  In this paper, we propose a new approach to the attitude control of
quadrotors, by which angular velocity measurements or a model-based observer
reconstructing the angular velocity are not needed. The proposed approach is
based on recent stability results obtained for nonlinear negative imaginary
systems. In specific, by constructing the respective storage functions, we
establish the nonlinear negative imaginary properties of the whole quadrotor
system and the quadrotor rotational subsystem. Then, an inner-outer loop method
will be implemented to synthesize a strictly negative imaginary controller.
This guarantees the robust asymptotic stability of the attitude of the
quadrotor about its reference signal in the face of modeling uncertainties and
external disturbances.
