For decades, the increasing applications in engineering field, such as space exploration, ocean sampling, aerial vehicle formation flight and so on, have made the cooperative control problem of multi-agent systems a research focus. In this thesis, we will further study the consensus problem, one of the basic cooperative control problems, for two classes of nonlinear multi-agent systems: the Euler-Lagrange systems and the rigid spacecraft attitude control systems. / An Euler-Lagrange system refers to a system whose motion equation is derived by the Euler-Lagrange equation. Euler-Lagrange systems can describe many practical systems, such as robot manipulators and AC motors. The rigid spacecraft attitude control system is usually embedded in the overall control system for a rigid spacecraft to perform attitude maneuver, such as spin stabilization or target acquisition. The consensus problems for these two classes of systems have many applications, like robot manipulators coordination and spacecraft formation flying. Being one of the basic cooperative control problems, consensus problem lays the foundation for some typical cooperative control problems, such as rendezvous, flocking and formation control. So far, two kinds of consensus problems have been investigated, i.e., the leader-following consensus problem and the leaderless consensus problem. For a multi-agent system, each agent will be viewed as a subsystem of the overall multi-agent system. The leader-following consensus problem aims to design a control law such that the state and/or output of each subsystem will asymptotically track a prescribed trajectory, which is usually generated by another subsystem called the leader system, while the leaderless consensus problem requires that the control law drive the state and/or output of each subsystem to a common trajectory. / The leader-following consensus problems for both the Euler-Lagrange systems and the rigid spacecraft attitude control systems present certain technical difficulties. On one hand, the controller is constrained by the communication network, which describes the information flow among all subsystems. On the other hand, both the Euler-Lagrange system and the rigid spacecraft attitude control system are nonlinear systems with complex kinematic or dynamic equations. To overcome these difficulties, we have employed the distributed observer approach, which consists of two steps: first, given a leader system, a distributed observer is designed for each follower subsystem to estimate the state of the leader system and recover the reference signal; second, by making use of the estimated reference signal, a certainty equivalence controller will be synthesized to control the overall multi-agent system. Whether or not the distributed observer approach will work depends on two crucial issues: the existence of the distributed observer for a given leader system and the stability of the closed-loop system subject to the certainty equivalence controller. / In the first part of this thesis, we have considered both the leader-following and leader-less consensus problems for multiple uncertain Euler-Lagrange systems under a switching communication network. The main results are summarized as follows: / 1. The leader-following consensus problem for multiple uncertain Euler-Lagrange systems under switching communication network was studied. Since the kinematic equation of the Euler-Lagrange system is linear, the leader system is linear. Given this leader system, we have established the distributed observers for all follower subsystems under a jointly connected communication network. To show the stability of the closed-loop system, we resorted to a generalized Barbalat’s lemma