Advanced control technologies for future storage devices

ADVANCED CONTROL TECHNIQUES FOR
FUTURE STORAGE DEVICES
THUM CHIN KWAN
NATIONAL UNIVERSITY OF SINGAPORE
2008
ADVANCED CONTROL TECHNIQUES FOR
FUTURE STORAGE DEVICES
THUM CHIN KWAN
(B.Eng. (Hons.), NUS)
A THESIS SUBMITTED
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
DEPARTMENT OF ELECTRICAL AND COMPUTER
ENGINEERING
NATIONAL UNIVERSITY OF SINGAPORE
2008
To
my parents, my sisters, my brothers-in-law,
and my friends
Acknowledgements
I would like to express my gratitude and respect to my advisor, Professor Ben
M. Chen for his guidance and support. His deep insights and broad views often
give me new and valuable ideas in my research. If not for him, the successful
completion of this research would be virtually impossible. I would also like to thank
Dr. Du Chunling of A*STAR Data Storage Institute (DSI) for her sincere and timely
assistance in addition to her beneficial instructions during the course of this research.
Her comments and suggestions have been extremely useful to me for improving the
quality of my research. Besides them, I am also deeply indebted to my co-advisor,
Dr. Ong Eng Hong of DSI as well as my former co-advisor, Dr. Guo Guoxiao of
Western Digital Technology for their guidances into the field of Hard Disk Drive
(HDD) technologies.
I am very grateful to many DSI staffs and students, namely, Dr. Teoh Jul Nee,

Mr. Lai Chow Yin as well as former DSI students, Dr. Pang Chee Khiang and
Dr. Zheng Jinchuan for how much I had benefited from so many of our discussions
on HDD servo technologies. I am also grateful to Dr. Peng Kemao and Dr. Lum
Kai-Yew of Temasek Laboratories, National University of Singapore (NUS), for their
valuable suggestions, which aided in the development of some ideas in this research.
To my family, I want to thank them for granting me the absolute freedom
to cho ose the career path I desire. To my friends, I wish to express my deepest
appreciation for their unconditional and relentless encouragement and emotional
i
ACKNOWLEDGEMENTS ii
support throughout the course of this research, especially during those difficult times
at work and love.
Finally, I want to thank NUS for offering me the NUS Research Scholarship
for my Ph.D. study in NUS, and DSI for providing the experimental platform and
equipment for my research as well as those two competition trophies that help to
decorate my cubicle quite nicely.
Contents
Acknowledgements i
Contents iii
Summary viii
List of Tables x
List of Figures xi
Nomenclature xix
1 Intro duction 1
1.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Hard Disk Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 HDD Servo Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1 Servo Information . . . . . . . . . . . . . . . . . . . . . . . . . 6
iii
CONTENTS iv

1.3.2 Servo Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.4 Servo Track Writing Technology . . . . . . . . . . . . . . . . . . . . . 14
1.5 Contributions of the Thesis . . . . . . . . . . . . . . . . . . . . . . . 14
1.6 Organization of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . 17
2 System Identification and Implementation Setup 18
2.1 HDD Implementation Setup . . . . . . . . . . . . . . . . . . . . . . . 18
2.2 Plant Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.1 HDD VCM Actuator Modeling . . . . . . . . . . . . . . . . . 21
2.2.2 STW Platform . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.3 Noise and Disturbance of STW platform . . . . . . . . . . . . . . . . 27
2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3 Servo Control Design for a High TPI Servo Track Writer with
Microactuators 30
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
3.2 Generalized KYP Lemma and Its Original Application in Sensitivity
Function Shaping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.3 An Enhanced Generalized KYP Lemma Based Sensitivity Function
Shaping Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.3.1 Selection of Poles of Q(z) and Simulation Results . . . . . . . 36
CONTENTS v
3.3.2 H
∞
Stability Robustness . . . . . . . . . . . . . . . . . . . . . 42
3.4 Rejecting Low-Frequency Disturbances for a STW Platform: Design
and Implementation Results . . . . . . . . . . . . . . . . . . . . . . . 44
3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
4 A Unified Control Scheme for Combined Seeking and track follow-
ing of a HDD Servo System 50
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.2 Unified Control Scheme for Track Seeking and Track Following and

