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

Abstract

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. This novel system is meant to be used for VTOL UAVs landing on moving platforms under severe environmental conditions. 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. The decoupling controller allows to fully actuate the UAV using the manipulator in addition to the UAV’s actuators. All three controllers are validated and compared in experiments using a KUKA/DLR light-weight robot on a non-moving 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.