Abstract:
A method for reducing the length of servo-wedges in a disk drive comprising a concentric tracks, each track comprising embedded servo-wedges each having a track identification field (TIF) and a servo-wedge identification field (WIF), wherein the embedded servo-wedges are grouped into servo-wedge groups comprising a first and second sub-group of servo-wedges. The method includes selecting a servo-wedge from a selected servo-wedge group; storing a first subset of a track identification data (TID) corresponding to a selected track in TIF of selected servo-wedge; storing a second subset of TID in a first portion of WIF of selected servo-wedge if selected servo-wedge is in second sub-group; storing a first wedge identification data (WID) corresponding to selected servo-wedge in a second portion of the WIF if selected servo-wedge is in second sub-group; and storing a second WID corresponding to selected servo-wedge in WIF if selected servo-wedge is in first sub-group.

Description:
FIELD OF THE INVENTION 
   This invention relates to servo-wedges on disks in a disk drive. More particularly, the invention is related to reducing the overhead associated with use of servo-wedges in a disk drive. 
   BACKGROUND OF THE INVENTION 
   Disk drives conventionally partition disk surfaces via a series of angularly-spaced embedded servo-wedges disposed on the disk surfaces between data-wedges which contain data tracks with data sectors recorded in the intervals between servo-wedges on each track. The servo-wedges are used in positioning and maintaining a head over a desired track during write and read operations. Typically, servo-wedges are sampled at regular intervals by a read/write channel, and are processed by a servo controller to provide position information to a microprocessor for positioning a head over a desired track. 
   While the servo-wedges are essential to the operation of the disk drive, their inclusion on the disk surface results in a reduction of the disk surface area available for the data-wedges which in turn translate into a reduction in data capacity of a disk drive. As such, the servo-wedges are considered an overhead in the storage of data on a disk drive. The constant demand for increased data capacity of a disk drive has resulted in an increased number of tracks per inch and/or bits per track on a disk surface. This, however, has also required an increased number of embedded servo-wedges disposed on the disk surface, and thus increased the associated overhead to the disk drive, hindering efforts to maximize the data capacity of a disk drive. 
   Accordingly, what is needed is a reduction in the overhead associated with the use of embedded servo-wedges on a disk surface of a disk drive. 
   SUMMARY OF THE INVENTION 
   This invention can be regarded as a method for reducing the length of the servo-wedge a disk drive comprising a plurality of concentric tracks, each track comprising an embedded servo-wedge having a track identification field and a servo-wedge identification field. The method includes storing a first subset of a track identification data corresponding to a selected track in the track identification field of the servo-wedge of the track; storing a second subset of the track identification data in a first portion of the wedge identification field; and storing a subset of a wedge identification data corresponding to the embedded servo-wedge in a second portion of the wedge identification field. 
   This invention can also be regarded as a method for reducing the length of a servo-wedge in a disk drive comprising a plurality of concentric tracks, each track comprising a plurality of embedded servo-wedges each having a track identification field and a servo-wedge identification field, wherein the plurality of embedded servo-wedges are grouped into at least one servo-wedge group comprising a first sub-group and a second sub-group of servo-wedges. The method includes selecting a servo-wedge from a selected servo-wedge group; storing a first subset of a track identification data corresponding to a selected track in the track identification field of the selected servo-wedge; storing a second subset of the track identification data in a first portion of the wedge identification field of the selected servo-wedge if the selected servo-wedge is in the second sub-group; storing a first wedge identification data corresponding to the selected servo-wedge in a second portion of the wedge identification field of the selected servo-wedge if the selected servo-wedge is in the second sub-group; and storing a second wedge identification data corresponding to the selected servo-wedge in the wedge identification field of the selected servo-wedge if the selected servo-wedge is in the first sub-group. 
   This invention can also be regarded as a disk drive comprising a plurality of concentric tracks, each track comprising a plurality of embedded servo-wedges each having a track identification field and a servo-wedge identification field, wherein the plurality of embedded servo-wedges are grouped into at least one servo-wedge group comprising a first sub-group and a second sub-group of servo-wedges. The disk drive further comprises the track identification field adapted to store a first subset of a track identification data corresponding to a selected track in a selected servo-wedge in a selected servo-wedge group. 
