  Battery endurance represents a key challenge for long-term autonomy and
long-range operations, especially in the case of aerial robots. In this paper,
we propose AutoCharge, an autonomous charging solution for quadrotors that
combines a portable ground station with a flexible, lightweight charging tether
and is capable of universal, highly efficient, and robust charging. We design
and manufacture a pair of circular magnetic connectors to ensure a precise
orientation-agnostic electrical connection between the ground station and the
charging tether. Moreover, we supply the ground station with an electromagnet
that largely increases the tolerance to localization and control errors during
the docking maneuver, while still guaranteeing smooth un-docking once the
charging process is completed. We demonstrate AutoCharge on a perpetual 10
hours quadrotor flight experiment and show that the docking and un-docking
performance is solidly repeatable, enabling perpetual quadrotor flight
missions.
