GRAFCET:
GRAFCET stands for Graphe de commande Etape Transition. It is a French word and its meaning is Step
transition Control Graph.
SFC:
SFC stands for sequential Function Chart.
It is nothing but a series of actions separated by transitions used to control sequential process.
GRAFCET is most popular in European notation and SFC is most popular in North American notation.
GRAFCET:
It is a functional diagram graph or standard, which allows to model the process to be automated,
contemplating inputs, actions to take, and intermediate processes that cause these actions. It
was initially proposed to document the sequential stage of process control systems with discrete
events.
It is emerged in France in the year of 1970, and was created by some manufacturers along with two
government agencies, AFCET (French Association for Cybernetics, Economic & Technical) and ADEPA
(National Agency for Development Automated production).
GRAFCET elements:
The GRAFCET represents the succession of phases or stages of a cycle. These phases are called Stages.
The stages have associated some actions to be implemented if the corresponding stages are active.
Switching between stages is called transition.
GRAFCET programming elements:
S No Symbol First Name Description
1 initial stage Indicates the beginning of GRAFCET diagram and is
activated when starting RUN the automata.
2 stage Its activation involves an action or wait.
3 Joints The joints are used to join each multistage
4 Transition It is condition for stopping the current stage and
activated the next stage. It is indicated by a line
perpendicular to a joint.
5 addressing indicates the activation of both stage as a function of
the condition is met. It is important to see that the
Difference between "or" and "and" in the GRAFCET is
what happens when closed.
6 simultaneous It shows the activation or deactivation of several stages
process at once.
7 Actions Actions taken to activate the stage wing belonging
associated
Basic principles:
To make the corresponding duty cycle in Grafcet language program should take into account the
following basic principles:
The process is broken down into stages that will be activated one after another.
Each stage is associated with one or more actions that will only be effective when the stage is
active.
A stage is activated when the transition condition is met and this activates the previous stage.
Compliance with a condition of transition involves the activation of the next stage and the
deactivation of the previous step.
There can never be two consecutive stages or conditions; always they must be placed
alternately.
Classification of the sequences:
In a GRAFCET we can find three types of sequences:
Linear
With or alternative routings
Simultaneous
Linear:
In the linear sequences cycle comprise a linear succession of stages as shown in the following
example GRAFCET.
With address:
In a GRAFCET with addressing the cycle can vary depending on the conditions are met.
Simultaneous:
Simultaneous sequences in various cycles can be running at once by simultaneous activation
stage.
GRAFCET stages:
Stages:
state of the system in which the outputs do not vary by varying the inputs. stable behavior.
initial stage
active
Inactive
Types stage:
normal stage
Initialisation step: those are activated at the beginning of the control algorithm
Drain stage: Stage that has no output transition
The stage is identified by the letter X followed by the number (eg X0, X3)
Actions associated with a stage:
Associated labels are indicated by the stage
Types of actions:
External: state change actuators
Internal: changes in memory, boot counters
Conditionals: if occur an additional logical condition
In a GRAFCET we can find one or more of the shares at one stage described below
The same action can be associated with different stages
Conditional actions:
The implementation of the action occurs when in addition to being active stage which is
associated, verify an additional logical condition (function Y):
Rules of evolution
Rule 1:
State or initial market: All GRAFCET must have at least one active stage in its initial state. This
initial marking must be specified in the SFC and usually represents the resting state of the
system.
Rule 2:
Transition postage: It is said that a transition is validated when all the immediately preceding
steps are active. A transition is crossed when being validated associated receptivity is true.
Rule 3:
Evolution of marking or active stages: when a transition is crossed, all the immediately
preceding steps are no longer active and all subsequent pass immediately be active. these
actions on and off occur simultaneously
Rule 4:
Simultaneous evolution: if at any given time several transitions meet the conditions to be
franked, this postage will occur simultaneously.
Rule 5:
Simultaneous activation and deactivation of a stage: If a stage is switched on and off
simultaneously, remained active.