   The wedge identification field further comprises a first portion adapted to store a second subset of the track identification data corresponding to the selected servo-wedge if the selected servo-wedge is in the second sub-group, a second portion adapted to store a first wedge identification data corresponding to the selected servo-wedge if the selected servo-wedge is in the second sub-group, and wherein the wedge identification field is further adapted to store a second wedge identification data corresponding to the selected servo-wedge if the selected servo-wedge is in the first sub-group. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary hard disk drive in which the present invention may be practiced. 
       FIG. 2  illustrate a disk formatted for use with a disk drive employing an embodiment of the present invention. 
       FIG. 3  is a flow chart illustrating a process used in an embodiment of the invention shown in  FIG. 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 1 , an exemplary hard disk drive  100  in which the present invention may be practiced is shown. As shown, the hard disk drive  100  includes a head disk assembly (HDA)  105  having one or more disks  102  with a magnetic media  101  formed on each surface  103  of a disk  102 . The HDA  105  further comprises a transducer head  114  mounted on a rotary actuator  116  that rotates about a pivot  120  via controlled torques applied by a voice coil motor  122 . While the disk drive  100  is in operation, the disk  102  rotates in an exemplary direction  113  about the axis of the spindle  104  at a substantially fixed angular speed such that the surface  103  of the disk  102  moves relative to the head  114 . 
   As shown in  FIG. 1 , a signal bus  124 , such as a flex cable, interconnects the HDA  105  to a control system  202  which can control the movement of the actuator  116  in a manner well known in the art. In addition, the control system  202  sends to and receives signals from the head  114  during read and write operations performed on the disk  102 . As also shown in  FIG. 1 , the control system  202  is interconnected to the interface control system  203  which is in turn interconnected to a host computer  138  by a bus  140  for transferring of data between the hard disk drive  100  and the host  138 . 
     FIG. 2  illustrate a disk  102  formatted for use with the disk drive  100  shown in  FIG. 1 . The disk  102  has a surface  103  that is partitioned into a series of angularly-spaced embedded servo-wedges  6 , such as W_ 0  through W_N, disposed on the disk surfaces  103  between data-wedges  7  which contain data tracks  8  with data sectors  9  recorded in the intervals between servo-wedges  6  on each track  8 . The servo-wedges  6  are used in positioning and maintaining the head  114  over a desired track  8  during write and read operations. In an embodiment of the present invention, each disk surface  103  is logically partitioned into wedge-groups  5 , with each wedge-group  5  having the same number of adjacent servo-wedges  6 , such as eight, as the other wedge-groups  5 . 
   For ease of illustrating the process of the present invention, an exemplary servo-wedge diagram  5   a  corresponding to the prior art, and an exemplary servo-wedge diagram  5   b  corresponding to the present invention is provided in  FIG. 2  and used throughout the detailed description. As shown by lines  205 , the servo-wedge diagrams  5   a  and  5   b  are each a linear representation of the angularly-spaced embedded servo-wedges  6  in a wedge-group  5  of a selected track  8  on the disk surface  103 . For exemplary purposes only, each of servo-wedge diagrams  5   a  and  5   b  represent a wedge-group  5  having eight servo-wedges  6  (W_ 0  through W_ 7 ), although wedge-groups of the other sizes are also contemplated to be within the scope of the present invention. 
   In the prior art servo-wedge diagram  5   a , each servo-wedge  6  comprises a track identification field  20  and a servo-wedge identification field  22 , wherein the servo-wedges  6  are grouped into at least one servo-wedge group  5  comprising a first sub-group of servo-wedges, such as sub-group  1 , and a second sub-group of servo-wedges, such as sub-group  2 . The determination as to which sub-group a particular servo-wedge  6  belongs is well known in the art and is generally made by obtaining a modulo of the wedge number (e.g.  0  through  7 ) divided by the number of wedges in the servo-group (e.g.  8 ), resulting in sub-group_ 1  comprising a single servo-wedge  6 , such as W_ 0 , and the sub-group_ 2  comprising the remaining seven servo-wedges  6 , such as W_ 1  through W_ 7  (partially shown) in the servo-group. 
