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 choose 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 Introduction 1.1 Background and Motivation . . . . . . . . . . . . . . . . . . . . . . . 1.2 Hard Disk Drives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 HDD Servo Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Servo Information . . . . . . . . . . . . . . . . . . . . . . . . . iii iv CONTENTS 1.3.2 Servo Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Servo Track Writing Technology . . . . . . . . . . . . . . . . . . . . . 14 1.5 Contributions of the Thesis . . . . . . . . . . . . . . . . . . . . . . . 14 1.6 Organization of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . 17 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 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 A Unified Control Scheme for Combined Seeking and track following 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 4.3 4.4 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 Simulation and Implementation Results . . . . . . . . . . . . . . . . . 69 4.3.1 Simulation Results . . . . . . . . . . . . . . . . . . . . . . . . 69 4.3.2 Implementation Results . . . . . . . . . . . . . . . . . . . . . 72 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 vi CONTENTS Mid-frequency Runout Compensation 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 5.4 Application to a HDD servo system . . . . . . . . . . . . . . . . . . . 97 5.5 . . . . . . . . . . . . . . . . . . . . . 93 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 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 An H∞ Disturbance Observer Design for High Precision Track Following 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 vii CONTENTS 6.3 Designs of nominal feedback controller C(z −1 ) and DOB . . . . . . . 122 6.3.1 6.4 Simulation and implementation results . . . . . . . . . . . . . 126 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 Conclusions 132 7.1 Findings and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 132 7.2 Suggestions for Future Research . . . . . . . . . . . . . . . . . . . . . 134 Bibliography 137 Author’s Publications 154 BIBLIOGRAPHY 140 [25] R. 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Wood, “Magnetic recording head positioning at very high track densities using a microactuator based, two stage servo system,” IEEE Transactions on Industrial Electronics, vol. 42, no. 3, pp. 222–233, 1995. Author’s Publications The research work related to this thesis has been reported in the following publications: Journal Publications 1. C. K. Thum, C. Du, J. Zhang, K. P. Tan, B. M. Chen, and E. H. Ong, “Servo control design for a high TPI servo track writer with microactuators,” IEEE Transactions on Magnetics. vol. 44, No. 9, pp. 2227-2234, September 2008. 2. C. K. Thum, C. Du, B. M. Chen, E. H. Ong, and K. P. Tan, “Mid-frequency runout compensation in hard disk drives via a time-varying group filtering scheme,” IEEE Transactions on Magnetics, vol. 44, no. 12, pp. 4769-4779, December 2008. 3. C. K. Thum, C. Du, B. M. Chen, E. H. Ong, and K. P. Tan, “A unified control scheme for combined seeking and track-following of a hard disk drive servo system,” accepted for future publication in IEEE Transactions on Control Systems Technology. 4. C. K. Thum, C. Du, F. L. Lewis, B. M. Chen, E. H. Ong, “H∞ disturbance observer design for high precision track-following,” accepted for future publication in IET Control Theory and Applications. 154 AUTHOR’S PUBLICATIONS 155 5. W. Lan, C. K. Thum, B. M. Chen, “A hard disk drive servo system design using composite nonlinear feedback control with optimal nonlinear gain tuning methods,” revised to IEEE Transactions on Industrial Electronics. 6. C. Du, H. Li, C. K. Thum, F. L. Lewis, and Y. Wang, “A general disturbance observer for disturbance compensation,” submitted to IET Control Theory and Applications. Conference Publications 1. C. K. Thum, C. Du, B. M. Chen, E. H. Ong, and K. P. Tan, “A linear timevarying control scheme and its application to a hard disk drive servo system,” in Proceedings of the 33rd Annual Conference of the Industrial Electronics Society, IECON, pp. 892-897, 2007. 2. C. K. Thum, C. Du, B. M. Chen, E. H. Ong and K. P. Tan, “Time-varying compensation for mid-frequency repeatable runout in hard disk drives via a linear feedback scheme,” appeared at the 17th IFAC World Congress, Seoul, Korea, July 2008. (Invited) [...]... market share for data storage systems in the foreseeable 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 control algorithms to improve the performance of HDD and servo track writing (STW) servo system for the... higher storage density can be achieved by pushing for higher track density, measured by track per inch (TPI) As for a significant reduction in data access time, track seeking times achieved using bounded control effort have to be shorten In summary, to assist HDDs to maintain a significant market share for desktop and home multimedia storage applications in future, new control strategies will need to be formulated... 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 reported that such high performance alternative storage devices threatened the future presence of conventional HDD as the leading product group in the global data storage. .. extra degrees of freedom in controller design so as to achieve better servo performance Several new control algorithms [54, 49, 55–58] that make use of the additional servo information providing by the additional sensors have been developed recently 5 Multi-rate Servo Control Similar to multi-sensing servo control, multi-rate servo control provides extra degrees of freedom in controller design Ignoring... the development of fast and high storage density HDDs In order to increase the track density of our STW platform, an enhanced sensitivity function shaping technique, which is based on the generalized Kalman-Yakubovic-Popov (KYP) lemma, has been 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... effective and practical control algorithms for fast and high storage density future HDDs In the rest of this chapter, to assist better understanding of the HDD servo technology, brief information about the history and present of HDD, servo track writing (STW) platform as well as their respective servo systems, will be studied 1.2 Hard Disk Drives A hard disk drive (HDD) is a non-volatile storage device which... 40], provide more intuitions and a greater ability to tune designs to achieve desired frequency domain properties for the closed-loop system than automated tools based on state-space formulation 2 Advanced Optimal Control Advanced model-based controller designing methods based on state-space formulation such as LQG/LTR [41, 42, 11], H2 [11, 32], H∞ [11, 14, 43, 44], mixed H2 /H∞ [45–48] as well as disturbance... 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... prevailing devices used for large and permanent data storage During the past half a century, HDD technology has made dramatic progress in terms of physical size, storage capacity, R/W speed and cost While the first HDDs of an 8” form factor had a capacity of 10 Megabytes (MB), costing over $100 per MB, nowadays, a 3.5” HDD of 500 Gigabytes (GB) costs less than half a dollar per GB In the mean time, the performance,... impossible to realize the maximum disturbance and noise attenuation performance with a fixed controller at all times Thus, various adaptive and self-tuning CHAPTER 1 13 control algorithms [49–53] have been developed to achieve robust performance as well as stability 4 Multi-sensing Servo Control While conventional HDD servo system achieves servo information with only the PES feedback signal, augmenting the servo . ADVANCED CONTROL TECHNIQUES FOR FUTURE STORAGE DEVICES THUM CHIN KWAN NATIONAL UNIVERSITY OF SINGAPORE 2008 ADVANCED CONTROL TECHNIQUES FOR FUTURE STORAGE DEVICES THUM CHIN. 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. 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