Patent Publication Number: US-2005128634-A1

Title: Systems and methods for improved worf calculation

Description:
CLAIM OF PRIORITY  
      This application claims priority from the following applications, which are hereby incorporated by reference in their entireties:  
      U.S. Provisional Patent Application No. 60/496,463, entitled SYSTEMS FOR IMPROVED WORF CALCULATION by Thorsten Schmidt, filed Aug. 20, 2003 (Attorney Docket No. PANA-01078US0).  
      U.S. Provisional Patent Application No. 60/496,538, entitled METHODS FOR IMPROVED WORF CALCULATION by Thorsten Schmidt, filed Aug. 20, 2003 (Attorney Docket No. PANA-01078US1). 
    
    
     FIELD OF THE INVENTION  
      The present invention relates to the ability to read and write data on rotatable storage media.  
     BACKGROUND  
      Advances in data storage technology have provided for ever-increasing storage capability in devices such as DVD-ROMs, optical drives, and disk drives. In hard disk drives, for example, the width of a written data track has decreased due in part to advances in reading, writing, and positioning technologies. Thinner data tracks result in higher density drives, which is good for the consumer but creates new challenges for drive manufacturers. As the density of the data increases, the tolerance for error in the position of a drive component such as a read/write head decreases. As the position of such a head relative to a data track becomes more important, so too does the placement of information, such as servo data, that is used to determine the position of a head relative to a data track.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a diagram showing components of a disk drive that can be used in accordance with embodiments of the present invention.  
       FIG. 2  is a diagram showing a typical pattern that can be written to a disc in the drive of  FIG. 1 .  
       FIG. 3  is a diagram showing servo information that can be written to the tracks shown in  FIG. 2 .  
       FIG. 4  is a diagram of a servo pattern that can be used with the drive of  FIG. 1 .  
       FIG. 5  is a diagram of a servo pattern containing WORF data in accordance with one embodiment of the present invention.  
       FIG. 6  is a diagram of a servo pattern containing WORF data and quadrant information in accordance with one embodiment of the present invention.  
       FIG. 7  is a diagram of a servo pattern containing WORF data and quadrant information for both a read and a write operation in accordance with one embodiment of the present invention.  
       FIG. 8  is a diagram of servo bursts that can be used in accordance with an embodiment of the present invention.  
       FIG. 9  is another diagram of servo bursts that can be used in accordance with an embodiment of the present invention.  
       FIG. 10  is a chart showing quadrants in accordance with one embodiment of the present invention.  
       FIG. 11  is another diagram of servo bursts that can be used in accordance with an embodiment of the present invention.  
       FIG. 12  is a flowchart showing a method that can be used with the system of  FIG. 1 .  
       FIG. 13  is a flowchart showing another method that can be used with the system of  FIG. 1 .  
    
    
     DETAILED DESCRIPTION  
      Systems and methods in accordance with one embodiment of the present invention can be used when servowriting, or self-servowriting, a rotatable storage medium in a data storage device, such as a hard disk drive. For example, a typical disk drive  100 , as shown in  FIG. 1 , includes at least one magnetic disk  102  capable of storing information on at least one of the surfaces of the disk. A closed-loop servo system can be used to move an actuator arm  106  and data head  104  over the surface of the disk, such that information can be written to, and read from, the surface of the disk. The closed-loop servo system can contain, for example, a voice coil motor  108  to drive the actuator arm, a spindle motor  112  to rotate the disk(s), a servo controller  110  to control the motors, and a disk controller  118  to accept information from a host  122  to provide to the servo controller. A host can be any device, apparatus, or system capable of utilizing the data storage device, such as a personal computer or Web server. The drive can contain at least one processor, or microprocessor  120 , that can process information for at least one of the disk controller  118  and servo controller  110 . The disk controller  118  can also provide user data to a read/write channel  114 , which can send data signals to a current amplifier or preamp  116  to be written to the disk(s)  102 , and can send servo signals to the servo controller  110 .  
      The information stored on such a disk can be written in concentric tracks, extending from near the inner diameter of the disk to near the outer diameter of the disk  200 , as shown in the example disk of  FIG. 2 . In an embedded servo-type system, servo information can be written in a servo wedge  202 , and can be recorded on tracks  204  that can also contain data  206 . In a system where the actuator arm rotates about a pivot point such as a bearing, the servo wedges may not extend linearly from ID to OD, but may be curved slightly in order to adjust for the trajectory of the head as it sweeps across the disk.  
      The servo information often includes bursts of transitions called “servo bursts.” The servo information can be positioned regularly about each track, such that when a data head reads the servo information, a relative position of the head can be determined that can be used by a servo processor to adjust the position of the head relative to the track. For each servo wedge, this relative position can be determined in one example as a function of the target location, a track number read from the servo wedge, and the amplitudes or phases of the bursts, or a subset of those bursts. The position of a head or element, such as a read/write head or element, relative to the center of a target track, will be referred to herein as a position-error signal (PES).  
