JPH07113626B2 - Steel plate flaw detection device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a flaw detection device for a steel sheet, and more particularly to improvement in detection accuracy of minute flaws.

[Prior Art] In order to detect a flaw generated in, for example, a steel plate or a steel pipe, an ultrasonic flaw detection method, an eddy current flaw detection method, a leakage magnetic flux flaw detection method or the like has been conventionally used.

Among these flaw detection methods, the magnetic flux leakage flaw detection method is conventionally used in every place because it can measure even in a bad environment compared to other methods.

FIG. 9 shows the structure of a conventional flaw detection apparatus for steel plates using the magnetic flux leakage flaw detection method. As shown in the figure, the flaw detection device includes a non-magnetic hollow roll 2 that rotates as the steel plate 1 moves,
The hollow roll 2 is provided with a coil 3 wound around a magnetized core 4 for magnetizing the steel plate 1, and a sensor 5 provided between the magnetized cores 4 and configured to detect a leakage magnetic flux, such as a Hall element. .

When detecting a flaw in the steel sheet 1, a direct current is passed through the coil 3 to generate a magnetic field. Then, the moving steel plate 1 is magnetized by the magnetizing core 4 via the roll 2. When the magnetized steel plate 1 has a flaw, a leakage magnetic flux is generated in the flaw portion. This leakage magnetic flux is detected by the sensor 5, and the flaw existing on the steel plate 1 is detected by the magnitude of the output signal from the sensor 5.

In this case, since the output signal of the sensor 5 contains a noise signal, in order to remove the influence of this noise, a threshold value of a certain level is set in advance, and the output signal of the sensor 5 exceeding this threshold value causes a flaw. Is detected to prevent false detection of defects.

[Problems to be Solved by the Invention] However, the above noise is caused by differences in steel types, rolling conditions, etc.
Alternatively, it has various noise levels due to the influence of other magnetic substances at the measurement location.

In order to remove these various noises and prevent false detection of defects,
Conventionally, the highest level of noise has been set as the threshold value. For this reason, although major flaws can be detected, minute flaws indicated by a signal below the threshold value output from the sensor 5 cannot be detected, and there is a disadvantage that the flaw measurement accuracy decreases.

On the other hand, the higher the sensitivity of the sensor 5 such as the magnetic sensor based on the principle disclosed in Japanese Patent Application Laid-Open No. 1-308982, the smaller the flaw (defect) can be detected, but various noises can be detected. The demand for noise removal and high-accuracy detection of defects increases as the tendency to pick up defects increases.

This invention has been made to solve the above disadvantages, and in particular, it eliminates the effects of changes in leakage magnetic flux and noise that occur almost simultaneously in the width direction of the steel sheet due to changes in rolling conditions of the steel sheet rather than defects, It is an object of the present invention to obtain a flaw detection device for a steel sheet, which is capable of detecting minute flaws with high sensitivity.

[Means for Solving the Problems] A flaw detection device for a steel sheet according to the present invention includes a plurality of magnetic sensors, and at least two sets of peak value detection means for receiving the outputs of the magnetic sensors to detect peak values. It has a calculating means for calculating the difference between the peak values and a comparing means for comparing the calculated result with a preset threshold value.

The magnetic sensor includes a coil wound around a ferromagnetic core, an AC power supply unit that supplies AC power of a constant frequency and a constant voltage to the coil through a fixed impedance, and a positive side of a voltage output from both ends of the coil. It comprises a DC voltage detecting means for detecting the voltage and the negative voltage, and a magnetic detecting means for calculating the difference between the positive voltage and the negative voltage detected by the DC voltage detecting means to detect the strength of the magnetic field. The coils wound around the ferromagnetic cores of the plurality of magnetic sensors are arranged along the width direction of the continuously moving steel plate.

The output signals from the plurality of magnetic sensors are divided into at least two groups and sent to the peak value detecting means, and the peak value detecting means detects the peak value of the detection signal for each magnetic sensor group.

The calculating means calculates the difference between the detected peak values of the plurality of groups and sends it to the comparing means.

Further, by supplying a pulse current added with a DC bias to the coil of each magnetic sensor, the span of measurement of the leakage magnetic flux can be expanded.

Further, the operating point of the magnetic sensor can be automatically compensated by variably controlling the DC bias added to the pulse current according to the output signal of each magnetic sensor.

