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This paper addresses the vibration reduction and disturbance attenuation problems of a flexible arm, which is modelled as a fourth-order partial differential equation. The main control objectives lie in reducing the influence of the disturbance, steering the flexible arm to the desired angular position and suppressing the vibration, simultaneously. To achieve the objectives, firstly, two different disturbance observers are designed. Secondly, we construct boundary controllers based on the two designed disturbance observers and boundary measurable signals. Then, with the two disturbance observers, the boundary disturbance compensation is achieved, and with the designed boundary controllers, the uniform boundedness of the flexible arm is guaranteed. Moreover, the well-posedness of the closed-loop system under the designed boundary controllers is discussed by means of the operator semigroup theory. Finally, the effectiveness and advantages of the proposed controllers are demonstrated through simulations and physical experiments. [ABSTRACT FROM AUTHOR]

In this article, the adaptive event-triggered fault-tolerant control (FTC) issue of uncertain nonlinear systems suffering from sensor and actuator faults as well as external disturbances is studied. A disturbance observer (DO) is constructed to compensate for external disturbances and approximation errors generated by neural networks (NNs). Then, switching event-triggered control and command filtering techniques are introduced to balance the communication frequency and tracking performance of the controlled systems while avoiding “explosion of complexity” issue caused by iterative derivations of virtual controllers. Furthermore, compensation signals are designed to eliminate filtered errors. Finally, it is proven that the developed FTC scheme can esure that all signals are bounded and free from Zeno phenomenon. Simulation results of a spring damping mechanical system verifies the merits of the designed control algorithm. [ABSTRACT FROM AUTHOR]

This paper suggests a new dynamic sliding mode controller incorporating a disturbance observer for controlling integrating processes with time delay. Here, a new reaching law has been introduced to expedite the system response without evoking chattering. The convergence of this law is also studied. Additionally, the whole system stability is proved using the Lyapunov stability theory. This paper also proposes a new disturbance observer to improve the disturbance rejection and to help suppress chattering further. The suggested observer has only one parameter to be designed. Therefore, this observer can easily be implemented. Several simulation examples and a real-time application are considered to prove the superiority of the suggested controller over the reported controllers in the literature. [ABSTRACT FROM AUTHOR]

In this paper, a predictive-based adaptive anti-saturation tracking control scheme is designed for uncertain multiple-input and multiple-output (MIMO) nonlinear systems subject to input constraints and external disturbance. Taking the safety of the MIMO nonlinear systems into account, a safety saturation threshold, which is less than the physical device saturation threshold, is given. In addition, since all signals of the auxiliary systems are not immediately employed to the systems when the input saturation happens, a new predictive-based auxiliary system is designed. Furthermore, to estimate the compound disturbance, with the help of the fuzzy control technology, the nonlinear disturbance observer (NDO) is used. Then, the stability of the closed-loop systems is proved under the designed controller. Finally, a simulation example is given to illustrate the feasibility of the designed anti-saturation controller. [ABSTRACT FROM AUTHOR]

In this paper, we study the problem of attitude trajectory tracking of rigid bodies subject to external disturbances. The attitude control problem has been studied in the last decades and novel solutions have been proposed. Nevertheless, most of the proposed controllers only achieve local or almost global asymptotic stability without considering external disturbances. Contrary to other works, we focus on the problem of achieving almost global exponential stability of the attitude tracking errors in the presence of external disturbances using continuous control laws. We propose an attitude trajectory tracking controller based on the exponential coordinates of rotation in combination with a disturbance observer that estimates the exogenous signals. To solve this problem, we adopt a hierarchical approach, where the proposed control law is divided into a kinematic controller (outer control loop) and a velocity tracking controller (inner control loop). To design the disturbance observer, it is assumed that exogenous signals can be generated by a linear exosystem, i.e. the magnitude and phase of the disturbance are unknown. The almost global exponential stability of the closed-loop dynamics' equilibrium point was proved by a strict Lyapunov function. The performance of the proposed approach is assessed by numerical simulations and experimental tests on a low-cost quadrotor. [ABSTRACT FROM AUTHOR]

This paper studies leader–follower cooperative control with the slowly-varying environment disturbances of the surface ships, which are mainly caused by wind, wave and current. A disturbance observer is constructed to estimate the slowly-varying environmental disturbances. Based on disturbance observer, the control law of the follower ship is designed by backstepping technique. The disturbance observer-based leader–follower cooperative control scheme is carried out via employing stochastic control theory to analyse the close-loop system. The scheme achieves the follower ship track the leader ship with a certain error range for cooperative movement and all signals are bounded in the mean square. Finally, the results of simulation are listed to verify the scheme. [ABSTRACT FROM AUTHOR]

