Abstract:
Disturbances to actuator control caused by external vibrations are corrected using feed-forward techniques that are selectively enabled or disabled based on signal values within a track follow loop. An increase in data integrity and reliability may be achieved by reducing off-track reads and writes caused by physical disturbances to disk drives during operation. Accelerometers detect external forces imparted on a disk and an acceleration feed-forward (AFF) signal is generated to compensate for disturbances to the location of a head caused by such external forces. Application of the AFF signal to an actuator may be based on whether signal values along the track follow loop exceed a certain threshold.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     Embodiments of the present invention relate to U.S. Provisional Application Ser. No. 60/772,339, filed Feb. 10, 2006, entitled “Vibration Detection for Acceleration Feed-Forward System”, the contents of which are incorporated by reference herein and which is a basis for a claim of priority. 
     BACKGROUND 
     Embodiments of the present invention relate generally to control systems, such as those used in magnetic storage systems and methods and, in specific embodiments, to systems and methods that correct for disturbances to coarse actuator control caused by external vibrations using feed-forward techniques that are selectively enabled or disabled based on track follow loop signals. 
     Magnetic storage systems, such as disk drives, are widely used in computers and other electronic devices for the storage and retrieval of data. Important design considerations for disk drive manufacturers generally include: (a) data storage capacity; (b) data transfer rate; (c) data integrity and reliability; and (d) manufacturing cost. 
     In general, related art disk drives comprise one or more disks for storing data, an actuator arm, and one or more transducers or heads. Each head is operable to read data from and write data to concentric circular tracks on a surface of a corresponding disk. The heads are typically attached to the actuator arm, and when a head performs a read or a write operation, the actuator arm is moved so that the head is positioned over the center of a selected track to perform the desired operation. 
     In recent years, disk drive manufacturers have sought to increase the data storage capacity of disk drives while controlling the manufacturing cost. One solution has been to increase track density by increasing the number of tracks per inch (TPI) on each disk. As TPI has increased, tracks have become narrower, and maintaining data integrity has become a greater design challenge because data errors can occur with smaller amounts of movement of a head away from a track center during a read or a write operation. 
     Movement of a head away from a track center can lead to an off-track read or an off-track write. An off-track read occurs when a head is positioned over a wrong track during a read operation and the head reads data from the wrong track. In such an instance, the incorrect data would have to be discarded, the head repositioned over the correct track, and the head would then have to read in the correct data. As a consequence, the data transfer rate of the disk drive would be reduced, because the time spent reading the wrong data would be wasted. Even worse than an off-track read is an off-track write. An off-track write occurs when a head is positioned over a wrong track during a write operation and the head writes data to the wrong track. As a result of an off-track write, data integrity is adversely affected, because existing data on the wrong track is improperly overwritten and is potentially lost. 
     Thus, to prevent data errors, it is preferable to maintain a head over a center of a selected track during a read or a write operation. In order to position a head during a read or a write operation, related art disk drives typically comprise a servo controller and have embedded servo sectors located in the tracks of each disk. The embedded servo sectors are located between data sectors and contain predetermined patterns from which a position of a head during an operation can be determined. 
     During read and write operations to a selected track, a head reads data from embedded servo sectors of the selected track and provides the data read from the embedded servo sectors as servo information to a servo controller. The servo controller receives the servo information provided by the head and determines a position error signal (PES) from the servo information. The PES is indicative of the position of a head relative to the center of the selected track. The PES is then fed into a compensator that produces an appropriate compensation signal so that the actuator arm will reposition closer to the center of the selected track. Once the actuator arm is repositioned, the process repeats as the head again reads data from the embedded servo sectors and provides updated positional information to the servo controller. This interplay between the PES, compensator, and the positional information regarding the actual location of the head, form the track follow loop. 