to deal with the discontinuity resulted from the switching of the communication network. / 2. Inspired by the idea of the distributed observer approach, we have employed an auxiliary system for each subsystem to further solve the leaderless consensus problem for multiple uncertain Euler-Lagrange systems under jointly connected communication network. When the communication network is undirected, we pointed out that the final common trajectory will be uniquely decided by the initial values and will not be affected by the communication network. / The second part of this thesis addresses the leader-following attitude consensus problem for multiple rigid spacecraft systems. Owing to the nonlinear kinematic equation of the rigid spacecraft attitude control system, the leader system is also nonlinear and we have established a nonlinear distributed observer for each follower subsystem to estimate the state of the leader system. Three scenarios were considered and are summarized as follows: / 1. In the first scenario, the controller uses the control feedback of both the attitude and angular velocity. We posed a technical lemma to show that the couplings between subsystems do not change the asymptotic behavior of each follower subsystem and hence the stability of the closed-loop system can be guaranteed. / 2. In the second scenario, only the attitude is available for control feedback. In the absence of the angular velocity, it is more difficult to prove the stability of the closed loop system. In order to do so, we have enhanced the performance of the distributed observers by making the convergent speed exponential. / 3. In the third scenario, we further solved the attitude consensus problem for multiple rigid spacecraft systems subject to unknown system parameters and external disturbances. For each follower subsystem, an adaptive law is used to estimate the unknown parameters and a dynamic compensator is attached to eliminate the influence of the external disturbances. / 近十年来，随着在空间探索，海洋监测，以及飞行器编队飞行等工程领域内的逐步应用，多智能体系统的协作式控制问题受到了广泛的关注。在本论文中，我们将进一步采用基于分布式观测器的方法来研究两类非线性多智能体系统，即Euler-Lagrange系统与刚性航天器姿态控制系统的一类基本协作式控制问题-趋同问题。 / Euler-Lagrange系统指代一类系统，其动态方程可由Euler-Lagrange方程导出。Euler-Lagrange系统可以描述许多实际系统，例如机器人手臂与交流电机。航天器姿态控制系统通常被嵌入航天器的全局控制系统中以调整航天器姿态，例如自旋镇定与目标跟踪。这两类系统的趋同问题有诸多应用，例如机器人手臂协作与航天器编队飞行。作为一类基本的协作式控制问题，趋同问题是一些典型的协作式控制问题的基础，例如蜂拥，聚类与编队。目前，趋同问题主要分为两类：有领导者的趋同问题与无领导者的趋同问题。在一个多智能体系统中，每一个智能体都作为全局系统的一个子系统。有领导者的趋同问题的控制目标是设计控制器使得所有子系统的状态和/或输出渐近地跟踪一个指定的轨迹，该轨迹通常由另一个被称作领导者的子系统产生，而无领导者的趋同问题则要求设计控制器使所有子系统的状态和/或输出渐近地趋于一个共同的轨迹。 / 目前，Euler-Lagrange系统与刚性航天器姿态控制系统的趋同问题都面临着技术上的挑战。一方面，控制器的设计受限于用于描述信息流动的通信网络。另一方面，Euler-Lagrange系统与刚性性航天器姿态控制系统都是非线性系统，且具有复杂的运动学方程或动力学方程。为了克服这些困难，我们采用了基于分布式观测器的方法。该方法包含两步：首先，为每一个跟随者设计一个分布式观测器以估测领导者的状态；其次，根据观测器的估测信号设计全局控制器。该方法能否成功取决于两个关键因素：一，对某个给定的领导者系统，分布式观测器是否存在；二，根据估测信号设计的确定性等价控制器是否能保证闭环系统的稳定性。 / 在本论文的第一部分中， 我们将分别考虑在通信网络联合连通条件下Euler-Lagrange 系统有领导者的和无领导者的趋同问题。主要结论总结如下 / 1. 我们将研究Euler-Lagrange系统的有领导者的趋同问题。因Euler-Lagrange系统的运动学方程是线性方程，其领导者系统也为线性系统。在联合联通的通信网络下，我们为每一个跟随者系统设计了线性的分布式观测器，并引用了一个广义Barbalat引理来证明闭环系统的稳定性。 / 2. 受分布式观测器方法的启发，我们将借由一个辅助系统来研究通信网络联合连通条件下Euler-Lagrange系统无领导者的趋同问题，并进一步指出，如果通信网络是无向的，那么系统的稳态将完全取决于系统的初态而与通信网络无关。 / 在本论文的第二部分中，我们将探讨刚性航天器系统的姿态趋同问题。由于刚性航天器系统的姿态运动学方程是非线性方程，其领导者系统也是一个非线性系统。我们为每一个跟随者系统设计了非线性的分布式观测器，并考虑了如下三类情况。 / 1. 在第一类情况下，控制器可同时采用角位置与角速度反馈。为确保子系统间的耦合不影响闭环系统稳定性，我们确立了一个引理用以说明此类耦合并不改变系统的稳态特性。 / 2. 在第二类情况下，控制器只能采用角位置反馈。这使得证明闭环系统的稳定性变得困难。为此，分布式观测器的收敛速度被提高至指数收敛。 / 3. 在第三类情况下，刚性航天器系统包含未知系统参数且受到外部干扰。我们采用了自适应控制技术以及动态补偿技术用以估计未知参数及抵消外部干扰。 / Cai, He. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 116-127). / Abstracts also in Chinese. / Title from PDF title page (viewed on 05, October, 2016). / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only.