Improvement of slider off-track vibration through design parameter modifications of a compatible FDB spindle system

Recently, the hard disk drive (HDD) industry has tried to use a compatible spindle system regardless of the number of disks because of the resulting cost reduction and standardization of components. The center of gravity (CG) location predominantly affects the disk and slider off-track vibration, which is why the rocking mode of a spindle system is affected by the CG. Any changes to the CG affect the operational vibration of the spindle system. In a compatible fluid dynamic bearing (FDB) spindle system, changing the number of disks may alter the CG. Nevertheless, research into the compatibility of FDB designs has not been undertaken. In this study, FDB design parameters were selected to reduce the slider off-track vibration with variations in the CG considering a compatible spindle system. First, a verified finite element (FE) model of a spindle system was constructed. The amplitude and frequency of the rocking mode were compared between a one-disk spindle system and a two-disk spindle system using the FE model, considering the relationship between the CG location, which is changed by the number of disks, and the location of the upper and lower journal bearings. HDD prototypes were then manufactured using the improved design. Based on the manufactured spindle system, the variations in the rocking mode characteristics and slider off-track vibration were measured and operational vibration tests were performed to verify the effect of the number of disks on the slider off-track vibration. An improved FDB spindle design was developed with a reduced rocking mode, and a compatible spindle system was proposed.     

Experimental study of the flow-structure interactions in an air- or helium-filled hard disk drive geometry

The increased rotating speed of the hard disk drive (HDD) causes an unsteady flow field between each stack of disks and leads to flow induced vibration on the slider suspension unit (SSU). This flow induced vibration can reduce the positioning accuracy of the SSU and lead to failure to read or write data. Therefore, reduction of turbulence kinetic energy around the SSU is an important step to improve the performance of the HDD. Several modifications have been investigated in air to decrease the direct effect of unsteady flows on the SSU (e.g. spoiler, damper, or divider in the region upstream of the arm). However, these methods are not fundamental solutions for reducing the vibrations on the SSU. Since the HDDs currently in use are filled with air, helium was selected to compare the flow pattern due to the differing inertial property. To visualize the flow pattern, particle image velocimetry (PIV) measurements were performed at the inter-disk mid-plane between a pair of disks near the arm and the SSU. The geometry is an expanded 2× model simulating Seagate cheetah 2.5-inch drive. For both the air and the helium filled drives, measurements have been performed for two different locations of the SSU for two different angular velocities of 1,000 and 3,000 rpm, corresponding to 5,000 and 15,000 rpm in the commercial drive. The results reveal that the flow patterns of the air and the helium flow are quite similar. However, with respect to the turbulence intensity around the SSU, the helium flow shows a drastic decrease compared with that of the air flow, resulting in much reduced positioning errors. As such, helium-filled drives have merit and should be looked into.     

Noise and vibration due to rotor eccentricity in a HDD spindle system

This paper numerically and experimentally investigates the characteristics of torque ripple and unbalanced magnetic force (UMF) due to rotor eccentricity and their effects on noise and vibration in a hard disk drive (HDD) spindle motor with 12 poles and 9 slots. The major excitation frequencies of a non-operating HDD spindle system with rotor eccentricity are the least common multiples (LCM) of pole and slot numbers of the cogging torque and the harmonics of slot number ±1 of the UMF. An experimental setup is developed to measure the UMF generated by rotor eccentricity and to verify the simulated UMF. In the operating HDD spindle motor, the harmonics of the commutation frequency of torque ripple (multiplication of pole and phase) are increased by the interaction of the driving current and rotor eccentricity, and they are the same as the LCM of pole and slot numbers for a HDD spindle motor with 12 poles and 9 slots. The major excitation frequencies of the UMF while operating condition are also the harmonics of slot number ±1 and the harmonics of commutation frequency ±1. We verify that the source of the harmonics of slot number ±1 and the harmonics of commutation frequency ±1 in acoustic noise and vibration is rotor eccentricity of the UMF through experiments.     

