  Drift control is significant to the safety of autonomous vehicles when there
is a sudden loss of traction due to external conditions such as rain or snow.
It is a challenging control problem due to the presence of significant sideslip
and nearly full saturation of the tires. In this paper, we focus on the control
of drift maneuvers following circular paths with either fixed or moving
centers, subject to change in the tire-ground interaction, which are common
training tasks for drift enthusiasts and can therefore be used as benchmarks of
the performance of drift control. In order to achieve the above tasks, we
propose a novel hierarchical control architecture which decouples the curvature
and center control of the trajectory. In particular, an outer loop stabilizes
the center by tuning the target curvature, and an inner loop tracks the
curvature using a feedforward/feedback controller enhanced by an
$\mathcal{L}_1$ adaptive component. The hierarchical architecture is flexible
because the inner loop is task-agnostic and adaptive to changes in tire-road
interaction, which allows the outer loop to be designed independent of
low-level dynamics, opening up the possibility of incorporating sophisticated
planning algorithms. We implement our control strategy on a simulation platform
as well as on a 1/10 scale Radio-Control~(RC) car, and both the simulation and
experiment results illustrate the effectiveness of our strategy in achieving
the above described set of drift maneuvering tasks.
