Derivative action reacts to how quickly its input changes. It can add damping and anticipate motion, but differentiation also magnifies high-frequency measurement noise.

Derivative on measurement

Differentiating the measured process instead of the error avoids a large derivative kick when the setpoint changes instantly. The disturbance response remains useful because process movement is still visible.

NOISE VIEW 01

Differentiation magnifies fast measurement changes

A small ripple on the measured process becomes a much larger high-frequency derivative contribution.

READ THE PLOTA smooth-looking PV can still create actuator chatter

The filter time constant

A first-order filter limits the derivative gain at high frequency. Too little filtering allows chatter; too much filtering removes the damping benefit and adds lag.

FILTER VIEW 02

The derivative filter trades chatter for lag

Filtering reduces rapid output movement while preserving enough early derivative action to add damping.

READ THE PLOTFilter enough to quiet noise—not enough to erase D

Judge the actuator, not only the PV

A process value can look smooth while the controller output moves rapidly. RMS effort and total controller variation reveal noise amplification that a response plot can hide.

KICK VIEW 03

Derivative on measurement avoids setpoint kick

Differentiating error sees an ideal setpoint step as an enormous slope. Differentiating measurement responds only when the process actually begins moving.

READ THE PLOTMeasurement mode separates setpoint commands from damping

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