Wednesday, April 25, 2012

Supervisory Control Theory of PLC

Currently, main PLC companies established the international standard IEC 61131-3 which classifies universal syntax and elements of some programming languages. Even though the standard is not stringent enough to realize the true portability of the code between different PLCs, the programs structure is united. Common computer supported design techniques can be built up, and adjusted to an exacting kind of PLC by a suitable code generator.

A PLC might be delighted as a separate event system, which adjusts its state in reaction to amends on its inputs and time. The process under control, linked to the controller through binary signals only, might be delighted equally. In the control theory there is a well reputable field - the supervisory control theory (SCT) – involving dissimilar portions of control of the logical separate event systems,e.g., systems where the events ordering is of the main concern within the controller action of SCT is read as a enabling and disabling mechanism events in the system. The theory allows a synthesis algorithmic of a supervisor, specified a process model and a model specification.

The SCT utilizes the framework of automata modeling, where a machine is read as a generator of a formal language. The supervisor’s synthesis use Petri nets has also been considered. Regardless of the theoretical foundation sound the above results application to PLC programming is not straightforward. The basic investigation issue within the SCT is the system’s restriction behavior. This is compatible for scheming interlocks that present an important element of discrete control logic. Nevertheless, this looks less proper although some applications are reported for the sequential part.

Rather than identifying permitted event sequences the preferred operation of controlled system is more logically explained by a type of flowchart. For this reason, the IEC 61131-3 classifies a specialized graphical programming language utilized for PLC programs structuring. It is equal to Grafcet, which can be read as a particular type of Petri net. This creates framework of Petri net a good nominee for identifying event sequences in a further compact document. A wide-ranging survey on the Petri net based techniques for separate event control design can be established in.

In recent times, a joint move toward has been proposed. The SCT is utilized to create the interlock element of the control logic. The sequential element is then planned by Petri nets, which are applied in a formal specification sense. The requirement is validated against the model of acceptable behavior obtained during the synthesis of interlock.


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