Harmonic current source based on fractional order FIR repetitive control

Harmonic current sources (HCS) that can provide variable-frequency and amplitude output current are widely used in automatic testing of the electrical equipments. Digital repetitive control (RC) can achieve zero steady-state error tracking of any periodic signal with known integer period, hence, it's suitable for harmonic control, but it usually fails to provide precise tracking of variable frequency periodical waveform with fractional time delay. In this paper, a fraction order delay FIR filter based on Lagrange interpolation is used to approximate the factional delay items in RC. The design criteria is summarized in a simple form, and the stability of Fractional order RC is also analyzed in this paper, experiments is conducted to verify the effectiveness of the proposed FORC strategy.