Addressing agent loss in vehicle formations and sensor networks

In this paper, we address the problem of agent loss in vehicle formations and sensor networks via two separate approaches: (1) perform a ‘self‐repair’ operation in the event of agent loss to recover desirable information architecture properties or (2) introduce robustness into the information architecture a priori such that agent loss does not destroy desirable properties. We model the information architecture as a graph G(V, E), where V is a set of vertices representing the agents and E is a set of edges representing information flow amongst the agents. We focus on two properties of the graph called rigidity and global rigidity, which are required for formation shape maintenance and sensor network self‐localization, respectively. For the self‐repair approach, we show that while previous results permit local repair involving only neighbours of the lost agent, the repair cannot always be implemented using only local information. We present new results that can be applied to make the local repair using only local information. We describe implementation and illustrate with algorithms and examples. For the robustness approach, we investigate the structure of graphs with the property that rigidity or global rigidity is preserved after removing any single vertex (we call the property as 2‐vertex‐rigidity or 2‐vertex‐global‐rigidity, respectively). Information architectures with such properties would allow formation shape maintenance or self‐localization to be performed even in the event of agent failure. We review a characterization of a class of 2‐vertex‐rigidity and develop a separate class, making significant strides towards a complete characterization. We also present a characterization of a class of 2‐vertex‐global‐rigidity. Copyright © 2008 John Wiley & Sons, Ltd.     

Closing ranks in rigid multi‐agent formations using edge contraction

This paper proposes a systematic approach to solve the closing rank problem for a rigid multi‐agent formation, viz. restoring rigidity after loss of an agent. The approach is based on a particular graph operation, the edge contraction operation. It is proven that when an agent is lost in an arbitrary two‐dimensional rigid formation, rigidity can always be restored by transferring all links to which this agent was incident on to one of its neighbors, though not in general any arbitrary one of them. From a graph theoretical point of view, this corresponds to contraction of a certain edge incident to the vertex representing the agent being lost. It is established, for any two‐dimensional rigid formation (graph), that there exists at least two such edges that can be contracted to solve the closing ranks problem. Later, it is demonstrated that any potential decentralized algorithm to check if an arbitrary edge is contractible would need to use information on vertices and edges that can be at arbitrarily large distance from the edge considered; and a set of rigid graph theoretical results are established for several general settings, which can be used in selection of the edge to contract in these settings in order to solve the corresponding closing ranks problems. Partial results are also obtained for three‐dimensional formations, and it is shown that the two‐dimensional results do not generalize as such to higher dimension. Copyright © 2010 John Wiley & Sons, Ltd.     

Distance‐based undirected formations of single‐integrator and double‐integrator modeled agents in n‐dimensional space

We study the local asymptotic stability of undirected formations of single‐integrator and double‐integrator modeled agents based on interagent distance control. First, we show that n‐dimensional undirected formations of single‐integrator modeled agents are locally asymptotically stable under a gradient control law. The stability analysis in this paper reveals that the local asymptotic stability does not require the infinitesimal rigidity of the formations. Second, on the basis of the topological equivalence of a dissipative Hamiltonian system and a gradient system, we show that the local asymptotic stability of undirected formations of double‐integrator modeled agents in n‐dimensional space is achieved under a gradient‐like control law. Simulation results support the validity of the stability analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

3D Multi‐Agent Formation Control with Rigid Body Maneuvers

In this paper, we propose a control scheme for the formation maneuvering problem of multi‐agent systems where the team of agents can translate and rotate as a virtual rigid body in 3D. Using the single‐integrator model, we formulate the basic control law which is comprised of a formation acquisition term, function of the graph rigidity matrix, and a rigid body maneuvering term. The control is dependent on the relative position of agents that are connected in an infinitesimally and minimally rigid framework in addition to the desired rigid body motion of the formation. To facilitate the design of the rigid body maneuver, one agent in the convex hull of the formation serves as the reference point for the rotation component. A simulation study demonstrates the formation controller.     

Bearing‐only control of directed cycle formations: Almost global convergence and hardware implementation

In this article, we study bearing‐only control of directed cyclic formations. First, we provide a necessary and sufficient condition on the bearing constraints so that the directed cycle formation of n‐agents in (n?1)‐dimensional space is infinitesimally bearing rigid. Second, a bearing‐only control law which only allows motions perpendicular to the desired bearing vector is proposed. Under this control law, the agents globally asymptotically converge to a desired formation which is fully determined from their initial positions and desired bearing vectors. Finally, the proposed formation control law is implemented on mobile robots to support the analysis.     

Formation control for multi‐agent systems through an iterative learning design approach

This paper deals with formation control problems for multi‐agent systems by using iterative learning control (ILC) design approaches. Distributed formation ILC algorithms are presented to enable all agents in directed graphs to achieve the desired relative formations perfectly over a finite‐time interval. It is shown that not only asymptotic stability but also monotonic convergence of multi‐agent formation ILC can be accomplished, and the convergence conditions in terms of linear matrix inequalities can be simultaneously established. The derived results are also applicable to multi‐agent systems that are subject to stochastic disturbances and model uncertainties. Furthermore, the feasibility of convergence conditions and the effect of communication delays are discussed for the proposed multi‐agent formation ILC algorithms. Simulation results are given for uncertain multi‐agent systems to verify the theoretical study. Copyright © 2012 John Wiley & Sons, Ltd.     

