PASSIVELY-SWITCHED VIBRATIONAL ENERGY HARVESTERS A Dissertation Presented By Tian Liu to The Department of Mechanical and Industrial Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the field of Mechanical Engineering Northeastern University Boston, Massachusetts May, 2017 ProQuest Number:10276237 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. ProQuest 10276237 Published by ProQuest LLC ( 2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, MI 48106 - 1346 ii ABSTRACT Vibrational energy harvesters capture mechanical energy from ambient vibrations and convert the mechanical energy into electrical energy to power wireless electronic systems. Challenges exist in the process of capturing mechanical energy from ambient vibrations. For example, resonant harvesters may be used to improve power output near their resonance, but their narrow bandwidth makes them less suitable for applications with varying vibrational frequencies. Higher operating frequencies can increase harvesters’ power output, but many vibrational sources are characterized by lower frequencies, such as human motions. This work presents the design, modeling, optimization, and demonstration of a new class of resonant vibrational energy harvesters that passively switch among dynamics with different characteristic frequencies to adapt to low-frequency excitations and changing vibrational environment. The passively-switched harvester consists of a driving beam that couples into ambient vibrations at low frequencies and a generating beam that converts mechanical energy into electrical energy at high frequencies. The interaction between the driving beam and the generating beam enables multiple characteristic dynamics of the system, namely coupled-motion dynamics and plucked dynamics. When the system passively switches between coupled-motion harvesting and plucked harvesting, its operational range is increased. The system is simulated in the time domain using a lumped element model that predicts power output. Based on the model, a iii passively-switched harvester in which the driving beam faces the generating beam is designed and experimentally tested over an operational range of 0.1g-2.6g and 4 Hz-27 Hz. The experimental results agree with the simulation that the harvester has increased operational range due to the system’s ability of passive switching among multiple dynamics. To create a more compact system, a second passively-switched harvester is created in which the beams are nested together. The nested-beam configuration achieves a 6X smaller device volume while retaining a similar system resonance. The nested-beam harvesters are experimentally tested within an operational range of 0.1g-2g and 5 Hz-22Hz, generating greater than 20 µW over a frequency range of 5.5 Hz-11 Hz at 0.8g. The nested- beam harvester retains a similar normalized power density of 148 µW/cm3g2 at 0.5g and 7Hz as compared with the facing-beam harvester. iv ACKNOWLEDGMENTS There are many people that I would like to acknowledge for their support and help in my PhD program. First of all, I would like to thank my research advisor, Professor Carol Livermore, for her mentoring and guidance throughout my PhD study. Her knowledge and wisdom are embedded in every email and meeting that I had with her, from which I learned not only the knowledge but also how to study and research. It is a great pleasure to work with her and I really enjoyed it. I would like to thank members of my thesis committee, Professor Sinan Muftu, and Professor Matteo Rinaldi, for their feedback and knowledgeable advice to improve my research. I would also like to thank Professor Mohammad E. Taslim for his academic advice at the beginning of the PhD program. I would like to acknowledge the prior student in this project, Ryan St. Pierre in University of Maryland, who started the project of passively-switched energy harvester and accomplished valuable results. I would also like to acknowledge prior student on energy harvesting, William Z. Zhu, for sharing experience and advice on energy harvesting. I would like to thank all members of Professor Livermore’s group for being great labmates and friends: to Sanwei for the sunburn that we had after kayaking on Charles River; to Xin for all the wonderful free food that he discovered on campus; to Chenye for all the great APPs and video games that he recommended; to Majid for the beautiful Irish bar that he found in Dublin; to Will for all the Popeyes chicken that we eat together at v lunch; to Phillipp for all the polymer that we spincoated in the cleanroom; to Ashley for all the origami that we fold by tweezers; to Chase for the delicious chocolate that she made; to all high school and community college students and teachers for playing card games together in Carol’s house. It is great pleasure and a lot of fun to work with everyone in the lab. Finally, I would like to thank my family: to my wife, Luchen Zhang, for her love and encouragement; to my parents, who have been supporting me all the time; and to my uncle Dr. Rongqin Sheng’s family for their caring and advice for my life in United States. vi TABLE OF CONTENTS 1. INTRODUCTION .........................................................................................................1 1.1. Energy harvesting for wireless sensor networks ...................................................3 1.2. Energy harvesting from the human body ..............................................................4 1.3. Thesis goal ............................................................................................................5 1.4. Agenda ..................................................................................................................6 2. LITERATURE REVIEW ..............................................................................................8 2.1. Ambient energy sources ........................................................................................8 2.1.1. Solar energy harvesting ................................................................................9 2.1.2. Thermal energy harvesting ..........................................................................9 2.1.3. RF energy harvesting .................................................................................10 2.2. Vibrational energy harvesters .............................................................................11 2.2.1. Transduction mechanisms of vibrational energy harvesters ......................14 2.2.1.1. Electromagnetic energy harvesters ...............................................14 2.2.1.2. Electrostatic energy harvesters .....................................................16 2.2.1.3. Piezoelectric energy harvesters ....................................................17 2.2.2. Energy capture by vibrational energy harvesters ......................................18 2.2.2.1. Frequency up-conversion .............................................................19 2.2.2.2. Resonance broadening ..................................................................21 vii 2.2.2.3. Resonance tracking .......................................................................23 3. PIEZOELECTRIC TRANSDUCTION .......................................................................26 3.1. Piezoelectric effect ..............................................................................................26 3.2. Governing equations of piezoelectric generator .................................................28 4. MODELING OF PASSIVELY-SWITCHED ENERGY HARVESTER ....................36 4.1. Overview .............................................................................................................36 4.2. Modeling approach .............................................................................................40 4.3. Simulation results................................................................................................52 4.4. Conclusions .........................................................................................................67 5. PASSIVELY-SWITCHED ENERGY HARVESTER: FACING CONFIGURATION ......................................................................................................................................68 5.1. Overview .............................................................................................................68 5.2. Experimental setup ..............................................................................................68 5.3. Experimental results ............................................................................................73 5.4. Conclusions .........................................................................................................89 6. PASSIVELY-SWITCHED ENERGY HARVESTER: NESTED CONFIGURATION ..................................................................................................................................... 91 6.1. Introduction .........................................................................................................91