Writing in Grafcet..?

I'm trying to write a few steps in Grafcet for an elevator program. But finding it hard to do so. Can anyone tell me how to upload a pic to show you what I've done..then maybe you can guide me in a good direction.

Thanks

Chris

Also how do you show NOT, AND, OR, NAND functions with Grafcet. I have done some web searching and found very little. I probably won't be doing PLC programming for life, so buying aother 50quid book doesn't seem worth it.

This is after a few attempts, its wrong..as I've just been reading more about it but so far the only page which has been clearer about how it works, is the one I'm reading now. But anyone out there can give me a clear insight abouthow this type of programming works. IF each action is an output, where do I put the Input's? are these the transitions?

http://static.flickr.com/37/105222423_e43c10fc38.jpg?v=0

Any help would be highly appreciated.

Chris

what is a quid?

Fred Raud

What is a buck ?

Hello Chris;

Have a look at the following diagram, showing an implementation in Grafcet of an automatic drill control:





Grafcet, also known as SFC (sequential file chart, in IEC-61131-3), uses Step/Transition sequences to control a sequential process.

Steps are shown as S1, S2... Here is where you organise your actions: Set a bit, Reset a bit, start a Delay timer, call a function block...

Transitions are shown as the snippets of ladder logic; they determine the conditions that allow passage from one step to the next.

You do not write Nand, Nor, Xor functions in Grafcet; what you do have is And or OR paths, called divergences: parralel paths that your logic can follow, either all at the same time (AND divergence) or selectively (OR divergence, used between S3 and S4).

Rigid conditions must be maintained for the Grafcet to be executable: only one step active at any time in a linear branch; to go to the next step, the preceding step must be active AND the transition conditions for that step must be true....

In order to write your function in SFC, start by illustrating the different states your process will occupy when functionning. These will be your steps. Then show what control actions you need for every step. Finally determine the logic for your transitions.


The example is from a Siemens manual for their S7-Graph add-on to Step7. Allen Bradley has manuals for their SFC functions. The main ressource for Grafcet would now be IEC-61131-3 standards.

Hope thios helps,

Daniel Chartier

10baseT said:

What is a buck ?

Click to expand...

a does husband

Fred Raud

Chris,

I'll try to make this easy.

Every state is a memory. The set of that memory is the combination of the state where you come from and the transition conditions. The reset of the state is the next state.

Not every action is an output, but the outputs are depending on the actions. If we look at the grafcet for traffic lights (yes, although we hate 'em here, I'll take that example), there are several different actions where a certain light will be on. Check out this grafcet to see what I mean. Extra signals, such as timers and counters, are also considered actions.

The inputs are used to make up the transitions amongst other things. Inputs can also be used to make intermediate signals, which in turn are used to make up the transitions.

To make everything crystal clear let's look at some examples taken from the above mentioned grafcet.

Step example:

Normal cycle Step 0 Set = Safety start Step 2 AND MR_GR OR Normal cycle Step 5 AND 3s

  Normal cycle Step 0 Reset = Normal cycle Step 1

  

Action example (main road orange light):

MR_OR =  Alarm mode AND 1Hz

        OR Safety start Step 0 AND 2Hz

        OR Safety start Step 1 AND 2Hz

        OR Normal cycle Step 1

  

As you can see, you can use grafcet to design everything that has memories and logical functions. I have used grafcet to design electronics, pneumatics, relay logic, PIC (microcontroller) programs, and yes, PLC programs too.


[EDIT]Damn Daniel, you beat me to it. But I see you stick to the Siemens Graph package. As stated in this post, you don't need a graph or SFC package to design using grafcet. I use it for every PLC I know, and (as stated) for other techniques as well[/EDIT]

Kind regards,

Fred Raud said:

what is a quid?

Fred Raud

Click to expand...

Pardon my English.. quid is a rough term for a pound sterling.

if there's two different outcomes which have to be achieved with similiar inputs but with OR functions for two of the inputs.. how do I show this?

Can you connect two SFC's together?

i tried writing it up as a flow diagram as has been suggusted to me, though I don't see my teacher until Wednesday.. It doesn't seem correct but then flow diagrams, SFC, I much prefer the Boolean.. numbers seem easier to process than these picturial diagrams. But unfortunately my course asks me to demonstrate much more one programming method

http://static.flickr.com/40/105260936_2104507109.jpg?v=0

Hello Jean-Pierre;

I only use the Siemens package for illustration purposes.
Mostly I will use Grafcet as a development technique (like flowcharts) that I can then easily program in ladder. The translation techniques (Grafcet to ladder) are well-known and simple to implement.
The few instances where I have used S7-Graph have been interresting but very costly in memory and processor time (on a Cpu S7-315 at least). When you program PCS7, it is a required package, as all CFC blocs are called with sequential graphs. But I still prefer ladder implementation. Easy to program, easy to modify, and cheaper (if you don't calculate your time too expensively...).
Ciao,
Daniel Chartier

Daniel,

I see you know what I ment. I agree with you all the way on this.

Kind regards,

Take a look at SFC Draw, a new Windows program for developing SFC's
The software is under development and ControlDraw Ltd is looking for testers
http://www.urswriter.com/SFCDraw/sfc_draw_home.shtml
You will also find good links here on SFC's
Francis