  Despite extensive research on sliding mode control (SMC) design for
quadrotors, the existing approaches suffer from certain limitations. Euler
angle-based SMC formulations suffer from poor performance in high-pitch or
-roll maneuvers. Quaternion-based SMC approaches have unwinding issues and
complex architecture. Coordinate-free methods are slow and only almost globally
stable. This paper presents a new six degrees of freedom SMC flight controller
to address the above limitations. We use a cascaded architecture with a
position controller in the outer loop and a quaternion-based attitude
controller in the inner loop. The position controller generates the desired
trajectory for the attitude controller using a coordinate-free approach. The
quaternion-based attitude controller uses the natural characteristics of the
quaternion hypersphere, featuring a simple structure while providing global
stability and avoiding unwinding issues. We compare our controller with three
other common control methods conducting challenging maneuvers like flip-over
and high-speed trajectory tracking in the presence of model uncertainties and
disturbances. Our controller consistently outperforms the benchmark approaches
with less control effort and actuator saturation, offering highly effective and
efficient flight control.