      For example, a centerline  300  for a given data track can be “defined” relative to a series of bursts, burst edges, or burst boundaries, such as a burst boundary defined by the lower edge of A-burst  302  and the upper edge of B-burst  304  in  FIG. 3 . The centerline can also be defined by, or offset relative to, any function or combination of bursts or burst patterns. This can include, for example, a location at which the PES value is a maximum, a minimum, or a fraction or percentage thereof. Any location relative to a function of the bursts can be selected to define track position. For example, if a read head evenly straddles an A-burst and a B-burst, or portions thereof, then servo demodulation circuitry in communication with the head can produce equal amplitude measurements for the two bursts, as the portion of the signal coming from the A-burst above the centerline is approximately equal in amplitude to the portion coming from the B-burst below the centerline. The resulting computed PES can be zero if the radial location defined by the A-burst/B-burst (A/B) combination, or A/B boundary, is the center of a data track, or a track centerline. In such an embodiment, the radial location at which the PES value is zero can be referred to as a null-point. Null-points can be used in each servo wedge to define a relative position of a track. If the head is too far toward the outer diameter of the disk, or above the centerline in  FIG. 3 , then there will be a greater contribution from the A-burst that results in a more “negative” PES. Using the negative PES, the servo controller could direct the voice coil motor to move the head toward the inner diameter of the disk and closer to its desired position relative to the centerline. This can be done for each set of burst edges defining the shape of that track about the disk. It should be understood that the pattern of  FIG. 3  is exemplary, and that many patterns can be used equally as well and can also take advantage of embodiments of the present invention.  
      The PES scheme described above is one of many possible schemes for combining the track number read from a servo wedge and the phases or amplitudes of the servo bursts. Many other schemes are possible that can benefit from embodiments in accordance with the present invention.  
      A problem that exists in the reading and writing of servo patterns involves the misplacement, or offset, of a read/write head with respect to the ideal and/or actual position of a track. It is impossible to perfectly position a head with respect to a track for each rotation of a disk, as there is almost always a noticeable offset between the desired position and the actual position of the head with respect to the disk. This can cause problems when writing servo patterns, as each portion of the pattern can be slightly misplaced. This can lead to what is referred to as written-in runout. Written-in runout can be thought of as the offset between the “actual” centerline, or desired radial center, of a track and the centerline that would be determined by a head reading the written servo pattern. Written-in runout can lead to servo performance problems, wasted space on a disk and, in a worst case, unrecoverable or irreparably damaged data.  
      It is possible using various methods, known to those of ordinary skill in the art, to determine the distance between the desired track centerline (either a read track centerline or write track centerline), having effectively removed at least a portion of the synchronous runout, and the apparent centerline obtained from demodulating the burst pattern. Examples of such methods can be found in U.S. Pat. No. 6,097,565 to Sri-Jayantha et al., entitled: “Repeatable runout free servo architecture in direct access storage device;” U.S. Pat. No. 6,061,200 to Shepherd et al., entitled “In-drive correction of servo pattern errors;” U.S. Pat. No. 5,978,169 to Woods et al., entitled “Repeated servo runout error compensation in a disc drive;” and U.S. Pat. No. 6,310,742 to Nazarian et al., entitled “Repeatable runout cancellation in sectored servo disk drive positioning system.” 
      This determined distance can be stored in the servo wedges for a track, such as after the servo bursts, and will be referred to herein as Wedge Offset Reduction Field (WORF) data. WORF data can be, for example, a digital number placed after a servo wedge on a given track that includes an amount that should be added to, or subtracted from, the PES value for that wedge obtained from demodulating the bursts. Alternatively, WORF data can also be stored in memory such as SRAM, DRAM, or flash.  
      A WORF value can be determined, for example, by observing a track of servo information over at least one revolution, if not several revolutions of the disk and combining the observed position information with the servo loop characteristics, which can be modeled, measured, or estimated. As an example, observed position information can be synchronously averaged to determine the synchronous runout, and can be combined with the servo loop characteristics. This combined information can be used to determine the misplacement of the burst edges used to determine a track centerline, for example. The servo can read the WORF value, “add” the value to the computed PES, and presumably follow a more accurate track. For instance, if a read/write head passes over a pair of servo bursts and determines a PES of +0.2, but it has been determined that the bursts are slightly misplaced and should have given a reading of −0.1 for that position of the read/write head, a −0.3 factor can be stored as WORF data at the end of the servo data such that the head knows to adjust the PES value by the WORF value.  
      The use of WORF information can cause problems, however, if the set of bursts used to compute PES, and to which the WORF value is added, is different than the set of bursts used to calculate the WORF value for a particular wedge. Applying an offset to this “incorrect” set of bursts can be enough to cause the head move by an amount that results in the head reading data from, or writing data to, an adjacent track. It can also cause write and read faults, where the drive determines that the head is far enough from the center of the track that the transfer of information should be stopped. Such occurrences can reduce the performance of the drive. Although the situation of using inconsistent WORF and burst sets can occur at any target position, it is more likely to occur when the target position is in the proximity of a boundary between two burst sets, or two quadrants.  