   As shown, each of track identification fields  20  in sub-group_ 1  and sub-group_ 2  comprises 18 bits of track identification data corresponding to the selected track  8  with a most significant bits (MSB) portion  20   a  of generally 6-bits in length, followed sequentially by a least significant bits (LSB)  20   b  portion of generally 12 bits in length. The servo-wedge identification field  22  in sub-group_ 1  and sub-group_ 2  comprises 9 bits in length. In sub-group_ 1 , the servo-wedge identification field  22  is adapted to store a wedge identification data WD 0  of 9 bits in length, containing a full address of the servo-wedge identification data. In sub-group_ 2 , however, each servo-wedge identification field  22  is partitioned into two portions  22   a  and  22   b  of 6-bit and 3 bits in length, respectively. Each 3-bits portion  22   b  is used to store a wedge identification data, such as one of WD 1  through WD 7  as shown on  FIG. 2 , each of which is a subset of the 9-bit WD 0 , suitably an index based on WD 0 , such as an incremental value added to WD 0 . The 6-bits portion  22   a , however, is left unused. Portions  22   a  in each of the seven servo-wedge identification fields  22  of sub-group_ 2  are therefore in essence wasted space on disk surface  103  and contribute to the over-head associated with the use of servo-wedges in on a disk  102 . 
     FIG. 3  in conjunction with  FIG. 2 , illustrates a process used in an embodiment of the invention for reducing in the overhead associated with the use of embedded servo-wedges  6  on a disk surface  103  of a disk  102 . For ease of illustrating the process of the present invention, the above described exemplary servo-wedge diagram  5   b  corresponding to the present invention is used in conjunction with  FIG. 3 . In the servo-wedge diagram  5   b , each servo-wedge  6  comprises a track identification field  24  and a servo-wedge identification field  26 , wherein the servo-wedges  6  are grouped into at least one servo-wedge group  5  comprising a first sub-group of servo-wedges, such as sub-group_A, and a second sub-group of servo-wedges, such as sub-group_B. 
   The determination as to which sub-group a particular servo-wedge  6  belongs is made by obtaining a modulo of the wedge number (e.g.  0  through  7 ) divided by the number of wedges in the servo-group (e.g.  8 ), resulting in sub-group_A comprising a single servo-wedge  6 , such as W_ 0 , and the sub-group_B comprising the remaining servo-wedges  6 , such as W_ 1  through W_ 7 , in the servo-group. As shown in  FIG. 2 , each track identification field  24  in sub-group_A and sub-group_B is adapted to store 12 bits of data. Each servo-wedge identification field  26  in sub-group_A and sub-group_B is adapted to store data up to 9 bits in length. In sub-group_A, the servo-wedge identification field  26  is adapted to store data of 9 bits in length. In sub-group_B, however, each servo-wedge identification field  26  is partitioned into two portions  26   a  and  26   b  of 6-bit and 3 bits in length, respectively. 
   Referencing  FIG. 3 , the process begins at block  310  in which a servo-wedge  6 , such as any one of servo-wedge W_ 0  through W_ 7  in diagram  5   b , is selected from a servo-wedge group  5 . Next in block  312 , a first subset of a track identification data corresponding to a selected track  8  is stored in the track identification field  24  of the selected servo-wedge  6 . Suitably, the first subset of the track identification data comprises the least significant portions, such as the least significant bits (LSB) of the track identification data, and comprises 12 bits of data. 
   Next, in block  316 , a second subset of the track identification data is stored in a first portion of the wedge identification field  26  of the selected servo-wedge if the selected servo-wedge is in the second sub-group. Suitably, the second subset of the track identification data comprises the most significant portions, such as the most significant bits (MSB) of the track identification data, and comprises 6 bits of data. In the exemplary servo-wedge diagram  5   b , if any of the servo-wedges W_ 1  through W_ 7  which are in sub-group_B (i.e. the second sub-group) is selected, then the most significant bits (MSB) of the track identification data are is stored in the portion  26   a  of the servo-wedge identification field  26  of a selected servo-wedge, such as W_ 1 . 
