A formal implementation of logic controllers for machining systems using Petri nets and Sequential Function Charts

In many machining processes, several machines work together with the characteristics of synchronization, concurrency, and cyclic sequences. A logic controller is a discrete event supervisory system which controls parallel and synchronized sequences of elementary operations of each machine to achieve the goal of the machining system. In this paper, a modular representation of the control logic is proposed. In the setting of a high volume transfer line, each station is considered as a module and a simple method to generate a Petri net representation from the timing bar chart of the transfer line is described. The modified reduction rule and locality properties are applied to compose the logic of Petri net modules. Using the well known properties of marked graphs, one of subclasses of Petri nets, the qualitative characteristics of the logic controller of the system such as liveness, safeness and reversibility can be verified. A live and safe marked graph can be directly transformed into Sequential Function Chart (which is one of the IEC1131-3 languages) and, using this SFC representation, a logic controller can be implemented directly. The reconfigurability properties of this approach to implementation of logic controllers are explored.