Dynamic microwaviness measurements of super smooth disk media used in magnetic hard disk drives

Recent technological advances in magnetic storage suggest the feasibility of extremely high-density magnetic recording up to 1 terabit per square inch (1 Tbit=10¹² bits) areal densities. Modelling indicates that approximately 3 nanometers (nm) of physical head-disk spacing is required for such high recording densities. When the recording slider is flying at such ultra low spacing over a high-speed rotating disk, it is experiencing disturbances from various different sources and of a wide frequency range. These disturbances may cause the recording slider to vibrate significantly, a condition that is known as fly height modulation (FHM), which may result in data loss. A significant source of excitation is from the surface irregularities of the rotating disk and is termed dynamic microwaviness. The term dynamic microwaviness has been introduced recently to differentiate from regular topographical features that are measured statically. In this paper, the procedure for making reliable dynamic microwaviness measurements of disk media used in hard disk drive (HDD) systems is described. Furthermore, such measurements are performed on different super smooth magnetic disks that are intended for extremely high recording densities using non-contact laser vibrometry. The root-cause of the dynamic microwaviness is investigated by measuring disk topographical features under static conditions and the interaction with system dynamics. It is found that dynamic microwaviness is primarily due to topographical features of spatial wavelengths ranging from 58.8 to 250 μm, and secondarily due to system dynamic effects.     

Analysis and optimization of dynamic response of air bearing sliders to disk waviness

Flying stability has been becoming more critical for air bearing sliders with extremely low flying height (FH). Therefore, the effects of disk waviness on flying height modulation (FHM) cannot be neglected. This paper presents an analytical study on the mechanism of FHM of air bearing sliders due to disk waviness, and a design optimization for increasing waviness following ability of sliders. An analytical three-degree-of-freedom (3-DOF) model is developed, where the air bearings are modeled as six lumped linear springs and dampers. The purpose of this model is to develop a quantitative understanding of how air bearing sliders respond to disk waviness. The dynamic characteristics of the slider-air bearing system are then analyzed, and the closed-form frequency resonance function (FRF) of FHM to disk waviness is derived. The impact of disk surface features and the positions of the trailing pad, the side pads, the leading pads and the negative pressure center on FHM are also investigated using parametric analysis. The analysis results show that the improvement of the roll-off characteristics of the disk surface waviness can also decrease the FHM. In addition, shortening the distance between the trailing pad pressure center and the head position, moving backward the side pads and leading pads and forward the negative pressure center can increase waviness following ability of the slider. Finally, an air bearing slider is designed according to the proposed design strategies for reducing the FHM due to disk waviness.     

Investigation of Lubricant Transfer between Slider and Disk Using Molecular Dynamics Simulation

A model for lubricant transfer from a rotating magnetic recording disk to a magnetic recording slider is developed using molecular dynamics simulation. The combined effect of disk velocity and local air-bearing pressure changes on lubricant transfer is investigated. The simulation results indicate that local pressure changes in the absence of disk circumferential velocity can cause lubricant redistribution on the disk, while local pressure changes on a moving disk can result in lubricant transfer from the disk to the slider. The amount of lubricant transferred from the disk to the slider and the lubricant buildup on the disk are a function of the local pressure change and disk velocity. The amount of lubricant transferred from the disk to the slider and the height of lubricant buildup on the disk surface decrease with an increase in the number of functional groups of the disk, a decrease in the local pressure change, and a decrease in the disk circumferential velocity.     

Interface technology of ultra-low flying height and highly stable head–disk interface for perpendicular magnetic recording

This paper reports authors’ efforts in slider and interface technologies with extremely small and very high stability head–disk spacing. The dual shallow step strategy is proposed in the femto form-factor slider design. It is found that the dual shallow step design is very effective in reducing flying height modulation (FHM) caused by disk waviness and enhancing the cooling effects on the read/write elements. A simple geometric model is built to explain the schematic of the improvement in FHM.     

