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：我们针对尾座式无人机轨迹生成与跟踪控制中的理论与实践问题展开研究。在理论层面，我们重点研究了融合实际无人机气动模型的全微分平坦特性，这为生成动态可行轨迹并实现高性能跟踪控制奠定了基础。通过设定协调飞行条件并选择飞行器位置作为平坦输出，我们发现尾座式无人机在整个飞行包线内具有精确气动模型的微分平坦性。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，实现精确的尾座式飞行。具体而言，我们提出了一种基于优化的尾座式轨迹规划器，在考虑运动动力学约束、无奇异性约束和作动器饱和约束的前提下，设计高质量平滑轨迹。该规划器将平坦输出轨迹实时转换为包含环境风影响的状态轨迹。为跟踪状态轨迹，我们开发了一种全局无奇异性、最小参数化的流形模型预测控制（MPC）方法，该方法充分利用精确气动模型，在整个飞行包线内实现高精度轨迹跟踪。通过室内外大量实飞实验验证了所提框架的有效性，实验包括：需特定姿态并以10m/s速度连续穿越窄窗的敏捷SE(3)飞行、最高速度20m/s的典型尾座式机动（过渡、平飞和盘旋），以及加速度达2.5g的极端特技机动（Wingover、Loop、Vertical Eight和Cuban Eight）。