To prevent flow maldistribution and imbalanced temperature, a disturbance observer-based control strategy is presented for microchannel cooling systems with multiple heat exchangers. A disturbance observer is designed using the extended Kalman filter for estimating the states and disturbance. The performance of the observer is validated using experimental data from a pumped liquid cycle testbed. Disturbance observer-based control is applied to systems with two and three heat exchangers. The findings demonstrate that the wall temperatures and the temperature differences for the heat exchangers could be maintained at the desired values. When the heat exchangers are subjected to heat loads in the form of square, sinusoidal, and sawtooth functions, the temperature fluctuation could be suppressed by 60%-80% and the temperatures could be maintained at the same values. Our research demonstrates that the proposed control strategy has acceptable accuracy and robustness for the system with multiple heat exchangers undergoing various operating situations. [ABSTRACT FROM AUTHOR]

Based on a line-of-sight (LOS) guidance law for a curve parametrized path, a finite-time backstepping control is proposed for the kinematic path-following of an underactuated autonomous surface vehicle (ASV). Finite-time observer is utilized to estimate the unknown external disturbances accurately. The first-order Levant differentiator is introduced into the finite-time filter technique, such that the output of filter can not only approximate the derivative of the virtual control, but also avoid the singularity problem of real heading control. The integral terminal sliding mode is employed to improve the tracking performance and converging rate in the surging velocity control. By virtue of Lyapunov function, all the signals in the closed-loop system can be guaranteed uniformly ultimate boundedness, and accurate path-following task can be fulfilled in finite time. The simulation results and comparative analysis validate the effectiveness and robustness of the proposed control approach. [ABSTRACT FROM AUTHOR]

A composite anti-disturbance stabilisation control scheme is presented for the ship-borne platform system with completely unknown disturbances. The completely unknown disturbances consist of external time-varying disturbances and norm-bounded unmodelled dynamics. The composite anti-disturbance control scheme is built through integrating the nonlinear disturbance observer and the robustifying term with the vectorial backstepping technique. The nonlinear disturbance observer is designed to provide the on-line estimations of external time-varying disturbances such that the disturbance compensation term is introduced into the control design. The robustifying term with the adaptive law is employed to attenuate norm-bounded unmodelled dynamics. On the basis of the vectorial backstepping technique, the composite anti-disturbance control law is derived. The composite anti-disturbance control structure consists of the outer layer with the nonlinear disturbance observer for external time-varying disturbance cancellation as well as the inner layer with the vectorial backstepping control together with the robustifying term for unmodelled dynamic attenuation. By means of the Lyapunov stability theory, it is proven that the composite anti-disturbance stabilisation controller can achieve the disturbance rejection and attenuation simultaneously. It is verified by illustrative simulations that the composite anti-disturbance stabilisation control scheme can effectively stabilise the ship-borne stabilisation platform. [ABSTRACT FROM AUTHOR]

This study deals with the stabilisation problem of fractional-order time-delay systems with polytopic uncertainties and multiple disturbances with the use of Lyapunov stability theory. Notably, the multiple disturbances comprising both the known and unknown signals, are respectively characterised by the norm-bounded and exogenous system which signifies the harmonic signals with modelling perturbations. The control protocol is configured by the integration of proportional-retarded controller with gain perturbations and the output of an anti-disturbance observer. Precisely, the disturbance observer is taken into account for attenuating the influence of disturbance signals. Moreover, the implementation of a time-delay term in the feedback loop replicates the dynamic features of a derivative action that relies only on position measurements and its time differencing capabilities make it to be minimally sensitive against noise and provide a smoothing effect. By virtue of these circumstances, the asymptotic stability of the considered system is assured. With the aid of a suitable Lyapunov function, a set of robust order-dependent sufficient conditions is derived and then solved through the MATLAB LMI toolbox. As a result, the controller gain matrices can be computed and two numerical examples, including an electrical circuit model with simulation results are presented to certify the inherent potential of the theoretical outcomes. [ABSTRACT FROM AUTHOR]

This paper considers the prescribed-time stabilisation of the linear system with matched unknown external disturbance. The disturbance is supposed to be generated by an exogenous system. In the case only the~output signal of the original system is available for feedback, a prescribed-time linear observer for the augmented system consisting of the original system and the exogenous system is designed. Then the prescribed-time stabilisation problem can be achieved by using the state-and-disturbance-observer-based periodic delay feedback. Furthermore, in the case the state of the original system is measurable, three classes of prescribed-time disturbance observers for the exogenous system are designed. The prescribed-time stabilisation problem can also be solved by applying the disturbance-observer-based periodic delay feedback. A numerical example is posted to demonstrate the effectiveness of the proposed approaches. [ABSTRACT FROM AUTHOR]

The anti-disturbance control problem is investigated for systems with multiple disturbances. For the disturbance with partially known information, an observer was designed to give the online estimation. Based on which, a fixed-time anti-disturbance control scheme was proposed by combing regional pole placement, the H ∞ control method and fixed-time stability theory. The proposed controller can make the closed-loop system globally stable in fixed time, which has the advantages of high control accuracy, fast convergence speed, and the convergence time is independent of the initial conditions. Finally, two simulation examples including a numerical example and a doubly fed induction generators system are provided to show the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, the composite control problem of continuous time semi-Markovian jump system with actuator saturation, multiple disturbances and unknown nonlinearity is investigated. A stochastic stability condition and a lower bound of the time between two consecutive events are derived to avoid Zeno behaviour, based on the combination of disturbance-observer-based control and event-triggered control. In addition, the passive control of the system based on the disturbance observer is studied. Finally, numerical simulation proves that the designed composite controller can effectively save communication resources, and can maintain the system to meet the passive performance indicators of the obtained closed-loop system under the influence of multiple disturbances and actuator saturation. [ABSTRACT FROM AUTHOR]

