Abstract:
A servo wedge on a recording medium for a disk drive includes a gray-code field that stores a micro-jog correction factor for a data track. The micro-jog correction factor compensates for the effect of accumulated track pitch variations. In one embodiment, the radial wedge formed by gray-code fields for adjacent data tracks is a continuous radial wedge similar to the gray-code field used for encoding track numbers.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention relate generally to disk drives and, more particularly, to systems and methods for reader-writer offset correction in such drives. 
         [0003]    2. Description of the Related Art 
         [0004]    A disk drive is a data storage device that stores digital data in concentric tracks on the surface of a data storage disk. The data storage disk is a rotatable hard disk with a layer of magnetic material thereon, and data is read from or written to a desired track on the data storage disk using a read/write head that is held proximate to the track while the disk spins about its center at a constant angular velocity. Typically there is a write head for writing data and a separate read head for reading data. The read and write heads are typically separated by some distance both in radial and tangential direction. 
         [0005]    To properly align the read/write head with a desired track during a read or write operation, disk drives generally use a closed-loop servo system that relies on servo data stored in servo sectors written on the disk surface when the disk drive is manufactured. These servo sectors form “servo wedges” or “servo spokes” from the outer to inner diameter of the disk, and are either written on the disk surface by an external device, such as a servo track writer, or by the drive itself using a self servo-writing procedure. The read/write head can be positioned with respect to the data storage disk by using feedback control based on servo information read from the servo wedges with the read head. Thus, the read head, which collects the servo information from the servo wedges, is used to position the heads relative to the disk for both reading and writing operations. 
         [0006]    Because the read and write heads are radially offset from each other with respect to the storage disk, the read head is generally positioned over a different track than the write head. This radial offset is referred to as “micro-jog,” and for disk drives employing a rotary actuator for moving the read/write head with respect to the disk, the magnitude of micro-jog varies across the surface of the disk. To read back data written by the write head, the read head must be accurately positioned over the center of the desired data track, which requires accurate knowledge of the micro-jog for that data track. Techniques are known for calculating micro-jog values, typically involving a micro-jog calibration that is performed at multiple locations across the disk surface during the post-manufacturing self-test process so that an interpolated curve can be constructed that defines the micro-jog for all data tracks on the disk. 
         [0007]    Such calibration schemes assume that track pitch, i.e., the radial distance between data tracks, is substantially constant. In reality, variation in track pitch from the nominal track pitch for the disk is common, albeit relatively small, e.g., on the order of ±1% of total track pitch. This level of variation is generally small enough to avoid significant data integrity problems for an individual track. However, even such small variation in track pitch adversely affects the accuracy of micro-jog values produced by an interpolated curve, since the small variations in track pitch can accumulate over the span of the reader-writer offset to produce a relatively large position error of the read head. For example, assume that the reader-writer offset, i.e., the micro-jog, for a particular location on the disk spans 10 tracks. If the track pitch for each of the 10 tracks between the read head position and the write head position is only 1% narrower than the nominal track pitch, the resulting inaccuracy of the micro-jog value provided by an interpolated calibration curve is 10% of the spacing between tracks. The 1% variation from nominal track width accumulates over the 10 tracks because the interpolated curve that provides the value of micro-jog assumes that all tracks have nominal track pitch and ignores the small variations in track pitch that can occur across the disk surface. Consequently, when the read head is moved the micro-jog distance dictated by the interpolated calibration curve, the read head is positioned 10% off-track, a radial position error large enough to prevent the read head from successfully reading data. 
