Control of discrete-time systems with actuator non-linearities: An LMI approach

This paper considers the problem of stabilization of discrete-time systems with actuator non-linearities. Specifically, full-state feedback and dynamic, output feedback control designs for discrete-time systems with time-varying, sector-bounded, input non-linearities are addressed. The proposed framework is based on a linear matrix inequality approach and directly accounts for robust stability and robust performance over the class of actuator non-linearities. Furthermore, it is directly applicable to actuator saturation control and provides state feedback and dynamic, output feedback controllers with guaranteed domains of attraction. The effectiveness of the approach is illustrated by two numerical examples.     

Fixed-architecture controller synthesis for systems with input–output time-varying nonlinearities

In this paper we develop a fixed-architecture controller analysis and synthesis framework that addresses the problem of multivariable linear time-invariant systems subject to plant input and plant output time-varying nonlinearities while accounting for robust stability and robust performance over the allowable class of nonlinearities. The proposed framework is based on the classical Luré problem and the related Aizerman conjecture concerning the stability of a feedback loop involving a sector-bounded nonlinearity. Specifically, we extend the classical notions of absolute stability theory to guarantee closed-loop stability of multivariable systems in the presence of input nonlinearities. In order to capture closed-loop system performance we also consider the minimization of a quadratic performance criterion over the allowable class of input nonlinearities. Our approach is directly applicable to systems with saturating actuators and provides full and reduced-order dynamic compensators with a guaranteed domain of attraction. The principal result is a set of constructive sufficient conditions for absolute stabilization characterized via a coupled system of algebraic Riccati and Lyapunov equations. The effectiveness of design approach is illustrated by several numerical examples. © 1997 John Wiley & Sons, Ltd.     

Quadratic stability in the circle theorem or positivity theorem

In this technical note, we study the quadratic stability problem for linear time-invariant multi-input, multi-output systems which have time-varying sector-bounded or positive real uncertainty feedback. It is known that the circle theorem and positivity theorem are sufficient conditions for robust stability, for sector-bounded and positive real feedback, respectively. We show that the circle theorem is necessary and sufficient condition for quadratic stability when the feedback is sector-bounded, and that the positivity theorem is necessary and sufficient condition for quadratic stability when the feedback is positive-real.     

基于时滞状态反馈的鲁棒容错控制

研究了一类线性不确定时滞系统的鲁棒容错控制问题。针对具有状态滞后,且假定状态和控制输入的不确定项均是范数有界的线性系统,基于Lyapunov稳定性理论和线性矩阵不等式方法,通过引入状态反馈和带时滞的状态反馈,得出了一个此类系统在对执行器失效或传感器失效两种情况下具有鲁棒容错性能的充分条件,并通过求解线性矩阵不等式组得到容错控制器设计结果。数值算例验证了这种控制器的设计方法的有效性和可行性。     

Reconfigurable control of Hammerstein systems after actuator failures: stability,tracking, and performance

A new method for the reconfigurable control of stable Hammerstein systems with sector-bounded static nonlinear input characteristics subject to actuator failures is described. It aims at the recovery of the nominal stability, setpoint tracking, disturbance rejection and performance properties by the reconfigured closed-loop system. This article extends the virtual actuator from linear systems to Hammerstein systems and provides sufficient linear matrix inequality conditions for closed-loop stability, and a corresponding synthesis algorithm. It is shown that the approach is robust against uncertainties of the static input nonlinearity in a small-gain sense, and universal in a certain sense. Feasible setpoints for the reconfigured closed-loop system are characterised, and infeasible setpoints are projected to feasible ones. An extension guarantees minimum performance loss. The method is successfully experimentally evaluated using a system of interconnected tanks.     

Actuator amplitude saturation control for systems with exogenous disturbances

We have developed fixed-order (i.e. full- and reduced-order) controllers for systems with actuator amplitude constraints and exogenous bounded energy L 2 disturbances. The actuator amplitude saturation and disturbance rejection constraints were embedded within an optimization problem by constructing a Riccati equation whose solution guaranteed closed-loop global asymptotic stability in the face of sector-bounded input nonlinearities and non-expansivity (gain boundedness) of the input-output system energy. The efficacy of the proposed framework is demonstrated via a numerical example.     

