Highly-accurate PLL speed control by adding P+D repetitive control

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 conventional PLL speed control systems. In this article, we propose a P+D repetitive controller to ease this condition. Rather simple implementation with an FPGA and effective experimental results revealing triple reduction in amplitude are shown.