Highly Precise Rotational Speed Control by a Hybrid of PLL and Repetitive Control

Abstract

PLL speed control system has excellent ‘macroscopic’, or long-term, steadiness. However, ‘microscopic’ speed fluctuation is another important issue. It is called ‘cogging,’ caused by electro-magnetic structural property of motor. Although repetitive control is useful for reducing such fluctuation, stability condition is hard to be satisfied for PLL which inevitably includes integrator. In this article, we propose a D repetitive controller to ease this condition. Rather simple implementation with an FPGA and effective experimental results revealing 60% reduction in fluctuation are shown.