     When operating in various environments, a disk drive may be subject to various external forces in the form of vibrations or shocks. Depending on the intensity and direction of these external forces, the actuator arm and head assembly can become displaced from their desired location over the center of a track. Translational forces will not have a significant impact on the position of the head if the actuator arm assembly is balanced. However, rotary forces acting in the plane of the disk may cause considerable head displacement. Although convergence of the track follow loop does provide some protection against such displacement by eventually re-positioning the head over the center of the track, the loop has a finite response time that might be too slow to correct for certain vibratory or other forces acting on the disk drive. 
     It has been proposed to use accelerometers to sense disruptive forces acting on a disk drive. The accelerometers generate signals representative of the intensity and direction of the forces acting on a disk drive, and these signals in turn can be used in a feed-forward architecture to make the disk drive more robust to such forces. 
     Various types of accelerometers, for example linear accelerometers and angular accelerometers, have been used in such compensatory schemes. Linear accelerometers detect forces acting in one direction (translational forces), whereas angular accelerometers detect rotational forces acting within some plane. As mentioned before, translational forces are not a particular threat to balanced actuator arm assemblies, and thus angular accelerometers are more useful. However, linear accelerometers may also be used in pairs to detect rotational force. The signal difference between two linear accelerometers affixed at opposite ends of a disk drive will yield a value close to zero in the presence of translational force since both accelerometers will notice acceleration in the same direction. However, in the presence of rotational force, each accelerometer will notice acceleration equal and opposite of the other since at any given moment they will be accelerating in opposite directions. Thus, in the presence of rotational force, the absolute value of the signal difference will constructively add. 
     Forces acting normal to the plane of rotation (z-axis) of disks of a disk drive are not a particular source of concern because the actuator arm assembly and head will not be displaced in a direction along the plane of disk rotation (x-y axes). Therefore, rotational accelerometers or linear accelerometers will be positioned such that their directions of sensitivity are parallel to the plane of the disk. Otherwise, the correctional information these accelerometers provide will contain components pertaining to disturbances parallel to the z-axis—information that may mislead the acceleration feed-forward system in trying to correct for a disturbance that is actually not affecting the position of the head. 
     Ideally, an accelerometer that is situated so as to only detect motion in the x-y axis will not generate a signal in response to forces being imparted on it from the z-axis. In reality however, an accelerometer situated so as to only detect motion in the x-y axis may still generate non-zero signal information in response to forces directed from the z-axis. In response to such signal information, an acceleration feed-forward system may move the head in order to compensate for what it erroneously thinks to be a threatening disturbance within the plane of the disk. In this way the acceleration feed-forward system can become a source of noise itself, and make it more difficult for the head to converge onto the center of the selected track. In extreme situations, this noise can cause off-track read or write errors, which is unacceptable. 
     SUMMARY OF THE DISCLOSURE 
     Embodiments of the present invention relate to correcting for disturbances to actuator control caused by external vibrations using feed-forward techniques that are selectively enabled or disabled based on signal values within a track follow loop. Particular embodiments may increase data integrity and reliability by reducing off-track reads and writes caused by physical disturbances to disk drives during operation. 
     Embodiment of the invention provide an AFF signal to compensate for any disturbances to a position of a head of a storage system relative to a selected track of a storage medium. In such embodiments, an AFF system may include an acceleration feed-forward (AFF) unit configured to selectively provide an AFF signal based on track follow loop signal samples. In further embodiments, the AFF unit is configured to selectively provide an AFF signal based on whether a storage system employing the acceleration feed-forward (AFF) system is in a read or write retry state. The AFF system further includes a track follow system for at least partially controlling motion of an actuator relative to the selected track of the storage medium, based on the AFF signal. 
     Further embodiments of the present invention relate to a method for providing an acceleration feed-forward (AFF) signal to compensate for any disturbances to a position of a head of a storage system relative to a selected track of a storage medium. In such embodiments, the method includes selectively providing an AFF signal based on (i) track follow loop signal samples and/or (ii) whether a storage system employing the method is in a read or write retry state. The method also includes at least partially controlling motion of an actuator relative to the selected track of the storage medium, based on the acceleration feed-forward (AFF) signal. 
