Vibration energy harvesting for low power and wireless applications

Vibration energy harvesting is an attractive technique for the potential powering of wireless sensors and low power devices. While the technique can be employed to harvest energy from ambient and mechanical vibrations, there are several generic requirements independent of the energy transfer mechanism that needs to be satisfied for efficient energy harvesting which are pursued here. For example, most energy harvesting devices developed are based on a single resonance frequency, and while recently efforts are being attempted to broaden the frequency range of the devices, lacking is a robust frequency tunable technique. In this work, a resonance frequency tunable mechanism employing magnetic force/stiffness technique is developed that allows the device frequency to increase or decrease based on the mode (attractive, repulsive) of the magnetic force applied. The developed technique provides the device to tune to approximately +/- 25% of its untuned resonance frequency allowing a wide frequency bandwidth. Further, this technique is developed into a self-tunable technique for autonomous device development. Another generic requirement is to match the electrical damping to the mechanical damping in the energy harvesting system for maximum efficiency. To satisfy this requirement, two independent energy harvesting techniques (piezoelectric and electromagnetic) are coupled through design, resulting in ∼30% and ∼65.5% increase in two different independent devices. Another key requirement is developing MEMS scale energy harvesting devices that will not only promises to enhance the power density but also allows potential integration with wireless sensors as an on-chip power source. Piezoelectric MEMS composite structures along with integrated silicon tip masses are fabricated using standard microfabrication techniques. Spray coating and spin coating techniques were explored to deposit zinc oxide as the piezoelectric material. Commercially available magnets were employed to demonstrate the frequency tuning/calibration of these structures resulting in ∼50% bandwidth. These approaches set the stage for a MEMS scale frequency tuning mechanism to be developed that would allow the integration of a self-tunability methodology for a completely autonomous MEMS scale energy harvesting devices for wide applications.