Data storage device attenuating thermal decay effect on fly height measurement

Description

BACKGROUND

Data storage devices such as 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 servo sectors. The 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 actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6₀-6_N recorded around the circumference of each servo track. Each servo sector 6_i 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 symbol 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 6_i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.

An air bearing forms between the head and the disk due to the disk rotating at high speeds. Since the quality of the write/read signal depends on the fly height of the head, conventional heads (e.g., a magnetoresistive heads) may comprise an actuator for controlling the fly height. Any suitable fly height actuator may be employed, such as a heater which controls fly height through thermal expansion, or a piezoelectric (PZT) actuator which controls the fly height through mechanical deflection. A dynamic fly height (DFH) servo controller may measure the fly height of the head and adjust the fly height actuator to maintain a target fly height during write/read operations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising servo tracks defined by servo sectors.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk, and a fly height actuator (FHA) for controlling a fly height of the head over the disk.

FIG. 2B is a flow diagram according to an embodiment for estimating a change in the fly height of the head in a manner that compensates for a thermal decay effect of a spacing pattern written on the disk.

FIG. 3A illustrates a thermal decay rate of different frequency spacing patterns according to an embodiment.

FIG. 3B illustrates FHA actuation curves for different frequency spacing patterns according to an embodiment.

FIG. 3C illustrates an embodiment for generating a first constant for an equation that estimates the change in fly height of the head according to an embodiment.

FIG. 3D illustrates an embodiment for generating a second constant for the equation that estimates the change in fly height of the head according to an embodiment.

FIG. 4 is a flow diagram according to an embodiment for generating the second constant.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive comprising a head 16 actuated over a disk 18, and control circuitry 20 configured to execute the flow diagram of FIG. 2B. A first spacing pattern is written to the disk and a second spacing pattern is written to the disk different from the first spacing pattern (block 22). A first fly height measurement (FHM1_1) is generated by reading the first spacing pattern and a first fly height measurement (FHM2_—1) is generated by reading the second spacing pattern (block 24). After an interval (block 26), a second fly height measurement (FHM1_—2) is generated by reading the first spacing pattern and a second fly height measurement (FHM2_—2) is generated by reading the second spacing pattern (block 28). A change in the fly height of the head is estimated (block 30) based on:
(ΔFHM2−B·ΔFHM1)(A−B)
where ΔFHM1 represents a difference between FHM1_—1 and FHM1_—2, ΔFHM2 represents a difference between FHM2_—1 and FHM2_—2, and A and B are constants.

In the embodiment of FIG. 2A, the disk 18 comprises a plurality of servo tracks 32 defined by servo sectors 34₀-34_N, wherein data tracks are defined relative to the servo tracks at the same or different radial density. The control circuitry 20 processes a read signal 36 emanating from the head 16 to demodulate the servo sectors 34₀-34_N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 20 filters the PES using a suitable compensation filter to generate a control signal 38 applied to a voice coil motor (VCM) 40 which 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. The servo sectors 34₀-34_N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern.

In the embodiment of FIG. 2A, the head 16 comprises a suitable fly height actuator (FHA) configured to control a fly height of the head 16 over the disk 18. Any suitable fly height actuator may be employed, such as a piezoelectric actuator that actuates through mechanical deflection, or a heater that actuates through thermal expansion. In one embodiment, it may be desirable to maintain the fly height of the head 16 at a target fly height during access operations (write/read) so as to optimize the recording quality and detection accuracy of the disk drive. Accordingly, the fly height of the head 16 may be measured and an FHA setting 44 (e.g., voltage or current) applied to the FHA of the head 16 may be adjusted until the measured fly height substantially equals a target fly height.

In one embodiment, the fly height measurement (FHM) may be computed using a harmonic ratio technique (Wallace spacing equation) that measures an absolute head-media spacing according to the ratio of the amplitude of the read signal at two different harmonics while reading a spacing pattern from the disk 18 (e.g., a test pattern or a pattern recorded in a servo sector). In one embodiment, this harmonic ratio can be generated by reading a spacing pattern at two different frequencies (e.g., a 2T and 4T pattern). However, for a given fly height of the head the FHM generated based on the harmonic ratio equation will change over time due to the spacing pattern degrading (referred to as thermal decay). Accordingly, in one embodiment the FHM is generated in a manner that compensates for the degradation of the spacing pattern over time.

