Automatic implementation of Petri net based control algorithms on PLC

Abstract

Signal interpreted Petri nets (SIPN) show good properties in modeling and analyzing control algorithms. With SIPN, logic controllers are modeled by places-setting output signals-and transitions between those places-depending on Boolean functions of input signals. This model has interesting dynamics: Firstly, several transitions can fire simultaneously. Secondly, there can be iterated firing of transitions before a new stable marking is reached. In standard PLC programming languages, there is no direct means to implement these dynamics. In this contribution a method for the generation of transparent PLC code from SIPN is presented (using instruction list or ladder diagram). The code is transparent because there is a one-to-one correspondence between SIPN elements and code segments. A new analytical method to guarantee the correct dynamics of the implemented code is presented and compared to existing approaches. The new method results in a special ordering of the code segments. The presented analysis of the SIPN shows if a transition t/sub a/ can fire in an iteration after transition t/sub b/. In this case, the code segment describing t/sub b/ has to be processed prior to the one of t/sub a/. A solution for the case, where the analysis results in an impossible ordering is also presented.