On robust stability of interval control systems

The authors develop results on the robust stability of a nonlinear control system containing both parametric as well as unstructured uncertainty. The basic system considered is that of the classical Lur'e problem of nonlinear control theory. A robust version of the Lur'e problem consisting of a family of linear time-invariant systems subjected simultaneously to bounded parameter variations and feedback perturbations from a family of sector-bounded nonlinear gains is presently treated. By using the Kharitonov theorem to develop some extremal results on positive realness of interval transfer functions (i.e. a family of rational transfer functions with bounded independent coefficient perturbations), the authors determine the size of a sector of nonlinear feedback gains for which absolute stability can be guaranteed. These calculations amount to the determination of the stability margin of the system under joint parametric and nonlinear feedback perturbations     

Fixed-structure controller design for systems with actuator amplitude and rate non-linearities

In this paper we develop output feedback controllers and fixed-order (i.e., full- and reduced-order) dynamic compensators for systems with actuator amplitude and rate saturation constraints. The proposed design methodology employs a rate limiter as part of the controller architecture. The problem of simultaneous control amplitude and rate saturation is embedded within an optimization problem by constructing a Riccati equation whose solution guarantees closed-loop global/local asymptotic stability in the face of sector bounded actuator amplitude and rate non-linearities. Application of the proposed framework is demonstrated via a flight control example involving actuator amplitude and rate saturation constraints.     

Model reference robust control of a class of SISO systems

A new control design technique, model reference robust control (MRRC), is introduced for a class of SISO systems which contain unknown parameters, possible nonlinear uncertainties, and additive bounded disturbances. The design methodology is a natural, nontrivial extension of model reference adaptive control (MRAC) which is essential to achieving robust stability and performance for linear time-invariant systems. The methodology also represents an important step toward achieving robust stability for time-varying and nonlinear systems. MRRC requires only input and output measurements of the system, rather than the full state feedback and structural conditions on uncertainties required by existing robust control results. MRRC is developed from existing model reference control (MRC) in a manner similar to MRAC. An intermediate result gives conditions under which MRRC yields exponentially asymptotic stability. The general result yielding uniformly ultimately bounded stability is then developed. A scalar example provides intuition into why the control works against a wide class of uncertainties and reveals the implicit learning capability of MRRC     

Anti-windup controller design using linear parameter-varying control methods

In this paper, we seek to provide a systematic anti-windup control synthesis approach for systems with actuator saturation within a linear parameter-varying (LPV) design framework. The closed-loop induced L2 gain control problem is considered. Different from conventional two-step anti-windup design approaches, the proposed scheme directly utilizes saturation indicator parameters to schedule accordingly the parameter-varying controller. Hence, the synthesis conditions are formulated in terms of linear matrix inequalities (LMIs) that can be solved very efficiently. The resulting gain-scheduled controller is non-linear in general and would lead to graceful performance degradation in the presence of actuator saturation non-linearities and linear performance recovery. An aircraft longitudinal dynamics control problem with two input saturation non-linearities is used to demonstrate the effectiveness of the proposed LPV anti-windup scheme.     

变采样网络控制系统的鲁棒控制

对于线性时不变控制对象,在控制器和控制对象都采用时间-事件驱动时系统就变成便采样网络控制系统,当网络时延不确定时,在小于或者等于一个变采样周期时,基于动态输出反馈对变采样网络控制系统进行建模,使用李雅普诺夫方法和线性矩阵不等式研究了系统的鲁棒稳定性,并设计了鲁棒控制器,最后给出实例证明在鲁棒控制器的控制下系统稳定。     

On matching conditions for adaptive state tracking control ofsystems with actuator failures

In this paper, new necessary and sufficient conditions are derived for actuator failure compensation for linear time-invariant systems with actuator failures characterized by unknown input signals at some unknown fixed values and time instants, for state tracking with state feedback. It is shown that the number of fully functional actuators is crucial in determining the actuation range, which specifies the compensation design conditions in terms of system actuation structure. Such conditions are required for both a nominal design using system knowledge and an adaptive design without system knowledge. An adaptive actuator failure compensation control scheme based on relaxed system actuation conditions is developed for systems with unknown dynamic parameters and actuator failure parameters including failure values, times, and patterns. Simulation results are presented to verify the desired system performance with failure compensation     

多变量反馈控制系统的完整性分析

以线性时不变多输入多输出反馈控制系统为研究对象，建立了基于ｋ个传感器和ｎ个执行器失效的故障系统数学模型，并提出了仍能保证系统稳定性的控制器必须满足的条件，从而为设计具有容错能力的控制系统提供了理论依据。     

考虑执行器动态和输入受限的近空间飞行器鲁棒可重构跟踪控制

针对多变量、不稳定的近空间飞行器姿态系统,在系统存在参数不确定和外部干扰的情况下,并考虑执行器动态和输入受限,提出一种鲁棒可重构跟踪控制策略.首先,利用二阶滑模干扰观测器分别重构姿态、角速率回路的复合干扰;其次,采用鲁棒二阶滑模积分滤波器的反推(backstepping)方法避免了控制器设计中微分项膨胀问题,利用鲁棒项抵消重构误差对系统的影响,以实现姿态控制器设计.然后,在考虑执行器动态、输入受限及舵面卡死故障下,给出一种线性矩阵不等式的在线优化舵面分配算法,以实现飞行器的姿态角渐近跟踪期望的制导指令.最后,仿真结果表明所提出的方法具有良好的跟踪控制性能.     

