Magnetic flux leakage inspection device and defect inspection method

Abstract

Provided are a magnetic flux leakage inspection device and a defect inspection method that can achieve miniaturization and weight reduction of an exciter, does not generate energy loss due to heat, can easily cut off excitation, and can maintain this state without using excessive energy. The magnetic flux leakage inspection device (1) comprises: a main exciter (2) in a gate shape, having an opening (2a) at a portion facing an inspection object, and having one or more first permanent magnets (3); a magnetic detector (4) arranged in the opening (2a) of the main exciter (2); an auxiliary exciter (5) connected to the main exciter (2) in a manner that sandwiches one or more first permanent magnets (3) of the main exciter (2), and having a second permanent magnet (6) that magnetizes the main exciter (2) in the same direction as the first permanent magnet (3); and an electromagnet coil (7) that generates magnetic flux by being supplied with a direct current, and can change the magnetization direction of the second permanent magnet (6) and reverse the magnetic pole of the second permanent magnet (6).

Description

Technical Field

[0001] The present invention relates to a magnetic flux leakage inspection device and a defect inspection method for detecting defects and the like by detecting leakage magnetic flux when magnetic flux passes through the interior of an inspection object. Background Art

[0002] In production lines for manufacturing soft magnetic thin steel sheets (strips), such as tinplate steel sheets for beverage cans and automotive steel sheets, a magnetic flux leakage inspection device that utilizes changes in leakage flux is known as a device for online and non-destructive inspection of defects on or within the steel sheets (e.g., Patent Document 1). In this technology, when a non-soft magnetic material is mixed into the steel sheet and presents as a defect, when magnetic flux passes through the steel sheet in a magnetically saturated state, the defect becomes an obstacle to the flow of magnetic flux, causing magnetic flux to leak from the steel sheet surface. Therefore, this leakage flux is detected and determined to be a defect. The steel sheet used as the inspection object is a thin steel sheet (strip) with a thickness of approximately 0.2 mm for beverage cans and approximately 0.8 mm for automotive steel sheets.

[0003] like Figure 9 As shown, this magnetic flux leakage inspection device comprises an exciter 101 that generates magnetic leakage flux to excite a conveyed steel plate 100, and multiple magnetic detectors 102 that detect the magnetic leakage flux. Multiple magnetic detectors 102 are arranged along the width of the plate. As shown, the exciter 101 is typically an electromagnet with a coil 104 wound around an iron core (yoke) 103. Coil-type elements, Hall elements, and the like are used as magnetic detectors 102 (for example, see Patent Document 2).

[0004] In the case of the electromagnet method, which uses an electromagnet as an exciter, a DC magnetic field is generated by passing a DC current through a coil. The strength of the magnet can be adjusted to a specified value by adjusting the number of coil turns and the current value. During testing, an external power supply continuously supplies current. However, the magnetic flux can be interrupted by shutting off the current flowing to the coil. The number of coil turns varies depending on the core wire diameter used, but is generally around 2500 to 3500 for a core wire diameter of 1.6 mm.

[0005] On the other hand, in recent years, a technique has been known to manufacture strong magnets and to impart the strong magnetic flux required for defect detection to the steel sheet using permanent magnets (e.g., Patent Document 3). The permanent magnet method has the advantage of being able to magnetize the inspection object without using an external power source.

[0006] In addition, in the case of the permanent magnet method, it is impossible to cut off the magnetic flux like an electromagnet, so there is a disadvantage that it is difficult to cut off the excitation force to the inspection object. However, as a technology that can eliminate this problem, a technology has been proposed that combines a permanent magnet with an electromagnet to make the magnetization performance of the magnetizer (exciter) variable (for example, Patent Document 4).

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Application Laid-Open No. 56-61645

[0010] Patent Document 2: Japanese Patent Application Laid-Open No. 2002-195984

[0011] Patent Document 3: Japanese Patent Application Laid-Open No. 2002-156363

[0012] Patent Document 4: Japanese Patent Application Laid-Open No. 2011-7570 Summary of the Invention

[0013] Summary of the Invention

[0014] Problems to be solved by the invention

[0015] Electromagnetic exciters require large coils to obtain sufficient leakage flux from defects in the steel plate being inspected, which in turn requires a long core, resulting in a heavy structure. On the other hand, for defect inspection, the distance between the tip of the yoke and the steel plate being inspected must be set to approximately 0.5 to 7 mm. However, in this case, the exciter must be retracted from the steel plate when the welded portion of the steel plate passes through the inspection device. However, if the inspection device is heavy, it takes a relatively long time to retract from the welded portion, etc., resulting in a longer uninspected area near the weld. Furthermore, a large amount of energy is required to function as a magnet, which increases heat generation.

[0016] Permanent magnet exciters are lighter than electromagnet exciters, do not require energy to function as magnets, and do not generate heat. However, since the magnetic flux cannot be cut off as mentioned above, it is difficult to clean the inspection equipment to remove iron powder, dust, etc. attached to the inspection equipment during inspection, or to cut off the excitation force to the inspection object during maintenance and inspection.

