  Quadrotors that can operate safely in the presence of imperfect model
knowledge and external disturbances are crucial in safety-critical
applications. We present L1Quad, a control architecture for quadrotors based on
the L1 adaptive control. L1Quad enables safe tubes centered around a desired
trajectory that the quadrotor is always guaranteed to remain inside. Our design
applies to both the rotational and the translational dynamics of the quadrotor.
We lump various types of uncertainties and disturbances as unknown nonlinear
(time- and state-dependent) forces and moments. Without assuming or enforcing
parametric structures, L1Quad can accurately estimate and compensate for these
unknown forces and moments. Extensive experimental results demonstrate that
L1Quad is able to significantly outperform baseline controllers under a variety
of uncertainties with consistently small tracking errors.
