Dear List,

For a long time, I have been doing plc programming using conventional Relay ladder logic method irrespective of the process..

Recently , I did some study on Sequential Function Charts (SFCs) programming and found it very useful especially for machines or processes that have repetitive cycles. Infact, a lot of machines I programmed earlier that operate on sequences or steps would have been much easier to program had I known about SFCs.

I would like members of the list that have had real life experiences on SFCs (good and bad ) to share with us.

Many thanks ,
Gabriel.
Email:[email protected]

 

Hello Gabriel;

I am very much in agreement with you. Our company ( we are systems integrators) has even developped a programming standard using GRAFCET (the original SFC)as a development interface for sequential controls (conveyors, palettizers...); something like a flowchart development. We then use standard techniques to translate the SFC to Ladder or any other PLC format. We have had much success with this; anyone in the company can easily read, interpret and modify a program developped for one of our custommers, without the hassle we used to have when someone else programmed wuith his own style.

Hope this helps,
Daniel Chartier

 

SFC is the best language to program a sequence in a PLC. I change 40 rungs in a TSX37 Modicon PLC for 2 sequencial charts using SFC. Now this machine makes the changes(sequence) very soft and it's more precise.

Ladder is useful but more complicated to understand and follow in a sequencing programs.

Additionally, wiht that kind of PLC, you can program one par of the program in ladder and another part in SFC.

Regards

Jose R Pabón.

 

I've used SFC on many occasions with NP PLC-5 controllers. It is an excellent programming tool. Unfortunatly it's missing on the SLC and Logix lines of controller. Also, unlike some other controllers that I've used, the fact that A-B SFC is stored in the controller and not massaged ladder packaged to look like SFC makes SFC much easier to use on PLC-5 systems. I feel that the IEC-61131 implementations by some manufacturers are too difficult to implement to be useful.

Finally, the only systems that I ever programmed that started up and ran properly on the 1st turn of the key were SFC based systems.

(8{)} ( .)

 

It is true that SFC's make the seuential kind
of programming lot easier where the equipments
step through predetermined steps in their service
cycle .

I worked with AB's SFC and ABB DCS systems and
using the SFC offered an easy way to write the
code for our process and also helped in tracking
the sequence while it was running online .

I guess its similar to object oriented
programming where the programm is broken into
small manageable blocks which are easier to handle.

Prash
10/24/02

 

I have been using Think and Do total flowchart programming for about 3 years now for both PC Control and the Koyo WinPLC embedded Windows CE controller. The advantages over Ladder and even Stage Sequence Ladder are numerous. With flexibility, functionality and the ease of analog management being at the top.

 

Hello Gabriel,

The SFC (plus the ST) is more powerful tool than the LD. Product dimensionalitys, quality of your programs will increase.

--
Have a nice day.
Vladimir E. Zyubin mailto:[email protected]
Thursday, October 24, 2002, 4:49:35 PM

 

I take it you hand translate from grafcet to ladder (or whatever)? I have been doing this also. For some applications state diagrams seem to work better than grafcet though.

What PLC code format do you typically use to implement this with? For example a typical step could be laid out as:

1) Set outputs. The actual outputs are directly addresses here.
2) Any subsidiary logic such as turning on various timers or counters, taking readings, etc. are addresses here.
3) Examine inputs for the desired state. This may be one rung of ladder, or it could be split into several rungs if one rung gets too long to conveniently read.
4) Resolve the transition to the next step based on the inputs (and any other conditions). This includes things like the logic for manual single step of the sequence, etc.

The above is repeated for as many steps as are present. Each step is self contained, so you can have parallel or alternate sequence paths (diverging and converging) without any side effects. You can tailor each step very easily to exactly what the process requires, rather than having to force fit the process into the sequential control model (drum controllers are bad for that).

The Siemens S7-200 series has a set of instructions specifically designed to support this method in ladder (and they document it in grafcet), although it can be easily implemented on virtually any PLC.

Is this essentially what you are referring to? If so, would you like to discuss the details of your methods?

************************
Michael Griffin
London, Ont. Canada
************************

 

SFC is a way to organize program sections. You do not write the programs themselves in SFC. The programs will be written using function block
diagram, ladder diagram, structured text or instruction list.

For regulatory control the function block diagram is my favorite. For logic I like ladder diagram because it is graphical, i.e. I can See how the power will flow, and in online mode I can See it execute. Many times it is easier to formulate logic in structured text, but you can't See it executing. A programming environment that allows you to mix the above so you can pick the right tool for the right task is best.

Jonas Berge
==================
[email protected]
www.smar.com

 

I am glad to see that many people have the happy experience of using SFC.
I like to use SFC,because it make plc programming easier.