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In this paper, a fixed-time sliding mode control scheme is developed to fulfill the trajectory tracking task of a marine surface vehicle with unknown dynamics. To restrain the adverse effects of the unknown dynamics including the parameter inaccuracy and exogenous disturbances, a fixed-time disturbance observer is designed to estimate the lumped uncertainties using the bi-limit homogeneous theory without requiring any knowledge of the model uncertainties. Then, a nominal tracking controller is proposed to stabilize the error dynamic model in the sense of fixed-time Lyapunov stability, based on which a novel integral-type sliding mode manifold with bi-limit homogeneity is constructed to drive tracking error convergence in fixed time. To enhance the robustness of the vessel control system, a disturbance observer–based fixed-time integral sliding mode tracking controller is finally proposed, and the chattering phenomenon is effectively alleviated by direct estimation compensations. The analysis of Lyapunov stability indicates that the closed-loop system is fixed-time stable. Numerical simulations on a model vessel are carried out to validate theoretical results of the proposed control scheme. [ABSTRACT FROM AUTHOR]

Robust controllers are attracting considerable interest in control of dynamic systems due to their capability of eliminating parameterized or unparameterized uncertainties. Therefore, model based robust control law is proposed in this study for ride comfort enhancement and applied on a 7 degree-of-freedom full-car suspension system with friction non-linearity. Inertia, spring and damping forces of the system are modelled with parameterized uncertainties while friction forces and external disturbances are considered as unmodelled dynamics, namely, unparameterized uncertainties. To better understand the effectiveness of proposed controller, a dry friction model that has non-linear characteristics is used for analysis. Closed-loop stability of the system is achieved by using well-known Lyapunov Stability Theorem. To better evaluate the effect of proposed robust controller on ride comfort enhancement with successful road holding, extensive numerical analysis is performed and the results are compared with those of previous similar controller and passive suspension system. The effectiveness of proposed control method has been confirmed. Consequently, satisfactory results have been obtained proving that the ride comfort of a vehicle that has both parameterized and unparameterized uncertainties has been further improved with reasonable power consumption values for a vehicle in terms of economic viability. [ABSTRACT FROM AUTHOR]

This study provides an adaptive fuzzy control scheme for the class of nonlinear uncertain strict-feedback systems with external disturbances. The backstepping and synergetic control theories are used to develop a robust control approach for this class of systems. The unknown behaviors of the system are estimated through fuzzy systems. The stability is analyzed using the Lyapunov method, which shows that the proposed control method ensures that all the signals of the closed-loop system are bounded, and the tracking error converges approximately to zero. Two simulation examples are given to demonstrate the efficiency of the developed control approach. [ABSTRACT FROM AUTHOR]

In this paper, the boundary feedback control problem for the Euler–Bernoulli beam with unknown time-varying distributed load and boundary disturbance is investigated. Based on the Lagrangian–Hamiltonian mechanics, the model of the beam is derived as a partial differential equation. To suppress the external disturbance, two disturbance rejection control approaches are adopted in the control design. Firstly, a disturbance observer is designed to estimate the boundary disturbance online. Thus, the effect of the boundary disturbance can be canceled directly in the feedback loop. A time-varying function is applied in the disturbance observer to prevent an excessively large control input. Secondly, a new observer is developed to estimate the upper bound of the disturbance and a sign function is introduced to suppress the influence of the disturbance without demanding the boundedness of the derivative of the boundary disturbance. The well-posedness and the uniform boundedness of the closed-loop system are proved using the operator semigroup theory and the Lyapunov method. Numerical comparisons with existing results are made for demonstrating the advantages of the proposed approaches. [ABSTRACT FROM AUTHOR]

A new method is developed for the robust stabilization of a Markov jump system with general discrete state time-delay and partly unknown transition probabilities. In contrast to most existing literature, a set of numerical testable conditions, rather than a huge matrix inequality, are established for the resulting closed-loop system in order to justify the stochastic stability and disturbance attenuation capability with the aid of a non-monotonic design approach. To be specific, by constructing a stochastic Lyapunov function via the application of an n -samples variation, sufficient conditions for the existence of a dissipative state feedback controller are derived with less conservativeness, that is, with larger stochastic stable region and better attenuation level. Three examples including a numerical example, a pest's structured population model and an F-404 test aircraft model are presented to demonstrate the advantage and practical potential of the proposed methodology. [ABSTRACT FROM AUTHOR]

