## Derivative Control

Derivative
control (also called rate control) responds to the rate of change of the
controlled variable. For example, if the temperature of a process is changing
rapidly, the controller must respond quickly to keep the temperature within
acceptable limits. In effect, derivative control action uses the rate of change
in a process to adjust the output of the controller. How much adjustment is
made is determined by how quickly the deviation from the set point is
occurring.

Taking the
derivative (or differentiation) is a mathematical computation that determines
the rate of deviation from the set point. Using mathematical symbols, this computation is expressed as d/dx
f(x).

Derivative
control is used in process control systems where lag time (the time it takes to
measure a change) is large. Derivative control is considered difficult to
implement and adjust; therefore, it is
used only when the amount of lag time is extensive. It is typically used in
combination with proportional plus integral control (referred to as PID
control) for temperature control and other slow applications. PID controllers
may be difficult to adjust.

Derivative control is rarely used with proportional control only. Proportional plus derivative control, referred to as PD control, is desirable in processes where there are several different lag times.

Derivative control is rarely used with proportional control only. Proportional plus derivative control, referred to as PD control, is desirable in processes where there are several different lag times.

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