To understand and thus improve the dynamic behaviour of the railway pantograph-catenary system, researchers have developed a number of approaches to the catenary modelling. The Finite Element (FE) approach provides a high degree of accuracy, but at the cost of significant computing time. In contrast, researchers in active pantographs generally use relatively simple (and fast running) lumped-mass catenary models. This paper aims to improve on the models available for the design and validation of active pantographs. Firstly, the veracity of the classic lumped-mass model is assessed and a new catenary model is proposed, by augmenting the lumped-mass model with a dynamics-matched mass and a disturbance which can be found using a disturbance observer (DO). Open-loop, passive pantograph, results show that the proposed model can represent the dynamic behaviour exhibited in the FE catenary very well. Secondly, two PID controllers are designed for an active pantograph, and tested with the FE, classic and DO-based lumped-mass catenary models. It is shown that if a controller is validated on the classic lumped-mass model, the credibility will be low. In contrast, the new DO-based model is found to deliver more robust results, providing efficient simulations for active pantograph design and validation. [ABSTRACT FROM AUTHOR]

This paper investigates the attitude tracking control of spacecraft under unified actuator dynamics. The inertia uncertainties, external disturbances, and input saturation are also considered. A novel robust attitude tracking control scheme is proposed by incorporating a disturbance observer and an auxiliary dynamic system into the command filtered backstepping control design. The command filtered backstepping control can avoid the 'explosion of complexity' problem inherently existing in the traditional backstepping control. The disturbance observer is designed to estimate the mismatched compound disturbance induced by inertia uncertainties and external disturbances. The auxiliary dynamic system is conducted to tackle the effect of input saturation. The uniform ultimate boundedness of the overall closed-loop system is rigorously proved. The proposed controller can guarantee the attitude tracking errors converge to the arbitrary small neighbourhood of zero even subject to actuator dynamics. Numerical simulations and comparisons illustrate the effectiveness and advantages of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper proposes three-dimensional cooperative guidance laws with prescribed performance. To avoid degradation in performance due to error in the estimation of the time-to-go, the relative distance between the missiles and the target is chosen as a coordination variable, and a fixed-time cooperative guidance law along the direction of the line-of-sight (LOS) is proposed. The guidance laws with prescribed performance for the elevation and direction of the azimuth of the LOS are then developed by combining the LOS angle tracking error with a performance constraint function to formulate a transformed error function. The problem of prescribed performance control is transformed into that of the boundedness of the transformed error function. A fixed-time convergence disturbance observer is designed to discuss the problem whereby information on aggregate uncertainty, including on the target, is unavailable. The results of simulations show that the proposed guidance laws are effective. [ABSTRACT FROM AUTHOR]

An adaptive neural network fault-tolerant control(FTC) scheme is proposed for nonlinear and nonstrict-feedback multi-agent systems (MASs) with directed fixed topology. Firstly, a disturbance observer is designed to estimate the unknown external disturbances in the systems, and realise the dynamic estimation of the disturbances. Secondly, the efficiency factor is estimated online, and then the FTC scheme is designed successfully under the backstepping framework. It is proved that all signals in the closed-loop systems are semi-globally uniformly bounded and the tracking error is controlled in a small range. Finally, an example is given to verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

Robustness analysis of disturbance observer (DOB)-based control systems under parametric uncertainty is addressed in this study. A spherical polynomial family-based approach is adopted to analyse how much uncertainty can be tolerated, and to capture a unified framework for different cases including non-minimum phase plant and different nominal and perturbed plant structures. Results are validated using the value set concept for spherical polynomial families. The study has shown that if the relative degrees of the perturbed plant and the nominal plant are equal, then robustness is achievable even if the plant model is low order. Although non-minimum phase zeros limit the selection of DOB bandwidth, the robustness margin can be exactly determined for a given DOB bandwidth. Furthermore, it is shown that when the plant numerator and denominator have uncertain parameters, the robustness margin is not increased as with the DOB filter bandwidth in general. [ABSTRACT FROM AUTHOR]

Coupled multiple-tank systems are very important for a wide range of industrial applications due to their unique uses. However, the liquid level control for the coupled two-tank multi-input multi-output (MIMO) system is quite challenging because it has strong nonlinearity and coupling, and it is susceptible to multiple external disturbances. For this process, this paper proposes a novel anti-disturbance control strategy consisting on a nonlinear composite hierarchical anti-disturbance predictive control (CHADPC). First, a model-based explicit nonlinear model predictive controller (ENMPC) is designed assuming that all disturbances are measurable and its global exponential stability is proved. Then, a nonlinear disturbance observer (DO) is designed to estimate the lumped disturbances. The composite controller handling the estimated disturbances is then proposed. Finally, simulation and experimental tracking control tests under perturbations and comparisons with recently reported works have been carried out to highlight the promising performance of the proposed ENMPC and CHADPC schemes. [ABSTRACT FROM AUTHOR]