In this paper, a model-free robust adaptive control scheme with finite-time convergence based on time-delay control is proposed for anti-sway and positioning control of two-dimensional underactuated overhead cranes. First, the whole overhead cranes system is simplified to an ultra-local model for time delay estimation (TDE). TDE brings a direct and effective model-free property but also an estimation error. Second, a sliding mode disturbance observer is designed to estimate and compensate for the TDE error. Third, sliding mode control (SMC) is used to enhance the robustness of the controller. An adaptive integral sliding surface is then designed to accelerate the sliding surface convergence rate and shorten the convergence time. To further optimize the selection of parameters, the parameter estimation is integrated to enhance the performance of model-free control. In the final analysis of the simulation, data yield that the introduction of parameter estimation increases the control performance by more than 20% on average, and the above facts verify the effectiveness of the scheme. Finally, the stability of the closed-loop control system is analyzed by using Lyapunov stability theory, and the effectiveness and robustness of the control scheme are verified through computer simulation results. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of preview sliding mode control for a class of nonlinear uncertain discrete-time singular systems with time-delays. A difference operator approach is used to deal with the controlled system such that the derived augmented systems including preview information of reference signal can be constructed. A suitable surface is proposed in the sense of admissibility by the free-weighed matrix technique. Based on this, a sliding mode controller with preview information is constructed incorporating with the tracking error to ensure that the closed-loop control system reaches the designed sliding mode surface within finite time. To apply the proposed control scheme to the practical engineering systems, the sliding mode tracking controller with preview information is developed for a DC motor based on the proposed control approach. The comparative experiment results demonstrate that the preview information of reference signal can improve the tracking performance. [ABSTRACT FROM AUTHOR]

This paper investigates nonlinear robust fault-tolerant control for turbofan engines with actuator faults. The nonlinear robust fault-tolerant control method is proposed in this paper, which can guarantee the fault tolerance and robustness of turbofan engines when faults and disturbances occur. The design method is implemented in three steps: Firstly, the dynamic effects caused by actuator faults on turbofan engines are analyzed and then the multivariable polynomial state-space model of turbofan engines with actuator faults is proposed. Secondly, appropriate design parameters are chosen according to fault information. Thirdly, the nonlinear robust fault-tolerant control law for turbofan engines with actuator faults is designed by solving an optimization problem. In addition, stability and L 2 gain-like for turbofan engines with bounded disturbances and actuator faults are guaranteed by the proposed nonlinear robust fault-tolerant control method. The distinctive characteristic of the paper is that the nonlinear robust fault-tolerant control method for turbofan engines against actuator faults is proposed which can relax the accuracy requirements of the fault diagnosis subsystem and improve the fault-tolerant performance of the closed-loop control system. Simulations show that the nonlinear robust fault-tolerant control method has better control performances including transient responses, disturbance rejection, and fault tolerance for the turbofan engine with actuator faults. [ABSTRACT FROM AUTHOR]

In this paper, trajectory tracking control of an underwater vehicle in three-dimensional (3D) space has been addressed. The assumed underwater vehicle has 6 degrees of freedom and the aim is to control all system rotations and displacements. In this paper, a finite-time sliding mode controller as a robust control method is proposed for an underwater vehicle with 6 degrees of freedom in 3D space using the method without simplifications or decouplings. Therefore, both system positions and orientations are controlled in the presence of disturbances and uncertainties. In previous research works, control of two-dimensional underwater vehicles is commonly studied. In this paper, a novel stable control algorithm is proposed for an underwater vehicle with 6 degrees of freedom. The stability of the closed-loop system is analyzed using the Lyapunov theory. The designed algorithm can cover 3D complicated tasks. Also, the designed algorithm as a robust control approach can attenuate external disturbances. The performance and stability of this approach are compared with the sliding mode controller. The numerical comparison results show that the proposed approach is effective and applicable in practice. [ABSTRACT FROM AUTHOR]

In this paper, an optimal robust adaptive fuzzy backstepping control is presented to the position control of the electro-hydraulic servo (EHS) system in the presence of structured and unstructured uncertainties. Initially, the robust control using the backstepping technique is presented to overcome the existing uncertainties in the dynamic equations. Mathematical proof demonstrates that the closed-loop system in the presence of uncertainties has a global asymptotic stability. Then, to overcome the chattering problem, a very simple fuzzy approximator is presented where it approximates the bounds of the uncertainties. Although the proposed robust fuzzy backstepping control has a desirable performance, it has no mathematical analysis to prove the stability of the closed-loop system. Therefore, to solve this problem, the proposed fuzzy approximator has been transformed into a one-law adaptive fuzzy approximator with a single-input single-output fuzzy rule. Mathematical analysis illustrates that the closed-loop system in the presence of uncertainties has a global asymptotic stability under the proposed robust adaptive fuzzy backstepping control. Furthermore, a novel modified harmony search algorithm (MHSA) has been developed, by using the original harmony search algorithm (OHSA) as an optimization technique, to achieve the optimal values of the membership functions and the control coefficients. Finally, a comparative study has been conducted between the proposed control scheme under the MHSA and the OHSA, and other existing advanced control approaches to verify the effectiveness of the proposed control. Results show that the proposed control scheme under the MHSA can suppress the chattering problem and reduce the disturbances effectively while ensuring that the performance is tracked. [ABSTRACT FROM AUTHOR]

This paper presents a novel robust adaptive dual layer sliding mode control (ADLSMC) for the problem of high accuracy tracking trajectory of robot manipulator in the presence of uncertainties and external disturbances. This new control scheme has a dual layer structure. The first layer drives the robot manipulator system reaches the global nonlinear sliding surface in finite time, and the second layer tackles the values of the two control gains overestimation problem. Moreover, compared with the traditional super-twisting with time delay estimation algorithm, the proposed controller can realize not only set-point tracking but also dynamic tracking, which is very widely used in practice. The stability of the closed–loop system and the finite time convergence are analyzed using Lyapunov techniques. The effectiveness of the proposed method is demonstrated by simulations and experimental studies. [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]

