Variable Frequency Inverter with Frequency Tracking Control for Ultrasonic Power Transfer across Metal Barriers

Abstract

In applications that require power transfer across metal barriers, traditionally a wired connection that physically penetrates the metal is used. However, in applications where a wired connection is not convenient or impossible, an emerging wireless power technique called Ultrasonic Power Transfer (UPT) is developed, which transfers power through mechanical vibrations. While the transmission of power across metal barriers has been extensively validated in academic literature, there is a significant lack of power converter development for Loose Contact UPT systems. Currently, theoretical models cannot accurately determine the optimal operating frequency of the Loose Contact UPT system that corresponds to maximum power transfer. In this thesis an empirical approach to guide practical development of Loose Contact UPT systems for optimal frequency tracking is proposed. The measured input admittances of the UPT system is used to characterise the variation in resonant frequencies which can guide the primary power converter and system control design. A practical Loose Contact UPT system is developed using 28 kHz piezoelectric transducers attached to aluminium plates and several Loose Contact setups at varying contact levels are proposed. The effect of loading on the resonant frequencies of the UPT system is investigated for each Loose Contact setup under open and short circuit secondary conditions. A resonant push-pull inverter which operates under zero voltage switching (ZVS) is developed for the primary converter. A switched capacitor network that can vary the operating frequency of the inverter while maintaining ZVS is developed and is improved by using additional capacitor networks for fine tuning the operating frequency of the inverter. To track the optimal operating frequency for maximum power transfer, a frequency sweep method is developed. The operating frequency of the push-pull inverter is varied by coarse tuning of capacitor banks and fine tuning with phase shift control. Experimental results have demonstrated that the proposed frequency sweep controller can accurately track the optimal operating frequency within a range of 26 kHz to 30 kHz under different Loose contact setups and secondary conditions.