Dynamics in the Bridged State of a Magnetic Recording Slider

A novel region of tribological interaction is explored by inducing near contact between the magnetic recording slider and disk. In this study, we performed frictional measurements over a wide range of subambient air pressure and disk rotation rate. Since the slider is supported over the disk by an air bearing, it has been found that cycling from ambient to subambient and then back up to ambient pressure over several minutes of time forms a frictional hysteresis loop. The high-friction branch of the loop, referred to as the bridged state, is characterized by an average frictional displacement and resonant vibration of the suspension mount assembly. The bridged state is currently employed for accelerated wear testing of magnetic slider/disk/lubricant systems. Future magnetic recording systems designed to operate at increasingly lower physical spacing will need to take into account these frictional forces which accompany the incipient contact between the lubricated disk and slider with finite surface roughness. A single degree of freedom model is solved to determine the equivalent dynamic friction force on the slider as an impulse series with random impulse frequency and amplitude from the measured frictional displacement in the bridged state. The mean slider-disk spacing in the bridged state is derived from the experimental friction force, the spacing probability density function, and the adhesion stress from the Lifshitz model for dispersion interaction energy.     

Experimental dynamic characterizations and modelling of disk vibrations for HDDs

Currently, the rotational speed of spindle motors in HDDs (Hard-Disk Drives) are increasing to improve high data throughput and decrease rotational latency for ultra-high data transfer rates. However, the disk platters are excited to vibrate at their natural frequencies due to higher air-flow excitation as well as eccentricities and imbalances in the disk-spindle assembly. These factors contribute directly to TMR (Track Mis-Registration) which limits achievable high recording density essential for future mobile HDDs. In this paper, the natural mode shapes of an annular disk mounted on a spindle motor used in current HDDs are characterized using FEM (Finite Element Methods) analysis and verified with SLDV (Scanning Laser Doppler Vibrometer) measurements. The identified vibration frequencies and amplitudes of the disk ODS (Operating Deflection Shapes) at corresponding disk mode shapes are modelled as repeatable disturbance components for servo compensation in HDDs. Our experimental results show that the SLDV measurements are accurate in capturing static disk mode shapes without the need for intricate air-flow aero-elastic models, and the proposed disk ODS vibration model correlates well with experimental measurements from a LDV.     

Numerical simulation of slider dynamics during slider–disk contact

In this paper, the dynamic behavior of pico slider in contact with the disk was calculated. The analysis model consists of a simplified suspension model, an air bearing model, and a slider–disk contact model. The contact model consists of two elements. One is surface roughness model measured by Atomic Force Microscope (AFM) and the other is micro-waviness model. The dynamic behaviors of the tri-pad slider are calculated at several rotation speeds to investigate slider vibration modes during slider–disk contact. The slider oscillation frequency depends on the rotation speed and it saturates about twice as much as eigen frequency of air bearing pitch mode.     

Ultra-low-flying-height design from the viewpoint of contact vibration

The design of a head-disk interface for ultra-low flying height has been studied from the viewpoint of contact vibration. It is known that a super-smooth disk is necessary for a slider to fly at an ultra-low flying height; however, such a disk increases the friction force, which potentially increases the vibration of the slider. To solve this problem, the head-disk interface must be optimized to reduce this increased vibration. It has been shown that a large pitch angle and center-pad-mounted read/write elements have advantages in terms of slider/disk contact. It has also been found that a micro-texture on the air bearing surface can prevent contact vibration. Moreover, a frequency-shift-damping slider was found to damp the vibration effectively. To further investigate these findings, numerical simulation and modeling of slider dynamics during contact have been performed. Their results revealed two zones of contact vibration: a stable zone and an unstable zone.     

斜动子与塔形定子构成的单驱双动超声电机

针对两相超声电机对模态频率一致性要求高及单相超声电机难于实现双向运动等问题,提出了一种单相驱动双向运动斜动子塔形直线超声电机。动子相对于塔形定子倾斜安装,利用塔形定子的面内对称模态或面内弯曲模态为工作模态,通过切换工作模态改变定子驱动足运动轨迹相对于动子的倾斜方向,实现动子正、反向运动。在分析电机工作原理及设计原则的基础上,推导了电机运行的导轨倾角适用范围,对设计制作的原理样机进行了模态实验和机械特性测试。实验表明,在导轨倾斜角为35°,激励电压为500 V,预压为力4.5 N的条件下,面内对称振动模态工作时的最大空载速度为79 mm/s,最大输出力为0.5 N;面内弯曲模态工作时的最大空载速度为756 mm/s,最大输出力为0.8 N。     

