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Platooning of vehicular systems has been considered as an effective solution for alleviating traffic congestion. However, the widely existing matched and mismatched disturbances can greatly affect the safety, efficiency, and comfort of the vehicular systems. In this study, to compensate for both the matched and mismatched disturbances while improving convergence performance, a distributed platoon control problem of vehicular systems is studied under the recursive terminal sliding mode control (RTSMC) framework. A finite-time disturbance observer (FTDO) based on the high-order homogeneous differentiator is proposed for accurate estimation of the matched and mismatched disturbances in finite time. Based on the nested sliding surface structure, an RTSMC scheme is developed for the platooning of vehicular systems. The salient features of the proposed control algorithm are that the fast convergence rate can be reached and the reaching phase is eliminated, which guarantees the safer, more efficient, and more comfortable platooning. Finally, simulations and experiments are conducted to demonstrate the effectiveness and efficiency of the proposed algorithm. [ABSTRACT FROM AUTHOR]

Summary: This paper presents a composite continuous fast nonsingular terminal sliding mode (CCFNTSM) trajectory tracking control method for Mars entry vehicle. Firstly, the trajectory tracking is transferred into the tracking of the pre‐designed drag acceleration. Secondly, the finite‐time disturbance observer (FTDO) is introduced to estimate the matched and mismatched disturbances. And then, the dynamical fast nonsingular terminal sliding mode manifold is designed based on the estimation information. Finally, the CCFNTSM controller is constructed and its continuity is guaranteed by employing the power function of sliding variable to replace the constant switching gain. Numerical simulation results validate the effectiveness of the proposed control method. [ABSTRACT FROM AUTHOR]

With the continuous development of synchrotron radiation light sources, higher requirements have been put forward for the stability of double-crystal monochromators in synchrotron radiation facilities. This paper designs an active adjustment mechanism for a double-crystal monochromator to improve its stability. Firstly, three spatial degrees of freedom are designed based on the active adjustment mechanism of flexible leaf spring parallel coupling, and the prototype of the mechanism is fabricated. Secondly, system identification experiments are carried out and the system transfer function curve is fitted by the nonlinear least squares method. Thirdly, the controller based on non-singular terminal sliding modes and a finite-time disturbance observer was designed for stability control and disturbance compensation. Finally, the effectiveness of the controller is verified by a model-in-the-loop approach based on the performance of the real-time target machine. The results show that the non-singular terminal sliding mode + finite-time disturbance observer control strategy can reduce the RMS value of the vibration displacement of Axis-1/Axis-2/Axis-3 by 81.25%, 78.53%, and 71.82%. [ABSTRACT FROM AUTHOR]

This paper investigates a problem of autonomous aerial refueling point parallel rendezvous guidance and flight control subject to exogenous wind field disturbances. The tanker and uncrewed receiver aircraft approach the common refueling rendezvous point according to their respective flight plans, ultimately maintaining a fixed altitude and the same speed. According to the relative position of refueling target point and uncrewed receiver, a new continuous terminal sliding-mode guidance law combined with the prescribed performance control theory is designed under the end angle and speed constraints. Meanwhile, a finite-time observer is proposed to estimate the parameter uncertainties and wake vortex disturbances during refueling rendezvous. Based on the estimated information, a backstepping double-integral sliding-mode controller is developed for the uncrewed receiver flight control system. In the process of the guidance and flight control system, the integral variable is introduced to improve the steady-state performance. The designed guidance and control system can enable the uncrewed receiver to achieve the refueling rendezvous task within finite time and improve the efficiency of uncrewed aerial refueling. Comparative numerical simulations are provided to demonstrate the effectiveness of the proposed guidance and control scheme. [ABSTRACT FROM AUTHOR]

Featured Application: This work addresses issues related to unknown disturbances and actuator saturation during quadrotor UAV flight. This paper introduces a nonlinear dynamic inversion control algorithm designed to address unknown disturbances and actuator saturation issues in unmanned aerial vehicle (UAV) attitude control. The algorithm is based on a combination of finite-time disturbance observer and anti-saturation auxiliary system, which ensures the rapid convergence of attitude tracking error. Firstly, based on the Newton–Euler equations, this paper establishes a model of the attitude system for quadrotor UAVs, and this paper eliminates the small-angle flight assumption. Secondly, considering the actuator saturation problem, an anti-saturation auxiliary control system is designed to shorten the time when the control volume is in the saturation interval and achieve finite-time convergence of the attitude error. And then, to improve the robustness of the controller, this paper proposes a disturbance observer based on the finite-time stability theory, which achieves a continuous smooth output of the observation results by introducing a hyperbolic tangent function in the observer, so that the observation error can be converged to zero in a finite time. Finally, it is demonstrated by Simulink simulation that the attitude error and the observation error converge quickly to zero. [ABSTRACT FROM AUTHOR]

ObjectiveA finite-time disturbance observer-based optimal backstepping control (FDO-OBC) method is proposed for the issue of unknown disturbances and formation changes in the formation trajectory tracking of unmanned surface vehicles (USVs).MethodsFirst, the USV formation control framework is established on the basis of the virtual structure method, and the kinematics and dynamics formation controllers are designed. Second, a finite-time disturbance observer is introduced to estimate and compensate for unknown environmental disturbances in real time. Further, a dynamic trajectory optimization strategy based on optimal backstepping control is proposed for the trajectory tracking issue of formation change, and the information of the disturbance observer is used to calculate the optimal control input and achieve the dynamic optimization of USV formation trajectory tracking. Finally, Lyapunov stability theory is used to demonstrate the stability of the designed formation control method.ResultsThe simulation results indicate that the proposed control strategy can effectively improve the accuracy and robustness of the USV formation system.ConclusionThe FDO-OBC method provides a new technical means for the design of USV formation control systems under unknown environmental disturbances.

