Evaluating the complexity of Petri nets and ladder logic diagrams for sequence controllers design in flexible automation

The varying control specifications of today's flexible and agile manufacturing systems are challenging the use of ladder logic diagrams (LLDs) and high-level programming language methods. More recently, Petri nets (PNs) are increasingly used to design sequence controllers. However, to establish PNs as an alternative to LLDs there is a need to justify the PN method. This can be partially accomplished by comparing the complexity of PN and LLD designs. This paper presents a methodology to evaluate the complexity of PNs and LLDs for sequence controllers design. Complexity of a design is characterized by the number of basic elements used to model the given control logic. More specifically, this paper presents analytical formulas to estimate the number of basic elements to model certain building blocks of logic modeling when PN and LLD are used. Also, by presenting a methodology to use these analytical formulas, this paper precludes the need for physically building the controllers by either PN or LLD for the comparison of their graphical complexity. The results are demonstrated by considering two examples of sequence controllers. Finally, the limitations of the present work are presented along with possible extensions for future research.