Enhanced bandwidth nonlinear resonance electromagnetic human motion energy harvester using magnetic springs and ferrofluid

An enhanced bandwidth nonlinear resonant electromagnetic energy harvester has been designed to harness low frequency energy from basic human motions. 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. The ferrofluid that self-assembles around the edges of the PM stack acts as its bearing system to minimize frictional losses during its movement. Copper wire is wrapped outside the tube to form the armature winding. The stiffness characteristic of the magnetic springs and the optimum equilibrium position and number of windings have been determined by finite element method analysis. As a proof of concept, a portable prototype of the proposed energy harvester that weighs 110 g and with a volume of only 37.7 cm³ is fabricated. A series of experiments is 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 10.15 and 32.53 mW, respectively, for walking and running. With ferrofluid, the maximum outputs are 17.72 and 54.61 mW, representing an increase of 74.58\% and 67.88\%, respectively. Furthermore, the prototype exhibits an average power density of 1.45 mW/cm³ during running motions, which compares favorably with existing harvesters used in low power wearable devices.