Hard Disk Drive

It is estimated that over 90% of all new information produced in the world is being stored on magnetic media, most of it on hard disk drives. Despite their importance, there is relatively little published work on the failure patterns of disk drives, and the key factors that affect their lifetime. Most available data are either based on extrapolation from accelerated aging experiments or from relatively modest sized field studies. Moreover, larger population studies rarely have the infrastructure in place to collect health signals from components in operation, which is critical information for detailed failure analysis.

This paper presents a tutorial on iterative learning control (ILC) and repetitive control (RC) techniques in hard disk drive (HDD) industry for compensation of repeatable runouts (RRO). After each tutorial, an application example is given. For ILC, a simple filtering-free implementation for written-in RRO compensation is presented. For the RC part, a new application of RC in dual-stage HDD servo is presented.

Abstruct-While the performance of flash memory exceeds hard disk drives in almost every category, the cost of flash memory must come down in order to gain wider acceptance in mass storage applications. This paper describes a 3.3 Vonly 32 Mb NAND flash memory that achieves not only high performance but also low cost with a 94.9 mm2 die size, improved yields, and a simple process With 0.5 pm CMOS technology. Die size is reduced by eliminating high voltage operation on the bitlines through a self boosted program inhibit voltage generation scheme. Incremental-step-pulse programming results in a 2.3 MB/s program data rate as well as improved process variation tolerance. Interleaved data paths and a boosted wordline results in a 25 ns burst cycle time and a 24 MB/s read data rate. Maximum operating current is less than 8 mA.

We study in this paper the theory and applications of a nonlinear control technique, i.e., the so-called composite nonlinear feedback control, for a class of linear systems with actuator nonlinearities. It consists of a linear feedback law and a nonlinear feedback law without any switching element. The linear feedback part is designed to yield a closed-loop system with a small damping ratio for a quick response, while at the same time not exceeding the actuator limits for the desired command input levels. The nonlinear feedback law is used to increase the damping ratio of the closed-loop system as the system output approaches the target reference to reduce the overshoot caused by the linear part. It is shown that the proposed technique is capable of beating the well-known time-optimal control in the asymptotic tracking situations. The application of such a new technique to an actual hard disk drive servo system shows that it outperforms the conventional method by more than 30%. The technique can be applied to design servo systems that deal with "point-and-shoot" fast targeting.

nature materials | VOL 2 | FEBRUARY 2003 | www.nature.com/naturematerials 85 A s fabrication technology pushes the dimensions of ferromagnetic structures into the nanoscale, understanding the magnetization processes of these structures is of fundamental interest, and key to future applications in hard disk drives, magnetic random access memory and other 'spintronic' devices 1-4 . Measurements on elongated magnetic nanostructures 5,6 highlighted the importance of nucleation and propagation of a magnetic boundary, or domain wall, between opposing magnetic domains in the magnetization reversal process. Domain-wall propagation in confined structures is of basic interest 7,8 and critical to the performance of a recently demonstrated magnetic logic scheme for spintronics 9 . A previous study of a 500-nm-wide NiFe structure obtained very low domain-wall mobility in a three-layer device 10 . Here we report room-temperature measurements of the propagation velocity of a domain wall in a single-layer planar Ni 80 Fe 20 ferromagnetic nanowire 200 nm wide. The wall velocities are extremely high and, importantly, the intrinsic wall mobility is close to that in continuous films 11 , indicating that lateral confinement does not significantly affect the gyromagnetic spin damping parameter to the extreme extent previously suggested 10 . Consequently the prospects for high-speed domain-wall motion in future nanoscale spintronic devices are excellent.

