Output control using feedforward and cascade controllers

Open-loop solution to the output control problem in SISO systems by means of feedforward and cascade controllers is investigated in this paper. A simple characterization of feedforward controllers which achieve steady-state disturbance rejection is given in a transfer-function setting. Cascade controllers which cause steady-state command tracking are also characterized. Furthermore, disturbance decoupling and command matching controllers are identified. Conditions for existence of feedforward and cascade controllers are also given. For unstable systems, it is shown that a stabilizing feedback controller can be employed without affecting the feedforward and cascade controllers used for output control; hence, the three controllers can be designed independently. Finally, output control by a combination of feedforward and feedback is discussed briefly.