Actual Disk Research Articles

Head-Positioning Control Using Virtual Resonant Modes in a Hard Disk Drive

A design method using a virtual resounant mode in the head-positioning systems of hard disk drives was developed. The virtual resonant mode is a digital filter that works like a mechanical resonant mode. In conventional control systems, it is difficult to compensate for the disturbance over the first-order mechanical resonant frequency. By using the proposed method, stable resonant modes in the control system can be designed with accuracy and it can decrease the disturbances above the first-order mechanical resonance. Application of this method to an actual hard disk drive showed that it can greatly decrease the disturbance in which the frequency is higher than first-order mechanical resonance.

A Numerical Study of Air-Bearing Slider Form-Factors

This paper presents a numerical study comparing the performance of air bearing slider form-factors. The air bearing slider and air bearing surface (ABS) design has gone through drastic changes in recent years in order to achieve the performance required by lower flying heights. In the past, improvements have been seen by scaling down the form-factors of air bearing sliders. The pico form-factor 1.25×1mm has been successfully used for several generations of products and the question arises—should the form-factor be scaled down further? The dynamic characteristics and flying-height modulation (FHM) performance of two different ABS designs in the pico and femto 0.82×0.66mm form-factors were numerically investigated. It was found that for the smaller form-factor designs, greater damping of the air bearing film and slider body system was achieved but with an undesirable decrease in modal frequencies. However, depending on the ABS design, beneficial dynamic properties can be achieved by scaling down the form-factor from pico to femto. Maximizing the total air bearing force (the sum of negative and positive) with a design featuring a large number of transverse pressure gradients can obtain high stiffness and damping. Geometric FHM was also investigated using both sinusoidal disk waviness and an actual measured disk topography. It was found that the FHM depends not only on the form-factor, but also on the ABS design. For long disk waviness wavelengths (longer than the slider body length, L), the FHM is proportional to Lβ where β was found to be between 2.6 and 4; hence, FHM is dependent on form-factor. For short disk waviness wavelengths, the FHM is a function of the ABS design and flying attitude and not form-factor. A disk waviness wavelength of 3 mm demarks the transition above which the FHM is a function of form-factor and below which the FHM is a function of the ABS design and the superposition of these two effects compose the geometric FHM. Simulations with an actual measured disk topography showed that the femto form-factor exhibited 22–32 percent less FHM than the pico form-factor for a similar design. However, by changing the ABS design, 35–40 percent less FHM was achieved within the same form-factor. By scaling down a pico slider to a femto slider, we do not necessarily achieve enhanced overall performance. Significant performance improvements in the pico form-factor can be attained if the ABS is properly designed. However, in designing a dynamically stable and low FHM air bearing slider a femto slider ultimately yields better performance when care is taken in designing the ABS.

Squeal Vibration in Disk Brake

This study presents experimental and analytical results of squeal vibration in the disk-pad-caliper system, which is modeled after an actual disk brake. The system consists of a disk clamped along an inner edge and a pad supported by a plate spring (caliper). The frequency of squeal becomes higher as the thickness of the plate spring becomes large, and as the thrust load grows large. The squeal vibration of the disk near the contact area between the disk and the pad has translational motion, and the squeal vibration of this pad-caliper has translational and rotational motion. The disk and the pad-caliper vibrate with the same amplitude and the same phase at a point in the contact area. Therefore we analyzed the disk-pad-caliper system as a pin-disk model, which combined the disk with the pad-caliper by the point.

614 ディスクブレーキの鳴きに及ぼす摩擦接触部の動剛性の影響(モデル化・摩擦機構,振動基礎(1))

This study presents experimental and analytical results of squeal vibration in the disk-pad-caliper system, which is modeled after an actual disk brake. The frequency of squeal becomes higher as the thrust load grows large. We think this phenomenon originates in that the stiffness of a contact region becomes hard according to load. Therefore we measured stiffness of a contact region in the experiment conditions imitating the situation which squeal vibration generated. And we analyzed the disk-pad-caliper system as a distributed spring model based on experiment result This model expresses that the mode of vibration and a sound pressure level is dependent on load.

