Abstract:
A process for continually compensating for the microjog error resulting from RRO. Typically, the 1F RRO is the most significant, but the method could be applied to the microjog error caused by RRO of other frequencies. The process continually determines an instantaneous microjog error based on the RRO and adjusts the read element target position throughout one revolution, such that the write element remains centered on its intended position.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to information storage devices and, more particularly, to information storage devices having read/write heads with spaced read and write elements. 
     BACKGROUND OF THE INVENTION 
     Most computers include a disk drive which is used for data storage. The disk drive includes a rotatable disk having a magnetic coating on at least one side thereof. A read/write head is disposed adjacent the surface, and an actuator can move the read/write head approximately radially with respect to the surface, so that the head can write data to the surface and/or read data from the surface. The surface on the disk is conceptually divided into a plurality of concentric data tracks, which can each store data. 
     Early disk drives included a read/write head having a single read/write element, which was used both for writing data and reading data. However, there has been a progressively increasing demand for disk drives with significantly higher storage densities, and one result is that new types of heads have come into common use, examples of which include the magneto-resistive (MR) head, and the giant magneto-resistive (GMR) head. These MR and GMR heads typically have one element for writing data and a separate element for reading data, and these read and write elements are physically spaced from each other. 
     As is known in the art, a head can be positioned with respect to a disk by using feedback control based on servo information read from the disk with a read element of the head. In a head with spaced read and write elements, the read element is used to position the head relative to the disk not only for reading, but also for writing. One aspect of this is that, as the head is moved relative to the disk, the orientation of the read and write elements varies with respect to the tracks on the disk, such that the write element is typically aligned with a track that is different from the track with which the read element is aligned. Consequently, in order to correctly position the write element over a selected track for the purpose of writing data to that track, the read element must be positioned at a location which is radially offset from the selected track. This radial offset is referred to as a “microjog”, and has a magnitude which varies as the head moves radially with respect to the disk. Techniques have been developed for calculating microjog values, and have been generally adequate for their intended purposes, but they have not been satisfactory in all respects. 
     As one aspect of this, there are existing disk drives in which the disk is rotatably supported in a removable cartridge, and in which the head is movably supported in a drive unit that can removably receive the cartridge. A given drive unit must be able to work with any of several similar and interchangeable cartridges, and any given cartridge must be capable of working in any of a number of compatible drive units. The removability of the cartridge introduces a number of real-world considerations into the system, and these considerations affect the accurate calculation of a microjog value. 
     For example, the cartridges have manufacturing tolerances which vary from cartridge. Thus, from cartridge to cartridge, there will be some variation relative to the cartridge housing of the exact position of the axis of rotation of the disk. As another example, two different cartridges may have slightly different mechanical seatings when they are inserted into the same drive unit. In fact, a given cartridge may experience different mechanical seatings on two successive insertions into the same drive unit. Real-world variations of this type cause small variations in the orientation of the read/write head with respect to the tracks on the disk, and thus affect accurate calculation of a microjog value. 
     One of the major components of head position error is called repeatable runout (RRO). RRO that occurs at the disk rotating frequency may be called 1F runout. There are several possible causes for 1F runout, such as an unbalanced spindle, or a non-ideal spindle bearing. 
     In order to realize higher data storage densities in systems of the type which utilize removable cartridges, it is desirable to be able to use read/write heads that facilitate high storage densities, especially read/write heads that have spaced read and write elements, such as MR and GMR heads. What is needed is a system that compensates for any changes in the microjog that may occur. 
     Further, if a removable cartridge is dropped, the disk may slip within the clamp, resulting in large RRO. As the head moves back and forth in order for the read element to follow the RRO, the write element, which is spaced some distance away from the read element, does not remain centered over the intended write position. If the track density is high enough, the microjog error caused by the RRO will increase, eventually resulting in degraded performance in reading the data. In addition, if the RRO changes once the disk has been written, a subsequent write may cause encroachment. What is needed is a system that can compensate for microjog error caused by the RRO. 
     SUMMARY OF THE INVENTION 
