A Clockless Synchronisation Framework for Cooperating Mobile Robots

Cooperating mobile robots are real-time systems that often require mutual synchronisation, either to carry out cooperative sensing and actuation, or to improve the quality of wireless communications. Concerning this last aspect, a common technique to improve the communication channel is to eliminate access collisions by allocating predefined disjoint time slots to robots, in a circular list, which is known as Time Division Multiple Access (TDMA). This technique typically requires a global clock to identify each slot. However, this method is not robust with respect to asynchronous transmissions generated by external or joining nodes. Consequently, this work proposes a global TDMA protocol that allows for real-time and guaranteed delivery of messages within deadlines, given its predictable schedule, and that: i) applies to dynamic mesh networks of cooperating mobile robots; ii) synchronises slots in a relative fashion using locally perceived delays of message exchanges that are globalised throughout the network, thus not relying on a global clock; and iii) tolerates external traffic and asynchronous joining robots using underneath standard ad-hoc wireless RF technologies that provide CSMA-type arbitration. We describe our protocol and prove that under common operating conditions all robots eventually reach synchronisation. We also propose a heuristic for the few cases that were not covered by the previous proof, which always led to consensus under extensive simulation testing. To the best of our knowledge, this is the first guaranteed clockless synchronisation approach for ad-hoc networks of mobile robots that works over commodity wireless protocols.