Which type of controller cannot maintain process variables exactly at the set point?

A proportional-only controller adjusts the control output in direct proportion to the error, which is the difference between the set point and the actual process variable. This means that while the controller will attempt to reduce the error, it will never completely eliminate it. The result is a steady-state error that persists, as the proportional response becomes less effective as the error approaches zero.

Only when the process variable aligns exactly with the set point would the controller output become zero, preventing further adjustment. Hence, there will always be a degree of error remaining at the steady state. This characteristic distinguishes the proportional-only controller from others like PID (which includes integral action to eliminate steady-state error) or On/Off controllers (which can switch fully on or off to quickly correct deviations). Similarly, the feedforward controller anticipates changes rather than solely responding to errors, allowing it to maintain set points more accurately under specific conditions.