Nonlinear technique and self-powered circuit for efficient piezoelectric energy harvesting under unloaded cases

Vibration energy harvesting using piezoelectric materials has been intensively studied over the last few years, because of the wide availability of vibrational sources and the good integration and conversion abilities of piezoelectric materials. However, the power output of Piezoelectric Energy Harvesters (PEH) is still limited due to the relatively low mechanical quality factor and coupling coefficient of the electromechanical structure. Recently, nonlinear processes have been introduced to increase the energy extraction abilities of PEHs, showing impressive increase of the power output under monomodal excitation. However, their performance is limited when the structure is subjected to more complex signals or when considering the charge of a single capacitor (the latter operation reflecting well the typical use of self-powered systems). The purpose of this paper is to expose an enhancement of this technique, based on disabling the nonlinear process if no harvesting event has been done previously. This therefore allows better energy conversion abilities under multimodal excitation as it allows a better control of the trade-off between the number of switching events and the voltage values at the switching instant, as well as increasing the harvested energy in monochromatic case, as the damping effect is limited. Finally, in order to dispose of realistic devices, a self-powered version of the enhanced technique is proposed and validated through simulation analysis and experimental implementation.