Sfb41

I have some questions about the SFB41:

about the inputs:
- TI: why does time has to be greater then the cycletime of OB35
- TD: why does time has to be greater then the cycletime of OB35
- TM_LAG: why does this factor has to be greater then (the cycletime of OB35)/2
-DISV, what does this mean and what has to put written here

about the ouputs:
- ER: what is this???

I hope that someone can help me with this problem.

great thanks.

Daisy

Hello, daisy;
Please make sure you specify what brand of PLC you are using, and give some details about your application. It will make it easier for us to know what you are talking about. I will presume, from the parameters you are asking about, that you use a Siemens S7-300/400 CPU, and that you need to know about the PID integrated function S_CONT (FB41).
First, CYCLE is a time parameter that you select (it is not equal to OB35 cycletime; recommended value is 1 sec); it sets the time interval between the calls of FB41 so that the requests are at fixed points in time. Why the other time parameters must be equal or greater than the CYCLE parameter is probably just a way for the original programmer of FB41 code to ensure that they can be treated within the time-span of the function block. In other words, thats the way the function was programmed at the beginning, and you have to live with it.
DISV is a disturbance value that you can enter if you need to program a Feedforward compensation. If you know the source of a steady (or recurrent) distubance in your process, you can enter here the address of the input that monitors it, or the estimated percentage of the disturbance (in comparision with the PV). The FB41 code will try to alter the output to take it into consideration.
ER is simply the error signal of the process: ER=PV-SP. it is used to calculate how each of the contol parameters (Kp, Ti and TD) will affect the output; different techniques of minimizing the error signal is a vast area of theoretical discussions in PID control.
Hope this helps,
Daniel Chartier

thank you very much

thanx thanx thanx and again and again ...

this helped me a lot.
I have also mailed this questions to siemens, but they didn't answer. I'm glad you did.
I don't know how I can thank you, but I will say it one more time: THANK YOU !!

big kiss from belgium.

Daisy