JPH0560545B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a surface inspection device using a laser beam, and in particular, to a surface inspection device that uses a laser beam, and in particular to a method for adjusting the level of a photoelectric conversion signal so as to appropriately extract defect information. It concerns things that are automatically controlled.

(Prior Art) There is a conventional surface inspection device shown in FIG. 7 for inspecting the presence or absence of defects on the surface of steel plates, aluminum plates, etc. That is, the laser light LS emitted from the laser light source 1 is reflected and deflected by a rotating polygon mirror 3 rotated by a motor 2, and linearly scans the surface to be inspected 4. The light reflected by the surface to be inspected 4 and carrying surface information is focused by a light receiving lens 5 onto a photomultiplier (hereinafter abbreviated as photomultiplier) 6, where it is photoelectrically converted.
The photocurrent signal S1 from the photomultiplier 6 is further subjected to current-voltage conversion by an amplifier circuit 7, and is applied to an AGC (automatic gain control) circuit 8. The AGC circuit 8 controls the high voltage power supply 9 applied to the photo signal 6 so as to keep the peak value of the incoming scanning signal S2 constant.

Here, the reason why the peak value of the scanning signal S2 is kept constant is to prevent changes in the reflectance of the surface to be inspected 4, fluctuations in the output of the light source 1, deterioration of the photoprint 6, etc.
This is to obtain constant optical information by keeping the scanning peak value constant.

The AGC circuit 8 used in this surface inspection apparatus has a detailed configuration shown in FIG. The scanning signal S2 passes through an input buffer circuit 11, further passes through an output buffer circuit 12, and is output to a main signal processing circuit (not shown) for defect inspection. Also, the scanning signal S
2 passes through a low-pass filter 13 and is peak-held in a peak-hold circuit 14. This peak hold circuit 14 does not completely hold the peak, but discharges with a slightly larger time constant. This output S
3 passes through the buffer circuit 15, and then in the differential amplifier circuit 16, the difference from the reference level set by the level setter 17 is determined, amplified, and given to the high voltage power supply 9 as a control signal S4. . The high voltage power supply 9 acts as an amplifier for the control signal S4 and applies high voltage to the photomultiplier 6. The level setter 17 has the function of not only creating a reference level but also setting a voltage that serves as an operating reference for the photomultiplier 6 at the same time.

This AGC circuit 8 has a defective signal as shown in Figure 9A.
Given a scanning signal S2 containing DEF,
Scanning signal S2A after passing through low-pass filter 13
In (FIG. 9B), the defect signal DEF has become smaller. This signal S2 is peak hold circuit 1
When the peak is held at 4, the output signal S
3 becomes as shown in FIG. 9C, and this output signal S
By taking the difference between 3 and the reference level, as shown in FIG. 9D, the control signal S4 becomes smaller as the peak value increases, increases as the peak value becomes smaller, and the control signal S4 does not respond to the spike-like defect signal DEF. can get.

(Problems to be solved by the invention) However, in the conventional AGC circuit 8,
For example, when a narrow metal plate is to be inspected, the level change period TT of the scanning signal S2 is 10th.
As the level is narrowed as shown in FIG. B, the level passed through the low-pass filter 13 becomes low, and therefore, the control signal becomes higher than in the normal case where the level change period TW is wide as shown in FIG. S4 is output.

Furthermore, as shown in FIG. 10C, the scanning signal S2
If part of the defect signal DEF1 with a large width and brightness is included, the level of the defect signal is
The AGC operation was slow and the original peak value level h was lower than the normal level hN.

Therefore, in the conventional AGC circuit 8, the level changes depending on the shape of the scanning signal S2.
The scanning signal S2 is set to a predetermined level that is a prerequisite for defect inspection.
There were cases in which it was not possible to adjust the values sufficiently.

The present invention has been made in consideration of the above points, and
It is an object of the present invention to provide a surface inspection apparatus that can accurately detect defect signals by keeping the gain of the defect signal included in the scanning signal constant without being affected by the shape of the scanning signal.