   Suitably, the second subset of the track identification data is stored sequentially to the first subset of the track identification data, such as shown by portion  26   a  and track identification field  24  in the exemplary servo-wedge diagram  5   b . In addition, the first subset of the track identification data (i.e. the 12 LSB) and the second subset of the track identification data (i.e. the 6 MSB) are suitably each separately encoded with a Gray Code. For example, for a complete 18-bit track identification data represented by the decimal 65536, the process converts the decimal 65536 into hexadecimal FFFF, and then into 16 bits binary string of 1111111111111111. Two zeros are then allocated to left of the 16 bits sequence to generate an 18 bits string of 001111111111111111 that conforms to the 18-bit track identification data storage format. The 18-bits string is then partitioned into two strings of a 6-bits MSB of 001111, and a 12 bits LSB of 111111111111. Encoding each string using Gray Code (well known in the art) results in a 6-bit MSB Gray-encoded string of 001000 that is then stored in portion  26   a , and a 12-bit LSB Gray-encoded string of 100000000000 that is then stored in the track identification field  24 . 
   Next, in block  318 , a first wedge identification data corresponding to the selected servo-wedge is stored in a second portion of the wedge identification field  26  of the selected servo-wedge if the selected servo-wedge is in the second sub-group. Suitably, the first subset of the wedge identification data comprises the least significant portions of the wedge identification data, and comprises 3 bits of data. In the exemplary servo-wedge diagram  5   b , if any of the servo-wedges W_ 1  through W_ 7  which are in sub-group_B (i.e. the second sub-group) is selected, then the 3-bits portion  22   b  of each servo-wedges is used to store one of WD 1  through WD 7  wedge identification data as shown on  FIG. 2 , each of which is a subset of the 9-bit WD 0  of portion  24 , suitably an index based on WD 0 , such as an incremental value added to WD 0 . Suitably, the first wedge identification data is stored sequentially to the second subset of the track identification data, such as shown by portions  26   b  and  26   a  in the exemplary servo-wedge diagram  5   b.    
   Next, in block  320 , a second wedge identification data corresponding to the selected servo-wedge is stored in the wedge identification field  26  if the selected servo-wedge is in the first sub-group. Suitably, the second wedge identification data comprises 9 bits of data comprising the least significant portions, such as the least significant bits (LSB), and the most significant portions such as the most significant bits (MSB) of the complete wedge identification data. In the exemplary servo-wedge diagram  5   b , if servo-wedge W_ 0  which is in sub-group_A (i.e. the first sub-group) is selected, then the most significant bits (MSB) and the least significant bits (LSB) of the wedge identification data (i.e. WD 0 ) are stored in the servo-wedge identification field  26  of the selected servo-wedge W_ 0 . Suitably, each of the first wedge identification data (i.e. WD 1  through WD 7 ) is a subset of the second wedge identification data (i.e. WD 0 ) suitably an index based on WD 0 , such as an incremental value added to WD 0 , and comprises the least significant portions of WD 0 . 
   During the operations of the disk drive  100 , the servo-wedges  6  are sampled at regular intervals by a read/write channel (not shown), and are processed by a servo controller (not shown). In the present invention, the servo controller and associated firmware suitably are adapted to provide position information to a microprocessor for positioning a head over a desired track based on first obtaining the least significant bits (LSB) and then the most significant bits (MSB) of the track identification data for each servo-wedges  6  in the sub-group_B of a servo-wedge group. Suitably, a track-estimator subsystem (not shown) well known in the art is used by the servo controller and associated firmware to compensate for lack of the most significant bits (MSB) of the track identification data for each servo-wedges  6  in the sub-group_A of a servo-wedge group. 
   One advantage of the foregoing feature of the present invention over the prior art is that by reducing the length of the track identification field  24  from 18 bits to just 12 bits in the manner described above, the overall length of a servo-wedges may be reduced. In this way, the present invention provides for a reduction in the overhead associated with the use of embedded servo-wedges on a disk surface of a disk drive. 
   It should be noted that the various features of the foregoing embodiments were discussed separately for clarity of description only and they can be incorporated in whole or in part into a single embodiment of the invention having all or some of these features.