Robust Autopilot for a Quasi-Linear Parameter-Varying Missile Model

A sideslip velocity autopilot is designed for a model of a tactical missile, and robust stability of the closed-loop system is investigated. The tail-controlled missile in the cruciform fin configuration is modeled as a second-order quasi-linear parameter-varying system. This nonlinear model is obtained from the Taylor linearized model of the horizontal motion by including explicit dependence of the aerodynamic derivatives on a state (sideslip velocity) and external parameters (longitudinal velocity and roll angle). The autopilot design is based on input-output pseudolinearization, which is the restriction of input-output feedback linearization to the set of equilibria of the nonlinear model. The design makes Taylor linearization of the closed-loop system independent of the choice of equilibria. Thus, if the operating points are in the vicinity of the equilibria, then only one linear model will describe closed-loop dynamics, regardless of the rate of change of the operating points. Simulations for constant lateral acceleration demands show good tracking with fast response time. Parametric and H sub( infinity ) stability margins for uncertainty in the controller parameters and aerodynamic derivatives are analysed using Kharitonov's approach. The analysis shows that the design is fairly robust with respect to both kinds of uncertainty.