Validation of the Measurement Performance of a 3D Vision Sensor by means of a Coordinate Measuring M

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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

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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
BS
MI
Single point clouds, Multiple point clouds, 3D
desctiptive models
Optical-Contact CAD
PD
CAM
RP
Re-styling
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The Optical-Contact CAD
Point clouds Descriptive models
Data elaboration
rough CAD model
Contact digitization
accurate CAD model
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The optical sensor OPL-3D

• Optical active triangulation
• Time-space coding of the meas. volume

Fringe projector
Video camera
Assembly system
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Point clouds
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Multi-view integration of point clouds
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Embedded Check procedure (1)

• Performed after the calibration
• Based on the use of control markers

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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?
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Monitoring the accuracy
Environment illumination, temperature Vibrations
Hardware principle of measurement, mechanical
stability..
Object Surface finishing, Colour and
texture, accessibility
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
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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
Point clouds
Accurate CAD Model
CAD-Point cloud Deviations
CMM Uncertainty
Accuracy evaluation of OPL-3D
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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

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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
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The optical acquisition
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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

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CAD-Point cloud Deviationssingle view
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CAD-Point cloud Deviationsmultiple views
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Bottlenecks

• Calibration master

• Alignment errors

• Need of skilled alignment strategies
• Need of view- fusion
• Need of accurate polygonal models