Quasiperiodic Disturbance Observer for Wideband Harmonic Suppression

Periodic disturbances composed of harmonics typically occur during periodic operations, impairing the 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 tradeoffs among harmonic suppression bandwidth, amplification of aperiodic disturbances, and deviation of harmonic suppression frequencies. This article proposes a quasiperiodic disturbance observer (QDOB) to compensate for quasiperiodic disturbances while simultaneously achieving the wideband harmonic suppression, nonamplification 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 QDOB 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. The experiments validated the frequency response and position-control precision of the QDOB while comparing it with conventional methods.