Critical and flutter speeds of optical disks

 Critical speeds and aerodynamic flutter instability of various optical disks are experimentally studied in this paper. The two nodal diameter modes of ASMO and CD/DVD disks have the lowest critical speeds at 3800 and 6900 rpm, respectively, where the backward natural frequency vanishes. As the rotational speed increases, aeroelastic disk flutter is observed. Experiments using ASMO disks show that the three nodal diameter mode causes flutter instability at 8750 rpm. At the flutter speed, the vibration amplitude of the flutter mode grows dramatically. The natural frequencies of multiple vibration modes remain almost constant in the post-flutter region, which is called frequency lock-on. CD/DVD disks do not experience flutter up to 14,000 rpm. Received: 5 July 2001/Accepted: 1 November 2001     

Mechanical vibration and critical speeds of rotating composite laminate disks

Since fiber-reinforced composite materials have high-specific strength and stiffness values, their application to rotating disks can enhance machinery performance by increasing the dynamic stability and reducing driving energy. There have been few works on the vibration characteristics of rotating multidirectional laminated disks made of fiber-reinforced composite materials. Most of the previous studies have been confined to single lamina disks. When a disk rotates, the centrifugal force causes the in-plane loads to affect the vibration characteristics of the rotating disks. In this paper, the exact expressions for the in-plane loads acting on rotating cross-ply laminate disks are presented. The vibration equation of rotating cross-ply laminate disks was solved by using Galerkin’s method. Using numerical examples, the natural frequencies and critical speeds of the rotating disks are discussed for various cross-ply ratios.     

Development of optical gap sensor application to near field recording

For applying near-field recording (NFR) technology to optical storage devices for the next generation, it is positively necessary to maintain a small air gap under about 100 nm. We design an apparatus to measure the air gap between the CISD type SIL [1] and the interface of dielectric substrate of the disk. And it consists of a prism, a polarizer and an analyzer. The air gap including the far-field as well as the near-field range is determined by measuring the intensity of polarized reflectance light. Through the Fresnel equation and Jones matrices, a mathematical model is established to understand the characteristics of a system according to design parameters. We can change measurement ranges/resolutions by adjusting an incident angle into the air interface. Experimental results for some specific cases are in good agreement with simulated ones and demonstrate the possiblity as a new optical gap detector.This work was funded by the Korea Science and Engineering Foundation (KOSEF) through the Center for Information Storage Device (CISD) Grant No. R11-1997-006101-0 and the Korea Sanhak Foundation.Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Local heat transfer characteristics in a single rotating disk and co-rotating disks

The present study investigates convective heat/mass transfer and flow characteristics inside rotating disks. The rotating disks are simulated on the commonly used 3.5 hard disk drives (HDD). The experiments are conducted for the various hub heights of 5, 10 and 15 mm in a single rotating disk and 4, 6 and 8 mm in co-rotating disks and for the various rotating Reynolds numbers of 5.53 × 10⁴, 8.53 × 10⁴ and 1.13 × 10⁵. To accommodate the general operating conditions of HDD, the experiments are also conducted with an obstruction of rectangular cross-section in the space, which simulates a read-write head arm. A naphthalene sublimation technique is employed to determine the detailed local heat transfer coefficients on the rotating disks using the heat and mass transfer analogy. Flow field measurements are conducted using laser Doppler anemometry (LDA) and numerical calculations are performed simultaneously to analyze the flow patterns induced by disk rotation. The results of a single rotating disk show that the heat transfer on the rotating disk is enhanced considerably according to the reduction of the hub height and the increase of the rotating Reynolds number. The head arm inserted in the cavity between the rotating disk and the cover enhances uniformity of the heat/mass transfer on the disk due to the deficit of the momentum in the average flow despite the enhancement of the tangential component of fluctuation velocity. The heat/mass transfer rates on the co-rotating disks have very low values near the hub in the inner region of the solid-body rotation and increase rapidly toward the outer region. The change of heat/mass transfer for various hub heights is negligible.The authors wish to acknowledge support for this study by the Ministry of Science and Technology through their National Research Laboratory program and by the KOSEF through the Center of Information Storage Device.     

