Patent Abstract:
The inventors have discovered that aerodynamic forces contribute to disk fluttering. If the flow of air about these disk surfaces is unstable, the resulting aerodynamic forces can mechanically excite the disk surfaces, causing fluttering. The invention includes media enclosures constraining such aerodynamic effects, methods of making disk drives with these enclosures, the disk drives. This includes disk drives of at most 13 millimeters in height.

Full Description:
This application is a Continuation of prior application No.: Ser. No. 10/100,960 filed on Mar. 18, 2002, now U.S. Pat. No. 6,771,458. 

   TECHNICAL FIELD 
   This invention relates to enclosures in a disk drive providing a reduced air gap around disk drive media surfaces within the enclosure. 
   BACKGROUND ART 
   Disk drives are an important data storage technology, which include several crucial components. Disk drive read-write heads directly communicate with a disk surface containing the data storage medium over a track on the disk surface. This invention involves improving the ability to position at least one read-write head over the track on the disk surface. 
     FIG. 1A  illustrates a typical prior art high capacity disk drive  10  including actuator arm  30  with voice coil  32 , actuator axis  40 , suspension or head arms  50 – 58  with slider/head unit  60  placed among the disks  12 . 
     FIG. 1B  illustrates a typical prior art high capacity disk drive  10  with actuator  20  including actuator arm  30  with voice coil  32 , actuator axis  40 , head arms  50 – 56  and slider/head units  60 – 66  with all but one disk  12  removed as well as including spindle motor  80 . 
   Since the 1980&#39;s, high capacity disk drives  10  have used voice coil actuators  20 – 66  to position their read-write heads over specific tracks. The heads are mounted on head sliders  60 – 66 , which float a small distance off the disk drive surface when in operation. Often there is one head per head slider for a given disk drive surface. There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator. 
   Voice coil actuators are further composed of a fixed magnet actuator  20  interacting with a time varying electromagnetic field induced by voice coil  32  to provide a lever action via actuator axis  40 . The lever action acts to move head arms  50 – 56  positioning head slider units  60 – 66  over specific tracks with speed and accuracy. Actuator arms  30  are often considered to include voice coil  32 , actuator axis  40 , head arms  50 – 56  and head sliders  60 – 66 . Note that actuator arms  30  may have as few as a single head arm  50 . Note also that a single head arm  52  may connect with two head sliders  62  and  64 . 
   Today, read-write head positioning errors are a significant point of failure and performance degradation. These positioning errors are caused in part by disk fluttering. Some fluttering problems for disks can be attributed to instabilities in the motor turning the disk, which are being addressed by the motor manufacturers. 
   The disk drive industry faces some significant challenges. As either recording densities or spindle speed increases, both head positioning accuracy and head-flying stability must increase. Note that competitiveness in the disk drive industry requires both requires both increased recording density and increased spindle speeds. Note that head-flying is the motion of the read-write head over the disk surface, which flies a short distance off that surface. 
   In order to achieve an even higher track density essential for meeting the higher recording density requirements, the allowable position error of the heads relative to registered data tracks is required to be less than 0.05 μm for the next few years. 
   New ways to improve head positioning and stabilize head-flying are needed to meet these challenges, as well as improve the reliability of existing disk drives. 
   SUMMARY OF THE INVENTION 
   The inventors have found that the above needs can be achieved through further reduction of disk fluttering and flow-induced vibration around actuator arms. High-speed rotation results in large amplitude vibration of the head-slider suspension and the arms. Thus the reduction of flow-induced vibration is essential to current and future disk drive to protect head-positioning failures. 
   Aerodynamics has been an area of active and continuing research since at least the nineteenth century. Prandtl defined boundary layers early in the twentieth century. The boundary layer concept was directly applicable to fluid flows involving air, water and other low viscosity fluids. The boundary layer is a fluid region near a surface with essentially no relative velocity with regards to that surface. This region is caused by the effect of friction between the solid surface and the fluid. The depth of this region is roughly proportional to the square root of the viscosity divided by the velocity of the surface. 
   The inventors have discovered that aerodynamic forces contribute to disk fluttering. If the flow of air about these disk surfaces is unstable, the resulting aerodynamic forces can mechanically excite the disk surfaces, causing fluttering. 
