Enhanced Bandwidth Nonlinear Resonance Electromagnetic Human Motion Energy Harvester Using Magnetic Springs and Ferrofluid

Abstract

An enhanced bandwidth nonlinear resonant electromagnetic energy harvester has been designed to harness low frequency energy from basic human motion. Some vertical stacked cylindrical permanent magnets (PMs) constitute the inertial mass of the proposed harvester, which is suspended axially by two magnetic-springs and circumferentially by ferrofluid within a carbon fiber tube. In order to widen the frequency band and improve harvesting efficiency, two PMs are respectively fixed on the two end caps of the carbon fiber tube, so as to form two magnetic-springs with variable stiffness by cooperating with the PM stack. The self-assembled ferrofluid around the PM stack acts as its bearing system to minimize any friction during its movement. Copper wire are wrapped outside the tube to form the armature winding. The stiffness characteristic of the magneticsprings and the optimum equilibrium position and number of windings have been determined by finite element method (FEM) analysis. As a proof of concept, a portable prototype of the proposed energy harvester that weighs 110g and with a volume of only 37.7cm 3 is fabricated. A series of experiments are carried out and the results show that the frequency band of the harvester becomes wider as the external vibration intensity increases. In addition, the effectiveness of ferrofluid in reducing friction is demonstrated under walking and running conditions. Without ferrofluid, the maximum average outputs are 10.15mW and 32.53mW respectively for walking and running. With ferrofluid, the maximum outputs are 17.72mW and 54.61mW, representing an increase of 74.58% and 67.88%, respectively. Furthermore, the prototype exhibits an average power density of 1.45mW/cm 3 during running motions, which compares favorably with existing harvesters used in low power wearable devices.

Tuning Techniques for Piezoelectric and Electromagnetic Vibration Energy Harvesters

“…Therefore, in practical applications, the increase of the effective operating frequency range of RVEHs is mandatory. In the literature, a number of papers describing control methods or architectures that are specifically designed with the aim of increasing the effective operating frequency range of RVEHs, has been proposed [87][88][89][90][91][92][93][94][95]. In particular, harvesters arrays, nonlinear harvesters, and mechanical tuning techniques (MTTs) are the most widely analyzed optimization methods.…”

Output Power Limit in Energy Harvesting Systems Based on Magnetic Induction Incorporating High-Frequency Effects

“…In (15), G is the proximity effect factor, given by a look-up table [45] according to r 0 and δ. D and b are the coil's outer diameter and length, respectively.…”

“…The operation principle is based on Faraday's law of induction where a voltage is induced in a coil by a time-changing magnetic flux. The source of the magnetic flux can be power lines [3][4][5][6][7][8][9][10], oscillating or vibrating permanent magnets [11][12][13][14][15][16][17][18][19][20][21][22][23][24], variable reluctance [25] or a nearby transmitter coil. Typical applications are condition monitoring sensors powered by power lines [3][4][5][6][7]9,[26][27][28], and powering of biomedical implants [2,14,17,[29][30][31][32].…”

A Piezo-Electromagnetic Coupling Multi-Directional Vibration Energy Harvester Based on Frequency Up-Conversion Technique

“…Researchers have also proposed many new structures for vibration energy harvesting. Li et al [22] designed an enhanced bandwidth nonlinear resonant electromagnetic energy harvester. The inertial mass of the proposed harvester is formed by four stacked ring permanent magnets (PMs), which is suspended axially by two magnetic springs and circumferentially by ferrofluid within a carbon fiber tube.…”

“…The magnetic springs are made up of two button PMs adhered, respectively, to the end cap at each end of the carbon fiber tube to provide varying repulsive forces to the PM stack, resulting in enhanced resonance frequency band and higher efficiency in energy harvesting. However, this kind of energy harvester can only collect vibration energy in a certain direction [15,22], which has no obvious advantage for human motion or swing. Therefore, many researchers use rotating mass structure to harvest human motion energy [21,23,24].…”