On robust asymptotic tracking: perturbations on coprime factors and parameterization of all solutions

The robust asymptotic tracking problem is analyzed in this paper relative to unstructured perturbations on each coprime factor of the transfer functions from the control input to the measured and controlled outputs. In each case, necessary and sufficient conditions for the existence of solutions are presented which are explicitly given in terms of problem data. Under such conditions, explicit parameterizations are given of all controllers which achieve robust asymptotic tracking, in terms of free, rational proper, and stable matrices