Evaluation Functions For Assembly Sequence Planning

Planning robotic manipulation operations is fundamental to the implementation of systems for assembly, disassembly, and repair of space-based equipment. Both teleoperated and fully-autonomous modes of operation will require the representation of feasible operations sequences which accomplish specified tasks. We have developed the AND/OR graph representation of assembly sequences and shown its completeness and correctness for assembly plan representation. The AND/OR graph partitions the state representation in a manner which facilitates local planning decisions. The use of alternative plan representations is coupled to the need for evaluation functions which guide the choice of desirable actions at any given state. In assembly planning, entropy evaluation functions provide a tool for assessment of the complexity of the current state in terms of the available degrees of freedom of motion of parts. Reducing the degrees of freedom of motion of the parts through the introduction of constraints corresponds to a decrease in entropy measures of the assembly. Assembly steps which couple the accuracy of the positioning device to the constraint clearances of the parts are selected using this measure. The entropy evaluation measure may be computed directly from attributes incorporated into a relational description of the parts geometry. The entropy measures must be coupled with other measures of manipulation complexity, availability of tools and fixtures, and speed and cost requirements for the development of general system planning systems.