Proportional control only attempts to return a measurement to the set point after a load upset has occurred. How ever it is impossible for a proportional controller to return the measurement exactly to the set point.
Use : It is normally used for level controls. It reduces the effect of a load change but it can not eliminate it. Proportional plus reset control:
Reset action is introduced to eliminate offset. It will integrate any difference between measurement and setpoint and cause the controller’s output to change until the difference between the measurement and set point is zero. Reset will act as long as the error exists.
Use: Proportional + Reset controllers are by far the common types used in industrial process control and where predominate dead times occur.
Proportional plus reset plus derivative:
Derivative or rate action helps the controller overcome system inertia and result in faster, more precise control. Derivative action occurs whenever the measurement signal changes. Under study conditions the rate action does not act. Derivative allows the controller to inject more corrective action.
Use : On temperature controls.