Leakage magnetic field inspection device, method for adjusting the leakage magnetic field inspection device, and defect inspection method

Abstract

To provide a leakage magnetism inspection apparatus which can accurately detect a defect in the direction orthogonal to the direction of a magnetic flux of an exciter.SOLUTION: A leakage magnetism inspection apparatus comprises: an exciter in which mutually-different magnetic poles are formed on one end side, and which has a pair of leg parts arrayed along the direction orthogonal to the one direction; a plurality of detectors which are arrayed from the one of the pair of leg parts to the other on the one end side of the pair of leg parts and detect the magnetic flux; and a defect detection unit which detects a defect in the inspection object on the basis of the detection signal of each of the plurality of detectors. The defect detection unit comprises: a bias part which applies a bias signal according to a magnetic environment between the pair of leg parts to each detection signal of the plurality of detectors; an amplifier part which performs amplification processing to the signal for each detector passing through the bias part; and a defect determination part which performs defect determination on the basis of the signal applied with the amplification processing.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a leakage magnetic field inspection device for detecting defects in, for example, thin steel plates, a method for adjusting the leakage magnetic field inspection device, and a defect inspection method. [Background technology]

[0002] Non-destructive detection of defects on the surface or inside of thin steel sheets is carried out on the production line (hereinafter also referred to as online) where soft magnetic thin steel sheets such as tinplate steel sheets for beverage cans and steel sheets for automobiles are manufactured.

[0003] One such inspection device known is a leakage magnetic field inspection device that detects magnetic flux resulting from defects in the thin steel plate that is the object of inspection. The leakage magnetic field inspection device includes an exciter (magnetizer) that saturates the magnetic flux in the inspection area of ​​the thin steel plate, and a detector that detects the magnetic flux leaking from the magnetized thin steel plate. In other words, the leakage magnetic field inspection device detects defects in the thin steel plate by detecting the magnetic flux resulting from defects that have leaked from the thin steel plate.

[0004] In C-direction magnetization, in which a thin steel plate is magnetized in its width direction to perform defect inspection, multiple detectors are arranged between the magnetic poles of an exciter. When detectors are arranged in this manner, it is preferable that the magnetic flux density between the magnetic poles has little variation, from the perspective of detection accuracy. Therefore, conventionally, efforts have been made to reduce the variation in magnetic flux between the magnetic poles of an exciter (see Patent Documents 1 to 3).

[0005] Incidentally, Patent Document 4 discloses a leakage magnetic field inspection device that detects various defects including defects extending in the longitudinal direction of thin steel plates. The defect discrimination unit described in Patent Document 4 employs L-direction magnetization, which magnetizes the thin steel plate in its longitudinal direction to inspect for defects. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 8-15227 [Patent Document 2] Japanese Patent Application Publication No. 7-22240 [Patent Document 3] Patent No. 6908213 [Patent Document 4] Japanese Patent Application Laid-Open No. 2014-106136 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even if the techniques described in Patent Documents 1 to 3 are applied to reduce the variation in magnetic flux between the magnetic poles of the exciter, it is difficult to completely uniformize the magnetic flux density.

[0008] Therefore, when C-direction magnetization is used, there is a problem that the sensitivity to defects varies among multiple detectors. In particular, defects extending in the longitudinal direction of thin steel plates tend to be detected as extremely low-frequency signals. If there is a difference in sensitivity to defects among each detector, there is a problem that the detection sensitivity to such low-frequency defect signals decreases.

[0009] In the leakage magnetic field inspection, the signal from the detector is amplified by an amplifier, and then signal processing is performed in which a predetermined band is cut off by a band-pass filter.

[0010] When the defect discrimination unit described in Patent Document 4 is used for defect inspection using C-direction magnetization, a DC signal is superimposed on the signal detected by the detector due to the influence of the magnetic gradient generated between the magnetic poles. Therefore, if the detection signal is uniformly amplified by an amplifier, saturation may occur depending on the detector. Therefore, the amplification of the detection signal by the amplifier is limited to a level that does not cause saturation, taking into account the detector that is most affected by the magnetic gradient.

[0011] As described above, defects extending in the longitudinal direction of a steel sheet tend to be detected as extremely low-frequency signals. Therefore, if the amplification of the detection signal by the amplifier is limited, it becomes difficult to separate the low-frequency noise contained in the detection signal from the defect signal, which results in a problem that defects extending in the longitudinal direction of the steel sheet cannot be properly detected.

[0012] The present invention has been made in consideration of the above problems, and aims to provide a leakage magnetic field inspection device, a method for adjusting a leakage magnetic field inspection device, and a defect inspection method that can accurately detect defects in an object to be inspected. [Means for solving the problem]

[0013] In order to solve the above problems, the present invention has the following features.

