Abstract:
Aerodynamic forces contribute to disk and actuator vibration leading to track positioning errors in storage devices such as hard disk drives. The invention provides a variety of dampening mechanisms and a method of dampening to alleviate these problems in single disk storage devices. This includes disk drives of at most 13 millimeters in height.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation in part of application Ser. No. 10/142,078, filed May 8, 2002, and of application Ser. No. 10/100,960, filed Mar. 18, 2002, and this application claims the benefit of U.S. Provisional Application No. 60/290,128, filed May 10, 2001. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to storage device enclosures that reduce vibrations in a disk rotating in such a storage device.  
         BACKGROUND ART  
         [0003]    Disk drives are an important data storage technology. Read-write heads directly communicate with a disk surface containing the data storage medium over a track on the disk surface.  
           [0004]    [0004]FIG. 1A illustrates a typical prior art hard disk drive, which may be a high capacity disk drive  10 . Disk drive  10  includes an actuator arm  30  that further includes a voice coil  32 , actuator axis  40 , suspension or head arms  50 . A slider/head unit  60  is placed among data storage disks  12 .  
           [0005]    [0005]FIG. 1B illustrates a typical prior art high capacity disk drive  10 . The actuator  20  includes actuator arm  30  with voice coil  32 , actuator axis  40 , head arms  50 , and slider/head units  60 . A spindle motor  80  is provided for rotating disk  12 .  
           [0006]    Since the 1980&#39;s, high capacity disk drives  10  have used voice coil actuators  20  to position their read-write heads over specific tracks. The heads are mounted on head sliders  60 , which float a small distance off a surface  12 - 1  of a rotating disk  12  when the disk drive  10  is in operation. Often there is one head per head slider for a given disk surface  12 - 1 . There are usually multiple heads in a single disk drive, but for economic reasons, usually only one voice coil actuator  20  for positioning head arms  50 .  
           [0007]    Voice coil actuators  20  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  to position head slider units  60  over specific tracks. Actuator arms  30  are often considered to include voice coil  32 , actuator axis  40 , head arms  50 , and swage mounts  70 . Swage mounts mechanically couple head sliders  60  to actuator arms  50 . Actuator arms  30  may have as few as a single head arm  50 . A single head arm  52  may connect with two head sliders  60  and  60 A (as shown in FIG. 1B).  
           [0008]    [0008]FIG. 1C illustrates a cross sectional view of a single platter prior art disk drive  10  and FIG. 1D illustrates a cross sectional view of a double platter prior art disk drive  10 . Each disk drive  10  includes a disk base  100  and cover  110  that encloses disks  12  that are rotated by the spindle motor  80 .  
           [0009]    Read-write head positioning errors are a significant point of failure and performance degradation. Positioning errors are caused in part by disk fluttering. Disk fluttering occurs when a disk flexes, or vibrates, as it rotates. Some fluttering problems for disks are due to instabilities in the motor turning the disk. Fluttering problems of this type are usually addressed by spindle motor manufacturers.  
           [0010]    There have been attempts to address disk flutter problems in the prior art. U.S. Pat. No. 6,239,943 B1, entitled “Squeeze film dampening for a hard disc drive” is directed to an attempt to address disk flutter problems. This patent discloses “a spindle motor . . . cause[ing] rotation of . . . a single or multiple disc or stack of disks . . . mounted in such a way that the rotating bottom or top (or both) disc surface is closely adjacent to a disc drive casting surface. The squeeze film action in the remaining air gap provides a significant dampening of the disc vibration. . . . Typical implementations use air gaps of 0.004-0.006″[inch] for 2½ inch [disk] drives and 0.006-0.010″[inch] for 3½| 0  inch [disk] dirves” (lines 12-21, column 2). “According to the theory presented . . . , the damping provided by the squeeze film effect between the disc and base plate should not be a function of the spinning speed.” (lines 53-55, column 5). “Significant reduction in the vibration of the top disc, in a two disc system, can be achieved by supplying squeeze film damping to the bottom disc alone. This is important because in a practical design, damping discs other than the bottom disc may be difficult.” (line 65 column 5 to line 2 column 6).  
           [0011]    While the inventors are respectful of U.S. Pat. No. 6,239,943, they find several shortcomings in its insights. It is well known that the combined relationship of read-write heads on actuators accessing disk surfaces of rotating disks brings operational success to a disk drive. There are significant aerodynamic forces acting upon a read-write head assembly and its actuator due to the rotational velocity of the disk(s) being accessed. These significant aerodynamic forces acting upon the actuator, the read-write head, or both, are unaccounted for in the cited patent. There are also significant gap distances that may relate to rotational velocity which are unaccounted for in the cited patent, as well as the inventors&#39; experimental evidence indicating larger air gap providing reductions in track position error than this patent or any other prior art accounts for. There are significant insights to be gained from seeing the development of wave related phenomena in the physical system, both acoustically and mechanically, which are unaccounted for in the cited patent.  
