IMPROVED PERFORMANCE OF HARD DISK DRIVE SERVOMECHANISM USING DIGITAL MULTIRATE CONTROL THAMBIRAJAH SUTHASUN ( B.Sc.Eng. (Hons.) ) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2003 Acknowledgments I express my sincere gratitude to my supervisor, Dr. Abdullah Al Mamun for his valuable guidance, support and encouragement throughout my study at the National university of Singapore. His patient guidance and encouragement is my greatest strength to fulfill this project. His invaluable advice and support helped me overcome the difficulties in the research. I am indeed indebted to him. I am deeply grateful to my co-supervisor Dr. S. Sri Jayantha (IBM, USA) for his guidance and intellectual honesty, both of which aided in the development of some ideas in this research. I owe a great depth of gratitude to Professor Iven Mareels (University of Melbourne) for his invaluable help on the subject related to the dual stage actuators which benefited me to develop many ideas in this thesis. I wish to express my thanks to A/Prof. Ben M. Chen (National University of Singapore) for allowing me to use laboratory facilities. During my candidature, as a graduate student I have had privilege and pleasure of meeting several renowned researchers. I am particularly grateful to Prof. M. Tomizuka (University of California, Berkeley) and Prof. R. Horowitz (University of California, Berkeley) and researchers whose research on hard disk drives have been a source of great inspiration as well as a foundation for my own work. i I also appreciate the self- giving help from Dr. Guoxiao Guo of Data Storage Institute, Singapore. He has provided assistance in the hardware. As a graduate student, I have enjoyed many helpful discussions with my finest Colleagues in Mechatronics and Automation Laboratory, National University of Singapore. Finally I want to thank some people who, more than others, have made it possible for me to finish my project. Staffs at NUS-Electrical workshop and Mr. A. Jalil have been invaluable when preparing the hardware for implementation. Without all your help, I would not complete this thesis so smoothly. Thank you. ii Contents Acknowledgements Summary i viii List of Figures xi List of Tables xvi 1. 2. Introduction 1.1 Background ………………… ………………………………………… .1 1.2 Trend in HDD Industry…… …………………………………………… 1.3 Overview of Multirate Systems……… ………………………………….9 1.4 Contribution and Organization of the Thesis…………………………….12 HDD Servo Mechanism 20 2.1 Introduction…………………………………………………………… 20 2.2 Servo Challenges in HDD………………………………………………29 2.3 Limitations with Single Actuator……………………………………….38 2.4 Benefit Dual-Stage Servo for Hard Disk Drives……………………… 40 2.5 Selection of Micro-actuators……………………………………………42 2.6 Conclusion………………………………………………………………45 iii 3. Digital Multirate Concept in Control Engineering 4. 46 3.1 Introduction…………………………………………………………… 46 3.2 Numerical prediction methods………………………………………… .49 3.3 Higher order hold prediction methods………………………………… .52 3.4 State space inter samples prediction methods……………………………58 3.5 Motivation of using multi- rate control in HDD………………………….62 3.6 Conclusion……………………………………………………………….67 System Modeling 68 4.1 Introduction…………………………………………………………… .68 4.2 Identifying the Plant Model from Frequency Response Data……………69 4.3 Experimental Setup………………………………………………………77 4.4 Modeling HDD Actuators……………………………………………… 78 4.4.1 Single-stage Actuated HDD……….…………………………… 78 4.4.2 Dual –Stage Actuated HDD…… ….…….…………………… .80 4.5 Conclusion………………………………………………………………… .90 5. Multirate Controller of Single Stage Actuator 5.1 91 Introduction………………………………………………………………91 iv 5.2 Proposed Design…………………………………………………………93 5.2.1 6. Selection of Multi rate Ratio (r)……….…………………………97 5.3 Selection of Digital Notch Filter……………………………………… 100 5.3 Modeling of Single Stage Actuator…………………………………… 105 5.4 Simulation Results…………………………………………………… .109 5.4.1 Different Schemes….………………………………………… .109 5.4.2 Comparison with Pub lished Works…….………………………115 5.5 Experimental Results………………………………………………… .116 5.8 Conclusion…………………………………………………………… .120 Feed Forward Control Design for Multi Mode Flexible Systems 121 6.1 Introduction…………………………………………………………….121 6.2 Overview of Existing Controllers………………………………………124 6.3 Standard Filters For Residual vibration problems…………………… .131 6.4 Robust Compensator Design……………………………………………131 6.5 Proposed Algorithm – Envelope Controller……………………… .….136 6.6 Results………………………………………………………………… 142 6.7 Conclusion…………………………………………………………… .146 v 7. Single-rate Control of Dual-Stage Actuator 147 7.1 Introduction……………………………………………………………147 7.2 Over view of Different Design Approaches……………………………149 7.3 Proposed Controller Design…………………………………………….155 7.4 Overview of the Micro-actuator Control.………………………………165 7.5 Simulation and Experimental Results…….…………………………….169 7.5. I Sensitivity Loop Transfer Function… ……….……………… 170 7.6 8. 7.5.2 Step Response… ………………………………………………171 7.5.3 Runout Disturbance Test……… ………………………………174 7.5.4 Experimental Setup……….…………………………………….177 7.5.5 Experimental Results…….…………………………………… 178 Conclusion……………….…………………………….……………….180 Multirate Controller for Dual Actuated Hard Disk Drives Servo Mechanism 182 8.1 Introduction………………… …………………………………………182 8.2 Controller Design…………………………………… ……………… .184 8.2.1 Control Architecture……… ………………………………… 184 8.2.2 Controller for Primary Actuator…… ………….………………187 8.2.3 Controller for Secondary Actuator…………….….