  We present a control framework for achieving encirclement of a target moving
in 3D using a multi-robot system. Three variations of a basic control strategy
are proposed for different versions of the encirclement problem, and their
effectiveness is formally established. An extension ensuring maintenance of a
safe inter-robot distance is also discussed. The proposed framework is fully
decentralized and only requires local communication among robots; in
particular, each robot locally estimates all the relevant global quantities. We
validate the proposed strategy through simulations on kinematic point robots
and quadrotor UAVs, as well as experiments on differential-drive wheeled mobile
robots.