[0014] [1] a conveying unit that conveys a plate-shaped inspection object in one direction; an exciter having a pair of legs each having a different magnetic pole formed on one end thereof and arranged along a direction perpendicular to the one direction; a plurality of detectors arranged on one end side of the pair of legs from one to the other of the pair of legs and configured to detect magnetic flux; a defect detection unit that detects defects in the inspection object based on detection signals from each of the plurality of detectors, The defect detection unit a bias unit that applies a bias signal corresponding to the magnetic environment between the pair of legs to each of the detection signals of the plurality of detectors; an amplifier unit that performs an amplification process on the signal for each of the detectors that has passed through the bias unit; a defect determination unit that determines whether there is a defect based on the signal amplified by the amplifier unit. [2] a bias signal setting unit that sets the bias signal output by the bias unit, The bias signal setting unit includes a setting information input unit that accepts input of setting values ​​for each of the bias signals, and sets the bias signals based on the setting values ​​accepted by the setting information input unit. [1] A leakage magnetic inspection device as described in. [3] a bias signal setting unit that sets the bias signal output by the bias unit, the bias signal setting unit includes a signal input unit that receives an input of an amplified signal corresponding to the output of each of the detectors amplified by the amplifier unit; The leakage magnetic field inspection device according to [1], wherein the bias signal is set in accordance with the intensity of the amplified signal input to the signal input unit. [4] The bias signal setting unit acquires a signal from each detector when the inspection object having no defects is magnetized, and sets the bias signal based on the signal. [2] or [3]. [5] The leakage magnetic field inspection device according to any one of [1] to [3], wherein the bias unit adds a bias signal corresponding to the magnetic gradient formed between one of the pair of legs to each of the detection signals of the plurality of detectors. [6] The leakage magnetic inspection device described in [4], wherein the bias unit adds a bias signal corresponding to the magnetic gradient formed between one of the pair of legs to each of the detection signals of the plurality of detectors. [7] A method for adjusting a leakage magnetic field inspection device, comprising: magnetizing an object to be inspected that does not have a defect using the leakage magnetic field inspection device according to [2] or [3], and setting the bias signal. [8] A defect inspection method that uses the method for adjusting a leakage magnetic field inspection device described in [7] to set a setting value for the bias signal, and maintains the set setting value of the bias signal to inspect defects in the inspection object. [Effects of the Invention]

[0015] According to the present invention, the bias section can mitigate the influence of the magnetic environment formed between the magnetic poles of the exciter. Therefore, the amount of amplification of the detection signal by the amplifier can be made uniform, making it possible to accurately detect, for example, low-frequency signals. As a result, it becomes possible to accurately detect defects formed in the longitudinal direction of thin steel plates, for example. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a diagram showing the configuration of a leakage magnetic field inspection device. [Figure 2] 1 is an explanatory diagram showing an aspect of defect inspection of an inspection object using a leakage magnetic field inspection device. FIG. [Figure 3] FIG. 2 is a block diagram showing the configuration of a defect detection unit in FIG. 1. [Figure 4] FIG. 10 is an explanatory diagram showing a manner in which each bias signal of the bias device is set. [Figure 5] 10 is a graph showing magnetic flux density at a position in the width direction between magnetic poles of an excitation yoke. [Figure 6] 10 is an explanatory diagram showing a mode in which a bias signal is added to a detection signal of a detector by a bias unit. FIG. [Figure 7] FIG. 10 is a block diagram showing the configuration of a defect detection unit according to a second embodiment. [Figure 8] 10 is a graph showing magnetic flux density at a position in the width direction between magnetic poles of an excitation yoke. [Figure 9] FIG. 11 is a block diagram showing the configuration of a defect detection unit according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0017] (First embodiment) Fig. 1 shows the configuration of a leakage magnetic field inspection device according to the present invention. As shown in Fig. 1, the leakage magnetic field inspection device 100 includes a conveying unit 10 that conveys a thin steel plate (hereinafter also referred to as a strip) SP, which is a plate-shaped object to be inspected, in one direction (a direction perpendicular to the plane of Fig. 1), an exciter 20 that magnetizes the strip SP, a plurality of detectors 31-36 that detect magnetic flux leaking from the strip SP, a defect detection unit 40 that detects defects in the strip SP based on the detection signals of each of the plurality of detectors 31-36, and a display unit 50 that displays the detection results by the defect detection unit 40.

[0018] Examples of the strip SP include soft magnetic materials such as tinplate, TFS (tin-free steel), galvanized steel, and raw sheets of galvanized steel. The thickness of the strip SP is preferably 0.1 to 3.2 mm, and more preferably 0.1 to 2.0 mm. For example, when the strip SP is for beverage cans, it should be about 0.2 mm, and when it is for automotive steel sheets, it should be about 0.8 mm.

[0019] FIG. 2 shows an embodiment of defect inspection of strip SP. As also shown in FIG. 2, conveying section 10 includes multiple non-magnetic rolls 11 made of a non-magnetic material. In this embodiment, two non-magnetic rolls 11 are arranged along the direction of the arrow, which is one direction (flow direction). Therefore, conveying section 10 can convey strip SP in one direction (flow direction). Note that another non-magnetic roll (not shown) may be arranged between the two non-magnetic rolls 11 of conveying section 10 so that strip SP is wound around it, and exciter 20 may be arranged on the opposite side of the contact surface between strip SP and the other non-magnetic roll.