[0017] For example, while cleaning and maintenance can be performed by pulling the exciter away from the object to prevent it from applying an excitation force to it, this requires a large mechanical force, requiring extensive disassembly and assembly equipment. Furthermore, methods for shutting off the permanent magnet's own excitation force include widening the gap between the core (yoke) and the object, or short-circuiting the permanent magnet's north and south poles through the core. However, these methods require additional mechanisms and structures.

[0018] The technology of Patent Document 4 adjusts the excitation force by weakening the magnetization of the permanent magnet using an electromagnet. This allows for reduced excitation force on the object being inspected to facilitate cleaning, maintenance, etc. However, this method requires continuous flow of current to the electromagnet to maintain the reduced excitation force, resulting in excessive energy consumption and safety issues during operation.

[0019] Therefore, an object of the present invention is to provide a magnetic flux leakage inspection device and defect inspection method that can achieve miniaturization and weight reduction of the exciter, does not generate energy loss due to heat, can easily cut off the excitation, and can maintain this state without using excessive energy.

[0020] Solutions to Problems

[0021] In order to solve the above-mentioned problems, the present invention provides the following (1) to (6).

[0022] (1) A magnetic flux leakage inspection device for inspecting an object to be inspected by exciting the object to be inspected and detecting leakage magnetic flux leaking from the object to be inspected, characterized in that it comprises: a main exciter in a gate shape, having an opening at a portion opposite to the object to be inspected, and having one or more first permanent magnets; a magnetic detector arranged at the opening of the main exciter; an auxiliary exciter connected to the main exciter in a manner that clamps the one or more first permanent magnets of the main exciter, and having a second permanent magnet that magnetizes the main exciter in the same direction as the first permanent magnet; and an electromagnet coil that generates magnetic flux by being supplied with a direct current and is capable of changing the magnetization direction of the second permanent magnet and reversing the magnetic pole of the second permanent magnet.

[0023] (2) The magnetic flux leakage inspection device according to (1) above, wherein the main exciter and the auxiliary exciter have yokes for forming a magnetic circuit, and the first permanent magnet and the second permanent magnet are sandwiched between the yokes.

[0024] (3) The magnetic flux leakage inspection device according to (1) or (2) above, wherein the first permanent magnet is a neodymium magnet.

[0025] (4) The magnetic flux leakage inspection device according to any one of (1) to (3) above, wherein the second permanent magnet is an alnico magnet or a samarium cobalt magnet.

[0026] (5) The magnetic flux leakage inspection device according to any one of (1) to (4) above, characterized in that the cross-sectional area of the electromagnetic coil calculated based on the diameter and number of turns of the conductive wire constituting the electromagnetic coil is 200 to 2000 mm 2 .

[0027] (6) A defect inspection method for inspecting defects of a steel strip conveyed as the inspection object using the magnetic flux leakage inspection device described in any one of (1) to (5) above, characterized in that:

[0028] During defect inspection of the strip, the current flowing to the electromagnet coil is stopped, so that the magnetic flux from the first permanent magnet is supplied from the main exciter to the inspection object. When the defect inspection is stopped, a DC current is supplied to the electromagnet coil to generate a magnetic flux, and the magnetic pole of the second permanent magnet is reversed by the magnetic flux, and the auxiliary exciter is magnetized in the opposite direction to the main exciter, thereby closing the magnetic flux from the first permanent magnet to the auxiliary exciter.

[0029] (7) The defect inspection method according to the above-mentioned (6) is characterized in that, when the defect inspection is restarted after the defect inspection is stopped, a direct current in the opposite direction to that when the defect inspection was stopped is supplied to the electromagnet coil, so that the magnetic pole of the second permanent magnet is reversed again and returns to the state before the defect inspection was stopped, and the auxiliary exciter is magnetized in the same direction as the main exciter.

[0030] Effects of the Invention

[0031] According to the present invention, the use of permanent magnets in the main exciter eliminates the need for excitation coils or long iron cores, thereby miniaturizing and lightweighting the exciter. Furthermore, the excitation coils do not need to be supplied with large amounts of energy to function as magnets, and heat generation issues are avoided. Furthermore, since an electromagnetic coil is provided that can change the magnetization direction of the second permanent magnet, the magnetic poles of the second permanent magnet can be reversed by flowing current through the electromagnetic coils for a specified period of time, thereby easily stopping the supply of magnetic flux from the main exciter to the inspection object. Even if the current to the electromagnetic coils is then cut off, the main exciter's supply of magnetic flux to the inspection object remains stopped, eliminating the need for excessive energy. It should be noted that since the electromagnetic coils only reverse the magnetic poles of the second permanent magnets, they can be small and do not hinder miniaturization and lightweighting of the exciter. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a longitudinal sectional view showing a schematic structure of a magnetic flux leakage inspection device according to one embodiment of the present invention.

[0033] Figure 2 This is a transverse cross-sectional view showing a schematic configuration of a magnetic flux leakage inspection device according to one embodiment of the present invention.

[0034] Figure 3This is a perspective view showing a schematic configuration of a magnetic flux leakage inspection device according to one embodiment of the present invention.

[0035] Figure 4 This diagram shows the magnetic field formed by the main exciter and the auxiliary exciter during normal operation (excitation is on).

[0036] Figure 5 This diagram shows the magnetic field formed by the main exciter and the auxiliary exciter when the operation is stopped (excitation cutoff).