     A process for continually compensating for the microjog error resulting from RRO. Typically, the 1F RRO is the most significant, but the method could be applied to the microjog error caused by RRO of other frequencies. The process continually determines an instantaneous microjog error based on the RRO and adjusts the read element target position throughout one revolution, such that the write element remains centered on its intended position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagrammatic view of an apparatus which is an information storage system that embodies aspects of the present invention; 
         FIG. 2  is a fragmentary diagrammatic view which shows a portion of the system of  FIG. 1  in a substantially enlarged scale; 
         FIG. 3  is a fragmentary diagrammatic view similar to  FIG. 2 , but showing a different operational position; 
         FIG. 4  is a fragmentary diagrammatic view similar to  FIGS. 2 and 3 , but showing still another operational position; 
         FIG. 5  is a diagrammatic view showing a geometric relationship between selected elements of the system of  FIG. 1 ; 
         FIG. 6  is a diagrammatic view showing different geometric relationships involving other elements of the system of  FIG. 1 ; and 
         FIG. 7  is a flowchart illustrating a method of positioning the read/write head based on an instantaneous RRO-induced microjog error. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatic view of an apparatus which is an information storage system  10 , and which embodies aspects of the present invention. The system  10  includes a receiving unit or drive  12  which has a recess  14 , and includes a cartridge  16  which can be removably inserted into the recess  14 . 
     The cartridge  16  has a housing, and has within the housing a motor  21  with a rotatable shaft  22 . A disk  23  is clamped on the shaft  22  for rotation therewith. The side of the disk  23  which is visible in  FIG. 1  is coated with a magnetic material of a known type, and serves as an information storage medium. This disk surface is conceptually divided into a plurality of concentric data tracks. In the disclosed embodiment, there are about 50,000 data tracks, not all of which are available for use in storing user data. 
     The disk surface is also conceptually configured to have a plurality of circumferentially spaced sectors, two of which are shown diagrammatically at  26  and  27 . These sectors are sometimes referred to as servo wedges. The portions of the data tracks which fall within these sectors or servo wedges are not used to store data. Data is stored in the portions of the data tracks which are located between the servo wedges. The servo wedges are used to store servo information of a type which is known in the art. The servo information in the servo wedges conceptually defines a plurality of concentric servo tracks, which have a smaller width or pitch than the data tracks. In the disclosed embodiment, each servo track has a pitch or width that is approximately two-thirds of the pitch or width of a data track. Consequently, the disclosed disk  23  has about 73,000 servo tracks. The servo tracks effectively define the positions of the data tracks, in a manner known in the art. 
     Approximately 60 of the data tracks, which are the radially innermost tracks, are set aside as a first reserved area  36 . Approximately 60 more data tracks, which are the radially outermost tracks, are set aside as a second reserved area  37 . The reserved areas  36  and  37  are not available to store user data, but instead are used for a special purpose which is discussed later. User data is stored in the many data tracks that are disposed between the reserved areas  36  and  37  (except in the regions of the servo wedges). 
     The drive  12  includes an actuator  51  of a known type, such as a voice coil motor (VCM). The actuator  51  can effect limited pivotal movement of a pivot  52 . An actuator arm  53  has one end fixedly secured to the pivot  52 , and extends radially outwardly from the pivot  52 . The housing of the cartridge  16  has an opening in one side thereof. When the cartridge  16  is removably disposed within the drive  12 , the arm  53  extends through the opening in the housing, and into the interior of the cartridge  16 . At the outer end of the arm  53  is a suspension  56  of a known type, which supports a read/write head  57 . In the disclosed embodiment, the head  57  is a component of a known type, which is commonly referred to as a giant magneto-resistive (GMR) head. However, it could alternatively be some other type of head, such as a magneto-resistive (MR) head. 
     During normal operation, the head  57  is disposed adjacent the magnetic surface on the disk  23 , and pivotal movement of the arm  53  causes the head  57  to move approximately radially with respect to the disk  23 , within a range which includes the reserved areas  36  and  37 . When the disk  23  is rotating at a normal operational speed, the rotation of the disk induces the formation between the disk surface and the head  57  of an air cushion, which is commonly known as an air bearing. Consequently, the head  57  floats on the air bearing while reading and writing information to and from the disk, without direct physical contact with the disk. However, the invention is not limited to systems in which the head is spaced from the disk by an air bearing, and can be used in systems where the head physically contacts the disk. 
     The drive  12  includes a control circuit  71 , which is operationally coupled to the motor  21  in the cartridge  16 , as shown diagrammatically at  72 . The control circuit  71  selectively supplies power to the motor  21  and, when the motor  21  is receiving power, the motor  21  effects rotation of the disk  23 . The control circuit  71  also provides control signals at  73  to the actuator  51 , in order to control the pivotal position of the arm  53 . At  74 , the control circuit  71  receives an output signal from the head  57 , which is commonly known as a channel signal. When the disk  23  is rotating, segments of servo information and data will alternately move past the head  57 , and the channel signal at  74  will thus include alternating segments or bursts of servo information and data. 
