Over the past several years, there has been increasing interest in harvesting energy from ambient vibrations in the environment by converting mechanical energy into electrical energy. A popular method is to use a piezoelectric cantilever beam. In order to harvest the most energy with the device, the beam's fundamental mode must be excited. However, this is not always possible due to manufacturing of the device or fluctuations in the vibration source. By being able to change the frequencies of the beam, the device can be more effective in harvesting energy. In this paper, a model for a three layered piezoelectric cantilever beam utilizing a shunt tuning circuit will be presented. The fundamental frequency of a cantilever beam is dependent on the stiffness and mass of the beam. Either adding a tip mass to the end of the beam or increasing the dimensions of the beam can alter the mass. The stiffness of the beam is a function of the geometry, mechanical properties, and the electromechanical coupling of the piezoelectric element. In this paper we prepare the use of a piezoelectric layer with an attached shunt circuit for tuning its stiffness, and thus the beam frequency. The piezoelectric coefficients of this layer and its shunt circuit determine the amount of electromechanical coupling. By varying the shunt circuit, the beam can be tuned to a certain frequency. This paper presents a study of the effects additional harvesting and tuning layers have on the amount of tuning and generated power in the beam. These additional layers will add more piezoelectric material as well as mass to the beam and therefore there will be a balance between the amount of harvested energy and the tunable frequency range. By quantifying the effects of these parameters, it will be easier to design a harvester to be used in a particular frequency range as well as to produce a certain level of power.
In this paper, a study is presented in which piezoelectric microbenders were fabricated and tested to demonstrate energy generating performance. Trapezoidal and diagonal (with respect to the substrate crystal directions) unimorph PZT cantilever benders with interdigitated electrode patterns were utilized. The interdigitated design is beneficial for microenergy harvesting devices because it utilizes the d33 mode, which can generate higher voltage than the d31 mode design. It can also eliminate the bottom electrode by only using an interdigitated top electrode, which facilitates fabrication, as opposed to the d31 mode design that requires both top and bottom electrodes. The micro-electromechanical system (MEMS) benders fabricated in this study consist of layers of SiO2/SiNx/ZrO2/PZT and Au/Cr interdigitated electrode on the top. The experimental results indicate that the fundamental frequencies of the microbenders are about 9.1 kHz for the trapezoidal bender and 18.48 kHz for the diagonal bender. The microtrapezoidal bender can generate power of approximately 1.4 μW into a 680 kΩ resistive load at the resonant frequency. The diagonal bender can generate power of about 18.2 μW into a 100 kΩ resistive load at the resonant frequency.
This paper demonstrates the tunability of resonant frequencies for MEMS piezoelectric resonators acting in the d 33 mode by experiment and theoretical analysis. Thin-film MEMS beams made by sol-gel PZT processing are first fabricated and tested to investigate the tuning capability. The three-layered interdigitated-electrode trapezoidal beams are then modeled by finite element analysis for validation. Beam curl and undercutting are also examined to present an alternative way of finding the stress gradient in comparison to Stoney's formula. The experimental and analytical results indicate that piezoelectric MEMS resonators have the ability to passively tune their resonant frequencies between open-circuit and short-circuit frequencies. Tuning of 0.2-0.6% is demonstrated experimentally, which compares with finite element predictions of 1.02-1.08%. Consideration is given to the reason for the differences in experimental percentages versus those predicted numerically, including the use of bulk PZT values in the ANSYS simulations, the undercut and curling effects of fabrication, and the low percentage of piezoelectric poling in the length direction.
The use of an interdigitated electrode configuration for tunable MEMS resonators is investigated. The tuning concept utilizes a shunt capacitor concept based on the fact that the mechanical compliance (stiffness) of the system is a function of both the mechanical properties and the electromechanical coupling of the piezoelectric element. Since the electromechanical coupling is dependant on the electrical impedance of the piezoelectric element and its shunt circuit, the circuit conditions applied to the piezoelectric tuning element can be varied in such a way as to tune the vibrational frequency of the resonator [1]. By utilizing an interdigitated electrode design to elicit the d33 response of the piezoelectric, a greater electromechanical coupling is achievable, corresponding to a wider range of tunability. In this paper, a model of the resonator is presented and then used in a study to determine the parameters which result in the highest range of tunability for the resonator.
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