JPH0718814B2 - Surface inspection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application] The present invention relates to a surface inspection apparatus suitable for inspecting a surface defect of a steel sheet in, for example, a steel rolling line.

(Prior Art) In a steel rolling line, the surface of the steel sheet is irradiated with light,
A surface inspection apparatus is in practical use for performing a predetermined signal processing on an electric signal corresponding to the reflected light and inspecting the presence or absence of defects on the surface of the steel sheet. That is, the defect information is included in the electric signal corresponding to the reflected light from the steel plate as a change in voltage, and the defect information is extracted by discriminating the electric signal with a predetermined threshold value and displayed as an image. Has become.

By the way, in general, fine irregularities are formed on the surface of an inspection object such as a steel plate, and the electrical signal corresponding to the reflected light has a fourth
A noise component N due to the unevenness is included as shown in FIG. The surface noise component N is constantly changing. Therefore, conventionally, in consideration of the surface noise component N, the threshold value L for obtaining the defect information is set to a higher level as shown in FIG. 4 (b) to discriminate the electric signal. .

However, with this threshold value L, it is possible to discriminate large defects whose electric signal level becomes high, but it is impossible to discriminate minute defects close to the surface noise component N.

(Problems to be Solved by the Invention) As described above, in the conventional surface inspection apparatus of this type,
It was difficult to detect minute defects with a low voltage level. For this reason, a decrease in surface inspection accuracy is unavoidable, leading to problems such as deterioration in quality of the object to be inspected.

Therefore, it is an object of the present invention to provide a surface inspection apparatus capable of reliably detecting even minute defects on the surface of the object to be inspected, capable of highly accurate surface inspection, and improving the quality of the object to be inspected.

[Structure of the Invention] (Means for Solving Problems) In order to solve the above problems and achieve the object,
Detects the reflected light from the surface of the object to be inspected and converts it into an electrical signal,
The noise upper limit average value is measured by inputting this electric signal so that the number of noise upper limit values for each unit area of the surface of the object to be inspected is a fixed number, and follows the fluctuation of the measured noise upper limit average value. A threshold value for discriminating the electric signal is set, the electric signal is discriminated by the threshold value, and the output thereof is image-processed to display an image of the surface defect of the inspection object. .

(Operation) By taking such a means, the threshold value for discriminating the electric signal is set according to the magnitude of the noise component due to the unevenness of the surface of the object to be inspected, thereby discriminating at a level close to the noise component. Is possible.

(Embodiment) FIG. 1 is a system diagram schematically showing the system configuration of an embodiment of the present invention. In the figure, 1 is a steel plate (inspection object) conveyed in the direction of the arrow by the steel rolling line,
Reference numeral 2 is a defect generated on the surface of the steel plate 1. Reference numeral 3 denotes a laser light source, and a laser beam 4 emitted from the light source 3 scans the surface of the steel sheet 1 by a rotating mirror 5 perpendicularly to the transport direction. Then, the reflected light 6 reflected on the surface of the steel plate 1 is condensed by the condenser lens 7 and input to the photodetector 9 via the spatial filter 8.

The photodetector 9 has a photoelectric conversion function of inputting the reflected light 6 and converting it into an electric signal. The electric signal S corresponding to the reflected light 6 output from the photodetector 9 is transmitted to the amplifier 10 and the high speed A noise level automatic measuring circuit (hereinafter abbreviated as noise level circuit) 12 and a discrimination circuit 13 through an automatic gain control circuit 11.
And given to. The noise level circuit 12 automatically measures the average value of the noise upper limit value per unit area of the steel plate 1 by processing the electric signal S, and discriminates the noise level signal NS corresponding to this noise upper limit average value. circuit
It outputs to 13. On the other hand, the discrimination circuit 13 sets a threshold value for discriminating the electric signal S based on the noise level signal NS, and discriminates the electric signal S at this threshold level. The output signal LS from 13 is given to the image processing unit 14. The image processing unit 14
The output signal LS is subjected to predetermined image processing to be converted into profile two-dimensional image data D and output to the display unit 15. The display unit 15 displays a profile two-dimensional image corresponding to the data D. It has become one.

