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Please use this identifier to cite or link to this item: http://hdl.handle.net/2031/6642

Title: Coordinated control in rendezvous tasks of multi-agent systems
Other Titles: Duo zhi neng ti xi tong ju ji ren wu zhong de xie tong kong zhi
多智能體系統聚集任務中的協同控制
Authors: Fan, Yuan ( 樊淵)
Department: Department of Manufacturing Engineering and Engineering Management
Degree: Doctor of Philosophy
Issue Date: 2011
Publisher: City University of Hong Kong
Subjects: Multiagent systems.
Notes: CityU Call Number: QA76.76.I58 F36 2011
x, 167 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2011.
Includes bibliographical references (leaves [142]-165)
Type: thesis
Abstract: Significant progress has been achieved for research on coordination of networked robotic systems in the past few years. Not only many theoretical results, but also a lot of experimental and practical results have been obtained, covering topics of consensus algorithms, graph theory, stochastic networks, animal swarming, agent flocking, robot formation, sensor coverage, information fusion, and so on. In all networked agent systems, the status of communication link is always a key factor in system analysis and control law design. As is well-known, the manner of information exchanging and sharing is determined by the communication topology of an agent system. Thus the status of communication links is crucial for system stability and task achievement. As a result, great effort has been devoted to investigation of the following two problems. One is under what communication link conditions can the networked system obtain the desired status, such as achieving an average consensus. The other is how to design a control scheme for agents to meet these conditions required by the specific practical tasks. In this thesis, we consider the problem of the coordinated control in a basic group task, called the rendezvous task. The goal of this task is to design distributed control algorithms to drive all the agents to arrive at the same position in the work space in a finite time or as time goes to infinity. Under the condition of limited communication range, some new approaches have been developed to solve the rendezvous related problems of multi-agent systems, including the weighted digraph balancing, the coordinated control with unreliable communication links, and connectivity control with bounded control inputs. These approaches enjoy significant advantages over existing ones in literature, for example, less design conservatism and easier implementation in practice. Moreover, some of the problems, such as multi-agent connectivity control with both unreliable communication and bounded control inputs, to the best of our knowledge, are considered for the first time in research. As is well-known, network consensus algorithms have extensive applications in coordination of robot groups and decision making of agents, such as rendezvous control and formation control. Among consensus algorithms, the so-called average consensus algorithm is important for its wide application in signal processing such as sensor fusion. Since balanced graphs play an important role in the average consensus problem and many other coordination problems for directed communication networks, we explore the conditions and algorithms for the digraph balancing problem. It is known that a digraph can be balanced if and only if it is strongly semiconnected. Based on this observation, we prove that a directed graph can be balanced if and only if the null space of its incidence matrix contains positive vectors. Then, based on this result and the corresponding analysis, two weight balance algorithms have been proposed. We also point out the relationship between the weight balance problem and the features of the corresponding underlying Markov chain. Compared with the existing balancing algorithms, our algorithms are easier to be implemented in practice. Another topic studied in this thesis is the rendezvous algorithms design problem for multi-agent systems with discrete time setups, that is, for agents with a waypoint control manner. Firstly, the agent system with limited communication ranges and reliable communication is considered. We present a novel rendezvous algorithm framework - the combination framework, which is named after the term convex combination in computational geometry. The convergence of the proposed algorithms is proved based on the novel tools from the recent results in graph theory and consensus study. It has been observed that the extensively studied circumcenter algorithm can be considered as a special case of the combination framework. Secondly, we investigate the coordination problem of multiple agents with limited communication ranges and unreliable communications. A novel coordination algorithm via the so-called proximity graph is developed so that the agent group can achieve rendezvous when the communication links satisfy an ergodic assumption. The convergence proof of the algorithm is established based on the tools from rooted graph theory. The rendezvous algorithms design problem for multi-agent systems with continuous time setups is also considered. Since in almost all existing studies on connectivity control, control inputs may go unbounded when agents tend to go out of the communication range of their neighbors, our attention is focused on design of connectivity control laws with bounded inputs for multi-agents with limited communication ranges. A general potential based control law is investigated and sufficient conditions for preserving connectivity are presented. A necessary condition for preserving connectivity with bounded control inputs is then obtained. Based on this condition, a novel control law with bounded inputs is proposed and the selection of appropriate parameters for the corresponding control law is also discussed. Obviously, compared with the existing control design approaches, the proposed control law with bounded inputs is easier to be implemented in practice. Finally, we revisit the rendezvous algorithms with way-point setup in the presence of unreliable communication and the connectivity preserving control approach with bounded inputs. To the best of our knowledge, this is the first time to study the rendezvous problem of multi-agent systems with limited communication range, unreliable communication, and bounded control inputs. Combining the reference point approach and the indicating function approach, we explicitly define a virtual neighbor for each pair of proximity neighbors, and develop a connectivity control law with bounded input for each agent. As a result, the proximity graph connectivity of the agent group has been preserved with controls that are easy for implementation in practice. It is also proved that the proposed control law can drive the agents to rendezvous.
Online Catalog Link: http://lib.cityu.edu.hk/record=b4086916
Appears in Collections:MEEM - Doctor of Philosophy

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