[Operation] The magnetic sensor according to the present invention is a ferromagnetic core that is wound around a ferromagnetic core to supply AC power of a constant frequency and a constant voltage to a coil, and that exhibits a hysteresis characteristic by a magnetic field generated by an AC current flowing through the coil. The impedance of the coil is changed according to the magnetic permeability of, and the output voltage across the coil is changed to a substantially rectangular waveform with positive and negative symmetry. The coil is arranged close to the steel plate, and the leakage magnetic flux from the steel plate changes the positive and negative voltage values of the output voltage across the coil. The changing positive side voltage and negative side voltage are respectively detected, the difference between the detected positive side voltage and negative side voltage is calculated, and the leakage magnetic flux of the steel sheet is detected as an electric signal. Thereby, the leakage magnetic flux can be detected with extremely high sensitivity.

A plurality of these magnetic sensors are used, the coils of each magnetic sensor are arranged along the width direction of the steel plate that moves continuously, and the output signals from each magnetic sensor are divided into at least two groups, and each magnetic sensor group The peak value of the detection signal of is detected. By calculating the difference in peak value for each magnetic sensor group, noise generated along the width direction of the steel sheet due to rolling conditions and the like is removed. Then, the flaw of the steel plate is detected by the detection signal from which the noise component is removed.

Further, by supplying a pulse current to which a DC bias is added to the coil of the magnetic sensor, the measurement span of the leakage magnetic flux can be expanded.

Further, the DC bias to be added to the pulse current is variably controlled according to the magnetic detection signal from the magnetic detection means, so that the operating point of the magnetic sensor is automatically compensated, and even if the measurement condition changes, it is always good. The measurement span can be secured.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention.
As shown in the figure, the flaw detection device includes a plurality of magnetic sensors 5,
Two peak value detecting means for inputting magnetic detection signals from a plurality of magnetic sensors 5 and detecting peak values of the magnetic detection signals
6a, 6b, the calculating means 7 for obtaining the difference between the peak values detected by the peak value detecting means 6a, 6b, and the difference between the peak value obtained by the calculating means 7 and the threshold setting means 9. Comparing means 8 and display means for comparing a threshold value and detecting flaws in the steel sheet
Have 10 and.

The magnetic sensor 5 includes a rod-shaped ferromagnetic core 11 and a ferromagnetic core.
A coil 12 wound around 11, an AC power supply means 13 for supplying a constant frequency and a constant voltage AC power to the coil 12 via a fixed impedance 14, a DC voltage detection means 15 connected to both ends of the coil 12, and a magnetic field. And a detection means 16. The DC voltage detecting means 15 has a positive polarity detector and a negative polarity detector, and detects the positive side voltage and the negative side voltage of the output voltage of the coil 12, respectively. The magnetic detection means 16 receives the output of the DC voltage detection means 15 and obtains the difference between the positive side voltage and the negative side voltage of the coil 12,
Detects magnetic field strength.

When the flaw detection device configured as described above is used to detect a flaw in the steel sheet 1, for example, as shown in FIG.
The coil 12 wound around the ferromagnetic core 11 of the magnetic sensor 5.
Is provided in a non-magnetic hollow roll 2 made of, for example, austenitic stainless steel or ceramics, which rotates with the movement of the steel sheet 1, and a coil 3 for magnetizing the steel sheet 1 is wound between the magnetized cores 4. Install. The coil 12 wound around the ferromagnetic core 11 of the magnetic sensor 5 is a hollow roll having a magnetized core 4 around which a coil 3 for magnetizing the steel plate 1 is wound, as shown in FIG. 2 (b). 2 and steel plate 1
Hollow rolls having the same diameter as the hollow roll 2 sandwiched between
It may be installed in 2a.

In explaining the operation of the flaw detection device configured as described above, first, the principle of the magnetic sensor 5 will be described with reference to the voltage waveform diagram of FIG. 3 and the magnetization characteristic diagram of the ferromagnetic core of FIG. To do.

When an alternating voltage E having a constant frequency and a constant voltage is supplied to the coil 12 as shown in the voltage waveform diagram of FIG.
The voltage E ₀ generated at both ends of the coil is determined by the resistance value R of the fixed impedance 14 and the impedance Z _S of the coil 12.

The impedance of the coil 12 changes in proportion to the magnetic permeability of the ferromagnetic core 11. That is, when an alternating current is passed through the coil 12 without applying an external magnetic field, the magnetic flux of the coil 12 magnetizes the ferromagnetic core 11. The magnetic permeability of the ferromagnetic core 11 is not constant but changes depending on the strength of the magnetic field, and the magnetization curve exhibits a hysteresis characteristic as shown in FIG. In FIG. 4, B is the magnetic flux density, n is the number of turns of the coil 2, and i is
Is the coil current.