     Yet further embodiments relate to storage systems and methods that include or employ an AFF system or method as described herein. Such embodiments of the present invention may include or employ a disk, a head, an actuator, a servo controller, accelerometers, and an AFF system as described herein. A surface of the disk has one or more tracks that include data sectors and servo sectors. The actuator allows for positioning of the head over a selected track of the one or more tracks of the disk. As the disk rotates, the head may read or write data information to the data sectors within a track. The head may also read servo sector information and relay that information to the servo controller. This information tells the servo controller the position of the head relative to the disk and to a given track. This information also serves to generate a position error signal that may be used to converge the head over the center of the desired track. The accelerometers provide signal information pertaining to forces acting within the plane of the disk to the acceleration feed-forward system. The acceleration feed-forward system generates a corresponding acceleration feed-forward signal that, when enabled, may correct the position of the actuator arm so as to compensate for any undesired displacement of the head caused by the forces. Embodiments may be used to control a course actuator arm. In other embodiments, the acceleration feed-forward signal may correct the position of a fine position actuator, such as, but not limited to a microactuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an exploded perspective view of a disk drive of an embodiment of the present invention; 
         FIG. 2  illustrates a functional block diagram of a disk drive of an embodiment of the present invention in communication with a host system; 
         FIG. 3  illustrates a block diagram of a track follow loop and an acceleration feed-unit forward control unit of a servo controller in accordance with an embodiment of the present invention. 
         FIG. 4  illustrates a block diagram of a servo vibe detect unit of an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an exploded three dimensional perspective view of a disk drive  1  of an embodiment of the present invention. The disk drive  1  comprises a base  2 , a printed circuit board assembly (PCBA)  3 , and a cover plate  4 . The PCBA  3  contains suitable control electronics and is rigidly mounted to an underside of base  2 . The cover plate  4  encloses essential components of the disk drive  1  in a cavity within the base  2  by attaching to the top side (the side opposite the PCBA  3 ) of the base  2 . 
     The disk drive  1  further comprises motion sensors  5  and  6  that are rigidly mounted relative to the base  2  such that the sensors  5  and  6  move with the base  2 . In  FIG. 1 , the sensors are mounted to the PCBA  3 , which in turn is rigidly mounted to the base  2 . It is possible, of course, to mount any one of the sensors  5  and  6  directly to the base  2 , or to mount them to any other structure that is rigidly attached to the base  2 . 
     The sensors  5 ,  6  may comprise, for example, piezoelectric linear accelerometers, or the like, and may be located at opposite ends of the PCBA  3 . These sensors  5  and  6  are oriented relative to each other so that the differential of the their signals will be ideally zero when the disk drive  1  is subjected to translational motion, and their outputs will constructively add in the presence of angular motion in the plane of the disk  7 . Since the purpose of the sensors  5  and  6  are to provide the disk drive  1  with information regarding angular motion in the plane of the disk  7 , any number of sensors may be used of varying types in accordance with this invention, not limited to piezoelectric linear accelerometers. These variations include angular sensors, or other types of multi-axis sensors. Such sensors and their various arrangements to detect motion in the plane of disks of a disk drive are well known to those of ordinary skill in the art. 
       FIG. 2  illustrates a functional block diagram of the disk drive  1  in communication with a host system  23  in accordance with an embodiment of the present invention. The disk drive  1  comprises a disk  7 , a transducer or head  11 , a coarse actuator  12 , a microactuator  13 , an actuator arm assembly  14 , an interface  15 , a main controller  16 , a spin motor  17 , a servo controller  18 , a read/write (r/w) channel  19 , and a motion sensing unit  10 . 
     The head  11  is mounted on one end of the actuator arm assembly  14 , and another end of the actuator arm assembly  14  is connected to the base  2  ( FIG. 1 ) of the disk drive  1  by a bearing  20 . The actuator arm assembly  14  comprises a first member  21  and a second member  22  that are interconnected by the microactuator  13 . During operation, the disk  7  spins around a central axis, and the head  11  reads data from or writes data to a surface of the disk  7 . The coarse actuator  12  rotates the actuator arm assembly  14  about the bearing  20  in order to control a position of the microactuator  13  and the head  11  over the disk  7 . The microactuator  13  moves the second member  22  of the actuator arm assembly  14  to position the head  11  over the disk  7 . 
