Differential Flatness Based Control for a Reconfigured Underactuated Hexacopter With Lateral Bi-Directional Thrust

Aerial docking of unmanned aerial vehicles (UAVs) is essential for advanced applications like persistent surveillance and in-flight recharging. Conventional underactuated multirotors, with only four control inputs for six degrees of freedom, lack independent control over their full pose. This fundamental limitation prevents them from achieving arbitrary attitudes and precise lateral forces required for docking maneuvers that impose specific angular constraints. While fully-actuated or over-actuated designs offer improvements, they often increase mechanical complexity or struggle with reliability. This letter addresses these limitations by introducing a reconfigured hexacopter design and tailored control approach. The proposed hexacopter features two dedicated lateral rotors providing bi-directional lateral thrust, significantly enhancing lateral force capability and decoupling horizontal control from the primary attitude loop. Crucially, we prove this hybrid design (combining longitudinal and lateral thrusters) is differentially flat. Leveraging this property, we develop a differential flatness-based control (DFBC) framework. Real-world experiments validate the platform and controller through representative demonstrations.