  Teams of UGVs patrolling harsh and complex 3D environments can experience
interference and spatial conflicts with one another. Neglecting the occurrence
of these events crucially hinders both soundness and reliability of a
patrolling process. This work presents a distributed multi-robot patrolling
technique, which uses a two-level coordination strategy to minimize and
explicitly manage the occurrence of conflicts and interference. The first level
guides the agents to single out exclusive target nodes on a topological map.
This target selection relies on a shared idleness representation and a
coordination mechanism preventing topological conflicts. The second level hosts
coordination strategies based on a metric representation of space and is
supported by a 3D SLAM system. Here, each robot path planner negotiates spatial
conflicts by applying a multi-robot traversability function. Continuous
interactions between these two levels ensure coordination and conflicts
resolution. Both simulations and real-world experiments are presented to
validate the performances of the proposed patrolling strategy in 3D
environments. Results show this is a promising solution for managing spatial
conflicts and preventing deadlocks.