This paper studies the trajectory tracking control problem of underactuated surface vessels (USVs) under false data injection attacks (FDIAs). We propose a concise nonlinear finite-time disturbance observer (FTDO) to compensate composite disturbances caused by FDIAs and external disturbances. An online approximator is designed using adaptive techniques to compensate for the dynamic uncertainties of the system. In addition, we consider the limit of input saturation at the control input. Under the framework of the backstepping method, this paper designs a practical finite-time control (FTC) method and performs a rigorous theoretical analysis of it through the Lyapunov stability theory. Finally, the effectiveness of the proposed control scheme was verified through simulation. This research provides a feasible solution to the trajectory tracking problem of underactuated vessels in FDIAs environment and provides theoretical support for further research in related fields.

The target can deceive the flight vehicle by releasing an infrared decoy to make the line-of-sight (LOS) angle rate deflect greatly, thus causing the flight vehicle to miss the target. Therefore, in order to accurately strike the target in complex adversarial scenarios, this paper proposes a finite-time convergence guidance law (FTCG) combined with a finite-time disturbance observer (FTDO). The complex adversarial scenario is established by combining the relative motion model between the flight vehicle and the target and the motion model of the infrared decoy. Based on this, considering the dynamic characteristic of the flight vehicle's autopilot, a guidance model is obtained. Utilizing sliding mode control theory and finite-time control theory, an FTCG of the LOS angle rate is designed. Then, the finite-time convergence of the guidance law is proved and the total convergence time is derived. Finally, for the target maneuvering that is difficult to measure in the guidance law, an FTDO is used to estimate and compensate for the target maneuvering in the guidance law. Simulation results show that the FTCG can make the LOS angle rate quickly converge and accurately strike the target in different scenarios, with a good guidance accuracy and robustness. Compared with the sliding mode guidance law (SMGL) and the adaptive sliding mode guidance law (ASMGL) based on an extended state observer (ESO), the advantages of the designed guidance law are illustrated. Finally, FTCG is extended to be three dimensional and compared with the proportional navigation guidance law (PNG) to further illustrate its effectiveness in a three-dimensional coordinate system. [ABSTRACT FROM AUTHOR]

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This article presents a disturbance‐observer‐based adaptive finite‐time dynamic surface control scheme, capable of guaranteeing transient behavior for the PMSM with arbitrary asymmetric time‐varying output constraint and unmatched external disturbance. The major challenge of this paper is devising efficient strategies to tackle the nonsymmetric output restraints with arbitrary characteristics and unmatched external perturbation for the system under the finite‐time backstepping framework. Given this, a nonlinear transformation function is adopted to coordinate from the output‐constrained dynamic model to an uncontained one, and a finite‐time disturbance observer is introduced to evaluate the unmatched external perturbation. Then, a dynamic surface control approach having adaptive properties for PMSM is conceived by combing a neural network to evaluate the nonlinear functions and a first‐order filter to handle the "explosion of complexity." Additionally, it is proved that the signals in the closed‐loop system can narrow down to a bounded region and the tracking error can merge in a limited time to tiny vicinity of zero by employing a fast finite‐time stability principle. Eventually, simulation cases and contrast results reveal the tracking performance and immunity to the disturbance of the devised controller. [ABSTRACT FROM AUTHOR]

This paper proposes a Finite-Time Disturbance Observer-based new Fast Terminal Sliding Mode Control (FTDO–FTSMC) for systems under disturbances and parametric uncertainties with input saturation. Our approach begins with the design of a finite-time observer to estimate disturbances, in which we show that the errors in disturbance estimation converge to zero in finite time. Then, a new fast terminal sliding surface is proposed for the fast and finite-time convergence of the tracking errors. The closed-loop system under the proposed control scheme has been proved to be asymptotically stable by using the Lyapunov theory. Finally, the proposed FTDO–FTSMC framework is applied to the attitude control of a three-DOF hover quadrotor under time-varying disturbances and parametric uncertainties. Simulation results show that the proposed control strategy achieves excellent performance in terms of trajectory tracking, disturbance rejection and robustness, even in the presence of external disturbances and model uncertainties, compared with the existing control approaches. [ABSTRACT FROM AUTHOR]

To address the search-and-docking problem in multi-stage prescribed performance switching (MPPS) scenarios, this paper presents a novel compound control method for three-dimensional (3D) underwater trajectory tracking control of unmanned underwater vehicles (UUVs) subjected to unknown disturbances. The proposed control framework can be divided into two parts: kinematics control and dynamics control. In the kinematics control loop, a novel parallel model predictive control (PMPC) law is proposed, which is composed of a soft-constrained model predictive controller (SMPC) and hard-constrained model predictive controller (HMPC), and utilizes a weight allocator to enable switching between soft and hard constraints based on task goals, thus achieving global optimal control in MPPS scenarios. In the dynamics control loop, a finite-time terminal sliding mode control (FTTSMC) method combining a finite-time radial basis function neural network adaptive disturbance observer (RBFNN-FTTSMC) is proposed to achieve disturbance estimation and fast convergence of velocity tracking errors. The simulation results demonstrate that the proposed PMPC-FTTSMC approach achieved an average improvement of 33% and 80% in the number of iterations compared with MPC with sliding mode control (MPC-SMC) and traditional MPC methods, respectively. Furthermore, the approach improved the speed of response by 35% and 44%, respectively, while accurately achieving disturbance observation and enhancing the system robustness. [ABSTRACT FROM AUTHOR]