This paper proposes a novel adaptive disturbance observer-based fast terminal global sliding mode tracking control for uncertain time-delay nonlinear systems in the presence of parameter uncertainties, multiple delayed state perturbations and external disturbances. The suggested fast terminal sliding mode disturbance observer guarantees the disturbance approximation error converges to zero in finite time. Under the proposed method, the tracking error trajectories converge to the sliding surface in finite time and the zero-tracking error is achieved. A new tracking control law is introduced and has the salient features such as no requirement of a priori knowledge of the upper bound of uncertainties as well as disturbances and the elimination of the undesirable chattering and reaching phase. Simulation results illustrate the effectiveness and advantages of the proposed control method. [ABSTRACT FROM AUTHOR]

This paper proposes a method based on Lyapunov theory to guarantee a definite exact convergence time for an adaptive sliding mode controller applied to a class of nonlinear uncertain systems subject to bounded perturbations. To achieve the objective, the gain adaptation law is enhanced with a sliding mode disturbance observer. The new control scheme ensures the sliding variable follows the defined segment-like trajectory during the reaching phase, thereby reducing the necessary control signal amplitude by avoiding the overestimation of the control gains. This novel approach allows a reduction of the design constraints on the system actuators while guaranteeing an exact desired convergence time under unknown bounded perturbations. The disturbance observer tuning methodology is also addressed in function of the expected perturbation signal type. To support the proposition, numerical simulations are performed to illustrate the fixed-time disturbance rejection capacities of the closed-loop system subject to constant, periodic, or stochastic perturbations on a tutorial example. Finally, the proposed controller is applied to drive the velocities of an unmanned surface vehicle model under realistic perturbations. [ABSTRACT FROM AUTHOR]

The three-phase four-leg inverter is widely used in microgrid recently. The control strategy is critical for the performance of nonlinear four-leg inverter system owing to the problems of coupling, disturbances of nonlinear/unbalanced loads, variable operating condition, etc. In view of the above problems, a nonlinear composite controller is proposed for four-leg inverter system under d q 0 reference frame by integrating a higher-order sliding mode observer (HOSMO) and finite-time feedback control technique. The HOSMO can observe the uncertainties and disturbances by means of the soft sensing manner for feedforward compensation, which economises some sensors for their states being inconvenient to measure. In addition, it is a novel way for the HOSMO to realise the decoupling of the inverter system under d q 0 frame. The finite-time feedback controller works in coordination with the HOSMO which provides more rapid convergence rate and further improves anti-disturbance capacity of the system. Finally, the simulation results with PI comparison illustrate the feasibility of the proposed method for stabilizing system. [ABSTRACT FROM AUTHOR]

This paper shows the effectiveness of disturbance observer (DO) based sliding mode control (SMC) scheme as compared to traditional SMC. Both the methods are simulated for variable speed wind turbine (VSWT) to ensure asymptotic tracking of rotor speed even in the presence of disturbances. Two mass model of VSWT is derived which includes the damping effect of the rotor's blade and generator. The stiffness and damping effect of low-speed shaft are also taken into account while modelling the system. The dynamic model of VSWT contains both matched and mismatched uncertainties. Traditional SMC can only attenuate the effect of match uncertainties. The DO-based SMC overcomes the drawback of traditional SMC by attenuating mismatch uncertainties. In the proposed approach, a sliding manifold is chosen based on the estimation of an uncertain parameter (aerodynamic torque), which drives the system's states to equilibrium point in a finite time. [ABSTRACT FROM AUTHOR]

This paper develops an observer-based predictive repetitive control system to track periodic reference signals or reject disturbances with bandlimited frequency content. The new design complements existing approaches to predictive control, where a model of the periodic disturbance is embedded in the controller. In particular, the new design, based on a novel combination of repetitive control and a disturbance observer, results in a significant improvement in design transparency and implementation simplicity. Although the design is undertaken using a state-space-model, frequency response analysis based on the sensitivity and complementary functions is used to demonstrate the characteristics of the repetitive control system for disturbance rejection, reference following and measurement noise attenuation. Moreover, operational constraints can be included in the design for applications where this feature is required. Simulation studies are given to highlight the closed-loop performance achievable in the presence of constraints. Experimental validation results from application to a two-joint robotic arm are also given and discussed. [ABSTRACT FROM AUTHOR]

This paper investigates the continuous composite control scheme for the tracking of nonlinear system with mismatched disturbances, input saturation and full-state constraints. Based on the fixed-time stability theory, a novel fixed-time disturbance observer is proposed, which switches from a uniform second-order sliding mode observer with high-degree term to the classical continuous super-twisting observer. Then, a command filtered backstepping control scheme is designed to enforce the tracking error into a small neighbourhood of the origin in fixed time while the full-state constraint and input saturation constraint are satisfied. To further reduce the communication burden, an event-triggered communication strategy between the controller and the actuator input is also developed. Furthermore, all the signals in the closed-loop system are proven to be bounded. Finally, the effectiveness of the proposed control strategy is illustrated by numerical simulations. [ABSTRACT FROM AUTHOR]

