JPH081458B2 - Optical magnetic field distribution measuring device - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art An apparatus for magnetizing a ferromagnetic material and detecting a leakage magnetic flux leaking from a defect by a hall element or a detection coil is
The inspection speed is higher than that of the magnetic particle flaw detection, and the detection output has a high correlation with the depth, and the surface roughness of the ferromagnetic material and the scale and penetration are higher than those of other equipment such as the eddy current flaw detector. Since it is less affected by variations in magnetic susceptibility, it is widely used in automatic flaw detection equipment for steel pipes. In addition, JP-A-2-227
As disclosed in Japanese Patent No. 683, an invention for applying the optical magnetic field measurement method to the detection of leakage magnetic flux has been made.

[0003]

However, since the conventional apparatus detects the leakage magnetic flux as the change of the magnetic field, the area where the magnetic field leaks rapidly becomes smaller as the size of the defect becomes smaller. In order to detect, it is necessary to arrange a large number of small magnetic field detection elements in close proximity to the surface of the material to be inspected and follow the partner. However, it was very difficult to manufacture a mechanism that follows a steel head or a thick plate that moves a sensor head having thousands of signal wires at high speed, and it was also very difficult in terms of cost. .

Further, the above-mentioned Japanese Patent Application Laid-Open No. 2-227683 relates only to point measurement for detecting small defects, and requires equipment cost as in the case of flaw detection of steel strip or thick plate having a large surface area. There is no prior art relating to means for resolving the inconsistency between inspection speed and inspection accuracy.

Further, in recent years, the quality requirements of customers' products have become stricter, and it has become necessary to guarantee a small defect that cannot be detected at the same magnetization level as that of the conventional magnetic flaw detector. It was said that a magnetization level of about 0.8 times the saturation magnetic flux density was optimal, but in the detection of small inclusions in a thin plate, for example, 40
Nearly double the magnetic field strength is required. Generally, a highly sensitive sensor can be used only in a weak magnetic field, but it is necessary to detect a weak magnetic field change distribution in a strong background magnetic field to enable detection of small defects.

In view of such a problem, the present invention radically solves this problem and enables a single detection unit to inspect a wide range of leakage magnetism, and enables high-speed and highly accurate detection of surface defects. It is an object of the present invention to provide an epoch-making optical magnetic field distribution measuring device.

[0007]

SUMMARY OF THE INVENTION The present invention is basically a magnetic field measuring apparatus using a magneto-optical effect, and a magneto-optical effect element having an easy axis of magnetization in a direction perpendicular to a film surface and linearly polarized light on the magneto-optical effect element. A device for projecting a fan-shaped light beam from above, an analyzer for converting the amount of rotation of the plane of polarization of the light specularly reflected from the bottom surface of the magneto-optical effect element film into a change in the amount of light, and a linear image sensor for measuring the amount of transmitted light. To be done.

Furthermore, in a preferred embodiment, the device for projecting the fan-shaped light beam from above is configured to focus the fan-shaped light beam on the photoelectric surface of the image sensor in a direction orthogonal to the spreading direction of the fan-shaped light beam. Then, the spreading direction of the fan-shaped light and the scanning direction of the image sensor are matched.

[0009]

First, the background of the invention will be described.

The inventor of the present invention has researched an apparatus for detecting a wide range of defects at high speed and even in minute defects by utilizing the magneto-optical effect.
As a result, there is a highly sensitive material in the rare-earth / iron / garnet (RIG) perpendicular magnetization film used in optical isolators and optical switches. It has an excellent characteristic as a leakage magnetic flux sensor that has a difficult magnetization property, is not magnetically saturated by a horizontal magnetic field for generating a leakage magnetic flux from a defect, and does not change the Faraday rotation in the vertical direction. It has been found.

