KR100285641B1 - Method for processing signals of magnetic sensor for detecting defects on steel sheet - Google Patents

Abstract

PURPOSE: A method is provided which accurately detects defects on a steel sheet and enables finding of absolute positions of the defects on the steel sheet by extracting signals due to the defects from signals due to other physical properties of the steel sheet, thereby increasing a sensitivity on defects of the sensor. CONSTITUTION: The method for processing signals of a magnetic sensor for detecting defects on a steel sheet comprises the steps of detecting defects on a magnetized moving steel sheet with a sensor(11); differentiating the space by inputting signals outputted from the sensor(11) into a differentiator(12)(S2); taking rapid variation components only in leakage flux for the space by inputting the differentiated signals into a high pass filter(13)(S3); and removing the signal components through a low pass filter(14) except for required components taken through the high pass filter(13)(S4), wherein a flux gate sensor using a space differentiation is used as the sensor, and the method further comprises the step of measuring a transfer speed of the steel sheet with an encoder, and setting frequency waves at the differentiator(12), the high pass filter(13) and the low pass filter(14) accordingly.

Description

Signal Processing Method of Magnetic Sensor for Scratches in Steel Sheets

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal processing method of a magnetic sensor for defect detection of a running steel sheet.

In order to detect flaws in the inner or surface of the thin steel sheet, a method of magnetizing the thin steel sheet and detecting leakage magnetic flux present in the magnetized thin steel sheet is used (for example, Korean Patent Publication No. 96-53641). .

Magnetic sensors can be classified into various types according to the area of the magnetic field to be measured, and also classified into various materials according to the materials used in the sensor configuration. Such magnetic sensors are steel sheets of the Pohang Institute of Industrial Science and Technology. This is described in the 1994 Research Report (93A123) on the inclusion of on-line flaw detection systems.

In general, in order to measure minute magnetic field changes of several tens of nT, a superconducting quantum interference device (SQUID), a search coil magnetometer, and a magnetometer by optical methods are used. Fluxgate sensors and magnetoresistance sensors are used for the measurement. In addition, when measuring a large magnetic field of about several tens of mT, a sensor such as a hall sensor, a magnetic diode (magnetodiode), a magnetic transistor (magnetotransistor) is used. Each of the above sensors has its own operating frequency bands. For example, Hall sensors have a low frequency of several hundred Hz, which makes them unsuitable for detecting fast magnetic signals above several KHz. Therefore, in order to detect flaws such as inclusions, roll marks, dull marks, dents, stickers, buckles, etc. present in the steel sheet during the production process, high-speed fine Sensors capable of detecting magnetic signals should be used. In addition, when detecting a change in the magnetic field using such a sensor, since it detects all the changes in the magnetic field that are not related to the flaw, which are caused by the difference in thickness, magnetic property, and tension of the thin steel sheet, The method of extracting only signals by means of detection is one of the important techniques for detecting flaws on steel sheets.

The signal processing method of magnetic sensor for flaw detection of thin steel sheet can reduce the claim rate and improve the added value of the product by evaluating and managing the quality of the product and providing the high quality product to the consumer. Big.

One conventional method using a peak-detection fluxgate to detect flaws in sheet steel (see EPO544911A1, Method and Device for Detecting Magnetic flux, NKK) is caused by factors other than flaw in sheet steel. By focusing on the distribution area of the magnetic field being several times wider than that of the defect, only the signal by the defect is extracted by subtracting the magnetic field measured by the two sensors. However, in this case, since the information of the signal due to the defect, that is, the direction of the magnetic field is forgotten, it is not a suitable method for the accurate flaw detection classification of the defect. In addition, defects cannot be detected when simultaneous defects pass through the two sensors, and absolute position detection of the defects is not possible.

The present invention has been studied and developed in this background. The flaw present in the thin steel sheet is extracted from the signal due to the different physical properties of the steel sheet to increase the sensitivity (S / N ratio) of the flaw of the sensor to accurately detect the flaw. In addition, the object of the present invention is to provide a method capable of determining the absolute position of the flaw existing in the steel sheet.

1 is a schematic structural diagram of a magnetizer for magnetizing a flaw object,

2 is a shape of the leakage magnetic flux generated in the object,

3 is a schematic process diagram of a signal processing method according to the present invention;

4 is a shape diagram of a signal by the method of the present invention.

<Explanation of symbols for main parts of drawing>

1: steel sheet 2: iron core

3: coil 4: magnetization power supply device

7: Hm 11: Sensor

12: Differentiator 13: High pass filter

14: low pass filter

The above object is to detect a flaw on a steel sheet with a sensor according to the present invention, the step of differentiating by inputting the output signal from the sensor to the differentiator, and the differential signal high pass filter (HPF) Taking only the signal components corresponding to the sudden change in the leakage magnetic flux by inputting to and removing the signal components other than the necessary components taken through the high pass filter through a low pass filter (LPF). It is achieved by providing a signal processing method of a magnetic sensor for flaw detection of sheet steel.

Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

In order to find the defect present on the steel sheet by magnetic method, it is necessary to magnetize the steel sheet 1 with an electromagnet made by winding the coil 3 around the ferrite core 2 as illustrated in FIG. 1. Do. Permanent magnets may be used to magnetize thin steel sheets, but the use of permanent magnets is not suitable because the magnetization strengths should be varied according to the thickness, physical properties, etc. of the steel sheet. The electromagnet is a method suitable for magnetization of a thin steel sheet to which the present invention is applied because the magnetization strength can be adjusted by the magnetizing current power supply 4 of FIG. 1. In the magnetization of the thin steel sheet through such a magnetizing means, it generally provides a current for magnetization such that the thin steel sheet is almost saturated. Since the thin steel plate made of magnetic material has nonlinear magnetic properties, the magnetization degree according to the magnetization current becomes nonlinear and is saturated at a constant magnetization current. When the flaws exist in the thin steel sheet, the shape of the leaked magnetic flux generated by the flaws is shown in FIG. 2. For example, the leaked magnetic fluxes 5 and 6 generated by the flaws 7 on the thin steel sheet 1 are the flaws ( It extends the signal to a fairly large area compared to the size of 7). For example, if a 0.3 mm diameter hole is made in a steel sheet, the area of the leakage flux signal on the sensor is extended to about 14 mm at a lift-off value of 2 mm, which is the distance between the sensor and the steel sheet. In the case of the through-hole in mm is about 12mm. On the other hand, the signal size in the leaker due to the combination of the two cases is about three times larger than that of the 0.3 mm hole than that of the 0.1 mm hole.

In order to measure the leakage magnetic flux, it is necessary to measure the magnetic field of the component perpendicular to the steel sheet or the horizontal component. In general, the vertical magnetic field is advantageous in terms of sensor placement and signal processing. In Fig. 2, the non-linear bending of the vertical component leakage magnetic flux at the center of the flaw 7 is a case where the lift-off (distance between the steel sheet and the sensor) is small, and when the lift-off is large, it is almost linear.

According to the method of the present invention, as shown in FIG. 3, the sensor 11 detects a flaw on the magnetized moving steel sheet 1, and outputs the output signal from the sensor 11 to the differentiator 12. Inputting the derivative to the space (S2), inputting the differentiated signal to the high pass filter 13 to take only the sudden change component of the leakage flux with respect to the space (S3), and the highpass filter 13 It is characterized in that it comprises a step (S4) of removing the signal components other than the necessary components taken through the low pass filter (14).

The sensor 11 used in the present invention is a flux gate sensor using a space differential, and the output signal from the sensor 11 has a shape in which signals of various frequencies are loaded as shown in FIG. Taking out only the signal component due to a defect is difficult because of the signal component due to the physical properties of the steel sheet, not due to the defect.

However, since the signal due to the defect has a larger spatial rate of change than the signal due to the change in the magnetic properties of the steel sheet itself, the differential signal is differentiated in the signal space using the differentiator 12, so that the signal due to the defect is well obtained as shown in FIG. Can be distinguished. However, if only the differential of the signal space is performed, it is difficult to obtain information such as the polarity or the interval of the peak to peak space with respect to the defect signal, which makes it difficult to distinguish the types of defects.

Therefore, if the signal is processed again by the high pass filter 13 and the signal components other than the necessary components are removed by the low pass filter 14, the signal due to the defect can be detected with high sensitivity as shown in FIG. Information on the defect can also be maintained. Here, the frequencies of the differentiator 12, the high pass filter 13, and the low pass filter 14 are determined as an appropriate frequency by measuring the moving speed of the steel sheet 1 with an encoder. For example, when the steel sheet 1 is moved at a speed of 5 m / sec, the frequency of the signal due to the defect is about 400 Hz, so that the frequency of the differentiator 12 is about 1 KHz, so that the high pass filter 13 The cutoff frequency is set to about 200 Hz, and the cutoff frequency of the low pass filter 14 is set to about 600 Hz.

The method of the present invention provides an effect of more accurately detecting the position, size and type of the defects on the steel sheet by processing the signal from the flaw detection magnetic sensor through the steps described above.

Claims (3)

Detecting a flaw on the magnetized moving steel sheet using a sensor, inputting and outputting an output signal from the sensor into a differentiator, and processing the differentiated signal through a high pass filter to correspond to a sudden change in leakage flux. And taking a signal component only, and removing a signal component other than the necessary component taken through the high pass filter through a low pass filter. 2. The signal processing method of the magnetic sensor for flaw detection of a steel sheet according to claim 1, wherein a fluxgate sensor using spatial differential is used as said sensor. The method according to claim 1 or 2, characterized in that the method further comprises the step of measuring the moving speed of the steel sheet with an encoder and accordingly setting the frequencies in the differentiator, high pass filter, low pass filter. Signal processing method of magnetic sensor for flaw detection of steel sheet.