Investigation of the dynamics of 50% sliders using laser Dopplervibrometry

The dynamics of 50% style sliders are studied using laser Doppler vibrometry. Variations of air bearing frequencies detected from the upper and front surface of the slider are investigated as a function of disk velocity, suspension preload, and yaw angle. The experimental data is compared with numerical simulation results, and the dependence of air bearing stiffness and damping is investigated as a function of disk velocity. Results for the 50% slider are compared with those of 70% style sliders and full-size 330 style sliders     

A study on the efficacy of flow mitigation devices in hard disk drives

We report on large eddy simulations of the turbulent flow of air in hard disk drives (HDDs) using a commercial CFD code. In particular, we focus on HDD casings in which flow-induced vibrations are reportedly reduced by small geometrical modifications. The modifications investigated are M1: a blocking plate situated between the disks, M2: a spoiler (or deflector) located behind (downstream of) the actuator arm, and M3: a similar deflector upstream of the arm. We observed that M1, M2, and M3 significantly modify the mean flow patterns in the drives. M1 reduces velocity magnitudes in most parts of the drive, the modification of M2 causes How reversal in regions close to the hub, while M3 causes the shedding of vortices upstream of the actuator arm. Our analysis points to M1 as the best candidate for mitigating the effects of turbulent airflow. This is because M1 is more effective in reducing the root-mean-square velocity fluctuations near the suspension. M1 is also more effective in reducing the pressure fluctuations near the base-plate and suspension region. This reduction, however, is at the cost of approximately 20% higher windage. Finally, we note that M3 has the adverse effects of increasing velocity and pressure fluctuations and hence is not the ideal candidate for mitigating airflow effects, among the modifications considered here.     

Impact force measurement of an actuator arm of a hard disk drive

Impact forces of an actuator arm of a hard disk drive (HDD) are measured by means of modifying the levitation mass method (LMM) whose basic concept was proposed by the first author. In the LMM, force is measured as the inertial force of a mass levitated with sufficiently small friction using an aerostatic linear bearing. The Doppler frequency shift of the laser beam reflecting from the mass is accurately calculated from the waveform recorded using a digitizer. The velocity, the position, the acceleration and the inertial force of the mass are calculated from the measured time-varying Doppler frequency shift. In the experiments, the mechanical response of an actuator arm of a HDD against an impact load and the inertial force of the actuator arm in free oscillation are measured. The importance and the problems concerning the present knowledge on the impact force of an actuator arm of a HDD are also discussed.     

Fly Height Measurement Based on Phase Comparison Michelson Interferometry Using Low-Coherence Light Source

We present a method for measuring head fly height in hard disk drives using phase comparison Michelson interferometry (PCMI), which compares the phases of two interference fringe patterns formed respectively on the inner surface of a glass disk and the air-bearing surface of a fly head slider through the glass disk. To suppress interference noise and further enhance measurement accuracy, we adopted a low-coherence light source as an illumination source to replace a high-coherence laser. The captured fringe images enabled us to extract ridge lines much more accurately than with a laser. We compared our measurements in the sub-10 nm spacing range with calculations and found excellent agreement.     

硬盘加载/卸载过程磁头悬臂接触面的动力学仿真

针对于微硬盘驱动器斜坡加载/卸载(Load/Unload)过程,对微硬盘的磁头/盘界面及磁头悬臂与支承件接触面的摩擦学及动力学特性进行建模,对加载及卸载过程中的浮动块、悬臂的动态特性和斜坡与提升臂的接触特性进行了仿真,分析了加载与卸载速度及斜坡倾角对斜坡与提升臂的接触变形与接触力及接触面间的摩擦能耗特性的影响。研究结果表明,加载及卸载速度的增加将导致斜坡接触面上的接触力及摩擦力的增大,增大斜坡倾角将使接触面摩擦能量损耗增加。研究结果对微硬盘磁头/盘界面的改进和加载/卸载技术的进一步应用提供依据。     

Voltage pulsing for localized clearance measurement

Mechanical clearance is a critical parameter of the head/disk interface (HDI) for sliders flying in close proximity in hard disk drives. It depends on a variety of HDI parameters, such as the air-bearing design, disk roughness, and the disk lubricant. An experimental setup was developed to determine the mechanical clearance locally at any location on the disk. Voltage pulsing is used to add an additional force component to the complex HDI equilibrium. The applied force is attractive in character and therefore reduces the mechanical spacing between the slider and the disk. If the magnitude of the applied voltage is changed, a slight contact between the slider and the disk can be realized. Simultaneous measurement of the induced slider displacement using a laser vibrometer allows the quantification of the mechanical clearance. The effect of disk roughness and microwaviness on the mechanical clearance is investigated.     