This paper is concerned with the problem of mode-dependent robust and non-fragile finite-time H ∞ control for a class of nonlinear singular Markovian jump systems (NSMJSs) with parameter uncertainties and time-varying norm-bounded disturbance. Some sufficient conditions ensuring the singular stochastic H ∞ finite-time boundedness (SS H ∞ FTB) are developed for the given system by using the stochastic analysis and linear matrix inequality techniques. Then, a finite-time H ∞ state feedback controller is designed, which can guarantee the H ∞ finite-time boundedness of the closed-loop systems. Furthermore, a robust and non-fragile finite-time H ∞ state feedback controller is also provided to ensure the H ∞ finite-time boundedness of the closed-loop systems when the controller gain has an additive perturbation. Finally, two numerical examples are given to illustrate the effectiveness of the obtained results. [ABSTRACT FROM AUTHOR]

A low-conservative composite nonlinear feedback controller is proposed for singular time-delay systems with time-varying delay. The proposed composite nonlinear feedback controller not only improves the transient responses of the closed-loop system but it also has less conservatism than other composite nonlinear feedback controllers. The gain of the linear part of the composite nonlinear feedback controller is obtained by precise mathematical calculation to depend not only on the upper bound of the delay but also on the delay range and rate of its changes. More advantages of the proposed composite nonlinear feedback controller are its accurate operation in the presence of actuator saturation, model uncertainties, and system singularities. The linear and nonlinear parts of the proposed controller are designed by solving a linear matrix inequality problem confirmed through a theorem using Lyapunov stability analysis. The theoretical achievements are endorsed by computer simulation through numerical and practical examples. [ABSTRACT FROM AUTHOR]

In this paper, the preview tracking control problem for Lipschitz nonlinear system, where future reference signals over a finite horizon can be previewed. First, an augmented error system including previewed information is constructed, which transforms a preview tracking control problem into a regulation problem. Furthermore, sufficient conditions on polytopic nonlinear systems, which guarantee the corresponding closed-loop system to be asymptotically stable, are derived by employing parameter-dependent Lyapunov function. A linear matrix inequality approach for designing preview controllers in state feedback and output feedback settings is presented. Finally, two numerical examples are provided to demonstrate the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

This paper is concerned with the H ∞ control problem for Lipschitz nonlinear systems in the finite-frequency domain. Both parameter uncertainties and external disturbances are considered. In contrast to existing full-frequency methods, the proposed approach takes into account of the frequency information of disturbances during the design proceeding. Sufficient analysis conditions for the closed-loop system are firstly derived. Then, synthesis conditions are formulated in terms of linear matrix inequalities (LMIs). In fact, the control gain is designed to attenuate the influence of disturbances in different frequency ranges (low, middle and high). Finally, the model of the Chua circuit is used to validate the effectiveness of the proposed finite-frequency approach and to prove its superiority compared to the full-frequency counterpart. [ABSTRACT FROM AUTHOR]

This paper studies a high-accuracy motion control method named output feedback adaptive robust control for a dc motor with uncertain nonlinearities and parametric uncertainties, which always exist in physical servo systems and deteriorate their tracking performance. As only position signal is measurable, a uniform robust exact differentiator (URED) for the unmeasurable states and adaptive control for the parametric uncertainties are integrated in the model compensation term; and the robust control is applied to handle uncertain nonlinearities and stabilize the system. Then, the stability of the closed-loop system is proved theoretically. Finally, simulation and experimental results are studied for a dc motor system to prove the control performance of the proposed control method. [ABSTRACT FROM AUTHOR]

Presented in this research paper is an integrated direct yaw moment control (DYC) and active front steering (AFS) for an uncertain vehicle lateral dynamics model considering network-induced communication delay, which is a time-varying continuous function with a known upper bound. Firstly, we consider tire cornering stiffness as a non-linear norm-bounded uncertain system that is modeled by fuzzy membership functions, and then vehicle lateral dynamics model is expressed by a set of linear Takagi-Sugeno (T-S) uncertain fuzzy models. Secondly, since the network-induced communication delay in vehicle control system is an inherent reason for stability and performance degradation, we derive a robust delay-dependent H ∞ control methodology via the Lyapunov-Krasovskii functional for stability and performance conditions of the closed-loop system. For the synthesis, the robust control method is employed within the T-S fuzzy-model-based analysis framework and formulations are performed based on the solution of delay-dependent linear matrix inequalities (LMIs). The simulation study is presented using MATLAB/Simulink to show the achieved improvements in tracking variables via the designed robust fuzzy H ∞ state-feedback controller. The proposed fuzzy robust delay-dependent controller is compared with a linear robust delay-dependent controller to clearly show the tracking improvements for different road conditions. Moreover, a performance-based analysis is carried out to demonstrate the advantage of the design with respect to different delay values. It is confirmed from the analysis results that the proposed fuzzy controller can successfully stabilize and possess improved tracking performance for vehicle lateral motion control. [ABSTRACT FROM AUTHOR]