         [0008]    In light of the above, there is a need in the art for a micro-jog correction method that compensates for variations in track spacing. 
       SUMMARY OF THE INVENTION 
       [0009]    One or more embodiments of the present invention provide a system and method for micro-jog correction in a disk drive that compensates for variations in track spacing. In the method, a micro-jog correction factor is determined for a data track of a disk drive and is encoded on the disk in “gray-code” format, an encoding of numbers in which two successive values differ in only one bit. The gray-code-formatted micro-jog correction factor is disposed in a position correction field of a servo wedge for the data track and is aligned with similar micro-jog position correction fields for adjacent data tracks to form a radial wedge. The use of gray-code format for micro-jog correction data allows recovery of the micro-jog correction data for any data track with an accuracy of ±one count. In one embodiment, the radial wedge formed by the micro-jog position correction fields is a continuous radial wedge similar to the gray-code field used for encoding track numbers, i.e., with substantially no gaps between data tracks. 
         [0010]    A method of positioning a read head of a hard disk drive above a data track of a recordable medium, according to an embodiment of the invention, includes the steps of determining a target position above the recordable medium, and moving the read head toward the target position and, during movement, reading one or more correction factors encoded in gray-code format and adjusting the target position based on the correction factors. 
         [0011]    A disk drive assembly comprising according to an embodiment of the invention includes a read head, a drive unit for moving the read head to a radial position above a recording medium, and a control unit for controlling the movement of the read head by issuing drive signals to the drive unit, wherein the control unit is programmed to determine a target position above the recording medium and issue a drive signal to the drive unit to move the read head toward the target position and, during movement, read one or more correction factors encoded in gray-code format and adjust the target position based on the correction factors. 
         [0012]    A recording medium for a disk drive assembly, according to an embodiment of the invention, comprises a plurality of servo wedges, wherein each of the servo wedges has reader-writer offset correction factors written therein in gray-code format. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0014]      FIG. 1  is a perspective view of a disk drive that can benefit from embodiments of the invention as described herein. 
           [0015]      FIG. 2  illustrates a storage disk with data organized in a typical manner known in the art. 
           [0016]      FIG. 3  is a graph demonstrating how track pitch for concentric data storage tracks may vary with respect to the nominal track pitch for a disk drive. 
           [0017]      FIG. 4  is a partial schematic diagram of a storage disk illustrating the trajectory of a read and write heads. 
           [0018]      FIGS. 5A-5C  are partial schematic diagrams of a storage disk and read and write heads during disk drive read and write operations, according to embodiments of the invention. 
           [0019]      FIGS. 6A-6B  are schematic diagrams of magnetic indicia written on two adjacent data storage tracks on a disk drive. 
           [0020]      FIG. 7  is a partial schematic diagram of a servo wedge on a storage disk that includes a micro-jog correction factor field, according to one or more embodiments of the invention. 
           [0021]      FIG. 8  is a flow chart that summarizes, in a stepwise fashion, a method for micro-jog correction in a disk drive that compensates for variations in track spacing, according to an embodiment of the invention. 
       
