JPH07318499A - Surface defect detecting device - Google Patents

Abstract

PURPOSE:To optically, efficiently and accurately detect a microscopic recess- projection defect caused on a surface of a steel plate used as an exterior trim material for an automobile or a domestic appliance or the like. CONSTITUTION:A device optically detects a microscopic recessprojection defect on a surface of a material 22 to be inspected. At least a single grid projector 21 is oppositely arranged on a surface of the carrying material 22 to be inspected in order to project grid stripes 23 having a constant stripe period obtained by interference of light in the width direction of the carrying material 22 to be inspected. A proper number of image pickup units 24 are arranged in close vicinity to the grid projector 21 in order to pick up an image of the grid stripes 23 projected on the material 22 to be inspected. A signal processor 25 is arranged to quantitatively determine a change in a stripe period in the width direction of the material 22 to be inspected from signals obtained by these image pickup units 24.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is capable of optically and efficiently removing minute depth concave defects or height convex defects generated on the surface of a steel sheet used as an exterior material for automobiles and home appliances. The present invention also relates to a detection device that can detect with high accuracy.

[0002]

2. Description of the Related Art For example, an exterior steel sheet used for a body of an automobile is subjected to processing such as press forming. Therefore, at the time of press forming, foreign matter adhered to the back surface side of a material to be inspected, which is a plate to be molded, or a material to be inspected. A minute defect occurs in the press-molded product due to a minute surface defect generated on the surface side of the, or a foreign substance adhered to the press die. Even if the depth is, for example, about several tens of μm, these minute defects appear in the appearance after coating and cannot be used as an exterior steel plate. Therefore, detecting such minute surface defects before and after press molding is one of the important items of quality control.

As a device for detecting this minute surface defect, an optical defect detection device has been conventionally developed.
The methods are roughly classified into the following two. Method to detect defects from the difference in the amount of reflected light from a sound area Method to detect defects based on the size of the unevenness of the defective area

Among these, as shown in FIG. 5, laser light emitted from the laser 1 is applied to the surface of the material 3 to be inspected while scanning with the rotating mirror 2, and the reflected light is received by the photoelectric converter 4. There is a method to do so ("Automatic technology of visual inspection" p44-46, published by Nikkan Kogyo Shimbun). In FIG. 5, 5 is a Fresnel lens and 6 is an interference filter. As for the above, a focus method for detecting a defect from a focus shift of an optical system caused by unevenness of the defect (Japanese Patent Laid-Open No. 6-
74903).

[0005]

The method of [Problem to be solved by the invention] is based on the principle of detecting the difference in the amount of reflected light between a sound portion and a defective portion, and therefore, a defect such as dirt or rust having a different reflectance from the sound portion, or a reflection such as a scratch or the like. This method is suitable for detecting a defect having a depth that causes a light amount difference. Moreover, since the inspection during transportation is relatively easy, it is generally used as an automatic inspection device for the surface of a steel sheet.

However, since the minute defect having a depth of about several tens of μm generated in the material used as the exterior material for automobiles and electric appliances as described above has a small difference in the amount of reflected light from the sound portion, the method of Its detection is difficult.

On the other hand, in the method (1), as shown in FIG. 6, the cut inspection material 3 is placed on the stage 7,
By moving the vertical direction of the automatic focusing optical system at each position while moving the
Since the unevenness of the surface of the material 3 to be inspected is measured, the measurement accuracy is good and minute unevenness defects can be detected, but on the other hand, there is a problem that the inspection takes a lot of time and online Not applicable. In FIG. 6, 8 is a surface height amount detecting device, 9 is a signal processing device, 10 is a display device, 11 is a horizontal plane direction scaler, and 12 is a height direction scaler.

As described above, at present, there is no device that can automatically and automatically detect the above-mentioned minute defects online with high precision and efficiency. Actually, the defect is judged by visually judging the difference in gloss between the defective part and the sound part caused by polishing the surface of the material to be inspected with a grindstone, or in the case of a convex defect by the tactile feel during polishing. Is.

Therefore, in addition to the problem that the inspection itself is very sensual and there are individual differences, the polishing work with a grindstone places a heavy burden on the inspector.

The present invention has been made in view of the problems in the above-described conventional surface defect detecting apparatus, and is made up of several tens μ.
It is an object of the present invention to provide a surface defect detection device capable of automatically detecting a minute irregularity defect of about m, automatically, with high accuracy and efficiency.

