A stiffness compensated piezoelectric energy harvester for low-frequency excitation

In this work, a stiffness compensated piezoelectric vibration energy harvester is modelled and tested for low-frequency excitations and large input amplitudes. Attracting magnets are used to introduce a negative stiffness that counteracts the stiffness of the piezoelectric beam. This results into a nearly statically balanced condition and makes the harvester a nonresonant device. A distributed parameter model based on modal analysis is used to model the output of the energy harvester. This model is extended by including the negative stiffness, endstop mechanics and force-displacement data to the model. The peak RMS power amounts 1.20 mW at 9 Hz and 3 g input acceleration. These are large inputs and serve to illustrate the case of having inputs larger than the device length. Furthermore, to benchmark the energy harvester in this work, the efficiency is evaluated in terms of generator figure of merit and is compared to prior art. This peak efficiency amounts to 0.567%, which is relatively large for its range of excitation. From the output that has been obtained with this design, it can be concluded that stiffness compensation can make a piezoelectric energy harvester competitive in terms of generator figure of merit at low-frequency excitation with input amplitudes exceeding the device length.