[0020] As shown in FIG. 1, the exciter 20 has an excitation yoke 21 disposed so as to face the strip SP, and an excitation coil 22 as a magnetic field generating means.

[0021] As shown in Fig. 2, the excitation yoke 21 has a pair of legs 21a formed in a columnar shape. The pair of legs 21a have different magnetic poles (N pole, S pole) formed on one end and are arranged along a direction perpendicular to one direction. The excitation yoke 21 has a beam-shaped connecting portion 23 connecting the other end sides of the pair of legs 21a. The material of the excitation yoke 21 may be any soft magnetic material. As a specific material for the excitation yoke 21, for example, a steel material such as a general structural rolled steel material (e.g., SS400) can be used in practical terms, taking into consideration economic efficiency.

[0022] The excitation yoke 21 is formed in a gate shape (U-shape) with an open bottom side (one end side) and a closed top side (the other end side) when viewed from the conveying direction (one direction) of the conveying section 10. Therefore, the excitation yoke 21 has an opening OP provided between one end sides of the pair of legs 21a.

[0023] One end of each of the pair of legs 21a is provided close to the surface of the strip SP. That is, the pair of legs 21a are provided on the surface of the strip SP so as to face the surface. In other words, the exciter 20 is provided at a position facing the strip SP.

[0024] Here, facing the exciter 20 and the strip SP means that one end of each leg 21a of the excitation yoke 21 is positioned facing the strip SP, and the angle that each leg 21a makes with respect to the surface of the strip SP is approximately perpendicular.

[0025] In this embodiment, the pair of legs 21a are arranged such that their axial direction is perpendicular to the surface of the strip SP. Each of the pair of legs 21a is provided with a gap between it and the surface of the strip SP.

[0026] The pair of legs 21a are preferably arranged so that the distance between one leg 21a and the surface of the strip SP is substantially the same as the distance between the other leg 21a and the surface of the strip SP. The distance from one end of each leg 21a to the surface of the strip SP is preferably about 0.5 to 7.0 mm.

[0027] The excitation coil 22 is wound around the connection portion 23 of the excitation yoke 21. When a direct current is supplied to the excitation coil 22, the excitation coil 22 generates a direct current magnetic field and magnetizes the excitation yoke 21. In other words, when a direct current is supplied to the excitation coil 22, the excitation coil 22 and the excitation yoke 21 become electromagnets.

[0028] The excitation coil 22 becomes a powerful electromagnet by increasing at least one of the number of turns and the current supplied thereto. The excitation coil 22 supplies magnetic flux to the inspection area of ​​the strip SP via the excitation yoke 21, magnetizing the strip SP.

[0029] The number of turns of the excitation coil 22 for each leg 21a is preferably about 400 to 2,000. The current supplied to the excitation coil 22 is preferably about 1 to 9 A. When the excitation yoke 21 is magnetized, different magnetic poles are formed at one end of the pair of legs 21a. Specifically, the tip of one leg 21a becomes an N pole, and the tip of the other leg 21a becomes an S pole.

[0030] The exciter 20 uses the DC magnetic field to supply a strong magnetic flux to the inspection area of ​​the strip SP, magnetizing the strip SP. Specifically, when the magnetic flux is supplied to the inspection area of ​​the strip SP, the magnetic flux density becomes approximately 1.7 T or more, and the inspection area of ​​the strip SP is in a magnetic flux saturated state or a state close to magnetic flux saturation.

[0031] The length of the exciter 20 in the depth direction (the length perpendicular to the paper surface in FIG. 1, the length along the flow direction in FIG. 2) can be set arbitrarily depending on the size of the strip SP. For example, the length of the exciter 20 in the depth direction is preferably about 40 to 100 mm.

[0032] If the depth of the exciter 20 is less than 40 mm, the magnetic field formed in the opening OP (between the pair of legs 21 a) tends to be non-uniform, which may affect the defect detection accuracy. Also, if the depth of the exciter 20 exceeds 100 mm, more excitation energy than necessary is required to detect defects, which tends to reduce energy efficiency.

[0033] 1, in this embodiment, the strip SP is excited (C-direction excitation) in a cross direction (C-direction, width direction of the strip SP) perpendicular to the transport direction (one direction) of the strip SP by the exciter 20. Therefore, when detecting long defects occurring in the transport direction (one direction) of the strip SP, excitation in the cross direction is effective.

[0034] In the case of C-direction excitation, the number of exciters 20 arranged in the width direction of the strip SP can be reduced by widening the opening OP of the excitation yoke 21 (increasing the distance between the pair of legs 21a), thereby reducing the equipment costs.

[0035] From this viewpoint, it is preferable that the interval L of the openings OP (the distance between the pair of legs 21a) is 40 to 600 mm. If the interval L of the openings OP is less than 40 mm, it is not possible to reduce the number of exciters 20 arranged in the width direction of the strip SP, and there is a risk that the equipment cost cannot be sufficiently reduced.