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     

Effects of thrust hydrodynamic bearing stiffness and damping on disk-spindle axial vibration in hard disk drives

This paper aims at investigating the effects of variations in thrust hydrodynamic bearing (HDB) parameters such as axial stiffness and damping coefficients on the axial vibration of disk-spindle systems in hard disk drives. For a parametric study, a closed-form axial frequency response function (FRF) of HDB spindle systems is derived as a function of the axial stiffness and damping coefficients of thrust HDBs. It is known that the axial vibration of the disk-spindle system is composed of two main parts: the vibration of the rigid hub in the axial direction and the disk deflection in the transverse direction. The results from this research clearly show that the vibration amplitudes at low frequency range is dominated by the axial vibration of the hub, and the amplitude of the unbalanced (0,0) mode is dominated by the disk deflection. The parametric study reveals that at low frequency range an increase in the bearing stiffness significantly reduces the hub axial vibration, and hence the axial vibration of the disk-spindle system. Surprisingly, a too much increase in the damping results in a higher amplitude of the unbalanced (0,0) mode. This is because a heavy damping constrains the hub vibration to nearly no motion, resulting in a direct transmission of vibration from the base to disk. To confirm the parametric study, a vibration test was performed on two HDB spindle motors with identical design but different fluid viscosity. The higher viscosity represents the higher axial stiffness and damping in the thrust bearing. The test result indicates that the spindle motor with higher viscosity has a larger unbalanced (0,0) mode amplitude when subjected to an axial base excitation.     

Transient dynamic analysis of ferro-fluid bearing spindle motor

 With areal recording density of hard disk drives (HDD) historically growing at an average of 60% per year and fast spindle speed to continue to reduce access time, it is becoming increasingly more difficult to maintain the precise positioning required of the GMR heads to read and write data. Any unexpected vibration will cause the data written to a wrong data track. Consequently, the dynamic behaviors of HDD spindle systems and their potential influences on track misregistration are key issues in disk drive design. With rapid advances in the emerging consumer device market, the fluid bearing spindle motors, which have low NRRO, low acoustic noise and high damping, are being developed as next generation spindles. This paper is to study transient dynamic performance of HDD ferro-fluid bearing spindle systems. The FEA based component mode synthesis method is used to reduce the overall spindle system dimensions. The effect of the unbalanced magnetic pulls (UMP) due to two different types of motor configurations (balanced and unbalanced configurations) on the dynamic behaviors of spindle system was investigated. The simulated results show that the motor with balanced configuration provides better spindle dynamic performance due to absence of UMP. The UMP derived from the unbalanced configuration can result in some frequency resonance interactions and adversely affect the HDD servo-tracking system. Received: 5 July 2001/Accepted: 17 October 2001     

Reduction of torque ripple and mechanical vibration of spindle motors in hard-disk drives using a sinusoidal driver

Mechanical vibration and acoustic noise are major obstacles in the development of high-density and high-spindle-speed hard disk drives. Torque ripple caused by the electrical driver is the main source of vibration and noise. This paper proposes a novel driver for spindle motors in hard disk drives based on the principle of position sensorless vector control. To reduce torque ripple, the proposed driver feeds the spindle motor in sinusoidal driving mode by which the sinusoidal current of the motor can be obtained. Experimental results of the proposed driver demonstrate the better driving performance in startup condition and fine sinusoidal current in steady state. Vibration testing shows significant improvement in the attenuation of vibration: the dominant vibration modes can be reduced to one tenth compared to that of a conventional driver. In addition, the mechanism of inducing torque ripple from time-harmonic currents is analyzed and the relationship between induced torque ripple and exhibited vibration modes is examined.     