Guaranteed domains of attraction for multivariable Luré systems via open Lyapunov surfaces

In this paper we provide guaranteed stability regions for multivariable Luré-type systems. Specifically, using the Luré–Postnikov Lyapunov function a guaranteed subset of the domain of attraction for a feedback system whose forward path contains a dynamic linear time-invariant system and whose feedback path contains multiple sector-bounded time-invariant memoryless nonlinearities is constructed via open Lyapunov surfaces. It is shown that the use of open Lyapunov surfaces yields a considerable improvement over closed Lyapunov surfaces in estimating the domain of attraction of the zero solution of the nonlinear system. An immediate application of this result is the computation of transient stability regions for multimachine power systems and computation of stability regions of anti-windup controllers for systems subject to input saturation. © 1997 by John Wiley & Sons, Ltd.     

Robust fault-tolerant controller design for linear time-invariant systems with actuator failures: an indirect adaptive method

In this paper, indirect adaptive state feedback control schemes are developed to solve the robust faulttolerant control (FTC) design problem of actuator fault and perturbation compensations for linear time-invariant systems. A more general and practical model of actuator faults is presented. While both eventual faults on actuators and perturbations are unknown, the adaptive schemes are addressed to estimate the lower and upper bounds of actuator-stuck faults and perturbations online, as well as to estimate control effectiveness on actuators. Thus, on the basis of the information from adaptive schemes, an adaptive robust state feed-back controller is designed to compensate the effects of faults and perturbations automatically. According to Lyapunov stability theory, it is shown that the robust adaptive closed-loop systems can be ensured to be asymptotically stable under the influence of actuator faults and bounded perturbations. An example is provided to further illustrate the fault compensation effectiveness.     

Robust fault-tolerant controller design for linear time-invariant systems with actuator failures: an indirect adaptive method

In this paper, indirect adaptive state feedback control schemes are developed to solve the robust faulttolerant control (FTC) design problem of actuator fault and perturbation compensations for linear time-invariant systems. A more general and practical model of actuator faults is presented. While both eventual faults on actuators and perturbations are unknown, the adaptive schemes are addressed to estimate the lower and upper bounds of actuator-stuck faults and perturbations online, as well as to estimate control effectiveness on actuators. Thus, on the basis of the information from adaptive schemes, an adaptive robust state feed-back controller is designed to compensate the effects of faults and perturbations automatically. According to Lyapunov stability theory, it is shown that the robust adaptive closed-loop systems can be ensured to be asymptotically stable under the influence of actuator faults and bounded perturbations. An example is provided to further illustrate the fault compensation effectiveness.     

Extensions to the design of sampled integrating regulators

The stability properties and design rules for a class of robust integrating regulators are considered. Passivity arguments are employed to define bounds on the parameters of an extended PID regulator and to establish the robustness of the closed-loop stability properties in the presence of sector-bounded non-linear elements in the feedback loop. This approach allows linear and non-linear single-input/single-output and multi-input/multi-oulput systems to be considered in the same framework.     

Robust controller synthesis for systems with input nonlinearities: a trade-off between gain variation and parametric uncertainty

A feedback control-system design problem involving input nonlinearities and structured plant parameter uncertainities is considered. Multivariable absolute stability theory is merged with the guaranteed cost control approach to robust stability and performance to obtain a theory of full- and reduced-order robust control design that accounts for input time-varying sector bounded nonlinearities. The principal result is a sufficient condition for characterizing dynamic controllers of a fixed dimension which are guaranteed to provide robust stability for plant parametric variations and absolute stabilization for input nonlinearities. The proposed framework provides a systematic design trade-off between classical robustness guarantees (i.e., gain and phase margins) versus parametric robustness. Furthermore, the framework is directly applicable to uncertain systems with saturating controls and provides fixed-order dynamic output feedback controllers with guaranteed domains of attraction. © 1998 John Wiley & Sons, Ltd.     

Stability of non-linear QFT designs based on robust absolute stability criteria

The paper is mainly devoted to the robust stability problem of non-linear QFT designs. The problem is first formulated for SISO non-linear systems, limited in practice to linear-memoryless sector-bounded non-linear interconnections, and in an I/O stability sense. The work investigates several possible robust adaptations of the Circle Criterion, depending on the type of resulting interconnection of linear and non-linear blocks, appearing in the feedback system. Also, the use of the Popov Criterion is investigated as an alternative less conservative than the Circle Criterion in some cases. The proposed techniques are given in usual QFT language, expressed as frequency conditions or boundaries in the Nichols Chart, allowing an easy integration with other design objectives. In addition, multivariable extensions using a conicity condition and the concept of maximal cone are adopted to give stability boundaries in interconnections of SIMO linear-MISO sector-bounded memoryless blocks. All the robust stability criteria are illustrated using significant examples to emphasize the practical application of the resulting techniques.