     The coarse actuator  12  may comprise a motor, such as a voice coil motor (VCM) or the like, and may provide for coarse positioning of the head  11  over the disk  7 . The microactuator  13  may comprise, for example, a piezoelectric actuator, an electromagnetic actuator, an electrostatic actuator, or the like. The microactuator  13  may provide for fine positioning of the head  11  over the disk  7 . A range of movement of the microactuator  13  may be small, such as moving the head  11  across a few tracks, while a range of movement of the coarse actuator  12  may be large, such as moving the head  11  across all tracks on the disk  7 . Other embodiments of the present invention may employ different disk drive configurations than that of the example shown in  FIG. 2 , including, but not limited to disk drive configurations which do not employ microactuators  13 . 
     The disk drive  1  is not limited to having only a single disk  7 , but may have a plurality of disks. Also, data may be written to both a top surface and a bottom surface of each disk, in which case a different head is required for each surface. The head  11  may have a single element for performing both reading and writing, or the head  11  may have separate elements for performing each of those operations, in which case the head  11  would comprise a read element and a write element. 
     In the following discussion, portions of the disk drive  1  are described with reference to functional blocks and not discrete hardware elements. The functions may be implemented using one or more of hardware, software, and firmware. In addition, more than one function, or different parts of functions, may be combined in a given hardware, software, or firmware implementation. 
     When the disk drive  1  is connected to the host system  23 , the interface  15  communicates with the host system  23  to receive, for example, data and commands, and to send, for example, data and status identifiers. The interface  15  also communicates with the main controller  16  and the r/w channel  19  to send and receive, for example, data and commands. When the main controller  16  receives a command from the interface  15  for a read or a write operation, the main controller  16  provides a signal to the spin motor  17  to cause the disk  7  to spin. 
     As shown in  FIG. 2 , the disk  7  has one or more tracks  24  for storing data. Each of the tracks  24  has a plurality of data sectors  25  and a plurality of embedded servo sectors  26 . During operation of the disk drive  1 , a data block may be read from or written to a data sector of the plurality of data sectors  25 . The plurality of embedded servo sectors  26  are written with servo patterns or data that are used for determining a position of the head  11  with respect to a track of the one or more tracks  24 . 
     The disk drive  1  is able to perform the operations of seeking and tracking. When the main controller  16  receives a read or write request from the host system  23  (via the interface  15 ), it may instruct the servo controller  18  to move the head  11  to a different track location so that the r/w channel  19  may initiate the read or the write. During this move from one track to another, the disk drive  1  is in seeking mode, and ideally, the head  11  comes to a rest directly over the center of the selected track. In reality, however, the head  11  may overshoot the desired track, requiring time for the head  11  to settle into the desired location. 
     When the servo controller  18  determines that the head  11  has settled over the selected track, the disk drive  1  is said to be in tracking mode, and the servo controller  18  may send a signal to the r/w channel  19  to start a read or write operation. It is also possible that the servo controller  18  sends the signal to the main controller  16 , rather than to the r/w channel  19 , in which case the main controller  16  would then send the signal to the r/w channel  19 . The r/w channel  19  also receives a command from the main controller  16  that specifies the type of operation to be performed. In the case of a read operation, the r/w channel  19  causes the head  11  to read the data and then sends the data to either the interface  15  or the main controller  16 . In the case of a write operation, the r/w channel  19  receives data from either the interface  15  or the main controller  16  and causes the head  11  to write the data. 
