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In this paper, a frequency-adjustable tuning fork electromagnetic energy harvester is introduced. The electromagnetic vibration energy harvester can adjust its natural frequency according to a change in the environmental excitation frequency without any change to the structure. In the frequency-adjustable range, it can make the energy harvester resonant with the environment excitation, and the output frequency stays the same. The frequency-adjustable tuning fork electromagnetic energy harvester significantly increases the range of frequencies used. The operating frequency of the centre can be easily switched from 9.2 to 20 Hz, enabling the application of multiple excitation frequencies. In addition, the output power and power density are significantly increased compared to a piezoelectric tuning fork energy harvester of the same size. The peak power is 23.59 mW at 9.2 Hz, the power increases by 14.85 mW, and the power density increases by 169.88%. The experimental results show that the electromagnetic tuning fork frequency-adjustable conversion energy harvester can make the LED lamp work.

The circular Halbach array is of great application value in electromagnetic energy conversion. The Halbach array can be formed with multiple magnets according to a specific method. The magnetic flux intensity of the magnet array is concentrated and enhanced in a certain area. The concentrated magnetic flux intensity could lead to a higher rate of change in the magnetic flux when the magnet and the coil move relatively. The spatial arrangement, structural dimensions, and residual flux density of the Halbach array are the main factors affecting its performance. Some areas with almost no magnetic induction in the Halbach array affect the performance of electromagnetic energy conversion. This paper conducted a comparative study of the Halbach array and the two-directional magnet array in terms of energy conversion. The root mean square (RMS) value and phase of the induced voltage in the coil were analyzed by a finite element method. Simulation shows that the RMS value of the induced voltage in the coil is equal when the gap is 1.55 mm and the REMEH adopts either of the two magnet arrays. The two-directional array has advantages in the energy harvesting performance when the gap is less than 1.55 mm, while the Halbach array has advantages in the energy harvesting performance when the gap is greater than 1.55 mm. The experimental verification of the designed prototype was implemented in this study. The experimental voltage is in good agreement with the simulation voltage. The threshold value of the gap in the experiment is about 2 mm, which is slightly higher than the simulation value. This characteristic has a certain reference value for the application of the Halbach array in the electromagnetic field.

In this paper, a novel tuning fork structure for self-frequency up-conversion is proposed. The structure has an in-phase vibration mode and an anti-phase vibration mode. The in-phase vibration mode is used to sense the environment vibration, and the anti-phase vibration mode is used for energy conversion and power generation. The low-frequency energy collection and the high-frequency energy conversion can be achieved simultaneously. Theoretical and experimental results show that the tuning fork frequency up-conversion energy harvester has excellent performance. This structure provides the energy harvester with excellent output power in a low-frequency vibration environment. At the resonant frequency of 7.3 Hz under 0.7 g acceleration, the peak voltage is 41.8 V and the peak power is 8.74 mW. The tuning fork frequency up-conversion energy harvester causes the humidity sensor to work stably. The structure has the potential to power wireless sensor nodes or to be used as a small portable vibration storage device, especially suitable for the monitoring of the environment related to human movement.

For a microelectromechanical system (MEMS) piezoelectric energy harvester consisting of double-clamped beams, the effects of both beam shape and electrode arrangement on the voltage outputs are analyzed. For two kinds of harvester structures including millimeter-scale and micro-scale, and different shapes including rectangular, segmentally trapezoidal and concave parabolic are taken into account. Corresponding electric outputs are calculated and tested. Their results are in good agreement with each other. The experimental results validate the theoretical analysis.