JP2617605B2 - Magnetic measuring device and diagnostic method for magnetic flaw detector - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a method for diagnosing a magnetic measuring device for detecting magnetic flux with a saturation type magnetic sensor, and a method for diagnosing a magnetic flaw detector incorporating the magnetic measuring device. .

[Prior Art] As a magnetic flaw detection apparatus for continuously detecting defects such as flaws and pinholes present inside or on a surface of a strip-shaped steel sheet while traveling using magnetism, as shown in FIG. 6, for example. An apparatus has been proposed (Japanese Utility Model Laid-Open No. 63-107849).

6 (a) and 6 (b) are schematic cross-sectional views of the magnetic flaw detector viewed from different directions. One end of a fixed shaft 2 penetrates a central shaft of a hollow roll 1 formed of a non-magnetic material, and the other end of the fixed shaft 2 is fixed to a frame of a building (not shown). The fixed shaft 2 is supported on the inner peripheral surfaces of both ends of the hollow roll 1 by a pair of rolling bearings 3a and 3b so as to be located at the center axis of the hollow roll 1. Therefore, the hollow roll 1 freely rotates about the fixed shaft 2 as a rotation center axis.

Within this hollow roll 1, magnetized iron cores 4 having a substantially U-shaped cross section are provided with magnetic poles 4a, 4b formed at their free ends.
Is positioned close to the inner peripheral surface of the hollow roll 1 and the support member 5
And is fixed to the fixed shaft 2 via the. The magnetized coil 6 is wound around the magnetized iron core 4. A plurality of magnetic sensors 7 are arranged in the axial direction between the magnetic poles 4a and 4b of the magnetized iron core 4. Each magnetic sensor 7 is fixed to the fixed shaft 2.

A power cable 8 for supplying an exciting current to the magnetizing coil 6 and a signal cable 9 for extracting each detection signal output from each magnetic sensor 7 are led out through the fixed shaft 2 to the outside. Therefore, the positions of the magnetized iron core 4 and each magnetic sensor 7 are fixed, and the hollow roll 1 rotates around the outer circumference of the magnetized iron core 4 and each magnetic sensor 7 with a small gap.

When the outer peripheral surface of the hollow roll 1 of the magnetic flaw detector having such a configuration is pressed with a predetermined pressure against one surface of the steel plate 10 running in the direction of arrow A, for example, the fixed shaft 2 is fixed to the frame. The hollow roll 1 rotates in the direction of the arrow.

When an exciting current is supplied to the exciting coil 6, a closed magnetic path is formed by the magnetized iron core 4 and the running steel sheet 10.
Therefore, the magnetized iron core 4 and the magnetized coil 6 constitute a magnetizer for magnetizing the steel plate 10. If the above-described defect exists inside or on the surface of the steel sheet 10, the magnetic resistance of the defect part increases, and a leakage magnetic flux is generated. This leakage magnetic flux is detected as a defect signal by the magnetic sensor 7 at the corresponding position.

Since the signal level of the detected defect signal corresponds to the size of the defect inside or on the surface of the steel sheet 10, the position of the defect in the width direction and the size of the defect existing inside or on the surface of the steel sheet 10 due to the change in the signal level of the defect signal Can be understood.

The magnitude of the defect existing inside or on the surface of such a steel sheet is determined based on the intensity of the leakage magnetic flux detected by each of the magnetic sensors 7 arranged linearly. Therefore, it is necessary to adjust the sensitivity of each magnetic sensor 7 so as to be uniform over all the magnetic sensors 7. In addition, it is necessary to match the sensitivities between the magnetic sensors 7 of other magnetic flaw detectors. In addition, it is necessary to periodically adjust for a change in sensitivity over time. Further, it is necessary to understand the correspondence between the detected amount and the absolute value of the defect size.

For this reason, conventionally, as shown in FIG. 7, a plurality of standard defect samples 12 each having a linear artificial defect 11 having a different scale are made of the same material as the steel plate 10, and each of the standard defect samples thus created is prepared. 12 was moved on the hollow roll 1 of the magnetic flaw detector, and at that time, the signal level of the defect signal of each magnetic sensor 7 caused by the artificial defect 11 was measured.

