Grafcet

tombobek

Member
Join Date
Apr 2002
Posts
2
Hi :)
I'm looking for good and free manuals for GRAFCET (in english, no in france). Thank you for yours help.
 
Hugh Jack's online book have a chapter regarding grafcet

2 different article's may also be of interest: PID auto-tuning using grafcet and Comparison between grafchart and grafcet

The basic of grafcet is easy:

Init step The start step after cold start/initialization

Step Action

Transition Condition to go to the next step

Macro-step Set of step and transitions

AND branch 1 transition leads to 2 or more steps

OR branch 2 or more alternative branches, each with a transition


Its said that only the french is following the standard, so check also websites after online manuals for spesific PLC brand.

Saia, Telemecanique and Siemens (Europe) and, as RSdoran showed, also AB (US) have grafcet. Im sure there is more.

Telemecanique e.g. have 3 types of action associated with steps:

action on activation actions executed once the step is activated

actions on deactivation actions executed once the step is deactivated

continuous actions actions executed cont. as long as the step is active


Other PLCs have only one type of step-action: "action on activation".

I also looked at Siemens S5 and grafcet once, it looked like a pain in the.... Depending on the structure, the prog had to declare some internal bits and timers, and if the structure was increased later...

So, chech the spesific PLC brand how grafcet is implementet (or not implemenet)

I use grafcet a lot. It is useful for many "sequence" tasks like communication and motion control. One example is a mashine in a assembly line with 11 axes. Grafcet is used both to position and coordination between axes. I dont think I would have done that assembly line without a PLC with grafcet.

One thing is rules how to put together steps and transitions, another topic is what to put in the steps/transitions.
I will only mention 2:
1: Avoid set/reset outputs inside the grafcet structure. Use internal bits instead and have the output code on the outside, with interlocks,fuse etc
2: The grafcet structure is fixed. There must be a way out if things hangs (else than the power switch). Code for reset structure e.g.
 

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