A 2-DOF MEMS Ultrasonic Energy Harvester

This paper reports a novel ultrasonic-based wireless power transmission technique that has the potential to drive implantable biosensors. Compared with commonly used radio-frequency (RF) radiation methods, the ultrasonic power transmission is relatively safe for the human body and does not cause electronic interference with other electronic circuits. To extract ambient kinetic energy with arbitrary in-plane motion directions, a novel 2-D MEMS power harvester has been designed with resonance frequencies of 38 520 and 38 725 Hz. Frequency-response characterization results verify that the device can extract energy from the directions of X, Y, and diagonals. Working in the diagonal direction, the device has a bandwidth of 302 Hz, which is twice wider than a comparable 1-D resonator device. A 1-mu F storage capacitor is charged up from 0.51 to 0.95 V in 15 s, when the harvester is driven by an ultrasonic transducer at a distance of 0.5 cm in the X-direction, and is biased by 60 Vdc, indicating the energy harvesting capability of 21.4 nW in the X-direction. When excited along the Y-axis, the harvester has an energy-harvesting capacity of 22.7 nW. The harvester was modeled and simulated using an equivalent electrical circuit model in Saber, and the simulation results showed good agreement with the experimental results. The ultrasonic energy harvesting was also investigated using a 1-D piezoelectric micro-cantilever.