Editorial

TL;DR: This special issue showcases recent advancements in advanced control and estimation techniques for aerial robotics, focusing on fault tolerance, disturbance rejection, and robust stability. The included papers demonstrate capabilities in handling complex environments, estimating physical parameters, and mitigating nonlinearities and uncertainties.

Abstract: The focus of this special issue is to report on some recent developments in the advanced control and estimation fields focussing on aerial robotics, especially those that successfully consider theoretical open problems, as well as efficient implementations in practical applications. The different methodological approaches and topics of study within this special issue demonstrate their capabilities of rejecting disturbances in complex environments, estimating the vehicle’s physical parameters or states in the uncertain environment, showing robustness against inherent nonlinearities and modeling uncertainties, and maintaining an acceptable level of robust stability in harsh conditions or even in the face of faults/failures. Fifteen papers are included in this special issue. Wang et al.1 proposed an adaptive sliding mode fault tolerant control (FTC) strategy for a quadrotor unmanned aerial vehicle (UAV) in the face of actuator faults and model uncertainties. In this strategy, the sliding mode reaching law, which measures the distance between the sliding variable and the designated boundary layer, was constructed to suppress chattering, whilst preserving system tracking performance. Furthermore, an adaptation scheme is developed to prevent overestimation of adaptive parameters during the process of fault compensation. To validate the effectiveness and superiority of the proposed control strategy, a comparative study was conducted via simulation. Zhao et al.2 considered the FTC problem for a class of quadrotors with variable mass and actuator faults. In this article, the variable mass and actuator fault are both estimated using adaptive fault observers. Using the knowledge of the estimated variable mass, a non-singular terminal sliding mode controller is created to ensure satisfactory trajectory tacking performance. Also, by utilizing the information of the actuator fault estimates, an integral sliding mode controller is designed to track the desired attitude. Simulation results show that the proposed FTC method guarantees good tracking performance in the presence of variable mass, faults and disturbances. Reis et al.3 addressed the problem of slung load transportation for an underactuated quadrotor requiring the damping of oscillations and external disturbances. The position control strategy involves a sliding mode controller that computes a vectored thrust actuation to damp out the system’s oscillations. To ensure good attitude tracking performance, conventional backstepping methods and a sliding mode controller built intrinsically in SO(3), are exploited to control the vehicle’s angular velocity and to produce torque commands, respectively. Simulation and experimental results validate the robustness of the proposed technique in a trajectory tracking scenario. Yu et al.4 investigated the fault-tolerant containment control (FTCC) problem for a group of fixed-wing UAVs and consider fault tolerance and collision avoidance simultaneously. In this article, a fractional-order (FO) FTCC scheme is proposed to steer all follower UAVs into the convex hull formed by the leader UAV with the involvements of FO calculus, disturbance observers (DOs), and interval type-2 fuzzy neural networks. In this scheme, FO sliding-mode surfaces with artificial potential functions are designed to revamp the filtered containment errors, and the DOs with FO calculus are constructed to estimate the FO lumped disturbances involving faults and external disturbances. To compensate for the DO estimation errors, IT2FNN learning mechanisms are introduced to improve the FTCC capability. Simulation results show that all follower UAVs successfully converge into the convex hull spanned by the leader UAVs without collisions even when a portion of the UAVs encounter faults. Labbadi et al.5 proposed a FO command filtered-based recursive finite-time control using a non-singular TSM technique for high-order uncertain nonlinear systems under disturbances with unknown bounds. The proposed control is developed to go beyond the limitations of existing finite-time tracking controllers. In this work, the reaching phase is removed, and a chattering alleviation property is achieved. This fractional-order control strategy has been applied to quadrotor UAVs in different scenarios affected by disturbances with increasing amplitudes and frequencies. Simulation results demonstrate the effectiveness of the proposal strategy even in comparison with an existing approach in the literature. Zhou et al.6 established a mathematical model of a hybrid quad-plane UAV characterized by complex structures, high nonlinearity, strong coupling, and various flight regimes involving hover, transition, and fixed-wing flight. A cascade flight