           [0012]    Increased recording density and increased spindle speeds are key factors to competitiveness in the disk drive industry. As recording densities and spindle speeds increase, both head positioning accuracy and head-flying stability must also increase. However, as spindle speeds increase, air flow-induced vibrations may also increase which may result in larger amplitude vibrations of the head-slider suspension causing read-write head positioning errors. Additionally, air flow-induced vibrations acting upon a rotating disk cause disk fluttering, which contributes to track positioning errors. Thus, reducing air flow-induced vibration is essential to reducing head-positioning and read-write errors.  
         SUMMARY OF THE INVENTION  
         [0013]    The present invention comprises a dampening mechanism reducing aerodynamic forces acting upon a disk rotating in a single disk storage device. The present invention achieves a reduction of disk fluttering and at least some forms of air flow-induced vibration around actuator arms, reducing head-positioning and read-write errors.  
           [0014]    The rotational velocity of a disk surface of the rotating disk may affect significant aerodynamic forces in an air cavity in which the disk rotates. These aerodynamic forces may act upon a read-write head assembly, its actuator, and the rotating disk causing disk fluttering, head-positioning errors and read-write errors.  
           [0015]    A boundary layer is defined herein as an air region near a solid 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 air. The depth of this region is roughly proportional to the square root of the viscosity divided by the velocity of the surface.  
           [0016]    Aerodynamic theory indicates the following: A rotating disk surface creates a rotating boundary layer of air. This boundary layer tends to rotate in parallel to the motion of the disk surface. A stationary surface, such as a base or cover, of the disk drive cavity facing the rotating disk surface also tends to generate a boundary layer. When the distance between the stationary surface and the disk surface is more than the boundary layer thickness of the rotating disk surface, a back flow is created against the direction of flow from the rotating disk surface. This back flow of air may act upon the disk surface, causing the disk to flutter, and may act upon the read-write head assembly, causing the head assembly to vibrate. This back flow of air, as well as other aerodynamic forces, may induce disk fluttering, head-positioning and read-write errors.  
           [0017]    It is useful to view the physical system of the rotating disks in a sealed disk enclosure as forming a resonant cavity for both acoustic and mechanical vibrations. Simulations and experiments by the inventors have found the resonant or natural frequencies for such cavities to be dampened based upon providing a dampening surface near a spinning disk at greater distances than either theory or the prior art report.  
           [0018]    The invented enclosure acts as a dampening mechanism including a stationary dampening surface positioned adjacent to a rotating disk surface at a distance, or air gap, between the dampening surface and the disk surface. Improvements in disk fluttering are noted for air gaps at or less than the boundary layer thickness. However, the inventors have also observed significant dampening effects in experimental conditions matching the sealed interior of an operational disk drive at larger air gaps than either theory or the prior art indicate.  
           [0019]    The reduced distance, or air gap, between the dampening surface of the dampening mechanism and rotating disk surface inhibits the creation of the back flow of air between the rotating disk surface and dampening surface. The air gap may also minimize the effects of the back flow of air and other aerodynamic forces acting upon the disk surface and the read-write head assembly, including its actuator. This reduces disk fluttering, improves head-positioning and aids the overall quality of disk drive performance.  
           [0020]    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.  
           [0021]    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  
       [0022]    [0022]FIG. 1A illustrates a typical prior art hard disk drive, which may be a high capacity disk drive  10 ;  
         [0023]    [0023]FIG. 1B illustrates a typical prior art high capacity disk drive  10 ;  
         [0024]    [0024]FIG. 1C illustrates a cross sectional view of a single platter prior art disk drive  10 ;  
         [0025]    [0025]FIG. 1D illustrates a cross sectional view of a double platter prior art disk drive  10 ;  
         [0026]    [0026]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;  
         [0027]    [0027]FIG. 2B illustrates a view of strong dynamic force (or pressure) near the outer-diameter region generated by the rotating air flow, leading to excitation of disk vibration;  
         [0028]    [0028]FIG. 2C 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;  
         [0029]    [0029]FIG. 2D 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;  
         [0030]    [0030]FIG. 3 illustrates disk vibration harmonics of rotation speed of a 3.5 inch conventional two platter disk drive  10  operating at 7200 revolutions per minute rotational velocity;  
         [0031]    [0031]FIG. 4 illustrates a head Position Error Signal (PES) spectrum experimentally determined as a Non-Repeatable Run Out (NRRO) PES spectrum in a conventional 57,000 Track-Per-Inch (TPI) disk drive system as disclosed in the prior art;  
         [0032]    [0032]FIG. 5 illustrates an exploded schematic view of a thin disk drive  10  using a single head and supporting various aspects of the invention;  
         [0033]    [0033]FIG. 6 illustrates a top schematic view of the thin disk drive  10  using the single head as illustrated in FIG. 5;  
         [0034]    [0034]FIG. 7 illustrates a top schematic view of disk drive  10  employing a dampening mechanism  120  in accordance with certain aspects of the invention providing over 180 degrees of radial coverage where the dampening surface (not shown) is within a first gap of the first disk surface of disk  12 ;  
