Patent Publication Number: US-2007097806-A1

Title: Disk drive that compensates for track radial pitch variation and methods thereof

Description:
RELATED APPLICATION  
      This application claims the benefit of and priority to U.S. Provisional Patent Application No. 60/733,074, filed Nov. 3, 2005, the disclosure of which is hereby incorporated herein by reference as if set forth in its entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to disk based storage devices and, more particularly, to positioning heads based on servo burst patterns on a disk.  
     BACKGROUND OF THE INVENTION  
      A simplified diagrammatic representation of a disk drive, generally designated as  10 , is illustrated in  FIG. 1 . The disk drive  10  includes a data storage disk  12  that is rotated by a spindle motor  14 . The spindle motor  14  is mounted to a base plate  16 . An actuator arm assembly  18  is also mounted to the base plate  16 .  
      The actuator arm assembly  18  includes a read/write head  20  mounted to a flexure arm  22  which is attached to an actuator arm  24  that can rotate about a pivot bearing assembly  26 . The actuator arm assembly  18  also includes a voice coil motor (VCM)  28  which moves the head  20  relative to tracks defined on the disk  12 . The spindle motor  14 , VCM  28 , and head  20  are coupled to a number of electronic circuits  30  mounted to a printed circuit board  32 . Although a single disk  12  is illustrated in  FIG. 1 , the disk drive  10  may instead include a plurality of disks with a head adjacent to each disk storage surface to read/write therefrom.  
       FIG. 2  is an exemplary top view of the disk  12 . Data is stored on the disk  12  within a number of concentric tracks  40  (or cylinders). Each track is divided into a plurality of radially extending sectors  42  of the disk  12 . Each sector  42  is further divided into a servo sector  44  and a data sector  46 . Information in the servo sectors  44  is used to, among other things, accurately position the head  20  so that host data can be properly written onto and read from the data sectors  46 .  
       FIG. 3  illustrates exemplary servo information  73  that can be stored in each of the servo sectors  44 . The servo information  73  can include a DC erase field  731 , a preamble field  732 , a servo address mark (SAM) field  733 , a track number field indicated by its least significant bits (LSBs)  734 , a spoke number field  735 , an entire track number field  736  which is recorded in at least one of the servo sectors  44 , and a servo burst field  737  of circumferentially staggered radially offset servo bursts (e.g., A, B, C, D servo bursts). The DC erase field  731  can indicate to the circuits  30  the onset of a servo sector  44 . The preamble  732  may be used by timing and gain loops in the circuits  30  to establish a gain and phase lock relationship for sampling the analog signal that is generated when reading the servo information through the head  20 .  
      A servo controller in the electronic circuits  30  determines the position of the head  20  relative to the tracks  40  in response to the servo information read from the servo sectors  44 . The servo controller uses the determined position to move the head  20  from an initial track to a target track (i.e., seek operation), and to maintained the head  20  aligned with the target track while data is read/written on the disk  12  (i.e., track following operation). During a seek operation, the track addresses are used as coarse positioning information to estimate the position of the head  20  as it is moved to the target track. During track following, the servo bursts are used as fine positioning information to precisely align the head  40  over the selected track.  
      A servo track writer (STW) can be used to the write the servo information  73  in the servo sectors  44  during a manufacturing process. To form the data tracks  40  across the disk  12 , the STW controls each head  20  to write servo information at locations that are distributed across the disk  12  with incremental radial steps (pitch) therebetween. An attempt is made to write the servo information  73  with a constant pitch, so that the resulting data tracks  40  will have a constant pitch across the disk  12 . As used herein, the term “pitch” is the radial distance between centers of adjacent regions on the surface of a disk  12 . For example, track pitch  48  (shown in  FIG. 2 ) is the distance between the centers of two radially adjacent tracks  40 .  
      During manufacturing, an attempt may be made to correct excessive track pitch variation by identifying the addresses for groups of tracks that have insufficient track pitch or excessive track pitch, and storing those track addresses in a table. Track addresses listed in the table are then not used during operation of the disk drive, which can be referred to as the tracks being “mapped out”.  
      The continuing need for higher capacity disk drives continues to drive higher track densities (i.e., smaller track pitch). With higher track densities, an acceptable margin for track pitch variation can correspondingly decrease and can cause a greater number of formatted disks to fail qualification tests and/or may result in reduced capacity and performance from the disk drive.  
