Cooperative Transport of a Slung Load Using Load-Leading Control

A system for cooperative transport of a slung load by a team of autonomous rotorcraft is described. The method described here is hierarchical and centralized, with the payload defined as the leader (here denoted as load-leading control): the payload uses knowledge of its current state and its desired trajectory to determine the net force and moment acting upon its center of gravity required to follow the desired trajectory. Using knowledge of the cable attachment geometry, the required cable forces are computed and transmitted to the rotorcraft. The cable force optimization problem is defined and shown to be nonconvex; constraints that make the problem convex are introduced. Cable forces computed using both the convex and nonconvex formulations are compared. Controllers for both payload and rotorcraft trajectory tracking are designed. Hardware implementation that uses small single-board computers carried onboard the payload and rotorcraft is described. Flight demonstrations conducted in an indoor motion capture studio using preplanned trajectories as well as human-in-the-loop control of the payload are described to show trajectory following and rejection of impulse disturbances. An outdoor flight test showing coordinated transport in the presence of light winds is described. All calculations are hosted on the computers carried on the payload and rotorcraft, and the system is shown to be capable of real-time control and path following both for fixed and in-flight changing formations.