Synchronization in heterogeneous networks of discrete‐time introspective right‐invertible agents

This paper studies output synchronization problem, formation problem, and regulated synchronization problem for a heterogenous network of discrete‐time introspective right‐invertible agents. We first propose a decentralized control scheme to solve the output synchronization problem for a set of communication topologies. Moreover, if the synchronization trajectories are assumed to be bounded, a universal controller can be constructed for all communication topologies, which contain a directed spanning tree. The design can be applied to solve the formation problem with arbitrary formation vectors. In the regulated synchronization problem, we assume only the root receives information from exosystem. We then design a decentralized controller to solve the problem for a set of communication topologies. Copyright © 2013 John Wiley & Sons, Ltd.     

Finite‐time formation control of multiple nonholonomic mobile robots

This paper considers finite‐time formation control problem for a group of nonholonomic mobile robots. The desired formation trajectory is represented by a virtual dynamic leader whose states are available to only a subset of the followers and the followers have only local interaction. First of all, a continuous distributed finite‐time observer is proposed for each follower to estimate the leader's states in a finite time. Then, a continuous distributed cooperative finite‐time tracking control law is designed for each mobile robot. Rigorous proof shows that the group of mobile robots converge to the desired geometric formation pattern in finite time. At the same time, all the robots can track the desired formation trajectory in finite time. Simulation example illustrates the effectiveness of our method. Copyright © 2012 John Wiley & Sons, Ltd.     

Containment control of multi‐agent systems by exploiting the control inputs of neighbors

This paper studies the containment control problem for multi‐agent systems consisting of multiple leaders and followers connected as a network. The objective is to design control protocols so that the leaders will converge to a certain desired formation while the followers converge to the convex hull of the leaders. A novel protocol is proposed by exploiting the control input information of neighbors. Both continuous‐time and discrete‐time systems are considered. For continuous‐time systems, it is proved that the protocol is robust to any constant delays of the neighbors' control inputs. For discrete‐time systems, a sufficient condition on the feedback gain for the containment control is given in terms of the time delay and graph information. Some numerical examples are given to demonstrate the results. Copyright © 2013 John Wiley & Sons, Ltd.     

浅谈绿色无线通信技术

目前,移动通信正朝着绿色通信的方向发展,网络中的基站节能是绿色无线通信的重点。该文简要介绍了网络节能的相关技术,包括认知无线电、协作中继、小区缩放及云计算等,这些技术将在绿色通信的发展中起到巨大的作用。     

304 Stainless steel oxidation in sulfate and sulfate+ bicarbonate solutions

The electrochemical characterization of 304 stainless steel in 0.1–0.5m Na2SO4 and Na2SO4 + NaHCO3 aqueous solutions at pH8 was done in combination with SEM surface analysis. Passivation of the surface film without any pitting events is observed for the current–potential and current–time experiments, and no anodic current spikes, which are associated with depassivation events, are distinguishable above the background current level. However, SEM pictures of an electrode surface polarized at potentials above 0.4V show microscopic pit nucleation. Even when the metal may be regarded as passive from a current–time or current–potential characteristics, passivity is not stable and localized film breakdown can still occur. The events are ascribed to pit nucleation at the active inclusion sites. The pits do not grow even into the metastable state but die through repassivation by metal salt precipitation immediately after birth. The effect is ascribed to the solubility of metal salts with the electrolyte produced by dissolution in the nucleation sites. The results show that pit nucleation and pit growth are two distinct processes. The importance of solution composition and the protective effect of bicarbonate ions is also discussed.     

Differentiating seawater and groundwater sulfate attack in Portland cement mortars

The study reported in this article deals with understanding the physical, chemical and microstructural differences in sulfate attack from seawater and groundwater. Portland cement mortars were completely immersed in solutions of seawater and groundwater. Physical properties such as length, mass, and compressive strength were monitored periodically. Thermal analysis was used to study the relative amounts of phases such as ettringite, gypsum, and calcium hydroxide, and microstructural studies were conducted by scanning electron microscopy. Portland cement mortars performed better in seawater solution compared to groundwater solution. The difference in performance could be attributed to the reduction in the quantity of the expansive attack products (gypsum and ettringite). The high Cl concentration of seawater could have played an important role by binding the C₃A to form chloroaluminate compounds, such as Friedel's salt (detected in the microstructural studies), and also by lowering the expansive potential of ettringite. Furthermore, the thicker layer of brucite forming on the specimens in seawater could have afforded better protection against ingress of the solution than in groundwater.     

混凝土受硫酸盐侵蚀的环境因素研究现状

硫酸盐侵蚀是水泥混凝土结构病害劣化的主要原因之一.海水、盐湖、地下水等环境中大多含有硫酸盐,混凝土组分本身也有可能带有硫酸盐,而它们在各种条件下对混凝土产生腐蚀作用,使其发生破坏.文章分析了阳离子、阴离子、硫酸盐浓度、温度、PH值、干湿循环及冻融循环、荷载作用等因素对混凝土受硫酸盐侵蚀的影响,综述了目前的研究现状,指出了当前研究成果中的成就与不足.     

社会网络的隐私保护研究综述*

某些网站将匿名处理后的社会网络数据公开发布,或者提供给科研机构、大学或者其他组织和个人使用,而这些数据往往侵犯了用户的隐私,但有关社会网络中个人信息安全和隐私保护的研究却处于起步阶段。综述了当前在线社会网络的研究成果,主要就社会网络及其隐私漏洞、信息泄露、再识别攻击、聚集攻击、推理攻击等进行了分析,并对今后的发展提出了预测,为社会网络的科研指出了可行的研究方向。     

一个新的基于口令的密钥协商协议

以不经意多项式计算作为核心组件,提出了一个基于口令的密钥协商协议PSKA-I,该协议能够抵抗字典攻击但只能工作于认证模型。为解决协议PSKA-I这一缺陷,根据BCK安全模型设计了消息传输认证器,将协议PSKA-I转换为非认证模型中的安全协议PSKA-Ⅱ。上述协议口令的安全性由不经意多项式计算予以保证。与GL协议相比,该协议的通信及计算复杂度明显降低。     

合作学习在英语写作教学中的应用

英语教学包括听、说、读、写四个方面的内容。大部分教师在教学过程中,重视前三项,而对于写作方面的教学,模式相对单一,效果不理想。但是,当我们将合作学习这种方式引进课堂时,学生积极参与,思维活跃,尤其是在写作教学方面,效果非常明显。合作学习,既能激发学生潜能,帮助学生形成良好的交际能力,又凸显了学生在课堂中的主体地位。本文主要研究在合作学习方式下,英语写作教学中合作小组的具体课堂活动形式,及对学生的评价方式。