Controller Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
4.2.1 Controller Structure . . . . . . . . . . . . . . . . . . . . . . . 55
4.2.2 Stability Issues . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.2.3 Design of Controller Parameters for HDD Servo System . . . . 62
4.2.4 Controller Design for HDD Application . . . . . . . . . . . . . 65
4.3 Simulation and Implementation Results . . . . . . . . . . . . . . . . . 69
4.3.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . 69
4.3.2 Implementation Results . . . . . . . . . . . . . . . . . . . . . 72
4.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
CONTENTS vi
5 Mid-frequency Runout Comp ensation in HDDs via a Time-Varying
Group Filtering Scheme 83
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
5.2 Design of “Add-on” Linear Time-Varying Group Filter for Mid-f
RRO Compensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
5.2.1 Group Filter Structure: Parallel Realization . . . . . . . . . . 87
5.3 Selection of Design Parameters . . . . . . . . . . . . . . . . . . . . . 93
5.4 Application to a HDD servo system . . . . . . . . . . . . . . . . . . . 97
5.4.1 Main Servo Compensator Design . . . . . . . . . . . . . . . . 97
5.4.2 Design of the LTV Group Filter for Mid-f RRO Compensation 98
5.4.3 Stability Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.4.4 Simulation and Implementation Results . . . . . . . . . . . . . 100
5.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6 An H
∞
Disturbance Observer Design for High Precision Track Fol-
lowing 116
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
6.2 Proposed DOB Design . . . . . . . . . . . . . . . . . . . . . . . . . . 118
6.2.1 DOB with nominal feedback controller . . . . . . . . . . . . . 118

6.2.2 H
∞
Q-filter design . . . . . . . . . . . . . . . . . . . . . . . . 121
CONTENTS vii
6.3 Designs of nominal feedback controller C(z
−1
) and DOB . . . . . . . 122
6.3.1 Simulation and implementation results . . . . . . . . . . . . . 126
6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
7 Conclusions 132
7.1 Findings and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 132
7.2 Suggestions for Future Research . . . . . . . . . . . . . . . . . . . . . 134
Bibliography 137
Author’s Publications 154
Summary
Latest solid state disks (SSDs) have shown read/write (R/W) performances that
are comparable those of the conventional hard disk drives (HDDs). And in the case
of 1.8” form factor, the SSDs matched storage capacity of its HDD counterparts as
well. Such high performance alternative storage devices are threatening the survival
of conventional HDDs in the global data storage systems market.
Nonetheless, as the multimedia world progressively enters into the high-definition
era, the demand of storage space increases dramatically for desktop and multime-
dia applications. With the latest developments in magnetic recording technologies,
HDD m anufacturers are producing HDDs of such a high storage density, which SSDs
manufacturers are incapable of achieving presently and in the near future. This par-
ticular advantage that conventional HDDs has over S SDs ensures that conventional
HDDs will possess a substantial market share for data storage systems in the fore-
seeable future. However, to gain a wider application in future data storage systems,
future HDDs will have to offer greater storage capacities as well as faster data access
time.

The objective of this dissertation is to provide new, effective and practical con-
trol algorithms to improve the performance of HDD and servo track writing (STW)
servo system for the development of fast and high storage density HDDs. In order
to increase the track density of our STW platform, an enhanced sensitivity func-
tion shaping technique, which is based on the generalized Kalman-Yakubovic-Popov
(KYP) lemma, has b een developed. As for improving the present HDD servo control
viii
SUMMARY ix
technology, a unified control scheme (UCS) for high performance track-seeking and
track-following in hard disk drives (HDDs). However, despite its effectiveness and
simplicity, the proposed UCS may not be appealing to the HDD industry, which
is generally conservative by nature, as it requires a HDD servo control design and
implementation overhaul. Hence, two more new control algorithms which can be
designed and implemented using an “add-o” fashion that improve different aspects
of the servo performance of existing HDD servo systems, while preserving the servo
performances as well as closed-loop stability margins achieved by the existing servo
design have been proposed too.
List of Tables
2.1 Parameters of the STW platform . . . . . . . . . . . . . . . . . . . . 25
4.1 Comparison of operation numbers of the two control schemes . . . . . 68
5.1 Design specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
5.2 Comparison of stability margins at steady state . . . . . . . . . . . . 105
x
List of Figures
1.1 Components of a 3.5” typical HDD. . . . . . . . . . . . . . . . . . . . 4
1.2 Embedded servo scheme. . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 An illustration of track seeking, track settling and track following of
a HDD servo system. . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 Mechanical structure of a typical HDD and the block diagram of its
VCM-actuated servo loop system. . . . . . . . . . . . . . . . . . . . . 9

