Modelling and PLC-based implementation of modular supervisory control

Abstract

Implementation of supervisory control in Programmable Logic Controllers (PLCs) typically requires that the underlying discrete event models of plant, specification and the supervisor itself fulfil a number of properties. These properties are discussed and methods for solving related problems are given. In particular, determinism and synchronisation issues are investigated. In a manufacturing system, for instance, there is typically a set of processes using a set of shared resources. Poor modelling may cause the system model to become nondeterministic. This can be circumvented by relabelling the relevant models. For industrial-sized models, though, this must be done automatically, something that we achieve by parameterising the models. Given a deterministic model, consisting of several interacting entities, we discuss its PLC-based implementation. In addition to the synchronisation of plant and supervisor, a modular supervisor must be internally synchronised. However, synchronisation of the submodules is not well defined in the PLC-world. This may be achieved by using event monitors and an immediate transit/immediate action execution model.