         [0035]    [0035]FIG. 8 illustrates a perspective view of certain preferred embodiments of dampening mechanism  120  comprised of at least one plate providing at least a first surface  122 , which, when assembled in disk drive  10 , provides a first gap near a first disk surface of rotating disk  12 , as further seen in FIGS.  11 A- 12 A;  
         [0036]    [0036]FIG. 9 illustrates a top schematic view of disk drive  10  employing an alternative embodiment dampening mechanism  120  of FIG. 7 providing less than 180 degrees of radial coverage where the dampening surface (not shown) is within a first gap of the first disk surface of disk  12 ;  
         [0037]    [0037]FIGS. 10A and 10B illustrate experimental results regarding track position errors obtained from an offline servo track write setup using an airflow stabilizer similar to the dampening mechanism  120  illustrated in FIGS. 8 and 9;  
         [0038]    [0038]FIGS. 11A and 11B illustrate cross section views of two alternative preferred embodiments of a single platter  12  disk drive  10  of the invention;  
         [0039]    [0039]FIG. 11C illustrates a cross section view of a preferred embodiment of a double platter  12  and  14  disk drive  10  of the invention;  
         [0040]    [0040]FIG. 12A illustrates a more detailed cross section view related with FIGS. 11A to  11 C;  
         [0041]    [0041]FIG. 12B illustrates theoretical results of the elasto-acoustic coupling effect regarding the damping coefficient of a vibrating disk surface  12  with regards to a normalized gap height Gap 1  of FIG. 12A;  
         [0042]    [0042]FIG. 12C illustrates theoretical results of the elasto-acoustic coupling effect regarding the damping coefficient of a vibrating disk surface  12  with regards to the normalized first dampening surface  122  of FIG. 12A;  
         [0043]    [0043]FIGS. 13A, 13B, and  14  illustrate the experimentally determined actuator vibration spectrum from 0 to 1K Hz at the inside diameter, middle diameter and outside diameter, respectively;  
         [0044]    [0044]FIGS. 15A and 15B illustrate experimental results of the elasto-acoustic coupling effect regarding the power spectrum of a vibrating disk surface  12  with regards to Gap  1  of FIG. 12A being 0.6 mm and 0.2 mm, respectively;  
         [0045]    [0045]FIGS. 16A and 16B illustrate experimental results of the elasto-acoustic coupling effect regarding the power spectrum of a vibrating disk surface  12  with regards to various values Gap  1  of FIG. 12A for disk rotational speeds of 7200 and 5400 revolutions per minute, respectively;  
         [0046]    [0046]FIG. 17 illustrates experimental results of the elasto-acoustic coupling effect regarding the displacement frequency spectrum of vibrating disk surface  12 , both with a dampening mechanism of 25 mm radial width  570  and without a dampening mechanism  560 ;  
         [0047]    [0047]FIG. 18 illustrates head Position Error Signal (PES) spectrum experimentally determined as a Non-Repeatable Run Out (NRRO) PES spectrum in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  580  and in a disk system employing a 25 mm dampening mechanism  590  providing a 30% reduction in PES;  
         [0048]    [0048]FIG. 19 illustrates head Position Error Signal (PES) spectrum experimentally determined as a Non-Repeatable Run Out (NRRO) PES spectrum in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  600  and in a disk system employing dampening mechanism with varying radial widths;  
         [0049]    [0049]FIG. 20 illustrates head Position Error Signal (PES) levels experimentally determined in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  600  and in a disk drive employing dampening mechanism with varying radial widths;  
         [0050]    [0050]FIG. 21 illustrates head Position Error Signal (PES) levels experimentally determined in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  600  and in a disk system employing dampening mechanism with varying coverage angles and radial width of one inch or 25 mms;  
         [0051]    [0051]FIG. 22 illustrates an extension of the material and analyses of FIGS. 2A and 12A for further preferred embodiments of the invention; and  
         [0052]    FIGS.  23 A- 23 E illustrate various shapes, edges, and materials for a plate used in dampening mechanism  120  of the previous Figures.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0053]    The rotational velocity of a disk surface of the rotating disk may affect significant aerodynamic forces in an air cavity in which the disk rotates. These aerodynamic forces may act upon a read-write head assembly, its actuator, and the rotating disk causing head-positioning and read-write errors and disk fluttering.  
         [0054]    As stated in the summary, a boundary layer is an air region near a solid 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 air. The depth of this region is roughly proportional to the square root of the viscosity divided by the velocity of the surface.  
         [0055]    [0055]FIG. 2A illustrates a cross section view of a spindal motor  80  and one disk  12  with air flow between the upper disk surface  12 - 1  and top disk cavity face, as well as air flow between the lower disk surface  12 - 2  and bottom disk cavity face. The disk surface is rotating at an essentially constant speed.  
         [0056]    Theoretically, a rotating disk surface tends to create a boundary layer of air rotating in parallel to the motion of the disk surface. A stationary surface, such as a base or cover, of the disk drive cavity facing the rotating disk surface will also tend to generate a boundary layer. When the distance between the stationary surface and the disk surface is more than the boundary layer thickness of the rotating disk surface, a back flow is created against the direction of flow from the rotating disk surface. This back flow of air may act upon the disk surface, causing the disk to flutter, and may act upon the read-write head assembly, causing the head assembly to vibrate. The faster the disk rotates the greater the aerodynamic effect upon the read-write head assembly and attached actuator.  
