Automatic Implementation of Petri net based Control Algorithms on PLC.
Proceedings of the American Control Conference ACC 2000, Chicago, June 28-30, 2000, pp. 2819-2823.
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 ta can fire in an iteration after transition tb. In this case, the code segment describing tb has to be processed prior to the one of ta. A solution for the case, where the analysis results in an impossible ordering is also presented.