Modeling of Sequential Control System with Cyclic Scan by Petri Net

Abstract

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 methods of sequential control systems. Especially, we propose the modeling method of Ladder Diagram (LD) with the cyclic scan of PC by Petri Nets (PNs). First, some experiments are conducted to examine the relationship between the cyclic scan and the behavior of LD. Second, PNs are used for the qualitative modeling of the behavior of LD, based on IEC61131-3. In addition to LD Boolean logic expressions, we consider the PC operation, described by the cyclic scan, peripherals, memories, scanning ladder rungs. As a result, 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 the firing sequences, and then confirmed in good qualitative agreement with that of the experimental results.