Abstract:
Dynamically detecting off-track errors in a disk drive by adjusting transducer head position control parameters in response to shock events. In an embodiment, shock events with large magnitude trigger a severe shock timer which lengthens the write fault before write operations resume. In another embodiment, a lower shock threshold is used following an initial shock event. In yet another embodiment, a lower shock threshold is used if an accumulated average position error of the transducer head is large.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 09/639,567 filed Aug. 15, 2000 now U.S. Pat. No. 6,882,489 which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to positioning and control of disk drive transducer heads. In particular, the present invention relates to protecting against track misregistration errors when the transducer head is off-track. 
     BACKGROUND OF THE INVENTION 
     Disk drives store information on magnetic disks. Typically, the information is stored in concentric data tracks on the disks. The tracks are usually divided into sectors. Information is written to and read from the disk by a transducer head. The transducer head may include a read head separate from a write head, or the read and write heads may be integrated. The transducer head is mounted on an actuator arm assembly that moves the transducer head radially over the disk. Accordingly, the movement of the actuator arm assembly allows the transducer head to access different tracks. The disk is rotated by a spindle motor at a high speed, allowing the transducer head to access different sectors within each track on the disk. 
     The actuator arm assembly is interconnected to a motor, such as a voice coil motor, to move the transducer head radially over the disk. The voice coil motor is controlled by a servo control system. The servo control system performs two functions: seeking and track-following. The seek function moves the transducer head from an initial position to a target track. The seek function is initiated when a host computer associated with the disk drive issues a command to read data from or write data to the target track. Because of increasingly high demands on the performance of computer storage devices such as disk drives, it is desirable to move the transducer head from its initial position to the target track as quickly as possible. Once the transducer head is sufficiently close to the target track, the track-following function is activated to center and maintain the transducer head on the target track until the desired data transfer is complete. 
     The transducer head will oscillate about the centerline of the target track for a time period following the transition from the seek mode to the track-following mode. Because data written while the transducer head is oscillating about the centerline of the target track may be unrecoverable during subsequent attempts to read the data, the write operations are prohibited for a time period following the transition from the seek mode to the track-following mode. In addition, because data from adjacent tracks may inadvertently be read, or may corrupt the read signal generated by the transducer head during a read operation attempted while the transducer head is oscillating, the read operations are also inhibited for a time period following the transition from the seek mode to the track-following mode. Settle time during which reading and writing by the transducer head is not allowed better ensures the integrity of data written to or read from the disk. 
     Disk drives are susceptible to data errors due to external shock events. This is because shock can cause the transducer head to deviate from a desired position over the centerline of a track. Therefore, it is important to prohibit the transfer of data to and from the disk during shock events. In particular, it is important to prohibit writing data to the disk when shock events occur to prevent unrecoverable errors when the data is written to unintended areas of the disk. 
     The “centerline” of the track does not necessarily coincide with the physical centerline of the track. Instead, the “centerline” may refer to the center of the intended data storage area of the track. Therefore, as used herein, the “centerline” of a track need not refer to the physical centerline of the track, and “centered” indicates that the transducer head is properly centered over the area within the track that is intended for data storage, regardless of whether that position coincides with the physical centerline of the track. 
     Track misregistration error occurs when the transducer head is not properly centered over the track. Read track misregistration error occurs when the read head of the transducer head is not properly centered over the track centerline. Likewise, write track misregistration error occurs when the write head of the transducer head is not properly centered over the track centerline. Write track misregistration errors are particularly troublesome because they can result in permanent data loss. For instance, data written while the write head is not centered over the track may be unrecoverable during subsequent read operations because the read head, traveling over the track centerline and looking for the data in the expected position, may not be able to retrieve the data written off-center. In addition, data written to adjacent tracks may be lost since data written off-center may overwrite or corrupt data in an adjacent track. Thus, it is important to detect shock events and prohibit writing while the transducer head is not properly centered over the target track. 
     A write fault occurs when the transducer head deviates a predetermined distance from the centerline of the target track and the servo control system is in the track-following mode. The disk drive may trigger a write fault in response to a shock event and maintain the write fault for a predetermined time period to allow oscillations caused by the shock event to dampen and disappear. While the write fault is in effect, the write operations are disabled. 
     In setting the distance that the transducer head must deviate from the track centerline (the magnitude of the tracking error) to trigger the write fault, and in setting the time during which the write operations are prohibited, consideration must be given to the data transfer rate of the disk drive. Delaying the write operations avoids track misregistration errors but also reduces the data transfer rate. Although the data transfer rate is of great concern, the integrity of the data is of paramount importance. 
