Panel surface flaw inspection

• US 4,920,385 A
• Filed: 04/06/1989
• Issued: 04/24/1990
• Est. Priority Date: 02/14/1984
• Status: Expired due to Term

• Current Assignee: Diffracto, Ltd. (LMI Technologies Inc.)
• Original Assignee: Diffracto, Ltd. (LMI Technologies Inc.)
• Inventors: Pryor, Timothy R., Clarke, Donald A., Reynolds, Rodger L.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US07/333,776
• Time in Patent Office: 383 Days
• Field of Search: 356/237, 356/446
• US Class Current: 356/237.2
• CPC Class Codes: G01N 21/88 Investigating the presence of flaws or contamination
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features

Abstract

This invention relates to electro-optical sensing of form type and other defects on surfaces such as sheet metal or plastic panels. Method and apparatus are disclosed for detection and quantification of defects such as dents, creases, low spots, flat spots, etc. which are a result of the manufacturing, material handling and assembly process. Surfaces of interest are generally those of automobile body panels, (e.g. hoods, fenders), refrigerator panels, furniture panels, and aircraft panels. Similar applications exist to dies and other formed metallic or plastic parts. Both automatic and human visual methods and apparatus are disclosed. The disclosed invention is also effective on paint defects such as orange peel encountered in automotive and other applications. Assemblies of panels, such as car bodies may also be inspected using the invention, and both fixed and moving (e.g. robotic) sensor versions are disclosed.

Claims

1. Method for inspecting surfaces for defects comprising the steps of;

illuminating said surface with a beam of light which is projected along a beam axis to said surface and reflected therefrom,

positioning a retro reflective material comprising a plurality of minute elements such that a zone of light is produced on said retro reflective material by the reflected beam of light and such that the light forming the zone is retro reflected from said retro reflective material and is re-reflected from said surface as a cone of light centered about said beam axis,

detecting a portion of said re-reflected cone of light and forming an image of the zone on a light sensing detector having a viewing axis at an angle to said beam axis so as to be substantially unresponsive to light re-reflected along the beam axis, and

determining from said detected image of said re-reflected light any defects in said surface.

2. A method according to claim 1 wherein said detection step includes detecting one of a change of position or a direction of movement of said image and further including the step of determining a slope direction of the defect from the change of position or direction of movement.

3. A method according to claim 1 wherein said beam is swept across said surface with a scanning mirror.

4. A method according to claim 1 wherein said illuminating light is polarized and the polarization of the returning light from said retro reflective material is rotated such that a polarizer at the detector will cause said light returning to be detected while light directly reflected from said surface is substantially attenuated.

5. A method according to claim 1 wherein said beam is optimized to determine smaller defects in said surface.

6. A method according to claim 1 wherein said beam is swept in a circle, eclipse, or other continuous curve.

7. A method according to claim 1 wherein said light is of an infra red wavelength providing suitable reflection from bare metal surfaces.

8. A method according to claim 1 wherein said beam is converged or diverged to obtain increased or decreased sensitivity to certain surface conditions or defects.

9. A method according to claim 1 wherein said light is collimated or converged in the sweep direction to provide smaller package size or to reduce the amount of retardation material or retro reflective material required for any given scan width on said surface.

10. A method according to claim 1 including the further steps of working on said surface and reinspecting said surface to determine if said defect is substantially removed.

11. A method according to claim 1 wherein a large aperture lens is utilized in order to accept the maximum displacement of said re-reflected beam.

12. A method according to claim 1 wherein first indication of a defect is detected and a rescan of said surface is made to obtain a more precise definition.

13. A method according to claim 12 wherein said first indication is provided in hardware and said precise definition is provided by software.

14. A method according to claim 12 wherein said rescan is made of signals from said surface stored in a memory.

15. A method according to claim 1 wherein said light is comprised of at least one substantially point source or linear light source.

16. A method according to claim 15 wherein said detector is the human eye.

17. A method according to claim 15 wherein said detector is a TV camera.

18. A method according to claim 17 wherein a videotape of said signals from said TV camera is made to allow said defect determination at a subsequent time.

19. A method according to claim 17 wherein said TV camera image is analyzed to determine the location, intensity, shape, area, or intensity distribution of said defect.

20. A method according to claim 17 wherein said TV image is provided of an area on said surface viewed directly by eye.

21. A method according to claim 1 wherein said reflective material and the means for said detector are scanned as a unit relative to said surface, so as to sweep out an area of said surface.

22. A method according to claim 21 wherein the direction of said scan is programmed to be different on different zones of said surface, or on sequential passes over the same zone.

23. A method according to claim 1 wherein said reflective material is fixed relative to said surface and the source of said light and the means for said detector are scanned as a unit over said surface.

24. A method according to claim 1 wherein said defect determination is obtained from a derivative of the signal detected as said beam is swept across said surface.

25. A method according to claim 1 wherein said defect determination is made by integrating the detected signal.

26. A method according to claim 1 wherein said defect determination is made by correlating the detected signal to known defect signals.

27. A method according to claim 2 wherein said defect determination is made from the signal corresponding to the maximum localized deviation in spot image centroid position.

28. A method according to claim 1 when the output of said detector is correlated to known types of flaw outputs.

29. A method according to claim 1 wherein the maximum shift in image is utilized to determine defect severity.

30. A method according to claim 29 wherein only those shifts greater than the background noise level are considered.

31. A method according to claim 1 including the further step wherein the derivative of the output of said detector is compared to give a severity number for said defect.

32. A method according to claim 1 wherein a cylinder lens is utilized to spread the beam in the direction of scan.

33. A method according to claim 24 wherein the value of said derivative is obtained only when the value of said signal exceeds the noise signal level of said surface or any coatings thereon.

34. Apparatus for inspecting a surface for defects comprising,

light source means for illuminating said surface with a beam of light projected along a beam axis to said surface and reflected therefrom,

retro reflector means for producing a zone of light thereon with the reflected beam of light and for retro reflecting the light forming the zone back to said surface where the light is reflected from the surface as a cone of light about said beam axis, said retro reflector means including a plurality of minute elements,

detector means to detect a portion of the re-reflected light from said surface by forming an image of the zone thereon, said detector means having a viewing axis at an angle to said beam axis so as to be substantially unresponsive to light reflected along the beam axis, and

analysis means to determine from said detected image of said zone of the re-reflected light any defects in said surface.

35. Apparatus according to claim 34 wherein said detector detects one of a change of position or a direction of movement of said image whereby a slope direction of the defect is determined.

36. Apparatus according to claim 34 including further acceptance means to accept for analysis substantially only the contribution re-reflected from said surface while substantially reducing the effect of light directly reflected from said surface.

37. Apparatus according to claim 36 wherein said acceptance means includes means for polarizing light from said light source, means for rotating the polarization of light reflected toward and returning to the surface from said retro reflector, and polarizer means in front of said photodetector.

38. Apparatus according to claim 34 wherein said analysis means includes a computer.

39. Apparatus according to claim 34 wherein said analysis means includes hardware circuits to determine flaw presence and/or magnitude.