[Structure of the invention] (Means for solving the problem) In order to solve the problem, in the present invention, the surface to be inspected 4 is scanned with a laser beam LS, and the reflected light is photoelectrically converted by the light receiving lens 5. In a surface inspection apparatus that focuses light on a means 6 and performs optical conversion, and extracts defect information on the surface to be inspected 4 from the obtained scanning signal S2,
An automatic gain control circuit 8 is provided which integrates the deviation between the scanning signal S2 and the reference value Vref2 only during the scanning period of the surface to be inspected, and controls the photoelectric conversion characteristics of the photoelectric conversion means 6 based on this integrated deviation value.

(Function) When the level of the scanning signal S2 during the scanning period is higher than the reference value Vref2, the deviation integral value also becomes larger, and conversely, the level of the scanning signal S2 becomes higher than the reference value Vref2.
If it is smaller, the deviation integral value will also be smaller. Therefore,
The scanning signal S2 is controlled based on the deviation integral value.
can be brought closer to the reference value Vref2.

In addition, since the deviation of the scanning period is integrated, even for spike-like pulses or pulses with partially different levels, the overall level is equal to the reference value Vref2.
If it is close to , the control will not be large and the response will be prevented from becoming excessive.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

In FIG. 1, a scanning signal S2 (FIG. 2A) via an input buffer circuit 11 is applied to an output buffer circuit 12, as well as a deviation integration circuit 20 and a level comparator 21. Level comparator 21
compares the reference signal Vref1 (FIG. 2A) set by the comparison level setting circuit 22 with the scanning signal S2, and determines a significant logic level only while the laser beam LS scans the surface to be inspected 4. The scanning period signal S5 (FIG. 2B) taken is output to the deviation integration circuit 20.

When the scanning period signal S5 takes a significant logic level, the deviation integrating circuit 20 opens the analog switch 23, obtains the deviation between the target wave height reference value Vref2 by the reference level circuit 24, and the scanning signal S2, and outputs the signal S6. is given to the integrating circuit 25.

This integrating circuit 25 integrates the incoming signal S6 to determine the positive area of the incoming signal S6.
SP (Fig. 3 B1 to B3) and the negative area SM
(B1 to B3 in FIG. 3), the integral value signal S7 is changed according to the difference between the two. This integrating circuit 25 has
An analog switch 26 is connected and is closed when the power is turned on to discharge the integrating capacitor C5 and initialize the integral signal S7 to zero.

The integral value signal S7 obtained in this way is inverted via the inverter circuit 28, and
It is output as a control signal S4 for the control signal S4. In addition,
AGC is included in the inverter circuit 28 having an operational amplifier configuration.
In order to determine the response speed of , a capacitor C6 is connected as a delay element.

In the above configuration, when the scanning signal S2 as shown in FIG. 2A is applied, the level comparator 21 forms the scanning period signal S5 as shown in FIG. 2B, and the deviation is determined according to the scanning signal S5. Integrating circuit 20
integrates the deviation between the reference level Vref2 and the scanning signal S2 for the scanning period to obtain a deviation integral signal S6 as shown in FIG.
and the inverter circuit 28, the peak value is brought to the reference level as shown in FIG. 2D.
A small control signal S4 is output when the peak value is higher than the reference level Vref2, and a large control signal S4 is output when the peak value is lower than the reference level Vref2.

Note that when the power is turned on, the integrating circuit 25 is cleared, so the high voltage becomes zero. Therefore, at this point, there is no scanning period signal S5, the analog switch 23 of the deviation integration circuit 20 is on, and the deviation integration signal S6 remains at zero. From this state, the high voltage at which the capacitor C5 is charged gradually increases by the charging circuit R13. When the scanning signal S2 appears when the high voltage has increased to a certain extent, the scanning period signal S5 is obtained, and the deviation integration circuit 20 operates.
AGC loop is enabled.

As described above, according to the embodiment described above, it is possible to obtain the control signal S4 that makes the peak value constant depending on the magnitude of the peak value. In doing so,
As shown in Figure 3 A2 and A3, normal waveform
Even if a scanning signal S2 is given that includes the defect signal DEF1 which is partially brighter than the NOR (A1 in Fig. 3) or has an abnormally shaped waveform with a narrow width, its deviation integral signal S6 (A1 in Fig. 3) is applied. B2, C2) are balanced in positive and negative as in the case of the normal waveform NOR (A2 in FIG. 3), making it possible to prevent excessive response.