     The servo controller  18  also receives information from the motion sensing unit  10 . In an embodiment of the present invention, the motion sensing unit  10  comprises the linear accelerometer motion sensors  5  and  6  ( FIG. 1 ), an analog to digital converter (A/D) (not shown) and various filters (not shown). A signal indicating a difference between the two linear accelerometers  5  and  6  is directly correlated to an angular displacement of the disk drive  1 , when the disk drive  1  is being subjected to a force in the plane of the disk  7 . This signal may then be digitized using an A/D, and then notch filtered to remove any resonant frequencies and noise inherent to the accelerometers themselves. The signal may also be gained appropriately for proper interfacing with the servo controller  18 . Again, the angular displacement signal may be generated using any number of and types of accelerometers, not limited to two linear accelerometers discussed herein. Moreover, the signal processing steps of digitizing and filtering the outputs of the motion sensors  5  and  6  are not limited to the techniques or order described above, as other types of filters may also be applied. 
       FIG. 3  shows a block diagram of a track follow loop  40  and an acceleration feed-forward (AFF) control unit  39  of the servo controller  18  (refer to  FIG. 2 ) in accordance with an embodiment of the present invention. The components illustrated in  FIG. 3  are just some of the components responsible for ultimately controlling the movement of the head  11 . In the following discussion, portions of the track follow loop  40  and the acceleration feed-forward control unit  39  are described with reference to functional blocks and not discrete hardware elements. The functions may be implemented using one or more of hardware, software, and firmware. More than one function, or different parts of functions, may be combined in a given hardware, software, or firmware implementation. 
     The track follow loop  40  comprises a coarse actuator path unit  30 , a first summing node  31 , a coarse actuator compensator  33 , a second summing node  34 , the coarse actuator  12 , and the head  11 . The coarse actuator path unit  30  is configured to provide a coarse actuator reference trajectory signal  80  that specifies a reference trajectory for the coarse actuator  12 . The reference trajectory for the coarse actuator  12  is a desired trajectory that the coarse actuator  12  should ideally follow to move the head  11  during a seek operation. In tracking mode, the coarse actuator path unit  30  serves to provide a reference signal to the coarse actuator  12  that helps maintain a position of the head  11  over a selected track. 
     According to  FIG. 2  and  FIG. 3 , while in tracking mode, the head  11  reads data from the servo sectors of the plurality of servo sectors  26  on the disk  7 , and provides the r/w channel  19  with a signal based on the read servo sector data. The r/w channel  19  processes this information and provides the servo controller  18  with information regarding the actual position of the head  11  with respect to a track of the one or more tracks. The actual position of the head  11  with respect to a track is then used as a feedback signal  41 . The feedback signal  41  is subtracted from the reference trajectory signal  80  generated by the coarse actuator path unit  30  at summing node  31 . The resulting output signal from summing node  31  is a position error signal (PES)  32 . Thus, the PES  32  indicates a difference between a desired reference position of the head  11  and an actual position of the head  11 . 
     The coarse actuator compensator  33  receives the position error signal  32  that is provided by the first summing node  31 . The coarse actuator compensator  33  is configured to provide a coarse actuator compensation signal  81  based on the PES  32  that can be used to control the coarse actuator  12 . The coarse actuator compensation signal  81  is then combined with a gated acceleration feed-forward (GAFF) signal  82  at the third summing node  34  to produce a current drive signal (CDS)  35  that controls the coarse actuator  12 , and ultimately the position of the head  11 . 
     The AFF control unit  39  comprises the motion sensing unit  10 , an AFF signal generator  37 , and a servo vibe detect unit  38 . The motion sensing unit  10  sends information regarding an angular disturbance that the disk drive  1  is being subjected to in the plane of the disk  7 , to the AFF signal generator  37 . The AFF signal generator  37  then calculates an appropriate acceleration feed-forward (AFF) signal  83  needed to compensate for the disturbances. However, use of this acceleration feed forward signal  83  depends on the state of the servo vibe detect unit  38 . If the servo vibe detect unit  38  is in an active state, the servo vibe detect unit  38  will allow the GAFF signal  82  to be equal to the AFF signal  83 , so as to be added to the coarse actuator compensation signal  81  via summing node  34 . If the servo vibe detect unit  38  is in an inactive state, the GAFF signal  82  will be equal to zero, and thus the coarse actuator will not be affected by the AFF signal  83 . 