     SUMMARY OF THE INVENTION  
      In some embodiments of the present invention, a disk drive includes a rotatable data storage disk having a plurality of radially distributed tracks, where the radial pitch between at least some of the tracks varies across the disk. A head is configured to read/write data on the tracks. An actuator is configured to position the head relative to the disk. A controller is configured to respond to a host read/write command identifying a track address on the disk by determining a corresponding shifted radial location on the disk that is radially offset from an actual location of the addressed track by a distance that at least partially compensates for the radial pitch variation between at least some of the tracks on the disk.  
      Some other embodiments of the present invention are directed to corresponding methods of positioning a head that is adjacent to a rotatable disk in a disk drive, and so as to at least partially compensate for radial pitch variation between at least some of the tracks on the disk.  
    
    
     DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a perspective view of a conventional disk drive.  
       FIG. 2  is a top view of a conventional disk and illustrates tracks and sectors.  
       FIG. 3  is a block diagram of conventional servo information fields in a servo sector.  
       FIG. 4  is a block diagram of a disk drive and illustrates electronic circuits of a disk drive that includes a data controller, servo controller, and a repository of track addresses and corresponding radial offsets in accordance with some embodiments of the present invention.  
       FIG. 5  is a flowchart of operations for at least partially compensating for radial pitch variation according to some embodiments of the present invention.  
       FIG. 6  illustrates six reference tracks and corresponding actual tracks that may be defined in data sectors on the disk by servo information in the servo sectors.  
       FIG. 7  illustrates an exemplary table of track addresses and track offset distances that may be defined by the repository according to some embodiments of the present invention.  
       FIG. 8  illustrates six reference tracks and corresponding actual tracks having a single track misplacement and which may be defined in data sectors on the disk by servo information in the servo sectors.  
       FIG. 9  illustrates six reference tracks and corresponding actual tracks having repetitive pairs of track misplacement and which may be defined in data sectors on the disk by servo information in the servo sectors.  
       FIG. 10  illustrates six reference tracks and corresponding actual tracks with one of the actual tracks being a narrowed track, and which may be defined in data sectors on the disk by servo information in the servo sectors.  
       FIG. 11  illustrates a portion of the disk on which the radial offset information is distributed across the disk within one or more of the servo sectors and radially aligned with the corresponding data tracks.  
       FIG. 12  is a flowchart of operations that may be carried out by the servo controller to respond to the radial offset information in the servo information so as to at least partially compensate for radial pitch variation among tracks.  
       FIG. 13  illustrates methods of micro-jogging the head to a shifted radial location relative to an addressed track N+1 in response to radial offset information in the adjacent servo information, such as that shown in  FIG. 11 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. However, this invention should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.  
      It also will be understood that, as used herein, the term “comprising” or “comprises” is open-ended, and includes one or more stated elements, steps and/or functions without precluding one or more unstated elements, steps and/or functions. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The term “and/or” and “/”includes any and all combinations of one or more of the associated listed items. In the drawings, the size and relative sizes of regions may be exaggerated for clarity.  
      Some embodiments of the present invention can provide disk drives, servo channels, and methods. Accordingly, the present invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.). Consequently, as used herein, the term “signal” may take the form of a continuous waveform and/or discrete value(s), such as digital value(s) in a memory or register. Furthermore, the present invention may take the form of a computer program product on a computer-usable or computer-readable storage medium having computer-usable or computer-readable program code embodied in the medium for use by or in connection with an instruction execution system.  
      The present invention is described below with reference to block diagrams and operational flow charts. It is to be understood that the functions/acts noted in the blocks may occur out of the order noted in the operational illustrations. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows.  
       FIG. 4  is a block diagram of electronic circuits  400  of a disk drive, which, for purposes of explanation, can be included within the circuits  30  of disk drive  10  of  FIG. 1 . The electronic circuits  400  include a data controller  402 , a servo controller  406 , a read/write channel  404 , and a repository  408  of track addresses and radial offset information according to some embodiments of the present information. The exemplary embodiment of the electronic circuits  400  has been illustrated with separate controllers  402 ,  406 , read/write channel  404 , and repository  408  for purposes of illustration and discussion only. It is to be understood that their functionality may be consolidated in fewer components or distributed among more and/or other components. The electronic circuits  400  respond to host read/write commands to control the head disk assembly (HDA)  410  to seek the head  20  to a track address and Logical Block Addresses (LBAs) identified by the host read/write command and to read/write data therefrom. The HDA  410  can include the actuator arm assembly  18 , the disk(s)  12 , the VCM  28 , and the spindle motor  14 .  
