Cooperative Control of Multi-Agent Systems
Creators: Junyi Yang
Publisher: name: University of Alberta Library
Year: 2022

<p>Cooperative control of multi-agent systems (MASs) has been a hot topic in control and communication community since it was proposed in last two decades. Cooperative control is applied to a wide range of real world problems, such as search and rescue, resource allocation and multiple robot formation. In order to realize a cooperative goal,  a communication network is introduced to facilitate the information  flow among MASs, which brings network-induced imperfections at the same time. In addition, the energy of onboard sensors and microprocessors, and the bandwidth of communication networks are limited. It is preferred to reduce the frequency of controller updates and data transmissions. Motivated by the above concerns, this thesis focuses on investigating the robustness of MASs against network-induced imperfections and proposing event-triggered mechanisms (ETMs) to reduce the network load and/or energy consumptions.<br>
Four research topics are considered. Firstly, an affine formation under fixed and switching topologies is studied. An ETM is proposed, such that the controller updates and data transmissions occur only when it is necessary to maintain system stability. To guarantee Zeno-freeness, an absolute term is introduced in the price of introducing steady-state errors. Secondly, a cooperative output regulation (COR) problem is studied. The problem is formulated in a hybrid system framework. By proposing a novel Lyapunov function, robustness against asynchronous samplings and time-varying delays are given in terms of maximally allowable transmission intervals (MATIs) and maximally allowable delays (MADs). Thirdly, a formation tracking problem of multiple nonholonomic systems without velocity measurements is considered. Two kinds of communication, namely, pull-based communication (PULC), which is enabled by agents&#39; onboard sensors, and push-based communication (PUSC), which is realized by data transmissions through networks, are considered separately. A periodic event-triggered mechanism (PETM) is proposed for PUSC, such that the closed-loop sampled-data system is robust to asynchronous samplings, at the same time, continuous monitoring and Zeno-behavior are avoided. In addition, a hierarchical structure is proposed, according to which, the followers are divided into two levels. Strongly integral input-to-state stability (iISS) is established for the closed-loop system. Finally, a distributed optimization-based formation problem is studied. The control protocol is design based on a modied Lagrangian-based (MLB) algorithm, under which, the agents can reach the global optimal solution and converge to the desirable formation structure simultaneously. A dynamic ETM is proposed to reduce network load. To guarantee Zeno-freeness in the presence of disturbances, an auxiliary variable is introduced to estimate the influence of disturbances. The closed-loop system is proved to be input-to-state exponentially stable (ISES) w.r.t. the disturbances.<br>
The effectiveness of the proposed methods are illustrated by numerical examples.  Under the proposed ETMs, unnecessary data transmissions and/or controller updates can be efficiently reduced. Zeno-freeness is guaranteed by event separation properties or computable positive minimum inter-event times. In addition, the proposed methods improve the robustness of closed-loop systems against network-induced imperfections.</p>