Edge scanning and swept surface approximation in reverse engineering

[摘要] ENGLISH ABSTRACT:Broadly speaking Reverse Engineering is the process of digitising a physical objectand creating a computer model of the object. If sharp edges formed by two surfacescan be extracted from a point cloud (which is the set of measured points) it can speedup the segmentation of the point cloud and the edges may also be used to constructswept surfaces (or various other types of surface that best captures the design intent).A strategy is presented to scan edges. The strategy simulates a CMM (CoordinateMeasurement Machine) as it would scan a sequence of short lines straddling the edge.Rather than measuring on a physical object, the algorithm developed in thisdissertation scans on the points in the point cloud. Each line is divided in two parts,or line sections, belonging to the surfaces fanning the edge. The points of the linesections are then approximated with polynomials. Each edge point is the intersectionof two such polynomials. In many engineering components sharp edges are replacedwith fillet radii or the edges become worn or damaged. This algorithm is capable ofreconstructing the original sharp edge without prior segmentation.A simple analytical model was developed to determine the theoretically achievableaccuracy. This Analytical accuracy was compared with the accuracy of edgesextracted from point clouds. A series of experiments were done on point clouds. Theinput parameters of the experiments were chosen using the technique of Design ofExperiments. Using the experimental results the parameters that most significantlyinfluences the accuracy of the algorithm was determined. From the Analytical andexperimental analysis guidelines were developed which will help a designer to specifysensible input parameters for the algorithm. With these guidelines it is possible to findan edge with an accuracy comparably with an edge found with the traditional methodof finding the edges with NURBS surface intersections.Finally the algorithm was combined with a swept surface fitting algorithm. Thescanned edges are used as rails and profile curves for the swept surfaces. Thealgorithms were demonstrated by reverse engineering part of another core box for aninlet manifold. If the edge detection parameters are specified according to the guidelines developedhere, this algorithm can successfully detect edges. The maximum gap size in the pointcloud is an important limiting factor, but its effect has also been quantified.

[发布机构] Stellenbosch University