We address the theoretical and practical problems related to the trajectory generation and tracking control of tail-sitter UAVs. Theoretically, we focus on the differential flatness property with full exploitation of actual UAV aerodynamic models, which lays a foundation for generating dynamically feasible trajectory and achieving high-performance tracking control. We have found that a tail-sitter is differentially flat with accurate aerodynamic models within the entire flight envelope, by specifying coordinate flight condition and choosing the vehicle position as the flat output. This fundamental property allows us to fully exploit the high-fidelity aerodynamic models in the trajectory planning and tracking control to achieve accurate tail-sitter flights. Particularly, an optimization-based trajectory planner for tail-sitters is proposed to design high-quality, smooth trajectories with consideration of kinodynamic constraints, singularity-free constraints and actuator saturation. The planned trajectory of flat output is transformed to state trajectory in real-time with consideration of wind in environments. To track the state trajectory, a global, singularity-free, and minimally-parameterized on-manifold MPC is developed, which fully leverages the accurate aerodynamic model to achieve high-accuracy trajectory tracking within the whole flight envelope. The effectiveness of the proposed framework is demonstrated through extensive real-world experiments in both indoor and outdoor field tests, including agile SE(3) flight through consecutive narrow windows requiring specific attitude and with speed up to 10m/s, typical tail-sitter maneuvers (transition, level flight and loiter) with speed up to 20m/s, and extremely aggressive aerobatic maneuvers (Wingover, Loop, Vertical Eight and Cuban Eight) with acceleration up to 2.5g.

翻译：我们针对尾坐式无人机在轨迹生成与跟踪控制中的理论与实际问题展开研究。在理论层面，我们重点探究结合真实无人机气动模型充分利用的微分平坦特性，该特性为生成动力学可行轨迹和实现高性能跟踪控制奠定基础。通过指定协调飞行条件并选择飞行器位置作为平坦输出，我们发现配备精确气动模型的尾坐式无人机在全飞行包线内具有微分平坦性。这一基本特性使我们能够在轨迹规划与跟踪控制中充分利用高保真气动模型，从而实现精确的尾坐式飞行。特别地，我们提出了一种基于优化的尾坐式轨迹规划器，在考虑动力学约束、无奇点约束和作动器饱和的条件下，可生成高质量平滑轨迹。平坦输出轨迹可实时转换为考虑环境风场的状态轨迹。为跟踪该状态轨迹，我们开发了一种全局无奇点且最小参数化的流形上模型预测控制器（MPC），该控制器充分利用精确气动模型，在全飞行包线内实现高精度轨迹跟踪。通过室内外实地测试中的大量真实世界实验，我们验证了所提框架的有效性：包括连续穿越需要特定姿态的窄窗的敏捷SE(3)飞行（速度达10m/s）、典型尾坐式机动（过渡飞行、平飞与盘旋，速度达20m/s）以及极度激进的特技机动（翼尖翻、筋斗、垂直8字与古巴8字，加速度达2.5g）。