# Patent

## Title
Data storage device maximizing areal density based on a target quality metric

## Abstract

A data storage device is disclosed comprising a head actuated over a disk. Data is written to the disk and read from the disk to measure a quality metric. A recording parameter is repeatedly adjusted and the quality metric is measured for each adjustment of the recording parameter. An operating value for the recording parameter is selected that corresponds to a substantially maximum areal density for the disk when the measured quality metric substantially matches a target quality metric.

## Background
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 60-6N recorded around the circumference of each servo 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 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 6i 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 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 is a flow diagram according to an embodiment wherein an operating value for a recording parameter is selected that corresponds to a substantially maximum areal density for the disk when a measured quality metric substantially matches a target quality metric.
FIG. 3A shows an embodiment wherein the recording parameter comprises the areal density for the disk.
FIG. 3B is a flow diagram according to an embodiment wherein a linear bit density (bits per inch (BPI)) and a radial density (tracks per inch (TPI)) are adjusted toward a maximum areal density when the measured quality metric substantially matches the target quality metric.
FIGS. 4A and 4B show an embodiment wherein the recording parameter may comprise a write current, laser power, overshoot, etc. which is adjusted relative to a target BPI/TPI ratio until the areal density reaches a maximum.
FIG. 4C is a flow diagram according to an embodiment wherein the BPI and TPI are adjusted until the measured quality metric substantially matches the target quality metric, and then the recording parameter (write current, laser power, overshoot, etc.) is adjusted and the process repeated until the areal density reaches a maximum.



