磁預壓節能型音圈馬達自動對焦致動器的設計

摘要：在影像系統中，對焦是其中最重要的功能之一，近年來將具有對焦或變焦功能的數位相機整合到手機裡已經躍升為市場的主流，各種廠牌的手機相機如雨後春筍般的出現在市場上。自動對焦致動器在相機系統中是基本配備，主要功能在於拍照時調整焦距，改善成像品質。常見的自動對焦致動器有：音圈馬達、步進馬達、壓電馬達及液態透鏡等，而音圈馬達因為有成本低、體積小及定位重複性高等優點，因此適合用於手機相機模組中。

傳統的音圈馬達分為開迴路與閉迴路控制兩種，開迴路控制是使用彈片來進行重複性定位，而閉迴路控制則是運用位置感測元件（如霍爾元件）所產生的位置回饋訊號來調整鏡頭的位置。然而產業界目前的問題之一是音圈馬達式的自動對焦致動器有彈片製造組裝困難、耐摔性差及能耗不易降低的缺點。本篇文章主要在提出一創新的無彈片開迴路式音圈馬達的設計來改善其缺點，該創新點在於利用磁預壓力結構設計以取代彈片的預壓力結構，以利往後在製造組裝上更為容易。由實驗結果顯示，本研究所提出利用磁預壓力可替代目前市面上較普遍的彈片式音圈馬達，並以一款8.5x8.5 mm音圈馬達致動器進行驗證發現，在相同對焦位移（0.4 mm）要求下，所需輸入電流下降15%以上，特別適合應用於後續高速動態響應對焦需求的超薄像機模組。

Abstract: Focusing is one of the essential functions in the image system. In recent years, mobile phones integrated with auto-focusing function have become mainstream, and a variety of types of mobile phones sprang to market. The auto-focusing actuator, used for adjusting foci and improving photo quality, is a fundamental module in digital cameras. Common choices of auto-focusing actuators for cell phone applications include VCMs, stepping motors, piezoelectric motors, liquid lens, enhanced crystal lens, polymer deformable membrane, and electroactive polymer actuators. VCMs prevail over other actuators due to low cost, small volume, and high positioning repeatability. Thus, VCMs are ideally suited for the camera module in a mobile phone.

The control schemes of existing VCM actuators can be broadly classified into two categories: open-loop and closed-loop position control. The former type makes use of spring foils to carry out repeated positioning while the latter utilizes feedback signals produced by sensors, such as Hall elements, to adjust lens positions. As we know, however, the existing VCM actuator has three chief drawbacks in its spring foils: difficulties in manufacturing and assembling, low robustness against dropping and power consumption limitation.

This study presents a novel auto-focusing VCM structure without spring foils to fix these drawbacks. The pre-compression magnetic force structure replaces the spring foils, making assembly easier. Experimental results show that the proposed structure can replace the commonly used VCM. An 8.5x8.5 mm VCM actuator demonstrates the required input current drops by at least 15% under an identical requirement of focusing displacement (0.4mm). This design especially improves the compactness and dynamic responses of the ultra-thin compact camera modules.

關鍵詞：致動器、音圈馬達、手機相機、相機模組、自動對焦

Keywords：Actuator, Voice Coil Motor, Cell Phone Camera, Camera Module, Autofocusing

前言
隨著科技的進步，近年來各家智慧型手機大廠均推出800萬畫素機種，以及手機相機模組的尺寸持續微型化，使得音圈馬達（voice coil motor, VCM）逐漸面臨可靠度有限、高耗電量、精密度不足等瓶頸，市面上200萬像素相機多只具備固定距離的對焦範圍，當使用者拍攝較遠或較近物體時，影像便會模糊不清，加上隨著鏡頭畫素提升、景深也將愈來愈窄，照片效果很容易因對焦不清而出現背景清楚、人像模糊的情形。當手機相機邁入300萬、500萬畫數時代之後，鏡頭必須再搭配自動對焦功能，且目前已經將具有自動對焦和變焦功能的相機模組整合到手機裡，根據圖1與圖2的市場調查資料顯示，在2015年全球預估有15億支配備有3百萬畫素以上高解析度鏡頭的相機手機，可見相機手機市場的未來潛力，全球的手機相機配備有3百萬畫素以上高解析度鏡頭的相機手機日益攀升，亦即相機手機高畫素的產品已經躍升為市場的主流。