   These aerodynamic forces act upon disk surfaces with respect to the air cavity in which the disk surfaces rotate. A rotating disk surface will tend to create a rotating boundary layer of air. This boundary layer will tend to rotate in parallel to the motion of the disk surface. The stationary surface of the disk drive cavity facing the disk surface will also tend to generate a boundary layer. The inventors discovered that when there is enough distance between the stationary surface and the disk surface for more than the boundary layer of the rotating disk surface, there is a back flow created against the direction of flow from the rotating disk surface. 
   The inventors have discovered that a significant reduction in disk surface mechanical fluttering results from reducing the air gap between stationary surfaces facing the disk surface to about the boundary layer thickness. The inventors have found that when the air gaps are approximately the boundary layer thickness, there is improved head positioning. When the air gaps are smaller fractions of the boundary layer thickness, there are further improvements in head positioning. These improvements are summarized for an operational rotating velocity of 5400 Revolutions Per Minute (RPM) in  FIG. 3 . Similar improvements are expected for other operating rotational velocities such as 7200 RPM, 10,000 RPM and over 14,000 RPM. 
   The invention includes not only the mechanical enclosures housing disk surfaces within a disk drive, but also the manufacturing methods, and the resulting disk drives. The disk drives may further be at most 13 millimeters in height. 
   These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  illustrates a typical prior art high capacity disk drive  10  including actuator arm  30  with voice coil  32 , actuator axis  40 , suspension or head arms  50 – 58  with slider/head unit  60  placed among the disks  12 ; 
       FIG. 1B  illustrates a typical prior art high capacity disk drive  10  with actuator  20  including actuator arm  30  with voice coil  32 , actuator axis  40 , head arms  50 – 56  and slider/head units  60 – 66  with all but one disk  12  removed as well as including spindle motor  80 ; 
       FIG. 2A  illustrates a cross section view of spindle motor  80  and one disk  12  with air flow between the upper disk surface  12  and top disk cavity face, as well as air flow between the lower disk surface  12  and bottom disk cavity face; 
       FIG. 2B  illustrates the air flow situation between the upper disk surface  12  and top disk cavity face of  FIG. 2A  showing the formation of two separate boundary layers; 
       FIG. 2C  illustrates the air flow situation between the lower disk surface  12  and bottom disk cavity face of  FIG. 2A  showing the formation of only one boundary layer; 
       FIG. 3  illustrates the relationship between the gap measured in millimeters along the horizontal axis and head positioning errors as a function of the gap for a disk surface rotating at 5400 RPM; 
       FIG. 4  illustrates an exploded schematic view of a thin disk drive  10  using a single head and supporting various aspects of the invention; 
       FIG. 5  illustrates a top schematic view of the thin disk drive  10  using the single head as illustrated in  FIG. 4 ; 
       FIG. 6A  illustrates a perspective view of voice coil actuator components  32 ,  40 ,  50 , and  60 , assembled with respect to the disk drive base  110  as illustrated in  FIGS. 4 and 5 ; and 
       FIG. 6B  illustrates a perspective view the assembled disk base  100 , spindle motor  80 , disk  12 , disk clamp  82 , and disk drive cover  110 , of  FIGS. 4 and 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2A  illustrates a cross section view of spindle motor  80  and one disk  12  with air flow between the upper disk surface  12  and top disk cavity face, as well as air flow between the lower disk surface  12  and bottom disk cavity face. 
   A qualitative description of air flow about a disk surface  12  is as follows. Because of the no-slip condition, fluid in contact with the surface rotates with the same angular velocity as the surface and experiences the same centripetal acceleration. At the start of motion, a boundary layer begins to form in the circumferential direction. Fluid in the boundary layer begins to spin but cannot maintain the same centripetal acceleration as the surface. It acquires an outward radial component. As the radial component increases in magnitude, a secondary layer develops in the radial direction with stresses centrally directed. These stresses exert an excitation force on rotating disk leading to the disk fluttering which impairs head-flying over that disk surface. 
     FIG. 2B  illustrates the air flow situation between the upper disk surface  12  and top disk cavity face of  FIG. 2A  showing the formation of two separate boundary layers. 
   In a conventional hard disk drive, the flow pattern has secondary flows, radially outward near the disk  202  and inward at the housing  200 , which dominate the air flow. They are connected by axial flows near the periphery and near the axle. When the gap  190  between disk and cover/base is even larger than that of boundary layer thickness, a significant quantity of fluid in the interior region is essentially isolated from the main flow. It rotates approximately as a rigid body at one-half the angular velocity of the disk. These flow characteristics make large vortex and accelerate disk-tilting effect, which results in a severe Position Error Signal (PES) problem. 
   It should be noted that in situations involving radial surface motion, the boundary layer is often formulated as proportional to the square root of the viscosity divided by radial velocity in radians per sec. 
   