     Previous methods of detecting shock events and triggering write faults use accelerometers and other devices that are not required for the basic functions of the disk drive. Accordingly, previous shock detection methods increase the cost of the disk drive. In addition, previous shock detection methods treat all shock events equally, regardless of the severity of the shock event, and therefore unnecessarily compromise the data transfer rate and data loss resistance of the disk drive. 
     It would be advantageous to provide a disk drive that reacts to shock events in different ways, depending on the severity of a particular shock event. In addition, it would be advantageous to provide a disk drive that registers the severity of shock events without a separate shock detector. Furthermore, it would be advantageous to provide a disk drive that protects against track misregistration errors without unduly limiting the data transfer rate. 
     SUMMARY OF THE INVENTION 
     The present invention dynamically alters transducer head position control parameters of a disk drive in response to shock events. 
     In one embodiment, the position error of the transducer head with respect to a target track is monitored, and where a large deviation from the centerline of the target track is observed, a severe shock is inferred and a severe shock timer is activated and enables a write fault to prohibit write operations while it is running. The severe shock timer supplements a normal shock timer triggered by smaller shock events. The severe shock timer also has a much longer running time than the normal shock timer. The long running time allows oscillations or ringing in the disk drive to dampen and disappear before the write operations are attempted again. In addition, because the ringing following a severe shock event has a relatively low frequency, the long delay introduced by the severe shock timer prevents the disk drive from repeatedly cycling between an off-track state in which writing is prohibited due to the normal shock timer and an on-track state which may be entered as the transducer head passes through the track centerline. 
     In another embodiment, shock events following an initial shock event more easily trigger a write fault. In response to a shock event, the shock threshold is lowered for a predetermined time period. Because shock events commonly occur in closely spaced intervals, the disk drive quickly responds to subsequent small shock events to better protect against track misregistration errors. 
     In another embodiment, if an accumulated average transducer head position error is large then the shock threshold is lowered. For example, if the transducer head is persistently traveling to one side of the track centerline, then the position error of the transducer head required to trigger a write fault is reduced since the transducer head is more vulnerable to being knocked out of position and corrupting data in the adjacent tracks due to a shock event. 
     Advantageously, the disk drive protects against track misregistration errors as appropriate at a given time while maintaining a high data transfer rate. When observed conditions indicate that track misregistration error is imminent, protection against track misregistration errors is increased. Otherwise, more relaxed protection against track misregistration errors is employed to increase the data transfer rate. 
     Additional objects, features and advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of a conventional disk drive with the cover removed; 
         FIG. 2  is a schematic representation of a disk; 
         FIG. 3  is a plan view of a transducer head in which the illustrated surface faces the disk; 
         FIG. 4  is a partial schematic illustration of a track; 
         FIG. 5  is a functional flow chart of dynamic shock detection using a severe shock timer according to an embodiment of the present invention; 
         FIG. 6A  is a time line illustration of write faults for the operation in  FIG. 5 ; 
         FIG. 6B  is a time line illustration of transducer head position relative to a track centerline for the operation in  FIG. 5 ; 
         FIG. 7  is a functional flow chart of dynamic shock detection using a hair trigger timer according to another embodiment of the present invention; 
         FIG. 8A  is a time line illustration of write faults for the operation in  FIG. 7 ; 
         FIG. 8B  is a time line illustration of transducer head position relative to a track centerline for the operation in  FIG. 7 ; 
         FIG. 9  is a functional flow chart of dynamic shock detection using an accumulated average position error according to yet another embodiment of the present invention; 
         FIG. 10A  is a time line illustration of the accumulated average position error for the operation in  FIG. 9 ; and 
         FIG. 10B  is a time line illustration of transducer head position relative to a track centerline for the operation in  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a disk drive  100  that includes a base  104  and a magnetic disk (or disks)  108  (only one of which is shown). The disk  108  is interconnected to the base  104  by a spindle motor (not shown) mounted within or beneath the hub  112  such that the disk  108  can be rotated relative to the base  104 . An actuator arm assembly  116  is interconnected to the base  104  by a bearing  120  and suspends a transducer head  124  at a first end. The transducer head  124  reads data from and writes data to the disk  108 . A voice coil motor  128  pivots the actuator arm assembly  116  about the bearing  120  to radially position the transducer head  124  with respect to the disk  108 . The voice coil motor  128  is operated by a controller  132  that is operatively connected to a host computer (not shown). By changing the radial position of the transducer head  124  with respect to the disk  108 , the transducer head  124  can access different tracks  136  on the disk  108 . 
