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One‐dimensional thrust vectoring turbojet vertical takeoff and landing (VTOL) aircraft have attracted research attention due to their high thrust‐to‐weight ratios and their solution for the slow speed response of turbojet engines. However, there are some uncertainties and physical limitations in their systems, such as external disturbances, unknown parameters and input saturation. To improve the accuracy and convergence speed of trajectory tracking, a fixed‐time control method that is robust to saturation is proposed. The system dynamics are established, and an auxiliary system is built within a fixed time control framework to address the influence of input saturation and increase the error convergence rate. Nonsingular fast terminal sliding mode technology is combined with a few adaptive laws to ensure the robustness of the closed‐loop system against dynamic uncertainties and improve the precision of steady‐state control. The stability of the control system is proven based on Lyapunov, and the controller parameters are optimized based on particle swarm optimization (PSO). The proposed method based on fixed‐time stability guarantees that the states of the closed‐loop system can reach the residual set around zero within the designed time, and an expression for the upper bound on the convergence time is given. Finally, numerical simulations demonstrate the superiority of the proposed method based on the error integral criterion. [ABSTRACT FROM AUTHOR]

Addressing the sensitivity of permanent magnet synchronous motors to external disturbances, a novel sliding mode control (NSMC) strategy is proposed to suppress sliding mode jitter and enhance speed regulation performance. First, an advanced nonsingular fast terminal sliding mode (ANFTSM) surface and a new adaptive power rate reaching law (NAPRRL) were developed. A new switching function replaces the conventional sign function to enhance the system's disturbance immunity and dynamic response speed. Then, the system's anti‐interference performance was further enhanced by introducing an improved novel sliding mode observer (INSMO) for feedback compensation of the aggregate disturbance. Finally, MATLAB/Simulink simulations and experimental validations demonstrate that the NSMC control strategy exhibits superior performance in both the start‐up response and load disturbance phases, with enhanced dither resistance, rapid dynamic response, and disturbance suppression capabilities. [ABSTRACT FROM AUTHOR]

In this study, to reduce the impact that permanent magnet linear synchronous motor (PMLSM) servo system on the sensitivity of external and internal interferences, an adaptive super-twisting-based nonsingular fast terminal sliding mode control (AST-NFTSMC) scheme is put forward. First of all, the mathematical model of PMLSM contains uncertainties is set up, and the control law is established to make position signal converge to given state asymptotically. Second, by introducing the super-twisting algorithm, the chattering caused by nonlinear switching function in traditional method is weakened. However, in actual industrial situations, it is difficult to determine the boundary of lumped disturbance, which makes the switching gains in super-twisting algorithm cannot be accurately selected. Therefore, an adaptive law is introduced to adjust the switching gains online, so that the gains can be dynamically adjusted within a limited range during operation, so as to further weaken chattering. Finally, the practicability of AST-NFTSMC is proved in theory and verified by system experiments. The experimental results prove that AST-NFTSMC can significantly weaken chattering, and result in robust control performance. [ABSTRACT FROM AUTHOR]

This study presents a combination of Type-2 fuzzy logic and nonsingular fast sliding mode technique to design a robust controller for a robotic system. The control law is composed of two signals. The first one called equivalent control law is dedicated to maintaining the system on the sliding surface and then converges to zero. Since the system is uncertain, a Type-2 fuzzy nominal model was constructed, deduced from linear local models, which allows a good approximation of the real robotic system. The second signal, whose objective is to force the system to attain the sliding surface, is deduced from stability analysis using Lyapunov theory. Several simulations were conducted to evaluate the efficiency of the proposed approach, showing good tracking performance for different reference signals despite the presence of uncertainties and external disturbances. [ABSTRACT FROM AUTHOR]

