JP2015040846A - Magnetic flux type leakage inspection device and inspection method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a magnetic flux type leakage inspection device and inspection method capable of highly accurately detecting defects in a wide range.SOLUTION: A magnetic flux type leakage inspection device includes: a magnetizer 10 which magnetizes a steel plate 1; a detector 13 which defects magnetic flux leakage due to a defect 2 existing in the steel plate magnetized by the magnetizer 10; and a defect detection unit 20 which processes a detection signal from the detector 13 to detect the defect 2 of the steel plate 1. The defect detection unit 20 includes: a wavelet transformation processing part 23; a comparison signal generation part 24 which generates a comparison signal with one or more frequencies; a signal processing part 25 which obtains a defect signal by performing signal processing of an output signal obtained by correlation arithmetic processing in the wavelet transformation processing part 23 using the comparison signal with one or more frequencies; and a defect information acquisition part 26 which uses at least a part of the defect signal obtained by the signal processing part to obtain information 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. 13, the leakage flux type inspection apparatus applies a strong magnetic field to the steel plate 1 as the object to be inspected, which is a transported ferromagnetic material, by the magnetizer 101, and exists inside 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, increasing the frequency on the low-pass filter side of the band-pass filter is affected by the influence of the induction signal of the surrounding electric equipment, and electrical noise is mixed, so it is impossible to select a high frequency.

  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 the defect by performing signal processing using a bandpass filter etc. due to the leakage magnetic flux generated from the defect, 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 sheet 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, the detection elements are selected and configured for the defects that need to be detected, and the constants of the bandpass filter for removing noise Must be set to an optimum value for a defect that needs to be detected. That is, it is necessary to prevent a defect shape having an extremely small inspection output from existing. In addition, a threshold value is determined in advance so that it can be determined whether the defect is harmful or harmless.

  As described above, the magnetic flux leakage inspection apparatus is desired to be adapted to as many defect shapes as possible. However, as shown in FIG. In the configuration arranged in the (steel sheet conveying direction), for example, a signal of a defect extending long in the direction in which the steel sheet as the inspection object moves tends to be small. Also, as described above, increasing the lower cutoff frequency on the high pass side is advantageous for detecting small defects, but it is disadvantageous for defects that are loose in change, 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 an object of the present invention is to provide a leakage magnetic flux type inspection apparatus and an inspection method capable of detecting a wide range of defects with high accuracy.

  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 for processing a detection signal from the detector to detect a defect present on the surface or inside of the object to be inspected, and the defect detection unit is subjected to correlation calculation processing. A wavelet transform processing unit that obtains a correlation between a test signal detected by the detector and a comparison signal, a comparison signal generation unit that generates a comparison signal of one or more frequencies, and the comparison signal of one or more frequencies A signal processing unit that obtains a defect signal by performing signal processing of one or more output signals obtained by correlation calculation processing by the wavelet transform processing unit, and a defect signal obtained by the signal processing unit. Providing leakage flux inspection apparatus characterized by having a defect information acquisition unit for obtaining information of a defect using at least a part.

  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 the inspection signal detected by the detector is supplied to a wavelet transform processing unit and a comparison signal A comparison signal having one or more frequencies generated by the generation unit is supplied to a wavelet transform processing unit, and the wavelet transform processing unit compares the inspection signal detected by the detector with the correlation calculation process and the one or more frequencies. By obtaining a correlation with the signal, one or more output signals are obtained, and a defect is obtained by using at least a part of the defect signals obtained by performing signal processing of the output signals. Providing leakage flux inspection method characterized by obtaining information of the defect by the broadcast acquisition unit.

  In the present invention, the wavelet transform processing unit may include a correlation calculation processing unit that performs a correlation calculation process using a comparison signal having one or more frequencies generated by the comparison signal generation unit. The comparison signal is best matched to the waveform of the inclusion to be detected. However, since it is possible to cope with various inclusions, one period of the signal that repeats continuously and the period can be defined. And a signal for which the time integration value of the signal for one period is 0 (zero), for example, one sine function wave.

  The defect information acquisition unit may obtain defect size information using a difference in output signal due to a difference in defect shape when a correlation calculation process is performed using comparison signals having different frequencies.

  It is preferable that the comparison signal generation unit changes a frequency of a comparison signal to be sent to the wavelet transform processing unit according to a conveyance speed of the object to be inspected.

