  In this paper, we consider the problem of collectively exploring unknown and
dynamic environments with a decentralized heterogeneous multi-robot system
consisting of multiple units of two variants of a miniature robot. The first
variant-a wheeled ground unit-is at the core of a swarm of floor-mapping robots
exhibiting scalability, robustness and flexibility. These properties are
systematically tested and quantitatively evaluated in unstructured and dynamic
environments, in the absence of any supporting infrastructure. The results of
repeated sets of experiments show a consistent performance for all three
features, as well as the possibility to inject units into the system while it
is operating. Several units of the second variant-a wheg-based wall-climbing
unit-are used to support the swarm of mapping robots when simultaneously
exploring multiple floors by expanding the distributed communication channel
necessary for the coordinated behavior among platforms. Although the
occupancy-grid maps obtained can be large, they are fully distributed. Not a
single robotic unit possesses the overall map, which is not required by our
cooperative path-planning strategy.
