Diode-less mechanical H-bridge rectifier for “zero threshold” vibration energy harvesters

In this paper the idea of a mechanical, diode-less, H-bridge voltage rectifier for applications to vibration energy harvesting systems is presented. Two piezoelectric bistable cantilevers that respond anti-symmetrically to environmental mechanical vibrations are used to generate an electrical voltage via piezoelectric transducers; they also, alternately close the electrical contacts between the generation and the storage stages of the harvester, thereby implementing the desired rectifying action. The piezoelectric voltage is, thus, rectified and current is transferred to the storage capacitor without having to overcome a threshold, as would be the usual case of diode bridge rectifiers. The H-bridge approach is therefore particularly suitable when low output voltages are generated: it works very well, in fact, both for low rms amplitude random vibrations in a low frequency band, and for micro and nanoscale systems. An analytic dynamical model of the anti-symmetric bistable cantilever system is presented, together with the results of numerical simulations; we also present the results of an experimental validation on a laboratory prototype.