      The read/write channel  404  can operate in a conventional manner to convert data between the digital form used by the data controller  402  and the analog form conducted through the head  20  in the HDA  410 . The read/write channel  404  provides servo positional information read from the HDA  410  to the servo controller  406 . The servo positional information can be used to detect the location of the head  20  in relation to LBAs on the disk  12 . The servo controller  406  can use LBAs from the data controller  402  and the servo positional information to seek the head  20  to an addressed track and block on the disk  12 , and to maintain the head  20  aligned with the track while data is written/read on the disk  12 .  
      The electronic circuits  400  are also configured to at least partially compensate for radial pitch variation between at least some of the tracks on the disk  12 , such as variation in pitch  48  between tracks  40  in  FIG. 2 . For purposes of explanation, the functionality for carrying out the radial pitch compensation is described in the context of being primarily carried out by the servo controller  406 , however, it may instead, or in addition, be carried out by the data controller and/or by another electronic component of the disk drive  10 .  
      The servo controller  406  responds to a track address on the disk  12  that is identified by a host read/write command by determining a corresponding shifted radial location on the disk that is radially offset from the track address by a distance that at least partially compensates for radial pitch variation between at least some of the tracks on the disk  12 . The repository  408  identifies track addresses and corresponding radial offset information. The servo controller  406  can access the repository  408  using the track address as a lookup pointer to obtain the corresponding radial offset information which may identify a radial distance that the head  20  needs to be moved from the addressed track centerline to at least partially compensate for track pitch variation. The servo controller  406  can determine therefrom the shifted radial location on the disk  12  to read/write data.  
      The repository  408  may be recorded on the disk  12  in the HDA  410  and/or it may reside in a semiconductor memory device within or otherwise accessible by the electronic circuits  400 . Moreover, as will be further explained below, the repository  408  may reside at a reserved location on the disk  12  and/or it may be radially distributed across the disk  12  so that the radial offset information may be recorded at locations that are radially aligned with the corresponding addressed tracks on the disk  12 . For example, the radial offset information may be included as part of the servo information within at least one of the servo sectors  44 .  
      exemplary flowchart of operations that may be carried out by the data controller  402  and/or servo controller  406  to at least partially compensate for radial pitch variation. At Block  500 , a read/write command from a host device is received. The read/write command identifies a track address on the disk  12 . At Block  502 , the repository  408  is accessed to determine the track offset distance. At Block  504 , a shifted radial location on the disk  12  for performing a read/write operation is determined in response to the commanded track address and the determined track offset distance. At Block  506 , the head is moved via a seek operation to the shifted radial location on the disk  12  and data is read/written along that location.  
      The repository  408  may, for example, identify radial offset information for each track address that has at least a threshold amount of pitch variation. However, identifying radial offset information for each track may result in a high storage space requirement for the repository  408  as the number of tracks on disk  12  increases. Various further embodiments of the present invention are directed to operations and methods for representing the radial offset information that can be used to compensate for radial pitch variation between tracks.  
       FIG. 6  is a diagram that illustrates six actual tracks and corresponding reference tracks that may be defined in data sectors  46  by servo information in the servo sectors  44 . The reference tracks T R1  to T R6  have the same radial track pitch therebetween and, accordingly, may represent tracks that a STW would preferably form on the disk  12 . The actual tracks T A1  to T A6  represent how the reference tracks T R1  to T R6  may actually have been formed on the disk  12 , and have been squeezed due to, for example, effects of runout during the servo sector formatting process. For example, tracks T A1  to T A6  each have a smaller track width than T R1  to T R6 . Consequently, the track pitch illustrated between actual tracks T A1  to T A6  differs from the desired track pitch illustrated between reference tracks T R1  to T R6 .  
      The repository  408  may contain a listing of each of the track addresses (T A1 , to T A7 ) and corresponding offset distances that the head  20  needs to be radially offset from the track centerline to at least substantially remove the squeeze (narrowness) present in tracks T A2  to T A6 .  FIG. 7  illustrates an exemplary table of track addresses (T A1  to T A6 ) and track offset distances (D 1  to D 5 ) that may be defined by the repository  408 . The track offset distances D 1  to D 5  may each represent the same radial offset distance. To avoid squeezing tracks that are adjacent the actual tracks T A1  to T A6 , one or more of the actual tracks may be mapped out by a corresponding indication in the repository  408  so that the mapped out track(s) will not be used to store data (e.g., track T A6  has been mapped out by the indication “Not Used”). The remaining squeezed tracks (i.e., tracks T A2  to T A5 ), which have not been mapped out, can then be expanded so as to use at least some of the radial disk space once reserved for the mapped out track(s) (i.e., track T A6 ). Accordingly, mapping out one or more tracks among a group of squeezed tracks may allow the rest of the group of tracks to be adjusted so as to provide a desired track pitch therebetween. For example, for n squeezed tracks, a number k of those tracks may be mapped out so that the remaining squeezed tracks (n-k) can be adjusted, thereby recovering (n-k)/n tracks.  
