Abstract:
A system, method, and apparatus for solving flow-induced track misregistration (TMR) problems in hard disk drives (HDD) are directed to breaking up large-scale eddies, and straightening air flows with honeycomb structures, woven wire screens, and guide vanes or holes. In addition, boundary layer manipulation techniques are applied to the airflow in the HDD, such as boundary layer suction with slots or holes, and wall damping techniques, such as an open honeycomb seal and Helmholtz resonators. These flow-conditioning solutions reduce the turbulence intensity throughout the HDD to reduce TMR. These solutions achieve these goals while minimizing increases in the running torque needed to overcome their inherent rotational drag.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates in general to an improved architecture for conditioning air flow inside data storage devices and, in particular, to an improved system, method, and apparatus for breaking up large-scale eddies and straightening air flow inside rotary disk storage devices. 
     2. Description of the Related Art 
     Data access and storage systems generally comprise one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, one to five disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm). Hard disk drives have several different typical standard sizes or formats, including server, desktop, mobile (2.5 and 1.8 inches) and micro drive. 
     A typical HDD also uses an actuator assembly to move magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk. 
     The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track. 
     One of the major hurdles in hard disk drive (HDD) development is track misregistration (TMR). TMR is the term used for measuring data errors while a HDD writes data to and reads data from the disks. One of the major contributors to TMR is flow-induced vibration. Flow-induced vibration is caused by turbulent flow within the HDD. The nature of the flow inside a HDD is characterized by the Reynolds number, which is defined as the product of a characteristic speed in the drive (such as the speed at the outer diameter of the disk), and a characteristic dimension (such as the disk diameter or, for some purposes, disk spacing). In general, the higher the Reynolds number, the greater the propensity of the flow to be turbulent. 
     Due to the high rotational speed of the disks and the complex geometries of the HDD components, the flow pattern inside a HDD is inherently unstable and non-uniform in space and time. The combination of flow fluctuations and component vibrations are commonly referred to as “flutter” in the HDD literature. The more precise terms “disk flutter” and “arm flutter” refer to buffeting of the disk and arm, respectively, by the air flow. Unlike true flutter, the effect of the vibrations in HDDs on the flow field is usually negligible. Even small arm and disk vibrations (at sufficiently large frequencies, e.g., 10 kHz and higher), challenge the ability of the HDD servo system to precisely follow a track on the disk. 
     Since the forcing function of vibrations is directly related to flow fluctuations, it is highly desirable to reduce any fluctuating variation in the flow structures of air between both co-rotating disks and single rotating disks. Thus, a system, method, and apparatus for improving the architecture for conditioning air flow inside data storage devices is needed. 
     SUMMARY OF THE INVENTION 
     One embodiment of a system, method, and apparatus of the present invention attempts to apply several techniques to solve track misregistration (TMR) problems in hard disk drives (HDD). Some of the solutions presented herein are related to straightening airflows in wind tunnel applications. Several key components for breaking up large-scale eddies or straightening HDD air flows are honeycomb structures, woven wire screens, and guide vanes or holes. The flow-conditioning solutions presented in the present application reduce the turbulence intensity throughout the HDD to reduce TMR. These solutions achieve these goals while minimizing increases in the running torque needed to overcome their inherent rotational drag. 
     Three of the solutions may be categorized as large-eddy break-up (LEBU) devices. By installing these devices inside an HDD, the turbulent energy generated by the devices is confined to a range of smaller eddies that are more easily dissipated. The LEBU devices are positioned in the flow stream between the disks. The passages in the devices are aligned tangentially along a framing structure. The framing structure extends radially between the disks such that the various sets of passages are interleaved relative to the disk stack. Moreover, the LEBU devices can be placed as a single unit or multiple units in series, depending upon the application. 
     Another solution affects the stability of the HDD flow and enables the flow to follow complex geometries and regions with adverse pressure gradients (i.e., increasing pressure in the direction of flow) without flow separation. Separated regions are a major source of flow fluctuations when the Reynolds number is sufficiently large. The latter is true for typical prior art HDD configurations. Suction inhibits turbulent mixing between the Ekman layers spun off the disk and their return flow. Reduced mixing leads to a reduction in the aerodynamic torque needed to spin the disk pack. 
     The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features and advantages of the invention, as well as others which will become apparent are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments. 
         FIG. 1  is a top plan view of one embodiment of a disk drive constructed in accordance with the present invention. 
         FIG. 2  is a top plan view of another embodiment of a disk drive constructed in accordance with the present invention. 
         FIG. 3  is a sectional side view of one embodiment of a interleaf structure taken along the line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a sectional side view of an alternate embodiment of the interleaf structure of  FIG. 3 . 
         FIG. 5  is a sectional side view of another alternate embodiment of the interleaf structure of  FIG. 3 . 
         FIG. 6  is a sectional side view of yet another alternate embodiment of the interleaf structure of  FIG. 3 . 
         FIG. 7  is a sectional side view of still another alternate embodiment of the interleaf structure of  FIG. 3 . 
         FIG. 8  is a partial isometric view of one embodiment of a boundary layer device for a disk drive enclosure. 
