Short presentation of GRAFCET

This presentation is extracted from :
Editorial of the special issue ŤAdvances in GRAFCETť of the revue ŤAutomatique Productique Informatique Industrielleť Volume 27 - 5, Hermes Editions FRANCE ; 1993

The ŤSequential Function Chartť, defined in the I.E.C. 848 Standard (ŤPreparation of function charts for control systemsť International Electrotechnical Comission Publication 848) inherits from the French item GRAFCET. What is it, where does it come from, where does it go ?

In 1975, fourty French-speaking researchers and industrial managers involved in complex discrete system control, used to meet every two months to compare and investigate models and methods for building sequential system control. They gathered their own experience : seventeen techniques were then used. Some used empirical questionnaries. Other used technological modules model. Still other used pure theoretical model derived from State Machine or Petri Nets. They decided to build a custom model, easier than actual ones, and more suitable to complex system and particulary manufacturing systems. After two years of hard and laborious meetings, talks, chats, and studies, they came to propose a model called GRAFCET. This name came from Ťgraphť because the model had a graphic basis, and AFCET (Association française de cybernétique économique et technique) from the scientific association which supported the work. The basic concepts of this discrete system model where, and remains today, quite clear and simple : the Ťstepť, the Ťactionť, the Ťtransitionť and the Ťcondition associated to transitionť.

The Ťstepť represents a partial state of system, in which an action was performed. The step can be Ťactiveť or Ťidleť. The associated Ťactionť is performed when the step is active, and remains asleep when the step is idle. The Ťtransitionť which links the previous step (one or several) of any transition and the following step (one or several), represents the fact that the action(s) of the previous step(s) is followed by the action(s) of the following one(s) and figures a decision of changing system state.Nevertheless, changing is under the control of two conditions :

every step previous to the transition must be active,
a boolean condition associated with the transition, must be true.

It is similar to the Ťfiring conditionť of the Petri net, to which a boolean constraint would be added. Mere over it is a synchronous model, and all the transitions which are firable at one time, are fired. Any time the conditions are verified for one transition (or several), the previous steps become Ťidleť, and the following steps become Ťactiveť. When in conflict at a step, activity had priority on idleness.

Additionnal features concerning the shape of the input and output variables, completes the model : so are defined rising and falling edge variables, different types of action, as output signals, and delayed variable. These additions are not theoretically necessary. The simple boolean variable would be sufficient. But they simplify description and writting of real industrial control.

This model mixed the ability of the Petri net model for the concurrent modelling, the softness of boolean function to represent complex decision function, and includes directly delayed signal definition. All the features were judged necessary to suit real industry requirements.
The dynamic bahaviour was first written as five evolution rules, mentionned in IEC 848 standard :

Rule 1 : Initial situation
The initial situation is characterized by the initial steps which are by definition in the active state at the beginning of the operation.There shall be at least one initial step.
Rule 2 : R2, Clearing of a transition
A transition is either enabled or disabled. It is said to be enabled when all immediately preceding steps linked to its corresponding transition symbol are active, otherwise it is disabled. A transition cannot be cleared unless: it is enabled, and its associated transition condition is true.
Rule 3 : R3, Evolution of active steps
The clearing of a transition simultaneously leads to the active state of the immediately following step(s) and to the inactive state of the immediately preceding step(s).
Rule 4 : R4, Simultaneous clearing of transitions
All simultaneous cleared transitions are simultaneously cleared.
Rule 5 : R5, Simultaneous activation and desactivation of a step
If during operation, a step is simultaneously actived and deactived, priority is given to the activation.

Later on, these rules were completed by an algorithm, which made it clearer.

During several years, it was tested in french private companies and educational system. It revealed rapidly to be very convenient to represent small and medium sequential system. So, it was introduced in the french educationnal programs. Simultanneously, it was proposed as a standard to the french association AFNOR. It was agreed in 1982.

Then, programmable controller builders and software producers choose the grafcet as an input langage for boolean sequential control and proposed implementations on computers or controllers. The industrial use grew up. Researchers began to study the theoretical and practical use of this model on the design methods ans tools. It became a standard required in contracts.

In 1988 it was adopted by the IEC as an international standard under the name of ŤSequential Function Chartť with reference to the number ŤIEC 848ť. Translators have existed for many years, to implement GRAFCET on real time computers or programmable controllers.