Data storage device calibrating parameter for heat assisted magnetic recording

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional U.S. Patent Application Ser. No. 62/156,136, filed on May 1, 2015, which is hereby incorporated by reference in its entirety.

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 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.

FIG. 2B shows a head according to an embodiment comprising a laser configured to heat the disk while writing data to the disk and a fly height actuator configured to actuate the head vertically over the disk.

FIG. 2C is a flow diagram according to an embodiment wherein during a calibration procedure a touchdown capable calibration power is applied to the laser for a first interval too short to cause the head to contact the disk.

FIG. 3A illustrates an embodiment wherein test data is written to the disk while applying the calibration power to the laser during the first interval, wherein the head would eventually contact the disk if the calibration power were applied to the laser for a second interval longer than the first interval.

FIG. 3B is a flow diagram according to an embodiment wherein test data is written to and read form the disk over a number of different calibration powers to generate a plurality of metrics, and a control signal applied to the fly height actuator is adjusted based on the metrics.

FIG. 4A illustrates an embodiment wherein the control signal applied to the fly height actuator is adjusted based on a difference between a measured peak in the metrics and a target peak.

FIG. 4B illustrates an embodiment wherein the control signal applied to the fly height actuator is adjusted based on a difference between a measured saturation in the metrics and a target saturation.

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 actuated over a disk 18, wherein the head 16 (FIG. 2B) comprises a laser 20 configured to heat the disk 18 while writing data to the disk 18. The disk drive further comprises control circuitry 22 configured to execute the flow diagram of FIG. 2C, wherein a write power for the laser is calibrated and applied to the laser while writing user data to the disk (block 24). A calibration power is applied to the laser for a first interval, wherein the calibration power is high enough to cause the head to contact the disk if applied for a second interval longer than the first interval (block 26). While applying the calibration power to the laser, test data is written to the disk during at least part of the first interval (block 28). The test data is read from the disk to generate a read signal, and a metric is generated based on the read signal (block 30).

In the embodiment of FIG. 2B, the head 16 comprises a suitable write element 32 (e.g., an inductive coil), a suitable read element 34 (e.g., a magnetoresistive element), and a suitable fly height actuator (FHA) 36 configured to actuate the head 16 vertically over the disk 18. Any suitable FHA 36 may be employed, such as a heater that actuates through thermal expansion, or a piezoelectric actuator that actuates through mechanical deflection.

In the embodiment of FIG. 2A, servo sectors 38₀-38_N define a plurality of servo tracks 30, wherein data tracks are defined relative to the servo tracks at the same or different radial density. In an embodiment where the servo sectors 38₀-38_N are recorded at the same data rate, the servo sectors 38₀-38_N form servo wedges that extend radially across the disk 18 as shown in FIG. 2A. Other embodiments may employ zoned servo sectors wherein the data rate may vary across the radius of the disk, thereby forming servo wedges within each servo zone. The control circuitry 22 processes a read signal 40 emanating from the head 16 to demodulate the servo sectors 38₀-38_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 22 filters the PES using a suitable compensation filter to generate a control signal 42 applied to a voice coil motor (VCM) 44 which rotates an actuator arm 46 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 38₀-38_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 (e.g., as shown in FIG. 1).

In one embodiment, it may be desirable to calibrate one or more parameters that affect the quality of the recorded data, and the therefore the accuracy in recovering the recorded data. For example, in one embodiment the quality of the recorded data may depend on the fly height of the head 16 over the disk 18 when writing the data. The fly height may affect the efficacy of the laser 20 to heat the surface of the disk 18 while maintaining a target track width (and corresponding density) for the data tracks. Accordingly, in one embodiment a control signal applied to the FHA 36 (FIG. 2B) may be calibrated in order to achieve a target fly height during write operations. In some embodiments, certain environmental conditions, such as the ambient temperature, or other factors (such as degradation of the laser), may affect the fly height of the head 16 over time. In one embodiment, it may be desirable to detect when the fly height of the head 16 has deviated from the target fly height so that the control signal applied to the FHA may be adjusted and/or so that the write power for the laser may be adjusted.

In one embodiment, a calibration power may be applied to the laser 20 during a calibration procedure, wherein during a first interval test data may be written to a target data track such as shown in FIG. 3A. At the end of the first interval, the writing of the test data terminates and the power applied to the laser is reduced in order to avoid the head 16 contacting the disk 18. That is, in one embodiment the calibration power applied to the laser 20 is high enough such that the thermal protrusion of the head components over time (i.e., for a second interval longer than the first interval) would eventually cause the head 16 to contact the disk 18 as illustrated in FIG. 3A. The test data is then read from the disk 18 to generate a suitable metric based on the read signal. For example, in one embodiment the metric may comprise the amplitude of the read signal after reading at least part of the test data as illustrated in FIG. 3A. In one embodiment, the process of writing/reading the test pattern may be repeated at different calibration power levels and a corresponding metric generated at each calibration power level. The resulting metrics may then be evaluated to calibrate a parameter of the disk drive, such as the control signal applied to the FHA 36 in order to maintain a target fly height of the head during write operations.