Attitude stability plays an important role in the safe flight of quadrotor unmanned aerial vehicle (UAV). This paper aims to address the problem of attitude control of quadrotor UAV. The mathematical model of quadrotor system subject to external disturbance is first introduced. Then, a finite-time disturbance observer (FTDO) is developed to effectively compensate for external disturbance. It is proved that the disturbance observation error can be guaranteed to converge to zero in finite time. Based on the designed FTDO, a backstepping sliding mode control technique is proposed to stabilize three attitude angles of a quadrotor UAV, which can eliminate the tracking errors of attitude channel to zero asymptotically. Moreover, by constructing an auxiliary equation, the bound of transient attitude tracking error in terms of L 2 norm is derived. Finally, the comparative simulations are carried out to illustrate the effectiveness of the proposed control scheme and several statistical indexes are given to quantitatively evaluate the performance in terms of observation error, tracking error and control signal. [ABSTRACT FROM AUTHOR]

Summary: Different formation controllers are to deal with different missions for unmanned aerial vehicle (UAV) swarm. The mission‐driven control scheme of the UAV swarm is necessary and worth exploring, leading to the main work of this article. Aiming at different requirements of UAV swarm, we proposed a mission‐driven control scheme, including a consensus‐based near‐optimal formation controller and a finite‐time precise formation controller. This control scheme can enable the swarm to reach its target position while maintaining consensus formation on the way. When the UAV swarm flies to the mission area, a consensus‐based near‐optimal formation controller based on adaptive neural networks is investigated to regulate the UAV states online. The controller drives the swarm from an arbitrary initial position and velocity to the desired target in an optimal manner while maintaining an expected formation with finite‐time consensus at the same time. When the UAV swarm approaches the mission area, a consensus‐based precise formation controller based on a fixed‐time disturbance observer (FDO) is designed to achieve an accurate and consensus formation within a limited time. The mission‐driven control scheme can better adapt to tasks and promote control optimization, formation precision, and system robustness. Finally, the efficiency of the proposed methodology is illustrated by numerical simulation. [ABSTRACT FROM AUTHOR]

This paper introduces a nonlinear dynamic inversion control algorithm designed to address unknown disturbances and actuator saturation issues in unmanned aerial vehicle (UAV) attitude control. The algorithm is based on a combination of finite-time disturbance observer and anti-saturation auxiliary system, which ensures the rapid convergence of attitude tracking error. Firstly, based on the Newton–Euler equations, this paper establishes a model of the attitude system for quadrotor UAVs, and this paper eliminates the small-angle flight assumption. Secondly, considering the actuator saturation problem, an anti-saturation auxiliary control system is designed to shorten the time when the control volume is in the saturation interval and achieve finite-time convergence of the attitude error. And then, to improve the robustness of the controller, this paper proposes a disturbance observer based on the finite-time stability theory, which achieves a continuous smooth output of the observation results by introducing a hyperbolic tangent function in the observer, so that the observation error can be converged to zero in a finite time. Finally, it is demonstrated by Simulink simulation that the attitude error and the observation error converge quickly to zero.

This paper deals with chaos control in an arch microelectromechanical system (MEMS) from the fractional calculus perspective. There is a growing need for effective controllers in various technological fields, and it is important to consider disruptions, uncertainties, and control input limitations when designing a practical controller. To address this problem, we propose a novel disturbance-observer-based terminal sliding mode control technique for stabilizing and controlling chaos in a fractional-order arch MEMS resonator. The design of this technique takes into account uncertainty, disturbances, and control input saturation in the fractional-order system. The proposed control technique is practical for real-world applications because it includes control input saturation. The equation for a fractional-order arch MEMS resonator is presented, and its nonlinear vibration and chaotic behavior are studied. The design process for the proposed control technique is then described. The Lyapunov stability theorem is used to prove the finite-time convergence of the proposed controller and disturbance observer. The proposed controller is applied to the arch MEMS resonator, and numerical simulations are used to demonstrate its effectiveness and robustness for uncertain nonlinear systems. The results of these simulations clearly show the effectiveness of the proposed control technique. [ABSTRACT FROM AUTHOR]

This paper proposed a novel finite-time disturbance observer-based recursive fractional-order sliding mode control (FTRFOSMC) algorithm under disturbances and input saturation for redundantly actuated cable driving parallel robots (RCDPRs). A recursive fractional-order sliding mode surface composed of the fractional-order non-singular fast terminal sliding mode function and an integral term is constructed, and the fast response convergence and high precision tracking performance can be obtained for the recursive characteristics of the proposed sliding mode surface; meanwhile, an auxiliary system is designed to overcome the adverse effects of the input saturation. Then, to compensate the model uncertainty and external disturbances, an adaptive finite-time disturbance observer is developed, and the estimation error can be stabilized in finite-time for unknown bound of the disturbance and its derivative. The stability of the proposed controller was investigated by the Lyapunov stability theory. Finally, numerical simulations with the software of the MATLAB/Simuink are conducted to verify the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

The target can deceive the flight vehicle by releasing an infrared decoy to make the line-of-sight (LOS) angle rate deflect greatly, thus causing the flight vehicle to miss the target. Therefore, in order to accurately strike the target in complex adversarial scenarios, this paper proposes a finite-time convergence guidance law (FTCG) combined with a finite-time disturbance observer (FTDO). The complex adversarial scenario is established by combining the relative motion model between the flight vehicle and the target and the motion model of the infrared decoy. Based on this, considering the dynamic characteristic of the flight vehicle’s autopilot, a guidance model is obtained. Utilizing sliding mode control theory and finite-time control theory, an FTCG of the LOS angle rate is designed. Then, the finite-time convergence of the guidance law is proved and the total convergence time is derived. Finally, for the target maneuvering that is difficult to measure in the guidance law, an FTDO is used to estimate and compensate for the target maneuvering in the guidance law. Simulation results show that the FTCG can make the LOS angle rate quickly converge and accurately strike the target in different scenarios, with a good guidance accuracy and robustness. Compared with the sliding mode guidance law (SMGL) and the adaptive sliding mode guidance law (ASMGL) based on an extended state observer (ESO), the advantages of the designed guidance law are illustrated. Finally, FTCG is extended to be three dimensional and compared with the proportional navigation guidance law (PNG) to further illustrate its effectiveness in a three-dimensional coordinate system.

