JP2014106136A - Magnetic leakage flux testing system and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic leakage flux testing system and method capable of accurately detecting defects in a wider defect detection range.SOLUTION: The magnetic leakage flux testing system includes a magnetizer 10 for magnetizing a steel plate 1, a detector 13 for detecting a magnetic leakage flux caused by a defect 2 existing on the magnetized steel plate 1, and a defect detection unit 20 for processing a detection signal from the detector 13 to detect the defect 2 of the steel plate 1. The defect detection unit 20 has first and second band pass filters 41 and 31 provided in parallel and includes a low frequency-side defect detection part 23 for allowing a signal in a relatively low frequency-side frequency range of the detection signal to pass through the first band pass filter 41 and detecting the defect from the frequency range and a high frequency-side defect detection part 22 for allowing a signal in a relatively high frequency-side frequency range of the detection signal to pass through the second band pass filter 31 and detecting the defect from the frequency range and detecting a size feature of the defect.

Description

  The present invention inspects defects by applying a strong magnetic field to an object to be inspected made of a ferromagnetic metal such as a steel plate and detecting a change in leakage magnetic flux caused by a defect existing inside or on the surface of the object to be inspected. The present invention relates to a leakage magnetic flux type inspection apparatus and inspection method.

  In a line that manufactures thin steel sheets such as tin steel sheets, automotive steel sheets, silicon steel sheets, etc., leakage as an inspection device for inspecting defects existing in or on the surface of a steel sheet, which is an object to be inspected made of a ferromagnetic metal, Magnetic flux type inspection devices are used (for example, Patent Documents 1 to 6).

  As shown in FIG. 12, the leakage magnetic flux type inspection apparatus applies a strong magnetic field to a steel plate 1 as an object to be inspected, which is a transported ferromagnetic material, by a magnetizer 101, and exists in or on the surface of the steel plate 1. This is a device that detects a change in leakage magnetic flux M leaking at a defect 102 portion using a magnetic flux detector (magnetic sensor) 103 installed near the surface of the object to be inspected. The magnetic field at this time is set to such a value that the magnetic flux can be passed through the steel plate until it seems to be saturated so that the leakage magnetic flux due to defects increases.

  As the magnetic flux detector (magnetic sensor) 103 for detecting the leakage magnetic flux, a magnetoresistive element, a detection coil, a magnet diode, and a Hall element are used. The detection signal is processed by the defect detection unit 110 shown in FIG. Perform detection. The conventional defect detection unit 110 amplifies the detected signal, a single band-pass filter (BPF) 112 that separates the noise signal from the amplified signal, and amplifies the signal and performs full-wave rectification. Amplifier / full-wave rectifier 113, sensitivity adjuster 114, and threshold processing unit 115 that performs signal threshold processing to determine the presence or absence of a defect.

  The signal amplified by the amplifier 111 is as shown in i), and this signal is supplied to the bandpass filter 112. The conventional band-pass filter 112 is a combination of a high-pass filter 112a and a low-pass filter 112b in series, and adjusts sensitivity by changing the lower-pass cutoff frequency on the high-pass side according to the size of the defect to be detected. That is, when the defect of the detection target is small, the lower limit cutoff frequency is changed to the high frequency region side and optimization is performed. In addition, the frequency of defects due to the movement of the inspection object is corrected by changing the cutoff frequency in accordance with the speed of the inspection object (speed tracking type bandpass filter). Adopted). However, since increasing the frequency of the bandpass filter is affected by the influence of the induction signal of the surrounding electrical equipment and electrical noise is mixed, it is not possible to select a high frequency unnecessarily.

  Further, since the magnitude of the leakage magnetic flux is approximately proportional to the size of the defect, the size of the defect is determined by amplifying the signal that has passed through the band-pass filter 112 by the amplifier / full-wave rectifier 113 as shown in ii). At the same time, full-wave rectification is performed to obtain the absolute value thereof as an inspection signal, and detection sensitivity adjustment (operation for drawing a calibration curve) is performed by the sensitivity adjuster 114 so that the defect signal has an appropriate signal level. The threshold processing unit 115 performs threshold determination on this signal as shown in iii), and sets “defect” only when a signal exceeding the threshold determined as harmful is obtained. It is also possible to determine the severity of harmfulness by setting the threshold in several stages.