A novel type of control scheme combining the disturbance-observer-based control (DOBC) with H ∞ control is proposed for a class of complex continuous models with disturbances. The disturbances are supposed to include two parts. One part in the input channel is generated by an exogenous system with uncertainty, which can represent the harmonic signals with modeling perturbations. The other part is supposed to have the bounded H 2 -norm. Parametric uncertainties exist both in concerned plant and in exogenous subsystem. The disturbance observers based on regional pole placement and D-stability theory are designed and integrated with conventional H ∞ control laws. The new composite DOBC and H ∞ control scheme is applied to complex continuous models for the case with known and unknown nonlinearity, respectively. Then the first type of disturbances can be estimated and rejected, and the second type can be attenuated; simultaneously, the desired dynamic performances can be guaranteed. Simulations for a flight control system are given to demonstrate the effectiveness of the results and compare the proposed results with the previous schemes.

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 capability of servo mechanisms to provide the necessary positioning accuracy. The desired characteristics of servo mechanisms for datastorage systems include robustness against variations of environmental parameters, high resolution, accuracy, stability, and fast response. This paper presents a comprehensive overview of advanced 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 optical data-storage systems. Moreover, newer holographic and nearfield 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 capacity, as required by the exploding demand in data-storage capacity for archival systems and massive multimedia data storage.

NAND Flash memory-based Solid-State Disks (SSDs) are becoming popular as the storage media in domains ranging from mobile laptops to enterprise-scale storage systems due to a number of benefits (e.g., lighter weights, faster access times, lower power consumption, higher resistance to vibrations) they offer over the conventionally popular Hard Disk Drives (HDDs). While a number of well-regarded simulation environments exist for HDDs, the same is not yet true for SSDs. This is due to SSDs having been in the storage market for relatively less time as well as the lack of information (hardware configuration and software methods) about state-of-the-art SSDs that is publicly available. We describe the design and implementation of FlashSim, a simulator aimed at filling this void in performance evaluation of emerging storage systems that employ SSDs. FlashSim is an event-driven simulator that follows the objected-oriented programming paradigm for modularity. We have validated the performance of FlashSim against a number of commercial SSDs for behavioral similarity. We have also used FlashSim to compare the performance of SSD devices employing different Flash Translation Layer (FTL) schemes, and analyzed the energy consumption of different FTL schemes in the SSD. FlashSim has been written to be inter-operable with the well-regarded DiskSim simulator, thus enabling the simulation of a variety of "hybrid" storage systems employing combinations of SSDs and HDDs. Given the current interest in such hybrid systems as opposed to systems with SSDs replacing HDDs (due to higher price), we believe this to be an especially useful feature of FlashSim. We have made FlashSim freely available for download with the hope that it would be of use to researchers exploring the design of SSD-based systems.

Improved methods are proposed for disk drive failure prediction. The SMART (Self Monitoring and Reporting Technology) failure prediction system is currently implemented in disk drives. Its purpose is to predict the near-term failure of an individual hard disk drive, and issue a backup warning to prevent data loss. Two experimentally tests of SMART showed only moderate accuracy at low false alarm rates. (A rate of 0.2% of total drives per year implies that 20% of drive returns would be good drives, relative to ≈1% annual failure rate of drives). This requirement for very low false alarm rates is well known in medical diagnostic tests for rare diseases, and methodology used there suggests ways to improve SMART.

Abstract responsibility for the conclusions of this paper. This paper addresses two puzzles related to industrial dynamics and competition. The first of these puzzles is that a high degree of globalization may well go hand in hand with increasing concentration. I show ...

Friction and nonlinearities result in large residual errors and deteriorate the performance of head positioning of hard disk drive (HDD) servo systems and other mechanical servo systems. Thus, it is highly desirable to characterize the behaviors of nonlinearities and friction in the servo systems. This paper presents a fairly comprehensive modeling and compensation of friction and nonlinearities of a typical voice-coil-motor (VCM) actuator used in commercial HDDs, and the design of an HDD servo system using an enhanced nonlinear control technique. Our contributions are two-fold: We will first obtain a complete model of the VCM actuator including friction and nonlinear characteristics through a careful examination of the configuration and structure of the actual system and through a thorough analysis of its physical effects together with its time-domain and frequency-domain responses. We will then proceed to design a servo system for the hard drive using an enhanced composite nonlinear feedback (CNF) control technique with a simple friction and nonlinearity compensation scheme. The enhanced CNF technique has a feature of removing the uncompensated portion of friction and nonlinearities without sacrificing the overall tracking performance. Simulation and experimental results for both the modeling and the servo design show that our approach is very effective and successful. In particular, our experimental results show that the enhanced CNF control has outperformed the conventional proportional-integral-derivative (PID) control in settling time by 76%. We believe that this approach can be adopted to solve other servomechanism problems.

