Reply To: PID questions

AdminFred Graham

Hi George, it seems there is something going on with the forum notification system…sorry for the late reply. Derivative may help tame the overshoot you are seeing but you are well advised to use it with caution. Introducing derivative into the process can quickly cause the system to tip into instability. As for the oscillating valves, I’d be interested in seeing the trends…is it a saw-tooth type oscillation which could imply some valve stiction.

You mention you are using Dependent form algorithm. Your Igain value may be too agressive which is why you are likely being forced into such a small Pgain. Remember in the dependent form Tau_subi is a 1 over relationship 1/ Tau_subi, meaning the smaller Igain value you put in the larger your Integral component. By having a Pgain smaller than 1 you are diminishing the Igain value which is counterintuitive. I would try making your Pgain some value slightly larger than 1 and then back off on the integral by making Tau_subi larger if needed.

Hope that makes sense and if you upload some trends that may be helpful.