A New QFT Design Methodology for Feedback Systems Under Input Saturation

Presented in this paper is a design methodology for robust stability and robust performance for a large class of plants subject to an input saturation nonlinearity. Based on Horowitz’s 3-DOF linear design method, a more general form for the additional loop transmission around the saturating element is proposed to design the third degree of freedom $Hs,$ for Type n plants, $n⩾1.$ Robust stability is established with the circle criterion and the describing function technique, and robust performance is improved by imposing nonovershooting conditions. All stability and performance requirements are then transferred into frequency domain design bounds which are easily obtained using QFT design techniques. This procedure applies to all SISO, Type n, stable plants with uncertainties (parametric or nonparametric), and to marginally stable and conditionally stable plants as well.