FIG. 2 is a block diagram showing a concrete circuit configuration of the noise level circuit 12. The gate circuit 21 inputs the sample width signal DA corresponding to the strip width of the steel sheet 1 and the line pulse signal LP synchronized with the constant movement amount of the rolling line, and samples only one scanning immediately after the generation of the line pulse signal LP. To generate a sampling signal SP for performing the sampling signal SP, and the sampling signal SP is a first and second AND gate circuit.
It is given to one of the input terminals of 22 and 23 respectively. In addition,
The cycle of the line pulse signal LP is sufficiently longer than the cycle of the sample width signal DA.

The clock signal CLK is input to the other input terminal of the first AND gate circuit 22, is ANDed with the sampling signal SP, and the output thereof is given to the first counter 24. The first counter 24 counts the number of pulses of the output signal from the first AND gate circuit 22, and the count content is latched by the first latch 25.

On the other hand, the comparator 26 compares the electric signal S input from the input terminal 27 with the signal level of the output signal OUTPUT of this circuit as a reference, and the output of the comparator 26 is the other of the second AND gate circuits 23. It is given to the input terminal and is ANDed with the output of the gate circuit 21. Second
The output of the AND gate circuit 23 is given to the second counter 28, where the number of pulses of the AND gate output is counted. The count content of the counter 28 is latched by the first latch 29.

The comparator 30 has an A input terminal for the first latch circuit.
The latch contents of 25 are input, and the latch contents of the second latch circuit 29 of the B input terminal are input and compared.
When "B>A", the count content of the third counter 31 is increased, and when "A>B", the count content of the third counter 31 is decreased. The count content of the third counter 31 is constantly given to the D / A converter 32. Here, after being converted into an analog signal, it is output from the output terminal 33 to the discrimination circuit 13 as the output signal OUTPUT, and the comparator 26
Be fed back to.

On the other hand, the fourth counter 34 counts the line pulse signal LP a certain number of times to generate a line synchronization signal LS according to a certain movement amount of the rolling line.
The output timing of the first and second latch circuits 25 and 29 is reached, and the signals delayed by the delay circuits 35 and 36 are the first and second signals.
It becomes the clear timing of the counters 24, 28 of.

Next, the operation of the noise level circuit 12 will be described. Now, it is assumed that the electric signal S as shown in FIG. 3 (a) is given to the input terminal 27. The sampling signal SP generated by the gate circuit 21 is shown in FIG.

The electric signal S is compared with the signal level of the output signal OUTPUT in the comparator 26, and the output is ANDed with the sampling signal SP in the second AND gate circuit 23. Then, a pulse signal as shown in FIG. 3 (c) is obtained.

On the other hand, the scanning speed of the laser beam 4 on the steel plate 1 is 2 mm / μse.
c, assuming that the clock signal CLK is 1MHz, the clock signal C
LK is a pulse signal generated 5 times per 10 mm on the scanning line. This pulse occurrence frequency can be regarded as the occurrence frequency of the noise upper limit value in the electric signal S. Therefore, the sampling signal obtained by the first AND gate circuit 22
The logical product of SP and the clock signal CLK becomes the pulse signal shown in FIG. 3 (d), and the pulse number is the second AND gate circuit.
It is considered to be the number of noise upper limit times of the electric signal S generated within the time when the pulse signal is obtained at 23.

The pulse numbers of the pulse signals obtained by the first and second AND gates 22 and 23 in this way are counted by the first and second counters 24 and 28, respectively, and the first and second latch circuits 25,29
Latched on. Then, the above-described processing is repeated until the fourth counter 34 counts the number of pulses of the line pulse signal LP a certain number of times and outputs the line synchronization signal LS. That is, the steel plate 1 has a constant surface area over a wide range.
The number of pulses of the pulse signal obtained by the first and second AND gate circuits 22 and 23 is counted and latched.

Next, when the line synchronization signal LS is output from the fourth counter 34, the contents of the first and second latches 25 and 29 are given to the A input terminal and the B input terminal of the comparator 30 and compared. At this time, if “B> A”, that is, if the count number of the second counter 28 is larger than the count number of the first counter 24, the signal level of the output signal OUTPUT of this circuit is too low, and the electric signal S Since it can be considered that the number of pulses obtained by discriminating the electric signal S by the comparator 26 is larger than the noise upper limit occurrence frequency of,
Set the output signal OUTPUT to a higher signal level.
The count value of the counter 31 of is increased.