JP 56-61645 A Japanese Patent Laid-Open No. 11-83808 JP 2002-195984 A JP 2002-257790 A JP 2004-354240 A JP 2007-64907 A

  In the process of detecting and determining defects by performing signal processing using a bandpass filter etc. from leakage magnetic flux generated from defects, the output is obtained roughly in proportion to the size of the defect, but the shape of the defect is greatly different, If the leakage state of the leakage magnetic flux is different, the same output may not be obtained. For example, a long defect and a wide defect have the same defect volume, but output is different if the magnitude of the magnetic flux in the steel plate is different. That is, a factor difference in the size perpendicular to the flow direction of the magnetic flux, such as the cross-sectional area of the defect, interferes with and affects the passage of the leakage magnetic flux.

  For this reason, the types and shapes of defects to be detected are arranged prior to the introduction of the device, and detection of the bandpass filter constants that select, configure, and remove noise for the defects that need to be detected can also be detected. It is necessary to set the optimum value for the required defect. That is, it is necessary to set a threshold value in advance so that the output of the shape of the defect to be detected does not become small and a necessary threshold value can be set.

  As described above, it is desired that the leakage magnetic flux type inspection apparatus is adapted to as many defect shapes as possible, but the magnetic poles that apply a magnetic field to the steel plate that is the object to be inspected are arranged in the flow direction of the defects (the conveyance direction of the steel plate). In the arrangement, for example, a defect that extends long in the moving direction of a steel plate that is an object to be inspected tends to have a smaller signal than a signal obtained by processing the same defect with another defect having the same defect volume. Also, as described above, increasing the lower cutoff frequency on the high pass side is advantageous for detecting small defects, but does not work advantageously for defects that change slowly, such as defects that are long in the moving direction. Therefore, it is extremely difficult to detect defects of all shapes with high accuracy.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a leakage magnetic flux type inspection apparatus and inspection method that can detect a defect with high accuracy by expanding the detection range of the defect.

  In order to solve the above problems, the present invention provides a magnetizer that magnetizes an object to be inspected made of a ferromagnetic metal, and a leakage magnetic flux caused by a defect present on or inside the object to be inspected that is magnetized by the magnetizer. And a defect detection unit that processes a detection signal from the detector to detect a defect existing on or in the surface of the inspection object, and the defect detection unit includes two or more defect detection units. A low-frequency-side defect detection unit configured to detect a defect from the frequency range by passing a signal in a relatively low-frequency side from the detection signal by at least the first band-pass filter in parallel with the band-pass filter; The high frequency side defect detection unit that detects a defect from the frequency range by allowing the second band pass filter to pass a relatively high frequency range from the detection signal. The a, the providing leakage flux inspection apparatus characterized by detecting the features of the size of the defect in the low-frequency side defect detecting portion and said high frequency side defect detection unit.

  Further, the present invention magnetizes an object to be inspected made of a ferromagnetic metal by a magnetizer, detects a leakage magnetic flux caused by a defect existing on the surface or inside of the object to be inspected, and detects from the detector A leakage magnetic flux type inspection method for processing a detection signal to detect a defect existing on the surface or inside of an object to be inspected, wherein two or more bandpass filters are arranged in parallel, and at least by the first bandpass filter A low-frequency-side defect detection unit that passes a signal in a frequency range relatively low in frequency from the detection signal and detects a defect from the frequency range; and a relatively high frequency from the detection signal by a second bandpass filter Leakage magnetic flux type characterized by detecting the feature of the size of the defect using the high frequency side defect detection unit that passes the frequency range on the side and detects the defect from the frequency range To provide a 査方 method.