In this paper, we consider the specified-time platoon control for the networked vehicles modelled by the third-order dynamics with uncertain external bounded disturbance. The vehicles are subjected to disturbance and parameter uncertainties. To ensure the platoon can converge within a predefined time, a novel disturbance observer with a time-varying dynamic gain is proposed. By the time transformation approach, the disturbance observer is proved to be uniformly ultimate bounded (UUB) stable in a specified time. The time-varying dynamic gain is also applied to design the novel specified-time controller for the vehicle platoon and rigorous proof is presented based on Lyapunov stability. Under the proposed disturbance observer and controller, the vehicle platoon can be stabilised within the predefined time without depending on any system parameters and initial conditions. Finally, some comparison simulation results are given to verify the effectiveness of the proposed protocols. [ABSTRACT FROM AUTHOR]

In this paper, a dynamic backstepping sliding mode controller with a fractional order sliding surface, which has a fuzzy boundary layer, is designed based on high-gain and disturbance observers for controlling the performance of a micro-electro-mechanical triaxial gyroscope. To compensate uncertainties of a system, a combination of sliding mode and robust nonlinear backstepping controller is used. In this design, in order to increase the degree of freedom of the controller, the sliding surface is selected to be of fractional order. in this paper, in order to significantly reduce the chattering phenomenon in the control signal, a new dynamic sliding surface in addition to the initial sliding surface and also fuzzy control theory to controllig the boundary layer are used. In addition, a high-gain observer and a disturbance observer are used to estimate the system states and incoming disturbances to the system. The asymptotic stability of the closed-loop system is proven by Lyapunov stability theorem. In order to evaluate the performance of the designed controller, this controller is compared with other sliding mode controllers. Simulation results show that the proposed controller has much less chattering phenomenon in control signal, increasing system stability, reducing the rise time and better tracking. [ABSTRACT FROM AUTHOR]

The problem of fault-tolerant control for the time-invariant system subject to actuator faults and multiple disturbances is focused. The multiple disturbances are composed of two kinds: one is a norm-bounded vector; the other described by an exogenous system is viewed as a harmonic signal with known amplitude and unknown frequency and phase. An improved disturbance observer is designed to obtain the modelled disturbance with partially known information. Based on this, two novel fault-tolerant control algorithms are put forward to the analysis and compensate two different types of actuator faults (one is norm-bounded, and the other is with an unknown boundary). The effectiveness and robustness of control schemes can be exhibited through some numerical simulations with different cases. [ABSTRACT FROM AUTHOR]

This paper studies event-triggered based adaptive neural network (NN) tracking control of a robotic manipulator with output constraints and disturbance. First, a novel asymmetric tan -type barrier Lyapunov function (BLF) is developed to satisfy the requirement of time-varying output constraints. Then, a fixed threshold event triggering is proposed to reduce the energy consumption, which avoids the happening of Zeno behaviour after analysis. Further, a disturbance observer (DO) and an adaptive neural network are devised to estimate the bounded disturbance and the unknown dynamics of the robotic manipulator. The proposed controller can achieve uniform boundness of the solution and adjustment of transient performance. Finally, the effectiveness of the presented methods is verified by related simulation results. [ABSTRACT FROM AUTHOR]

In this study, we propose an adaptive tracking dynamic surface back-stepping control based on Nussbaum disturbance observer for uncertain high-order strict-feedback MIMO nonlinear systems with external disturbances, unknown parameters and modelling uncertainties. It is assumed that the control action is affected by input saturation; so to compensate the resulted undesirable effects, the tanh(.) function is used as an approximation to the saturation function. To tackle the unknown external disturbances and uncertainties, a Nussbaum disturbance observer is proposed. Moreover, an appropriate adaptive law is presented to deal with the unknown parameters. Then, the dynamic surface control method is used to overcome the inherent problem 'explosion of complexity' that appeared in the conventional back-stepping method. The Lyapunov stability analysis proves that the closed-loop system is semi-globally uniformly bounded. At last, reasonable effectiveness and applicability of the adaptive tracking dynamic surface control are illustrated by simulation results. [ABSTRACT FROM AUTHOR]

This study focuses on the vibration control and boundary output constraint problems for a flexible wing system. The coupled twist-bending dynamic model of the flexible wing contains several partial differential equations (PDEs) and ordinary differential equations (ODEs). With the help of prescribed performance functions, an innovative boundary controller is designed to achieve the vibration suppression without the violation of prescribed performance. The proposed controller can make the boundary deformations converge to an arbitrarily small residual set. In addition, a disturbance observer is proposed to eliminate the adverse effect caused by unknown external disturbances. Then the asymptotic stability of the closed-loop system is justified by the Lyapunov's direct method. Finally, numerical simulations are carried out to demonstrate the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