         [0057]    [0057]FIG. 2A may also provide insight into the tendency of such physical systems to display both acoustic and mechanical resonance. It is useful to view the physical system of the rotating disks, in the enclosure of operating hard disk drive, as forming a resonant cavity for both acoustic and mechanical vibrations. Simulations and experiments by the inventors have found the resonant or natural frequencies for such cavities to be dampened based upon providing a dampening surface near a spinning disk at greater distances than either theory or the prior art report.  
         [0058]    [0058]FIG. 2B was adapted from a presentation by Professor Dae-Eun Kim entitled “Research and Development Issues in HDD Technology: Activities of CISD” at the International Symposium on HDD Dynamics and Vibration, Center for Information Storage Device (CISD), Yonsei University, Seoul, Korea on Nov. 9, 2001, and illustrates a view of strong dynamic force (or pressure) near the outer-diameter region generated by the rotating air flow, leading to excitation of disk vibration. The air flow near the outer diameter, between disks  12  and  14  experiences unsteady periodic vortices, causing resonant harmonic mechanical vibrations, fluttering the disks  12  and/or  14 . Additionally, near the enclosure region formed by the disk base  100  and/or cover  110  (best seen in FIGS. 1C and 1D), a region of strong, turbulent air forms. FIGS. 2C and 2D discuss this phenomena further.  
         [0059]    [0059]FIG. 2C illustrates the typical air flow between a disk surface and a non rotating surface showing the formation of two separate boundary layers.  
         [0060]    In a conventional hard disk drive, the flow pattern has secondary flows, radially outward near the disk and inward at the housing, which dominate the air flow. The air flows are connected by axial flows near the periphery and near the axle. When the gap between disk and a stationary surface is larger than that of the boundary layer thickness, a significant quantity of air in the interior region is essentially isolated from the main flow. The isolated air rotates approximately as a rigid body at one-half the angular velocity of the disk. These flow characteristics make a large vortex and accelerate the disk-tilting effect, which results in a severe Position Error Signal (PES) problem.  
         [0061]    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 shows boundary layer thickness to Revolutions Per Minute (RPM).  
                                         TABLE 1                                   RPM   Boundary Layer Thickness (mm)                                        5400   0.7           7200   0.55           10,000   0.45                      
 
         [0062]    [0062]FIG. 2C tends to indicate the existence of 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 . Near the rotating disk surface, toward its rim, air flow velocities 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.  
         [0063]    Removing the vortex adjacent the disk surface has been found to improve mechanical stability. By making the gap too narrow for secondary flows to exist, as illustrated in FIG. 2D, the air adopts a Couette flow pattern with a nearly straight-line, tangential velocity profile between the housing and the disk.  
         [0064]    Accordingly, in one embodiment of the invention, a dampening mechanism is positioned adjacent to the surface of a rotating disk to significantly reduce the distance between a stationary surface and the rotating disk surface. This reduced distance, or air gap, between the dampening mechanism and the disk surface may be approximately the boundary layer thickness of the rotating disk. Alternatively, the air gap may be less than the approximate boundary layer thickness.  
         [0065]    The reduced distance, or air gap, between the dampening mechanism and rotating disk surface may inhibit the creation of the back flow of air between the rotating disk surface and stationary surface. The air gap may also minimize the effects of the back flow of air and other aerodynamic forces acting upon the disk surface and the read-write head assembly, including its actuator. This may reduce disk fluttering and may improve head-positioning. When the air gap is a smaller fraction of the boundary layer thickness, there may be further improved in head positioning and reduced disk fluttering.  
         [0066]    [0066]FIG. 3 graph showing disk vibration as harmonics of a rotation speed of a 3.5 inch conventional two platter disk drive (configured as seen in FIGS. 1D and 2B) operating at 7200 revolution per minute rotational velocity, wherein the disks  12  and  14  are 1.27 mm thick aluminum disks driven by a fluid-dynamic bearing motor  80 . The measurements are of axial disk vibration at the outside diameter of the top disk as measured by a laser Doppler velocity meter. The vertical axis indicates displacement of the outside diameter as measured in meters on a logarithmic scale from 100 pico-meters to 100 nano-meters. The peaks circled on the left represent Harmonics of a rotation speed, while the peaks circled on the right represent disk vibration modes.  
         [0067]    [0067]FIG. 4 is a graph showing a head Position Error Signal (PES) spectrum experimentally determined as a Non-Repeatable Run Out (NRRO) PES spectrum in a conventional 57,000 Track-Per-Inch (TPI) disk drive system as disclosed in the prior art. The left axis indicates NRRO PES in nano-meters, and the right axis equivalently indicates NRRO PES in percentage of track pitch. The trace indicates the readings within three standard deviations for PES, which is roughly 35.7 nano-meter or seven percent of the track pitch. The PES peak  400  is caused by flow-vortex induced effects. The PES peaks within region  410  are induced by disk vibration.  
         [0068]    Both FIGS. 3 and 4 indicate resonant or standing wave phenomena. The resonant frequencies of the disk vibration modes of FIG. 3 have a high correlation to the PES peaks within region  402  of FIG. 4.  
