Sunday, April 22, 2012

Control Logic Synthesis of PLC

Based on the two standard paradigms combination applied in the field of separate event systems: supervisory control theory, applied to create the interlock logic, and Petri nets, applied for requirement of the sequential logic and certification of non-blocking.

Multi-stage approach
It is to design of the control logic is schematically. The requirements are divided in two elements. The first element engages prevention of undesired behavior. It is created called as interlocks that realize measures to guarantee safety, organize sub-processes, etc. The second element of requirements deals with the sequential behavior and identifies prescribed tasks order. The sequencing element of the control logic is only created after the interlock element has been intended.

The control logic Interlock element
The interlock set of supervisors is planned within the framework of SCT. Other than the supervisors state machine models that may be simply realized in programming software of PLC, the synthesis result is also a admissible model behavior, e.g., all possible event model of sequences in the managed system that fulfill with the interlock requirement. One of the approach key points is that this model is applied as a model of open-loop process when scheming the sequencing element of the control logic.

The control logic Sequencing element
The sequencing controller works a dissimilar role than the supervisors of interlock. Instead of allowing or disable the happening of occurrences in the system it has to aggressively activate events that effect in a state alteration of the actuating parts of the process. The sequencing part design of the control logic might also be executed within the SCT, e.g. but the design advance is not as easy as with the interlock element. An option way is to make official the requirement, e.g. by a Petri net, and to officially validate preferred properties.

An benefit of the Petri net symbol is the simple path from the developed requirement models to the industrial execution. This is because of the closed connection between Grafcet, Petri nets, and SFC which allows a SFC to be straight redrawn from a model of Petri net and a number of the traditional analysis methods of Petri nets can be practical also to SFCs. The part synthesis of is executed manually throughout modeling the sequential requirement by the extension of RTPN (Real-time Petri nets) Petri nets. Once the model of RTPN is acquired, it is algorithmically confirmed for non-blocking against the permissible behavior model acquired in the design stage of interlock.


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