With the development of robotics technology, the application of robotics has been extended more and more widely. The robot is required not only to complete some simple tasks, but also to achieve complex tasks with complex and unknown environment. For many complex tasks, it is hard or impossible for single robot to accomplish by itself. Therefore the system composed of multiple robots is considered and the tasks are performed through coordination and cooperation among multiple robots. Multi-robot system has been paid more and more attention because of its advantages such as flexibility, parallelism and robustness that single robot cannot compare. This thesis focuses on the formation problems of coordination and control of multi-robot system with unknown environment. Firstly, the advantages and application of multi-robot system are introduced. The research development and main research directions of multiple robots system are reviewed. The background and structure of this thesis are also introduced. Secondly, a layered control architecture for individual mobile robot is proposed based on the achievement of task-level cooperation of multiple robots system, the abilities the mobile robot should have and the consideration of more and more complex environment and task demand. The architecture is composed of three layers: system-monitoring layer, cooperation-planning layer and behavior-control layer. Thirdly, the strategy based on the behavior selection mechanism used to complete the formation-keeping task of multi-robot system is discussed. The behaviors' description, reasonable conditions of behavior selection and "Leader changing" approach are given in order to ensure the smooth completion of the task. Fourthly, in order to solve the problem that the behaviors' control parameters of robot adopting motor-schema based architecture with the formation control task are hard to be chosen, genetic algorithm is adopted to optimize the control parameters of robots. The robots with the optimized control parameters of behaviors move in a specified formation harmoniously. Fifthly, considering the problems that robot adopting fixed control parameters used to behaviors' combination has lower adaptability to environment and the difficulty of collision avoidance is increased because of dynamic obstacles during the research on multi-robot formation control with dynamic and unknown environment, a method that the behaviors' control parameters of robot adjust automatically with the conditions is proposed, and Parabola prediction model as well as recurrence least square algorithm with restricted scale are adopted to achieve the prediction of moving obstacles' positions. Sixthly, the problem that multiple mobile robots cooperatively hunt the invader that has safety motion strategy with unknown environment is discussed. A series of strategies are proposed on the basis of task modeling in order to ensure that multiple robots find and cap
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