Landing a quadrotor UAV on a moving platform with sway motion using robust control

Autonomous landing of quadrotor unmanned aerial vehicles (UAVs) is a critical and challenging issue, especially on a moving target under complex terrain. This paper proposes a practicable solution that consists of a reference trajectory and a robust control strategy. More specifically, a continuous reference trajectory is planned to ensure the safety, where a feasible landing constraint is provided by introducing the sway motion of the target platform. Then, a robust hierarchical control strategy that exploits a saturated force controller and an attitude constraint torque controller is developed, where a dynamics estimator is employed to counteract the uncertain dynamics. Stability analysis indicates that the overall closed-loop system is uniformly ultimately bounded. Since the global positioning system (GPS) fails to provide an acceptable measurement for the precise landing, an ultra-wide band (UWB) positioning system is designed which possesses a 20-centimeter level of accuracy. To simulate a realistic scenario over the rough ground or sea condition, a motion platform with sway motion is utilized in the experiment facility. Flight experiments are performed to validate and assess the proposed strategy.