Design of Stable Controllers Attaining Low [formula omitted] Weighted Sensitivity

This note is focused on strong stabilization and sensitivity reduction for a linear time-invariant SISO system. Stable controllers are practically desirable in the presence of possible actuator or sensor failures. It is verified precisely that the optimal controller of weighted sensitivity minimization is almost always unstable when the plant has two or more zeros in the right-half plane. An algorithm for attaining low sensitivity property by stable controllers is presented, based on the Nevanlinna-Pick interpolation.