Short presentation of GRAFCET,  GRAFCET Elements, Rules of evolution

Repesentation of multiple sequences

Short presentation of GRAFCET

The «Sequential Function Chart», defined in the I.E.C. 848 Standard («Preparation of function charts for control systems» 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 «graph» 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 «step», the «action», the «transition» and the «condition associated to transition».

GRAFCET ELEMENTS

Steps, transition and links

The STEP and the associated ACTION

The «step» represents a partial state of system, in which an action was performed

The step can be «active» or «idle»

A Step is either active or idle and at a given moment the situation of the automated system is entirely defined by the total set of active steps. Situation S = (X15, X100) means the steps 15 and 100 are active
For each step are defined the actions to be carried out which are characteristic of this situation. Such actions are only effective when the step is active

These actions to be carried out when the step is active are described literally or symbolically within one or more rectangles of any size linked to the right hand side of the symbol step.

These actions may be very different in nature.
Moreover, the carrying out of these actions may be subject to other logical variables, as a fucntion of input variables, of internal variables or of the active or inactive state of the other steps.

When step 27 is active, it is necessary to:

- turn on L1 if DEF is present
- turn on L4 if PP is absent
- shut trap door n°2 if step 15 is active (X15=1)
- start timing for ten seconds, etc...

NB. the boolean variable corresponding to the active state of step i is written xi

The associated «action» is performed when the step is active, and remains asleep when the step is idle

The TRANSITION

The «transition» 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,
                     step 5 activated
• a boolean condition associated with the transition, must be true.
                      a = 1

Directed links

The Links indicate the lines of development of the state of the GRAFCET.

The links are horizontal or vertical

The general direction of the description is from top to bottom. Input and  output   at or from the step are represented vertically, the arrival being above. If in a few exeptional cases the input should be below, an arrow would be mandatory.

Arrows should be used every time they facilate a better understanding and every time the normal direction is not respected.

In order to avoid any ambiguity, it is preferable to avoid direct crossings of the link lines

Rules of evolution

• 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.

  Disabled transition	Enabled transition	Cleared  transition
  The transition 10 - 11 is disabled, step 10 being idle	The transition 10 -11 is still disabled, step 10 being active, but cannot be cleared because the receptivity a.( b+ c) = 0	The transition is cleared because  the receptivity a.( b+ c) = 1

• 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).

Disabled transition	Enabled transition	Cleared  transition
The transition is disabled, step 9 is idle	The transition is still disable, steps 9, 13 and 22 being active, but may not be cleared because receptivity a + b.c = 0	The transition is cleared as soon as the receptivity a + b.c = 1

• 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.

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