[0036] Increasing the spacing L between the openings OP is desirable because it widens the widthwise range that can be detected by a single exciter 20. However, the longer the spacing L between the openings OP, the weaker the magnetic flux supplied to the strip SP. Therefore, if the spacing L exceeds 600 mm, there is a risk that the defect detection sensitivity will decrease. Furthermore, if the spacing L between the openings OP exceeds 600 mm, the weight of the exciter 20 will increase, and the accompanying equipment for holding it will also tend to become larger.

[0037] From the viewpoint of ensuring sufficient magnetic flux supplied to the strip SP, the interval L of the openings OP is preferably 300 mm or less, more preferably 200 mm or less, and even more preferably 100 to 150 mm.

[0038] When inspecting a wide strip SP, it is preferable to install a plurality of exciters 20 in the width direction so that the entire width of the strip SP can be inspected.

[0039] The detectors 31 to 36 detect magnetic flux (magnetic lines of force) leaking from the strip SP in a magnetically saturated state. The detectors 31 to 36 include coil elements and Hall elements.

[0040] 1, a plurality of detectors 31 to 36 are arranged in the opening OP in the direction in which the strip SP is magnetized. In other words, the detectors 31 to 36 are arranged on one end side of the pair of legs 21a from one to the other of the pair of legs 21a.

[0041] It is preferable that a plurality of detectors 31 to 36 are provided according to the distance between the magnetic poles, i.e., the interval between the pair of legs 21a. There is no particular limit to the number of detectors 31 to 36 provided, but in this embodiment, six detectors are provided at predetermined intervals from each other.

[0042] When each of the detectors 31 to 36 detects a magnetic flux, it transmits to the defect detection unit 40 an analog voltage signal whose magnitude corresponds to the magnetic flux.

[0043] The defect detection unit 40 detects that the strip SP has a defect based on the detection signals transmitted from the detectors 31 to 36. That is, the defect detection unit 40 has a function of detecting a defect signal corresponding to a defect in the strip SP. When the defect detection unit 40 detects that the strip SP has a defect, it causes the display unit 50 to display the detection result.

[0044] The display unit 50 displays the detection results transmitted from the defect detection unit 40. For example, a display or the like can be used as the display unit 50. The detection results can be displayed, for example, by displaying the words "Defect Found."

[0045] 3 shows the configuration of the defect detection unit 40. As shown in Fig. 3, the defect detection unit 40 includes a bias unit 41 that individually applies bias signals to detection signals transmitted from the multiple detectors 31 to 36 arranged in the opening OP, an amplifier unit 42 that performs an amplification process on the signals for each of the detectors 31 to 36 that have passed through the bias unit 41, an A / D conversion unit 43 that converts the signals for each of the detectors 31 to 36 amplified by the amplifier unit 42 into digital signals, a filter unit 44 that cuts off a predetermined band of the digital signals that have passed through the A / D conversion unit 43, and a defect determination unit 45 that performs defect determination using the signals that have passed through the filter unit 44. Note that the filter unit 44 is not an essential component, and the defect detection unit 40 may be configured as appropriate depending on the embodiment.

[0046] The bias unit 41 has bias devices 411 to 416. Each of the bias devices 411 to 416 applies a preset bias signal to the detection signals output from the detectors 31 to 36. The bias signal applied by each of the bias devices 411 to 416 is determined according to the magnetic environment formed at the opening OP, i.e., the magnetic gradient.

[0047] Specifically, the bias unit 41 includes a plurality of bias devices 411 to 416 that apply individually set bias signals to the detectors 31 to , respectively.

[0048] The bias devices 411 to 416 are provided corresponding to the respective detectors 31 to 36. The bias devices 411 to 416 may be configured by adders or subtractors that individually add bias signals to the sensor signals (detection signals) of the detectors 31 to 36.

[0049] That is, the biasers 411 to 416 receive signals from the detectors 31 to 36 and a constant voltage value as an external signal. The biasers 411 to 416 output bias signals corresponding to the results of addition or subtraction of these signals to the amplifier unit 42.

[0050] The amplifier section 42 amplifies the detection signals output from the respective bias devices 411 to 416. For example, the amplifier section 42 amplifies each of the signals output from the bias devices 411 to 416 by the same amplification factor.

[0051] The amplification factor may be set, for example, with the maximum voltage set to 5 V, and a large amplification factor may be set that is within the range of the maximum voltage and that can detect defects in the strip SP with high sensitivity. The leakage magnetic field inspection device 100 of this embodiment reduces the influence of the magnetic environment formed at the opening OP by the bias unit 41, i.e., the magnetic gradient, and therefore can set a larger amplification factor than in the prior art.

[0052] The A / D conversion unit 43 reduces the influence of noise that becomes a disturbance when determining defects detected in the strip SP. This allows the defect determination unit 45 to accurately determine defects. The signal processing performed by the A / D conversion unit 43 may be set to, for example, about 96 Hz. By setting the A / D conversion unit 43 in this way, defects can be detected effectively even when the transport speed of the strip SP is set to 700 m / min.

[0053] The filter unit 44 blocks predetermined low-frequency components and high-frequency components from the time-series digital signal. Specifically, the filter unit 44 is disposed between the A / D conversion unit 43 and the defect determination unit 45 as needed.

