What is the primary operational difference between a feedforward and a feedback control loop in an industrial process?

The primary operational difference between feedforward and feedback control loops lies in how they respond to disturbances affecting a process. A feedback control loop measures the output of a process and compares it to a desired setpoint. If there is a difference, known as an error, the controller adjusts a manipulated variable to bring the output back to the setpoint. For example, in a temperature control system for a water tank, a temperature sensor measures the water temperature, and if it deviates from the setpoint, the controller adjusts the steam valve to either increase or decrease the steam flow, thus correcting the temperature. The key is that feedback control reacts to errors *afterthey have already affected the process output. Conversely, a feedforward control loop measures a disturbance *beforeit affects the process output. Knowing the disturbance, the controller calculates and adjusts the manipulated variable proactively to counteract the disturbance's effect. For instance, if the feed flow rate to a distillation column changes, a feedforward controller would immediately adjust the reboiler heat duty to compensate for the change in feed composition, preventing the column's overhead product composition from deviating from its setpoint. In essence, feedforward control attempts to anticipate and prevent errors, while feedback control corrects errors after they have occurred.