Hypersonic Cruise using a Disturbance Attenuation Controller Adaptive through a Nonlinear Modified Gain Observer

A disturbance attenuation controller coupled with a nonlinear modified gain observer is developed and applied to an air-breathing hypersonic vehicle, subject to large aerodynamic uncertainties. The nominal nonlinear longitudinal dynamics of the hypersonic vehicle are linearized to yield a nominal linearized model at a specific trim condition. The large uncertainties in aerodynamic coefficients induce error in both the nominal equilibrium point and the nominal linearized model. Hence a stabilizing controller for the true longitudinal dynamics of the aircraft cannot be generated base on the nominal perturbation model without parameter adaptation. The nonlinear modified gain observer estimates system parameter used to adapt the disturbance attenuation controller in generating a stabilizing worst case input for the hypersonic vehicle. The control scheme is validated using a simulation of the nonlinear longitudinal dynamics, demonstrating the stability and performance of the disturbance attenuation controller in presence of large aerodynamic coefficient uncertainties. In terms of transient response and overall input cost, the worst case controller results appears to be superior in comparison to those obtained from the Sum-of-Squares method as well as the nonlinear dynamic inversion approach. Although the aerodynamic uncertainties are large, the small perturbation approach combined with the worst case input still leads to a stable steady state close loop system.