Does a controller scan time will have a big impact in PID Tuning Parameters?

Suppose if i am changing my control system from a old 1990's system to a new advanced control system whether I can use the same PID Tuning Parameters?

I know for sure I need to compare the PID algorithm used in the two control system. If I match the algorithm and calculate the new settings with the old one will it work the same way? or since we are changing the controller I need to do the tuning again?

Thanks all for your reply.

 

Does the scan time have a big impact? - Yes it does. But if you are scanning faster it will usually make things better not worse and you will probably not need to worry about it. However, there is one potential problem you must check for...

Some types of PLC's may not control the repeat interval of the PID correctly (due to bad programming practice). Examples of this could be some older Allen Bradley using PID instead of PIDE or Siemens S5 / S7 not running the PID function blocks in the cyclic interrupt task. In this case the execution interval is not controlled properly and the Integral term will have been mis-tuned to compensate for this.

Can you use the "same" tuning parameters ? - Yes you can and they will give you a good starting point BUT (as you already know) - you will probably need to translate them. For example your old controller might use Repeats per minute and your new controller might use seconds per repeat. To save the trouble og manually doing this, look here -> http://www.expertune.com/TuningTranslator.html

(remove any spaces from the link)

Rob
www[.]lymac.co.nz

 

Real process control systems, including all DCS, have synchronous scheduling for the PID function blocks. You do not need to worry about speed of the controller.

On the other hand, if you are using a PLC to do the PID, you should make sure that you have scheduled that function block for synchronous execution.

As you pointed out, the tuning constants are quite different across the different instantiations of the PID function block. Please pay close attention to this.

Dick Caro
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Richard H. Caro, CEO, CMC Associates
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