     The state of the servo vibe detect unit  38  may be a function of one or more signals within the track follow loop  40 . In an embodiment of the present invention, the track follow loop signals that affect the state of the servo vibe detect unit  38  are the PES  32  and the CDS  35 , although any signal along the track follow loop  40  may be used in determining when to enable or disable use of the AFF signal  83 . 
       FIG. 4  shows a block diagram of how the state of the servo vibe detect unit  38  may be changed to active, enabling use of the AFF signal  83  (refer to  FIG. 3 ). In the following discussion, the servo vibe detect unit  38 , and its operation, is described with reference to functional blocks and not discrete hardware elements. The functions may be implemented using one or more of hardware, software, and firmware. More than one function, or different parts of functions, may be combined in a given hardware, software, or firmware implementation. 
     With reference to  FIGS. 2 ,  3 , and  4 , while the disk drive  1  operates in tracking mode, the PES  32  and the CDS  35  values are generated and updated as the head  11  reads servo sector information pertaining to the actual location of the head  11  over the disk  7 . These signal values may be updated at least once per servo sector of the plurality of servo sectors  26  located on the disk  7 . An absolute value of each new PES  32  sample value and CDS  35  value generated is calculated and provided by PES absolute value unit  45  and CDS absolute value unit  46 , respectively. An output of the PES absolute value unit  45 , which represents an absolute value of the PES  32  sample value, is then fed into the PES summation unit  47 . The PES summation unit  47  adds together the absolute values of the PES  32  sample values output by the PES absolute value unit  45 . 
     A count of the PES counter  49  is decremented each time the PES summation unit  47  adds to its total another value output by the PES absolute value unit  45 . The PES counter  49  starts counting down from a value that may be set to be equal to a number of servo sectors located along one track  24  around one revolution of the disk  7 . The PES counter  49  expires when the count reaches zero, and upon expiration, the PES counter  49  sends a signal to the PES summation unit  47  to: (1) output its present total value to the PES filter unit  51 ; and (2) clear its total value to start anew for a next counter cycle counted by the PES counter  49 . Thus, the PES summation unit  47  generates a sum value representing the total value of the absolute values of the PES  32  sample values output by the PES absolute value unit  45  within one PES counter  49  cycle. 
     Similarly, an output of the CDS absolute value unit  46 , which represents an absolute value of the CDS  35  sample value, is then fed into the CDS summation unit  48 . The CDS summation unit  48  adds together the absolute values of the CDS  35  sample values output by the CDS absolute value unit  46 . A count of the CDS counter  50  is decremented each time the CDS summation unit  48  adds to its total another value output by the CDS absolute value unit  46 . The CDS counter  50  starts counting down from a value that may be set to be equal, for example, to the number of servo sectors located around one revolution of one track  24  on disk  7 . The CDS counter  50  expires when the count reaches zero, and upon expiration, the CDS counter  50  sends a signal to the CDS summation unit  48  to: (1) output its present total value to the CDS filter unit  52 ; and (2) clear its total value to start anew for a next counter cycle counted by the CDS counter  50 . Thus, the CDS summation unit  48  generates a sum value representing the total value of the absolute values of the CDS  35  sample values output by the CDS absolute value unit  46  within one CDS counter  50  cycle. 
     The PES filter unit  51  may be any low pass filter, such as, an integrator, a Chebyshev filter, a Butterworth filter, or the like. The output of the PES filter unit  51  is then input into the PES comparator unit  53 . The PES comparator unit  53  checks to see if any filter output value from the PES filter unit  51  exceeds some particular threshold value. The threshold value may be set manually to some fixed value, or may automatically be set and changed by the disk drive  1  to a higher or lower value depending on the hostility (strength and frequency of occurrence of vibratory events) of the environment the disk drive  1  is in. As long as the filter output values from the PES filter unit  51  remain less than the threshold value, the enable AFF switch  55  will not allow the GAFF signal  82  to be set equal to the AFF signal  83 . However, if the threshold value is exceeded, a servo vibration condition is said to be detected. Then, the enable AFF switch  55  will allow the GAFF signal  82  to be equal to the AFF signal  83 , so that it may be added to the coarse actuator compensator signal  81  via summing node  34 . 
