For automatic machining of a free-form surface, an effective tool of surface-modeling is needed to generate the numeric data of the desired surface. Computer-aided modeling technique is nowadays widely available, but this software modeling often turns out to be unproductive when the surface is of a complex shape designed from aesthetic viewpoints. In this case, reverse engineering technique can be usefully incorporated in that an 1:1 or reduced model of the desired surface is prefabricated with clay or wood and then a digitization process is performed against the model.
A CNC coordinate measuring machine equipped with a conventional contact probe may be efficiently used for digitization of reverse engineering. A simple way of performing digitization is to sample height data at fixed xy-coordinates which may be predertermined either merely by equispaced grid points or by some statistical approaches. However, this passive sampling tends to result in a collection of data that is too dense with abundant points of no significance or too sparse to satisfy a specified geometric tolerence of approximation. This problem can be improved if sample points are selected adaptively during digitization by estimating the actual surface profile continuously from the data being sampled.
In this thesis, we present an operational algorithm of digitization which can supervise the movement of s contact probe in an autonomous manner. The surface is discretely sampled and fitted by using a polyhedral surface model of triangular patches. The vertex points of the polyhedron are adaptively annexed as digitization proceeds to represent the surface with a minimum number of data satisfying a specified geometric tolerence. The database structure which represent the polyhedron is LTL structure. An improved algorithm of the triangulation, which reforms the triangular patch to equi-lateral triangle, is included to prevent deformity of triangular patch. A real-time path planning of the probe is also provided to reduce total measuring time. Three sample objects are measured for algorithm efficiency test. As the result of actually measuring, proposed autonomous digitization saved the measuring time and measuring points.
The point data which is gained from digitization is not correspond with the measuring surface because probe ball has finite radius. So compensation algorithm of the probe radius is needed and three algorithms are provided to find out the actual contact point. In three compensation algorithms, the contact point is calculated by presuming tangent plane and coordinate rotating method is showed best results through several simulations.