Magnetically induced vibration of a flexible rotating disk-spindle system due to the internal magnetic force arising from the spindle motor of a HDD

This paper investigates the magnetically induced vibration of a flexible rotating disk-spindle system and stationary stator-base due to the internal excitation of the local magnetic force arising from the spindle motor of a HDD. A three-dimensional magnetic finite element model of the spindle motor is developed, and the Maxwell stress tensor method is applied to calculate the local magnetic force acting on the stationary teeth and rotating permanent magnet of the spindle motor. Also, a three-dimensional structural finite element model is developed and local magnetic force is applied to teeth and permanent magnet. The simulated forced vibration of the base plate matched well with the measured one. The dominant frequency component of local magnetic force is the 12th harmonic corresponding to the number of poles, but the dominant frequency component of vibration is the 36th harmonic corresponding to the least common multiple of the number of poles and slots because the 12 and 24th harmonics in local force are canceled out when they are summed up along the air gap. The 12th, 24th and 36th harmonics of the axial vibration are mostly affected by the axial magnetic force, and the amplitudes of those harmonics are increased with the increase of stator eccentricity.     

Reduction of torque ripple and mechanical vibration of spindle motors in hard-disk drives using a sinusoidal driver

Mechanical vibration and acoustic noise are major obstacles in the development of high-density and high-spindle-speed hard disk drives. Torque ripple caused by the electrical driver is the main source of vibration and noise. This paper proposes a novel driver for spindle motors in hard disk drives based on the principle of position sensorless vector control. To reduce torque ripple, the proposed driver feeds the spindle motor in sinusoidal driving mode by which the sinusoidal current of the motor can be obtained. Experimental results of the proposed driver demonstrate the better driving performance in startup condition and fine sinusoidal current in steady state. Vibration testing shows significant improvement in the attenuation of vibration: the dominant vibration modes can be reduced to one tenth compared to that of a conventional driver. In addition, the mechanism of inducing torque ripple from time-harmonic currents is analyzed and the relationship between induced torque ripple and exhibited vibration modes is examined.

     

Turbulence intensity inversion induced by the mass-reducing hole in an air or helium filled hard disk drive

Since its introduction, the mass-reducing hole in the middle of an actuator arm has been investigated as a source of turbulent flow in the space between a pair of co-rotating disks in an air-filled Hard Disk Drive (HDD). The present study investigates the effect of the mass-reducing hole by performing Particle Image Velocimetry (PIV) and numerical calculation in both air and helium. Helium was selected as an alternative medium due to its high kinematic viscosity which is expected to stabilize the turbulent flow around the actuator arm. A double-scale experimental apparatus was built to simulate a commercial drive. The same model was simulated numerically. The investigations were performed for two different positions of the actuator arm and two angular speeds of the disk (1,000 and 3,000 rpm; corresponding to 4,000 and 12,000 rpm in a 3.5-inch commercial drive). Experimental data was collected at the inter-disk mid-plane and ensemble-averaged to compute the turbulence intensity. The results show that, as expected, the helium flow induces lower turbulence intensity than the air flow at low speeds of rotation. In particular, the helium flow stabilizes the turbulent flow around the Slider Suspension Unit (SSU) more effectively than the air flow. However, at high speeds of rotation, the helium flow generates a higher level of turbulence intensity immediately behind the mass-reducing hole than the air flow. The physical mechanism of the switch is explained.     

Thermal analysis of helium-filled enterprise disk drive

Enterprise hard disk drives (HDDs) are widely used in high-end storage systems for data center. One of key performance requirements for enterprise HDDs is data access rate, which demands very high rotational speed (e.g. 15 k rpm or more) to permit fast access time. To reach such high speed, the disk spindle motor draws more power to spin and hence the temperature of HDD enclosure increases due to large windage loss. It has been known, temperature rise is one of the most fundamental factors that affect the reliability of the disk drive. In order to develop high reliable enterprise HDDs, thermal management of enterprise HDDs needs to be optimized to improve heat dissipation. One possible approach is to fill disk drive with helium because of its lower density and higher thermal conductivity. This paper investigates thermal performances of helium-filled enterprise disk drives through FEM simulations with experimental validations. Windage loss and heat convection of the HDD filled with helium and air are analysed. The simulated and measured temperature distributions of one commercial enterprise HDD with helium-filled and helium-air mixture are compared with those of an air-filled one. The results show 41% reduction of temperature rise of HDD enclosure can be achieved by filling with helium in comparison with that of air-filled HDD. It is also projected that in terms of equivalent cooling capability like air-filled HDD at 15 k rpm, helium-filled HDD spindle can spin up to 19 k rpm, which will greatly increase data access rate by 25% for future enterprise applications.     

