V. CONCLUSION AND TECHNICAL CONTRIBUTIONS OF PRESENTED WORK
8. Acknowledgements
The authors would like to thank the National Science Foundation (Grant #1035627) for providing financial support that made the presented research possible. The authors would also like to thank Mark E.
Hofacker for his work on the design and assembly of the experimental harvester setup, as well as his contribution to the theory and subsequent development of the power harvesting ratio.
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APPENDIX B: SUBMITTED JOURNAL PAPER
Assessing Stability and Predicting Power Generation of Electromagnetic Vibration Energy Harvesters Using Physical Bridge Vibration Data
Alexander V. Pedchenko, Janette J. Meyer, and Eric J. Barth
Vanderbilt University Nashville, TN
Submitted for publication to IEEE/ASME Transactions on Mechatronics on Novemer 19th, 2015
54 1. Abstract
This paper presents the use of the power harvesting ratio (PHR) approach for evaluating the power harvesting capabilities of an electromagnetic vibration energy harvester. This was done for different electrical loads and measured bridge vibration data displaying multiple frequency components. Bridge vibration data was collected and characterized. The modes of the bridge were determined using a model sledge hammer, and the response of the bridge to a single vehicle was measured. Analysis of the data revealed that several of the modes contributed toward a response with multiple non-negligible frequency components. Measured bridge time-series data was then replayed on an experimental setup with an electromagnetic vibration energy harvester. Six electrical loads were implemented on the experimental platform: four passive loads and two active loads. The PHR approach was used to predict the average power from each load. Experimentally measured average power was within 6% of the predicted average power.
The PHR approach was also used to successfully predict harvester instability for the active load dictated by the maximum power transfer theorem, and validated experimentally. This paper demonstrates the utility of the PHR approach in evaluating harvester stability and performance for multifrequency excitations and sophisticated electrical loads including active loads.
2. Introduction
According to the most recent data collected by the National Bridge Inventory, in 2013 approximately 25% of U.S. bridges were classified as “structurally deficient” or “functionally obsolete” [1]. Most bridges are inspected only once every 24 months with the caveat that “structures with advanced deterioration or other conditions warranting close monitoring may be inspected more frequently” [3]. The poor state of many of the nation’s bridges coupled with the need to inspect those in particularly bad condition more often results in a large demand for frequent structural monitoring that cannot be fulfilled in a timely manner. One way to address this demand is through implementation of autonomous structural monitoring.
There have been multiple sensing options proposed for structural health monitoring. Strain gauge placement at key locations [5], pattern recognition of bridge acceleration data [6], and acoustic emissions monitoring [7] are just some of the suggested techniques to detect structural deterioration. These sensing methods share one advantageous characteristic – they can be implemented to function remotely.
Ultimately, remote condition monitoring requires that the sensors acquire pertinent data concerning the state of the bridge, record it, and transmit it to a location where the data can be analyzed and processed. All of these processes require electrical power, which ideally, in order to make the sensor network truly autonomous, would come from a source requiring no maintenance. Hard-wiring power and data transfer cables to such a network is difficult and expensive [8]. Additionally, it is preferable for this power source
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to be “install and forget,” which makes batteries a subpar solution due to the need for periodic replacement or recharging. One suitable solution is the implementation of an energy harvesting device capable of scavenging power from its surroundings.
Although there may be multiple domains from which energy can be harvested, some, like solar and wind energy, possess some drawbacks for this particular application. Explicitly, a bridge can be exposed to a series of overcast or windless days, resulting in insubstantial power generation from these types of power sources. Conversely, since regular bridge use is accompanied by some degree of mechanical vibrations, electrical power generation via electromechanical vibration energy harvesting does not feature this drawback. This type of energy harvesting would ensure that when the bridge is being used, at least some amount of power is being generated by the power supply for the sensor.
