REGULATION AS AN INTERPOLATION PROBLEM

The design of a controller such that the closed-loop system will track reference signals or reject disturbance signals from a specified clas s is known as the ''servomechanism problem'' or the ''regulator proble m.'' We show here that the regulator problem can be looked at as an in terpolation problem for a subspace-valued function that can be viewed as a multivariable version of the Nyquist curve. The result is applied to obtain a simple parametrization of all solutions. (C) Elsevier Sci ence Inc., 1997.