[0054] The filter section 44 has a low frequency cutoff section 44a that cuts off preset low frequency components of the digital signal, and a high frequency cutoff section 44b that cuts off preset high frequency components.

[0055] The low frequency cutoff section 44a and the high frequency cutoff section 44b may be band pass filters (BPFs). Specifically, a high pass filter may be used as the low frequency cutoff section 44a. A low pass filter may be used as the high frequency cutoff section 44b. The filter section 44 is preferably configured by combining a high pass filter and a low pass filter.

[0056] The lower limit of the cutoff frequency of the bandpass filter should be set to a frequency lower than the frequency of a sine wave whose wavelength is the time it takes for the defect length to be detected. The cutoff frequency band should be selected so that the S / N ratio is high for defects that are long in the longitudinal direction of the strip SP.

[0057] For example, the lower limit of the cutoff frequency of the bandpass filter may be set to 0.01 to 100.00 Hz, preferably 0.10 to 10.00 Hz. The lower limit of the cutoff frequency of the bandpass filter may be determined according to the line speed of the production line that inspects the strip SP for defects, and the lower limit of the cutoff frequency may be changed according to changes in the line speed.

[0058] The upper limit of the cutoff frequency of the bandpass filter should be set to 100 to 200 Hz. By setting the upper limit of the cutoff frequency in this range, it is possible to eliminate the effects (noise) caused by flapping of the strip SP when the line speed is increased.

[0059] The defect determination unit 45 may have the same functions as those used in conventional defect detection units. That is, the defect determination unit 45 has a computer (not shown) with a CPU, ROM, and RAM. The defect determination unit 45 compares the defect signal with a threshold value, and if the defect signal exceeds the threshold value, determines that the strip SP has a defect.

[0060] Specifically, the defect determination unit 45 determines that a strip SP has a defect when the defect signal exceeds a predetermined threshold. The threshold may be set based on the signal strength of the defective strip SP. For example, the defect determination unit 45 may identify the strength of a defect signal determined to be a defect based on past operational results, use the strength of the defect signal as a reference value, and set the threshold based on the reference value.

[0061] When a signal exceeding the threshold value is obtained through such defect determination processing, the defect determination unit 45 generates display information indicating that a defect exists, and outputs the display information to the display unit 50.

[0062] The defect determination unit 45 may determine the size, severity, etc. of the detected defect through information processing by a computer, and display the determination results on the display unit 50. Specifically, the defect determination unit 45 generates defect degree information indicating the size and severity of the defect based on the difference between the defect signal and a threshold value, and displays the defect degree information on the display unit 50.

[0063] Fig. 4 shows a manner in which the bias signals of the bias devices 411 to 416 are set. As shown in Fig. 4, the leakage magnetic field inspection device 100 includes a bias signal setting unit 46 that sets the bias signals output by the bias devices 411 to 416 of the bias unit 41.

[0064] For example, a ladder-type bias circuit can be used as the bias signal setting unit 46. The ladder-type bias circuit can be configured by connecting multiple fixed resistors R-0 to R-5 in series and connecting one end of the fixed resistor R-5 and one end of the fixed resistor R-0 to a bias power supply that applies a constant voltage.

[0065] By using a ladder-type bias circuit as the bias signal setting unit 46, different bias voltages are extracted between the fixed resistors R-5 to R-0, and the bias voltages are input to the bias devices 411 to 416. In other words, the bias signal setting unit 46 inputs the respective bias voltages to the bias devices 411 to 416, thereby setting the bias signal to be output by the bias unit 41.

[0066] Specifically, when the bias signal setting unit 46 is configured as a ladder-type bias circuit, the set value of the bias signal can be set by adjusting the resistance values ​​of each fixed resistor arranged in series with each other.

[0067] The resistance value of each fixed resistor is preferably adjusted so that the sensor outputs of all the detectors 31 to 36 become approximately zero when a defect-free strip SP is magnetized.

[0068] Such resistance value adjustment should be performed during the manufacture of the leakage magnetic detection device or before use in online defect detection. The magnetic environment formed at the opening OP, i.e., the magnetic gradient, is determined by the arrangement of the exciter 20. Therefore, readjustment of the bias signal is basically unnecessary unless the configuration of the exciter 20 and the detectors 31-36 is changed. However, since the behavior of the magnetic gradient may change due to changes in the characteristics of the excitation coil 22 and the like over time (changes in the magnetic environment), it is preferable to perform adjustment about once every three months.

[0069] Fig. 5 shows the magnetic flux density according to the position in the width direction of the excitation yoke 21. Note that the position in the width direction in Fig. 5 indicates the distance from the center of the interval L. In the leakage magnetic field inspection device 100, when the interval L of the opening OP is increased, a magnetic gradient is likely to occur at the opening OP. For example, as shown in Fig. 5, the magnetism in the excitation yoke 21 varies depending on the position in the width direction, and a magnetic gradient of -60 to 60 mT is formed.

[0070] Specifically, a magnetic gradient of approximately 60 mT occurs at a position of about 40 mm in the width direction of the opening OP. If the bias unit 41 is not provided, when such a magnetic gradient occurs, a similar magnetic gradient is superimposed on the voltage output by the detector.

