Abstract:
The present invention is directed to utilizing the capabilities of multi-stage actuators in disk drives to write servo tracks that have a variable number of tracks-per-inch (TPI) from head to head, thereby improving both the performance of the drive and manufacturing yields. An appropriate TPI for a particular head has been found to depend on a number of factors that vary from head to head. Consequently, the initial step is to determine an appropriate TPI for at least one of the heads or, more preferably, all of the heads in the drive. The determination typically involves measurements such as read and write width measurements and off-track performance tests for the relevant heads. Once an appropriate TPI format has been determined, the servo tracks are written according to the TPI formats using either a servo track writer or self servo track writing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to disk drives and, in particular, to the writing of servo tracks on a disk within the disk drive. 
     BACKGROUND OF THE INVENTION 
     A disk drive is a device that is commonly employed in computer systems to store data. Typically, a disk drive includes: (1) one or more disks that each have a plurality of concentric tracks on which data is stored; (2) a spin motor for rotating the disk or disks; (3) one or more heads that are each capable of writing and/or reading data to/from a track on a disk; (4) an actuator for moving the head or heads to a desired location adjacent to a disk so that data can be written to the disk or read from the disk; and (5) circuitry for transferring data between a disk and a portion of a host computer system that is exterior to the disk drive, such as a random access memory (RAM). 
     A disk drive also typically includes a servo system that operates to move a head over a defined track on a disk surface and maintain the head over the defined track until directed to move the head over a different track. The servo system maintains the position of the head over a defined track based upon information that is read from a servo track. In one type of drive, the servo tracks are embedded in or coincident with the user data tracks, i.e., the servo track and the user data track form a single physical track with the servo data interspersed among the user data. Typically, the servo track: (1) identifies the particular track over which a head is positioned; and (2) provides data from which the position of the head relative to the center line of the track can be determined. The identification of the particular track is primarily used when the head is being moved from one track to another track (which is commonly known as a seek operation) to determine when the head is positioned over the desired track. Once the head is over the desired track, the data indicating the position of the head relative to the center line of the track is determined and used to maintain the head over the desired track (which is commonly known as a tracking operation). For example, if the data indicates that the head is positioned to one side of the center line, the servo system causes the actuator to move the head towards the center line. 
     Presently, the servo tracks are written on the disk surfaces of a disk drive during the manufacturing process by a servo track writer. The servo track writer uses a “push pin” to move the actuator arm and thereby position the heads for writing the servo tracks. To elaborate, the servo track writer uses the “push pin” to move the actuator and, as a consequence, position the heads for the writing of first servo tracks (one per disk surface). Once the first servo tracks have been written, the servo track writer uses the “push pin” to move the actuator and thereby reposition the heads for the writing of the second servo tracks. This process is repeated until all of the servo tracks have been written. As an alternative to using a servo track writer, the drive itself can be used to write the servo tracks in what is known as self servo writing. In this case, a motor associated with the actuator is used to move the actuator arm in discrete steps to write each servo track. In either case, for at least a band or section of contiguous tracks, the heads are either: (1) moved such that the arc that the heads move through from one track to the next track is substantially equal, which results in the track density changing over the band of tracks and is known as the “equal-arc drive format”; or (2) moved such that the distance between adjacent tracks remains substantially constant over the band of tracks, which is known as the “equal-length drive format.” 
     Regardless of whether a servo track writer or self servo writing is used to establish the servo tracks in a drive or the track format (equal-arc or equal-length) used, the track density measured in tracks per inch (TPI) at a give radius is the same for all of the disk surfaces in the drive. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes that the optimal servo track density at a given radius can vary from disk surface to disk surface within the drive and that the present methods of writing servo tracks do not provide for writing the servo tracks on different disk surfaces with different densities at a given radius. The present invention is directed to using a multi-stage actuator within the drive to write servo tracks on two or more disk surfaces within a drive with the track density on each surface at a given radius approaching the optimal track density for that surface. The multi-stage actuator includes a primary actuator for coarsely positioning a head and a secondary actuator for finely positioning the head. 
