A retrofitted energy harvester for low frequency vibrations

Piezoelectric-based energy harvesting is an efficient way to convert ambient vibration energy into usable electric energy. However, its output power drops steeply with reducing excitation frequency. To improve the harvesting performance at low frequencies, a multi-impact harvester is proposed in this paper. The proposed design consists of a hung mass and two stiff piezoelectric cantilever beams. A series of impacts between the mass and cantilever beams are involved during the vibration of the mass, which triggers high frequency vibrations on the cantilever beams. Four equations of motion corresponding to different system statuses are presented to simulate the vibration of the harvester. Based on the linear piezoelectric theory, a distributed-parameter electromechanical model is used for the bimorph cantilever beam and its output power is calculated. For comparison, a conventional cantilever-beam-based harvester and a single-impact harvester are introduced and their output powers are also calculated. Under the same sinusoidal excitation, the modeling result shows that the power of the proposed harvester is more than three times larger than the ones from the conventional cantilever beam harvester and the single-impact harvester. The multi-impact harvester also occupies less space than the conventional one. A parametric study is presented in this paper for the multi-impact harvester considering different external resistances, hung masses, cantilever beam thicknesses and excitation frequencies.