     
       
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               illustrates the boundary layer thickness to 
             
             
               Revolutions Per Minute (RPM). 
             
           
        
         
             
                 
               RPM 
               Boundary Layer Thickness (mm) 
             
             
                 
                 
             
           
        
         
             
                 
               5400 
               0.7 
             
             
                 
               7200 
               0.55 
             
             
                 
               10,000 
               0.45 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 2B  reveals a large vortex over the area of the top disk of a disk stack, which may have just one disk. This vortex provides a mechanical force acting to excite disk fluttering. This is the situation found in all hard disk drives the inventors are aware of. 
   The inventors found that removing this large vortex in the area of top disk improves the mechanical situation. 
   Near the rotating disk surface, toward its rim, air flow velocities close nearing 10 meters (m) per second (sec) have been found in simulations. At the edge of the boundary layer, about one boundary layer thickness from the disk surface, air velocity is about 0. Further from the disk surface, a back flow forms due to the friction with the stationary surface. 
     FIG. 2C  illustrates the air flow situation between the lower disk surface  12  and bottom disk cavity face of  FIG. 2A  showing the formation of only one boundary layer. 
   By making the gap  190  too narrow for secondary flows to exist as illustrated in  FIG. 2C , the fluid adopts a Couette flow pattern  204  with a nearly straight-line, tangential velocity profile between the housing and the disk. 
     FIG. 3  illustrates the relationship between the gap measured in millimeters along the horizontal axis and head positioning errors as a function of the gap for a disk surface rotating at 5400 RPM. 
   The vertical axis is a percentage scale, with 100% being the current head position error rates with contemporary gaps of about 1.2 mm. 
   When the gap is made less than the boundary layer thickness of 0.7 mm, errors in head positioning are about 75% compared to conventional error rates. Note that when the gap is about 0.4 mm, the head positioning errors are 50% of conventional rates. 
     FIGS. 4 to 6B  illustrate various schematic views a thin disk drive  10  using a single head and supporting various aspects of the invention. 
   Note that a thin disk drive may be preferred in certain applications, such as multi-media entertainment centers and set-top boxes. Note that the thin disk drive using only a single head may further be preferred, allowing further reduction in the gap between the base  100  and the disk surface  12 . The use of a single head in the thin disk drive aids in reducing manufacturing costs and increasing manufacturing reliability. 
     FIG. 4  illustrates an exploded schematic view of a thin disk drive  10  using a single head and supporting various aspects of the invention. 
   Disk drive  10  includes a printed circuit board assembly  120 , a disk drive base  100 , a spindle motor  80 , a disk  12 , a voice coil actuator  30 , a disk clamp  82  and a disk drive cover  110 . Voice coil actuator  30  may further include a single read-write head on a head/slider  60 . Disk drive cover  110  may further include at least one region  112  providing a top stationary surface close to disk  12  upper surface. 
     FIG. 5  illustrates a top schematic view of the thin disk drive  10  using the single head as illustrated in  FIG. 4 . 
   Note that region  112  may be essentially outside the region traveled by the actuator arm(s)  50  and head sliders  60  of voice coil actuator  30  when assembled and in normal operation. 
     FIG. 6A  illustrates a perspective view of voice coil actuator components  32 ,  40 ,  50 , and  60 , assembled with respect to the disk drive base  110  as illustrated in  FIGS. 4 and 5 . 
     FIG. 6B  illustrates a perspective view the assembled disk base  100 , spindle motor  80 , disk  12 , disk clamp  82 , and disk drive cover  110 , of  FIGS. 4 and 5 . 
   The preceding embodiments have been provided by way of example and are not meant to constrain the scope of the following claims.

Technology Classification (CPC): 6