And/Or Graph Representation of Assembly Plans

Luiz Homem de Mello, A. C. Sanderson

This paper presents a compact representation of all possible assembly plans of a given product using AND/OR graphs. Such a representation forms the basis for efficient planning algorithms which enable an increase in assembly system flexibility by allowing an intclligcnt robot to pick a course of action according to instantaneous conditions. Two applications are discussed: the selection of the best assembly plan (off-line planning), and opportunistic scheduling (on-line planning). An example of an assembly with four parts illustrates the use of the AND/OR graph representation to find the best assembly plan based on weighing of operations according to complexity of manipulation and stability of subassemblies. In practice, a generic search algorithm, such as the AO* may be used to find this plan. The scheduling efficiency using this representation is compared to fixed sequence and precedence graph representations. The AND/OR graph consistently reduces the average number of operations.

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