A novel transformable UAV: HOFA modeling, control, and experimental validations

• A new transformable UAV design between underactuated and fully actuated flight modes • Three-servo, six-rotor design improves reliability, scalability, and efficiency • HOFA-based controller unifies UA/FA modeling and ensures smooth transitions • Disturbance observers mitigate modeling errors and external disturbances in flight • Simulations and indoor tests confirm robust pose tracking and mode transitions This paper presents modeling, control, and experimental validation of a novel transformable unmanned aerial vehicle (UAV) capable of switching between underactuated (UA) and fully actuated (FA) flight modes. The UAV can transition from an energy-efficient coplanar hexacopter in the UA mode to a fully pose-tracking platform in the FA mode. The transformable design utilizes three servos and six rotors, offering improved reliability and scalability over the single-servo prototype. To address complex nonlinear dynamics and external disturbances, a high-order fully actuated (HOFA) control framework is developed with disturbance observers for compensation. A new smooth transition strategy is developed to blend the high-order UA controller with the low-order FA controller via a sigmoid blending function. Crucially, the resulting hybrid dynamics during the transition phase are formulated as a unified HOFA system with bounded uncertainties, ensuring stable mode switching and avoiding the destabilizing effects during controller switching. The effectiveness of the proposed mechanical design and control framework is validated through both simulation and experimental results, demonstrating robust performance and the execution of desired maneuvers in both the UA and FA modes, as well as a seamless transition between them.