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Abstract
Abstract. Understanding the performance of large high performance manufactured structures can require highly accurate dimensional measurement across large volumes with the often conflicting capability to record critical parts of the structure in fine detail. Examples include turbine blades, aircraft wings and off-site manufactured modular structures assembled on-site for city, energy and transport infrastructure. Established large-volume industrial metrology systems such as laser trackers and photogrammetry partially meet the need through the measurement of targets and reflectors, but are limited in capability to record high density local detail needed to capture the finest manufactured features. Whilst large-volume surface sensing is possible with laser radar, photogrammetric pattern projection and contact probing for example, the detail required at a local level typically demands local sensing which generally takes the form of a tracked sensor such as a triangulation laser scanner or hand held touch probe. Local sensing systems face challenges where surfaces have fine detail of similar magnitude to the local sensing system sampling capability and particularly for optical sensors where the light reflected back to the sensor by the surface includes specular reflections compounded by local geometry. This paper investigates how Reflection Transformation Imaging (RTI) with a dome camera and lighting system might be calibrated, characterised and tracked as an alternative technology that is more robust to material surface properties and capable of very fine surface detail capture. Laboratory results demonstrate the capability to characterise and locate the dome to sub-millimetric accuracy within a large-volume tracked space to achieve local surface sampling at the 30 μm × 30 μm level. A method utilising sparse touch probe points to seed conversion of low and high frequency normal maps into a common 3D surface is explored with local agreement with laser tracker surface probe check points to the order of 30 μm.
Highlights
Large high performance manufactured structures comprising assemblies of smaller structures utilise varying joining methods to connect individual components together
Work described in MacDonald et al (2016) on alternative sensors for surface metrology compared the accuracy of 3D surface reconstruction between image sets of a small metric test object taken in an illumination dome by photometric stereo and improved structure-from-motion (SfM) against reference point cloud data from a 3D colour laser scanner
This investigation has updated the imaging equipment in UCL’s Reflection Transformation Imaging (RTI) imaging dome, calibrated it to take into account the internal geometry of its full frame DSLR camera and 105mm macro lens, determined the geometry of its 64 internal light sources to ~250 m and the pose of its internal camera in a common network adjustment that connects internal and external features defined by coded targets to a precision of ~50 m
Summary
Large high performance manufactured structures comprising assemblies of smaller structures utilise varying joining methods to connect individual components together. Whilst large volume surface sensing is possible, for example with: laser radar; photogrammetric pattern projection and; contact probing, the detail required at a local level often demands local sensing which generally takes the form of a tracked laser scanner or hand-held probe. These systems all face challenges, for example where surface detail is of the same order of magnitude as the probing capability, or where light received at the sensor includes specular components compounded by complex local geometry and varying surface finish
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