結合聲子晶體樑共振腔之壓電能量擷取裝置研製

Abstract

聲子晶體(Phononic crystals)是由一種或多種材料週期性排列的結構，其具有頻溝(band gap)的特性，可阻擋某頻段之彈性波傳遞。依據產生頻溝機制的不同可分為布拉格散射和局部共振型式，其中局部共振型式主要取決於共振子的結構與頻率，因此可實現以較小尺寸阻擋較大的波長。在完美週期的聲子晶體中，將其中一單胞移除會引致局部共振模態，本論文即利用此方法設計出聲子晶體共振腔。利用聲子晶體頻溝的特性使彈性波能量得以聚集，再透過壓電效應將振動能轉換為電能。並設計低頻風能轉換敲擊力的機制，以此激發出高頻聲波。 本論文使用有限元素法軟體(COMSOL)來模擬板波在聲子晶體樑結構中之行為，並透過幾何尺寸分析設計出具有低頻頻溝(7-11.5 kHz)之聲子晶體共振腔，在9.04 kHz有一共振模態。另外，模擬結果顯示當聲子晶體層數達到四層以上時有較佳的共振振幅。在壓電聲晶體的模擬中，可以發現壓電層厚度與共振腔底板比例為0.3、壓電層長度為8 mm時會有較佳的輸出電壓，故以此做為最終設計。 實驗的量測結果顯示，本研究之聲子晶體結構在5.9 kHz至9 kHz聲波會有較大的衰減，證明其阻擋波傳的能力。同時也對敲擊源的激發能量頻率進行量測與分析，以設計出最佳的敲擊源。在聲子晶體共振腔量測的部分，以螺絲配合AT-3黏膠製成的共振腔其共振頻率為8.28 kHz，加入壓電層後的共振頻率為7.25 kHz，平均功率為0.77 mW，最佳阻抗為8.27 kΩ。與無頻溝之對照組平板相比功率可達到7倍之多，證明聲子晶體對於能量擷取的效益。 最後配合電源管理模組與風力敲擊裝置進行實驗，將四個共振腔同時於風速 4 m/s下運作，平均每1.06秒可使設計電路中的電容充滿至標準電壓，並放電至後端負載，實踐壓電能量擷取裝置之研究。

Phononic crystals (PC) are composed of one or more materials which distributed periodically. One of the main properties of the phononic crystals is band gap which prevents the wave of a specific frequency range propagating. The band gap based on local resonant could get lower frequency band structures. A resonant cavity is formed by removing a unit from perfect PCs. And the energy of elastic wave at the resonant frequency can be trapped in the cavity. In this study, the piezo energy harvester is based on converting the acoustic energy into electric energy by putting a piezoelectric ceramics film. Besides, we design a mechanism to transforming low-frequency wind energy into impact, so we can get the high-frequency acoustic waves as source. In this study, we numerically calculate the dispersion of lamb wave in phononic crystals strip by finite element method (FEM). The PC resonator with a band gap at 7-11 kHz and resonant frequency at 9.04 kHz was formed. Moreover, the simulated results show that more than 4 rows of PC grating can get larger resonant amplitude. And the maximum voltage output occurs at the ratio of thickness of piezoelectric ceramics film and PC plate in 0.3. The length of piezoelectric ceramics film is 8mm. In the experiment, the acoustic wave attenuated at 5.9-9 kHz in a PC strip. And we design impact source by analyzing different materials contact with thin plate. The piezo energy harvester with resonant frequency at 7.25 kHz could get a maximum power 0.77 mW under a load resistance 3.9 kΩ. Compared with the control group, the output power is 7 times lager. It shows that PC is helpful to trapping energy. Finally, we connect the output voltage to power-management circuit to observe capacitance discharging. When four PC resonators work at the same time, the average time of capacitance discharging is 1.06 seconds with the wind velocity of 4 m/s. It validates that the piezoelectric energy harvester could generate electric power from wind energy.