  According to the present invention, the wavelet transform processing unit obtains the correlation between the inspection signal detected by the detector by the correlation calculation process and the comparison signal having one or more frequencies and obtains one or more output signals. A defect signal can be taken out, and a signal corresponding to the size of the defect can be obtained for both large and small defects. For this reason, a wide range of defects can be detected with high accuracy.

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 which generate | occur | produces from the edge part of a defect (border of a defect and a steel plate) among the inspection signals, and the frequency component which generate | occur | produces from the center of a small defect. 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 and two bandpass filters corresponding to each frequency band are used. It is a figure which shows the example of the waveform after each band pass filter at the time of providing three types of band pass filters from which a frequency range differs. It is a figure which shows the isolation | separation condition at the time of performing frequency isolation | separation by changing the frequency of a high pass filter about the defect from which a magnitude | size differs. It is a figure which shows the change of the output at the time of changing a gain locally in the frequency range centering on 8 kHz using a band pass filter, (a) shows the condition which changed the frequency of the high pass filter, (b) Shows the S / N ratio and output of the inspection signal. It is a figure which shows the change of the output at the time of changing a gain locally in the frequency range centering on 2 kHz using a band pass filter, (a) shows the condition which changed the frequency of the high pass filter, (b) Shows the S / N ratio and output of the inspection 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 for demonstrating the principle of the correlation calculation process in a wavelet transformation process part. It is a figure which shows the intensity distribution simulation result of the leakage magnetic flux which leaked in the defective part about 1 mm in length and width 0.1mm which exists in a steel plate, and thickness 0.01mm. It is a figure which shows the test | inspection output when changing the frequency of a comparison signal and detecting the defect from which a magnitude | size differs. 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.
The frequency component in the defect signal may differ depending on the feature of the defect, and in such a case, separation processing that matches the signal component with the feature of the defect is required.

  As frequency components, there are a low frequency (FIG. 1) corresponding to the length of the defect and a high frequency (FIG. 2) at the center of a small defect or a change in the edge 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. .

  In order to make the contents of the present embodiment easier to understand, a technique for detecting a defect from a frequency component of an inspection signal that has been conventionally performed will be described first, and then the present embodiment will be described.

(1) Method for Detecting Defects from Frequency Components of Inspection Signal Frequency components commensurate with the size of the defect such as the defect length, defect width and thickness shown in FIG. 1 are included in the inspection signal from the defect. As shown in FIG. 2, there is a frequency component generated from the edge portion of the defect (between the defect and the steel plate) and a high frequency component in the central portion of the small defect, and there is a difference in the frequency included in each. Can be easily understood. That is, the frequency component generated from the edge portion includes a higher frequency component with a shorter time period.

  Conventionally, a band-pass filter is used alone, and the high frequency generated at the edge and the frequency generated in the entire defect are adjusted by increasing / decreasing at the same time. As a specific method, as shown in FIG. 14 described above, an unnecessary low frequency including noise such as vibration, power supply frequency, induction from a drive motor, and the like by a high pass filter in a band pass filter in order 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 unnecessary frequencies that are higher than the frequency range that the defect has is selected in consideration of prevention of noise in a wide area.

  On the other hand, FIG. 3 is a diagram showing 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. From this figure, FIG. In order to obtain a corresponding signal, it is effective to use two types (plural types) of bandpass filters in combination with a highpass filter (HPF) and a lowpass filter (LPH) corresponding to FIGS. I understand.

  FIG. 4 shows an example of a waveform after passing through each bandpass filter when three types of bandpass filters having different frequency ranges are provided. These are merely examples, and the waveforms vary depending on the elements and configuration used. The amount of surrounding noise is also an example.

  However, although it is possible to effectively separate signals by providing a plurality of bandpass filters having different frequency ranges as shown in FIG. 3, a normal frequency filter is a circuit designed for continuous sine waves. If the signal is not generated continuously as in the case of a defect, even if a plurality of bandpass filters are provided, it may be difficult to appropriately separate a high frequency of defects.

  FIG. 5 shows the frequency separation situation of defects of different sizes. The frequency separation method in FIG. 5 obtains an output by changing the frequency of the HPF. Since the output becomes lower when the frequency is increased, it becomes impossible to discriminate with the same threshold as when the frequency is lower. Usually, sensitivity calibration is performed using a 0.1 mmφ drill hole as a standard sample so that the output of each frequency can be determined with the same threshold. Sensitivity calibration uses a standard sample to gain individual sensitivity of each detection element by gain so that the same inspection output can be obtained when inspecting a wide range of inspection objects by arranging multiple detectors (detection elements) in the width direction. It is also work to adjust.