A parametric study on rocking vibration of rotating disk/spindle systems with hydrodynamic bearings: rotating-shaft design

 The system studied in this paper is a rotating disk/spindle assembly supported by hydrodynamic bearings with a rotating shaft design. Based on an experimentally verified mathematical model [1, 2], this paper presents how various spindle parameters affect critical vibration modes of the system, such as half-speed whirls and (0, 1) unbalanced modes (i.e., rocking modes). The parameters studied include number of disks, hub/shaft interface stiffness, shaft rigidity, thrust bearing location, radial bearing stiffness, radial bearing damping, and radial bearing locations. To simulate operational tests, the numerical study focuses on frequency response functions (FRF) of rotating disk/spindle systems subjected to linear base excitations. Simulation results show that 1-disk configuration has smaller FRF amplitude than the 4-disk configuration. In addition, the amplitude of half-speed whirl is primarily controlled by the radial bearing stiffness. In contrast, the amplitude of (0, 1) unbalanced modes is dominated by hub/shaft interface stiffness. Finally, radial bearing locations significantly affect the amplitude of half-speed whirls and (0, 1) unbalanced modes simultaneously. Received: 16 October 2001/Accepted: 31 December 2001     

Linear protrusion structure on carriage-arm surface for reduction of flow-induced carriage vibration in hard disk drives

One of the major contributors to head positioning errors is carriage vibration in low frequency due to an air flow caused by disk rotation. It is necessary to suppress the disturbance for Hard Disk Drives (HDDs) to have more capacity. We experimentally studied a reduction in the flow-induced carriage vibration using linear protrusion structures by putting wires on carriage arms. This study was carried out using 2.5?inch HDD which has high rotational speed of 10,000?rpm. We measured position error signals (PES) and compared with a conventional carriage. From the experimental results, we found that the linear protrusion structure was effective to reduce the carriage vibration. Leading edge wire configuration and 2 linear protrusion configuration improved average non-repeatable position errors (NRPE) by 6.9% and 6.4%, respectively.     

On investigation of vibro-acoustics of FDB spindle motors for hard disk drives

The present work investigates vibro-acoustic behaviors of the fluid dynamic bearing (FDB) spindle motors for hard disk drives (HDD) through the sound spectra and the frequency response functions (FRF) of the motor structure. The quantitative evidence on the significance of the acoustic noise originated from the electromagnetic source is deduced from the sound spectra that were measured in two distinct cases of the spinning motor: in the normal operation and at the moment immediately after the power supply was disconnected. It is found that the effect of electromagnetic noise source is more dominant than the combined effect of the mechanical and aerodynamic sources. In addition, it is identified that, within the audible range of frequency, the frequency range of 13.4–20 kHz deems important to the noise problem as it is the main contributor to the acoustic noise for the FDB spindle motors. Moreover, the structural resonances that can be identified via the FRF are found to play an important role in the noise emitted by the motors. The concurrence of resonance and excitation frequencies clearly intensifies the sound spectrum, resulting in high discrete peaks, hence higher decibel level.     

