Abstract:
To alleviate excessive non-repeatable runout (NRRO) on heads between discs, an original multi-disc design is adapted by (a) reducing the number of discs in the stack and (b) configuring the disc drive of the adapted design with a larger nominal inter-disc separation than that of the original design. In one embodiment, the larger separation is maintained by an increased number of disc spacers between each consecutive pair of the discs in the disc stack of the modified design. The result of such a design adaptation is a disc drive with better performance characteristics than would exist by the original design.

Description:
RELATED APPLICATIONS  
       [0001]    This application claims priority of U.S. provisional application Serial No. 60/303,581, filed Jul. 5, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This application relates generally to data storage devices and more particularly to an apparatus and method for improving disc drive actuator servo performance.  
         BACKGROUND OF THE INVENTION  
         [0003]    Computers commonly use disc drives or tape drives to store large amounts of data in a form that can be readily accessed by a user. A disc drive generally includes a stack of vertically spaced magnetic discs that are rotated at high speed by a spindle motor. The surface of each disc is divided into a series of concentric, radially spaced data tracks in which the data are stored in the form of magnetic flux transitions. Typically, each data track is divided into a number of data sectors that store data blocks of a fixed size.  
           [0004]    Data are stored and accessed on the discs by an array of read/write heads mounted to a rotary actuator assembly, or “E-block.” Typically, the E-block includes a plurality of actuator arms which project outwardly from an actuator body to form a stack of vertically spaced actuator arms. The stacked discs and arms are configured so that the surfaces of the stacked discs are accessible to the heads mounted on the interleaved stack of actuator arms.  
           [0005]    Head wires included on the E-block conduct electrical signals from the heads to a flex circuit, which in turn conducts the electrical signals to a flex circuit bracket mounted to a disc drive basedeck. For a discussion of some modern E-block assembly techniques, see U.S. Pat. No. 5,404,636 entitled “Method of Assembling a Disk Drive Actuator” issued Apr. 11, 1995 to Stefansky et al., and assigned to the assignee of the present invention.  
           [0006]    The actuator body pivots about a cartridge bearing assembly which is mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The actuator assembly includes a voice coil motor which enables the actuator arms and the heads attached thereto to be rotated about the cartridge bearing assembly so that the arms move horizontally (i.e. in a plane parallel to the surfaces of the discs) to selectively position a head adjacent to a preselected data track.  
           [0007]    The voice coil motor includes a coil mounted radially outwardly from the cartridge bearing assembly, the coil being immersed in the magnetic field of a magnetic circuit of the voice coil motor. The magnetic circuit comprises one or more permanent magnets and magnetically permeable pole pieces. When current is passed through the coil, an electromagnetic field is established which interacts with the magnetic field of the magnetic circuit so that the coil moves in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces.  
           [0008]    Each of the heads is mounted to an actuator arm by a flexure which attaches to the end of the actuator arm. Each head includes an interactive element such as a magnetic transducer which either senses the magnetic transitions on a selected data track to read the data stored on the track, or transmits an electrical signal that induces magnetic transitions on the selected data track to write data to the data track. Air currents are caused by the high speed rotation of the discs. A slider assembly included on each head has an air bearing surface which interacts with the air currents to cause the head to fly at a short distance above the data tracks on the disc surface.  
           [0009]    There is a generally recognized trend in the industry to increase track density, making more and more accurate track following necessary. At the same time, increasing disc rotation speeds have resulted in more and more noise energy being transferred to each arm and head-gimbal assembly by wind. This acts as a disturbance having energy distributed across a wide spectrum of frequencies. This makes accurate track following difficult, especially when it includes significant energy at any of the resonance frequencies of the arms. Thus, there is a need for an improved technique for reducing wind-induced disturbances upon arms and head-gimbal assemblies of the disc drive.  
           [0010]    Meanwhile, there has been a trend among disc drive manufacturers to construct drives of various capacities, for a given housing size or “form factor.” Once a design is complete for a “fully populated” disc drive, one or more modified designs of so-called “de-populated” disc drives are hastily generated. This widespread approach merely scales down the fully populated design, failing to recognize the synergy achievable by optimizing a de-populated design. Thus, there is a need for ways to take appropriate advantage of this opportunity.  
           [0011]    The present invention provides a solution to these and other problems, and offers other advantages over the prior art.  
