Understanding Far Field Ultrasonic Power Transmission for Automobile Sensor Networks in Free Space

Ultrasonic Power Transmission (UPT) has gained attention for powering implanted diagnostic devices due to its non-invasive properties. However, UPT in free-space is still less exploited with considerable potential for powering Wireless Sensor Networks (WSNs). An important challenge in understanding the significant parameters for a UPT system is to comprehend the losses and quantify the limitations of the technology in terms of distance, frequency and transmission power (ISO226). In this work, the authors attempt to model the transmission link of an UPT system, identifying variables which can be modified for obtaining maximum power output from the wireless power transfer through formulation and experimental results. Ultrasonic transducers of two varying frequencies were used in free space power transmission to understand the absorptive and geometric attenuation of sound waves in air, experimentally. A measured power of 6.3$\mu$W(40kHz) and 8. 3nW(100kHz) was observed over 30cm. These observations may enable acoustic powered WSNs in automobiles powering multi-nodes using single transmitter.