Sub-g weak-vibration-triggered high-efficiency energy harvesting for event identification

Abstract

A micro-vibration energy harvester employing two-stage mutual ferromagnetic interaction is proposed and developed to frequency up-convert sub-g weak-vibration energy into electricity, with high efficiency. The two-stage structure consists of a low-frequency cantilever (stage 1) with a magnet and seismic mass attached at the cantilever end, as well as a magnetized high-frequency nickel cantilever (stage 2) with a piezoelectric thin film bonded for electric generation. Through magnetic repelling between stage 1 and the magnetized nickel stage 2, a magnetic barrier is formed between the two stages. Once ambient vibration reaches a certain acceleration threshold, the stage 1 cantilever can overcome the magnetic barrier to trigger the piezoelectric stage 2 into frequency up-conversion resonance and electricity generation. Otherwise, the harvester remains in an idle state. Thus, this threshold triggered harvester (TTH) can be used directly as a sensor to autonomously monitor vibrational events. Herein, the seismic mass on the stage 1 cantilever is proposed to set and adjust the sub-g vibration threshold; thereby, the developed sub-g TTH with fixed small gap distance can reliably induce strong dual-stage magnetic interaction under sub-g vibration. Using this approach, the threshold-triggered generating efficiency for sub-g vibration is much higher than in traditional TTH, where the threshold is set by the gap distance between two magnets on the two stages. The relation of threshold versus seismic mass is obtained by both theoretical analysis and numerical calculation, and the accuracy of the design model is confirmed by experimentally testing the microfabricated TTH for thresholds of 0.25, 0.5, and 0.75 g. A comparison is made between the sub-g and traditional TTH, and the switch-on triggered generating power and power density under 0.25 g weak-vibration of the sub-g TTH are 0.72 and 0.48 µW cm−2, respectively, four times higher than in the traditional TTH. Therefore, the novel sub-g TTH is promising in various applications such as monitoring for building structure vibration and people movement.