磁头面内运动的测量与抑制

为了进一步提高读写头的定位精度,实验研究了磁头稳定飞行时的面内运动.在将主轴系统与磁头加载机构分离开后,利用激光多普勒测振仪测量磁头面内运动和盘片偏摆大小,通过幅值谱对测量信号进行频谱分析,讨论了面内运动与盘片偏摆的相关性.结果表明,磁头面内运动主要是由盘片偏摆引起的,其运动幅值与盘片偏摆的幅度呈正相关,当偏摆幅值从6.9624μm减小到3.3330μm时,偏离磁道方向的运动振幅从140.6nm降低到59.1nm.通过抑制盘片的偏摆幅度可以有效地抑制磁头偏离磁道方向的运动幅值,有助于提高磁头定位的精确度.     

A novel laser Doppler velocimeter

The idea of using a reference-beam laser Doppler velocimeter (LDV) to measure the velocity of a vehicle for an inertial navigation system is proposed. In order to reduce the measurement error produced by the vehicle's jolt, a novel laser Doppler velocimeter (LDV) based on the Janus configuration is presented. The system mounted at the bottom of the vehicle is composed of two single reference-beam subsystems, one of the laser probes looks forward and the other looks backward. Each of the subsystems transmits a laser at 532?nm with the same inclination angle and detects its own Doppler frequency from the scattered light. The pitch angle of the vehicle is calculated by the two detected Doppler frequencies and the inclination angle, and then the measured velocity of the vehicle is compensated. The results of the theory analysis and experiments show that the vehicle's jolt affects conventional reference-beam LDV strongly. LDV with a Janus configuration is insensitive to the inclination angle, and its measurement accuracy is much better than that of a conventional reference-beam LDV. Comparing with the DZL-1 electronic speedometer, the measurement mean error is less than 0.9%, so it is suitable to offer the parameter of velocity for a vehicle self-contained inertial navigation system.     

Modeling and simulation of hard-particle interaction in head/disk interfaces

A hard particle interacting with a slider and a disk in the head/disk interface of hard-disk drives can produce a scratch on the disk and result in data loss. Our work emphasizes modeling and simulation of slider, particle, and disk interactions, scratch generating mechanisms, and linking of scratch to interface design parameters. Two models are presented. The Monte Carlo method was used to calculate scratch probabilities and parametric studies were performed to determine the effect on scratch probability of various parameters, such as friction coefficients, particle mean size, slider wall angles, the ion-milling (IM) etch depth from the air-bearing surface (ABS) of the slider, and the efficacy of additional ABS structures acting as particle shields. Simulation results show that the friction coefficient is a dominant parameter. If the disk friction coefficient is smaller than the slider friction coefficient, scratches are not produced. Using shallow IM depths is an effective and practical way to reduce the scratch probability. Experimental results are very close to the simulation predictions.     

Use of an upwind finite volume method to solve the air bearing problem of hard disk drives

In this paper we present an upwind higher order finite volume numerical scheme over unstructured triangular mesh to solve the slider air bearing problem of hard disk drives. The scheme is nodal based, which uses the median dual as the control volume. The convection part of the generalized Reynolds equation is modeled by the flux difference splitting technique. Higher order accuracy in space is achieved by a linear reconstruction technique with a flux limiting technique incorporated to prevent oscillation in the high-pressure gradient regions. A linear Galerkin method is used to discretize the diffusion terms. In addition, a non-nested multi-grid iteration technique is used to increase the convergence rate. Finally, the steady state flying attitude of a slider subject to pre-applied suspension force and torques is obtained by a Quasi–Newton iteration method, and the results of this scheme are compared with two other schemes. Received 16 July 2000     

