Entrapment/Escorting and Patrolling Missions in Multi-Robot Cluster Space Control Ignacio Mas, Steven Li, Jose Acain and Christopher Kitts Abstract— The tasks of entrapping/escorting and patrolling around an autonomous target are presented making use of the multi-robot cluster space control approach. The cluster space control technique promotes simplified specification and monitoring of the motion of mobile multi-robot systems of limited size. Previous work has established the conceptual foundation of this approach and has experimentally verified and validated its use for 2-robot, 3-robot and 4-robot systems, with varying implementations ranging from automated trajectory control to human-in-the-loop piloting. In this publication, we show that the problem of entrapping/escorting/patrolling is trivial to define and manage from a cluster space perspective. Using a 3-robot experimental testbed, results are shown for the given tasks. We also revise the definition of the cluster space framework for a three-robot formation and incorporate a robot- level obstacle avoidance functionality. Index Terms— cluster space, multi-robot systems, formation control, robot teams, escorting. I. I NTRODUCTION Robotic systems offer many advantages to accomplishing a wide variety of tasks given their strength, speed, precision, repeatability, and ability to withstand extreme environments [1]. While most robots perform these tasks in an isolated manner, interest is growing in the use of tightly interacting multi-robot systems to improve performance in current appli- cations and to enable new capabilities. Potential advantages of multi-robot systems include redundancy, increased cover- age and throughput, flexible reconfigurability and spatially diverse functionality. For mobile systems, one of the key technical considerations is the technique used to coordinate the motions of the individual vehicles. A wide variety of techniques have been and continue to be explored. Behavioral methods [2], [3], [4] are based on designing a set of actions or behaviors for each element in the group such that desirable group behavior emerges as a result. Another approach makes use of potential fields to represent the desired formation pattern and trajectory [5]. An alternative method uses leader- follower patterns [6], [7]. A variant of this is leader-follower chains, in which follower robots control their position rela- tive to one or more local leaders which, in turn, are following This work has been sponsored through a variety of funding sources to include Santa Clara University Technology Steering Committee grant TSC131 and National Science Foundation Grant No. CNS-0619940. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. The authors are with the Robotic Systems Laboratory, Santa Clara University, 500 El Camino Real, Santa Clara CA 95053, United States. {iamas,sxli,jacain,ckitts}@scu.edu other local leaders in a network that ultimately are led by a designated leader [8]. Despite the wide range of multi-robot coordination meth- ods, to our knowledge only one paper explicitly addressed the escort/entrapment task [9]. Our work explores this issue using the cluster space control method, a specific centralized approach for robot clusters of limited size (on the order of ones to tens) and locale (such that global communication is available). The motivation of the cluster space [1] approach is to pro- mote the simple specification and monitoring of the motion of a mobile multi-robot system. This strategy conceptualizes the n-robot system as a single entity, a cluster, and desired motions are specified as a function of cluster attributes, such as position, orientation, and geometry. These attributes guide the selection of a set of independent system state variables suitable for specification, control, and monitoring. These state variables form the system’s cluster space. Cluster space state variables may be related to robot-specific state variables, actuator state variables, etc. through a formal set of kinematic transforms. These transforms allow cluster commands to be converted to robot-specific commands, and for sensed robot-specific state data to be converted to cluster space state data. As a result, a supervisory operator or real- time pilot can specify and monitor system motion from the cluster perspective. Our hypothesis is that such interaction enhances usability by offering a level of control abstraction above the robot- and actuator-specific implementation de- tails. Previous work presented a generalized framework for de- veloping the cluster space approach for a system of n robots, each with m degrees of freedom (DOF)[1]. This frame- work was successfully demonstrated for both holonomic and non-holonomic two-robot systems, including several cluster- space-based versions of regulated motion [10], automated trajectory control [11], [12], human-in-the-loop piloting [13], [14], and potential field-based obstacle avoidance [15]. The method was also implemented for three-robot [16] and four- robot [17] non-holonomic planar systems, as well as two- and three-robot surface ship [18] and aerial systems [19], [20]. In this publication, we show that the problem of entrap- ping/escorting/patrolling is trivial to define and manage from a cluster space perspective. Using a 3-robot experimental testbed, results are shown for such tasks. Additionally, a robot-level obstacle avoidance functionality is added in order to prevent collisions between the robots and the tracked target. The 2009 IEEE/RSJ International Conference on Intelligent Robots and Systems October 11-15, 2009 St. Louis, USA 978-1-4244-3804-4/09/$25.00 ©2009 IEEE 5855