      Systems and methods in accordance with one embodiment of the present invention address the problem of applying WORF values to improper servo burst pairs by combining quadrant position information with the WORF information stored in a servo wedge. For example, the non-repeatable runout (NRRO) suffered by a read/write (R/W) head during self-servowriting can be written into the servo bursts. This can cause each servo burst to be misplaced relative to the desired centerline of a track. A similar problem can occur during servo write and media write processes.  
      For example, it can be seen in  FIG. 3  that the centerline of a track  300  is defined by a series of A-B burst pairs, where for each pair the bottom edge of the A-burst  302  and the top edge of the B-burst  304  are used to define the centerline position. This can be referred to as an A-burst/B-burst boundary, designating that the bottom edge of the A-burst and the top edge of the B-burst are to be used to define the centerline position.  
      Due to irregularities in the disk and in the writing mechanism, for example, each burst pair can be misplaced relative to the desired or optimal position. One way to account for the misplacement of the complimentary edges of each burst pair, or burst boundary, is to determine the “average” location of the burst boundaries, or to determine an improved centerline by examining the burst pairs over at least one revolution of the disk, then determining how far each burst pair is from the location of the center of the improved track. This information distance, which can include WORF data, can be stored such as by writing to the track or storing in memory.  
      In the section of exemplary servo information  400  shown in  FIG. 4 , there are four data track centerlines shown  402 ,  404 ,  406 , and  408 . While the majority of the track centerlines are positioned approximately along the companion edges or boundaries of the appropriate servo bursts, it can be seen that the A-burst for track centerline  406  is misplaced by an appreciable amount. For this wedge of the data track, the PES signal would give a reading that would cause a R/W head to be positioned above the desired head position for the track in the Figure. For such a track, WORF data can be added to the servo information in this servo wedge to account for the misplacement.  
      In the servo pattern  400  of  FIG. 4 , it can be seen that there is a data track centerline for every third burst boundary. For example, data track centerline  402  is defined by an A-burst/B-burst boundary. The next boundary  412  down in the Figure is defined by a D-burst/C-burst boundary, with the following boundary  414  defined by a B-burst/A-burst boundary. These boundary lines can be referred to as burst lines. When the synchronous or repeatable runout is removed from the pattern, it is possible that the desired track center position passes closer to one of the adjacent boundaries than the “intended” boundary. For example, when the servo pattern was written, it was intended that the position of data track centerline  402  be defined by the A-burst/B-burst boundary. It is possible, however, that once the repeatable runout is accounted for, such as may be due to eccentricities or other characteristics of the disk, the “corrected” data track centerline would pass closer to either the C-burst/D-burst boundary defining line  410 , or the D-burst/C-burst boundary defining line  412 . In such a case, it would be improper to apply a WORF adjustment to the PES signal obtained from one of those boundary burst pairs that was calculated for the A-burst/B-burst boundary.  
      One approach to using WORF values in such a situation utilizes the concept of servo quadrants. A servo quadrant is defined herein to represent a radial area that is a portion, or radial subdivision, of a burst cycle. A quadrant can be thought of as a radial extent over which the fractional servo position error signal is a given function of the bursts. For example, the radial position error signal (PES) can be determined by combining the track number and a function of the bursts. The function used can depend upon the radial location and the values of the bursts. The region over which the function is the same is defined as a quadrant.  
      Quadrants can repeat as often as the servo pattern. In some patterns, a quadrant can be that area, along a data track, that is closest to a given servo burst boundary. The term quadrant is used in the industry to refer to any such radial area, regardless of the servo pattern. This means that the term “quadrant” is used as abroad, generic term that can include, for example, sextants and octants. When a quadrant is referred to herein, it should be understood that the reference is not limited to a four-burst pattern.  
      For example, in  FIG. 4  quadrant zero (Q 0 ) is the area of the servo pattern for which line  414  defined by the B-burst/A-burst boundary is closer than the line defined by any other boundary. The fractional PES for this quadrant would be defined by the amplitude given by the A-burst minus the amplitude given by the B-burst, which could be designated as A-B. The fractional PES can then be multiplied by an appropriate gain constant. Quadrant one (Q 1 ) is the area that is closest to line  404 , defined by the C-burst/D-burst boundary, which would give a PES value of D-C or −(C-D). Quadrant two (Q 2 ) is closest to line  402  defined by the A-burst/B-burst boundary (PES of B-A or −(A-B)), and quadrant three (Q 3 ) is closest to line  412  defined by the D-burst/C-burst boundary (PES of C-D). There is also a quadrant one closest to line  410  (also a PES of D-C or −(C-D)). It should be understood that the concept of quadrants is exemplary, and can be extended to other servo patterns or to any PES schemes that use more or less than four different combinations of burst values to determine the fractional PES.  
