Data storage device measuring laser protrusion fly height profile

Description

BACKGROUND

Data storage devices such as disk drives may 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 controller to control the actuator arm as it seeks from track to track.

Data is typically written to the disk by modulating a write current in an inductive coil to record magnetic transitions onto the disk surface in a process referred to as saturation recording. During readback, the magnetic transitions are sensed by a read element (e.g., a magnetoresistive element) and the resulting read signal demodulated by a suitable read channel. Heat assisted magnetic recording (HAMR) is a recent development that improves the quality of written data by heating the disk surface with a laser during write operations in order to decrease the coercivity of the magnetic medium, thereby enabling the magnetic field generated by the write coil to more readily magnetize the disk surface.

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.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of servo sectors defining servo tracks.

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.

FIG. 2B shows a head according to an embodiment comprising a laser configured to heat the disk during write operations based on a laser power.

FIG. 2C is a flow diagram according to an embodiment wherein after increasing the laser power for a first interval, the laser power is decreased and a fly height pattern is read from a first data segment to measure a fly height of the head.

FIG. 2D shows an embodiment wherein each servo sector comprises a servo preamble recorded at a first frequency, and the fly height pattern comprises a second frequency different from the first frequency.

FIGS. 3A and 3B show an embodiment wherein multiple fly height measurements are taken to characterize a protrusion fly height profile for the head at the beginning of write operations.

FIGS. 4A-4C illustrate an embodiment wherein a control setting applied to a fly height actuator (FHA) is adjusted in response to the measured fly heights in order to maintain a target fly height.

FIG. 5 shows a control setting profile for the FHA that achieves a target fly height at the beginning of write operations according to an embodiment.

FIGS. 6A and 6B illustrate an embodiment wherein the protrusion fly height profile is measured over multiple data segments spanning multiple servo sectors.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16 comprising a plurality of tracks 18, where each track comprises a plurality of servo sectors 20₀-20_N and at least one data segment between consecutive servo sectors. The disk drive further comprises a head 22 (FIG. 2B) actuated over the disk 16, the head 22 comprising a laser 24 configured to heat the disk 16 during write operations based on a laser power. Control circuitry 26 is configured to execute the flow diagram of FIG. 2C, wherein during a first revolution of the disk when the head reaches a first data segment (block 28), the laser power is first increased (block 30) over a first interval (block 32) to cause at least part of the head to protrude toward the first data segment. After the first interval, the laser power is decreased (block 34) and a fly height pattern is read from the first data segment to first measure a fly height of the head (block 36).

In the embodiment of FIG. 2A, the servo sectors 20₀-20_N define a plurality of servo tracks 18, wherein data tracks are defined relative to the servo tracks at the same or different radial density. The control circuitry 26 processes a read signal 38 emanating from the head 22 to demodulate the servo sectors 20₀-20_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 26 filters the PES using a suitable compensation filter to generate a control signal 40 applied to a voice coil motor (VCM) 42 which rotates an actuator arm 44 about a pivot in order to actuate the head 22 radially over the disk 16 in a direction that reduces the PES. The servo sectors 20₀-20_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. 2B, the head 22 comprises a suitable write element 46 (e.g., an inductive write coil), a suitable read element 48 (e.g., a magnetoresistive element), and a suitable fly height actuator (FHA) 50 (e.g., a thermal element). Any suitable laser 24 may also be employed in the head 22, such as a suitable laser diode, together with any other suitable optical components for implementing HAMR, such as a suitable waveguide and a suitable near field transducer for focusing the laser beam onto the surface of the disk 16. In one embodiment, when the laser power is increased, there is a heating effect that causes at least part of the head 22 to protrude toward the disk 16. That is, the heating effect due to increasing the laser power results in a protrusion fly height profile of the head 22 an example of which is shown in FIG. 3B. In one embodiment, the protrusion fly height profile due to the heating effect of the laser is measured and a corresponding control setting profile for the FHA 50 is generated in order to compensate for the change in fly height, thereby achieving a substantially constant fly height throughout a write operation.