In one embodiment, the change in the FHM over time may vary depending on the frequency of the spacing pattern due to a varying rate of thermal decay at different frequencies of the spacing pattern. This embodiment is illustrated in FIG. 3A wherein the FHM decay rate may increase as the frequency of the spacing pattern increases. In one embodiment, a FHM may be generated for two spacing patterns written at different frequencies (e.g., 2T/4T and 2T/6T) and the result used to generate a final FHM that compensates for the thermal decay of the spacing patterns.

In one embodiment, for a given fly height of the head, the FHM generated based on the harmonic ratio technique may also vary based on the frequency of the spacing pattern used to generate the FHM. This embodiment is illustrated in FIG. 3B which shows actuation curves for different spacing patterns, wherein the x-axis represents different FHA settings corresponding to different physical fly heights for the head and the y-axis represents the corresponding FHM generated for different frequency spacing patterns. The actuation curves shown in FIG. 3B may be converted into a correlation of FHM for two different spacing patterns as shown in FIG. 3C. That is, for two different frequency spacing patterns (e.g., 2T/4T and 2T/6T) a constant A may be computed representing a linear correlation between the FHM generated for each spacing pattern. This constant A may be used to represent a relationship between the change in FHM for the spacing patterns relative to a physical change d in the fly height of the head:
ΔFHM2(d)=A·ΔFHM1(d)  Eq. (1)

A similar correlation between the thermal decay impact on the FHM computed for two different frequency spacing patterns may be generated by measuring the FHM for each frequency spacing pattern at different intervals. An example of this embodiment is illustrated in FIG. 3D wherein an FHM is generated at three different intervals for five different frequency spacing patterns. From these measurements, a linear relationship may be generated between the thermal decay impact on the FHM generated for two different frequency spacing patterns relative to time t for the same physical fly height of the head:
FHM2_—TD(t)=B·FHM1_—TD(t)  Eq. (2)

From the above equations (1) and (2), when the FHM changes due to a physical change d in the fly height of the head as well as due to thermal decay of the spacing pattern over time t, the following relationships may be derived:
ΔFHM1(d,t)=ΔFHM1(d)+FHM1_—TD(t)
ΔFHM2(d,t)=ΔFHM2(d)+FHM2_—TD(t)
ΔFHM2(d,t)=A·ΔFHM1(d)+B·FHM1_—TD(t)  Eq. (3)
From the above equation (3), a change in the fly height of the head may be estimated in a manner that compensates for the thermal decay of the spacing patterns based on:
ΔFHM1(d)=(ΔFHM2−B·ΔFHM1)/(A−B)  Eq. (4)
That is, when the physical fly height of the head changes by d, in one embodiment the change in the FHM may be measured based on the change in the FHM generated for the first spacing pattern ΔFHM1 and the change in the FHM generated for the second spacing pattern ΔFHM2 which are input to the above equation (4), wherein the resulting estimate of the change in FHM takes into account the impact that thermal decay has on the spacing patterns.

FIG. 4 is a flow diagram according to an embodiment for generating the constant B in the above equation (4). At a first operating temperature T1, the first spacing pattern and the second spacing pattern are written to the disk (block 46) and a first fly height measurement (FHM1_T1) is generated by reading the first spacing pattern and a first fly height measurement (FHM2_T1) is generated by reading the second spacing pattern (block 48). After generating FHM1_T1 and FHM2_T1, the operating temperature of the disk drive is increased to T2 and the disk drive is operated at the increased temperature T2 for a period of time (block 50), such as a period of hours, in order to enhance the thermal decay effect of the spacing patterns. After the interval at the second temperature T2, a second fly height measurement (FHM1_T2) is generated by reading the first spacing pattern and a second fly height measurement (FHM2_T2) is generated by reading the second spacing pattern (block 52). The B constant is then generated at block 54 according to:
B=(FHM2_—T2−FHM2_—T1)/(FHM1_—T2−FHM1_—T1).

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, 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 operations 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 a SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform 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.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.