Formation Control for Non-Holonomic Mobile Robots: A Hybrid Approach

Many cooperative tasks in real world environments, such as exploring, surveillance, search and rescue, transporting large objects and capturing a prey, need the robots to maintain some desired formations when moving. Formation control refers to the problem of controlling the relative position and orientations of robots in a group, while allowing the group to move as a whole. Problems in formation control that have been investigated include assignment of feasible formations, moving into formation, maintenance of formation shape (Desai et al., 2001) and switching between formations (Desai et al., 1999; Fierro et al., 2002). The work in (Das et al., 2002) is a very good example of the state of the art in robot formation control, in which it is presented a complete framework to achieve a stable formation for car-like and unicycle-like mobile robots. A feasible solution to address these problems is by using hybrid control systems in formation control. In fact, several papers can be found in the literature using hybrid control systems: including a discrete event system at the supervisory level and continuous controllers to give the control actions (Desai et al., 1999; Ogren & Leonard, 2003; Chio & Tarn, 2003; Ogren, 2004; Shao et al., 2005). In this chapter, a hybrid approach for the autonomous navigation of a mobile robots team in a specified formation is developed considering a centralized leader-follower controller (Gava et al., 2007) (see for example (Shao et al., 2005) for a review on the leader-following method) and the non-holonomic constraint of the unicycle-like mobile robots (Gulec & Unel, 2005). In this last paper, the authors state that a complicated coordinated task can be interpreted in terms of simpler coordinate tasks that are to be manipulated sequentially. The leader robot of the team, which navigate independently according to its own control laws, has a laser range-finder, odometry sensors and an omnidirectional camera, whereas the followers have odometry and collision (sonar) sensors. The laser range-finder and odometry sensors of the leader robot are used to implement the leader robot controller (Toibero et al., 2007); and the omnidirectional camera is used to identify the follower postures relative to the leader coordinate system needed in the implementation of the centralized formation controller (Gava et al., 2007). The existence of such a relative sensor is not a constraint since the leader could get access to these positions using another absolute position sensor such as, for instance, a GPSs or odometry and then convert them to the framework attached to the leader robot. In addition, the centralized control architecture, where the control actions for all the followers are generated by the leader, could be decentralized by allowing the O pe n A cc es s D at ab as e w w w .in te hw eb .c om