Adsorbed Water Film and Heat Conduction from Disk to Slider in Heat-Assisted Magnetic Recording

In heat-assisted magnetic recording (HAMR), a tiny area of magnetic recording media has to be heated up to a high temperature with laser to lower the coercivity temporarily for information to be written on the area. In a humid environment, some of the water vapor molecules adsorb on the disk surface to form a water film. In HAMR writing, the adsorbed water film on the disk surface will desorb instantly from the high-temperature laser heating area to become high-temperature high-pressure water vapor. The water vapor molecules will transfer extra heat from the high-temperature laser heating area on the disk surface to the slider, which makes the temperature of the slider surface higher in a humid environment than that in dry air. The heat transfer increases dramatically with relative humidity and with the decrease in slider–disk spacing.     

Rotordynamic analysis and experimental validation of a high speed induction motor made by copper die casting process

A copper die casting induction motor can obtain an energy saving effect of about 2–3% compared to previous aluminum die casing induction motors. In addition, copper die casting motors can reduce the size of motors and reduce material costs. The critical speed and unbalance response of high speed machines need to be verified by rotordynamic analysis for dynamic stability of the rotors. The critical speed analysis, harmonic analysis and transient analysis by unbalance are performed for dynamic stability. The unbalance analysis results are compared with the experiment considering allowable vibration displacement (API 611) and balancing grade (ISO 1940-1). This paper dealt with the design, analysis and experimental validation of a high speed induction motor. The dynamic stability of the prototype is verified successfully, and two experimental methods by ISO 10816-3 are suitable and reliable for the allowable vibration evaluation of rotating machine.     

电主轴的安装对其动态精度的影响

电主轴前、后端的动态精度对工件表面的加工质量和精度很重要,本文主要研究了电主轴的安装对其动态精度的影响。实验中采集了电主轴前、后端的振动信号,并对其频谱图加以分析,找出了影响电主轴前、后端振动的主要因素,通过实验研究提出了电主轴尾部安装的方法,实验表明:电主轴前、后端振动加速度幅值较小,振动平稳。验证了改进的安装方法是正确的,保证了电主轴的动态精度。     

Identification Method of Dynamic Coefficients of Fluid Film Bearings for HDD Spindle Motors

Fluid film bearings are widely used as support elements of rotating shaft for HDD （hard disk drive） spindle motors. Recently, the opportunity for the HDD spindle motors exposed to external vibration has been increasing because the HDDs are used for various information related equipments such as mobile PCs, car navigation systems. Hence, the rotating shaft has a possibility to come in contact with the bearing and it causes wear or seizure to the bearing surface. In order to avoid the problems, it is extremely important to enhance the dynamic characteristics of the fluid film bearings for spindles. However, verification from both theory and experiment of dynamic characteristics such as spring coefficients and damping coefficients is rare and few. In this paper, the bearing vibration characteristics when the HDD spindle is oscillated are investigated theoretically and experimentally. And then the identification method ofoil film coefficients of fluid film bearing spindles is described.     

Investigation of Head/Disk Contacts in Helium–Air Gas Mixtures

The shock response of a pico-type magnetic recording slider in different helium–air gas mixtures is investigated numerically. A finite element-based air bearing simulator and a slider/disk contact model including van der Waals and friction forces are coupled to determine the contact characteristics between slider and disk. The minimum flying height and the maximum contact force are studied as a function of helium percentage and disk velocity. The results show that the dynamic performance of the slider is not affected substantially as long as the helium percentage is <50 % but is increasingly more affected if the helium percentage becomes larger than 50 %.     

Air Bearing Spindle Motor for Hard Disk Drives

This article discusses the possibility of using air-bearing spindle motors to replace hydrodynamic-bearing spindle motors used in hard disk drives through comparing the advantages and disadvantages of air-bearing spindle motors with those of hydrodynamic-bearing spindle motors. Then the dynamic characteristics of the air-bearing system are analyzed and optimum parameters are proposed. The prototypes based on the obtained parameters are fabricated and tested. The test result is presented and compared with the numerical prediction; good agreement is observed.     