The problem of H ∞ control is investigated for turbofan systems with uncertain parameters and multiple disturbances in this paper. Some disturbances with partly known information are described via an external system, and other disturbances are assumed to be L 2 norm bounded. According to the disturbance-observer-based-control (DOBC) method and resilient H ∞ control technique, a robust resilient controller is designed to reject and attenuate the influence of these disturbances, and guarantees that closed-loop systems are asymptotically stable with H ∞ performance. Some solvable sufficient conditions are obtained based on the linear matrix inequality (LMI) technique and Lyapunov stability theory. Finally, a simulation is presented to show the robustness and effectiveness of the developed resilient anti-disturbance H ∞ control method. [ABSTRACT FROM AUTHOR]

The complexity of the closed-loop system, short transient response time, and fast synchronization error convergence rates are the three basic parameters that limit hacking in the data encryption and secure the communication systems. This paper addresses the following two challenges: The full-order synchronization (FOS) of two parametrically excited second-order nonlinear pendulum (PENP) chaotic systems with uncertain parameters. The reduced-order synchronization (ROS) between the canonical projection part of an uncertain third-order chaotic Rossler and the uncertain PENP systems. This article designs a new robust adaptive synchronization control (RASC) algorithm to address the above two challenges. The proposed controller achieves the FOS and ROS in a shorter transient time, and the synchronization error signals converge to the origin with faster rates in the presence of bounded unknown state-dependent and time-dependent disturbances. The Lyapunov direct method verifies this convergence behavior. The paper provides parameters updated laws that confirm the convergence of the uncertain parameters to some fixed values. The controller does not cancel the nonlinear terms of the plant; this property of the controller keeps the nonlinear terms in the closed-loop that results in the enhanced complexity of the dynamical system. The proposed RASC strategy is successful in synthesizing oscillation free convergence of the synchronization error signals to the origin for reducing the transient time and guarantees the asymptotic stability at the origin. The simulation results endorse the theoretical findings. [ABSTRACT FROM AUTHOR]

This paper presents a disturbance observer-based robust optimal flight control strategy for near space vehicle (NSV) attitude system with external time-varying disturbance generated by an exogenous system. For the purpose of eliminating the effect of the disturbance, nonlinear disturbance observer (NDO) technique is used and the disturbance estimation error is guaranteed to be globally exponential convergence. Then, based on the disturbance estimation result and desired trajectory signal, a steady state control input is presented and the optimal tracking problem of original system with external disturbance can be converted into the optimal regulation problem of a nominal error system. Furthermore, a single network-based adaptive dynamic programming (ADP) method is applied to obtain the corresponding optimal feedback control law. Finally, all the signals in closed-loop system are proved to be uniformly ultimately bounded (UUB) and the tracking error can converge to a sufficiently small bound. Simulation tests about NSV attitude system are given to verify the effectiveness of proposed robust optimal flight control scheme. [ABSTRACT FROM AUTHOR]

This work presents a novel robust control design approach for missile longitudinal autopilot under multiple disturbances and uncertainties. The uncertainties and disturbances of the missile dynamics are treated as a lumped disturbance based on the concept of equivalent input disturbance. Then a generalized extended state observer is employed to estimate the system states and the equivalent input disturbance in an integrated manner. These estimates are used to construct the state-feedback controller as well as to attenuate the effect of the exogenous disturbances and endogenous uncertainties. The state-feedback controller is obtained by solving the linear matrix inequalities of mixed H 2 / H ∞ control problem, which provides an impressive flexibility to tune the controller to compromise between H 2 performance and H ∞ performance. Closed-loop stability of the system under the presented controller-observer structure is also established. The proposed design tactfully circumvents the engineering implementation problems encountered by mixed H 2 / H ∞ control, achieves strong robustness against disturbances and uncertainties, and does not involve any complicated nonlinear control methodologies. Numerical simulation results of nominal and perturbed performance comparisons with classic methods sufficiently demonstrate the feasibility and robustness of the proposed method. [ABSTRACT FROM AUTHOR]

Complex-order differintegral (COD) is the extended version of fractional-order one in which the differintegral order can be a complex number rather than a real number. In comparison with fractional-order differintegral (FOD), the distinguishing feature of the COD is that the phase slope of its Bode diagram is a function of imaginary part of the complex order of the COD. In this paper, by the use of this property of the COD, a robust control system is proposed. The design procedure and the realization of the proposed COD-based closed-loop control system are discussed. Since the phase of COD's frequency response is a nonsymmetric function of frequency, stability analysis of the proposed control system is considered a problematic task. It is proven that for the stability of the control system, it is essential that the COD be applied in a limited frequency band that is derived by the use of the Nyquist stability criterion. Finally, some numerical examples are given to demonstrate the validity and superiority of the proposed complex-order control system. [ABSTRACT FROM AUTHOR]