ObjectivesThis paper presents a non-singular terminal sliding mode track control method based on a finite-time disturbance observer to solve the problem of accurately tracking and controlling an the 3D trajectory of an unmanned underwater vehicle under complex external disturbances.MethodsA non-singular terminal sliding mode track controller is designed to ensure that the tracking error converges to zero accurately within a limited time. A finite-time disturbance observer is designed to improve the anti-jamming ability of the system under external multidimensional time-varying disturbances.ResultsThe Lyapunov function is used to prove that the designed control strategy can remain stable for a limited time. MATLAB is used for the simulation experiment, and a comparison with the backstepping sliding mode control method under step disturbance shows that the method presented in this paper achieves accurate trajectory tracking.ConclusionsThe results of this paper can provide a solution for accurately tracking the 3D trajectories of unmanned underwater vehicles.

The quadrotor system's nonlinear behaviour, uncertain modelling parameters and the presence of exogenous disturbances in the surrounding environment make its flight control a critical and challenging job. Several linear and nonlinear control methodologies have been proposed so far for this purpose during the past few decades which still need improvements. In this paper, variable exponential sliding mode control (ESMC) is proposed for the attitude stabilization and altitude tracking of a quadrotor having parametric uncertainties and being exposed to exogenous disturbances. The multi-layer perceptron (MLP) neural network is incorporated with the ESMC to accommodate adaptively the effect of parametric uncertainties. The conventional weight update law of the neural network is replaced by the sliding mode effect, thereby enhancing the learning efficiency of the network without computational complexities. A finite-time disturbance observer (FTDO) is integrated with the control law to make the controller robust to exogenous disturbances and minimize the chattering issue as well. The stability of the proposed control scheme is checked and verified by the Lyapunov theory. Numerous simulations of the proposed control algorithm are carried out on a quadrotor model suffering from exogenous disturbances and parametric uncertainties in MATLAB SIMULINK and the results are compared with the SMC. The remarkable performance of the proposed control strategy demonstrates and substantiates its validity. [ABSTRACT FROM AUTHOR]

In this paper, a composite passivity-based control method based on a finite-time disturbance observer (FTDO) and a passivity-based control (PBC) is proposed to improve the stability of the Boost converters with constant power loads in DC Microgrids. The FTDO improves the robustness and rapidity of the system by accurately estimating system disturbances. The PBC ensures the stability of the system via its transient energy dissipation. The FTDO operates parallel to the PBC, and compensates the observed value to the PBC through a feedforward channel. When compared with other controls, the proposed composite passivity-based control has the advantages of a fast dynamic response and accurate tracking of system disturbances in a wide working range. Finally, the control method proposed in this paper is verified by MATLAB/Simulink simulations and hardware in the hard-ware-in-loop experiments. [ABSTRACT FROM AUTHOR]

In recent years, the agricultural applications of unmanned vehicles have garnered significant attention thanks to the rapid development of global positioning systems, inertial navigation technology, and control theory. In this study, a novel sliding mode controller for farm vehicles lateral path tracking control in the presence of unknown disturbances is created. Based on the standard kinematic model and the study of agricultural circumstances, the kinematic error model with unknown external disturbances and severe nonlinearity is initially constructed. To deal with the disturbances that exist in the lateral path tracking system, this work offers a finite-time disturbance observer-based composite terminal sliding mode control (FDOB-CTSMC). Meanwhile, the finite-time disturbance observer-based composite adaptive terminal sliding mode control (FDOB-CATSMC) is developed on the basis of the sliding mode filter and the adaptive control technology, which will significantly reduce the controller chattering issue. Using the Lyapunov theory, the finite-time convergence of the lateral deviation and the sliding variable can be verified. The numerical simulations demonstrate that the proposed controller is far better than the traditional path tracking controllers. [ABSTRACT FROM AUTHOR]

In this paper, a finite-time control method has been proposed for underactuated unmanned surface vehicles (USVs) with external disturbances in order to implement trajectory tracking control. Considering the complexity of the marine environment and the high-accuracy and rapidity required by USVs to complete complex marine missions, such as water quality detection, underwater pipe-laying, rescue operations and so on, a novel hierarchical sliding mode base on non-singular terminal sliding mode control (NTSMC) method is designed for underactuated USVs to ensure that all tracking error can faster converge to a neighbourhood around zero within finite time and address effectively the singularity problem which always exist in terminal sliding mode control (TSMC). The underactuated problem is addressed by hierarchical sliding mode technique. Meanwhile, the dynamic surface control (DSC) is employed to address the explosion problem of computational complexity in traditional method. Further, a novel finite-time disturbance observer (FDO) is devised to estimate accurately the unknown environmental disturbances and for practicality, a saturation constraint function is used to limit the input of the controller. Finally, the effectiveness and stability of the proposed method are validated by simulations and comparisons. [ABSTRACT FROM AUTHOR]

Tristable energy harvesters (TEHs) have been proposed to achieve broad frequency bandwidth and superior low-frequency energy harvesting performance. However, due to the coexistence of three potential wells and the sensitivity to system conditions and external disturbances, the desired high-amplitude inter-well oscillation in the TEHs may be replaced by the chaotic or intra-well oscillations with inferior energy output. Specifically, the chaos has an unpredictable trajectory and may cause system damages, lessen the structural durability as well as require a more complicated circuit for power management. Therefore, in this paper, we firstly propose an adaptive finite-time disturbance observer (AFTDO) for performance enhancement of TEHs by detecting the external disturbances that induce the chaos, and reject them for the recovery of the desired inter-well motion. The proposed AFTDO eliminates the need to know in advance the upper bounds of imposed perturbations in conventional observers by means of the proposed adaptive protocols, leading to the higher efficacy of estimation. The mathematical model of the piezoelectric TEH system and the AFTDO is provided. To demonstrate the effectiveness of the AFTDO, a series of numerical simulations have been performed. Results show that for both cases with sinusoidal and impulsive disturbances, the AFTDO can successfully track the trajectories of the disturbance signals with the adaptive gain, and reject the disturbance to enable the TEH to sustain the periodic inter-well oscillation with effective energy harvesting performance. [ABSTRACT FROM AUTHOR]

