以滑動模式控制用於近場光碟機讀寫頭飛行高度之控制

Abstract

本論文旨在發展近場光碟機之飛行高度控制設計。近場光碟機應用近場光學理論之飛行讀寫頭設計，突破了以往光碟機技術瓶頸，可以大幅的提高光碟片儲存容量和密度；為了達到近場光學之效果，讀寫頭之物鏡與碟片記錄層之距離，亦即所謂的飛行高度，必須維持在次波長之近場光學高度內。因此本研究應用飛行高度控制器結合壓電彎片驅動器與滑動器，在飛行高度方向上執行迴授控制，用於追隨碟片模態與傾斜所造成之碟片高度變化，以維持要求的穩定飛行高度。 針對飛行高度控制中的重現誤差部分，本研究首先提出結合滑動模式控制特性與學習控制特性，發展以滑動模式為基礎之學習控制，用於消除重現性誤差。滑動模式對模型不確定性及外界干擾具有獨特的強健性。學習法則的補償項是使用形狀函數來趨近積分項中的影響函數，並且估測控制輸入。此類學習控制器能夠學習所需之重複控制訊號輸入，消除重現誤差。 本研究還提出整合頻域修整(Frequency shaping)於滑動模式控制方法的頻域修整滑動模式控制器，在應用線性二次法(Linear quadratic)之最佳化滑動模式控制設計中，採用頻域修整性能指標函數，本控制設計能將所需的頻域補償器整合至滑動模式之中，進一步的消除特定頻率之誤差訊號或高頻干擾。 本研究發展上述兩種控制法則，對所設計的飛行讀寫頭，進行飛行高度之控制實驗。相較於傳統的控制器，實驗結果證明所提出之方法能在碟片振動之干擾下達成穩定的飛行高度控制。

This study aims to control the flying height of pickup heads in near-field optical disk drives. Near-field optical disk drives apply the near-field optics theory to replace conventional optical disk drives, thereby substantially increasing data storage capacity. In order to achieve near-field optical recoding, the distance between the objective lens and disk surface, i.e. the flying height must keep within the range of sub-wavelength. As a flying height actuator, a piezoelectric bender is used in this study to complement an air bearing at the head/disk interface to perform the feedback control in order to decrease flying height variation due to disk vibration and tilt. Firstly, this study aims to develop a sliding-mode based learning controller for eliminating repetitive error in flying height control. It incorporates characteristics of sliding mode control into learning control. The reason for using sliding mode control is attributed to its robust properties dealing with model uncertainty and disturbances. The learning algorithm utilizes shape functions to approximate influence functions in integral transforms and estimate the control input to reduce repetitive error. Thus, the repetitive component in the flying error can be compensated and a stable flying height can be maintained under a periodic disk vibration. In addition, this study presents a discrete frequency-shaped sliding mode control law for flying height control of near-field optical disk drives. Frequency shaping is conducted such that filtered state variables are penalized at the disk vibration frequency using inverse notch filters in LQR weighting functions. Hence, the controller can achieve stable flying height in the presence of certain frequency vibration of optical disks. Using the proposed methods, this study carries out experiments for flying height control with the pickup head. According to experimental results, the proposed methods are validated in comparison with conventional controllers.