         SUMMARY OF THE INVENTION  
         [0012]    It has been observed that disc drives with more than one disc show considerably more non-repeatable runout (NRRO) on inner heads (i.e. between discs). To alleviate this, an original multi-disc design is adapted by (a) reducing the number of discs in the stack and (b) configuring the drive of the adapted design with a larger nominal inter-disc separation than that of the original design. In one embodiment, the larger separation is maintained by an increased number of disc spacers between each consecutive pair of the discs in the disc stack of the modified design. The result of such a design adaptation is a disc drive with better performance characteristics than would exist by the original design. For example, because of a reduced level of windage-induced NRRO, the disc drive of the adapted design can permit data tracks servo-written at a higher density than would otherwise be feasible.  
           [0013]    These and various other features as well as additional advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 shows a partially exploded, fairly complete view of a data storage device constructed in accordance with the present invention.  
         [0015]    [0015]FIG. 2 shows a method of the present invention for constructing and operating disc drives according to an improved design.  
         [0016]    [0016]FIG. 3 shows key portions of a prototype disc drive and of a modified-design disc drive made according to the method of FIG. 2.  
         [0017]    [0017]FIG. 4 shows how NRRO is much worse for “inner heads” in a fully-populated disc drive but not for a particular drive of the present invention.  
         [0018]    [0018]FIGS. 5&amp;6 show an air flow model of a closely-spaced pair of discs, illustrating why windage causes NRRO for inner heads in conventional fully-populated and de-populated disc drives.  
         [0019]    [0019]FIGS. 7&amp;8 show an air flow model for a pair of discs maintained at a greater inter-disc separation, illustrating how the present invention can be used to create higher-performance disc drives of a de-populated design. 
     
    
     DETAILED DESCRIPTION  
       [0020]    Although the examples below show more than enough detail to allow those skilled in the art to practice the present invention, subject matter regarded as the invention is broader than any single example below. The scope of the present invention is distinctly defined, however, in the claims at the end of this document.  
         [0021]    Numerous aspects of data storage device technology that are not a part of the present invention (or are well known in the art) are omitted for brevity, avoiding needless distractions from the essence of the present invention. For example, this document does not include much detail about servo track writing or controlling track density. Neither does it include methods for constructing discs or alternating discs with spacers on clamped disc stacks.  
         [0022]    Definitions and clarifications of certain terms are provided in conjunction with the descriptions below, all consistent with common usage in the art but some described with greater specificity. A “design” of a disc drive, for example, refers herein to a mechanical and electrical description of the disc drive that is generally considered sufficient to permit the disc drive to be constructed without significant creativity. A so-called “de-populated” disc drive conventionally refers to one having a lower number of discs and/or heads than an otherwise similar “original” disc drive design upon which its design is based. “NRRO” conventionally refers to 3-sigma non-repeatable runout, three times a calculated standard deviation of radial positions measured relative to an ideal track center. Except as noted, NRRO dimensions are given in microinches, which is standard in the disc drive industry.  
         [0023]    Turning now to FIG. 1, there is shown a data storage device  100  constructed in accordance with a preferred embodiment of the present invention. Device  100  is a disc drive including base  102  to which various components are mounted. Top cover  123  cooperates with base  102  conventionally to form a sealed chamber. The components include a spindle motor which rotates data storage discs  110  at several thousand revolutions per minute. Information is written to and read from tracks  112  on discs  110  through the use of an actuator assembly  161 , which rotates during a seek operation about a bearing shaft assembly  130  positioned adjacent discs  110 . Actuator assembly  161  includes a plurality of actuator arms which extend above and below each disc  110 , with one or more flexures extending from each of the actuator arms. Mounted at the distal end of each of the flexures is a transducer head  134  which includes an air-bearing slider enabling transducer head  134  to fly in close proximity above the corresponding surface of associated disc  110 .  
         [0024]    Servo and user data travels through transducer head  134  and flex cable  180  to control circuitry on controller board  106 . Flex cable  180  maintains an electrical connection by flexing as heads  134  traverse tracks  112  along their respective radial paths  138 . By “radial,” it is meant that path  138  is substantially aligned with a radius of the disc(s)  110 , although their directions may be offset from a perfectly radial direction (such as  115 ) by up to about 20 degrees due to head skew, as is understood in the art.  