[0011]

In order to achieve the above-mentioned object, a surface defect detecting apparatus of the present invention is an apparatus for optically detecting a minute defect on the surface of a material to be inspected and having a width of the material to be inspected. Direction, in order to project a lattice fringe having a constant fringe period obtained by the interference of light, at least one grating projector disposed facing the surface of the inspection material and the width direction of the inspection material. An appropriate number of imagers arranged close to the lattice projector to image the projected lattice fringes, and signal processing for quantifying the variation of the fringe period in the width direction of the material to be inspected from the signals obtained by these imagers. It is equipped with a container.

In the present invention, the interference fringes are projected onto the surface of the material to be inspected as lattice fringes, as is clear from the principle, and a fine lattice fringe period necessary for detecting the above-mentioned minute defects can be obtained. In addition to the following 2
For one reason.

First, in the case of projecting a grid fringe (projecting a shadow of a mold grid) using a normal mold grid, the measurement pitch of irregularities and the measurement accuracy are limited by the pitch of the grid used. However, in the case of interference fringes, the grating fringe period can be easily changed by adjusting the interference optical system, so the measurement pitch and measurement accuracy can be easily changed according to the inspection target.

Secondly, in the case of using an ordinary die grid, the die grid and the die grid are in contact with each other due to the bending of the inspected material when applied to the cut out inspected material and the change of the path line when the inspected material is conveyed. Since the distance of the inspection material changes, the lattice fringes are blurred and the measurement accuracy is reduced, but when the interference fringes are lattice fringes, there is no such concern in the coherent range of the light used.

Further, in the present invention, the lattice projector projects the lattice fringes on the entire width of the material to be inspected, and all the lattice fringes projected on the entire width of the material to be inspected are imaged at once by an appropriate number of image pickup devices. Then, it is possible to continuously inspect the material to be inspected without any uninspected area.

[0016]

The surface defect detecting apparatus of the present invention has a constant fringe period in the width direction of the material to be inspected, which is obtained by the interference of light from at least one grating projector arranged to face the surface of the material to be inspected. The checkered pattern is projected on the material to be inspected. Then, the lattice fringes projected in the width direction of the inspection material are imaged from different angles by an appropriate number of image pickup devices arranged close to the lattice projector. Finally, the signal processor quantifies the variation of the fringe period in the width direction of the material to be inspected from the signals obtained by these image pickup devices.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface defect detecting apparatus of the present invention will be described below based on an embodiment shown in FIGS. FIG. 1 is a schematic diagram showing the overall configuration of the device of the present invention, FIG. 2 is a detailed view of a grating projector that constitutes the device of the present invention, FIG. 3 is an explanatory diagram of fringe period fluctuation due to a defect, and FIG. 4 is a diagram of the device of the present invention. 6A and 6B are explanatory diagrams of a fringe period variation calculation method in the signal processor, in which FIG. 7A is a diagram of a lattice fringe viewed from the optical axis direction of the image pickup device, and FIG. It is the figure showing the signal.

In FIG. 1, reference numeral 21 is a material to be inspected 2 being conveyed.
In order to irradiate the lattice stripes 23 having a constant stripe period (bright / dark pitch) in the surface width direction of 2 from the oblique direction,
It is a grating projector in which, for example, one unit is arranged at an angle φ relative to the surface of 2.

This grating projector 21 is, for example, as shown in FIG. 2, a laser projector 21a and this laser projector 21a.
A magnifying lens system 21b that magnifies the emitted laser spot, a beam splitter 21d that divides the laser light flux 21c expanded by this magnifying lens system 21b into two directions, and a laser light flux 21c that is split by this beam splitter 21d. Reflecting reflection mirrors 21e, 2
Each of the laser light fluxes 21c reflected by the reflection mirrors 21e and 21f passes through the beam splitter 21d again and is irradiated from the direction of the angle φ to the surface width direction of the inspection target material 22 being conveyed.

Here, if only the reflection mirror 21f is tilted by an angle θ from the direction perpendicular to the incident direction of the laser light beam 21c, an optical path difference corresponding to the angle θ is generated in the laser light beam 21c divided into two parts, and the inspection target material 22 is obtained. The lattice stripes 23 having a constant stripe period are radiated in the width direction.