………… .188 vi 8.3 8.4 Results…………………………………………………………….……192 8.3.1 Simulation Results…………… ……………………………….192 8.3.2 Experimental Results……………………………………… ….195 Conclusion……………………………….…………………………… 198 9. Conclusions and Suggestions 200 9.1 Conclusion……………………………………………………………200 9.2 Suggestions for Future Research…………………………………… 204 Bibliography 207 Appendix A. Author's Publications 220 vii Summary The two main functions of the head positioning servomechanism in hard disk drives (HDD) are track seeking and track following. Track seeking moves the read write (R/W) head from the current track to a specified destination track in minimum time using a bounded control effort. Track following maintains the head as close as possible to the center of the destination track while information is being read from or written to the disk. It is suggested that on a disk surface, tracks should be written as closely spaced as possible to maximize the usage of the disk surface. This means an increase in the track density, which subsequently means a more stringent requirement on the allowable variations of the position of the heads from the true track center. The prevalent trend in the design of hard disk drive servomechanism is towards smaller hard disks with increasingly larger capacities. This implies that the track width has to be smaller leading to lower error tolerance in the positioning of the head. The controller for track following has to achieve tighter regulation in the control of the servomechanism. In hard disk drives, the head position is detected from the servo signal embedded with data. The choice of sampling frequency for the servomechanism depends on the rotational speed of the disks and the number of servo sectors per track. Since servo sectors occupy part of the storage area, it is desired to keep the number of servo sectors per track low to maximize storage efficiency. This restriction on the sampling frequency often makes it difficult to achieve the performance demanded from the servomechanism. viii However, the selection of the frequency at which the control signal can be updated is not restricted by the problem of storage efficiency. The frequency of updating the control signal can be set faster than the sampling frequency. In other words the output sampling frequency is different from control updating frequency, and the servomechanism represents a multirate system. The main objective of this thesis is to study the advantages achieved through the use of a multirate control in HDD servomechanism, and to design a multirate controller for the system. In particular, this thesis introduces a number of newly proposed techniques for designing multirate controller: 1. Single input single output (SISO) approach for multirate design, 2. Robust feedforward controller for systems with multiple flexible modes, 3. Partial estimation approach for Dua l input single output systems, and 4. Multirate control for dual stage actuated systems. The SISO approach for multirate systems presented in this thesis is a combination of a multirate observer and a all pass filter. This approach reduces the complexity in designing multirate control for any system where the sampling frequency is constrained. It has become increasingly more difficult to position a magnetic head right on top of narrow data tracks with high accuracy by using a conventional voice-coil motor (VCM). A dual-stage actuator (DSA) system in HDDs is a prospective solution for boosting servo bandwidth to satisfy the future requirement of ultra-high track density (Track Per Inch, TPI). Main issue in designing the controller for the light-weight secondary actuator in an HDD servomechanism lies in the presence of lightly damped resonant modes. In this thesis, we propose a new design concept for designing compensators by preconditioning the signal for expected variation of parameters. Signal processing optimization tools are used to find a better preconditioning feedfoward compensator for multimode flexible systems. ix Bibliography [1] Y. Miura," HDD Technology: Past, Present and Future", Digest of the Asia Pacific Magnetic Recording Conference August 2002. 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Journal Publications 1. T. Suthasun, I. Mareels, A. Al Mamun, "System Identification and Controller Design for Dual Actuated Servomechanism", to be published in Control Engineering Practice. 2. A. Al Mamun, T.H. Lee, Pan Ling, T. Suthasun, "Robust Controller for Dual Stage Actuator in Hard Disk Drive", submitted for publication in ASME Journal of Dynamic Systems, Measurement and Control. 220 3. T. Suthasun, A. Al Mamun, T.H. Lee, Pan Ling, S.M. Sri-Jayantha, "Robust Multirate Controller for Dual Actuated Servomechanism", submitted for publication in ASME Journal of Dynamic Systems, Measurement and Control. Conference Publications 1. A. Al-Mamun, T.H. Lee, Pan Ling, T. Suthasun, "Internal model control for the dualstage actuator in hard disk drive", Proceedings of the 28th Annual Conference of the IEEE Industrial Electronics Society, IECON’02, Sevilla, Spain, Nov, 2002. 2. A. Al Mamun, T.H. Lee, P. Ling, T. Suthasun, "Design of controller for dual-stage actuator in hard disk drive using internal model approach", World Congress on Intelligent Control and Automation, Shanghai, China, June 2002 3. T. Suthasun, I. Mareels, A. Al Mamun, "System identification and control design for dual actuated disk drive", International Conference on Control Technology and Application 2001, Pretoria, South Africa, December 12-15, 2001 4. A. Al Mamun, T. H. Lee, T. Suthasun, "Multi-rate tunable filter and Controller design and implementation for actuators in hard disk drives." 