Correctness analysis of Petri net based logic controllers

Petri nets are able to express the causality as well as the concurrency of a control algorithm. To model logic controllers Signal Interpreted Petri Nets (SIPN) show good properties. In SIPN the firing of a transition depends on input signals from the environment and the SIPN influences the environment via output signals. Since the function of a logic controller is basically determined by software, the question of software quality arises in this area. ISO/IEC 9126 [ISO 1991] defines six characteristics of software that can be used as quality criteria. In [FREY AND LITZ 2000] the six characteristics are set in the framework of controller design: The functionality and reliability of the software rely on the correctness of the algorithm. In this contribution criteria for formal correctness of SIPN control algorithms are given and it is shown how they can be evaluated using the SIPN reachability graph. Due to a close relation between Sequential Function Chart (SFC) according to IEC 1131-3 standard [IEC 1992] and SIPN the criteria are also valid for SFC.