This paper originates from two well-accepted challenges in the control of a quadrotor with a cable-suspended payload: 1) designing a refined controller based on high-precision payload swing modeling to achieve the quantized prescribed robustness; and 2) resolving the trajectory tracking performance degradation issue caused by input saturation. To combat these challenges, we start with establishing a precise payload swing model. The experimental investigation reveals the fact that neglect of realistic factors like cable-joint dry friction and cable elasticity has significant impacts on the precision of the existing models, especially under small swing angles. Therefore, we propose a new integrated drag model including a novel sign of the payload airspeed-dependent term, which lumps all payload swing damping factors together. This model is experimentally verified to be precise to provide the payload swing disturbance spectrum for quantitatively designing the bandwidth of the uncertainty and disturbance estimator (UDE). Furthermore, we resolve the input saturation issue by augmenting the classic UDE-based controller with a tracking differentiator (TD). Rigorous performance analysis derives a clear relationship between the control performance and the UDE parameter, which forms a simple yet effective parameter tuning guideline for practical applications to ensure the prescribed robustness and trajectory tracking accuracy. The effectiveness and advantage of the proposed controller are verified via comparative experiments in different flight scenarios. Note to Practitioners —The control input saturation issue and multi-parameter optimization are frequently encountered in engineering practices. When applying the TD to address the input saturation, the selection of parameter $r$ in the TD depends on the reference continuity. Specifically, if the reference is continuous, then $r$ can be large; otherwise $r$ should be small to smooth the reference to avoid input saturation. For the multi-parameter optimization, the feedback gains $k_p$ and $k_d$ should be tuned first to guarantee the system stability, followed by decreasing the UDE parameter $T$ to improve the system robustness. However, the feasible range of $T$ may be restricted by the measurement noise and actuator bandwidth in practice. The proposed algorithm is applicable not only in aerial transportation systems but also in addressing other disturbance rejection problems.

In practice, the consensus performance of a multi-UAV system can degrade significantly due to the presence of measurement noise and disturbances. However, simultaneously rejecting the noise and disturbances to achieve high-precision consensus tracking control is rather challenging. In this paper, to address this issue, we propose a novel distributed consensus tracking control framework consisting of a distributed observer and a local dual-estimator-based tracking controller. Each UAV’s distributed observer estimates the leader’s states and generates the local reference, functioning even under a switching communication topology. In the local tracking controller design, we reveal that classic uncertainty and disturbance estimator (UDE)-based control can magnify the noise. By combining the measurement error estimator (MEE) with UDE, a local robust tracking controller is designed to reject noise and disturbances simultaneously. The parameter tuning of MEE and UDE is unified into a single parameter, and the monotonic relationship between this parameter and system performance is revealed by the singular perturbation theorem. Finally, the validity of the proposed control framework is verified by both simulation and comparative real-world experiments.

In the quadrotor-suspended-payload system, simultaneously achieving trajectory tracking and payload anti-swing control is essential but challenging. We reveal that the fast maneuver required by the anti-swing control may not be effectively executed by the classic quadrotor dual loop control framework, due to the inherent non-ideal inner loop actuation bandwidth in practice. To address this issue, we propose a novel hierarchical control framework incorporating the uncertainty and disturbance estimator (UDE)–based backstepping control. The proposed framework consists of three loops, where the outer loop tracking and anti-swing controller and the inner loop attitude controller are both designed in the conventional way, and more importantly, a newly proposed intermediate loop is induced between them. In the intermediate loop, the robust dynamic compensator is developed based on an experimentally verified actuator model to extend the quadrotor actuation bandwidth, such that the outer loop control bandwidth requirements are quantitatively satisfied. Furthermore, the system stability is proved by the Lyapunov method along with the system performance analysis based on the singular perturbation theorem. Finally, comparative simulation and experimental results are presented to demonstrate the feasibility and effectiveness of the proposed controller.