On the relationship of natural frequency,damping, and feedforward acceleration with transient vibrations of track-seek control

The relationship between natural frequency, damping, the waveform of a feedforward acceleration input, and the transient vibrations of track-seek control is derived theoretically and a target for vibration design of the head actuator of a hard disk drive (HDD) is discussed from the viewpoint of the transient vibrations. When the acceleration feedforward input of the track-seek control is expressed as a polynomial function of time, the transient response of a single degree of freedom system is solved by applying integral by parts to the Duhamel’s integral repeatedly. The results show the transient vibrations are generated at the initial and terminal times of the input and their amplitude depends on the lowest orders of non-zero derivatives of the polynomial. They also show that increasing the damping ratio effectively reduces the transient vibrations for fourth and sixth order polynomials but not for fifth and seventh order polynomials. The results were confirmed by performing a continuous time simulation and analyzing the shock response spectrum. Also discussed is the relationship between the transient vibrations and the seek time and track pitch, which are key parameters associated with the data access time and the HDD capacity. This relationship indicated the requirements needed for vibration design of the head actuator to increase HDD performance and capacity.     

A study of non-operational dynamic responses of disk in 3.5 in. hard disk drive to impact load

Hard disk drives have to be designed to sustain operational and non-operational shock. There are many analytical models and numerical schemes proposed and many experiments conducted for analyzing the transient impact responses of hard disk drives. The existing researches have been focused on the slider-suspension responses at the head-disk interface in which the linear models have been used and the effects of spindle motor have been ignored. In this study, the complex vibrations of disk of 3.5 in. hard disk drive (HDD) under shock are experimentally investigated. The hammer impact test and linear drop test are conducted for the HDD to study the effect of shock on the disk responses. The results show that the nonlinear rock modes substantially contribute to the vibrations of disk when HDD is under shock impact. The nonlinear properties of the disk responses and the mode damping ratio are evaluated by using empirical mode decomposition approach.     

Numerical analysis of off-track vibration of head gimbal assembly in hard disk drive caused by the airflow

The problem of flow-induced vibration of head gimbal assembly (HGA) in hard disk drive (HDD) is analyzed by means of numerical models. Flow field is calculated in a fraction of physical domain called extended wedge-like domain. For this purpose, appropriate inlet and outlet boundary conditions are specified. The aerodynamic disturbances generated by the slider are studied by performing flow calculations for the cases of with and without slider. It is observed that the slider blocks the airflow causing a high-pressure region in the proximity of the leading surface of the slider. In addition, significant vortical structures are found being generated by the side surfaces of the slider. A finite element model is developed for calculating the response of HGA to the aerodynamic excitations. It is found that the flow disturbances generated by the slider play a significant role in the off-track vibration of the HGA.     

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.     

A novel contact slider/gimbal to raise recording density in flexible disk drives

 We developed the soft and full-contact head/gimbal assembly (HGA) for card-size flexible disk drive. We used a 150 μg slider and a flexible beam of BeCu alloy to realize soft contact. We designed our HGA using FEM simulation and optimized the load force, the load pressure, and the stress on the beam. We report the mechanical characteristics, read-write signals, and the vibration of the HGA. A new type loading/unloading mechanism was developed for the removable flexible medium, and confirmed the reliability of the loading/unloading mechanism by the deformation of the HGA and the loaded medium. Received: 5 July 2001/Accepted: 1 November 2001     

A frequency-shaping methodology for discrete-time sliding mode control

This paper proposes a frequency shaping methodology for performance enhancement of discrete-time sliding mode control for systems with narrow band disturbances. This approach augments the sliding mode surface as a dynamic feedback system and designs an ‘optimal’ shaping filter in the feedback loop based on H-infinity synthesis, which assures both the stability and the desired frequency-response characteristics of the sliding mode surface. Furthermore, this frequency-shaping technique is advanced to be parameter-dependent to improve the efficiency and flexibility. The methodology enables the design of sliding mode control in frequency domain, which is critically important for high-precision systems that are subjected to high-frequency disturbances. The proposed frequency-shaped sliding mode control is applied onto a hard disk drive in the presence of vibrations with multiple peak frequencies; effective vibration suppression is demonstrated through comprehensive simulation study.     