In one embodiment shown in FIG. 3A, when the head 22 is over a target data segment during a first revolution of the disk, the control circuitry 26 configures the disk drive into a write mode including to increase the laser power to a write power as well as apply a write current to the write element 46 in order to write data to the first data segment. The combined heating effect of the laser 24 and the write element 46 causes at least part of the head 22 to protrude toward the first data segment for a first interval of the protrusion fly height profile shown in FIG. 3B. After the first interval, the control circuitry 26 configures the disk drive into a read mode (decreases the laser power and disables the write current) in order to read a suitable fly height pattern (FHP) from the first data segment. The control circuitry 26 processes the resulting read signal 38 to measure the fly height of the head 22 at a corresponding first point of the protrusion fly height profile as shown in FIG. 3B. In one embodiment, reading the fly height pattern almost immediately after terminating the write mode results in a fairly accurate fly height measurement before the protrusion recedes (due to thermal cooling). After measuring the fly height, the control circuitry 26 allows the head 22 to cool so that the thermal protrusion recedes to the initial fly height shown in FIG. 3B. During a second revolution of the disk, the control circuitry 26 again configures the disk drive into the write mode (increases the laser power and applies the write current) for a second interval longer than the first interval as shown in FIG. 3A. After the second interval, the control circuitry 26 configures the disk drive into the read mode (decreases the laser power and disables the write current) in order to read the FHP and generate a second fly height measurement for the protrusion fly height profile as shown in FIG. 3B. This process is repeated for incrementally longer intervals as shown in FIG. 3A until a sufficient number of fly height measurements have been taken in order to characterize the protrusion fly height profile as shown in FIG. 3B.

In one embodiment, the control setting applied to the FHA 50 is adjusted based on a control setting profile that compensates for the thermal protrusion of the head at the beginning of a write operation. For example, the control setting profile applied to the FHA 50 may attempt to maintain the head 22 at the same fly height prior to configuring the disk drive into the write mode. FIG. 4A illustrates an embodiment wherein the fly height measurement taken after the first interval of the write operation is used to generate a first FHA setting (FIG. 5) that attempts to compensate for the change in fly height. During a second revolution of the disk, the first FHA setting is applied to the FHA 50 during the first interval and a second fly height measurement is taken. If the second fly height measurement differs from the target fly height beyond a threshold, the first FHA setting is again adjusted and the process repeated until the first FHA setting causes the measured fly height to substantially match the target fly height. The control circuitry 26 then configures the disk drive into the write mode for the second interval as shown in FIG. 4B in order to make a corresponding fly height measurement. During the first interval of FIG. 4B, the control circuitry 26 applies the first FHA setting previously calibrated to the FHA 50 so that the fly height of the head 22 remains near the target fly height during the first interval. Accordingly, the fly height measurement taken after the second interval corresponds to the fly height deviation after the first interval until the end of the second interval. The fly height measurement after the second interval is used to adjust the second FHA setting of the profile shown in FIG. 5. During a subsequent revolution of the disk, the control circuitry 26 applies the first FHA setting during the first interval, and the second FHA setting during the remaining of the second interval in order to re-measure the fly height. This process is repeated until the first and second FHA settings result in a fly height that substantially matches the target fly height at the end of the second interval. This process is then repeated for the remaining intervals as shown in FIG. 4C in order to generate the complete control setting profile for the FHA 50 shown in FIG. 5.

In another embodiment, the control setting profile for the FHA such as shown in FIG. 5 may be generated directly from the measured protrusion fly height profile such as shown in FIG. 3B. In yet another embodiment, the control setting profile for the FHA 50 may be initialized based on the measured protrusion fly height profile such as shown in FIG. 3B, and then each control setting tuned until the measured fly height after each interval substantially matches the target fly height as described above. In another embodiment, the control circuitry 26 may generate the control setting profile for the FHA 50 with a suitable phase lead for one or more of the control settings in order to optimize the control setting profile.

