# Patent

## Title
Disk drive employing fly height calibration tracks to account for magnetic entropy and thermal decay

## Abstract

A disk drive is disclosed comprising a head actuated over a disk comprising a plurality of tracks including a first fly height calibration track and a second fly height calibration track. The disk drive further comprises a fly height actuator operable to control a fly height of the head in response to a fly height control signal. When a refresh event is detected, calibration data is written to the second fly height calibration track. After writing calibration data to the second fly height calibration track, the first fly height calibration track is read in order to calibrate the fly height control signal applied to the fly height actuator. After calibrating the fly height control signal, the second fly height calibration track is read in order to recalibrate the fly height control signal applied to the fly height actuator.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority from provisional U.S. Patent Application Ser. No. 61/376,009, filed on Aug. 23, 2010, the specification of which is incorporated herein by reference.


BACKGROUND
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
During a write operation, a current is applied to a write element of the head (e.g., a write coil) to create a magnetic field which magnetizes the surface of the disk by orienting the direction of magnetic grains (e.g., horizontally in longitudinal magnetic recording, or vertically in perpendicular magnetic recording). The orientation of the grains exhibits hysteresis thereby generating their own magnetic field when the write magnetic field is removed. During a read operation, a read element of the head (e.g., a magnetoresistive element) transduces the magnetic field emanating from the disk surface into a read signal that is demodulated into an estimated data sequence.
The hysteresis of the magnetic grains is not permanent meaning that over time the grains will orientate into random directions (magnetic entropy) until the magnetic field is no longer sensed reliably (leading to data errors during reproduction). Magnetic entropy may also be precipitated by various factors, such as increasing ambient temperature. That is, at higher temperatures the uniform alignment of the grains will degrade faster. Another factor that precipitates magnetic entropy is a phenomenon referred to as adjacent track interference (ATI) wherein when writing data to a target track, the fringe field from the write element degrades the uniform alignment of the grains recorded in an adjacent track. The fringe field from the write element may also adversely affect a wider range of tracks with respect to the written track, a phenomena referred to as wide area track erasure or WATER. The degrading effect of ATI on the adjacent tracks as well as WATER on the near adjacent tracks compounds over time with each write operation to the target track. Eventually, the magnetic field emanating from the disk surface will deteriorate to the point that the data is no longer recoverable.
FIG. 1 shows a prior art disk format 2 comprising a number of data tracks 4 defined by concentric servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art disk format comprising a plurality of servo sectors that define a plurality of tracks.
FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head actuated over a disk, a fly height actuator, and control circuitry.
FIG. 2B shows an embodiment of the present invention wherein the disk comprises a first fly height calibration track and a second fly height calibration track.
FIG. 2C is a flow diagram according to an embodiment of the present invention wherein when a refresh event is detected, calibration data is written to a second fly height calibration track after which a first calibration track is used to calibrate a fly height control signal at least once before using the second fly height calibration track to calibrate the fly height control signal.
FIG. 3A is a flow diagram according to an embodiment of the present invention wherein after writing calibration data to the second fly height calibration track, using the second fly height calibration track is delayed for a period of time to account for thermal decay.
FIG. 3B shows an embodiment of the present invention showing that the thermal decay may vary based on an ambient temperature of the disk drive.
FIG. 3C is a flow diagram according to an embodiment of the present invention wherein the delay interval before using the second fly height calibration track is adjusted based on a measured ambient temperature of the disk drive.
FIG. 4 shows an embodiment of the present invention wherein a plurality of refresh zones are defined on the disk, and each refresh zone comprises first and second fly height calibration tracks.
FIG. 5 is a flow diagram according to an embodiment of the present invention wherein prior to refreshing a refresh zone, calibration data is written to the second fly height calibration track.



DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head 16 actuated over a disk 18 comprising a plurality of tracks 20 including a first fly height calibration track and a second fly height calibration track (FIG. 2B). The disk drive further comprises a fly height actuator operable to control a fly height of the head 16 in response to a fly height control signal 22, and control circuitry 24 operable to execute the flow diagram of FIG. 2C. When a refresh event is detected (step 26), calibration data is written to the second fly height calibration track (step 28). After writing calibration data to the second fly height calibration track, the first fly height calibration track is read in order to calibrate the fly height control signal applied to the fly height actuator (step 30). After calibrating the fly height control signal, the second fly height calibration track is read in order to recalibrate the fly height control signal applied to the fly height actuator (step 32).
The disk 18 shown in the embodiment of FIG. 2A comprises a plurality of servo sectors 340-34N that define the tracks 20. The control circuitry 24 processes a read signal 36 emanating from the head 16 to demodulate the servo sectors 340-34N into a position error signal (PES) representing a position error of the head relative to a target data track. The control circuitry 24 comprises a servo compensator for filtering the PES to generate a control signal 38 applied to a voice coil motor (VCM) 40 that rotates an actuator arm 42 about a pivot in order to actuate the head 16 radially over the disk 18 in a direction that reduces the PES.
Any suitable fly height actuator may be employed, such as a heater which controls fly height through thermal expansion, or a piezoelectric (PZT) actuator integrated with the head 16. In one embodiment, an operating fly height control signal used during normal operation of the disk drive is determined by decreasing the fly height of the head 16 (by adjusting the fly height control signal 22) until the head 16 contacts the disk 18 (touchdown). The operating fly height control signal may then be determined, for example, relative to the fly height control signal applied to the fly height actuator that causes the touchdown. In one embodiment, the operating fly height control signal is determined by backing off the fly height control signal that causes the touchdown. In another embodiment, a suitable technique may be employed to measure the fly height of the head 16 relative to when the touchdown is detected, and then select the operating fly height control signal that achieves a target fly height. For example, the fly height control signal applied to the fly height actuator is correlated with the measured fly height based on when touchdown is detected so that an operating fly height control signal can be selected that will achieve a target fly height.
During the touchdown operation, the touchdown of the head may be detected in any suitable manner. In one embodiment, a microactuator may be integrated with the head (e.g., in a gimbal that couples a read/write element to a slider), or integrated with a suspension that couples a gimbal to the actuator arm 42, or mounted on the actuator arm 42, etc. The microactuator may be dedicated for detecting head touchdown, or it may also be used to actuate the head over the disk in fine movements during normal operation. Other embodiments may detect head touchdown by evaluating a temperature signal generated by a temperature sensor integrated with the head, wherein when head touchdown occurs, the temperature signal will exceed a threshold. Other techniques for detecting head touchdown include evaluating the amplitude of the read signal (e.g., when reading the servo bursts), evaluating the PES for serving the head, and/or evaluating a servo wedge-to-wedge timing, all of which may be affected by head touchdown.
After executing the touchdown operation in order to initialize the operating fly height control signal 22, a fly height calibration track is read in order to initialize a fly height reference that corresponds to the initial operating fly height control signal. Over time, it may be necessary to recalibrate the operating fly height control signal in order to adjust the operating fly height of the head 16 to compensate for fly height deviation due, for example, to changing environmental conditions (e.g., temperature or pressure), or to changes in the response of the fly height actuator. The operating fly height control signal may be recalibrated in response to any suitable event, such as over a periodic interval, or if a touchdown of the head is detected during normal operation, or if an environmental sensor detects a change in an environmental condition that affects the fly height (e.g., a change in temperature or pressure). In order to recalibrate the operating fly height control signal, the fly height calibration track is read to estimate the fly height of the head relative to the fly height reference calibrated after the touchdown operation. However, the fly height calibration track may degrade over time due, for example, to general magnetic entropy or to magnetic interference from write operations of near adjacent tracks.
