What is the best practice in selecting the execution time strategy for control loops, modules running in process control industry?

 

Control loop execution times should be selected based upon the dominant process dynamics. Typically, you will choose the PID execution time to be no slower than 1/3 to 1/10 of the process deadtime.

In process plants, some loops have faster dynamics than others. Liquid
flow and pressure loops, for example, are typically quite fast. Temperature and composition loops tend to be slow.

PID tuning software should be able to recognize and report issues with sampling interval. To see an example of how this is done, visit http://www.expertune.com/r2.asp?f=AList&l=TimeLine.html

In any case, you should be sure that the sample time is consistent and repeatable... not usually a problem on DCS platforms, but something that must be actively managed on PLC platforms.

- George
George.Buckbee (at) expertune.com