     The control circuit  71  includes a channel circuit of a known type, which processes the channel signal received at  74 . The channel circuit includes an automatic gain control (AGC) circuit, which is shown at  77 . The AGC circuit  77  effect variation, in a known manner, of a gain factor that influences the amplitude of the channel signal  74 . In particular, the AGC circuit uses a higher gain factor when the amplitude of the channel signal  74  is low, and uses a lower gain factor when the amplitude of the channel signal  74  is high. Consequently, the amplitude of the channel signal has less variation at the output of the AGC circuit  77  than at the input thereof. 
     The control circuit  71  also includes a processor  81  of a known type, as well as a read only memory (ROM)  82  and a random access memory (RAM)  83 . The ROM  82  stores a program which is executed by the processor  81 , and also stores data that does not change. The processor  81  uses the RAM  83  to store data or other information that changes dynamically during program execution. 
     The control circuit  71  of the drive  12  is coupled through a host interface  86  to a not-illustrated host computer. The host computer can send user data to the drive  12 , which the drive  12  then stores on the disk  23  of the cartridge  16 . The host computer can also request that the drive  12  read specified user data back from the disk  23 , and the drive  12  then reads the specified user data and sends it to the host computer. In the disclosed embodiment, the host interface  86  conforms to an industry standard protocol which is commonly known as the Universal Serial Bus (USB) protocol, but could alternatively conform to any other suitable protocol, including but not limited to the IEEE 1394 protocol. 
       FIG. 2  is a fragmentary diagrammatic view which shows, in a substantially enlarged scale, a portion of the structure of  FIG. 1 , including the head  57 , the suspension  56 , and portions of the arm  53  and disk  23 . It should be understood that the depiction of all of these components is highly diagrammatic. Reference numerals  101 – 108  identify eight adjacent data tracks on the disk  23 , which are close to but not within the reserved area  37  ( FIG. 1 ). The tracks  101 – 108  are circular and concentric but, due to the degree of enlargement involved in  FIG. 2 , the curvature is sufficiently gradual that these tracks appear to be straight lines. The read/write head  57  has a read element  112  and a write element  113 , which are shown diagrammatically, and which are spaced from each other. The write element  113  has a length which is somewhat longer than the length of the read element  112 . 
     As the disk  23  rotates, segments of servo information and segments of data on the disk alternately move past the read element  112 . The read element  112  produces the channel signal, which is supplied at  74  to the control circuit  71  ( FIG. 1 ), and which includes alternating bursts of data and servo information. By analyzing the successive bursts of servo information received from the read element  112 , the control circuit  71  can make an accurate determination of the current radial position of the read element  112 . In particular, the control circuit  71  can use the servo information to accurately determine the radial position of the read element  112  with respect to the not-illustrated servo tracks. Since the positions of the data tracks are defined by the servo tracks, knowledge of the radial position of the read element  112  with respect to the servo tracks also constitutes knowledge of the radial position of the read element  112  with respect to the data tracks. Thus, in  FIG. 2 , the control circuit  71  knows from servo information read by the read element  112  that the read element  112  is currently disposed at a location straddling data tracks  104  and  105 , with slightly more of the read element over track  105  than over track  104 . Using this servo information read by the read element  112 , the control circuit can affect feedback control to maintain the read element in a given radial position, or to radially reposition the read element  112 . 
     Positioning the head  57  with respect to the disk  23  for the purpose of reading data is relatively straightforward, because the read element  112  is used both to read the data of interest and also to read the servo information which is used to position the read element  112 . On the other hand, the write element  113  is used to write data to the disk  23 , but does not read any information from the disk  23 . Consequently, in order to write data to the disk  23 , the write element  113  must be positioned indirectly, through the approach of positioning the read element  112  using the servo information which it is reading from the disk, and knowing where the write element  113  is in relation to the read element  112 . A degree of complexity is introduced by the fact that the write element  113  is typically not aligned with the same data track as the read element  112 . In fact, the radial position of the write element  113  in relation to the read element  112  is not a constant, but varies as the head  57  is moved radially of the disk. 
     In  FIG. 2 , for example, due to the angle of the actuator arm  53  with respect to the disk  23 , the write element  113  is offset in a radial direction by approximately 2.33 data tracks from the read element  112 . As explained above, there are 1.5 of the not-illustrated servo tracks for each illustrated data track, and so the offset can also be expressed as 2.33 data tracks×1.5=3.5 servo tracks. This radial offset, which is also referred to as a “microjog”, is indicated diagrammatically at  116  by an arrow. Consequently, if the control circuit  71  wants to use the write element  113  to write data to the data track  107 , the control circuit must use the servo information received through the read element  112  to accurately position the read element  112  so that it straddles data tracks  104  and  105  in the manner shown in  FIG. 2 , thereby centering the write element  113  over the data track  107  so that the write element can be used to write data to the data track  107 . 
