Conversion of LD Program Into Augmented PN Graph

Abstract

This article presents a process of converting the ladder diagram (LD) into the Petri net (PN) by analysing the attributes of the LD program and the characteristics of the programmable logic controller (PLC) operation. It also presents a new representative conversion method of obtaining a PN graph from an LD program. It is mainly developed to equip LD programs with the formal analysis capability normally available in the Petri net theory. In order to represent the PLC operations with the PN graph, the concepts of preservation arc, buffer place, and reset arc are introduced, and the conversion table that converts the core LD functions into the PN graph is constructed. In order to analyse mathematically the converted PN, the augmented Petri net state equation is created using firing condition function. The process of a material transport car (MTC) is used as an example to demonstrate the validity of the presented PN. The simulation results for the process of the MTC demonstrate that the presented PN is a valid approach to represent PLC programs.