Quaternion-based Flight Control of Fixed-wing UAV via ESO-augmented Dynamic Inversion

Based on the quaternion description, a flight control scheme combining extended state observer (ESO) and nonlinear dynamic inversion (NDI) is developed in this work. The nonlinear model of the fixed-wing unmanned aerial vehicle (UAV) is first established, and all the angles are replaced by quaternions to avoid possible singularity. A two-loop cascade NDI controller is then designed to track the wind angles. This requires perfect knowledge of the UAV model. To improve the controller's ability to handle modeling uncertainties and disturbances, the ESO is introduced to complement the pure NDI controller. The ESO estimates the unmeasurable lumped disturbance of the model, which can be used to compensate the NDI designed control signal. A series of simulations are performed to evaluate the two proposed controller types, and the ESO-augmented NDI controller is found to be superior in terms of robustness.