And/Or Graph Representation of Assembly Plans

'Ibis paper prescnts 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 cnablc an increase in asscmbly system Ilexibility by allowing an intelligent 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 cxample of an assembly with four parts illustrates the use of the AND/OR graph representation to find the best assembly plan based on wcighing of operations according to complexity of manipulation and stability of subassemblies. In practice, a generic search algorithm, such as the AO* may he used to find this plan. The scheduling efficiency using this representation is compared to fixed sequence and precedence graph represcnations. [he AND/OR graph consistently reduces the average number of operations.