Study On Some Problems Of Hybrid CAD Modeling In Reverse Engineering

As one of the most important components of advanced manufacturing technologies, reverse engineering has become an essential tool for product innovative design and new product rapid development. With the rapid development of computer and modeling techniques, reverse engineering concentrates more on design intent rather than product copy. The CAD model should comprise both feature surfaces and freeform surfaces. Some problems of hybrid CAD modeling in reverse engineering are intensively explored in this dissertation.The hybrid CAD modeling method is based on feature-based method. It also comprises freeform-based method. During the process of model reconstruction, the feature surfaces and freeform surfaces are expressed in different forms. The surfaces are reconstructd using section feature based method and surface feature.based method The fundamental theories and techniques of hybrid CAD modeling are introduced, and the reconstruction of freeform curves and surfaces will be researched in detail.A practical algorithm for B-spline curve fitting to a dense data point set is presented. Almost of all the feature points, which are interpolated by the fitting curve, are identified automatically before the curve fitting process. More knots are inserted iteratively to make the fitting error less than the predefined error.After the segmentation of some complicated point cloud during hybrid CAD modeling in reverse engineering, some n-sided regions may appear. Two methods are proposed to fill these n-sided regions, using multiple non-trimmed B-spline surfaces and single trimmed B-spline surface respectively. For the convenience of calculation, discrete boundary constraints are used in stead of the continuous ones in both methods during the surface fitting process and the final model is approximate G~1 continuous. Two methods are applicable to different cases and each method has the advantages of its own.During the process of data acquisition, the measuring points are sometimes missing under the influence of various factors. A practical algorithm for boundary surface fitting with the constraints of incomplete point cloud is presented. The base surface is firstly constructed according to its boundary type, and the weakness of each control point is calculated using a new criterion. Then, the weakly defined control points are constrained and discrete boundary constraints are added during the surface fitting process. The result surface is close to the incomplete point cloud while preserving good quality of its own. Furthermore, it satisfies approximate G~1 continuity with adjacent surfaces.Finally, some advanced topics for future work are addressed and discussed briefly.