This paper investigates the fixed-time attitude tracking control problem for rigid spacecraft subject to external disturbance and output constraints. First, the state transformed function (STF) technique is employed to convert the constrained spacecraft error dynamics into an unconstrained one. Subsequently, a fixed-time disturbance observer (FXTDO) is designed to estimate and reconstruct the lumped disturbance of unconstrained system. Combined with the developed STF, FXTDO and fixed-time integral terminal sliding mode (FITSM) surface techniques, the proposed fixed-time control law provides zero-error attitude tracking with high control precision and chattering avoidance, while the output constraints are never transgressed. The fixed-time stability of the closed-loop system is conducted via the Lyapunov technique and bi-limit homogeneity theory, and the expression of convergence time is also presented. Simulations illustrate the efficiency of the investigated controller.

This work focuses on the safety of learning-based control for unknown nonlinear system, considering the stability of learned dynamics and modeling mismatch between the learned dynamics and the true one. A learning-based scheme imposing the stability constraint is proposed in this work for modeling and stable control of unknown nonlinear system. Specifically, a linear representation of unknown nonlinear dynamics is established using the Koopman theory. Then, a deep learning approach is utilized to approximate embedding functions of Koopman operator for unknown system. For the safe manipulation of proposed scheme in the real-world applications, a stable constraint of learned dynamics and Lipschitz constraint of embedding functions are considered for learning a stable model for prediction and control. Moreover, a robust predictive control scheme is adopted to eliminate the effect of modeling mismatch between the learned dynamics and the true one, such that the stabilization of unknown nonlinear system is achieved. Finally, the effectiveness of proposed scheme is demonstrated on the tethered space robot (TSR) with unknown nonlinear dynamics.

In this article, the event-triggered fixed-time tracking control is investigated for uncertain strict-feedback nonlinear systems involving state constraints. By employing the universal transformed function (UTF) and coordinate transformation techniques into backstepping design procedure, the proposed control scheme ensures that all states are constrained within the time-varying asymmetric boundaries, and meanwhile, the undesired feasibility condition existing in other constrained controllers can be removed elegantly. Different from the existing static event-triggered mechanism, a dynamic event-triggered mechanism (DETM) is devised via constructing a novel dynamic function, so that the communication burden from the controller to actuator is further alleviated. Furthermore, with the aid of adaptive neural network (NN) technique and generalized first-order filter, together with Lyapunov theory, it is proved that the states of closed-loop system converge to small regions around zero with fixed-time convergence rate. The simulation results confirm the benefits of developed scheme.

This paper develops a disturbance observer-based discrete-time sliding mode control (DOB-DSMC) method to address the trajectory tracking problem for nonholonomic robots subject to disturbances. Firstly, a reduced-dimensional tracking error system is derived to overcome the disadvantage of the underactuated robot system. In the light of reduced-dimensional system, a disturbance observer is studied to estimate and compensate for disturbances. Then, inspired by the continuous terminal sliding mode surface, a novel discrete-time terminal sliding surface is designed. Further, the arctangent function is inducted to improve the tracking accuracy and attenuate the chattering phenomenon. Moreover, the asymptotic stability of system is proved via the Lyapunov method. Finally, the effectiveness of the exploited algorithm is verified by a numerical simulation.

This paper presents a dynamic event-triggered communication mechanism to mitigate limited communication in multi-UAV systems. This mechanism aims to reduce resource consumption within these systems by enabling UAVs to communicate only when specific triggering conditions are met. Unlike static event-triggered communication mechanisms, our approach incorporates a dynamically adjusted triggering threshold through a carefully designed dynamic rule. As a result, the system performance is enhanced while communication resource utilization in multi-UAV systems is reduced. Building upon this dynamic event-triggered communication mechanism, we propose a distributed formation control strategy. Furthermore, we outline a criteria for designing the relevant control parameters. To validate the proposed approach, numerical simulation is conducted.

This paper investigates the fixed-time control issue for the post-capture space tethered robot system in the presence of uncertainty and external disturbances. A new form of fixed-time nonsingular terminal sliding mode surface (FNTSMS) is constructed by introducing the power function to traditional terminal sliding mode surface (TSM), thus the system fast response and singularity circumvention are ensured gracefully. With the aid of FNTSMS, a new fixed-time control scheme is proposed for post-capture space tethered robot, which can achieve the high-precision tracking performance and fast response rate after the space targets are captured. Moreover, the proposed scheme is compared with the traditional boundary layer sliding mode control method. The simulation results verify the effectiveness of the proposed control scheme.

This article addresses the finite-time attitude formation-containment control problem for networked uncertain rigid spacecraft under directed topology. A unified distributed finite-time attitude control framework, based on the sliding-mode control (SMC) principle, is developed. Different from the current state of the art, the proposed attitude control method is suitable for not only the leader spacecraft but also the follower spacecraft, and only the neighbor state information among spacecraft is required, allowing the resulting control scheme to be truly distributed. Furthermore, the proposed method is inherently continuous, which eliminates the undesired chattering problem. Such features are deemed favorable in practical spacecraft applications. In addition, upon using the proposed neuro-adaptive control technique, the attitude formation-containment deployment can be achieved in finite time with sufficient accuracy, despite the involvement of both the uncertain inertia matrices and external disturbances. The effectiveness of the developed control scheme is confirmed by numerical simulations.

This article proposes two kinds of terminal sliding mode control (TSMC) strategies for implementing the finite-time attitude tracking of spacecraft under environmental disturbances and model uncertainties. First, the integral disturbance observer (IDO) is designed to estimate the disturbances and uncertainties. Second, the IDO-based continuous TSMC (CTSMC) method is developed to achieve active disturbance rejection and better tracking performance. Third, to further mitigate the chattering, a modified TSMC (MTSMC) method is constructed by employing an adaptive method and incorporating a piecewise smooth function. Finally, simulations are performed to show the feasibility of the proposed TSMC laws.

This article is concerned with the finite-time tracking control problem for a class of strict-feedback nonlinear systems involving state constraints, unknown nonlinearities, and nonvanishing disturbances. Unlike the literature that mainly focuses on a C0finite-time controller, in this article, a novel C1smooth finite-time adaptive neural network (NN) controller is proposed by employing a smooth switch between the fractional and cubic form state feedback. The proposed controller not only avoids the singularity but also makes it possible to implement the dynamic surface control (DSC) technique. By applying the adaptive NN control technique, together with barrier Lyapunov functions (BLFs) and a generalized first-order filter including both linear and fractional terms, the desired fast finite-time control performance of the closed-loop nonlinear systems can be guaranteed, and meanwhile, the state constraints are never violated. Under the proposed control scheme, the tracking control problems of nonlinear systems with output constraint and full-state constraints are, respectively, discussed. It is explicitly shown that all the internal error signals are driven to converge into small regions in a finite time. Finally, the effectiveness of the control scheme is also confirmed by the applications to the control of a second-order nonlinear system and an uncertain ship autopilot.

The problem of spacecraft attitude tracking control for actuator faults/failures and external disturbances is addressed in this paper. Assuming that information on external disturbances is unknown, a finite time sliding mode (FTSM) control law is developed to handle actuator faults/failures. Under the constraint conditions including the actuator faults and external disturbance, stability analysis shows that the designed controller not only ensures the attitude tracking convergence in finite time, but also has superior to actuator fault tolerance. Finally, in order to illustrate the efficiency and robustness of the proposed control law, the numerical simulations are carried out for spacecraft under actuator faults and disturbances.