1.5 Media-level servo track writing. . . . . . . . . . . . . . . . . . . . . . 15
2.1 Implementation setup of HDD servo system. . . . . . . . . . . . . . . 20
2.2 Frequency responses of the VCM actuator (LDV range 2 µm/V). . . . 22
2.3 Photo of a dissected 3.5” HDD. . . . . . . . . . . . . . . . . . . . . . 22
2.4 Overview of the STW platform with dual PZT microactuators. . . . . 24
2.5 Zoom-in head-disk-spindle assembly of the STW platform. . . . . . . 24
2.6 Servo mechanism of the MSTW setup (MicroE loop uses optical sens-
ing signal as the feedback signal to a PID controller. We focus on t he
control design for PZT microactuator with readback PES as the feed-
back signal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
xi
LIST OF FIGURES xii
2.7 Frequency responses of the PZT microactuator. . . . . . . . . . . . . 27
2.8 Measured RRO power spectrum of the STW platform prior to servo
control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.9 Measured NRRO power spectrum of the STW platform prior to servo
control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3.1 A simplified block diagram of HDD servo loop with disturbance and
noise injected. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.2 Case 1 : |S| and |T |. . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3.3 Case 1 : |T
Q
|. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4 Case 2 : |S| and |T |. . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
3.5 Case 2 : |T
Q
|. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3.6 Case 3 : |S| and |T |. . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.7 Case 3 : |T
Q

|. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.8 Multiplicative uncertainty ∆ of the microactuator. . . . . . . . . . . . 46
3.9 |S| and |T |. Proposed scheme further reduces |S| at frequency < 200
Hz and maintains an equivalent low gain at around 650 Hz and 3.8
kHz while |T | at frequency > 1.5 kHz (esp. after 15 kHz) improves
instead of getting worse. . . . . . . . . . . . . . . . . . . . . . . . . . 47
3.10 Measured NRRO power spectrum with servo control. Sudden appear-
ance of peaks at higher frequency is due to the humps in the |S| from
1.2 to 7 kHz with the respectively servo controllers. . . . . . . . . . . 48
LIST OF FIGURES xiii
3.11 |T W
U
|. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
4.1 A block diagram of a typical HDD servo system without any controller. 53
4.2 Block diagram of the proposed NLTV control scheme. . . . . . . . . . 56
4.3 Internal stability analysis diagram of ˜x
p
. . . . . . . . . . . . . . . . . 60
4.4 Nyquist plot of G
1
(s). . . . . . . . . . . . . . . . . . . . . . . . . . . 67
4.5 Simulation results: Normalized responses un der a single conventional
CNF control law whose design parameters are optimized for the nom-
inal 2 µm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4.6 Simulation results: Normalized responses under a single UCS control
law whose design parameters are optimized for the nominal 2 µm. . . 71
4.7 Simulation results: Nominal seeking performance robustness of the
proposed scheme against plant uncertainty. . . . . . . . . . . . . . . . 71
4.8 CNF: 2 µm seek, Ch1: LDV-measurement (2 µm/V), r, Ch3: VCM-
driver input, u, y(t), Ch4: Seek Command Input (2 µm/V). . . . . . 72

