Structural Optimization Design Of Piezoelectric Haversters And Transformers

Recently, advances in low power Very Large Scale Integration(VLSI) have reduced power requirements to the range of tens to hundreds of microwatts. Such low power dissipation makes it possible to power the small electronic devices by scavenging ambient energy from the environment. Due to their strong electromechanical coupling effects, piezoelectric materials are natural candidates for designing devices that scavenge various ambient power by converting mechanical energy into electric energy for powering small electronic devices of a very lower power requirement. Sch a device is termed as a piezoelectric energy harvester, which can be viewed as a half of the structure of a piezoelectric transformer in which the energy converts twice, i.e., from electrical-to-mechanical and then from mechanical-to-electrical. In this paper, four types of piezoelectric harvester models and a new type of piezoelectric transformer are presented. The performance of the piezoelectric devices is theoretically analyzed, and several feasible methods to improve the performance are presented. The results gotten in this paper propose a set of guidelines for us to design piezoelectric harvester and transformer with optimal structure. Specific contents are as follows:(1) Based on linear piezoelectricity theory, we obtain the analytical solution for the output voltage and power of the harvester, which is a combination of the physical and geometrical parameters of the scavenging structure, the frequency of the ambient vibration and the load impedance. The numerical results illustrate the considerably enhanced performances achieved by adjusting the load impedance and the physical and geometrical parameters of the scavenging structure, which is very important for us to design a piezoelectric harvester with excellent performance while work at a certain operating frequency.(2) Comparision of the four piezoelectric harvesters is presented, which indicates each of them can only work efficiently at a certain range of operating frequency. A harvester in the flexural mode can scavenge considerable energy at a circumstance of low frequency and weak vibration strength, while a harvester in the thickness-shear mode must work at a circumstance of high frequency and strong vibration strength. Compared with a haverter in the flexural mode, a harvester in the thickness-shear mode can obtain much more energy only if the vibration is strong enough. These results present a theoretical instruction for us to choose the most suitable harvester according to the operating environment.(3) The effect of polymer layer on the performance of piezoelectric harvester in thickness-shear vibration is analyzed in this paper. Numerical results show that the use of polymer layer can considerably reduce the system resonant frequency, which broadens the application domain of the harvester.(4) A Rosen transformer model with attached end mass is proposed in this paper. Analytical solutions for free and forced vibrations are obtained and numerical calculations based on the analytical solutions are presented. The results indicate that the effects of end masses are significant on the location of the nodal point of the operating mode and the transforming ratio. Which end the end mass should be attached at is determined by the operating mode of the transformer. If the transformers work at the first resonance mode, driving end masses should be our first choice. Only under the condition that the transformer has to be operated at the second resonance mode should we attach end masses to the receiving end. These effects can be explored for designing new Rosen transformers operated at different resonance mode with better or optimal performance.