JPH0430540Y2 - - Google Patents

Description

[Detailed description of the invention] [Field of industrial application] The present invention relates to a device for detecting defects such as non-metallic inclusions that detects defects while running a thin steel plate running at speeds that vary over a wide range. This is intended to be uniform over the speed range.

[Conventional technology]

For detecting flaws in cold-rolled steel sheets, one coil is approximately 6000m long.
A common off-line inspection method is to take approximately 20 samples of 0.5m long steel plates, apply liquid magnetic powder to them, and visually inspect them. There is also online inspection, in which the steel plate is moved intermittently and automatically inspected while it is stopped or moving at low speed.

Magnetic flaw detection is widely used for automatic detection of flaws in magnetic materials, and includes the leakage flux method and the eddy current method. The leakage flux method uses a coil to detect leakage magnetic flux that is generated at a flaw when the object is magnetized, and is suitable for flaw detection on a running steel plate. This leakage flux method is also used for online flaw detection in the above-mentioned low-speed moving state, and an example of its circuit is shown in FIG. 11 to 1n are sensors, 21 to 2n are amplifiers, 31 to 3n are bandpass filters, 41
-4n are square circuits, and n pieces of these are provided (n channels are provided). Reference numeral 5 denotes an analog aggregate circuit, which is composed of n diodes (maximum value detection circuit) whose input terminals are connected to respective channels and whose output terminals are commonly connected. 6 is a comparator, and 7 is a reference value setter.

The sensor 11 (as well as other sensors) includes a U-shaped core 11a and coils 11b and 11c wound around both legs of the U-shaped core 11a as shown in FIG. When the steel plate S is magnetized and has a flaw d, leakage magnetic flux is generated in the flaw d portion. Since the steel plate S is traveling, the leakage magnetic flux passes through the iron core 11a, and at this time, the magnetic flux of the iron core 11a increases, and then decreases (the leakage magnetic flux passes through the iron core 11a). As a result, the coils 11b and 11c induce a voltage. These coils are connected in series so that the induced voltages are summed, and this summed voltage becomes the sensor output voltage.

The sensors 11 to 1n are arranged in the width direction of the steel plate S, so that they can detect flaws in any part of the steel plate in the width direction. Note that the sensor needs to be small to detect minute flaws, and therefore, a large number of sensors are required for full-surface flaw detection of a wide steel plate. The output voltage of each sensor 11 to 1n is amplified by amplifiers 21 to 2n, frequencies other than the flaw signal frequency are cut by BPFs 31 to 3n (improvement of S/N), and squared by squaring circuits 41 to 4n. The positive and negative polarities are removed and the peak is sharpened (this processing is not necessarily required), and the signals are collected in an analog collection circuit 5 (ored in digital terms) and input to a comparator 6. Comparator 6 is collective circuit 5
When the output exceeds the reference value from the setting device 7, a flaw detection output is generated.

[Problem that the invention attempts to solve]

In the defect detection apparatus shown in FIG. 5, the center frequencies of the bandpass filters (BPF) 31 to 3n are fixed, partly because the steel plate is detected at a constant low speed. However, the frequency component of the sensor output is
The frequency changes with the moving speed, and as the moving speed increases, the frequency increases. Therefore, if the center frequency of the BPF is constant, it cannot cope with the case where the moving speed of the steel plate S changes. It is appropriate to perform flaw detection on thin steel plates on a coil preparation line in a steelworks, where the steel plate moving speed varies over a wide range of 60 mpm to 1200 mpm.
Further, when the steel plate moving speed changes, the sensor power voltage, that is, the induced voltage of the coils 11b and 11c changes, and as the moving speed increases, the induced voltage becomes large, and then gradually peaks and decreases. The circuit shown in FIG. 5 cannot cope with such changes in the sensor output voltage.

The present invention aims to improve these points and provide a defect detection device whose detection ability remains unchanged (has the same detection characteristics) even when the steel plate moving speed changes.

[Means for solving problems]

The present invention is a device for detecting defects in a running thin steel plate, and includes a sensor having a coil that induces a voltage by interlinking with leakage magnetic flux generated from a defective part of the magnetized steel plate, and an output voltage of the sensor. A memory that stores a correction coefficient corresponding to the steel plate speed to make it independent of the steel plate speed, a frequency-voltage converter to which a pulse with a frequency proportional to the steel plate speed is input, and a frequency-voltage converter that converts the output voltage of the sensor into the output voltage. The output of the device is A/
a multiplier or a divider that processes a correction coefficient read from the memory using a D/A converted signal; and an output of the multiplier or divider is input;
A switched capacitive filter is used in which the center frequency of the passband can be changed in proportion to the speed of the steel plate by the high frequency pulse obtained by inputting the output of the frequency-voltage converter to the voltage-frequency converter. The present invention is characterized in that it includes a bandpass filter that has been used as a filter, and a comparator that produces a defect detection output when the output of the filter is equal to or higher than a reference value.