4.9 UCS: 2 µm seek, Ch1: LDV-measurement (2 µm/V), r, Ch3: VCM-
driver input, u, y(t), Ch4: Seek Command Input (2 µm/V). . . . . . 73
4.10 UCS: 0.2 µm seek, Ch1: LDV-measurement (2 µm/V), r, Ch3: VCM-
driver input, u, y(t), Ch4: Seek Command Input (2 µm/V). . . . . . 74
4.11 UCS: 20 µm seek, Ch1: LDV-measurement (2 µm/V), r, Ch3: VCM-
driver input, u, y(t), Ch4: Seek Command Input (2 µm/V). . . . . . 75
4.12 Experimental results: Conventional CNF performance robustness against
±10% variations in the nominal seek length. . . . . . . . . . . . . . . 76
LIST OF FIGURES xiv
4.13 Experimental results: Conventional CNF performance robustness against
±20% variations in the nominal seek length. . . . . . . . . . . . . . . 77
4.14 UCS performance robustness against initial velocity of 3 mm/s. Ch1:
LDV-measurement (2 µm/V), Ch4: Trigger. Indicates the start of a
nominal 2 µm seek. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.15 UCS performance robustness against initial velocity of 6 mm/s. Ch1:
LDV-measurement (2 µm/V), Ch4: Trigger. Indicates the start of a
nominal 2 µm seek. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.16 UCS performance robustness against initial velocity of 12 mm/s. Ch1:
LDV-measurement (2 µm/V), Ch4: Trigger. Indicates the start of a
nominal 2 µm seek. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
4.17 UCS performance robustness against initial acceleration of 4.5 ×10
3
mm/s
2
. Ch1: LDV-measurement (2 µm/V), Ch4: Trigger. Indicates
the start of a nominal 2 µm seek. . . . . . . . . . . . . . . . . . . . . 80
4.18 UCS: Measured nominal 2 µm step response under the effect of dis-
turbance and noise. Ch1: LDV-measurement (2 µm/V), Ch4: Seek
Command Input (2 µm/V). . . . . . . . . . . . . . . . . . . . . . . . 80
4.19 CNF: Measured nominal 2 µm step response under the effect of dis-

turbance and noise. Ch1: LDV-measurement (2 µm/V), Ch4: Seek
Command Input (2 µm/V). . . . . . . . . . . . . . . . . . . . . . . . 81
4.20 Measured frequency response of open-loop function (LDV range 2
µm/V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.21 Measured frequency gain response of sensitivity function . . . . . . . 82
4.22 Power spectrum of measured PES NRRO with servo control. . . . . . 82
LIST OF FIGURES xv
5.1 Open-loop block diagram of a typical HDD servo loop precompen-
sated with a main servo compensator. . . . . . . . . . . . . . . . . . . 86
5.2 Block diagram of the proposed LTV group filter, F
t
, add ed onto the
main feedback loop via parallel realization. . . . . . . . . . . . . . . . 87
5.3 Maps of eigenvalues of A
0
cl
+A
∞
∆
. ‘×’: Nominal plant; ‘◦’: Resonances
variations: -10% frequency and damping shift; ‘+’: Resonances vari-
ations: +10% frequency and damping shift. . . . . . . . . . . . . . . 100
5.4 Simulated disturbance responses at 700 Hz. Approx. transient set-
tling time, F
t
: 3 ms; F
LT I1
(z): 2.5 ms; F
LT I2
(z): 6.5 ms. . . . . . . . 101

5.5 Simulated disturbance responses at 2 kHz. Approx. transient settling
time, F
t
: 1.8 ms; F
LT I1
(z): 1.5 ms; F
LT I2
(z): 6.5 ms. . . . . . . . . . 102
5.6 Simulated combined disturbance responses at, respectively, 2 kHz and
700 Hz. Approx. transient settling time, F
t
: 3 ms; F
LT I1
(z): 2.5 ms;
F
LT I2
(z): 6.5 ms. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
5.7 Simulated sensitivity function magnitudes with respective RRO com-
pensation scheme at the steady state. Note: The notch widths
with F
t
are equally wide as those with F
LT I1
(z) and their
sensitivity function humps are equally sized at t = 0. . . . . . 103
5.8 Simulated open-loop frequency response with respective RRO com-
pensation scheme at the steady state. . . . . . . . . . . . . . . . . . . 104
5.9 Proposed LTV method, F
t
: Measured disturbance responses at 700

Hz. Ch1: LDV-measurement (2 µm/V), y, Ch2: VCM-driver input,
u. Approx. transient settling time: 3 ms . . . . . . . . . . . . . . . . 108
LIST OF FIGURES xvi
5.10 Proposed LTV method, F
t
: Measured disturbance responses at 2 kHz.
Ch1: LDV-measurement (2 µm/V), y, Ch2: VCM-driver input, u.
Approx. transient settling time: 1.5 ms . . . . . . . . . . . . . . . . . 108
5.11 Proposed LTV method, F
t
: Measured disturbance responses at 700
and 2 kHz. Ch4: LDV-measurement (2 µm/V), y, Ch3: VCM-driver
input, u. Approx. transient settling time: 3 ms . . . . . . . . . . . . 109
5.12 Conventional LTI method, F
LT I2
(z): Measured disturbance responses
at 700 Hz. Ch1: LDV-measurement (2 µm/V), y, Ch2: VCM-driver
input, u. Approx. transient settling time: 7 ms . . . . . . . . . . . . 110
5.13 Conventional LTI method, F
LT I2
(z): Measured disturbance responses
at 2 kHz. Ch1: LDV-measurement (2 µm/V), y, Ch2: VCM-driver
input, u. Approx. transient settling time: 6 ms . . . . . . . . . . . . 110
5.14 Measured sensitivity functions with respective compensation scheme
at the steady state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
5.15 Measured open-loop frequency responses with respective compensa-
tion scheme at the steady state (LDV range 2 µm/V). . . . . . . . . . 111
5.16 Simulated sensitivity functions with respective compensation scheme
at the steady state. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.17 Simulated disturbance responses of 0.4 µm at 700 Hz. Approx. tran-