This paper focuses on the problem of reliable finite–time stochastic stability (FTSS) for uncertain networked control systems (NCSs). A Markovian jump system (MJS) model with partly unknown transition probabilities (TPs) for the NCSs with random delays, data packet dropouts (disorders as well) and stochastic actuator faults is established to describe the closed–loop system. A mode-dependent static output feedback controller is designed taking only the measured outputs into account. A new criterion is also derived in terms of linear matrix inequalities (LMIs) to ensure reliable FTSS of the closed–loop system, based on the stochastic stability theory. Simulation studies on a benchmark numerical example, as well as an unstable numerical example can verify the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This paper investigates the problem of designing a non-fragile polynomial fuzzy controller for a class of polynomial fuzzy models subjected to persistent bounded disturbances and system uncertainty. The proposed controller is considered to satisfy the input saturation constraint. Furthermore, the resilient controller is affected by linear fractional uncertainties. The overall structure of the controller is obtained by the polynomial parallel distributed compensation concept. The sufficient conditions guarantee an L1 performance of the perturbed system and exponential stability of the non-perturbed system is achieved in terms of sum-of-squares decomposition conditions. Subsequently, the sum-of-squares-based conditions lead to minimization of the L1 performance such that the above-mentioned hard constraints on the control input and robustness of the closed-loop system against the system and controller uncertainties will be guaranteed. The controller gain matrices will be achieved by numerically solving the sum-of-squares-based conditions with the third-party MATLAB toolbox 'SOSTOOLS'. Finally, extensive studies and hardware-in-the-loop simulations on a ball-and-beam system are presented, which illustrate the proposed controller can accurately and smoothly track the set point frequency. Furthermore, the proposed control approach is more robust over prior-art controllers for all the simulation scenarios. [ABSTRACT FROM AUTHOR]

Sloshing is a complex nonlinear dynamic phenomenon which has a significant influence on the stability of structure–fluid systems. In this study, the dynamic equation of sloshing based on Hamilton principle is established and linearized into a state space equation. Considering the uncertainty of the system, a robust H infinite guaranteed cost control method is proposed to mitigate the response of fluid wave height to horizontal acceleration of the tank body. Simulation results are given to demonstrate the closed-loop performance of the nonlinear dynamic modeling and linear optimal control method. [ABSTRACT FROM AUTHOR]

In this paper, finite-time robust control problems are considered for a class of uncertain systems subject to exogenous disturbances, and an input–output finite-time stabilization (IO-FTS) method is presented with finite-time boundedness (FTB) constraints. For an uncertain system, a state feedback controller is designed, via linear matrix inequalities (LMIs), to ensure the IO-FTS and FTB of the closed-loop system. By using Lyapunov stability theory, sufficient conditions for IO-FTS and FTB are given. These conditions are expressed as a feasibility problem of a set of LMIs. The direct current motor control problem and roll autopilot design of a skid-to-turn missile are considered for a case study. Simulation results are presented to show the effectiveness of the proposed method as a promising approach to controlling similar uncertain systems. [ABSTRACT FROM AUTHOR]

This paper is concerned with the problem of observer design for a class of nonlinear systems with time-varying delay and bounded disturbances. For the stabilization problem, attention is focused on the design of a quantized observer that ensures stability of the closed-loop system. For the robust H∞ control problem, a quantized observer is designed such that, in addition to the requirement of the robust stability, a prescribed disturbance attenuation level also needs to be achieved. The nonlinearity is assumed to satisfy the local Lipschitz condition. A more general case is considered, differing from the previous results where the Lipschitz constant is fixed and predetermined. Finally, a numerical example is provided to show the effectiveness of our method. [ABSTRACT FROM AUTHOR]

This paper points out some technical errors in a recent paper that appeared in Transactions of the Institute of Measurement and Control entitled ‘An LMI approach to non-fragile robust optimal guaranteed cost control of uncertain 2-D discrete systems with both state and input delays’ by Akshata Tandon and Amit Dhawan (http://dx.doi.org/10.1177/0142331216667476). We reveal that the upper bound of the closed-loop cost function provided by their Lemma 4 is erroneous. Some critical issues associated with the system initial conditions assumed in their paper are highlighted. The closed-loop cost bound claimed by their Theorem 1 is found to be incorrect. The optimization problem formulated in their Theorem 2 for the selection of an optimal guaranteed cost controller is erroneous. Finally, the corrections over their results are made available. [ABSTRACT FROM AUTHOR]

µ-control synthesis offers the advantage of maintaining robust performance and stability in the presence of plant uncertainties. Expressions relating the 2-norm of the vector of regulated outputs and closed-loop modal damping to the weights used for µ-control synthesis are derived in this paper. By using these expressions, the amount of damping added to each mode can be controlled individually while maintaining equal importance for each controlled mode. Hence, the selection of weights for synthesizing a suitable µ-controller is greatly simplified. Furthermore, a comparative study of µ-controllers designed by the proposed procedure and positive position feedback (PPF) controllers designed by analytically derived optimal parameters is also presented. The techniques are compared for their vibration attenuation, energy utilization, and stability characteristics, in the presence of parametric and unmodeled dynamics uncertainties. Results for both time domain and frequency domain simulations are presented. As in some cases (sandwich plate structures etc.) the noncollocation of actuator/sensor becomes crucial for achieving good performance, the above-mentioned characteristics are evaluated for both collocated and noncollocated sensor locations. [ABSTRACT FROM AUTHOR]

