Abstract: In complex Programmable Logic Controller (PLC) control system,traditional flow sheet is very hard to describe complex events such as concurrent,coordination and competition.It constructs models of Liquefied Natural Gas (LNG) cylinder line production control system by using the ability of Petri nets in graph and model.Through hierarchical model and analyzes,potential problems are eliminated such as deadlock and conflict.Logic expression is listed based on Petri net’s condition of transition and change of place.It makes a connection between logic expression and PLC Ladder Logic Programming Language (LAD).According to the Petri net’s model,PLC program can be designed clearly and readably.Through practical operation,the system is stable and reliable.

Key words: Petri net, model, Programmable Logic Controller (PLC), control