What characteristic defines a proportional controller?

A proportional controller is fundamentally defined by its ability to control a system based on the feedback error, which is the difference between a measured process variable and a desired set-point. This type of controller continuously adjusts the control output in proportion to the error. Consequently, when the error is larger, the output response is greater, and when the error is smaller, the output is less aggressive. This relationship allows the system to respond dynamically to changes, enabling effective regulation of the process variable.

The characteristic of maintaining set-point precisely implies perfect control, which is not achievable typically in real-world systems due to constant disturbances and inherent lags — thus, proportional controllers work to minimize the error rather than eliminate it completely. The option indicating binary outputs reflects the nature of on/off control systems, which is distinct from the continuous control provided by a proportional controller. Lastly, the notion that controllers only act during process disturbances would limit their functionality; proportional controllers operate continuously, making adjustments in response to any existing error regardless of whether a disturbance is perceived. This continuous modulation is crucial for maintaining stable operation in many applications.