Experimental and numerical investigation of self-heating effects on the through-metal ultrasonic power transfer efficiency

Ultrasonic waves can be efficiently used to power electronic devices sealed inside a metallic enclosure, such as small sensors and other electrical components. Ultrasonic power transfer (UPT) can provide energy to these electronics while maintaining structural integrity in situations where perforating the metallic barrier for tethered charging or battery replacement is not possible. A typical through-metal UPT system consists of two piezoelectric transducers bonded symmetrically on either side of a metallic barrier, where the transducers transmit and receive elastic waves and convert the mechanical energy into electrical energy at the receiver end. For continuous and optimal performance of a UPT system, it is imperative to understand the losses that affect its efficiency. That is, dielectric and mechanical losses within UPT systems cause self-heat generation that can significantly affect the efficiency of the UPT system. In this work, the effects of temperature on UPT efficiency are studied using both numerical and experimental methods. We drive the piezoelectric transducer continuously at various input power levels to study the power transmission efficiency and temperature of the UPT system over time. Multiphysics (coupled electro-mechanical-thermal) finite-element simulations are performed to estimate the temperature profile of the UPT system. We quantify how the power transmission efficiency of continuously-driven UPT systems decreases over time as temperature rises. In addition, numerical simulations show that mechanical losses are the dominant source of losses for the self-heat generation in the UPT system (as compared to dielectric etc.). Our reported results provide insights for optimizing UPT system design and for understanding the self-heat generation characteristics.