     In one embodiment, a disk drive includes at least two separate and substantially parallel disk surfaces that are capable of storing data. Associated with each disk surface is a head for transferring data between the disk surface and the exterior environment. A multi-stage actuator is used to move the heads to desired positions over the disk surfaces for the transfer of data. The multi-stage actuator includes a primary actuator for coarsely positioning the heads relative to the disk surfaces. Associated with each head is a secondary actuator that permits the position of the head to be more finely controlled. The data transfer circuitry of the disk drive, which is normally used to write/read user data to/from the disk, is also adapted to write the servo tracks on the track surfaces. Initially, the tracks per inch (TPI) format that is appropriate for each disk surface on which servo tracks are to be established is determined. While the TPI format may be the same for each disk surface, it is more likely that the TPI format will be at least slightly different for each disk surface. Typical measurements from which the TPI format for a particular surface is determined include the read head width, the write head width and off-track performance based upon a bit error rate and read channel quality factor. Once the TPI format for each disk surface has been determined, the primary actuator is used to position each of the heads for the writing of the first servo track on each of the disk surfaces. The secondary actuators are also used to position each of the heads for the writing of the first servo tracks. Once the heads have been positioned, the first servo tracks are written on each of the disk surfaces. After the first servo tracks are written, the heads are repositioned to write the second set of servo tracks on the disk using the secondary actuators. Because the secondary actuators are capable of operating independently of one another, a different TPI format can be implemented for each disk surface. Once most or all of the servo tracks that can be written for a given position of the primary actuator by using the secondary actuators to move the heads have been written, the primary actuator is repositioned and the process is repeated. 
     In one embodiment, the position of the actuator arm that carries the heads is adjusted using the actuator motor in a self servo writing operation. In this case, the actuator motor is used to position the actuator arm associated with the primary actuator. 
     In another embodiment, the position of the primary actuator and, more specifically, the actuator arm that carries the heads (as is part of the primary actuator) is adjusted by a servo writer. In this case, a “push pin” associated with the servo track writer contacts the actuator arm and pushes the arm such that the heads are positioned over the desired locations on the disk surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a typical disk drive with a multi-stage actuator; 
     FIG. 1B is a functional side view of certain components in the drive illustrated in FIG. 1A; 
     FIG. 2 is a functional diagram that shows the secondary actuators that are used to finely position heads relative to the disk surfaces; 
     FIG. 3 is a function block diagram of certain elements of the disk drive that are used in self servo writing; 
     FIG. 4 illustrates the initial servo track written on a disk; and 
     FIG. 5 illustrates the self writing of servo tracks on the first and second surfaces of a disk with the TPI on the first surface of the disk being different than the TPI on the second surface of the disk. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1A and 1B illustrate a typical disk drive  20  that includes a plurality of disks. To simplify the description of the invention, it is only necessary to consider a single magnetic disk  22 . It should, however, be appreciated that the invention is adaptable to disk drives that include multiple disks. The disk  22  is capable of storing data in concentric tracks located on a first surface  24 A and a second surface  24 B of the disk  22 . A spin motor  26  is used to rotate the disk  22  about a central axis  28  at a substantially constant rotational velocity. 
     A first head  30 A is provided for transferring data between the first surface  24 A of the disk  22  and the exterior environment. Similarly, a second head  30 B is provided for transferring data between the second surface  24 B of the disk  22  and the exterior environment. The first and second heads  30 A,  30 B each include a write element for writing data to the disk  22  and a read element for reading data from the disk  22 . 
     To position the first and second heads  30 A,  30 B over the tracks on the first and second surfaces  24 A,  24 B of the disk  22  so that data can be transferred, a multi-stage actuator  32  is provided. Included in the multi-stage actuator  32  is a primary actuator  34  for coarsely positioning the first and second heads  30 A,  30 B over desired locations on the first and second surfaces  24 A,  24 B of the disk  22 . The primary actuator  34  is comprised of a carriage  36  that includes first and second arms  38 A,  38 B for holding, respectively, the first and second heads  30 A,  30 B. Typically, the first and second arms  38 A,  38 B each include a rigid portion and a flexible, suspension portion. The suspension portion is located between the head and the rigid portion. A voice coil motor  40  is provided for rotating the first and second arms  38 A,  38 B about an axis  42 . To prevent the primary actuator  34  from moving the heads beyond the outer edge of the disks and contacting the interior of the disk housing (not shown), a crash stop  43  is provided. 
