Quasiperiodic Disturbance Observer for Wideband Harmonic Suppression

Abstract: Periodic disturbances composed of harmonics typically occur during periodic operations, impairing performance of mechanical and electrical systems. To improve the performance, control of periodic-disturbance suppression has been studied, such as repetitive control and periodic-disturbance observers. However, actual periodic disturbances are typically quasiperiodic owing to perturbations in each cycle, identification errors of the period, variations in the period, and/or aperiodic disturbances. For robustness against quasiperiodicity, although wideband harmonic suppression is expected, conventional methods have trade-offs among harmonic suppression bandwidth, amplification of aperiodic disturbances, and deviation of harmonic suppression frequencies. This paper proposes a quasiperiodic disturbance observer to compensate for quasiperiodic disturbances while simultaneously achieving the wideband harmonic suppression, non-amplification of aperiodic disturbances, and proper harmonic suppression frequencies. A quasiperiodic disturbance is defined as comprising harmonics and surrounding signals. On the basis of this definition, the quasiperiodic disturbance observer is designed using a periodic-pass filter of a first-order periodic/aperiodic separation filter for its Q-filter, time delay integrated with a zero-phase low-pass filter, and an inverse plant model with a first-order low-pass filter. The periodic-pass filter achieves the wideband harmonic suppression while the zero-phase and first-order low-pass filters prevent the amplification of aperiodic disturbances and deviation of harmonic suppression frequencies. For the implementation, the Q-filter is discretized by an exact mapping of the s-plane to the z-plane, and the inverse plant model is discretized by the backward Euler method.