Evaluating Track-Following Servo Performance of High-Density Hard Disk Drives Using Patterned Media

In recent years, track densities of magnetic hard disks have continued to grow. A promising approach to continuing the trend to ultrahigh density is using bit-patterned media (BPM). However, the implementation of BPM in hard disk drives (HDDs) to achieve high recording density is challenging and requires various new techniques, such as new servo pattern designs and position error signal (PES) decoding schemes. In applying BPM in HDDs, it is important to select a servo pattern providing sufficient PES quality for head positioning. In this paper, we discuss evaluation of PES quality and servo pattern performance from a closed-loop (servo) point of view in order to evaluate the quality of several servo patterns. We consider three servo patterns (the amplitude pattern, chevron pattern, and differential frequency pattern) as case studies. We developed a PES simulation tool to provide a realistic HDD track-following simulation. Because of PES nonlinearity in the amplitude servo pattern, we considered time-based servo patterns as alternatives. For time-based servo patterns, we found that readback signal sampling and transition jitter greatly affect PES quality. Therefore, we conclude that the differential frequency servo pattern is superior to other patterns, since it is less sensitive to transition jitter and readback signal sampling.      

Study of Lithographically Defined Data Track and Servo Patterns

We report on fabrication of discrete tracks on perpendicular magnetic recording (PMR) media with an e-beam lithographical process. We studied the recording performance of the e-beam media on a spinstand in parallel with conventional PMR media. Discrete track media show significant reduction in adjacent track erasure (ATE). We studied and quantitatively measured the source of the ATE improvement, and developed a triple track geometrical model to calculate achievable track density for both discrete track recording (DTR) and continuous media. From the model, we identify two factors of DTR that contribute to reaching a higher TPI. Using the same fabrication technique, we also studied servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts with servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better position error signal sensitivity and support higher TPI in the future.Discrete tracks are fabricated on conventional PMR media with an e-beam litho graphical process. The recording performance is studied on a spinstand in parallel with conventional PMR media. Discrete track media shows significant reduction in adjacent track erasure (ATE). The source of the ATE improvement is studied and quantitatively measured. A triple track geometrical model is developed to calculate achievable track density for both DTR and continuous media. From the model, we identify two factors of DTR, which contribute to reaching a higher TPI. Using the same fabrication technique, we also study servo burst design and its playback waveform quality. At 250 ktpi, we compare DTR servo bursts and servo bursts written with a conventional method. DTR servo bursts show better edge definition, which can translate to better PES signal sensitivity and support higher TPI in the future.     

PC-based position error signal generation and servo system for a spinstand

We describe a high-performance servo control system for enhancement of spinstand servo performance. This PC-based system uses a multifunction I/O card for controller output and a high-speed digitizer card capable of sampling the frequency-encoded servo pattern at 500 MS/s. The PC, running under the Linux operating system, uses the Goertzel algorithm decoding scheme to calculate the position error signal (PES) at a 15-kHz update rate for feedback control. The control signal drives a custom-made lead zirconate titanate (PZT) actuator, which moves the suspension and thus the read/write head. A proportional and integral (PI) type servo controller supports a 1.1-kHz servo bandwidth, producing a 21.9% improvement of the positioning accuracy of the spinstand.     

Synchronous servo scheme using maximum-likelihood detectors

For magnetic disk drives, we have developed a scheme for synchronous servo with maximum-likelihood (SSML) detection that is synchronous around the entire circumference of the track and utilizes ML estimation to detect head position, clock mark phase, and track addresses. Compared to the conventional asynchronous sector servo scheme, the SSML scheme has many, much shorter, servo areas-typically several hundred-distributed on the disk. This results in higher servo information rate, which will be needed for future high-track-density disk drives with a wide servo bandwidth. We analyze the performance of the SSML scheme, including the format efficiency. We discuss a limitation of the synchronous servo scheme with respect to the number of servo areas, and we show an appropriate region for its application. Feasibility of the SSML scheme has been demonstrated in prototype 2.5-in hard disk drives.     

