Dear sir, I am a process control technologist of Thaioil Refinery. My questions is below: Q: I am confused the use of PI or PID. What kind of process, which suits for PI or PID? PI or PIT controller is suited for dominant dead time( timeconstant > dead time), isn't it? and PID controller is suited for dominant time constant and no noise from the transmitter, isn't it?

This is my first time visiting your web site and I would like to thanks for your group to contribute the control knowledege to people. Kind Regards, Mongkol Janchookiat Process Control Technologist/TNPP-4 Thai Oil Refinery

-- Mongkol Janchookiat (mongkol@thaioil.co.th), December 22, 1999

Answers

You are correct. Use D when the loop has low noise. If the loop is very noisy, D will make the output jittery.

Do not use D on a pure dead time process - it will make the loop unstable. As long as there is some amount of lag or first order time constant in the process, D can help.

See background articles on http://www.expertune.com/articles.html

-- John Gerry (john.gerry@expertune.com), October 09, 2000.

what is the effect that each controller term has on the accuracy and stability of the control system output.conclude your answer with observation as to the importance of having a correctly tuned control loop and include a description of two practical procedures that are used to tune a control loop.

-- khled mohamed (khled72@yahoo.com), December 16, 2000.

I agree with John Gerry said. Just another comment, the D term is most often used on temperature loops. Most temperature loops can make use of derivative because they are low noise, are not pure dead time, and usually have two or more lags (process lag + combination of t/c and well lags). Of course, there are exceptions, but that is a good rule of thumb. John Shaw www.jashaw.com

-- John Shaw (john@jashaw.com), December 17, 2000.