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

Abstract

Flexible manu facturing workcells for discrete production usually exhibit the characteristics of a discreteevent system (DES).In the past, workcells have been sufficiently simple that intuitive and ad-hoc control solutions have been adequate.However, the increasing complexity o f these systems h a s created a need for forma1 approaches for their analysis and control.A variety of methodologies, such as Petri nets and Rama&=-Wonham automata have been proposed to address the control o f DESs within a formal theoretical framework. \Traditionally, h e objective o f 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, deterrnined 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 arnount of time.In a Flexible-Manufachiring Workcell (FMC), however, it is cornmon to frequently change the set o f part types to be mnufactured andlor the set of machines used to produce the parts, Le.. changing the set of specifications.Since such changes usually occur on a temporarily b a i s , the eff~ciency of traditional workcells can be increased via the utilization of virtual flexible-manufachiring 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 prion 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.. .- 111The proposed control techniques use Extended-Moore Automata (EMA) and Rarnadge-Wonham theories to synthesize deadlock-fiee supervisors.The use of EMA also allows the minimization of the state-space of supervisors, which has been a fndarnental problem in DES control.Moreover, the proposed supervisor techniques allow the automatic implernentation 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 achial implementation using an expenmental robotic workcell.Different EMAbased supervisors have been successflly implemented using a user-friendly supervisory-control software package developed in our laboratory for MS-Windows environrnents.