抓爬式靜電微致動器之模擬及低電壓區輸出特性的改善

Abstract

抓爬式靜電微致動器(SDA)有微牛頓等級輸出力、在數十奈米等級步進尺寸下可行進達數毫米。 迄今，此類致動器的幾何尺寸選擇主要依賴經驗，驅動電壓及輸出力亦需經由實驗測試後方有依據。為減少試誤法造成成本負擔，發展系統化的解析模型有其必要性。此外傳統矩形抓爬式微致動器的啟動電壓常超過80伏特，容易造成介電層崩潰之可靠度問題。因此開發新設計以降低操作電壓、能源消耗、並維持足夠輸出力及可靠度是另一研究重點。

本研究第一部份旨在建立抓爬式微致動器的較符合現實情況之解析模型，並以面型鎳基微加工技術製作。由於抓爬式微致動器的運動以單一方向為主，假設其沿寬度方向的變形及應力分佈一致；因此可依據Euler-Bernoulli樑的模型，配合適當的邊界條件以解出相關的積分常數，再依序推導如未接觸長度、貼底(priming)電壓、變形曲線、輸出力、彎矩及最大正向應力等特徵。上述輸出力可設為單自由度運動方程式的輸入。在長80微米、寬65微米的尺寸下，以80到120伏特為輸入範圍，其步進尺寸預測值在22到41奈米之間。以500Hz電壓測試結果發現，1500次輸入脈波所得總行程位移介於5.9到13.9微米；每個抓爬式微致動器輸出力介於12.4 到30.2微牛頓，其測試結果與預測值誤差小於10%，顯示本文提出之解析模型具有可預測輸出力的較佳實用性。

本研究第二部份提出一種新式可在較低電壓驅動的抓爬式微致動器設計，主要是將撓性接頭加入傳統矩形板的中間適當處。在同樣總長80微米和寬65微米的尺寸下，製作並測試具有不同撓性接頭尺寸及位置的設計，其結果發現啟動電壓最小可降低到約40伏，遠低於傳統同尺寸抓爬式微致動器所需要的80伏特。當輸入電壓在40到120伏之間時，其步進尺寸預測值在9到67奈米之間，而實測輸出力在4.9到19.6微牛頓之間。證實所提出的新設計可在傳統抓爬式微致動器無法操作的低電壓區有效操作。但有限元素法分析結果亦顯示，在高電壓區的最大輸出力，會受到撓性接頭最大應力達到降伏應力的限制。在此提出的融合撓性接頭的設計，將可提供抓爬式微致動器更大的設計空間，以滿足不同輸出特性的要求。

An improved model is proposed to reduce the cost due to trial and error in selection of Scratch Drive Actuator (SDA) geometry, driving voltage and output force level. Besides, a novel SDA design to reduce driving voltage from at least 80 volts to 40 volts to get better reliability with comparable output is also proposed. It is assumed that the deformation and property of SDA is same along the width direction, as the main plate may be treated by a beam model. Governing equation based on Euler-Bernoulli beam is first constructed. Solving this equation with proper boundary conditions, key SDA characteristics can be determined analytically, such as non-contact length, priming voltage, deflection curve, output force, bending moment and stress. The output force just stated is the input of SDA dynamic model of single degree-of-freedom including friction. To verify proposed model, electroplated nickel SDA arrays of 80 micron in length and 65 micron in width with suspended spring are fabricated and tested. The average travel distance after 1500 input pulses from 80 to 120 volts are measured to be from 5.9 to 13.9 micron, while average measured output forces are from 12.4 to 30.2 micro-N per SDA. Deviations between simulated and measured results are less than 10%, showing the superior ability of the proposed integrated SDA model for better performance prediction. A low-voltage scratch drive actuator (LVSDA) is also proposed here by incorporating flexible joint into the conventional rectangular SDA to improve performance in low-voltage region. Experimental results show that, at the same total plate length of 80 micron and width of 65 micron, the proposed LVSDA can be actuated as low as 40 volts, much lower than 80 volts, the minimum required input voltage of conventional SDA. From nonlinear finite element analysis conducted by CosmosWorks, yielding effect is found to be a critical factor. Before yielding, LVSDA can provide better performance than SDA at the same input voltage. However, the yielding stress in flexible joint would limit the achievable maximum output force in high-voltage region. By varying joint length, width, or location, LVSDA has been operated in low-voltage region where the conventional SDA can not be operated, and can still provide comparable performance as SDA in high-voltage region. The proposed LVSDA design can provide more flexibility in design selection to meet different performance requirements.