     With reference to FIG. 2, the multi-stage actuator  32  includes secondary actuators  44 A,  44 B for fine positioning of, respectively, the first and second heads  30 A,  30 B. The two secondary actuators are independently controllable. Independent control allows one of the secondary actuators to be implementing a seek operation with one of the heads (i e., moving a head from one track to another track) while the other secondary actuator is implementing a tracking operation with the other head (i.e., maintaining the position of the other head over a desired track). Further, independent control permits each of the secondary actuators  44 A,  44 B to be simultaneously implementing either a tracking function or a seeking function. For purposes of the description, the secondary actuators  44 A,  44 B are both rotary types of actuators. An example of such a secondary actuator can be found in U.S. Pat. No. 5,521,778. It should, however, be appreciated that the invention is equally applicable to a disk drive that uses a secondary actuator that moves a head in a linear manner. 
     The disk drive  20  further includes a data transfer device that operates in conjunction with the multi-stage actuator  32  to write servo tracks on the disk  22 . With reference to FIG. 3, an embodiment of the data transfer device  46  is illustrated that operates to: (1) use the primary actuator  34  and secondary actuators  44 A,  44 B to position the first and second heads  30 A,  30 B for the writing of servo tracks on the disk  22 ; (2) write the initial servo track on the first surface  24 A of the disk  22 ; (3) use the initial servo track as a reference for writing one or more servo tracks on the second surface  24 B of the disk  22 ; (4) use a servo track written on the second surface  24 B of the disk  22  as a reference for writing further servo tracks on the first surface  24 A of the disk adjacent to the initial servo track; (5) use a servo track, other than the initial servo track, written on the first surface  24 A of the disk  22  as a reference for writing further servo tracks on the second surface  24 B of the disk  22 ; and (6) repeat steps (4) and (5) until all of the servo tracks have been written on the first and second surfaces  24 A,  24 B of the disk  22 . 
     The device  46  includes some, if not all of the circuitry normally used to read and write user data to and from the disk  22 . Specifically, the device  46  includes an interface  48  that is capable of transferring data between the disk drive  20  and the exterior environment (typically, a host computer). The device  46  also includes a servo pattern generator  49  for providing the servo data that is written to the disk  22 . 
     The data transfer device  46  also includes channel processing circuitry  50  that is normally used to process and/or manage user data that is to be written to the disk  22  by one of the heads  30 A,  30 B and that has been read from the disk by one of the heads  30 A,  30 B. For writing servo tracks on the disk  22 , the channel processing circuitry  50  is capable of: (1) processing and/or managing servo data that is read from a servo track on the disk  22  by one of the heads  30 A,  30 B and providing the servo data to a servo system; and (2) while the servo data is being read and provided to a servo system, write a servo track to the disk  22  using one of the heads  30 A,  30 B and servo data provided by the servo pattern generator  49 . 
     The data transfer device  46  further includes a servo system  52  that is normally used in the writing of user data to: (1) control the primary actuator  34  to coarsely posit first and second heads  30 A,  30 B at a desired location over, respectively, the first and second surfaces  24 A,  24 B of the disk  22 ; and (2) control the secondary actuators  44 A,  44 B to finely position the first and second heads  30 A,  30 B, respectively. For the purpose of writing servo tracks, the servo system  52  serves the same functions with the only difference being that servo data and timing data rather than user data is written to the disk  22 . The servo system  52  is susceptible to a number of different approaches, including the parallel loop, master-slave loop, dual feedback loop, master-slave with decoupling approaches. 
     The data transfer device  46  further includes a controller  54  for coordinating the operation of the interface  48 , servo pattern generator  49 , channel processing circuitry  50 , and servo system  52 . With respect to the servo system  52 , the controller  54  operates to identify the tracks that the primary actuator  34  and each of the secondary actuators  44 A,  44 B should either be moving the heads  30 A,  30 B towards (i.e., seeking) or following (i.e., tracking). As is seen, the controller  54  is connected to the interface  48 , servo pattern generator  49 , channel processing circuitry  50  and servo system  52 . 
     Having described the disk drive  20 , the writing of servo tracks on the first and second surfaces  24 A,  24 B of the disk  22  is described. Initially, measurements are taken that provide a basis for determining the desirable or optimal TPI for each surface. Among the possible measurements are the read head width, i.e., the width of the track established by a read head or a read/write head when in the read mode of operation. Other possible measurements include the write head width and off-track performance based upon bit error rate and/or the read channel quality factor (which are typically shown in what are known as “bathtub” and “747” curves). 
