Investigation of Energy Harvest Using a Piezoelectric Unimorph Post-Buckled by a Stretching Spring

Abstract

Pre-stressed piezoelectric unimorphs show enhanced actuation displacements and high efficiency of energy harvesting compared with conventional unimorphs. A method to increase the amount of stored energy by injecting elastic energy to energy harvesting system consisting of the THUNDER device is described in this paper. A stretching spring is mounted on the two tabs of THUNDER device in order to inject energy to the system. The mechanical stress applied on THUNDER device results in an increase in the initial stored mechanical and elastic energy, which contribute to the improved response of the modified device. In experiment, two different springs were added on the THUNDER device: one's initial length is 17mm with k=45N/m and another is 33mm with k=145N/m. For the THUNDER device with a spring of k=145N/m and a proof mass of 8.2g, the maximum open circuit VRMS was 29.4V, and output power of 4.53mW was obtained by a load resistor of 90 kΩ at vibration frequency of 51Hz. Compared with standard device, the energy density or the output power at resonance frequency increased by 74.4%. The displacement performance of the modified devices was larger than that of the standard device. Through measurements and analysis, after a stretching spring was attached to the THUNDER device, dielectric constant did not change obviously, while d31 increased a lot. We can conclude that the improvement of energy harvesting is mainly due to the increase of d31 and stress distribution in the THUNDER device. Furthermore, the use of an initial energy injection mechanism based on a nonlinear approach can artificially enhance the conversion abilities of piezoelectric materials.