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Is derivative control just used on temperature loops? John A. Shaw Friday, November 14, 2003 Question: Almost all controllers on my chemical plant DCS are PI with no derivative. There are a few on temperature control that do have derivative. All, PI and PID, seem to be working well. Is derivative appropriate for temperature control? Answer: Yes, derivative is appropriate for temperature control. Back in the days of analog control, you could order a PI controller or a PID controller for slightly more money. You had to choose up front, and justify even a small difference in cost. Many engineering firms specified PID control for temperature loops and PI control for all other loops. With a DCS you can choose, at the time you are tuning the loop, whether or not to use derivative. However, the old rule of thumb: derivative on temperature, is still often followed. There are several reasons for the old rule of thumb: derivative usually should be used on loops with very little noise. Temperature loops were usually very smooth and without noise. Derivative helps most when there are multiple lags in the process, again, typical of temperature loops. Derivative can be used on other loops, and, with DCSs it does not add any cost or require a decision before the startup. You can try it. If it helps; use it; if it hurts, don't use it. |