This SoA report was an in-depth version of a wider SoA report on portable measurement technologies delivered to Airbus in February. 2009 by a consortium of ...
Department of Civil, Environmental and Geomatic Engineering
Free-form surface measurement under review at Airbus UK Stephen Kyle, Stuart Robson (UCL) Michael McCarthy (NPL) Richard Burguete, Amir Kayani (Airbus)
Background •
Airbus UK has a strong interest in precision, freeform surface measurement for a variety of applications
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A commissioned State-of-the-Art (SoA) review of surface scanning systems was delivered to Airbus by UCL in April 2010 f This was supported with a system evaluation undertaken by NPL
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This SoA report was an in-depth version of a wider SoA report on portable measurement technologies delivered to Airbus in February 2009 by a consortium of organizations lead by the University of Bath
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The current work also builds on work by UCL and UKAEA, presented by Brownhill at LVMC 09
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The presentation reports on the results from this work and plans for further work.
Themes to be presented •
The difficulties in comparing systems f Limitations in manufacturers’ specifications f Limitations in current guidelines for performance testing
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Surface form errors due to tracking errors f Relevant to laser line scanners which must be tracked in space as the laser
line is “painted” across a surface
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Surface form errors due to other surface properties, e.g. f Polished or matt, black or white – characteristics which affect scanned shape
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Next work f Status and proposals
What’s the problem? •
Airbus UK has a range of freeform surface measurement requirements f Coverage of small and large areas, e.g. part of wing box, or across a wing f Required accuracies from a few tens of microns to a millimetre or so
So let’s just buy a surface scanning system .. •
Choosing a system f Comparison is difficult due to limited and/or inconsistent specifications f Standardized performance tests are not sufficiently comprehensive
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Using a system f Surface properties such as colour can distort the measured shape f Errors in tracking the scan head may appear in the resultant shape
Hmm .. not so easy ..
Examples of reviewed systems •
Line scanners (mid-volume operation) f Spatially located by CMM arms and fixed-
base cameras
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Area (patch) scanners f Cameras combined with structured light
projection (patterns, fringes)
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Large volume scanner f Laser-tracked line scanners f Single-beam, direct ranging-systems
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Note modest project scope and resourcing f Limited the extent of the review but
revealed the essential problems f Survey-class scanners (Terrestrial laser scanners) excluded due to lower accuracy
Example comparison of system specifications •
It is not easy to compare systems from manufacturers’ specifications f Researchers in Mainz were reporting this in 2003 – no change in 2010 f Manufacturing specifications likely to be best-case laboratory measurements
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Scan head tracked by camera (1) f Head accuracy: ±30µm (no statement of measurement percentage) f Tracking system: 90µm + 10µm/m (ditto)
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Scan head tracked by camera (2) f Single point accuracy: 50µm at 1σ (68% of measurements) o Relates to radial errors when scanning a sphere f Length accuracy: ±70μm + 25 x L μm at 2σ (95% of measurements) o Relates to separation of centres of two scanned spheres
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Area scanner (1): nothing in product brochure,10μm in case study Area scanner (2): z resolution in product brochure, 25μm in case study
System specifications – problem areas •
No consistency in the presentation of accuracies and capabilities
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Sometimes no accuracy information is given “Resolution” is sometimes stated instead but can be misinterpreted: f Does it relate to the significant figures in the output measurement? f Does it indicate a system’s real ability to detect fine detail?
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“Repeatability” is sometimes stated instead: f A guide to a system’s capability but still not a measure of accuracy.
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Hybrid systems (e.g. arm + scan head) may quote accuracies for the full system or for individual system components
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Statements explicitly related to accuracy of surface shape are infrequent f They often only relate to spherical surfaces and sphere centres
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Factors causing variability are not usually addressed (see later slides)
Not all bad news, e.g. Faro 2009
Not all bad news, e.g. Leica specifications
System testing for evaluation data • •
Two Airbus artefacts, representing different materials and surface finishes and including some surface steps, were available for testing Two different test systems, representing the generic technologies of line and area scanning, were used to measure both artefacts f Owned and applied by NPL, manufacturer and model not revealed
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Comparisons only between tests, not against reference data f This was a small-scale project so reference measurements, e.g. on CMM,
were not part of the evaluation
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The Airbus artefacts were material samples not originally designed as artefacts for surface measurement testing f Additional target spheres were attached around the perimeter in order to
have reference locations for fitting scans to a common coordinate system
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Artefact descriptions and some sample results follow which illustrate the difficulty of making consistent measurements with different systems
Small artefact Material Aluminium Aluminium Composite
Finish Final overcoat -
gloss
Pre-final overcoat - matt Pre undercoat -
exposed aluminium, composites
Large artefact Material Composite Metallic
Finish Final overcoat -
gloss
Pre-final overcoat - matt Pre undercoat -
exposed aluminium composites
Presence of tracking errors
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Three passes of line scanner show offsets of around 40μm which is attributable to errors in 6DOF tracking of the scan head
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Similar results have been seen in other UCL projects so the problem is likely to be generic and not specific to a particular system.
