ical Petri Net Framework for the sentation and Analysis of Assembly

This paper uses Petri nets as a representation and analysis framework for automated assembly. It shows how the hierarchical nature of assembly is captured by Petri nets in representing assembly plans and lower level control plans. The tasks in a control plan correspond to assembly robot operations and can be viewed as a lower level representation of tasks at the assembly plan level. Petri nets provide a single unified framework for representing the hierarchical nature of assembly. This paper presents an approach to deriving performance and throughput measures of assembly on the basis of critical path analysis of the Petri net representation of plans. I. INTRODUCTION SSEMBLY can be represented in successive phases hi- erarchically and in increasing detail. This paper is con- * Assembly Planning level. An assembly plan is a sequence of assembly tasks that start in a state where all parts are unconnected and terminates in a state representing the final assembled product. At this level each task is specified in terms of assembly parts or subassemblies, with no reference to production line resources or tools. 8 Control Planning level. At this level each assembly task is broken down into steps, each one corresponding to an assembly robot operation. Each task at this level is specified in terms of assembly parts (or subassemblies) and production system resources, and therefore, control plans require knowledge about system resources. The choice of representation of the assembly hierarchy influences the design and implementation of the assembly planner and the control system. It is desirable to have a single unified scheme to represent assembly plans and control plans for better integration of design, implementation, and functioning of the system. The representation scheme should possess a sufficient degree of formality for rigorous analysis of plans. The ability to incorporate time, so that performance and throughput measures of assembly can be obtained, is also essential for any representation framework. However, a representation framework alone is not sufficient for automating assembly because an intelligent system must be able to analyze these plans and derive new and vital information necessary for the furtherance of the assembly. For example, essential measurements such as performance cerned with two of the levels in the assembly hierarchy.