JP2932307B2 - Method and apparatus for surface inspection and distortion measurement using retroreflection - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for surface inspection and strain measurement using retroreflection.

Prior art and its problems This application is a patent application to the United States of America, "Improvement of D site" (application No. 711646) and "I site" (application no.
No. 868736), the present invention is directed to a method for forming a higher quality image of a surface to be inspected by using irradiation on a retroreflective surface, and a practical measurement of the surface. And a method for utilizing the image.

Inspection methods utilizing DiffractSight (trademark D-site) technology are effective in observing, evaluating and classifying defects in metal sheet and plastic body panels. The inspection can use a standard television camera and a high intensity light source together with a retroreflective screen to convert various small slopes on the surface to be inspected into dimly shining information. It is. The advantages of this inspection system are that it has high sensitivity to changes in the contour shape, low sensitivity to changes in the position of the portion to be inspected, and a wide field of view. This technology has an excellent effect on improving process control and product quality control in the printing, assembly and molding industries. Indoor and outdoor experimental results confirm the advantage of the D site for surface quality inspection. For an introduction to the D site, see El. OLHageniers, "DiffractSight-a new form of surface analysis"
analysis) ", SPIE 814, 193-197 1987.

The D-site technique "enhances" a defect by converting a partial change in the slope of the defect on the surface to be inspected into a change in light intensity.
The theory describing the process of manifestation described in US Pat. No. 4,693,319 is briefly described in the literature and includes a summary of a number of parameters that influence the level of manifestation.

In order for an ideal D site image to be formed,
The surface to be inspected generally needs to have a minimum surface roughness of 2 microns. A surface that does not reflect as such can be given a temporary intense gloss by covering it with a fluid film, a process called highlighting.

In the following description, the term “retroreflective surface image (Retr
"Oreflection Surface Image" (what is called a "D-site image") is used to represent an image that occurs when a light source illuminates a surface as follows: a light source. A surface is irradiated with light from the surface, and the light from the surface is irradiated on a myriad of small retroreflective elements, reflected by the elements to the surface, reflected again from the surface and returned to an observation point (or observation surface). The resulting image, shown in the "I-Site" of the aforementioned U.S. patent application, is a retroreflected index distortion image.
n index distortion image). A common term that encompasses both types of images is "Retroreflection Image".

SUMMARY OF THE INVENTION The main parts and improvements of the present invention are as follows.

1. Eliminate the effects of obstructions in specific directions by using a diffuse light source (consisting of multiple elements or a single large element) to substantially illuminate the surface from different angles.

2. The use of the diffuse light source enhances the clarity of the returned light image, thereby obtaining the exact location of the defect or distortion on the material surface.

3. Use optical positioning equipment to improve sensitivity and achieve other objectives.

4. By selecting or controlling the beam spread from the retroreflector
Modify or enhance the surface image by retro-reflection.

5. Utilize sequential retro-reflection, especially where infrared or other light sources are used (where retro-reflector structures tend to be expensive).

6. Use the combined equipment for the inspection system.

Embodiments Hereinafter, the present invention will be described with reference to the accompanying drawings.

FIG. 1a is a basic embodiment of the invention showing the effect of both a "point" light source and a wide area light source. The advantages provided by the broad-area light source in providing a more reliable image of the distortion of the surface of the object and in more accurately describing the actual position of the distortion on that surface are shown. FIG. 1b is an enlarged detail of a portion of FIG. 1a, and FIG. 1c is a graphical representation of the intensity of light reflected from differently sized retroreflective elements.

FIG. 2 shows another method of operation of the present invention and the effect of varying the spread of the beam reflected from variously sized retroreflective elements.

FIG. 3 shows an embodiment of the present invention. In such an embodiment, a group of retroreflective elements is moved through a field of light illuminating the surface, and a retroreflective surface image is stored by a camera or its memory (optical or electronic storage of the image).

FIG. 4 shows an apparatus according to the invention in which the reference retroreflective surface images accumulated for the object to be inspected are alternately displayed on a monitor, allowing accurate repositioning of the inspection part as well as the light source and the camera. Adjustments are assisted with respect to focus, optical magnification or light intensity.