基于音圈电机作动器的主轴振动LQG控制研究

为克服被动动力吸振器偏离最优状态时抑振效果严重降低的不足,针对动刚度较低的铣削加工机床的主轴振动控制,设计了一种混合动力吸振器的主动振动控制系统。该吸振器以音圈电机为作动器,以位移和速度作为状态反馈信号,直接对铣削刀具施加控制力,从而达到抑制主轴振动的目的。在分析音圈电机驱动特性的基础上,建立了两自由度的铣刀与主轴振动力学模型,推导出系统的状态方程,并采用线性二次高斯控制（LQG）最优控制方法对振动控制模型进行了仿真,最后在实际的数控雕铣机床上进行了相关的铣削主轴振动控制实验。结果表明,该方法能有效降低主轴切削振动,基于振动位移反馈的抑振效果优于基于振动速度反馈的抑振效果,但基于振动速度反馈能更有效地抑制高频的共振峰值,实际系统应根据振动反馈信号实时调整主动控制参数。     

ENGINEERING CHARACTERISTICS AND ITS MECHANISM EXPLANATION OF VIBRATORY SYNCHRONIZATION TRANSMISSION

Vibratory synchronization transmission (VST) is a kind of special physical phenomenon ininertia vibration mechanical systems. For an inertia vibration mechancal system driven by one pair ofmotors runs in step, even the power supply of one motor is cut off, the motor can continue to keeprotating state under the vibration exciting of the machine body driven by only one other motor. And itsrotating frequency will be the same as that of the other one. The transient process of this wonderfulphysical phenomenon has not been described quantitatively according to current-existing mechanicalmodels. On the basis of investigation of the engineering characteristics of VST, a mechanical and elec-trical coupling mathematical model of a two-shaft inertia vibration machine is established. With thismodel, the transient process of VST is recurred quantitatively and successfully, and a reasonable ex-planation is given.     

Some considerations of slideway friction characteristics by observing stick-slip vibration

It is important to clarify the frictional characteristics of a slideway and to prevent unstable vibration, such as stick-slip vibration, for the improvement of kinematic performance and for precise positioning. In this research, the relations among the dynamic friction characteristics, the pitching motion or the floating up of the slider, the surface roughness of a slideway, and the lubricant property are investigated experimentally. As a result, some points necessary for the kinematic performance improvement of the slider are clarified.     

Stability of Gas Bearing Sliders for Large Bearing Number: Convective Instability of the Tapered Slider©

The dynamics and stability of tapered air bearing sliders used for computer hard disk drive magnetic recording heads is examined using analytical methods. Lubrication theory is applied to determine the lift on the slider from the Reynolds equation in the limit of large bearing number. The dynamics of the slider are given by a nonlinear integro-differential equation. Linear stability analysis of this model yields valuable insight into the behavior of the slider. Most significantly, it is found that convective effects can not be neglected and yield either damping or instability depending on the slider configuration. This analysis is also applied to determine the response of the slider motion due to deviations in the disk surface.     

采用模拟退火算法对皮米磁头进行形状优化

采用优化设计方法优化一种皮米磁头的形状,能够降低磁头飞高,提高硬盘的存储密度及磁头的飞行稳定性。优化设计以磁头俯仰角和形状尺寸为约束,以磁头在磁盘半径方向内侧、中间、外侧稳定飞行时的飞行高度和侧倾角与优化目标值间的波动最小为优化目标建立优化模型,采用模拟退火算法对优化模型求解。优化结果表明,优化后磁头的飞浮高度可从7 nm降低到5 nm,而且磁头寻轨时磁头的稳态飞行波动得到明显改善,能够满足工程上对飞高的波动性要求。最后,比较了提高磁盘转速后初始磁头和优化磁头的稳态飞行特性,结果表明磁盘转速对优化结果有影响,改变磁盘转速建立新的优化模型可以得到适合新条件的优化磁头。