S7-300 PID in TIA Portal

[IdealDan]

Click here: LMGTFY.

Or here: Gerry@PLCtalk.


That depends on the character of the dynamics of the process being controlled.

Is this a level control? Temperature control? Flow control? Pressure control?

If you make a step change in the CV, with the controller in manual,

• how long does it take for the PV to move through half or two-thirds of its response to that step change?
• Is there any deadtime, or does the PV start to move immediately when the CV changes?

Think about how the PID works; that, plus the dynamics of the system response, should tell you how to set the update time.

Thanks for input Bro;
I have Level, Flow and Pressure Control, and I have not yet known their dynamics.
From the above link you attached, I saw 10ms suggestion; Could you give me an idea e.g. For fast step response 10ms or more/less;
For Slow step response 20ms or more/less etc..

 

[Lare]

Thanks for this great information;
Please, lastly, what is the best CYCLE time for the PID CONT_C Block and that of the OB35?

Cycle input needs time interval between PID block calls. AS OB35 is cyclic interupt with 100ms cycle, you need to specify 100ms to input if PID is called at every OB35 cycle.



If you need slower or even faster control locate FB41 call to different OB-block.


http://www.plccenter.cn/Siemens_Step7/bas00082.htm

 

[drbitboy]

...Could you give me an idea e.g. ...

No, because I don't know your process.

I suspect flow and liquid pressure applications will tend to be fast (10-100ms); flow dynamics might even be dominated by the control element response to changes in CV. Level or gas pressure applications tend to be slow, assuming large cross-sectional areas or volumes, and even 1s (1000ms) or more might be acceptable.

Those are very general statements, and I am sure there are exceptions. You need to evaluate your own process.

It would not be a waste of time to write a coarse model of a simple process e.g. put the equivalent of this

PV = ((1-K) * PV) + (K * CVnew)
​

on a rung in the continuous task, where K is between 0 and 1: smaller Ks will model a slower process e.g. K=0.001 and a mean per-scan time of ~10ms yields a first order time constant of about 10s, so a 1000s PID update period would probably be okay; larger Ks will model a faster process e.g. K=0.1 and a mean scan time of ~10ms yield a first-order time constant of about 100ms, so a 10ms PID update period might be needed.

I don't know how PID is implemented on a S7-300, but for best results ensure that the PID instruction executes at the same update period with which you configure it; it is all too common that people configure the PID with a long update period, say 1000ms or 10s, but let the PID instruction run in the continuous task, where the actual update period is more like 10ms or less.