Tunable Energy Harvesting From Ambient Vibrations

Most civil structures have a low vibration response frequency range, generally one to two orders of magnitude lower than the operating frequency spectrum of most piezoelectric energy scavenging devices, which is dictated by the device’s design and the used materials. This considerably limits the levels of harvestable power under ambient vibrations. In this paper, the improvement of the energy harvesting characteristics of a bimorph cantilever lead zirconate titanate (PZT) piezoelectric beam through the application of initial pre-stress loading conditions is studied. A generalized model that can take into account all the vibration modes of the beam as well as the back coupling effect is derived using the Hamiltonian principle. The model describes the effect of the pre-stress parameters on the harvestable energy levels. Results showing the variations of the natural frequency, amplitude, and efficiency of the piezoelectric device with varying preload are presented. Vibration recordings from a bridge under ambient loading are used to show variations of the harvested power with different pre-stress conditions. Increases of up to 250% in the output power levels are shown possible through the application of 8N of compressive axial loading for a system with a 15g vibrating mass. Experimental verification of the model is also performed. The time and frequency domain responses of a piezoelectric bimorph are measured and compared to theoretical results.