Optolab, University of Brescia LIGM, University of Padua

Validation of the Measurement Performance

of 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, Italy2Laboratory of Industrial and Geometrical Metrology,

DIMEG – Dept. of Mechanical Engineering and Management

University of Padova, Italy

Optolab, University of Brescia LIGM, University of Padua

Optical-Contact CAD

The optical sensor: OPL-3D

View alignment

Single point clouds, Multiple point clouds, 3D desctiptive models

Volumetric fusion

CAM RP Re-styling

Development of a novel methodology for the reverse engineering of complex, freeform surfaces, combining three-dimensional vision systems and Coordinate Measuring Machines

BS

Context of the work

MI

PD

Optolab, University of Brescia LIGM, University of Padua

‘rough’ CAD model

The Optical-Contact CAD

Contact digitization

‘accurate’ CAD model

Data elaboration

Point clouds

Descriptive models

CAM RP Re-styling

Optolab, University of Brescia LIGM, University of Padua

The optical sensor: OPL-3D• Optical active triangulation

• Time-space coding of the meas. volumeFringe projector

Video camera

Assembly system

Optolab, University of Brescia LIGM, University of Padua

Point clouds

Optolab, University of Brescia LIGM, University of Padua

Multi-view integration of point clouds

Optolab, University of Brescia LIGM, University of Padua

Embedded ‘Check’ procedure (1)

• Performed after the calibration

• Based on the use of control markers

Calibration master

Optolab, University of Brescia LIGM, University of Padua

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?

Optolab, University of Brescia LIGM, University of Padua

…Monitoring the accuracy

MeasuremeMeasurement nt

UncertaintyUncertainty

Hardware:

principle of measurement,

mechanical stability..

Environment:

illumination,

temperature

Vibrations…

Extrinsic factors:

Operator skill,

Surface cleanliness

Measurement time

Clamping system

Measuring strategy:

measuring field,

calibration procedure

coordinate system

Object:

Surface finishing,

Colour and texture,

accessibility

Data processing:

Filtering,

Registration of multiple views

Algorithm sophistication

No standard available!

Optolab, University of Brescia LIGM, University of Padua

Evaluation of the accuracy of OPL-3D: substitution approach

Accuracy evaluation of OPL-3D

Point cloudsAccurate CAD Model

CAD-Point cloud Deviations

CMM Uncertainty

Calibrated Object

CMM used as the link of the traceability chain

Full optical digitization by means of OPL-3D

Optolab, University of Brescia LIGM, University of Padua

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

Optolab, University of Brescia LIGM, University of Padua

The accurate CAD Model•Touch probe CMM• MPE = 2.2 + L/300 µm (L: mm)

CA

D m

odel

from

the

CM

M

Accuracy of the CAD model

Optolab, University of Brescia LIGM, University of Padua

The optical acquisition

From the Check 100m

From the Check 70m

Single view acquisition: 240mm x 180mm

Multiview acquisition (3 v)120mm x 89mm

Optolab, University of Brescia LIGM, University of Padua

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

Optolab, University of Brescia LIGM, University of Padua

CAD-Point cloud Deviations:single view

Optolab, University of Brescia LIGM, University of Padua

CAD-Point cloud Deviations:multiple views

Optolab, University of Brescia LIGM, University of Padua

Bottlenecks

Scanning Number

Planarity (mm)

1 0,2485

2 0,2470

3 0,2460

average 0,2472

•Calibration master •Alignment errors

•Need of skilled alignment strategies

•Need of view- fusion

•Need of accurate polygonal models