[0037] Figure 6 Graphs showing the magnetic flux density distribution and magnetic flux (magnetic lines of force) distribution in the excitation-on state in the embodiment.

[0038] Figure 7 Graphs showing the magnetic flux density distribution and magnetic flux (magnetic lines of force) distribution in the excitation cutoff state in the embodiment.

[0039] Figure 8 (a) means Figure 1 FIG. 1 is a diagram showing an embodiment of a magnetic flux leakage inspection device having a structure shown in FIG. Figure 8 (b) and Figure 8 (c) means Figure 8 (a) is a diagram of the second permanent magnet and the first permanent magnet in the magnetic flux leakage inspection device shown.

[0040] Figure 9 This is a cross-sectional view showing an example of a conventional magnetic flux leakage inspection device using an electromagnet. DETAILED DESCRIPTION

[0041] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0042] <Magnetic flux leakage inspection device>

[0043] Figure 1 1 is a longitudinal sectional view showing a schematic structure of a magnetic flux leakage inspection device according to one embodiment of the present invention. Figure 2 is its transverse cross-sectional view, Figure 3 It is its stereogram.

[0044] like Figure 1 As shown, a magnetic flux leakage inspection device 1 according to one embodiment of the present invention is configured to detect defects in a conveyed steel strip (thin steel plate) 10 serving as an inspection object. The device comprises a main exciter 2 having one or more first permanent magnets 3; a plurality of magnetic detectors 4; an auxiliary exciter 5 having a second permanent magnet 6; and an electromagnetic coil 7. The main exciter 2, auxiliary exciter 5, and electromagnetic coil 7 constitute an exciter.

[0045] [Main exciter]

[0046] The main exciter 2 is arranged so as to face the steel strip (thin steel plate) 10 being transported, which is the object to be inspected. It is gate-shaped (with a U-shaped cross section) and has an opening 2a in the portion facing the steel strip 10, which is the object to be inspected. Here, "facing" means that the tip of the leg 8a of the yoke 8 is arranged approximately perpendicularly to the steel strip 10, which is the object to be inspected. A plurality of magnetic detectors 4 are provided in the opening 2a. The main exciter 2 includes a first permanent magnet 3 and a yoke 8 for forming a magnetic circuit, and the yoke 8 is magnetized by the first permanent magnet 3. The yoke 8 includes a pair of legs 8a and a linear center portion 8b connecting the base ends of the pair of legs 8a. The opening 2a is formed between the tip ends of the pair of legs 8a, and the tip ends of the legs 8a are arranged close to the steel strip 10. The tip end of one leg 8a serves as the north pole, while the tip end of the other leg 8a serves as the south pole. The first permanent magnet 3 is sandwiched in the middle of the center portion 8b of the yoke 8. The first permanent magnet 3 and the yoke 8 form a magnetic circuit sandwiching the magnetic detector 4, so that the inspection portion of the steel strip 10 is brought into a magnetic flux saturation state or a state close to magnetic flux saturation.

[0047] Examples of the steel strip 10 to be inspected include soft magnetic materials such as tinplate steel sheets, TFS (tin-free steel sheets), galvanized steel sheets, and original galvanized steel sheets. The thickness thereof is preferably in the range of 0.1 to 3.2 mm.

[0048] As the first permanent magnet 3, a magnet having a magnetic force capable of causing the inspection portion of the steel strip 10 to be inspected to be in a magnetic flux saturation state is selected. As such a magnet, any permanent magnet such as a neodymium magnet, a samarium-cobalt magnet, a ferrite magnet, or an alnico magnet can be used. In particular, a neodymium magnet having a strong magnetic force is suitable. The first permanent magnet 3 may be one, but may also be a plurality of magnets. Figure 1 As shown, the first permanent magnet 3 is divided into two adjacent legs 8a, or it can be divided into three or more. In this case, the first permanent magnet 3 is preferably arranged in a bilaterally symmetrical position. By dividing the first permanent magnet 3 into two, the magnetic flux density can be made more uniform. The strength of the first permanent magnet 3 can be determined by its thickness ( Figure 1 The horizontal length of the central portion 8b in the permanent magnet is adjusted. For permanent magnets, "thickness" refers to the length in the direction of magnetic flux flow.

[0049] The yoke 8 only needs to be a soft magnetic material. In practical terms, it is preferable to use steel such as SS400 for economical considerations. The yoke 8 needs to be formed to a thickness that does not become a resistance in order to allow the necessary magnetic flux to pass through (the "thickness" of the yoke refers to the thickness of the yoke). Figure 1The thickness of the leg portion 8a and the central portion 8b of the yoke 8 in the cross section shown is measured in a direction perpendicular to the direction of magnetic flux flow. ) In this embodiment, it is preferably 10 to 20 mm. If it is thicker than necessary, the weight increases. In an exciter using an electromagnet, a large coil is required to excite the inspection object, and the yoke needs to be extended to wind the coil. However, in this embodiment, since the first permanent magnet 3 is used, the length of the leg portion 8a of the yoke 8 can be shorter than when an electromagnet is used. For example, the length of the leg portion 8a of the yoke 8 of the electromagnet method needs to be approximately 150 to 250 mm, while in this embodiment, it can be set to approximately 50 to 100 mm.

