Robust and Adaptive Backstepping Control for Hexacopter UAVs

Abstract

A nonlinear robust and adaptive backstepping control strategy is hierarchically proposed to solve the trajectory tracking problem of hexacopter UAVs. Due to the under-actuated and coupled properties of the hexacopter dynamics, the nominal backstepping control approach is fully designed as the main controller. Considering the model uncertainties and external disturbances perturbing the system stability, a robust $2^{nd}$-order linear extended state observer (LESO) with more reliable velocity feedback is devised to observe and suppress the instabilities, and peaking phenomena in the observation are removed. Usually, large observer gains are selected to reduce the tracking errors but will amplify the measurement noise. To further enhance the system robustness, an adaptive switching function based compensator is introduced to eliminate the observation errors, through which the requirement on large observer gains is relaxed, and high gain behaviors of the LESO are avoided. Stability analysis proves that the nonlinear control scheme can ensure the hexacopter UAV asymptotic tracking along the designated trajectory. Comparative simulations under different controllers are carried out to demonstrate the efficiency and superiority of the proposed control scheme.