The Power Harvesting Ratio: Design and Power Estimation of Vibration Energy Harvesters

Abstract

Due to approximately one quarter of the bridges in the United States being classified as “functionally obsolete” or “structurally deficient”, there is currently a large demand for frequent bridge inspection. This demand can be addressed by remote structural health monitoring. The sensors and data transmission equipment necessary to implement remote monitoring requires electrical power, and wiring sensor networks to bridge power lines is expensive, while batteries require regular maintenance/replacement. By expanding the characteristically narrow operational bandwidth of conventional vibration energy harvesters, these devices can serve as local power sources for structural monitoring networks by harnessing the mechanical energy from vibrations produced during typical bridge use. The presented research contributes to the developing field of multifrequency vibration energy harvesting by introducing two new techniques for linear harvesters. The first is a method for analyzing the effect of an electrical load on the dynamic stability of a harvester. This technique aids the recent research interest in the use of active loading to increase power generation by serving as a means to determine whether the chosen active load results in stable dynamics and, therefore, can actually be utilized. The second introduced tool is a technique for estimating the average power generation of a vibration energy harvester from the device’s dynamics and the discrete Fourier transform (DFT) of its excitation. This method, termed the power harvesting ratio (PHR), presents the power output of a particular harvester/electrical load combination as a function of frequency and shows the power contribution of each frequency component comprising the excitation. The stability assessment tool and PHR are experimentally validated using a custom electromagnetic vibration energy harvester. The stability assessment tool is shown to accurately predict whether a certain active load will lead to stable or unstable overall system dynamics. PHR is demonstrated to accurately predict power generation for a variety of excitations (including typical bridge vibrations) and electrical loads (both, passive and active). The PHR technique is also used to investigate potential benefits of active electrical over passive loading, optimization of power yields of different architectures of vibration energy harvesters, and effects of frequency and amplitude variations in the excitation on power yield.