       FIG. 3  is a fragmentary diagrammatic view similar to  FIG. 2 , but showing a different operational position. In particular, the actuator arm  53  has been rotated counterclockwise from the position shown in  FIG. 2 , so that in  FIG. 3  the head  57  is near but not within the reserved area  36 .  FIG. 3  shows eight data tracks  121 – 128 . It will be noted that the write element  113  is centered over the data track  122 , and the read element  112  straddles the data tracks  124  and  125 , with slightly more of the read element over the track  124  than the track  125 . Thus, when the control circuit  71  wants to use the write element  113  to write data to the track  122 , it uses servo information read by the read element  112  to accurately position the read element  112  so that the read element straddles the tracks  124 – 125  in the manner shown in  FIG. 3 . 
     In this situation, the read element  112  is offset by approximately 2.33 data tracks (3.5 servo tracks) from the write element  113 , which is the microjog indicated by the arrow  131  in  FIG. 3 . However, it will be noted that the radial direction of the arrow  131  in  FIG. 3  is opposite to the radial direction of the arrow  116  in  FIG. 2 . Stated differently, in order to position the write element  113  over the track  107  in  FIG. 2 , the control circuit  71  must position the read element  112  so that it is disposed 2.33 data tracks (3.5 servo tracks) in a direction radially inwardly from the track  107 . In contrast, in order to position the write element  113  over the track  122  in  FIG. 3 , the control circuit  71  must position the read element  112  so that it is disposed 2.33 data tracks (3.5 servo tracks) in a direction radially outwardly from the track  122 . 
       FIG. 4  is a fragmentary diagrammatic view similar to  FIGS. 2 and 3 , but showing yet another operational position. In  FIG. 4 , the actuator arm  53  is disposed approximately halfway between the positions shown in  FIGS. 2 and 3 .  FIG. 4  shows eight data tracks  141 – 148 . The read element  112  and the write element  113  are both relatively accurately centered over the same data track  144 . Thus, in  FIG. 4 , the microjog value is zero, because the read element  112  does not need to be positioned with an offset from the track  144  in order to center the write element  113  over the track  144 . Stated differently, if the read element  112  is radially centered over the track  144 , the write element will also be radially centered over the track  144 . 
     With reference to  FIGS. 2–4 , it will be noted that, as the actuator arm  53  is pivoted and moves the head  57  radially across the disk, the appropriate microjog value varies progressively from a positive value through zero to a negative value. This is due in part to the spacing between the read element  112  and the write element  113 , and is also due in part to the fact that there is variation in the angle of the read and write elements with respect to the tracks on the disk as the head is moved radially with respect to the disk. Consequently, when the write element  113  is to be used to write data to any given data track, an appropriate microjog value must be determined for that data track in order to know where to position the read element  112  while that write operation is carried out. 
     As discussed above in association with  FIG. 1 , the cartridge  16  can be removed from the drive  12 . In fact, the drive  12  is designed with the intent that any one of a number of similar cartridges can be interchangeably inserted into the drive  12 , and that the drive  12  will work reliably and accurately with any of the cartridges. The removability of the cartridge  16  introduces additional considerations into the determination of an appropriate microjog value, because there will be factors that vary from cartridge to cartridge, and factors that vary from insertion to insertion. For example, there will be mechanical tolerances involved in how different cartridges seat within the recess  14  within the drive  12 . In fact, if a given cartridge is disposed in the drive  12 , and is then removed and reinserted, the mechanical seating may change somewhat. If that cartridge is then removed and replaced with a different cartridge, the replacement cartridge may seat differently than the original cartridge. Consequently, the exact position of the disk with respect to the head may vary from one cartridge insertion to another, for either the same or different cartridges. 
     Further, internal variations can exist from cartridge to cartridge. For example, due to mechanical tolerances, the physical location of the motor shaft  22  with respect to the housing of its cartridge may be slightly different in one cartridge as compared to another cartridge. These tolerance and/or seating variations can cause variation in the distance between the motor spindle  22  and the actuator pivot  52 , which in turn can affect the appropriate microjog value. 
     A further consideration is that the servo information on the disk in one cartridge may have been written to the disk at the factory by one servo-writer machine, while the servo information on the disk in a different cartridge may have been written by a different servo-writer machine. As a result, each track on one disk may not be in precisely the same radial location as the equivalent track on another disk. 