Degradation of the fly height calibration track can be overcome by refreshing the track (rewriting the calibration data). However, after rewriting the fly height calibration track the calibration data will begin to degrade immediately due to thermal decay which induces error in the measured fly height until the thermal decay settles out. To compensate for this thermal decay, in one embodiment of the present invention at least two fly height calibration tracks are employed (FIG. 2B). When the first calibration track begins to degrade significantly (due to magnetic entropy or interference from writes to near adjacent tracks), the second fly height calibration track is refreshed while continuing to use the first calibration track to perform fly height calibration. After the thermal decay interval associated with refreshing the second fly height calibration track, the fly height calibration process switches from the first to the second fly height calibration track. A similar process is then repeated when the second fly height calibration track degrades significantly, that is, the first fly height calibration track is refreshed and then the fly height calibration process switches to using the first fly height calibration track after the thermal decay interval.
This embodiment is understood with reference to the flow diagram of FIG. 3A wherein the fly height calibration is initially configured to use the first fly height calibration track. When a refresh event is detected (step 26), the second fly height calibration track is refreshed by writing calibration data to the track (step 28). The refresh event may be detected in any suitable manner, such as by detecting the expiration of a refresh interval. In another embodiment, the refresh event is detected when degradation of the first fly height calibration track is detected. The degradation may be detected in any suitable manner, such as by detecting performance degradation of the disk drive (e.g., increased error rate or degrading channel metrics) after calibrating the fly height control signal. After refreshing the second fly height calibration track (step 28), the first fly height calibration track is used to calibrate the fly height control signal at least once (step 30) while waiting for the thermal decay to settle out for the second fly height calibration track (step 44). The fly height calibration process then switches to using the second fly height calibration track before the first fly height calibration track degrades too much (step 32).
FIG. 3B illustrates an embodiment of the present invention wherein the thermal decay interval after refreshing the fly height calibration track may vary based on the ambient temperature of the disk drive. In one embodiment shown in the flow diagram of FIG. 3C, after refreshing the second fly height calibration track (step 28) the ambient temperature of the disk drive is measured (step 46) and the delay interval corresponding to the expected thermal decay is adjusted based on the measured ambient temperature (step 48). After waiting for the adjusted delay interval (step 50), the fly height calibration process switches to using the second fly height calibration track (step 32).
Any suitable technique may be employed to estimate the fly height by reading the fly height calibration track, including to generate a fly height estimate representing the fly height reference after executing the touchdown operation. In one embodiment, the fly height estimate is generated using a harmonic ratio (HR) method. After writing an NT pattern (e.g., 2T pattern) to the fly height calibration track, a fly height estimate is generated by reading the fly height calibration track and measuring an amplitude of the read signal at a first harmonic (VA) and at a second harmonic (VB). A log of the ratio (VA/VB) is then multiplied by (λ/4π), where λ is the frequency of the NT read signal in the spatial domain (i.e., velocity/write_frequency). In one embodiment, the first harmonic is the fundamental harmonic of the NT read signal, and the second harmonic is the third harmonic of the NT read signal. The NT pattern may be written in the user data area between the servo sectors (FIG. 1A) and the HR fly height estimate generated at any suitable resolution (e.g., once per data sector).
FIG. 4 shows an embodiment of the present invention wherein a plurality of refresh zones are defined on the disk, and each refresh zone comprises a plurality of tracks including first and second fly height calibration tracks. The data recorded in the tracks will degrade over time due to magnetic entropy and interference from writes to adjacent and near adjacent tracks. To compensate for this degradation, the control circuitry refreshes the data in each refresh zone when needed, for example, after a refresh interval and/or after the number of write operations in a refresh zone exceeds a threshold. Prior to refreshing one of the refresh zones, calibration data is written to the second fly height calibration track so that the second fly height calibration track can be used to calibrate the fly height control signal prior to refreshing the zone.
This embodiment is understood with reference to the flow diagram of FIG. 5 wherein the fly height control signal is calibrated using the first fly height calibration track (step 52). Data is written to the tracks of the refresh zones (step 54), and when one of the refresh zones nears a refresh event (step 56), calibration data is written to the second fly height calibration track (step 58). The fly height calibration process continues to use the first fly height calibration track (step 60) in order to write data to the refresh zone (step 62) during the thermal decay interval. When the refresh zone is ready to be refreshed (step 64), the fly height control signal is calibrated using the second fly height calibration track (step 66) and the refresh zone is refreshed (step 68).
Any suitable control circuitry may be employed to implement the flow diagrams in the embodiments of the present invention, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain steps described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into an SOC.
In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the steps of the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.

