Propeller failure is a major cause of multirotor Unmanned Aerial Vehicles (UAVs) crashes. While conventional multirotor systems struggle to address this issue due to underactuation, over-actuated platforms can continue flying with appropriate fault-tolerant control (FTC). This paper presents a robust FTC controller for an over-actuated UAV platform composed of quadcopters mounted on passive joints, offering input redundancy at both the high-level vehicle control and the low-level quadcopter control of vectored thrusts. To maximize the benefits of input redundancy during propeller failure, the proposed FTC controller features a hierarchical control architecture with three key components: (i) a low-level adjustment strategy to prevent propeller-level thrust saturation; (ii) a compensation loop for mitigating introduced disturbances; (iii) a nullspace-based control allocation framework to avoid quadcopter-level thrust saturation. Through reallocating actuator inputs in both the low-level and high-level control loops, the low-level quadcopter control can be maintained with up to two failed propellers, ensuring that the whole platform remains stable and avoids crashing. The proposed controller's superior performance is thoroughly examined through simulations and real-world experiments.

翻译：螺旋桨故障是多旋翼无人机坠毁的主要原因。传统多旋翼系统因欠驱动特性难以应对此问题，而过驱动平台可通过适当的容错控制实现持续飞行。本文针对由被动关节连接四旋翼构成的过驱动无人机平台，提出鲁棒容错控制器。该平台在高层飞行控制与低层矢量推力控制层面均具备输入冗余。为在螺旋桨故障时最大化利用输入冗余优势，所提容错控制器采用分层控制架构，包含三个关键模块：(i) 防止螺旋桨级推力饱和的低层调整策略；(ii) 抑制引入扰动的补偿回路；(iii) 基于零空间的控制分配框架以避免四旋翼级推力饱和。通过高低层控制回路的执行器输入重分配，可使低层四旋翼控制在最多两个螺旋桨失效时仍保持稳定运行，确保整个平台避免坠毁。通过仿真与真实实验对所提控制器的优越性能进行了全面验证。