     Yet another consideration is that the foregoing discussion has focused on how a particular drive must be able to accurately and reliably work with any of a number of different cartridges, but the converse is also true. In particular, a given cartridge must be able to reliably and accurately work in a number of different drives. 
     Still another consideration is that the spacing between and orientation of the read and write elements  112  and  113  may vary from head to head (and thus from drive to drive), for example due to process variations involved in manufacturing the head. In order to be able to use exactly the same firmware program for the processor  81  in each drive  12 , without customization for each drive, the firmware must be capable of accommodating real-world variations such as variations from one read/write head to another. 
     Consequently, in the context of a removable cartridge, there are a variety of factors, including those discussed above, which can affect proper calculation of an accurate microjog value. One feature of the present invention relates to techniques that allow accurate determination of a microjog value, despite factors of this type. These techniques for accurately calculating microjog are explained in detail below. First, however, an overview is provided. 
     In particular, with reference to  FIG. 1 , the control circuit  71  of the drive  12  responds to insertion of a cartridge  16  by erasing data in at least part of the reserved area  36 , and then positioning the write element  113  approximately over a central portion of the reserved area  36 , using servo information read by the read element  112 . The control circuit  71  then uses the write element  113  to write some predetermined data in the reserved area  36 . The control circuit  71  then moves the head  57  radially while searching for this data with the read element  112 , until the control circuit  71  determines a radial position in which the read element  112  would be radially centered over this data. Based on servo information read by the read element  112  while the data is being written, and also servo information read by the read element  112  while the same data is later being read, the control circuit  71  knows the exact radial position of the read element  112  when the data was being written, and the exact radial position of the read element  112  when the data was being read. The control circuit  71  can then take the difference between these two positions, in order to accurately determine an actual microjog value for one specific data track within the reserved area  36 . 
     The control circuit  71  then carries out a similar sequence of operations for the other reserved area  37 . This results in a very accurate determination of an actual microjog value for one specific data track within the reserved area  37 . The information obtained in this manner, which includes the two actual microjog values, serves as compensation information that is specific to the particular cartridge  16  that has been inserted into the drive  12 , and the particular current seating of that cartridge. 
     Thereafter, when the control circuit  71  needs to write data to a selected data track on the disk  23 , it carries out a two-step procedure. First, it uses a predetermined translation technique, which is independent of the particular cartridge and its present seating, to determine a nominal or ideal microjog value for the selected track. Second, the control circuit  71  uses the compensation information to adjust the nominal microjog value, in order to obtain an actual microjog value which accurately takes into account the particular cartridge and its current seating, thereby permitting the write element  113  of the head  57  to be accurately positioned over the selected track. This microjog value is the calibrated microjog value. The specific manner in which this is all carried out will now be described in greater detail. 
       FIG. 5  is a diagrammatic view of selected components from the system of  FIG. 1 , including the motor spindle  22 , the actuator pivot  52 , and the head  57 . Line  201  extends radially from the motor pivot  22  to the head  57 . Line  202  extends radially between the motor spindle  22  and the actuator pivot  52 , and has a length M. Line  203 , which corresponds conceptually to the actuator arm  53 , extends radially from the actuator pivot  52  to the write element  113  on the head  57 , and has a length A. The angle between the lines  202  and  203  is identified by θ. The angle formed by the line  201  with the read and write elements is identified by φ, and can be referred to as head skew angle. It will be noted that the angles θ and φ are not constant, but vary as the arm  53  is pivoted by the actuator  51  to move the head  57  toward or away from the motor spindle  22  in  FIG. 5 . 
       FIG. 6  is a diagrammatic view showing the read and write elements  112  and  113  of the head  57 . As mentioned above, line  203  extends radially from the actuator pivot  52  to the write element  113 , and in particular to the center of the write element  113 . The distance between the read element  112  and the write element  113 , in a direction parallel to the line  203 , is a distance S which is identified in  FIG. 6  by a double-headed arrow. 
     In  FIG. 6 , the center of the read element  112  is depicted as being laterally offset from the line  203  by a distance δ, which is identified by reference numeral  212 . The head  57  in the disclosed embodiment is designed so that, in theory, the read element  112  should have its center disposed on the line  203 , such that δ=0. However, the offset δ is depicted in  FIG. 6  because, due to practical considerations such as manufacturing process variations, the read element  112  may not actually be centered accurately on the line  203 . In  FIG. 6 , the total microjog amount is indicated at MJ, and is made up of two portions, which are respectively MJ 1  and MJ 2 . Using standard trigonometric principles, MJ 1  and MJ 2  can be expressed as:
 