Therefore, the output voltage E ₀ generated at both ends of the coil 12 has a substantially rectangular waveform with positive and negative symmetry, as shown in FIG. 3 (b). And in the state where no external magnetic field is applied, the positive voltage
V ₁ and the negative side voltage V ₂ become equal.

When an external magnetic field is applied to the coil 12 in this state, the ferromagnetic core
The magnetic flux crossing 11 is a composite magnetic flux of the magnetic flux generated in the coil 12 and the magnetic flux of the external magnetic field. As a result, the waveform generated at both ends of the coil 12 has a difference between the positive voltage V ₁ and the negative voltage V ₂ as shown in FIG. 3 (c). The external magnetic field can be indirectly measured by comparing the positive side voltage V ₁ and the negative side voltage V ₂ of the output voltage generated at both ends of the coil 12 and obtaining the difference.

Then, when the magnetic flux density of the external magnetic field is measured by the magnetic sensor 5, as shown in FIG. 5, a high output voltage of 0 to 500 mV can be obtained for a minute magnetic flux density of 0 to 10 Gauss. The magnetic flux density can be measured with a much higher detection sensitivity than the output voltage of the Hall element, which is several millivolts.

Next, the operation of the flaw detection device using the magnetic sensor 5 based on the above principle will be described.

For example, as shown in FIG. 2 (a), a ferromagnetic core is provided between the magnetized cores 4 of the hollow roll 2 provided in the width direction in the vicinity of the surface of the steel plate 1 that moves continuously. A plurality of coils 12 wound around 11 are installed. Coil in this state
12 through the fixed impedance 14 from the AC power supply means 13 to a constant frequency as shown in the waveform diagram of FIG.
Supply constant voltage AC power. The coil 12 generates a magnetic field by the supplied alternating current, and its magnetic flux causes the magnetic core 11 to move.
Is magnetized until the magnetic flux density B becomes saturated as shown in FIG. The magnetic field strength of this ferromagnetic core 11 is the coil
The hysteresis characteristic is repeated by the alternating current flowing in 12,
The impedance of the coil 12 is changed at the same frequency as the frequency of the alternating current. On the other hand, a leakage magnetic flux of the magnetic flux generated in the magnetized core 4 provided in the hollow roll 2 and passing through the steel plate 1 is applied to the coil 12. The leakage magnetic flux from the steel plate 1 changes depending on the flaw and the size of the flaw of the steel sheet 1. Therefore, the third
As shown in FIG. 7C, a difference occurs between the positive side voltage V ₁ and the negative side voltage V ₂ of the output voltage on both sides of the coil 2.

Therefore, the DC voltage detecting means 15 detects the positive side voltage V ₁ and the negative side voltage V ₂ of the coil 12 and sends them to the magnetic detecting means 16. The magnetic detection means 16 calculates the voltage difference between the positive side voltage V ₁ and the negative side voltage V ₂ sent to obtain a magnetic detection signal proportional to the leakage magnetic flux of the steel plate 1.

The magnetic detection signals of the plurality of magnetic sensors 5 thus detected are divided into, for example, two groups on the left and right centering on the widthwise central portion of the steel plate 1 and sent to the peak value detecting means 6a, 6b for each group. . The peak value detecting means 6a, 6b respectively detect the peak values of the magnetic detection signals divided into the left and right two groups for a certain period of time or for each certain length of the steel sheet 1, and send them to the calculating means 7.

The calculation means 7 calculates the difference between the peak values of the two left and right groups sent. In this way, by obtaining the difference between the peak values of the left and right two groups divided in the width direction of the steel sheet 1, changes in the leakage magnetic flux and noise components that occur almost simultaneously along the width direction of the steel sheet depending on the steel type, rolling conditions, etc. The effect of can be removed.

The signal from which the noise component has been removed is sent from the calculating means 7 to the comparing means 8 and compared with a threshold value set in advance by the threshold value setting means 9, and the signal exceeding the threshold value is flawed on the steel sheet 1. It is displayed on the display means 10 as a detection signal.

