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 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. We 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. We 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.