PN PLC: A methodology for designing, simulating and coding PLC based control systems using Petri nets

Flexible manufacturing cells (FMCs) are well established as a means of improving manufacturing efficiency in many batch production industries. They consist of an integrated group of computer numerically controlled (CNC) machine tools, programmable logic controller (PLC) based work/tool handling equipment and a control system to synchronize their operation. The safe and reliable operation of FMCs is clearly essential for their efficient use. However, FMCs are complex systems with elements that operate concurrently and interact at irregular times, depending upon the components to be produced. This inherent complexity demands intricate and sophisticated control systems which require development methodologies that are both comprehensive and rigorous. In this paper the authors introduce a methodology (PN PLC) which uses Petri nets for producing correct programmable logic controller (PLC) programs directly from the specifications. The methodology is specifically contrasted with the benefits, and problems, encountered in using tools such as 'ladder diagrams' (LDs) and 'sequential function charts' (SFCs) for designing logical control systems. PN PLC has advantages over ladder logic in clarity, which allows the program to be checked, maintained and updated reliably. Furthermore, the Petri net graph is a one-to-one equivalent of a ladder logic diagram, and thus either representation may be algorithmically translated to the other without loss of information. Finally, the rules for creating Petri net graphs, and subsequently translating them, are both simple and robust in use.