PLC Programming for Hybrid Systems via Signal Interpreted Petri Nets
To appear in: Proceedings of the 4<super>th</super> International Conference on Automation of Mixed Processes ADPM, Dortmund, Germany, September 18-19 2000, pp. 189-194.
To analyze control algorithms for hybrid processes a purely discrete event approach is-in most cases-not sufficient. First of all, hybrid processes often produce analog sensor signals, these signals have to be converted to events to be used in the discrete controller. Furthermore the controller has to set analog output signals to influence the process. An often used approach to include analog values in discrete controllers is to program pre- and post-processors for the external input signals. With this approach on the one hand the controller remains purely discrete but on the other hand the pre- and post-processors can not be included in the analysis of the controller. To overcome this problem, in this contribution Signal Interpreted Petri Nets (SIPN), which are a Petri net model with binary input and output signals suited for the specification of PLC programs are extended to include analog signals. Furthermore, SIPN are extended to timed SIPN. It is shown how the extended model can be analyzed, using a reachability graph that includes timing information in the form of time intervals, and how it can be implemented on PLC using Sequential Function Chart according to IEC 61131 standard.