A multi-stable rotational energy harvester using a rolling sphere and magnetic coupling for ultra-low frequency motions

Limited power supply is a continuing challenge with wireless sensors due to depleting nature of batteries. Vibration energy harvesters provide a promising alternative solution, but they also face limitations when harnessing broadband and low frequency energy sources, such as that available from wind turbine tower vibrations and human body motions. This paper presents a novel multi-stable broadband rotational energy harvester using magnetic coupling and a rolling sphere to harness ultra-low frequency motions. A spherical magnet, placed in a circular path, is used as an inertial mass to absorb ambient vibrations. Eight tethering magnets are placed underneath the path to create eight equilibria of the rolling sphere. The same number of coils are placed above the rolling path and are aligned with the tethering magnets, such that the rolling magnet can provide the maximum rate of magnetic flux change. A prototype was fabricated and tested on a mechanical slider. The device can operate in the cross-well motions over a wide frequency bandwidth (1.5 - 2.5 Hz). With a 44 Ω external load, the device is capable of transferring 8.1mW output power under 2.5 Hz and 0.9g excitations. The self-powered sensing was also tested using the harvester. 1.91 mJ of energy is stored in a 330 μF capacitor within 116 s.