On the other hand, when "A>B", that is, when the count number of the first counter 24 is larger than the count number of the second counter 28, the signal level of the output signal OUTPUT of this circuit is too high,
Comparing the noise upper limit occurrence frequency of the electrical signal S
Since it can be considered that the number of pulses obtained by discriminating the electric signal S at 26 is smaller, the count value of the third counter 31 is decreased so that the signal level of the output signal OUTPUT becomes lower. Let Then, the count value of the third counter 31 is converted into an analog signal by the D / A converter 32, output as the output signal OUTPUT, and fed back to the comparator 26.

As a result, the level of the output signal OUTPUT becomes an average value of the noise upper limit value of the electric signal S, and changes according to the fluctuation of the noise upper limit value. Therefore, in the discrimination circuit 13, the output signal OUTPUT, that is, the noise level signal NS
Since the threshold level for discriminating the electric signal S is set based on the above, the threshold level fluctuates according to the average value of the noise upper limit values of the electric signal.

As described above, in this embodiment, the threshold level in the discrimination circuit 13 automatically changes in accordance with the change in the noise upper limit value of the electric signal S. Therefore, since it is not necessary to consider the variation of the noise upper limit value, a threshold value close to the noise upper limit value can be set. As a result, it becomes possible to discriminate the minute defect signal having a level close to the noise upper limit value, the discriminated signal is given to the image processing unit 14 and subjected to predetermined signal processing, and the defect image is displayed on the display unit 15. It

Thus, according to this embodiment, not only large defects but also minute defects can be detected stably, so that highly accurate surface inspection can be performed and the quality of the steel sheet 1 can be improved.

The present invention is not limited to the above embodiment.
For example, in the above-described embodiment, the case where the invention is applied to the surface inspection of the steel plate in the steel rolling line is shown.
By changing the frequency of the clock signal CLK according to the frequency component of noise, it becomes possible to automatically measure the noise level in other types of fields, and it is needless to say that it can be applied as a surface inspection device for various inspected objects. Yes. In addition, it goes without saying that various modifications can be made without departing from the scope of the present invention.

[Effects of the Invention] As described above in detail, according to the present invention, the reflected light from the surface of the object to be inspected is detected and converted into an electric signal, and the electric signal is input to the unit of the surface of the object to be inspected. Measure the noise upper limit average value so that the number of noise upper limit values for each area is a fixed number of times, set a threshold value that discriminates the electric signal by following the fluctuation of the measured noise upper limit average value, Since the electric signal is discriminated by this threshold value and the output thereof is subjected to image processing so as to display the image of the surface defect of the inspection object, it is possible to surely detect even minute defects on the surface of the inspection object, and the accuracy is improved. It is possible to provide a surface inspection device capable of high-quality surface inspection and capable of improving the quality of an inspection object.

[Brief description of drawings]

1 to 3 (a) to (d) are diagrams showing an embodiment of the present invention. FIG. 1 is a system diagram showing a system configuration, and FIG. 2 is a specific configuration of a noise level circuit. 3A to 3D are signal waveform diagrams for explaining the operation of the noise level circuit, and FIGS. 4A and 4B are diagrams for explaining conventional problems. 1 ... Steel plate (inspection object), 2 ... Defect, 3 ... Laser light source, 9
...... Photodetector, 12 ... Noise level circuit (noise level automatic measuring circuit), 13 ... Discrimination circuit, 14 ... Image processing unit, 15 ... Display, 21 ... Gate circuit, 22, 23 ... First and second AND gate circuit, 24, 28, 31, 34 ... First to fourth counters, 25, 29 ...
First and second latch circuits, 26, 30 ... Comparator, 32 ... D / A
Converter, 35, 36 ... Delay circuit.

Claims (1)

[Claims] 1. A unit area on the surface of the object to be inspected by inputting an electric signal obtained by the photoelectric conversion means for detecting reflected light from the surface of the object to be inspected, converting the light into an electric signal and outputting the electric signal. Noise upper limit average value measuring means for measuring the noise upper limit average value such that the number of times of noise upper limit value for each is a fixed number, and the electric signal is tracked according to the fluctuation of the noise upper limit average value measured by this measuring means. Discriminating means for setting a threshold for discrimination and discriminating the electric signal by this threshold, and image processing means for image-displaying the surface defect of the inspected object by image-processing the output from the discriminating means. A surface inspection apparatus comprising: