HDD에서의 외란분석 및 TMR 최소화를 위한 제어 이득 결정 = Disturbance identification and control gain determination for minimum TMR in HDD

국문 초록 (Abstract)

HDD(Hard Disk Drive)에서의 외란은 TMR(Track MisRegistration)의 원인이며, 트랙밀도 향상에 장애가 된다. 본 논문에서는 HDD에 작용하는 외란을 PES(Position Error Signal)만을 이용하여 정량적으로 분석해낸다. PES를 이용한 외란 분석 방법은 HDD 하드웨어의 변경을 최소화하기 때문에, 분석 결과가 더욱 정확하다는 장점이 있다. 외란은 크게 RRO(Repeatable Run-Out)와 NRRO(Non-Repeatable Run-Out)로 구분하며, NRRO는 입력 외란, 출력 외란, 노이즈로 분리한다. 또한, 외란 분석에 필요한 VCM(Voice Coil Motor) 구동부의 주파수 응답함수를 시간 지연과 0차 홀더를 고려하여 폐루프 주파수응답함수로부터 계산한다. 본 논문에서 제안한 외란 분석 방법은 다양한 제어 이득에 대한, PES의 파워 스펙트럼 예상값을 실험값과 비교함으로써 검증하며, 그 결과로부터 분석된 외란을 서보의 성능 예측에 사용할 수 있음을 확인한다.
PID 제어기를 실험에 사용된 HDD 트랙추종서보 시스템의 기본 제어기로 삼아 PES의 표준편차를 예측함으로써 제어 이득이 트랙추종서보의 성능에 미치는 영향을 고찰한다. 이 때, 제어 이득을 효율적으로 결정하기 위해 교차주파수와 위상여유 등을 설계 사양으로 하여 해석적인 방법으로 PID 이득을 계산한다. 끝으로, 최적화 기법을 이용하여 임의의 제어기에 대해 최대 성능을 얻을 수 있는 제어 이득을 결정할 수 있음을 보인다. 여기서, 최적화 문제의 목적함수는 PES NRRO의 파워 스펙트럼으로부터 계산된 표준편차이며, 설계변수는 제어 이득이 된다.

다국어 초록 (Multilingual Abstract)

Disturbances, the main source of TMR (Track MisRegistration) being barrier to high TPI (Track Per Inch) of hard disk drives are identified quantitatively using PES (Position Error Signal) only, without extra sensor, for the purpose of increasing the accuracy by minimizing the hardware alternation. Disturbances are separated into RRO (Repeatable Run-Out) and NRRO (Non-Repeatable Run-Out), and then NRRO is separated into input disturbance, output disturbance and noise in the frequency domain. Further, plant FRF (Frequency Response Function) is calculated from the experimentally measured closed loop FRF with the influence of the zero-order-hold and the time delay taken into account. The suggested disturbance identification method is verified by comparing the predicted PSD (Power Spectral Density) of PES and the PSD of measured PES for various control gains, which convinces that identified disturbances can be used to predict servo performance accurately.
Without loss of generality, it is assumed that PID controller is used as a base controller in the track following servo system used in the experiment. Then, the effect of gain changes on the performance of the track following servo system is examined by predicting the standard deviation of PES. Here, analytic method of calculating PID gains from design specifications such as the gain crossover frequency and the phase margin is introduced in order to efficiently determine control gains without trial and error. Finally, it is shown that control gains can be determined using optimization method under the condition that any type of controller can be used in HDD track following servo systems, where the standard deviation calculated from predicted PSD of PES NRRO becomes the object function and design variables are composed of controller gains.

목차 (Table of Contents)

• TABLE OF CONTENTS = i
• LIST OF TABLES = iii
• LIST OF FIGURES = iv
• NOMENCLATURE = vii
• ABSTRACT = viii
• Chapter 1. Introduction = 1
• 1.1 Research Background and Object = 1
• 1.2 Previous Research Review = 3
• 1.3 Research Overview = 4
• Chapter 2. Track Following Servo System = 5
• 2.1 Structure of HDD = 5
• 2.2 Head Position Detection Method in HDD = 7
• 2.3 Calculation of PES = 10
• 2.4 Servo System in HDD = 13
• 2.5 Track Following Servo in HDD = 14
• 2.5.1 Contributors to PES = 14
• 2.5.2 Input-Output Relation = 15
• 2.5.3 RRO, NRRO, WRTMR and WWTMR = 16
• Chapter 3. Experiment Setup = 17
• Chapter 4. Measurement of Plant Frequency Response Function = 19
• 4.1 Measurement of Plant Frequency Response Function = 19
• 4.2 Plant Modeling = 24
• Chapter 5. Measurement and Analysis of PES = 27
• 5.1 Separation of PES RRO and PESNRRO = 27
• 5.2 Power Spectral Density = 28
• 5.3 Standard Deviation = 30
• Chapter 6. Identification of Disturbances by Measuring PES Only = 31
• 6.1 Identification of Output Disturbance and Noise = 31
• 6.2 Identification of Input Disturbance = 36
• 6.3 Results of Disturbance Identification = 39
• Chapter 7. Verification of Identified Disturbances = 42
• Chapter 8. Determination of Controller Gains = 57
• 8.1 Performance Limitation = 57
• 8.2 On the controller Design = 62
• 8.3 Discrete Controller Design Using Analytic Method = 64
• 8.4 Determination of Control Gains = 67
• 8.5 Optimization Method for Determining the Gains = 74
• 8.6 Further Considerations = 76
• Chapter 9. Conclusion = 78
• References = 80