Abstract:
The present invention is directed to a method and apparatus for reducing velocity errors when writing spiral servo information onto a disk surface of a disk drive. In one embodiment, a servo track writer is provided for moving a write head at a controlled velocity. The write head is used to write spiral servo information onto a disk surface. The write head is moved, using the servo track writer, at an actual velocity trajectory to simulate writing one spiral of spiral servo information. Differences between the actual velocity trajectory and a desired velocity trajectory are measured on a control sample by control sample basis over a window of control samples. The differences are integrated over the window. A gain, associated with the servo track writer controlling the velocity of the write head, is adjusted using the integrated differences.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed from U.S. Provisional Patent Application Ser. No. 60/475,048 filed Jun. 2, 2003, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to data storage devices, such as disk drives. More particularly, the present invention relates to a method and apparatus for reducing velocity errors (e.g., velocity overshoot and velocity undershoot) when writing spiral servo information onto a disk surface of a disk drive. 
     BACKGROUND OF THE INVENTION 
     Computer disk drives store information on magnetic disks. Typically, the information is stored on each disk in concentric tracks that are divided into sectors. Information is written to and read from a disk by a transducer that is mounted on an actuator arm capable of moving the transducer radially over the disk. Accordingly, the movement of the actuator arm allows the transducer to access different tracks. The disk is rotated by a spindle motor at high speed which allows the transducer to access different sectors on the disk. 
     A conventional disk drive, generally designated  10 , is illustrated in  FIG. 1 . The disk drive comprises a disk  12  that is rotated by a spin motor  14 . The spin motor  14  is mounted to a base plate  16 . An actuator arm assembly  18  is also mounted to the base plate  16 . 
     The actuator arm assembly  18  includes a transducer  20  mounted to a flexure arm  22  which is attached to an actuator arm  24  that can rotate about a bearing assembly  26 . The actuator arm assembly  18  also contains a voice coil motor  28  which moves the transducer  20  relative to the disk  12 . The spin motor  14 , voice coil motor  28  and transducer  20  are coupled to a number of electronic circuits  30  mounted to a printed circuit board  32 . The electronic circuits  30  typically include a read channel chip, a microprocessor-based controller and a random access memory (RAM) device. 
     The disk drive  10  typically includes a plurality of disks  12  and, therefore, a plurality of corresponding actuator arm assemblies  18 . However, it is also possible for the disk drive  10  to include a single disk  12  as shown in  FIG. 1 . 
       FIG. 2  is a functional block diagram which illustrates a conventional disk drive  10  that is coupled to a host computer  33  via an input/output port  34 . The disk drive  10  is used by the host computer  33  as a data storage device. The host  33  delivers data access requests to the disk drive  10  via port  34 . In addition, port  34  is used to transfer customer data between the disk drive  10  and the host  33  during read and write operations. 
     In addition to the components of the disk drive  10  shown and labeled in  FIG. 1 ,  FIG. 2  illustrates (in block diagram form) the disk drive&#39;s controller  36 , read/write channel  38  and interface  40 . Conventionally, data is stored on the disk  12  in substantially concentric data storage tracks on its surface. In a magnetic disk drive  10 , for example, data is stored in the form of magnetic polarity transitions within each track. Data is “read” from the disk  12  by positioning the transducer  20  above a desired track of the disk  12  and sensing the magnetic polarity transitions stored within the track, as the track moves below the transducer  20 . Similarly, data is “written” to the disk  12  by positioning the transducer  20  above a desired track and delivering a write current representative of the desired data to the transducer  20  at an appropriate time. 
     The actuator arm assembly  18  is a semi-rigid member that acts as a support structure for the transducer  20 , holding it above the surface of the disk  12 . The actuator arm assembly  18  is coupled at one end to the transducer  20  and at another end to the VCM  28 . The VCM  28  is operative for imparting controlled motion to the actuator arm  18  to appropriately position the transducer  20  with respect to the disk  12 . The VCM  28  operates in response to a control signal i control  generated by the controller  36 . The controller  36  generates the control signal i control  for example, in response to an access command received from the host computer  33  via the interface  40  or in response to servo information read from the disk surface  12 . 
