Abstract
Cooperative control of multi-agent systems has been attracting tremendous research interest in control and robotics communities due to their broad applications in many fields including formation control of unmanned aerial vehicles, rendezvous of multiple mobile robots, deployment of sensor networks, and so on. Various cooperative control problems including, but not limited to, consensus, cooperative output regulation, formation, and containment, have been widely investigated. A key feature of most existing works on cooperative control of multi-agent systems is that the parameters of agents are assumed to be known a priori. However, in many real-word applications, the parameters of agent dynamics might be unknown due to either their dynamics complexity or their time-varying nature. This motivates researchers to investigate and develop distributed adaptive control approaches to multi-agent systems with unknown agent parameters. Note that, most of those existing works focus on minimum phase multi-agent systems without external disturbances or unmodeled dynamics, which may not be the case for many real-world applications. In fact, real-word multi-agent systems are often of non-minimum phase agent dynamics, also often subject to external disturbances and/or unmodeled dynamics. Thus, cooperative control problems of unknown non-minimum phase multi-agent systems without or with external disturbances and/or unmodeled dynamics deserve further investigation.On another front, communication networks of multi-agent systems in practice are often exposed to cyber-attacks, in addition to commonly-known communication constraints such as time-delays and packet dropouts. One of typical cyber-attacks is the so-called denial-of-service attacks, which tend to influence the timeliness of information transfer by blocking or destroying the connectivity of networks. This motivates researchers to investigate and develop distributed resilient control approaches to multiagent systems under denial-of-service attacks in the past few years. Note that, in most existing works on cooperative control of multi-agent systems under denial-of-service attacks, the developed distributed resilient control strategies address cooperative control problems asymptotically or exponentially. However, many engineering applications in cooperative control of multi-agent systems call for finite-time or fixed-time convergence instead of exponential or asymptotical convergence, even in the case of denial-of-service attacks. Thus, finite-time/fixed-time cooperative control problems of multi-agent systems under denial-of-service attacks deserve further investigation.
Motivated by these observations, this thesis investigates distributed adaptive control and resilient control problems of multi-agent systems.
Part I: Distributed adaptive control of multi-agent systems. This part starts with the leader-following consensus problem. Firstly, the leader-following output consensus problem of heterogeneous linear multi-agent systems with unknown agent parameters under directed graphs is studied. The dynamics of followers are allowed to be non-minimum phase with unknown arbitrary individual relative degrees. This is contrary to many existing works on distributed adaptive control schemes where agent dynamics are required to be minimum phase and often of the same relative degree. A distributed adaptive pole placement control scheme is developed, which consists of a distributed observer and an adaptive pole placement control law. It is shown that under the proposed distributed adaptive control scheme, all the signals in the closed-loop system are bounded and the outputs of all the followers track the output of the leader asymptotically. As practical systems are often subject to external disturbances and/or unmodeled dynamics, the robust leader-following output consensus problem is then addressed for unknown heterogeneous non-minimum phase linear multi-agent systems with external disturbances and/or unmodeled dynamics under directed graphs. The system matrix of the leader and the parameters of the nominal dynamics of the followers are assumed to be unknown. A novel distributed robust adaptive pole placement control scheme consisting of an adaptive distributed observer and a robust adaptive pole placement controller is developed. It is shown that the robust leader-following output consensus problem of the concerned multi-agent system can be solved by the proposed distributed robust adaptive control scheme under some sufficient conditions. It is also shown that the exact output consensus can be achieved if the unmodeled dynamics and external disturbances vanish as time goes to infinity.
Next, the robust adaptive leaderless consensus problem of heterogeneous uncertain non-minimum phase linear multi-agent systems over directed communication graphs is investigated. Each agent is assumed to be of unknown nominal dynamics and also subject to external disturbances and/or unmodeled dynamics. A novel distributed robust adaptive control strategy is proposed. It is shown that the robust adaptive leaderless consensus problem is solved with the proposed control strategy under some sufficient conditions.
Part II: Distributed resilient finite-time/fixed-time control of multi-agent systems. This part focuses on problems of finite-time/fixed-time cooperative output regulation. Firstly, the resilient finite-time cooperative output regulation problem of heterogeneous linear multi-agent systems under denial-of-service attacks is investigated. A novel distributed resilient finite-time observer is first proposed for each agent to estimate the state of the exosystem by taking into consideration of denial-of-service attacks. Then, a novel distributed resilient finite-time controller is further developed. It is shown that under the proposed distributed controller, the resilient finite-time cooperative output regulation for heterogeneous linear multi-agent systems under denial-of-service attacks can be achieved for any initial conditions. As the upper bounds for their settling times of finite-time cooperative control strategies depend on their initial conditions, the problem of resilient global practical fixed-time cooperative output regulation of uncertain nonlinear multi-agent systems subject to denial-of-service attacks is then studied. A novel distributed resilient adaptive fixed-time control strategy is proposed, which consists of a novel distributed resilient fixed-time observer with a chain of nonlinear filters and a novel distributed resilient adaptive fixed-time controller. It is shown that the problem of resilient global practical fixed-time cooperative output regulation can be solved by the proposed control strategy.
Furthermore, the effectiveness of the proposed distributed control schemes in this thesis is demonstrated by several simulation examples via MATLAB.
| Date of Award | 28 Aug 2024 |
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| Original language | English |
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| Supervisor | Gang Gary FENG (Supervisor) & Lu LIU (Co-supervisor) |