Autonomous Quadrotor Landing on Inclined Surfaces Using Perception-Guided Active Asymmetric Skids

We present an autonomous quadrotor capable of safely landing on sloped surfaces up to 40
$^{\circ }$
, intended for emergency scenarios where the only terrain available for landing may be sloped. This system uses a downward-facing depth perception system to determine the direction, angle, and smoothness of the slope. Two robotic landing skids of different lengths actively conform to the slope in order to maintain level body attitude upon landing. We developed an analytical model to design leg lengths, conform to the slope surface angle, and ensure both zero tilt angle of the quadrotor body and clearance of the propellers from the surface. The selection of skid angles is framed as an optimization to match the slope angle while maximizing the buffer between the propellers and the surface. An eigenvalue decomposition of the point cloud covariance matrix provides a surface normal vector, which is used to determine the proper skid angle and yaw angle of the quadrotor. The ratio of the eigenvalues is used to determine whether the surface is sufficiently smooth for safe landing. The proposed system and method were validated in a motion capture environment by conducting five autonomous takeoff and land missions over different sloped surfaces ranging from 0
$^{\circ }$
to 40
$^{\circ }$
. The detected slope angle and direction of all trials were within 1
$^{\circ }$
and 3.3
$^{\circ }$
, respectively, of ground truth. No failures or crashes occurred during testing, which demonstrates the viability and robustness of this system for use in real-world scenarios.