In this paper, an adaptive robust controller with uniform robust exact differentiator has been proposed for a class of nonlinear systems with structured and unstructured uncertainties. The adaptive robust controller is integrated with an uniform robust differentiator to handle the problem of the incalculable part of the derivative of virtual controls and the differential explosion happened in backstepping techniques. The stability of the closed loop system is demonstrated via Lyapunov method ensuring a prescribed transient and tracking performance. Simulation and experimental results are carried out to verify the advantages of the proposed method. [ABSTRACT FROM AUTHOR]

A robust nonfragile observer-based controller for a linear time-invariant system with structured uncertainty is introduced. The H∞ robust stability of the closed-loop system is guaranteed by use of the Lyapunov theorem in the presence of undesirable disturbance. For the sake of addressing the fragility problem, independent sets of time-dependent gain-uncertainties are assumed to be existing for the controller and the observer elements. In order to satisfy the arbitrary H2-normed constraints for the control system and to enable automatic determination of the optimal H2/H∞ bound of the performance functions in disturbance rejection control, additional necessary and sufficient conditions are presented in a linear matrix equality/inequality framework. The H2/H∞ observer-based controller is then transformed into an optimization problem of coupled set of linear matrix equalities/inequality that can be solved iteratively by use of numerical software such as Scilab. Finally, concerning the evaluation of the performance of the controller, the control system is implemented in real time on a mechanical system, aiming at vibration suppression. The plant under study is a multi-input single-output clamped-free piezo-laminated smart beam. The nominal mathematical reduced-order model of the beam with piezo-actuators is used to design the proposed controller and then the control system is implemented experimentally on the full-order real-time system. The results show that the closed-loop system has a robust performance in rejecting the disturbance in the presence of the structured uncertainty and in the presence of the unmodeled dynamics. [ABSTRACT FROM AUTHOR]

This paper aims to deal with the problem of fault detection in a closed-loop mode for a three-axis gyro-stabilized camera mount under the consideration of unknown disturbances. First, the influences of potential actuator and sensor faults are analysed and, based on this, the faults model as well as an equivalent additive fault input are introduced. For the purpose of fault detection, an observer-based fault detection filter is considered to generate the residual. The sensitivity of the residual to fault is evaluated by the non-zero singular values of the transfer function from fault to residual, whereas the robustness to disturbance is represented by the H∞-norm of the transfer function from disturbance to residual. To obtain a maximum sensitivity/robustness ratio criterion, the design of fault detection filter is formulated as an Hi/H∞ optimization problem and the unified solutions for the optimization criterion are given. For residual evaluation, a window evaluation function is developed and an analysis of threshold selection is given to reduce the conservativeness of fault detection. Finally, experimental results are presented to demonstrate the effectiveness of the proposed method. It is demonstrated that the proposed method can effectively detect the provided fault as soon as possible. [ABSTRACT FROM AUTHOR]

The robustly resilient memory control problem is addressed for a class of switched systems with time delay under asynchronous switching. By resorting to the piecewise Lyapunov-Krasovskii functional, an asynchronous memory controller is designed to ensure the exponential stability of the closed-loop system. Here, the piecewise Lyapunov-Krasovskii functional means that for the activated subsystem, the associated Lyapunov-Krasovskii functional on the unmatched interval is different from that on the matched interval. Then an asynchronously resilient memory controller is derived to guarantee that the corresponding closed-loop system is robustly exponentially stable for all the admissible uncertainties. All the conditions are cast into the form of linear matrix inequalities. Finally, a numerical example is provided to illustrate the validity of the proposed results. [ABSTRACT FROM AUTHOR]

In this work, the Uncertainty and Disturbance Estimation (UDE) approach is employed for the design of a robust flight controller for high performance aircraft. The UDE estimated uncertainty is used to robustify an input–output linearization-based controller designed for nominal system. The UDE theory employing a first-order filter and α-filter is used for the estimation of the composite uncertainty. To address the issue of output derivatives, an observer which too employs the UDE estimated uncertainties for the purpose of robustness is designed. Closed-loop stability for the overall controller-observer system is established. Uncertainties and disturbances in terms of parametric variations, wind gust, unmodeled actuator dynamics and measurement noise are considered to evaluate and compare performances of the two filter-based designs. Further, a performance comparison with some of the existing designs is carried out and the results are presented to demonstrate the efficacy of the proposed work. A notable feature of the proposed design is that the approach neither requires an accurate plant model nor any information about the uncertainties and disturbances. [ABSTRACT FROM AUTHOR]

This paper presents simulation and real-time implementation of input-shaping schemes with a distributed delay for control of a gantry crane. Both open-loop and closed-loop input-shaping schemes are considered. Zero vibration and zero vibration derivative input shapers are designed for performance comparison in terms of trolley position response and level of sway reduction. Simulation and experimental results have shown that all the shapers are able to reduce payload sway significantly while maintaining satisfactory position response. Investigations with different cable lengths that correspond to ±20% changes in the sway frequency have shown the distributed delay-based shaper has asymmetric robustness behaviour. The shaper provides highest robustness for the case of 20% increase in the sway frequency but lower robustness for the case of 20% decrease. However, other schemes give symmetric robustness behaviour for both cases. [ABSTRACT FROM AUTHOR]