     The read/write channel  38  is operative for appropriately processing the data being read from/written to the disk  12 . For example, during a read operation, the read/write channel  38  converts an analog read signal generated by the transducer  20  into a digital data signal that can be recognized by the controller  36 . The channel  38  is also generally capable of recovering timing information from the analog read signal. During a write operation, the read/write channel  38  converts customer data received from the host  33  into a write current signal that is delivered to the transducer  20  to “write” the customer data to an appropriate portion of the disk  12 . As will be discussed in greater detail, the read/write channel  38  is also operative for continually processing data read from servo information stored on the disk  12  and delivering the processed data to the controller  36  for use in, for example, transducer positioning. 
       FIG. 3  is a top view of a magnetic storage disk  12  illustrating a typical organization of data on the surface of the disk  12 . As shown, the disk  12  includes a plurality of concentric data storage tracks  42 , which are used for storing data on the disk  12 . The data storage tracks  42  are illustrated as center lines on the surface of the disk  12 ; however, it should be understood that the actual tracks will each occupy a finite width about a corresponding centerline. The data storage disk  12  also includes servo information in the form of a plurality of radially-aligned servo spokes  44  (or wedges) that each cross the tracks  42  on the disk  12 . The servo information in the servo spokes  44  is read by the transducer  20  during disk drive operation for use in positioning the transducer  20  above a desired track  42  of the disk  12 . Among other things, the servo information includes a plurality of servo bursts (e.g., A, B, C and D bursts or the like) that are used to generate a Position Error Signal (PES) to position the write head relative to a track&#39;s centerline during a track following operation. The portions of the track between servo spokes  44  are used to store customer data received from, for example, the host computer  33  and are referred to as customer data regions  46 . 
     It should be understood that, for ease of illustration, only a small number of tracks  42  and servo spokes  44  have been shown on the surface of the disk  12  of  FIG. 3 . That is, conventional disk drives include one or more disk surfaces having a considerably larger number of tracks and servo spokes. 
     During the disk drive manufacturing process, a special piece of equipment known as a servo track writer (STW) is used to write the radially-aligned servo information which forms servo spokes  44 . A STW is a very precise piece of equipment that is capable of positioning the disk drive&#39;s write head at radial positions over the disk surface, so that servo information is written on the disk surface using the disk drive&#39;s write head with a high degree of positional accuracy. 
     In general, a STW is a very expensive piece of capital equipment. Thus, it is desirable that a STW be used as efficiently as possible during manufacturing operations. Even a small reduction in the amount of data needed to be written by the STW per disk surface can result in a significant cost and time savings. 
     A STW is used to write servo information, by controlling the position of the disk drive&#39;s write head, on a disk surface in a circumferential fashion at each radius at which the disk drive&#39;s write head is positioned. During drive operation, the servo information is used to position the transducer of the disk drive over the appropriate data track and data sector of the disk. Accordingly, as the number of tracks per inch (TPI) increases, the amount of time necessary to write servo information increases. That is, the number of circumferential passes that a STW must make over a disk surface increases as TPI increases. Thus, unless more STWs are supplied, manufacturing times will continually increase as the TPI increases. 
     Instead of using a STW to write servo information in a circumferential fashion at each radius, the assignee of the present invention presently uses a STW to write servo information in a spiral fashion (in at least some of its disk drives). Specifically, the STW moves the write head in a controlled manner (e.g., at a constant velocity or along a velocity profile) from a location near the outer diameter of the disk to a location near the inner diameter of the disk (or visa-versa) as the disk spins. 
       FIG. 4  is a diagrammatic representation of a disk surface  210  having a first spiral of servo information  215  written thereon. The dashed line, identified by reference numeral  220 , represents a track. The first spiral of servo information  215  may make multiple revolutions around the disk surface  210  (roughly two revolutions as shown in  FIG. 4 ), but only crosses track  220  once. 
       FIG. 5  is a diagrammatic representation of a disk surface  210  having a first spiral of servo information  215  and a second spiral of servo information  225  written thereon. As shown in  FIG. 5 , the first and second spirals  215 ,  225  are interlaced with one another and are written approximately 180 degrees apart. Again, each spiral crosses track  220  only once. 
