Extended Moore automata for the supervisory part-flow control of virtual manufacturing workcells

grantor: University of TorontoFlexible manufacturing workcells for discrete production usually exhibit the characteristics of a discrete-event system (DES). In the past, workcells have been sufficiently simple that intuitive and ad-hoc control solutions have been adequate. However, the increasing complexity of these systems has created a need for formal approaches for their analysis and control. A variety of methodologies, such as Petri nets and Ramadge-Wonham automata have been proposed to address the control of DESs within a formal theoretical framework. Traditionally, the objective of a workcell supervisor has been to control the discrete-event system according to manufacturing constraints and parts to be produced. In the DES supervisory-control theory, the main challenge is to synthesize a supervisor that confines the DES to a specific behavior, determined by a set of specifications. Using this theory, when any of the system's specifications changes, a new supervisor must be synthesized, which may take a considerable amount of time. In a Flexible-Manufacturing Workcell (FMC), however, it is common to frequently change the set of part types to be manufactured and/or the set of machines used to produce the parts, i.e., changing the set of specifications. Since such changes usually occur on a temporarily basis, the efficiency of traditional workcells can be increased via the utilization of virtual flexible-manufacturing workcell concept. Unlike traditional workcells, virtual workcells may be re-configured by either (i) adding to or removing from it machines, or (ii) rerouting through it parts which have not been a priori planned for. Past workcell-supervisor-synthesis methodologies have not considered the supervisory control of such workcells. This thesis address four basic supervisory-control problems for the autonomous control of workcells that can cope with parts having alternative routes which may share resources with other workcells, and be temporarily expanded by the introduction of either new part types or new machines into their nominal configurations. These are the building blocks necessary for the control virtual workcells. The proposed control techniques use Extended-Moore Automata (EMA) and Ramadge-Wonham theories to synthesize deadlock-free supervisors. The use of EMA also allows the minimization of the state-space of supervisors, which has been a fundamental problem in DES control. Moreover, the proposed supervisor techniques allow the automatic implementation of the (EMA-based) supervisors using commercial controllers such as industrial Programmable-Logic Controllers (PLCs). The supervisor synthesis methodologies proposed in this thesis have been verified to be viable techniques through their actual implementation using an experimental robotic workcell. Different EMA-based supervisors have been successfully implemented using a user-friendly supervisory-control software package developed in our laboratory for MS-Windows environments.Ph.D