---
title: GRAFCET
lang: en
type: notion
tags:
  - eee
  - notion
prerequis:
  - algorithme-en
aa:
  - RA-EEE-C03-2/EEE/5
phases:
  - concept
  - preuve-de-concept
draft: false
source_fr: embarque/algorithme/grafcet.md
source_sha256: facd47a455a2c574010ced11b68e154a51d4d2fbdffe93122f057156df933f2e
---

**GRAFCET** (*GRAphe Fonctionnel de Commande Étapes-Transitions*, the French acronym for a step-and-transition control chart) is a **standardised** graphical language (**IEC 60848**) that describes the sequential behaviour of an automated system. It represents a control [[algorithme-en|algorithm]] as **steps** (what the system does) joined by **transitions** carrying the conditions for moving on, and it excels wherever several sequences advance **in parallel**: it is the tool of industrial automation.

![The symbols of a GRAFCET chart: an initial step as a double square, numbered square steps carrying actions to their right, transitions as horizontal bars with their transition conditions, and an arrowed return link.](/ressources/img/grafcet/generique.svg)

## What is it for?

For a process that runs through phases (advance, drill, retract, eject), GRAFCET describes the sequence in a way that is **unambiguous and shareable**. Its strengths:

- it is **standardised** (IEC 60848) — a GRAFCET chart reads the same way to any automation engineer, it is a common language;
- it cleanly separates **what the system does** (the actions of the steps) from **what makes it move on** (the conditions on the transitions);
- it handles **parallelism** — several simultaneous sequences, something a [[logigramme-en|flowchart]] or a simple [[machine-a-etats-en|state machine]] struggle to express;
- it translates directly into programming a **PLC** (the SFC language is very close to it).

It is the representation to prefer as soon as you are talking about a **machine cycle** or about **automation**. For a plain chain of decisions with no parallelism, a flowchart stays lighter.

## How to read a GRAFCET chart

GRAFCET strictly alternates **steps** and **transitions**, joined by **links** read from top to bottom.

- **Step** (numbered square) — a situation of the system. The **initial step** (double square) is active at start-up. An active step holds a token. The **actions** attached to it (boxes to its right) are carried out **for as long as the step is active**.
- **Transition** (horizontal bar) — separates two steps and carries a **transition condition**, called a *réceptivité* in French: the boolean condition for moving on (a sensor, a button, a logical combination written with `·` for AND and `+` for OR).
- **Clearing rule**: a transition is cleared when **the step above it is active AND the condition is true**. On clearing, the step above goes inactive and the step below goes active — in the same instant.

A **return link** (arrowed when it goes back up) closes the cycle. On a plain sequence, only one step is active at a time. Parallelism goes through divergences instead (see *Special case*).

## Example — Drilling station

A station that drills a workpiece then ejects it, in a cycle. At rest (step 0), nothing moves. The chart then runs through four situations.

The diagram is the French one, so its labels are in French: `départ`, `pièce présente`, `Descendre broche`, `Remonter`, `Éjecter` and `pièce évacuée`, that is start, workpiece present, lower the spindle, raise, eject and workpiece cleared.

![Cyclic GRAFCET chart of the drilling station: initial step 0 at rest, cleared on "start and workpiece present" to step 1 Lower spindle, then "spindle down" to step 2 Raise, "spindle up" to step 3 Eject, and "workpiece cleared" looping back to rest.](/ressources/img/grafcet/percage.svg)

The condition on the first transition, start AND workpiece present, is a **logical combination**: the cycle only starts if the operator presses start **and** a workpiece is in place. Each step does one thing and only hands over on the next explicit condition. The sequence is readable, and transcribes as it stands onto a PLC. It is that standardised clarity that makes GRAFCET preferable to a flowchart for a machine cycle.

## Pitfalls

**Mixing up step and action.** The step is a *situation* ("on the way down"). The action is *what you do* while in it ("lower the spindle"). The action lives in a box to the right of the step, not inside the square.

**A forgotten transition condition.** A transition with no condition is cleared straight away. The chart "falls" from one step to the next without waiting for anything. Every transition carries a condition.

**A missing initial step.** With no double square, the chart does not know which situation to start in. One and only one initial step per chart (parallelism aside).

**Several steps active by mistake.** On a plain sequence, only one step should be active at a time. Several tokens at once, where none were intended, give away a badly placed divergence or a badly closed loop.

**Mixing up the chart and the PLC code.** GRAFCET describes the *requirements* for the behaviour (IEC 60848). It is very close to a PLC's SFC language, but it stays a specification, not the program itself.

**An AND divergence never closed.** What opens in parallel (AND divergence) has to be resynchronised (AND convergence). Forgetting the convergence leaves branches that never meet again.

## Special case — AND and OR divergences

Two structures set GRAFCET apart from purely sequential representations. The **OR divergence** (sequence selection): several transitions leave the same step, and depending on the conditions **only one** branch is taken, an exclusive choice. The **AND divergence** (parallelism): a single transition activates **several steps at once**, launching parallel sequences, which will have to be resynchronised by an AND convergence. It is that ability to handle parallelism that makes GRAFCET what it is. A [[logigramme-en|flowchart]] or a simple [[machine-a-etats-en|state machine]] do not express it naturally.

![The two divergences of GRAFCET: on the left the OR divergence — from one step, two branches each carrying its own transition, only one being taken according to which condition is true; on the right the AND divergence — a single transition then a double bar activating two steps in parallel, resynchronised lower down by a double-bar convergence followed by a single transition.](/ressources/img/grafcet/divergences.svg)

## See also

- [[algorithme-en|Algorithm]] — the parent page, which places GRAFCET among the representations
- [[machine-a-etats-en|State machine]] — close in spirit, but with no standardised parallelism and no PLC framing
- [[logigramme-en|Flowchart]] — lighter for a chain of decisions with no cycle and no parallelism
- [[chronogramme-en|Timing diagram]] — complementary, for checking the timing of a cycle's signals
- [[preuve-de-concept-en|Proof of concept]] — where the sequence gets implemented and tested