A shift in paradigm from dedicated to open communication channels in power systems has made them prone to constant and time-varying delays. This paper tackles a novel load frequency control (LFC) problem in the presence of constant and time-varying delays in state and control input under load disturbances. The presence of time delays can deteriorate the performance of the controller or even destabilise the system. The above problem is addressed through a prediction-based super twisting sliding mode control (ST-SMC) using a state and disturbance observer. The proposed design achieves finite-time convergence of frequency and tie-line power deviation. The said design is validated under ramp and random step disturbance, with nonlinearities like generation rate constraints and governor deadband, with an integration of renewable energy and also with IEEE 39 bus power system. The closed-loop stability is proven thanks to candidate Lyapunov function and verified by simulations. [ABSTRACT FROM AUTHOR]

This paper investigates fault-tolerant control algorithm for diesel engine air path system with consideration of system parametric uncertainties, external disturbances and actuator faults. Initially, a finite-time disturbance observer is developed for estimating the synchronised uncertainties including parametric uncertainties and external disturbances. Based on the reconstructed information from the observer, a fault-tolerant control law is designed by using sliding mode control incorporated adaptive method. Furthermore, to attenuate the chattering problem caused by conventional sliding mode control method, the hyperbolic tangent function is utilised. A novel sliding variable is employed for improving the performance of the modified chattering-free control approach. The proposed fault-tolerant control algorithm is feasible for implementing and unnecessary caculations can be avoided, because the synchronised uncertainties and the actuator faults are tackled separately. Besides, the observer does not require the priori knowledge of the upper bounds of the synchronised uncertainties. Finally, simulation results and comparison are carried out and analysed for illustrating the effectiveness and validness of the proposed control schemes. [ABSTRACT FROM AUTHOR]

This paper investigates the problem of event-based disturbance compensation control for discrete-time surface-mounted permanent magnet synchronous motor (SPMSM) subject to both mismatched external disturbances and the limited communication bandwidth. The mismatched external disturbances make it impossible to transform the original model into the multiple-step-ahead predictor model by using the existing predictor technology. To tackle such a challenge, a novel backstepping-based disturbance compensation control framework is firstly proposed for the discrete-time SPMSM with known system dynamics. In the proposed framework, the disturbance observer is designed to compensate for the external disturbances, and the novel variable substitution/decoupling technology is proposed to design the event-based controller for the original controlled system. The proposed controller is composed of the event-based feedback control signal and the feedforward disturbance compensation signal, thereby improving the system disturbance rejection ability and mitigating the communication resource. Subsequently, an event-based adaptive neural control scheme is proposed by combining a modified neural disturbance observer. The proposed scheme ensures that the tracking error converges to a small neighbourhood of the origin, all the signals in the closed-loop system are bounded and meanwhile the communication resources are greatly reduced. The effectiveness of the proposed controller is illustrated through the simulation results. [ABSTRACT FROM AUTHOR]

Motivated by the conservatism and complexity of the existing robust stability analysis method for disturbance observer (DOB)-based control system, a robust stability analysis method, addressing parametric uncertainty, is proposed for different relative order processes with time delay. By Bounded Phase Condition, a simplified Generalised Kharitonov Theory is proposed for stability analysis of inner loop, replacing the redundant stability verification of generalised Kharitonov segment polynomial by stabilising the end points of the segment polynomial. Then, based on the simplified theorem and the stability equation method, all the feasible low pass filter (LPF) parameters that stabilised the inner-loop can be obtained. It is the first time that all the feasible LPF parameters are given for different relative order. Furthermore, the proposed method is extended to ensure the stability of outer loop and obtain the admissible range of controller parameter. Simulation studies present how to obtain the stability region and demonstrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

Heavy-duty vehicle platooning has received much attention as a method to reduce fuel consumption by keeping inter-vehicle distance short. When a platoon follows a fuel-optimal velocity profile calculated using preview road slope information, significant improvement in the fuel economy occurs. To calculate the optimal velocity in the existing method, however, platoon should acquire expensive road slope data in advance. As an alternative, we propose a road slope estimation method, which enables platoon to calculate the optimal velocity profile without the usage of actual road slope data. Other major challenges in platoon operation include overcoming the effect of the vehicle model uncertainties and external disturbances for ensuring the control performance. The most significant part of the disturbances arises from slopes along a route. Existing method for reducing the effect of the slope employs a feed-forward type compensation in the control loop by combining the vehicle position acquired from GPS and the slope database. However, this method exhibits limitations: the mass of the vehicles in the platoon is uncertain which lowers the accuracy of the feed-forward compensation, and the platoon requires the pre-acquired slope database. To overcome these limitations, we propose an alternative method employing disturbance observer. Simulations of various scenarios are conducted to show the efficacy of the proposed method using the actual road slope data of a Swedish highway. [ABSTRACT FROM AUTHOR]

