JP5640298B2 - Magneto-optical flaw detection method and apparatus used therefor - Google Patents

Description

  The present invention relates to a magneto-optical flaw detection method for detecting a surface defect of a cylindrical inspection object using a magneto-optical film and an apparatus used therefor. This technique is useful, for example, for defect inspection of a battery outer case.

  Conventional flaw detection methods for detecting surface defects of a cylindrical battery outer case (battery can) made of a magnetic material include a method using a magnetic sensor and a method using a coil. In the flaw detection method using a magnetic sensor, a defect is detected by bringing the magnetic sensor close to the battery outer case and measuring the leakage magnetic field from the surface of the excited battery outer case (see Patent Document 1). In the method using a coil, a coil through which an alternating current or a pulse current is passed is brought close to the battery outer case, and the defect is determined based on an impedance change or a voltage change due to a defect present in the battery outer case (see Patent Document 2). .

  In these flaw detection methods, it is necessary to scan the magnetic sensor and the coil over the entire side surface of the battery outer case. Usually, the battery outer case is rotated around its central axis, and the magnetic sensor and the coil are moved in a direction parallel to the rotation axis. Therefore, a mechanism for rotating and holding the battery outer case and a mechanism for moving the magnetic sensor and the coil in a direction parallel to the rotation axis while holding the state close to the battery outer case are required, and the structure becomes complicated. In addition, above all, since the magnetic sensor and the coil are scanned over the entire side surface of the battery outer case, there is a drawback that it takes time to inspect each battery outer case.

  Incidentally, a flaw detection method using a magneto-optical film is known as a flaw detection technique for magnetic materials. This is a method of exciting a magnetic material and magnetically transferring a leakage magnetic field generated in the vicinity of a defect on the surface of the material to a magneto-optical film, and detecting the defect using the magneto-optical effect (see, for example, Patent Document 3).

  In general, a magneto-optical element is used in which a magneto-optical film is formed on a transparent substrate and a metal reflective film is formed thereon by sputtering or vapor deposition. The reflective film is arranged so as to face the surface of the object to be inspected, light is irradiated from the transparent substrate side, the light passing through the transparent substrate and the magneto-optical film is reflected by the reflective film, and the magneto-optical film and the transparent substrate are again mounted. Light exiting through is detected. At this time, since the leakage magnetic field from the surface of the object to be inspected is magnetically transferred to the magneto-optical film, the Faraday rotation angle changes locally depending on the presence or absence of the leakage magnetic field. By combining the polarizer and the analyzer, the leakage magnetic field Can be detected as a two-dimensional light intensity distribution. Therefore, compared with the flaw detection method using a magnetic sensor or a coil, there is an advantage that the presence or absence of defects in a wide area can be observed instantaneously, which is used for flaw detection of steel strips and steel plates having a large surface area.

  In such a magneto-optical flaw detection method, the leakage magnetic field strength on the surface of the inspection object is detected. Therefore, when the distance from the surface of the inspection object to the magneto-optical film increases, the sensitivity rapidly decreases. Therefore, in order to improve the defect detection sensitivity, it is necessary to observe the magneto-optical film by arranging it so as to maintain a constant interval (for example, about several tens to one hundred μm or less) in the very vicinity of the surface of the inspection object. However, when the object to be inspected is cylindrical, such as a battery outer case, unlike the case of a flat plate, the entire side surface cannot be inspected with the conventional apparatus configuration because the side surface is curved.

  Of course, the surface of the object to be inspected can be inspected with the naked eye or a camera if it is large to some extent. However, since the battery outer case is molded by pressing, oils and the like are adhered to the surface immediately after molding, and it cannot be inspected with a camera or the like unless it is cleaned to remove dirt. However, for product management and the like, there is a demand for development of a method that can detect defects easily and quickly even if it remains dirty without being washed immediately after molding.

JP 2006-153856 A JP 2009-252644 A Japanese Patent No. 2672912

  The problem to be solved by the present invention is to make it possible to detect a surface defect of a cylindrical inspection object easily and efficiently in a short time using a magneto-optical film.

The present invention provides a flaw detection method in which a magnetic field is applied to an inspection object, and a leakage magnetic field generated in a defect portion on the surface of the inspection object is magnetically transferred to a magneto-optical film disposed close to the inspection object and detected using the magneto-optical effect. in the method, the inspection object has a cylindrical shape, in Rukoto is rolled the cylindrical inspection object in the lateral direction at the top of the magneto-optical element having a reflective film on the upper surface of the magneto-optical film, the object to be The side surface of the inspection object is sequentially brought into contact with the magneto-optical element over the entire circumference, whereby the leakage magnetic field on the side surface of the inspection object is sequentially magnetically transferred to the magneto-optical film, and the polarizer is passed from below the magneto-optical element. Two-dimensional light intensity in which the entire side surface of the inspection object is developed by irradiating the reflected light and observing the light reflected by the reflection film through the analyzer, so that the defect portion existing on the entire side surface of the inspection object is developed. The feature is to detect as distribution That is a magneto-optical flaw detection method.

