31-08-2012, 11:52 AM
DESIGN AND SIMULATION OF MEMS VIBERATIONAL ENERGY HARVESTER FOR MICRO POWER GENERATION
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ABSTRACT
In recent years, energy harvesting using piezoelectric materials has become a very popular research topic. Various device sizes and structures have been tested, but it is difficult to compare power measurements as device fabrication and experimental methods vary from paper to paper. In an effort to standardize comparisons in spite of these changing parameters, the dependence of generator power output on device dimensions has been investigated. Though MEMS scale devices have been produced, comparatively little work has been done using aluminum nitride (AlN). This project utilizes AlN due to its ease in processing and potential for on-chip integration. By operating at a MEMS scale, the benefit is that arrays of piezo generators can be placed on the same die. With the process advantages of AlN, a long term goal of an integrated power-harvesting chip becomes feasible. However, theoretical results of scaling predict that raw power output and even power per unit
volume will decrease with scaling. This indicates that a single large generator, taking up the same area as several small generators, would produce a noticeably larger power output. Due to time constraints, no new generators could be fabricated within the time span of the project. An existing piezoelectric cantilever was used to verify the theoretical predictions of resonant frequency and static deflections under applied voltage. These predictions agreed quite closely with the observed results. However, no measurable electrical response could be found while exciting the beam with an electromagnetic shaker device. A similar experiment was performed using an AFM to directly excite the beam, but again the electrical response was difficult to characterize. While the results of the experiments were not optimal, the difficulty in measuring the electrical response of the beam demonstrates the design challenges involved with energy harvesting on a small scale. Piezoelectric generators rely on resonance to generate useful quantities of power, and power output is highly sensitive to the frequency of the physical vibrations applied. Whilegenerators of this type could be useful if targeted to a specific application if the frequency ofenvironmental vibrations is known, a more versatile approach would use a different design toreduce the frequency sensitivity. Broad-band designs, using either non-resonant or self-tuningstructures, would be able to harvest energy much more efficiently in changing environments.