Simulation And Experimental Study Of Multi-dimensional And Low-frequency Piezoelectric Vibration Energy Harvester

In recent years,the fusion of sensors and wireless communications has promoted the rapid development of wireless sensor networks(WSN).There are many types of sensors in WSN,and they perform different functions.They can detect temperature,pressure,humidity,light intensity,and so on.The number of these sensors is huge,and the energy supply is still battery in most cases.Although they consume less power,the battery capacity is limited and the service life is short.When the battery power is exhausted,the sensor cannot work properly,which greatly limits the function of WSN.In order to solve this problem,some researchers have proposed a feasible solution to harvest mechanical energy from the environment to replace the traditional power supply.Generally,mechanical energy conversion mechanisms mainly include piezoelectric,electromagnetic,electrostatic,and triboelectric.Among these energy conversion mechanisms,piezoelectric energy harvesters have received extensive attention due to their stable output performance,high power density,and easy integration with microsystems.Currently,the piezoelectric vibration energy harvester(PVEH)is based on a spring-mass system.When the natural frequency of the PVEH matches the environmental vibration frequency,PVEH has the best output performance.When the natural frequency of the PVEH does not match the environmental vibration frequency,the output performance of PVEH drops sharply.In addition,when the vibration direction in the environment deviates from the polarization direction of the piezoelectric material,the output performance of PVEH drops sharply.In this dissertation,a tower-shaped three-dimensional multi-mode piezoelectric vibration energy harvester and a frequency-tuning piezoelectric vibration energy harvester based on a solid-liquid mixture are designed to harvest vibration with variable frequencies and directions in the environment.The main work of this dissertation is as follows:1.This dissertation proposes a design scheme of a tower-shaped three-dimensional multi-mode piezoelectric vibration energy harvester(TS-PVEH).Its structure includes acrylic substrate,piezoelectric film,mass block and bent cantilever beam.The use of bent beams in the structure instead of rectangular cantilever beams,not only improves the performance of TS-PVEH multi-directional energy harvesting,but also makes TS-PVEH have three vibration modes in the low frequency range,thereby improving TS-PVEH’s broadband performance.COMSOL software is used to perform static analysis,modal analysis and harmonious response analysis of TS-PVEH.The simulation results show that the natural frequencies corresponding to the first three modes of TS-PVEH are 10.406 Hz,10.7 Hz,and 25.78 Hz,respectively.The mode shape corresponding to the first two modes is the vibration in the X-Y plane,and the mode shape corresponding to the third mode is the vibration in the X-Z plane,and the horizontal stiffness of TS-PVEH is smaller than the vertical stiffness.The optimized results show that as the thickness and width of the bending beam increase,the natural frequency of TS-PVEH increases,while the output voltage decreases;on the contrary,as the height of the mass increases,the natural frequency of TS-PVEH decreases,and the output voltage increases.2.The vibration test system is used to perform a comprehensive performance test on TS-PVEH.First,the amplitude-frequency response of TS-PVEH is tested and analyzed.The results show that the natural frequencies of TS-PVEH are 9.6 Hz and 24.6 Hz,and the natural frequencies of TS-PVEH are not related to acceleration,but only related to the structural parameters of TS-PVEH.Secondly,the impact of vibration direction on the output performance of TS-PVEH was tested.The results show that the angular bandwidth of TS-PVEH is about180°in the X-Y plane,while in the X-Z plane,the angular bandwidth of TS-PVEH is about 40°.In addition,under the different load resistances,the output voltage and power of TS-PVEH were measured.Under the excitation of vibration with acceleration of 2 m/s~2 and frequency of9.6 Hz,the load voltage of a single PVDF can reach 35 V,and the maximum power is 49.6μW.Under the excitation of vibration with acceleration of 3 m/s~2 and frequency of 24.6 Hz,the load voltage of a single PVDF can reach 29.3V and the maximum power is 51.5μW.The influence of the PVDF connection method and acceleration on the charging effect was discussed.Experimental results show that the greater the acceleration,the faster the voltage across the capacitor rises.When PVDF is connected in parallel,the charging effect is the best.3.A design scheme of a frequency-tunable piezoelectric vibration energy harvester(FT-PVEH)based on a solid-liquid mixture is proposed,which mainly includes a cylindrical cantilever beam,the piezoelectric film,an acrylic container and filler.Cylindrical cantilever beams are used instead of traditional rectangular cantilever beams,and PVDF is pasted on the surface of the cantilever beam in a circular array,which improves the performance of FT-PVEH to harvest multi-directional vibration and increases the overall output of FT-PVEH.The influence of the state and weight of the filler on the frequency bandwidth of FT-PVEH is studied.When the filler is liquid and the volume ratio is 0.5,the greater the acceleration,the greater the frequency bandwidth of FT-PVEH,and the optimal bandwidth is about 4.9 Hz.Compared with the solid filler,the frequency bandwidth of FT-PVEH is increased by 91%.When the filler is a solid-liquid mixed state and the volume ratio is 0.25,the greater the acceleration,the greater the frequency bandwidth of FT-PVEH,and the optimal bandwidth is about 6.5 Hz.Compared with the solid filler,the frequency bandwidth of FT-PVEH is increased by 182%.