Giovanna Sansoni 1 , Simone Carmignato 2 , Enrico Savio 2

1. Validation of the Measurement Performanceof a 3-D Vision Sensor by means of a Coordinate Measuring Machine Giovanna Sansoni1, Simone Carmignato2, Enrico Savio2 1Laboratory of Optoelectronics,DEA - Dept. of Electronics for the Automation University of Brescia, Italy 2Laboratory of Industrial and Geometrical Metrology, DIMEG – Dept. of Mechanical Engineering and Management University of Padova, Italy

2. Context of the work Development of a novel methodology for the reverse engineering of complex, freeform surfaces, combining three-dimensional vision systems and Coordinate Measuring Machines The optical sensor: OPL-3D View alignment Volumetric fusion MI BS Single point clouds, Multiple point clouds, 3D desctiptive models Optical-Contact CAD PD CAM RP Re-styling

3. CAM RP Re-styling The Optical-Contact CAD Point clouds Descriptive models Data elaboration ‘rough’ CAD model Contact digitization ‘accurate’ CAD model

4. The optical sensor: OPL-3D • Optical active triangulation • Time-space coding of the meas. volume Fringe projector Video camera Assembly system

5. Point clouds

6. Multi-view integration of point clouds

7. Calibration master Embedded ‘Check’ procedure (1) • Performed after the calibration • Based on the use of control markers

8. Embedded ‘Check’ procedure (2) • Master dependent • Calibration masters for computer-vision applications often priviledge low-cost aspect rather than the accuracy • Designed for single view acquisition • Unable to grasp any information about the influence of the alignment for multi-view acquisition What about the accuracy of OPL-3D?

9. …Monitoring the accuracy Environment: illumination, temperature Vibrations… Object: Surface finishing, Colour and texture, accessibility Hardware: principle of measurement, mechanical stability.. No standard available! Measurement Uncertainty Extrinsic factors: Operator skill, Surface cleanliness Measurement time Clamping system Measuring strategy: measuring field, calibration procedure coordinate system Data processing: Filtering, Registration of multiple views Algorithm sophistication

10. Evaluation of the accuracy of OPL-3D: substitution approach Calibrated Object CMM used as the link of the traceability chain Full optical digitization by means of OPL-3D Accurate CAD Model Point clouds CAD-Point cloud Deviations CMM Uncertainty Accuracy evaluation of OPL-3D

11. The calibrated object • Stainless steel, long term stability • Volume: 230mm x 100mm x 20 mm • Free form surface • Representative of a typical measurement application • Spheres for accurate alignment • White painted • Substitution approach valid only for measurement tasks similar to that one represented by the calibrated object • Reasonable estimate of the accuracy for a wider range of applications

12. The accurate CAD Model • Touch probe CMM • MPE = 2.2 + L/300 µm (L: mm) Accuracy of the CAD model CAD model from the CMM

13. Single view acquisition: 240mm x 180mm Multiview acquisition (3 v)120mm x 89mm From the Check 70mm From the Check 100mm The optical acquisition

14. CAD-Point cloud Deviations • Performed by Imageware Surfacer (SDRC Corporation) • Evaluation of local deviations between the accurate CAD model from the CMM and the point clouds from OPL-3D • Criticality of the alignment of the point clouds to the CAD model • Best fit using all measured points • Best fit using the sphere points • Best fit using the points on the freeform surfaces

15. CAD-Point cloud Deviations:single view

16. CAD-Point cloud Deviations:multiple views

17. Bottlenecks • Calibration master • Alignment errors • Need of skilled alignment strategies • Need of view- fusion • Need of accurate polygonal models