Many methods are proposed to improve the flux estimation
and reduce the torque ripple. A full order state observer [7] is
established for stator flux of the PMSM to substitute traditional
pure integrator stator flux observer, but the method only can
accurately estimate the flux linkage in the low speed region of
the PMSM system and the error of the flux linkage also
impossibly tends to zero. The method of zero voltage space
vectors [8] is employed in the DTC system of the PMSM,
having the limited effect of reducing the torque ripple.
Intelligent control theories also are considered to improve the
system performance, such as the fuzzy control [9,10]. The
fuzzy control is applied to the DTC, but the huge computation,
the complex algorithm and the difficulty to realize real-time
control are the limitations of the method