Control loops can be either manual or automatic. A manual control loop requires a human being to observe the value of the controlled variable. If this variable is not at the setpoint, the human observer adjusts a manipulated variable (see Figure 300-1).
An automatic control loop employs a controller to keep the controlled variable at the setpoint. In Figure 300-2, the controller receives a signal from a transmitter (the circled X) representing the condition of the controlled variable, and sends an output signal to a valve regulating the manipulated variable.
In a refinery furnace, a controller monitors the outlet temperature (controlled variable). If the outlet temperature is not at the desired value (setpoint), the controller changes the fuel flow (manipulated variable) by changing the position of the fuel valve (final control element). Automatic control may be open loop (feed forward) or closed loop (feedback).