In the embodiment of FIG. 3A, the fly height pattern (FHP) is recorded in a data segment that spans the area between consecutive servo sectors (SS). In this embodiment, the protrusion fly height profile may be measured without interrupting the write mode by reading a servo sector (and without cooling the head while reading the servo sector). In another embodiment, the protrusion fly height profile may be measured across a servo sector including to take into account the cooling effect when reading the servo sector. A control setting profile may also be generated for the FHA 50 corresponding to when a servo sector is encountered during a write operation. That is, during normal operation the control circuitry 26 may select the control setting profile for the FHA 50 based on when a write operation begins relative to the first encountered servo sector.

FIG. 6A shows an embodiment wherein the protrusion fly height profile may span two or more servo sectors (SS). That is, the protrusion of at least part of the head 22 at the beginning of a write operation due to thermal expansion may last through a number of servo sectors before settling to a steady state. Accordingly, in one embodiment the above described process for measuring the protrusion fly height profile may include intervals that span multiple servo sectors, including the cooling effect of reading each servo sector. Also in this embodiment the protrusion fly height profile may be measured at different starting locations relative to the first encountered servo sector so that the cooling effect of the servo sectors on the protrusion fly height profile as shown in FIG. 6A may be taken into account.

FIG. 6B illustrates an embodiment that may facilitate measuring the protrusion fly height profile at any given offset relative to the first encountered servo sector. In this embodiment, a suitable fly height pattern is written in the data segments of a first data track 52. The control circuitry 26 then positions the read element 48 over the first data track 52 and the write element 46 over a second data track 54. During subsequent revolutions of the disk, the control circuitry 26 configures the disk drive into the write mode over varying intervals and then reads the fly height pattern to generate the protrusion fly height profile as described above. Since the fly height pattern is written over multiple data segments of the first data track 52, the fly height measurement may be taken at essentially any desired offset relative to the first encountered servo sector, and therefore in one embodiment the protrusion fly height profile may be measured at a number of different circumferential offsets relative to the servo sectors. Since in one embodiment the write element 46 is offset radially from the read element 48, the write element 46 does not erase the fly height pattern in the first data track 52 while writing data to the second data track 54.

Any suitable fly height pattern may be written to the disk in the embodiments described above, and any suitable technique may be used to measure the fly height by reading the fly height pattern. In one embodiment, the fly height pattern comprises a sequence of periodic transitions that results in one or more sinusoids in the read signal 38. The read signal 38 may be processed, for example, by evaluating a ratio of harmonics in the read signal according to a Wallace spacing equation in order to measure the fly height of the head over the disk. In one embodiment, writing the fly height pattern to a data segment of a data track enables the control circuitry 26 to write any desired pattern at any desired frequency (or frequencies) in order to facilitate the fly height measurement. For example, FIG. 2D illustrates an embodiment wherein the control circuitry 26 may write a fly height pattern to the disk comprising a frequency that differs from a frequency of the servo preamble in a servo sector. Accordingly in this embodiment the control circuitry 26 may generate the fly height measurement by reading any desired fly height pattern rather than be constrained to the frequency of the servo preamble. In addition, reading the fly height pattern from a data segment allows the cooling effect of the servo sectors on the protrusion fly height profile to be measured at any desired circumferential offset as described above.

In the embodiment described above with reference to FIG. 3A, the control circuitry 26 may measure the protrusion fly height profile by increasing the laser power over a data segment between consecutive servo sectors (at varying write intervals). This embodiment allows the head 22 to cool after terminating the write mode while the disk finishes the current revolution before again increasing the laser power during a subsequent revolution. In another embodiment, the head 22 may cool sufficiently within a partial revolution of the disk, and therefore the control circuitry 26 may make multiple fly height measurements (after different write intervals) during a single revolution of the disk. That is, the control circuitry 26 may read a fly height pattern from different parts of the same data track during a single revolution of the disk in order to expedite measuring the protrusion fly height profile as described above.

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.