Direct Coupling of Electromagnetic Wave to Resonant Piezoelectric Transformer for RF Energy Harvesting

The number of devices connected to the internet is estimated to have reached three times the world population in 2023. Machine-to-machine (M2M) connections, also known as internet-of-things (IoT), are growing rapidly. These include wearables for health monitoring, video surveillance, manufacturing, and tracking devices.

M2M devices are estimated to have reached 14.7 billion in 2023 with a staggering 4.4 billion being mobile. The M2M is the fastest growing mobile category -- even faster than smartphone category. One of the biggest questions related to this growth is: How will mobile M2M devices be powered in the future?

There is currently no solid technological solution to address the power needs for mobile IoT devices. The objective of this project is to explore and study an innovative solution to power future mobile M2M devices. The proposed innovative microsystem enables wireless power transfer missions to sensors (chemical, physical or biological) in remote regions operating under low- or no-power conditions.

The microsystem is comprised of an antenna array integrated with a resonator on one substrate. Therefore, the device offers a unique platform for studying the direct coupling of electromagnetic and acoustic waves. Two types of sensing mechanisms are made possible with the new microsystem. 1) IoT sensing powered by RF energy harvester: Examples include intruder detection, inventory tracking, wearables, and monitoring patients. 2) Direct detection using on-board antenna or resonator: Examples include angle of arrival measurement, wireless sensor networks, radar, and remote sensing.

Enabled by direct coupling of an antenna array with a resonant piezoelectric transformer (RPT), the new microsystem architecture pursued in this project will potentially replace the rectifying antennas (rectennas) that have dominated the RF energy harvesting research landscape for more than 50-years but still have not achieved high efficiency at low RF input power range (less than −30 dBm or 1 microwatt). On the other hand, advances in micro- and nano-mechanical devices, have resulted in the realization of high-Q acoustic resonators (Q>1000) operating in the 1-10 GHz frequency band.

As opposed to a conventional rectenna architecture in which an antenna is coupled to rectifying diodes, in the proposed architecture, energy is coupled from the electromagnetic domain to the mechanical domain using an antenna array, transmission line, and an array of high-Q RPT. The new microsystem will be the first demonstration of direct coupling of electromagnetic energy into the mechanical domain for energy harvesting purposes.

Once the energy is transferred efficiently from free space to a resonator, it is processed in the mechanical domain for unique advantages. Specifically, high passive voltage gain (on the order of 100) can be achieved using resonators of high electromechanical coupling coefficient. As a result of this voltage amplification, RF rectifying diodes are properly biased at higher voltage to operate efficiently.

Therefore, RF energy harvesting with high efficiency under low power condition can be achieved. In addition, it can lead to energy conversion in a wide frequency band by coupling two or more efficient RPTs with asymmetric ports that result in a transformer filter with a significant boost in the output voltage to enable much higher efficiency in RF rectifying diodes placed after the RPT transformer filter.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.