       FIG. 2  illustrates the disk  108  in more detail. The tracks  136  are divided into data fields  204   a - 204   h  and servo sectors  208   a - 208   h . The data fields  204   a - 204   h  store user data and the servo sectors  208   a - 208   h  store servo information to provide the transducer head  124  with the position of the transducer head  124  relative to the centerline of a target track  136 . 
     Although the disk  108  is illustrated having a relatively small number of tracks  136  and servo sectors  208 , a typical disk contains a very large number of tracks and servo sectors. For example, disks having over 14,000 tracks per inch and 240 servo sectors per track are presently available. In addition, alternate configurations of the disk  108  are possible. For example, one surface of the disk  108  can be dedicated to servo information while the other surface of the disk  108  (and any remaining disks  108  in the disk drive  100 ) can exclusively store user data. 
       FIG. 3  illustrates the transducer head  124  in more detail. The transducer head  124  includes a write head  304  and a magnetoresistive read head  308 . 
     The write head  304  includes a write pole  312  and a shared shield  316 . The write pole  312  and the shared shield  316  are joined magnetically to form a yoke about which a coil of wire is wrapped (not shown). During a write operation, current is passed through the coil to produce a magnetic field within the yoke. At a write gap  320  between the write pole  312  and the shared shield  316 , the magnetic field spreads out because the magnetic permeability of the write gap  320  is less than that of the yoke. The write gap  320  is positioned in close proximity to the disk  108 , allowing the magnetic field to magnetize the disk  108  so that digital information in the form of magnetic flux transitions is encoded on the disk  108 . A “1” is encoded on the disk  108  by reversing the direction of the current in the coil, thereby reversing the direction in which the disk  108  is magnetized. A “0” is encoded on the disk  108  by the absence of a change in magnetic polarity. Of course, these conventions can be reversed. 
     The read head  308  includes the shared shield  316 , a magnetoresistive strip  324  and a read shield  328 . The magnetoresistive strip  324  is connected to a current source (not shown). Because the electrical resistance of the magnetoresistive strip  324  varies with the strength and direction of an applied magnetic field, the magnetic flux transitions from the disk  108  are sensed by a change in voltage drop across the magnetoresistive strip  324 . Furthermore, the magnetic shields  316  and  328  limit the effect of magnetic flux transitions in adjacent tracks  136  on the magnetoresistive strip  324 . The transducer head  124  can contain other types of read heads such as an inductive, giant magnetoresistive, spin valve or dual spin valve read head. 
       FIG. 4  shows the track  136  in more detail. The track  136  has a track width  400  of about 1 micrometer and a track centerline  404  perpendicular thereto. The write pole  312  is about ⅔ the track width  400 . By centering the data on the track  136  about the track centerline  404 , the maximum distance between the data on the adjacent tracks  136  is maintained and overwriting or corrupting the data on the adjacent tracks  136  is reduced. The disk drive  100  expects the data on the track  136  to be positioned about the track centerline  404  during read operations. When the transducer head  124  does not precisely follow the track centerline  404 , the position error may be expressed in terms of the track width  400 . For instance, a normal shock threshold  408  is 15 percent of the track width  400  (as measured from either side of the track centerline  404 ) and a severe shock threshold  412  is 35 percent of the track width  400  (as measured from either side of the track centerline  404 ). 
       FIG. 5  is a flow chart  500  of dynamic shock detection using a severe shock timer according to an embodiment of the present invention. Initially, a shock event is detected by a deviation between the write head centerline  332  and the track centerline  404  of 15 percent of the track width  400  as the write head centerline  332  reaches the normal shock threshold  408  (step  504 ). Immediately thereafter, the write operations are prohibited (step  508 ). 
     The magnitude of the shock event is considered (step  512 ). If the shock event is a severe shock, which is detected by a deviation between the write head centerline  332  and the track centerline  404  of 35 percent of the track width  400  as the write head centerline  332  reaches the severe shock threshold  412 , then an off-track timer is initialized with a severe shock time value (step  516 ). Otherwise, the shock event is normal (mild) and the off-track timer is initialized with a normal shock time value that is shorter than the severe shock time value (step  520 ). While the off-track timer is running, write operations by the transducer head  124  are disabled. Thus, the severe shock time value prohibits the write operations for a longer time period than the normal shock time value, thereby allowing oscillations in the hub  112  caused by a severe shock event (but not a normal shock event) to dampen. Accordingly, the write operations are inhibited only as long as necessary, thereby increasing the data transfer rate. 