This paper focuses on the trajectory tracking of quadrotors under bounded external disturbances. An optimised robust controller is proposed to drive the position and attitude of a quadrotor converge to their references quickly. At first, nonsingular fast terminal sliding mode control is developed, which can guarantee not only the stability but also finite-time convergence of the closed-loop system. As the parameters of the designed controllers play a vital role for control performance, an improved beetle antennae search algorithm is proposed to optimise them. By employing the historical information of the beetle's antennae and dynamically updating the step size as well as the range of its searching, the optimising is accelerated considerably to ensure the efficiency of the quadrotor control. The superiority of the proposed control scheme is demonstrated by simulation experiments, from which one can see that both the error and the overshooting of the trajectory tracking are reduced effectively. [ABSTRACT FROM AUTHOR]

In this paper, a non-negative adaptive mechanism based on an adaptive nonsingular fast terminal sliding mode control strategy is proposed to have finite time and high-speed trajectory tracking for parallel manipulators with the existence of unknown bounded complex uncertainties and external disturbances. The proposed approach is a hybrid scheme of the online non-negative adaptive mechanism, tracking differentiator, and nonsingular fast terminal sliding mode control (NFTSMC). Based on the online non-negative adaptive mechanism, the proposed control can remove the assumption that the uncertainties and disturbances must be bounded for the NFTSMC controllers. The proposed controller has several advantages such as simple structure, easy implementation, rapid response, chattering-free, high precision, robustness, singularity avoidance, and finite-time convergence. Since all control parameters are online updated via tracking differentiator and non-negative adaptive law, the tracking control performance at high-speed motions can be better in real-time requirement and disturbance rejection ability. Finally, simulation results validate the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

For accurate trajectory tracking of robotic manipulators with actuators, a novel nonsingular fast terminal sliding mode control (NFTSMC) strategy based on radial basis function neural network (RBFNN) is put forward and investigated in this paper. Because of the existence of nonsingular fast terminal sliding mode (NFTSM) manifold, the controller possesses high precision and fast convergence. Considering that it is difficult to obtain accurate model parameters owing to modeling errors or external disturbances, RBFNN is used to approximate the nonlinear uncertainties due to its simple structure and great generalization ability. A new adaptive law is designed to adjust RBFNN. In order to compensate the estimation errors and suppress other unstable factors, a robust term is introduced. A new adaptive law is developed to flexibly adjust the robust term. Then, Lyapunov theory is applied to prove the system stability and finite-time convergence. Finally, a small-sized industrial robotic manipulator Epson LS3-401S with its first two joints is taken as the simulation plant, and several simulations between the proposed controller and the other two controllers are performed. External disturbances and other two conditions are considered to simulate the real environment, and the corresponding results verify the effectiveness and superiority of the proposed controller. [ABSTRACT FROM AUTHOR]

In this paper, a novel fault-tolerant control tactic for robot manipulator systems using only position measurements is proposed. The proposed algorithm is constructed based on a combination of a nonsingular fast terminal sliding mode control (NFTSMC) and a novel high-speed third-order sliding mode observer (TOSMO). In the first step, the high-speed TOSMO is proposed for the first time to approximate both the system velocity and the lumped unknown input with a faster convergence time compared to the TOSMO. The faster convergence speed is obtained thanks to the linear characteristic of the added elements. In the second step, the NFTSMC is designed based on a nonsingular fast terminal sliding (NFTS) surface and the information obtained from the proposed high-speed TOSMO. Thanks to the combination, the proposed controller–observer tactic provides excellent features, such as a fast convergence time, high tracking precision, chattering phenomenon reduction, robustness against the effects of the lumped unknown input and velocity requirement elimination. Especially, the proposed observer does not only improve the convergence speed of the estimated signals, but also increases the system dynamic response. The system's finite-time stability is proved using the Lyapunov theory. Finally, to validate the efficiency of the proposed strategy, simulations on a PUMA560 robot manipulator are performed. [ABSTRACT FROM AUTHOR]