FIG. 5a shows a single retroreflective element with different dispersion characteristics and side faces at right angles to each other. No.
FIG. 5b shows one side of a retroreflective element having a corrugated motion form. FIG. 5c shows one side of a retroreflective element having countless small diffractive members.

Rodger Reynolds, Omer Hageniers et al., “Optical Enhancement of Surface Changes for Contou on Metal Sheet and Plastic Panel Inspection.
r Variations for Sheet Metal and Plastic Panel Ins
under the heading “SPIE”, held on June 28, 1988 in Dearborn, Michigan, USA, on SPIE International Symposium on Optical Technology and Industrial Value for Advanced Manufacturing Technology (SPIE).
International Symposium) is a reference sample.

 Next, these figures will be described in detail.

FIG. 1a shows the creation of a surface image by means of a retroreflector and the depiction of the "primary" and "secondary" images detailed in the above-mentioned document by Reynolds and Hageniyas et al. An example is shown.

Here, three preferred types of light sources discovered to date are point light sources, broad light sources, and collective light sources of point light sources that effectively produce surface illumination from multiple angles, such as wide light sources ( a multiple collection Of point sou
rces) will be described in more detail.

"Point" light source (10) illuminating the surface (11) with illuminating light
Think about the use of.

The light emitted from the point light source is, after reflection by the surface (11), retroreflected by a retroreflector (12) (typically a screen) having a large number of small retroreflective elements. Retroreflective elements are typically small for the surface area to be measured (eg, 50 micron diameter elements for 50 mm or more of the surface area). Light rays returning from these retroreflective elements, for example (15),
Again, the surface is directed toward the camera, and the camera utilizes the returning light to form an image of the surface. Camera (25) is “off axis” (γ ≠ θ)
, The position may be “on-axis” (γ = θ).

For example, small upward projections (ridges, protrusions, etc.)
Is present, the rays reflected from the projecting slope of the surface are reflected at points (eg, P _A ,
P _B ). This point is a different location than would be illuminated by light reflected from the surface if the surface did not have this protrusion. Rays from this point and all points on the screen return from these points of arrival to the entire area of the surface, but the typical angular spread α of the rays returned from the screen is ± 1/2 °. Therefore, it covers an area that is usually larger than the protruding portion. If the slope of the protrusion is large enough (relative to the spread of the screen), the light beam (eg point
P rays proceeding _A) is deviates from within the system, also possible that rays of the return is not quite or almost (i.e., light that is to be returned, hits the screen at the appropriate angle for all or retroreflected Not get).

In the figure, when γ> θ, rays hitting an undistorted surface (eg, points S ₁ and S ₂ ) near or around the special defect are reflected on the screen (eg, on the screen shown in FIG. 1a). Proceeding to points P ₁ and P ₂ ), which impinge on the surface, causing an angular spread of the light beam and re-irradiating the surface, enveloping the protruding part (or other distortion) of the defect and producing a backlight (front light) effect Is important.

According to a typical experimental setup using a 50-75 micron beam of light (according to a 3M Campani Scotchlite type 7615), the beam spread α of the retroreflective element is It is about ± 1/2 ± (with respect to the distribution midpoint).

For a screen facing a distance "R" set to a value of 1-2 m, the actual area of the surface re-illuminated by each micro retroreflective element is large compared to the typical defect area. Things. For example, a small defect in a metal sheet may be only 1 mm in diameter, while the diameter "D" of the area of the surface that is irregularly re-reflected by each point on the screen has a value of 50 mm at R = 2 m.

Light Source Size Effect It is interesting to consider what happens when using a "point" light source with a small spread over the surface. If a person is looking directly at the retroreflective screen through the panel surface and the optical system is focused on the screen, the redistribution of the light rays caused by the distortion in the first reflection from the surface will cause the screen to become distorted. Various bright and dark patterns can be observed above. For example, upward protrusion of the surface, ie, protrusion of the surface, results in a relatively dark area on the screen. That is, the intensity level of light reaching the screen is reduced relative to its surroundings than would be had in a defect-free state at that location.

As shown in Figure 1a, the projecting portion acts like a small convex mirror, directed in the direction of P _A and P _B as a whole large amount of light rays.