However, the RIG perpendicular magnetization film has a band-shaped 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, a large spot, The average value is Faraday
Although there is no problem because rotation is detected, the width of the band is wider for higher sensitivity materials, so if the light spot for inspection is made small to detect small defects, it will become a large noise source and difficult to detect accurately. I understand.

Further, magnetic domain growth occurs when a perpendicular magnetic field is applied to the film, but the magnetic domain structure that does not normally move even when a large in-plane magnetic field is applied is affected by the magnetic field in the direction of hard magnetization in the presence of the 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 by optical scanning.

Next, the behavior of the magneto-optical effect element under a strong in-plane magnetic field, which is a new finding obtained by the inventor by the inventor, will be described with reference to FIGS. 3, 4 and 5.

FIG. 3 shows a magnetic effect element observed with a polarization microscope in a state where no horizontal magnetic field is applied, and black and white stripes extending in random directions can be observed. This is a magnetic domain pattern of the perpendicular magnetization film, and the area ratio is 1: 1.
Is. When this is photoelectrically converted, the white part becomes a positive voltage,
The black part has a negative voltage. Conventionally, there was no problem because the magnetic field was measured as an average value of the whole. The width of the stripe is about 40 μm, which is a size that cannot be ignored because the width of the cracking defect is about 100 μm, and the defect detection sensitivity decreases when the measurement pixel is increased in order to reduce the noise of the magnetic domain. The first problem has come up.

FIG. 4 shows a magnetic domain pattern similarly observed by applying a horizontal magnetic field of about 500 Oersted toward the left and right, but the area ratio of black and white does not change.
It can be seen that many stripes are arranged in the left-right direction as compared with FIG. Even though this element is a highly sensitive material that causes magnetic saturation with only Hs = 40 Oersted in the vertical direction, even under such a strong magnetic field in the in-plane direction, the average value in the direction perpendicular to the plane is It can be seen that the characteristic of does not change. But,
Randomness remains because alignment occurs partially,
When scanning in the width direction with a minute spot, a low frequency component is generated, so that there is a second problem that the SN ratio cannot be improved only by the low pass filter.

FIG. 5 shows a state in which a strong horizontal magnetic field is applied to a ferromagnetic material having vertical stripe defects to generate a leakage magnetic flux, and a magnetic effect element is brought close to the surface of the ferromagnetic material and observed by a polarization microscope. Although the black portion 8 is enlarged on the left side of the defect extending in the vertical direction and the white portion 9 is enlarged on the right side, it can be seen that there is a vertical vertical magnetic field in this portion. Further, it can be observed that the stripes are aligned in the magnetic field direction in the portion where the stripe width is changed. On the other hand, in the portion where the vertical magnetic field does not exist, the directions of the magnetic domain patterns are random, and even horizontal alignment does not occur. The material used for this measurement is a material that causes magnetic saturation at 200 Oersteds in the vertical direction. As described above, by selecting the material, it is possible to deal with the second problem in the region where there is no defect, and it is possible to improve the defect revealing property.

However, in the defective portion, alignment of magnetic domains occurs in the magnetic field direction, and the width of this stripe is 5
It is about 50 μm, which is not so different from the width of cracks and the width of minute inclusions. Further, since the pixels of the image sensor are of the same size, the detection voltage greatly varies depending on the degree of overlap between them.

The present invention has been made based on the above findings.

[0019]