  In the present invention, the low frequency side defect detection unit may extract a frequency component corresponding to the length of the defect to detect a defect that is long in the moving direction of the inspection object. In this case, the length of the defect is determined using the output of the low frequency side defect detection unit, and the defect is determined using the output of the high frequency side defect detection unit and the output of the low frequency side defect detection unit. The size including the length can be determined. Further, the low frequency side defect detection unit adds a signal of a frequency component corresponding to the size of the defect over time, forms an upwardly convex or downwardly convex signal, and sets the length of the signal. The length of the defect may be calculated based on the corresponding time. Further, in the second band pass filter of the high frequency side detection unit, it is preferable that the lower limit cutoff frequency on the high pass side is equal to or higher than a frequency of a sine wave with a passing time of a defect length to be detected as one wavelength. . Since it varies somewhat depending on the device, it is recommended to determine the optimum value by experiment.

  According to the present invention, two or more band-pass filters are arranged in parallel, and at least the first band-pass filter allows a signal in a frequency range relatively lower than the detection signal to pass through to detect defects from the frequency range. By the low frequency side defect detection unit for detecting, and the high frequency side defect detection unit for detecting the defect from the frequency range by passing the relatively high frequency side frequency range from the detection signal by the second band pass filter, Detect defects with different characteristics. For this reason, the defect detection range can be expanded to detect defects that have been difficult to detect in the past, and defects can be detected with higher accuracy than in the past.

It is a schematic diagram which shows the frequency component corresponding to the length of the defect among inspection signals. It is a schematic diagram which shows the frequency component generate | occur | produced from the edge part (defect and steel plate boundary) of a defect among inspection signals. FIG. 3 is a diagram illustrating an example in which the frequency band of the frequency component of FIG. 1 and the frequency band of the frequency component of FIG. 2 are arranged on the frequency axis. It is a figure which shows the example of the waveform after passing the band pass filter of various frequency ranges. It is a figure which shows the situation where the test output changes when the frequency (low frequency side) of the high pass filter of one band pass filter is changed (the lower limit is provided), and the situation where the frequency of the high pass filter is changed to (a) (B) shows the S / N ratio and output of the signal. It is a schematic block diagram which shows the magnetic flux leakage type inspection apparatus which concerns on embodiment of this invention. It is a block diagram which shows the structure of the defect detection unit in the leakage flux type inspection apparatus of FIG. It is a figure which shows the signal at the time of adding the signal of the frequency component suitable for the magnitude | size of the defect shown in FIG. 1 according to time passage. It is a figure which shows the specific process in a low frequency side defect detection part. It is a figure which shows the time between the peaks of the signal before adding once in the length extraction part of the low frequency side defect detection part. It is a figure for demonstrating the characteristic direction of a defect. It is a figure for demonstrating the measurement principle in the conventional leakage magnetic flux type inspection apparatus. It is a block diagram which shows the defect detection unit in the conventional leakage flux type inspection apparatus.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
Since the frequency components in the defect signal may have different component contents depending on the feature of the defect, if the signal component is separated according to the feature of the defect, processing for each feature becomes possible.

  The characteristics of the frequency component include a low frequency (FIG. 1) corresponding to the defect length and a high frequency (FIG. 2) which is a change in the edge portion of the defect.

  In this way, it is possible to detect defects of various shapes by processing each characteristic frequency flexibly and obtaining each output value, and obtaining a calibration curve between the degree of harm such as the defect volume and the signal. .

  Although the present embodiment is based on the above points, a conventional defect detection method will be described first so that the contents of the present embodiment can be more easily understood.

(1) Conventional Defect Detection Method The inspection signal from the defect includes a frequency component corresponding to the length of the defect as shown in FIG. 1 and an edge portion (defect and defect) as shown in FIG. It can be easily understood that there are frequency components generated from the boundaries of the steel plates, and there is a difference in the frequency that each contains. That is, the frequency component generated from the edge portion includes a higher frequency component with a shorter time period.

  FIG. 3 is a diagram illustrating an example in which the frequency band of the frequency component of FIG. 1 and the frequency band of the frequency component of FIG. 2 are arranged side by side on the frequency axis. In FIG. 3, in order to obtain signals corresponding to FIGS. 1 and 2, two types (plurality) of band-pass filters corresponding to FIGS. 1 and 2 may be used.

