Robot-Assisted Landing of VTOL UAVs: Design and Comparison of Coupled and Decoupling Linear State-Space Control Approaches

This paper addresses the problem of landing a VTOL UAV using a serial robotic manipulator fixed to the landing surface, which assists the UAV during the last, most challenging, landing phase. In this phase, UAV and manipulator are connected via a universal hinge, which decouples the flying vehicle’s and the robot’s end-effector orientation. The main contribution of the paper is the design of a linear state-space controller for position and orientation of the UAV while it is fixed to the manipulator. Furthermore, we compare a coupled and a decoupling realization of the model-based controller with a model-free controller. Both model-based controllers consider the dynamics of an attitude-controlled aerial vehicle and use the acceleration of the robot’s end-effector as control input. All three controllers are validated and compared in experiments using a KUKA/DLR light-weight robot on a nonmoving base and an AR.Drone 2.0 quadrotor. The experimental results show that decoupling is superior to the coupled and the model-free approach, since the orientation of the UAV rotorcraft is controlled more precisely.