Landing of VTOL UAVs using a stationary robot manipulator: A new approach for coordinated control

This paper deals with the landing of VTOL UAVs under severe conditions. We present a novel system based on a robot manipulator, which is used to support a UAV in the last, most challenging, landing phase. In this phase, the aerial vehicle is connected to the robot by means of a universal hinge. This allows to decouple the UAV's orientation from the robot's end-effector orientation. The main contribution of the paper is a new control approach for the whole system composed of a VTOL UAV and a manipulator. A combination of a backstepping controller accounting for the UAV's dynamics with an impedance controller for the manipulator is used for coordinated control of the whole system. The proposed approach allows to independently control position and orientation of a VTOL UAV, whereby an arbitrary stable attitude controller can be used for the flying vehicle. The advantage of the presented approach is that the interaction forces between robot and UAV are taken into account explicitly and that a Lyapunov stability proof for the UAV subsystem can be derived directly. Robustness and performance of the control approach are investigated in simulation and experiments. The experimental results for an AR.Drone quadrotor and a DLR/KUKA light-weight robot with seven degrees-of-freedom are presented in the paper.