Reducing tracking error •
Verbal reporting suggests that scan heads located by laser tracker show less tracking error than heads located by CMM arms
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Potentially test the same head with different location systems? E.g: f Nikon MMDX scanner usable with either CMM arm (from different
manufacturers) or fixed-base camera f Steinbichler scan head used in camera-tracked system (NDI/Steinbichler T-Scan) and laser-tracked system (Leica T-Scan)
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For laser trackers, recent developments in photogrammetry, for example, may offer improved accuracy angle tracking, hence reduced spatial errors on scanned surface
Effect of surface properties unrelated to shape •
Not a new problem which still remains, e.g. f Clark and Robson were reporting on how colour affects shape in 2005
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Problems are to be expected: f A scanner’s projected light requires diffuse reflection at the surface in
order to create an image at the sensor f Polished, mirror-like surfaces cause specular reflection and send most of the projected light away from the sensor f Black surfaces are hard to “see” because they absorb the light energy and create no image on the sensor
Boundary and viewing angle errors
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• • • • •
A boundary between two materials in the small artefact was examined by line scanner (green dots), area scanner with perpendicular (90°) view angle (purple dots) and area scanning with 45° view angle (blue dots) Diagram shows “edge-on view” of 15mm wide point cloud along boundary 90° scan indicates flat surface (not confirmed against reference) In comparison, 45° scan has clear systematic error Line scan shows possible boundary effects or systematic error (not further clarified) Note grid size is 10µm.
Reducing form errors Surface scanning systems offer various areas where improvements could be investigated: •
Errors due to characteristics unrelated to surface form, e.g. reflectance, colour, texture, material f Data processing based on measured behaviour of surface f Improved hardware design
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Errors due to scanning geometry f Data processing to apply corrections for known scanning angles f Improved hardware design
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Errors caused by local topography, e.g. spurious reflections f Improved hardware design
Performance tests – standards and limitations •
If manufacturers’ specifications are a limited guide to system performance then why not use a recognized standard guideline instead?
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Best available guideline is the German VDI/VDE 2364: f Part 1 – Optical systems with point-by-point probing f Part 2 – Optical systems with single-view, area scanning f Part 3 – Optical systems with multiple-view, area scanning
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This standard defines a particular way of measuring a reference artefact which typically defines a spatial length and simple forms (sphere, plane) to a high accuracy
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Only parts 2 and 3 are relevant to freeform surface scanning but are not yet considered sufficiently comprehensive (see next slides)
Limitations to guideline /1 Guideline states • .. applies to optical 3-D measuring systems based on area scanning, whose function is based on triangulation • .. does not cover .. repositioning the sensor .. by means of measuring translatory and/or rotary positioning axes • Systems in which the sensor is positioned by measuring translational .. axes such as on coordinate measuring machines .. are not covered Comment • It is clear that the guidelines are designed for area scanners and not line scanners although the latter are in common use • The guidelines do not cover surface scanning where a second, location, system contributes to the errors f Location by arms and CMMs is explicitly excluded and location by
cameras and laser trackers is excluded by implication
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Part 3 of the guideline permits a scanner to be moved in order to cover a wider area in many patches but a second system is not involved.
Limitations to guideline /2 Guideline states: • .. calibrated artefacts .. shall be designed so that their properties have no significant effect on the quality parameter to be determined Comment • Surface properties which are unrelated to form but affect form measurement, such as reflectance, are not covered by the guidelines.
Limitations to guideline /3 Guideline states: • The quality parameters for .. systems based on area scanning are f probing error: o range of radial distance between measured points and best-fit sphere. f sphere spacing error: o difference between the measured and calibrated values of the distance
between the centres of two spheres. f flatness measurement error: o the range of the signed distances of the measuring points from the best-fit plane
Does this answer questions such as: • Effect of angle of view on measured surface form? • Effect of surface topography on surface form, e.g. reflections in corners? • Ability of system to detect surface detail, e.g. a small step?
Possible improvements for Airbus • • •
Design artefacts and associated system performance tests which are directly related to Airbus applications Artefacts would represent materials and surface finishes in actual use Specific surface topography could be included, e.g. edges, holes, steps
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Develop in cooperation with a national standards laboratory – NPL
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Image shows a prototype NPL reference artefact with a wide variation in surface form
Next steps for Airbus •
Airbus is extending its cooperation with NPL and UCL to: f Create a better test artefact for Airbus to evaluate system performance f Define associated measurement procedures
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In parallel, Airbus is supporting a UCL CASE studentship (an industrial Ph D programme) to look at wider issues such as improvements to measurement system operation
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Ideally, we should convince manufacturers to adopt a standardized approach to the presentation of system specifications, even if procedures quantifying system operation are still under development or not widely accepted
For more about freeform measurement ..
Agenda • • • • • • • • •
Measurement Network and the FreeForm Network Confidence in the Equipment but What About Measurement Traceability? Advances in the Development of Fringe Projection Systems CMM mounted fringe projector for freeform shape measurement International FreeForm Measurement Intercomparison of an NPL Artefact Laser Scanners - where are they going? Future Opportunities in Laser Scanning for the Auto Industry Open Forum Discussion Lab tours
