GRAFCET

	DIN EN 60848
Area	automated production systems
title	defines a graphic design language
Brief description:	GRAFCET - specification language for function charts of the sequence control
Latest edition	2014-12
ISO	IEC 60848: 2014

GRAFCET (acronym from “  GRA phe F onctionnel de C ommande E tapes / T ransitions”), EN 60848 , is a specification language for viewing process descriptions. It is mainly used in automation technology , but also in process engineering .

The standard is the successor to DIN 40719-6 function plan and replaced it on April 1, 2003. Compared to the function plan, many things are more clearly defined in GRAFCET. Many parts of the command scope have been simplified and some new options have been added.

In contrast to the function diagram that has been replaced, the sequence language of IEC 61131-3 is expressly mentioned in the GRAFCET standard as a possible implementation of a GRAFCET diagram and is still a valid standard.

The function diagram of the Siemens programming language STEP 7 largely corresponds to the function block language of IEC 61131-3 . The representation type S7 graph is based on GRAFCET.

purpose

Grafcet is a graphic planning and documentation tool for sequence controls, more precisely for the " sequence part of a control system".

Outline of a Grafcet

With Grafcet the representation of a sequence control is structured horizontally and vertically: The vertical arrangement is made in the structure and functional part of the sequence control. The horizontal structure takes place in transitions and steps .

Breakdown into structure and functional part

Simplest structure of a Grafcet

The structure consists of

• Steps
• Transitions
• Active compounds
• and - if desired - comments and step / transition names

The structure thus graphically shows the sequence of movements in a sequence control. Step and transition names must be unique, comments must be in quotation marks. The step name (the number in the picture) defines the step variable according to the following pattern: Xstep name . An "X" is placed in front of the step name. The so-called step variable can be used in the effect part, for example.

The planning of a sequence control begins with the structure of the Grafcet with sensible comments so that the motion sequence of the control can be recognized.

Structure of a Grafcet with comments

The functional part of a Grafcet consists of

• Transition conditions,
• Actions
• and - if desired - comments.

The effect part can only be created if there is a structure.

Complete (very simple) Grafcet with structure and functional part

Structure in steps and transitions

The decisive elements of a Grafcet are steps (with actions) and transitions (logical conditions for the transition from the previous to the next step). Steps and transitions are connected by the functional connections. The basic rules for this are:

• Steps and transitions always alternate. An operative connection can only connect a step with a transition or a transition with a step, but never step with step or transition with transition.
• Active connections always work from top to bottom or from left to right. If other effective directions are required, they must be marked with an arrow.
• A Grafcet often has an initialization step (with a double border) which defines where / with which step the step chain begins. However, included steps are an exception to this. The double frame can be dispensed with here, because in the case of included steps, the step is marked with an asterisk (*), which becomes active exactly when the enclosing step becomes active. In addition, a partial Grafcet can be activated by a forced control command (double frame in the action box). An initial step can also be dispensed with in this case.

• Most Grafcets are closed Grafcets, so they have a jump from the end back to the beginning. This results from the purpose of Grafcet, the representation of sequence controls in production: In order to be able to manufacture several parts, step chains have to be repeated.

The transition supplies a Boolean signal, i.e. only TRUE or FALSE ('1' or '0'). Boolean links can be represented both “mathematically” and graphically. The AND link with the *, the OR link with the +, the negation with the overline, the rising edge with the arrow pointing upwards and the falling edge with the arrow pointing downwards. Times are set before (switch-on delay) or after (switch-off delay).

The step has a unique variable name which - with the help of the identifier X - can be queried as a Boolean variable in transitions or conditions. If z. For example, if the step has the name 17, the variable name is X17. One or more actions can be assigned to the step (but need not). There are also steps without actions; these are accordingly referred to as spaces.