〔Example〕

FIG. 1 shows an embodiment of the present invention, and the same parts as in FIG. 5 are given the same reference numerals. As is clear from comparing these figures, in the present invention, the amplifiers 21 to
Multipliers 81 to 8n are inserted after 2n, and bandpass filters 31 to 3n are of a variable center frequency type. A suitable filter for this type of filter is a switched capacitor filter, which consists of an electronic switch and an internal capacitor so that the filter center frequency of the signal follows the external clock frequency in sync, and the clock frequency can be sufficiently separated from the signal frequency. It is commercially available as an integrated circuit designed to be 50 times or 100 times larger. 100 is a pulse generator which outputs pulses with a frequency proportional to the line speed (steel plate running speed). This pulse frequency is converted into a voltage by a frequency-voltage converter 101. 9 is a correction coefficient voltage generation circuit;
A/D converter 91, memory element 92, and D/D converter 91, memory element 92, and D/D converter 91;
It consists of an A converter 93. The memory element 92 stores coefficients as shown by the curve _C1 in FIG. 2 in correspondence with the speed v. The curve _C1 is an inverse curve of the sensor output voltage curve with respect to the steel plate speed v, and if the sensor output voltage is multiplied by this coefficient, the result becomes independent of the steel plate speed. The A/D converter 91 is a memory element 92
Since the address (v) of F/V converter 101 is generated, the analog output (proportional to v) of the F/V converter 101 is converted into a digital value. The D/A converter 93 is a memory element 92
converts the coefficients (digital values) stored in
The output of n is corrected by the coefficient to make it independent of the steel plate speed v.

The output of the F/V converter 101 is also applied to a voltage/frequency converter 102 and converted into a clock frequency. Since this clock frequency can be separated from the signal component, it should be set to an appropriate frequency for controlling BPF31 to 3n.
The frequency is about 100 times higher than the center frequency of the BPF. As shown in Figure 4, the center frequency ₀ of this BPF band is equal to the width of the U-shaped iron core of the sensor.
Then, the period of the sensor output (curve C ₂ ) generated when the defect d passes is 2l, so the steel plate moving speed is v (mpm) and ₀ = 1/2l x v/60 [KHz]. good. The output of the V/F converter 102 performs this change in ₀ due to v. FIG. 3 shows changes in the center frequency ₀ and band width W of the bandpass filters 31 to 3n controlled in this manner with respect to the steel plate speed v. Note that the same effect can be obtained even if the multiplier is used as a divider and the inverse coefficient is stored in the memory.

[Effect of idea]

As explained above, according to the present invention, the defect detection characteristics of the defect detection device can be kept uniform even if the steel plate speed changes over a wide range.
It is extremely effective because it allows full-width and full-length inspection of steel plates directly on high-speed, variable-speed production equipment, instead of the conventional off-line fluid magnetic particle inspection using the sampling sampling method or the online inspection method using intermittent operation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2 and 3 are graphs explaining correction coefficients and center frequency changes, FIG. 4 is an explanatory diagram of the sensor, and FIG.
The figure is a block diagram illustrating a conventional device. In the drawing, S is a steel plate, d is a defect, 11b, 11c
is a coil, 11 to 1n are sensors, 92 is a memory,
81 to 8n are multipliers, 31 to 3n are band pass filters, and 6 is a comparator.

Claims (1)

[Scope of Claim for Utility Model Registration] (1) A device for detecting defects in a running thin steel plate, comprising a sensor having a coil that induces a voltage by interlinking with leakage magnetic flux generated from a defective part of the magnetized steel plate; , a memory that stores a correction coefficient corresponding to the steel plate speed that makes the output voltage of the sensor independent of the steel plate speed; a frequency-voltage converter to which a pulse with a frequency proportional to the steel plate speed is input; a multiplier or a divider that processes the correction coefficient read from the memory using a signal obtained by A/D converting the output of the frequency-voltage converter; and an output of the multiplier or divider. is input, and the center frequency of the passband is changed in proportion to the steel plate speed by the high-frequency pulse obtained by inputting the output of the frequency-voltage converter to the voltage-frequency converter.
1. A defect detection device for a thin steel sheet, comprising: a bandpass filter using a switched capacitor filter; and a comparator that produces a defect detection output when the output of the filter is equal to or higher than a reference value. (2) The sensor coil is wound around a U-shaped core, and the center frequency of the bandpass filter is ₀ [KHz].
Utility model registration claim 1, characterized in that ₀ = 1/2 l x v/60, where the distance between the legs of the core is l [mm] and the speed of the steel plate is v [m/min]. The described thin steel plate defect detection device. ”