PETRI-NET-BASED SUPERVISORY CONTROL OF DISCRETE EVENT SYSTEMS AND THEIR LADDER LOGIC DIAGRAM IMPLEMENTATIONS

Abstract

The last decade has witnessed rapid developments in computer technology, which in
\nreturn, has found widespread applications in manufacturing systems, communication
\nnetworks, robots etc. Such systems are called Discrete Event Systems (DESs), in which
\nproperties such as non-determinism, conflict and parallelism are exhibited. As DESs
\nbecome more complex, the need for an effective design tool and its implementation
\nbecomes more important. Supervisory control theory, based on finite state machines
\n(FSM) and formal languages, is a well established framework for the study of DESs. In
\nsupervisory control, given a model of the system and the desired system behaviour
\nspecifications, the objective is to find a supervisor (controller) such that the controlled
\nbehaviour of the system does not contradict the specifications given and does not
\nunnecessarily constrain the behaviour of the system. In general, the classes of
\nspecifications that have been considered within the supervisory control fall into two
\ncategories: the forbidden state problem, in which the control specifications are expressed
\nas forbidden conditions that must be avoided, and the desired string problem, in which
\nthe control specifications are expressed as sequence of activities that must be provided.
\nIn supervisory control, there are some problems when using FSMs as an underlying
\nmodelling tool. Firstly, the number of states grows exponentially as the system becomes
\nbigger. Secondly, FMSs lack from graphical visivalisation. To overcome these problems
\nPetri nets have been considered as an alternative modelling tool for the analysis, design
\nand implementation of such DESs, because of their easily understood graphical
\nrepresentation in addition to their well formed mathematical formalism.
\nThe thesis investigates the use of Petri nets in supervisory control. Both the forbidden
\nstate problem and the desired string problem are solved. In other words, this work
\npresents systematic approaches to the synthesis of Petri-nets-based supervisors
\n(controllers) for both the forbidden state problem and the desired string problem and
\nintroduces the details of supervisory design procedures. The supervisors obtained are the form of a net structure as oppose to supervisors given as a feedback fiinction. This
\nmeans that a controlled model of the system can be constructed and analysed using the
\ntechniques regarding to Petri net models.
\nIn particular the thesis considers discrete manufacturing systems. The results obtained
\ncan be applied to high level control of manufacturing systems, where the role of the
\nsupervisor is to coordinate the control of machines, robots, etc. and to low-level control
\nof manufacturing systems, where the role of the supervisor is to arrange low-level
\ninteractions between the control devices, such as motors, actuators, etc.
\nAn approach to the conversion from the supervisors to ladder logic diagrams (LLDs)
\nfor implementation on a programmable logic controller (PLC) is proposed. A discrete
\nmanufacturing system example is then considered. The aim of this is to illustrate the
\napplicability, strengths and drawbacks of the design techniques proposed.