         [0025]    During a seek operation, the overall track position of transducer heads  134  is controlled through the use of a voice coil motor (VCM), which typically includes a coil  122  fixedly attached to actuator assembly  161 , as well as one or more permanent magnets  120  which establish a magnetic field in which coil  122  is immersed. The controlled application of current to coil  122  causes magnetic interaction between permanent magnets  120  and coil  122  so that coil  122  moves. As coil  122  moves, actuator assembly  161  pivots about bearing shaft assembly  130  and transducer heads  134  are caused to move across the surfaces of discs  161  between the inner diameter and outer diameter of the disc(s)  161 . Fine control of the position of head  134  is optionally made with a microactuator (not shown) that operates between the head  134  and the actuator arm. Finally, a side view indicator  300  is shown to illustrate how actuator assembly  161  is situated with respect to discs  110  in conjunction with an embodiment of the invention depicted in FIG. 3.  
         [0026]    [0026]FIG. 2 shows a method  200  of the present invention comprising steps  205  through  265 . A first disc drive is constructed with several discs, a successive pair of which are separated by a first nominal distance  210 . Tracks are written into the drive at a first track density  220 . A second disc drive is constructed similar to the first but with a larger disc separation distance and only one head per actuator arm  225 .  
         [0027]    It should be noted that a typical rotary actuator can move enough to change the head-to-head skew by several tracks (if radial) and/or several bits (if circumferential). These shifts can be caused by mechanical disturbances such as thermal variations. This shift was not especially significant under a prior art method, which simply used each head&#39;s position (with a static offset) to derive an initial estimate for a position of a consecutive head&#39;s position. Unfortunately, this method is not generally effective for estimating a position of a head having a more substantial vertical separation between heads, such as exist with the use of the present invention.  
         [0028]    To address this problem, a method is presented that is effective for estimating head position during a head switch during servo writing, certification or normal operation. A first horizontal shift S is detected between first and second transducer heads (see  331 , 332  of FIG. 3). To prepare for a head switch to a third transducer head (see  333  of FIG. 3), a position estimate is derived from the shift S and a ratio of the known vertical distances V 1  &amp; V 2 . In one embodiment, this dynamic mechanical shift estimate (S×V 2 /V 1 ) is simply added to a conventional initial estimate of the third transducer head&#39;s position.zf  
         [0029]    Before or after the optional head switch adjustment method of steps  235  &amp;  240 , the a second disc drive is servo written  250 . (Note that a higher track density will generally be possible by virtue of the reduced NRRO resulting from the larger inter-disc spacing in the second drive.) Additional disc drives are then made according to the second disc drive  255 .  
         [0030]    Turning now to FIG. 3, there are shown key portions of a prototype disc drive  301  and of a modified-design disc drive  302  made according to the method of FIG. 2. Prototype disc drive  301  has four discs  311  rotating about axis  351 . The discs each have nominal thickness  316  and nominal radius  318 . Between each two consecutive discs in the stack, there is a nominal separation distance  317 . Outer arms  312  are shown above the top disc and below the bottom disc, each supporting one transducer head. Three inner arms  313  are shown, each interleaved between two consecutive discs, each inner arm  313  supporting two heads facing in opposite directions to access a respective data surface.  
         [0031]    Modified-design disc drive  302  has two discs  321  rotating about axis  352 . The discs each have nominal thickness  326  (equal to  316 ) and nominal radius  328  (less than  318 ). Between discs  321  is a nominal separation distance  327  several times larger than  317 . Two inner &amp; two outer arms  322  each supports only one transducer head  331 , 332 , 333 . With this structure, each of the arms experiences a similar amount of windage-induced NRRO, and each performs similarly.  
         [0032]    In accordance with the preferred embodiment described above with reference to steps  235  &amp;  240  of FIG. 2, vertical offset values between heads are given. It should be understood that V 1  of step  240  can be estimated as nominal thickness  326 , and that V 2  of step  240  can be estimated as separation distance  327 .  
         [0033]    Turning now to FIG. 4, head number  402  is plotted against NRRO  401 . Data plot  473  shows the performance of a fully populated disc drive like that of item  301  in FIG. 3. To varying degrees, heads between the discs (i.e. heads  1  through  6 ) generally suffer much more windage-induced NRRO than end-arm heads  0  and  7 . (As used herein and consistent with industry usage, “much more” means at least about 10% more.) Data plot  449  shows a comparable performance indicator of a de-populated disc drive like that of item  302  in FIG. 3. (Note that heads  2  through  5  are absent, those corresponding to the two middle discs that have been removed in the present de-populated design.) The worst-case head (i.e. head  7 ) of the de-populated design has an NRRO not much more than that of the best-case head (i.e. head  0 ).  