         [0069]    [0069]FIG. 5 illustrates an exploded schematic view of a typical thin disk drive  10  using a single head and supporting various aspects of the invention. A thin disk drive may be preferred in applications such as multi-media entertainment centers and set-top boxes. The thin disk drive may preferably use only a single head, allowing further reduction in the gap between surfaces if base  100  and a surface of disk  12 . Using a single head in the disk drive may reduce manufacturing costs and increases manufacturing reliability.  
         [0070]    In the typical configuration shown in FIG. 5, drive  10  includes a printed circuit board assembly  102 , 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 , and Disk drive cover  110  may further include at least one region  112  providing a top stationary surface close to an upper surface of disk  12 .  
         [0071]    [0071]FIG. 6 illustrates a top schematic view of the thin disk drive  10  of FIG. 5.  
         [0072]    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. Region  112  may provide a connected surface, without breaks. Region  112  may further provide a simply connected surface, lacking any perforations or holes.  
         [0073]    [0073]FIG. 7 illustrates a top schematic view of disk drive  10  employing a dampening mechanism  120  in accordance with certain aspects of the invention providing over 180 degrees of radial coverage where the dampening surface (not shown) is within a first gap of the first disk surface of disk  12 .  
         [0074]    [0074]FIG. 8 illustrates a perspective view of certain preferred embodiments of dampening mechanism  120  comprised of at least one plate providing at least a first surface  122 , which, when assembled in disk drive  10 , provides a first gap near a first disk surface of rotating disk  12 , as further seen in FIGS.  11 A- 12 A. Note that various embodiments of the invention may provide more than one dampening surface to other disk surfaces, which may or may not belong to other disks.  
         [0075]    [0075]FIG. 9 illustrates a top schematic view of disk drive  10  employing an alternative embodiment dampening mechanism  120  providing less than 180 degrees of radial coverage where the dampening surface (note shown) is within a first gap of a surface of disk  12 .  
         [0076]    In some embodiments the dampening surfaces may form one or more plates. The dampening surfaces indicated in FIGS. 7 and 9 may each preferably form essentially a truncated annulus or “C” shape, comprising an inner boundary  140  and an output boundary  142  facing toward and away from the spindle motor, respectively. Dampening surfaces may further include first  144  and second  146  non-radial boundaries. Various preferred plates are illustrated in FIGS.  23 A- 23 E.  
         [0077]    Dampening mechanism  120  is also referred to herein as a disk damper, a disk damping device, a dampening means, and an airflow stabilizer. Dampening mechanism  120  may further include a shroud or wall arranged away from the axis of rotation, in certain preferred cases to be further discussed in FIG. 22, rigidly attached to at least one of the plates shown in FIG. 8.  
         [0078]    [0078]FIGS. 10A and 10B show experimental results regarding track position errors obtained from an offline servo track write setup using an airflow stabilizer similar to the dampening mechanism  120  illustrated in FIGS. 8 and 9.  
         [0079]    The vertical axis of FIG. 10A indicates track position root mean square errors in micro-inches. Box  520  indicates the experimental track position error results without dampening mechanism  120 , indicating 0.056 micro-inches root mean square errors. Box  522  indicates the experimental track position error results using dampening mechanism  120 , indicating 0.036 micro-inches root mean square errors.  
         [0080]    The vertical axis of FIG. 10B indicates the probability density per micro-inch. The horizontal axis indicates track position errors in micro-inches. Trace  524  indicates the probability density at various positional errors without the use of dampening mechanism  120 . Trace  526  indicates the probability density at various positional errors with the use of dampening mechanism  120 .  
         [0081]    [0081]FIGS. 11A and 11B illustrate cross section views of two alternative embodiments of a single platter  12  disk drive  10  of the invention.  
         [0082]    [0082]FIG. 11C illustrates a cross section view of an embodiment of a double platter  12  and  14  disk drive  10  of the invention.  
         [0083]    FIGS.  11 A- 11 C illustrate dampening mechanism  120  may include a plate providing at least one dampening surface  122  close to a first disk  12  at essentially a first gap. FIG. 11C illustrates dampening mechanism  120  further providing a second dampening surface  124  close to a second disk  14  at essentially a second gap.  
         [0084]    [0084]FIG. 12A illustrates a more detailed cross section view related to FIGS. 11A to  11 C, and more specifically to FIG. 11B, of the dampening mechanism  120  and adjacent disks  12  and  14 . Dampening mechanism  120  includes first dampening surface  122  separated from first disk surface  12 - 1  of disk  12  by essentially air layer Gap  1  as shown in FIGS. 11A to  11 C.  
         [0085]    Note that in FIG. 11A, the first disk surface  12 - 1  is the bottom disk surface of disk  12 . In FIGS. 11B and 11C, the first disk surface  12 - 2  is the bottom disk surface of disk  12 .  
         [0086]    Dampening mechanism  120  may further include a second dampening surface  124  separated from a second disk surface  14 - 1 , in this case, of a second disk  14  by essentially air layer Gap  2 , as shown in FIGS. 11C and 12A.  
         [0087]    Each of these gaps is at most a first distance, which is preferably less than 1 mm. Each of these gaps is preferably greater than 0.3 mm. It is further preferred that each of these gaps be between 0.35 and 0.6 mm.  
         [0088]    One or more of these gaps may preferably be less than the boundary layer thickness. In certain embodiments, one or more of these gaps may preferably be less than a fraction of the boundary layer thickness.  
