Feed-forward control measures a disturbance before it can affect the controlled variable, and changes the manipulated variable to compensate for the disturbance. Of course, for feed-forward control to work properly, the magnitude and timing of the effect on the controlled variable must be known. The process might be worse off if the manipulated variable is changed too much or too quickly.
In Figure 300-18, a gas-fired furnace process is equipped with a temperature controller (TC), a feed-forward controller (FFC), and a summer, which adds the two controller outputs together. The feed-forward controller, also called a flow fraction controller, operates like a simple multiplier: The output of the FFC consists of its input (from the flow transmitter FT) multiplied by a ratio entered by the operator.
Figure 300-19 shows what might happen in a real furnace as the feed rate is changed. In the top graph, the feed rate to the furnace is raised at time 1. By time 2, the furnace outlet temperature begins to drop below setpoint. The fuel valve then begins to open and raises the outlet temperature back to the setpoint by time 3. In the bottom graph, the fuel valve has begun to open by time B, and by time C the furnace temperature is back to the original setpoint. With feed-forward and feedback control, the process has recovered from the feed rate disturbance much faster than with feedback control alone. Note that the temperature’s period of oscillation is the same in both cases. This period is a dynamic characteristic of the furnace and cannot be changed by the control system. However, the feed-forward controller has been able to reduce the size of the temperature disturbance and has speeded up the recovery.
Feed-forward control should not be used by itself, but always with feedback control, because the rate and magnitude of the reaction of a process to a disturbance is rarely consistent.