The Symbolic Obdd Scheme For Generating Mechanical Assembly Sequences

摘要

Assembly sequence planning is one of typical combinatorial optimization problems, where the size of parts involved is a significant and often prohibitive difficulty. The compact storage and efficient evaluation of feasible assembly sequences is one crucial concern. Ordered binary decision diagram (OBDD) is a canonical form to represent and manipulate the Boolean functions efficiently, and appears to give improved results for large-scale combinatorial optimization problems. In this paper, assembly knowledge models of liaison graph and translation function are formulated by OBDDs, and OBDD-based representation of assembly sequences is proposed. A novel OBDD-based procedure was presented to generate all geometrically feasible assembly sequences from the OBDDs of liaison graph and translation relation. This procedure can be used conveniently on the computer and all the feasible sequences can be derived. The great advantage of OBDD-based scheme is that the storage space of OBDD-based representation of feasible assembly sequences does not increase with the part count of assembly dramatically so quickly as that of AND/OR graph does. We developed the prototype tool for generating assembly sequence using Visual C++ and CUDD package, and undertake some experimental tests. It was shown that the OBDD scheme generated feasible assembly sequences correctly and completely.