Disturbance observer-enhanced nonlinear dynamic inversion control for unmanned helicopter path tracking

In this paper, an integrated methodology incorporating both guidance law and improved nonlinear dynamic inversion is addressed to design a robust nonlinear controller for the unmanned helicopter with unmodeled dynamics and unknown external disturbances, and an inner-outer loop control strategy is carried out, which separates the unmanned helicopter control problems into an outer guidance loop for path following and an inner control loop for attitude control. The position controller, which acts as the outer-loop is designed by use of a nonlinear guidance law for providing the reference attitude angles for the inner-loop, and the attitude controller is designed as the inner-loop based on an improved nonlinear dynamic inversion (NDI) by applying the sliding mode control approach, which can follow the commands generated by the outer guidance loop fleetly and accurately. Meanwhile, the fast super-twisting sliding mode observer is introduced to estimate the uncertainties and compensate them in real time to improve the robustness of unmanned helicopter to model uncertainties and external disturbances. Six degrees of freedom (6-DOF) simulation results illustrate that unmanned helicopter can track three-dimensional path accurately, and demonstrate the feasibility and reasonability of the proposed method.