  However, conventionally, a band-pass filter is used alone, and the high frequency generated at the edge and the frequency generated at the entire defect are adjusted by increasing / decreasing at the same time. As a specific method, as shown in FIG. 13 described above, an unnecessary low frequency including noise such as vibration, power supply frequency, induction from the drive motor, and the like by a high-pass filter in a band-pass filter to separate noise. To cut. Even after cutting noise such as vibration, power supply frequency, induction from drive motor, etc., there is formation noise generated from the magnetically non-uniform part of the steel sheet. It becomes. Therefore, in order to cut the ground noise of the steel sheet, the value of the frequency to be cut is further increased, and adjustment is performed so that the S / N is maximized.

  A low-pass filter (low-pass frequency) that cuts off high frequencies is selected in consideration of prevention of noise in a wide area.

  FIG. 4 shows examples of waveforms after passing through bandpass filters in various frequency ranges. These are merely examples, and the waveforms vary depending on the elements and configuration used. The amount of surrounding noise is also an example.

  FIG. 5 is a diagram showing a situation in which the inspection output changes when the frequency (low frequency side) of the high-pass filter of one band-pass filter is changed (a lower limit is provided), and (a) shows the frequency of the high-pass filter. The changed situation is shown, and the S / N ratio and output of the signal are shown in (b). The signal of FIG. 5 is a signal change state of a drill hole of about 0.1 mmφ. As shown in FIG. 5B, it can be seen that the cut-off at the lower limit frequency of 1 kHz has high inspection output but poor S / N. It can be seen that the cut-off at the lower limit frequency of 3 kHz improves the S / N but decreases the inspection signal.

  4 and 5, when it is desired to find a small defect of about 0.1 mmφ drill hole, the lower cutoff frequency on the high pass side is increased to, for example, 2 kHz or higher, and the frequency band in which the S / N with a small defect is increased. Select. At this time, as the lower limit cutoff frequency of the high-pass filter is increased, the signal value becomes smaller, so that necessary gain adjustment is also performed. In this case, while confirming that the edge signal of a large defect does not become extremely small, the noise ratio of the steel plate to be inspected and the signal ratio of the defect to be found and the optimum value of the filter are obtained.

  However, in such a conventional method, the magnetic poles that apply a magnetic field to the steel plate that is the object to be inspected are arranged in the flow direction of the defect (the conveying direction of the steel plate), for example, the direction in which the steel plate that is the inspected object moves Defects that extend longer tend to have a smaller signal than signals that have been processed for other shapes of defects of the same defect volume. In addition, increasing the cutoff frequency on the high-pass side is advantageous for detection of small defects, but it does not work advantageously for defects that change slowly, such as defects that are long in the moving direction. Therefore, with the conventional method, it has been extremely difficult to detect a defect that is particularly long in the moving direction of the steel sheet with high accuracy.

(2) Leakage magnetic flux type inspection apparatus according to the present embodiment The output of the first bandpass filter (BPF1) on the low frequency side shown in FIG. 3 often includes defect length information. In the embodiment, signal processing in the frequency range on the relatively high frequency side corresponding to relatively small defects and signal processing in the frequency range on the relatively low frequency side including long defects are separately performed, and each output value Therefore, defects of various shapes can be detected with high accuracy.

  6 (a) and 6 (b) are schematic configuration diagrams showing the leakage flux type inspection apparatus according to the embodiment of the present invention, and FIG. 7 is a block diagram showing the configuration of the defect detection unit in the leakage flux type inspection apparatus of FIG. is there. The leakage magnetic flux type inspection apparatus (FIG. 6) of the present embodiment is provided near the surface of the steel plate 1 corresponding to the magnetizer 10 and a plurality of magnetizers 10 provided along the width direction of the steel plate 1 being conveyed. It has a detector (magnetic sensor) 13 for detecting the leakage magnetic flux, and a defect detection unit 20 for detecting the defect 2 from the leakage magnetic flux signal detected by the detector 13. 6A shows an example in which the magnetizer 10 and the detector 13 are arranged on the opposite side with the steel plate 1 sandwiched therebetween, and in FIG. 6B, these are arranged on the same side of the steel plate 1. An example is shown. Among these, the apparatus of FIG. 6A has the advantage that the detector 13 can be densely arranged because the magnetizer 10 does not exist on the detector 13 side.