     Additional spirals of servo information may be written on the disk surface  210  depending upon the servo sample rate (that is, the number of servo samples required for each track  220  to keep the disk drive&#39;s transducer sufficiently on-track). For example, if a servo sample rate of 120 equally-spaced servo sectors per track was required, 120 equally-spaced spirals would be written on the disk surface  110 . Accordingly, by writing servo information in a spiral fashion, the time necessary to write servo information on disk surface  110  using the STW is a function of the servo sample rate (i.e., the number of spirals of servo information to be written) rather than the number of tracks. 
     Referring again to  FIGS. 4 and 5 , the spirals of servo information are written by moving the disk drive&#39;s write head using the STW in a generally radial direction (more accurately, in a radial direction along an arc due to the position of the bearing assembly), while both the disk is spinning and the write head is enabled. The direction of disk rotation is indicated by an arrow as shown in each of  FIGS. 4 and 5 . 
     The disk drive&#39;s write head is enabled for nearly its entire stroke (i.e., from a position near the OD to a position near the ID or visa-versa) while under the control of the STW. As a result, a continuous spiral of servo information is written. 
     Each of the spirals of servo information includes sync marks written at fixed time intervals by the disk drive&#39;s write head. As mentioned above, the STW is used to move the disk drive&#39;s write head at some fixed velocity (or velocity profile) in a generally radial direction across the disk surface. If the time interval between sync marks is known and the velocity of the disk drive&#39;s write head is known, the distance between sync marks along a spiral can be determined. Specifically, the following formula may be applied: Distance=(STW Velocity)(Time), where Distance represents the radial distance between sync marks, Velocity represents the radial velocity of the disk drive&#39;s write head (under control of the STW) and Time represents the interval between sync marks. 
     For example, the interval between sync marks may be set at 1 microsecond, while the write head may be controlled to move at a radial velocity of 10 inches per second along its stroke. Thus, the radial distance between sync marks can be calculated to be 1 microinch along each spiral. 
     Each sync mark along a given spiral corresponds to a unique radius. Accordingly, the sync marks may be used to accurately position a transducer of a disk drive over the disk surface. 
     While it might be desirable to write spiral servo information across the entire disk surface (as shown in  FIGS. 4 and 5 ), in practice, this is not the case. Instead, the servo track writer brings the write head from zero velocity to a constant velocity over a relatively short period of time. Spiral servo information is not written until the write head is brought to a constant velocity. Accordingly, there is some portion of the disk surface onto which spiral servo information cannot be written because the write head has not been brought to a constant velocity. 
     It is desirable for the servo track writer to bring the disk drive&#39;s write head to a constant velocity as soon as possible, so that more of the disk surface may be used to write spiral servo information (and, hence, so that more of the disk surface may be used to write final servo information, if final servo patterns are self-servo written using spiral servo information). Using a servo track writer in this manner presents new problems, as prior servo track writers only were required to move the disk drive&#39;s write head in small radial steps across the disk surface. 
     One problem relates to velocity overshoots and undershoots. Specifically, in bringing the write head from zero velocity to a constant velocity, the servo track writer&#39;s pushpin is designed to follow a velocity trajectory. (As is well-understood by those skilled in the art, the servo track writer&#39;s pushpin provides a mechanical linkage between the servo track writer and the disk drive.) However, because of gain variation and other errors, the velocity of the servo track writer&#39;s pushpin deviates from the desired velocity trajectory at a transition point between an initial acceleration region and a zero acceleration region, resulting in either velocity overshoots or velocity undershoots. These velocity errors are written into the spiral servo information (e.g., sync marks are not placed at their proper locations). When the spiral servo information is used for self-servo writing, the velocity errors may be propagated into the final servo patterns. 
     Accordingly, it would be desirable to develop a technique for reducing velocity overshoots and undershoots when transitioning from the initial acceleration region to the zero acceleration region. 
     SUMMARY OF THE INVENTION 
     The present invention is designed to meet some or all of the aforementioned, and other, needs. 
     The present invention is directed to a method and apparatus for reducing velocity errors when writing spiral servo information onto a disk surface of a disk drive. 
