Nonlinear optimal control for the 6-DOF fixed-wing UAV

The dynamic model of the 6-DOF fixed-wing unmanned aerial vehicle (UAV) is non-affine in the input and the solution of the associated nonlinear control and stabilisation problem is a non-trivial task. In this article, a novel nonlinear optimal control method is applied to the above-noted model of the 6-DOF fixed-wing UAV. First, it is proven that the dynamic model of the fixed-wing aircraft is differentially flat. Next the state-space model of the fixed-wing UAV undergoes approximate linearisation around a temporary operating point that is recomputed at each time-step of the control method. The linearisation is based on Taylor series expansion and on the associated Jacobian matrices. For the linearised state-space model of the fixed-wing UAV, a stabilising optimal (H-infinity) feedback controller is designed. This controller stands for the solution of the nonlinear optimal control problem under model uncertainty and external perturbations. To compute the controller's feedback gains, an algebraic Riccati equation is repetitively solved at each iteration of the control algorithm. The stability properties of the control method are proven through Lyapunov analysis. The proposed nonlinear optimal control approach achieves fast and accurate tracking of setpoints under moderate variations of the control inputs and a minimum dispersion of energy by the propulsion and steering system of the UAV.