applied. The power amplifier 64 controls so that a constant current having the analog signal level output from the D / A converter 77 as a target value flows through the magnetizing coils 12 and 13.

Since the polarity of the electric power applied to the magnetizing coils 12 and 13 changes periodically, the electrifying coils 12 and 1
The magnetic pole generated in the magnetizer 10 by 3 also changes periodically. Since the period of this polarity change is very short, the skin effect appears, and the magnetic flux in the thick steel plate 1 concentrates on the surface layer portion. As a result, it is possible to magnetize only the portion of the thick steel plate 1 necessary for detecting a defect with a relatively small electric power.

The image pickup device 50 scans the two-dimensional image light incident through the analyzer 40 to pick up an image. Imaging device 5
The image signal output by 0 is converted into a digital signal by the A / D converter 51 and applied to the polarity converter 52.
The polarity converter 52 is controlled by the output of the polarity detector 63 that detects the polarity of the square wave signal output by the oscillator 61. The image signal output from the polarity converter 52 is applied to the frame buffer 53 and the adder 54. The adder 54 is
The image signal output by the polarity converter 52 and the image signal output by the frame buffer 53 are added, and the result is output to the image processing device 55. The image processing device 55 subjects the image signal to predetermined image processing, and the result of the processing is applied to the defect discriminator 56.
Output to. The defect discriminator 56 identifies whether there is a defect. The monitor TV 57 can display an image before image processing, an image after image processing, a defect discrimination result, etc. on its two-dimensional display screen.

An example of a two-dimensional image taken by the image pickup device 50 is shown in FIGS. These are images obtained when the Faraday effect element 20 is opposed to the thick plate surface portion including the artificially produced surface defect, and FIG.
FIG. 6 is an image when the magnetization polarity of the thick steel plate 1 is positive, and FIG. 6 is an image after a half cycle of the magnetization polarity change period, that is, when the magnetization polarity is negative. The pattern seen over the entire area in the image is the magnetic domain pattern of the Faraday effect element 20.

The polarity converter 52 is a circuit for switching the polarity (positive / negative) of the image signal. The polarity converter 52 switches the positive / negative in accordance with the signal output from the polarity detector 63, and the positive / negative is switched every half cycle of the magnetization polarity change cycle. Images and negative images are output alternately. Therefore, for example, a positive image is output when the magnetization polarity of the thick steel plate 1 is positive, and a negative image is output when the magnetization polarity of the thick steel plate 1 is negative.

FIG. 7 shows a negative image of the image of FIG. Referring to FIG. 7, it can be seen that the black and white of the magnetic domain pattern are reversed, and the black and white ratio of the defective portion is the same as in the image with the opposite magnetization polarity (FIG. 5). Therefore, in this embodiment, the image shown in FIG. 5 and the image shown in FIG. 7 are added by the adder 54 to cancel the magnetic domain pattern component which is noise.

The frame buffer 53 simultaneously outputs an image stored so far and writes a new image every half cycle of the magnetization polarity change cycle. Therefore, the latest image output from the polarity converter and the image before the half cycle are simultaneously applied to the adder 54, but one of them is the inverted negative image and the other is the non-inverted positive image. It is an image, and since the magnetization polarities of both are opposite,
The image shown in FIG. 5 and the image shown in FIG. 7 can be added at the same timing.

The image obtained by this addition is, for example, as shown in the uppermost portion of FIG. 8, and the magnetic domain pattern components are almost eliminated, and the contrast of the defective portion is doubled.
The waveform shown in the center of FIG. 8 shows the signal level change of one line of the image obtained by addition, but it is necessary to further process the signal in order to identify the presence or absence of a defect.

Therefore, the next image processing apparatus 55 carries out various kinds of signal processing. That is, low-pass filtering,
Vertical averaging of images and high-pass filtering are performed. The residual component of the magnetic domain pattern is further attenuated by the low-pass filter process (smoothing), the S / N ratio of the defect signal is improved by the averaging process in the vertical direction, and the gradual grading by the magnetizer is performed by the high-pass filter process. It is possible to reduce the uneven brightness of the image caused by the vertical magnetic field distribution. As a result of this processing, the signal waveform shown at the bottom of FIG. 8 is obtained, and this signal is applied to the next defect discriminator 56. The defect discriminator 56 identifies a relatively large level change, such as the signal shown at the bottom of FIG. 8, as a defect.

