Date of Award

Summer 8-2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Abstract

Continuous improvement in electronics manufacturing has led to the deployment of low-power sensors, which has resulted in an urgent need for developing energy harvesters capable of generating electric power using abundant and free energy sources such as ambient vibrations. The work presented here is motivated by the growing interest in targeting nonlinear energy harvesting through magnetic interactions, which are compatible with ambient vibration energy sources that are often characterized by a broadband frequency spectrum and can be particularly rich with low frequencies. In this work, experimental and theoretical studies were performed to investigate a magnetic- levitation-based vibration energy harvester that can be switched from a mono-stable to a bi-stable configuration. A mono-stable configuration consists of an oscillating magnet that is levitated between two stationary top and bottom magnets. A cluster of peripheral solid magnets is fixed around the harvester casing and results in a bi-stable configuration. Traditionally, magnetic forces in magnetic-levitation-based harvesters are represented using polynomial functions that are integrated into the equation of motion. In this work, analytical models describing the interaction between magnets were developed and integrated into the equation of motion. Results suggested that, for the bi-stable configuration, the analytical model of magnetic force provides more accurate results compared to those obtained using polynomial functions. Results showed that a variety of load-deflection characteristics can be obtained by changing geometric ratios of the peripheral magnets in the bi-stable configuration. During dynamic operation, the bi-stable configuration exhibits inter-well, chaotic, and intra-well motion at different accelerations. Thinner peripheral magnets are favorable for the bi-stable design, especially at lower acceleration levels. Thinner peripheral magnets yield lower energy barriers, improved frequency responses, and exhibit approximately zero stiffness near equilibrium position. The use of thinner peripheral magnets caused the harvester to move towards monostability; therefore, implying that mono-stability is the favorable mode for vibration energy harvesting under harmonic excitation. Normalized power densities of 5.0 mW cm−3 g −2 at 1.25 g m s −2 and 0.35 mW cm−3 g −2 at 2.5 g m s −2 were measured for mono-stable and bi-stable configurations, respectively.

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