On the characterization of a minimal order robust servocompensator

In recent papers, [1] - [4], Davison et al. have given the characterization of a minimal-order robust feedback controller containing an error-driven compensator which achieves asymptotic tracking and disturbance rejection using state space methods. More recently, Desoer and Wang, [7] - [8], rederived the minimal order of a similar controller, [5], using frequency domain methods. It is the purpose of this note to give a detailed derivation that the latter minimal order controller must contain a Davison servocompensator, [4], using frequency domain methods. This adds perspective and information to [7], [8, Proof of Corollary IV. 1].