Automatic aerial retrieval of a mobile robot using optical target tracking and localization

In this paper we present a system for automatic deployment and retrieval of a mobile ground robot using a helicopter UAV. Our system allows using a mobile outdoor robot in areas that cannot be reached other than from the air and aerial measurements alone are not sufficient. For example a ground robot can perform in situ measurements and even take samples that can later be analyzed when the robot is returned by the aerial system. We use a helicopter UAV with a rotor diameter of 1.8m and a takeoff mass of 11kg as a proof-of-concept platform. The UAV is equipped with our modular autopilot system. The real time control and navigation is done by the flight control computer. The target detection is done by the image processing computer connected to a downward looking camera. In addition to the autopilot payload the helicopter can carry an extra mass of around 2kg. The ground robot we used had a mass of 1.1kg and is equipped with a GPS sensor and a communication system that is used to send its current position estimate to the UAV. The aerial system is using a high precision hover position controller and a multi-sensor fusion module which is used for detection and precise localization of the mobile robot. It combines GPS-based localization for obtaining an initial estimation of the ground robot location and a vision-system for its accurate localization. We use a known optical marker on the ground robot for its precise localization relative to the aerial system. All control and sensor processing and fusion are performed on board of the UAV. The docking system we developed is very similar to the probe-and-drogue aerial refueling system. It is used to compensate position disturbances of the UAV during the docking maneuver. Results from multiple successful outdoor flight experiments will be presented.