The multilevel programming algorithm is based on a program-and-verify (P&V) technique with the aim of creating stable intermediate regions of the cell. Each program pulse is current-controlled, thus speeding up the Joule effect directly to the point where its ...

Two novel discrete anti-windup (AW) techniques are applied to a dual-stage actuator of an experimental hard disk drive system. The techniques, one low order, the other full order, employ convex 2 -performance constraints in combination with linear-matrix-inequality-optimization methods. It is shown that the AW compensators can improve the performance of the nominal dual-stage servo-system when the secondary actuator control signal saturates at its allowable design limits. Also, stability is achieved despite saturation of both the secondary actuator and the voice-coil-motor actuator. Practical results show that the performance of each AW compensator is superior to another well-known ad-hoc AW technique, the internal model control AW scheme. The main contribution of the paper is the application of theoretically rigorous AW methods to an industrially relevant servo system. Index Terms-Anti-windup (AW) control, coarse-fine control, discrete control, hard-disk servo-system.

Dynamic power management (DPM) work proposed to date places inactive components into low power states using a single DPM policy. In contrast, we instead dynamically select among a set of DPM policies with a machine learning algorithm. We leverage the fact that different policies outperform each other under different workloads and devices. Our algorithm adapts to changes in workloads and guarantees quick convergence to the best performing policy for each workload. We performed experiments with a policy set representing state of the art DPM policies on a hard disk drive and a WLAN card. Our results show that our algorithm adapts really well with changing device and workload characteristics and achieves an overall performance comparable to the best performing policy at any point of time.

This paper discusses two mechatronic innovations in magnetic hard disk drive servo systems, which may have to be deployed in the near future, in order to sustain the continuing 60% annual increase in storage density of these devices. The first is the use of high bandwidth dual-stage actuator servo systems to improve the precision and track-following capability of the read/write head positioning control system. The second is the instrumentation of disk drive suspensions with vibration sensing strain gages, in order to enhance airflow-induced suspension vibration suppression in hard disk drives.

Tunneling magnetoresistive (TMR) reading heads at an areal density of 80-100 Gbit/in 2 in a longitudinal magnetic recording mode have for the first time been commercialized for both laptop and desktop Seagate hard disk drive products. The first generation TMR products utilized a bottom TMR stack and an abutted hard bias design. These TMR heads have demonstrated three times the amplitude of comparable giant magnetoresistive (GMR) devices, resulting in a 0.6 decade bit error rate gain over GMR. This has enabled high component and drive yields. Due to the improved thermal dissipation of current-perpendicular-to-plane geometry, TMR runs cooler and has better lifetime performance, and has demonstrated the similar electrical static discharge robustness as GMR. TMR has demonstrated equivalent or better process and wafer yields compared to GMR. The TMR heads is proven to be a mature and capable reader technology. Using the same TMR head design in conjunction with perpendicular recording, 274 Gbit/in 2 has been demonstrated. Advanced design can reach 311 Gbit/in 2 .

