Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.572902
Title: Dynamic inspection of specular freeform surfaces
Author: Wedowski, Raphael David
Awarding Body: University of the West of England, Bristol
Current Institution: University of the West of England, Bristol
Date of Award: 2011
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Abstract:
This thesis provides a review of the state-of-the-art in vision systems and methodologies and an introduction of important surface attributes and representations. Then three novel methods for the dynamic inspection of specular freeform surfaces are presented. These comprise two novel machine vision systems as well as a novel high-speed, multi-scale line tracing algorithm. Both of the novel systems employ a reciprocal deflectometric arrangement. The reflection of a laser line from a surface is monitored on a translucent screen. Here, a complex curve, known as the 'specular signature', is formed that contains all the information on the surface. Methods for extracting and interpreting this information are presented and incorporated into the two vision systems. Prototype demonstrators were designed and assembled to verify the presented methodologies. Extensive experimental validations of all three contributions are shown and the results are compared to ground truth data. Statistical validations of the systems are also presented. Also, the optical and angular resolutions as well as the limitations and the allowable ranges of surface characteristics for both systems, were calculated and presented. It is shown that they are applicable to a range of surface geometries and roughnesses that is comparable to those of existing techniques. The first of the two novel systems is designed for the robust and qualitative detection, classification and localisation of surface defects. It was validated using various real defects on specular freeform surfaces. It is shown that any discontinuity on a surface will be detected and can be classified as long as one criterion regarding the smallest radius of concave curvature on the surface is fulfilled. It is known that this criterion will be fulfilled for a very wide range of common surfaces. The second proposed vision system serves the purpose of a complete, quantitative reconstruction and digitalisation of moving, specular freeform surfaces. While the first system only requires the information from the specular signature, the second system also uses traditional and highly inaccurate surface height data, gathered through laser triangulation. These two data sets, computed from the diffuse as well as the specular reflection, are fused together to generate highly accurate surface bump (gradient) maps. Through the reverse engineering of several real specular specimens and the comparison to ground truth, it is shown that the standard deviation of the error of the height map reaches micrometer levels while that of the angular accuracy reaches levels below one degree. As a third original contribution to knowledge, a novel, high speed, multi-scale line extraction algorithm was developed. Intended for the rapid extraction of the specular signature from the screen images, it combines the processing speed of crude edge detectors with the versatility and accuracy of complex differential geometrical line extractors. It is also multi- scale, with best match scale space being chosen fully automatically. By combining the formerly separated steps of line point detection and line point linkage, the new algorithm is able to increase the processing speed of existing line extractors by up to 50 times. The time requirement is of the same order of magnitude as for crude edge detection algorithms such as Canny. The novel algorithm can also be implemented without the need for any global thresholds as it defines itself a variable local threshold, thereby increasing the sensitivity drastically.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.572902  DOI: Not available
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