應用可變焦式液態透鏡於光學共焦距離量測系統之開發

Abstract

本文基於雷射共焦理論探討在距離量測應用上的設計與評估，本研究在距離量測系統中加入一個特殊的可變焦式液態透鏡，成為一個具有可變焦功能的共焦距離量測系統；利用此可變焦式液態透鏡會因電腦對其輸入電流的不同而快速變焦之特性，取代一般傳統共焦測距法所需的Z軸方向垂直移動機制，藉此建立精確快速的距離量測系統，如此不僅能節省系統元件的複雜度及降低成本，還能避免Z軸方向運動時造成的各種誤差或干擾，而且搭配移動平台的二維移動可望完成三維的座標量測功能。本研究亦利用先前文獻推導出的共焦測距方程式配合電腦控制程式初步完成一個即時的能量曲線擬合(curve fitting)系統，用以加速量測速度及精準度，測量一點距離的時間由6.4秒進步至0.25秒，具有明顯的量測效率改善也具體展示本研究可變焦式精密量測系統的實用效益。實際樣品的仿真掃描之中也獲得平均精度約為7.28μm的表面形貌量測誤差，證明其三維掃描的可行性，未來再經由校正後可望有更佳的量測表現。量測系統最前端設計可容易置換不同倍率的物鏡，經實驗證實提高物鏡倍率可以加強整個系統對入射光源的聚焦能力以達到更穩定精準的距離量測效果。再者，嘗試選用不同倍率物鏡，倍率從5X到100X不等，發現不同倍率的物鏡提供距離量測範圍，可依不同的量測需求調整從30μm到23mm不等。對可變焦式液態透鏡藉由每單位1mA的輸入電流改變可達由0.1μm至 0.1mm不等的焦距變化，配合共焦反射光束能量的量測，確認當時量測物件的距離。本研究也利用不同材質物件的量測結果比較確認此距離量測系統的可行度與實用性在大部分的材料中都可以有一定程度的量測效果，有利於未來的各項應用。

In this study, a confocal displacement sensor was developed and analyzed. A varifocal liquid lens was equipped into this confocal displacement sensor to generate the displacement data converting from focal length without the axial moving of the lens. By controlling the driving current of this varifocal lens in this specific confocal displacement sensor, the displacement in z axis of detected samples could be analyzed efficiently and precisely without the movement in z axis. As a result, the displacement sensor could be smaller and less expensive. The power curve fitting method for assisting the searching of peak value of power curve was proposed to reduce the measuring time. A real time curve fitting program was presented based on previous researches. The measuring time in measuring distance was reduced from 6.4s to 0.25s by using this program. The measuring efficiency was improved. It is shown that this sensor is suitable to be applied in a 3-D scanning process with a 2-D stage because of the small error in a 3-D scanning test of a designed sample. The scanning accuracy is about 7.28μm. And measuring precision is expected to be improved with this program in the future. Disturbances and vibrations caused by the movement in z-axis of traditional confocal sensors are also prevented. A 3-D scanning system may be constructed by combining this displacement sensor and a 2-D stage. The front end of this displacement sensor was designed to equip an objective lens to improve the measuring precision and stability. Five different objective lenses from 5X to 100X were used, respectively. With equipping different objective lens, the measuring range could be from 30μm to 23mm and the variety in focal length with changing each 1mA driving current to the varifocal liquid lens may from 0.1μm to 0.1mm. Distance in z axis of measured samples is measured by analyzing the power of reflective light beam. It is confirmed by several experiment that this sensor can measure distance of different samples made of different materials. This result make the varifocal confocal displacement sensor can be applied in different measuring conditions. The scanning accuracy of this varifocal confocal displacement sensor is expected to be improved by more experiment in the future.