Consensus of multi‐agent systems with fixed inner connections

This paper characterizes a unified consensus region for multi‐agent systems, where there exist fixed physical connections with information exchange. The notions of synchronization region in complex networks and consensus region in multi‐agent systems can be explained under this unified framework. The effect of the coupling terms on the consensus regions in different situations is analyzed specifically. Furthermore, necessary and sufficient conditions for consensus of agents under both distributed state feedback and observer‐based output feedback control are established. On the basis of a parameter‐dependent Lyapunov function, a 2‐step controller design procedure is proposed, which can reduce the conservativeness to some extent in comparison with the conventional direct Lyapunov method. In addition, for the case with disturbance, the robustness of the system is investigated. Finally, some numerical examples are presented to illustrate the theoretical results.     

无线传感器网络定位理论和算法

定位技术作为网络协议和应用的基础,已经成为无线传感器网络重要的支撑技术,是传感器网络研究的核心问题之一.系统地总结了近年来定位理论和算法的最新研究进展.全面阐述了定位问题的形式化定义、定位问题复杂度分析、基于刚性理论的定位理论和定位问题可计算性研究的最新成果.通过对定位理论的研究可以更好地揭示定位技术的本质,回答很多定位技术相关的基本问题.此外,还深入分析了近年来典型的定位算法,介绍每种算法的设计思想,分析其适用范围和不足.最后给出定位理论和定位算法未来的研究方向.     

Distributed containment control of multi‐agent systems with general linear dynamics in the presence of multiple leaders

This paper considers the containment control problems for both continuous‐time and discrete‐time multi‐agent systems with general linear dynamics under directed communication topologies. Distributed dynamic containment controllers based on the relative outputs of neighboring agents are constructed for both continuous‐time and discrete‐time cases, under which the states of the followers will asymptotically converge to the convex hull formed by those of the leaders if, for each follower, there exists at least one leader that has a directed path to that follower. Sufficient conditions on the existence of these dynamic controllers are given. Static containment controllers relying on the relative states of neighboring agents are also discussed as special cases. Copyright © 2011 John Wiley & Sons, Ltd.     

Optimal Deployment of Mobile Sensors for Target Tracking in 2D and 3D Spaces

This paper proposes a control strategy to autonomously deploy optimal placements of range-only mobile sensors in 2D and 3D spaces. Based on artificial potential approaches, the control strategy is designed to minimize the intersensor and external potentials. The inter-sensor potential is the objective function for optimal sensor placements. A placement is optimal when the inter-sensor potential is minimized. The external potential is introduced to fulfill constraints on sensor trajectories. Since artificial potential approaches can handle various issues such as obstacle avoidance and collision avoidance among sensors, the proposed control strategy provides a flexible solution to practical autonomous optimal sensor deployment. The control strategy is applied to several optimal sensor deployment problems in 2D and 3D spaces. Simulation results illustrate how the proposed control strategy can improve target tracking performance.      

Formation Maneuvering and Target Interception for Multi‐Agent Systems via Rigid Graphs

In this paper, we introduce control laws for multi‐agent formation maneuvering and target interception problems. In the target interception problem, we consider that the target velocity is unknown. Using a single‐integrator agent model, the proposed controls consist of a formation acquisition term, dependent on the graph rigidity matrix, and a formation maneuvering or target interception term. The control laws are only a function of the relative position of agents in an infinitesimally and minimally rigid graph, and either the desired maneuvering velocity of the formation or the target's relative position to the leader. The target interception control includes a continuous dynamic estimation term to identify the unknown target velocity. A Lyapunov‐like stability analysis is used to prove that the control objectives are met.     

Distributed MPC for Tracking and Formation of Homogeneous Multi‐agent System with Time‐Varying Communication Topology

The distributed model predictive control (MPC) is studied for the tracking and formation problem of multi‐agent system with time‐varying communication topology. At each sampling instant, each agent solves an optimization problem respecting input and state constraints, to obtain its optimal control input. In the cost function for the optimization problem of each agent, the formation weighting coefficient is properly updated so that the adverse effect of the time‐varying communication topology on the closed‐loop stability can be counteracted. It is shown that the overall multi‐agent system can achieve the desired tracking and formation objectives. The effectiveness of the results is demonstrated through two examples.     

Node‐to‐node consensus of multi‐agent networks with event‐triggered control and packet losses

This paper studies the node‐to‐node consensus problem for multi‐agent networks possessing a leaders' layer and a followers' layer via the pinning control. In order to realize the consensus and reduce the update frequency of the controller, a suitable event‐triggered mechanism is introduced into the control strategy. Furthermore, the phenomenon of packet loss is considered in the designed controller. Based on the M‐matrix theory and Lyapunov stability theory, this paper presents the sufficient conditions for the node‐to‐node consensus of networks. Meanwhile, it is proved that the Zeno behaviour is excluded. Finally, two numerical simulations are provided to demonstrate the effectiveness of the theoretical results.     