This work proposes a novel robust nonlinear control method for trajectory tracking of a quadrotor unmanned aerial vehicle using the prescribed performance control technique and disturbance observers, in the presence of external disturbances and uncertainties of physical parameters. By exploiting the cascaded structure of the position subsystems and the attitude subsystems, the controller is designed in a backstepping manner, and the dynamic surface technique is adopted to avoid the explosion of the controller complexity. For each axis of the subsystem with uncertainties, the prescribed performance control technique is adopted to ensure a moderate control performance. Then a disturbance observer is constructed to enhance the corresponding controller to achieve a sufficiently small ultimate error. Not only the control performance of the overall control system, but also the behaviour of all the internal error signals are analysed rigorously. Finally, the performance of the proposed controller is confirmed by simulation studies. [ABSTRACT FROM AUTHOR]

In this paper, a robust hierarchical controller is proposed for trajectory tracking of a quadrotor in the presence of disturbances and parametric uncertainties. The hierarchical controller has an outer-inner loop structure. The outer loop controls the position of the quadrotor and generates desired roll and pitch angles for the inner loop corresponding to desired position, velocity and heading angle. The inner loop controls the attitude of the quadrotor. A disturbance observer based backstepping controller is designed for the outer loop to compensate for disturbances and guarantee robust position tracking. An adaptive non-singular terminal sliding mode controller is designed for the inner loop to ensure the attitude angles converge to their desired values in finite time and deal with the parametric uncertainties. The stability and online estimating capability of the proposed controller are proved by using Lyapunov theory. Finally, numerical simulation results are presented to validate the effectiveness of the proposed controller. [ABSTRACT FROM AUTHOR]

In this paper, photovoltaics (PV) outputs are not only limited constantly but also controlled by bidirectional communications to compensate short period power fluctuations. In this case, if qualities of communication systems are not high, communication delays may not be ignored. The effect of communication delays is evaluated. If communication delays are not very small, a system reaches a stability limit, and a PV output may oscillate continuously. Then, a compensation method of them is proposed. The compensation method uses a disturbance observer. In this method, it is a merit that models of communication delays are not necessary and only a PV output model is necessary. It is found that the proposed method can compensate communication delays to some extent by simulating step responses. Furthermore, compensation of short period power fluctuations in a micro grid is considered by using the proposed method. Short period power fluctuation may be reduced by the compensation method of communication delays. [ABSTRACT FROM AUTHOR]

For the straight-line tracking of unmanned surface vehicle (USV), most researchers pay much attention to the course control. In the present work, a strategy of decoupling and control of the speed and yaw of the USV is proposed to maintain a constant speed and a desired course, which ensures the high efficiency of ship-borne sonar during a seabed exploration. Since USV is 3-DOF underactuated system, the Lyapunov direct method is combined with the hierarchical back stepping control method to achieve the desired speed and yaw angle of USV under the external disturbances, such as wind, waves and currents. A disturbance observer is then constructed to compensate the oscillation by predicting changes in external disturbances. The control system is further proved to be asymptotically stable by the Lyapunov theory. The validity and robustness of this decoupling controller is verified by applying to a practical USV in the straight-line tracking. [ABSTRACT FROM AUTHOR]

This paper investigates the fixed-time prescribed performance tracking control for the n-DOF uncertain manipulator. First, a novel Barrier Lyapunov Function (BLF) is proposed to guarantee the prescribed performance for the manipulator tracking error. Then, we introduce a disturbance observer to estimate the system uncertainty and disturbance accurately in a predefined time. Next, a composite controller based on the nonsingular fast integral terminal sliding mode is constructed. It is strictly proved that the closed-loop system is stable in fixed-time, which is independent of the initial conditions. Moreover, both transient and steady-state performances of the outputs can be preserved. Finally, numerical simulations and experimental studies are presented to demonstrate the effectiveness of the proposed methods. [ABSTRACT FROM AUTHOR]

This paper proposes a novel, simultaneous bipedal locomotion method using haptics for remote operation of biped robots. In general, traditional biped walking methods require very high computational power and advanced controllers to perform the required task. However, in this proposed method, a master exoskeleton attached to the human's lower body is used to obtain the trajectory and haptic information to generate the trajectory of the slave biped robot in real time. Lateral motion of the center of mass of the biped is constrained in this experiment. Also, it is considered that no communication delay is presented in between the two systems in this experiment, and they are not discussed in this paper. Since a direct motion transmission is used in the proposed method, this method is quite straight forward and a simultaneous walking can be realized at the same time with high performance. Also, it does not require an exact dynamic model of the biped or specific method to plan the trajectory. The gait pattern of the biped is directly determined by that of the human. Also, the operator can feel the remote environment through the exoskeleton robot. Results obtained from the experiments validate the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, boundary and distributed controllers are proposed to reduce the vibration of a flexible string with external disturbance. To accurately represent the flexible string's behaviour, partial differential equations are introduced for the flexible string modelling. Boundary and distributed disturbance observers, which are proved exponentially stable, are designed to compensate unknown disturbance. Based on the proposed disturbance observers, the boundary and distributed controllers are developed by linear matrix inequality and the asymptotic stability of the system with controllers is demonstrated by Lyapunov's direct method. Extensive numerical simulations are presented to validate the good performance of the proposed control laws. [ABSTRACT FROM AUTHOR]

