A new approach to evaluate ladder logic diagrams and Petri nets via the IF-THEN transformation

Abstract

The `basic element' approach has been proposed for evaluating the complexity of sequence control designs using the ladder logic diagram (LLD) and Petri net (PN). Whereas basic elements in the LLD stand for push buttons, limited switches, relay coils, timers, counters, solenoids, and lines, they are places, transitions, and arcs in the PN. Since basic elements for different structures have different physical meanings, the basic element approach may lead to misleading comparison and unreliable results. Thus, a suitable and unified measure for analyzing the LLD and PN is pursued. In this paper, a new approach via the IF-THEN transformation is proposed. The same measure obtained from the IF-THEN rules and corresponding logic operators can then be suitably applied to both LLD and PN. The provided example is of five sequences with increasing complexity for a stamping process. It indicates that the proposed IF-THEN transformation is more appropriate than the basic element approach in real applications. Moreover, results clearly show that the PN is more suitable than LLD for sequence control design as processes become more complex.