Position error signal estimation at high sampling rates using data and servo sector measurements

The sampling rate in most hard disk drives (HDDs) is limited by the number of servo burst sectors that have prewritten position error signal (PES) data and by the spinning speed of the HDD. The paper presents an algorithm that can generate estimates of the PES during read at high sampling rates by processing the dedicated servo burst measurements and the mean square error (MSE) path metric data used to choose the correct bit sequence using the Viterbi decoding process of the read signal. The MSE metric is a nonlinear function of the absolute value of the true position of the head relative to the center of the written data. The position estimation algorithm is nonlinear and is based on several logic statements together with the use of two state observers. The estimation algorithm is tested on an actual disk drive and demonstrated to provide values of the PES at a frequency four times higher than that generated by the servo burst measurements.

Composite nonlinear feedback control for linear systems with input saturation: theory and an application

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.

Numerical simulation of positioning error caused by air-flow-induced vibration of head gimbals assembly in hard disk drive

Unsteady airflow in an actual hard disk drive (HDD) which has a rotary actuator is numerically simulated by using the large eddy simulation technique, and the aerodynamic force affecting head gimbal assembly (HGA) is estimated from the simulation. The calculated force is fed to a structure analysis to predict the airflow-induced vibration of the HGA. The authors calculate the head positioning error or the head displacements for the different HGA orientations to explain and classify the mechanism of airflow-induced positioning error (i.e., windage) in HDD.

Quench crack behavior of nickel-base disk superalloys

There is a need to increase the temperature capability of superalloy turbine disks to allow higher operating temperatures in advanced aircraft engines. When modifying processing and chemistry of disk alloys to achieve this capability, it is important to preserve the ability to use rapid cooling during supersolvus heat treatments to achieve coarse grain, fineγ′ microstructures. An important step in this effort is an understanding of the key variables controlling the cracking tendencies of nickel-base disk alloys during quenching from supersolvus heat treatments. The objective of this study was to investigate the quench cracking tendencies of several advanced disk superalloys during simulated heat treatments. Miniature disk specimens were rapidly quenched after solution heat treatments. The responses and failure modes were compared and related to the quench cracking tendencies of actual disk forgings. Cracking along grain boundaries was generally observed to be operative. For the alloys examined in this study, the solution temperature, not alloy chemistry, was found to be the primary factor controlling quench cracking. Alloys with high solvus temperatures show greater tendency for quench cracking.

Quench Crack Behavior of Nickel-Base Disk Superalloys

There is a need to increase the temperature capability of superalloy turbine disks to allow higher operating temperatures in advanced aircraft engines. When modifying processing and chemistry of disk alloys to achieve this capability, it is important to preserve the ability to use rapid cooling during supersolvus heat treatments to achieve coarse grain, fineγ′ microstructures. An important step in this effort is an understanding of the key variables controlling the cracking tendencies of nickel-base disk alloys during quenching from supersolvus heat treatments. The objective of this study was to investigate the quench cracking tendencies of several advanced disk superalloys during simulated heat treatments. Miniature disk specimens were rapidly quenched after solution heat treatments. The responses and failure modes were compared and related to the quench cracking tendencies of actual disk forgings. Cracking along grain boundaries was generally observed to be operative. For the alloys examined in this study, the solution temperature, not alloy chemistry, was found to be the primary factor controlling quench cracking. Alloys with high solvus temperatures show greater tendency for quench cracking.

MC-13 REALIZATION OF AN ADAPTIVE VOLTAGE DRIVER FOR VOICE COIL MOTOR

When the usual current command for the voice coil motor (VCM) is been replaced by a simpler voltage command, the same servo performance can be obtained by placing an adaptive pre-filter between servocontroller output and voltage driver input. The adaptation is required in order to compensate for heat-induced variations of VCM resistance. The paper reports the experimental results obtained with an actual disk during operation, in which an Extended Kalman Filter (EKF) is used as VCM resistance estimator and implemented on a DSP, in a computationally efficient way.

Vibration Servo Control Design for Mechanical Resonant Modes of a Hard-Disk-Drive Actuator

A design method that can suppress the vibration from high-order resonant modes in a head-positioning system of a hard disk drive was developed. This method uses the vector locus of an open-loop transfer function to design both the controller and the structure simultaneously. Application of the method to an actual hard disk drive showed that positioning accuracy under high-order resonant modes can be improved by 55%. In addition, when all resonances are in-phase, the developed method can suppress vibrations caused by all mechanical resonances. It is concluded that the method can design a control system with high robustness and high following-control performance.