On Taylor vortices and Ekman layers in flow-induced vibration of hard disk drives

This paper is about flow-induced vibration (FIV) of disks in hard disk drives (HDD) influenced by two classical flow structures in fluid dynamics, Taylor Couette vortices (TCV) and Ekman layers. FIV is computed with a fully coupled commercial aerodynamics/structural code. The emphasis is on FIV of disks and geometries under conditions typical for high speed, server HDDs. In typical server drives computational fluid dynamic (CFD) analysis predicts the occurrence of TCVs in the disk to shroud clearance. TCVs typically do not occur in mobile and desktop drives. The main controlling non-dimensional parameters are the Reynolds number, the Taylor number and the aspect ratio of the disk to shroud clearance. The existence of Ekman layers on the disk surfaces is persistent. The Ekman layers and their radial return flow interact in a complex manner with the flow in the disk to shroud clearance. The turbulent viscosity between shrouded disks results from “bursting” phenomena that are typical for the flow field near the disk rims and shroud. The details of a turbulent burst are presented together with its momentary disk excitation effect. The benchmark case used is a fully shrouded set of two disks with a disk to shroud clearance and a disk thickness to shroud aspect ratio such that TCVs occur in the disk to shroud clearance. The TCVs interact with the Ekman layers such that the outer TCVs are continuously destroyed and recreated. An example is presented of fully coupled FIV of a two-disk axi-symmetric benchmark case. The two co-rotating shrouded disks attract aerodynamically: they deflect statically inward. The results also show the dynamic disk deformation dominated by the disk (0,0) “umbrella” mode. In addition, there is random disk deflection caused by the turbulent bursting. At server drive conditions and a 70 mm diameter disk the peak to peak deflection is approximately 20% of the mean deflection. Three dimensional effects are also presented such as wavy TCVs. In another benchmark with a cavity the flow near unshrouded disk edges is shown. In that case the pressure fluctuations can be an order of magnitude greater than in shrouded regions.     

Aerodynamic effect on natural frequency and flutter instability in rotating optical disks

 Experimental studies on the aerodynamic coupling effect on natural frequencies and flutter instability of rotating disks are investigated in this paper. The experiments performed using a vacuum chamber and optical disks give two main results. One is that the aerodynamic effect by surrounding air reduces the natural frequencies and critical speeds of the vibration modes in pre-flutter regions. The other is that the natural frequency of the disk rotating at ambient atmospheric pressure is equal to that in vacuum at the flutter onset speed where the disk experiences aero-induced flutter. In post-flutter regions, the aerodynamic coupling between the disk and surrounding air increases the natural frequencies of the disk. Received: 17 June 2002/Accepted: 7 October 2002 The work was supported by Grant No. R11-1997-042-090001-0 of the Center for Information Storage Devices designated by the Korea Science & Engineering Foundation. Paper presented at the 13th Annual Symposium on Information Storage and Processing Systems, Santa Clara, CA, USA, 17–18 June, 2002     

Improving multimedia systems performance using constant-density recording disks

Multimedia systems store and retrieve large amounts of data which require extremely high disk bandwidth and their performance critically depends on the efficiency of disk storage. However, existing magnetic disks are designed for small amounts of data retrievals geared to traditional operations; with speed improvements mainly focused on how to reduce seek time and rotational latency. When the same mechanism is applied to multimedia systems, overheads in disk I/O can result in dramatic deterioration in system performance. In this paper, we present a mathematical model to evaluate the performance of constant-density recording disks, and use this model to analyze quantitatively the performance of multimedia data request streams. We show that high disk throughput may be achieved by suitably adjusting the relevant parameters. In addition to demonstrating quantitatively that constant-density recording disks perform significantly better than traditional disks for multimedia data storage, a novel disk-partitioning scheme which places data according to their bandwidths is presented.     

