What does a reverse-acting controller do?

• A. As input increases, output decreases.
• B. As input increases, output increases.
• C. The controller output remains constant regardless of input.
• D. It only works for temperature control.

Answer: (A)

A reverse-acting controller moves its output in the opposite direction to the input signal. So, when the input rises, the output falls. This inverse relationship is used when increasing the measured variable would otherwise push the process further from the setpoint, so the controller reduces the actuator output to bring the process back toward the target. For example, in systems where more input would mean more of the desired effect, you’d instead back off the actuator as the input increases. The other options describe either a direct increase with input, no change, or an unnecessary limitation to temperature control, which doesn’t define what a reverse-acting controller is.