In practical applications, for highly nonlinear systems, how to implement control tasks for dynamic systems with uncertain parameters is still a hot research issue. Aiming at the internal parameter fluctuations and external unknown disturbances in nonlinear system, this paper proposes an adaptive dynamic terminal sliding mode control (ADTSMC) based on a finite-time disturbance observer (FTDO) for nonlinear systems. A finite-time disturbance observer is designed to compensate for the unknown uncertainties and a dynamic terminal sliding mode control (DTSMC) method is developed to achieve finite time convergence and weaken system chattering. Moreover, a dual hidden layer recurrent neural network (DHLRNN) estimator is proposed to approximate the sliding mode gain, so that the switching item gain is not overestimated and optimal value is obtained. Finally, simulation experiments of an active power filter model verify the designed ADTSMC method has better steady-state and dynamic-steady compensation effects with at least 1% THD reduction in the presence of nonlinear load and disturbances compared with the simple adaptive DTSMC law. [ABSTRACT FROM AUTHOR]

To address the search-and-docking problem in multi-stage prescribed performance switching (MPPS) scenarios, this paper presents a novel compound control method for three-dimensional (3D) underwater trajectory tracking control of unmanned underwater vehicles (UUVs) subjected to unknown disturbances. The proposed control framework can be divided into two parts: kinematics control and dynamics control. In the kinematics control loop, a novel parallel model predictive control (PMPC) law is proposed, which is composed of a soft-constrained model predictive controller (SMPC) and hard-constrained model predictive controller (HMPC), and utilizes a weight allocator to enable switching between soft and hard constraints based on task goals, thus achieving global optimal control in MPPS scenarios. In the dynamics control loop, a finite-time terminal sliding mode control (FTTSMC) method combining a finite-time radial basis function neural network adaptive disturbance observer (RBFNN-FTTSMC) is proposed to achieve disturbance estimation and fast convergence of velocity tracking errors. The simulation results demonstrate that the proposed PMPC-FTTSMC approach achieved an average improvement of 33% and 80% in the number of iterations compared with MPC with sliding mode control (MPC-SMC) and traditional MPC methods, respectively. Furthermore, the approach improved the speed of response by 35% and 44%, respectively, while accurately achieving disturbance observation and enhancing the system robustness.

This article investigates the distributed attitude consensus tracking control for spacecraft formation flying with unknown external disturbances and model uncertainties. First, a terminal sliding mode disturbance observer (TSMDO) is constructed to estimate the generalized disturbances including external disturbances and model uncertainties. The finite-time convergence of the estimation errors using TSMDO is analyzed. Second, a variable structure control law is developed to avoid introducing initial errors of the TSMDO. Third, a novel adaptive nonsingular fast terminal sliding mode (ANFTSM) control law based on TSMDO is proposed to ensure the convergence of attitude tracking errors to zero. Based on theoretical analysis, the finite-time stability can be guaranteed by Lyapunov theory. Finally, the effectiveness of the developed control law is verified via numerical simulations. [ABSTRACT FROM AUTHOR]

This article investigates the fast finite‐time exact tracking problem for nonlinear systems with time‐varying disturbances. A new composite controller with a finite‐time disturbance observer is proposed to achieve fast finite‐time exact tracking. Furthermore, a new time‐varying barrier power integrator technique is developed to design the controller. This can ensure not only asymmetric/symmetric time‐varying full‐state constraints but also singular‐free continuous control effort. To reduce the complexity of computation, fuzzy logic systems are adopted to approximate complex nonlinearities. Finally, an example of single‐link robot systems subject to time‐varying disturbances is presented to show the validity of the proposed method. [ABSTRACT FROM AUTHOR]

A learning-based online optimal sliding-mode control strategy is developed for space circumnavigation missions subject to input constraints, and the mismatched uncertainties caused by measurement uncertainties are also considered in this infinite-horizon optimal control problem. The logarithmic hyperbolic cosine function is used to design the optimal value function to overcome the weakness that the derivative of the adaptive weight of neural network (NN) changes too fast when the value of the sliding-mode function is too large, and another suitable nonquadratic function is used to incorporate input constraints into the optimal control framework. To approximate the Hamiton-Jacobi-Bellman equation corresponding to the novel optimal value function, an actor-critic (AC) architecture is introduced with NNs, and a finite-time disturbance observer (FTDO) is employed to estimate the mismatched uncertainties in the plant. The simulation results verify the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

This work considers a finite-time command filtered course tracking problem of ships with unknown bounded disturbances, unmodelled dynamics and input saturation. By melting command filter technique into backstepping design, a finite-time control scheme is proposed, which overcomes the 'explosion of complexity' and 'singularity' problems resulted from the calculation of repeated derivatives of virtual control inputs in the conventional backstepping method. A hyperbolic tangent function is adopted to estimate saturation with a bounded estimation error in order to guarantee that the saturation function acting on the rudder angle can be differentiable, where the rudder angle is considered as a new state variable whose initial value is zero. The finite-time disturbance estimator is constructed to provide the estimation of compounded uncertain item derived from unmodelled dynamics and external disturbances. Simulation results and comprehensive comparisons are used to demonstrate the effectiveness and superiority of the proposed control scheme. [ABSTRACT FROM AUTHOR]