  The magnetizer 10 includes a yoke 11 and an exciting coil 12 wound around the yoke 11, and a current is passed through the exciting coil 12 to excite the steel sheet 1 and generate a leakage magnetic flux M. As the detector (magnetic sensor) 13, a magnetoresistive element, a detection coil, a magnet diode, and a Hall element are used. The magnetizer 10 and the detector 13 are integrated to form a detection head. In addition, the code | symbol 14 shown to Fig.6 (a) is a hollow roll.

  In the present embodiment, the defect detection unit 20 amplifies the signal detected by the detector 13, as shown in FIG. 7, in order to individually and flexibly process the frequency bands separated corresponding to the feature of the defect. An amplifier 21 for detecting a defect from a relatively high frequency signal among the amplified signals, and detecting a defect from a relatively low frequency signal among the amplified signals. And a low-frequency side defect detection unit 23 to be performed.

  The high-frequency side defect detection unit 22 includes a high-frequency side bandpass filter (BPF2) 31 (corresponding to BPF2 in FIG. 3), an amplifier / full-wave rectifier 32 for amplifying the signal and full-wave rectifying, and sensitivity. It has the adjuster 33 and the threshold value processing part 34 which performs the threshold value process of a signal and detects a defect. Only the configuration of the high frequency defect detection unit 22 is the same as that of the conventional defect detection unit 110 shown in FIG.

  The band-pass filter (BPF2) 31 is a combination of a high-pass filter and a low-pass filter, and the lower limit cutoff frequency is equal to or higher than the frequency of a sine wave with the passing time of the defect length to be detected as one wavelength, for example, 2 kHz As described above, the frequency band in which the S / N with a small defect becomes high is selected. The signal amplified by the amplifier 21 passes through the band-pass filter (BPF2) 31 and is then amplified and full-wave rectified by the amplifier / full-wave rectifier 32, and the defect signal is set to an appropriate signal level by the sensitivity adjuster 33. Thereafter, the threshold processing unit 34 performs threshold determination on this signal, and “defect” is determined only when a signal exceeding the threshold determined as harmful is obtained. The threshold processing unit 34 outputs “output 1” which is the same output as the conventional one.

  Next, the low frequency side defect detection unit 23 includes a band pass filter (BPF1) 41 (corresponding to BPF1 in FIG. 3) on the low frequency side, a signal decomposition unit 42 that decomposes the input signal at the 0 V position, A time width selecting unit 43 for extracting and selecting only a predetermined time width, a signal adding unit 44 for adding adjacent time signals, a length extracting unit 45 for extracting a length from the added signals, a defect A size calculation unit 46 that calculates the size including the length, a size determination unit 47 that includes the length, and threshold processing of the defect length signal extracted by the length extraction unit 45 performs defect processing. And a length threshold processing unit 48 for detecting the length of. The band-pass filter (BPF1) 41 is also a combination of a high-pass filter and a low-pass filter, and the lower limit cutoff frequency is, for example, about 500 Hz.

  The low-frequency defect detection unit 23 passes a band-pass filter (BPF1) 41 that allows a relatively low-frequency signal to pass therethrough, and a signal of a frequency component corresponding to the size of the defect shown in FIG. 8 (a)) is added as time elapses, and a signal convex in the positive direction (convex upward) as shown in FIG. 8B (in some cases, convex in the negative direction (convex downward)) Signal). And the time between 0V and 0V of the signal after the time-series addition processing is measured, the length of the defect is calculated from the moving speed of the steel plate, the calculated length is judged, and whether or not an abnormality has occurred Is detected.

  The bandpass filter (BPF2) of the high frequency side defect detection unit 22 and the bandpass filter (BPF1) of the low frequency side defect detection unit 23 are provided in parallel.

