In this paper, the uncertainty and disturbance estimator (UDE)-based position controllers are developed to achieve the robust position control of a quadrotor using only onboard sensing. Firstly, in order to accurately estimate the positions of the quadrotor in GPS-denied environments, an open source high speed optical flow sensor PX4FLOW is adopted. Secondly, the UDE-based controllers are developed to handle the challenges brought by the highly nonlinear quadrotor position dynamics, including underactuation, coupling, nonaffine inputs, model uncertainties and external disturbances. Real flight experiments, including hover and disturbance rejection are carried out to demonstrate the effectiveness of the developed UDE-based controllers.

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