Next, the microcomputer C shown in FIG.
The operation of the PU will be described. This micro computer
The CPU mainly executes the magnetic flux density determination processing 73 and the magnetizing current determination processing 74, the specific contents of which are shown in FIG. That is, the magnetic flux density determination process 73
Then, the gap G1 is input, and then the table 1 prepared in advance in the memory is referenced, and the value M1 associated with G1 is set.
Is determined as the optimum magnetic flux density.

FIG. 9 shows data obtained by experiments, which shows an axis showing the size (G1) of the gap between the Faraday effect element and the surface of the steel sheet to be inspected and an axis showing the magnetic flux density in the steel sheet. 3 shows a group of equal S / N curves and an optimum magnetization curve on the two-dimensional space of. Referring to FIG. 9, it can be seen that the S / N ratio of the defect detection signal greatly changes depending on the gap and the magnetic flux density. That is, the highest S / N ratio can be obtained by changing the magnetic flux density along the optimum magnetization curve along with the change in the gap G1.

Therefore, in the table 1 of the micro computer CPU, the relations between various gaps corresponding to the optimum magnetization curve of FIG. 9 and the magnetic flux densities corresponding to them are stored in advance. Therefore, the optimum magnetic flux density M1 can be immediately obtained by referring to the table 1 using the input G1 as a parameter.

Referring again to FIG. In the magnetizing current determination processing 74, first, the gap G2 is input from the gap sensor 72,
Information about dimensions and steel grade is input from the process computer 78. In the next step 85, the table 2 prepared in advance in the memory is referred to, the optimum magnetic flux density M1 is corrected by the gap G2, and the corrected value is set as M2. Further, in the next step 86, the table 3 prepared in the memory in advance is
, The optimum magnetic flux density M depending on the size of the steel sheet to be inspected
2 is corrected and the corrected value is set to M3. Further, in the next step 87, the optimum magnetic flux density M3 is corrected according to the steel type to be inspected by referring to the table 4 prepared in advance in the memory, and the corrected value is set as M4. In the following step 88,
The current value C + associated with the magnetic flux density M4 is obtained by referring to the table 5 prepared in advance in the memory. Then, in the final step 89, the current values C + and C- are latched by the latch 7
Output to 5. The current value C- is a negative value obtained by inverting only the sign of C +.

FIG. 11 shows the results of measurement of the relationship between the magnetic flux density inside the steel sheet and the gap (G2) between the magnetic pole and the steel sheet surface when artificial defects of various depths are present in the steel sheet.
With reference to FIG. 11, it can be seen that the magnetic flux density changes with the change in the gap G2 even if the currents flowing through the electric coils 12 and 13 are constant. Therefore, in order to maintain the magnetic flux density at the target value. Must compensate for the current flowing through the electric coils 12 and 13 as the gap G2 changes. The conversion table for performing this compensation is table 2.

With respect to the dimensions of the steel sheet, especially the thickness thereof, the change thereof leads to the change of the magnetic flux distribution in the steel sheet. Therefore, even if the current flowing through the electric coils 12 and 13 is constant,
The magnetic flux density changes as the dimensions change. Therefore, in order to maintain the magnetic flux density at the target value, it is necessary to perform compensation according to changes in dimensions. The conversion table for applying this compensation is table 3. Similarly, if the type of steel sheet changes,
Since the relative permeability changes, the magnetic flux density changes. Therefore, in order to maintain the magnetic flux density at the target value, it is necessary to perform compensation according to the change in steel grade. The conversion table for applying this compensation is table 4.

The magnitude of the current flowing through the electric coils 12 and 13 and the magnetic flux density have a correlation as shown in FIG. 10, for example. Information on such correlation is stored in the table 5 in advance. Therefore, the value C + of the current to be set can be immediately obtained by referring to the table 5 with the target magnetic flux density M4 as a parameter.

In the above embodiment, various compensations are carried out by referring to the table in which various information is stored in advance.
For example, the same compensation can be performed by using a predetermined calculation formula according to the result of the experiment. In the embodiment, the current value is obtained from the result of compensating the optimum magnetic flux density, but the current value may be compensated after obtaining the current value from the optimum magnetic flux density before compensation.

Further, in the embodiment, a single copying mechanism having a relatively simple structure is used so that the entire defect inspection apparatus follows the surface of the steel sheet, but the magnetic poles (11e, 11f).
When the Faraday effect element and the Faraday effect element are supported by independent copying mechanisms, the change in the gap between the Faraday effect element and the steel plate can be reduced, which is practical.

