Trajectory Generation and Tracking Control for Aggressive Tail-Sitter Flights

TL;DR

This paper presents a comprehensive framework for trajectory generation and high-precision tracking control of tail-sitter UAVs, leveraging differential flatness and aerodynamic models for agile, aggressive flight in complex environments.

Contribution

It introduces a novel differential flatness property for tail-sitters with aerodynamic models, enabling advanced trajectory planning and control in the entire flight envelope.

Findings

Successful real-world tests including narrow window flights and aerobatic maneuvers.

High-accuracy trajectory tracking achieved with the proposed MPC approach.

Demonstrated robustness in indoor and outdoor environments with speeds up to 20m/s.

Abstract

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.