ABSTRACT
Aclassical problem in control theory is to impose, via feedback, a prescribed steady-state response to every external command in a given family. This may include, for instance, the problem of having the output of a controlled plant asymptotically track any prescribed reference signal in a certain class of functions of time, as well as the problem of having this output asymptotically reject any undesired disturbance in a certain class of disturbances. In both cases, the issue is to force a suitably defined tracking error to zero, as time tends to infinity, for every reference output and every undesired disturbance ranging over prescribed families of functions of time.
