Docking Multirotors in Close Proximity Using Learnt Downwash Models

Unmodeled aerodynamic disturbances pose a key challenge for multirotor flight when multiple vehicles are in close proximity to each other. However, certain missions require two multirotors to approach each other within 1-2 body-lengths of each other and hold formation --we consider one such practical instance: vertically docking two multirotors in the air. In this leader-follower setting, the follower experiences significant downwash interference from the leader in its final docking stages. To compensate for this, we employ a learnt downwash model online within an optimal feedback controller to accurately track a docking manoeuvre and then hold formation. Through real-world flights with different manoeuvres, we demonstrate that this compensation is crucial for reducing the large vertical separation otherwise required by conventional/naïve approaches. Our evaluations show a tracking error of less than 0.06m for the follower (a 3-4x reduction) when approaching vertically within two body-lengths of the leader. Finally, we deploy the complete system to effect a successful physical docking between two airborne multirotors in a single smooth planned trajectory. Full paper, presented at International Symposium on Experimental Robotics (ISER) 2023, can be found here: https://doi.org/10.48550/arXiv.2311.13988