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

FIG. 1 shows the overall structure of the apparatus used in the present invention. Reference numeral 1 denotes a fan-shaped light source, and 2 denotes fan-shaped light emitted from the light source. Reference numeral 3 is a polarizer, 4 is a semi-transmissive mirror, 5 is a magneto-optical effect element, 6 is an analyzer, and 7 is a linear array image sensor. The polarizer 3 is a fan-shaped light 2
Only the vibration component in the direction perpendicular to the plane created by the fan-shaped light is linearly polarized. The semi-transmissive mirror 4 transmits half of the fan-shaped light straight, and the rest is vertically reflected and projected on the magneto-optical effect element 5. The magneto-optical effect element 5 is a rare-earth iron garnet perpendicular magnetization film, has a non-magnetization characteristic except in a direction perpendicular to the plane, and moves a magnetic domain or causes magnetic saturation in a horizontal magnetic field of about 500 to 1000 oersteds. Use the one that does not happen.
Also on the upper surface of the film nonreflective co - computing, on the bottom surface total reflection co - coating has been applied, the light incident on the film going out is reflected at the bottom surface in the vertical direction. Due to the Faraday effect, the polarization plane rotates in proportion to the product of the distance of reciprocal transmission in the film, the Verdet constant of the film, and the magnetic field strength in the vertical direction at the position where the film exists. The reflected light passes through the semitransparent mirror 4 and reaches the analyzer 6. The polarization axis of the analyzer 6 is inclined by 45 ° with respect to the polarizer 3. 7 is a linear array image
It is a disensor and photoelectrically converts incident fan-shaped light and outputs it as a voltage time-series signal.

Now, the fan-shaped light 2 is focused light in a direction perpendicular to the fan-shaped plane, and its focal point is the photoelectric conversion surface of the image sensor. On the other hand, FIG. 2 is a schematic diagram for explaining how fan-shaped light spreads in the optical system of FIG. 1, and the same elements corresponding to those of FIG. 1 are designated by the same reference numerals.

In FIG. 2, L ₁ [mm] is the distance from the focus position of the fan-shaped light 2 inside the fan-shaped light source 1 to the magneto-optical effect element 5, and L ₂ [mm] is the magneto-optical effect element. It is the distance to the linear image sensor-7.
Now, if K ₁ = (L ₁ + L ₂ ) / L ₁ (1), an image of width W ₁ [mm] on the magneto-optical effect element 5 is K at the position of the linear image sensor-7. _{After being} multiplied by ₁ , W ₂ = K ₁ · W ₁ ········· (2). On the other hand, in FIG. 1, when the diameter of the outlet of the fan-shaped light source 1 in the vertical direction is D ₀ [mm] and the diameter in the same direction on the magneto-optical effect element 5 is D ₁ [mm], the above L ₁ [ mm] is replaced by the distance from the outlet of the fan-shaped light source 1 to the magneto-optical effect element 5, K ₂ = (L ₁ + L ₂ ) / L ₁ (3) D ₁ = 1 / K ₂・ D ₀・ ・ ・ ・ ・ ・ (4) The diameter D ₂ [mm] at the position of the linear image sensor 7 is determined by the diffraction limit of the laser D ₂ = 1.22 · λ · (L ₁ + L ₂ ) / D ₀ ··· (5 ) Is represented. Where λ [mm] is the laser wavelength. However, in reality, the aberration of the lens is added to this.

This D ₂ can be selected to be smaller than the pixel of the linear image sensor-7. Therefore, Riniaime - Jisensa - when the size of the pixel is assumed to be square having a side with a [mm], in practice the surface of the magnetic effect element for detecting the magnetic field, 1 in the width direction of the K ₁ minute, Conversely, it becomes D ₁ / a times in the vertical direction, and it is possible to have an elongated rectangular field of view.

Therefore, the optical system used in the present invention is shown in FIG.
When applied to the target shown in, when the direction of spreading the fan-shaped light and the direction of applying the horizontal magnetic field are made to coincide, the visual field of the image sensor is elongated in the direction of the flaw and is orthogonal to the alignment direction of the stripes. It is possible to reliably average a large number of black and white stripes.

Since the defects of the rolled metal generally extend in the rolling direction, about 1 mm can be taken by applying a horizontal magnetic field in the direction orthogonal to the rolling direction, and a sufficient number of magnetic domains can be averaged. , Variation is reduced.
The scanning direction of the image sensor is 10 μm.
The following resolutions can be secured, and in addition, an ideal defect detection device with high precision and high resolution can be realized.

