用於自動對焦數位相機之音圈馬達及其伺服控制IC之分析與設計

Abstract

摘 要 本篇論文說明用於行動電話之數位相機自動對焦控制系統之音圈馬達設計與建模，並針對此自動對焦模組提出控制器設計方法，使用FPGA實現全數位之定位控制IC。在音圈馬達中，由於其機械結構及磁路架構，力常數及反抗電動勢常數呈現高度非線性之特性。傳統透過幾何結構導出磁力線分佈以鑑別馬達之等效參數的建模方式過於複雜且不切實際。針對此特殊用途之音圈馬達，基於其機械結構及材料，藉由Ansoft公司之電磁模擬軟體Maxwell 2D，發展一套建立其數學模型之流程。本文所使用之音圈馬達乃非傳統使用於手機相機自動對焦系統致動器之步進馬達或需要彈簧之音圈馬達。為了達到在受限的體積之下，具有最大之力常數之音圈馬達設計，本篇論文提出一套利用模擬軟體輔助設計之方法。最後模擬與實驗結果之比對將呈現模型之正確性及其所滿足之效能。根據音圈馬達之建模與設計，其伺服驅動控制IC設計規劃中包含數位伺服控制器、數位電流迴路及全橋式直流-直流轉換器。針對高效能、輕薄型手機相機及錄像機之鏡頭自動對焦定位控制，此伺服控制IC提供全數位化之方法。此自動對焦系統直線運動範圍全行程為0.6 mm，在30 ms內定位，且控制解析度在30 μm之內。馬達最大驅動力輸出為30 mN，輸出之峰值電流為120 mA。

This thesis presents the modeling of a voice coil motor (VCM) used for the auto-focusing control of a digital camera in applications to high-performance mobile phones. The force constant and its associated back-EMF constant of a VCM can be highly nonlinear with the mechanical structure. Conventional modeling technique using the derived flux distribution based on geometrical structure is quite involved and impractical to identify its equivalent parameters. A modeling procedure for a specially designed VCM is developed based on its mechanical structure and used material by using an electromagnetic simulation software─the Ansoft’s Maxwell 2D. An iterative optimal design process is then proposed to maximize the force constant of the VCM with a specified volume. Simulation results with experimental verification are given to illustrate that the proposed design procedure can achieve a satisfactory performance. According to the modeling and design of the VCM, this thesis presents the design methodology to implement a fully digital control IC for the position control of the auto-focus (AF) lens module in applications to mobile phone camera using the field programmable gate array. As compared with the conventional stepping motor control with a spring return fixture, the AF lens module is driven by a voice coil motor with a digital servo drive IC, which includes a digital servo controller, a digital current controller, and a full-bridge dc-dc converter. The designed digital servo drive IC provides a total digital solution to the position control of the AF lens module in applications to high-performance light-weight slim-type mobile phone camera and video. The designed AF lens module can reach a control range of 0.6 mm within 30 ms with a control resolution less than 30 μm, a peak driving force of 30 mN, and a peak output current of 120 mA.