Insights to flow induced vibrations in computer hard disks

Transcription

1 Insights to flow induced vibrations in computer hard disks T. H. Yip, M. A. Suriadi, S. C. M. Yu & E. H. Ong Data Storage Institute, A-STAR Singapore MPE School, Nan. Tech. U., Singapore Abstract Read/write operations in computer hard disk drives (HDD) are supported by a rotary actuator. Air movements in the drive are a consequence of still air dragged along by the flat surfaces of the spinning circular disks. Air flowing in the enclosed HDD induces severe mechanical vibration of the actuator that causes Track Mis-registration error (TMR) and limits the achievable TPI, an indicator of the storage capacity of a computer hard disk drive. The velocity fields of an enclosed HDD model were measured with a Laser Doppler Anemometer non-intrusively. The model retained most of the common dimensions and basic features of a ½ HDD. The disks were spun at 0,000 rpm. From the spatial derivatives of the Reynolds shear stresses spatially distributed in the HDD model, the aerodynamic force distribution in the flow field was obtained. It was apparent that the highly fluctuating aerodynamic forces from the surrounding air acted on the arms and could have caused the tips of the arms to yaw back and forth. The aerodynamic action maybe closely related to the flow induced vibrations on arms widely reported by the hard disk drive industries. By modifying the cross sectional shape of the actuator arms, the attenuation of aerodynamic forces in the flow field was not observed. Introduction Computer hard disk drives (HDDs) are one of the most widely used storage devices of electronic data. Commercial HDDs exist in several sizes for different 00 WIT Press, ISBN

2 6 Advances in Fluid Mechanics V applications. Among them, ½ HDD could be found in almost every desktop computer in offices and homes. Besides storage capacity, one of the most important features of HDD is the access time. That is the time required to retrieve stored data from the magnetic media disks or to deposit new or updated data onto the disks. As the size of the electronic files grow larger with the ferocious advancement of software development, access time becomes excessively lengthy. By increasing the rotational speed of the disks, access time will be shortened considerably. As a consequence, the rotational speed of the magnetic disks has increased progressively, from 0 rpm to over 0 rpm. However, the increase in rotational speed of the magnetic disks bring along unexpected problems, arising from the air contained inside the HDD. Inside the enclosed space of the ½ hard disk, still air between the co-rotating disks is dragged along by the fast spinning surfaces of the magnetic disks and circulates inside the enclosure. Vibrations of the actuator and fluttering of magnetic disks have been attributed to the circulating airflow [,]. Airflow induced vibrations on the rotary arm contribute to track mis-registration (TMR) as well as possible failure of the magnetic read/write head and even contact to the disk surface. There is little published literature on the airflow characteristics of ½ HDD that spins at 0,000 rpm. In an attempt to provide insights to the airflow fields of the actual ½ HDD which runs at 0, 000 rpm, a HDD model was fabricated from Perspex material. The HDD model follows the dimensions of a standard ½ format HDD closely. Basically, the model consists of a stack of three transparent co-rotating disks, a flat transparent cover and a dummy actuator with arms of a rectangular cross section, as shown in Fig (a). The suspension and the head-gimbal assembly are not included on the dummy rotary arm. The indentations and curved surfaces on the inner walls and the top cover are neglected. The experiments were conducted in two stages. In stage, the rotary actuator has a rectangular cross section, as showed in Fig (b)(i). The sharp corners of the rectangular cross section were chamfered in stage, as showed in Fig (b)(ii). The disks were mounted on a disk drive motor co-axially, separated from one another with space rings. The aspect ratio (S) of the axial distance between the co-rotating disks (h) and the disk radius (R) is, S=h/R~0.08. The co-rotating disks were spun at 0, 000 rpm, which gives a tip speed Reynolds number ΩR Re = =.x0 5, where Re is the υ Reynolds number based on the radius of the disk ; Ω is the angular rotation speed of the disk and ν the kinematic viscosity of air. Experimental methodology A two beam one-component fiber-optic (TSI Incorporated IFA 750 series) Laser Doppler Anemometer system with a W Argon water-cooled laser operating in a backward scatter mode, as shown in Fig is used in the experiment. A 8 mm focusing lens with a focal length of 50 mm provided a measuring probe volume 00 WIT Press, ISBN

3 Advances in Fluid Mechanics V 7 of 0.09 mm x 0.09 mm x. mm with a.7 µm fringe spacing. Bragg shifting of frequency up to 0 MHz on the streamwise direction was used for directional discrimination purposes. The fiber-optic probe was mounted on a fully automated three-dimensional traversing system with a positional accuracy of ± 0.0 mm. Y Direction of Rotation X Motor Disk Arm Actuator Pivot (b)(i) (a) (b)(ii) Figure : General description of the HDD model. PC Oscilloscope Z-direction TSI IFA 750 Digital Burst Correlator Transparent HDD model Color Burst Optics W Argon Laser Figure : Schematic layout of experimental setup. Flow velocities were measured with the laser light beamed from the top of the ' ' transparent top cover. Velocity properties of air (U, u and V, v ) were measured from X and Y directions only. Reynolds shear stresses ( u 'v' ) in the flow field were measured with the one-component LDA by rotating the anemometer fringe patterns at 5 and 5 with respect to the normal orientation [5]. Smoke particles obtained by boiling liquid glycerine were used to 00 WIT Press, ISBN

