# A Review of MEMS Scale Piezoelectric Energy Harvester
## 2 Research Progress of PEH
### Optimization of Energy Harvesters' Structure
#### 2.4.2 Frequency Up-Converting  
In some situations, it might be difficult to reduce the resonant frequency of the PEHs. Therefore, in recent years, people propose a new technology called frequency up-conversion to match the frequency between the external excitation and the energy harvester. The technology was firstly applied to the electromagnetic energy harvester designed by [PERSON] in 2008 [67]. The system had two resonating structures. The top resonator is a plate suspended with a soft spring and has a low resonance frequency that is adjusted for the target application (1-100 Hz). It supports a magnet for mechanical frequency up-conversion. The bottom resonator is a cantilever beam which has a higher resonance frequency, and supports a coil for power generation, and a magnetic tip that could be attracted to the large magnet. As the large magnet resonates in response to external vibration, it gets closer to the cantilever beam located beneath. The distance between them is adjusted such that the magnet catches the cantilever at a certain point of its movement, pulls it up, and releases it at another point. According to [PERSON] and [PERSON]'s study [24], the generated power produced by such a system is proportional to the cube of the frequency of vibration, and thus the electromagnetic energy harvester is able to generate more power. In 2017, [PERSON] et al. [68] designed the PEH using frequency up-conversion technology (as shown in Figure 8). This energy harvester included a lower frequency piezoelectric bimorph (LFPB) and a higher frequency piezoelectric bimorph (HFPB). Under low-frequency vibration, the displacement of the lower frequency piezoelectric bimorph free end was larger than that of the higher frequency piezoelectric bimorph free end. As shown in Figure 9, the external excitation of 29 Hz vibrational frequency and 1 g acceleration were applied to the PEH. In the AB stage, the two piezoelectric bimorphs combined and moved together. After that, the two piezoelectric bimorphs separated, the higher frequency piezoelectric bimorph vibrated along the CE at a frequency of 153.8 Hz and the lower frequency piezoelectric bimorph vibrated along the DE at a frequency of 29 Hz. The operating frequency band of the PEH was between 25 Hz and 30 Hz. With an excitation of 1 g and 29 Hz, the highest output voltage and power generated by the higher frequency piezoelectric bimorph were 16.5 V and 2.62 mW, respectively. By contrast, the maximum output voltage and output power of the lower frequency piezoelectric bimorph were 37.8 V and 0.58 mW, respectively. There is a difference between [PERSON]' study [67] and [PERSON]'s study [68], even though they both apply the frequency up-conversion technology, i.e., the energy harvester used a non-contact way to transfer mechanical energy between lower frequency oscillator and higher frequency oscillator in [PERSON]' study [67] while it used a contact way in [PERSON]'s study [68]. The energy harvester of non-contact mode can avoid the energy loss caused by collision, so it has higher conversion efficiency. In 2017, [PERSON] et al. [69] developed frequency up-conversion hybrid energy harvester by utilizing piezoelectric and electromagnetic conversions simultaneously. The structure was composed of two cantilevers (inter and external) on a flexible substrate. The piezoelectric material was bonded to the backside of internal cantilever and electromagnetic coil was mounted on the end of external cantilever. When external bending was applied to the substrate, as the internal cantilever was bound to the substrate by magnetic attraction force, it was deformed along with the substrate and the external cantilever remained in a flat state. When the bending radius of the cantilever became smaller than the threshold radius of curvature, it was released and oscillated at its resonant frequency. During this oscillation, the relative motion between the internal cantilever and the external cantilever occurred.