Linescan System Design for Robust Web Inspection

Transcription

1 Linescan System Design for Robust Web Inspection Vision Systems Design Webinar, December 2011 Engineered Excellence 1

2 Introduction to PVI Systems Automated Test & Measurement Equipment PC and Real-Time Vision Systems (Line and Area Scan) System Solution Provider Factory and Lab Automation Engineered Excellence Consulting Engineering Services Turnkey Solutions & Product Development 2

3 Outline Background Calibration Method Calibration Target Alignment Hardware Summary Engineered Excellence 3

4 Why Use Multiple Line Scan Cameras? Applications requiring high resolution combined with large Field of View (FOV) solved using multiple line scan cameras: Wide continuous web applications in paper, film or metal processing Discrete parts such as large glass or metal panels Engineered Excellence 4

5 Multiple LS Camera Models Multiple cameras used to minimize Working Distance (WD) Overlapping FOV s Engineered Excellence 5

6 Top and Bottom Linescan Cameras Top and bottom imaging with four (4) linescan cameras each Top line light set for bright field Bottom light set for dark field Engineered Excellence 6

7 Multiple Linescan Camera System Engineered Excellence 7

8 Alignment Considerations Alignment of the cameras to each other, the product, and lighting Adjustments need to be easy for operators, yet semi-permanent for stability Several degrees of freedom usually required for accurate alignment Engineered Excellence 8

9 Calibration System calibration comprises several functions: Spatial (X & Y) Intensity (Gain and Black Level) Focus Alignment (Angular and Translation) Engineered Excellence

10 Why Calibrate and Align? Spatial calibration for accurate defect measurement Gain and black level to equalize camera and lighting across pixels Focus for maximum image clarity and contrast Roll and pitch for both lighting and pixel alignment Translation for cross-axis pixel alignment Engineered Excellence 10

11 Gain and Black Level Gain has at least two parts: Many linescan cameras have multiple taps and can store a white level correction to match the tap gains Line lights typically have gentle fall off in level away from lengthwise center of light Black level adjusts for the totally dark response; essentially the offset level Engineered Excellence 11

12 Alignment Tools Hardware Calibration target Camera adjustments Software Adjustment aids Engineered Excellence 12

13 Calibration and Alignment using Custom Target Engineered Excellence 13

14 Adjustment Diagrams Downstream Translation Cross Track Translation Pitch Roll Colored lines are placeholders for the line scan pixels Cameras out of adjustment for mode shown in images Ideal location for each camera is on the horizontal and between the vertical grey lines Engineered Excellence 14

15 Novel Calibration Target A new calibration target for easy alignment of multiple line scan cameras to each other and to lighting in roll, pitch and translation Spatial calibration and focus adjustment also enabled Engineered Excellence 15

16 Single Camera Section of Target Array of dots used to measure spatial calibration Focus using high contrast lines Roll alignment using chevrons Translation alignment using variable width bars Pitch alignment using diamonds Tap gain and lighting compensation using light bars Engineered Excellence 16

17 Focus Sharpness of line edges along single line (shown in BLUE) is Figure of Merit (FOM) High number of edges for improved strength of FOM No translation >> rapid response at linescan rate Non-critical positioning; any part of the pattern serves as useful focus target Engineered Excellence

18 Roll Alignment Roll FOM is difference between spacing of left and right half of chevron lines, which is linearly proportional to roll angle Use of opposing angles simplifies FOM to linear vs trig relationship Sign of error tells operator which direction to adjust Handles ±4 of misalignment (more with longer lines) Engineered Excellence

19 Pitch Alignment Section a1 b1 a2 b2 Ideal pitch is achieved when measured width of diamond, a is at maximum and spacing between diamonds, b is at minimum Options for FOM include maximizing diamond width, minimizing spacing or some combination of the two Feedback using meter or strip chart, with operator adjusting pitch to obtain peak in FOM Engineered Excellence 19

20 Translation Alignment Overlap Region Measured Offset Pattern of variable width stripes spanning camera overlap region Adjacent cameras reference same features due to overlap Variable widths enables identification of reference locations Position of stripes relative to camera pixels using edge detection methods gives the cross-axis translation error to sub-pixel resolution Engineered Excellence 20

21 Calibration Target for 4 Cameras Additional features such as 2D bar code containing detailed calibration target information and direction of travel indicators White (light bar) region for pixel to pixel gain correction Engineered Excellence 21

22 Camera & Lighting Alignment Hardware Engineered Excellence 22

23 System with Four Cameras Four cameras mounted in independent roll, pitch, and translation (along motion axis) adjusters. Lens focus provides the Z axis motion. Line light with adjustable pitch and translation (along motion axis). Bright field orientation in this case. Engineered Excellence 23

24 Mount with Roll & Pitch Adjust Jack screws with opposing springs for both roll and pitch Locking nuts to eliminate movement during normal operation Engineered Excellence 24

25 Lighting Alignment Angle is adjusted using external electronic level, fixed using locking screw (lower picture). Level adjustment used in a factory and during installation Position is adjusted using pinion gears on each end; one side driven with the other following Engineered Excellence 25

26 Summary Multiple line scan cameras can solve challenging simultaneous large FOV and high resolution defect detection applications Careful alignment in roll, pitch and translation of all camera and lights New calibration target developed to make focus, roll, pitch and translation adjustments easier Camera mounting hardware designed to make adjustments easy for operators following simple gauges and strip charts on screen Engineered Excellence 26

27 For Additional Information PVI Systems, Inc Engineered Excellence 27