         [0089]    Some inventors describe the dampening of disk  12  vibrations by an elasto-acoustic coupling effect between an elastic-vibration wave field of disk  12  and an acoustic pressure wave field of the adjacent air medium in the gap separating the first disk surface  12 - 1  and first dampening surface  122 . These inventors define the elasto-acoustic coupling effect as a coupling generated between the elastic-vibration wave field of disk  12  and the acoustic pressure wave field in the gap between first disk surface  12 - 1  and first dampening surface  122 .  
         [0090]    Experimental results by these inventors point to the acoustic-pressure wave of the air layer gap providing a strong damping force to the elastic-vibration wave of disk  12 . These inventors additionally describe the dampening of disk  14  vibrations by a similar elasto-acoustic coupling effect between an elastic-vibration wave field of disk  14  and an acoustic pressure wave field of the adjacent air medium in the gap separating the second disk surface  14 - 1  and second dampening surface  124 .  
                                                                   Rotation                           Disk Size   Rate in       Radial           Disk   (Number   RPM       Width(s)   Coverage       Figure   Material   of   Tracks Per   Gap(s)   Inches   angle(s) in       Number   (Thickness)   Platters)   Inch (TPI)   Mms   (mm)   degrees                    3   Al   3.5 in   7200 RPM   Not   Not   Not       (prior   (1.27 mm)   2   Not relevant   relevant   relevant   relevant       art)        4   Al   3.5 in   7200 RPM   Not   Not   Not       (prior   (1.27 mm)   2   (57,000   relevant   relevant   relevant       art)           TPI)       10A   Al   3.5 in   7200 RPM   0.6 mm   1 in   180           (1.27 mm)   3   Not       (25 mm)                   relevant       10B   Al   3.5 in   7200 RPM   0.6 mm   1 in   180           (1.27 mm)   3   Not       (25 mm)                   relevant       12B   Theoretical   Arbitrary   Any RPM   See Figure   Arbitrary   Arbitrary           Lumped   Arbitrary   Not relevant           Mass           Model       12C   Theoretical   Arbitrary   Any RPM   See Figure   Arbitrary   Arbitrary           Lumped   Arbitrary   Not relevant           Mass           Model       13A   Al   3.5 in   7200   0.5 mm   2/3 in   180           (1.27 mm)   2           (17 mm)       13B   Al   3.5 in   7200   0.5 mm   2/3 in   180           (1.27 mm)   2           (17 mm)       14   Al   3.5 in   7200   0.5 mm   2/3 in   180           (1.27 mm)   2           (17 mm)       15A   Al   3.5 in   7200 RPM   0.6 mm   1 in   200           (1.27 mm)   2   Not relevant       (25 mm)       15B   Al   3.5 in   7200 RPM   0.6 mm   1 in   200           (1.27 mm)   2   Not relevant       (25 mm)       16A   Al   3.5 in   7200 and   0.2-1.8 mm   1 in   200           (1.27 mm)   2   5400 RPM       (25 mm)                   Not relevant       16B   Al   3.5 in   7200 and   0.2-1.8 mm   1 in   200           (1.27 mm)   2   5400 RPM       (25 mm)                   Not relevant       17   Al   3.5 in   7200 RPM   0.5 mm   0 and 1 in   200           (1.27 mm)   2   (57,000       (25 mm)                   TPI)       18   Al   3.5 in   7200 RPM   0.5 mm   0 and 1 in   200           (1.27 mm)   2   (57,000       (25 mm)                   TPI)       19   Al   3.5 in   7200 RPM   0.5 mm   0 to 1 in   200           (1.27 mm)   2   (57,000       (25 mm)                   TPI)       20   Al   3.5 in   7200 RPM   0.5 mm   0 to 1 in   200           (1.27 mm)   2   (57,000       (25 mm)                   TPI)       21   Al   3.5 in   7200 RPM   0.5 mm   1 in   0-200           (1.27 mm)   2   (57,000       (25 mm)                   TPI)                  
 
         [0091]    [0091]FIG. 12B illustrates theoretical results of the elasto-acoustic coupling effect regarding the damping coefficient of a vibrating disk surface  12  with regards to a normalized gap height Gap 1  of FIG. 12A.  
         [0092]    The normalized gap height is in dimensionless units corresponding to a range roughly from 0 to 10. The damping coefficient is defined as used in theoretical vibration theory. In viscous damping, the damping force is proportional to the velocity of the vibrating body. The viscous damping coefficient c is expressed by c=−F/v where F is damping force and v is the velocity of the vibrating body. The negative sign indicates that the damping force is opposite to the direction of velocity of vibrating body.  
         [0093]    [0093]FIG. 12C illustrates theoretical results of the elasto-acoustic coupling effect regarding the damping coefficient of a vibrating disk surface  12  with regards to the normalized first dampening surface  122  of FIG. 12A. The horizontal axis shows the ratio of dampening surface  122  area to disk surface  12  area multiplied by a factor of ten, which is best seen in the top views of FIGS. 7 and 9.  