      Using the concept of quadrants in this example, it is possible to store the quadrant information along with the WORF data. For example, a WORF field can include both a misplacement adjustment for a burst boundary and the quadrant associated with that burst boundary. The servo system can then determine that the WORF field should be used only if the servo uses the matching quadrant. In another embodiment, WORF values for the adjacent quadrants can also be written to the disk or stored in memory, such that if the trajectory of the head takes it to one of these quadrants, a proper adjustment can be made. If WORF information is not stored for these adjacent quadrants, then a drive can decide to simply not apply a WORF adjustment, or can decide to not read or write for that revolution of the disk. The drive can then return to the data track at a later time, or, if writing data, can decide to no longer use that track or data sector. In another embodiment, the drive can force the position algorithm to use the burst set associated with the WORF value. There may be various other ways to use the quadrant information written with the WORF value. Referring again to  FIG. 1 , the WORF information could be read and included in the read signal sent from the head  104  through the read/write channel  114  and, after being processed by the microprocessor  120 , ultimately sent to the servo controller  110  as servo data to be used in driving the actuator arm  106  to position the head  104 .  
      An example of WORF data being added to a servo pattern is shown in  FIG. 5 . Taking the pattern of  FIG. 4 , WORF information is added for each data track corresponding to centerlines  402 ,  404 , and  406 . In the pattern shown, the servo information is “trimmed” or partially overwritten on subsequent passes of a write head, or in subsequent servowriting steps, such that the width of each track of servo information is thinner than the width of information otherwise written by the write head. As shown in  FIG. 5 , the width of WORF data can as large as the width of the data track, but is typically somewhat less wide than the data track. In the Figure, it can be seen that there is a preamble, or sync mark, at the beginning of the WORF field. The WORF preamble is exemplary and may not be necessary. It can be seen, then, that it would be possible to read an adjacent or “incorrect” boundary and still read a WORF value intended for another boundary. A WORF value can be recorded for the position of a head when writing the track, the position of a head when reading the track, or both. Each WORF value may include, for example, a preamble and at least one data value.  
       FIG. 6  shows an example in which quadrant information has been added to the WORF information for each data track. In addition to the WORF data, another section of information (WQ) is added to designate the quadrant, or WORF Quadrant, for which the WORF value was calculated. While the WQ information is shown at the end of the servo information for that wedge/track, it should be understood that the quadrant information can be placed at any available location in the wedge, outside the wedge, or could be incorporated with other information.  
       FIG. 7  also shows an example in which quadrant information has been added to the WORF information for each data track. In  FIG. 7 , however, it can be seen that WORF data and WORF quadrant information is also written for a read operation. WORF data can be stored for both read and write operations, and can be stored in any order. WORF data in a wedge can also contain WORF information for subsequent wedges. While the WORF data for write operations may often be located at or near the radial position of a data track, the WORF data for read operations may be radially offset relative to a data track or servo track. If a read/write head is used that contains offset read and write elements, the radial separation between the elements will vary as the head sweeps across the surface of the disk. Therefore, the WORF offset can similarly vary radially across the disk.  
       FIG. 8  shows a burst pattern for three servo wedges  500 ,  502 ,  504  on a data track. The track centerline is defined by the A-burst/B-burst boundaries of each wedge. The trajectory of a head without the use of WORF information can be seen. For the burst pattern in wedge  500 , the bursts are all written in approximately the correct location, such that a PES value of about zero would be read for the wedge. For wedge  502 , the A-burst/B-burst boundary is misplaced by about 10% toward the ID. This could result in a PES value of about −10%. In order to account for the PES value, the trajectory of the head without the use of WORF information would then improperly be adjusted toward the ID. By wedge  504 , even though the A-burst/B-burst boundary is written in approximately the correct location, the improper trajectory would cause a PES value of about 10% to be read.  
      In order to improve the trajectory of the head, the WORF information can be processed for each wedge. For wedge  500 , the PES value of zero would correspond to a WORF value of zero, as the boundary was not misplaced. For wedge  502 , the WORF value would take into account the 10% misplacement of the A/B boundary, such that when combined with the −10% PES value that would otherwise be calculated, the PES value would be about zero. It can be seen that the effect of wedge  502  on the trajectory is much more favorable when using the WORF value than without the WORF value. By the time the head gets to wedge  504 , the WORF value is again zero because there is no misplacement, but there is also a PES value of zero because the trajectory of the head causes it to straddle the A/B boundary.  
       FIG. 9  shows a similar pattern, except that the target position is 25% removed from track center. This can be due to a read/write offset that is a fraction of a data track. It can also be seen that there is no misplacement of bursts in wedge  600 , but the A/B boundary is misplaced about 10% toward ID in wedge  602  and the C/D boundary is misplaced about 10% toward ID in wedge  604 . As above, it can be seen that the trajectory without WORF is affected by the misplacement of wedge  602 . The PES value that is then read for wedge  604  is about zero, such that the drive thinks the trajectory is along the target even though it is actually misplaced relative to the target.  