    
    
       [0022]    For clarity, identical reference numbers have been used, where applicable, to designate identical elements that are common between figures. It is contemplated that features of one embodiment may be incorporated in other embodiments without further recitation. 
       DETAILED DESCRIPTION 
       [0023]      FIG. 1  is a perspective view of a disk drive  110  that can benefit from embodiments of the invention as described herein. For clarity, disk drive  110  is illustrated without a top cover. Disk drive  110  includes a storage disk  112  that is rotated by a spindle motor  114 . Spindle motor  114  is mounted on a base plate  116 . An actuator arm assembly  118  is also mounted on base plate  116 , and has a slider  120  mounted on a flexure arm  122  with a read head  127  and a write head  129 . For clarity, read head  127  and write head  129  are omitted from  FIG. 1 , and are instead shown in FIGS.  4  and  5 A- 5 C. Flexure arm  122  is attached to an actuator arm  124  that rotates about a bearing assembly  126 . Voice coil motor  128  moves slider  120  relative to storage disk  112 , thereby positioning read and write heads  127  and  129  over the desired concentric data storage track disposed on the surface  112 A of storage disk  112 . Spindle motor  114 , read and write heads  127  and  129 , and voice coil motor  128  are coupled to electronic circuits  130 , which are mounted on a printed circuit board  132 . The electronic circuits  130  include a read channel, a microprocessor-based controller, and random access memory (RAM). For clarity of description, disk drive  110  is illustrated with a single storage disk  112  and actuator arm assembly  118 . Disk drive  110 , however, may also include multiple storage disks  112  and multiple actuator arm assemblies  118 . In addition, each side of disk  112  may have an associated read and write heads  127  and  129 , both of which are collectively coupled to the rotary actuator  130  such that both read and write heads  127  and  129  pivot in unison. The invention described herein is equally applicable to devices wherein the individual heads are configured to move separately some small distance relative to the actuator using dual-stage actuation. 
         [0024]      FIG. 2  illustrates storage disk  112  with data organized in a typical manner after servo wedges  244  have been written on storage disk  112  by either a media writer or by disk drive  110  itself via self servo-write (SSW). Storage disk  112  includes concentric data storage tracks  242  located in data sectors  246  for storing data. Concentric data storage tracks  242  are positionally defined by servo information written in servo wedges  244 . Each of concentric data storage tracks  242  is schematically illustrated as a centerline, but in practice occupies a finite width about a corresponding centerline. Substantially radially aligned servo wedges  244  are shown crossing concentric data storage tracks  242  and have servo sectors containing servo information that defines the radial position and track pitch, i.e., spacing, of concentric data storage tracks  242 . Such servo information includes a reference signal that is read by read head  127  during read and write operations to position the read and write  127  and  129  above a desired track  242 . Servo wedges  244  are described in greater detail in conjunction with  FIG. 7 , below. In practice servo wedges  244  may be somewhat curved, for example, configured in a shallow spiral pattern. Typically, the actual number of concentric data storage tracks  242  and servo wedges  244  included on storage disk  112  is considerably larger than illustrated in  FIG. 2 . For example, storage disk  112  may include hundreds of thousands of concentric data storage tracks  242  and hundreds of servo wedges  244 . 
         [0025]    As noted above, servo wedges  244  are written on storage disk  112  by either a media writer or by disk drive  110  itself via an SSW process. In either case, due to fluctuations in writing head position that occur during the process of writing servo wedges  244 , the majority of concentric data storage tracks  242  vary slightly from the nominal track pitch for disk drive  110 , i.e., most of concentric data storage tracks  242  are either wider or narrower than the nominal track pitch. In addition, because writing head fluctuations are generally low frequency fluctuations in the 10-100 Hz range, track pitch for concentric data storage tracks  242  does not vary randomly from track to track. Instead, track pitch varies in a somewhat periodic fashion, with alternating groupings of narrower and then wider tracks. 
         [0026]    By way of illustration,  FIG. 3  is a graph  300  demonstrating how track pitch for concentric data storage tracks  242  may vary with respect to the nominal track pitch for disk drive  110 . In graph  300 , the abscissa represents track number and the ordinate represents actual track pitch of an exemplary storage disk. As shown, the actual track pitch of tracks  1 - 100  varies from 4 or 5 nm greater than to 4 or 5 nm less than the ideal track pitch of 100 nm in a somewhat periodic and gradual fashion. Blocks of ten or more adjacent tracks having wider than nominal track pitch generally alternate with blocks of ten or more adjacent tracks having narrower than nominal track pitch, so that over a large number of tracks, e.g., 20 or more, the average track pitch is substantially equal to the nominal track pitch. 
         [0027]    When disk drive  110  is in operation, actuator arm assembly  118  sweeps an arc between an inner diameter (ID) and an outer diameter (OD) of storage disk  112 . Actuator arm assembly  118  accelerates in one angular direction when current is passed through the voice coil of voice coil motor  128  and accelerates in an opposite direction when the current is reversed, allowing for control of the position of actuator arm assembly  118  and the attached read and write heads  127  and  129  with respect to storage disk  112 . Voice coil motor  128  is coupled with a servo system known in the art that uses positioning data read from storage disk  112  by read head  127  to determine the position of read and write heads  127  and  129  over concentric tracks  242 . The servo system determines an appropriate current to drive through the voice coil of voice coil motor  128 , and drives said current using a current driver and associated circuitry. As is known in the art, as actuator arm assembly  118  sweeps an arc between the ID and the OD of storage disk  112 , the skew angle between slider  120  and storage disk  112  varies, where skew angle is defined as the angle between the longitudinal axis of slider  120  and the direction of tangential velocity of storage disk  112 . 