4 8 Advances in Fluid Mechanics V seed the flow. The smoke particles were injected into the enclosed stack of disks by a medical syringe via a Ø 0.8mm hole drilled at the bottom of the D shaped trough. The astigmatism errors associated with multiple light refraction and offaxis alignment of LDA probe [8,9] was minimised by using a receiving lens with a large focal length (50 mm) in the fiber-optic probe. Prior to making any velocity measurements, the motor and laser were allowed to run for a minimum of 0 minutes. Air velocities were measured at 750 points spread across the horizontal cross sectional planes between the nd and rd disk, as shown in Fig. The dummy actuator was positioned at the Middle-Disk (MD) position. At every measurement point,000 samples were taken. With the exception at some regions far away from the co-rotating disks, data rates of 500~000 Hz were normally attainable. With the above precautions taken, a careful appraisal of the errors associated with the LDA system was conducted. The sources of error mainly stemmed from velocity biasing, velocity gradient broadening [], the signal processor accuracy (precision limitation), the finite sampling size [7] and the multiple seeding in the control volume []. Based on 95 % level of confidence, the expected uncertainty of the measured velocity components and the corresponding fluctuations are summarized as follows, the mean velocities U and V (normalized by U max = the maximum disk tip speed) is expected to contained.5% uncertainty. The rms of the velocity fluctuations The Reynolds shear stresses an uncertainty of 7.%. Results and discussion ' u and ' v (normalized by U max ) is.7%. u 'v' (normalized by U max ) are expected to carry For stage of the experiment, Fig. shows the mean resultant velocity vectors of the airflow between the nd and the rd rotating disks. The bulk direction of airflow corresponds with the direction of motor rotation. Flow between the hub of the motor and the actuator was accelerated due to blockage effects of the ' u v intruding actuator arms. The bulk levels of and in the flow field U max U max was about ~5% respectively. At any one point in the flow field, the interaction between the respective components of velocity fluctuations ' ' u i, j with vorticity fluctuation ω i, j, k would produce aerodynamic lift force f. However, the strength of the resultant lift forces depended largely on the radial distribution of Reynolds shear stresses in the flow field. The magnitude of the fluctuating aerodynamic forces would only become significant at points where large Reynold shear stress gradients were located [6]. Fig. 5(a) and 5(b) show the distribution of aerodynamic forces f = f x + f y (indicated with arrows in the HDD model in the X and Y directions respectively). 00 WIT Press, ISBN

5 Advances in Fluid Mechanics V 9 Figure : Measurement grid mid-way between the nd and rd disks. Figure : Mean resultant velocity between the nd and rd disk measured in stage I. 00 WIT Press, ISBN

6 50 Advances in Fluid Mechanics V (a) (b) Figure 5: Distribution of aerodynamic forces between nd and rd disk measured in stage. (a) (b) Figure 6: Distribution of aerodynamic forces between nd and rd disk measured in stage. 00 WIT Press, ISBN

7 Advances in Fluid Mechanics V 5 The aerodynamic forces f were calculated from the Reynolds shear stress ( u' v') gradients distributed in the flow field. Large aerodynamic forces are x i, j found behind the trailing edge of the actuator arms as shown in Fig. 5(a) and 5(b). The bulk of f x and f y are acting on a point about / the length of the arm from the tip respectively. It must be reminded that the magnitudes of aerodynamic forces were constantly fluctuating with time. The continuous action of fluctuating aerodynamic force on one particular point of the arm would cause rapid and successive yawing of the actuator. These micro-displacements at the tip of the actuator are suspected to be closely associated to the flow-induced suspension-head unit vibrations widely reported in the hard disk drive industries. In stage, the actuator arms were modified to have chamfered leading and trailing edges. As a consequence, the bulk of f x and f y shift towards the tip of the actuator arms, as shown in Fig. 6(a) and 6(b). However, the magnitude of the aerodynamic forces remains unchanged. Therefore, the attenuation of aerodynamic forces with actuator modification has not been observed from the current study. The yawing movement of the actuator arm may have been magnified by the shifting of the bulk of actuator arms. Concluding remarks f x and f y towards the tip of the In this paper, the turbulent flow characteristics in a hard disk drive model have been experimentally investigated. The results show that large aerodynamic forces are concentrated behind the trailing edge of the actuator arms. By modifying the cross sectional shape of the arm geometry the aerodynamic forces migrate to the tip of the arms. The modification has not brought about any noticeable attenuation of aerodynamic forces in the hard disk drive model under study. These aerodynamic forces maybe closely associated with the flowinduced vibration problems or windage problems widely reported in the industries. References [] Driver, D.M., Hebbar, S.K., 987, Experimental study of a three dimensional, shear driven turbulent boundary layer., AIAA Journal Vol. 5, pp. 5-. [] Durst, F., Melling, A. and Whitelaw, J.H., 98, Principles and Practice of Laser Doppler Anemometry., Academic Press, London. 00 WIT Press, ISBN

8 5 Advances in Fluid Mechanics V [] Humphrey J. A. C., Haj-Hariri, H. Iwasaki, M., Kazemi, M. and Rosales, L., 00, Modeling and controlling flow-induced suspension-head unit vibrations in hard disk drives, Microsystem Technologies, Vol. 8, pp [] McAllister J., 997, Disk Flutter: Causes and potential cures, J. Data Storage, pp. 9- [5] Melling, A. & Whitelaw, J.H., 976, Turbulent flow in a rectangular duct. J. Fluid Mech. Vol. 78, pp [6] Tennekes, H., and Lumley, J. L., 97, A First Course in Turbulence, MIT Press, Cambridge, MA. [7] Yanta, W. J. and Smith, R. A.,97, "Measurement of turbulent transport properties with a laser-doppler velocimeter", AIAA paper no [8] Zhang, Z. and Eisele, K.,998, Further considerations of the astigmatism error associated with off-axis alignment of an LDA probe., Experiments in Fluids, Vol., pp [9] Zhang, Z. and Eisele, K.,998, On the overestimation of the flow turbulence due to fringe distortion in LDA measurement volumes., Experiments in Fluids, Vol. 5, pp WIT Press, ISBN