[0050] It should be noted that the length of the exciter in the depth direction ( Figure 1 The length of the exciter in the direction perpendicular to the paper (the length perpendicular to the paper) can be arbitrarily set according to the size of the inspection object. For example, when magnetizing the steel strip 10 in the conveying direction (L-direction magnetization), the depth length of the exciter is preferably longer than the width of the steel strip 10 being inspected. On the other hand, when magnetizing the steel strip 10 in the width direction (C-direction magnetization), the depth length of the exciter can be approximately 40 to 100 mm.

[0051] The spacing between the north and south poles at the front ends of the legs 8a of the yoke 8 (the width of the opening 2a) is affected by the structure of the magnetic detector 4 positioned therebetween. However, for L-direction magnetization, a spacing of 10 mm to 30 mm is generally preferred. However, for C-direction magnetization, the spacing can be set between 40 mm and 600 mm, preferably between 100 mm and 150 mm. Increasing the spacing requires a more powerful exciter.

[0052] [Magnetic detector]

[0053] The magnetic detector 4 is a structure for detecting the magnetic flux (magnetic lines of force) leaked from the steel strip 10 in a magnetic saturation state, and a coil element, a Hall element, etc. can be used. When the steel strip 10 is magnetized in the conveying direction (L direction magnetization), as shown in FIG. Figure 2 As shown, the main exciter 2 is set to be larger than the width of the steel strip 10 being inspected, and the magnetic detectors 4 are arranged along the width of the steel strip, for example, at intervals of 1 mm. The magnetic detectors 4 are arranged within the range of the width + α (+α is a margin to account for the meandering of the steel strip 10) along the width of the steel strip. The margin α is preferably set to approximately 5-20% of the width of the steel strip. For example, if the steel strip is 1000 mm wide and the magnetic detectors 4 are spaced 1 mm apart, 1000 magnetic detectors 4 + α are arranged along the width of the steel strip. In this case, the depth of the main exciter 2 also needs to be greater than the width of the steel strip, reaching a length of at least 1000 mm.

[0054] [Auxiliary exciter]

[0055] The auxiliary exciter 5 is gate-shaped (with a U-shaped cross section), and is arranged to overlap on the side of the main exciter 2 opposite to the strip 10, and is connected to the main exciter 2. The auxiliary exciter 5 can also be formed integrally with the main exciter 2. The auxiliary exciter 5 has a second permanent magnet 6 and a yoke 9 for forming a magnetic circuit, and the yoke 9 is magnetized by the second permanent magnet 6. The yoke 9 has a pair of legs 9a and a straight central portion 9b connecting the base ends of the pair of legs 9a, and the front ends of the pair of legs 9a are connected to the central portion 8b of the main exciter 2. At this time, the legs 9a of the yoke 9 are connected to the main exciter 2 in a manner that clamps the two first permanent magnets 3 of the main exciter 2. That is, the pair of legs 9a of the auxiliary exciter 5 are connected to the outside of the two first permanent magnets 3. The second permanent magnet 6 is sandwiched in the middle of the yoke 9. Figure 1 In the example shown, the second permanent magnet 6 is separately arranged on the pair of leg portions 9a, but it can also be arranged in the central portion 9b. When arranged in the central portion 9b, it can be a single magnet or divided into two. When the second permanent magnet 6 is divided into two, it is preferably arranged in bilaterally symmetrical positions.

[0056] As the second permanent magnet 6, a permanent magnet having a magnetic force roughly equivalent to that of the first permanent magnet 3 of the main exciter 2 is used. Any permanent magnet, such as a neodymium magnet, a samarium-cobalt magnet, a ferrite magnet, or an alnico magnet, can be used for the second permanent magnet 6. However, the second permanent magnet 6 is preferably a magnet having a smaller coercive force than the first permanent magnet 3 and easily reversing its magnetic poles. Alnico magnets or samarium-cobalt magnets are suitable as such magnets. The magnetic force of an alnico magnet is approximately one-third that of a neodymium magnet of the same size. Therefore, when using a neodymium magnet as the first permanent magnet 3 and an alnico magnet as the second permanent magnet 6, the alnico magnet is set to be three times the size of the neodymium magnet. However, alnico magnets have a relatively small coercive force, making their magnetic poles easily reversible. On the other hand, samarium-cobalt magnets have the advantage of excellent stability even under harsh inspection environments such as high-temperature atmospheres. Here, the second permanent magnet 6 is arranged with the same polarity as the first permanent magnet 3.

[0057] The yoke 9 can be made of a soft magnetic material, similar to the yoke 8. In practical terms, it is preferable to use steel such as SS400 for economical reasons. The yoke 9 is set to a thickness that does not cause resistance to the magnetic flux required for the auxiliary exciter 5. As will be described later, during normal operation, the magnetic flux is supplied from the leg 9a of the yoke 9 to the leg 8a of the yoke 8. Therefore, it is preferable to Figure 1 As shown, the thickness of the leg portion 8 a of the yoke 8 is thicker than the thickness of the leg portion 9 a of the yoke 9 .