 MJ 1 =S ·sin(φ)
 
 MJ 2=δ·cos(φ)
 
     Consequently, the microjog amount MJ can be expressed as: 
     
       
         
           
               
             
               
                 
                   
                     MJ 
                     = 
                       
                     ⁢ 
                     
                       MJ1 
                       + 
                       MJ2 
                     
                   
                 
               
               
                 
                   
                     = 
                       
                     ⁢ 
                     
                       
                         S 
                         · 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             ϕ 
                             ) 
                           
                         
                       
                       + 
                       
                         δ 
                         · 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             ϕ 
                             ) 
                           
                         
                       
                     
                   
                 
               
             
           
         
       
     
     The microjog amount MJ can be normalized with absolute dimensions to the track pitch TP of the servo tracks, thereby yielding a microjog distance MJD in servo tracks, as follows: 
     
       
         
           
             
               
                 
                     
                   
                     
                       
                         
                           MJD 
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 S 
                                 · 
                                 
                                   sin 
                                   ⁡ 
                                   
                                     ( 
                                     ϕ 
                                     ) 
                                   
                                 
                               
                               + 
                               
                                 δ 
                                 · 
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     ϕ 
                                     ) 
                                   
                                 
                               
                             
                             
                               
                                 cos 
                                 ⁡ 
                                 
                                   ( 
                                   ϕ 
                                   ) 
                                 
                               
                               · 
                               TP 
                             
                           
                         
                       
                     
                     
                       
                         
                           = 
                             
                           ⁢ 
                           
                             
                               
                                 S 
                                 · 
                                 
                                   tan 
                                   ⁡ 
                                   
                                     ( 
                                     ϕ 
                                     ) 
                                   
                                 
                               
                               + 
                               δ 
                             
                             TP 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     With reference to  FIG. 5 , it can be shown with trigonometry that: 
     
       
         
           
             
               
                 
                   
                     tan 
                     ⁡ 
                     
                       ( 
                       ϕ 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         A 
                         M 
                       
                       - 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           θ 
                           ) 
                         
                       
                     
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         θ 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Inserting Equation (2) into Equation (1) yields: 
     
       
         
           
             
               
                 
                   
                     
                       MJD 
                       nom 
                     
                     ⁡ 
                     
                       ( 
                       track 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         S 
                         · 
                         
                           ( 
                           
                             
                               
                                 A 
                                 M 
                               
                               - 
                               
                                 cos 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     θ 
                                     ⁡ 
                                     
                                       ( 
                                       track 
                                       ) 
                                     
                                   
                                   ) 
                                 
                               
                             
