Optics: measuring and testing – Inspection of flaws or impurities – Surface condition
Reexamination Certificate
1997-04-10
2002-10-08
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
Inspection of flaws or impurities
Surface condition
C356S370000, C356S446000, C348S128000, C250S559480
Reexamination Certificate
active
06462813
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns the field of automated inspection systems, and, particularly, a system and method useful for scanning a painted surface and detecting defects thereon.
DESCRIPTION OF THE RELEVANT PRIOR ART
Quality control is an important component of automated production processes. Like the products of the production processes which it scrutinizes, quality control has become increasingly automated over time. For example, automated visual inspection systems, rather than human inspectors, are increasingly being employed to perform repetitive visual scanning of workpieces in order to detect any flaws therein.
In particular, a number of prior art automated scanning systems have been developed which are specifically intended to inspect the surface of a workpiece to detect defects and blemishes in the painted surface thereof. For example, U.S. Pat. No. 5,367,378 to Harding et al. discloses an optical inspection system and method for evaluating the surfaces of large contoured panels. The method involves providing an illumination means for projecting a pattern of lines having a periodic configuration with features having a separation period. A camera records an image of the pattern of lines as reflected from the surface being inspected. The recorded image is evaluated and quantified by calculating a slope of a defect observed by the camera using a specified mathematical relationship. The distance between the illuminated pattern and the surface are used to calculate the defect slope, and a defect depth value is generated using a specific relationship dependent on the length of the defect area visually recognizable from the reflected image and the calculated defect slope.
U.S. Pat. No. 5,389,794 to Allen et al. discloses a surface pit and mound detection and discrimination system. The system scans a beam of radiation over the surface to be inspected, and includes a mechanism for separately sensing radiation scattered from the surface in the near and far specular regions in order to differentiate between pits and mounds and other types of defects. The system includes means for detecting a local slope on the surface from radiation scattered from the surface in the near specular region and for differentiating between whether said beam of radiation is scanning a pit or a mound.
U.S. Pat. No. 5,461,474 to Yoshii et al. discloses an inspection apparatus for detecting foreign matter on a surface having first and second spaced lines. The surface is scanned by a light beam in a direction orthogonal to the lines, and the size of the light beam is less than the interval between the two lines. The system includes detection means for detecting light reflected from the surface from the scanning and producing first and second signals corresponding to the scan of the first surface line and the second surface line, respectively. Signal processing means are provided for processing a correlation between the first and second signals and detecting whether foreign matter exists on the surface. This system is particularly useful for inspecting semiconductor devices having a circuit pattern formed thereon. The “first and second lines” on the surface to be inspected are, thus, part of the circuit pattern. The system has no particular utility for inspecting surfaces which do not include orthogonal patterns of lines and which contain defects which do not isotopically scatter impinging light. Such limitations render this system particularly unsuitable for inspecting irregular parts having large painted surfaces.
U.S. Pat. No. 4,918,321 to Klenk et al. discloses a method for the detection of blemishes on the surface of the object. The system uses a strip of light which is moved over the surface of the workpiece. Striplike sections of the surface being inspected are in each case recorded stepwise in the region of the strip of light, the step size of successive recordings being smaller than the width of the strip of light. The light reflected from the surface is directed onto an opto-electronic video camera by means of a movable reflecting means which controls the increment of movement between adjacent images. Since the Klenk system relies on a movable reflecting means, this creates the difficult problem of coordinating movement of the reflecting means over the surface of the workpiece if the surface is curved and complex.
U.S. Pat. No. 5,237,404 to Tanaka discloses a surface defect inspection system which interrogates the workpiece surface with radiating light having a predetermined change pattern. A camera is arranged for receiving an image of the irradiating, structured light which is reflected by the surface. The structured light contains gradations in intensity. The system looks for disruptions in the pattern of the structured light by taking partial derivatives of the wave form of the video signal.
Ideally, an automated system useful for inspecting workpieces having painted surfaces, such as vehicle body panels, should possess several important characteristics. First, the system should be able to isolate and detect individual flaws in the painted surface such as are caused by dirt or other foreign substances, pinholes or scratches in the paint finish, etc. Thus, it is important that the system be able to distinguish between defects which are raised or elevated above the paint surface (often caused by dirt or other foreign particles) and defects which are depressed below the nominal paint surface (such as caused by interruptions in the paint coat). Furthermore, it would be highly advantageous if such a system were able to characterize the defects by size so that only those body parts having defects larger than a certain reference size would need to be rejected.
Many painted workpieces such as vehicle body panels have certain structural features, such as, for example, cutouts for the door handle on a side body panel. Each such panel can be expected to have the same cutout in the same location. Thus, it would be highly desirable for an automated inspection system to be able to distinguish between these expected interruptions in the surface and true defects.
SUMMARY OF THE INVENTION
The present system and method have been designed to provide the desirable characteristics of an automated inspection system described above. Disclosed herein is a system and method for detecting defects on a surface, and particularly on a painted surface such as an automobile body panel. The system is capable of characterizing surface defects in a number of ways, including by size of the defect and by surface elevation of the defect (that is, whether it is a crater or a piece of foreign matter). The system is also capable of “learning” to recognize known features on the workpiece, such as, for example, a cutout for a door handle on a side auto body panel.
The system of the present invention comprises three main parts. The first part is the optical components, including a light source capable of projecting a line of light having leading and trailing edges or transitions from dark to bright. In one embodiment, the light source is a diffuse light source which radiates light through a slit to produce a line of light. The width of the slit is chosen with some care to meet the dual criteria of being wide enough that the surface roughness of the paint on the workpiece to be inspected (the “orange peel”) does not throw a shadow, yet narrow enough that the more severely sloped dirt particles and craters do throw shadows. The system uses differences in the intensity of the light reflected back from the surface of the workpiece to detect dirt or other foreign particles in the paint. It would be undesirable for the system to mistake natural surface roughness of the paint for actual defects. In one embodiment, the system detects shadows cast by defects. In an alternate embodiment, the system detects light backscattered by dirt or other foreign particles in the paint. In yet another embodiment, the system detects both shadows cast by defects and the light backscattered by defects.
A moving substrate carries a succession
Czubko Myron
Haven G. Neil
Harness & Dickey & Pierce P.L.C.
Perceptron, Inc.
Pham Hoa Q.
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