         [0094]    [0094]FIGS. 13A, 13B, and  14  illustrate the experimentally determined actuator vibration spectrum from 0 to 1K Hz at the inside diameter, middle diameter and outside diameter, respectively obtained using laser Doppler vibrometer readings taken of an actuator operating in a 3.5 inch disk drive rotating two platters at 7200 RPM. The actuator was a fully assembled actuator including suspension mechanism, head-gimbal assembly and four channel read-write heads.  
         [0095]    Traces  530  and  532  illustrate actuator vibration through the frequency range respectively without and with dampening mechanism  120 . Dampening mechanism  120  is a plate as illustrated in FIGS. 7, 8 and  11 C, positioned within a gap of 0.5 mm from the respective disk surfaces of the two disks  12  and  14 . The plate has a radial width of two thirds of an inch, or about 17 mm.  
         [0096]    Peak  534  is a vortex-sound induced actuator resonance at approximately 258 Hz in trace  530 , which is almost completely eliminated in trace  532 . Peak  536  is a vortex-sound induced actuator resonance at approximately 346 Hz in trace  530 , which is almost completely eliminated in trace  532 . The removal of these resonance peaks is advantageous to the overall track positioning capability of the actuator with regards to the disk surfaces.  
         [0097]    [0097]FIGS. 15A and 15B illustrate experimental results of the elasto-acoustic coupling effect regarding the power spectrum of a vibrating disk surface  12  with regards to Gap  1  of FIG. 12A being 0.6 mm and 0.2 mm, respectively. The vertical axis indicates displacement of the outside diameter as measured in meters on a logarithmic scale from 100 pico-meters to 100 nano-meters.  
         [0098]    Peaks in regions  540  and  550  are considered by the inventors to be attributable to disk vibration. Peak  542  at a gap of 0.6 mm reduces to peak  552  when the gap decreases to 0.2 mm.  
         [0099]    [0099]FIGS. 16A and 16B illustrate experimental results of the elasto-acoustic coupling effect regarding the power spectrum of a vibrating disk surface  12  with regards to various values Gap  1  of FIG. 12A for disk rotational speeds of 7200 and 5400 revolutions per minute, respectively. The reported vibration data are the measured axial disk vibration made at the outside diameter of the top disk as measured by a laser Doppler velocity meter.  
         [0100]    [0100]FIG. 17 illustrates experimental results of the elasto-acoustic coupling effect regarding the displacement frequency spectrum of vibrating disk surface  12 , both with a dampening mechanism of 25 mm radial width  570  and without a dampening mechanism  560 .  
         [0101]    [0101]FIG. 18 illustrates head Position Error Signal (PES) spectrum experimentally determined as a Non-Repeatable Run Out (NRRO) PES spectrum in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  580  and in a disk system employing a 25 mm dampening mechanism  590  providing a 30% reduction in PES.  
         [0102]    The left axis indicates NRRO PES in nano-meters. The right axis equivalently indicates NRRO PES percentage of track pitch. Trace  580  indicates readings within three standard deviations for PES of roughly 36 nano-meters or equivalently, 7 percent track pitch. Trace  590  indicates readings within three standard deviations for PES of roughly 24 nano-meter or equivalently, 4.7 percent of track pitch.  
         [0103]    [0103]FIG. 19 illustrates head Position Error Signal (PES) spectrum experimentally determined as a Non-Repeatable Run Out (NRRO) PES spectrum in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  600  and in a disk system employing dampening mechanism with varying radial widths.  
         [0104]    Results from dampening mechanisms  120  of 25, 17 and 12.5 mm radial width are indicated by traces  602 ,  604 , and  606 , respectively.  
         [0105]    [0105]FIG. 20 illustrates head Position Error Signal (PES) levels experimentally determined in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  600  and in a disk drive employing dampening mechanism with varying radial widths.  
         [0106]    In the experiments illustrated by FIGS. 19 and 20, the pitch of one data track is 0.44 micrometers. The vertical axis indicates the PES level at three standard deviations. Box  600  indicates the experimental results when no dampening mechanism is used. Boxes  602 ,  604 , and  606  indicate the experimental results when dampening mechanisms of one inch, two-thirds inch and one half inch in radial width, respectively, are used. Dampening mechanism  120  was a plate as illustrated in FIG. 23E.  
         [0107]    The experimental results indicate that the 25 mm radial width dampening mechanism has the lowest PES level, supporting the hypothesis that the wide-width dampening mechanism reduces the PES more than the narrow-width dampening mechanism.  
         [0108]    [0108]FIG. 21 illustrates head Position Error Signal (PES) levels experimentally determined in a conventional 57,000 Track-Per-Inch (TPI) disk drive system  600  and in a disk system employing dampening mechanism with varying coverage angles and radial width of one inch or 25 mms.  
         [0109]    In these experiments, the pitch of one data track is 0.44 micrometers. The vertical axis indicates the PES level at three standard deviations. Box  600  indicates the experimental results when no dampening mechanism is used. Boxes  612 ,  614 , and  616 , indicate experimental results when a dampening mechanism with a coverage angle of 200, 130, and 80 degrees, respectively are used.  
         [0110]    The experimental results illustrated in FIG. 21 support the hypothesis that wide-angle dampening mechanisms reduce PES more than narrow-angle dampening mechanisms.  
