On the execution of sequential function charts

The transition between the supervisory control theory (SCT) and its implementation in programmable logic controllers (PLCs) is not straightforward. This is mainly due to the fact that the SCT is stated in an event-based asynchronous setting whilst PLCs are signal-based, synchronous and sequential. Based on Petri nets, the PLC programming language sequential function chart (SFC) seems to be the ideal choice for this transition. However, the transition requires detailed knowledge of how the PLC and SFCs work. Different execution models for SFCs are presented and compared with the international standard IEC 61131-3. Often, the application of the SCT to a control problem results in a set of modules between which mutual exclusion and synchronisation are important aspects. The results indicate, however, that for modular SFC programs it seems generally impossible to simultaneously achieve both mutual exclusion between modules as well as synchronisation of modules without using workarounds that may be either manual or suffer from state space explosion. Nevertheless, one of the presented execution models, the immediate transit/immediate action model, together with a handshaking procedure is shown to give a simple solution to both mutual exclusion and synchronisation.