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Closed‐loop Performance and Stability

This chapter first discusses closed‐loop stability by examining the locations of closed‐loop poles and the applications of the Routh‐Hurwitz stability criterion. It then presents the Nyquist stability criterion based on a frequency response analysis, which leads to conclusions of closed‐loop stability by examining the frequency response of the loop transfer functions, particularly convenient for analysis of systems with time delay. Next, the chapter introduces control system structures with one and two degrees of freedom, which are related to the proportional integral derivative controller realization to reduce the overshoot in reference response. The external signals such as reference signal, disturbance signals and measurement noise are important in the analysis of control system performance. The chapter discusses these signals in relationships with the sensitivity functions. It also examines the key issues existed in a feedback control system that are related to reference following, disturbance rejection, and noise attenuation from the angles of sensitivity analysis.