  We propose a combined method for the collaborative transportation of a
suspended payload by a team of rotorcraft. A recent distance-based
formation-motion control algorithm based on assigning distance disagreements
among robots generates the acceleration signals to be tracked by the vehicles.
In particular, the proposed method does not need global positions nor tracking
prescribed trajectories for the motion of the members of the team. The
acceleration signals are followed accurately by an Incremental Nonlinear
Dynamic Inversion controller designed for rotorcraft that measures and resists
the tensions from the payload. Our approach allows us to analyze the involved
accelerations and forces in the system so that we can calculate the worst case
conditions explicitly to guarantee a nominal performance, provided that the
payload starts at rest in the 2D centroid of the formation, and it is not under
significant disturbances. For example, we can calculate the maximum safe
deformation of the team with respect to its desired shape. We demonstrate our
method with a team of four rotorcraft carrying a suspended object two times
heavier than the maximum payload for an individual. Last but not least, our
proposed algorithm is available for the community in the open-source autopilot
Paparazzi.
