Abstract
An energy harvester composed of a microcantilever beam with a tip mass and a fixed electrode covered with an electret layer is investigated when subject to an external harmonic base excitation. The tip mass and fixed electrode form a variable capacitor connected to a load resistance. A single-degree-of-freedom model, derived based on Newton’s and Kirshoff’s laws, shows that the tip mass displacement and charge in the variable capacitor are nonlinearly coupled. Analysis of the eigenvalue problem indicates the influence of the electret surface voltage and electrical load resistance on the harvester linear characteristics, namely the harvester coupled frequency and electromechanical damping. Then, the frequency–response curves are obtained numerically for a range of load resistance, electret voltage and base excitation amplitudes. A softening nonlinear effect is observed as a result of decreasing the load resistance and increasing the electret voltage. It is found that there is an optimal electret voltage with the highest harvested electrical power. Below this optimal value, the bandwidth is very small, whereas the bandwidth is large when the electret voltage is above this optimal value. In addition, it is noted that for a certain excitation frequency, the harvested power decreases or increases as a function of electrical load resistance when the coupled frequency is closer to short- or open-circuit frequency, respectively. However, when the coupled frequency is between the short-circuit and open-circuit frequencies, the harvested power has an optimal resistance with the highest power. Increasing the excitation amplitude to raise the harvested power could be accompanied with dynamic pull-in instability and/or softening behavior depending on the electrical load resistance and electret voltage. However, large softening behavior would prevent the pull-in instability, increase the level of the harvested power, and broaden the bandwidth. These observations give a deeper insight into the behavior of such energy harvesters and are of great importance to the designers of electrostatic energy harvesters.
Highlights
Power generation for low-energy, portable and miniature equipment attracts wide attention due to its potential in wireless and remote sensing, mobile electronics, wearable health devices, independent assistive technology, and space and military applications [1]
The remainder of the study is organized as follows: Section 2 shows the derivation of SDOF charge-based model, static and dynamic analyses and the eigenvalue problem of the model, Section 3 discusses the performance of the electrostatic energy harvester for a variety of operating conditions and electrical load resistances, and Section 4 summarizes the main findings and conclusions of this work
The above results obtained in this work showed that the effectiveness of electrostatic energy-harvesting, which is mainly due to the nonlinear coupling between the displacement and charge, increases for higher base acceleration and higher electret voltage due to introducing softening behavior to the harvester performance
Summary
Power generation for low-energy, portable and miniature equipment attracts wide attention due to its potential in wireless and remote sensing, mobile electronics, wearable health devices, independent assistive technology, and space and military applications [1]. These stacks determine the elastic restoring force of the harvester and, its resonance frequency Another compact MEMS device comprises proof mass with groove pattern suspended by springs facing electret strips on the bottom surface is demonstrated in [38]. In [41], an analytical tool is proposed to formulate coefficients of lumped-parameter models for electret-based energy harvesters without consideration of strong electric fields, but including fringing fields, parasitic capacitance, and proof mass displacement. The remainder of the study is organized as follows: Section 2 shows the derivation of SDOF charge-based model, static and dynamic analyses and the eigenvalue problem of the model, Section 3 discusses the performance of the electrostatic energy harvester for a variety of operating conditions and electrical load resistances, and Section 4 summarizes the main findings and conclusions of this work
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