  Most simply, the cylindrical inspection object is configured to roll while contacting the upper surface of the magneto-optical element with its own weight. Alternatively, the magneto-optical element is fitted in the recess of the holding member so that the upper surface of the magneto-optical element is lower than the upper surface of the holding member, and the cylindrical inspection object rolls while contacting the upper surface of the holding member with its own weight. By moving, the inspection object can be brought into a non-contact state with respect to the magneto-optical element.

  Most typically, the cylindrical inspection object is a battery outer case. By attaching a permanent magnet to the outside of the bottom surface, a magnetic field can be applied to the object to be inspected.

  As an apparatus for performing the above-described magneto-optical flaw detection method, a magneto-optical element having a reflective film provided on the upper surface of the magneto-optical film, and a cylindrical inspection object positioned above the magneto-optical element are arranged in the horizontal direction. A comb conveyor for transporting and an optical system located below the magneto-optical element, wherein the comb conveyor is provided with a plurality of grooves each containing a cylindrical object to be inspected, and the depth of each groove is The optical system has a structure that is shallower than the diameter of an object to be inspected, and the optical system has a configuration in which light from a light source irradiates the magneto-optical element through a polarizer and the reflected light is imaged by a CCD camera through the analyzer.

  In the flaw detection method according to the present invention, a cylindrical inspection object is rolled laterally above a magneto-optical element having a structure in which a reflective film is provided on the upper surface of the magneto-optical film, and observed from below the magneto-optical element. Therefore, the leakage magnetic field generated by the defect on the side surface of the inspection object is transferred to the magneto-optical film while the inspection object makes one rotation while keeping the distance between the inspection object and the magneto-optical film constant. Magnetic transfer can be performed, whereby the entire side surface can be observed, and surface defects can be detected easily and efficiently in a short time.

  In the present invention, even when the cylindrical inspection object rolls while contacting the upper surface of the magneto-optical element, it is only in contact with its own weight, so that the reflection film or the protective film thereon is less likely to be damaged. . Further, when the magneto-optical element is fitted into the recess of the holding member so that the upper surface of the magneto-optical element is lower than the upper surface of the holding member, the cylindrical object to be inspected is in contact with the upper surface of the holding member by its own weight. Even after rolling, the object to be inspected floats with respect to the magneto-optical element and is in a non-contact state, so that the durability of the magneto-optical element is improved and it can be used for a long time.

  When the cylindrical inspection object is a battery outer case, it has a bottomed structure. Therefore, by attaching a disc-shaped permanent magnet magnetized in the thickness direction on the outside of the bottom surface, a magnetic field is applied to the inspection object. Can be applied. If it does in this way, since a permanent magnet will be individually attached to each to-be-inspected object, even if it conveys it laterally while rolling, it will not become the obstacle of the conveyance. In addition, there is little possibility that a magnetic field generated by a permanent magnet directly acts on the magneto-optical film.

  In order to transport a cylindrical inspection object in a horizontal direction, when a comb conveyor having a structure in which a plurality of grooves each containing an inspection object are arranged in parallel and the depth of each groove is shallower than the diameter of the inspection object is used. The inspection object can be observed by rolling the upper part of the magneto-optical element one by one at a predetermined interval.

Explanatory drawing of the magneto-optical flaw detection method which concerns on this invention. The front view of the principal part. Explanatory drawing which shows an example of the conveying method of the to-be-inspected object used by this invention. The enlarged view which shows the example of the positional relationship of a to-be-inspected object and a magneto-optical element, the holding structure of a magneto-optical element.

  The present invention applies a magnetic field to an object to be inspected, magnetically transfers a leakage magnetic field generated in a defect portion on the surface of the object to a magneto-optical film disposed close to the object to be inspected, and detects the magnetic field using the magneto-optical effect. This is an optical flaw detection method. The object to be inspected in the present invention is a cylindrical inspection object 10 as shown in FIG. 1, typically a battery outer case. The battery outer case is also called a battery can and is made of an iron-based magnetic material (for example, a thin steel plate subjected to surface treatment such as nickel plating) and has a bottomed cylindrical shape. One end is open and the other end (bottom) is closed to form a flat surface, which is usually formed by press drawing. Therefore, various defects may occur during processing.