         [0028]      FIG. 4  is a partial schematic diagram of storage disk  112  illustrating the trajectory  410  of read and write heads  127  and  129  as actuator arm assembly  118  moves between ID  421  and OD  422  of storage disk  112 . When actuator arm assembly  118  positions read and write heads  127  and  129  at ID  421 , a first skew angle  401  is formed between the longitudinal axis  423  of slider  120  and the direction of tangential velocity of storage disk  112  at the point on storage disk  112  directly adjacent to read and write heads  127  and  129 . Similarly, when actuator arm assembly  118  positions read and write heads  127  and  129  at OD  422 , a second skew angle  402  is formed that is greater than first skew angle  401 . Thus, as read and write heads  127  and  129  move along trajectory  410 , the skew angle between read and write heads  127  and  129  varies. As the skew angle varies, the radial offset with respect to storage disk  112  between read and write heads  127  and  129 , i.e., the micro-jog, also varies. 
         [0029]    During post manufacturing self-test process, the micro-jog is measured for each of the concentric tracks  242 . Then, for each of a plurality of zones  320  along trajectory  410 , the micro-jog is averaged. Using the average micro-jog values, each of which represents one of the zones  320 , curve fitting and interpolation techniques known in the art are used to produce a calibration curve that provides an estimated micro-jog value for any concentric track  242 . A representation of this calibration curve is stored in tables in memory for use by the servo system. Also, during post manufacturing self-test process, micro-jog correction factors for concentric tracks  242  are written onto the corresponding data tracks of storage disk  112 . A micro-jog correction factor represents the difference in the actual micro-jog value as measured and the micro-jog value as estimated from the calibration curve. 
         [0030]      FIGS. 5A-5C  are partial schematic diagrams of storage disk  112  and read and write heads  127  and  129  during read and write operations of disk drive  110 , according to embodiments of the invention. In  FIGS. 5A-5C , the track positions of a number of concentric tracks as defined by servo bursts  510 , i.e., Tracks  0 - 4 , are illustrated. Due to the variations in track pitch described above in conjunction with  FIG. 3 , the actual track pitch of Tracks  0 - 4  may be slightly smaller or larger than the nominal track pitch for storage disk  112 , for example on the order of 1 or 2 nm. 
         [0031]    In  FIG. 5A , write head  129  performs a write operation with the read head  127  positioned above Track  0 . During this write operation, read head  127  reads position information from Track  0  so that the servo loop of disk drive  110  holds read head  127  and write head  129  in the desired position relative to storage disk  112 . As shown, read head  127  and write head  129  are separated by a distance  512 . Because of this separation, read head  127  has to move by distance  512  to read the data that has been written by write head  129  while read head  127  was positioned above Track  0 . 
         [0032]    Distance  512  is the actual micro-jog corresponding to Track  0 . An estimate of this micro-jog is obtained from the calibration curve described above in conjunction with  FIG. 4 . This estimate is represented in  FIG. 5B  as  513  and is an example of a micro-jog value of 4 tracks. However, at this position, read head  127  may not be able to read the data written on a data track corresponding to Track  0 . A correction by distance  514  is needed to properly read the data written on a data track corresponding to Track  0 . 
         [0033]    In the embodiments according to the invention, a gray-code field  520  containing micro-jog correction factors is used. As read head  127  moves from Track  0  to an estimated target track position, it reads micro-jog correction factors contained in gray-code field  520  and continuously reads and applies the micro-jog correction factors as it moves towards the estimated target track position. The final, converged position of read head  127  is shown in  FIG. 5C . 
         [0034]    Table 1 provides a series of 4-bit gray-codes that may be used in embodiments of the invention to store micro-jog correction factors, where the gray-codes are written on data tracks of storage disk  112 . The first column of Table 1 presents the number of counts for the adjustment. In such an embodiment, one “count” is scaled to be equal to the maximum value by which track pitch can vary between two adjacent data tracks of a disk drive, e.g., 1 nm, 2 nm, etc. The second column presents the corresponding gray-code binary bit sequences, or gray-code “words,” that may be written on a data track in the form of magnetic indicia. Inspection of Table 1 reveals that for each step count in the first column, there is a unique gray-code word in the second column. In addition, it should be noted that any two adjacent gray-code words change only one bit at a time, i.e., adjacent words in Table 1 are identical except for one bit position change. By using gray-codes rather than conventional binary bit sequences, the micro-jog correction factor for any particular data track can be read by a read head with an accuracy of ±1 count, even if the read head is positioned substantially between that particular data track and an adjacent data track. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Example Gray-codes 
               