Also, instead of the standard defect sample 12, a columnar reference electromagnet 13 having a known magnetic field strength is placed on a magnetic flaw detector so as to cover all the magnetic sensors 7 at once, as shown in FIG. The reference electromagnet 13 is forcibly mounted to generate pseudo leakage magnetic flux corresponding to a defect, and the signal level of a defect signal of each magnetic sensor 7 caused by the pseudo leakage magnetic flux is measured.

[Problem to be Solved by the Invention] However, in the diagnostic method for checking the operation of each magnetic sensor 7 using the standard defect sample 12, the engraving accuracy of the artificial defect 11 formed on the standard defect sample 12 becomes a problem. That is, it is very difficult to form a uniform artificial defect 11 having a groove depth or groove width of, for example, 0.5 mm or 0.2 mm over a width direction exceeding, for example, 1 m. Even if a standard defect sample 12 of an artificial defect 11 can be manufactured with high accuracy, since it is formed of the same material as the steel plate 10, there is a possibility that corrosion or careless handling may cause a time-dependent change such as damage or deterioration. It is difficult to maintain the same defect scale over a long period of time.

Further, in the diagnostic method using the reference electromagnet 13, the magnetic flux due to the magnetic field formed by the reference electromagnet 13 needs to be uniformly distributed to all the magnetic sensors 7 under the same conditions. However, the detection signal obtained from each magnetic sensor 7 is
Since it greatly depends on the relative positional relationship between each magnetic sensor 7 and the reference electromagnet 13, the work of positioning the reference electromagnet 13 with respect to the magnetic flaw detector needs to be performed with great care. Therefore, a large amount of time and labor is required for the first diagnostic work, and there is a problem that the work efficiency of the diagnostic work of the magnetic flaw detector is greatly reduced.

In addition, although the magnetic flaw detector which detects the defect of the steel plate with the leakage magnetic flux has been described, it is not particularly limited to the defect detection.
The above-described problem also exists in a general magnetometer for measuring the magnetic field strength.

The present invention has been made in view of such circumstances, and by adding a bias current to an AC exciting current flowing through a detection coil of each saturation type magnetic sensor, the output signal of the magnetic sensor crosses the magnetic sensor. A diagnostic method of a magnetic measuring device and a magnetic measuring device capable of detecting a signal level change amount corresponding to a magnetic flux density, as a result, easily confirming the operation of a magnetic sensor in a short time and greatly improving diagnostic work efficiency It is an object of the present invention to provide a method for diagnosing a magnetic flaw detector in which is incorporated.

[Means for Solving the Problems] In order to solve the above problems, a first aspect of the present invention is to bring a magnetic sensor having a detection coil wound around a ferromagnetic core close to a measurement magnetic field to reduce an AC exciting current. The voltage is applied to the coil of the magnetic sensor through the fixed impedance to magnetize the core to the saturation region, and the positive and negative peak values of the voltage generated at both ends of the detection coil of the magnetic sensor are detected, respectively. A diagnostic method applied to a magnetic measurement device that adds a value and detects a magnetic flux density of a measured magnetic field with the added value, and adds a bias current corresponding to a standard magnetic flux density to an AC exciting current, and adds the resultant value to the added value. This is to confirm that a predetermined change occurs.

Further, the second invention is an application of the first invention to a magnetic measurement device using a plurality of magnetic sensors.

Further, a third invention is an application of the first invention described above to a magnetic flaw detector in which the magnetic measuring device is incorporated. That is, a magnetic field is crossed over the steel plate by a magnetizer, a magnetic sensor formed by winding a detection coil around a ferromagnetic core is brought close to the steel plate, and an AC exciting current is applied to the coil of the magnetic sensor via a fixed impedance. Then, the core is magnetized to the saturation region, the positive and negative peak values of the voltage generated at both ends of the detection coil of each magnetic sensor are detected, and the detected positive and negative peak values are added. This is a diagnostic method applied to a magnetic flaw detector that detects the density of leakage magnetic flux generated due to a defect of the magnetic field, and adds a bias current corresponding to a leakage magnetic flux density caused by a standard defect to an AC exciting current, and adds the result. This is to confirm that a predetermined change occurs in the value.

Further, the fourth invention is an application of the third invention to a magnetic flaw detector using a plurality of magnetic sensors.

