**1. Introduction**

In recent years, with the rapid development of micro electromechanical systems (MEMSs) and the Internet of things (IoT), various micro wireless sensor nodes (WSNs) have been developed. These nodes are widely used in military surveillance, structural health monitoring, road traffic monitoring, and so on [1–4]. However, the limited lifetime of traditional batteries restricts the application of WSNs in complex environments and increases the working load of changing the batteries periodically. To overcome this restriction, some environmental energy harvesters dedicated to collect solar, thermal, wind, ocean wave, and vibration energies have been developed [5]. Among these, vibration energy is ubiquitous, such as structural vibrations, human activities, and fluid flows. The mechanical vibration energy can be converted into electrical energy through four transduction mechanisms, which are electromagnetic [6,7], piezoelectric [8–10], triboelectric [11,12], and electrostatic [13,14]. Piezoelectric vibration energy harvesters (PVEHs) have received significant attention due to their simple configuration, high energy conversion efficiency, and precision controllability of the mechanical response [15–17].

Some piezoelectric materials are widely used in MEMS energy harvesters, which are aluminum nitride (AlN) [18,19], zinc oxide (ZnO) [20,21], and Pb(ZrxTi1-x)O3 (PZT) [22–27]. Among these, PZT has a higher electromechanical coupling coe fficient compared with AlN and ZnO. Cui et al. [26] developed a multi-beam energy harvester with a PZT thin-film layer using a sol-gel deposition method. The maximum output power of 16.74 nW was obtained under an acceleration of 1 g and resonant frequency of 1400 Hz. Generally, PZT thin-film deposition requires a specific and complicated fabrication recipe, and the output performance of the PZT thin-film is limited. Therefore, PZT thick-film-based energy harvesters were developed. Xu et al. [28] proposed a screen-printed PZT/PZT thick-film bimorph cantilever for energy harvesting. However, the screen-printed PZT thick films are not dense enough, which means their piezoelectricity is low compared with that of bulk PZT. Thus, preparing a high-quality PZT thick film on silicon (Si) substrate through wafer bonding of bulk PZT has been proposed [29–31]. Janphuang et al. [30] demonstrated a wafer-level fabrication process of piezoelectric energy harvester using a spin-on polymetric adhesive WaferBOND as a bonding layer between bulk PZT and Si. The harvester exhibited an average power of 82.4 μW under an excitation of 1 g at 96 Hz. The above studies indicate that the MEMS PVEHs with thinned bulk PZT thick films have the potential for high output performance.

Another challenge for MEMS PVEHs is that the resonant frequencies of piezoelectric cantilevers are higher than most ambient vibration sources. Most of the natural vibration sources are random and at a low-frequency, typically ranging from 30 to 200 Hz [32–34]. In order to e ffectively utilize the low-frequency environmental vibrations, lowering the resonant frequency and widening the operating bandwidth have been the major target for the small-scale PVEHs. The frequency up-conversion mechanisms provided a good solution to address these issues and have aroused grea<sup>t</sup> research interest [35–38]. In general, the frequency up-conversion technologies can be divided into non-impact and impact types. Galchev et al. [36] demonstrated a non-impact piezoelectric generator that utilized a magnetic latching mechanism to convert the ambient low frequency to a higher internal operation frequency. However, the average power of the device was 3.25 μW at 1 g. Improvement of the output power needs to be considered. Jung et al. [39] introduced an energy harvester that uses the snap-through buckling action of a pre-buckled beam for frequency-up conversion instead of magnetic coupling. A maximum output power of 131 μW was generated usinga3g acceleration. Andò et al. [40] proposed a snap-through buckling based vibrational energy harvester by adopting a flexible buckled beam, which was able to generate power in the excess of 400 μW under an acceleration of 13.35 m/s2. However, large accelerations are generally required to drive the beam to induce snap-through buckling, and it is di fficult to fabricate the buckled beam with standard technologies. In addition to these non-impact frequency up-conversion approaches, Umeda et al. [37] first demonstrated the impact-based frequency up-conversion approach for energy harvesting by investigating the power transformation of a steel ball impacting on a piezoelectric membrane. Halim et al. [38] proposed a mechanical impact-driven PVEH consisting of two series-connected PZT cantilevers and a flexible driving cantilever. A peak power of 734 μW from two series connecting PZT beams was achieved at the resonant frequency of 14.5 Hz. The impact-driven frequency up-conversion technology e ffectively increases the output power of the energy harvester at low frequency. Liu et al. [8] developed a PZT thin-film MEMS-based frequency up-converted PVEH system by utilizing the periodic impact between an S-shaped, low-frequency driving cantilever and a straight, high-frequency PZT generating cantilever. The PVEH system realized a low operating frequency under 37 Hz and the volume was very small. However, the maximum output power was only 0.12 μW with a 0.8 g acceleration. So far, there have been few studies on silicon-based PVEH fabricated at the scale of MEMS for harvesting energy from low-frequency vibration through an impact-based frequency up-conversion mechanism.

Therefore, this study has carried out research and discussion targeting a low-frequency MEMS PVEH by using a frequency up-conversion mechanism. First, a new wafer-level micromachining process for fabricating the PZT thick-film cantilever energy harvester was put forward. Then, the piezoelectric energy harvesting system (PEHS) with a low-frequency S-shaped stainless-steel cantilever (SSC) and a high-frequency straight piezoelectric cantilever (SPC) was incorporated. The output performances of the system and the single SPC were investigated and compared by using a vibration control and testing system. The experimental results indicated that the impact-based frequency up-conversion mechanism was able to improve the output performance of the harvester under a low-frequency and low-acceleration vibration environment.

#### **2. Design and Simulation**
