Abstract
In this paper, a robust disturbance observer based controller is presented for glideslope regulation of aircraft in turbulence and with uncertainties in the aerodynamic model. A significant challenge in designing disturbance observers for such system arises from the nonlinear disturbance to state coupling. This state dependent coupling limits the application of disturbance observer based control without resorting to system approximation. Instead of model simplification, this work explicitly accounts for the disturbance to state coupling and leverages the polynomial nature of the system dynamics to design an exponentially convergent disturbance observer. The underlying principle behind synthesis of stable disturbance observers is sum-of-squares (SOS) optimization and in particular, polynomial matrix inequalities (PMI). Through exponential convergence of disturbance estimate, the wind components and aerodynamic uncertainties can be rapidly estimated and then compensated by deploying control surfaces. The efficacy of the proposed approach is demonstrated using the disturbance observer with a nominal dynamic inversion controller for glideslope regulation of an aircraft based on the F/A-18 High Angle of Attack (HARV) model.