Conversely, a concave portion of the surface at the same location on the surface can concentrate light rays on the screen by an effect like a "concave mirror". An observer looking at the screen in reflection through the surface sees the effect of these rays (what is referred to in the reference as the "primary" signature of the distortion). It is possible.

However, the first depiction, for example, of a surface protrusion, appears to have been moved from the exact location of the defect. The first depiction appears to be farther (or closer) from the observer due to the light source making a smaller (or larger) angle with the surface than the observer. The amount of this movement is proportional to the angle difference between the axes of the illumination and viewing directions. “Off axis”
For operation at (γ ≠ θ), this means that if one wants to determine the location of the defect on the surface (and correct the defect using, for example, a device or grinder for polishing the defect area in a metal sheet panel). It makes it difficult to use the first depiction.

Because the image of the first depiction exhibits a clear light / dark contrast, the point light source detects the sharpest contrasting defect and goes through a process of finding small defects with steeper slopes (at least other processes). (When there is almost no defect).

However, point light sources generally should not be used if it is desired to make the contours accurate by brightening the positive slopes and darkening the negative slopes. The point light source is accurate in the "off-axis" mode if one wants to know exactly where on the panel the defect indication is.

Use a single global light source (or larger light source), such as in a camera flash, or a series of single light bulbs, such as (10), (20) and (21), to size the light source. In the extended case, the illumination angle for each defect on the surface is different at each position of each light source (or many "continuous" points on the global light source). In this case, any "first depiction" on the screen will be substantially "erased" by light rays reaching the screen from other undistorted surface locations (positions illuminated by other parts of the light source). The actual "first" effect on the screen can be "washed out" with little or no effect when viewing the screen through the panel.

Focusing on the test surface (especially with a camera lens that can easily maintain focus unlike the human eye) and viewing the surface under the illumination of the broad light source generally results in a second effect Can be seen. The effect is that rays that have not hit a defect or other distortion in the surface will appear as "backlighting" (if γ> θ) to the defect. In this case, a pattern having a unique and accurate contour expression is generated even for a defect having a relatively large inclination. However, the blurring of the illuminating light from the screen caused by the arrangement of the plurality of light sources reduces the sensitivity to smaller defects.

In the case of a defect with a small inclination, if little movement occurs for the second image and the first image, the rays from the second image will overwhelm the rays of the first image in intensity, so that in practice It is important that only the second image is visible. Hence, the main difference between the point light source effect and the wide area light source effect lies in the case where there is a larger slope (eg, a 0.001 inch high, 0.020 inch diameter protrusion). This is because highlight oil (high
When applying light oil) and when using something with a substantially sloping waveform, this waveform is more "noisy" when viewed with a point light source than a wide area light source.
y) ". In these cases, a wide area light source is preferred.

As a condition of size for using a 1m square surface,
Typically, the wide area light source is a group of 10 point light bulbs in a 5 cm x 10 cm area or with a camera flashgun in the area of 2 cm x 4 cm, so the size of the screen is It is larger than the surface receiving light reflected from the screen. Smaller (larger)
For the surface, typically a smaller (larger) global light source and screen may be used.

Next, the operation method of the device of the present invention will be described through at least one theory.

The light rays generated by the broad-area light source (10) illuminate the surface area around the defect, and the light rays reflected from that area and re-reflected by the retroreflective element re-illuminate said defect and are substantially observed by the camera. Provide a large portion of the irradiation range for defects. The light rays reflected back by the surface are in this embodiment directed to a camera which is mounted on-axis as shown. (In the Figure 2, the beam splitter (50) is used the light from a point source (51) to advance along the axis of said camera (52).), Such as S ₁ and S ₂ Light rays from undistorted surface areas
The further away from the defect, the progressively the actual light intensity for the defect decreases. This is because rays emanating from individual small retroreflectors, such as (15), with diffusion are enlarged in the area to be covered, i.e., the divergence of Gaussian rays from each of the small retroreflective elements, and the camera. Alternatively, it may be reduced by both the angular spread in moving away from the defect.

This re-irradiation from multiple points on the screen to any point on the surface (and its distortion, if any) results in a "second image" referred to in the reference.