[Operation] First, the operation principle of the magnetic measurement device to which the diagnostic method is applied will be described. That is, the AC exciting current is always passed through the detection coil constituting the magnetic sensor. The core is always magnetized to the saturation region. Therefore, this magnetic sensor is a saturation type magnetic sensor, and the amplitude of the output signal has a predetermined amplitude value determined by the saturation magnetic field. When the magnetic flux of the external magnetic field intersects the magnetic sensor, the magnetic flux is added to the saturation magnetic field, so that the output signal waveform is shifted upward or downward. Accordingly, as described above, the amount of shift (the amount of change in signal level) of the output signal waveform is determined by detecting the positive and negative peak values of the voltage generated at both ends of the detection coil of the magnetic sensor. The magnetic flux density can be evaluated by adding the values.

In such a magnetometer, when the bias current is added to the AC exciting current flowing through the detecting coil, the exciting current flowing through the detecting coil has a waveform in which the AC component is superimposed on the bias component. Therefore, since the magnetic field of the bias current is added to the magnetic field of the AC exciting current in the core, the output signal waveform of the magnetic sensor shifts to one side by the bias current. This shift amount (signal level change amount) corresponds to the signal level amount caused by the external magnetic field. Therefore, if the relationship between the magnetic field strength and the bias current value is grasped in advance, the bias current can be used as a pseudo magnetic field.

In the method of diagnosing a magnetic measurement device in which a plurality of magnetic sensors are incorporated, it is sufficient to confirm that each signal level change amount coincides with each signal level change amount value of each magnetic sensor.

Next, in a magnetic flaw detector in which the magnetic measuring device is incorporated, if a defect exists in the steel plate, a leakage magnetic flux corresponding to the size of the defect is generated. Then, the leakage magnetic flux crosses the magnetic sensor.

Therefore, if the relationship between the defect size and the bias current value is grasped, the bias current can be used as a pseudo defect. That is, by adding a bias current corresponding to a leakage magnetic flux density caused by a defect of a known scale to the AC exciting current flowing through the magnetic sensor, and confirming that a predetermined signal level change occurs in the output signal of the magnetic sensor. Good.

In a method of diagnosing a magnetic flaw detector in which a plurality of magnetic sensors are incorporated, it goes without saying that it is sufficient to confirm that a predetermined signal level change occurs in the output signal of each magnetic sensor.

Embodiment A diagnosis method according to an embodiment of the present invention will be described below with reference to the drawings. In this embodiment, a case will be described in which this diagnostic method is applied to a magnetic flaw detector in which a magnetic measuring device using a plurality of magnetic sensors is incorporated.

FIG. 1 is a block diagram showing a control circuit configuration of a magnetic flaw detector to which the diagnostic method of the embodiment is applied. Since the mechanical configuration other than the control circuit is almost the same as that of the conventional magnetic flaw detector shown in FIG. 6, detailed description of the overlapping parts will be omitted.

In the figure, reference numeral 21 denotes a rectangular wave generating circuit.
12 outputs as the AC exciting current, a high frequency rectangular wave signal e ₁ having a frequency f. The high-frequency square wave signal e ₁ output from the square wave generation circuit 21 is differentiated by the next differentiating circuit 22,
The waveform is converted into a trigger-like high-frequency pulse signal e ₂ having a period 1 / f, and is input to one end of the adder 23. This adder
A DC bias voltage Eb is input from a DC bias circuit 24 to the other end of 23. Addition circuit 23 waveform synthesizing a pulse signal e ₂ and DC bias voltage Eb. Adder output signal 24, i.e., the magnetic sensors 7 _1, 7 _2, ..., the excitation signal e ₃ for 7 _N are input to the resistor 26 and the multiplexer 27 as an impedance element after being amplified by the amplifier 25. Multiplexer 27, in synchronization with the synchronizing signal sent from the external control circuit 28, the input via a resistor 26 the excitation signal e ₃ to the magnetic sensors 7 _1, 7 _2, ..., sequentially applies to 7 _N To go.

The magnetic sensors 7 ₁ to _7-N is formed by winding a detection coil 30 on the rod-shaped core 29 as shown. One end of the detection coil 30 is grounded, and the other end is connected to the resistor 26 via a multiplexer 27.

Output signal e ₀ of the magnetic sensors 7 ₁ to _7-N that is taken out from the connection point 31 between the resistor 26 through the multiplexer 27 is input to the level variation detection circuit 32. This level fluctuation detection circuit
The DC level change amount _VL output from 32 is converted into a defect size and displayed on a display device 33 such as an oscilloscope. The display device 33 includes the external control circuit.
A synchronization signal is input from 28.