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. The disk drive further comprises control circuitry 20 configured to execute the flow diagram of FIG. 2B, wherein data is written to the disk and read from the disk to measure a quality metric (block 22). A recording parameter is repeatedly adjusted and the quality metric is measured for each adjustment of the recording parameter (block 24). An operating value for the recording parameter is selected that corresponds to a substantially maximum areal density for the disk when the measured quality metric substantially matches a target quality metric (block 26).
In the embodiment of FIG. 2A, servo sectors 280-28N define a plurality of servo tracks 30, 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 32 emanating from the head 16 to demodulate the servo sectors 280-28N 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 34 applied to a voice coil motor (VCM) 36 which rotates an actuator arm 38 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 280-28N 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 one embodiment, the control circuitry 20 may configure one or more recording parameters in order to achieve a target recording quality while also substantially maximizing the areal density of the disk. The areal density of the disk may be configured by adjusting a linear bit density (bits per inch (BPI)) for data recorded along a data track, and/or by adjusting a radial density of the data tracks (tracks per inch (TPI)). In one embodiment, the recording parameter adjusted by the control circuitry may be the areal density; that is, the areal density may be adjusted until it reaches a maximum when the measured quality metric substantially matches the target quality metric.
Any suitable quality metric may be measured by writing and reading data to/from the disk. Example quality metrics may include one or more of an off-track read capability (OTRC), error rate, squeeze margin, etc., wherein the measured quality metric varies relative to the areal density of the disk. For example, in one embodiment the OTRC quality metric may decrease as the linear density (BPI) increases or as the radial density (TPI) increases. FIG. 3A illustrates an embodiment where the measured quality metric matches a target quality metric (represented by curve 40) for different BPI and TPI settings. That is, there are specific BPI and TPI settings where the measured quality metric will substantially match the target quality metric along curve 40, and there is a particular BPI and TPI setting along curve 40 where the areal density will reach a maximum (i.e., where BPI×TPI will reach a maximum).
FIG. 3B is a flow diagram according to an embodiment wherein the BPI and TPI settings are adjusted until the areal density reaches a maximum when the measured quality metric substantially matches a target quality metric (e.g., the maximum areal density point along curve 40 in FIG. 3A). The BPI and TPI are configured to initial settings (block 42) which may be determined in any suitable manner, such as nominal initial settings established for a family of disk drives. Data is then written and read to/from the disk in order to measure the quality metric of the recorded data at the current BPI and TPI settings (block 44). If the measured quality metric is greater than the target quality metric, then the BPI is increased (block 46), and if the measured quality metric is less than the target quality metric, then the BPI is decreased (block 48). The quality metric is again measured at block 44 for the adjusted BPI setting. The process is repeated by adjusting the BPI until the measured quality metric substantially matches the target quality metric. Once the measured quality metric substantially equals the target quality metric, a determination is made (block 50) as to whether the maximum areal density has been reached. In one embodiment, the maximum areal density may be detected when the areal density reaches a peak value (e.g., begins to decrease after increasing). If the maximum areal density has not been reached at block 50, then the TPI is adjusted (block 52) and the process is repeated by adjusting the BPI until the measured quality metric again matches the target quality metric for the new TPI. The process is repeated until the maximum areal density is detected at block 50, after which an operating value is selected for the recording parameter (BPI and TPI).
In one embodiment illustrated in FIG. 3B, after adjusting the TPI setting at block 52 the control circuitry may make a corresponding adjustment to the BPI (block 54) based on a nominal function that estimates a relationship between the BPI and TPI and the target quality metric. Referring to the example shown in FIG. 3A, the curve 40 representing the target quality metric for different BPI and TPI settings may be represented as a nominal function of BPI and TPI. For example, the curve 40 may be determined for a subset of disk drives in order to determine a nominal curve 40 and a corresponding nominal function for a family of disk drives. In one embodiment, the first BPI and TPI setting that achieves the target quality metric may be used to generate the nominal function. For example, in one embodiment the curve 40 shown in FIG. 3A may vary proportionally across the disk drives such that a single BPI and TPI point may define the function that represents curve 40 for each disk drive. In another embodiment, the control circuitry may determine two or more points along curve 40 (two or more different BPI and TPI settings that achieve the target quality metric) and then curve fit the points to generate the nominal function used at block 54 to adjust the BPI after adjusting the TPI at block 52. In one embodiment, employing a nominal function to adjust the BPI after adjusting the TPI significantly reduces the calibration time by reducing the number of BPI adjustments made at block 46 and/or bock 48 before the measured quality metric converges to the target quality metric.
Any suitable recording parameter may be adjusted in order to maximize the areal density for a target quality metric. Examples of recording parameters other than BPI and TPI may include the write current applied to the write element of the head during write operations, a write power applied to a laser configured to heat the disk during write operations, an overshoot parameter, etc. FIG. 4A illustrates an embodiment wherein a number of curves 40 representing the target quality metric may correspond to a number of different settings for the recording parameter (e.g., different write current or laser write power settings). For example, curve 40A may correspond to a low write current setting and curve 40B may correspond to a higher write current setting. As the setting is increased the curves may expand outward until reaching curve 40C which may correspond to the optimal setting for the recording parameter that achieves the maximum areal density. That is, if the setting for the recording parameter is increased further, the curve will begin to contract for example to curve 40D.
In one embodiment, a target ratio for BPI/TPI is configured by the control circuitry when calibrating the recording parameter. An example BPI/TPI ratio is represented by line 56 in FIG. 4A which, in one embodiment, may pass through the maximum areal density point of each of the target quality metric curves 40A-40D. Accordingly, for the target BPI/TPI ratio along line 56 there is a setting for the recording parameter (e.g., write current) that will maximize the areal density for the disk, such as the setting that generates point 58 along curve 40C in the example of FIG. 4A. FIG. 4B illustrates the relationship between the areal density (BPI×TPI) and the setting for the recording parameter, wherein each point in the curve of FIG. 4B corresponds to a point along line 56 of FIG. 4A.
FIG. 4C is a flow diagram according to an embodiment wherein the BPI and TPI are set to initial settings that also correspond to a target ratio of BPI/TPI (block 60). The recording parameter (write current, laser write power, etc.) is set to an initial setting (block 62), wherein the initial settings for BPI and TPI as well as the recording parameter may be established in any suitable manner, such as nominal initial settings established for a family of disk drives. Data is then written and read to/from the disk in order to measure the quality metric of the recorded data at the current BPI and TPI and recording parameter settings (block 64). If the measured quality metric is greater than the target quality metric, then the BPI and TPI are increased proportionally (block 66), and if the measured quality metric is less than the target quality metric, then the BPI and TPI are decreased proportionally (block 68) so as to maintain the target BPI/TPI ratio. The quality metric is again measured at block 64 for the adjusted BPI and TPI settings (i.e., for the adjusted areal density). The process is repeated by adjusting the BPI and TPI proportionally until the measured quality metric substantially matches the target quality metric. Once the measured quality metric substantially equals the target quality metric, a determination is made (block 70) as to whether the maximum areal density has been reached. If the maximum areal density has not been reached at block 70, then the recording parameter is adjusted (block 72) and the process is repeated by adjusting the BPI and TPI proportionally until the measured quality metric again matches the target quality metric for the new TPI. The process is repeated until the maximum areal density is detected at block 70, after which an operating value is selected for the recording parameter (write current, laser write power, etc.).
In one embodiment, the BPI and TPI may be adjusted at block 74 based on a nominal function that estimates a relationship between the recording parameter setting and the areal density (BPI×TPI). That is, the curve shown in FIG. 4B may be estimated using a nominal function, and when the recording parameter is adjusted at block 72, a corresponding adjustment may be made to the areal density (BPI×TPI) at block 74 based on the nominal function. In one embodiment, the nominal function may be predetermined and/or it may be determined based on one or more data points measured for the curve by executing the flow diagram of FIG. 4C one or more times for one or more recording parameter settings.
In one embodiment, the line 56 shown in FIG. 4A may not pass through the point of each target quality metric curve 40A-40D at the point where the areal density is maximum. For example, the target quality metric curves 40A-40D may not scale proportionally along the x and y axis relative to the recording parameter setting such that the line 56 may not always pass through each curve 40A-40D at the maximum areal density. Even if the target quality metric curves 40A-40D scale proportionally along the x and y axis, the line 56 representing the target BPI/TPI ratio may not pass through the maximum areal density point of each curve due to variations across disk drives. Accordingly, in one embodiment after executing the flow diagram of FIG. 4C in order to calibrate the optimal recording parameter setting (write current, laser write power, etc.) that substantially maximizes the areal density along line 56, the final point along line 56 may not correspond to the maximum areal density of the final target quality metric curve 40C. In this case, the control circuitry may execute the flow diagram of FIG. 3B after executing the flow diagram of FIG. 4C in order to make a final adjustment to BPI and TPI in order to maximize the areal density for the calibrated recording parameter setting. That is, the flow diagram of FIG. 3B may be executed using the calibrated recording parameter setting as well as the final BPI and TPI settings determined by the flow diagram of FIG. 4C. For example, the final BPI and TPI settings after executing the flow diagram of FIG. 4C may be used as the initial BPI and TPI settings at block 42 of FIG. 3B.
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.

