Automatic Decompositions of Assembly Sequence Plans

Abstract

This paper describes an approach to robotic task sequence planning which decomposes task sequences into operations sequences for an assembly workcell. The approach provides a framework for robust execution of tasks through properties of traceability - implicit mapping of operations to task representation, and viability - retaining multiple paths for execution. Given the descriptions of the objects in this system and all feasible geometric configurations and relationships among these objects and combinations of objects, an AND/OR net which describes the relationships of all feasible geometric states and associated feasibility criteria for net transitions is generated. This AND/OR net is mapped into a Petri net which incorporates all feasible sequences of high level operations. The resulting Petri net is then decomposed in a stepwise manner into a lower level Petri net of which each transition can be directly implemented by control commands or command sequences in an object-oriented framework to control and coordinate the operations of devices and objects in the system. All possible task sequences are found using a systolic stack algorithm. Each feasible sequence is pumped out of the stack and could be stored sequentially. A shortest sequence may be chosen as an output of the hierarchical planning system to efficiently implement the desired tasks.