[0071] That is, when no defects exist in the strip, it is preferable that the sensor output from each detector be 0 V. However, due to the influence of the magnetic gradient, the detector outputs a voltage in the range of +αV to -αV.

[0072] In conventional defect detection units, the sensor output from the detector is amplified directly by the amplifier section, so if the amplification factor is large, the output of the detector, which is outputting, for example, +αV, may become saturated (for example, +5V).

[0073] If the analog signal amplified by the amplifier saturates, it becomes impossible to detect magnetic flux fluctuations during defect inspection of the strip SP, making it difficult to properly detect defects. This requires setting an upper limit on the amplification factor of the amplifier, which results in the detector's sensor output not being sufficiently amplified, reducing the sensitivity of defect detection. Furthermore, when multiple detectors are arranged in the C-direction, it is difficult to apply a uniform magnetic flux density throughout the entire opening OP.

[0074] 6 shows how the bias unit 41 adds bias signals to the detection signals of the detectors 31 to 36. As shown in Fig. 6, the bias signal setting unit 46 sets the bias signals output by the bias devices 411 to 416 so that the bias signals (voltages) output from the bias devices 411 to 416 are 0V when there is no defect in the strip SP.

[0075] Therefore, by providing the bias unit 41, the leakage magnetic field inspection device 100 of this embodiment can reduce to a certain extent the influence of the magnetic environment generated at the opening OP, i.e., the magnetic gradient, on the individual detectors 31 to 36. As a result, when no defects occur in the strip SP, the outputs of the detectors 31 to 36 can be brought closer to zero as a whole. As a result, when the detectors 31 to 36 detect a defect, even if the sensor signal is amplified in the amplifier unit 42, the signal will not be saturated, and the accuracy of defect detection can be improved.

[0076] The leakage magnetic field inspection device 100 may set appropriate bias signal setting values ​​for the bias devices 411 to 416 depending on the magnetic environment, that is, the characteristics of the magnetic gradient, which differ depending on the configuration of the exciter 20.

[0077] For example, when inspecting a defect-free strip SP using the exciter 20, the bias signal setting unit 46 may acquire the measurement values ​​of the detectors 31 to 36 output from the amplifier unit 42 and set the bias signal so that each measurement value becomes zero. In other words, the bias signal setting unit 46 acquires the signal from each of the detectors 31 to 36 when a defect-free strip SP is magnetized, and sets the bias signal based on the signal.

[0078] This allows the sensor outputs of the detectors 31 to 36 to approach zero individually when no defects have occurred in the strip SP. Therefore, the leakage magnetic field inspection device 100 can cancel the influence of the magnetic gradient on each of the detectors 31 to 36 even when the magnetic gradient generated at the opening OP is not linear. In other words, the leakage magnetic field inspection device 100 can reduce the influence of the magnetic environment of the opening OP.

[0079] During defect inspection (operation) of the strip SP, it is advisable to maintain the bias signal output from the bias signal setting unit 46 at the set bias signal setting value. By doing so, even if the sensor signal is uniformly amplified in the amplifier unit 42, signal saturation can be suppressed, and the sensitivity of defect detection can be improved.

[0080] In this way, by using a ladder-type bias circuit and maintaining the set value of the bias signal during operation, it is possible to reduce operating costs with a simple configuration.

[0081] The bias signal setting unit 46 may also use a ladder-type bias circuit using variable resistance elements. In this case, the sensor outputs of the detectors 31 to 36 are individually recorded when there are no defects in the strip SP, and the resistance values ​​of the variable resistance elements of the bias circuit can be individually adjusted so that the output from the bias unit 41 approaches zero. Configuring the bias signal setting unit 46 in this way allows for a simple circuit configuration, which reduces manufacturing costs.

[0082] As described above, according to the leakage magnetic field inspection device 100 of the present invention, even if the magnetic flux density at the opening OP is non-uniform, it is possible to detect defects with high sensitivity.

[0083] (Second embodiment) The bias devices 411 to 416 may be connected to a computer that sets the bias signals, for example, and may add the bias signals based on signals from the computer.

[0084] The defect detection unit of this embodiment converts a preset bias signal setting value into a voltage signal for detection signals from the multiple detectors 31 to 36 and applies the voltage signal to the bias section 41. Note that the same components as those in the first embodiment are denoted by the same reference numerals and description thereof will be omitted.

[0085] Fig. 7 shows the configuration of a defect detection unit 60 according to this embodiment. As shown in Fig. 7, the defect detection unit 60 has a bias signal setting unit 61. The bias signal setting unit 61 has a setting information input unit 62 that receives an input of a setting value for the bias signal, a setting unit 63 that adjusts the setting value of the input bias signal, and a D / A conversion unit 64 that provides an analog voltage as a bias signal to the bias unit 41. Note that the D / A conversion unit 64 is not an essential component and may be used as appropriate depending on the embodiment.

[0086] The setting information input unit 62 is an input device that allows the user to input characters, numbers, etc. As the setting information input unit 62, for example, a keyboard, a touch panel, etc. can be used.