The level fluctuation detection circuit 32 is provided for each input magnetic sensor.
A positive polarity detector 34a for detecting a peak value Va equal to or higher than 0 potential of the signal waveform in the output signal e ₀ of 7 _{1 to} 7 _N ;
And the negative detector 34b for detecting a zero potential following the peak value -Vb ₀ signal waveforms, each detector 34a, the direct current output from 34b detection value Va, the level change value by adding the -Vb V _L [= Va + (− V
b)], and an adder 35 for calculating.

The external control unit 28 includes a multiplexer 27 and each detector 34.
a, by sending 34b, and the display unit 33 a synchronization signal to clear the detector 34a in synchronism with the switching timing of the magnetic sensors 7 ₁ to _7-N by the multiplexer 27, 34b of the detection value (hold value) In addition, the display position of the size of the defect corresponding to the level fluctuation amount displayed on the display device 33 is moved.

The magnetizing coil 6 constituting the magnetizer has an exciting power source 36.
Is supplied with an exciting current at all times.

Next, the operation of the diagnostic method for the magnetic flaw detector configured as described above will be described.

First, when the DC bias voltage Eb is 0 output from the DC bias circuit 24 (Eb = 0), the multiplexer 27 as a high frequency pulse signal e ₂ as it is the excitation signal e ₃ as AC excitation current output from the differentiating circuit 22 1 selected in
This is supplied to the detection coils 30 of the magnetic sensors 7. In addition,
The current value of the excitation signal e ₃ is determined by the core 29 around which the detection coil 30 is wound.
Is a value magnetized to the saturation region.

Accordingly, in a state where the defects in the steel plate 10 does not occur, since the core 29b of the magnetic sensor 7 is magnetized to a predetermined value alternately at excitation signal e ₃ of the high frequency, the connection point
Output signal e ₀ output from the 31, as shown in the left side waveform of FIG. 2, 0 potential and the positive side peak value Va and the negative electrode side peak value Vb becomes equal output signal e _0A each other around the. Therefore, the output signal e _0A is detected by the positive polarity detector 34a and the negative polarity detector 34b, and the peak values Va and −Vb are detected, and the level change amount _VL added by the adder 35 is 0. Become.

Next, when the DC bias voltage Eb (current Ib) is output from the DC bias circuit 24 (Eb ≠ 0), the high-frequency pulse signal e ₂ output from the differentiating circuit 22 is converted by the adder 23 into a DC bias voltage Eb. Is added. As a result, the signal waveform of the excitation signal e ₃ outputted from the adder 23 becomes the high frequency pulse waveform signal e ₂ is superimposed on a DC bias voltage Eb. Therefore, the exciting current flowing through the detection coil 30 of the magnetic sensor 7 selected by the multiplexer 27 has a waveform in which the high-frequency pulse signal component is superimposed on the DC component. Therefore, the DC bias voltage is applied to the AC magnetic field of the AC exciting current in the core 29.
A DC magnetic field due to the DC bias current Ib corresponding to Eb is added. Therefore, the output signal e _{0B of the} magnetic sensor 7
Are shifted in one direction as shown on the right side of FIG. Therefore, the level change amount _VL obtained by performing DC conversion by a pair of detectors and adding by the adder 35 is not 0,
It becomes a constant value [Va + (− Vb)] determined by the DC bias current value Ib (DC bias voltage Eb). Therefore, the DC bias current Ib can be indirectly measured by measuring the signal level variation _VL [= Va + (− Vb)].

The DC bias current Ib and the strength of the magnetic field generated around the magnetic sensor 7 have a one-to-one correspondence with a linear relationship. The magnetic flux density of the leakage magnetic flux corresponds to the size of the defect. Therefore, if the relationship between the defect size and the DC bias current Ib is grasped in advance, it can be considered that a defect having a scale corresponding to the current Ib exists when the DC bias current Ib is applied. Therefore,
If a predetermined signal level change amount _VL is obtained with the DC bias current Ib applied, it can be evaluated that the magnetic sensor 7 is normal.

Specifically, by driving the multiplexer 27 with a synchronizing signal from the external control circuit 28, sequentially analyzes the output signal e ₀ of the magnetic sensors 7 ₁ to _7-N in the level variation detection circuit 32, the magnetic sensors The level change amounts V _{L1 to} V _LN of 7 _{1 to} 7 _N are displayed on the display device 33 in the form of a graph or a list.

