Since the magnet array consists of 4 magnets, flexible coil array with 10 coils connected in series (5 on top semi-cylinder and 5 on bottom semi-cylinder, as shown in Fig. The gradient of the magnetic flux reaches its maximum value when the coil center is located at the boundary between the magnets. In order to prevent the magnet array to rotate in the acrylic tube, grooves are engraved on the wall of the tube with laser and small magnets installed on the magnet array and fit in these grooves. This arrangement of magnets provides magnetic field with large spatial gradient, and can improve the energy conversion efficiency. The cylinder magnet array is formed by four NdFeB disc magnets with alternating north and south poles. The ferrofluid-based liquid suspension is formed in an enclosed chamber made by laser-cut acrylic tube. The schematic of the energy harvester based on ferrofluid liquid spring with cylinder magnet is shown in Fig. One potential application of this low-frequency energy harvester is to power sensors from vibrations of building walls or bridges with a vibration frequency of tens Hz. We have present an energy harvester based on liquid spring with a volume of 0.33 cc and a resonant frequency of 340 Hz in MEMS 2015, 25 in this paper, less condensed water-based ferrofluid is used to significantly reduce the resonant frequency to 16 Hz. Ferrofluid used as a spring offers small volume, low resonant frequency, and robustness under strong vibration for an energy harvester. Thus, a sturdy suspension, such as liquid spring, which has a low resonant frequency while maintaining small volume is highly desired. And energy harvester with sloshing ferrofluid requires a large volume for a low resonant frequency. Moreover, a rigid spring or magnetic spring is prone to break under a strong vibration or in the long run because of fatigue. It is difficult with a rigid spring or magnetic spring to achieve a low resonant frequency while maintaining a small volume for an electromagnetic energy harvester, especially when its coil is made with microfabrication process. 20 But the large distance between the magnets which is necessary for a low resonant frequency increases the volume of the energy harvester and sets a limit to how low the resonant frequency can be achieved for a given harvester size. 19 Another 25.6 cc energy harvester with a magnetic spring formed by a magnetic attractive force and ferrofluid beneath the magnet to suspend it was reported to have a resonant frequency of 4.5 Hz and produce 0.27 mW power from 0.3 g acceleration. 18 A smaller version occupying 3.8 cc and weighing 8.5 g with microfabricated coils and magnetic spring can generate 0.6 μW power from 0.6 g input acceleration at a resonant frequency of 8 Hz. 15 The performance can be improved by adopting a magnet array with rapidly changing magnetic flux, 16,17 and 32 mW power can be produced from a slow-running motion with an energy harvester occupying 120 cc and weighing 180 g with a resonant frequency of 4 Hz. An energy harvester occupying 40.18 cc with one moving magnet between two fixed magnets housed in a teflon tube is reported to have a resonant frequency of 7–10 Hz and produce 52.02 μW/cm 3 for input acceleration of 0.5 g. Instead of a rigid spring, a magnetic spring using a repulsive force between magnets to suspend a proof mass can be used to reduce the resonant frequency.
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