     After the off-track timer is initialized, the disk drive  100  determines whether the transducer head  124  is within an on-track threshold, which is detected by a deviation between the write head centerline  332  and the track centerline  404  of less than 15 percent of the track width  400  as the write head centerline  332  fails to reach the normal shock threshold  408  (step  524 ). If the transducer head  124  is not within the on-track threshold, then the process returns to step  512  to determine the magnitude of the shock event and proceeds to step  516  or  520  as appropriate. 
     The time value held by the off-track timer can only be increased at steps  516  and  520 . For example, if the time value held by the off-track timer is 5 milliseconds when step  520  is entered, and the normal shock time value is 130 microseconds, then the off-track timer is not reset to 130 microseconds. Instead, the time value remains at 5 milliseconds. However, if the initial time value at step  516  or  520  is greater than the time value held by the off-track timer when step  516  or  520  is entered, then the off-track timer is reset to the initial time value. For example, if the time value held by the off-track timer is 50 microseconds when step  520  is entered, and the normal shock time value is 130 microseconds, then the off-track timer is reset to 130 microseconds. 
     If the transducer head  124  is within the on-track threshold (step  524 ), then the off-track timer is decremented as each servo sector  208  in the track  136  passes beneath the transducer head  124  (step  528 ). The disk drive  100  determines whether the time value held by the off-track timer is zero (step  532 ). If not, the write operations continue to be prohibited and the process returns to step  524  to determine whether the transducer head  124  is within the on-track threshold. Otherwise, the transducer head  124  is declared “on-track” and the write operations are enabled (step  536 ). 
     The off-track timer disables the write operations by decrementing the time value as the servo sectors  208  pass beneath the transducer head  124 . For example, the severe shock time value sets the off-track timer to expire after 120 servo sectors  208  pass beneath the transducer head  124 , and the normal shock time value sets the off-track timer to expire after two servo sectors  208  pass beneath the transducer head  124 . 
     The off-track timer in another approach disables the write operations for an initialized time period. As examples, the severe shock time value disables the write operations until the disk  108  has made at least ¼ of a revolution but less than a full revolution, or until the disk  108  has made at least ½ of a revolution but not more than ¾ of a revolution, or for the time necessary for the disk  108  to make about ¾ of a revolution, or for approximately six milliseconds. 
     The time period, the number of servo sectors  208  that pass beneath the transducer head  124  and the amount the disk  108  rotates during which the write operations are prohibited in response to a severe shock event may be greater than that determined by the severe shock time value if the transducer head  124  does not return to the on-track state by the next position sample. Likewise, the time period, number of servo sectors  208  that pass beneath the transducer head  124  and the amount the disk  108  rotates during which the write operations are prohibited in response to a non-severe shock event may be greater than that determined by the normal shock time value if the transducer head  124  does not return to the on-track state by the next position sample. 
     Although a single off-track timer initialized to the severe shock time value or the normal shock time value has been described, the off-track timer can be initialized to additional time values of varying lengths in response to shocks of varying magnitudes. In addition, multiple off-track timers can be initialized to separate time values. For example, a severe shock timer can be initialized to the severe shock time value in step  516  and a normal shock timer can be initialized to the normal shock time value in step  520 . 
       FIGS. 6A and 6B  are time line illustrations of write faults and the transducer head  124  position relative to the track  136 , respectively, for the flow chart  500 . The transducer head position  604  indicates the position of the write head centerline  332  with respect to the track centerline  404  in terms of the track width  400 . The normal shock threshold  616  is 15 percent of the track width  400  (as measured from either side of the track centerline  404 ) and the severe shock threshold  608  is 35 percent of the track width  400  (as measured from either side of the track centerline  404 ). Furthermore, a severe shock timer is initialized to the severe shock time value and a normal shock timer is initialized to the normal shock time value. 
     The transducer head position  604  is near the track centerline  404  and the timers and the write fault are inactive and the write operations are allowed at time A. The transducer head position  604  has a large deviation from the track centerline  404  and crosses the severe shock threshold  608  at time B, thereby indicating a severe shock event at time B and activating the severe shock timer from time B to time G as shown by the severe shock timer operation  612 . The transducer head position  604  has a series of relatively large amplitude, low frequency oscillations that follow the severe shock event at time B and continue to time E. Because the severe shock timer is active and therefore the write fault is active and no write operations occur from time B to time G, track misregistration errors due to the relatively long ringing between times B and E are avoided. 
