Collaborative Robot Exploration and Rendezvous: Algorithms, Performance Bounds and Observations

We consider the problem of how two heterogeneous robots can arrange to meet in an unknown environment from unknown starting locations: that is, the problem of arranging a robot rendezvous. We are interested, in particular, in allowing two robots to rendezvous so that they can collaboratively explore an unknown environment. Specifically, we address the problem of how a pair of exploring agents that cannot communicate with one another over long distances can meet if they start exploring at different unknown locations in an unknown environment. Ours is the first work to formalize the characteristics of the rendezvous problem, in particular aspects relevant to heterogeneous robot systems. We approach it by proposing several alternative algorithms that the robots could use in attempting to rendezvous quickly while continuing to explore. These algorithms exemplify different classes of strategy whose relative suitability depends on characteristics of the problem definition. The algorithms are based on the assumption that potential rendezvous locations, called landmarks, can be selected by the robots as they explore; these locations are based on a distinctiveness measure computed with an arbitrary sensor. We consider the performance of our proposed algorithms analytically with respect to both expectedand worst-case behaviour. We then examine their behaviour under a wider set of conditions using numerical analysis. This numerical analysis is confirmed using a simulation of multi-agent exploration and rendezvous. We also examine the exploration speed, and show that a multi-robot system can explore an unknown environment faster than a single-agent system, even with the constraints of performing rendezvous to allow communication. We conclude with a demonstration of rendezvous implemented on a pair of actual robots.