  In this paper, an exponentially stable trajectory tracking controller is
proposed for multi-UAV payload transport. The multi-UAV payload system has a
2-DOF magnetic spherical joint between the UAVs and the vertical rigid links of
the payload frame, so the UAVs can roll or pitch freely. These vertical links
are rigidly attached to the payload and cannot move. An input-output feedback
linearized model is derived for the complete payload-UAV system along with
thrust vectoring control for trajectory tracking of the payload. The
theoretical analysis on tracking control laws shows that control law is
exponentially stable, thus guaranteeing safe transportation along the desired
trajectory. To validate the performance of the proposed control law, the
results for a numerical simulation as well as a high-fidelity Gazebo real-time
simulation are presented. Next, the robustness of the proposed controller is
analyzed against two practical situations: External disturbance on the payload
and payload mass uncertainty. The results clearly indicate that the proposed
controller is robust and computationally efficient while achieving
exponentially stable trajectory tracking.