In order to solve the problems of high cost, low reliability, unstable performance, and difficult maintenance in the complex working conditions brought about by traditional mechanical sensors to the permanent magnet synchronous motor (PMSM) control system, a continuous hyperbolic tangent function sigmoid is first proposed as the variable-structure model of the switching function. It refers to the method of the model reference adaptive system (MRAS) observer to estimate the motor speed and rotor position. Secondly, in view of the inherent chattering and poor robustness of the traditional sliding mode control (SMC), a speed-loop control strategy of nonsingular fast terminal sliding mode control (NFTSMC) is proposed. On this basis, a new type of reaching law with an improved terminal attractor that can eliminate negative exponential terms is designed. This control technology can quickly converge under conditions of sudden load changes and motor parameter changes, and improve the dynamic response of the system. Finally, simulation and experimental verification are carried out based on PSIM software and the PTS-3000 experimental platform. The results prove that MRAS positionless control can accurately estimate the rotor speed and position, and the NFTSMC based on the improved terminal attractor has superior control performance. [ABSTRACT FROM AUTHOR]

In the presence of complex unknowns consisting of system dynamics and environmental disturbances, it is rather meaningful to exactly track an unmanned surface vehicle (USV) to the desired trajectory in practical scenarios including routing inspection, marine survey, and guard patrol, etc. In this paper, the exact trajectory tracking problem is solved by establishing a finite‐time extended state observer‐ (FESO) based exact tracking control (FESO‐ETC) scheme. By virtue of nonsmooth analysis, the FESO is firstly devised by only requiring continuous differentiability and is incorporated into the nonsingular fast terminal sliding mode control framework, and thereby further enhancing disturbance rejection and tracking accuracy. Moreover, global finite‐time stability of the entire FESO‐ETC closed‐loop system is derived from rigorously theoretical analysis, and thereby contributing to a model‐free finite‐time control paradigm. Simulation studies and comparisons demonstrate that the proposed FESO‐ETC approach can achieve exact trajectory tracking in the presence of complex unknowns. [ABSTRACT FROM AUTHOR]

This paper proposes a novel robust model-free adaptive interval type-2 fuzzy sliding mode control (MF-AIT2FSMC) for the proton exchange membrane fuel cell (PEMFC) system using nonlinear disturbance observer. The main control objective is to adjust the oxygen stoichiometry at its reference trajectory under load disturbances and parameter uncertainties, to avert the oxygen starvation problem and to obtain maximum net power. The referred MF-AIT2FSMC control strategy is composed of three parts: first, a nonlinear disturbance observer is proposed to estimate both oxygen and nitrogen partial pressures which are considered as unmeasurable variables. Second, a nonlinear disturbance observer-based intelligent proportional-integral (NDBO-iPI) control is derived, whereas the NDBO is used to estimate the unmodeled system dynamics via the knowledge of input and output signals. Third, adaptive interval type-2 fuzzy nonsingular fast terminal sliding mode control is inserted to NDBO-iPI control to compensate the NDBO estimation error, enhance the control performance and ensure the global controlled system stability. Furthermore, the entire controlled system stability is verified via the Lyapunov approach. Finally, the corresponding numerical simulation on the PEMFC system is obtained to demonstrate the effectiveness and efficiency of the proposed MF-AIT2FSMC technique by comparing with the NDBO-iPI and standard PID controller. [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]

Taking into account almost all kinds of variations and uncertainties to which AC island photovoltaic (PV) microgrid is often subjected, this paper proposes a new nonsingular fast terminal sliding mode control (NFTSMC) strategy for two-stage converters to enhance robustness against those disturbances and improve system dynamic performance. Firstly , to effectively decrease the adverse effect on the system from intermittent solar irradiation and uncertain environmental temperature, a NFTSMC cascaded with perturb and observe (P&O) scheme is put forward for fore-stage DC/DC converter, and PV output voltage is controlled to track its reference and thus maximum power point tracking (MPPT) performance is improved. Then , to significantly decrease the adverse effect on the system from variations of load and LC filter parameters, by incorporating an extended state observer (ESO) into NFTSMC, an ESO-based NFTSMC scheme is proposed for post-stage DC/AC converter, and the load voltage is regulated to track its reference and thus system robustness is strengthened, in which ESO is designed to replace load current sensor and thus reduce hardware cost. By using global terminal attractor without discontinuous switching function, the proposed NFTSMC strategy can not only strengthen system robustness but also improve system dynamic performance. Lastly , on account of the variations of PV generation and load demand, a conventional double-loop PI scheme is put into use for bidirectional DC/DC converter and thus system power balance is ensured. Comprehensive MATLAB simulation and StarSim/Hardware experiment results show that the proposed NFTSMC strategy for two-stage converters can overcome negative effects caused by various internal and external disturbances, and thus enable island PV microgrid to operate reliably and supply high-quality AC power. • For PV island microgrid subjected to various disturbances, the proposed NFTSMC strategy can enhance system robustness • The proposed NFTSMC cascaded with P&O for forestage DC/DC effectively improves PV MPPT performance under various disturbances. • An ESO is designed and incorporated into NFTSMC scheme to remove load current sensor and thus reduce hardware cost. [ABSTRACT FROM AUTHOR]

