  Collision detection and recovery for aerial robots remain a challenge because
of the limited space for sensors and local stability of the flight controller.
We introduce a novel collision-resilient quadrotor that features a compliant
arm design to enable free flight while allowing for one passive degree of
freedom to absorb shocks. We further propose a novel collision detection and
characterization method based on Hall sensors, as well as a new recovery
control method to generate and track a smooth trajectory after a collision
occurs. Experimental results demonstrate that the robot can detect and recover
from high-speed collisions with various obstacles such as walls and poles.
Moreover, it can survive collisions that are hard to detect with existing
methods based on IMU data and contact models, for example, when colliding with
unstructured surfaces, or being hit by a moving obstacle while hovering.