  In the present invention, this cylindrical inspection object 10 is inspected by rolling (indicated by arrow B) in the lateral direction (indicated by arrow A) above the magneto-optical element 12. A magnetic field is applied to the object to be inspected 10 by attaching a disk-shaped permanent magnet 14 magnetized in the thickness direction to the outside of the bottom surface. If there is a defect on the surface of the inspection object 10, a change in the leakage magnetic field occurs in the vicinity of the defect. Since the permanent magnets 14 are directly attached to the inspected objects 10, there is little possibility that a magnetic field from the permanent magnets 14 is directly applied to the magneto-optical element 12.

  The magneto-optical element 12 used in the present invention has a structure in which a reflective film 22 is provided on the upper surface of the magneto-optical film 20. For example, as shown in FIG. 2, a magneto-optical film 20 such as iron garnet is grown on a transparent single crystal substrate 24 by a liquid phase epitaxial method or the like, and a reflective film 22 made of metal or the like is formed by sputtering or vapor deposition. A protective film may be further provided on the upper surface of the reflective film. Here, the magneto-optical element 12 has a quadrangular shape, and one side thereof (side in the direction orthogonal to the arrow A) is slightly longer than the length of the inspection object 10 and the other side (side in the same direction as the arrow A). Is slightly longer than the entire circumference of the inspection object 10. The inspection object 10 rolls in the lateral direction on the magneto-optical element 12 while being in contact with the reflective film 22. As a result, the entire surface of the object to be inspected 10 contacts the magneto-optical element 12 during one rotation, and the leakage magnetic field generated by the defect is magnetically transferred to the two-dimensional planar magneto-optical film 20.

  The light emitted from the light source 30 is converted into linearly polarized light through the polarizer 32, the optical path is bent by the beam splitter 34, and the magneto-optical element 12 is irradiated from below. The light reflected by the reflection film 22 passes through the beam splitter 34, passes through the analyzer 36, and is observed by the CCD camera 38. A half mirror may be used instead of the beam splitter. In this way, the light emitted from the light source 30 irradiates the entire magneto-optical element 12, and the CCD camera 38 images the entire magneto-optical element 12.

  Here, the magnetization direction in the magneto-optical film 20 is in the in-plane direction in a natural state. In this case, when the magneto-optical film 20 is irradiated with linearly polarized light, the polarization plane of the reflected light by the reflective film 22 does not rotate. Therefore, if the transmission axes of the polarizer 32 and the analyzer 36 are orthogonal to each other, the reflected light cannot pass through the analyzer 36 and the image taken by the CCD camera 38 becomes dark (black). If the magnetization direction in the magneto-optical film 20 is perpendicular to the film surface, the polarization plane of the reflected light by the reflection film 22 rotates when the linearly polarized light is applied to the magneto-optical film 20. Therefore, if the transmission axes of the polarizer 32 and the analyzer 36 are orthogonal to each other, the reflected light passes through the analyzer 36 and the image taken by the CCD camera 38 becomes bright (white).

  As the inspection object 10 rolls laterally above the magneto-optical element 12, the leakage magnetic field is magnetically transferred to the magneto-optical film 20 with the side surface of the inspection object 10 spread over the entire circumference. That is, if there is a defect on the side surface of the inspection object 10, the leakage magnetic field changes, and when the inspection object 10 rolls and the defect is positioned directly below, the magnetization direction of the magneto-optical film 20 is changed by the leakage magnetic field. Since the magnetization direction of the magneto-optical film 20 is locally different depending on the presence or absence of defects, there is a difference in the intensity of light transmitted through the analyzer 36 at that location, and the image taken by the CCD camera 38 appears to shine white. For example, in the case of a linear scratch that is not parallel to the central axis, a moving image is obtained in which white dots run obliquely with the rotation of the inspection object (in other words, with time). Therefore, the shape and size of the defect can be determined from the shape and degree of light and darkness (two-dimensional light intensity distribution) in the image photographed by the CCD camera 38. In this way, while the inspection object 10 makes one rotation, it is possible to detect a linear scratch or U-shaped scratch on the side surface, or a defect such as a wrinkle or a crack as a two-dimensional image.

  As described above, the cylindrical inspection object 10 such as the battery outer case is placed on the magneto-optical element 12, so that the entire side surface is sequentially magnetized during one rotation by rolling in the lateral direction. 12 is contacted. Accordingly, it is possible to keep the distance between the object to be inspected 10 and the magneto-optical element 12 constantly constant and extremely small. When the object to be inspected 10 is in contact with the magneto-optical element 12, there is a concern about the deterioration of the reflective film 22 or the protective film covering it, but an excessive load is applied in the direction perpendicular to the film surface only by contact with its own weight. The magneto-optical element 12 is hardly deteriorated because it does not slide and rolls instead of sliding. Furthermore, even when the object to be inspected is eccentric, it can be inspected because it is always kept in contact.