      WORF information could be used to correct the trajectory as in  FIG. 7 , but trajectory with WORF information is non-ideal in this case, because a WORF value calculated for the C/D burst quadrant in wedge  604  would have a value of about 10%. After using the WORF value for wedge  602 , however, the trajectory of the head would no longer take the head closest to the C/D boundary in wedge  604 , but closest to the A/B boundary. If the 10% WORF value is improperly applied to the A/B boundary, which is actually in about the correct place, the trajectory of the head will improperly be adjusted toward the OD of the disk. If, however, WORF quadrant information is used, such that the drive knows the WORF value is for the C/D quadrant, the WORF value will not be improperly applied to the A/B quadrant and the trajectory will be closer to the target.  
      One way in which to determine the current quadrant is to examine the absolute magnitude of the boundaries. For example,  FIG. 10  shows an example of a boundary curve for boundary pair A/B and boundary pair C/D. It can be seen that when the head is in Q 0 , the absolute value of A-B is smaller than the absolute value of C-D. Therefore, the head is either in Q 0  or Q 2 . Since the sensed amplitude of C is greater than the sensed amplitude of D, then the head knows it is in Q 0 . Accordingly, the PES signal can be calculated from A-B. In quadrant  1 , the absolute value of C-D is less than the absolute value of A-B. Therefore, the head is in either Q 1  or Q 3 . Since the sensed amplitude of A is larger than the sensed amplitude of B, the head knows it is in Q 1 . When the absolute value of A-C and B-D are the same, a decision will need to be made as to which curve to use to record WORF values. As an alternative, when |A-C| =|B-D| WORF can be recorded for both boundaries.  
      Improving Results  
      Improved servo positioning results can be obtained in other embodiments utilizing the calculated WORF values while taking into account the quadrant information described above. Such an approach can reduce quadrant switching problems that can be associated with WORF calculations, and can improve the accuracy of the WORF calculations. Improving WORF accuracy can improve the TMR of the drive, which can thereby improve the overall quality of the drive.  
      Using one of the processes described above for determining WORF information, a number (N) of revolutions of PES data measurements can be taken to determine the synchronous runout. It can be important in such a calculation to consistently use the same quadrant for any particular wedge. For example,  FIG. 11  shows a diagram having servo information for five different wedges  700 ,  702 ,  704 ,  706 ,  708 . In a simple example, it can be assumed that five revolutions of the disk, or passes of a read/write head over the servo information, are used to collect the PES data for calculating synchronous runout. In one situation, the first four passes of the read write head might read a burst transition in quadrant  2  (Q 2 ) for each of the five wedges in the Figure. In the fifth pass, shown by path  710  in the Figure, the PES in wedges  700 ,  704 ,  706 , and  708  is determined using quadrant  2 . For wedge  702 , the head reads servo position information from quadrant  1  (Q 1 ). If the PES values for each pass are used to determine the synchronous runout, then the resulting WORF value for wedge  702  will likely be inaccurate, as the collected values came from different quadrants. The problem stems from the fact that the position of the head as it passes over wedge  702  relative to quadrant  1  can be quite different than when passing over quadrant  2 . In  FIG. 11 , PES information would indicate that the head was too “high” in the Figure, being above the A-B transition, but would indicate the head was too “low” in the Figure, being below the C-D transition.  
      In such a situation, several solutions can be used to account for the quadrant variation. In one approach, a drive system can continue to collect PES data until the synchronous runout can be calculated using quadrant-consistent revolutions, or “consistent” revolutions, such as 5 total revolutions using the same quadrants in the example above. This could be a sixth revolution in the example, if the next pass over the five wedges uses quadrant  2 . Alternatively, the drive system can continue collecting PES data on subsequent revolutions until five consecutive revolutions yield PES data from quadrant  2 . The latter approach can require additional servowriting time, but can yield more stable results. For example, if the head is alternating between quadrants  1  and  2  on subsequent passes, such as due to a vibration or other oscillation-inducing event, the former approach would use the PES values collected as soon as five values are collected for either quadrant  1  or quadrant  2 , even though the path of the head might be quite irregular.  
