Research On Piezoelectric Energy Harvester Based On MEMS Technology

The technologies of sensor/actuator network, embedded system, RFID, wireless communication etc. have been developed recently. With development of these technologies, their power supply should be featured with small/micro volume, long life, replacing needless and self-service. Traditional battery can’t satisfy all the demands apparently. Energy harvesting from environmental vibration is an effective approach. Its power life is unbounded and it needn’t manually recharge. On the other hand, if MEMS technology is adopted in the energy harvester fabrication, the integration process of energy harvester and electronic components/devices could be realized, which will decrease the whole volume and cost, and then wireless node in network is anticipated to become a miniaturized, integrated and self-powered cell.In this thesis, MEMS-based piezoelectric energy harvester is studied; the piezoelectric effect of piezo-material is introduced to transfer vibration mechanical energy to electricity. The main contents and conclusions are listed below:1. A piezoelectric cantilever/mass composite structure is proposed to harvest vibration energy in resonance mode. The structure is designed for low-frequency (less than 1000Hz) vibration in common environment. It includes silicon host layer, silicon oxide dielectric, PZT layer sandwiched by electrodes and a mass attached in cantilever tip end. The mass is introduced to decrease natural frequency of the whole structure. Thereafter, an equivalent electrically-induced-damping spring-mass model and a mechanical-electrical coupling model of piezoelectric cantilever are utilized to analyze the relation between electrical performance and vibration /structure parameter. Analysis results indicate that the electrical power transferred from vibration is proportioned to square of vibration acceleration, to structure mass, and the piezoelectric voltage output is proportioned to vibration acceleration, to PZT thickness and to piezoelectric constant. It also makes clear that electrical performance of the energy harvester is related to its mechanical damping: when the electrically-induced-damping is equal to the mechanical damping, the maximum value of electrical power transferred from vibration can be obtained.2. Based on 3D finite element model of composite structure, the natural frequency, piezoelectric voltage, tip displacement and stress distribution are computed, and their dependence on structure parameters such as length/width/height of the the cantilever and mass are analyzed. Then PSpice equivalent circuit model is explored to resolve the voltage and power delivered to external resistance and release the output performance of the energy harvester. According to the analysis, a range of structure parameter is selected: cantilever beam length L, 2000~3500μm; width, 500~1000μm, mass length, 1/6L~2/3L, PZT effective length, 1/5~1L; and PZT thickness, thicker is better, at least 1μm; silicon thickness, 8~15μm. If the structure parameters are controlled in above range, the natural frequency of structure can be set into 100~ 1000 Hz and piezoelectric voltage can reach above 0.2Volt, at the same time, the structure can suffer gravity and at least1g vibration shock. On the other hand, the internal equivalent resistance is about 10K~100K?, when optimal external resistance is connected, maximum deliverable power is aboutμW level.3. A serial of energy harvester with different structure parameters are fabricated by using MEMS technology. The involved technologies include Sol-gel process for PZT film preparation, electrode deposition, optical lithography and etching process for functional films patterning, UV-LIGA based SU8 process, micro-electroforming, etc. The detail process parameters are studied, and related process control and improvement issue is discussed.4. A testing system is used to measure the performance of fabricated prototypes. The system includes wave-form generator, power amplifier, vibrator, accelerator, oscillograph, displacement sensor etc. The natural frequency, voltage output, maximum deliverable power, tip displacement, rectification and capacitance storage experiment etc are measured or carried out. Testing results are compared with related theory analysis and simulation. Good agreement between them is obtained. According to testing results, the best performance of piezoelectric-composite-cantilever energy harvester is about: the natural frequency is 229Hz, under 0.5g acceleration resonant excitation, its AC voltage output is 3.93V, DC voltage across capacitance is 1.57V, and maximum power output is 2.55μW, and power density is about 625μW/cm3.5. To improve electrical performance, PZT thickness and PZT effective length issues and method of serial/parallel connection are studied. Furthermore, a multi-degree-of-freedom structure and trapezium/triangle structure type are proposed to widen energy harvester’s bandwidth or enhance its electrical output. Ultimately, a semi-active RFID tag and a low cycle duty sensor node are illustrated to analyze the application feasibility of our present energy harvester performance. As a result, our research result is promising in ultra-low-power consumption devices application.