      The track offset information for removing the squeeze in tracks T A1  to T A5  may be represented in the repository  408  more compactly by storing in the repository  408  the range of the squeezed group of tracks and the mapped out track(s). For example, if 10 tracks corresponding to track  100 - 110  were squeezed, the repository may store the information ( 100 ,  109 ,  110 ). The servo controller  406  can then determine from this information that track  110  has been mapped out and that the other tracks  100  to  109  are to be expanded by the radial distance ( 110 - 100 )/ 109 - 100 ) or  1 . 11 . Thus, to seek to track  105 , the servo controller  406  positions the head  20  at the following shifted radial location to remove the effect of track squeeze:  
         shifted   ⁢           ⁢   radial   ⁢           ⁢   location   ⁢           ⁢   for   ⁢           ⁢   track   ⁢           ⁢   105     =       100   +       [       (     105   -   100     )     *     (     110   -   100     )       ]       (     109   -   100     )         =     105.56   .           
 
      Accordingly, the LBA of a host read/write command is converted into a track address, which is used as a reference pointer in the repository  408  to determine the track offset distance and, therefrom, the shifted radial location of the track on the disk  12 . Through a seek operation, the head  20  is positioned over the shifted radial location of the track while data is read from or written to the disk  12 .  
      The track offset information may be developed and stored within the disk drive  10  during the manufacturing of the disk drive  10 . For example, after a STW writes servo information on the disk  12  to define track locations, the pitch between the tracks across the disk  12  can be tested. When the pitch variation exceeds defined thresholds, track offset information can be defined for individual tracks and groups of tracks. Some tracks may be mapped out from use so that their space can be used for adjusting track pitch among other tracks.  
       FIG. 8  is a diagram that illustrates the desired reference tracks T R1  to T R6  and the actual tracks T A1  to T A6  having a single track misplacement. As shown in  FIG. 8 , actual tracks T A1 , T A2 , T A5 , and T A6  have the same width, while track T A3  is narrower and radially adjacent track T A4  is wider than the other tracks T A1 , T A2 , T A5 , and T A6 . Accordingly, only a single track T A3  is misplaced out of the group of tracks. As explained above, if the track pitch variation were left uncompensated, the performance of read/write operations to track T A3  may be significantly degraded and the reliability of read/write operations to track T A4  may be significantly decreased, along with other possible effects of carrying out read/write operations to those tracks. To compensate for the resulting radial pitch variation, an offset can be applied to track T A3  so as to expand its track width to correspond to the widths of tracks T A1 , T A2 , T A5 , and T A6 . The corresponding radial offset information stored in the repository  408  may identify the track number(s) that are to be widened. The servo controller  406  may then widen the identified track numbers and reduce the width of the radially adjacent next greater track address. Thus, with reference to  FIG. 7 , the repository  408  may identify track T A3 . The controller  408  can then increase the width of track T A3  and decrease the width of adjacent track T A4  by the same defined distance. A plurality of such individually misplaced tracks may thereby be separately defined in the repository  408  in this manner.  
       FIG. 9  is a diagram that illustrates the desired reference tracks T R1  to T R6  and actual tracks T A1  to T A6  having a repetitive pairs of track misplacement. As shown in  FIG. 9 , actual tracks T A1 , T A3 , and T A5  have narrow widths, while tracks T A2 , T A4 , and T A6  have expanded widths. Moreover, each narrow track is followed by a wide track in pairs that repeat as tracks T A1  and T A2 , T A3  and T A4 , T A5  and T A6 . The track pairs can thereby be efficiently represented in the repository  408  by an indication of the range of tracks over which offsets can be repetitively applied to compensate for the radial track variation. For example, if  10  tracks corresponding to track  100 - 110  were squeezed and stretched, the repository may identify information ( 100 ,  110 ). The servo controller  406  can then determine from this information that any host read/write commands to track addresses within the range of  100  to  110 , that, beginning with track address  100 , the head  
      alternately radially offset by 10% in a direction to increase the width of a track followed by a radial offset of 10% in an opposite direction to decrease the width of the next adjacent track to decrease the width of that track (i.e., increase width of tracks T A1 , T A3 , and T A5 , and decrease width of tracks T A2 , T A4 , and T A6 ). Accordingly, track pitch variation among tracks T A1  to T A6  can be corrected without mapping out (removing from use) any of those tracks.  