A specific process in the low frequency side defect detection unit 23 is shown in FIG.
First, the signal (input signal) that has passed through the bandpass filter (BPF1) 41 shown in (a) is decomposed at the 0V position by the signal decomposition unit 42 as shown in (b). In this way, the signal is decomposed (separated) at a portion where the signal crosses 0V so that it can be handled as data.

  Next, as shown in (c), the time width selection unit 43 determines whether or not the time width between 0 V and 0 V is half or more of the predetermined time t, and more than half of the predetermined time t. Only the time width of is extracted and selected. The predetermined time t is the number of relations of the passage time of the detection element corresponding to the defect length (time in FIG. 8). It is determined in advance in consideration of the time expansion due to the size of the detector and the detection distance from the defect and the time lag when the signal peaks at the edge. Also, since there is a change due to the difference in shape between the tip and tail, an arbitrary value can be selected.

  Next, as shown in (d), the signal addition unit 44 performs adjacent time addition on temporally adjacent ones of the decomposed signal blocks. When the end of addition is greatly deviated from 0V or significantly different from t, it is excluded from the target. A method may be used in which areas are obtained by using time as a horizontal component and output as a vertical component, the sizes are compared, and the same level is selected. When a plus and minus pair of signals having an appropriate size can be selected, the length extraction unit 45 obtains the time between the peaks of the signals before adding once as shown in FIG. The defect length feature amount is obtained from the time and the moving speed. As a simple method for starting / stopping the length feature amount, there is a method of performing inspection level determination (threshold value larger than noise) as in the case of inspection output threshold determination.

  Only when the threshold value processing unit 48 performs threshold processing on the defect length feature quantity signal extracted by the length extraction unit 45 and a signal exceeding the length threshold value determined as harmful is obtained. “There is a defect” and “output 3” which is the output of the defect length feature quantity is output from the length threshold processing unit 48. By performing defect length feature amount determination by threshold processing as described above as needed, it is possible to determine defect length abnormality.

  On the other hand, the defect signal extracted by the length extraction unit 45 and the defect signal from the threshold processing unit 34 of the high frequency side defect detection unit 22 are calculated by the size calculation unit 46 to include the defect length. That is, a signal having a feature amount of defects is obtained. Then, the size determination unit 47 determines whether the size of the defect including the length feature obtained by the size calculation unit 46 is equal to or larger than the size of the defect determined as harmful, and determines the size of the defect. “Output 2” is output.

  As shown in FIG. 11, the defect feature direction includes a thickness feature direction, a width feature direction, and a length feature direction. Since the conventional inspection output is a signal mainly corresponding to the thickness and width of the defect, the volume size is determined by adding the length feature amount as in the present embodiment to the conventional signal. It becomes possible. That is, by obtaining a long characteristic signal based on the present invention, it is possible to easily detect a defect of a feature that has been difficult to find in the past.

  As described above, conventionally, the lower limit cutoff frequency of the bandpass filter is increased to detect a defect from a high frequency signal. However, in the present invention, a relatively high frequency signal is relatively The feature of the defect size can be captured from at least two frequency ranges of the low frequency signal. For example, the feature of the defect length that has been difficult to detect in the past can be captured. For this reason, the defect detection range can be expanded to detect defects that have been difficult to detect in the past, and defects can be detected with higher accuracy than in the past.

  The present invention can be variously modified without being limited to the above embodiment. For example, in the above embodiment, the detection signal is divided into a relatively high frequency range and a relatively low frequency range using two bandpass filters, and these are processed separately. However, the frequency range may be divided into three or more. Moreover, although the example which captures the feature of the length of the defect from the frequency range on the relatively low frequency side has been described in the above embodiment, the present invention is not limited to this. Furthermore, the signal processing method is an example, and various other methods can be employed. Further, the leakage magnetic flux type inspection apparatus is not limited to FIGS. 6A and 6B, and the present invention can be applied to various systems.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Defect 10 Magnetizer 11 Yoke 12 Excitation coil 13 Detector (magnetic sensor)
14 Hollow roll 20 Defect detection unit 21 Amplifier 22 High frequency side defect detection unit 23 Low frequency side defect detection unit 31 Band pass filter (BPF2)
32 Amplifier / Full Wave Rectifier 33 Sensitivity Adjuster 34 Threshold Processing Unit 41 Band Pass Filter (BPF1)
42 Signal Decomposition Unit 43 Time Width Selection Unit 44 Signal Addition Unit 45 Length Extraction Unit 46 Size Calculation Unit 47 Size Determination Unit 48 Length Threshold Processing Unit M Leakage Magnetic Flux

