Robustness assessment and control design using a relay feedback approach

In this paper, a relay feedback approach for the assessment of robustness in control systems is first proposed. The approach uses a relay in series with a time-delay element, where the amount of time delay is swept over a range to automatically generate a number of sustained oscillations. From these oscillations, a systematic set of procedures is developed to yield estimates of the maximum sensitivity and stability margins. If these parameters are assessed to be unsatisfactory, some means to automatically retune the controller would be necessary and useful. In this paper, an approach for the design of the PI controller is, thus, proposed to concurrently satisfy user specifications in terms of maximum sensitivity and stability margins, based on the information available from the robustness assessment. Guidelines are given, in the paper, to assist the user to select generally satisfactory parameters to meet robust design objectives. The proportional-integral ( PI) control parameters are then obtained, via the minimization of objective functions, so that the robustness specifications can be met as closely as possible. A simulation study on commonly encountered processes and results from a real-time experiment are furnished in the paper to illustrate the effectiveness of the proposed design scheme.