       FIG. 12  shows a flowchart for one such process. In this process, servo information for a track can be read over a number of revolutions  800 . This can include reading servo information for each wedge of information along the track. The misplacement of at least a portion of the servo information for that track can be determined for each revolution  802 , such as the misplacement of servo bursts for at least a subset of the servo wedges. The corresponding servo quadrant containing each portion of servo information being used to determine the misplacement can be identified  804 . A determination can be made as to whether the total number of “good” revolutions has been met  806 . A “good” revolution can be any revolution for which the anticipated quadrant was detected for each misplacement determination. A number of “good” revolutions can be set for each track to meet, such as five good revolutions, such that a system will take the number of revolutions necessary to obtain that number of good revolutions. If the total number of good revolutions has not been met, servo information can be read for another revolution  812  and another determination  802  can take place. Once the total number of good revolutions has been met, the synchronous runout of the track can be determined by looking at the quadrant information and misplacement for each good revolution  808 . Information about the synchronous runout can then be stored, such that the information can be used in a read or write operation using that track for position information  810 . Information can be stored to the disk as WORF data, for example.  
      In another embodiment, only “good” data, or data collected for the appropriate quadrant, can be used in a runout calculation. In the example above, the “good” data could be considered to be the PES information collected on the first four passes, which collected information using only quadrant  2 . In such a situation, the system could simply use the four “good” values instead of the nominal five values. Limitations can be placed on such an approach to improve results. For example, if only one or two of the five values use the appropriate quadrant, then the system may not choose to calculate runout on just those values, as the radial position of the head may have been fairly unstable. In such a situation, the system may decide to collect PES data using one of the additional revolution approaches described above, or can instead choose to take another five revolutions and examine the quadrant results again. This process, as well as the ones described above, can either be repeated until acceptable results are obtained, until a maximum number of retries is reached, or for a maximum amount of time. If acceptable values are not obtained, the drive can deal with the data track using any of the methods known and/or used in the art to deal with tracks and/or wedges having potentially unreliable servo information.  
       FIG. 13  shows one such process. Similar to the process of  FIG. 12 , servo information can be read for a track over a number of revolutions  900 , and the displacement of at least a portion of the servo information can be determined for each revolution  902 . The servo quadrant containing the portion of the servo information used to determine the displacement can be identified  904 . A minimum number of “consistent” revolutions can be set, with a “consistent” revolution being defined in one embodiment as a revolution for which the identified servo quadrants are as expected, or are consistent with the majority of other revolutions. If the minimum number of revolutions has not been met, such as at least three revolutions out of five, the process can start over for that track at step  900 . If the minimum number has been reached  906 , then the synchronous runout of the track can be determined by looking at the quadrant information and misplacement for each consistent revolution  908 . Information about the synchronous runout can then be stored for later use in a read or write operation using that track for position information  910 .  
      Problems with inconsistent quadrant usage can occur in any target position, but can occur more often when the target position is near a quadrant “switch point.” A switch point can occur, for example, when servo data for adjacent wedges along a track use different quadrants as a reference for WORF information. Near a switch point, the contribution from the two quadrants can be about equal, such that any misplacement of a read head or read element can result in the wrong quadrant information being used if the quadrant information is not otherwise tracked.  
      Although embodiments described herein refer generally to systems having a read/write head that can be used to write bursts on rotating magnetic media, advantages of the present invention can be obtained for other media storage devices. For example, a laser writing information to an optical media can utilize WORF data and position information to account for irregularities in positioning information. Any media, or at least any rotating media, upon which information is written, placed, or stored, may be able to take advantage of embodiments of the invention, as variations in optical, electrical, magnetic, mechanical, and other physical systems can be made by varying a drive signal or other control mechanism in order to account for misplacement.  
      In some embodiments, a system for adjusting the position of a head relative to a track on a rotatable storage medium can include: a reference pattern on a surface of a rotatable medium, the reference pattern containing a plurality of concentric tracks containing radial position information; a head containing at least one of a read element capable of reading information from the surface and a write element surface capable of storing information to the surface; and a control mechanism adapted to rotate the rotatable medium and position the head relative to the rotatable medium such that the misplacement of one of a plurality of positioning patterns for a track on the rotatable medium can be determined for each of a number of revolutions and the quadrant containing the respective positioning pattern for each revolution can be identified, the rotating medium having a plurality of quadrants extending radially across a surface of the rotatable medium, the information about the misplacement and quadrant for each revolution capable of being used to determine the synchronous runout of the track, the determination of synchronous runout accounting for any variation in the quadrant containing the respective positioning pattern for each revolution, the control mechanism further causing the write element to store information about the synchronous runout to the rotatable medium.  
      In some embodiments, the rotatable medium in the system can be selected from the group consisting of magnetic disks, optical disks, and laser-recordable disks.  
      In some embodiments, the radial position information can include at least one phase burst pair.  
      In some embodiments, the system can further include read circuitry adapted to accept information from a read element and determine the position of the respective read/write head.  
      In some embodiments, the system can further include a write mechanism adapted to write information about the misplacement and quadrant to the quadrant on the rotatable medium containing the positioning pattern.  
      In some embodiments, the control mechanism in the system can be further adapted to determine the misplacement by determining a position error signal for the positioning pattern.  
      In some embodiments, the system can further include a servo controller capable of determining a position error signal.  