In this way, the detection signal of the leakage magnetic flux from the steel sheet 1 detected by the plurality of magnetic sensors 5 with high detection sensitivity is divided into two groups on the left and right in the width direction of the steel sheet 1, and the peaks of the divided detection signals of the leakage magnetic flux are divided. By calculating the difference between the values, it is possible to remove the noise component generated along the width direction of the steel plate,
It is possible to detect small flaws as well as major flaws, and it is possible to significantly improve the flaw detection accuracy of the steel sheet.

Furthermore, when measuring the magnetic flux leakage, the temperature fluctuates and the coil 12
Even if the winding resistance and the magnetic permeability of the ferromagnetic core 11 change, the impedance of the coil 12 due to this change changes equally with the positive and negative polarities of the magnetizing current, so they compensate each other.
Drift does not occur in the difference between the positive side voltage and the negative side voltage of the coil 11 due to the temperature change, and the leakage magnetic flux can be accurately measured without the influence of the temperature change.

In addition, by magnetizing the ferromagnetic core 11 until it is sufficiently saturated with the current flowing through the coil 12, the output voltage generated at both ends of the coil 12 is clipped to a constant value, and the output voltage at both ends of the coil depends only on the strength of the external magnetic field. Since the amplitude and phase of the positive electrode and the negative electrode are changed, the AC power supply means 13
Even if the output voltage and the resistance value of the fixed impedance 14 change a little, the detection sensitivity does not change, and the leakage magnetic flux can be measured with high sensitivity.

In the above embodiment, the case where the detection signals of the leakage magnetic flux of the plurality of magnetic sensors 5 are divided into the left and right two groups in the width direction of the steel plate 1 to obtain the peak value has been described. Even if the detection signal of the leakage magnetic flux is divided into alternate groups or divided into two or more groups, the same operation as in the above embodiment can be achieved.

In the above embodiment, the AC power supply means is provided to the coil 12 wound around the ferromagnetic cores 11 of the plurality of magnetic sensors 5.
The case of supplying continuous AC power as shown in FIG. 3 (a) from 13 has been described, but the AC power supply means 13
Pulse current is output from the pulse current, and the DC bias is added to this pulse current to change the reference point of the magnetization curve of the ferromagnetic core 11 to shift and move the measurement span for detecting the magnetic flux density to detect a flaw. The accuracy can be further improved.

FIG. 6 shows a magnetic sensor 5 of an embodiment in the case of supplying a pulse current with a DC bias applied to the coil 12. As shown in the figure, a high-frequency voltage pulse current is supplied from the AC power supply means 13 to the adder 21, and a DC bias is supplied from the DC power supply 22 to the adder 21. In the adder 21, a DC bias is added to the high frequency voltage to generate a combined voltage, and the combined voltage is amplified by the power amplifier 23 and supplied to the coil 12 via the fixed impedance 14 composed of a resistor.

In this example, the value of the DC bias current from the DC power supply 22 was changed to 0 mA, 50 mA, 100 mA, 150 mA, and 200 mA, and the output voltage of the coil 12 was measured with respect to the change in the magnetic flux density of the external magnetic field. The detection sensitivity characteristics shown in the figure were obtained.

As can be seen from FIG. 7, when the DC bias current is added to the pulse current, the output voltage can obtain a very good linear characteristic even if the value of the DC bias current is changed. The range where the characteristics are obtained is about 0 to 40 Gauss, and the DC bias current is
The measurement span of the magnetic flux density can be moved compared to 0 to 20 Gauss at 0 mA, and the measurement area can be easily changed.

Further, the DC bias current from the DC power supply 22 may be made variable according to the detection signal S output from the magnetic detection means 16 so that a good measurement span can always be obtained even if the measurement conditions change.

FIG. 8 is a block diagram showing an embodiment in which the DC bias current is varied. As shown in the figure, the magnetic detection means 16
Detection signal S of the differential amplifier through the low-pass filter 24
Send to 25. The differential amplifier 25 is a detection signal S and a reference voltage generator.
The reference voltage from 26 is compared and the difference voltage is compared with the DC power supply 22
Send to. The DC power supply 22 changes the DC bias current by this difference voltage and sends it to the adder 21.

In this way, the magnetic detection means 16 detects the operating point of the magnetic sensor 5 by the detection signal S, and the differential voltage from the reference voltage of the reference voltage generator 26 is obtained by the differential amplifier 26 to obtain the DC bias from the DC power supply 22. By the control, the detection signal S of the magnetic detection means 16 in the state where there is no defect is automatically compensated so that it is always 0V.

