A graphical simulation tool for programmable logic controllers

Ladder diagrams have been widely used in the design of programmable logic controller (PLC) software. The ladder diagram is suitable for modelling sequential control systems, but when ladder diagrams are used to model a concurrent system, the relations among the concurrent processes are difficult to follow. In this paper, the Petri net is proposed as a substitute for a ladder diagram. The semantics of the pure Petri net is sufficient for the modelling purpose. However, the modelled systems are cluttered. An extended Petri net, the programmable logic controller net (PLCNet), is therefore defined, which simplifies the structure of the Petri net by defining more complex semantics. A set of basic structures are identified on which any control system can be constructed. A set of operation rules are formally synthesized in order to implement the semantics of the PLCNet. The operation rules constitute the rules in the rule base of the simulation tool. The database of the simulation tool is composed of the underlying models in the first-order predicate. The simulation tool resides in a user-friendly graphical environment