         [0111]    [0111]FIG. 22 illustrates an extension of the material and analyses of FIGS. 2A and 12A for further preferred embodiments of the invention.  
         [0112]    As in FIGS. 11A and 12A, dampening mechanism  120  includes first dampening surface  122  separated from first disk surface  12 - 1  of disk  12  by essentially air layer Gap  1  as shown in FIGS. 11A to  11 C. Dampening mechanism  120  further includes a second dampening surface  124  separated from a second disk surface  12 - 2 , in this case, of first disk  12  by essentially air layer Gap  2 .  
         [0113]    Dampening mechanism  120  includes a “vertical-plane” disk damper containing a first vertical surface  130  separated from an outer edge  12 - 3  of disk  12  by essentially HGap  1 . The horizontal gap between first vertical surface  130  and the outer edge of disk  12  creates an enclosing disk-edge wave field in the air medium, further contributing to stabilizing the disk  12 .  
         [0114]    As in FIG. 12A, each of these Gaps  1 - 4  is at most a first distance, which is preferably less than 1 mm. Each of the gaps is further preferably greater than 0.3 mm. Each of the gaps is further preferred between 0.35 mm and 0.6 mm.  
         [0115]    One or more of these gaps may preferably be less than the boundary layer thickness. In certain embodiments, one or more of these gaps may preferably be less than a fraction of the boundary layer thickness.  
         [0116]    The invention contemplates using the disk cover  110  to provide at least first dampening surface  122  as part of the dampening mechanism  120  and also using disk cover  110  to further provide first vertical surface  130 .  
         [0117]    [0117]FIG. 22 further illustrates dampening mechanism  120  including a third dampening surface  126  separated from a third disk surface  14 - 1  belonging to a second disk  14  by essentially a third gap, Gap  3 .  
         [0118]    Dampening mechanism  120  may also include the “vertical-plane” disk damper containing a second vertical surface  132  separated from the outer edge  14 - 3  of disk  14  by essentially HGap  2 . The horizontal gap between second vertical surface  132  and outer edge  14 - 3  of disk  14  create an enclosing disk-edge wave field in the air medium, further contributing to stabilizing the disk  14 .  
         [0119]    Dampening mechanism  120  may also include a fourth dampening surface  128  separated from a fourth disk surface  14 - 2  by a fourth gap, Gap  4 .  
         [0120]    Each of the horizontal gaps is at most a second distance, which is preferably less than 1 mm. Each of the gaps is further preferably greater than 0.3 mm. Each of the gaps is further preferred between 0.35 mm and 0.6 mm. One or more of these horizontal gaps may preferably be less than the boundary layer thickness. In certain embodiments, one or more of these horizontal gaps may preferably be less than a fraction of the boundary layer thickness.  
         [0121]    The invention also contemplates using the disk base  100  to provide at least fourth dampening surface  128  as part of the dampening mechanism  120  and also using disk base  100  to further provide second vertical surface  132 .  
         [0122]    FIGS.  23 A- 23 E illustrate various shapes, edges, and materials for a plate used in dampening mechanism  120  of the previous Figures.  
         [0123]    Note that boundaries  140 - 146  are only indicated in FIG. 23E to simplify the other Figures and is not meant to limit the scope of the claims.  
         [0124]    [0124]FIG. 23A illustrates an aluminum plate  120  including a sharp step edge on boundaries  140 ,  144  and  146  with perforations. The perforations are preferably about 5 mm is diameter to optimally reduce actuator vibration. FIG. 23B illustrates a hard plastic, preferably a polycorbonate material such as LEXAN®, plate  120  including a wedge type edge on boundaries  140 ,  144  and  146 . FIG. 23C illustrates a hard plastic plate  120  including a sharp step edge on boundaries  140 ,  144  and  146 . FIG. 23D illustrates an aluminum plate  120  including a round chamfer edge on boundaries  140 ,  144  and  146 . FIG. 23E illustrates an aluminum plate  120  including a sharp step edge on boundaries  140 ,  144  and  146 . In embodiments using an aluminum plate, the plates may preferably include a coating of Aluminum Plus on one or more surfaces.  
         [0125]    The invention further contemplates plates such as illustrated in FIGS.  23 A- 23 E further including fingers formed to disrupt formation of vortices in the neighborhood of the actuator and its components.  
         [0126]    The disk drive system employing dampening mechanisms  120  as illustrated in the previous Figures also benefits from reduced noise levels. Table 3 below illustrates experiments conducted upon several disk drives employing two disks rotating at 7200 revolutions per minute. The experiments used a preferred dampening mechanism  120  illustrated in FIG. 23D with a Gap of 0.5 mm, radial width of ⅔ in, or 17 mm, and a coverage angle of 200 deg.  
                                 TABLE 3                           Acoustic Noise with no   Acoustic noise with           dampening mechanism   dampening mechanism       Drive No.   (Sound power level: dB)   (Sound Power Level: dB)                   1   27.8   25.6       2   28.3   26.1       3   28.6   26.1       4   28.4   26.1       5   26.9   24.9       Average value   28.0   25.8            Average Reduction   2.2                  
 
         [0127]    The preceding embodiments have been provided by way of example and are not meant to constrain the scope of the following claims.