[0058] [Solenoid coil]

[0059] The electromagnet coil 7 is configured to generate magnetic flux by supplying a DC current, thereby changing the magnetization direction of the second permanent magnet 6 and reversing the magnetic pole of the second permanent magnet. The electromagnet coil 7 can reverse the magnetic pole of the second permanent magnet 6 by adjusting the number of turns and the DC current supplied, thereby reversing the magnetic pole of the second permanent magnet 6 and reversing the magnetization. Figure 1 and Figure 3 As shown, the electromagnetic coil 7 is preferably arranged in a manner covering the second permanent magnet 6 of the auxiliary exciter 5. The electromagnetic coil 7 is a structure that only reverses the magnetic pole of the second permanent magnet, so it does not need a large structure like the electromagnetic coil used in the electromagnet type exciter (for example, a structure consisting of about 2500 to 3500 turns when the core wire diameter of the electromagnetic coil conductor is 1.6 mm relative to the length of the yoke leg of 200 mm). The number of turns of the electromagnetic coil 7 can be, for example, about 500 to 2000 relative to the core wire diameter of the same conductor, or it can be set to about 500 to 1000. It should be noted that when divided into multiple electromagnetic coils, the number of turns of the electromagnetic coil 7 represents the total number of turns.

[0060] At this time, the cross-sectional area S (mm) of the electromagnet coil is calculated based on the core wire diameter D (mm) and the number of turns N of the conductive wire constituting the electromagnet coil. 2 ) is preferably 200 to 2000 mm 2 However, the cross-sectional area S is expressed by the following formula.

[0061] S=π·(D / 2) 2 ·N

[0062] Cross-sectional area S is less than 200mm 2 If the excitation current is about 2A, it is sometimes difficult to reverse the magnetic pole of the second permanent magnet. On the other hand, when the cross-sectional area S exceeds 2000mm 2 When the cross-sectional area S is within the above range, the weight of the electromagnet constituting the auxiliary exciter increases, and the overall weight of the magnetic flux leakage inspection device 1 increases. Therefore, by keeping the cross-sectional area S within the above range, the function of the auxiliary exciter in this embodiment can be achieved while reducing the weight of the device. It should be noted that if the core diameter of the conductive wire constituting the electromagnet coil is small, the overall weight will not increase even if the number of turns of the coil is increased.

[0063] [Arrangement relationship between the main excitation unit and the auxiliary excitation unit]

[0064] like Figure 3As shown, in the case of L-direction magnetization, the pair of leg portions 8a of the yoke 8 of the main exciter 2 are integrally formed and extend along the width direction of the steel strip 10. However, the central portion 8b of the yoke 8 of the main exciter 2 and the auxiliary exciter 5 can be divided into multiple pieces spaced apart along the width direction of the steel strip 10. This facilitates assembly of the wiring and the electromagnetic coil 7. In this case, the thickness of the first permanent magnet 3 and the second permanent magnet 6 is determined to compensate for the magnetic force of the central portion 8b of the yoke 8 and the portion where the auxiliary exciter 5 is not present.

[0065] [Magnetic flux density, etc.]

[0066] The inspection portion of the steel strip 10 is primarily excited by the DC magnetic field of the first permanent magnet 3 of the main exciter 2, achieving magnetic flux saturation. However, during operation, the direction of the magnetic field of the auxiliary exciter 5 is aligned with that of the main exciter 2, thus also contributing to the excitation of the steel strip 10. To achieve magnetic flux saturation or near-saturation in the inspection portion of the steel strip 10, the DC magnetic fields of the first and second permanent magnets 3 and 6 create a magnetic flux density within the steel strip 10, the inspection target, equivalent to or greater than 1.7 Tesla. While increasing the distance from the steel strip 10 decreases the magnetic flux within the steel strip 10, even in this case, the first permanent magnet 3 is selected to ensure a magnetic flux of at least 1.7 Tesla within the strip. The distance between the tip of the leg 8a and the steel strip 10 varies depending on the device configuration, but is typically set within a range of 0.1 to 7 mm. A value of 0.5 to 3 mm is preferred.

[0067] <Inspection method using magnetic flux leakage inspection device>

[0068] Next, an inspection method using the magnetic flux leakage inspection device configured as described above will be described.

[0069] [Normal operation (excitation on)]

[0070] In normal operation, when the current supply to the electromagnet coil 7 is stopped, Figure 1 In this way, the magnetic flux leakage inspection device 1 is operated with the first permanent magnet 3 of the main exciter 2 and the second permanent magnet 6 of the auxiliary exciter 5 being set to the same polarity. Figure 4 As shown, a magnetic field is formed within the yoke 8 of the main exciter 2 such that magnetic flux flows from the tip end on the south pole side toward the tip end on the north pole side. A magnetic field is also formed within the yoke 9 of the auxiliary exciter 5 such that magnetic flux in the same direction is generated. Therefore, the magnetic flux emitted from the tip end of the main exciter 2 into the air excites the inspection portion of the steel strip 10 being inspected, and the magnetic flux density in that portion can be brought to a state of magnetic flux saturation or close to magnetic flux saturation, exceeding 1.7 Tesla.