Aerodynamically induced power loss in hard disk drives

In this paper, the effects of rotational speed, form factor and enclosure conditions on power dissipation in hard disk drives are presented. The aerodynamically dissipated power losses by 3.5, 2.5 and 1 in. hard disks are experimentally measured using a vacuum chamber and compared to theoretical estimations. Experiments in open air without enclosure agree well with theoretical predictions; a 3.5-in. disk satisfies the turbulent model but 1 and 2.5-in. disks match the laminar one, which is inversely proportional to the half power of Reynolds number. Experiments using a single 3.5-in. disk in enclosure show that aerodynamic power loss is proportional to the second power of rotational speed and the fourth power of disk radius, which agrees with the laminar theory rather than turbulent one. It is also shown that the aerodynamic power loss is reduced as the axial gap and radial clearance of enclosure decrease.     

Fast sliding-mode seek control of hard disk servos

A nonlinear controller is presented for both seeking control and following control in hard disk servos. Usually, a switching mechanism is used for mode transit from seeking to following. As a result, the seeking time and settling time are lengthened very much, especially for short seeks. Based on time-varying sliding surfaces, the proposed nonlinear control approach eliminates the switching mechanism and thus realizes the smooth mode transit. Its effectiveness is demonstrated in simulation and experiments. In particular, the seek time is improved up to 50%.Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Transient analysis of sliding contact of layered elastic/plastic solids with rough surfaces

 Based on the boundary element method (BEM) and a variational principle, a numerical model is developed to analyze the time – transient sliding contact of two layered elastic/plastic solids. Two cases are considered: one is the loading/sliding/unloading of a rough surface on a smooth surface, and the other is of two rough surfaces. Contact statistics, contact pressure profile and stress distribution are predicted at each time step with updated surface roughness. The results are used to study the effect of surface roughness, physical properties of the layer and the substrate, and lubricant film thickness on friction, stiction, and wear. Discussion on the integration of this contact model into advanced tribological models, e.g., wear model, is also presented. Received: 28 June 2002/Accepted: 23 October 2002 Currently at: Seagate Technology, Pittsburgh, PA Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Vibration analysis of a contact slider/gimbal of a flexible disk drive

 To design contact-type gimbal system for a newly developed removable flexible disk drive called Card size flexible disk drive (CFDD) (Shinpuku et al., 2001; Ryoson et al., 2001), the gimbal's vibration characteristics in the disk-rotation direction were measured and simulated using FEM. To determine the relationship between vibration and friction in the head disk interface (HDI), a hard disk was also used. The friction characteristics were inserted into the FEM model based on the results. The gimbal was modified to suppress vibration amplitude to half in both simulation and experiment. Received: 5 July 2001/Accepted: 1 November 2001 Paper presented at the 12th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 28–29 June, 2001.     

Parametric study of the aeroelastic response of mistuned bladed disks

Mistuning, imperfections in the cyclical symmetry of bladed disks, is unavoidable due to many factors, including manufacturing tolerances and in-service wear and tear. Even small mistuning levels can cause large changes in the dynamics of bladed disks because mistuning destroys cyclic symmetry. In particular, mistuning can cause mode localization and a drastic increase of the maximum forced response level. To study mistuning, classical finite element analyses (FEA) approaches become computationally expensive because mistuning requires the analysis of the full disk model. Recently, compact and accurate reduced order models (ROMs) of mistuned bladed disks considering only structural coupling between sectors have been developed. However, the incorporation of unsteady aerodynamic effects into these ROMs is limited despite the fact that accurately accounting for the aerodynamic blade-to-blade coupling is essential for predicting mistuning effects in turbomachinery. Previous studies by the authors have shown that aerodynamic coupling can have important effects on the dynamics of bladed disks for subsonic flows. In particular, differences in the aerodynamic damping affect significantly the forced response amplification factor, and aeroelastic mistuned mode shapes differ significantly from the structural ones. These phenomena are accentuated in the transonic regime, where the presence of the shock creates a stronger blade-to-blade coupling than in the subsonic regime, as demonstrated herein. Parametric studies are conducted for various inflow Mach numbers and incidence angles. It is shown that the effect of aerodynamic coupling diminishes when the inflow Mach number is smaller, while the differences in the aerodynamic damping values still affect significantly the dynamics of mistuned bladed disks. Also, severe mistuned mode localizations are observed for modes having separated frequencies due to frequency veering.     