## Claims

1. A disk drive comprising:
a disk comprising a plurality of tracks including a first fly height calibration track and a second fly height calibration track;
a head actuated over the disk;
a fly height actuator operable to control a fly height of the head in response to a fly height control signal; and
control circuitry operable to:
when a refresh event is detected, write calibration data to the second fly height calibration track;
after writing calibration data to the second fly height calibration track, read the first fly height calibration track in order to calibrate the fly height control signal applied to the fly height actuator; and
after calibrating the fly height control signal, read the second fly height calibration track in order to recalibrate the fly height control signal applied to the fly height actuator.

2. The disk drive as recited in claim 1, wherein the control circuitry is further operable to detect the refresh event by detecting a degradation of calibration data recorded in the first fly height calibration track.

3. The disk drive as recited in claim 1, wherein the disk further comprises guard band tracks adjacent at least one of the first and second fly height calibration tracks.

4. The disk drive as recited in claim 1, wherein prior to reading the second fly height calibration track the control circuitry is further operable to wait for a delay interval corresponding to a thermal decay of the second fly height calibration track.

5. The disk drive as recited in claim 4, wherein the control circuitry is further operable to:
measure an ambient temperature; and
adjust the delay interval based on the measured ambient temperature.

6. The disk drive as recited in claim 1, wherein:
the control circuitry is further operable to define a plurality of refresh zones, wherein each refresh zone comprises a plurality of tracks;
each refresh zone comprises first and second fly height calibration tracks; and
the control circuitry is further operable to refresh data recorded in the tracks of a refresh zone by reading and rewriting the data.

7. The disk drive as recited in claim 6, wherein the control circuitry is further operable to write the calibration data to the second fly height calibration track of a first refresh zone prior to refreshing the first refresh zone.

8. The disk drive as recited in claim 7, wherein after writing the calibration data to the second fly height calibration track, the control circuitry is further operable to wait for a delay interval prior to refreshing the first refresh zone.

9. The disk drive as recited in claim 8, wherein the delay interval corresponds to a thermal decay of the second fly height calibration track.

10. The disk drive as recited in claim 8, wherein after the delay interval the control circuitry is further operable to read the second fly height calibration track in order to calibrate the second control signal applied to the fly height actuator.

11. A method of operating a disk drive comprising a head actuated over a disk comprising a plurality of tracks including a first fly height calibration track and a second fly height calibration track, and a fly height actuator operable to control a fly height of the head in response to a fly height control signal, the method comprising:
when a refresh event is detected, writing calibration data to the second fly height calibration track;
after writing calibration data to the second fly height calibration track, reading the first fly height calibration track in order to calibrate the fly height control signal applied to the fly height actuator; and
after calibrating the fly height control signal, reading the second fly height calibration track in order to recalibrate the fly height control signal applied to the fly height actuator.

12. The method as recited in claim 11, further comprising detecting the refresh event by detecting a degradation of calibration data recorded in the first fly height calibration track.

13. The method as recited in claim 11, wherein the disk further comprises guard band tracks adjacent at least one of the first and second fly height calibration tracks.

14. The method as recited in claim 11, wherein prior to reading the second fly height calibration track the method further comprises waiting for a delay interval corresponding to a thermal decay of the second fly height calibration track.

15. The method as recited in claim 14, further comprising:
measuring an ambient temperature; and
adjusting the delay interval based on the measured ambient temperature.

16. The method as recited in claim 11, wherein:
the method further comprises defining a plurality of refresh zones, wherein each refresh zone comprises a plurality of tracks;
each refresh zone comprises first and second fly height calibration tracks; and
the method further comprises refreshing data recorded in the tracks of a refresh zone by reading and rewriting the data.

17. The method as recited in claim 16, further comprising writing the calibration data to the second fly height calibration track of a first refresh zone prior to refreshing the first refresh zone.

18. The method as recited in claim 17, wherein after writing the calibration data to the second fly height calibration track, the method further comprises waiting for a delay interval prior to refreshing the first refresh zone.

19. The method as recited in claim 18, wherein the delay interval corresponds to a thermal decay of the second fly height calibration track.

20. The method as recited in claim 18, wherein after the delay interval the method further comprises reading the second fly height calibration track in order to calibrate the second control signal applied to the fly height actuator.