[0071] [Operation stop (excitation cut-off)]

[0072] When the inspection device is stopped for cleaning, maintenance, or other purposes and the excitation to the steel strip 10 being inspected is cut off, a direct current in the direction of magnetic pole reversal of the second permanent magnet 6 flows to the electromagnet coil 7. The current is increased until the magnetic flux generated by the electromagnet coil 7 reaches a value that allows magnetic pole reversal. This current is then supplied for a period of time until the magnetic pole reversal occurs, thereby reversing the magnetic pole of the second permanent magnet 6. By reversing the magnetic pole in this manner, the second permanent magnet 6 can be magnetized in the reversal direction (reverse magnetization). At this time, a current that makes the magnetic force of the second permanent magnet 6 equal to the magnetic force of the electromagnet coil 7 is set as a reference current. Preferably, a current (direct current) that is 2 to 3 times the reference current is allowed to flow for, for example, 1 to 30 seconds to stabilize. More preferably, it is allowed to flow for 1 to 10 seconds. The magnetic pole reversal can be sufficient even in a relatively short period of time, thereby suppressing the heat generated by the coil.

[0073] By reversing the magnetic pole of the second permanent magnet 6, Figure 5 As shown, the auxiliary exciter 5 is magnetized in the opposite direction to that during normal operation, forming a magnetic field such that the yoke 9 of the auxiliary exciter 5 absorbs the magnetic flux from the first permanent magnet 3, thereby confining the magnetic flux from the first permanent magnet 3 to the auxiliary exciter 5. Specifically, the magnetic flux from the first permanent magnet 3 is supplied to the auxiliary exciter 5, while almost no magnetic flux is supplied to the leg portion 8a of the yoke 8 of the main exciter 2. Consequently, almost no magnetic flux is emitted from the tip of the leg portion 8a of the main exciter 2 into the air or the steel strip 10 being inspected, resulting in an excitation-off state where only weak magnetic flux is emitted.

[0074] To return to normal operation (excitation-on) from the excitation-off state, a reverse current is supplied to the electromagnet coil 7 at the same current as that used to return to the excitation-off state, thereby returning the magnetic pole orientation of the second permanent magnet 6 to its original state. At this time, current is supplied to the electromagnet coil 7 for, for example, 1 to 30 seconds, as in the excitation-off state.

[0075] <Defect Inspection Method>

[0076] The defect inspection method of this embodiment is a defect inspection method for inspecting defects of a steel strip 10 being transported as an inspection object using the above-mentioned magnetic flux leakage inspection device 1. When inspecting defects of the steel strip 10, the current flowing to the electromagnet coil 7 is stopped, and the magnetic flux from the first permanent magnet 3 is supplied from the main exciter 2 to the steel strip 10 as the inspection object. When the defect inspection is stopped, a direct current is supplied to the electromagnet coil 7 to generate a magnetic flux, and the magnetic pole of the second permanent magnet 6 is reversed by the magnetic flux, and the auxiliary exciter 5 is magnetized in the opposite direction of the main exciter 2, so that the magnetic flux from the first permanent magnet 3 is limited to the auxiliary exciter 5.

[0077] Furthermore, when defect inspection is restarted after having been stopped, a direct current in the opposite direction to that when the defect inspection was stopped is supplied to the electromagnet coil 7, so that the magnetic poles of the second permanent magnet 6 are reversed again and return to the state at the time of defect inspection, and the auxiliary exciter 5 is magnetized in the same direction as the main exciter 2.

[0078] <Effects of Implementation>

[0079] As described above, in the magnetic flux leakage inspection device 1 of this embodiment, the use of the first permanent magnet 3 in the main exciter 2 eliminates the need for an excitation coil or a long iron core. Therefore, compared to conventional electromagnet methods, even taking into account the use of an auxiliary exciter, the exciter as a whole can be made smaller and lighter. For example, the length of the yoke 8 of the main exciter 2 of this embodiment can be shortened compared to conventional electromagnet methods, and the total weight of the wire of the coil used in the auxiliary exciter 5 can be reduced to approximately half. This reduces the weight of the magnetic flux leakage inspection device by more than 50%. Furthermore, there is no need to continuously supply high amounts of energy to the excitation coil, as is the case with electromagnet methods, and there is no problem of heat generation.

[0080] Since the exciter can be miniaturized, the installation of the device is simple. Moreover, since the exciter can be lightweight as a whole, even in the case of a protruding step such as the welded portion of the strip steel 10 to be inspected, the retreat and recovery of the welded portion when passing through the exciter can be carried out quickly. As a result, the uninspected time can be shortened, and the uninspected area before and after the welded portion can be narrowed. Moreover, the problem of heating caused by the coil does not arise, thereby significantly reducing the temperature imbalance of the magnetic detector (sensitivity change, zero-point drift, detector degradation) caused by the heating of the coil in the conventional electromagnet-type exciter. In addition, a large excitation coil is not required, and therefore a large DC power supply for supplying DC current is not required.

[0081] Furthermore, since the electromagnetic coil 7 is provided to change the magnetization direction of the second permanent magnet 6, the magnetic pole of the second permanent magnet 6 can be reversed by flowing current through the electromagnetic coil 7 for a predetermined time, thereby achieving reverse magnetization. This makes it easy to disconnect the excitation of the main exciter 2. Even after the current is disconnected, the excitation of the main exciter 2 remains disconnected, eliminating the need for excessive energy.