         FIG. 9  is a partial isometric view of an alternate embodiment of the boundary layer device of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , one embodiment of a system, method, and apparatus for reducing track misregistration in disk drives is shown. This embodiment employs an information storage system comprising a magnetic hard disk file or drive  111  for a computer system. Drive  111  has an outer housing or base  113  (e.g., an enclosure) containing at least one magnetic disk  115 . Disk  115  is rotated by a spindle motor assembly having a central drive hub  117 . An actuator  121  comprises a plurality of parallel actuator arms  125  (one shown) in the form of a comb that is pivotally mounted to base  113  about a pivot assembly  123 . A controller  119  is also mounted to base  113  for selectively moving the comb of arms  125  relative to disk  115 . 
     In the embodiment shown, each arm  125  has extending from it at least one cantilevered load beam and suspension  127 . A magnetic read/write transducer or head is mounted on a slider  129  and secured to a flexure that is flexibly mounted to each suspension  127 . The read/write heads magnetically read data from and/or magnetically write data to disk  115 . The level of integration called the head gimbal assembly is head and the slider  129 , which are mounted on suspension  127 . The slider  129  is usually bonded to the end of suspension  127 . 
     Suspensions  127  have a spring-like quality, which biases or urges the air bearing surface of the slider  129  against the disk  115  to enable the creation of the air bearing film between the slider  129  and disk surface. A voice coil  133  housed within a conventional voice coil motor magnet assembly is also mounted to arms  125  opposite the head gimbal assemblies. Movement of the actuator  121  (indicated by arrow  135 ) by controller  119  moves the head gimbal assemblies radially across tracks on the disk  115  until the heads settle on their respective target tracks. The head gimbal assemblies operate in a conventional manner and always move in unison with one another, unless drive  111  uses multiple independent actuators (not shown) wherein the arms can move independently of one another. 
     Referring now to  FIGS. 1 and 2 , drive  111  further comprises a flow-conditioning device  141  that is mounted to the enclosure  113  adjacent to the disk  115 . The flow-conditioning device  141  comprises one or more “flow straighteners” that may be either symmetrically arrayed or asymmetrically about the disk  115 , depending upon the application.  FIG. 2  illustrates the symmetrical arrangement. Each flow-conditioning device  141  comprises a foundation or support post  145  that is mounted to the enclosure  113 . As shown in  FIG. 3 , one embodiment of support post  145  is mounted to and extends between both portions of the enclosure  113 : base plate  113   a  and top cover  113   b.    
     The flow-conditioning device  141  includes at least one projection or finger  143  (e.g., five shown for four disks  115  in  FIG. 3 ) having passages  147 . The fingers  143  extend radially with respect to the disks  115  and their axis  116 , and parallel to the surface  118  of the disks  115 . When two or more fingers  143  are used, the adjacent fingers  143  define a slot  144  that closely receives the two parallel surfaces of the disk  115 . The fingers  143  originate at the support post  145  and preferably extend to or near the disk hub  117 . However, there is no contact between any portion of the flow-conditioning device  141  and the disks  115 . 
     Each finger  143  comprises a small, generally rectangular frame having a plurality of the passages  147  that permit air flow to move all the way through the finger  143 . The passages  147  are formed in the finger  143  in directions that are axially and radially transverse (e.g., perpendicular) with respect to the disks  115 . Each of the fingers  143  has the passages  147  to reduce air flow turbulence intensity and track misregistration. 
     The fingers  143  are positioned in the air flow stream generated by the disks  115  so that, as the disks  115  rotate, the passages  147  are aligned with the air flow stream and reduce an air flow turbulence intensity and track misregistration between the heads on the sliders  129  and the read/write tracks on the disks  115 . The turbulent energy generated by the flow-conditioning device(s)  141  is confined to a range of smaller eddies that are more easily dissipated within the disk drive  111  than prior art large eddies. Each finger  143  has an angular or arcuate width in a range of approximately 5 degrees or less. Each finger  143  also can be configured to have a constant width along the radial direction of the disk  115 . 
     As shown in  FIGS. 3-7 , the passages  147  may comprise many different configurations or combinations thereof. For example, in  FIG. 3 , the passages  147  are configured in a honeycomb structure  151  having a tight array of one or more hexagonal feature(s) that extend across the entire face of the fingers  143 . In one embodiment, the passages  147  (honeycomb cell size) are on the order of five times smaller than the axial disk spacing. In addition, the fingers  143  may have an arcuate width that is approximately equal to said axial distance minus a mechanical clearance on the order of 0.5 mm. 
     In another embodiment ( FIG. 4 ), the passages are formed from wire screen walls  153 , which may be woven, mounted on a framing structure. The wire screen dimensions are dictated by the size of the disk diameter and spacing. Typically, the wire screen walls  153  comprise at least two or three passages across the vertical direction. In one version, the wire screen walls  153  have a thickness on the order of 0.1 mm. 
     In  FIG. 5 , the passages comprise sets of guide vanes  155  that extend axially with respect to the disk  115 . In the embodiment shown, the guide vanes  155  are grouped in small sets of three that radially offset from each other. The individual vanes in guide vanes  155  are parallel to each other. Other guide vane configurations are possible, including those of the slanted type. The guide vanes have a thickness that is sufficient to ensure mechanical stability and ruggedness, which may be on the order of 0.3 mm. 