sient settling time, F
t
: 3 ms; F
LT I1
(z): 2.5 ms; F
LT I2
(z): 6.5 ms. . . . 112
5.18 Proposed LTV method, F
t
: Measured disturbance responses of 0.4
µm at 700 Hz. Ch4: LDV-measurement (2 µm/V), y, Ch3: VCM-
driver input, u. Approx. transient settling time: 3 ms. . . . . . . . . 113
LIST OF FIGURES xvii
5.19 Simulated disturbance responses of 0.1 µm at 3 kHz. Approx. tran-
sient settling time, F
t
: 0.9 ms; F
LT I1
(z): 0.7 ms; F
LT I2
(z): 10 ms.
Note: F
t
= F
3
t
+ F
4
t
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

5.20 Simulated sensitivity functions with respective RRO compensation
scheme at the steady state. Note: F
t
= F
3
t
+ F
4
t
. . . . . . . . . . . . . 114
5.21 Proposed LTV method, F
t
: Measured disturbance responses of 0.1 µm
at 3 kHz. Ch4: LDV-measurement (2 µm/V), y, Ch3: VCM-driver
input, u. Approx. transient settling time: 1 ms. Note: F
t
= F
3
t
+ F
4
t
. 114
5.22 Simulated disturbance responses of 0.4 µm at 1 kHz. Approx. tran-
sient settling time, F
t
: 2.3 ms; F
LT I1
(z): 2 ms; F
LT I2

(z): 4 ms. Note:
F
t
= F
3
t
+ F
4
t
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
6.1 Block diagram of DOB with the nominal feedback controller. . . . . . 119
6.2 Approximately simplified block diagram of Fig. 1. . . . . . . . . . . . 120
6.3 Frequency responses of respective Q-filters. . . . . . . . . . . . . . . . 125
6.4 Illustration of attenuation of n
c
with DOB using Nyquist plots of
(Q
conv
− 1) and (Q
prop
− 1). . . . . . . . . . . . . . . . . . . . . . . . 126
6.5 Nyquist plot of T
ol
in (6.20) for the nominal and perturbed plant with
Q
prop.
Gain margin: 8.5 dB (Nominal), 6.5 dB (−10 % perturbation),
7 dB (+10 % perturbation); Phase margin: 50 deg (Nominal), 45 deg
(−10 % perturbation), 47 deg (+10 % perturbation) . . . . . . . . . . 127
6.6 Simulated sensitivity gain for the nominal and perturbed plant with

Q
prop.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
6.7 Open-loop frequency responses with the nominal feedback controller
C without DOB. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
LIST OF FIGURES xviii
6.8 Frequency responses of T
ol
with DOB designed using the conventional
method, Q
conv.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.9 Frequency responses of T
ol
with DOB designed using the proposed
method, Q
prop.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.10 Measured gain of the sensitivity transfer function. . . . . . . . . . . . 130
6.11 Power spectrum of measured PES NRRO with resp ective metho ds. . 131
7.1 Photo of a dual-stage actuator with a PZT-actuated suspension. . . . 136
Nomenclature
Throughout this thesis, the following abbreviations and notations, which are fairly
standard, are adopted.
ARE Algebraic Riccati equation
BPI Bits Per Inch
CNF Composite Nonlinear Feedback
DOB Disturbance Observer
FFT Fast Fourier Transform
FIR Finite Impulse Response

