  Periodic disturbances composed of harmonics usually appear during periodic
operation, impairing performance in mechanical and electrical systems. To
improve the performance, control for periodic-disturbance suppression has been
studied, such as repetitive control and periodic-disturbance observer. For
robustness against perturbations in each cycle, slight changes over cycles,
slight variations in the period, and/or aperiodic disturbances, although
wideband harmonic suppression is expected, the conventional methods have
trade-offs among the wideband harmonic suppression, non-amplification of
aperiodic disturbances, and deviation of harmonic suppression frequencies. This
article proposes a quasi-periodic disturbance observer to estimate and
compensate for a quasi-periodic disturbance. The quasi-periodic disturbance is
defined to consist of harmonics and surrounding signals, based on which the
quasi-periodic disturbance observer is designed using a periodic-pass filter of
a first-order periodic/aperiodic separation filter, time delay integrated with
a zero-phase low-pass filter, and an inverse plant model with a first-order
low-pass filter. For the implementation of the proposed observer, its Q-filter
is discretized by the exact mapping of the s-plane to the z-plane, and the
inverse plant model is discretized by the backward Euler method. The
experiments validated the frequency response and position-control precision of
the quasi-periodic disturbance observer in comparison with conventional
methods.