By automatically compensating the operating point in the center of the measurement span of the magnetic sensor 5 as described above, a good measurement span can always be ensured even if the measurement conditions change, and the flaw detection performance can be further improved.

As described above, the magnetic sensor according to the present invention is
AC power of constant frequency and voltage is supplied to the coil wound around the ferromagnetic core, and the magnetic field generated by the alternating current flowing in this coil changes the impedance of the coil by the permeability of the ferromagnetic core that exhibits hysteresis characteristics. , The output voltage across the coil is changed into a substantially rectangular wave with positive and negative symmetry. The coil is arranged close to the steel plate, and the leakage magnetic flux from the steel plate changes the positive and negative voltage values of the output voltage across the coil. The changing positive side voltage and the negative side voltage are detected respectively, the difference between the detected positive side voltage and the negative side voltage is calculated, and the leakage flux of the steel sheet is detected as an electric signal. It can be detected with sensitivity.

A plurality of these magnetic sensors are used, the coils of each magnetic sensor are arranged along the width direction of the steel plate that moves continuously, and the output signals from each magnetic sensor are divided into at least two groups, and each magnetic sensor group By detecting the peak value of the detection signal and calculating the difference between the peak values for each magnetic sensor group, the noise generated along the width direction of the steel sheet due to the rolling conditions etc. is removed, so the noise component is removed. The microscopic flaw of the steel plate can be detected by the detected signal, and the flaw detection accuracy of the steel plate can be improved.

Further, by supplying a pulse current added with a DC bias to the coil of the magnetic sensor, the measurement span of the leakage magnetic flux can be expanded and the flaw detection accuracy can be further improved.

In addition, the DC bias added to the pulse current is variably controlled according to the magnetic detection signal from the magnetic detection means, so that the operating point of the magnetic sensor is automatically compensated, and even if the measurement condition changes, it is always good. The measurement span can be secured, and the flaw detection accuracy can be further improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 (a) and 2 (b) are layout diagrams of the coils of the above embodiments, and FIGS. 3 and 4 are of the magnetic sensor of the above embodiments. Showing the principle,
3 (a), (b), and (c) are voltage waveform diagrams,
FIG. 4 is a magnetization characteristic diagram of the ferromagnetic core, FIG. 5 is an output characteristic diagram of the magnetic sensor of the above embodiment, FIG. 6 is a block diagram showing another embodiment, and FIG. 7 is shown in FIG. FIG. 8 is a block diagram showing the output characteristic of the embodiment, FIG. 8 is a block diagram showing the third embodiment, and FIG. 9 is a sectional view showing the conventional example. 1 ... steel plate, 5 ... magnetic sensor, 6a, 6b ... peak value detecting means, 7 ... computing means, 8 ... comparing means, 11 ... ferromagnetic material core, 12 ... coil, 13 ... AC Power supply means, 14
...... Fixed impedance, 15 ...... DC voltage detection means, 16
...... Magnetic detection means, 21 …… Adder, 22 …… DC power supply, 25
...... Differential amplifier, 26 …… Reference voltage generator.

Claims (3)

[Claims] 1. A coil wound around a ferromagnetic core, AC power supply means for supplying AC power having a constant frequency and a constant voltage to the coil through a fixed impedance, and a voltage output from both ends of the coil. DC voltage detecting means for detecting the positive side voltage and negative side voltage of the magnetic field, and magnetic detecting means for calculating the difference between the positive side voltage and the negative side voltage detected by the DC voltage detecting means to detect the strength of the magnetic field. A flaw detection device for a steel plate having a plurality of magnetic sensors, wherein the coils wound around the ferromagnetic cores of the plurality of magnetic sensors are divided into a plurality of groups along the width direction of the continuously moving steel plate. And a peak value detecting means for detecting the peak value of the detection signal for each magnetic sensor group by inputting the output signals from the plurality of magnetic sensors into at least two groups, and the peak value detecting means. Multiple detected A calculating means for calculating a difference between the peak value of flaw detection device steel sheet characterized by comprising a comparison means for comparing a preset threshold with the results calculated by said calculating means. 2. The flaw detection apparatus for a steel sheet according to claim 1, wherein the AC power supply means of the magnetic sensor outputs a pulse current, and a DC bias adding means for adding a DC bias to the pulse current is provided. 3. A flaw detection device for a steel sheet according to claim 2, further comprising control means for variably controlling a DC bias added by said DC bias adding means in accordance with a magnetic detection signal from said magnetic detection means.