     Similarly, the CDS filter unit  52  may be any low pass filter, such as, an integrator, a Chebyshev filter, a Butterworth filter, or the like. The output of the CDS filter unit  52  is then input into the CDS comparator unit  54 . The CDS comparator unit  54  checks to see if any filter output value from the CDS filter unit  52  exceeds some particular threshold value. The threshold value may be set manually to some fixed value, or may automatically be set and changed by the disk drive  1  to a higher or lower value depending on the hostility (strength and frequency of occurrence of vibratory events) of the environment the disk drive  1  is in. As long as the filter output values from the CDS filter unit  52  remain less than the particular threshold value, the enable AFF switch  55  will not allow the GAFF signal  82  to be equal to the AFF signal  83 . However, if the particular threshold value is exceeded, a servo vibration condition is said to be detected. Consequently, the enable AFF switch  55  will allow the GAFF signal  82  to be set equal to the AFF signal  83 , so that it may be added to the coarse actuator compensator signal  81  via summing node  34 . 
     It is important to note that the processes of summing and filtering the PES  32  and CDS  35  sample values can be altered or replaced with any variety of processes that yield representations of the PES  32  and CDS  35  sample values that are less indicative of fleeting noise-like variations, and more indicative of actual vibrations or rotational forces being imparted on the disk drive  1 . For example, instead of summing the absolute values of the PES  32  sample values and/or CDS  35  sample values, a variance calculating unit may generate variance values of the PES  32  sample values and/or the CDS  35  sample values. These variance values may then be filtered and then compared to threshold values. Also, the counter values of the PES counter  49  and CDS counter  50  can be changed from a number of servo sectors  26  along one revolution of a disk  7  to some ratio of a number of servo sectors  26  along one revolution of a disk  7 . In addition, the threshold values of the PES comparator unit  53  and the CDS comparator unit  54  may be variable, that is, they may be increased or decreased either manually or automatically depending on the hostility of the environment the disk drive  1  is in, or the amount of protection the user desires. 
     In some embodiments of the present invention, only the PES  32  sample values are processed (summed, filtered, and compared to a threshold value) in determining whether to allow the GAFF signal  82  to be set equal to the AFF signal  83 . In other embodiments, only the CDS  35  sample values are processed in determining whether to allow the GAFF signal  82  to be set equal to the AFF signal  83 . Yet, in other embodiments, both the PES  32  sample values and the CDS  35  sample values are processed in determining whether to allow the GAFF signal  82  to be set equal to the AFF signal  83 . In some embodiments of the present invention, both the PES filter  51  output values and the CDS filter  52  output values must exceed specific threshold values for the GAFF signal  82  to be set equal to the AFF signal  83 . In yet other embodiments, if either the PES filter  51  output values or the CDS filter  52  output values exceed a specific threshold value, the GAFF signal  82  may be set equal to the AFF signal  83 . 
     In an embodiment of the present invention, processing of the PES  32  and the CDS  35  sample values are suspended during seek operations. Processing may again be resumed from the same counter count of the PES counter  49  and the CDS counter  50  once the disk drive  1  re-enters tracking mode, without having to discard previously calculated PES  32  and CDS  35  sample values of a partial disk rotation. To illustrate, the head  11  might be situated over a particular track, among the one or more tracks  24  on the disk  7 , performing a read operation. During this time, the PES  32  and CDS  35  sample values may be processed, for example, for half a disk revolution as the head  11  completes its read task and moves to a different track. While the head  11  is seeking a new track, the counter values of the PES counter  49  and the CDS counter  50  and processing of the PES  32  and CDS  35  sample values may be suspended. Once the head  11  settles on its new track destination, processing of the PES  32  and CDS  35  sample values may resume at the same counter count, such that only half a disk revolution is left to produce a total value output from the PES summation unit  47  and a total output value from the CDS summation unit  48 . Thus, processing of the PES  32  and CDS  35  values for one revolution of the disk  7  may not be fixed to disk position because processing may resume from a different location on the disk  7  from where it originally left off. Resuming processing in this fashion may be more efficient in that the summation process would not have to restart from a value of zero every time the head  11  moves to a different track. 