     Once the TPI for each surface has been determined, the servo tracks are written on the first and second surfaces  24 A,  24 B with the desired TPIs using either a servo track writer or self servo writing. In the case of self servo track writing, operation commences with the receipt of a command from an exterior device, such as a microprocessor, at the interface  48  directing the disk drive  20  to perform the self servo writing operation. In response to the command, the controller  54  directs the servo system  52  to position the primary actuator  34  against the crash stop  43  for writing the initial servo track  56 A (FIG. 4) on the first surface  24 A of the disk  22  and adjacent to the edge  58  of the disk  22 . By positioning the primary actuator  34  against the crash stop  43 , any positional error in the initial servo track  56 A is substantially reduced, i.e., the end of the track should meet the start of the track with little, if any, radial offset. This, in turn, reduces any positional error in the servo tracks that are subsequently written on the disk  22 , the quality of which is dependent upon the initial servo track  56 A. The controller  54  also directs the servo system  52  to cause the secondary actuator  44 A to position the head  30 A for writing the initial servo track  56 A on the first surface  24 A of the disk  22 . Likewise, the controller  54  causes the head  30 B to be positioned with the secondary actuator  44 B for writing the initial servo track  56 B on the second surface  24 B of the disk  22  (FIG.  5 ). 
     Once the first head  30 A has been positioned, the controller  54  causes the servo data for the initial servo track  56 A to be transferred from the servo pattern generator  49  to the first head  30 A for writing on the disk. The content and the location of the servo data in the initial servo track  56 A is dependent upon the particular servo mechanisms being implemented in the drive. In one embodiment, the servo data is in the form of servo sectors that are typically located at positions  60 A- 60 H that are regularly spaced from one another. In one embodiment, the servo sectors for a servo track include an index that defines the beginning of the track, a track address, a segment addresses (i.e., a portion of the track), and data (i.e., servo bursts) that can be used to facilitate the following of the track. Included in the servo data of the initial servo track is a clock or timing signal. The clock signal is used by the servo system  52  to locate each servo sector along the track. In particular, the clock signal provides pulses or cycles that can be counted and the count used to establish predetermined spacing between the servo sectors in a servo track. 
     Once the initial servo track  56 A has been written and the second head  30 B positioned for writing the initial servo track  56 B on the second surface  24 B of the disk  22 , the controller  54  causes the servo data from the initial servo track  56 A to be read and used by the servo system  52  so that the first head  30 A tracks or follows the initial servo track  56 A. By having the first head  30 A follow the initial servo track  56 A and by maintaining the position of the second head  30 B relative to the first head  30 A, the second head  30 B follows a path over the second surface  24 B that substantially mirrors the initial servo track  56 A. In addition, the controller  54  causes the clock or timing signal from the initial servo track  56 A to be used to establish the desired spacing between the servo sectors of the initial servo track  56 B. The controller  54  also causes the servo data for the initial servo track  56 B to be transferred from the servo pattern generator  49  to the second head  30 B for writing on the disk and thereby establish the servo track  56 B on the second surface  24 B of the disk  22 . 
     In one embodiment, the servo data in servo track  56 A is used as a reference to write not only the initial servo track  56 B but also several other servo tracks on the second surface  24 B of the disk  22 . In this case, once the initial servo track  56 B has been written, the controller  54  causes the servo system  52  to use the secondary actuator  44 B to adjust the position of the second head  301 B for writing the next servo track  56 B′ on the second surface of the disk with the desired TPI. This process is typically repeated to establish a group of servo tracks  59  on the second surface  24 B of the disk with the specified TPI. For a given position of the primary actuator  34 , the number of servo tracks that can be established on the second surface  24 B is limited to the point or close to the point at which the multi-stage actuator  32  is incapable of adequately tracking the initial servo track  56 A and using the information from the initial servo track  56 A as a reference for writing servo tracks on the second surface  24 B of the disk  22 . Generally, this occurs, for a given position of the primary actuator  34 , when the secondary actuator  44 A is at or near the limit of its motion in one radial direction and the secondary actuator  44 B is at or near the limit of its motion in the opposite radial direction. 