FPC Flexible Printed Circuit
GES Global Exponential Stability
G-KYP Generalized Kalman-Yakubovich-Popov
GUS Global Uniform Stability
GUAS Global Uniform Asymptotic Stability
HDD Hard Disk Drive
HAMR Heat Assisted Magnetic Recording
IIR Infinite Impulse Response
IVC Initial Value Compensation
KYP Kalman-Yakubovich-Popov
LMI Linear Matrix Inequality
LDV Laser Doppler Vibrometer
xix
NOMENCLATURE xx
LQG Linear Quadratic Gaussian
LTI Linear Time-invariant
LTR Loop Transfer Recovery
LTV Linear Time-varying
MISO Multi-Input, Single-Output
MSC Mode Switching Control
MSTW Media-level Servo Track Writing
NLTV Nonlinear Time-varying
NRRO Nonrepeatable Runout
NLSO Nonlinear Least Squares Optimization
NLTV Nonlinear Time-varying
PES Position Error Signal
PID Proportional-Integral-Derivative
PMR Perpendicular Magnetic Recording
PTOS Proximate Time Optimal Servomechanism
PZT Lead-Zirconate-Titanate (Piezoelectric)

RRO Repeatable Runout
R/W Read/Write
SISO Single-Input, Single-Output
SNR S ignal-To-Noise Ratio
SSD Solid State Disk/Drive
STW Servo Track Writing
TMR Track Misregistration
TPI Track Per Inch
UCS Unified Control Scheme
VCM Voice Coil Motor
ZOH Zero Order Hold
Chapter 1
Introduction
1.1 Background and Motivation
In November 2007, Samsung, the world’s top company in global electronic industry
launched a new generation of solid state disks (SSD) that used native SATA-II
interface [1]. Those new SSDs were coming in 1.8” and 2.5” form factor. They
featured 960 Mbps sequential read and 800 Mb ps sequential write speed. Their
power consumptions were rated at a mere 0.7W. Those were the very first SSDs,
in the entire storage industry, that not just rivaled, but surpassed conventional
hard disk drive (HDD) in read/write (R/W) speed. And in the case of 1.8” form
factor, the SSDs matched storage capacity of its HDD counterpart as well. It had
been rep orted that such high performance alternative storage devices threatened
the future presence of conventional HDD as the leading product group in the global
data storage systems market.
Major HDD manufacturers, namely Seagate Technology, Western Digital Tech-
nology as well as Hitachi Global Storage Technologies, are finding it hard to face
off the SSD in term of performances in power consumption, random memory access
time, robustness against shock and production of heat and acoustic noise. As a
1

CHAPTER 1 2
result, the deployment of SSDs in mobile applications, in which power consump-
tion and resistance against shock disturbances are serious matters of concern, has
been very successful over the last few years. Global computer makers such as Apple
Computers, Samsung and DELL Computers are already selling their latest high-end
notebook personal computers (PC) that use SSDs. According to one of the latest
reports produced by IDC, the premier global market intelligence firm, it is pre-
dicted that SSDs will go mainstream as advances in solid state memory technology
and dipping price points drive worldwide SSD adoption for mobile applications [2].
Going forward, the PC market presents the biggest demand for storage devices and
the PC market is transitioning from one being dominated by desktop PC shipments
to one being dominated by notebook PC shipments. This transition increases the
importance of low power consumption and shock resistance requirements, which are
dynamics that align very well with the benefits of SSDs.
While HDD manufacturers are still unable to produce HDDs that match SSDs
in general performance, they are still capable of ever-increasing the storage density
of HDD and have already packed as much as 1 Terabytes (TB) of data into a 3.5”
HDD [3] by using the latest perpendicular magnetic recording (PMR) technology [4]
that significantly increases bit density, which is measured by bits per inch (BPI). As
the PMR technology matures, higher BPI, consequently higher storage density, will
be achieved. The development of newer recording technologies such as heat assisted
magnetic recording (HAMR) [5] as well as patterned media magnetic recording [6]
are in the progress. Once completed, they will be ready to replace PMR and pushing
for even higher BPI as well as and hence, higher storage density. By far, the rate of
increase of storage density of HDD is still ahead its SSD counterpart [7].
As the multimedia world progressively enters into the high-definition era [8], the
increasing demand for even more storage space for desktop and home multimedia
applications has already become the norm [9]. This ensures that HDD will possess a
substantial market share for desktop and home multimedia applications. Moreover,
for these applications, other than having a faster random access memory R/W speed