  As shown in FIG. 5, in the result of obtaining the frequency characteristics of defects using HPF, the difference in inspection output from the calibration hole tends to open as the higher frequency is used, and there is room for improvement in order to obtain higher sensitivity. There is. FIGS. 6 and 7 show the results of confirming the change in output by using a bandpass filter in a wide frequency range and increasing or decreasing the gain of the local frequency portion. 6 shows the output change when the gain is locally changed (increased) in the frequency range centered on 8 kHz, and FIG. 7 shows the output change when the gain is locally changed (increased) in the frequency range centered on 2 kHz. In both figures, (a) shows the situation where the HPF frequency is changed, and (b) shows the S / N ratio and output of the inspection signal. As shown in these figures, the output tends to increase in both cases, and the tendency is slightly larger when the gain of 2 kHz is increased. Thus, the gain of the local frequency portion is increased by the HPF of an arbitrary frequency. The method of increasing the sensitivity by increasing / decreasing is not preferable because there is a possibility that quality cannot be continuously guaranteed.

(2) Leakage magnetic flux type inspection apparatus according to this embodiment In this embodiment, a wavelet transform method is used to solve the problems of the method for detecting defects from the frequency components of the inspection signal using the bandpass filter described above. Thus, even a high frequency defect signal can be discriminated.

  Here, the wavelet transform is a wavelet function obtained by multiplying a mother wavelet by a in the time direction and moving by b in the time direction. This wavelet function and a signal to be arbitrarily confirmed are each subjected to Fourier transform, This is a technique for obtaining the inner product and determining how many components of the wavelet function are included. In the conventional signal analysis FFT (Fourier transform), the frequency distribution can be extracted, but the information in the time direction is lost. Wavelet transform can extract information without losing information in the time direction. In addition, the Fourier transform has a conflicting problem that the precision of the data is relatively lost if the local part is precisely performed, but the wavelet transform has a local part in terms of time and information. It has features such as precise processing.

  In order to use the wavelet transform process as it is, the Fourier transform process is required. Therefore, the Fourier transform process is used as it is, and it is returned to the real time in the processing stage after the signal analysis, and the necessary frequency information and output value are extracted. However, here, a method of determining a wavelet function that is a characteristic of the wavelet transform without performing Fourier transform and obtaining an inner product is used. That is, the frequency and waveform of the wavelet function are determined in advance (the waveform function and b moved in the time direction and b moved in the time direction), and an arbitrary signal (detection from a detection element including an inclusion defect signal) is detected. Signal) and the inner product performed in the original wavelet transform processing are performed as correlation calculation processing, and how much the defined wavelet function contains is calculated in real time. This makes it possible to perform processing capable of extracting a signal having a necessary frequency at high speed. The calculation at this time adds the product of the data for each sample frequency and the product of the calculated wavelet function. That is, data for each sample frequency is obtained by performing the same number of products as the number of data of the wavelet function and the addition value thereof (correlation calculation processing by product-sum calculation).

  FIGS. 8A and 8B are schematic configuration diagrams showing a leakage magnetic flux type inspection apparatus according to an embodiment of the present invention, and FIG. 9 is a block diagram showing a configuration of a defect detection unit in the leakage magnetic flux type inspection apparatus of FIG. is there. The leakage magnetic flux type inspection apparatus (FIG. 8) of the present embodiment is provided in the vicinity of 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 to be 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. 8A shows an example in which the magnetizer 10 and the detector 13 are arranged on the opposite side across the steel plate 1, and FIG. 8B shows that these are arranged on the same side of the steel plate 1. An example is shown. Of these, the apparatus shown in FIG. 8A 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.8 (a) is a hollow roll.

  As shown in FIG. 9, the defect detection unit 20 includes a pre-amplifier 21 for amplifying a signal (sensor signal) detected by the detector 13, a low-frequency noise cut unit 22 for cutting low-frequency noise, and an amplification. A wavelet transform processing unit 23 for performing correlation calculation processing on the processed signal, a comparison signal generating unit 24 for generating a plurality of comparison signals having different frequencies, and performing signal processing of the output signal subjected to correlation calculation processing to generate a defect signal A signal processing unit 25 to be obtained, and a defect information acquisition unit (overall determination unit) 26 to obtain defect information from the signal processed by the signal processing unit 25.

  The wavelet transform processing unit 23 performs correlation calculation processing using a plurality of comparison signals having different frequencies generated by the comparison signal generation unit 24. Specifically, the wavelet transform processing unit 23 includes a plurality of correlation calculation processing units 31, and a plurality of comparison signals having different frequencies generated by the comparison signal generation unit 24 are respectively sent to the plurality of correlation calculation processing units 31. The correlation calculation processing unit 31 performs correlation calculation processing using the comparison signal.

  The signal processing unit 25 amplifies the output signal obtained by the correlation calculation processing by each correlation calculation processing unit 31 and performs full-wave rectification, and a plurality of amplifiers / full-wave rectifiers 32, and each amplifier / full-wave rectifier 32. It has a plurality of sensitivity adjusters 33 for adjusting the sensitivity of signals, and a plurality of threshold processing units 34 for detecting a defect by performing threshold processing of each signal after sensitivity adjustment. The defect information acquisition unit (overall determination unit) 26 comprehensively determines the results of the plurality of threshold processing units 34 and finally acquires defect information.

As shown in FIG. 10, each correlation calculation processing unit 31 of the wavelet transform processing unit 23 obtains an output by obtaining the correlation between the inspection signal that is the sample source waveform and the comparison waveform (comparison signal) by the correlation calculation processing. The expression shown in the lower part of FIG. 10 is an expression indicating a general wavelet transform, in which f (x) is an inspection signal, Ψ _{a, b} (x) is a comparison waveform (wavelet function), and W (a, b ) Is a wavelet coefficient.

  Normally, in this type of processing, the correlation is obtained by Fourier transform, but in the present invention, wavelet transform is used to obtain the correlation between waveforms shown on the time axis. The correlation can be obtained by processing the comparison waveform while shifting it in accordance with the time axis of the inspection signal. As the time step for shifting, the step of sampling timing can be used. In order to shorten the processing time, resampling may be performed to roughen the time step.

  As a comparative waveform (wavelet function) used for the wavelet transform, a Gabor function, a Dobeshi function, a Meyer function, or a Mexican hat function can be used, but these are for a continuous waveform. On the other hand, as a comparative waveform (wavelet function), the signal is one period of a signal that can be defined and repeated continuously, and the time integration value of the signal for one period is 0 (zero). By using a signal such as a simple sine function (one wave), it becomes easy to detect a non-continuous defect.

  This will be described with reference to FIG. FIG. 11 is a diagram showing a simulation result of the intensity distribution of leakage magnetic flux leaked at a defective portion having a length of 1 mm, a width of 0.1 mm, and a thickness of about 0.01 mm existing in the steel plate. Here, an example of leakage flux vertical magnetic flux intensity distribution at a position about 1 mm away from the steel plate is shown. As shown in FIG. 11, it can be seen that the strength of the vertical component of the leakage magnetic flux includes a waveform close to a sine wave from a high frequency to a low frequency. However, as a matter of course, the intensity distribution waveform varies depending on the size of the defect.

  The comparison signal generation unit 24 generates a plurality of comparison signals having different frequencies and sends them to each correlation calculation processing unit 31 of the wavelet transform processing unit 23. In this embodiment, the comparison signal having a frequency of 1 kHz, 2 kHz, 4 kHz, 8 kHz,..., N kHz is sent to each correlation calculation processing unit 31, and this is the line speed (speed of the steel plate to be inspected). ) Is an example of 300 mpm. The comparison signal generator 24 preferably changes the frequency of the comparison signal following the line speed when the line speed is changed. Thereby, even if the line speed is changed, the same inspection output can be obtained.

  In the leakage flux type inspection apparatus of the present embodiment, the leakage magnetic flux M generated by exciting the steel plate 1 by the magnetizer 10 is detected by the detector (magnetic sensor) 13, and the detection signal (sensor signal) is detected by the preamplifier. Amplified at 21, low frequency noise is cut by the low frequency noise cut unit 22, and sent to the wavelet transform processing unit 23.

  Then, the comparison signal generation unit 24 generates a plurality of comparison signals from a low frequency to a high frequency according to the line speed, and sends them to each correlation calculation processing unit 31 of the wavelet transform processing unit 23. In this example, the line speed is 300 mpm, and a comparison signal having a frequency of 1 kHz, 2 kHz, 4 kHz, 8 kHz,..., N kHz is used.

  In the wavelet transform processing unit 23, each correlation calculation processing unit 31 performs the correlation calculation processing as shown in FIG. 10 to obtain the correlation between the inspection signal and the comparison signal of a predetermined frequency. The output signal from each correlation calculation processing unit 31 is amplified and full-wave rectified by the amplifier / full-wave rectifier 32, and further the sensitivity is adjusted (gain processing) by the sensitivity adjuster 33, so that the defect signal has an appropriate signal level. Thereafter, in threshold processing by the threshold processing unit 34, threshold determination is performed on the signal to obtain a defect signal (defect detection). Then, defect detection results corresponding to a plurality of output signals having different frequencies of the comparison signal are sent to the defect information acquisition unit (overall determination unit) 26, and the selection of the frequency of the comparison signal, the utilization ratio, etc. are determined, and the defect information is acquired To do. FIG. 9 illustrates an output signal when the comparison signal is 1 kHz and 8 kHz, but the output signal of the correlation calculation processing in each correlation calculation processing unit 31 of the wavelet transform processing unit 23 varies depending on the frequency of the comparison signal. I understand.

  In the present embodiment, the output signal is processed with the same processing actual signal (time axis) as in the prior art. It is also possible to detect a defect by giving a gain with digital processing without converting the actual signal (time axis) back to analog data and comparing the absolute value with a threshold value.

  FIG. 12 is a diagram showing inspection output when defects having different sizes are detected by changing the frequency of the comparison signal according to the present embodiment. Here, sensitivity calibration was performed using a 0.1 mmφ drill hole as a standard sample so that the output of each frequency could be determined with the same threshold. A sine function (one wave) was used as the comparison signal. As shown in FIG. 12, even if the frequency is changed, it can be seen that the inspection output difference from the calibration hole is almost constant for any size defect, and the sensitivity is high. In this example, for example, even if the comparison signal is one circuit of 8 kHz, the inspection performance is sufficient. Further, the output of the signal from 1 kHz to 2 kHz differs depending on the shape of the defect, and it is possible to obtain information on the size of the defect by using the output difference depending on the shape of the defect. In consideration of detecting defects, it is desirable that the frequency of the comparison signal be high as long as the S / N is not lowered, but it is preferable to select an optimum frequency in consideration of the calculation processing time.

  Conventionally, defects are detected from the frequency components of the inspection signal using a filter (HPF) that passes a high frequency corresponding to the frequency components of the defects, but an optimum HPF that matches the size of the defects is obtained. It was difficult. For example, when a filter that passes a high frequency in order to detect a small defect in a high frequency region, a sufficient S / N ratio and a sufficiently large output cannot be obtained for a large defect ( (See FIG. 5).

  On the other hand, in the present embodiment, by providing the wavelet transform processing unit 23 and using a plurality of comparison signals having different frequencies, it becomes possible to take out a defect signal in a wide range. It is possible to obtain a signal corresponding to the size of. In particular, small defect detection can be optimized by inspecting at a high frequency.

  That is, since the frequency of the comparison signal can be selected in a wide range, a defect having a high S / N ratio can be detected at an arbitrary frequency, and a wide range of defects can be detected with high accuracy.

  At this time, by using a sine function (one wave) as a comparison signal, a defect signal appearing discontinuously can be detected with high accuracy.

  In addition, since the output signal of the correlation calculation processing from each correlation calculation processing unit 31 varies depending on the frequency of the comparison signal, the size and shape of the defect can be determined by selecting a frequency corresponding to the shape of the defect using this. It is possible to obtain defect information including the defect information. That is, by using the defect information acquisition unit (total determination unit) 26, selection of the frequency of the comparison signal and the usage ratio can be determined, and information on the size and shape of the defect can be obtained based on the selection.

  Furthermore, since the comparison signal generation unit 24 has a function of changing the frequency of the comparison signal following the line speed, the same inspection output can be obtained even if the line speed is changed.

  The present invention can be variously modified without being limited to the above embodiment. For example, in the above embodiment, the frequency of the comparison signal is 1 kHz, 2 kHz, 4 kHz, 8 kHz,..., N kHz, but is not limited to this, and is appropriately set according to the expected defect shape and size. do it. Further, the comparison signal may be one or more frequencies.