     The transducer head position  604  oscillations extend beyond the normal shock threshold  616  but not the severe shock threshold  608  at time C and subside within the normal shock threshold  616  at times D and E, thereby activating the normal shock timer from time C to time D as shown by the normal shock timer operation  620  without resetting the severe shock timer. However, the write fault is not affected by the normal shock timer operation  620  since it occurs during the severe shock timer operation  612 . Of course, the normal shock timer need not be activated while the severe shock timer is in operation. 
     The transducer head position  604  has another deviation from the track centerline  404  and crosses the normal shock threshold  616  but not the severe shock threshold  608  at time F, thereby indicating a normal shock event at time F and activating the normal shock timer from time F to time H as shown by the normal shock timer operation  620  without reactivating the severe shock timer. Although the write fault is not affected by the normal shock timer operation  620  from time F to time G since it occurs during the severe shock timer operation  612 , the write fault is activated by the normal shock timer operation  620  from time G to time H. 
     The transducer head position  604  oscillations subside within the normal shock threshold  616  at times G and H, and therefore the timers and the write fault are inactive at time H at which time the write operations are allowed again provided the transducer head  124  is otherwise adequately centered relative to the track  136 . 
       FIG. 7  is a flow chart  700  of dynamic shock detection using a hair trigger timer according to another embodiment of the present invention. Initially, a shock event is detected (step  704 ). The shock event can be detected in a variety of ways. For example, the shock event is detected by a predetermined deviation between the write head centerline  332  and the track centerline  404 . As another example, the shock event is detected by a minimum velocity at which the transducer head  124  exceeds a predetermined distance (such as 10 percent of the track width  400 ) from the track centerline  404 . 
     After the shock event is detected, a hair trigger timer is activated (step  708 ) and the shock threshold is lowered from its normal value to a small value (step  712 ). For example, the normal shock threshold is 20 percent of the track width  400  and the small shock threshold is 10 percent of the track width  400 . As another example, the normal shock threshold is a normal velocity of the transducer head  124  and the small shock threshold is a small velocity of the transducer head  124  which is lower than the normal velocity. In any case, the hair trigger timer is activated in response to the shock event, and when the hair trigger timer is running, the small shock threshold replaces the normal shock threshold and therefore the disk drive  100  is more sensitive to shock events and activates the write fault in response to small shock events that are within the normal shock threshold. 
     The hair trigger timer is decremented as each servo sector  208  in the track  136  passes beneath the transducer head  124  (step  716 ). When the hair trigger timer is zero or has otherwise run its course (step  720 ) then the small shock threshold is replaced by the normal shock threshold (step  724 ). 
     Thus, the hair trigger timer increases shock sensitivity to prohibit the write operations more easily for a limited time period after a shock event is detected to better protect against small shock events in close temporal proximity to the initial shock event that might otherwise go undetected and lead to track misregistration errors. In addition, the increased protection against track misregistration errors has little affect on the data transfer rate of the disk drive  100 . 
     Although the write fault is activated in response to the hair trigger timer, and therefore remains active while the hair trigger timer is running, other approaches are available. For example, the write fault can be activated by a separate shock timer. 
       FIGS. 8A and 8B  are time line illustrations of writes faults and the transducer head  124  position relative to the track  136 , respectively, for the flow chart  700 . The transducer head position  804  indicates the position of the write head centerline  332  with respect to the track centerline  404  in terms of the track width  400 . The normal shock threshold  808  is 15 percent of the track width  400  (as measured from either side of the track centerline  404 ) and the small shock threshold  816  is 7 percent of the track width  400  (as measured from either side of the track centerline  404 ). 
     The transducer head position  804  is near the track centerline  404  and the hair trigger timer and the write fault are inactive and the write operations are allowed at time A. The transducer head position  804  has a large deviation from the track centerline  404  and crosses the normal shock threshold  808  at time B, thereby indicating a shock event at time B and activating the hair trigger timer from time B to time D as shown by the hair trigger timer operation  812 . The transducer head position  804  has a series of relatively small amplitude, high frequency oscillations that follow the shock event at time B and subside before time C. Because the hair trigger timer is active and therefore the write fault is active and no write operations occur from time B to time D, track misregistration errors due to the relatively short ringing between times B and C are avoided. 