Method for In Situ Motion Measurement of Head-Slider in Both Flying Height and Off-Track Directions

We present a novel method to determine the motion of the slider's flying height and off-track direction simultaneously in a hard disk drive. The results show the relative movement of the slider in two dimensions, both during thermal actuator controlled contact and full flying condition. The method separates the position error signal (PES) and the flying height signal from the readback signal. The method includes writing dual-frequency pattern tracks adjacently, with the readback signals from the tracks filtered for the in situ motion measurement. The harmonic ratio method is used to determine the flying height variation of the head, whereas the conventional servo burst detection method is used to determine the PES.     

A track locating servo system utilizing the data heads as absolute position transducers

Magnetic transitions may be compactly written in the normally unused deadspace between the data sectors on each data surface of a moving head disc drive, which may be read by ordinary data heads to obtain absolute position information for use by the track locating servo. A track following system which complements the track locating system is briefly described. Implementation of these techniques eliminates both the need for a servo surface or external encoder, and troublesome reference surface to data surface mechanical tolerance problems.     

High track density for magnetic disk drives with an "Embedded servo" positioning system

The CII-HB "Embedded servo" original technique for disk drive units (such as the D 160) allows operation at high track densities ( 800 TPI) and fast access time. The overall information needed for head displacement and positioning is provided by the data head itself, without any other transducer. Servo transitions, written between the data sectors, are arranged to provide gain information, track logical address, and fine position. This information is used by a microcomputer program to move the head rapidly to the desired track and to lock it in position within an instantaneous error of less than 3 μm (120 μm inches). Practical results are presented and compared with a theoretical model. Limitations and improvements are discussed.     

An adaptive filter for the processing of position error signal in disk drives

A novel circuit used to process analog position error information in a disk drive head-positioning servo system drastically reduces noise in the signal and compensates for effects due to insufficient position sample rate at high carriage velocities. The processed signal does not suffer from envelope droop at high carriage velocities, and lends itself well to track counting algorithms. Because the filter's bandwidth and low-frequency gain are controlled by carriage velocity information, the filter characteristics adapt to environmental conditions for near optimal reduction of noise.     

Evaluation of test procedures for determining servo compatibilityof heads and media in magnetic disk drives

We evaluate a testing specification proposed by the National Storage Industry Consortium's (NSIC) Extremely High Density Recording (EHDR) group for evaluating head and media compatibility for servo performance in magnetic disk drives. These tests use average amplitude and average noise profile measurements across isolated tracks to predict the shape, linearity, noise, and long-term stability of position error signal (PES) patterns. We compare the predictions from these tests to measurements from null and amplitude PES patterns written on a spin-stand. Results show average PES-profile prediction errors of 1%-2% track width and noise level prediction within a factor of 2. We present data from tests for long-term stability of the magnetoresistive (MR) read element after repeated write cycles by the inductive write head. In the set of heads we tested, the MR head's center and effective width changed only slightly. Although we evaluated the NSIC specification for MR read elements, the specification should be equally valid for other read head types also, as long as the PES patterns are similar     

Adaptive compensation for position error signal nonlinearity

The magnetoresistive (MR) magnetic head is a poor positioning transducer for a disk file's servo control system because its positioning response is nonlinear with radial displacement. This paper shows how the MR head's poor positioning properties are alleviated by a self-adjusting adaptive algorithm that allows a disk file to linearize its own servo position error signal (PES). The adaptive linearizer uses a nonlinear state estimator whose nonlinearity adjusts to match the nonlinearity of the PES. As the match between the two nonlinearities adaptively improves, the state estimator gives increasingly accurate estimates of the true actuator position     

Head positioning servo design for the IBM 3344/3350 disk files

The 3344/3350 disk files have significantly increased both the recording areal density and total storage capacity per spindle in comparison to previous disk file products, such as the 3340. Parameters directly related to the head positioning systems are compared. Three areas of the head positioning servo system that required design changes for increased performance are described: (1) the encoding used on the servo disk to obtain position information, (2) the compensation technique employed in the track following controller to reduce steady state positioning error and (3) implementation of the phase plane trajectory for improved accuracy and settling as well as decreased access time.     

