  In this paper, we present a Nonlinear-Proportional Integrator (N-PI)
disturbance observer (DOB) to enhance the motion tracking of the performance of
a surface-mounted Permanent Magnet Synchronous Motor (SPMSM) in rapidly speed
varying regions. By presenting an N-PI-DOB for load torque estimation with
torque modulation technique, we show that the tracking error dynamics of
angular position/velocity are coupled with tracking errors of currents loop and
estimation errors. After analyzing disturbances of currents tracking error
dynamics, we design the N-PI-DOB and Lyapunov-based nonlinear currents
controller to enhance the motion tracking performances. With these N-PI-DOBs
and motion controllers, we analyze the stability of motion tracking error
dynamics and estimation error dynamics. We experimentally perform a comparative
study with/without the N-PI-DOB to verify the effectiveness of the proposed
method in the condition of the unknown load torque and rapidly speed-varying.
