296 IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY, VOL. 20, NO. 2, MARCH 2012 Control Methods in Data-Storage Systems Giovanni Cherubini, Fellow, IEEE, Chung Choo Chung, Member, IEEE, William C. Messner, Senior Member, IEEE, and S. O. Reza Moheimani, Fellow, IEEE Abstract—The recording performance of data-storage devices, in which write/read elements move relative to a storage medium to reliably store and retrieve information, depends on the capa- bility of servo mechanisms to provide the necessary positioning ac- curacy. The desired characteristics of servo mechanisms for data- storage systems include robustness against variations of environ- mental parameters, high resolution, accuracy, stability, and fast response. This paper presents a comprehensive overview of ad- vanced servo-control methods for data storage. The applications are to well-established recording technologies, including magnetic tape and magnetic disk systems as well as CD/DVD/Blue-Ray op- tical data-storage systems. Moreover, newer holographic and near- field optical systems and the emerging probe-storage technology are also addressed. Emphasis is given to the potential exhibited by the technologies considered for achieving ultra-high storage ca- pacity, as required by the exploding demand in data-storage ca- pacity for archival systems and massive multimedia data storage. Index Terms—Control systems, hard-disk drive, holographic data storage, optical-disk drive, scanning-probe data storage, servomechanism, tape drive. I. INTRODUCTION S ERVO control systems are essential to achieve high throughput and near-optimum capacity in data-storage devices in which accurate positioning of write/read elements is required to reliably store and retrieve information. In today’s increasingly interconnected world, there is an exploding de- mand in data-storage capacity for archival as well as real-time applications, as a staggering volume of digital data is being produced at an ever increasing pace. According to an Inter- national Data Corporation study [1], the amount of digital information produced in 2011 is expected to approach 1.8 zettabytes, corresponding to bytes, or 10 times that produced in 2006, with a compound annual growth rate of about 60%, i.e., significantly faster than the growth rate of storage capacity. Furthermore, new regulatory requirements demand that a larger fraction of this data be preserved, for example, in the government and healthcare sectors. Manuscript received December 05, 2010; revised October 24, 2011; accepted November 05, 2011. Manuscript received in final form November 15, 2011. Date of publication December 13, 2011; date of current version February 01, 2012. Recommended by Associate Editor G. Guo. G. Cherubini is with IBM Research—Zurich, 8803 Rüschlikon, Switzerland (e-mail: cbi@zurich.ibm.com). C. C. Chung is with the Division of Electrical and Biomedical Engineering, Hanyang University, Seoul 133-791, Korea. W. C. Messner is with the Department of Mechanical Engineering, Carnegie Mellon University, Pittsburg, PA 15213 USA. S. O. R. Moheimani is with the School of Electrical Engineering and Com- puter Science, The University of Newcastle, Callaghan NSW 2308, Australia. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCST.2011.2176942 Servo control is becoming increasingly important for sys- tems such as hard-disk drives (HDDs), tape drives, and optical disk drives (ODDs). Significant new technologies are emerging in this field in response to challenging requirements for future storage systems. Examples are described in the programs of the Information Storage Industry Consortium (INSIC), which are aimed at achieving extremely high-density recording in HDDs, on the order of a few Tb/in , and extremely high tape capac- ities, on the order of a few tens of terabytes, within the next decade. The INSIC also envisages significantly higher capaci- ties for ODDs, up to the terabyte range, that can be achieved by resorting to near-field recording. For an overview of nanopositioning technologies and devices emphasizing the key role of advanced control techniques in im- proving precision, accuracy, and speed of operation of these sys- tems in various applications, the reader is referred to [2]. To our knowledge, the present paper provides the first com- prehensive survey of control techniques that find application in storage technologies. Advanced servo-control design methods are reviewed, with emphasis on those elements that are consid- ered essential for achieving ultra-high storage capacities. For example, in HDDs, robust control systems for dual-stage servo actuators are being considered to perform fine head positioning [3], [4]. Feedback-control systems for nanoprobe-based heating in heat-assisted magnetic recording are envisaged to control the nanoscale gap between the probe and the recording medium [5]. In tape drives, new control methods are needed to com- pensate the dynamic skew of the head relative to the moving tape by means of tilting rollers and/or actuators with more than one degree of freedom [6]. High-bandwidth piezoelectric actua- tors are considered for enhancing the performance of track-fol- lowing loops in the presence of high-frequency lateral tape mo- tion (LTM) [7]. In ODDs, near-field recording aims at reducing the size of the beam spot by increasing the numerical aperture of a focusing lens using solid immersion lens technology [8], [9]. When using a blue laser diode as light source, the gap distance should be controlled to less than 100 nm, with tight margins. Stable servo systems are needed to prevent collisions between the lens and the disk surface and to achieve good system perfor- mance [10]. Emerging data-storage systems are also addressed that will inherently require high-performance position control. Holographic data storage holds the potential for a volumetric density greater than 100 Gb/cm and throughput larger than 1 Gb/s [11]. The tremendous potential of scanning-probe storage devices to achieve ultra-high areal density has also been recog- nized [12]–[15]. In these devices, nanopositioning techniques to navigate a scanner over the storage medium with nanometer accuracy play a fundamental role [16], [17]. 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