A feedback controller responds only after it detects a deviation in the value of the controlled output from its desired set point. On the other hand, a feedforward controller detects the disturbance directly and takes an appropriate control action in order to eliminate its effect on the process output.

Consider the distillation column shown in Fig (V.1) The control objective is to keep the distillate concentration at a desired set point despite any changes in the inlet feed stream.

(a) Feedback control configuration

(b) Feedforward control configuration

Fig V.1: Feedback and Feedforward control configuration of a distillation column