I. Introduction
In Wireless Sensor Nodes applications, when vibrations are available, Vibration Energy Harvesters can be used to replace batteries or at least to limit their drawbacks (expensive maintenance/replacement, insufficient or unpredictable duration of operational life, not eco-friendly disposal due to the hazardous chemical content) [1] – [3] . Most Vibration Energy Harvesters belong to one of the following three main categories: piezoelectric, electrostatic and electromagnetic. This paper is focused on resonant Piezoelectric Vibration Energy Harvesters (PVEHs) that are based on the Piezoelectric Effect. Piezoelectric Effect is the ability of some materials (notably crystals and certain ceramics) to generate a voltage when they are subject to a mechanical stress. The electrical polarization is proportional to the applied strain [4] – [5] .