Date of Award

5-2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

John R Wagner, Committee Chair

Committee Member

Gregory Mocko

Committee Member

Todd Schweisinger

Abstract

Experts estimate that approximately one third of the worldwide population currently owns a smartphone, and subscriptions continue to grow. Compared to mobile devices of the past decade, smartphones provide desktop computer-level processing power in a palm-sized package. However, the high computing power and 24 hours - 7 days a week connectivity results in a shorter battery life, often forcing the user to rely on portable battery packs. Worldwide energy consumption statistics show that the electric power grid depends primarily on fossil fuels. Thus, a renewable power source based on human motion energy harvesting offers a potential solution to power portable communication devices and may help reduce dependence on the power grid.

A novel wrist-worn energy-harvester, based on an automatic winding mechanism, was designed, fabricated and experimentally tested. The mechanism frequently employed in wrist and pocket watches dates back to the 18th century, and is one of the oldest examples of mobile human energy harvesting. In this project, the prototype device contains a rotary pendulum connected to a DC generator through a planetary gear train. An electronics module consisting of a rectifier and boost converter filters the generator output, supplying regulated DC output to charge a battery, and/or power an electrical load. An onboard microcontroller broadcasts the voltage, current, and power data wirelessly for data collection during testing.

Numerical and experimental validations were conducted for the energy harvester. A mathematical model for human arm swing dynamics was developed based on a triple pendulum system, and the device’s behavior was studied for both walking and running activities. The mechanical energy output from the rotary harvester pendulum was predicted to be 0.42 mJ and 2.06 mJ for simulated walking and running sequences over a period of 5 seconds (without load). A subsequent mathematical model was developed incorporating the electromechanical behavior of the generator and attached electronics module. A simulated running sequence with a representative electrical load yielded 1.72 mJ of electrical energy output over 5 seconds. The prototype was experimentally validated over the same conditions, resulting in an unregulated energy output of 1.39 mJ and a regulated energy output at 5 VDC of 1.16mJ for 5 seconds. Experimental testing successfully demonstrated the harvester’s potential as a mobile energy source for portable consumer electronics. Future steps shall focus on implementing efficient components for increased power output and designing for improved ergonomics.

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