Feedforward controllers and tracking accuracy in the presence of plant uncertainties

In the absence of plant parameter uncertainty feedforward controllers can be synthesized to achieve perfect continuous tracking. When a plant has uncertainties it is in general impossible to achieve such perfect tracking. Investigated in this paper is the role played by feedforward controllers in the presence of plant uncertainties. The authors show that the use of feedforward controllers cannot improve the tracking error beyond what is achievable with a properly designed feedback loop, over all plant uncertainties. Including preview in the feedforward will not alter the situation either. The authors present two methods of designing robust compensators so that the tracking error due to uncertainties will be made small in some sense in the frequency domain and will have zero error in the steady state.