Passive Fault Tolerance through Tension-to-Thrust Feed-Forward: Hybrid Input-to-State Stability for Decentralized Multi-UAV Slung-Load Transport under Abrupt Cable Severance

A new paper from researchers Hadi Hajieghrary and Paul Schmitt tackles a critical problem in autonomous drone logistics: what happens when a cable breaks mid-flight during multi-UAV slung-load transport. Traditional controllers rely on coordinated force allocation and peer-state exchange, leaving no certified recovery mechanism for sudden cable severance. The authors propose a passive architecture that routes each vehicle's measured cable tension directly into its altitude thrust command (T_i^ff = T_i), while a surrounding proportional-derivative, anti-swing, and projection cascade preserves local tracking feasibility. The key innovation is a conditional hybrid practical input-to-state stability certificate that composes slack-excursion-bounded taut-cable reduction, bounded post-severance Lyapunov jumps, inter-fault decay, and per-fault-cycle contraction into an explicit recovery envelope.

The controller was validated in Drake multibody simulation with five vehicles, a 10 kg payload, Kelvin-Voigt cables, and Dryden wind under single- and dual-severance schedules. Results show the closed loop attains 0.312-0.328 m RMSE, 76.1-95.2 mm peak sag, and recovery within one payload-pendulum period. Critically, disabling the tension feed-forward inflates cruise error by 34-39% and peak sag by 3.6x-4.0x, identifying local tension feed-forward as the dominant passive recovery mechanism. The paper is submitted for review at IEEE Transactions on Control Systems Technology.