Robust nonlinear control design of a hypersonic flight vehicle using minimax linear quadratic Gaussian control

For a nonlinear uncertain rigid body model of an air-breathing hypersonic flight vehicle (AHFV), a robust feedback linearization method together with a minimax optimal linear quadratic Gaussian (LQG) controller is proposed. In the framework of robust feedback linearization with a selected diffeomorphism, it is not possible to measure or manipulate all of the states algebraically. The minimax LQG control approach solves this problem by using output feedback and estimating the unmeasured states in the presence of the uncertainties. Here, a problem of constructing a guaranteed cost controller which minimizes a guaranteed cost bound has been considered and the tracking of velocity and altitude is achieved under inertial and aerodynamic uncertainties. The procedure consists of the simplification of a nonlinear model to achieve full vector relative degree and then use of the robust feedback linearization method to linearize the nonlinear dynamics. This linearization process, followed by a robust minimax LQG control design, provides a robustly stable closed loop system.