1122 ディスクブレーキの鳴き発生のモデリング

This study presents experimental and analytical results of squeal in the disk-pad-caliper system, which is modeled after an actual disk brake. The system consists of a disk clamped along an inner edge and a pad supported by a plate spring (caliper). It is shown that the squeal vibration of the disk near a contact area has translational motion, and the squeal vibration of pad-caliper has translational motion and rotational motion. Squeal vibration of the disk-pad-caliper system near a contact area has same amplitude and phase at a point. Therefore we analyzed the disk-pad-caliper system as a pin-disk model, which combined the disk with the pad-caliper by a point.

A Design Framework for Multi-Resolution Video Servers

Video can be encoded into multiple-resolution format in nature. A multi-resolution or scalable video stream is a video sequence encoded such that subsets of the full resolution video bit stream can be decoded to recreate lower resolution video streams. Employing scalable video enables a video server to provide multiple resolution services for a variety of clients with different decoding capabilities and network bandwidths connected to the server. The inherent advantages of the multi-resolution video server include: heterogeneous client support, storage efficiency, adaptable service, and interactive operations support. For designing a video server, several issues should be dealt with under a unified framework including data placement/retrieval, buffer management, and admission control schemes for deterministic service guarantee. In this paper, we present a general framework for designing a large-scale multi-resolution video server. First, we propose a general multi-resolution video stream model which can be implemented by various scalable compression techniques. Second, given the proposed stream model, we devise a hybrid data placement scheme to store scalable video data across disks in the server. The scheme exploits both concurrency and parallelism offered by striping data across the disks and achieves the disk load balancing during any resolution video service. Next, the retrieval of multi-resolution video is described. The deterministic access property of the placement scheme permits the retrieval scheduling to be performed on each disk independently and to support interactive operations (e.g. pause, resume, slow playback, fastforward and rewind) simply by reconstructing the input parameters to the scheduler. We also present an efficient admission control algorithm which precisely estimates the actual disk workload for the given resolution services and hence permits the buffer requirement to be much smaller. The proposed schemes are verified through detailed simulation and implementation.

Heat transfer and thermal behaviour of a rotating disk passed by a planar air stream

The aerodynamics of a thin, but finite, free rotating disk in an outer forced flow field with constant mean velocity parallel to the disk plane are investigated numerically. The heat transfer coefficients and their distributions on the isothermal disk surface are calculated for several velocities in case of air. Appropriate correlations are derived for the Nusselt number as function of the involved similarity numbers. The idealised model is faced also with the heat transfer problem of an actual motorbike disk brake. The transient heat conduction within rotating disks are treated analytically using the integral-transform technique. This enables even for heat sources which are very short in space a certain estimation of the azimuthal variations of the temperature field due to the rotation.

HIL simulation system for HDD servo firmware

For the purpose of debugging and evaluating quality of the servo-control program used for hard disk drives (HDDs), we have developed a hardware-in-the-loop simulation (HILS) system that uses a computer-based virtual mechanism instead of an actual disk drive mechanism. As the characteristics of the virtual mechanism (e.g., mechanical resonance, disk shift) can be tuned continuously, the capability and tolerance of the servo-control program can be measured quantitatively. In addition, the coded program for a new mechanism can be developed before an actual proto-drive is assembled. We are now applying this HILS system to the development of HDD products.

Measuring rail/wheel contact points of running railway vehicles

It has been difficult to detect lateral contact point of rail and wheel, though technology of instrumented wheelset has advanced in the last 20 years. Therefore, it is interesting and worth while to study on method of measuring rail/wheel contact point and contact force of running railway vehicles, since many things in real rail/wheel contact phenomenon are still unknown. Recently, we have tried to develop a method of measuring rail/wheel contact point by improving conventional method of measuring wheel load and lateral force in which strain of the disk surface is used for measuring these forces. Firstly surface strain on disk type wheel is analyzed by an FEM program, NASTRAN, under the condition that vertical, lateral, or longitudinal force separately acts on various points along the wheel profile. By the calculated result, it shows possibility of detecting the rail/wheel contact point by means of measuring lateral distribution of compress strain which changes according to the load position, i.e. rail/wheel contact point. Secondly the idea above is verified by static load experiments using an actual disk type wheelset. The location of an area where lateral force affects little on surface strain is investigated, since the lateral distribution of strain should be measured in such an area to avoid the interference of bending by lateral force. And the theory for the measurement is modified for on-track testing and the best position and direction for strain gauges is proposed in accordance with the calculated and experimental results. Finally, two wheelsets for the measurement are made and field running test using this method is performed to collect data in various kind of running conditions.