[0082] Furthermore, since the excitation of the main exciter 2 remains cut off, the inspection device body can be easily moved to the maintenance position. Even if iron powder or the like adheres to the inspection device, it can be easily cleaned, resulting in good maintainability and excellent durability.

[0083] Furthermore, since the excitation of the main exciter 2 can be turned on and off by flowing current through the electromagnetic coil 7 , remote operation can be performed via the electric circuit.

[0084] <Other Applications>

[0085] Although the embodiments of the present invention have been described above, these are merely examples and are not restrictive, and various omissions, substitutions, and changes may be made without departing from the spirit of the present invention.

[0086] For example, the inspection object is not limited to steel strips, and any thin soft magnetic material can be applied. In addition, the structure of the main exciter and auxiliary exciter is not limited to Figures 1 to 3 The structure shown.

[0087] Example 1

[0088] Here, a neodymium magnet is used as the first permanent magnet, an alnico magnet is used as the second permanent magnet, and a steel material (SS400) is used as the yoke to form the main exciter and the auxiliary exciter, and an electromagnetic coil and a magnetic detector are assembled to manufacture a Figure 1 The magnetic flux leakage inspection device shown.

[0089] <Excitation-on state>

[0090] First, the current to the electromagnet coil is stopped, and the magnetic pole of the second permanent magnet (alnico magnet) of the auxiliary exciter is set to Figure 4 The excitation is turned on in the direction shown. The magnetic flux density distribution and magnetic flux (magnetic lines of force) distribution at this time are as follows Figure 6 It should be noted that although the magnetic flux density is highest in the inspection portion of the inspection object, this portion is not shown in the figure because it is very small.

[0091] Figure 6 This is a figure that expresses the color illustration in black and white, but even considering this point, it can be seen that the magnetic flux exists more in the main exciter, and the magnetic flux (magnetic lines of force) leaks to the outside from the yoke of the main exciter, and most of the magnetic flux (magnetic lines of force) leaking to the outside is toward the inspection object.

[0092] <Excitation cut-off state>

[0093] Next, the magnetic flux leakage inspection device is set to a state of excitation cutoff. A direct current is made to flow through the electromagnet coil, and the magnetic pole of the second permanent magnet is reversed by the magnetic flux generated from the electromagnet coil, thereby generating a state of excitation cutoff. Specifically, a direct current in a direction that reverses the magnetic pole of the second permanent magnet is made to flow through the electromagnet coil, and the current is increased. The current that makes the magnetic force of the second permanent magnet and the magnetic force of the electromagnet coil the same is set as the reference current, and a current three times the reference current is made to flow for 9 seconds to reverse the magnetic pole of the second permanent magnet, thereby performing reverse magnetization. The magnetic flux density distribution and the magnetic flux (magnetic lines of force) distribution at this time are as shown below. Figure 7 shown.

[0094] Figure 7 also with Figure 6 Although the color drawings are rendered in black and white, even considering this point, it is clear that the polarity reversal of the second permanent magnet causes nearly all of the main exciter's magnetic flux to be attracted by the auxiliary exciter, suppressing the flow of magnetic flux toward the inspection object. Furthermore, it was confirmed that the magnetic flux density of the auxiliary exciter's yoke was equivalent to 1.2 T, indicating that the auxiliary exciter's yoke attracted nearly all of the main exciter's magnetic flux. Furthermore, it was confirmed that the auxiliary exciter (second permanent magnet) was reversely magnetized.

[0095] Example 2

[0096] Instructions used Figure 1 The second embodiment of the magnetic flux leakage inspection device of the structure shown in FIG. Figure 8 As shown in (a), the distance L between the openings of the yoke legs of the main exciter is 30 mm, and the yoke width is ( Figure 1 The main exciter 2 has a permanent magnet 3 which is a neodymium magnet. Figure 8 As shown in (c), two magnets, each 8 mm thick, 12 mm wide, and 40 mm deep, are sandwiched midway between the center portion 8b of the yoke 8. In this configuration, the width (depth) of the center portion 8b of the yoke of the main exciter 2 is narrower than the width (yoke width) of the leg portions 8a, with the center portion 8b positioned approximately in the center of the yoke width. However, the strong magnetic force of the neodymium magnets used in the main exciter 2 ensures that sufficient magnetic flux is supplied to the entire yoke width at the yoke's tip.

[0097] The height of the yoke 8 of the main exciter is set to 60 mm. Since there is no need to provide an electromagnet in the main exciter, the length of the yoke leg portion can be shortened and the thickness can also be reduced.

[0098] On the other hand, the auxiliary exciter 5 is arranged to overlap the main exciter 2 on the side opposite to the strip 10 and is connected to the main exciter 2. In the auxiliary exciter 5, the second permanent magnet 6 is sandwiched between the leg 9a of the yoke 9 of the auxiliary exciter 5. The second permanent magnet 6 is an alnico magnet, such as Figure 8 As shown in (b), two magnets with a thickness of 24 mm, a width of 12 mm, and a depth of 40 mm are sandwiched between the legs 9a of the yoke 9. Here, the depth of the auxiliary exciter 5 is set to the same dimension as the depth of the center portion 8b of the yoke 8 of the main exciter 2. In this case, the height of the auxiliary exciter's yoke 9 is 82 mm. Although the auxiliary exciter is equipped with an electromagnet coil, the number of turns of the wire can be relatively small, so the height of the yoke 9 can also be reduced.