Event‐triggered Control for Cluster Consensus in Multi‐agent Networks

This paper considers the issue of cluster consensus for multiple agents in fixed and undirected networks. Agents in a network are supposed to split into several clusters, and a fraction of the agents in each cluster are pinned by virtual leaders. According to the Lyapunov stability theory and graph theory, some appropriate event‐triggered protocols are developed for consensus of the agents belonging to the same cluster, which can greatly reduce both the number of communication updates and that of control actuation updates. Finally, a numerical example is shown to demonstrate the effectiveness of the proposed theoretical results.     

一种抗独立攻击的WSN三维安全定位算法

为解决无线传感器网络中节点定位缺乏安全性和定位精度不高的问题,基于最速下降法与牛顿迭代法提出了一种新的三维安全定位算法。该算法首先给出三维空间下未知节点定位的计算方法和独立攻击模型,然后利用最速下降法和恶意锚节点检测技术选出最优锚节点数据,并结合牛顿迭代法实现了快速高精度的三维安全定位。实验结果表明,在危险的环境下,该算法能有效抵抗独立攻击以及在定位精度和未知节点可成功定位率方面都具有较好的优势。     

Distributed consensus of multi‐agent systems with general linear node dynamics and intermittent communications

Without assuming that the mobile agents can communicate with their neighbors all the time, the consensus problem of multi‐agent systems with general linear node dynamics and a fixed directed topology is investigated. To achieve consensus, a new class of distributed protocols designed based only on the intermittent relative information are presented. By using tools from matrix analysis and switching systems theory, it is theoretically shown that the consensus in multi‐agent systems with a periodic intermittent communication and directed topology containing a spanning tree can be cast into the stability of a set of low‐dimensional switching systems. It is proved that there exists a protocol guaranteeing consensus if each agent is stabilizable and the communication rate is larger than a threshold value. Furthermore, a multi‐step intermittent consensus protocol design procedure is provided. The consensus algorithm is then extended to solve the formation control problem of linear multi‐agent systems with intermittent communication constraints as well as the consensus tracking problem with switching directed topologies. Finally, some numerical simulations are provided to verify the effectiveness of the theoretical results. Copyright © 2013 John Wiley & Sons, Ltd.     

多智能体的联盟形成理论与应用研究

结合合作博弈的思想阐述了多智能体的联盟形成的基本内涵,介绍了迄今为止国际上稳定联盟的静态形成和动态形成的一些主要理论和方法,分析了其联盟结构的产生和联盟成员之间的利润分配。最后重点讨论了多智能体的联盟形成方法在分布式智能控制系统中的应用。     

Optimal sensor placement for target localisation and tracking in 2D and 3D

This paper analytically characterises optimal sensor placements for target localisation and tracking in 2D and 3D. Three types of sensors are considered: bearing-only, range-only and received-signal-strength. The optimal placement problems of the three sensor types are formulated as an identical parameter optimisation problem and consequently analysed in a unified framework. Recently developed frame theory is applied to the optimality analysis. We prove necessary and sufficient conditions for optimal placements in 2D and 3D. A number of important analytical properties of optimal placements are further explored. In order to verify the analytical analysis, we present a gradient control law that can numerically construct generic optimal placements.     

A backstepping design for directed formation control of three‐coleader agents in the plane

This paper deals with a directed formation control problem of three agents moving in the plane, where the agents have a cyclic ordering with each one required to maintain a nominated distance from its neighbor, and each agent is described by a double integrator. Firstly, a directed formation control law based on the knowledge only of the neighbor's direction is designed by using the integrator backstepping technique, which can not only accomplish the desired triangle formation but also ensure that speeds of all agents converge to a common value without collision between each other during the motion. Then, with the purpose of relaxing and even overcoming the restriction of initial conditions of the agents owing to collision avoidance, we introduce the inter‐agent potential functions into the design. The convergence of the proposed control algorithms is proved by using tools from LaSalle's invariance principle. Simulation results are provided to illustrate the effectiveness of the control laws. Copyright © 2008 John Wiley & Sons, Ltd.     

基于姿态校正与姿态融合的2D/3D骨架动作识别方法

针对现有的人体骨架动作识别方法对肢体信息挖掘不足以及时间特征提取不足的问题,提出了一种基于姿态校正模块与姿态融合模块的模型PTF-SGN,实现了对骨架图关键时空信息的充分利用。首先,对骨架图数据进行预处理,挖掘肢体和关节点的位移信息并提取特征;然后,姿态校正模块通过无监督学习的方式获取姿态调整因子,并对人体姿态进行自适应调整,增强了模型在不同环境下的鲁棒性;其次,提出一种基于时间注意力机制的姿态融合模块,学习骨架图中的短时刻特征与长时刻特征并融合长短时刻特征,加强了对时间特征的表征能力;最后,将骨架图的全局时空特征输入到分类网络中得到动作识别结果。在NTU60 RGB+D、NTU120 RGB+D两个3D骨架数据集和Penn-Action、HARPET两个2D骨架数据集上的实验结果表明,该模型能够有效地识别骨架时序数据的动作。