This paper mainly focuses on the attitude tracking controller design for a vertical take-off and vertical landing (VTVL) reusable launch vehicle in regardless of uncertain parameters, un-modeled dynamics, external disturbances and actuator faults. Starting from the kinematics and dynamics model of the VTVL, the attitude tracking error model subject to grid fin faults is built. A novel fixed-time extended state observer (FxTESO) is presented to estimate the derivative of the attitude tracking error and the lumped disturbances including grid fin faults and other uncertainties. Subsequently, a novel nonsingular fast terminal sliding mode surface (NFTSMS) is designed using the FxTESO, which has faster convergence performances than the traditional sliding mode surface. Finally, an adaptive fault-tolerant control scheme is proposed by utilizing the FxTESO, the proposed NFTSMS and the nonsingular fast terminal sliding mode control with an adaptive gain. The proposed controller achieves faster convergence speed and shorter reaching time. Both adaptive fault-tolerant controller and NFTSMS are proved by Lyapunov theory. The performances of the proposed control scheme are demonstrated by numerical simulations. • A new VTVL reusable launch vehicle was studied. • A novel FxTESO was presented to simplify the control system and reduce the knowledge of the model parameters. • A novel NFTSMS was designed to achieve faster convergence performances and better singularity-free property. • An adaptive fault-tolerant control scheme with an adaptive gain was proposed. [ABSTRACT FROM AUTHOR]

In this paper, a disturbance observer based nonsingular fast terminal sliding mode control (NFTSMC) scheme is proposed for the trajectory tracking control of an underactuated autonomous underwater vehicle (AUV). A nonlinear disturbance observer (DO) is designed to estimate complex external disturbances and incorporated into a NFTSMC. In constructing sliding surfaces, the parameter selection in the exponential terms in NFTSMC is expanded. Lyapunov's second method proves the uniformly ultimately bounded stability of the proposed DO based NFTSMC. Simulation is performed to demonstrate the effectiveness of the proposed controller. Compared with the existing NTSMC methods, the convergence rate is improved by the proposed NFTSMC. Moreover, the proposed controller has good robustness to smooth external disturbances. For random disturbances along with an impulse at some time, the proposed controller can also track the disturbance well and achieve good robustness. • An improved sliding surface is proposed to deal with the finite selection problem of exponential parameters. • The unknown time-varying disturbances are estimated by disturbance observer and compensated in the control laws. • Compared with the existing NTSMC methods, proposed control strategy realizes faster and accurate tracking. [ABSTRACT FROM AUTHOR]