     At this point, one of the servo tracks established on the second surface  24 B of the disk  22  needs to be used as a reference to write further servo tracks on the first surface  24 A of the disk  22  with the desired TPI, which may be different than the TPI of the servo tracks that have been established on the second surface  24 B. Typically, the last servo track  56 B″ written on the second surface  24 B is the furthest from the edge  58  and is used as the reference for writing further servo tracks on the first surface  24 A of the disk  22 . Since the servo track  56 B″ is going to be used as a reference, the track includes the clock signal that is used to position the servo sectors during the writing of the additional servo tracks on the first surface  24 A of the disk  22 . It should, however, be appreciated that any of the servo tracks established on the second surface  24 B can be used as the reference provided the servo track includes the clock signal. In any event, the controller  54  causes the process to be repeated to write a group of servo tracks  61  on the first surface  24 A of the disk  22  with the desired TPI. Specifically, the servo data from the servo track  56 B″ on the second surface  24 B of the disk  22  is used by the servo system  52  to maintain the position of the second head  30 B over the servo track  56 B″, the secondary actuator  44 A positions the first head  30 A over the first surface  24 A to establish the group of servo tracks  61  on the first surface  24 A of the disk  22  with the appropriate TPI for the first surface  24 A. The group of servo tracks  61  contains the same number of servo tracks as the group of servo tracks  59 . In addition, the group of servo tracks  61  includes servo track  62 . After the primary actuator  34  has been repositioned to write a new band of servo tracks, the servo track  62  is used to write a second band of servo tracks on the second surface  24 B in the same manner that the initial track  56 A was used in writing the first band of servo tracks on the second surface  24 B. Consequently, servo track  62  includes the clock signal. Likewise, while primary actuator  34  remains in a fixed position (except for track following adjustments), one of the servo tracks in the second band of servo tracks on the second surface  24 B that includes the clock signal is then used to write another band of servo tracks on the first surface  24 A in the same manner that servo track  56 B″ was used to write servo tracks on the first surface  24 A. 
     Once all of the servo tracks have been written on the first and second surfaces  24 A,  24 B of the disk  22  for a given position of the primary actuator  34 , the controller  54  causes the primary actuator  34  to be repositioned and the process is repeated. By using the last servo track written on the first surface  24 A of the disk  22  (i.e., the servo track  62 ), repositioning of the primary actuator  34  is substantially reduced, thereby reducing a potential source of error. The controller  54  causes the servo track writing process of using a reference on one surface of the disk  22  to facilitate the writing of servo tracks on the other surface of the disk  22  in an alternating manner to continue until all of the servo tracks have been established on both surfaces  24 A,  24 B of the disk  22 . 
     After all of the servo sectors have been established on the first and second surfaces  24 A,  24 B of the disk  22 , the clock or timing signal is no longer needed to establish desired spacing between the servo sectors. Consequently, the clock or timing signal present between the servo sectors can be overwritten with user data. 
     As an alternative to self servo writing, the servo tracks can be written with the use of a servo track writer. In this case, the push-pin of the servo track writer (rather than the actuator motor) is used to establish the position of the primary actuator  34 . Further, the servo pattern generator used to produce the servo data is within the servo track writer rather than the disk drive  20 , and thus, the disk drive  20  need not include servo pattern generator  49 . Otherwise, the process for writing the servo tracks is substantially identical to that described in the self servo writing method. 
     All of the servo tracks on first and second surfaces  24 A and  24 B of the disk  22  can be coincident with user data tracks. Alternatively, some of the servo tracks (e.g., tracks  56 A and  56 B) closest to edge  58  of disk  22  can be outside the data regions of first and second surfaces  24 A and  24 B that contain the user data tracks, in which case these servo tracks will contain exclusively servo information. 
     The invention is applicable or adaptable to disk drives in which: (1) there are two or more heads associated with a single surface of a disk and secondary actuators are associated with two or more of the heads; (2) there are two or more heads associated with disk surfaces that are, in turn, associated with different disks within the drive; (3) there are two or more primary actuators, rather than the single primary actuator described above; (4) a primary actuator is utilized that moves a head in a linear manner; (5) a secondary actuator is employed that moves a head in a linear manner; (6) primary and secondary actuators are utilized that involve combinations of rotary and linear actuators; (7) the servo track data is received from the exterior environment; (8) the servo tracks are written away from the center of the disk(s) rather than towards the center of the disk(s); (9) the servo tracks are written out of sequence; (10) only one servo track is written on a surface of a disk at a time; (11) the device  46  that cooperates with the multi-stage actuator to write the servo tracks is a separate device from the device used to write and/or read user data on the disk(s); and (12) a multi-stage actuator is employed that has more than two stages. 
     The foregoing description of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and the skill or knowledge in the relevant art are within the scope of the present invention. The preferred embodiment described above is further intended to explain the best mode known of practicing the invention and to enable others skilled in the art to utilize the invention in various embodiments and with the various modifications required by their particular applications or uses of the invention. It is intended that the appended claims be construed to include alternate embodiments to the extent permitted by the prior art.