What defines a direct acting controller?

• A. Output decreases as input increases
• B. Output is independent of input
• C. Output increases as input increases
• D. Output inversely proportional to input

Answer: (C)

Direct acting means the controller’s output moves in the same direction as its input. In other words, when the input signal to the controller increases, the controller responds by increasing its output. This behavior helps push the manipulated variable in the direction needed to bring the process variable toward the setpoint. For example, if the error (setpoint minus process value) grows larger, a direct-acting controller increases heating or valve opening to raise the process value. The opposite behavior—output decreasing as input increases—would be reverse acting. If the output didn’t depend on input, there would be no controlling action. So the defining feature is that output rises as input rises.