A study of acoustic noise generating mechanisms of small form factor HDDs

A combined experimental and numerical study of the acoustic noise from a small form factor hard disk drive (HDD) is made to investigate the relative contribution of structure-borne idle noise to the total generated noise. Initially, the idle noise of a 1.8″ HDD was measured in an anechoic chamber, and a clear high-frequency peak is found in its total idle noise frequency spectrum. Then the modeling and simulation (M&S) of the top cover vibration and the associated sound radiation are performed to identify the dominant source and transmission path causing this noise peak. The M&S process consists of a 3D structural finite element (FE) modeling of the HDD to calculate the frequency-domain vibration response of the top cover, and a boundary element (BE) modeling of the HDD for calculating the radiated sound pressure. The loading specified in the FE model is motor torque ripple: the dominant electromagnetic excitation of fluid dynamic bearing spindle motor for HDDs. Finally, the obtained acoustic BE results of the sound pressure levels at a selected field point are compared to those measured physically in the chamber. It is shown that for the HDD considered, the coincidence of a high-frequency resonant mode with the fifth harmonic frequency of motor torque ripple is responsible for the high-frequency peak noise in the idle noise spectrum.     

Numerical study of the flow-structure interactions in an air- or helium-filled simulated hard disk drive

Since its invention, the Hard Disk Drive (HDD) has been the most widely-used device for data storage. Recently, the volume of data is getting larger and the corresponding rotation speed of the HDD is increasing to allow for better data transfer. The decreasing size of the disk is increasing the density of data on the disk surface. As a result, the positioning accuracy of the Suspension Slider Unit (SSU), where the magnetic head is mounted, is the problem that has to be overcome for better performance of the HDD. Additionally, the increased rotating speed of the disk induces unsteady flow between each pair of disks. This unsteady flow becomes turbulent around the SSU and induces vibrations on the SSU which deteriorate the performance of the HDD. There have been many investigations to understand the fluid mechanics phenomena inside the HDD filled with air. Additionally, many modifications have been tried to minimize the flow-induced vibration on the SSU by placing a blockage upstream of the arm to generate a low velocity region. However, none of these investigations have explored the effect of using gases other than air. In this work, the flow physics in the HDD is investigated numerically with the drive filled with air or helium. Numerical analyses were performed using the commercial code (ANSYS/CFX) with an expanded 2 × model simulating Seagate cheetah 2.5-inch drive. Despite obvious un-addressed issues in sealing the HDD, the unsteady characteristics of the flow are dissipated sufficiently faster in helium than in air so as to warrant further studies addressing the more practical issues of working with helium. Of particular importance is the unsteady flow around the SSU. This leads to lower levels of flow-induced vibration in the case of helium flow. As such, HDD performance may be improved by using helium to improve the dynamics of the HDD at higher rotation speeds. For both air- and helium-filled drives, calculations have been performed with two different locations of the SSU and two different angular velocities, 1,000 and 3,000 rpm corresponding to 5,000 and 15,000 rpm in 3.5-inch commercial drive. Not only is it shown that the helium-filled drive suffers lower positioning errors, but also the underlying flow physics responsible for such improvement are explained.     

Optimization of 3.5-in. HDD spindle motors for OP-vibration performance: theoretical prediction and experimental verification

The purpose of this paper is to optimize OP-vibration performance of 3.5-in. hard disk drive (HDD) spindle motors through theoretical prediction and experimental verification. OP-vibration performance of HDD is closely related to the first rocking vibration of spindle motors because excited frequencies of 3.5-in. HDD from the environment are mostly below 500 Hz and the first rocking vibration is the only resonance in the corresponding frequencies. Therefore, minimizing first rocking vibration leads to improve OP-vibration performance of the spindle motors. In order to minimize the first rocking vibration key parameters of FDB spindle motors were selected from a previous work done by Heo and Shen (Microsyst Technol 11:1204–1213, 2005). Then, the selected parameters have been optimized to minimize the first rocking vibration through a theoretical model developed at University of Washington. Then, experiments with ten prototype FDB spindle motors have been conducted to verify the theoretical results. Each prototype motor has different spindle parameter configurations including bearing coefficients, bearing locations, and center of gravity location, etc. Also, this paper demonstrated that radial measurements of spindle rocking vibration have better correlation with OP-vibration performance than axial measurements through PES measurements. Finally, the optimized design has been manufactured by a motor maker and has also successfully verified the theoretical prediction experimentally.