Reference numeral 24 denotes the material to be inspected 2 being conveyed as described above.
2, a suitable number of CCD lines, for example, are installed vertically above the material 22 to be inspected in the vicinity of the grid projector 21 in order to capture an image of the grid fringes 23 having a constant fringe period irradiated in the surface width direction. It is an imager having a field of view in the width direction like a sensor. The lattice fringes 23 imaged by these image pickup devices 24 have a fringe period calculated by the signal processor 25. If the surface of the inspected material 22 has irregularities, the fringe period is different from that of a sound portion, and thus the defect is a defect. Is determined to exist.

The variation of the fringe period of the lattice fringes 23 that occurs when the surface of the inspection material 22 has a concave defect will be described in detail below with reference to FIG. In FIG. 3, the material 22 to be inspected
The fringe period Po of the lattice fringes 23 radiated on the surface of is expressed by the following mathematical formula 1 using the angle θ in FIG. 2 where the wavelength of the laser used is λ.

[0023]

[Equation 1] Po = λ / (2 × tan θ)

Next, assuming that the lattice fringes 23 are irradiated onto the inspection target material 22 at an angle φ as shown in FIG. 2, in the optical axis direction of the image pickup device 24 installed at a position facing the inspection target material 22. The fringe period Pn of the lattice fringes 23 in the sound portion is expressed by Equation 2 below.

[0025]

[Formula 2] Pn = Po / sin φ

Now, considering the case where the inspection target material 22 has a concave defect 26 having a depth d, the bright point of the lattice fringe 23 is originally at the point A in FIG. 3 at the edge portion of the concave defect 26. What should have existed, the bright points of the lattice fringes 23 move to point A '. Therefore, the fringe period Pd of the lattice fringes 23 at the edge portion of the concave defect 26 is represented by the following mathematical formula 3 when viewed in the optical axis direction of the image pickup device 24, and is different from the fringe period Pn of the lattice fringes 23 in the sound portion. .

[0027]

[Formula 3] Pd = Pn + d / tan φ

As described above, since the fringe period of the lattice fringes 23 imaged on the image pickup device 24 in the concave defect portion 26 is different from that of the sound portion, the defect can be obtained by calculating the variation of the fringe period (d / tan φ). It is clear that the detection of This also applies to the convex defect portion.

Next, with respect to the grating projector 21 (each parameter of the optical system) in the present invention, it is possible to detect defects having unevenness with a depth or height of several tens of μm by using actual set values. Explain that. Laser wavelength λ = 0.488
μm (Ar laser), the inclination angle θ of the reflection mirror 21f =
If 0.05 °, Po = 0.28m from the above formula 1.
m. Further, if the irradiation angle φ of the lattice stripes 23 is 5 °, then Pn = 3.2 mm from the above mathematical formula 2. Therefore,
The lattice stripes 23 are irradiated at a pitch of 3.2 mm in the width direction of the material 22 to be inspected. However, since the area of a fine unevenness defect is usually several tens of mm in diameter, the sampling pitch in the plate width direction for calculating the unevenness is obtained. It seems that there is no problem.

Next, the resolution for measuring the unevenness of the defect in the image pickup device 24 will be described. As the imager 24, C
A CCD line sensor with the number of CD elements 24a of 5000 pixels is used, and the field of view in the width direction of the inspected material 22 is 160 m.
Assuming that the camera lens 24b having m is used,
In the visual field, the lattice fringes 23 of 160 mm / 3.2 mm = 50 cycles are imaged. At this time, one period Pn of the lattice stripes 23
The number of pixels corresponding to is 5000 pixels / 50 cycles = 100
It becomes pixel.

Therefore, assuming that the resolution for measuring the irregularities of the defect in the image pickup device 24 is Δd, Δd is given by the above-mentioned formula 3
In case of Pd = 2 × Pn, the defect depth d is set to 1
Since it is equivalent to the value divided by 00, Δd = Pn × tan φ
/100=2.8 μm. This is a resolution sufficient to detect a defect having unevenness of several tens of μm.

The above description is an example of the parameter setting values of the optical system, and it goes without saying that it is not always necessary to use the above values, and the optimum parameters can be set according to the shape (concavity and convexity, area) of the defect to be detected. Just decide. Further, the basic fringe period Po can be easily changed only by changing the deflection angle θ of the reflection mirror. Furthermore, when vertical fluctuations occur during the transportation of the material 22 to be inspected, the bright and dark positions of the lattice stripes 23 change but the stripe cycle does not change. Therefore, in detecting the defect by calculating the cycle fluctuation, There is no problem.