      In some embodiments, the information stored about the misplacement can include a digital number that indicates amount the position error signal should be adjusted for that positioning pattern.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and the control mechanism can be further adapted to determine the misplacement of a positioning pattern by examining the position of each of the plurality of positioning patterns in the quadrant to determine an average positioning pattern position about the track.  
      In some embodiments, the control mechanism in the system can be further adapted to determine the misplacement of a positioning pattern by determining the misplacement of the positioning pattern relative to the average position of positioning patterns about the track.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and the control mechanism can be further adapted to identify the quadrant containing the respective positioning pattern by examining the relative position of each of the plurality of positioning patterns.  
      In some embodiments, the control mechanism can be further adapted to store information about the misplacement and quadrant by writing the information in the quadrant containing the positioning pattern.  
      In some embodiments, the control mechanism can be further adapted to determine a total number of consistent revolutions to be used in determining synchronous runout for a track.  
      In some embodiments, the control mechanism can be further adapted to take at least one additional revolution if the total number of consistent revolutions has not been reached.  
      In some embodiments, the control mechanism can be further adapted to determine a minimum number of consistent revolutions to be used in determining synchronous runout for a track.  
      In some embodiments, the control mechanism can be further adapted to take at least one additional revolution if the minimum number of consistent revolutions has not been reached.  
      In some embodiments, the control mechanism can be further adapted to take at least one additional revolution before using information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track.  
      In some embodiments, a system for adjusting the position of a head relative to a track on a rotatable storage medium can include: a rotatable medium including at least one surface having a servo pattern contained thereon, the servo pattern containing a plurality of concentric quadrants and a plurality of servo wedges; a positioning mechanism adapted to determine any misplacement of a portion of the servo pattern in one of the servo wedges, and the quadrant containing that portion; and read/write circuitry adapted to: determine, on each of a plurality of revolutions of a rotating medium, the misplacement of one of a plurality of positioning patterns for a track on the rotating medium; identify the quadrant containing the respective positioning pattern for each revolution, the rotating medium having a plurality of quadrants extending radially across a surface of the rotating medium; use information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track, the determination of synchronous runout accounting for any variation in the quadrant containing the respective positioning pattern for each revolution; and store information about the synchronous runout to be used in any of a read operation and write operation that determines position using that positioning pattern, such that the misplacement information is only used for the respective quadrant.  
      In some embodiments, the system can further include a write mechanism adapted to write information about the misplacement and quadrant to the quadrant on the rotatable medium containing the positioning pattern.  
      In some embodiments, the read/write circuitry can be further adapted to determine the misplacement by determining a position error signal for the positioning pattern.  
      In some embodiments, the system can further include a servo controller capable of determining a position error signal.  
      In some embodiments, the information stored about the misplacement can include a digital number that indicates amount the position error signal should be adjusted for that positioning pattern.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and the read/write circuitry is further adapted to determine the misplacement of a positioning pattern by examining the position of each of the plurality of positioning patterns in the quadrant to determine an average positioning pattern position about the track.  
      In some embodiments, the read/write circuitry can be further adapted to determine the misplacement of a positioning pattern by determining the misplacement of the positioning pattern relative to the average position of positioning patterns about the track.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and the read/write circuitry is further adapted to identify the quadrant containing the respective positioning pattern by examining the relative position of each of the plurality of positioning patterns.  
      In some embodiments, the read/write circuitry can be further adapted to store information about the misplacement and quadrant by writing the information in the quadrant containing the positioning pattern.  
      In some embodiments, the read/write circuitry can be further adapted to determine a total number of consistent revolutions to be used in determining synchronous runout for a track.  
      In some embodiments, the read/write circuitry can be further adapted to take at least one additional revolution if the total number of consistent revolutions has not been reached.  
      In some embodiments, the read/write circuitry can be further adapted to determine a minimum number of consistent revolutions to be used in determining synchronous runout for a track.  
      In some embodiments, the read/write circuitry can be further adapted to take at least one additional revolution if the minimum number of consistent revolutions has not been reached.  
      In some embodiments, the read/write circuitry can be further adapted to take at least one additional revolution before using information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track.  
      In some embodiments, a system for two-step self-servowriting can include: means for determining, on each of a plurality of revolutions of a rotating medium, the misplacement of one of a plurality of positioning patterns for a track on the rotating medium; means for identifying the quadrant containing the respective positioning pattern for each revolution, the rotating medium having a plurality of quadrants extending radially across a surface of the rotating medium; means for using information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track, the determination of synchronous runout accounting for any variation in the quadrant containing the respective positioning pattern for each revolution; and means for storing information about the synchronous runout to be used in any of a read operation and write operation that determines position using that positioning pattern, such that the misplacement information is only used for the respective quadrant.  
