Surface defect inspection system and method

• US 6,462,813 B1
• Filed: 04/10/1997
• Issued: 10/08/2002
• Est. Priority Date: 04/12/1996
• Status: Expired due to Fees

• Current Assignee: Autospect, Inc. (Atlas Copco AB)
• Original Assignee: Perceptron Incorporated (Atlas Copco AB)
• Inventors: Haven, G. Neil, Czubko, Myron
• Primary Examiner(s): Pham, Hoa Q.
• Application Number:
  US08/833,926
• Time in Patent Office: 2,007 Days
• Field of Search: 356/237, 356/394, 356/371, 356/445, 356/446, 356/429-431, 356/237.1, 356/237.2, 250/559.47, 250/559.01, 250/559.05, 250/559.06, 250/559.48, 348/125, 348/126, 348/128
• US Class Current: 356/237.2
• CPC Class Codes: G01N 2021/8829 Shadow projection or structured background, e.g. for deflectometry three-dimensional metrology of surfaces G01B11/25
• CPC Class Codes: G01N 21/8806 Specially adapted optical and illumination features
• CPC Class Codes: G01N 21/95607 using a comparative method
• CPC Class Codes: G06T 2207/10008 from scanner, fax or copier
• CPC Class Codes: G06T 2207/30108 Industrial image inspection
• CPC Class Codes: G06T 7/0002 Inspection of images, e.g. flaw detection
• CPC Class Codes: G06T 7/0004 Industrial image inspection

Abstract

A system and method for detecting defects on a painted workpiece surface. The system includes an elongated line of light having a pair of sharp transition zones. The line of light falls onto a moving substrate containing a plurality of workpieces to be inspected. An area detector receives light scattered by the workpiece surface. A first image is captured proximate one edge of the line of light and a second image is captured proximate the other. A defect on the workpiece produces characteristic shadow or backscatter patterns which vary between the first and second images. The shadow and backscatter patterns cause variations in the amplitude of light detected by the detector. The image data is then digitized and electronically processed by software which recognizes each defect, and characterizes the defect by type and/or size.

Claims

1. A system for detecting defects on a coated surface, the system comprising:

a source of diffuse light arranged for emitting an elongated line of light having a leading edge and a trailing edge onto a moving substrate having a direction of travel substantially perpendicular to said line of light such that said substrate passes therethrough, a workpiece to be inspected being supported by said moving substrate;

an area detector for viewing said substrate and capturing images of the received light scattered by a surface of said workpiece, said detector being operative to view a portion of the surface of the workpiece including that illuminated by said line of light, said detector having a frequency of operation selected so that each portion of the surface of said workpiece is imaged at least twice during said workpiece'"'"'s traversal through the field of view of the detector;

means for generating digital signals, including first and second sets of digital signals corresponding to the amplitude of scattered light received by said detector during said two captured images of a portion of said workpiece; and

means for comparing said first and second sets of digital signals to isolate and amplify variations in the amplitude of received light caused by a defect in said workpiece surface, wherein the means for comparing said first and second sets of digital signals includes substracting first and second sets of digital signals one from the other to form a composite set of digital signals and applying a finite impulse response filter to the composite set of digital signals such that the filter is designed to detect correlation in accordance with the speed of the workpiece and the time between the two captures images.

2. The system of claim 1 wherein said defect characteristic size.

3. The ststem of claim 1 wherein said defect characteristic surface elevation.

4. The system of claim 1 wherein said detector is a charge coupled device detector.

5. The system of claim 1 further comprising means for comparing said isolated and amplified signal with stored values representing known workpiece surface irregularities so that the system is able to distinguish between known surface irregularities and a surface defect.

6. The system of claim 1 wherein said detector is a CCD camera, said two captured images of a portion of said workpiece occurring during the traversal of said portion of the workpiece through said line of light.

7. The system of claim 6 further comprising means for comparing said isolated and amplified signal with stored values representing known workpiece surface irregularities so that said system is able to distinguish between known surface irregularities and a surface defect.

8. The system of claim 1 wherein said detector is a OCD camera, said two captured images of a portion of said work piece occurring during the traversal of said portion of the workpiece outside of the line of light.

9. The system of claim 8 further comprising means for comparing said isolated and amplified signal with stored values representing known workpiece surface irregularities so that said system is able to distinguish between known surface irregularities and a surface defect.

10. The system of claim 1 further including means for processing said isolated and amplified signal to identify at least one characteristic of said surface.

11. A method for detecting defects on a painted surface, the method comprising the steps of:

emitting an elongated line of light having a leading edge and a trailing edge;

passing a moving substrate through said line of light in a direction of travel substantially perpendicular thereto, said substrate supporting at least one workpiece to be inspected;

imaging with a detector an area of said substrate that includes said line of light to receive light scattered by a surface of said workpiece;

operating the system at a frequency of operation selected so as to image each portion of said workpiece at least twice during said workpiece'"'"'s traversal through the field of view of the detector;

generating digital signals, including first and second sets of digital signals corresponding to the amplitude of scattered light received by said detector during, respectively, said first and second images;

substracting first and second sets of digital signals one from the other to form a composite set of digital signals; and

applying a finite impluse response filter to the composite set of digital signals such that the filter is designed to detect correlation in accordance with the speed of the workpiece and the time between the two captures image, thereby isolating variations in the amplitude of received light caused by a defect in said workpiece surface to identify a surface detect.

12. The method of claim 11 wherein the step of identifying defect further comprises the step of identifying the size of said defect.

13. The method of claim 11 wherein the step of identifying a surface defect further comprises the step of identifying the surface elevation of said defect.

14. The system of claim 11 wherein the step of imaging said substrate further comprises the step of imaging said substrate with a charge coupled device detector.

15. The system of claim 11 further comprising the step of comparing said isolated and amplified signal with stored values representing expected workpiece surface irregularities to distinguish between expected surface irregularities and a surface defect.