Optimal tracking performance for unstable tall plant models

This article focuses on the best achievable tracking performance of unstable tall plant models. The work is presented for discrete time, LTI systems, when an exponentially decaying signal is considered as reference. Closed form expressions for the best tracking performance for one and two degree of freedom control architectures are presented. As an application of those results, they are used in an example to compute the performance gains in the control of originally tall systems which have been squared-up by adding control input channels.