Furthermore, when a CCD line sensor is used as the image pickup device 24, the scanning frequency of the CCD line sensor is usually several kHz, which is higher than the pass line fluctuation frequency (normally several Hz to several tens Hz). Since the frequency is quite high, the lattice stripes 2 during one scan of the CCD line sensor.
It is unlikely that the light and dark positions of 3 will change extremely,
It doesn't matter. In the case of inspecting, for example, the entire width of the inspected material 22, if the above-mentioned parameter setting value is mentioned, assuming that the inspected material 22 has an overall width of 1600 mm, it is 1
It suffices to install 0 CCD line sensors.

Finally, a method of calculating the fringe period variation in the signal processor 25 will be described with reference to FIG. FIG. 4A is a view of the lattice stripes 23 viewed from the optical axis direction of the image pickup device 24. From this figure, it can be seen that the stripe period of the edge portion particularly varies in the defective portion. The field of view of the image pickup device 24 is the inspection object 2
4 is changed from “a” to “b” in FIG. 4A when 2 is conveyed, but FIG. 4B shows the output signal of the image pickup device 24 in each visual field.

In the signal processor 25, as shown in FIG.
After binarizing the output signal as shown in (b) with an appropriate threshold value, the fringe period is calculated, and a portion having a variation in the fringe period is determined as a defect. Here, as the fringe period which is the reference of the presence or absence of fluctuation, an average fringe period within one visual field of the image pickup device 24 may be used, or an average fringe period obtained from a plurality of visual fields in the traveling direction of the inspection target material 22. May be used.

In the above description, the defect detection during the transportation of the inspected material 22 has been described, but the grating projector 21 and the image pickup device 24 are used for the inspected material 22 even when the inspected material 22 is stationary. It goes without saying that scanning may be performed in the longitudinal direction. In addition, in the present embodiment, one grid projector 21 projects grid fringes on the entire width of the material 22 to be inspected.
For example, a plurality of grid projectors 21 may be arranged in a staggered pattern, and the grid stripes may be projected on the entire width of the inspection target material 22 by the plurality of grid projectors 21. Further, in this embodiment, the material to be inspected 2
Although the grid pattern is projected on the entire width of No. 2, it is not necessary to project the grid pattern on the entire width.

[0037]

As described above, according to the surface defect detecting apparatus of the present invention, a certain amount of light can be obtained by the interference of light from at least one grating projector arranged to face the surface of the material to be inspected. After projecting a lattice fringe having a fringe period in the width direction of the inspected material, the lattice fringes projected on the inspected material are imaged from different angles by an appropriate number of image pickup devices arranged close to the lattice projector, and then these Since the signal processor quantifies the fluctuation of the fringe period in the width direction of the material to be inspected from the signal obtained by the imager, it is possible to inspect the minute unevenness defects that were difficult to detect by the conventional optical defect detection device. It is possible to detect automatically and with high accuracy even during the transportation of the material. Further, there is no individual difference such as visual detection.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the overall configuration of a device of the present invention.

FIG. 2 is a detailed view of a grating projector which constitutes the device of the present invention.

FIG. 3 is an explanatory diagram of fringe period variation due to a defect.

4A and 4B are explanatory diagrams of a fringe period variation calculation method in a signal processor constituting the device of the present invention, FIG. 4A is a diagram of a lattice fringe seen from the optical axis direction of the image pickup device, and FIG. In
It is a figure showing the output signal of the image sensor of b.

FIG. 5 is an explanatory diagram of a conventional surface defect detection device.

FIG. 6 is an explanatory diagram of a conventional surface defect detection device.

[Explanation of symbols]

 21 Lattice Projector 22 Inspected Material 23 Lattice Stripe 24 Imager 25 Signal Processor

Claims (1)

[Claims] 1. A device for optically detecting microscopic defects on the surface of a material to be inspected, for projecting lattice fringes having a constant fringe period obtained by interference of light in the width direction of the material to be inspected. At least one that is arranged relative to the surface of the material to be inspected
Table grating projector, a suitable number of image sensors arranged in close proximity to the grating projector to image the lattice stripes projected in the width direction of the material to be inspected, and the signal to be inspected from these image sensors A surface defect detecting device comprising a signal processor for quantifying a variation of a fringe period in a material width direction.