                             
                               sin 
                               ( 
                               
                                 θ 
                                 ⁡ 
                                 
                                   ( 
                                   track 
                                   ) 
                                 
                               
                             
                           
                           ) 
                         
                       
                       + 
                       δ 
                     
                     TP 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Given a particular value of the angle θ, which corresponds to a particular data track and an associated servo track, Equation (3) can be used to determine the nominal or ideal microjog distance in servo tracks (MJD nom ), which is the radial offset in servo tracks that the read element  112  must have from the selected data track in order to center the write element  113  over the selected data track. Equation (3) basically represents circumstances in an ideal system that is not subject to various real-world factors of the type discussed above, such as those relating to removability. The exception is the presence in Equation (3) of δ, which in an ideal system would be zero. 
     A calibrated microjog distance can be determined from the nominal microjog obtained in Equation (3). The calibrated microjog is determined based on the function calc_calibrated_microjog (head, track) which calibrates the nominal microjog for the position and geometry of the head and drive. A technique for obtaining this value is disclosed in U.S. patent application Ser. No. 10/612,810, filed Jul. 2, 2003, the contents of which are hereby incorporated by reference. 
       FIG. 7  is a flowchart illustrating a process  300  of positioning the read/write head based on an instantaneous RRO-induced microjog error. The process begins in START block  305 . Proceeding to block  310 , the process  300  calculates the microjog distance, assuming no RRO, for the intended head and write target position as described above. The microjog distance is an offset, which is summed with the write target position to determine a read target position. The read target position is the initial position of the read element, such that on average, the write element is over the write target position. 
     Proceeding to block  315 , the process  300  obtains a measurement of the peak RRO in tracks, and its phase with respect to an index that occurs once per revolution. The RRO measurement may be obtained by analyzing sector to sector timing measurements taken during a revolution. Or the measurement could be calculated from coefficients in the RRO cancellation algorithm and the loop gain at the desired frequency. This RRO measurement may be obtained once during spin-up calibrations of the disk drive, or it may be obtained each time the head is positioned for a write. For simplicity, it is assumed that the magnitude and phase of the RRO remain constant for all tracks on the disk. In one embodiment, it may be determined that the RRO measurement is too high to safely perform data writes. In this circumstance, the write functionality may be disabled, allowing a user to access data on the disk but not write any data to the disk. 
     Proceeding to block  320 , the process  300  calculates the microjog distance assuming the head has moved the distance of the known RRO in tracks. The difference between the microjog distance with and without RRO represents the peak microjog error caused by the RRO:
 
calc_calibrated_microjog(head,track+RRO)−calc_calibrated_microjog(head,track)
 
     Note that if the RRO as measured in tracks is 0, then the peak microjog error caused by the RRO is 0. 
     Proceeding to block  325 , the process  300  determines the instantaneous microjog error due to the RRO as the disk rotates as:
 
sin(w)*(calc_calibrated_microjog(head,track+RRO)−calc_calibrated_microjog(head,track))
 
     where w refers to the angle through which the disk rotates, synchronized with the phase of the RRO previously measured. 
     Proceeding to block  330 , the process  300  feeds the instantaneous estimate of the microjog error into the track-follow control loop, which is trying to keep the read element over the read target position defined previously. Now, the read element is no longer centered on the read target position, but it moves back and forth with the estimate of the RRO-induced microjog error. The result is that the write element remains centered on the write target position. The process  300  then terminates in END block  335 . 
     The process  300  described up to this point applies mainly to a disk drive with a removable disk. It is important that any drive be able to write and re-write any data track on any disk. So keeping the write element centered on the write target position is critical. But in a drive with fixed media, it may be desirable to keep the read element centered on the read target position during the write. As was described previously, the write element will wander back and forth about some average position if RRO is present. When the read element is eventually used to read back the data, there may be some signal loss because the data is not always centered throughout the revolution. In a modification to the method described previously, the estimate of the RRO-induced microjog error could be introduced into the track-follow loop at the time the data is read. Thus, the read element would move in a position very close to where the write element actually wrote the data, improving the signal loss problem. However, if there is a chance that the disk could ever slip and significantly change the RRO, then it would be wise to keep the data centered during the write. 
     Although one embodiment has been illustrated and described in detail, it will be understood that various substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the following claims.