The action contains instructions on what should happen when the associated step is active or becomes active or is deactivated.

scope

The processes are divided into steps and transitions ( step enabling conditions). The alphanumeric identification can be found in the title block. Comments can be added as required, they must be in quotation marks. The transition condition is to the right of the transition. It can be supplemented by a transition name on the left side of the transition. It must be in brackets. The step enabling condition is usually expressed as a Boolean equation . The mal point (alternatively also asterisk) describes an AND link. The plus sign is an OR link. The following rule applies: AND before OR (dot before dash, as in mathematics). Negations are indicated by a line above the variable name, rising or falling edges with an up or down arrow in front of the variable name. Temporal events are described in the form t1 / variable name / t2. The event becomes true after the time t1 after the rising edge of the variable designated with the variable name and the time t2 after the falling edge still remains true. The time t1 or t2 and the associated slash can optionally be omitted. Actions can have different behaviors. This becomes visible through additions to the actions. A distinction is made between:

• continuously acting actions (former N-command)
• Actions with assignment condition (former C command)
• Retentive actions when step activation or deactivation (former S command)
• Retentive actions in the event of an event
• delayed actions (former D command)
• time-limited actions (former L command)

With GRAFCET, it is possible to structure processes hierarchically in order to model complex relationships in a clear manner. This is useful , for example, to display operating modes such as manual / automatic or emergency stop .

For this purpose, positive control commands and enclosing steps are used.

Difference to S7-GRAPH

• GRAFCET is a technology-independent specification language for planning and describing sequence controls.
• S7-GRAPH is the STEP 7 variant of the standardized PLC programming language Sequential Function Chart (AS).

The S7-GRAPH programming language does not reflect the GRAFCET standard EN 60848. This sequence language from Siemens AG is only one possible implementation of a GRAFCET plan. S7-GRAPH fulfills the PLC standard EN 61131-3 (programming with sequential function chart). It is essential to distinguish between these two standards (logical planning / documentation versus real implementation).

GRAFCET editor

Several GRAFCET editors are now available so that a GRAFCET plan no longer has to be created as a drawing.

• The free open source drawing program DIA can also be used to create GRAFCET plans.
• sfcedit is a small, pure editor for GRAFCET from France (available in German, English and French). The program can also be used as a portable version. B. can be used from a USB stick without installation under Windows.
• FluidSIM (V4.2 and higher) contains a GRAFCET editor that allows you to simulate a GRAFCET plan.
• WinErs is a process control system with soft PLC. It is also used as a training simulation system for process technology. It contains a GRAFCET editor with which process engineering systems can be planned and simulated.
• Omegon Teachware offers a free Grafcet version of the Omegon Fluid Technology OFT2 software that can be used to create Grafcet plans.
• With the help of the LaTeX package "grafcet", GRAFCET plans can be created within a LaTeX document.
• GRAFCET Studio is an editor with which GRAFCETs can be created and run. A transfer to a PLC is also possible.

literature

• Christian Duhr: GRAFCET. EUROPA-Lehrmittel Bildungsverlag, 2nd edition. Work book. 2018, ISBN 978-3-8085-3769-5
• Documentation in electrical engineering, presentation rules. DIN-VDE-Taschenbuch 530. Beuth Verlag, Berlin 2004, ISBN 3-410-15932-0 . This includes DIN EN 60848 GRAFCET
• Gerhard Schmidt: GRAFCET. Festo Didactic, 2nd edition, Esslingen 2015. Order no. 548678, ISBN 978-3-427-54867-6 .
• Bernhard Plagemann: GRAFCET crash course. Dr.-Ing. Paul Christiani GmbH, Constance 2008. Order no. 82459, ISBN 978-3-86522-441-5

Individual evidence

1. ↑ sfcedit ( Memento from December 28, 2017 in the Internet Archive )
2. ↑ FluidSIM
3. ↑ WinErs
4. ↑ OFT2 ( Memento of the original dated February 2, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
5. ↑ [1]
6. ↑ GRAFCET Studio