Preserving Full 6-DOF Actuation Under Abrupt Total Rotor Failures: Passive Fault-Tolerant Flight Control Using a Biaxial-Tilt Hexacopter

2026-06-03

AI Summary

This study addresses the challenge of severe degradation in force and torque authority—and the consequent loss of full six-degree-of-freedom controllability—faced by multirotor aerial vehicles under sudden complete rotor failures. Focusing on a dual-tilt overactuated hexacopter, the work proposes a passive fault-tolerant control architecture that operates without explicit fault detection or mode switching. The approach integrates a high-order fully actuated controller with a linear extended state observer and employs a momentum-estimation-based model reference adaptive control allocation strategy. To quantify fault tolerance, an inscribed sphere metric incorporating transient force/torque jump terms is introduced. Simulations and real-flight experiments demonstrate stable hover and accurate six-degree-of-freedom trajectory tracking under single and multiple rotor failures, with the bi-tilt-overactuated (BTO) configuration exhibiting superior recovery margin and environmental robustness compared to single-tilt and coplanar layouts.

Abstract

Conventional multirotors suffer from a rapid collapse of attainable wrench space (AWS) under abrupt total rotor failures, rendering full 6-DOF recovery physically impossible. This paper addresses passive fault-tolerant flight of a biaxial-tilt overactuated hexacopter (BTO) under abrupt total rotor failures that are a priori unknown to the controller. The control design and analysis focus on representative abrupt rotor-failure cases for which the post-failure system remains fully actuated, while no explicit fault detection, isolation, or fault-mode switching is assumed. First, we extend the inscribed-sphere metric of the AWS by incorporating the transient-wrench-jump term, enabling quantitative feasibility assessment under up to three simultaneous rotor failures and benchmarking against uniaxial-tilt and coplanar hexacopters. Second, we develop two computationally efficient passive schemes without relying on fault detection or online optimization. One scheme operates at the controller layer by combining a high-order fully actuated (HOFA) controller with a linear extended state observer (LESO) for lumped-disturbance rejection. The other scheme operates at the allocator layer by using model-reference adaptive control allocation with momentum-based wrench estimation to compensate for control-allocation biases. Simulations and flight experiments validate stable hovering and 6-DOF trajectory tracking under single and multiple rotor failures. Further systematic comparisons confirm that the BTO provides larger recovery margins than uniaxial-tilt and coplanar designs. Additional onboard-sensor-only experiments, including indoor tracking under wind disturbance, outdoor tracking under extreme conditions, narrow-frame traversal, and contact-based aerial writing, further validate the robustness of the proposed framework in complex operational environments.

Problem

• 6-DOF actuation
• rotor failure
• fault-tolerant flight
• wrench space
• multirotor

Innovation

• passive fault-tolerant control
• biaxial-tilt hexacopter
• 6-DOF full actuation
• wrench space feasibility
• adaptive control allocation