     In an embodiment of the present invention, processing of the PES  32  and the CDS  35  sample values are suspended during an error condition. An error condition may be present anytime the disk drive  1  enters a state deemed to be inoperable, for example, the head  11  exceeds a specific boundary along a track  24  during a read or write operation. Processing of the PES  32  and the CDS  35  sample values may resume from a same counter count of the PES counter  49  and of the CDS counter  50 , such as described above for suspension during seek operations, once the error condition is removed. 
     Yet in other embodiments, the state of the servo vibe detect unit  38  may be allowed to become active only when the disk drive  1  is in a read or write retry state. The disk drive  1  may enter a read or write retry state when it has previously failed to perform a requested read or write operation, and is reattempting the same operation. In alternative embodiments, the state of the servo vibe detect unit  38  may become active only when both the disk drive  1  is in a read or write retry state and a servo vibration condition is detected. This would allow for restricting the use of the AFF signal  83  to situations where the external disturbances acting on the disk drive  1  are the likely cause for the failed read or write attempts. 
     In some embodiments of the present invention, once a servo vibration condition is detected and the state of the servo vibe detect unit  38  is set active, the state of the servo vibe detect unit  38  may become inactive after a certain number of consecutive summed and filtered PES and CDS values fall below some specific threshold value. In some embodiments this specific threshold value is a same value as a value used to detect whether a servo vibration condition existed. In other embodiments this specific threshold value may be higher or lower than the original threshold value used to detect whether a servo vibration condition existed. 
     In various embodiments of the present invention, once a servo vibration condition is detected and the state of the servo vibe detect unit  38  is set active, the state of the servo vibe detect unit  38  may become inactive after a certain percentage of summed and filtered PES and CDS values within some counter period fall below some specific threshold value. In some embodiments this specific threshold value is a same value used to detect whether a servo vibration condition existed. In other embodiments this specific threshold value may be higher or lower than the original threshold value used to detect whether a servo vibration condition existed. 
     In yet other embodiments of the present invention, once a servo vibration condition is detected and the state of the servo vibe detect unit  38  is set active, the state of the servo vibe detect unit  38  will remain active for some period of time known as a timeout period. After expiration of the timeout period, the servo vibration condition must be re-detected before the GAFF signal  82  is set equal to the AFF signal  83  so as to be added to the coarse actuator compensator signal  81 . The timeout period may start at a default period of, for example,  30  seconds or the like, and then later be raised or lowered by either the user, or automatically by the disk drive  1  itself if the disk drive  1  is subjected to a hostile environment (one with frequent or strong disruptive forces). Such a timeout period reduces a probability of the state of the servo vibe detect unit  38  from toggling back and forth between active and inactive states. It also may obviate the complication of having to add hysteresis to the PES comparator unit  53  and CDS comparator unit  54  threshold values. 
     Hysteresis is a method of temporarily lowering the threshold values of the comparator units  53 ,  54  until the PES filter  51  output values and the CDS filter  52  output values fall below the temporarily lowered threshold values. This typically helps to prevent toggling between active and inactive states by providing some margin for the filter  51 ,  52  output values to vary in. 
     In yet other embodiments, the GAFF signal  82  may be set equal to the AFF signal  83  by processing other track follow loop signals besides the PES  32  samples and CDS  35  samples, for example, the coarse actuator compensator signal  81 . 
     The embodiments disclosed herein are to be considered in all respects as illustrative, and not restrictive of the invention. The present invention is in no way limited to the embodiments described above. Various modifications and changes may be made to the embodiments without departing from the spirit and scope of the invention. The scope of the invention is indicated by the attached claims, rather than the embodiments. Various modifications and changes that come within the meaning and range of equivalency of the claims are intended to be within the scope of the invention.