5th International Conference on Mechatronics Technology, Singapore, June 6-8, 2001. 221 5. A. Al Mamun, T. Suthasun, T.H. Lee, "Simplified multi- rate control for reduced residual vibration - frequency domain analysis", International Conference on Measurement and Control, MECO 2001, Pittsburgh, USA, May 16-18, 2001. 6. T. Suthasun, A. Al Mamun, T.H. Lee, "Digital multi-rate control for improved performance in presence of mechanical resonance", Students’ Conference on Research and Development, SCOReD, 2001, Malaysia, February 20-22, 2001. 222 [...]... …………………………………………………… 24 2.3 Block diagram of hard disk drive …………………………………… 25 2.4 The hard disk drive R/W scheme…………………………………….… 28 2.5 Dedicated and sectored servo systems………………………………… 29 2.6 The performance of the hard disk drives……………………………… 36 2.7 Photograph of the entire dual-stage actuator setup………………… ….42 2.8 Photograph of the piezoelectric micro-actuator…… …………… …….45 3.1 Block diagram of multi rate concept……………... Design of controller using multirate sampling is a practical solution to achieve improved performance in systems where sampling rate is restricted due to some practical reasons 1.4 Contribution and Organization of the Thesis Track density in hard disk increases as demand increases for capacity Improved performance of the read/write head positioning servomechanism is an essential factor in the effort of. .. the control architecture 5 Use of multirate technique results in a design of less complexity that achieves better performance without increasing the cost of manufacturing In chapter 2, we review the definitions used in hard disk drives and evo lutionary path of magnetic data storage It includes the challenges in disk drives, constraints and improvement for future hard disk drives There are serious limitations... plant parameters This function of the servo controller is known as tracking controller or track following controller in HDD As was mentioned, the PES is the only source of feedback information in the HDD servo controller The sampling frequency for digital control of HDD servomechanism is determined by the number of sectors on each track and the speed of the Motor The number of sectors is kept at the minimum... stability and transient performance The controller update must be adequate enough to design a controller Under a constraint on the sampling rate of the measurement, the input- updating rate may be set higher for improvement of performance; this scheme is referred to as multirate control system General concept on multirate control is summarized below 1.3 Overview of Multirate Systems Multirate and periodic... practicality of the solution One of the primary objectives of this research is to push the performance of the servomechanism to a higher level with no additional cost Major contributions of this thesis are summarized below 1 Design simplicity and robust design for resonance are addressed to enhance the performance of the servomechanism in the presence of restrictions on sampling rate of the measurement... (1) availability of model, (2) sampling frequency, (3) dynamic performance, and (4) complexity of the controller architecture This chapter provides adequate information for selecting new algorithms to overcome those constraints with simple controllers Finally the motivations 14 of using multirate controller for single actuated and dual actuated HDD drives are explained Identification of the primary voice... proven floppy disk drives to the newest removable storage systems In hard disk drives industry, the density of recording doubled every two years in the last decade Many technologies have been at work in the disk drives In this chapter, we examine some prevalent technologies in the hard disk industry The practice of storing large amounts of data on magnetic media traced back to the early 1950s It was IBM’s... families of computers: mobile computers 4 Introduction of redundant array of independent drives (RAID), which consisted of more than one drive 5 Greatly increased size of software products (operating systems (OS), graphic files, multimedia files, video, etc.) For example, the older DOS 4 operating system required only 1.44 MByte floppy disk while the modern Windows 2000 OS requires a compact disc (CD) As often... systems 9 Fig.1.3 Generalized digital control system General architecture of a digital control system is shown in Fig.1.3, where G(s) is a continuous time plant, C[z] is a discrete-time controller implemented in digital computer, d(t) and n(t) are disturbance and measurement noise, respectively The discrete-time controller has to deal with continuous-time signal in the digital control systems It need to . diagram of hard disk drive …………………………………… 25 2.4 The hard disk drive R/W scheme…………………………………….… 28 2.5 Dedicated and sectored servo systems………………………………… 29 2.6 The performance of the hard disk drives………………………………. IMPROVED PERFORMANCE OF HARD DISK DRIVE SERVOMECHANISM USING DIGITAL MULTIRATE CONTROL THAMBIRAJAH SUTHASUN ( B.Sc.Eng. (Hons.). allowable variations of the position of the heads from the true track center. The prevalent trend in the design of hard disk drive servomechanism is towards smaller hard disks with increasingly