In this paper, a novel fixed-time controller (FTC) strategy based on leader–follower mechanism and finite-time disturbance observer (FDO) is proposed for surface vehicles (SVs) formation suffering from complex unknowns. The excellent features of designed strategy are shown below: (1) A fixed-time tracking control (FTTC) approach combining with integral sliding mode (ISM) technology is devised for a nominal leader SV such that fixed-time stability can be ensured; (2) To achieve formation efficiently, a fixed-time formation controller (FTFC) strategy incorporating with backstepping technology is proposed for coordinating follower SVs; (3) Considering complex disturbances in the entire formation system, finite-time disturbance observers (FDOs) are injected into the FTFC framework which in turn contributes to accurate formation control with fixed-time convergence. Finally, simulation results demonstrate remarkable performance of the proposed FDO-FTFC scheme. [ABSTRACT FROM AUTHOR]

This study aims to build a novel tracking control algorithm using a finite-time disturbance observer which obtains fast convergence within a predetermined amount of time and strong stability for a class of second-order nonlinear systems. Firstly, a nonlinear sliding mode manifold with fast finite-time convergence is introduced. Then, according to the designed manifold for the guarantee of finite-time convergence and robustness stabilization, a nonlinear control algorithm based on theory of finite-time control is developed. Specifically, the information of the lumped uncertainty was achieved by a new finite-time Disturbance Observer (DO). Thanks to the synthetic advantages of the above techniques, the designed controller marked with powerful features including a practical design, fast convergence rate, high precision, a convergence of the control errors in finite-time, along with impressive small chattering in the control actions. Furthermore, the control proposal also eliminates the necessity of the upper boundary of the uncertainties affecting the system and its finite settling time can be estimated in advance by designating the appropriate design parameters. The finite-time stability of the proposed DO, sliding surface, and control algorithm has been fully confirmed by Lyapunov principle. Trajectory tracking simulation for a 3-DOF manipulator and trajectory tracking experiment for a Maglev System (MLS) has been performed under different operating conditions using MATLAB/SIMULINK to testify the effectiveness and feasibility of the suggested strategy.

This paper proposes a continuous nonsingular terminal sliding mode (NTSM) control approach for nonlinear systems subject to mismatched terms in order to achieve finite time exact tracking and disturbance rejection. The controller is constructed using a composite method that utilizes a power integrator and combines the output regulation theory, disturbance observation technique, feedback domination, and sliding mode control technique. The performance analysis demonstrates that the proposed continuous NTSM controller can drive the system output to the desired reference signal in a finite time in the presence of mismatched time‐varying disturbances and nonsmoothed nonlinearities in each channel. The finite time Lyapunov theory is utilized to ensure finite‐time convergence of the closed‐loop system. The simulation results validate the effectiveness of the proposed continuous NTSM controller. [ABSTRACT FROM AUTHOR]

Space manipulator is considered as one of the most promising technologies for future space activities owing to its important role in various on-orbit serving missions. In this study, a robust finite-time tracking control method is proposed for the rapid and accurate trajectory tracking control of an attitude-controlled free-flying space manipulator in the presence of parametric uncertainties and external disturbances. First, a baseline finite-time tracking controller is designed to track the desired position of the space manipulator based on the homogeneous method. Then, a finite-time disturbance observer is designed to accurately estimate the lumped uncertainties. Finally, a robust finite-time tracking controller is developed by integrating the baseline finite-time tracking controller with the finite-time disturbance observer. Rigorous theoretical analysis for the global finite-time stability of the whole closed-loop system is provided. The proposed robust finite-time tracking controller has a relatively simple structure and can guarantee the position and velocity tracking errors converge to zero in finite time even subject to lumped uncertainties. To the best of the authors' knowledge, there are really limited existing controllers can achieve such excellent performance under the same conditions. Numerical simulations illustrate the effectiveness and superiority of the proposed control method. [ABSTRACT FROM AUTHOR]

Summary: In this article, the problem of angular velocity trajectory tracking for converter‐fed DC motor system with mismatched disturbances/uncertainties is investigated using a continuous nonsingular terminal sliding mode control. The fourth order dynamic model of the system is quite higher than the second order of the traditional one, which makes the control design more challenging in terms of availability of various signals and tuning of the overall control parameters. Also, the actuator dynamics of the converter not only increase the system order but also generate mismatched disturbances/uncertainties, which is difficult to counteract through baseline sliding mode controller. The disturbance estimation information obtained from the finite time disturbance observer is incorporated into the design of nonsingular terminal sliding manifold. Then, a nonsingular terminal sliding mode controller is designed to achieve finite time tracking performance despite matched and mismatched disturbances/uncertainties. In addition, it retains the fine properties of nominal performance recovery as well as chattering alleviation. Finally, the superiority of the proposed method is shown both through numerical simulation and experimental results. [ABSTRACT FROM AUTHOR]

This article investigates the robust tracking issue for the longitudinal dynamics of hypersonic vehicles subjected to mismatched uncertainties, and a novel sliding mode control approach is proposed to achieve the fixed-time convergence of tracking errors and satisfactory robustness against mismatched uncertainties. Establishing the control-oriented hypersonic vehicle model as velocity and altitude subsystems with mismatched uncertainties, the article introduces the nonlinear finite-time disturbance observer technique to estimate the uncertainties precisely. With the estimated uncertainties from the observer, the fixed-time sliding mode control is presented to track the velocity and altitude references. Consequently, the effect of the mismatched disturbances can be eliminated and the tracking performance can be improved. The stability of the closed-loop system is also analyzed. Numerical simulation results demonstrate the validity and superiority of the proposed control. [ABSTRACT FROM AUTHOR]