     In one embodiment, a servo track writer is provided for moving a write head at a controlled velocity. The write head is used to write spiral servo information onto a disk surface. The write head is moved, using said servo track writer, at an actual velocity trajectory to simulate writing a spiral of spiral servo information. Differences between the actual velocity trajectory and a desired velocity trajectory are measured on a control sample by control sample basis over a window of control samples. The differences are integrated over the window. A gain, associated with the servo track writer controlling the velocity of the write head, is adjusted using the integrated differences. After the gain is adjusted a plurality of times by repeatedly simulating spirals of spiral servo information, the write head writes spiral servo information onto the disk surface, while said servo track writer controls the velocity of the write head. 
     Other embodiments, objects, features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation illustrating a conventional disk drive with its top cover removed; 
         FIG. 2  is a functional block diagram which illustrates a conventional disk drive that is coupled to a host computer via an input/output port; 
         FIG. 3  is a diagrammatic representation of a top view of a magnetic storage disk illustrating a typical organization of data on a disk surface; 
         FIG. 4  is a diagrammatic representation of a disk surface having a spiral of servo information written thereon, along with a circular data storage track; 
         FIG. 5  is a diagrammatic representation of a disk surface having two spirals of servo information written thereon, along with a circular data storage track; 
         FIG. 6  is used to describe one embodiment of the present invention, and includes a diagrammatic representation of a desired velocity trajectory, a velocity trajectory with overshoot and a velocity trajectory with undershoot; and, 
         FIG. 7  is a flowchart used to describe one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiments in many different forms, there are shown in the drawings and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
     Prior to writing spiral servo information onto the disk surface, the write head may be positioned with its actuator arm against a crash stop. That is, the write head will not be over the disk surface or will be in a position very close to the inner diameter or outer diameter of the disk surface. 
     In order to quickly bring the write head to a constant velocity over the disk surface, a gain associated with the servo track writer is set so that the servo track writer&#39;s pushpin follows a desired velocity trajectory. A diagrammatic representation of a desired velocity trajectory  610  is shown in  FIG. 6 . It should be understood that  FIG. 6  is provided for illustrative purposes only and does not represent actual data. 
     As shown in  FIG. 6 , the desired velocity profile includes an initial acceleration region (i.e., where the desired velocity values are changing) and a zero acceleration region (i.e., where the desired velocity is constant value, shown as V). The zero acceleration region may also be referred to as the constant velocity region, as shown in  FIG. 6 . A transition point  620  is identified along the desired velocity trajectory. 
     As mentioned above, because of gain variation and other errors, the velocity of the servo track writer&#39;s pushpin deviates from the desired velocity trajectory when transitioning from the initial acceleration region to the zero acceleration region. A diagrammatic representation of a velocity trajectory with overshoot is identified by reference numeral  630 , while a diagrammatic representation of a velocity trajectory with undershoot is identified by reference numeral  640 . Again, the trajectories  630 ,  640  are provided for illustrative purposes only and do not represent actual data. 
     As can be seen in  FIG. 6 , if the servo track writer&#39;s pushpin actually followed the velocity profile with overshoot, the velocity of the servo track writer&#39;s pushpin would be greater than constant velocity V over some period in the constant velocity region. Likewise, if the servo track writer&#39;s pushpin actually followed the velocity profile with undershoot, the velocity of the servo track writer&#39;s pushpin would be less than constant velocity V over some period in the constant velocity region. 
     The disk drive&#39;s write head is designed to write spiral servo information onto the disk surface at time T. That is, it is assumed that the write head, under control of the servo track writer&#39;s pushpin, is moving at a constant velocity at time T, because the servo track writer&#39;s pushpin is following the desired velocity profile. When the servo track writer&#39;s pushpin experiences velocity errors and does not follow the desired velocity profile, these velocity errors are written into the spiral servo information. 
     Before writing spiral servo information, warm-up spirals are performed to account for thermal changes that may occur during the process of writing spiral servo information onto the disk surface. That is, the servo track writer&#39;s pushpin follows (or attempts to follow) the desired velocity profile and moves the disk drive&#39;s write head. However, the disk drive&#39;s write head does not write any spiral servo information onto the disk surface. A predetermined number of warm-up spirals (e.g., 20 warm-up spirals) may be performed. An example thermal warm-up regimen is described in U.S. patent application Ser. No. 10/788,242 entitled “Methods And Apparatuses For Writing Spiral Servo Patterns Onto A Disk Surface” filed Feb. 26, 2004, which is incorporated herein by reference. 
