  In practical applications, autonomous quadrotors are still facing significant
challenges, such as the detection and avoidance of very small and even dynamic
obstacles (e.g., tree branches, power lines). In this paper, we propose a
compact, integrated, and fully autonomous quadrotor system, which can fly
safely in cluttered environments while avoiding dynamic small obstacles. Our
quadrotor platform is equipped with a forward-looking three-dimensional (3D)
light detection and ranging (lidar) sensor to perceive the environment and an
onboard embedded computer to perform all the estimation, mapping, and planning
tasks. Specifically, the computer estimates the current pose of the UAV,
maintains a local map (time-accumulated point clouds KD-Trees), and computes a
safe trajectory using kinodynamic A* search to the goal point. The whole
perception and planning system can run onboard at 50Hz with careful
optimization. Various indoor and outdoor experiments show that the system can
avoid dynamic small obstacles (down to 20mm diameter bar) while flying at 2m/s
in cluttered environments. Our codes and hardware design are open-sourced on
Github.
