Magnetically coupled dual-beam energy harvesters have been presented by several researches to achieve a broader response bandwidth. However, in the low frequency operating environment with limited space, the dimension of the piezoelectric beam is limited which leads to high stiffness of the generated beam. Therefore, the output of dual-beam system is relatively low under low frequency and amplitude vibrations. In this paper, a nonlinear magnetic and torsional spring coupling energy harvester is proposed. The stiffness of the torsional spring can be easily adjusted to make the designed harvester suitable for low frequency vibrations. The introduction of torsion springs provides an extra degree of freedom to the system and broadens the response band of the system from 8.7 Hz to 15 Hz. Furthermore, the presence of the 1:3 internal resonance phenomenon in the harvester considerably achieves frequency enhancement and bandwidth extension, significantly improving the harvesting performance in low frequency environments. Numerical analysis, simulation, and experimental are carried out to verify that the proposed energy harvester is capable of harvesting low frequency wideband vibration energy. The results show that magnetic and torsional spring coupling piezoelectric energy harvester is capable of broadband energy harvesting in the range of 5-20 Hz (i.e., 15 Hz bandwidth).

Up to now, many nonlinear techniques such as bistable structure have been used to harvest vibration energy from the environment to achieve a wide response bandwidth. However, most bistable harvesters require two opposing magnets to form the potential energy function, which limits the miniaturization of the harvesters. In this paper, a two-degree-of-freedom U-shaped single magnet bistable energy harvester (SMBEH) based on the internal resonance technique is proposed. The harvester consists of a U-shaped beam, a magnet and a tip mass. The governing equations of the system are derived and the output performance of the harvester is obtained through numerical simulation and experiments which are in good agreement. The proposed SMBEH can achieve a wide band and large amplitude output due to the 1:2 internal resonance and bistable characteristic. When the excitation amplitude is equal to 0.4g, the SMBEH can produce significant output in two frequency ranges from 7Hz to 7.7Hz and 11.7Hz to 15.73Hz. In the end, the output performance of SMBEH at different resistances and the charging performance were verified, respectively.

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In recent years, smart bearing technology is being developed for the purpose of prolonging the reliability and the service time of bearing by detecting the early faults and regulating the working condition. However, powering wireless monitoring devices integrated within the smart bearing is still an uphill task due to the limited power capacity and lifetime of traditional power cells. In this paper, a variable reluctance energy harvester (VREH) is proposed to scavenge the rotating energy from the bearing, which includes a toothed spacer and an m-shape electric steel with a coil and a permanent magnet on each leg. The key parameters of the proposed harvester are analyzed and discussed via the theoretical study and the finite element analysis. In comparison with the VREH without the middle magnet, the proposed harvester greatly improves the energy harvesting performance and reduces the magnetic flux leakage to the ferromagnetic environment. Simulation results show that for the air and ferromagnetic environments, the average output power improvement of the VREH with the middle magnet reaches 33.89% and 61.6%, respectively. A fabricated prototype is tested using the high-speed railway bearing testing machine. The influence of the impedance matching circuit and the coil connection on the VREH is investigated with the rotating speed varying from 600 r/min to 1800 r/min. An energy management and storage circuit is designed and fabricated to test the energy conversion performance of the proposed harvester. Under the impedance matched condition, the optimal RMS output power is of 72.13- 414.86 mW. It is shown that the proposed electromagnetic harvester is capable of collecting the rotational energy in the ferromagnetic environment and can be used for the high-speed smart bearing applications.