Collision-free Path Planning and Control of Coordinated UAVs with Tethered Payloads

Unmanned aerial vehicles (UAVs) have been found in various fields, in particular, with emphasis on the load transport function. As maintaining the load integrity is crucial, these applications benefit from controllers enabling stable and safe flights. State-of-the-art methods for path planning and control of slung-load UAVs are often concerned with swing-minimizing trajectories or control of either the load or the vehicle to enable agile maneuvers. Optimization methods have found more success in uniting the planning and control for slung-load UAVs as a whole. In this paper, we employ a nonlinear model predictive control (NMPC) framework to navigate a pair of slung-load UAVs around an obstacle. In the preliminary simulations conducted, NMPC is first used to reach a desired goal state considering the two independent system dynamics concurrently. This approach yields an unsuccessful mission under certain parameter settings. An alternative method separates the planning and control to use NMPC as a tracking controller of a pre-generated trajectory. The advantages of the latter approach are demonstrated.