Claims (10)

A magnetizer for magnetizing a test object made of a ferromagnetic metal;
A detector for detecting a leakage magnetic flux caused by a defect existing on the surface or inside of the inspection object magnetized by the magnetizer;
A defect detection unit that processes a detection signal from the detector to detect a defect existing on or inside the surface of the inspection object;
The defect detection unit has two or more bandpass filters arranged in parallel, and at least the first bandpass filter allows a signal in a frequency range relatively lower than the detection signal to pass through to detect defects from the frequency range. A low-frequency-side defect detection unit that detects a defect, and a high-frequency-side defect detection unit that detects a defect from the frequency range by passing a relatively high-frequency side frequency range from the detection signal by a second bandpass filter. A leakage magnetic flux type inspection apparatus, wherein the low frequency side defect detection unit and the high frequency side defect detection unit detect a feature of a defect size.   The leakage flux type inspection apparatus according to claim 1, wherein the low frequency side defect detection unit extracts a frequency component corresponding to the length of the defect and detects a defect that is long in the moving direction of the inspection object.   The output of the low frequency side defect detection unit is used to determine the length of the defect, and the output of the high frequency side defect detection unit and the output of the low frequency side defect detection unit are used to determine the length of the defect. The leakage magnetic flux type inspection apparatus according to claim 2, wherein the size is included.   The low frequency side defect detection unit adds a signal of a frequency component corresponding to the size of the defect over time, forms an upward or downward convex signal, and corresponds to the length 4. The leakage magnetic flux type inspection apparatus according to claim 2, wherein the length of the defect is calculated based on time.   The second band-pass filter of the high-frequency side detection unit is characterized in that a lower-limit cutoff frequency on the high-pass side is equal to or higher than a frequency of a sine wave with a passing time of a defect length to be detected as one wavelength. The leakage magnetic flux type inspection apparatus according to any one of claims 1 to 4. An object to be inspected made of a ferromagnetic metal is magnetized by a magnetizer, leakage magnetic flux caused by defects existing on the surface or inside of the object to be inspected is detected by a detector, and a detection signal from the detector is processed. A leakage magnetic flux type inspection method for detecting defects existing on the surface or inside of an inspection object,
Two or more band pass filters are arranged in parallel, and at least the first band pass filter passes a signal in a frequency range relatively low from the detection signal to detect a defect from the frequency range. Using a defect detection unit and a high frequency side defect detection unit that passes a relatively high frequency range from the detection signal by the second band pass filter and detects a defect from the frequency range, the size of the defect The leakage magnetic flux type inspection method characterized by detecting the feature of the length.   The leakage flux type inspection method according to claim 6, wherein the low-frequency side defect detection unit detects a defect that is long in the moving direction of the inspection object by extracting a frequency component corresponding to the length of the defect.   The output of the low frequency side defect detection unit is used to determine the length of the defect, and the output of the high frequency side defect detection unit and the output of the low frequency side defect detection unit are used to determine the length of the defect. The leakage magnetic flux type inspection method according to claim 7, wherein the size of the inclusion is determined.   The low frequency side defect detection unit adds a signal of a frequency component corresponding to the size of the defect over time, forms an upward or downward convex signal, and corresponds to the length 9. The leakage magnetic flux type inspection method according to claim 7, wherein the length of the defect is calculated based on time.   The second band-pass filter of the high-frequency side detection unit is characterized in that a lower-limit cutoff frequency on the high-pass side is equal to or higher than a frequency of a sine wave with a passing time of a defect length to be detected as one wavelength. The leakage magnetic flux type inspection method according to any one of claims 6 to 9.

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