Discrete time, 25 KTPI track-following servos were designed for magnetic hard disk drive dual stage actuators using the μ-synthesis methodology. The design methodology was tested on two microactuator models. The first is a model of a piezoelectrically actuated suspension, under development by Hutchinson Technology Incorporated. The second is a model of an electrostatically actuated MEMS microactuator, under development by the IBM Almaden Research Center. Low order controllers were successfully designed for both models, which achieved the prescribed robustness and performance requirements

In a typical disk drive servo system, two or more types of controllers are used for track seeking, track following, and track settling modes. This leads to the problem of mode switching among these controllers. We present in this paper a unified control scheme, the discrete-time composite nonlinear feedback control, which can perform all the above functions in hard disk drive (HDD) servo systems with actuator saturation. The proposed scheme is composed by combining a linear feedback law and a nonlinear feedback law. The linear feedback law is designed to yield a fast response, while the nonlinear feedback law is used to increase the damping ratio of the closed-loop system as the system output approaches the command input. In the face of actuator saturation, this control law not only increases the speed of closed-loop response, but also improves the settling performance. Implementation results show that the proposed method outperforms the conventional proximate time-optimal servomechanism by about 30% in settling time.

In recent years, many systems have employed NAND flash memory as storage devices because of its advantages of higher performance (compared to the traditional hard disk drive), high-density, random-access, increasing capacity, and falling cost. On the other hand, the performance of NAND flash memory is limited by its ¿erase-before-write¿ requirement. Log-based structures have been used to alleviate this problem by writing updated data to the clean space. Prior log-based methods, however, cannot avoid excessive erase operations when there are frequent updates, which quickly consume free pages, especially when some data are updated repeatedly. In this paper, we propose a hybrid architecture for the NAND flash memory storage, of which the log region is implemented using phase change random access memory (PRAM). Compared to traditional log-based architectures, it has the following advantages: (1) the PRAM log region allows in-place updating so that it significantly improves the usage efficiency of log pages by eliminating out-of-date log records; (2) it greatly reduces the traffic of reading from the NAND flash memory storage since the size of logs loaded for the read operation is decreased; (3) the energy consumption of the storage system is reduced as the overhead of writing and reading log data is decreased with the PRAM log region; (4) the lifetime of NAND flash memory is increased because the number of erase operations are reduced. To facilitate the PRAM log region, we propose several management policies. The simulation results show that our proposed methods can substantially improve the performance, energy consumption, and lifetime of the NAND flash memory storage1.

Flash memory solid-state disks (SSDs) are replacing hard disk drives (HDDs) in mobile computing systems because of their lower power consumption, faster random access, and greater shock resistance. We describe Hydra, a high-performance flash memory SSD architecture that translates the parallelism inherent in multiple flash memory chips into improved performance, by means of both bus-level and chip-level interleaving. Hydra has a prioritized structure of memory controllers, consisting of a single high-priority foreground unit, to deal with read requests, and multiple background units, all capable of autonomous execution of sequences of highlevel flash memory operations. Hydra also employs an aggressive write buffering mechanism based on block mapping to ensure that multiple flash memory chips are used effectively, and also to expedite the processing of write requests. Performance evaluation of an FPGA implementation of the Hydra SSD architecture shows that its performance is more than 80 percent better than the best of the comparable HDDs and SSDs that we considered.

Rapid change in the geographical location of production raises important questions regarding the welfare, development potential, and competitive position of different countries and regions. This paper explores in detail the geography of economic activity in a specific industry, the hard disk drive (HDD) component of the computer industry. Firms in the HDD industry are breaking the production system into ever smaller distinct steps, and spreading the physical location of these steps around the world. Firms from the United States dominate the industry. Our findings suggest that globalization has enabled US firms to sustain their dominant position in the industry, preserve employment in the United States (and possibly expand it), and increase employment worldwide, most notably in Southeast Asia.

Magnetoresistive sensors using spin valves and magnetic tunnel junctions are reviewed, considering applications as readers in hard disk drives, as well as applications where the ultimate field detection limits are required (from nT down to pT). The sensor noise level in quasi-DC or high-frequency applications is described, leading to sensor design considerations concerning biomedical and read head applications. Magnetic tunnel junction based sensors using MgO barriers appear as the best candidates for ultra-low field (pT) detection, either in the high-frequency regime, or for quasi-DC applications.