      a diagram that illustrates the desired reference tracks T R1  to T R6  and actual tracks T A1  to T A6 , with track T A3  more narrow than the desired width of each of tracks T R1  to T A6 . The width of track T A3  may be expanded by mapping-out one of the other tracks (e.g., mapping out one of T R1 , T R2 , T R4 , T R5 , or T R6 ) and shifting the tracks between track T A3  and the mapped out track to provide the desired width. Alternatively, when a nearby track is wider than the preferred width, the width of track T A3  can be expanded by shifting the tracks between track T A3  and the wider track so as to add the excessive width of the wider track to the narrow track. For example, if track T A7  (not shown) is sufficiently wider than the desired track width to allow expansion of track T 3 , tracks T A4  to T A6  can be shifted toward track T A7  so that the track T A3  is widened and tracks T A4  to T A6  maintain the same width. The repository may represent this desired shifting by identifying track offset information that can include the address for T A3  and the address for T A6 . The servo controller  406  may then respond to the track offset information by shifting track T A4  to T A6  in a direction that expands track T A3  and narrows the track immediately following the defined range (i.e., narrows track T A7 ). Accordingly, radial track pitch variation caused by the narrow track T A3  and by the wide track T A7  may be substantially removed by shifting some of the tracks as described.  
      As will be appreciated, one or more of the these processes may be carried out to compensate for radial track pitch variation. Moreover, some groups of tracks may be mapped out so as not to be used for data storage because of, for example, excessive track pitch variation, while other individual ones or groups of tracks may be shifted and/or selectively mapped out to allow compensation for track pitch variation associated with those tracks.  
      As explained above, the repository  408  may be consolidated at a defined location on the disk  12 , in a semiconductor memory in the electronic circuits  30 , and/or it may be distributed across the disk  12  with relevant portions of the radial offset information being aligned with the corresponding tracks.  FIG. 11  illustrates an exemplary embodiment in which the radial offset information is distributed across the disk  12  within one or more of the servo sectors  44  and radially aligned with the corresponding data tracks. Accordingly, in this exemplary embodiment, the repository  408  is located on and distributed across the disk  12 . As will be appreciated, the content and order of the servo information can vary from the exemplary embodiment shown in  FIG. 11 .  FIG. 12  is a flowchart of operations that may be carried out by the servo controller  406  to respond to the radial offset information in the servo information, such as shown in  FIG. 11 , so as to at least partially compensate for radial pitch variation among tracks.  FIG. 13  illustrates methods of micro-jogging the head  20  to a shifted radial location relative to an addressed track N+1 in response to radial offset information in the adjacent servo information, such as that shown in  FIG. 11 .  
      Referring to  FIGS. 11-13 , exemplary operations of the servo controller  406  will be described for at least partially compensating for radial pitch variation between some tracks. A read/write command is received (Block  1200 ) which identifies a track address (e.g., track number N+1 in  FIG. 11 ) from which data is to be read/written on the disk  12 . The servo controller  406  responds to the read/write command by seeking the head  20  to an initial radial location (Block  1202 ) that corresponds to the track address. One or more servo sectors  44  are read (Block  1204 ) to obtain the radial offset information for the track address (e.g., partial or complete radial offset information for track N+1). For example, the complete radial offset information may be stored in one or more of the servo sectors  44 , or the partial information may distributed across a plurality of servo sectors  44  so that the plurality of sectors  44  need to be read to allow the controller  406  to generate complete radial offset information therefrom.  
      The controller  406  determines (Block  1206 ) a shifted radial location on the disk  12  to read/write data in response to the radial offset information that was read among the servo information. The controller  406  micro-jogs (Block  1208 ) the head  20  a determined radial distance to align the head  20  with the shifted radial location on the disk  12 . Data is then read/written (Block  1210 ) on the disk  12  along the shifted radial location to carry out the host read/write command.  
      As will be appreciated, distributing partial portions of the radial offset information among a plurality of servo sectors  44  may decrease the amount of storage space needed in the each servo sector  44  for the radial offset information, however it may also result in a longer delay between when the head  20  arrives on track and when it can be micro-jogged to a final position to allow reading/writing along the shifted radial position.  
      In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.