[0040]

As described above, according to the present invention, the first gap detecting means for detecting the size of the gap between the surface of the inspection object and the Faraday effect element means, the surface of the inspection object and the magnetizing means. A second gap detecting means for detecting the magnitude of the gap between the magnetizing means and the magnetizing means is provided, and the magnitude of the exciting current for energizing the magnetizing means is controlled in accordance with the magnitude of the gap detected by the second means. Therefore, even when inspecting an inspection object whose surface position constantly changes due to its vibration, such as a thick steel plate, deterioration of the S / N ratio of the detection signal can be prevented, and defects can always be detected with high reliability. It can be detected.

[Brief description of drawings]

FIG. 1 is a perspective view showing an appearance of a magnetizer 10 used in an example.

FIG. 2 is a front view showing the external appearance of the main part of the defect inspection apparatus of the embodiment.

3 is a side view showing an appearance of a main part of the defect inspection apparatus of FIG.

FIG. 4 is a block diagram showing a configuration of an electric circuit of the defect inspection apparatus.

5 is a plan view showing an example of an image captured by the image capturing device 50. FIG.

6 is a plan view showing an image after the half cycle of FIG. 5. FIG.

FIG. 7 is a plan view showing a negative image of the image of FIG.

FIG. 8 is a waveform diagram showing changes in brightness of an image before and after processing.

FIG. 9 is a graph showing an equal S / N ratio curve and an optimum magnetization curve.

FIG. 10 is a graph showing a correlation between a magnetizing current and a magnetic flux density.

FIG. 11 is a graph showing the correlation between the magnetic flux density and the gap G2 between the magnetic pole and the steel plate.

FIG. 12 is a flowchart showing the operation of the CPU.

[Explanation of symbols]

1: Thick steel plate 1a: Surface 2: Wheel 10: Magnetizer 11: Core 11a, 11b, 11
c, 11d: Sides 11e, 11f: Protrusions 11g: Openings 12, 13: Electric coil 20: Faraday effect element 30: Projector 31: White lamp 32: Diffusion plate 33: Polarizer 40: Analyzer 45: Bandpass filter 50: Imaging device 51: A / D converter 52: Polarity converter 53: Frame buffer 54: Adder 55: Image processing device 56: Defect discriminator 57: Monitor TV 60: Light receiver 61: Oscillator 62: Strobe power supply 63: Polarity detector 64: Power amplifier 65: Phase shifter 71, 72: Gap sensor 73: Magnetic flux density determination process 74: Magnetization current determination process 75: Latch 76: Data selector 77: D / A converter 78: Process Computer CPU: Microcomputer

Claims (3)

[Claims] 1. A magnetizing means for magnetizing at least a surface layer portion of an inspection object; arranged in the vicinity of a magnetized surface of the inspection object, and rotating a polarization plane of light passing therethrough according to a magnetic field received by the magnetization means. Faraday effect element means; Illuminating means for irradiating the plane of the Faraday effect element means with linearly polarized light; Reflected light from the Faraday effect element means is received through an opening in the center of the frame. An analyzer for detecting the defect on the inspection object from the intensity of the light passing through the analyzer, the defect detecting means supporting the magnetizing means and the Faraday effect element means, and Means for locating the object near the surface of the object to be inspected; first gap detecting means for detecting the size of the gap between the surface of the object to be inspected and the Faraday effect element means; surface of the object to be inspected and magnetizing means The size of the gap between Second gap detecting means for detecting
A magnetic flux density determining means for determining a target magnetic flux density based on the size of the gap detected by the first gap detecting means; and a target magnetic flux density determined by the magnetic flux density determining means and the second
A magneto-optical defect inspection device comprising: an exciting current determining means for determining an optimum exciting current based on the size of the gap detected by the gap detecting means, and energizing the magnetizing means with the determined exciting current. 2. The magnetic flux density determining means sets a calculation formula or a constant table for obtaining the magnitude of the magnetic flux density when the S / N ratio of the defect detection signal becomes maximum with respect to the size of each gap. 2. The magnetic flux density which is held in advance and which maximizes the S / N ratio of the defect detection signal with respect to the size of the gap detected by the first gap detecting means, and which is used as the target magnetic flux density. Magneto-optical defect inspection device. 3. The magneto-optical defect inspecting apparatus according to claim 1, wherein the exciting current determining means further inputs information on the size and steel type of the inspection object and determines an optimum exciting current according to the information.