Therefore, the operator by outputting a constant DC bias current Ib by operating the DC bias power source 24, monitors the level variation V _L1 ~V _LN displayed on the display device 33, an abnormal level change amount V _{If L} exists, it can be determined that the magnetic sensor 7 has a failure or a poor gain adjustment.

FIG. 3 is a magnetic detection sensitivity characteristic diagram of the magnetic sensor 7 used in this embodiment. That is, when a magnetic flux measuring device is separately provided near the magnetic sensor 7 and the excitation power supply 36 is controlled to sequentially change the magnetic flux density of the external magnetic field generated near the magnetic poles 4a and 4b of the magnetized iron core 4. The relationship between the magnetic flux density (Gauss) measured by the magnetic flux measuring device described above and the signal level variation _VL obtained from the level variation detection circuit 32 was experimentally determined. Further, the value of the DC bias current Ib is set to Ib = 0,5 mA, 10 m
The change of the output voltage (signal level change amount V _L ) when changing to four levels of A and 20 mA was examined.

As is clear from the experimental results, the DC bias current
When Ib is changed, the output voltage characteristic moves up and down in parallel. In other words, it can be understood that changing the DC bias current Ib is equivalent to changing the applied magnetic field.

Therefore, it is determined whether or not each magnetic sensor 7 is normal.
By driving the DC bias circuit 24, it can be immediately grasped from the value of the output signal which can be regarded as having a defect in each magnetic sensor 7. Thus, the DC bias current Ib
Since it can be considered that the leakage magnetic flux is generated due to the defect, the use of the standard defect sample or the standard electromagnet is not necessary. Therefore, compared to the conventional method,
The work efficiency of the diagnostic work can be significantly increased.

In addition, since signal processing is performed by regarding the electric signal as a defect, diagnostic accuracy can be significantly improved.

Further, since the operator does not need to manually attach or detach the standard defect sample or the standard electromagnet by manual operation, the above-described diagnostic processing can be automatically executed using a microcomputer or the like. Therefore, diagnosis can be performed more easily.

FIG. 4 is a block diagram showing a main part of a magnetic flaw detector to which a diagnostic method according to another embodiment of the present invention is applied. The same parts as those in FIG. 1 are denoted by the same reference numerals, and the description of the overlapping parts will be omitted.

In this embodiment, the DC bias circuit 24 shown in FIG.
In the place of, it uses a low-frequency oscillation circuit 24a which outputs an AC bias signal e _b having a low frequency sinusoidal. That is, in this embodiment, the pulse signal e ₂ output from the differentiating circuit 22 in the adder 23 is replaced by
e _b is synthesized. Therefore, the output signal e ₀ output from the magnetic sensors 7, as shown on the right side in FIG. 5, an amplitude-modulated waveform at the AC bias signal e _b of the waveform. As a result, the output signal of the level fluctuation detection circuit 32 becomes a sine wave signal having the level change amount _VL as an amplitude as shown in the figure. Therefore, the quality of the magnetic sensor 7 can be evaluated by evaluating the level change amount _VL .
Therefore, substantially the same effects as in the previous embodiment can be obtained.

Further, in this embodiment, as shown in FIG. 5, since the level change amount _VL is detected by the amplitude value of the output signal of the level change detection circuit 32, the output signal e due to the ambient temperature change or the like is output. _{It is possible} to remove the influence of zero point fluctuation or the like. Therefore, diagnosis accuracy can be further improved without providing a separate temperature compensation circuit.

Note that the diagnostic method of the present invention is not limited to the above embodiments. In the embodiment, the case where the present invention is applied to a magnetic flaw detector incorporating a magnetic measuring device has been described. However, it is needless to say that the present invention can also be applied to a general magnetic measuring device which measures a normal magnetic field strength other than the leakage magnetic flux.

Furthermore, a high-frequency excitation signal was used as an AC excitation current applied to a detection coil of a magnetic sensor of a magnetic measurement device and a magnetic flaw detection device to which the diagnosis method of the embodiment can be applied. For example, when a DC magnetic field is measured, When the moving speed of the steel sheet is low, the frequency of the generated leakage magnetic flux waveform is low, so that sufficiently high diagnostic accuracy can be obtained even if a low-frequency exciting current is used as the AC exciting current.

