THE REGULATOR PROBLEM WITH ROBUST STABILITY

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 problem'. For the regulator problem to be solvable with robust closed-loop stab ility, the plant obviously needs to be such that the regulation proble m and the robust stabilization problem are solvable separately. In thi s paper we determine the extra conditions that are necessary and suffi cient for the two problems to be solved simultaneously. It turns out t hat these conditions can be given a simple geometric interpretation in terms of a multivariable version of the Nyquist curve of the plant.