Calibration and Recalibration of machine vision systems. Lars Fermum Chief instructor STEMMER IMAGING GmbH

Transcription

1 Calibration and Recalibration of machine vision systems Lars Fermum Chief instructor STEMMER IMAGING GmbH

2 AN INTERNATIONAL CONCEPT STEMMER IMAGING customers in UK Germany France Switzerland Who? What? Theoretical and applied knowledge General machine vision topics Specific products (hardware / software) Tailored to your needs At our four European sites At your premises Online / mobile Where? Why? Learn how to use machine vision even more efficiently Benefit from the extensive knowledge of the experts at STEMMER IMAGING

3 System setup and Calibration

4 THE KEY TO PERFECT INSPECTIONS IDEA OF REPEATABLE INSPECTION RESULTS Perform a system setup with repeatable methods Have a good documentation of these properties Calibrate your measured values Check your repeatability and precision Re-calibrate the system (if necessary) Have a good documentation of parameters and settings! A good calibration routine is telling you what s wrong with your system.

5 SETTING UP THE SYSTEM CORRECTLY PROPER SYSTEM SETUP & DOCUMENTATION Mechanical adjustment of the camera and illumination installation position Documentation of all relevant working distances and angles; make photos, markings etc. Perfect focus and f-stopping of the lenses: Documentation of f-number Setting up the illumination and flash (strobe) controller Store reference images to see intensity and lighting distribution, documentation of special flash controller settings Exposure time, camera gain, white balance of camera Store software projects or settings, like GenICam grid information etc. Setup software application Setup routines for focussing & positioning, calibration routines, machine vision application

6 Work Equipment Test Chart for calibration purposes

7 TEST CHART FOR CALIBRATION ON THE REVERSE OF THE HANDBOOK A universal test chart for quick and convenient system set-up and checking. Possible areas of use: Exact focussing of the lenses Checking the system resolution System calibration and alignment Adjustment of the camera Test measurements and system checking Decoding and grading of 1D and 2D bar codes

8 UNIVERSAL TEST CHART FOR IMAGING AND MACHINE VISION Exact focussing of the lenses with precision structures

9 UNIVERSAL TEST CHART FOR IMAGING AND MACHINE VISION Checking the system resolution using line pair patterns and Siemens star

10 UNIVERSAL TEST CHART FOR IMAGING AND MACHINE VISION Alignment of colour and contrast settings to cameras Good white balance Poor white balance

11 UNIVERSAL TEST CHART FOR IMAGING AND MACHINE VISION Test measurements and system checking on reference objects Different objects with known measurements to check the initial values of different measurement tools for imaging and machine vision (edge, blob, etc.)

12 UNIVERSAL TEST CHART FOR IMAGING AND MACHINE VISION Decoding and grading of 1D-bar codes and datamatrix codes

13 UNIVERSAL TEST CHART FOR IMAGING AND MACHINE VISION System calibration and camera adjustment using geometric grid structures Useful for calculating origin, scale x, scale y and orientation

14 Re-calibration of the system

15 (RE)CALIBRATION IDEA OF (RE)CALIBRATION Make sure, that input values into mv system (test object etc.) and output values (measurement results) will correlate Convert pixel values to international or national measurement units (ISO 9001) (Re)calibrate brightness of defined region of interests Color/ white balancing Remove lens distortion Compensate temperature effects etc.

16 SYSTEM CALIBRATION (1) CONVERSION OF PIXEL VALUES INTO REAL WORLD VALUES Simple scaling factor, e.g. pixel => millimetre, if necessary keep separate for X and Y values. Please calculate scale factors with most long reference distance. Coordinate transformation: the origin of the image and pivot location are also redefined (calibration using 3 points).

17 SYSTEM CALIBRATION (2) CONVERSION OF PIXEL VALUES INTO REAL WORLD VALUES Non-linear coordinate transformation: non-linear distortions (barrel-shaped or pincushion optical distortion, non-perpendicular acquisition with trapezoidal image field) are eliminated. Image coordinates corrected, the image remains distorted. Non-linear image coordinate transformation: the image is re-interpolated and the image perspective is eliminated. A new undistorted image will be created.

18 SYSTEM CALIBRATION (3) EXP. FOR NON-LINEAR IMAGE COORDINATE TRANSFORMATION Exp.: Correction of barrel-shaped lens distortion Exp.: Correction of perspective lens distortion

19 SYSTEM CALIBRATION (4) 3D CALIBRATIONS Special 3D calibration bodies with known reference surfaces and angles allow metric 1-Click-calibration in combination with special software packages. Can be used for the simultaneous calibration of one or more cameras. In addition to metric calibration also a plane fit for alignment of 3D point clouds is possible. This is important for 3D matching and for easy processing of range map images.

20 Repeatable inspection results

21 THE KEY TO PERFECT INSPECTIONS STATISTICAL ANALYSIS Monitor your machine vision results and make test measurements with a reference object or special calibration body. You can do this periodically to guarantee measuring tool capability. Many machine vision systems will offer you extra statistical information, like: min, max, mean, std. dev., cp and cpk of measured values.

22 STATISTICAL ANALYSIS STATISTICAL ANALYSIS OF INSPECTION RESULTS (2) Accuracy Multiple measurements of the same part. Difference between mean value and real value is called bias Repeatability Multiple measurements within a short time period with same part, operator and test equipment. The inspected part will be removed each time and returned by operator or feeding mechanism Reproducibility Measurements with different operators, at different places, with different (similar) test equipment

23 STATISTICAL ANALYSIS STATISTICAL ANALYSIS OF INSPECTION RESULTS (3) Stability Perform measurements with same equipment at the same place, same operator but at different certain times Linearity Multiple measurements with multiple parts, covering the full tolerance range (min./ max.). Calculate the accuracy (bias) or each part and check, if accuracy is similar for different (sized) parts => linearity of test equipment Please check the special test and measurement regulations and standards like GUM (Guide to the Expression of Uncertainty in Measurement, DIN V EN 13005), AIAG MSA 4, ISO 9001:2008, VDA and other international, national and customer standards

24 THE KEY TO PERFECT INSPECTIONS QUALIFICATION OF YOUR MV SYSTEM Different statistic methods (type 1-study, type-2 study, type-3 study) will check accuracy, repeatability & reproducibility. Good way to start: Perform a series of 30 measurements of the same part with automatic or individual part feeding. The variation of your results measuring the same part should be 5% of your allowed tolerance. If your accuracy and repeatability is getting worse, you have to check system setup, hardware, environmental conditions and perform a proper recalibration of your system.

25 Thanks a lot for your attention! STEMMER IMAGING LTD The Old Barn, Grange Court Tongham, Surrey, GU10 1DW, UK info@stemmer-imaging.co.uk Phone: Fax: Your contact: Lars Fermum, Jon Vickers