Analysis of dynamic characteristics of a HDD spindle system supported by ball bearing due to temperature variation

 This paper presents a method to investigate the characteristics of a ball bearing and the dynamics of a HDD spindle system due to temperature variation. Finite element model is developed for the rotating and stationary parts of a HDD spindle system separately to determine their thermal deformations by using ANSYS, a finite element program. Then, the relative position of the rotating part with respect to the stationary part is determined by solving the equilibrium equation of the contact force between upper and lower ball bearings. The validity of the proposed method is verified by comparing the theoretical natural frequencies of a HDD spindle system with the experimental ones before and after temperature variation. The proposed method makes it possible to predict the characteristics of a ball bearing and the dynamics of a HDD spindle system due to temperature variation. It shows that the elevated temperature results in the increase of contact angle and the decrease of bearing deformation, contact force and bearing stiffness, which result in the decrease of the natural frequencies of a HDD spindle system. Received: 20 June 2002 / Accepted: 28 August 2002     

Writing-induced heating and its effects on air bearing performance

As extremely low and stable flying height will be needed as the technology approaches even higher areal density, investigations are made on the effects of the writing current on the flying height stability and the slider deformation. Experimental results indicate that increasing writing current to 50 mA can induce a flying height reduction up to 0.6 nm, such a reduction also occurs with increase in writing frequencies. Measurements on static deformation slider reveal a 2–3 nm crown reduction of the slider over the same writing current range. Thermal analysis and computer modeling show that the internal heat generation from the writing current can lead to a crown distortion up to 2.5 nm, which implies a corresponding reduction in flying height of about 0.6 nm.Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Vibro-acoustic interaction of components in hard disk drive under seek process

With the increasing use of electro-mechanical systems such as hard disk drives, CD-Rom drives, and DVD drives in the consumer electronics industry, there is a growing demand for quieter products. The noise emitted from these devices may originate from the vibration of mechanical components in operation, such as bearings, gears and actuators. The vibration is then transmitted to other parts of the devices, such as the cover, and the noise emitted may then be amplified. This paper aims to develop a method for identify the acoustic noise emitted from hard disk drive seek process. In order to determine effective noise and vibration control of seek noise during the seek process, we first investigate the dynamic characteristic of the head actuator assembly under the different electrical drive signal using finite element method. In this paper, a bearing stiffness matrix (6 × 6) is adopted and performed using ANSYS element (Matrix 27). Using this method, we can understand the vibro-acoustic characteristics of hard disk drives and their components, and the mechanisms of vibration and sound transmission. Experiments are being conducted to demonstrate its validity through experiments. The method will also enable design optimisation for noise to be carried out at the design stage, before a new product is built, or help guide design changes on an existing device.Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Attenuation of rotating disk vibrations with sector-shaped plates in disk storage devices

A parametric study has been conducted to understand the mass loading contribution to rotating disk vibrations in disk storage devices. With sector-shaped plates retaining air between the two plates and a rotating disk, results have shown frequency crossing of disk flutter modes with non-zero nodal diameters where significant modal pattern switching is observed. Extending the range of the analysis to sector geometries provides insight into the disk vibrations on the degree of frequency separation and mode switching as a span angle of the sector-shaped plates and disk rotating speed. Design guidelines are formulated to minimize this mode crossing phenomenon and thus minimize the number of spindle disturbances present during track follow or the servotrack writing process.     

Advanced optical and magneto-optical recording techniques: a review

Optical data storage is advancing rapidly as we move forward in the 21st Century. Most people today are familiar with CDR and CD-RW as forms of optical data storage, but DVD-R, with considerably higher capacity, will challenge CD-R as the preferred storage medium soon. This review paper looks at the current state of optical recording and some of the current techniques which offer increased magneto-optical storage density, such as Magnetic Super Resolution (MSR), which is already on the market as the GIGAMO disk, and other methods under development. The latter includes Magnetically AMplified Magneto-Optical System (MAMMOS) and Domain Wall Displacement Detection (DWDD). To complement these technologies are a variety of optical techniques, such as near-field recording, which will enable higher storage densities by overcoming the diffraction limit. Holographic storage has promised much over the years, and has largely remained in the research domain. However, new materials and multiplexing techniques show considerable promise for this volumetric storage medium. The overlapping optical and magneto-optical technologies have yielded the concept of hybrid recording and this will also be addressed in this paper. The paper concludes with a look at alternative storage mechanisms, which utilise electronic, as opposed to thermal transitions, to offer improved data rates and improved storage efficiency, and this may dramatically change the way data is stored.David Jenkins would like to thank the Royal Academy of Engineering for financial assistance for attending the Information Storage and Processing Systems Conference where this paper was presented. Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