We report on an approach to designing high-speed, low-voltage programmable continuous-time filters with an embedded variable-gain amplifier (VGA). The methods we describe here are aimed at implementation in ultra-short-channel, lowvoltage CMOS technologies. The seventh-order equiripple filter and VGA combination described here has a 03-dB frequency programmable from 30 to 100 MHz, gain programmable from 0 to 17 dB and 12 dB of boost.

This paper examines trends in magnetic spacing in the hard disk drive industry for the last 15 years or so, as we transitioned from inductive to magnetoresistive (MR), giant magnetoresistive (GMR), and tunneling magnetoresistive (TMR) heads, and from longitudinal to perpendicular recording. Historical data show surprisingly good scaling relationships between areal density, linear density, track density, bit aspect ratio (BAR), and magnetic spacing. These trends have held from below 1 Gb/in 2 until today's densities, with no significant discontinuities when crossing major technology changes. In particular, it is found that magnetic spacing has scaled with the length of the bit, and more specifically has equaled about half the bit length. Magnetic spacing components, on the other hand, show larger degree of deviations from the trend. Projections to 1 and 10 Tb/in 2 are made.

A decoupled control design structure and discrete time pole placement design method are proposed for MEMS-based dual-stage servo control design in magnetic disk drives. Dual-stage track following controllers are designed using a decoupled three-step design process: the voice coil motor (VCM) loop design, the microactuator (MA) inner loop design, and the MA outer loop design. Both MIMO (when the MA relative position sensing is available) and SIMO (when the MA relative position sensing is not available) designs are considered. The effect of MA resonance mode variations on the stability and performance of the controllers are analyzed. Self-tuning control and online identification of the MA model are developed to compensate for the variations in the MA's resonance mode.

For extreme-scale high performance computing systems, system-wide power consumption has been identified as one of the key constraints moving forward, where the DRAM main memory systems account for about 30-50% of a node's overall power consumption. Moreover, as the benefits of device scaling for DRAM memory slow, it will become increasingly difficult to keep memory capacities balanced with increasing computational rates offered by next-generation processors. However, a number of emerging memory technologies - nonvolatile memory (NVM) devices – are being investigated as an alternative for DRAM. Moving forward, these NVM devices may offer a number of solutions for HPC architectures. First, as the name, NVM, implies, these devices retain state without continuous power, which can, in turn, reduce power costs. Second, certain NVM devices can be as dense as DRAM, facilitating more memory capacity in the same physical volume. Finally, NVM, such as contemporary NAND flash memory, can be less expensive than DRAM in terms of cost per bit. Taken together, these benefits can provide opportunities for revolutionizing the design of extreme-scale HPC systems. Researchers are investigating how to integrate these emerging technologies into future extreme-scale HPC systems, and how to expose these capabilities in the software stack and applications. Current results show a number of these strategies may offer high-bandwidth I/O, larger main memory capacities, persistent data structures, and new approaches for application resilience and output post-processing, such as transaction-based, incremental-checkpointing and in-situ visualization, respectively.

Fixed-delay tree search with decision feedback (FDTSIDF) has been proposed for retrieving data from hard disk drives. One problem with the algorithm as originally posed is that 4 additions and 1 multiplication sets the critical path delay through the detector. This paper explores the decision space in FDTSIDF using linear discriminants. On recording channels using 2/3(1, 7) run-length limited coding, a detector achieving the performance of FDTSIDG can be implemented with a 2-tap transversal filter. The feedback loop can be rearranged so that this transversal filter no longer resides in the forward path of the feedback loop. Instead its transfer function is incorporated into the specification of the forward and backward equalizers. This modification leads to the simpler architecture of decision feedback equalizer (DFE), where the slicer performs binary decisions on a multi-level signal. Implementation issues pertaining to phase detection, gain detection, dc detection and adaptive equalization using a least-means squared technique will also be addressed. Simulation results demonstrate adaptive equalization, where the desired model is generated by a 3-tap transversal filter consisting of only adders, delays, multiplexers and 1 programmable coefficient.

