  We present an online centralized path planning algorithm to cover a large,
complex, unknown workspace with multiple homogeneous mobile robots. Our
algorithm is horizon-based, synchronous, and on-demand. The recently proposed
horizon-based synchronous algorithms compute all the robots' paths in each
horizon, significantly increasing the computation burden in large workspaces
with many robots. As a remedy, we propose an algorithm that computes the paths
for a subset of robots that have traversed previously computed paths entirely
(thus on-demand) and reuses the remaining paths for the other robots. We
formally prove that the algorithm guarantees complete coverage of the unknown
workspace. Experimental results on several standard benchmark workspaces show
that our algorithm scales to hundreds of robots in large complex workspaces and
consistently beats a state-of-the-art online centralized multi-robot coverage
path planning algorithm in terms of the time needed to achieve complete
coverage. For its validation, we perform ROS+Gazebo simulations in five 2D grid
benchmark workspaces with 10 Quadcopters and 10 TurtleBots, respectively. Also,
to demonstrate its practical feasibility, we conduct one indoor experiment with
two real TurtleBot2 robots and one outdoor experiment with three real
Quadcopters.