Laser Doppler Velocimetry for Joint Measurements of Acoustic and Mean Flow Velocities: LMS-Based Algorithm and CRB Calculation

This paper presents a least-mean-square (LMS) algorithm for the joint estimation of acoustic and mean flow velocities from laser Doppler velocimetry (LDV) measurements. The usual algorithms used for measuring with LDV purely acoustic velocity or mean flow velocity may not be used when the acoustic field is disturbed by a mean flow component. The LMS-based algorithm allows accurate estimations of both acoustic and mean flow velocities. The Cramer-Rao bound (CRB) of the associated problem is determined. The variance of the estimators of both acoustic and mean flow velocities is also given. Simulation results of this algorithm are compared with the CRB, and the comparison leads us to validate this estimator.     

Air bearing parameter effects on take-off velocity

An air bearing simulation analysis of a taper flat slider is presented for predicting the effects of slider/suspension parameters on the take-off velocity of the slider. These take-off velocities are determined through the use of a computer program that solves the Reynolds gas lubrication equation for the pressure distribution in the air bearing. Of the many parameters considered, crown (curvature along the length of the air bearing surface) has the most impact on take-off velocity, and therefore on the sliding distance that the slider is in contact with the disk during a contact start/stop operation. The nonlinear relationship between crown and take-off velocity is shown and discussed. Several improved take-off designs are presented     

Real-time measurements of spatial velocity distribution with a laser Doppler imaging system

A new laser Doppler imaging system with a TV camera has been constructed, which brightly displays 1-D velocity distribution on a monitor. Some characteristics of this system have been experimentally investigated from measurements of simple velocity distribution on a constantly rotating ground glass disk. From an adaptation to fluid flow, it has been shown that the measurements of spatial velocity distribution can be achieved in real time.     

Monochromatic heterodyne fiber-optic profile sensor for spatially resolved velocity measurements with frequency division multiplexing

Investigating shear flows is important in technical applications as well as in fundamental research. Velocity measurements with high spatial resolution are necessary. Laser Doppler anemometry allows nonintrusive precise measurements, but the spatial resolution is limited by the size of the measurement volume to approximately 50 microm. A new laser Doppler profile sensor is proposed, enabling determination of the velocity profile inside the measurement volume. Two fringe systems with contrary fringe spacing gradients are generated to determine the position as well as the velocity of passing tracer particles. Physically discriminating between the two measuring channels is done by a frequency-division-multiplexing technique with acousto-optic modulators. A frequency-doubled Nd:YAG laser and a fiber-optic measuring head were employed, resulting in a portable and flexible sensor. In the center of the measurement volume of approximately 1-mm length, a spatial resolution of approximately 5 microm was obtained. Spatially resolved measurements of the Blasius velocity profile are presented. Small velocities as low as 3 cm/s are measured. The sensor is applied in a wind tunnel to determine the wall shear stress of a boundary layer flow. All measurement results show good agreement with the theoretical prediction.     

三叉杆式万向联轴器的平均传动效率分析

为获得三叉杆式万向联轴器的平均传动效率，分析了三叉杆式万向联轴器输入轴上滑块的运动规律。首先，在滑块中心建立辅助坐标系，求出滑块在它与导向槽接触面中心相对导向槽的滑动速度以及滑块中心相对导向槽的滑动速度；其次，利用摩擦功率损失与速度的关系式，推导出三叉杆式万向联轴器的瞬时传动效率；最后，通过积分得出三叉杆式万向联轴器平均传动效率的表达式。通过分析发现：三叉杆式万向联轴器的平均传动效率与摩擦系数、输入轴与输出轴偏转角、叉杆半径和回转半径有关，平均传动效率随着摩擦系数、偏转角、叉杆半径的增大而减小，随着回转半径的增大而增大。该结果可为三叉杆式万向联轴器的设计和应用提供重要理论依据。     

Noninvasive Vibration Diagnostics to Detect Head–Disk Contact in Hard Disk Drives