[0087] The setting unit 63 is a computer including a CPU, a ROM, and a RAM. The setting unit 63 is connected to the setting information input unit 62 and the D / A conversion unit 64 so as to be able to communicate data with them. Therefore, the setting unit 63 can input the setting value of the bias signal received by the setting information input unit 62 to the bias unit 41 via the D / A conversion unit 64.

[0088] According to the leakage magnetic field inspection device 100 of this embodiment, it is possible to provide the set value of the bias signal inputted by the setting information input unit 62 as the bias signal for each of the bias devices 411 to 416.

[0089] FIG. 8 shows the magnetic field depending on the position in the width direction of the excitation yoke 21. Note that the position in the width direction in FIG. 8 indicates the distance from the center of the interval L. In the leakage magnetic field inspection device 100, when the interval L of the opening OP is increased, the magnetic flux density is likely to change depending on the position. For example, as shown in FIG. 8, the magnetic field in the excitation yoke 21 varies depending on the position in the width direction, and is approximately 1690 mT near 0 mm, which is the center of the interval L, but is approximately 1745 mT at positions -60 mm and 60 mm. That is, in FIG. 8, the magnetic flux density is represented by a curve that is convex downward.

[0090] Therefore, by inputting a setting value corresponding to such a magnetic flux density via the setting information input unit 62, the bias signal setting unit 61 can set the bias signal so as to bring the outputs of the detectors 31 to 36 closer to zero.

[0091] According to the leakage magnetic field inspection device 100 of this embodiment, the outputs of the detectors 31 to 36 can be individually adjusted so as to approach zero. Therefore, the leakage magnetic field inspection device 100 of this embodiment can improve the sensitivity of defect detection not only when a linear magnetic gradient occurs at the opening OP as shown in Fig. 5, but also when a curved magnetic flux density distribution occurs as shown in Fig. 8. In other words, the leakage magnetic field inspection device 100 of this embodiment can improve the sensitivity of defect detection by reducing the influence of the magnetic environment occurring at the opening OP.

[0092] Furthermore, similarly to the first embodiment, when a defect-free strip SP is inspected by the exciter 20, the bias signal setting unit 61 acquires the measurement values ​​of the detectors 31 to 36 output from the amplifier unit 42 and sets the bias signal so that each measurement value becomes zero. Specifically, the bias signal setting unit 61 sets the bias signal by referring to the setting value input from the setting information input unit 62 so that each measurement value becomes zero.

[0093] The set value of the bias signal may be set based on the output from a voltmeter connected to the amplifier unit 42. For example, the set value of the bias signal may be set to the same absolute value as the output from the amplifier unit 42, but with different positive and negative signs.

[0094] By setting the set value in this way, it is possible to eliminate the influence of disturbances such as drift and offset caused by temperature changes in elements such as the detectors 31 to 36 and the amplifier unit 42. This allows the amplifier unit 42 to perform stable amplification.

[0095] (Third embodiment) The bias signal setting unit may set the bias signal based on the digital signal converted by the A / D conversion unit 43. Note that the same components as those in the first and second embodiments are denoted by the same reference numerals and description thereof will be omitted.

[0096] Fig. 9 shows the configuration of a defect detection unit 70 according to this embodiment. As shown in Fig. 9, the defect detection unit 70 has a bias signal setting unit 71. The bias signal setting unit 71 has a signal input unit 72 that receives an input of an amplified signal corresponding to the output of each of the detectors 31 to 36 amplified by the amplifier unit, a setting unit 73 that adjusts the setting value of the bias signal, and a D / A conversion unit 74 that provides an analog voltage as a bias signal to the bias unit 41. Note that the D / A conversion unit 74 is not an essential component and may be used as appropriate depending on the embodiment.

[0097] The signal input unit 72 is an interface for transmitting the signal output from the A / D conversion unit 43 to the setting unit 73 .

[0098] The setting unit 73 is a computer including a CPU, a ROM, and a RAM. The setting unit 73 is connected to the signal input unit 72 and the D / A conversion unit 74 so as to be able to perform data communication. Therefore, the setting unit 73 can input a setting value based on a signal input to the signal input unit 72 to the bias unit 41 via the D / A conversion unit 74.

[0099] The setting unit 73 may set a set value that has the same absolute value as the signal input to the signal input unit 72 but has a different sign, based on the signal. By the setting unit 73 setting the set value in this manner, the value of the signal output from the A / D conversion unit 43 can be made closer to zero. Therefore, according to the leakage magnetic inspection device 100 of this embodiment, an appropriate set value of the bias signal can be automatically set in accordance with the characteristics of the magnetic gradient, which differ depending on the configuration of the exciter 20.

[0100] It is preferable to use a defect-free strip SP for the setting by the setting unit 73. In other words, the bias signal setting unit 71 acquires signals from the detectors 31 to 36 when the defect-free strip SP is magnetized, and sets the bias signal based on the signals.

