Improved energy harvesting from low-frequency small vibrations through a monostable piezoelectric energy harvester

Scavenging energy from low-frequency and low-level excitations has always been a huge challenge for the piezoelectric energy harvesting since the frequencies of ambient excitations are usually below the device's operating frequency and small excitations may fail to actuate the device to produce usable electricity. To remedy this key issue, a piezoelectric energy harvester with stoppers (PEHS) has been proposed by the authors. The stoppers and the magnetically attractive coupling employed in the PEHS make the device monostable, removing the requirement for overcoming the potential barrier that normally appears in a bistable or tristable system. A theoretical model for the PEHS is established and experimentally validated, with which the PEHS is investigated under both harmonic excitations and random excitations. The results indicate that the operating frequency range of the PEHS can be tuned toward the lower frequency by changing the (mass-magnet) gap between the tip mass and the external magnets, making the efficient energy harvesting from low-frequency excitations possible. For a given harmonic excitation, the PEHS can provide a larger power output and wider operating bandwidth than the linear PEH no matter what way the frequency sweep is conducted. Moreover, compared with the linear PEH, improved power output can also be attained under the Gaussian white noise with a small intensity, enabling the PENS to deliver useful power even in the presence of small random excitations. Although the optimal PEHS configuration in terms of mass-magnet gap is found to vary slightly with the excitation levels, there exists a certain gap that can guarantee the optimal or near optimal performance of the PEHS under both low-level harmonic excitations and low-intensity random excitations, demonstrating the harvester's superior adaptation to the ambient excitations with variable strengths. (C) 2018 Elsevier Ltd. All rights reserved.