Edge scanning and swept surface approximation in reverse engineering

Broadly speaking Reverse Engineering is the process of digitising a physical object and creating a computer model of the object. If sharp edges formed by two surfaces can be extracted from a point cloud (which is the set of measured points) it can speed up the segmentation of the point cloud and the edges may also be used to construct swept 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 (Coordinate Measurement Machine) as it would scan a sequence of short lines straddling the edge. Rather than measuring on a physical object, the algorithm developed in this dissertation "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 line sections are then approximated with polynomials. Each edge point is the intersection of two such polynomials. In many engineering components sharp edges are replaced with fillet radii or the edges become worn or damaged. This algorithm is capable of reconstructing the original sharp edge without prior segmentation. A simple analytical model was developed to determine the theoretically achievable accuracy. This Analytical accuracy was compared with the accuracy of edges extracted from point clouds. A series of experiments were done on point clouds. The input parameters of the experiments were chosen using the technique of Design of Experiments. Using the experimental results the parameters that most significantly influences the accuracy of the algorithm was determined. From the Analytical and experimental analysis guidelines were developed which will help a designer to specify sensible input parameters for the algorithm. With these guidelines it is possible to find an edge with an accuracy comparably with an edge found with the traditional method of finding the edges with NURBS surface intersections. Finally the algorithm was combined with a swept surface fitting algorithm. The scanned edges are used as rails and profile curves for the swept surfaces. The algorithms were demonstrated by reverse engineering part of another core box for an inlet manifold. 1 Stellenbosch University http://scholar.sun.ac.za