Online Nonlinear Model Predictive Control for tethered UAVs to perform a safe and constrained maneuver

This paper focuses on the design and implementation of a two layer hierarchical controller for a tethered aerial vehicle. In particular, a challenging problem of smoothly landing on a inclined plane is considered. Such problem yields nonlinear control objective and constraints which require advanced control techniques like Nonlinear Model Predictive Control (NMPC), that is known to be computationally demanding. A control architecture composed by two layers is proposed: an NMPC operating at 100Hz is employed to generate feasible trajectories on-line and a simplified local controller, working at 500Hz, is used to track them. The effectiveness of this hierarchical control structure is demonstrated by a closed-loop simulation. Moreover, the rejection of external disturbances, such as the wind, is a peculiar characteristic of the proposed control strategy.