HAMR HDD shock responses

Heat assisted magnetic recording (HAMR) and slim mobile hard disk drives (HDD) are being developed parallelly to maintain cost advantage over the solid state drive. Operational shock and non-operational shock capabilities are seriously challenged for the slim HDDs due to reduced stiffness (thickness). It is worse for slim HAMR drives due to additional laser diode (LD) and other necessities being added on slider. Shock tests are part of the key performance matrices that must be passed in HDD reliability tests, and the concerns for HAMR mobile drives are, (1) slider lift-off G-level degradation during op-shock, and (2) LD back-to-back hitting during non-operational shock. We studied a few potential HAMR HGA designs, also analyzed a design that improves drive op-shock performances.     

Feedback control of one-link flexible manipulator with gear train

This article reports experimental results in control of a one-link flexible manipulator. A d.c. drive with gear train rotates the link with a tip-mass in the horizontal plane. A “stiff” hardware servo-system tracks the commanded drive velocity. We describe the experimental setup and develop a dynamic model for the one-mode flexible vibration control. We design and experimentally test controllers for active damping of the vibrations and stabilization of the link-tip position. Results for continuous control implemented with an analog computer and a sampled-data, digital-computer control are reported. A high control performance is achieved despite the gear-train friction influence. In addition, sampled-data digital controllers for tracking control of the link reference motion are designed and tested.     

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.     

Differential evolution based high-order peak filter design with application to compensation of contact-induced vibration in HDD servo systems

Sensitivity loop shaping using add-on peak filters is a simple and effective method to reject narrow-band disturbances in hard disk drive (HDD) servo systems. The parallel peak filter is introduced to provide high-gain magnitude in the concerned frequency range of open-loop transfer function. Different from almost all the known peak filters that possess second-order structures, we explore in this paper how high-order peak filters can be designed to improve the loop shaping performance. The main idea is to replace some of the constant coefficients of common second-order peak filter by frequency-related transfer functions, and then differential evolution (DE) algorithm is adopted to perform optimal design. We creatively introduce chromosome coding and fitness function design, which are original and the key steps that lead to the success of DE applications in control system design. In other words, DE is modified to achieve a novel design for hard disk drive control. Owing to the remarkable searching ability of DE, the expected shape of sensitivity function can be achieved by incorporating the resultant high-order peak filter in parallel with baseline feedback controller. As a result, a seventh-order peak filter is designed to compensate for contact-induced vibration in a high-density HDD servo system, where the benefits of high-order filter are clearly demonstrated.     

基于FPGA的新型虚拟逻辑分析仪的设计

提出了一种基于FPGA的虚拟逻辑分析仪的设计.该系统对采集到的模拟或数字信号进行存储、处理和逻辑分析.通过FPGA控制数据单次或连续采集、缓冲,通过PCI总线将缓冲区数据转移到硬盘管理卡,由硬盘管理卡将数据存入海量硬盘.     

Microdrive operational and non-operational shock and vibration testing

Commercially available microdrives were tested using linear and rotary shock and vibration testing equipment. Several microdrives designed with different slider and disk configurations were tested to track hard and soft errors as well as head/disk failures. The shock amplitude for operational and non-operational shock was gradually increased to determine the maximum shock that a microdrive could withstand before failure. After failure, the microdrive was examined to determine whether a mechanical failure occurred or whether the failure was due to a magnetic hard/soft error. During a shock event, the displacement and frequency of the vibrations of the microdrive were examined at various locations on the arm and suspension. A scanning laser Doppler vibrometer (LDV) was also used to determine the amplitude and frequency of the vibrations of the front cover and to investigate whether these vibrations contribute to failure of the head/disk interface. A finite element model of the disk drive was also developed to simulate the shock response. The maximum amplitude for failure to occur was determined numerically for operational and non-operational conditions using a pulse width of 2 ms. A comparison of experimental and numerical results is given.     

A design based on final-state control for residual vibrationless seeking in hard disk drives

A new design method - based on a final-state control (FSC) - for short-span seeking in a hard-disk drive (HDD) has been developed. The short-span seeking is performed by two-degree-of-freedom (2-DOF) control, which uses a feedforward (FF) control input along with a reference trajectory. The design method can directly generate the FF control input, whose derivative at a specified order is minimized and whose power spectrum amplitude is reduced at a specified frequency. The residual vibration caused by mechanical resonance can therefore be reduced by the generated FF control input. Test with a 2.5-in form-factor HDD experimentally confirmed that the developed seeking control significantly reduces the residual vibration in a HDD.     

