Hierarchical Geometric Path-Following Control for UAV Slung Load Transport

This paper addresses the motion control problem of a slung load system (SLS), which consists of an unmanned aerial vehicle (UAV) carrying a pendulum. The pendulum is pivoted at the UAV's center of mass through a 2-degree-of-freedom rotational joint. The other end of the pendulum carries a payload to be transported by the SLS. The control design is based on a geometric SLS model, where UAV and pendulum rotations evolve in SO(3) and S2, respectively. Hence, the control design is geometric in nature and does not adhere to the parameterization of SO(3) and S2. The objective is to control payload position and UAV heading without using timed reference trajectories. Instead, the control design stabilizes the so-called path set to control payload motion using a hierarchical control structure. Utilizing the Reduction Theorem, the paper proves that the path set is almost globally asymptotically stable. The Reduction Theorem simplifies the stability proof as no Lyapunov function is needed and allows for modularity in individual control-loop designs. Numerical simulations are presented to validate the result and show its almost global asymptotic nature.