This paper presents a robust constraint backstepping control scheme for high performance aircraft attitude tracking in the presence of physical constraints, model uncertainties and unsteady effects. The six-degree-of-freedom aircraft model with unsteady aerodynamics is first established, of which the characteristics are investigated through open-loop analysis. Based on the immersion and invariance approach, a state observer is developed to estimate the unmeasurable unsteady aerodynamic states. The unsteady effects are then compensated in the controller design where the bounds of estimate errors and model uncertainties are used to increase robustness. In addition, a command filter is introduced to impose physical constraints on states and control inputs and overcome the ‘explosion of terms’ problem. An auxiliary system of which the states are applied to the controller design is constructed as well to evaluate the constraint effects. Furthermore, stability of the closed-loop system and convergence of the tracking error are proven by Lyapunov stability theorem. Finally, several numerical simulations are performed to verify the effectiveness and robustness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

A sliding mode control (SMC) strategy is proposed for a second-order non-linear system using fractional calculus and adaptive fuzzy compensator, named fuzzy fractional-order SMC (FFOSMC). The major contribution of this research includes the formulation of a robust fractional-order controller based on a novel non-integer sliding manifold. The proposed controller utilizes reduced discontinuous switching gain using a fuzzy logic system. A fractional-order adaptation law is derived to update the fuzzy logic system, which has more degrees of freedom to achieve the desired performance. The stability of the closed-loop system is guaranteed using fractional-order Lyapunov theory. The parameters of the proposed control method are optimized using Simulink response optimization. Finally, the performance of the FFOSMC is fully investigated and compared with a conventional SMC controller and an integral SMC controller. From simulation results, it is concluded that the proposed algorithm is effective and more robust. [ABSTRACT FROM AUTHOR]

In this article, an algorithm combining two different variance constrained control strategies (i.e. output variance constrained: OVC controller and input variance constrained: IVC controller) is developed. Performance of the resulting novel control technique called as combined output and input variance constrained controller is evaluated on helicopters. Their linearized state-space models are used to see feasibility and efficiency of this new strategy (i.e., output and input variance constrained). Closed loop responses of some outputs and inputs of interest and also some outputs out of interest are investigated while using output and input variance constrained for air vehicle flight control system. Then, closed loop performance of output and input variance constrained is compared with respect to the two other classical control techniques (i.e. outputs variance constrained and input variance constrained). Finally, robustness of output and input variance constrained with respect to the modeling uncertainities is examined. [ABSTRACT FROM AUTHOR]

This paper addresses the performance recovery of explicit model predictive control (eMPC) when states of the system are not available via measurement. It is shown that the performance of the closed-loop system under the eMPC controller can be degraded when using a state observer instead of measurement. To moderate this degradation, an optimization-based approach is proposed that enables the designer to systematically design an optimal observer. It is proved that the proposed approach leads to the maximum achievable performance (in some sense) when compared to the desired performance (i.e. when states are available via measurement). Simulation results illustrate the performance and effectiveness of the proposed approach compared to a set of observers designed using a robust pole-placement approach. [ABSTRACT FROM AUTHOR]

This paper addresses the problem of platoon control. The stability and the string stability of a platoon of vehicles in the simultaneous presence of the time delay and the lag are investigated. The contribution of this paper is twofold. First, the paradigm of the cluster treatment of characteristic roots is utilized in order to reveal the stabilizing parametric regions in the domain of the delays to render the stability of a closed-loop system. Second, the restrictions which need to be imposed on the control parameters are considered in order to make the vehicle behave monotonically in the forward direction without collision as well as the requirement for the string stability. Finally, an example of multiple-vehicle platoon control is presented, which demonstrates the effectiveness and the robustness of the proposed method. [ABSTRACT FROM PUBLISHER]

Self-sensing actuation has been extensively used in vibration control of flexible structures over the last three decades. Positive position feedback controller has been commonly used in this field due to the robustness against spillover phenomena. Piezoelectric clamped capacitance is the most important parameter that affects the performance of this technique. In this study, the effect of capacitance change on performance and stability of a self-sensing system with positive position feedback controller is investigated. Based on this analysis, some modifications are suggested to increase the stability of the closed-loop system against capacitance change. An online Fourier transform–based capacitance measurement method is used, which guarantees good performance and stability of closed-loop system in the presence of capacitance change. Experimental results are also presented to show the effectiveness of this method in vibration control of cantilever beam. [ABSTRACT FROM PUBLISHER]

A large number of applications in industrial domains are concerned to slow processes, including flow, pressure, temperature and level control. Synthesis and control of such applications in the presence of uncertainty have been studied by many authors in recent decades. In this paper, a robust flow controller has been designed using H∞ techniques to define and regulate the flow of pipe. The most robust method is synthesized using a state-space approach by taking into account the structured (parametric) perturbations in the plant coefficients. The proposed approach ensures internal stability, satisfying both frequency and time domain requirements, and obtaining minimal performance H∞-norm of the closed-loop system. The simulations are carried out using MATLAB and the results are compared with the classical PID, linear–quadratic–Gaussian and H2 methods, and they indicate that the overall system output performance can be improved using the proposed H∞ robust controller. [ABSTRACT FROM AUTHOR]