扩频通信中数字窄带干扰抑制的仿真分析

由于对新兴的个人通信业务缺乏可分配频率 ,已提出在这种业务中使用扩频信号 ,这将出现与现存的频带占有者互相交叠的情况。为使其不干扰与其共存的窄带用户 ,这种扩频信号的功率将有所限制。近来对抑制干扰商用扩频通信系统的强窄带干扰的研究已重新引起人们的兴趣。在以前的研究中 ,窄带用户被模拟为正弦信号或自回归信号。该论文表明 ,当窄带干扰实际是一个数字通信信号时 ,预测滤波器技术抑制这种窄带信号的效果与从多用户通信中得出的技术相比要差。该论文特别研究了当把窄带用户模拟为数字信号时 ,恢复扩频比特流的最优和接近最优的 (低背景噪声 )线性算法。并对其性能进行了仿真分析。     

Active vibration isolation based on model reference adaptive control

High-performance instruments are very sensitive to vibrations and jitters. In this article, a new approach towards multi-degree-of-freedom (DOF) active vibration isolation and its application in spacecraft jitter suppression are presented. Model reference adaptive control (MRAC) with acceleration feedback is used to isolate random disturbances. However, a side effect of this algorithm is that displacements at low frequencies are amplified. Thus, the MRAC is augmented with proportional–integral–differential (PID) displacement feedback to suppress vibration displacements. The MRAC-PID composite control is applied to a 4-leg platform to isolate vibrations and suppress tip/tilt jitters. The scheme is also used to isolate 6-DOF vibrations and steer the payload of a flexible spacecraft. Satisfactory performance of vibration isolation and jitter attenuation has been observed.     

Dynamics simulation of MEMS device embedded hard disk drive systems

Currently, hard disk drives (HDD) use rotating disks to store digital data and magnetic recording heads are flying on the disk to read/write data. The recording heads are mounted on a slider–suspension assembly, which makes heads move from one track to another on the disk. The heads movement is controlled by close-loop feedback servo systems. It is well known that dynamic behaviors of head–slider–suspension-assembly (HSA) systems are of great influence on the track per inch capacity of HDD [1, 2]. As the problem is structurally complex, it is usually investigated using experimental methods or finite element simulation models [3]. Furthermore, the dual-stage servo system has been commonly considered as one promising solution to increase the servo bandwidth of the recording positioning system for high TPI HDDS. In particular, MEMS device embedded systems are superior to others in batch-fabrication. However, this dual-stage system has also resulted in more difficulties in predicting HDD dynamic performance. This paper presents the study of the problem using the macromodeling simulation approach. It applies efficient FEM based sub-structuring syntheses (SSS) [4] and fast boundary element method (BEM) approaches incorporated with system dynamics technology to investigate the dynamic characteristics of MEMS actuator embedded HSA systems for HDD.This research is funded by the Agency for Science, Technology and Research of Singapore, Strategic Research Program. Also, the authors would like to thanks Miss Jia Wenhui, who is a Research Student with ECE Department at National University of Singapore, Mr. Lim Boon Buan, the former research engineer with Data Storage Institute, for the MEMS actuator modeling and analytical work.     

Analysis of slip characteristics between dimple and flexure in hard disk drives

A mobile hard disk drive (HDD) is often faced with inevitable mechanical problems because of the portability of the drive. Then, almost HDD use much faster emergency parking system to protect the system from these problems. However, very fast emergency parking causes a large ramp contact, which could create dimple-flexure interactions such as a dimple-flexure slip, head-gimbal assembly vibration, unexpected slider motion. These dimple-flexure interactions largely affect the flyability of a slider. As such, dimple-flexure interactions are one of the most important factors in designing a mobile HDD. Therefore, in this study, we characterized the dimple-flexure slip among dimple-flexure interactions and analyzed it using a combination of experiments and finite element modeling. We evaluated the feasibility of dimple-flexure slip and verified the main cause of dimple-flexure slip. We also investigated the relationship between dimple-flexure slips and ramp contact during emergency parking. Finally, we designed the finite element ramp contact simulation, and analyzed dynamic characteristics for the dimple-flexure slip.     

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.     

Piezoelectric film attached suspension for detecting disk flutter and reducing track misregistration

To increase the recording density of hard disk drives, high accuracy of head positioning is required. But disk flutter, which is airflow induced vibration, is a major contributor to head positioning error. For detecting disk flutter, we proposed a suspension to which a PVDF film sensor was attached, and built the suspension into an actual HDD. Comparing the power spectrum of disk vertical vibration with that of the PVDF film sensor output, disk flutter was well detected. We also implemented a feedforward controller and confirmed that the influence of disk flutter on position error signal is considerably suppressed.     

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.