**5. Conclusions**

In summary, this work presented the design, fabrication, and experimental testing of a MEMS PEHS. As a parallel structure, the PEHS consisted of a piezoelectric cantilever, a stainless-steel S-shaped cantilever with proof mass, and supporting frames. By employing the parallel-cantilever structure, the bottom low-frequency SSC would impact on the top SPC during vibration and realize a frequency up-conversion. The piezoelectric cantilever chip used in the harvester was fabricated using a PZT thick film MEMS fabrication process. Furthermore, the key techniques during fabrication were Cu-Sn-Cu eutectic bonding, mechanical lapping, and electrode layer etching. Experimental results showed that the SPC vibrated at an acceleration of 0.3 g could generate the maximum output power of 0.15 μW at the resonant frequency of 1012 Hz, and the normalized power density was 0.006 <sup>μ</sup>W·g<sup>−</sup>1·Hz−1·cm<sup>−</sup>3. The output performances of the PEHS were also investigated under di fferent initial gap distances and accelerations. Under a gap distance of 0.9 mm, the normalized power density of the PEHS was measured to be 0.216 <sup>μ</sup>W·g<sup>−</sup>1·Hz−1·cm<sup>−</sup><sup>3</sup> at an acceleration of 0.3 g and resonant frequency of 40 Hz, which was much higher than that of the SPC. Moreover, the half-bandwidth of the PEHS broadened to 5 Hz due to the collision between SSC and SPC. It was proven that combining the PEHS with a

frequency up-conversion mechanism can increase the output power under low frequency and low acceleration vibrations.

**Author Contributions:** Conceptualization, M.H. and H.L.; methodology, M.H. and C.H.; software, M.H. and Y.L.; formal analysis, M.H. and T.C.; writing – original draft, M.H., H.L. and C.H.; writing – review & editing, H.L., M.H., F.W. and Z.Y.; project administration, H.L. and F.W.; funding acquisition, H.L., G.T. and L.S.

**Funding:** This research was funded by the National Natural Science Foundation of China (Grant Nos. 51875377, 41527901 and 51565038).

**Conflicts of Interest:** The authors declare no conflict of interest.
