Research On Characteristics Of Piezoelectric Energy Harvesting Based On Stress Adjustment

Vibrational energy is the renewable energy sources, which is independent of climate. Piezoelectric vibration generator can work as power supply for low power electronics, sensors in the wireless sensor network and MEMS products. There isn’t electric network source and it don’t need to replace the batteries, what will make vibration testing point more durable and easier to facilitate large-scale networks. However, the main problem on piezoelectric energy harvesting is that energy density and efficiency of energy transfer is not satisfactory, and this paper focuses on how to enhance the output power and voltage of piezoelectric material through stress adjustment.First, a stress tunable structure by adjusting the micropositioners to shorten the distance between the ends of the energy harvester was designed in this paper. The goal was to judge the effects of stress on actuation displacement. Then, the stress adjustment structure was further optimized. The stress adjustment structures based on adding springs with different elastic coefficient were designed for the THUNDER simply supported beam. The experimental results for THUNDER unimorph under simple supported boundary condition showed that when the elastic coefficient of spring is145N/m, the output power at resonance frequency increased by74.4%. The stress adjustment structures based on regulating the length of spring was designed for the THUNDER cantilever and initial stress-free plain unimorphs. An analytical model based on Hamiltonian theory was developed used to evaluate the effects of compressive force under the cantilever boundary condition. An expression for voltage charge generation was then derived and compared with the experimental results. The output voltage can be improved when the preload was below0.495N. If the preload exceeded this level, the output voltage would decrease. Meanwhile, the resonance frequency of energy harvester decreases steadily with the increase of compressive force. In order to study the effects of stress adjustment on energy harvesting characteristics of initial stress-free unimorphs, the experiments were designed to investigate the energy harvesting of piezoelectric ceramics PZT-5A rectangular cantilever, PVDF piezoelectric film rectangular and triangular cantilever. The experiment results showed that the improvement of PZT-5A energy harvester was obviously and the output voltage increased by374%and that of PVDF triangular energy harvester increased by41.5%. However, the output voltage of PVDF rectangular energy harvesters under stress adjustment structure almost didn’t change. Finally, the displacement and equivalent capacitance were measured and the change rules of piezoelectric coefficient and dielectric constant were summarized:with the increase of compressive force the piezoelectric coefficient became bigger but dielectric constant didn’t change. The improvement of energy harvesting was due to the increase of piezoelectric coefficient.The results show that the energy harvesting of pre-stress unimorph and initial stress-free plain unimorphs can be improved under stress adjustment structure. It’s concluded that with the increase of compressive force, the measured output voltage of piezoelectric energy harvester first increased and then decreased. This showed that the effects of compressive force on energy harvesting had function limit.