Petri-net-based supervisory control of discrete event systems and their ladder logic diagram implementations
, PhD thesis, University of Salford.
The last decade has witnessed rapid developments in computer technology, which in
return, has found widespread applications in manufacturing systems, communication
networks, robots etc. Such systems are called Discrete Event Systems (DESs), in which
properties such as non-determinism, conflict and parallelism are exhibited. As DESs
become more complex, the need for an effective design tool and its implementation
becomes more important. Supervisory control theory, based on finite state machines
(FSM) and formal languages, is a well established framework for the study of DESs. In
supervisory control, given a model of the system and the desired system behaviour
specifications, the objective is to find a supervisor (controller) such that the controlled
behaviour of the system does not contradict the specifications given and does not
unnecessarily constrain the behaviour of the system. In general, the classes of
specifications that have been considered within the supervisory control fall into two
categories: the forbidden state problem, in which the control specifications are expressed
as forbidden conditions that must be avoided, and the desired string problem, in which
the control specifications are expressed as sequence of activities that must be provided.
In supervisory control, there are some problems when using FSMs as an underlying
modelling tool. Firstly, the number of states grows exponentially as the system becomes
bigger. Secondly, FMSs lack from graphical visivalisation. To overcome these problems
Petri nets have been considered as an alternative modelling tool for the analysis, design
and implementation of such DESs, because of their easily understood graphical
representation in addition to their well formed mathematical formalism.
The thesis investigates the use of Petri nets in supervisory control. Both the forbidden
state problem and the desired string problem are solved. In other words, this work
presents systematic approaches to the synthesis of Petri-nets-based supervisors
(controllers) for both the forbidden state problem and the desired string problem and
introduces 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
means that a controlled model of the system can be constructed and analysed using the
techniques regarding to Petri net models.
In particular the thesis considers discrete manufacturing systems. The results obtained
can be applied to high level control of manufacturing systems, where the role of the
supervisor is to coordinate the control of machines, robots, etc. and to low-level control
of manufacturing systems, where the role of the supervisor is to arrange low-level
interactions between the control devices, such as motors, actuators, etc.
An approach to the conversion from the supervisors to ladder logic diagrams (LLDs)
for implementation on a programmable logic controller (PLC) is proposed. A discrete
manufacturing system example is then considered. The aim of this is to illustrate the
applicability, strengths and drawbacks of the design techniques proposed.
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