Modelling and Analysis of Close Contact Ultrasonic Power Transfer Systems with Metal Barriers

Abstract

Traditional methods of powering electrical loads across sealed metallic containers require physical penetration of the metallic body to feed an electrical cable through it, thus reducing the structural integrity of the container. Consequently, wireless power transfer technologies are employed to retain the structural integrity of metallic containers. An emerging wireless power technology that is not limited by metal barriers is Ultrasonic Power Transfer (UPT), using mechanical particle vibrations and electromechanical conversion to enable the electrical power transfer. Power transfer through metal barriers has already been validated in the extant academic literature, but there are many behaviours that have not yet been explored. This thesis endeavours to enhance the understanding of the UPT technology with an emphasis on metal barriers by developing appropriate models that characterize the system behaviour and predict the output characteristics for various mechanical configurations. In this thesis, UPT systems are characterized from a structural perspective rather than by the conventional wave theory employed by existing models. The characteristics of mechanical resonance and coupling for UPT systems are presented, establishing a core understanding of the power transfer mechanism. Mechanical features determining the power transfer capability are identified, which include the attachment contact level, the material compatibility of the medium to transducers, and the transducer misalignment. A particular emphasis is placed on the contact interface, which classifies systems as tight, loose, or no-contact (‘close-contact’ is the term given to systems with tight or loose contact interfaces). The structural and contact models are proposed to describe a variety of close-contact UPT systems, each supported by experimental results. The structural model resembles a tightly coupled magnetic transformer, while the contact model is similar to the equivalent circuit model of an inductive power transfer system, which is acknowledged as a loosely coupled transformer. The mechanical features incorporated in the proposed models can thus be easily understood by drawing parallels to the magnetic-coupled system. The optimal operating frequency for the systems is found to correlate to both the maximum power transfer and efficiency. The proposed models establish the theory of UPT, and provide valuable information to guide the design and advance the UPT technology.