A hierarchical controller for quadrotor unmanned aerial vehicle transportation systems

Quadrotor unmanned aerial vehicles (UAVs) have the natural advantage of transferring payloads in complex environment considering its excellent agility. However, the payload sway motion cannot be controlled directly, and the quadrotor itself possess the underactuated characteristic, which makes it a difficult job to realize accurate quadrotor positioning and payload swing suppression simultaneously. In this paper, we present a hierarchical control scheme to achieve the aforementioned objectives. Specifically, the system model is first transformed into an inner-outer loop interconnected form. Subsequently, an energy storage function for the outer loop is established so as to obtain the applied thrust and the desired roll angle for the quadrotor. The desired roll angle is in fact the reference input for the inner loop, which helps to derive the applied torque for the system. As the quadrotor's orientation dynamics converge faster than its translational dynamics, the time-scale separation principle is utilized to indicate the feasibility of the proposed method. Finally, simulation results are provided to illustrate the control scheme's superior performance and good robustness.