Research On Multiple Mobile Robot Formation Control And Cooperative Transport

Multiple mobile robots system provides an unchallenged incentive to all researchers in the past two decades, with its distribution characters in time and space, as well as its high efficiency. Great interests have been attracted to application domains including industry, military, national defense, daily life, and deep space, etc. In the research of specific application tasks, such as transporting heavy object, arresting target and multi-sensors cooperative map exploration, etc., robots are required to form and maintain specific shapes of formation during the tasks'execution. And these lead to a rush in the research of multi-robot formation control, which is one of the key techniques in multi-robot cooperation and coordination. Takes the heavy object transport problem as application background, problems in formation control are discussed in this thesis, from low level behavior-based motion control, motion planning, to high level formation task planning.Contributions of this thesis can be summarized in several points illustrated below:1. As the foundation of robot movement and formation control, the basic robot motion behavior and the behavior structure are designed at first. In the existing works on behavior-based control, behaviors are typically designed with focus on specific tasks or experiments, most of which are high level descriptive behaviors. The design of basic behavior and its reusability are less mentioned. In this thesis, the behaviors of multi-robot formation are proposed, with the posture deviation between robot and virtual target robot in formation as its input, and the driven wheel rotation velocity as its output. Service-oriented architecture (SOA) with reusability is introduced to the design of the basic motion behavior structure. This structure can satisfy new conditions with proper modification, extension or combination. The feasibility and reusability of the proposed service-based motion behavior structure are proved in both simulation and experiment. The structure is used in researches on multi-robot formation control and cooperative transport tasks.2. On the basis of basic motion behavior, formation initialization and movement planning are studied. An auction based formation initialization planning method with time optimal index is proposed. The formation problem has been transformed to task allocation problem, that is, the mobile robots in follower roles bid for the position where the virtual target robots is in formation, and the robot in leader role performs a centralized planning algorithm to allocate the positions to all followers. The integration of the centralized planning and distributed biddings can accomplish the position allocation task while reducing the complex of computation dimension. Besides, the real-time monitor mechanism raised at the bidding moment can help the leader to plan the formation shape again as soon as robots break down, or task requires. This mechanism can maintain the completeness of the formation. Simulation and experiments in formation control are taken to illustrate the feasibility of the proposed auction-based time optimal strategy. The formation time-optimal strategy is compared with single robot time shortest strategy and the strategy that robot and target with the same suffix are preferred. Results show that the proposed strategy cost less time and has higher efficiency in formation generation.3. Obstacle avoidance strategy in formation generation and movement is studied with the proposed basic motion behavior service and monitor mechanism. A multiple dynamic virtual targets tracking based obstacle avoidance strategy is proposed, takes both the formation information and the obstacle information into consideration. The proposed obstacle avoidance strategy combined with the motion behavior can accomplish the task that the robot in formation escapes from U-shape trough and returns to the position in formation. Monitor mechanism ensures the formation shape would be kept while robot is avoiding obstacle. Experiment of obstacle avoidance in formation illustrates the effectiveness of the proposed multiple dynamic virtual targets tracking based obstacle avoidance strategy.4. Multi-robot cooperative object transport task is studied on the basis of formation control, with the basic motion behavior service and obstacle avoidance strategy. Being different from the existing reactive behavior based cooperative transport method, the cooperative transport problem with robots in the relationship of pusher-watcher is transformed into the multiple mobile robots formation control problem. Limits on real robot sensors and actuators are also considered. In order to reduce the defects of the repetition movement controlled by reactive behaviors, geometric relation is considered in the path planning of the movement and obstacle avoidance of the formation. The virtual target in the path is set for robot formation to track, so robots can accomplish the transport task. Box pushing task with compare to reactive behavior-based approach in Microsoft Robotics Developer Studio (MRDS) is designed to illustrate the feasibility of the proposed cooperative transport strategy.5. Multiple mobile robot transport problem in the environment with undulating terrain is studied. With the restraint in terrain, as well as slipping of the robot, deformation of wheel, friction etc., it is more difficult for robots transport task. Navigation strategies of both Navigate Straight Forward to Destination (NSFD) and Navigate Plain First to Destination (NPFD) are proposed in this thesis. Formation rearrangement strategy is designed when pusher robots slipped. Simulation of the box pushing in the environment with undulating terrain is performed to illustrate the validity of the proposed strategy. The cooperative transport strategies with NSFD and NPFD are compared in different environment. The results show that the one with NPFD is more suitable for cooperative transport task in the complex environment.6. On the basis of the multiple mobile robot formation control and the single task allocation problem, task allocation problem of multi-task robots-multi-robot tasks (MT-MR) is studied. Considering the energy resource limits and the different sensors equipped in robot, a dynamic coalition structure generation based on credit mechanism (CoSGCrM) approach is proposed with sub-optimal solutions to the coalition formation problem. Compared with the first-price auction method, the feasibility and the higher efficiency in accomplishing tasks of the proposed multi-robot coalition formation task allocation approach has been proved.