Robust Stability of Networked Load Frequency Control Systems with Time-Varying Delays

In deregulated power system scenario coupled with smart grid technology, for networked load frequency control (LFC), an open communication structure, owing to low cost and flexibility, is preferred over dedicated networks in the feedback control loop for transmitting/receiving the data between the geographically displaced power system and the control center. In such a control scheme, closing the feedback loop through an open communication channel, in turn, introduces two additive time-varying delays of dissimilar characteristics in the feedback path. These delays degrade the performance of the closed-loop system, and exert a destabilizing effect on the overall system. In this paper, using Lyapunov-Krasovskii functional approach, a less conservative stability criterion is presented to ascertain delay-dependent stability of such network-controlled LFC systems with two additive time-varying delays in the feedback path in the presence of uncertain load disturbance conditions. Unlike the existing results, which are derived by combining these two delays into one, the proposed result considers the two delays as separate entities thereby imparting more generalization into the stability analysis. The effect of unknown exogenous load disturbance is incorporated by mathematically modeling them as a bounded non-linear time-varying function of current and delayed state vectors.     

一种提高全向水平极化天线带宽的设计

设计了一种新型带微扰的宽带全向水平极化天线,通过天线正面的弧形偶极子的阶梯状边缘和背面的正方形微扰提升天线带宽.利用CST仿真得到最优的参数解,并制作实物进一步验证天线带宽.四对弧形偶极子组成一个圆环,每对弧形偶极子对应一个方向,并通过宽带巴伦均匀同相馈电,模拟载有均匀同相电流的圆环以实现全向水平极化.天线相对带宽50％(1.71～2.85GHz),并且在1.71～2.55GHz间拥有较高增益(2～4dBi).该天线结构简单、紧凑、低剖面,特别适合于室内基站建设.     

A recent survey on computational techniques for solving singularly perturbed boundary value problems

This survey paper contains a surprisingly large amount of material and indeed can serve as an introduction to some of ideas and methods of singular perturbation theory. In continuation of a survey performed earlier, this paper limits its coverage to some standard numerical methods developed by numerous researchers between 2000 and 2005. A summary of the results of some recent methods is presented and this leads to conclusions and recommendations regarding methods to use on singular perturbation problems. Because of space constraints, we considered one-dimensional singularly perturbed boundary value problems only.     

基于参数转移的岸-舰双基地波超视距雷达发射阵幅相扰动估计

 岸-舰双基地波超视距雷达采用岸基阵列发射不同载频的信号进行几乎全向辐射,舰载单根天线接收,接收端利用综合脉冲孔径技术得到等效发射方向图.本文结合该雷达系统特点,利用接收到的直达波信号,提出基于参数转移的幅相扰动估计方法.该方法引入新的未知量,通过参数转移,将由接收站方位误差引起的相位扰动估计误差转移到新的未知量中.论文详细推导了基于参数转移的发射阵幅相扰动估计模型以及幅相扰动估计的克拉-美罗界.计算机仿真表明了所提方法的有效性.     

含非线性扰动的混合变时滞中立系统鲁棒稳定性新判据

中立型时滞系统在工程实际中有着广泛应用背景.本文研究了一类含非线性扰动的混合变时滞中立型系统地鲁棒稳定性问题.基于直接Lyapunov泛函方法,通过构造一类新的包含时滞区间上下界的三重积分项Lyapunov-Krasovskii泛函,并结合积分不等式方法、自由权矩阵技术和凸组合处理方法,建立了一种新的线性矩阵不等式形式的离散时滞和中立时滞均相关的鲁棒稳定性判据.最后,通过数值算例验证了新判据的有效性和优越性,和一些已有文献相比,本文提出的判据具有更低的保守性.     

Buoyancy effects on the laminar boundary layer heat transfer along vertically moving cylinders

Abstract

The effects of buoyancy forces on the laminar boundary layer flow and heat transfer along vertically moving cylinders are analyzed for the cases of prescribed surface temperature and prescribed wall heat flux in power of streamwise distance. Local similarity solutions are obtained to show the effects of buoyancy parameters and the transverse curvature of the cylinder on the surface friction and heat transfer rate.     

Stability analysis for T–S fuzzy systems with interval time‐varying delays and nonlinear perturbations

This paper is concerned with the problem of the robust stability for T–S fuzzy systems with interval time‐varying delay and nonlinear perturbations. The key features of the approach include the introduction of uncorrelated augmented matrix items into the Lyapunov functional and the use of some appropriate integral inequalities. Unlike the existing methodologies, the proposed approach involves neither free weighting matrices nor any model transformation. It can, however, lead to much less conservative stability criteria than the existing ones for the systems under consideration. The cases of perturbations are assumed to both nonlinear time‐varying perturbations and time‐varying uncertainties in linear fractional form. Numerical examples show that the obtained results are less conservative than the existing ones in the literature. Copyright © 2009 John Wiley & Sons, Ltd.     

剪切扰动对液晶聚合物流变行为影响的数值模拟

采用有限体积数值方法模拟了剪切扰动条件下液晶聚合物的复杂流变性特性。其中剪切扰动系统通过给速度梯度施加一项额外小量而实现。首先,考虑了扰动为零的简单剪切流情形,分别得到了各种典型的分子运动状态,不但为剪切扰动的定性分析作了准备,而且也验证了数值算法的有效性。然后,重点研究了剪切扰动对分子运动行为转变的影响,数值结果表明流场中微小的剪切扰动在低剪切率和高剪切率下对分子演化状态的转变机理并不相同。此外,进一步预测了液晶聚合物流变行为及微观特性的变化,结果发现流场的扰动作用也会显著地改变其相应的物性函数。     

The nonlinear response of inverted “V” flames to equivalence ratio nonuniformities

Recent studies indicate that heat release fluctuations generated by equivalence ratio perturbations may constitute sources of instability with effects similar to those induced by acoustic perturbations. The present article addresses this issue by considering the dynamics of an inverted laminar dihedral (“V”) flame spreading in an open geometry when this flame is submitted to equivalence ratio modulations. The problem is investigated with numerical simulations by first establishing a steady state flame which then evolves in a uniform flow transporting a fixed level of equivalence ratio perturbations. The flame features wrinkles of increasing amplitude locking on the convected composition perturbations. The wrinkling amplitude grows with distance from the injector. For sufficiently large wrinkle amplitudes, the flame interacts with the fresh mixture outer boundary, giving rise to sudden disruptions of the flame sheet. The rapid burning of fresh mixture pockets generates a nonlinear heat release signal with abrupt changes in the waveform. It is found that high levels of modulation induce axial velocity perturbations, which in turn interact with the flame and modify the response. Calculations described in this article may serve to guide analytical modeling of the response of combustion to equivalence ratio inhomogeneities. A simple model is devised on this basis to distinguish regimes corresponding to weak and strong interactions.     

Robust stability of Metzler operators under parameter perturbations

In this paper we study how the spectral bound of Metzler operators changes under parameter perturbations. Characterizations of the stability radii of Metzler operator with respect to this type of disturbances are established. The results generalize those obtained in (Vietnam J. Math. 2006; 34 :357–368; Vietnam J. Math. 1998; 26 :147–163). Copyright © 2009 John Wiley & Sons, Ltd.