Firms need to access skills, capital and customers to enter into an industry initially and the choices they make to access these resources are likely to exert path dependent influences over subsequent entry behavior into new sub-markets. This paper explores how firms configure themselves to access skills, capital and customers and reports data on their association with whether and when firms enter new sub-markets in the worldwide hard disk drive (HDD) industry. While, as with other studies, there appear to be geographic differences between US and Japanese firms in sub-market entry behavior, these are shown not to simply reflect differences in region: US startup firms with former IBM personnel and Japanese incumbent firms with keiretsu linkages to their customers, exhibit different entry behavior than other firms in the same region. The analysis suggests that entry decisions are influenced by firms' configuration choices to access needed resources and that the menu of configuration options available to firm managers varies across different institutional settings.

Abstract. Often we hear controversial opinions in digital forensics on the re- quired or desired number of passes to utilize for properly overwriting, some- times referred to as wiping or erasing, a modern hard drive. The controversy has caused much misconception, with persons commonly quoting that data can be recovered if it has only been overwritten once or twice. Moreover, referencing that it actually takes up to ten, and even as many as 35 (referred to as the Gut- mann scheme because of the 1996 Secure Deletion of Data from Magnetic and Solid-State Memory published paper by Peter Gutmann) passes to securely overwrite the previous data. One of the chief controversies is that if a head posi- tioning system is not exact enough, new data written to a drive may not be writ- ten back to the precise location of the original data. We demonstrate that the controversy surrounding this topic is unfounded.

This paper deals with the problem of a servo system design for a conventional hard disk drive with a single voice-coil-motor (VCM) actuator using a so-called robust and perfect tracking (RPT) approach. We will first model the physical system and then formulate it into a robust and perfect tracking problem, in which a measurement feedback controller can be obtained to achieve a robust and perfect tracking for any step reference, i.e., the -norm of the resulting tracking error with 1 can be made arbitrarily small in the presence of external disturbances and initial conditions. Some tradeoffs are then made in order for the RPT controller to be implementable using the existing hardware setup. The implementation results of the RPT controller are compared with those of a proportional integral derivative (PID) controller. The results show that the servo system with our RPT controller has much better performance than the PID one has. Our servo system has faster settling time, lower overshoot and higher accuracy.

Position sensors other than the R/W heads are not embedded into current hard-disk drives (HDDs) due to cost, resolution, and signal-to-noise ratio (SNR) issues. Moreover, the "optimal" location for placing these sensors is still unknown. In this paper, the Pb-Zr-Ti (PZT) elements in the PZT suspension-based microactuator are used as a secondary actuator and a displacement sensor simultaneously with self-sensing actuation. The proposed displacement estimation circuit produces an estimated PZT microactuator's displacement with high SNR and nanometer resolution comparable to that measured from the laser doppler vibrometer (LDV). A robust active mode damping controller is then designed to damp the PZT microactuator suspension's torsion modes and sway mode, as well as decoupling the HDD dual-stage servo into two distinct loops for individual sensitivity optimization. Our results show attenuation of PZT microactuator's suspension modes by 5 dB and sway mode by 30 dB with low sensitivity. A reduction of up to 20% in 3σ position error signal is also observed.

The paper presents a new proximate time optimal seek control approach with discretetime variable structure control theory for a hard disk drive (HDD) servo. It incorporates the time optimal control idea into the time-varying sliding mode control design and derives a class of proximate time optimal control algorithms with robustness. The proposed approach provides one unified framework for both track seeking and track following controller design, it achieves smooth transition from seek control to track-following control naturally, and it eliminates the common chattering and overshoot problem as well. A systematic method is developed for the choice of controller parameters. Simulation results are demonstrated to show the robustness of the proposed method to disturbances, unmodelled dynamics and parameter variations with near time optimal performance.

Trends in the main application area of rare earth permanent magnets, data storage, are presented with the focus on miniaturization. Three examples of magnet applications, a CD-lens actuator, two spindle motors and the voice coil motor of hard disk drives, are discussed in detail and a brief sketch of other application areas is given. With respect to material properties we will focus on magnetization, high temperature use, shaping and shape integrity.

Solid-State Drives (SSDs) offer significant performance improvements over hard disk drives (HDD) on a number of workloads. The frequency of garbage collection (GC) activity is directly correlated with the pattern, frequency, and volume of write requests, and scheduling of GC is controlled by logic internal to the SSD. SSDs can exhibit significant performance degradations when garbage collection (GC) conflicts with an ongoing I/O request stream. When using SSDs in a RAID array, the lack of coordination of the local GC processes amplifies these performance degradations. No RAID controller or SSD available today has the technology to overcome this limitation. This paper presents Harmonia, a Global Garbage Collection (GGC) mechanism to improve response times and reduce performance variability for a RAID array of SSDs. Our proposal includes a high-level design of SSD-aware RAID controller and GGC-capable SSD devices, as well as algorithms to coordinate the global GC cycles. Our simulations show that this design improves response time and reduces performance variability for a wide variety of enterprise workloads. For bursty, write dominant workloads response time was improved by 69% while performance variability was reduced by 71%.

We consider in this paper the design and implementation of a dual-stage actuated hard disk drive (HDD) servo system, in which an additional piezoelectric actuator is mounted on top of the conventional voice-coil-motor (VCM) actuator to provide a faster and finer response. More specifically, the dual-stage actuated HDD servo system consists of a primary VCM actuator and a secondary suspension piezoelectric microactuator. The VCM actuator is used to quickly move the read/write (R/W) head of the HDD servo system to a target track, whereas the microactuator is used to fine-tune the R/W head position when it is getting closer to the target. In our design, the primary actuator is controlled by a newly developed composite nonlinear feedback (CNF) control law, which has fast rise time and small overshoot, and the microactuator is controlled through a simple static gain together with a low-pass filter and a notch filter. Simulation and implementation results show that in track following, the dual-stage actuated HDD servo system has outperformed the single-stage actuated counterpart by more than 47% in settling time and by 39% in position error signal (PES) tests.

In this paper, we present a new seek algorithm formulated by combining the time-optimal control and input shaping methods. The algorithm achieves near optimal bang-bang performance with minimal excitation of the resonance mode. The computation requirements are minimal, and therefore, the algorithm is attractive for practical design and applications. We shall focus on the application of hard disk drives.

We propose a compensator-based strategy for design of a track-seeking and track-following control system for a dual-stage servo actuator in hard disk drives. A well-known decoupling structure is employed to disconnect the control of the primary voice coil motor (VCM) actuator from the loop for a secondary high-bandwidth actuator. The compensator is placed in the secondary loop and suitably combined with a saturation nonlinearity in order to obtain actuator signal boundedness. The design procedure consists of four steps: 1) design of an established nonlinear seek-settle-track following controller for the VCM; 2) design of a linear track following controller for the secondary actuator; 3) observer design; and 4) design of a compensator to retain global stability and to improve performance. The proposed control system improves performance of both long-span seeking (proximate-time-optimal controller) and short-span seeking. In addition, it achieves high-bandwidth track following performance. The experimental results show good track-following performance, and short-span/long-span-seeking performance with fast settling time. The overshoot during track seeking can be made negligible for a suitably tuned VCM-actuator control loop.

In a modern large-capacity magnetic hard disk drive (HDD), both fast track seeking and perfect positioning of the read/write head are required. Mode switching control (MSC) approaches are widely used to meet these requirements. This paper proposes a new approach in designing MSC with an application to HDD servo systems. The proposed scheme uses a proximate time-optimal servomechanism in the track seeking mode, and a robust perfect tracking (RPT) controller in the track following mode. Unlike the conventional MSC approaches, the new method does not require an initial value compensation during mode switching. This is because the RPT controller in the second stage is capable of rendering the L p -norm ð1ppoNÞ of the resulting tracking error arbitrarily small in the presence of external disturbances and initial conditions. Simulation and experimental results show that the proposed method improves the seek and settling time by about 10% over the conventional approaches. r