Discovery and Implications of a New Large-Scale Stellar Bar in NGC 5248

For decades, the grand-design SAB spiral galaxy NGC 5248 has been postulated to host a short bar of semimajor axis 22'' (1.6 kpc). From dynamical and morphological arguments, however, we argue that its spiral structure is being driven by a large-scale bar whose corotation radius lies at ~115'' (8.6 kpc). Our estimate is based partially on a deep R-band image, which reveals that the feature previously thought to be an inclined disk is in fact an extended stellar bar. The bar is embedded within a fainter outer disk visible out to a radius of 230'' (17.2 kpc). The bar has a deprojected ellipticity of 0.44 and a semimajor axis of 95'' (7.1 kpc). The classical grand-design spirals of NGC 5248, prominent in B, R, and K light, lie on the leading edge of the large-scale stellar bar and are accompanied by concave dust lanes out to at least 70''. The offset between the dust and young stars is consistent with our understanding of gas flows in barred galaxies, where shocks along the leading edges of a moderately strong bar compress the gas to form massive young stars. While in many strongly barred galaxies, optical spiral arms are prominent outside the bar but not within it, NGC 5248 illustrates how intense star formation along a moderately strong bar can lead to conspicuous open spiral arms within the bar itself. NGC 5248 also provides a clear example of how a large-scale stellar bar embedded within a faint outer optical disk can be misidentified as an inclined disk when imaging studies lack the sensitivity to detect the actual outer disk. We discuss the implications for the estimated bar fraction at higher redshifts.

A Method for Designing a Vibration-Servo-Control System to Reduce the Effect of Mechanical Resonant Modes in a Hard Disk Drive.

Regarding the control of head positioning in hard disk drives, one of the barriers to achieve high positioning accuracy is the effect of the vibrations from the several mechanical resonant modes of the actuator. Thc resonances of high order modes are regarded as uncertainty, so that the conventional control system cannot reduce vibrations caused by high-order resonant modes. In this study, our purpose was to design the control system that can suppress the vibration from high-order resonant modes. The proposed method can do this by designing both the controller and the structure simultaneously using vector locus of open-loop characteristics. The experimental results from application of this method to actual hard disk drives are presented. The measured data show that positioning accuracy at high-order resonant modes can be greatly improved by proposed method.

Measuring Rail/Wheel Contact Points of Running Railway Vehicles.

In this paper, a method of measuring contact points of running railway vehicles is proposed and evaluated. The new method is developed by improving conventional method of measuring wheel load and lateral force. Firstly, surface strain on disk type wheel is analyzed by FEM under the conditions that vertical, lateral and longitudinal force acts separately on various points along the wheel profile. The calculated result shows possibility of detecting rail/wheel contact point by means of measuring lateral distribution of compress strain which changes according to the load position i.e. rail/wheel contact point. Secondly, this idea is verified by static load experiments using an actual disk type wheelset. The location of an area where lateral force affects little on surface strain is investigated, since the lateral distribution of strain should be measured in such an area to avoid the interference of disk bend by lateral force. Finally, the theory for the measurement is modified for on-track testing.

A study of interface dynamics for stiction-free slider and super-smooth disk

This paper reports results of experimental investigations into the head–disk interface dynamics for the combination of the stiction free slider with super-smooth disk and the influences of disk roughness and lubricant thickness on the flying performance of stiction-free slider. The experiments were carried out with an optical head–disk interaction tester with which the tribologically interface used in the tester is still identical to the actual disk drive case The results indicate that the pad height on the air-bearing surface of the slider is one of the key parameters that determines the take-off and landing velocity of the slider. The flying performance of the slider strongly depends on the lubricant thickness when the disk speed is around the take-off velocity.