Vibration energy harvesters (VEHs) can transform ambient vibration energy to electricity and have been widely investigated as promising self-powered devices for wireless sensor networks, wearable sensors, and applications of a micro-electro-mechanical system (MEMS). However, the ambient vibration is always too weak to hinder the high energy conversion efficiency. In this paper, the integrated frame composed of piezoelectric beams and mechanical amplifiers is proposed to improve the energy conversion efficiency of a VEH. First, the initial structures of a piezoelectric frame (PF) and an amplification frame (AF) are designed. The dynamic model is then established to analyze the influence of key structural parameters on the mechanical amplification factor. Finite element simulation is conducted to study the energy harvesting performance, where the stiffness characteristics and power output in the cases of series and parallel load resistance are discussed in detail. Furthermore, piezoelectric beams with variable cross-sections are introduced to optimize and improve the energy harvesting efficiency. Advantages of the PF with the AF are illustrated by comparison with conventional piezoelectric cantilever beams. The results show that the proposed integrated VEH has a good mechanical amplification capability and is more suitable for low-frequency vibration conditions.

The piezoelectric stack energy harvester (PSEH) is often used to transform ambient large amplitude vibrational energy, such as road vibrations and pressure fluctuations, because of its ability to withstand large external excitation force. However, due to the high natural frequency of the piezoelectric stack, it is difficult to generate resonance phenomenon, which leads to low energy efficiency for the energy harvester. In this paper, a piezoelectric stack energy harvester with force magnification frame (FMF) is proposed and studied for energy harvesting from low-frequency pressure ripples. First, a force magnification frame is designed to enhance the pressure transferring ability. The FMF is able to, not only amplify the exciting force acting on the piezoelectric stack but also decrease the natural frequency of the PSEH. Based on finite element method (FEM) analysis, the natural frequency of the PSEH with FMF is obtained. The natural frequency of the PSEH is successfully reduced, and the open-circuit voltage output of the PSEH under low-frequency resonance conditions is much higher than for off-resonance conditions. The results validate that the PSEH with FMF is efficient for lower frequency vibration environments.