The design of the autopilot is one of the most important algorithms of missiles. Performance of the autopilot and its robustness are significant matters to hit a target accurately. The autopilot should satisfy the desired performance under disturbances. In the scope of this study, three autopilots were offered for tracking pitch acceleration command using different control methods: three-loop classic control, pole-placement control and receding horizon predictive control. The aim of the autopilot designed by employing receding horizon predictive control is to minimize the flight control effort, and to make the close-loop system insensitive against modelling uncertainties and stochastic shattering factors. This study comes up with that the missile is able to move in desired performance under disturbances such as control surface misplacement, thrust misalignment, wind and aerodynamic uncertainties with more robustness, less control effort and minimum miss distance and terminal time using an alternative control method instead of classic and pole-placement control methods which are generally referred by the defence industry. [ABSTRACT FROM PUBLISHER]

We address stabilization problems of networked control systems in the presence of bounded packet losses and time-varying network-induced delays. When the delay is less than the transmission interval, we prove that the stability of the closed-loop system is implied by that of a switched system. Based on theories on switched systems, we present stabilizing conditions by using packet-loss-dependent Lyapunov functions, and design multiple controllers depending on the packet loss and the network-induced delay. Furthermore, based on the stabilizing conditions, we discuss the robustness of the stabilization against the packet loss and the network-induced delay. Several numerical examples and simulations are worked out to demonstrate the effectiveness of the proposed design approach. [ABSTRACT FROM AUTHOR]

To alleviate mechanical parts fatigue and provide a comfortable ride for passengers, car mechanics require desired dynamic behavior of car suspensions. This paper suggests state feedback controllers that, with system uncertainties, guarantee the closed loop system satisfying the desired pole region, thus achieving satisfactory oscillation damping and settling time, and having the guaranteed cost performance simultaneously. A state feedback controller is applied to the design of an active suspension model for a quarter-car vehicle. The controller's parameters are obtained using the linear matrix inequalities optimization, also allowing considering the model uncertainty represented by a norm-bounded structure. Simulations are carried out taking into account the presence of uncertain parameters. The results are shown to be better than the classical linear quadratic regulator. [ABSTRACT FROM PUBLISHER]

The problem of observer-based finite-time exponential l2 − l∞ control is studied in this paper for a class of discrete-time switched systems with time delay and uncertainties. The observer-based controller is designed to guarantee that the developed closed-loop system is robust finite-time exponential l2 − l∞ stabilizable for all admissible uncertainties. By resorting to the average dwell time approach and Lyapunov–Krasovskii functional technology, some new delay-dependent criteria guaranteeing finite-time boundedness and stabilizability with exponential l2 − l∞ performance are developed. The observer-based finite-time exponential l2 − l∞ controller is also designed by virtue of a cone complement linearization method. Finally, a numerical example is provided to demonstrate the effectiveness of the proposed results. [ABSTRACT FROM AUTHOR]

It is a well known fact that system parameters of flexible structures keep on changing because of several reasons. Ordinary controllers lose their effectiveness in changed situations and do not guarantee the stability of the closed loop (CL) system. However, controllers designed based on robust control theory not only maintain the CL stability of the perturbed system for a large variation in system parameters but also maintain best performance. In this work, H∞ optimization based controllers are designed and implemented experimentally on a flexible beam with active constrained layer damping treatment. It is observed that H∞ loop shaping design procedure based controllers outperform linear quadratic Gaussian and standard H∞controllers both in terms of robust stability and robust performance. Relative merits and demerits of the controllers designed using μ- synthesis technique are also discussed. It has also been observed that time domain results also explain some of the important facts. Certain time domain parameters of the CL system prove the relative superiority of these controllers in terms of control energy utilization and robust performance. [ABSTRACT FROM PUBLISHER]

In this article, a new design based on the extended state observer (ESO) technique for the pitch autopilot of a tail-controlled, skid-to-turn missile is proposed. The pitch-plane dynamics with the angle of attack as output is significantly uncertain. The ESO simultaneously estimates the state and the uncertainty. The estimated uncertainty is used to robustify the input–output–linearization-based controller designed for the nominal system. Closed-loop stability of the observer–controller combination is proved. The notable feature of the proposed design is that it neither requires accurate plant model nor any information about the uncertainty. The effectiveness of the ESO in estimation of the uncertainties and states and in meeting the specified tracking performance in the presence of significant uncertainties is illustrated by simulation. Finally, to demonstrate the efficacy of the proposed design, comparison of its performance with some of the well-known existing controllers is presented. [ABSTRACT FROM PUBLISHER]

The applicability of the basic relay tuning method is extended by a robust on-line method to tune proportional–integral controllers without breaking the closed-loop control. A relay is connected in parallel to the controller to induce self-oscillation and then simple measurements are made only on the half period of limit cycle output to obtain a first-order plus dead time model of the process dynamics. A non-iterative optimum tuning formulae is developed in order to reduce the control signal variations. The simulation and experimental studies are presented to validate the design method. [ABSTRACT FROM PUBLISHER]