There has been a significant amount of research aimed at improving the power output of vibration energy harvesters. The need for this research stems from the fact that conventional vibration energy harvesters are capable of performing well (producing significant power) only at a single frequency [15, 19, 21]. Several techniques have been proposed to address this shortcoming and enable vibration energy harvesting across a wide frequency bandwidth.
For example, research conducted in [23] and [24] demonstrates the effectiveness of generator arrays.
These generator arrays are devices in which multiple conventional cantilever beam generators with marginally different natural frequencies are used in a parallel configuration. The natural frequencies of the incorporated generators thereby comprise a frequency bandwidth within which energy capture can occur.
Another approach to addressing the single effective operation frequency problem is to enable the harvester’s natural frequency to be tunable. This technique was studied by Zhu et al., [9], Challa et al., [16], and Constantinou et al., [41]; though differing in specifics of implementation, all three research groups proposed tuning methods which relied on a controllable separation between grounded and harvester- mounted magnets to alter the stiffness, and thereby, the natural frequency of their respective harvesters.
Peters et al., [14] showed that the stiffness/natural frequency tuning technique can also be accomplished by using piezoelectric actuators. Some research has also been carried out in tuning methods that obviate the need for physical actuation. Cammarano et al., [15] and Bowden et al., [26] propose tuning the harvester’s natural frequency by adjusting the electrical load.
Generator arrays and natural frequency tuning are two major subgroups of solutions for expanding harvesters’ operational bandwidths. However, the literature also features other unique solutions that are not classified by these two methods. For example, Tang and Li’s two-stage vibratory structure employs multimode vibration coupling to achieve a wide frequency band [42].
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Despite a large number of emerging techniques being proposed for developing multifrequency energy harvesting, there is a lack of general analytical tools that can be used to drive their development and assess their feasibility and efficacy. Two such tools have been introduced in [43].
The first of the two tools aids in assessing the stability of harvester dynamics given a particular choice of electrical load. Section 6.1 will demonstrate the importance of including this assessment whenever there is consideration of using an active electrical load for the harvester.
The second tool was developed to address the need for a better metric for harvester power generation.
Beeby et al., argued that current power metrics are not “ideal” as they “…ignore important factors such as bandwidth,” stating that “insufficient data exists in literature to enable this (bandwidth considerations) to be included” [28]. This literature gap was the driving factor for the development of a power metric named the “power harvesting ratio” (PHR) in [43]. This power harvesting ratio assists in the quantification of the amount of power delivered by a harvester to a particular electrical load at any specified frequency component of the excitation. The plot resulting from employing PHR enables prompt identification of frequency components which provide significant contributions to generated power in multifrequency excitation.
The purpose of the present manuscript is to demonstrate the practical applicability and usefulness of these tools via experimental validation using real-world bridge data. As will be shown in Section 4, bridge vibrations can be comprised of multiple substantial frequency components. In such cases, it becomes important to identify and quantify potentially significant power contributions when evaluating a candidate load. The manuscript will show that this identification and quantification process is more easily accomplished using the proposed power harvesting ratio than conventional power-estimating methods, which usually consider only a single, presumably dominant frequency.
The rest of this manuscript is organized as follows: Section 3provides a review of the two analytical tools introduced in [43]. Section 4 describes the process of acquiring and analyzing real-world bridge data; the section also presents the two excitation time series data sets used in the experimentation. Section 5 describes the experimental platform used for validation. This section includes descriptions of a custom-made electromagnetic vibration energy harvester and the electrical loads used for power generation analysis.
Section 6 describes the methodology; it describes how to apply the analytical tools described in Section 3 to the dynamics of the experimental harvester, the excitation data, and the employed electrical loads to draw conclusions about harvester stability and average power generation. Section 7 describes and presents the results of the experimentation carried out to validate and assess the accuracy of the claims and techniques presented in Section 6. Section 8 summarizes the findings of the research described in this manuscript.
Lastly, Section 9 comprises a short list of individuals and organizations that the authors would like to thank for their valuable assistance.
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