       FIG. 7  is used to describe one embodiment of the present invention. First, an initialization is performed, where variable N is set to 1 (step  705 ). Then, warm-up spiral N is performed (step  710 ). 
     Next, the difference between the measured velocity and the desired velocity is integrated over a predefined window of control samples (step  715 ). Reference is made to  FIG. 6 , which shows a window  650 . The window  650  is shown for illustrative purposes only and does not represent the size of an actual window. The size of an actual window is determined by trial-and-error. 
     As shown  FIG. 6 , the window begins in the initial acceleration region and the window ends in the zero acceleration region. The upper shaded area (between the velocity trajectory with overshoot and the desired velocity trajectory) represents the magnitude of an integration sum for an overshoot situation, which is calculated by integrating the difference between the measured velocity (curve  630 ) and the desired velocity (curve  620 ) over the window  650 . Similarly, the lower shaded area (between the desired velocity trajectory and the velocity trajectory with undershoot) represents the magnitude of an integration sum for an overshoot situation, which is calculated by integrating the difference between the desired velocity (curve  620 ) and the measured velocity (curve  640 ) over the window  650 . The sign of the integration sum depends upon whether an overshoot condition exists or an undershoot condition exists. 
     The magnitude and sign of the integration sum are used to adjust the feedback gain of the servo loop associated with the servo track writer (step  720 ). It should be understood that the above measurements do not provide an exact indication of the how much the gain should be adjusted. Instead, the magnitude and the sign of the integration sum can be thought of as pushing the gain in the right direction in small steps. Since the process will be repeated for many warm-up spirals, as described below, the gain will eventually converge. 
     In order to ensure that the gain is adjusted in small steps, a gain adjustment factor may be multiplied with the integration sum. The gain adjustment factor is determined by trial-and-error. 
     Steps  715  and  720  can be expressed by the following mathematical equation: 
             G   =     G   +       [       ∑     i   =   winstart     winend     ⁢     (       V   ⁢           ⁢     measured   i       -     V   ⁢           ⁢     desired   i         )       ]     ×   AdjustFactor             
where G is the Gain, Vmeasured i  is the measured velocity at the i th  sample, Vdesired i  is the desired velocity at the i th  sample, winstart represents a sample at the beginning of the window, winend represents a sample at the end of the window, and AdjustFactor is an adjustment factor having a value that is determined by trial-and-error.
 
     Returning to  FIG. 7 , a determination is made as to whether there are any more warm-up spirals to be performed, where W is the total number of warm-up spirals (step  725 ). If there are more warm-up spirals to be performed, then N is incremented (step  730 ) and the process returns to step  710 . 
     On the other hand, if there are no more warm-up spirals to be performed, then no more adjustments are made to the gain G. That is, the gain G is frozen while spiral servo information is written onto the disk surface (step  735 ). 
     As will be appreciated by those skilled in the art, overshoot is typically a second order response (damped sine wave) where the overshoot is generally followed by a period of undershoot. Accordingly, in another embodiment, a second integration window is provided to measure this undershoot. More specifically, the undershoot is measured using the same formula as above with the sign reversed to integrate the sum of the undershoot error over a second window immediately following the first window. The two integration sums are then added together and the result is used to adjust the gain. This increases the sensitivity of the measurement since the integration is being performed over a larger area. The size of the second window is determined by trial-and-error. 
     In one embodiment, a dead zone is provided, wherein gain adjustments are not made if the integration sum is less than a predetermined value (preferably, close to zero). Again, this predetermined value may be determined by trial-and-error. 
     In one embodiment, 20 warm-up spirals are written. Accordingly, in step  725 , the variable W equals 20. 
     While an effort has been made to describe some alternatives to the preferred embodiment, other alternatives will readily come to mind to those skilled in the art. Therefore, it should be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not intended to be limited to the details given herein.