[Link]Understanding Derivative in PID Control

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Derivative action at work

John Ziegler and Nathaniel Nichols, the fathers of PID loop tuning, recognized as far back as 1942 that derivative action dampens the control effort. They discovered through trial and error that setting the derivative time to at least half of the process deadtime would slow the controlled response to a step change in the setpoint.

The top trend chart shows a typical first-order process responding to a setpoint change under the influence of a full, three-term PID controller tuned according to the Zeigler-Nichols rules. The middle trend chart shows the same response with the brakes off; that is, with derivative action disabled. The rate at which the process variable climbs after the setpoint change and the degree by which it overshoots the setpoint have both increased.

Ironically, overshoot can be eliminated entirely by leaving the derivative action disabled, decreasing the proportional gain and increasing the integral time. The bottom trend chart shows the process’s setpoint response under the influence of a PI-only controller tuned according to the critical damping rules that call for a 69% smaller proportional gain and an integral time lengthened by a factor equal to the process gain. No derivative action is required.


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