     Alternatively, the passages may be formed by cylindrical tubes  157 , as shown in  FIG. 6 . The cylindrical tubes  157  may comprise many different configurations, such as side-by-side in a flat array having a single row of the axially parallel cylindrical tubes  157 . In another embodiment ( FIG. 7 ), the cylindrical tubes  159  form a plurality (two shown) of parallel rows that are configured in an alternating pattern of upper and lower positions. 
     Referring now to  FIG. 8 , an inner wall of the enclosure  113  may be configured with a boundary layer device  161 . The boundary layer device  161  is designed to manipulate the air flow inside the disk drive  111  to provide aerodynamic and acoustic damping that promote viscous dissipation of turbulent fluctuations. 
     The boundary layer device may comprise many different forms. For example, in  FIG. 8 , a suction plenum  163  having an array of suction apertures  165  is shown. The apertures  165  may comprise slots, holes, and/or combinations thereof, and are used to evacuate air flow from the interior of the disk drive  111  into the suction plenum  163 . The air flow (see arrows  167 ) is then reintroduced into the interior of the disk drive  111  at a suitable location, such as at perforations  169  in hub  117  (for clarity, disks  115  are not shown). Moreover, the suction air also may be passed through an air filter  171  before being reintroduced into the pack of disks  115 . 
     An alternative embodiment of the boundary layer device is shown in  FIG. 9  as a lining of cavities  173  on the inner wall of enclosure  113 . In one version, the cavities  173  comprise a honeycomb of hexagonal walls  175 , each of which is perforated by a small orifice  177 . Collectively, these cavities  173  form a close-packed array of Helmholtz resonators. Because these resonators can be tuned, they are particularly effective in suppressing narrow-band turbulence fluctuations. In particular, the Helmholtz resonators may be tuned to act as acoustic notch filters or certain prominent frequencies in the file. For example, one such frequency is the vortex shedding frequency associated with the actuator arm. In addition, the cavities may comprise closed and open cell acoustic foam. 
     The present invention also comprises a method of reducing track misregistration in a disk drive. In one embodiment ( FIG. 1 ), the method comprises providing a disk drive  111  having an enclosure  113 , a disk  115  having a surface  118  with tracks, and an actuator  121  having a head for reading from and writing to the tracks. The method further comprises positioning a flow-conditioning device  141  ( FIG. 3 ) adjacent to the surface  118  of the disk  115 , rotating the disk  115  to generate an air flow, flowing the air flow through passages  147  in the flow-conditioning device  141 , and thereby reducing air flow turbulence intensity and track misregistration between the head and the tracks on the disk  115 . 
     The method also may comprise positioning the disk  115  in an elongated slot  144  in the flow-conditioning device  141 . The method may further comprise orienting the passages  147  at axially and radially transverse positions with respect to the disk  115 , and forming the passages in a configuration selected from the group consisting of: a honeycomb structure ( FIG. 3 ), wire screen walls ( FIG. 4 ), guide vanes ( FIG. 5 ), and cylindrical tubes ( FIGS. 6 and 7 ). In another embodiment, the method may further comprise forming a symmetrical array ( FIG. 2 ) of the flow-conditioning devices  141  about the disk  115 . 
     Alternatively, or in combination with any of the foregoing steps of the method, the method may further comprise forming a boundary layer device  161  ( FIGS. 8 and 9 ) on an inner surface of the enclosure  113 , and manipulating the air flow inside the disk drive  111  with the boundary layer device  161  to provide aerodynamic and acoustic damping that promote viscous dissipation of turbulent fluctuations. The method may comprise evacuating air flow from an interior of the disk drive  111  into a suction plenum  163  ( FIG. 8 ), and reintroducing the air flow into the disk drive  111 . In addition, the method may comprise configuring the boundary layer device as a lining of walled cavities  173  ( FIG. 9 ), each having a small orifice  177  in communication with the interior of the disk drive  111 . 
     The present invention has several advantages, including the ability to reduce TMR problems in hard disk drives HDDs. These solutions break up large-scale eddies, straighten air flows, and manipulate the boundary layers. As a result, the turbulence intensity is reduced throughout the HDD to reduce TMR while minimizing increases in the running torque needed to overcome rotational drag. The turbulent energy generated by the devices is confined to a range of smaller eddies that are more easily dissipated. The LEBU devices can be used individually or as multiple units in series. 
     The present invention also enables the flow to follow complex geometries without flow separation. Suction inhibits turbulent mixing between the Ekman layers spun off the disk and their return flow. Reduced mixing leads to a reduction in the aerodynamic torque needed to spin the disk pack. In addition, turbulent fluctuations are dampened via the dissipation generated by the special linings, some of which can be tuned to suppress narrow-band fluctuations. The application of these special linings along the interior walls of the HDD provide aerodynamic and acoustic damping. In particular, the Helmholtz resonators may be tuned to act as acoustic notch filters or certain prominent frequencies in the file. 
     While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.