Reliability of modern hard disk drives (HDDs) is a major concern especially with decreasing head-to-disk spacing in order to attain higher recording densities. One of the reasons for HDD failure is the occurrence of intermittent or sustained head–disk contacts. It is therefore important to be able to detect such head–disk contacts early on so that backup safety schemes can be implemented before catastrophic HDD failure and permanent data loss. In the study reported here, we used noninvasive vibration diagnostics and multiaxial vibration measurements to assess the condition and functionality of HDDs. We chose a miniature triaxial accelerometer for the diagnostic study, given that it is relatively inexpensive and easy to use. We measured contact vibration, induced by a scratch on the top surface of the rotating disk, on the outside of the HDD by attaching an accelerometer on the actuator shaft. We found that contact vibrations between the head and disk inside the HDD can reliably be measured on the outside of the HDD with an accelerometer. We also found that a miniature accelerometer does not cause significant mass loading, as verified by noncontacting laser vibrometry.     

Efficiency Measurement of Water Jet Orifices by a Novel Electrooptical Technique

The problem of the efficiency evaluation of water jet nozzles is dealt with in this paper, and a novel laser Doppler velocimetry (LDV) technique is proposed to acquire high-speed water velocity at the exit of the cutting head nozzles. A standard LDV technique has been implemented for the first experimental trials, with the aim of validating the analytical results with directly measured velocity values. The novel LDV technique has been proposed, with the outlying of the critical aspects to be taken into account for future developments and improvements in measurement dynamic range, resolution, and accuracy.     

Analog sensor design proposal for laser Doppler velocimetry

Laser Doppler velocimetry (LDV) has been widely used for many years in fluid mechanics to measure particle velocity. However, in most applications, i.e., in industrial processes, such a system is often too expensive. This paper discusses a technique based on the use of an analog phase-locked loop and an analog integrator system for processing laser Doppler velocimeter data to infer particle velocity. This method appears to be suitable for designing low-cost integrated LDV sensors. A SIMULINK program has been written in order to validate the method for velocities in the 10-80 mm/s range. Finally, the performance of the estimator is illustrated by Monte-Carlo simulations obtained from synthesized Doppler signal.     

低压流场速度测量的不确定度分析

激光多普勒测速技术被广泛应用于流速测量领域。在搭建好的真空气流检测实验平台基础上,运用二维激光多普勒测速仪对真空腔室进行流速测量;为了减小测速系统的测量误差,对真空腔室流速测量不确定度的来源及评定进行了分析与说明,得到了真空腔室流速测量的合成相对标准不确定度和相对扩展不确定度。研究表明:LDV重复性测速和进气量估读引起的不确定度较大,而LDV系统误差和其它因素引起的不确定度较小。研究结果对后期真空腔室流速测量有一定指导意义。     

Setup and Test of a Laser Doppler Velocimeter for Investigations of Flow Behaviour of Polymer Melts

The flow behaviour of a low-density polyethylene melt is investigated in a specifically developed flow channel by means of Laser Doppler Velocimetry (LDV). The used flow channel is a slit die with a planar contraction of 14:1. The investigation of the velocity fields was performed in the steady state of flow. The optics of the LDV system as well as the used frequency analyser proved to be reliable for measurements of velocities down to 250μm/s. By adding TiO₂ tracer particles to the pellets the signal quality as well as the signal frequency were improved. It is demonstrated that the Laser Doppler Velocimeter is suited to detect velocities of polymer melts with an error of a few per cent by comparing the measured volume flow rate to the directly determined mass flow rate. Using simple fluid mechanics the viscosity function is obtained by measuring only one velocity profile within the fully developed flow in the slit die. Over a wide range of shear rates the viscosity function obtained via LDV measurement corresponds with the viscosity function which was determined by the classical mass-flow-rate method. Both resulting viscosity functions were additionally checked by performing measurements with a capillary rheometer. The LDV setup described in this paper is a powerful experimental tool to investigate the flow behaviour of polymer melts. Its accuracy and the high spatial and temporal resolution opens a way to get more quantitative insight into the flow of polymer melts and to check the validity of model calculations. This revised version was published online in August 2006 with corrections to the Cover Date.