Contactless Ultrasonic Energy Transfer: Acoustic-Piezoelectric Structure Interaction Modeling and Performance Enhancement

The harvesting of ambient vibrations for powering wireless electronic components has been heavily researched over the last decade. As long as sufficient vibrational energy is readily available in the neighborhood of small electronic devices, it is possible to achieve mechanical-to-electrical energy conversion by means of a proper transduction mechanism and thereby enable self-powered wireless electronic systems. An alternative scenario is the case in which the wireless electronic component has little or no vibrational energy available in its environment, yet wireless charging of its battery is still of great interest. Examples to this scenario range from medical implants to underwater sensor networks. The goal in such systems is to charge the electronic component by contactless transfer of energy without relying on mechanical or electrical connection to the wireless component. The commonly used transduction methods of wireless energy transfer are known to be inductive, capacitive, far-field electromagnetic, and optical coupling. Acoustic energy transfer based on the propagation of energy at ultrasonic frequencies is a recently explored alternative. As this field receives growing attention, it is required to develop fully coupled modeling framework to quantify the energy transfer characteristics with a focus on the transmitter, receiver, fluid medium, as well as geometric and material parameters. In this paper, we present multiphysics modeling of contactless ultrasonic energy transfer for wireless electronic components submerged in liquid medium. The source is a pulsating sphere and the receiver is a piezoelectric transducer operating in the 33-mode of piezoelectricity with fundamental underwater resonance frequency above the audible frequency range. The goal is to quantify the electrical power transmitted from the source to the electrical load across the electrodes of the receiver in terms of the source strength for a given distance between the transmitter and the receiver. Both analytical and finite-element models are developed for the resulting acoustic-piezoelectric structure interaction problem. Fixed-free and free-free mechanical boundary conditions are considered for the fluid loaded receiver to validate the analytical model against multiphysics finite-element simulations. For the electrical boundary conditions across the electrodes, resistive and resistive-inductive loads are considered. Specifically broadband power transfer is achieved by optimal resistive-inductive load tuning for performance enhancement. Effects of various system parameters on the electroelastic response are explored. The analytical model developed in this work can be used to predict and optimize the multiphysical system dynamics with very good accuracy and substantially improved computational efficiency as compared to finite-element modeling using commercial packages.