When a large area is inspected using the present invention, it is necessary to generate a strong and constant horizontal magnetic field over a long distance while suppressing the vertical magnetic field. As shown in Japanese Utility Model Laid-Open No. 62-111539, a completely identical rod-shaped electromagnet that generates a uniform horizontal magnetic field as shown in Japanese Patent Laid-Open No. 59-160750 is disclosed.
If two materials are arranged in parallel and the material to be inspected is arranged along the line of axial symmetry, the vertical magnetic fields cancel each other out and the horizontal magnetic fields reinforce each other, so that a strong and uniform horizontal magnetic field can be obtained. By doing so, it is possible to generate a strong horizontal magnetic field while suppressing the vertical magnetic field in the width direction of the strip-shaped inspection material.

Further, in the present invention, since the scanning is performed electronically , the scanning speed can be made sufficiently faster as compared with the case where the target member is vibrated, and the same signal processing as a normal image signal is applied in the post-processing. For example, the effect of the magnetic field change due to the lift-off change, which is a problem in Japanese Patent Laid-Open No. 2-227683, can be removed only by a simple spatial differentiation process, and further, the pattern recognition of defects and the defect type determination can be performed. It will be possible.

[0028]

In recent years, the field of optoelectronics has made remarkable progress, and high-sensitivity Faraday rotating materials for optical output semiconductor lasers and isolators, large contact sensors for large facsimiles, etc. are cheaper than conventional ones. Since a wide-width fan-shaped light source, a high-sensitivity perpendicular magnetized film, and a wide-width image scanner can all be realized at low cost, the device of the present invention has many conventional small sensors arranged in parallel. The number of circuits is reduced to one hundredth of that of the conventional one, and the economical efficiency can be improved epoch-making.

Further, by changing the magnification of the optical system, it becomes possible to detect a defect smaller than before, and the detection accuracy is improved.

Since the number of constituent parts is reduced, the number of failures is reduced and the maintainability is remarkably improved.

The apparatus of the present invention can be applied to all areas where the accurate leakage magnetic flaw detection method could not be applied due to economical difficulty, and has a great effect on productivity improvement through labor saving and automation.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 2 is a schematic diagram showing the device of FIG.

FIG. 3 is a plan view sketching a polarization microscope photograph of a magnetic effect element when a magnetic field is not applied.

FIG. 4 is a plan view sketching a polarization micrograph of a magnetic effect element when a magnetic field is applied in the left-right direction.

FIG. 5 is a plan view in which a polarization microscope photograph of a magnetic effect element is sketched when a strong horizontal magnetic field is applied to a ferromagnetic material having streaky defects to bring the magnetic effect element close to the surface thereof.

[Explanation of symbols]

1: Fan-shaped light source 2: Fan-shaped light 3: Polarizer 4: Semi-transmissive mirror 5: Magneto-optical effect element 6: Analyzer 7: Linear array image scanner-8: Black stripe enlarged portion 9: White stripe enlarged portion

Claims (2)

[Claims] In the magnetic field measuring apparatus using the method according to claim 1 magneto-optical effect, magneto-optical effect element and, this linearly polarized light magneto disable the fan beam from above having an axis of easy magnetization in the perpendicular direction to the film plane
A device that simultaneously projects over the entire width of the element, an analyzer that converts the amount of rotation of the polarization plane of the light specularly reflected from the bottom surface of the magneto-optical effect element film into a change in the amount of light, and the full width of the magneto-optical effect element.
And a linear image sensor for measuring the amount of transmitted light. 2. A device for projecting a fan-shaped light beam from above is configured to focus the fan-shaped light beam on a photoelectric surface of an image sensor in a direction orthogonal to a spreading direction of the fan-shaped light beam,
2. The optical magnetic field distribution measuring device according to claim 1, wherein the direction of spread of the fan-shaped light and the scanning direction of the image sensor coincide with each other.