Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Electrical Engineering

Department

Electrical Engineering

First Advisor

Zeynep Çelik-Butler

Abstract

Vibrational energy harvesters convert kinetic energy from environment to useful electrical energy to power wireless sensor networks. These devices are finding applications in robotics, defense, medical, structural health monitoring, aerospace and wearables. The goal of this dissertation is to improve the design of vibrational electrostatic energy harvesters. Wideband, out-of-plane gap-closing, electret-based electrostatic energy harvesters using nonlinear springs and mechanical stoppers are designed. A unique mechanism is developed to introduce nonlinearity in the force-displacement relationship by varying the spring anchor height relative to the level of the proof-mass in the vertical direction. Both spring softening and hardening responses are achieved. A mathematical model of the electrostatic energy harvester is derived and a linear energy harvester (with linear spring) is optimized. The system is then built using CoventorWare/MEMS+?, where the linear springs are replaced by the novel nonlinear springs. Force-displacement curves are obtained by finite element analysis and fitted to obtain the nonlinear stiffness coefficients. Mechanical stoppers are also designed and integrated with the hardening spring system to further enhance the operational bandwidth of the device. For comparison purposes, two different softening spring and one hardening spring systems, each with the same linear stiffness as the linear energy harvester are designed. The linear energy harvester obtained an acceleration- normalized power density of 7.5 µWs4/cm3-m2 with full width at half maximum bandwidth of 11 Hz, whereas, a softening spring energy harvester produced a normalized power density of 6.6 µWs4/cm3-m2 and bandwidth of 82 Hz. Hardening system with mechanical stoppers produced the highest bandwidth of 231 Hz with a normalized power density of 8.9 µWs4/cm3-m2. Wideband energy harvesters produced significant improvement in bandwidth over the linear counterpart. Moreover, arrays of four out-of-plane, gap-closing, electret-based electrostatic energy harvesters capable to harvest over a wide frequency range are designed. The novel design consists of energy harvesters with linear springs, softening springs, hardening springs and mechanical stoppers. This unique method of designing array with combination of linear and nonlinear generators allows the system to harvest significant amount of power even with shift in vibration frequency. A dynamic model of the system is derived and optimized using MATLAB. A maximum power of 110 nW is obtained over a frequency range of 600-1350 Hz. The design also takes 89 % less volume compared to an array of linear energy harvesters to generate over the same frequency range.

Keywords

Wideband energy harvesting, Nonlinear springs, Mechanical stoppers, Electrostatic energy harvester array

Disciplines

Electrical and Computer Engineering | Engineering

Comments

Degree granted by The University of Texas at Arlington

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