GRAFCET - A POWERFUL TOOL FOR SPECIFICATION OF LOGIC CONTROLLERS

Basically, a logic controller is a discrete-event system whose purpose is to control the behavior of a process which is itself (seen by the controller as) a discrete-event system, taking into account the state of this process and other information coming from an operator or from other systems. In the early 1970's, the need to describe increasingly complex logic controllers was becoming evident, since the programmable logic controllers (PLC) were becoming more powerful and more extensively used, This paper shows how Grafcet can be used for this purpose. Grafcet is a tool, drawing its inspiration from Petri nets (a general purpose mathematical tool allowing various discrete-event systems to be described), whose aim is the specification of logic controllers. It is the basis of the Sequential Function Chart (SFC), an International Standard in 1987. This paper, introductory in nature, explains the Grafcet model using the classical-state table model and presents a method for interpreting grafcets.