Unity feedback compensation of unstable plants

The compensation of a plant (possibly unstable and/or non-minimum phase) via unity feedback with internal stability is studied in this paper. The restrictions imposed on the designer by unstable poles and zeros of the plant are described and discussed, and the whole class of appropriate compensators needed for a particular design is given. The hidden modes which might be introduced in the closed loop system are fully characterized; and the sensitivity of the compensated system, as well as the stability and properness of the compensator and its relation to state observers, are also treated.