Nonrepeatable Run-out Rejection Using Online Iterative Control for High-Density Data Storage

The spectra of disturbances and noises affecting precise servo positioning for ultrahigh-density storage in future hard disk drives are time-varying and remain unknown. In this paper, we propose an online iterative control algorithm that sets the measured position error signal (PES) into the servo system to achieve high track densities by minimizing the square of the H₂-norm of the transfer function from nonrepeatable run-out (NRRO) disturbances to the true PES. It is not necessary to solve any algebraic Riccati equations and linear matrix inequalities. The algorithm constructs an online repeatable run-out estimator to extract NRRO components for gradient estimates, thereby preventing the controller parameters from being trapped in a local minima. Experimental results on a PC-based servo system for a spinstand show an improvement of 22% in 3sigma NRRO and suppression of baseline NRRO spectrum     

Distortion effects on servo position error transfer function

Two kinds of nonlinear distortion of hard-disk-drive position error signal (PES) transfer function are examined and modeled. The first distortion is the saturation of the PES due to narrow read width. The second distortion is the on-track dead band at the center of the transfer function due to wide erase band. Several formulas are developed to describe the relationships between the distortion effects and the design parameters: read width, track spacing, and erase width. These distortions are highly undesirable for the track following sectored servo system. However, they are predictable and avoidable if certain conditions are satisfied. The geometrical analysis of the distortion phenomenon is presented here     

Repetitive controller design for minimum track misregistration in hard disk drives

Repetitive control is a widely used technique for the compensation of repeatable error in systems that contain rotating mechanisms or repeat a trajectory. Generally, it includes delay chains and a low-pass filter in the positive feedback loop, which generate a periodic signal. The controller has typically been implemented in a plug-in fashion and designed heuristically with the simplest form of the filter. However, this design approach is somewhat ambiguous in the selection of controller parameters because of its influence over nonharmonic frequencies. Also, it leaves the possibility for further improvement. This paper presents an improved design method for the repetitive controller that provides minimum track misregistration (TMR) in a hard disk drive (HDD). For TMR prediction, the method identifies disturbances acting on an HDD and estimates servo performance, using the identification result. We have confirmed the identification and estimation procedure through experiments. In our method, first the basic tracking controller is designed and later the repetitive controller is designed in conjunction with a Q filter. A cost function based on Parseval's theorem, reflecting the servo performance as TMR, is defined. Then the servo performance is estimated from the identified disturbance, and the plant and designed controller's frequency response are modified as necessary by changing the parameters of the controller, whose optimization is carried out with a commercial nonlinear optimization tool. The design strategy facilitates the controller design by providing an accurate estimation for the attainable servo performance and design criteria under the optimization framework.     

Identification and decentralized control of a dual-actuator hard disk drive system

The dual-actuator hard disk drive with two voice coil motor actuators mounted in the diagonal corners of the baseplate allows faster data access with relatively slow spindle speed. However, the mechanical interaction between the actuators tends to affect the head positioning accuracy. In this paper, we use the circle-fit modal testing method to identify the dual-input dual-output frequency response model of the dual-actuator plant. We discuss the decentralized control scheme for the dual-actuator tracking servo. The servo loop decomposition reveals that the interaction induces narrow-band vibrations near the crossover frequency. We propose a phase-lead interaction filter with a slight change to the loop stability to suppress the vibrations. Experimental results on a dual-actuator prototype show that the position error signal 6/spl sigma/ value due to interaction vibrations is reduced by 61% with the interaction filter in combination with the decentralized control scheme.     

Novel method for minimizing track seeking residual vibrations ofhard disk drives

Track access time is an important parameter for high-performance hard disk drives (HDDs). Presently, the resonances of the head actuator caused by the pivot-bearing stiffness are an even more urgent problem, since these resonances have become the main design factor that limits further reduction of track access time and improvement of the servo control bandwidth in HDDs. Our objective in this paper is to investigate, from a mechanics and dynamics viewpoint, the influence of the driving force waveforms on residual vibrations and to identify the characteristics of the waveform, which will lead to zero or very low residual vibrations. Using a simplified model of a head actuator assembly and finite-element simulation, we show that the residual vibration of the head actuator assembly depends strongly on the waveform and the time duration of the driving impulse force. If suitable waveform and time duration of the seeking impulse force are appropriately chosen, very low track seeking residual vibrations and quick track access can be achieved     

交流伺服系统在卷烟机中的应用

在卷烟机中采用交流伺服驱动代替传统传动方式,实现多电机同步驱动,采用速度环和位置环嵌套的闭环控制技术进行实时跟踪,以减小传动误差、减小噪音,增强机组的控制精度与稳定性,简化机电系统的维护与修理.     

The MD 122 dual 3 Mbyte flexible disk drive

The dual 3Mbyte flexible disk drive provides 6 Mbyte of formatted data storage in a compact package designed for in-built use. The two disks may be inserted or removed independently and share a common double ended spindle. A common actuator is used to position the four recording heads. High areal recording density is achieved using advanced technology recording heads and media in association with a microprocesor controlled environmentally compensated servo positioner. The drive incorporates an intelligent controller which provides sophisticated data handling functions and controls the drive in such a manner that the interface to the host is free of drive dependant parameters.     

基于改进FNN-CCC的双伺服压力机同步控制策略研究

目的 改善双伺服压力机同步控制策略的动态响应性能和鲁棒性,提升双伺服压力机的单轴跟踪精度和双轴同步精度,实现成形过程的高精度位置控制。方法 建立双伺服压力机驱动系统数学模型,分析系统同步误差来源,结合模糊神经网络单轴控制算法,引入迭代学习律,设计一种改进模糊神经网络-交叉耦合（FNN-CCC）同步控制器。基于系统控制模型进行单轴阶跃响应特性与双轴正弦跟随特性仿真,搭建嵌入式双伺服压力机驱动系统试验平台,在偏载干扰条件下进行双轴同步控制试验,验证所提出理论的有效性。结果 仿真结果表明,与模糊控制算法和BP神经网络控制算法相比,该控制器单轴控制算法的超调量分别减少了11.5%和25.5%,调节时间分别减少了48.8%和34.4%,具有更好的动态响应性能。与原控制器相比,改进后的交叉耦合同步控制器最大双轴同步误差降低了65.7%,同步控制精度有所提高。试验结果表明,与传统PID-交叉耦合控制器相比,改进的FNN-CCC控制器有更好的控制性能,在热冲压合模成形阶段,单轴跟踪误差分别减小了81.8%和75.0%,双轴同步误差减小了69.2%。结论 所提出的同步控制策略在偏载干扰条件下具有较好的动...     

Adjustable balancer with electromagnetic release of balancing members

Adjustable balancers are useful in applications where rotational imbalance is frequently varied. For example, in media level servo track writers used in the hard disk drive industry, a set of blank disks is loaded on a spindle motor for the purpose of servo writing. Since servo writing is performed at high speeds, imbalance introduced by disks and spacers results in significant vibrations that can affect the quality of the servo writing process. This paper proposes a new configuration of adjustable balancer that enables to compensate for rotational imbalance each time a new set of blank disks is loaded into the servo track writing machine. The adjustable balancer is ring shaped with a number of grooves displaced around axis of rotation. Steel balls are placed inside the grooves and held by permanent magnets. Imbalance compensation is achieved by electromagnetically activated release of the steel balls. The main advantage of the proposed configuration is its large compensation range. Presented three-dimensional finite-element analysis results and experimental results demonstrate and prove the effectiveness of this novel configuration.