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In repetitive control (RC), the enhanced servo performance at the fundamental frequency and its higher order harmonics is usually followed by undesired error amplifications at other frequencies. In this paper, we discuss a new structural configuration of the internal model in RC, wherein designers have more flexibility in the repetitive loop-shaping design, and the amplification of nonrepetitive errors can be largely reduced. Compared to conventional RC, the proposed scheme is especially advantageous when the repetitive task is subject to large amounts of nonperiodic disturbances. An additional benefit is that the transient response of this plug-in RC can be easily controlled, leading to an accelerated transient with reduced overshoots. Verification of the algorithm is provided by simulation of a benchmark regulation problem in hard disk drives, and by tracking-control experiments on a laboratory testbed of an industrial wafer scanner.
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... However, it should be noted that the non-integer delay coefficients approximated by the Lagrange interpolation are changed with the non-integer delay parameter so that the fractional filter should be redesigned for signals with different frequencies. Furthermore, to enhance the flexibility when implementing RC, a spectrum-selection filter was integrated in RC to balance the convergence speed and robustness to external disturbances [34,35]. However, the delay number should be an integer when implementing the above controllers. ...
... For the general RC [21] or oddharmonic RC [22], the main drawback is that the errors or disturbances can be amplified if the frequency spectrum is located at the non-harmonics of the fundamental frequency due to the waterbed effect. In [34], the RC scheme using spectrum-selection filter was proposed to make a trade-off between the convergence speed and robustness. ...
... To generate notches at the fundamental frequency and its harmonics, and to eliminate the distortion in the passband, a spectrumselection filter is designed as [34,35] ...
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