  FIG. 3 shows an example of a method for conveying a cylindrical inspection object. Here, the inspection object 10 is conveyed in the lateral direction by a comb conveyor 40 positioned above the magneto-optical element 12. The comb-shaped conveyor 40 includes a plurality of grooves 42 in which one cylindrical inspection object 10 is inserted, and the depth of each groove 42 is shallower than the diameter of the inspection object 10. The width of the groove 42 is the inspection object. The structure is larger than 10 diameters. The inspection object 10 is placed in each groove 42, and the comb conveyor 40 is moved in the lateral direction. Accordingly, the inspection object 10 moves on the magneto-optical element 12 while rolling in the groove 42 and is observed by the optical system positioned below.

  The object to be inspected 10 may be in direct contact with the magneto-optical element 12, but if a very slight constant gap is always formed with respect to the magneto-optical element 12, the magneto-optical element 12 is worn away. Can be prevented. For this purpose, as shown in FIG. 4, the magneto-optical element 12 may be mounted in the recess 52 of the holding member 50 so that the upper surface of the magneto-optical element 12 is slightly lower than the upper surface of the holding member 50. . The holding member 50 has a frame shape having an opening at the center so that light can be irradiated from below. Accordingly, the inspection object 10 is configured to roll on the holding member 50, that is, the upper surface of the frame portion, and the interval s between the inspection object 10 and the magneto-optical element 12 can be kept constant. For example, if the interval s is set to about 20 μm, the leakage magnetic field intensity from the surface of the object to be inspected can be detected with high sensitivity, and the uppermost reflective film (or protective film) of the magneto-optical element can be detected. It can also prevent damage.

  When the magneto-optical element 12 is configured to be held at a low position by the holding member 50 as described above, the outer diameter of the cylindrical inspection object is slightly changed in the axial direction (for example, in a battery outer case) Even after the battery element is accommodated, the vicinity of the opening end may be slightly larger in diameter (even if the vicinity of the opening end may be slightly larger in diameter), the correction may be made to roll straight using the difference in height between the holding member and the magneto-optical element. Can do.

  A 2 μm thick Bi-substituted iron garnet film (magneto-optic film) is grown on a 700 μm thick SGGG single crystal substrate (transparent substrate) by liquid phase epitaxy, and a 200 nm thick Pt sputtered film (reflective film) is formed thereon. A magneto-optical element was formed. A permanent magnet was mounted on the bottom surface of the battery outer case, placed on the magneto-optical element, and rolled laterally. A green LED was used as the light source, the light was converted into linearly polarized light using a polarizer, irradiated from below the magneto-optical element, and the reflected light was received by the CCD camera through the analyzer to obtain an image.

  When the surface of the battery outer case was observed with such an apparatus configuration, even a minute linear scratch having a width of about 200 μm could be reliably detected.

DESCRIPTION OF SYMBOLS 10 Test object 12 Magneto-optical element 14 Permanent magnet 20 Magneto-optical film 22 Reflective film 30 Light source 32 Polarizer 34 Beam splitter 36 Analyzer 38 CCD camera

Claims (4)

In a flaw detection method in which a magnetic field is applied to an object to be inspected, and a leakage magnetic field generated in a defect portion on the surface thereof is magnetically transferred to a magneto-optical film disposed close to the object to be inspected, and detected using the magneto-optical effect.
The inspection object has a cylindrical shape, and the cylindrical inspection object is laterally rolled on the top of the magneto-optical element provided with a reflection film on the top surface of the magneto-optical film, thereby side surfaces of the inspection object. Is sequentially contacted with the magneto-optical element over the entire circumference, thereby sequentially transferring the leakage magnetic field on the side surface of the inspection object to the magneto-optical film, and irradiating light from below the magneto-optical element through a polarizer. Then, by observing the light reflected by the reflective film through the analyzer, the defect portion existing on all the side surfaces of the inspection object is detected as a two-dimensional light intensity distribution developed on the entire side surface of the inspection object. A magneto-optical flaw detection method characterized by the above.   The magneto-optical flaw detection method according to claim 1, wherein the cylindrical inspection object rolls while contacting the upper surface of the magneto-optical element with its own weight. 3. The magneto-optical flaw detection method according to claim 1, wherein the cylindrical inspection object is a battery outer case, and a magnetic field is applied to the inspection object by mounting a permanent magnet outside the bottom surface. An apparatus used for carrying out the magneto-optical flaw detection method according to claim 3 , wherein a magneto-optical element having a reflective film provided on an upper surface of the magneto-optical film, and a cylindrical inspection object positioned above the magneto-optical element are horizontally disposed. A comb conveyor that conveys in a direction and an optical system positioned below the magneto-optical element. The comb conveyor includes a plurality of grooves each containing a cylindrical object to be inspected. The optical system has a configuration in which light from a light source irradiates the magneto-optical element through a polarizer and the reflected light is imaged by a CCD camera through the analyzer. A magneto-optical flaw detector.

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