## Claims

1. A data storage device comprising:
a disk;
a head actuated over the disk; and
control circuitry configured to:
write data to the disk and read the data from the disk to measure a quality metric;
repeatedly adjust a recording parameter and measure the quality metric for each adjustment of the recording parameter; and
select an operating value for the recording parameter that corresponds to a substantially maximum areal density for the disk when the measured quality metric substantially matches a target quality metric.

2. The data storage device as recited in claim 1, wherein the recording parameter comprises the areal density for the disk.

3. The data storage device as recited in claim 1, wherein the recording parameter comprises a write current applied to a write element.

4. The data storage device as recited in claim 1, wherein the recording parameter comprises a write power applied to a laser configured to heat the disk during write operations.

5. The data storage device as recited in claim 1, wherein the control circuitry is further configured to adjust the recording parameter based on a nominal function that estimates a relationship between the recording parameter and the target quality metric.

6. The data storage device as recited in claim 5, wherein the control circuitry is further configured to generate the nominal function based on at least one recording parameter setting where the corresponding measured quality metric substantially matches the target quality metric.

7. The data storage device as recited in claim 6, wherein the control circuitry is further configured to generate the nominal function based on at least two recording parameter settings where the corresponding measured quality metric for each recording parameter substantially matches the target quality metric.

8. The data storage device as recited in claim 1, wherein the quality metric comprises an off-track read capability.

9. The data storage device as recited in claim 1, wherein the quality metric comprises an error rate.

10. The data storage device as recited in claim 1, wherein the quality metric comprises a squeeze margin.

11. A method of operating a data storage device, the method comprising:
writing data to a disk and reading the data from the disk to measure a quality metric;
repeatedly adjusting a recording parameter and measuring the quality metric for each adjustment of the recording parameter; and
selecting an operating value for the recording parameter that corresponds to a substantially maximum areal density for the disk when the measured quality metric substantially matches a target quality metric.

12. The method as recited in claim 11, wherein the recording parameter comprises the areal density for the disk.

13. The method as recited in claim 11, wherein the recording parameter comprises a write current applied to a write element.

14. The method as recited in claim 11, wherein the recording parameter comprises a write power applied to a laser configured to heat the disk during write operations.

15. The method as recited in claim 11, further comprising adjusting the recording parameter based on a nominal function that estimates a relationship between the recording parameter and the target quality metric.

16. The method as recited in claim 15, further comprising generating the nominal function based on at least one recording parameter setting where the corresponding measured quality metric substantially matches the target quality metric.

17. The method as recited in claim 16, further comprising generating the nominal function based on at least two recording parameter settings where the corresponding measured quality metric for each recording parameter setting substantially matches the target quality metric.

18. The method as recited in claim 11, wherein the quality metric comprises an off-track read capability.

19. The method as recited in claim 11, wherein the quality metric comprises an error rate.

20. The method as recited in claim 11, wherein the quality metric comprises a squeeze margin.