Quasi-Static Fault-Tolerant Feedback Control of a Quadrotor Carrying a Slung Payload under Rotor Failure

Abstract

This paper presents a nonlinear control law for a slung payload system (SPS) consisting of a quadrotor unmanned aerial vehicle (UAV) carrying a slung payload. We propose a fault-tolerant control (FTC) design for the case of a complete single-rotor failure, i.e., only three functioning propellers. The failure is not modeled as a disturbance to be handled by estimation. Rather, we model the faulty SPS in control-affine form and design a quasi-static feedback law directly for the faulty dynamics. The controller achieves local exponential convergence of the payload position to a geometric path in space, rather than tracking a timed reference trajectory. Moreover, the transverse dynamics are exponentially stable, and the path-following manifold is rendered controlled invariant by feedback. Controlled invariance allows motion along the path to be prescribed independently through a tangential command while the state stays on the path-following manifold. The UAV body angular velocity remains bounded under bounded applied torque despite the failure. To validate the control design, numerical simulations apply the controller derived from the faulty SPS model in coordinates to the coordinate-free SPS model. Code and animations are available at https://gradslab.github.io/qs-sl/.