Aug 2016
What’s the Deal With Derivative?
What is it that derivative is supposed to help accomplish when tuning proportional-integral-derivative (PID) loops? Most of us get by most of the time with simply using the P and I components when tuning a loop. And if the loop is reasonably fast, such as a flowmeter or speed adjustment or even pressure, we can get the job done and not have to deal with derivative.
When we’re dealing with loops that have long response times such as temperature loops for tanks or dryers, there is a tendency to over-damp those loops to avoid massive overshoot of the process variable.
As an example, think about driving your car. Pretend that you are the PID loop control, the speedometer is your process variable, and the speed limit (at least in your mind) is the setpoint. If you only use P to govern how you apply the accelerator, then you’ll never reach setpoint because P needs some error in order to have an output. And since this is a linear direct proportion (gain being the proportionate factor), it is also immediately responsive. You would get a lot of accelerator when the car was first at zero speed, and then as the speed increased the accelerator would be decreased in proportion to the difference between the current speed and the desired speed (setpoint).

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