Low Frequency Microscale Energy Harvesting
Apo, Daniel Jolomi
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The rapid advancement in complimentary metal-oxide-semiconductor (CMOS) electronics has led to a reduction in the sizes of wireless sensor networks (WSN) and a subsequent decrease in their power requirements. To meet these power requirements for long time of operation, energy harvesters have been developed at the micro scale which can convert vibration energy into electrical energy. Recent studies have shown that for mechanical-to-electrical conversion at the mm-scale (or micro scale), piezoelectric mechanism provides the best output power density at low frequencies as compared to the other possible mechanisms for vibration energy harvesting (VEH). However, piezoelectric-based VEH presents a fundamental challenge at the micro scale since the resonance frequency of the structure increases as the dimension decreases. Electromagnetic induction is another voltage generation mechanism that has been utilized for VEH. However, the electromagnetic induction based VEH is limited by the magnet and coil size and the decrease in power density at the micro scale. Hybrid energy harvesting is a novel concept that allows for increased power response and increased optimization of the generated voltage. The work in this field is currently limited due to integration challenges at small dimensions. An effective design for low frequency piezoelectric VEH is presented in this work. A unique cantilever design called arc-based cantilever (ABC) is presented which exhibits low natural frequencies as compared to traditional cantilevers. A general out-of-plane vibration model for ABCs was developed that incorporated the effects of bending, torsion, transverse shear deformation and rotary inertia. Different configurations of micro ABCs were investigated through analytical modeling and validation experiments. ABC structures were fabricated for dual-phase energy harvesting from vibrations and magnetic fields. Next, a levitation-induced electromagnetic VEH concept based on double-repulsion configuration in the moving magnet composite was studied. Computational modeling clearly illustrated the advantages of the double-repulsion configuration over the single-repulsion and no-repulsion configurations. Based on the modeling results, an AA battery-sized harvester with the double-repulsion configuration was fabricated, experimentally characterized and demonstrated to charge a cell phone. The scaling analysis of electromagnetic energy harvesters was conducted to understand the performance across different length scales. A micro electromagnetic harvester was developed that exhibited softening nonlinear spring behavior, thus leading to the finding of nonlinear inflection in magnetically-levitated electromagnetic harvesters. The nonlinear inflection theory was developed to show its causal parameters. Lastly, a coupled harvester is presented that combines the piezoelectric and electromagnetic voltage mechanisms. The advantages of each mechanism were shown to positively contribute to the performance of hybrid harvester. The cantilever provided low stiffness, low frequency, and pure bending, while the magnetic system provided nonlinearity, broadband response, and increased strain (and thus voltage).
- Doctoral Dissertations