A novel drag test to evaluate media tribology design on head wear

 By changing the head's Z-height, a new accelerated drag test has been developed to characterize head wear under high interference conditions. Compared to contact start stop (CSS) and other drag tests, heads wear faster in our approach, due to the stressed condition at the tilted trailing edge. The optimized Z-height has been chosen so that the head-disk interference and acoustic emission are maximized. The wear volume can be estimated by optical microscopy. The influence of lube bonded ratio and X1P additive amount on head wear has been studied for AM3001 lubricant. Lower bonded ratio and higher X1P additive were found to give rise to less head wear, which is in agreement with the existing results of contact start-stop and other drag tests. In media manufacturing, tape burnish processes were found to significantly affect the bonded ratio in the AM3001/X1P lubricant system, which consequently affects head wear. Short tape burnish times were found to reduce the media's head wear. Received: 16 July 2002/Accepted: 23 October 2002 The authors would like to thank Dr. Ken E. Johnson for his extensive support in this project. Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Simulation of the head disk interface for discrete track media

This paper investigates the effect of discrete tracks on the steady state flying behavior of sub ambient proximity sliders. A finite element based air bearing simulator is used to simulate the flying characteristics of sliders over a grooved disk surface. Sliders flying over discrete track disks “see” a disk surface that consists of ridges and grooves. The air bearing pressure build-up for sliders flying over discrete track disks is different from that for sliders flying over plane disks. Low air bearing pressure can be expected for those regions of the slider that are positioned over grooves, while high air bearing pressure exists over ridges. The air bearing characteristics are determined for several pico and femto-type air bearing sliders flying over discrete track disks. An empirical equation is obtained describing the loss of flying height of a slider flying over discrete track disks.     

Minimization of tape transient disturbances in track following tape drives with a disturbance observer

The primary purpose of the head servo in the track following tape drives is to track follow as accurately as possible during read/write operations in spite of disturbances such as tape edge movements or tape transients, internal and external vibrations transmitted to the head actuator and signal noise and dropouts. The track following and fine positioning is done using a position error signal (PES). Feedback controller design using proportional-integral-derivative (PID), lead-lag or estimator/controller techniques are used for maintaining accurate track following. This paper assumes that in addition to the PES signal, a head position signal is available for long seek, short seek, and track identification tasks, etc. In this paper, the development of a disturbance observer that supplements the nominal head servo controller in minimizing the effects of the tape transients is presented. The PES and the head position error signals are blended to provide a measure of the tape transient disturbance. This paper presents the theory of the disturbance observer and analyses to show the range of frequencies over which the disturbance observer will be effective. Finally, simulation results are presented for tape transients of various frequencies.Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Investigation of wear resistance and lifetime of diamond-like carbon (DLC) coated glass disk in flying height measurement process

Flying height has been greatly reduced to less than 10 nm to achieve high-density magnetic storage. This leads to significant disk wear especially, glass disks used in flying height measurement process. This paper reports the utilization of diamond-like carbon (DLC) overcoat to increase the wear resistance and lifetime of commercial glass disks in a flying height tester. Wear resistance of the DLC coated glass disks was investigated in wear test using a triboindentor. The results showed significant wear resistance improvement of the coated disk where the wear depth reduced from 62.2 nm on an uncoated disk to 5–7 nm on 15-nm thick DLC coated disks. Furthermore, lifetime measurement of the coated disk has been performed in a flying height tester. As a result, lifetime of the coated disk has been drastically improved by more than 30 times in comparison to an uncoated disk.     

Asymmetric responses of rotating,thin disks experiencing large deflections

An analytical nonlinear solution for the asymmetric mode vibration of rotating disks is given in this paper through a recently developed, accurate plate theory instead of the von Karman model. The nonlinear solution can reduce to the linear one when nonlinear effects vanish. The symmetrical response is also recovered when the nodal diameter vanishes. The natural frequency varying with rotation speed and deflection amplitude is investigated through a 3.5-inch diameter computer memory disk. From this investigation, it is found that the softening of rotating disks may occur for larger nodal-diameter numbers. The methodology given in this paper can be applied to nonlinear responses in structures such as rotating shafts and traveling plates.