      In some embodiments, a method for adjusting the position of a head relative to a track on a rotatable storage medium can include: determining, on each of a plurality of revolutions of a rotating medium, the misplacement of one of a plurality of positioning patterns for a track on the rotating medium; identifying the quadrant containing the respective positioning pattern for each revolution, the rotating medium having a plurality of quadrants extending radially across a surface of the rotating medium; using information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track, the determination of synchronous runout accounting for any variation in the quadrant containing the respective positioning pattern for each revolution; and storing information about the synchronous runout to be used in any of a read operation and write operation that determines position using that positioning pattern, such that the misplacement information is only used for the respective quadrant.  
      In some embodiments, the method of determining the misplacement can include determining a position error signal for the positioning pattern.  
      In some embodiments, the position error signal can be determined by a servo controller.  
      In some embodiments, the information stored about the misplacement can include a digital number that indicates amount PES should be adjusted for that positioning pattern.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and determining the misplacement of a positioning pattern can include examining the position of each of the plurality of positioning patterns in the quadrant to determine an average positioning pattern position about the track.  
      In some embodiments, determining the misplacement of a positioning pattern can further include determining the misplacement of the positioning pattern relative to the average position of positioning patterns about the track.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and identifying the quadrant containing the respective positioning pattern can include examining the relative position of each of the plurality of positioning patterns.  
      In some embodiments, storing information about the misplacement and quadrant can include writing the information in the quadrant containing the positioning pattern.  
      In some embodiments, the method can further include determining a total number of consistent revolutions to be used in determining synchronous runout for a track.  
      In some embodiments, the method can further include taking at least one additional revolution if the total number of consistent revolutions has not been reached.  
      In some embodiments, the method can further include determining a minimum number of consistent revolutions to be used in determining synchronous runout for a track.  
      In some embodiments, the method can further include taking at least one additional revolution if the minimum number of consistent revolutions has not been reached.  
      In some embodiments, taking at least one additional revolution can occur before using information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track.  
      In some embodiments, a method for reducing written-in runout in a servo pattern on a magnetic hard disk can include determining the misplacement of a servo burst pair on a rotating hard disk for a number of revolutions; identifying the quadrant containing the servo burst pair for each revolution and determining if each revolution is a consistent revolution, the rotating hard disk having a plurality of quadrants extending radially across a surface of the disk; determining the misplacement of, and identifying the quadrant containing, a servo burst pair for at least one additional revolution if a minimum number of consistent revolutions has not been met; and storing information about the misplacement of the burst pair and the quadrant to be used in any of a read operation and write operation that determines position using that burst pair, such that the misplacement is only used for that quadrant.  
      In some embodiments, determining the misplacement can include determining a position error signal for the burst pair.  
      In some embodiments, the position error signal can be determined by a servo controller.  
      In some embodiments, the information stored about the misplacement can include a digital number that indicates an amount the position error signal should be adjusted for that servo burst pair.  
      In some embodiments, a quadrant can include a plurality of additional servo burst pairs spaced about a track on the hard disk; and determining the misplacement of a servo burst pair includes examining the position of each of the plurality of additional servo burst pairs in the quadrant to determine an average burst pair position about the track.  
      In some embodiments, determining the misplacement of a servo burst pair further can include determining the misplacement of the burst pair relative to the average burst pair position about the track.  
      In some embodiments, a quadrant can include a plurality of additional positioning patterns spaced about a track on the rotating medium; and identifying the quadrant containing the respective positioning pattern can include examining the relative position of each of the plurality of positioning patterns.  
      In some embodiments, storing information about the misplacement and quadrant can include writing the information in the quadrant containing the servo burst pair.  
      In some embodiments, storing information further can include writing the information in the servo wedge containing the servo burst pair.  
      In some embodiments, storing information further can include writing the information after the servo burst pair in the servo wedge containing the servo burst pair.  
      In some embodiments, the method can further include storing information about a misplacement of at least one additional burst pair and the additional quadrant containing the additional burst pair.  
      In some embodiments, the additional quadrant can be adjacent to the quadrant containing the servo burst pair.  
      In some embodiments, the method can further include reading the stored information about the misplacement of the burst pair and the quadrant and using that information to position a head relative to the servo burst pair.  
      In some embodiments, the method can further include not applying the misplacement information if another servo burst pair from another quadrant is used for position information.  
      In some embodiments, a method of manufacturing a self-servowriting drive can include: providing means for determining, on each of a plurality of revolutions of a rotating medium, the misplacement of one of a plurality of positioning patterns for a track on the rotating medium; providing means for identifying the quadrant containing the respective positioning pattern for each revolution, the rotating medium having a plurality of quadrants extending radially across a surface of the rotating medium; providing means for using information about the misplacement and quadrant for each revolution to determine the synchronous runout of the track, the determination of synchronous runout accounting for any variation in the quadrant containing the respective positioning pattern for each revolution; and providing means for storing information about the synchronous runout to be used in any of a read operation and write operation that determines position using that positioning pattern, such that the misplacement information is only used for the respective quadrant.  
      The foregoing description of preferred embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations will be apparent to one of ordinary skill in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications that are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalence.