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     

Optical methods of the head positioning in magnetic disk systems

This paper reports a new method of head positioning by applying optics to magnetic disk systems. To increase the accuracy of head positioning, several methods in which a servo disk is not used have been investigated. Until now these methods were unsuccessful because of interference between the head positioning signal and the data signal, both of which were recorded on the same data surface. In the method explained here, the head can be positioned on the disk by an optically detected positioning signal without disturbing operation of the magnetic disk system. A unique structure for both the disk and head was investigated. Optical patterns depicting position of the data tracks were formed by coloring the anode oxidized surface layer of an aluminum substrate, and by forming a magnetic film over that. Three optical fibers were imbedded in a hole in the central rail of a Winchester-type ferrite head and were used to read the optical pattern. A stable positioning signal was optically detected from the rotating disk. By using the signal to assemble a simple servo loop, the head could be positioned with ±3 μm accuracy. Potential accuracy was found to be ±0.7 μm.     

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.     

Mechanical and servo design of a 10 inch disk drive

Design concepts of the mechanical and servo system of a newly-developed compact disk drive are discussed. This new drive features 10 inch diameter disks with a capacity above 400 Mega-bytes. The mechanical structure, with its own closed self-circulating air system , is designed to minimize thermal offtrack and undesired vibration. A new rotary type head actuator is accommodated, and servo system design is based on an optimal control theory to achieve fast, precise positioning.     

Digital processing of dual-frequency servo burst in hard disk drives

The trend of increasing areal density, i.e., the number of bits recorded in unit area of the media, continues in the hard disk drive (HDD) industry. This growth is sustained by increasing both the track density and bit density. The track density that can be achieved in a drive depends on many factors such as the dimensions of read/write head and the performance of the head positioning servomechanism. The higher the servo bandwidth, the better the precision achieved in controlling the position of the head. Most of the drives use an embedded servo scheme, which puts a physical constraint on the frequency at which the position error is sampled, and hence, on the achievable bandwidth. The need for a better position error sensing (PES) scheme is the motivation behind exploring alternative methods of servo encoding. This paper addresses different aspects of decoding position error from the dual-frequency servo bursts. It is shown, using both simulation and experimental results, that the proposed scheme offers a feasible solution for generating the PES signal in high-performance HDD.     

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.      

A minimum time seek controller for a disk drive

A time optimal controller design is proposed for the purpose of steering the read/write head of a magnetic disk drive during a seek manuever. The design includes the development of a switch-on-time (SOT) open loop slew control using a third order model of the voice coil motor. The SOT algorithm employs a look up table of switch times versus angle. A perturbation correction stage is included to tailor the switch times to adjust for variations in the operating characteristic of the disk drive (due, for example to heat buildup, wear, etc.). The design also includes a terminal stage servo control that employs an optimal feedback/feedforward controller which is used to provide precise positioning at the end of the seek maneuver     

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.     

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     

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.     

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.     

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     

Dynamic Modeling and Control of a Piezo-Electric Dual-Stage Tape Servo Actuator

We present a data-based approach for modeling and controller design of a dual-stage tape servo actuator. Our method uses step response measurements and a generalized realization algorithm to identify a multivariable discrete-time model of the actuator. The data acquisition and modeling can be implemented in the servo firmware of a tape drive. We have designed a dual-stage controller, based on the model, using loop shaping techniques adopted for multivariable control problems. We applied the procedure to the prototype of a dual-stage actuator tape head to reduce the effect of lateral tape motion. The prototype consists of a conventional voice coil motor for coarse positioning and a micro-actuator for fine positioning. The micro-actuator, which is mounted on the voice coil motor, uses a piezo crystal to follow high-frequency lateral tape motion (up to the kilohertz regime), while the voice coil motor follows only low-frequency lateral tape motion. Compared to a single-stage design, the dual-stage servo design provides a 25% bandwidth improvement and a voice coil motor control signal that is much smaller in magnitude.      

Development of a servowriter for magnetic disk storage applications

The design, development, and application of a research servowriter, which is necessary to record magnetic position encoding schemes, are presented. The servowriter is designed to evaluate emerging disk drive technologies. Submicron positioning accuracy of the disk actuator is achieved by integrating the state-of-the-art electronic, optic, electromechanical, and digital signal processing hardware. The servowriter also provides flexible and sophisticated signal encoding schemes through a programmable pattern generator and has several modules of customized electronic hardware, each controlled by a supervisory computer. The position of the R/W (read/write) head and actuator assembly is measured by a laser interferometric system. The measured position is used to estimate the velocity of the R/W head, which is then used to implement a state-variable-based proportional-integral-derivative controller on the Intel (8086/8087) microprocessor chip set. Both systematic and random error sources associated with phase and amplitude encoding schemes are described. Environmental factors such as seismic vibration and temperature change, which indirectly contribute to servowriting errors, are identified, and steps to contain these error sources are described     

Tilt correction in an optical disk system

A servo system for the correction of disk tilt in optical disk data storage is proposed, and its basic concepts are demonstrated by the use of a static system in which the disk does not spin. Because disk tilt produces primarily coma in the beam focused onto the disk, the system uses a variable coma generator to produce an equal and opposite amount of coma as that caused by the tilted disk. The magnitude and direction of disk tilt are detected by the use of the light reflected from the front facet of the disk substrate.     

The effect of spring pad loading and head penetration on the flight of flexible disk over a spherical head

A flexible disk, spinning at relatively low speed (80-600 r/min) and locally sandwiched between a spherical head and a spring loaded pressure pad, is investigated by stroboscopic white light interferometry. The contours of the disk in the vicinity of the head in general, and the minimum head/disk spacing in particular, are determined for various values of parameters such as disk speed, head penetration, and the pad loading force. Based on the topology of the contours, various modes of flight with distinctive characteristics are classified, and the effects of the head penetration and the pad loading force are thoroughly analyzed. Qualitative explanations of the observed phenomena are given and the problems associated with the optimum head design are discussed.     

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     

Longitudinal recording in a stray field

We have studied the sensitivity of soft error rate (SER) to an external magnetic field in longitudinal hard disk drives. We found that during writing SER is generally quadratic with field amplitude and symmetric with field direction. The application of an external field during writing also shifts the center of the track. In contrast, during reading SER is predominantly linear with field amplitude and antisymmetric with field direction. There is no shift in the track center when the field is applied during reading up to 100 Oe. We explain the difference in the track center shift between writing and reading in terms of the effect of the stray field on the servo positioning system.     

Design of a high performance rotary positioner for a magnetic disk memory

The design described in this paper covers a non-traditional, rotary positioner for a 14 inch, mid-range magnetic disk memory. It was intended to provide high performance and track density while retaining traditional advantages found in rotary positioners such as high reliability, and low cost, power, weight, and size. The paper describes positioner motor architecture and design, head support arm architecture and design, performance achieved by the total servo system including positioner, and finally, conclusions reached as a result of this design approach. Extensive use of computer modelling and computer assisted testing and analysis is highlighted. In particular, modal analysis results obtained from theoretical studies are described and compared with positioner performance measurements.     

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.