Next, a second embodiment of the present invention will be described in detail with reference to FIG. In FIG. 4, in which parts corresponding to those in FIG. A first mono multivibrator 30 is triggered and a second mono multivibrator 31 is triggered on a rising edge.

The mono multivibrator 30 receives the scanning period signal S
A pulse signal S9 (FIG. 5C) having a pulse width slightly longer than the scanning period indicated by 5 (FIG. 5B) is generated and applied to the switch 23 of the deviation integration circuit 20. In this embodiment, the deviation integration circuit 20 and the integration circuit 25
A sample and hold circuit 32 is inserted between the sample and hold circuit 32, and the second mono multivibrator 31 receives a sampling pulse signal S10 for the sampling switch 33 of the sample and hold circuit 32.
(Fig. 5D) is formed. The rest of the configuration is the same as that of the first embodiment, and a description of the configuration will be omitted.

In this embodiment, the deviation integration circuit 20 integrates the deviation between the scanning signal S2 (A in FIG. 5 and the reference level signal Vref2) over a period slightly wider than the scanning period, and at the end of the scanning period, the sample and hold circuit 32 The deviation integral signal S6 (Fig. 5E) is sampled.Thus, as shown in Fig. 5F, the sample-and-hold output signal S11 takes a smaller value with a delay of one scanning period when the peak value is larger than the reference level signal Vref2. , when the peak value is smaller than the reference level signal Vref, a larger value is obtained with a delay of one scanning period.Thus,
This signal S11 is integrated via the integrating circuit 25, inverted via the inverter circuit 29, and provided to the high voltage power supply 9 as a control signal S4 (FIG. 5G).

Therefore, also in this embodiment, the scanning signal S2
The wave height level is set to the target level according to the wave height value of
It is possible to realize an AGC loop in which the voltage is Vref2, and even if a scanning signal S2 containing a partially bright defect signal DEF1 as shown in FIG. 6B arrives, the normal scanning signal S2 (6 AGC operation can be performed in the same way as in Figure A).

In the above embodiment, the deviation integration circuit 20, the level comparator 21, the integration circuit 25, the inverter circuit 29, etc. are configured using operational amplifiers, but the specific configuration is limited to this. There's no need.

[Effects of the Invention] As described above, according to the present invention, the control signal for gain control is formed based on the integral value of the deviation between the scanning signal and the target level during the scanning period. It is possible to easily obtain a surface inspection device that can control the peak value of the scanning signal to be constant regardless of the waveform shape.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the first embodiment of the present invention, FIG. 2 is a signal waveform diagram of each part thereof, FIG. 3 is a schematic diagram for explaining the operation of the circuit in FIG. 1, and FIG. A block diagram showing the second embodiment of the invention, FIG. 5 is a signal waveform diagram of each part thereof, FIG. 6 is a schematic diagram for explaining the operation of the circuit shown in FIG. 4, and FIG. 7 is an overall configuration of the surface inspection device. 8 is a block diagram showing a conventional automatic gain circuit portion, FIG. 9 is a signal waveform diagram of each part thereof, and FIG. 10 is a schematic diagram for explaining the drawbacks of the conventional circuit. 1... Laser light source, 4... Surface to be inspected, 5... Light receiving lens, 6... Photo multiplier, 8...
AGC circuit, 9...High voltage power supply, 20...Difference integration circuit, 21...Level comparator, 25...Integrator circuit, 28...Inverter circuit, S2...Scanning signal, S4...Control signal.

Claims (1)

[Scope of Claims] 1. A surface to be inspected is scanned with a laser beam, the reflected light is focused on a photoelectric conversion means by a light receiving lens and photoelectrically converted, and defect information on the surface to be inspected is obtained from the obtained scanning signal. In the surface inspection apparatus for extracting the above-mentioned scanning signal and the reference value, an automatic system that integrates the deviation between the above-mentioned scanning signal and the reference value during the period when the above-mentioned surface to be inspected is being scanned, and controls the photoelectric conversion characteristics of the above-mentioned photoelectric conversion means based on this integrated deviation value. A surface inspection device comprising a gain control circuit.