  Further, the leakage magnetic flux type inspection apparatus and the defect detection unit used therefor are not limited to the configurations shown in FIGS. Specifically, an HPF used for the purpose of canceling the stray magnetic field described in JP-A-8-5610 may be applied. That is, Japanese Patent Laid-Open No. 8-5610 discloses a frequency lower than the frequency of defects, such as a gradual change in the magnetic field generated by a change in the speed of the steel plate, a change in the thickness of the steel plate, and a shape change, in order to cancel the stray magnetic field. A high-pass filter that cuts (generally, the line speed is 300 mpm and 500 Hz or less) is described, but such a high-pass filter wastes its input range when an A / D converter is installed in front of the defect detection unit. This is effective for suppressing excessive expansion. In addition, since such a low frequency is a frequency band lower than the frequency range of the comparison signal used in the correlation calculation processing intended in the present invention, it is not mentioned in the above embodiment, but such a low frequency is detected. A high-pass filter as described above can be installed for the purpose of cutting from the signal.

  Further, in the above-described embodiment, the wavelet transform processing unit has a plurality of correlation calculation processing units, and a comparison signal having a different frequency is sent to each of the correlation calculation processing units, and the correlation calculation processing is performed in each correlation calculation processing unit. However, the present invention is not limited to this, and the correlation calculation processing may be performed by sending comparison signals having different frequencies to one correlation calculation processing unit.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Defect 10 Magnetizer 11 Yoke 12 Excitation coil 13 Detector (magnetic sensor)
DESCRIPTION OF SYMBOLS 14 Hollow roll 20 Defect detection unit 21 Previous stage amplifier 22 Low frequency noise cut part 23 Wavelet transformation process part 24 Comparison signal generation part 25 Signal process part 26 Defect information acquisition part (general judgment part)
31 Correlation calculation processing unit 32 Amplifier / full wave rectifier 33 Sensitivity adjuster 34 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 is
A wavelet transform processing unit for obtaining a correlation between the inspection signal detected by the detector and the comparison signal by correlation calculation processing;
A comparison signal generator for generating a comparison signal of one or more frequencies;
A signal processing unit that obtains a defect signal by performing signal processing of one or more output signals obtained by performing correlation calculation processing by the wavelet transform processing unit using the comparison signal of the one or more frequencies;
A leakage magnetic flux type inspection apparatus comprising: a defect information obtaining unit that obtains defect information using at least a part of the defect signals obtained by the signal processing unit.   2. The leakage magnetic flux according to claim 1, wherein the wavelet transform processing unit includes a correlation calculation processing unit that performs a correlation calculation process using a comparison signal having one or more frequencies generated by the comparison signal generation unit. Type inspection device.   The comparison signal is a signal for one period of a continuously repeating signal whose period can be defined, and a signal whose time integration value of the signal for the one period is 0 (zero). The leakage magnetic flux type inspection apparatus according to claim 1 or 2.   2. The defect information acquisition unit according to claim 1, wherein the defect information acquisition unit obtains defect size information using a difference in output signal due to a difference in defect shape when a correlation calculation process is performed using comparison signals having different frequencies. The leakage magnetic flux type inspection apparatus according to claim 1.   The said comparison signal production | generation part changes the frequency of the comparison signal sent to the said wavelet transformation process part according to the conveyance speed of the said to-be-inspected object, The any one of Claims 1-4 characterized by the above-mentioned. Leakage magnetic flux type inspection device. 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,
In addition to supplying the inspection signal detected by the detector to the wavelet transform processing unit, supplying the comparison signal of one or more frequencies generated by the comparison signal generating unit to the wavelet transform processing unit, in the wavelet transform processing unit, One or more output signals are obtained by obtaining a correlation between the inspection signal detected by the detector by the correlation calculation process and the one or more comparison signals, and the defect signal obtained by performing the signal processing of the output signal A leakage magnetic flux type inspection method characterized in that at least a part of the defect information is obtained by a defect information acquisition unit.   The leakage flux according to claim 6, wherein the wavelet transform processing unit includes a correlation calculation processing unit that performs a correlation calculation process using a comparison signal having one or more frequencies generated by the comparison signal generation unit. Expression inspection method.   The comparison signal is a signal for one period of a continuously repeating signal whose period can be defined, and a signal whose time integration value of the signal for the one period is 0 (zero). The leakage magnetic flux type inspection method according to claim 6 or 7.   The defect information acquisition unit obtains defect size information by using a difference in output signal due to a difference in defect shape when correlation calculation processing is performed using comparison signals having different frequencies. The leakage magnetic flux type inspection method according to claim 8.   The said comparison signal production | generation part changes the frequency of the comparison signal sent to the said wavelet transformation process part according to the conveyance speed of the said to-be-inspected object, The any one of Claims 6-9 characterized by the above-mentioned. Leakage magnetic flux type inspection method.

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