  Planning a path for a nonholonomic robot is a challenging topic in motion
planning and it becomes more difficult when the desired path contains narrow
passages. This kind of scenario can arise, for instance, when quadcopters
search a collapsed building after a natural disaster. Choosing the quadcopter
as the target platform, this paper proposes the Kinodynamic Aggressive
Trajectory (KAT) motion planning algorithm, which aims to compute aggressive
trajectories for narrow passages under nonholonomic constraints. This type of
maneuvers is necessary because the dynamics of quadcopters entail that some
narrow passages can only be traversed at high speed.
  To find the best path, the KAT uses RRT to determine a holonomic path first
and then adjusts it to satisfy the nonholonomic constraints. The innovations in
this process are: 1) The states of the robot are divided into near-holonomic
set and non-holonomic set, which makes the constraints local rather than
global; 2) For each of the most confined waypoints in the path, KAT plans
forward and backward simultaneously around the waypoint to find a feasible
local trajectory traversing the narrow passage. Our approach thus transforms a
globally-constrained planning problem into a problem with local constraints,
and as a result, the computation becomes tractable. We evaluate KAT by applying
it to a quadcopter flying through two inclined holes that require aggressive
maneuvers in a simulated environment. The average computation time to
successfully find a solution for passing two 50 degree inclined holes is around
32 seconds.
