Anders Hellgren

PLC-based implementation of supervisory control for discrete event systems

The supervisory control theory is a general theory for automatic synthesis of controllers (supervisors) for discrete event systems, given a plant model and a specification for the controlled behavior. Though the theory has for over a decade received substantial attention in academics, still very few industrial applications exist. The main reason for this seems to be a discrepancy between the abstract supervisor and its physical implementation. This is specifically noticeable when the implementation is supposed to be based on programmable logic controllers (PLCs), as is the case with many manufacturing systems. The asynchronous event-driven nature of the supervisor is not straightforwardly implemented in the synchronous signal-based PLC. We point out the main problems of supervisor implementation on a PLC, and suggest procedures to alleviate the problems.

Modelling and PLC-based implementation of modular supervisory control

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.

Modular implementation of discrete event systems as sequential function charts applied to an assembly cell

Modular implementation of discrete event models in programmable logic controllers (PLCs) using sequential function charts (SFCs) is investigated. Synchronisation of such models, typically finite state automata of Petri nets, is not well defined in the synchronous PLC-world. We develop an algorithm that computes event monitors for synchronising concurrently executing SFCs. The methodology is applied to an assembly cell.

Desco — a Tool for Education and Control of Discrete Event Systems

Within the Control and Automation Laboratory at Chalmers, there has been developed a software suite to facilitate the manipulation of state automata and Petri nets for supervisor calculation (among other things). This suite of software tools includes a graphical automata/Petri net drawing tool, a commandline based automata/Petri net manipulation tool and a graphical visualisation tool. The two first tools (jointly named Desco, for Discrete Event Systems Controller), consisting of the N’gin (that is, the mathematical manipulation engine) and the GUI (the graphical user interface) have been developed at the Control and Automation Laboratory. The third tool is a general graph drawing software, GrapViz, from AT&T research (see http://www.research.att.com/sw/tools/graphviz/) which is closely integrated with Desco.

On the execution of discrete event systems as sequential function charts

The transition between the supervisory control theory (SCT) and its implementation in programmable logic controllers (PLCs) is not straightforward. This is mainly due to the fact that the SCT is stated in an event based asynchronous setting whilst PLCs are signal based and synchronous. Based on Petri nets the PLC programming language, the sequential function chart (SFC), is an ideal choice for this transition. However, the transition requires detailed knowledge of how the PLC and SFCs work. We present different execution models for SFCs and compare them with the international standard IEC 1131-3.

Controllability Revisited: A Generalization for the Modular Approach

Abstract In the Supervisory Control Theory, originally presented by Ramadge and Wonham, Ramadge and Wonham (1989), an important property is controllability. The definition of controllability as originally presented assumed equal event sets, alphabets, in the specification and the plant. When this assumption does not hold, it has been argued that the differences in the alphabets can easily be removed. However, no matter how we choose to handle the differences, pursuing verification with the original definition will lead to false conjectures in some cases. A few examples on this is presented in this paper. In order to overcome the problem, a generalized controllability definition, allowing non-equal alphabets, is presented. It is of great importance to be able to verify controllability of subsystems with different alphabets when exploiting the modularity of discrete event systems for computationally efficient verification and synthesis.

Deadlock detection and controller synthesis for production systems using partial order techniques

Discrete event systems can be used to model the behaviour of production systems. The supervisory control theory is a suitable tool for synthesising controllers that coordinate the resource utilisation of concurrent products in the production system. Due to the combinatorial state space explosion, computations become intractable for most real life systems. To alleviate this problem, methods that do not enumerate the entire state space are needed. One method that has proven valuable for the verification of concurrent systems is based on partial order principles. For a certain class of production systems it is shown how such ideas may be used to synthesise non-blocking discrete event controllers.

Modeling primitives and specification structures for supervisory control

Three different modeling languages for discrete event systems are compared, automata, Petri nets and process algebra, and it is shown how a couple of basic primitives can be modeled in these languages. Based on these modeling primitives an architecture for a general routing and resource booking problem is presented. The architecture is based on general models for a set of resources, desired routing specifications for a set of objects (products, data packets, vehicles) and a controller that synchronizes the objects utilization of the available resources. High level graphical routing specifications for the objects are also introduced, together with corresponding Petri nets, in order to simplify the specification of desired routes. As applications of the suggested architecture we consider cell controllers for flexible manufacturing systems and multi-purpose batch plants in chemical processing industry.

Database Design for Machining Cell Resource Models

Abstract To promote flexibility of manufacturing systems a reference architecture, CHAMP, has been developed at Chalmers University of Technology. It uses object oriented principles and a database to model separate parts of a production system. Here, the resource models of the control system and the corresponding parts of the database are explored and described. It is emphasised on the interaction between the internal resource objects, where a generalised handshake protocol is proposed.

Prioritised Synchronous Composition of Inhibitor ARC Petri Nets

The supervisory control theory includes constructing a set of models of discrete event systems. These models are to operate concurrently and interact with each other. In this paper the prioritised synchronous composition (PSC) of Petri nets is discussed. The PSC is an important composition formalism because it captures controllability without requiring the supervisor to be complete, for instance. PSC for Petri nets requires testing the number of tokens to be less than the arc weight. One way to realise such tests is to use weighted inhibitor arcs.