[0099] The auxiliary exciter 5 is configured to change the magnetization direction of the second permanent magnet 6, thereby reversing the magnetic pole of the second permanent magnet. The electromagnetic coil 7 is arranged to cover the second permanent magnet 6. In this embodiment, the electromagnetic coil 7 is formed by providing a coil with 800 turns for each magnet, with a wire having a diameter of 1.6 mm covering the second permanent magnet. The electromagnetic coil 7 includes a power supply unit connected to a DC power supply and capable of controlling the current to a predetermined value.

[0100] Magnetic detectors are arranged at the opening of the yoke 8 of the main exciter 2. The magnetic detectors are Hall elements, and 80 of them are arranged at intervals of 1 mm along the depth direction of the leg portion 8a of the yoke 8.

[0101] Using the magnetic flux leakage inspection device constructed as described above, normal operation (excitation on) was carried out. As a result, a uniform magnetic flux saturation state (1.7 T or more) was achieved in the center of the opening of the main exciter 2 and in the depth direction of the leg 8a of the yoke 8. Meanwhile, in the above-mentioned device, the reference current for achieving the same magnetic force between the second permanent magnet 6 and the electromagnetic coil 7 is 3 A. Therefore, a current of 9 A, three times the reference current, was applied to the electromagnetic coil 7. As a result, after the current was applied for 9 seconds and then stopped, the magnetic flux density at the opening of the leg 8a of the yoke 8 reached 0.1 T. The short-term application of current caused a reversal of the magnetic poles, enabling a stopped state (excitation off). Furthermore, at this time, the temperature rise of the exciter was negligible, less than 1°C.

[0102] It should be noted that after the leakage magnetic inspection device of this embodiment is temporarily stopped (excitation cut off), a current of 9A is applied to the electromagnet coil 7 for 9 seconds in the opposite direction to when the operation was stopped, thereby returning the direction of the magnetic pole of the second permanent magnet 6 to its original state, and enabling it to enter a normal operation state (excitation turned on).

[0103] Description of labels

[0104] 1 Magnetic flux leakage inspection device

[0105] 2 Main exciter

[0106] 3. First permanent magnet

[0107] 4 Magnetic detector

[0108] 5 Auxiliary exciter

[0109] 6 Second permanent magnet

[0110] 7 Electromagnetic coil

[0111] 8, 9 Magnetic yoke

[0112] 8a, 9a Legs

[0113] 8b, 9b central part

[0114] 10 Strip steel (inspection object).

Claims

1. A magnetic flux leakage inspection device for inspecting an object by exciting the object to be inspected and detecting magnetic flux leakage from the object to be inspected, characterized in that: have: a gate-shaped main exciter having an opening at a portion facing the inspection object and having one or more first permanent magnets; a magnetic detector disposed at the opening of the main exciter; an auxiliary exciter connected to the main exciter so as to sandwich the one or more first permanent magnets of the main exciter, and having a second permanent magnet that magnetizes the main exciter in the same direction as the first permanent magnet; and The electromagnet coil generates magnetic flux by being supplied with a direct current, thereby changing the magnetization direction of the second permanent magnet and reversing the magnetic pole of the second permanent magnet.

2. The magnetic flux leakage inspection device according to claim 1, characterized in that: The main exciter and the auxiliary exciter include yokes for forming a magnetic circuit, and the first permanent magnet and the second permanent magnet are interposed between the yokes.

3. The magnetic flux leakage inspection device according to claim 1 or 2, characterized in that: The first permanent magnet is a neodymium magnet.

4. The magnetic flux leakage inspection device according to claim 1 or 2, characterized in that: The second permanent magnet is an Alnico magnet or a SmCo magnet.

5. The magnetic flux leakage inspection device according to claim 1 or 2, characterized in that: The cross-sectional area of the electromagnet coil calculated based on the diameter and number of turns of the wire constituting the electromagnet coil is 200 to 2000 mm 2 .

6. A defect inspection method, comprising inspecting defects of a steel strip being conveyed as the inspection object using the magnetic flux leakage inspection device according to any one of claims 1 to 5, wherein: When inspecting defects of the steel strip, the current flowing to the electromagnet coil is stopped, and the magnetic flux from the first permanent magnet is supplied from the main exciter to the inspection object. When the defect inspection stops, a direct current is supplied to the electromagnet coil to generate a magnetic flux, through which the magnetic pole of the second permanent magnet is reversed, and the auxiliary exciter is magnetized in a direction opposite to the main exciter, thereby closing the magnetic flux from the first permanent magnet to the auxiliary exciter.

7. The defect inspection method according to claim 6, characterized in that: When the defect inspection is restarted after being stopped, a direct current in the opposite direction to that when the defect inspection was stopped is supplied to the electromagnet coil, so that the magnetic pole of the second permanent magnet is reversed again and returns to the state before the defect inspection was stopped, and the auxiliary exciter is magnetized in the same direction as the main exciter.

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