         [0034]    [0034]FIG. 5 shows a finite element model (generated by FLUENT 5.0 software) of a disc drive  500  having two discs  509  supported on a spindle  543  rotating within a chamber  509  and about an axis  528 . Direction lines show substantially inward air flow along the edges of the chamber  509  and substantially outward air flow along the surfaces of the discs  509 . From the same model, FIG. 6 shows the regions of fastest air flow. Only within region  530  does the flow speed exceed 30 meters per second, and only within region  535  does the flow speed exceed 35 meters per second. In present-day disc drives, flow speeds of these magnitudes induce a significant disturbance upon actuator arms, particularly when the heads are near their innermost tracks.  
         [0035]    [0035]FIGS. 7&amp;8 show a similar finite element model of a disc drive  700 , likewise generated by FLUENT 5.0 software but with a much larger vertical separation between discs. Note that no air flow exceeding 35 meters per second is expected, and that only two small pockets of air flow exceeding 30 meters per second are expected.  
         [0036]    Alternatively characterized, a first embodiment of the present invention is a method for making a modified-design disc drive having a plurality of (N) coaxially stacked data storage discs. An original disc drive (such as  301 ) is first constructed (such as by step  210 ) with more than N original-type discs (such as  311 ) are within a first housing (such as  102  with  123 ), a successive pair of the original-type discs being separated by a first nominal distance D (such as  317 ). Then, the modified-design disc drive(s) are constructed (such as by step  225 ) so that each consecutive pair of the discs are separated by a greater nominal distance (such as  327 ).  
         [0037]    In a second embodiment, the original disc drive further includes an inner head (such as  313 ) that suffers much more windage-induced non-repeatable runout than an outer head (such as  312 ) suffers. The modified-design disc drive has an inner head (such as  332 ) that does not suffer much more windage-induced NRRO than an outer head (such as  331 ) suffers. Preferably, the method for constructing the modified-design disc drive includes a step placing more spacers between the discs, and does not include any step of supporting two oppositely-facing transducer heads on an actuator arm of the modified-design disc drive (such as that of FIG. 3).  
         [0038]    In a third embodiment, a first horizontal shift in a position of a first transducer head relative to a second transducer head is detected (e.g. by step  235 ). An estimated horizontal position of a third transducer (such as  333 ) is determined from the shift and from the predetermined estimates of the vertical offsets (such as  326  &amp;  327 ) from the second head to the first and third heads. This is useful for accounting for actuator tilt when the distanc to the third head is several times larger than the distance to the first head, especially during a power-on calibration of the static (horizontal) offsets between heads. It can optionally be accomplished by scaling the detected shift linearly based upon indicators of the vertical distances V 1  and V 2 .  
         [0039]    In a fourth embodiment, thousands of tracks are servo-written into each of the two disc drives, the modified-design disc drive being servo-written at a higher nominal track density. This results in a modified-design disc drive that capitalizes on the improved performance resulting from the present drive design modification method. A de-populated disc drive having two discs can thus replace a fully populated disc drive having four, reducing the capacity by less than 50%.  
         [0040]    In a fifth embodiment, the modified-design disc drive is constructed with discs thin enough so that D/H is larger for the modified-design disc drive than for the original disc drive, where D is the nominal distance between consecutive discs and where H is the nominal disc thickness. More preferably, the modified-design disc drive&#39;s D/H is also greater than about 2.5 (i.e. that of separation  317  relative to thickness  316 ). Most preferably, the modified-design disc drive&#39;s D/H is at least about 9.5 (i.e. that of separation  327  relative to thickness  326 ).  
         [0041]    In a sixth embodiment, the modified-design disc drive is constructed with discs small enough so that D/R is larger for the modified-design disc drive than for the original disc drive, where D is the nominal distance between consecutive discs and where R is the nominal disc radius. More preferably, the modified-design disc drive&#39;s D/R is greater than about 0.024 (i.e. that of separation  317  relative to radius  318 ). Most preferably, the modified-design disc drive&#39;s D/R is at least about 0.23 (i.e. that of separation  327  relative to radius  328 ).  
         [0042]    All of the structures and methods described above will be understood to one of ordinary skill in the art, and would enable the practice of the present invention without undue experimentation. It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only. Changes may be made in the details, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, adaptations of an original disc drive descibed herein can be used to improve new “fully populated” disc drive designs, as well as those of the prior art, without departing from the scope and spirit of the present invention. In addition, although the preferred embodiments described herein are largely directed to magnetic disc drives, it will be appreciated by those skilled in the art that many teachings of the present invention can be applied to optical and magneto-optical disc drives without departing from the scope and spirit of the present invention.