             
          
           
               
                   
                 Number of 
                 Gray-code 
               
               
                   
                 Counts 
                 Word 
               
               
                   
                   
               
               
                   
                  0 
                 0000 
               
               
                   
                  1 
                 0001 
               
               
                   
                  2 
                 0011 
               
               
                   
                  3 
                 0010 
               
               
                   
                  4 
                 0110 
               
               
                   
                  5 
                 0100 
               
               
                   
                  6 
                 0101 
               
               
                   
                  7 
                 0111 
               
               
                   
                  8 
                 1111 
               
               
                   
                  9 
                 1110 
               
               
                   
                 10 
                 1100 
               
               
                   
                 11 
                 1101 
               
               
                   
                 12 
                 1001 
               
               
                   
                 13 
                 1011 
               
               
                   
                 14 
                 1010 
               
               
                   
                 15 
                 1000 
               
               
                   
                   
               
             
          
         
       
     
         [0035]      FIG. 6A  is a schematic diagram of magnetic indicia written on two adjacent data storage tracks on a disk drive, where the indicia are written in conventional binary code format and the value of the binary word in each data track represents a micro-jog correction factor. In this example, the binary code is a 4-bit word, where the first column indicates a value of 2 3 , the second column indicates a value of 2 2 , the third column indicates a value of 2 1 , and the fourth column indicates a value of 2°. The presence of a bar represents a “1” value for a particular column and the absence of a bar represents a “0” value for the column. As shown, data track N has a binary word (0-1-0-1) representing a value of 5 stored therein and adjacent data track N+1 has a binary word (0-1-1-0) representing a value of 6 stored therein. If read head  127  is attempting to read data track N, but is initially positioned substantially between data tracks N and N+1, the value that read head  127  will read is unpredictable. Namely, read head  127  may only read the magnetic indicia of data track N (0-1-0-1), which indicates a correction of 5 counts. Alternatively, read head  127  may only read the magnetic indicia of data track N+1 (0-1-1-0), which indicates a correction of 6 counts. Furthermore, read head  127  may read a combination of both data tracks (0-1-1-1) or (0-1-0-0), which indicate corrections of 7 counts and 4 counts, respectively. Thus, the use of binary code format to store a micro-jog correction factor is of little value unless reader  127  can be aligned accurately with the desired data track, in which case the micro-jog correction factor is not really needed. 
         [0036]      FIG. 6B  is a schematic diagram of magnetic indicia written on two adjacent data storage tracks on a disk drive, where the indicia are written in gray-code format and the value of the gray-code word in each data track represents a micro-jog correction factor. In this example, the gray-code is a 4-bit word from Table 1 that corresponds to a unique value from 0-15 and, unlike binary code, is not a sum of the columns. As shown, data track N has a gray-code word representing a value of 5 stored therein and adjacent data track N+1 has a gray-code word representing a value of 6 stored therein. If read head  127  is attempting to read data track N, but is initially positioned substantially between data tracks N and N+1, the value that read head  127  reads is either 5 or 6. This is because any two adjacent gray-code words change by one bit and because the micro-jog correction factor represented by the gray-code words is scaled so that the maximum amount the micro-jog correction factor can change between adjacent data tracks is one count. Whether read head  127  reads the magnetic indicia of data track N, of data track N+1, or a combination of both, the only possible values to be read are 5 or 6. Thus, the use of gray-codes to store micro-jog correction factor on a disk, such as track pitch variation micro-jog, allow the retrieval of the micro-jog correction factors regardless of the accuracy of the positioning of read head  127 . 
         [0037]      FIG. 7  is a partial schematic diagram of a servo wedge  700  on storage disk  112  that includes a micro-jog correction factor field  701 , according to one or more embodiments of the invention. Servo wedge  700  includes servo information for each of concentric tracks  242  on storage disk  112 . A portion of servo wedge  700  is illustrated that includes servo information for three of concentric data storage tracks  242 , specifically Track N, Track N+1, and Track N+2. Servo wedge  700  is substantially similar to servo wedges  244 , described above in conjunction with  FIG. 2 , and includes a preamble field  702 , a track identification field  703 , a servo burst field  704 , a repeatable run-out (RRO) field  705 , and micro-jog correction factor field  701 . Preamble field  702 , track identification field  703 , servo burst field  704 , and repeatable run-out (RRO) field  705  are features commonly employed in servo wedges of disk drives and are known in the art. Micro-jog correction factor field  701  according to embodiments of the invention includes a gray-code word having a value that indicates how many “counts” a micro-jog value should be modified to compensate for accumulated track pitch variation for the given data track. 
         [0038]    In one embodiment, micro-jog correction factor field  701  is written with substantially no gap between data tracks. In such an embodiment, the lack of gap  710  maximizes the amplitude of a signal read from micro-jog correction factor field  701  by read head  127  when read head  127  is positioned between data storage tracks. 
         [0039]    In another embodiment, the micro-jog correction factor field  701  employs an extra bit to indicate whether the correction factor is a positive or negative value. Alternatively, the calibration curve described in conjunction with  FIG. 4  is adjusted so that only positive correction factors are used. 
         [0040]      FIG. 8  is a flow chart that summarizes, in a stepwise fashion, a method  800  for micro-jog correction in a disk drive, according to an embodiment of the invention. Method  800  is described in terms of a disk drive substantially similar to disk drive  110  in  FIG. 1 . However, other disk drives may also benefit from the use of method  800 . The commands for carrying out steps  801 - 805  may reside in the disk drive control algorithm and/or as values stored in the electronic circuits of the disk drive or on the storage disk itself. As described above in conjunction with  FIG. 4 , prior to the first step of method  800 , a micro-jog calibration curve is constructed and tables of values representing the micro-jog calibration curve are stored in memory. 
         [0041]    In step  801 , a request to read from a particular track is received. In step  802 , micro-jog value corresponding to this particular track is determined from tables in memory. In step  803 , read head  127  is moved toward a target track based on the micro-jog value. If according to step  804 , read head  127  is near the target track, e.g., 5 or so tracks away, micro-jog correction factor is continuously read and applied. In this method, convergence occurs substantially above the desired data track and data is read from the desired data track. 
         [0042]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.