[0101] The automatically set bias signal setting value may be stored in, for example, a memory unit (not shown) provided in the bias signal setting unit 71. The setting unit 73 may read the setting value stored in the memory unit when inspecting the strip SP for defects (during operation) and set the bias signal using the setting value. Setting the setting value in this manner makes it possible to prevent signal saturation even when the signal is amplified in the amplifier unit 42, thereby increasing the sensitivity of defect detection.

[0102] During an operation in which defect inspection is being performed, the setting unit 73 may be configured to use a set value stored in a storage unit (not shown). Even in this case, the signal input unit 72 may receive an input of a signal from each of the detectors 31 to 36 that has been amplified by the amplifier unit 42.

[0103] Note that the characteristics of each device constituting the defect detection unit 40 may change over time, causing changes in the magnetic environment, i.e., the characteristics of the magnetic gradient. For this reason, it is advisable to reset the bias signal after the leakage magnetic field inspection device 100 has been operating for a certain period of time. For example, it is advisable to reset the bias signal after the leakage magnetic field inspection device 100 has been operating continuously for about three months. To perform such resetting, it is advisable that the setting unit 73 be able to switch between a "resetting mode" in which the bias signal is reset and an "operating mode" in which the set value is maintained.

[0104] According to the leakage magnetic field inspection device 100 of this embodiment, similarly to the second embodiment, the outputs of the detectors 31 to 36 can be individually set to approach zero. Therefore, the leakage magnetic field inspection device 100 of this embodiment can improve the sensitivity of defect detection not only when a linear magnetic gradient occurs at the opening OP as shown in Fig. 5, but also when a curved magnetic flux density distribution occurs as shown in Fig. 8. In other words, the leakage magnetic field inspection device 100 of this embodiment can improve the sensitivity of defect detection by reducing the influence of the magnetic environment occurring at the opening OP.

[0105] As in the second embodiment, the set value of the bias signal may be set based on the output from a voltmeter connected to the amplifier unit 42. For example, the set value of the bias signal may be set to the same absolute value as the output from the amplifier unit 42, but with different positive and negative signs.

[0106] Even if the set value is set in this way, as in the second embodiment, the influence of disturbances such as drift and offset due to temperature changes in elements such as the detectors 31 to 36 and the amplifier unit 42 can be eliminated, thereby enabling the amplifier unit 42 to perform stable amplification.

[0107] In this embodiment, a digital signal converted by the A / D conversion unit 43 is input, but an analog signal amplified by the amplifier unit 42 may be input to the signal input unit 72. [Explanation of symbols]

[0108] 100 Leakage magnetic inspection device 10 Conveying section 20 exciter 21a Legs 31~36 Detectors 40 Defect Detection Unit 41 Bias section 42 Amplifier section 45 Defect Judgment Section 46 Bias signal setting section 61 Bias signal setting section 62 Setting information input section 71 Bias signal setting section 72 Signal input section

Claims

1. a conveying unit that conveys a plate-shaped inspection object in one direction; an exciter having a pair of legs arranged in a direction perpendicular to the one direction, and different magnetic poles formed on one end sides of the legs; a plurality of detectors arranged on one end side of the pair of legs from one to the other of the pair of legs and configured to detect magnetic flux; a defect detection unit that detects defects in the inspection object based on detection signals from each of the plurality of detectors, The defect detection unit a bias unit that applies a bias signal to each of the detection signals of the plurality of detectors according to a magnetic environment between the pair of legs and between the magnetic poles; an amplifier unit that performs an amplification process on the signal for each of the detectors that has passed through the bias unit; a defect determination unit that performs defect determination based on the signal amplified by the amplifier unit; A leakage magnetic field inspection device comprising a bias signal setting unit that sets the bias signal output by the bias unit.

2. The bias signal setting unit of the leakage magnetic inspection device described in Claim 1 includes a setting information input unit that accepts input of setting values ​​for each of the bias signals, and sets the bias signals based on the setting values ​​accepted by the setting information input unit.

3. The bias signal setting unit includes a signal input unit that receives an input of an amplified signal corresponding to the output of each of the detectors amplified by the amplifier unit, and 2. The leakage magnetic field inspection device according to claim 1, wherein the bias signal is set in accordance with the intensity of the amplified signal input to the signal input section.

4. 4. The leakage magnetic field inspection device according to claim 2, wherein the bias signal setting unit acquires a signal for each detector when the inspection object having no defects is magnetized, and sets the bias signal based on the signal.

5. The leakage magnetic field inspection device according to any one of claims 1 to 3, wherein the bias unit adds a bias signal corresponding to a magnetic gradient formed between one of the pair of legs and the other to each of the detection signals of the plurality of detectors.

6. 5. The leakage magnetic field inspection device according to claim 4, wherein the bias unit applies a bias signal to each of the detection signals of the plurality of detectors according to a magnetic gradient formed between one of the pair of legs and the other.

7. 4. A method for adjusting a leakage magnetic field inspection device, comprising: magnetizing the inspection object having no defect using the leakage magnetic field inspection device according to claim 2 or 3; and setting the bias signal.

8. A defect inspection method for setting a set value of the bias signal using the method for adjusting a leakage magnetic field inspection device according to claim 7, and inspecting defects in the inspection object while maintaining the set value of the bias signal.

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