Propeller failure is one major reason for the falling and crashing of multirotor Unmanned Aerial Vehicles (UAVs). While conventional multirotors can barely handle this issue due to underactuation, over-actuated platforms can still pursue the flight with proper fault-tolerant control (FTC). This paper investigates such a controller for one such over-actuated multirotor aerial platform composing quadcopters mounted on passive joints with input redundancy in both the high-level vehicle control and the low-level quadcopter control of vectored thrusts. To fully utilize the input redundancies of the whole platform under propeller failure, our proposed FTC controller has a hierarchical control architecture with three main components: (i) a low-level adjustment strategy to avoid propeller-level thrust saturation; (ii) a compensation loop to attenuate introduced disturbance; (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 at most two failed propellers and the whole platform can be stabilized without crashing. The proposed controller is extensively studied in both simulation and real-world experiments to demonstrate its superior performance.

翻译：螺旋桨故障是多旋翼无人机坠落和坠毁的主要原因之一。传统多旋翼无人机因欠驱动特性几乎无法应对此问题，而过驱动平台仍可通过合适的容错控制实现飞行。本文针对一种由安装于被动关节上的四旋翼组成的过驱动多旋翼飞行平台，研究其在高阶飞行控制与低阶四旋翼矢量推力控制中均具有输入冗余特性的容错控制器。为充分利用螺旋桨故障下整个平台的输入冗余，本文提出的容错控制器采用分层控制架构，包含三个主要部分：(i) 避免螺旋桨级推力饱和的低阶调整策略；(ii) 抑制引入扰动的补偿回路；(iii) 基于零空间的避免四旋翼级推力饱和的控制分配框架。通过在低阶与高阶控制回路中重新分配执行器输入，可在最多两个螺旋桨故障时维持低阶四旋翼控制，并实现整个平台的稳定飞行而不坠毁。通过仿真与真实世界实验的广泛验证，本文所提控制器展现了优越性能。