[Effects of the Invention] As described above, according to the diagnostic method of the present invention, the magnetic flux is artificially applied to the magnetic sensor by adding the bias current to the AC exciting current flowing through the detection coil of each saturation type magnetic sensor. Can be applied. Therefore, the amount of change in the signal level corresponding to the magnetic flux density can be detected from the output signal of the magnetic sensor, and as a result, the operation of the magnetic sensor can be easily confirmed in a short time, and the diagnostic work efficiency can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a schematic configuration of a magnetic flaw detector to which a diagnostic method according to an embodiment of the present invention is applied, FIG. 2 is a time chart showing an operation of the method of the embodiment, and FIG. FIG. 4 is a block diagram showing an essential part of a magnetic flaw detector to which a diagnosis method according to another embodiment of the present invention is applied, and FIG. 5 is a block diagram showing the detection method of the embodiment. FIG. 6 is a time chart showing the operation, FIG. 6 is a sectional view showing the configuration of a general magnetic flaw detector, and FIG. 7 is a view showing a conventional diagnostic method. 4 ... magnetized iron core, 6 ... magnetized coil, 7 ... magnetic sensor, 10 ... steel plate, 21 ... rectangular wave generation circuit, 22 ... differential circuit, 23 ... adder, 24 ... DC bias circuit, 24a ……
Low frequency oscillator, 26 ... resistor, 27 ... multiplexer, 29
... core, 30 detection coil, 32 level fluctuation detection circuit, 33 display device.

Claims (4)

(57) [Claims] A magnetic sensor comprising a detection coil wound around a ferromagnetic core is brought close to a measurement magnetic field, and an AC exciting current is applied to the coil of the magnetic sensor via a fixed impedance to saturate the core. The positive and negative peak values of the voltage generated at both ends of the detection coil of the magnetic sensor are detected, and the detected positive and negative peak values are added. A diagnostic method applied to a magnetic measuring device for detecting a bias current corresponding to a standard magnetic flux density to the AC exciting current, and confirming that a predetermined change occurs in the added value. Diagnostic method for a magnetometer. 2. A plurality of magnetic sensors each having a detection coil wound around a ferromagnetic core, each approaching a measurement magnetic field, and applying an AC exciting current to the coils of each of the magnetic sensors via a fixed impedance. The core is magnetized to the saturation region, positive and negative peak values of the voltage generated at both ends of the detection coil of each of the magnetic sensors are respectively detected, and the detected positive and negative peak values are added. A diagnostic method applied to a magnetic measurement device that detects each magnetic flux density of the measurement magnetic field, wherein a bias current corresponding to a standard magnetic flux density is added to the AC exciting current, and a predetermined change is added to each of the added values. A method for diagnosing a magnetic measurement device, comprising: confirming that a magnetic field is generated. 3. A magnetic sensor crossing a steel sheet by a magnetizer, a magnetic sensor having a detection coil wound around a ferromagnetic core is brought close to the steel sheet, and an AC exciting current is applied to the magnetic field via a fixed impedance. The core is magnetized to the saturation region by applying to the coil of the sensor, and the positive and negative peak values of the voltage generated at both ends of the detection coil of each magnetic sensor are respectively detected, and the detected positive and negative peak values are added. A diagnostic method applied to a magnetic flaw detector that detects a density of a leakage magnetic flux caused by a defect of the steel sheet with the added value, wherein the AC exciting current corresponds to a leakage magnetic flux density caused by a standard defect. A method for diagnosing a magnetic flaw detection apparatus, comprising: adding a bias current obtained as described above; and confirming that a predetermined change occurs in the added value. 4. A magnetic field is crossed over a steel plate by a magnetizer, a plurality of magnetic sensors having a detection coil wound around a ferromagnetic core are brought close to the steel plate, and an AC exciting current is passed through a fixed impedance. The cores are applied to the coils of the magnetic sensors to magnetize the core to a saturation region, and the positive and negative peak values of the voltage generated at both ends of the detection coils of the magnetic sensors are respectively detected. Add the values,
A diagnostic method applied to a magnetic flaw detector which detects a density of each leakage magnetic flux caused by a defect of the steel sheet with each of the added values, wherein the AC exciting current corresponds to a leakage magnetic flux density caused by a standard defect. A method for diagnosing a magnetic flaw detection apparatus, comprising: confirming that predetermined changes occur in the respective added values by adding the bias currents.