Impact-driven broadband piezoelectric energy harvesting using a two-degrees-of-freedom structure

Impact-driven piezoelectric energy harvesters are more capable of harvesting vibration energy at low frequency than bending-type ones. However, an impact-driven harvester usually relies on either a free moving object or a cantilevered driving beam to provide impact. The former is difficult for developing a precise model to predict its frequency responses while the latter can only achieve high power output about the first resonant frequency of the driving beam. In this paper, a novel impact-driven two-degree-of-freedom piezoelectric energy harvester is introduced to scavenge vibration energy harvesting in a wide frequency range that includes its two resonant frequencies. The proposed harvester consists a two-degrees-of-freedom beam structure and a piezoelectric diaphragm. The beam structure is designed to have two close resonant frequencies with similar amplitudes of tip displacement under excitations at a fixed acceleration magnitude. A proof mass is fixed to the end of the beam to hits and deforms the piezoelectric diaphragm to generate electricity under base excitations. The diaphragm also works as a stopper to introduces the piecewise linear hardening effect and improve the frequency range under up sweeps. The theoretical model is developed based on the stiffness influence coefficients method and validated with experimental results. The amplitudes of the voltage output at the first two resonant frequencies are similar under excitations with a fixed acceleration magnitude. Different excitation levels are tested to investigate the performance of the harvester. Parametric analysis of the beam lengths is conducted to investigate their influence on the performance of the harvester. The proposed harvester demonstrates the capability of harvesting vibration energy in a wide frequency range that covers the two resonant frequencies. The hardening effect caused by the stopper further improves the bandwidth under up sweeps.