  This paper presents a system for autonomous cooperative wall building with a
team of Unmanned Aerial Vehicles (UAVs). The system was developed for Challenge
2 of the Mohamed Bin Zayed International Robotics Challenge (MBZIRC) 2020. The
wall-building scenario of Challenge 2 featured an initial stack of bricks and
wall structure where the individual bricks had to be placed by a team of three
UAVs. The objective of the task was to maximize collected points for placing
the bricks within the restricted construction time while following the
prescribed wall pattern. The proposed approach uses initial scanning to find a
priori unknown locations of the bricks and the wall structure. Each UAV is then
assigned to individual bricks and wall placing locations and further perform
grasping and placement using onboard resources only. The developed system
consists of methods for scanning a given area, RGB-D detection of bricks and
wall placement locations, precise grasping and placing of bricks, and
coordination of multiple UAVs. The paper describes the overall system,
individual components, experimental verification in demanding outdoor
conditions, the achieved results in the competition, and lessons learned. The
presented CTU-UPenn-NYU approach achieved the overall best performance among
all participants to won the MBZIRC competition by collecting the highest number
of points by correct placement of a high number of bricks.
