UDE-Based Robust Control of a Quadrotor-Slung-Load System

This article addresses the robust trajectory track- ing problem for a Quadrotor-Slung-Load System (QSLS), which consists of a point-mass load and a rigid-body quadrotor connected by an inelastic cable. To construct the controller, we employ the backstepping technique and propose an Uncer- tainty and Disturbance Estimator (UDE) to compensate for uncertainties arising from imprecise model parameters and exogenous time-varying disturbances affecting both quadrotor and load. The main feature of the UDE is its ability to convert the robust control problem into a low-pass filter design in the frequency domain, which generates an estimate of lumped uncertainties. To streamline the design process, we utilize a coordinate transformation strategy that converts the QSLS into a configuration that resembles the dynamics of a typical quadrotor system. The proposed controller ensures uniformly ultimate boundedness of closed-loop errors in the presence of time-varying exogenous disturbances, while guaranteeing asymptotic stability when disturbances are zero. Finally, we present comprehensive simulation and experimental results to validate the effectiveness and robustness of the proposed solution.