Study on the performance of tailored spring elements for piezoelectric MEMS energy harvesters

Wireless Sensors Network (WSN) is a promising technology based on devices, e.g. nodes, wirelessly connected within a networked infrastructure. This technology could be used for countless applications, from the Internet of Thing (IoT) to telemedicine and eHealth. One of the main constraints limiting the effectiveness of WSN is the energy requirement. Energy harvesting is a valuable technology that can extend the lifetime of WSN, and even offer the potential to replace battery-based solutions in some cases. There are different environmental sources of energy commonly exploited for scavenging purposes, i.e. solar, Radio Frequency (RF), kinetic and thermal energy. Nevertheless, kinetic energy is the most widespread source making its harvesting a research topic of relevant interest. In order to harvest vibrational energy there are basically three transduction mechanisms: electromagnetic, electrostatic and piezoelectric. The latter one offers some desirable features, e.g. ease of integration in common micro fabrication processes and high output power density. This work aims to study the performances of whip elements [1-2], or tapered cantilevers for energy harvesting applications, realized in MEMS technology. The effectiveness of those geometries has already been proven in macroscopic designs [3], but such an approach was never considered in MEMS energy harvesters due to different technology constraints. In macro whips the stiffness of the cantilever is lowering along the beam length by reducing both thickness and width. Differently from the macro counterpart, in micro scale it is difficult to obtain a layer with a defined thickness gradient. The presented geometries are designed to reproduce a similar stiffness pattern circumventing the constraints imposed by the micro fabrication process, by implementing a specific perforation pattern.