Application of H∞ and μ synthesis techniques for Reusable Launch Vehicle control

The primary objective of Reusable Launch Vehicle mission is to evaluate autonomous navigation, guidance and control schemes under the environment of reentry from a high Mach number to touchdown. This necessitates the design of a controller such that the closed loop system remains stable for all possible plant perturbations and the response is admissible for every disturbance and command in the prescribed sets. This paper examines the applicability of H_∞ and μ-synthesis control to the design of automatic flight control systems for RLV. In H_∞ optimization problems, design specifications and goals are reflected as closed loop transfer function weightings. After setting up the appropriate input and output weighting functions, a stabilizing controller that satisfies a bound on H_∞ norm of the weighted system transfer function is determined from the solution of two algebraic Riccati equations. μ-synthesis combines μ-analysis and H_∞ synthesis to yield a control law that will desensitize performance to structured plant uncertainty. The impact on performance, of the degree of conservatism inherent in each approach, is examined. It is seen that H_∞ with μ-synthesis provides a superior framework for the design of robust controllers.