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.

In this work, a piezoelectric beam is stiffness compensated through adding a negative stiffness formed by attracting magnets. The mechanism's purpose is low-frequency energy harvesting. The effect of deformation speed on the beam's stiffness is investigated by force-displacement measurements taken at different speeds and with different load resistors connected. The effect of the load resistance on the beam's stiffness has been found to be strongly dependent on the deformation speed. A load that results in the same stiffness as in a closed circuit at low deformation speed results in a stiffer response at a faster deformation speed. Also, when the beam is brought close to static balance with a certain load resistance connected, alteration of the load resistance has a great influence on the attained stiffness level. Furthermore, memory effects in the hysteresis found in piezoelectric actuators, related between input voltage and displacement, were also confirmed between displacement and force in sensor application.