Cooperative quadrotors carrying a suspended load

This work proposes the representation of the high level dynamic modelling of a formation of two quadrotor UAVs cooperatively carrying a suspended load. The two quadrotors are attached to the load by massless and stretchable cables, which can slack and taut during the task accomplishment. The cable is modeled as a high stiffness spring with a damping coefficient, according to the Hooke's law, and its behavior is taken into account in the disturbance estimation. A fictitious repulsion force is also introduced to minimize the risk of a collision between the UAVs during lateral displacements. Finally, to dealt with disturbance rejection caused by the load compensation and the cable tension variations, a nonlinear controller is proposed to guide the UAVs cooperatively during positioning and trajectory-tracking tasks, considering that the movement of the UAVs is restricted to the XZ-plane. Finally, simulated results are presented, in order to validate the proposal.