Closed loop control, also known as feedback control, is the most widely used type of automatic control. If feedback control were used in Figure 300-3, the controller would open the drain valve only when the liquid level rose above the controller setpoint and would continue to adjust the valve as needed to keep the liquid drained from the vessel.
The gas separation process in Figure 300-4 has a feedback (closed loop) level control system in which the controller LC receives a signal from the level transmitter LT. The controller compares this measurement with the setpoint and adjusts the outlet valve as necessary. The difference between the controller measurement and controller setpoint is the error signal. When the error is not zero, the level controller opens or closes the outlet valve to return the level to the setpoint.