Multi-vehicle Cooperative Control for Load Transportation *

This work proposes a cooperative control solution to the problem of transporting a suspended load using multiple quadrotor vehicles. The problem is addressed for two quadrotors, with a methodology that can be generalized for any number of quadrotors. A dynamic model of the system is developed considering a point-mass load, rigid massless cables, and neglecting aerodynamic effects of the cables. The concept of differential flatness is explored and a new set of flat outputs, which can be used to fully characterize the state of the system, is proposed. A nonlinear Lyapunov-based controller in cascaded form is derived, by defining adequate mappings between the cable tension vectors and the quadrotor thrust vectors and exploring the analogy with the problem of controlling a single quadrotor. Simulation results are presented for tracking of load trajectories. Comparisons are made with a free-flying quadrotor control scheme to highlight the enhanced performance of the proposed scheme.