Modeling of a sequential control system with cyclic scan by Petri net

Programmable Controllers (PCs) are widely applied in the control of manufacturing systems. The purpose of this paper is to give some basic concepts for the verification of sequential control systems. In particular, we propose the modeling method of the Ladder Diagram (LD) with cyclic scanning of the PC by Petri Nets (PNs). First, some experiments are conducted to examine the relationship between the cyclic scan and the behavior of the LD. Second, PNs are used for qualitative modeling of the behavior of the LD, based on IEC61131-3. In addition to LD Boolean logic expressions, we consider PC operation, described by the cyclic scan, peripherals, memories, and scanning ladder rungs. It is shown that the difference between the LD description and the behavior caused by the cyclic scan can be represented by the proposed PNs. Moreover, the behavior of the PNs can be analyzed by firing sequences, and is confirmed to be in good qualitative agreement with the experimental results. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 139(4): 79–86, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.1169