Maximally robust feedback controller design

This paper contains results on the design of integral feedback controllers for discrete-time SISO plants, with maximal stability robustness against real time-invariant coefficient uncertainty in the plant numerator. The main result is the characterisation of nominal plants into two classes, according to whether or not maximal robustness is limited by the need to avoid a cancellation between the perturbed plant numerator and the integrator pole. Optimal stability margins and remarks about robust design are given for both kinds of plant