When producing an image of a surface under such illumination, if the camera is at an angle that is farther from the surface than the light source, the positive tilt (as viewed from the screen) will be To the camera (away from the camera), and a negative tilt is the opposite. And, clearly, the undistorted panel has a cone-shaped spread with an angle α that results in a disc-shaped light (70) and a Gaussian distribution with a diameter of about 20 to 40 mm with respect to the aforementioned experimental parameters. ), Keep sending the light background towards the light source. This disk relates to an air diffraction disk as it appears in the far field diffraction pattern of the retroreflective element. The components in any screen are uniform in size, and in this case diffraction rings are also observed.

As described above, both on-axis and off-axis observation operations are possible. The effect that occurs in the case of on-axis is to irradiate the light ray more distant than in the case where there is no defect, due to the effect of the protrusion defect, so that the area on the screen where the defect is displayed is displayed. The effect is to create a darkening effect. The steeper the slope of the defect, the darker the area on the screen.

In the case of off-axis, the light from the defect (on re-irradiation by the multiple retroreflective elements of the screen)
Depending on the inclination of the defect and whether the defect is positive or negative with respect to the surrounding surface, more or less rays are directed towards the camera.

Mixing the size and / or color of the retroreflective elements can mitigate the ring effect due to overlapping diffraction patterns. By changing the size, wavelength or optical dispersion, the photosensitivity can be changed technically.

For example, the rate of increase in light going to the camera due to the defect having a positive slope is proportional to the slope, and the light intensity is proportional to the divergence of the Gaussian light beam, ie the spread angle α. For larger elements or smaller wavelengths, the angle α decreases, but sensitivity can typically increase for fixed diameter elements. A tunable light source such as (51) controlled by a variably controlling light wavelength (100) (eg, a prism) can be used to change the sensitivity. Also, a test surface sample with a known surface slope can be used and used to determine the effect on wavelengths that need to be matched with the readings and to perform calibration. These theories also apply to the refractive index gradient in the case of the aforementioned "I sight" U.S. patent application.

As another example, a retroreflective sheeting with a 0.1 mm diameter corner cube is typically a 0.075 mm glass bead.
Type screen (Scotchlight 7615). The diameter of the Gaussian disk at the observation position increased from 10 cm to about 7.5 cm, and the sensitivity was improved. In this way, the sensitivity is improved by reducing the angle α until conditions such as the diameter of the retroreflected light illuminating the surface approach the magnitude of the characteristic of the object to be detected (eg, surface protrusion). You.

FIG. 3 illustrates another embodiment of the present invention, in which a smaller group of retroreflective elements moved in space are used to sequentially build up an image of the retroreflective element surface.

Referring to FIG. 2 relating to the method of operation, for any defect, the area that contributes to irradiating the defect and emitting light is the area relatively close to the defect and the corresponding area of the screen. It is. For a typical device used today for the inspection of automotive panels, given that the distance "R" and the angular spread α of the retro-reflected beam are similar to those described above, a panel that is constantly used to produce an effect and surrounds a small defect The surface area is about 50mm to 75mm in diameter
Irradiating the screen corresponding to this area of the panel surface.

Thus, the effect can be reproduced simply by using a group of retroreflective elements large enough to act on rays from the effective area of the surface (always).

For example, a 70 mm diameter disk (300) of FIG. 3, which is a retroreflective screen with a number of retroreflective elements,
Attached to the movable manipulator device (320), it is moved from the surface (350) in the reflected ray field of the light source (340) and reproduces the image on the camera in time sequence. The images can be stored in memory and added digitally to the memory or optically converted and added simply by using a camera (eg, a CCD) that functions integrally in its entirety.

If a larger group of retroreflective elements is used (eg, the screen described above), no manipulator is needed. Thus, this sequential technique is generally more complex, slower, and less available. However, this sequential technique has the advantage that when used as a screen, it is easy to use expensive retroreflective elements that are very expensive and larger than the size of the test surface.

This is especially noticeable when special infrared retroreflectors are used and infrared irradiation is used (glass beads are generally considered ineffective above 2 microns).

