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Workpiece localization and computer aided setup system

Authors Xiong, Zhenhua
Issue Date 2002
Summary Workpiece localization refers to the following problem: assuming a rigid workpiece is arbitrarily fixtured to a work table, determine the position and orientation of the workpiece frame relative to some known world reference frame. In this dissertation, we focus on the development of a computer-aided setup (CAS) system. Several important building blocks of such a system are discussed, including robust algorithms, accurate compensation method and a computer-aided probing strategy. Two computer-aided setup systems, on the basis of both an open architecture CNC (Computer Numerical Control) machine and a conventional CNC machine, are built and tested. The contributions of this dissertation are as follows: First, we presented a new problem found in previous symmetric localization algorithms, specifically, the problem of discrete multiple solutions for symmetric localizations. We showed that the problem was caused by some missing components of the symmetry subgroups G0 in previous research. Modified symmetry subgroup G0s of symmetric features were given. Simulation results with the modified symmetric localization algorithm showed that the discrete multiple solutions problem no longer appeared. To make the localization algorithms more applicable to a computer-aided setup system, a unified updating algorithm was presented which can be applied to both 3D general workpieces and symmetric workpieces. This actually supplies a robust localization to the implementation of a CAS system. Next, to make online measurement practical, we addressed the probe radius compensation problem. Different kinds of compensation methods were reviewed. We proposed a new compensation method for workpiece localization application. We compared the performance of our proposed method with other existing methods. It was shown that our proposed method had the best performance both in terms of accuracy and computational efficiency, which makes the compensation method applicable to a computer-aided setup system. We presented the optimal planning problem for workpiece measurement. A sequential optimization algorithm was introduced to obtain an optimal-determinant solution. Then, based on reliability analysis of localization solutions and the sequential optimization algorithm, a computer-aided probing strategy was proposed. With this strategy, given the desired translation and orientation error bounds and the desired confidence limit, we could experimentally find the least number of points needed to measure, while both error bounds were satisfitid. Simulation results showed the computer-aided probing strategy supplies it basic method for online probing. Finally, two computer-aided setup systems were built on the basis of both an open architecture CNC machine and a conventional CNC machine. The integrated systems consisted of three main modules: a touch-trigger probe systtlrn for data collection, a user graphic interface for model viewing and data inspecting, and compensated localization algorithms. Experimental results showed that the system is suitable for real-time implementation in manufacturing process.
Note Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2002
Language English
Format Thesis
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