An example of this embodiment is understood with reference to the flow diagram of FIG. 3B in view of the read signal amplitude waveforms shown in FIGS. 4A and 4B. After calibrating the write power for the laser (block 48), the calibration power for the laser is initialized to a value different than the write power, such as less than the write power (block 50). While applying the calibration power to the laser for a first interval (block 52), test data is written to the disk during at least part of a first interval (block 54). The test data is then read to generate a read signal, and the amplitude of the read signal after reading at least part of the test data is measured and saved (block 56). The calibration power is then adjusted (block 58), such as by increasing the calibration power, and the process is repeated from block 52 until a sufficient number of read signal amplitude measurements have been taken (block 60). The control signal applied to the FHA is then adjusted based on the saved read signal amplitudes (block 62).

Referring to the example shown in FIG. 4A, the dashed waveform 64 may represent the read signal amplitude described above measured by reading the test data from the disk written using different calibration power levels. When the calibration power applied to the laser is low, the read signal amplitude is low due to an undersaturation of the media. As the calibration power increases, the read signal amplitude increases, and as shown in the example embodiment of FIG. 4A, the read signal amplitude may eventually reach a peak at a calibration power that is greater than the write power for the laser (i.e., the write power calibrated for normal write operations). In the embodiment of FIG. 4A, the read signal amplitude begins to decrease as the calibration power applied to the laser is further increased. As the calibration power is increased past the write power, the calibration power will reach a level that will cause the head to contact the disk if applied to the laser for too long (past the first interval) as described above. Accordingly, in one embodiment the first interval is selected to ensure the head 16 will not contact the disk 18 while enabling the test data to be written at higher laser powers so that the desired metric (e.g., read signal amplitude) may be measured.

In one embodiment, at least part of the dashed waveform 64 shown in FIG. 4A may be generated by executing the flow diagram of FIG. 3B after calibrating the control signal applied to the FHA to achieve the target fly height as well as calibrating the write power for the laser. In this manner the peak in the dashed waveform 64 may correspond to a target inflection point when the head 16 is flying over the disk 18 at the target fly height during write operations. In one embodiment, the flow diagram of FIG. 3B may be re-executed periodically or reactively in order to verify that the head is still flying at the target fly height during write operations. For example, in some embodiments the control circuitry may react to a temperature sensor, or to a quality metric associated with the read signal, when deciding whether to re-execute the flow diagram of FIG. 3B. In one embodiment, the control circuitry may seek the head to a calibration track in order to re-execute the flow diagram of FIG. 3B so that previously recorded user data is not overwritten by the test data.

The solid waveform 66 in FIG. 4A shows an embodiment wherein the inflection point (measured peak in this example) that is generated after re-executing the flow diagram of FIG. 3B shifts left from the target inflection point (target peak in this example) due to the head flying too low. That is, the calibrated power corresponding to the measured peak in waveform 66 occurs at a lower power than the calibrated power corresponding to the target peak in waveform 64. In one embodiment, the difference in the calibration powers (i.e., the delta) may be processed in order to adjust the control signal applied to the FHA 36. Alternatively, the delta exceeding a threshold may trigger a recalibration of the control signal applied to the FHA 36 using any suitable technique.

In another embodiment, the amplitude of the read signal at the measured peak in the waveform 66 may deviate from the amplitude at the target peak in the target waveform 64 as shown in the example of FIG. 4A. Accordingly, in one embodiment the difference in the read signal amplitude at the measured peak relative to the target peak may be used to adjust the control signal applied to the FHA 36 or trigger a recalibration of the control signal.

FIG. 4B illustrates another embodiment wherein the read signal amplitude waveforms (e.g., target waveform 68 and deviant waveform 70) may exhibit an inflection point other than a peak. In this example, the inflection point comprises a saturation point of the read signal amplitude which may be measured in any suitable manner, such as by measuring when the slope of the read signal amplitude approaches zero. Accordingly in this embodiment the control signal applied to the FHA 36 may be adjusted, or a recalibration of the control signal triggered, based on the delta between the target saturation point and the measured saturation point after re-executing the flow diagram of FIG. 3B.

In one embodiment, the metric generated at block 30 of FIG. 2C may be used to adjust the write power for the laser, or may be used to trigger a recalibration of the write power using any suitable technique. That is, in one embodiment the deviation in the inflection point such as shown in the examples of FIGS. 4A and 4B may be compensated by adjusting the write power applied to the laser instead of, or in addition to, adjusting the control signal applied to the FHA 36. For example, if a change in the ambient temperature causes the head to deviate from the target fly height, it may be desirable to adjust the physical fly height of the head (by adjusting the control signal applied to the FHA 36) to maintain the target fly height as well as adjust the write power for the laser since the ambient temperature may also affect the output power of the laser. Accordingly, in one embodiment the control circuitry may be configured to adjust (or recalibrate) the write power for the laser after adjusting the control signal applied to the fly height actuator.

The metric generated at block 30 of FIG. 2C may be used for any suitable reason, such as to adjust the control signal applied to a FHA 36 and thereby adjust the fly height of the head 16 as described above. In one embodiment, one or more other parameters may be adjusted instead of, or in addition to, the control signal applied to the FHA 26. For example, the write current applied to the write element 32 (FIG. 2B) may affect the fly height of the head 16 and therefore in one embodiment the write current may be adjusted in response to the metric generated at block 30 of FIG. 2C. In yet another embodiment, the metric may be used to calibrate the write power for the laser 20 or the write current for the write element 32. In other embodiments, the metric generated at block 30 of FIG. 2C may be evaluated to characterize the performance of the laser 20 for quality control and/or to provide feedback to improve the fabrication process for the head 16.

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.