In particular, the second reason for using infrared light is that when a special light source such as a laser must be used, it is cost-effective to irradiate a small area. This effect is concentrated in an area that is much larger than the defect, but relatively small with respect to the panel surface, so that a light source (eg, 370) with an illumination axis (371) can be used with a mirror axis with two axes of rotation. A scanner (375) can scan the surface sequentially with the retroreflector array scan. Precalculation, teaching or servo control can be used to maintain the placement of the retroreflector in the reflected field of the swept light beam.

The various arrangements of FIG. 3 can replace groups of retroreflective elements (300) with many small or single large aperture retroreflective elements. The aperture retroreflective element may have a surface (eg, circle 379) that is substantially larger than, for example, the defect (380).
Of the beam so as to cover the area (for example, α =
1/2 ゜), and an aperture large enough to collect light rays coming from a small divergence angle from any selected broad-area light source, not just point light sources It has a width of A suitable single element retroreflector in the embodiment of FIG.
For example, a 3 inch diameter corner cube with a diffusion of 0.1-0.5 °. Such retroreflectors are much inferior to normal retroreflectors for use in other optical studies.

For example, a "cloud" as an individual retroreflective element can be used instead of an element fixed to a screen or manipulator. This is the method that is used for very large surfaces, as the airplane passes through a surface or fluid (e.g. air, water) that has a shape or index of refraction, such as a suitable ramp, and the like. Can be dropped.

FIG. 4 illustrates a novel method for assisting an operator utilizing the present invention. It is often desirable to pick up the test portion and return it to its original position, for example, by storing the retroreflective surface image of the original master portion digitally, for example, on disk (405). Parts are used for reference. In comparing the test image with the master image by eye or computer program, if possible, the magnification of the lens / focal length of the lens, the aperture of the lens, the light intensity and the position of the part are all used for comparison. It is desired to return to the original state to get the master image.

The master image in this case may be the same portion captured at different times, a similar portion known to be in high quality, or an image of what is owned as a reference.

To do this, the present invention performs an alternating combination of the master image and image data into the currently targeted image. This allows for noticeable flicker in fields where the two images do not match each other due to any of the changes described above.
ker). As the change decreases, a state of less flicker is obtained when performing the measurement. To achieve the combination, a frame grabber of the processor (400) reads the master image from the disk (405) and inserts test images from the camera (25) in alternating fields.

As there are many variables, the preferred way to manipulate them is chosen. When the angle of incidence of the camera unit with respect to the fixed part is the same, the inventor has found that the following is preferable.

1. Move the panel to the approximate position.

2. Adjust the camera's zoom lens (if provided) until the image is the same size as the stored image.

3. Adjust the light intensity (this can be easily done with a substantial flicker effect because the test image and the stored image have different brightness levels).

4. Fine-tune the position of the panel to get the magnification of the lens. For example, in FIG. 4, the test panel image (425) is moved a distance "W" to match the stored image (430).

Other ways of achieving this were considered, including being provided briefly over the entire field of each image with a high flicker frequency rather than an alternating combination.

More importantly, as shown in FIG. 3, the retroreflector group (300) can be square, or other shapes, in addition to a circle. Also, the camera (360) can view the entire surface at the same time, and the camera (390) is positioned (eg, via a beam splitter (391)) to view a more limited area of the surface. Importantly, can be swept with the light source by a mirror scanner or other sweeping device (375).

FIG. 5a shows a novel construction of a single retroreflector with suitable dispersion and usable for ultraviolet, infrared and visible wavelengths. As one example, a single corner cube is made such that at least one side has a suitable diffusing surface. It is important that different angles α can be achieved in different directions by providing right-angled sides with different dispersion characteristics. This is useful for generating retroreflective images using single or multiple retroreflective elements.

The corner cube type retroreflector (500) is composed of one of three reflecting surfaces (501) having dispersion characteristics. The retroreflector, in this example, has a corrugated foam (505) and also has an aluminized surface (5).
This has been achieved by the use of a myriad of individual small refractive elements such as holes in 06). These holes have a diameter on the order of the size of the glass beads in the screen used in the previous example (eg, 0.05-0.1 m).
m).