This paper investigates the finite-time attitude stabilization of spacecraft under multiple uncertainties. Besides inertia uncertainties, external disturbances, and actuator faults, the measurement uncertainties are particularly considered. When including the measurement uncertainties, the spacecraft attitude control system becomes a mismatching system, which brings a great challenge to the control design. To resolve this problem, a novel robust finite-time attitude control approach is proposed by using a dual-disturbance-observer-based structure. First, two finite-time disturbance observers are developed to estimate the mismatched and matched lumped disturbances in the spacecraft attitude kinematics and dynamics, respectively. Then, the robust finite-time attitude controller is synthesized by incorporating the two finite-time disturbance observers into the adding a power integrator technique. It is strictly proved that the proposed robust finite-time attitude controller can ensure the actual attitude and angular velocity stabilize to the small neighborhoods around the origin in finite time even subject to multiple uncertainties. Benefiting from the feedforward disturbance compensation, the proposed controller is not only robust against inertia uncertainties, external disturbances, and actuator faults, but also insensitive to measurement uncertainties. Lastly, the effectiveness and advantages of the proposed control approach are validated through simulations and comparisons. [ABSTRACT FROM AUTHOR]

This article proposes a finite‐time disturbance observer‐based modified super‐twisting algorithm (FDO‐STA) for disturbed high‐order integrator‐chain systems under matched and mismatched disturbances. We first design a finite‐time observer for disturbance estimation, in which we show the finite‐time convergence of disturbance estimation errors to zero. Second, by employing the estimates of disturbances and their derivatives, a new dynamic sliding surface is derived, which ensures the finite‐time convergence of the controlled output to zero in the sliding phase. Then, based on the estimates of disturbances and their derivatives, the designed sliding surface, and a modified super‐twisting algorithm, we develop the FDO‐STA, which guarantees the finite‐time convergence of the sliding variable to zero in the reaching phase. Rigorous analysis is provided to show the finite‐time stability of the overall closed‐loop system under the proposed control scheme. We finally apply the proposed FDO‐STA framework to the path following control for fixed‐wing UAVs under wind disturbances. Various simulation results are provided to show the effectiveness of the proposed controller, compared with the existing control approaches. [ABSTRACT FROM AUTHOR]

Summary: The increasing dependence on marine resources has encouraged the rapid development of dynamic positioning (DP) technology in ships and other marine vessels. This study proposes a novel DP scheme for ships subjected to comprehensive disturbances (unknown environmental disturbances and thruster faults). An integral nonsingular fast terminal sliding mode control (INFTSMC) scheme is initially designed without accounting for environmental disturbances. This scheme has a higher convergence rate and robustness against unknown environmental disturbances than the NFTSMC scheme. Furthermore, a new finite‐time disturbance observer is developed to adapt to the changes in the comprehensive disturbances and ensure that the observed errors converge within a small region around the origin in finite time. The INFTSMC scheme is then combined with the finite‐time observer to create a finite‐time observer‐based thruster fault‐tolerant control (FTOAFTC) scheme. Detailed simulation studies and quantitative analyses are carried out on the traditional sliding mode control (SMC), NFTSMC, and FTOAFTC schemes. The FTOAFTC scheme's transient and steady‐state performances, robustness against environmental disturbances, and fault‐tolerance ability are found to be superior to those of the other schemes. [ABSTRACT FROM AUTHOR]

In this paper, a novel disturbance-observer-based approximate dynamic inversion (ADI) approach is developed for pure-feedback nonaffine-in-control nonlinear systems (PFNNSs) in the presence of both high-order mismatched disturbances and actuator saturation. Finite-time disturbance observers (FTDOs) are utilized to estimate the disturbances and their derivatives. Then, we rebuild the system with the outputs of FTDOs. Thereafter, ADI is employed to derive the desired virtual and actual control of the nominal system, where no disturbances and saturation are presented. Furthermore, an augmented intermediate subsystem is constructed for the reduced slow subsystem to compensate for the difference between inputs with and without saturation by approximating its inversion. The stability of the closed-loop system is studied using Tikhonov's theorem. The proposed method is applied to a numerical example and a one-link robotic system with a brush DC motor. The simulation results demonstrate the validity of the presented approach.

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In this study, the distributed finite‐time optimisation problem is investigated for second‐order multi‐agent systems subject to mismatched disturbances. The whole control design is conducted within a feedforward–feedback composite control framework, which consists of two main sessions. In the first session, based on homogeneous systems theory, a finite‐time optimisation control algorithm is presented for a nominal second‐order single system with a quadratic cost function. In the second session, via the result in the first session and by combining finite‐time disturbance observer, homogeneous systems theory, integral sliding‐mode control, and distributed finite‐time estimator techniques together, a distributed finite‐time optimisation control algorithm is proposed for second‐order multi‐agent systems with mismatched disturbances and quadratic local cost functions. Under the proposed control algorithm, the outputs of all the agents reach finite‐time consensus on the minimiser of the global cost function. Simulations demonstrate the effectiveness and superiority of the proposed distributed finite‐time optimisation control algorithm. [ABSTRACT FROM AUTHOR]

This work addresses the issue of trajectory tracking of helicopters under disturbances and input constraints via the proposed fast finite-time backstepping framework. Backstepping with the property of fast finite-time convergence exhibits fast transient convergence both at a distance from and at a close range of the equilibrium. Moreover, finite-time disturbance observers based on multivariable super-twisting are employed to counteract the effect of perturbations. Aiming at the adverse effect of input saturation, a novel auxiliary system is developed to avoid the singularity by transforming auxiliary variables for three times. In addition, the framework is also applied into helicopter control problem, in which thrust magnitude and thrust direction are used as intermediate control variables, connecting external translational dynamics and internal angular dynamics. A rigorous proof of finite-time stability of the closed-loop system is derived from Lyapunov theory. Finally, the effectiveness and superiority of the proposed framework are verified by comparative simulation. [ABSTRACT FROM AUTHOR]

