mychinkee

To all Expert:

Has anyone known how to re-program or convertthe PID Block which is in FB41 (CONT_C)to FCXX, the reason I needed it is because my FC block could not call FB block?

Any goodsuggestion how to do it?

Thanks for your help!

Hi

What did you mean with "my FC block could not call FB block? This FB41 is better to call in same (fixed) time interval.(for example 100ms). Call this in some timing OB. This FAQ is also helpfull: Where can you find descriptions and examples of PID control with FB 41, FB 42 and FB 43, and SFB 41, SFB 42 and SFB 43?

best regards

Thanks for your reply.

Actually my problem is, I develop my programin such a way thatI am doing my logicscontrol inside the FC block, but unfortunately inside the FC block, I am not being able to call out the FB41, that's the reason why I need to know whether there is a possbile way to develop a PIDcontrol like FB41 but in FC format. By doing so, I could use call out the FC (PID Control) inside my FC block.

Thanks!

mychinkee

Thanks for your reply.

Actually my problem is, I develop my programin such a way thatI am doing my logicscontrol inside the FC block, but unfortunately inside the FC block, I am not being able to call out the FB41, that's the reason why I need to know whether there is a possbile way to develop a PIDcontrol like FB41 but in FC format. By doing so, I could use call out the FC (PID Control) inside my FC block.

Thanks!

SFB/FB "CONT_C" (continuous controller) is used on SIMATIC S7 programmable logic controllers to control technical processes with continuous input and output variables....

Note: The calculation of the values in the control blocks is only correct if the block is called at regular intervals. For this reason, you should call the control blocks in a cyclic interrupt OB (OB30 to OB38). Enter the sampling time in the CYCLE parameter.

You cannot garantee the repeatativity and accuracy of the FB 41 calculations if you do not call it cyclically at very precise and known intervals, for example, calling it in OB 35 set at 100 ms, and writing T#100ms for the CYCLE parameter value. This is not a property of Siemens Controllers; it is a basic function for any PID implementation in any numeric calculator; even Matlab required time setting for its different calculations of controllers.

As far as programming your own PID FC, I am sure you will find thew experience interesting...
I did something of the kind, using TurboC, developping a numerical control program on a PC. Good luck.
Look for "numerical PID" or "discrete PID algorithms" on the web, you will get the basic equations. One example is:
http://lorien.ncl.ac.uk/ming/digicont/digimath/dpid1.htm
But you will have to take care of the operational mode, anti-windup (integral windup) and other little functional details. This can take a little more time.

Hope this helps,
Daniel Chartier

By the way, all of the hits you will find will discuss the time constant of the calculator....