Harnessing optimal robustness towards PI regulators of linear perturbed unstable systems

In this paper, we investigate how a regulator can be implemented for gaining the optimal robustness margin of second-order plants in the presence of uncertain perturbations. We are primarily interested in an intricate system, i.e., the second-order unstable plant with a zero relative degree that is accompanied by at least one unstable pole and non-minimum phase dynamics, both implemented by the proportional and proportional–integral (PI) type regulators. More specifically, we shall examine three scenarios of the system ranging from the worst case of two unstable poles and two minimum phase zeros to the case of one unstable pole and one minimum phase zero. For each class of systems, we furnish the bi-dimensional feasible horizons for the controller parameters of PI regulators. Drawing upon the feasible domains, we derive the explicit expressions of the optimal robustness margin against unknown uncertainties. Beyond that, we determine the regulator parameters for accomplishing the maximum margins, i.e., the optimum proportional and integral gains. At last, we extend to explore the optimum robustness margin achievable with a PI regulator for third-order non-minimum phase systems. Our results illustrate that the optimal robustness margins obtainable are dependent on the locations of the system’s unstable poles and non-minimum phase dynamics while attaining the optimized margin is relevant to the control parameters of the PI regulator, thus casting insight upon the design and adaptation of the regulator parameters.