In this work, an efficient robust control scheme is proposed for position control of induction motors based on field-oriented control (FOC). The proposed control scheme consists of three interior loops. In the outer loop, the central controller based on the nonsingular fast terminal sliding mode control (NFTSM) is designed. The objective of this loop is to provide the required stator current in the q-axis for regulating the electromagnetic torque. In the inner loops, two fractional-order PI (FOPI) controllers are designed to provide the stator voltages. Besides, since induction motors have a complex mathematical model, time-varying dynamics, and non-linear structure, to deal with these challenges in the controller design, the quantum neural network (QNN) is presented with a novel framework and employed to estimate the lumped uncertainties. This makes the main contribution of the paper. In addition, to further improve system performance, the parameters of the FOPI controllers are tuned by employing the artificial bee colony (ABC) optimization algorithm. The stability of the proposed control approach is proved by Lyapunov's stability theory. According to the results of the comparative simulation, the superiority of this approach is confirmed. • Addressing the position control of the IM using an effective control approach. • Presenting the QNN with a novel framework. • Estimating the lumped uncertainties using the adaptive QNN. • Combining the merits of the FOPI controllers, and NFTSMC. • Tuning the FOPI controllers using the ABC optimization algorithm. [ABSTRACT FROM AUTHOR]

An adaptive nonsingular fast terminal sliding mode control scheme consisting of an adaptive control term and a robust control term for electromechanical actuator is proposed in this article. The adaptive control term with an improved composite adaptive law can estimate the uncertain parameters and compensate for the modelled dynamical uncertainties. While the robust control term, which is based on a modified nonsingular fast terminal sliding mode control method with fast terminal sliding mode (TSM) reaching law, provides fast convergence of errors, and robustifies the design against unmodelled dynamics. Furthermore, the control method eliminates the singular problems in conventional TSM control. On the basis of the finite-time stability theory and the differential inequality principle, it is proved that the resulting closed-loop system is stable and the trajectory tracking error converges to zero in finite time. Finally the effectiveness of the proposed method is illustrated by simulation and experimental study. [ABSTRACT FROM AUTHOR]

In this paper, a finite-time extended state observer-based nonsingular fast terminal sliding mode control (NFTSMC) is proposed for the trajectory tracking of autonomous underwater vehicles (AUVs) with various hydrodynamic uncertainties and external disturbances. First, a proportional–integral velocity variable based third-order fast finite-time extended state observer is designed to estimate the lumped uncertainties and their first derivatives. The concepts of finite-time stability and a new proposition are utilized to prove the finite-time uniformly ultimately boundness of the proposed observer. Then, based on the disturbance estimation, an NFTSMC is developed for AUVs. The inverse tangent function and linear control term are used to provide robustness against estimation error and avoid chattering in the thrusters. The position and attitude trajectories are proved to be fast finite-time stable via the Lyapunov theorem. To verify the theoretical analysis, comparative numerical simulations are provided to demonstrate the effectiveness of the proposed control scheme. • A novel FFTESO is developed to accurately estimate the disturbances. • FFTESO-based NFTSMC scheme is designed to achieve the desired trajectory tracking. • The finite-time stability of the proposed scheme can be guaranteed. • The proposed control scheme ensures fast convergence and less chattering. [ABSTRACT FROM AUTHOR]

In order to improve the performance in the practical engineering applications including so called low-speed video tracking and large-angle swing scanning imaging at the same time for a three-axis universal inertially stabilized platform (UISP), we propose an adaptive nonsingular fast terminal sliding mode control (ANFTSMC) strategy subjected to the uncertain disturbances and input saturation constraints. First of all, a second-order dynamic model is established with uncertain disturbances and input saturation constraints. Secondly, a nonsingular fast terminal sliding mode controller (NTSMC) is constructed to ensure the system error converges to zero fast in a finite time; meanwhile, a novel reaching law based on a modified normal distribution function is designed to adjust the control gain. Thirdly, an adaptive control law is designed to online estimate the parameters of the lumped uncertain disturbances. Additionally, the stability of the control system is proved by Lyapunov theory. Finally, extensive comparative simulations and experiments are carried out, the results comprehensively show the effectiveness and superiority of the proposed control method, which can accelerate convergence, weaken the chattering, and has the better control accuracy and robust performance both in the low-speed tracking and large-angle swing scanning applications. Moreover, the exact dynamic model and the prior knowledge of the upper bounds of the disturbances are not required during the procedure of the controller design, which make it have more extensive application value in practical engineering. [ABSTRACT FROM AUTHOR]