In this paper, a sampled-data adaptive output feedback controller is proposed for a class of uncertain nonlinear systems with unmeasured states, unknown dynamics and unknown time-varying external disturbances. To approximate uncertain nonlinear functions, radial basis function neural networks (RBFNNs) are employed. The state observer and the disturbance observer (DO) are constructed to estimate the unmeasured state and the external disturbance, respectively. Then, the sampled-data adaptive output feedback controller and adaptive laws are designed by using the backstepping design technique. The allowable sampling period T is derived to guarantee that all states of the resulting closed-loop system are semi-globally uniformly ultimately bounded. Finally, two simulation examples are presented to illustrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

This article proposes a proportional-type cascade output voltage control algorithm equipped with disturbance observers for a three-phase AC/DC converter considering a model-plant mismatch. The advantages of the proposed method are grouped into two. First, a simple first-order disturbance observer is devised for both inner and outer-loops, not only to recover the target closed-loop performance, but also to eliminate steady-state offset errors. Second, it is thoroughly verified that the entire closed-loop system driven by the proposed proportional-type cascade controller is stable. The efficacy of the proposed method is experimentally investigated using a 3 − kW AC/DC converter. [ABSTRACT FROM AUTHOR]

This paper addresses the disturbance-observer-based linear quadratic regulation (LQR) problem of singularly perturbed systems subject to harmonic disturbances. To reduce the computational burden and facilitate the on-board implementation, a recursive matrix decoupling method is proposed to decompose the stiff singularly perturbed model into the block-diagonal form. Then, a robust LQR controller is designed based on the equivalent decoupled system, which consists of a state feedback controller for performance optimisation and a robust controller for disturbance compensation. Moreover, the designed controller is integrated with a finite frequency disturbance observer to attenuate the effect of harmonic disturbances. Finally, a simulation example on a DC motor system is provided to verify the effectiveness of the proposed design method. [ABSTRACT FROM AUTHOR]

This work considers the problem of distributed consensus tracking control of second-order uncertain nonlinear systems under a directed communication graph which contains a spanning tree, where the leader node is the root. It is assumed that the followers receive only the relative positions from the neighbours. For the purpose of consensus tracking controller design, in each follower, a group of K-filters is introduced so that the necessity of velocity estimating is avoided. Then we can express each follower's tracking error dynamics as a second-order system with mismatched uncertainties. And hence we can design a robust consensus tracking controller for each follower by using the combination of the backstepping design and the disturbance observer based control using only relative position information. Theoretical analysis is performed to show that the DOBs' estimation errors can be made to decay to be sufficiently small very quickly before the system states escape from the feasible region. Then we show that all the followers' states track those of the leader with arbitrarily small ultimate error bounds. And simulation examples are provided to demonstrate the performance of the proposed method. [ABSTRACT FROM AUTHOR]

The attitude tracking control issue for hypersonic reentry vehicle subjected to bounded uncertainties is investigated in this paper. The uncertainties are estimated by a disturbance observer with a simple structure whose parameter tuning methodology is provided to guarantee the system robustness. The coupling effect of the hypersonic reentry vehicle is analysed with a binary form based on a coupling effect indicator. The positive or negative value of indicator represents the harmful or beneficial coupling effect on the system. Then, a coupling effect-triggered control is developed to cancel the harmful couplings while utilising the beneficial couplings. The trigger point in the controller occurs when the property of the coupling effect happens to change. The specific criterion is introduced to quantify the performance improvement via the proposed scheme. Application to the hypersonic reentry tracking is presented to demonstrate the validity of the proposed method. [ABSTRACT FROM AUTHOR]

This article investigates a new current control strategy for grid connected doubly fed induction generators (DFIG) under unbalanced grid voltage conditions. DFIG dynamic model is represented in synchronously rotating symmetrical component dq frames of references. A proportional controller which feed forwards estimated disturbance terms is proposed for power control in unbalanced voltage conditions to accomplish positive sequence power demand and also to dissipate negative sequence components which cause double frequency oscillations in power. DFIG parameters are not required to be known since it estimates the machine parameters relevant nonlinear terms with a disturbance observer (DOB). The novel controller structure is implemented by using dSPACE ds1103 digital controller which controls a 1.1 kW DFIG experimental setup. [ABSTRACT FROM AUTHOR]

This article suggests a robust cascade speed control algorithm for a permanent magnet synchronous motor (PMSM) combining the classical feedback linearising (FL) method and the disturbance observers (DOBs) without the integrators. The contributions of this method are twofold. The first one is to provide the simple DOBs for not only guaranteeing the closed-loop performance recovery property but also removing the steady-state errors without the integrators with respect to the tracking errors. The second one is to prove that the inner and outer loops are stabilised by the proposed cascade-type controller, simultaneously. The simulation and experimental results reveal that the proposed method maintains the speed tracking performance to be satisfactory for a wide operating region with the fixed control gain despite a plant-model mismatch where a 3-kW interior PMSM is utilised. [ABSTRACT FROM AUTHOR]