The combination of DC-DC buck power converters with DC motors for generating the so-called smooth start of drives has many advantages in engineering practice. Achieving high performance of such systems is, however, limited by the influence of disturbances/uncertainties of multiple sources. Some of the disturbances are mismatched, which makes them even more difficult to handle. Furthermore, the relatively high order of system dynamics makes the control design challenging. In this paper, a control structure with continuous dynamic sliding mode controller with a finite-time disturbance observer is proposed to address these practical issues. First, a special state transformation is applied, aggregating the acting disturbances/uncertainties in a sole perturbing term of the system expressed in new coordinates. Then, the observer estimates in real time the information about the lumped disturbances based on already available input/output signals and the obtained estimated signals (and their high order time-derivatives) are used to construct a sliding surface. Finally, the sliding mode controller is applied to achieve high performance of the resultant plant dynamics and to robustify the governing scheme against modelling discrepancies. The stability of the closed-loop system is proved here using Lyapunov stability theory and the efficiency of the proposed control method is validated through a multi-criteria numerical simulation. [ABSTRACT FROM AUTHOR]

The three-degree-of-freedom (3D) pendulum has been used as a benchmark in the field of nonlinear dynamics and control. Nonetheless, the attitude control of 3D pendulum is still an open problem presently since some issues remain not well addressed. In this paper, a robust adaptive finite-time attitude control method is proposed for the attitude tracking control of a 3D pendulum with external disturbance. First, a baseline finite-time attitude controller is designed based on the adding a power integrator technique. Then, a finite-time disturbance observer is designed to exactly estimate the unknown external disturbance. Finally, a robust adaptive finite-time attitude controller is constructed by integrating the baseline finite-time attitude controller with the finite-time disturbance observer. The proposed robust adaptive finite-time attitude controller can guarantee the global finite-time stability of the whole closed-loop system even in the presence of external disturbance owing to the feedforward dynamic compensation. Numerical simulations and hardware experiments are both performed to illustrate the effectiveness and superiority of the proposed control method. [ABSTRACT FROM AUTHOR]

This paper investigates the path following control problem for underactuated unmanned surface vehicle (USV) in the presence of unmodeled dynamics, external disturbances and input saturation. A novel adaptive robust path following control scheme is proposed by employing trajectory linearization control (TLC) technology and finite-time disturbance observer, which is composed of a concise yaw rate controller and a surge speed controller. The salient features of the proposed scheme include: a path following guidance law is designed to ensure USV effectively converging to and following the desired path; TLC is introduced into the field of USV motion control as new effective technique, and it is the first time used to design path following controller for underactuated USV; a finite-time nonlinear tracking differentiator is constructed not only to avoid the signal jump caused by derivation, but also to filter noise and high frequency interference. A finite-time disturbance observer (FDO) is devised to exactly observe the uncertain dynamics and unknown external disturbances, which improves the tracking accuracy and precise disturbance rejection of the proposed controller; then, an auxiliary dynamic system that is governed by smooth switching function is developed to compensate for the saturation constraint on actuator. Stability analysis verifies that all signals in the closed-loop system are uniformly ultimately bounded. Finally, simulation results and comparisons illustrate the superiority of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Summary: This article proposes fault‐tolerant finite‐time attitude tracking control of a rigid spacecraft actuated by four reaction wheels without unwinding problem in the presence of external disturbances, uncertain inertia parameter, and actuator faults. First, a novel antiunwinding finite‐time attitude tracking control law is derived with a designed control signal which works within a known actuator‐magnitude constraint using a continuous nonsingular fast terminal sliding mode (NFTSM) concept. Second, a finite‐time disturbance observer (FTDO) is introduced to estimate a lumped disturbance due to external disturbances, uncertain inertia parameter, actuator faults, and input saturation. Third, a composite controller is developed which consists of a feedback control based on the continuous NFTSM method and compensation term based on the FTDO. The global finite‐time stability is proved using Lyapunov stability theory. Moreover, the singularity and unwinding phenomenon are avoided. Simulation results are conducted under actuator constraints in the presence of external disturbances, inertia uncertainty, and actuator faults and results are illustrated to show the effectiveness of the proposed method. In addition, to show the superiority of the proposed control method over the recently reported control methods, comparative analysis is also presented. [ABSTRACT FROM AUTHOR]

This paper aims to develop a new finite‐time convergent guidance law for intercepting maneuvering targets accounting for second‐order autopilot lag. The guidance law is applied to guarantee that the line of sight (LOS) angular rate converges to zero in finite time and results in a direct interception. The effect of autopilot dynamics can be compensated based on the finite‐time backstepping control method. The time derivative of the virtual input is avoided, taking advantage of integral‐type Lyapunov functions. A finite‐time disturbance observer (FTDOB) is used to estimate the lumped uncertainties and high‐order derivatives to improve the robustness and accuracy of the guidance system. Finite‐time stability for the closed‐loop guidance system is analyzed using the Lyapunov function. Simulation results and comparisons are presented to illustrate the effectiveness of the guidance strategy. [ABSTRACT FROM AUTHOR]

In this paper, for the position control problem of permanent magnet linear motors, a fast nonsingular terminal sliding mode control (FNTSMC) method based on the finite-time disturbance observer (FTDO) is proposed. By employing a fast nonsingular terminal sliding surface, the FNTSMC is designed. Besides, a FTDO is applied to estimate the disturbance and the estimation is served as compensation for the controller. A rigorous analysis based on the Lyapunov stability theory is provided to prove that the proposed control method can achieve faster dynamic response characteristic and higher steady accuracy than the linear sliding mode control method and the PID control method. Numerical simulation results are explored to illustrate the superiority of the proposed approach. [ABSTRACT FROM AUTHOR]