In extreme cases, the retroreflector can be as large as the surface (or media) to be imaged. Thereby, the basic device of FIG. 1 can be replaced. Also,
Lines rather than holes can be used to produce the appropriate dispersion by diffraction.

Light rays used in the present invention include, for example, electromagnetic waves of all wavelengths that can be reflected and retroreflected, such as soft X-rays whose wavelengths are in millimeters.

[Brief description of the drawings]

1A is a schematic view of a basic embodiment showing the effects of a point light source and a wide area light source, FIG. 1B is an enlarged view of a part of FIG. 1A, and FIG. Is a graph showing the intensity of light rays reflected from the retroreflective element, FIG. 2 is a schematic diagram showing another operation method, FIG. 3 is a schematic diagram showing still another embodiment, and FIG. FIG. 5a is a schematic view of a single reflective element, and FIGS. 5b and 5c are side views of a retroreflective element. (10), (340), (370) ... Light source (11), (350) ... Surface (12) ... Retroreflector (15) ... Small retroreflective element (20), (21) ... Light bulb (25), (52), (360), (390) ... Camera (50), (391) ... Beam splitter (51) ... Tunable wavelength light source (70) ... Disc-shaped light (100) ... Variable control of light wavelength (300) ... Retroreflector group (320) ... Movable manipulator device (371) ... Irradiation axis (375) ... Mirror scanner or other sweeping device (379) ... Circle (380)… Defect (400)… Processor (405)… Disk (425)… Test panel image (430)… Stored image (501)… Reflective surface (505)… Corrugated form

Continuation of front page (72) Inventor Timothy R. Pliers Canada N8N2L9 Ontario Tecumseh R.F. Field surveyed (Int.Cl. ⁶ , DB name) G01B 11/30 G01N 21/88

Claims (16)

(57) [Claims] Providing a group of retroreflective elements; illuminating the surface such that a light field from an inspection surface illuminates the group elements;
And by moving at least a portion of the light field,
A method of forming a retroreflective image, comprising the step of re-irradiating the surface by moving and sequentially repositioning the group elements to be irradiated. 2. The method of claim 1, wherein said light field is larger than said group element. 3. The method according to claim 2, wherein the axis of illumination and the group elements are correspondingly moved so as to sequentially illuminate the inspection surface and irradiate retroreflectively. 4. The method according to claim 3, wherein the axis of the camera lens is moved relative to the axis of the illumination. 5. In forming a retroreflection image by the method according to claim 1, an angle distribution of energy retroreflected from the retroreflection element is selected so as to obtain a sensitivity suitable for the degree of unevenness of the inspection surface. Sensitivity control method of reflection image. 6. The method according to claim 5, wherein the angular distribution is controlled by selecting the wavelength of the light beam used. 7. The method of claim 5, wherein the distribution is controlled by a size, type, or geometric distribution characteristic of the retroreflective element. 8. A method of forming a retro-reflective image using a test sample having a known surface slope, changing an intensity distribution at each angle of a light beam retro-reflected from the retro-reflective element, and the intensity distribution. Evaluating the retro-reflective image of the test sample that changes in response to the change of the intensity of the test sample, and selecting an intensity distribution so as to obtain appropriate sensitivity. 9. A calibration system for forming a retroreflective image, comprising: inserting an inspection surface; deforming the inspection surface by a known amount; and measuring a change in the retroreflective image as a function of the amount. Method. 10. The method of claim 1, wherein a single retroreflective element is used instead of a group. 11. The method according to claim 10, wherein said single element has a beam spread of 0.05 ° or more. 12. The step of alternately displaying an inspection image and a reference image, and adjusting positions, optical variables or other variables affecting the inspection image until the inspection image and the reference image similarly appear. Performing the step of reshaping the conditions for forming the image by retro-reflection. 13. A method for illuminating and shining a surface with diffused light and a single retroreflective element for reflecting back light arriving at a certain angular range from a light source illuminating the diffused light through the surface. Using the device to retro-reflect light from the surface. 14. The method according to claim 13, wherein the retroreflective element has the additional property of diffusing light reflected on any small area of the retroreflector. 15. The diverging angle of at least one plane is 0.1 ° or more.
The method described in. 16. The method of claim 14, wherein said diffusion is caused by a number of diffractive elements.