光碟機的聚焦控制為伺服控制的首要步驟，控制系統須將雷射光點精確的聚焦在碟片表面以利數位資料的寫入或讀取，而後續的偱軌伺服才能正常運作。本論文主要是建立使用像散法的自動聚焦控制系統，可運用於DVD-RAM光碟機。使用適應性濾波演算法LMS來設計控制器(FXLMS)，以主動式雜訊控制方式，由控制器產生前饋反向信號源，迅速地抑制外界的擾動，特別是低頻信號造成的聚焦誤差。
論文中使用數位信號處理器(DSP)實現適應性控制器，主要目的在消除100Hz以下的擾動。控制信號用以消除單頻或寬頻擾動信號。實驗結果對聚焦致動器之共振頻率與其諧振模式至少可以消除大約15 dB左右，驗證了主動式前饋控制可消除DVD光碟機因外界干擾所造成的聚焦誤差。

To begin with the servocontrol of DVD drive system, the controller is specified to manipulate precisely the laser focus spot on disc surface with highspeed data access. That is, autofocus control is a prerequisite for track following and other control functionalities. In this thesis, we set up an autofocus control system for DVD drive with the astigmatic optical system, and then design a digital filter based on FXLMS algorithm to implement an active noise controller. The feedforward signal generated by the adaptive controller as an antinoise can efficiently suppress the primary source from the external disturbance.
The adaptive feedforward controller is implemented on a 32-bits floating point digital signal processor (DSP). We are aiming at suppressing disturbance with frequencies under 100Hz. Experimental results demonstrate that external disturbance characterized as the resonance frequency and its harmonics of the focus actuator is successfully decreased at least 15dB.

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