A Cooperative Aerial System of A Payload Drone Equipped with Dexterous Rappelling End Droid for Cluttered Space Pickup

TL;DR

The paper tackles payload pickup in cluttered environments by introducing a cooperative aerial system composed of a payload drone with a winch and a dexterous rappelling end droid connected by a tether. It leverages a catenary cable model with parameters $T_0$ and $\mu$ to enforce safe cable-length bounds $L_{ ext{min}}(t) \le L(t) \le L_{ ext{max}}(t)$, and develops a MINCO-based trajectory optimization framework that respects these constraints and obstacle avoidance. End-droid dynamics are treated within a differential-flatness framework to enable smooth, dynamically feasible paths from the current to the target location, while the cable model accounts for sag and taut states. The approach is validated in Gazebo simulations and indoor real-world experiments, demonstrating safe, efficient pickup and passive retrieval during lifting, thereby expanding the operational workspace beyond conventional UAV grabbing methods.

Abstract

In cluttered spaces, such as forests, drone picking up a payload via an abseil claw is an open challenge, as the cable is likely tangled and blocked by the branches and obstacles. To address such a challenge, in this work, a cooperative aerial system is proposed, which consists of a payload drone and a dexterous rappelling end droid. The two ends are linked via a Kevlar tether cable. The end droid is actuated by four propellers, which enable mid-air dexterous adjustment of clawing angle and guidance of cable movement. To avoid tanglement and rappelling obstacles, a trajectory optimization method that integrates cable length constraints and dynamic feasibility is developed, which guarantees safe pickup. A tether cable dynamic model is established to evaluate real-time cable status, considering both taut and sagging conditions. Simulation and real-world experiments are conducted to demonstrate that the proposed system is capable of picking up payload in cluttered spaces. As a result, the end droid can reach the target point successfully under cable constraints and achieve passive retrieval during the lifting phase without propulsion, which enables effective and efficient aerial manipulation.

Figures (7)

• Figure 1: Overall architecture and typical pick-up scenario of the proposed cooperative aerial system. The system consists of a payload drone and a dexterous rappelling end droid. In cluttered spaces such as forests, a payload drone with sufficient carrying capacity cannot enter to pick up the target. The dexterous rappelling end droid could freely pass through narrow spaces and locate the target.
• Figure 2: Platform of the payload drone equipped with a dexterous rappelling end droid.
• Figure 3: Coordinate system setup and catenary-based modeling of the tether cable between the payload drone and the end droid.
• Figure 4: Force analysis diagram of the entire cable in two equilibrium states. (a) Taut cable. (b) Slack cable.
• Figure 5: Trajectory planning results and corresponding cable length profiles of the end droid under three target point configurations. (a)-(c) The planned position trajectories of the end droid along x, y, and z axes under three target point settings: (2, 0, 0) m, (2, 0, 1) m, and (2, 0, 2) m, respectively. The initial position is fixed at (0, 0, 0) m. (d)-(f) The corresponding cable length variations, including the minimum, maximum, and current cable lengths. In all cases, the planned motion remains within the allowable cable length bounds, validating the feasibility of the proposed planner under tether constraints.