     The transducer head position  804  has another deviation from the track centerline  404  and crosses the small shock threshold  816  but not the normal shock threshold  808  at time C, thereby indicating a small shock event at time C and resetting the hair trigger timer so that the hair trigger timer is active from time C to time E as shown by the hair trigger timer operation  820 . Although the write fault is not affected by the hair trigger timer operation  820  from time C to time D since it occurs during the hair trigger timer operation  812 , the write fault is activated by the hair trigger timer operation  820  from time D to time E. 
     The transducer head position  804  oscillations subside within the small shock threshold  816  at times D to F, and therefore the hair trigger timer and the write fault are inactive at times E and F at which times the write operations are allowed again provided the transducer head  124  is otherwise adequately centered relative to the track  136 . 
       FIG. 9  is a flow chart  900  of dynamic shock detection using an accumulated average position error according to yet another embodiment of the present invention. Initially, N position error samples of the write head centerline  332  relative to the track centerline  404  are taken as N servo sectors  208  in the track  136  pass beneath the transducer head  124 , with one sample taken per servo sector  208 , and the position error samples are added together (step  904 ). The sum of the position error samples is divided by N to obtain an accumulated average position error (step  908 ). The accumulated average position error is compared to a position error threshold (step  912 ). If the accumulated average position error is greater than the position error threshold then a small shock threshold (which is lower than the normal shock threshold) is set (step  916 ), otherwise the normal shock threshold is set (step  920 ). 
     The next position error sample is then taken (step  924 ). The difference between the accumulated average position error in step  912  and the most recent position error sample in step  924  is calculated, and the result is divided by N and then added to the accumulated average position error to obtain a new (updated) accumulated average position error (step  928 ). The process then returns to step  912  to compare the accumulated average position error in step  928  with the position error threshold, set the shock threshold to the normal value or the small value as appropriate in steps  916  and  920  and then update the accumulated average position error using another position error sample in steps  924  and  928 . 
     Thus, the shock sensitivity is increased to prohibit the write operations more easily while the accumulated average position error is large to better protect against small shock events that might otherwise go undetected and lead to track misregistration errors since the transducer head  124  is persistently positioned on one side of the track centerline  404  and therefore more vulnerable to shock events. 
     Although the accumulated average position error is updated as each servo sector  208  passes beneath the transducer head  124 , other approaches are available. For example, the accumulated average position error can be computed periodically as every fourth servo sector  208  passes beneath the transducer head  124 . In addition, the accumulated average position error can be determined from a continuously collected position error. Furthermore, the accumulated position error need not be an average value. For example, an accumulated gross position error can be compared to an appropriate position error threshold. 
       FIGS. 10A and 10B  are time line illustrations of the accumulated average position error and the transducer head  124  position relative to the track  136 , respectively, for the flow chart  900 . The accumulated average position error  1004  is shown in terms of the track width  400 , and the transducer head position  1008  indicates the position of the write head centerline  332  with respect to the track centerline  404  in terms of the track width  400 . The position error threshold  1012  is a predetermined amount of the track width  400  (as measured from either side of the track centerline  404 ). Furthermore, the position error samples are taken periodically at the sample times  1016 . 
     The accumulated average position error  1004  is unknown at time A because the requisite number of position error samples have not yet been collected, and therefore the write fault is inactive and the write operations are allowed. The accumulated average position error  1004  is known and within the position error threshold  1012  and therefore the write fault remains inactive and the write operations remain allowed at time B. 
     The accumulated average position error  1004  crosses the position error threshold  1012  shortly before time C and is detected by the position error sample at time C, thereby indicating that the transducer head  124  has been positioned on one side of the track centerline  404  for too long, and therefore the shock threshold is lowered from its normal value to a small value. While the small shock threshold replaces the normal shock threshold, the disk drive  100  is more sensitive to shock events and activates the write fault in response to small shock events that are within the normal shock threshold. 
     The accumulated average position error  1004  subsides within the position error threshold  1012  shortly before time D and is detected by the position error sample at time D, and therefore the shock threshold is returned to its normal value. 
     The timers described above can be implemented with a count value that is decremented by each sector  208  on the track  136  that passes beneath the transducer head  124 , or alternatively, as a clock timer based on a time count. Furthermore, the timers can be implemented as software routines in the controller  132  or as hardware devices in communication with the controller  132 . 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. Further, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, within the skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments herein are further intended to explain the best mode presently known of practicing the invention and to enable others skilled in the art to utilize the invention in such or in other embodiments and with various modifications required by their particular application or use of the invention. It is intended that the appended claims include alternative embodiments to the extent permitted by the prior art.