Method of writing a preamble field on a disk drive to reduce track squeeze

Preamble fields are written on a storage disk in a hard disk drive in a way that reduces track squeeze. Preamble fields for a particular data storage track on the storage disk are written over multiple revolutions of the storage disk to eliminate low-frequency variations of the preamble stitch line from an ideal position of the preamble stitch line. By writing the preamble fields for one data storage track over multiple revolutions, and by writing the preamble fields in each revolution to non-consecutive servo wedges, low-frequency variations of the preamble stitch line from its ideal position can be converted to high-frequency variations of the preamble stitch line that do not produce low-frequency track squeeze.

BACKGROUND

1. Field of the Invention

Embodiments of the present invention relate generally to disk drives and, more particularly, to a method of writing a preamble field on a disk drive to reduce track squeeze.

2. Description of the Related Art

A disk drive is a data storage device that stores digital data in concentric tracks on the surface of a data storage disk. The data storage disk is a rotatable hard disk with a layer of magnetic material thereon, and data is read from or written to a desired track on the data storage disk using a read/write head that is held proximate to the track while the disk spins about its center at a constant angular velocity.

To properly align the read/write head with a desired track during a read or write operation, disk drives generally use a closed-loop servo system that relies on servo data stored in servo sectors written on the disk surface when the disk drive is manufactured. The servo sectors are written between user data fields on the track of interest. These servo sectors form “servo wedges” or “servo spokes” from the outer to inner diameter of the disk, and are either written on the disk surface by an external device, such as a servo track writer, or by the drive itself using a self servo-writing procedure. The read/write head can be positioned with respect to the data storage disk by using feedback control based on servo information read from the servo wedges with the read element of the read/write head. The servo sectors provide position information about the radial location of the read/write head with respect to the disk surface in the form of servo patterns or “servo bursts.”

During the process of writing the servo wedges on a disk, servo sectors are typically written on the disk one data track at a time. Due to fluctuations in read/write head position while writing the servo patterns for a given data track, the servo patterns for the data track do not form a perfect circle on the disk. Instead, each servo pattern is generally written at a location having a certain displacement, or “runout,” from the ideal track position. This displacement of servo patterns from the ideal track position is referred to as “written-in” repeatable runout (RRO).

As is known in the art, written-in RRO that produces high-frequency disturbances in the position of a read/write head can be readily compensated for during normal operation by implementing correction factors for each servo sector to facilitate smooth and controllable travel of the read/write head along a data track of a storage disk. However, low-frequency fluctuations in head position also generally occur as the servo patterns are written for a specific data track, and therefore produce low-frequency disturbances in the position of the read/write head during normal operation. Such low-frequency disturbances in head position, e.g., fluctuations having a frequency of less than about 500 Hz, are difficult to compensate for and can produce track squeeze, as illustrated below inFIG. 1.

FIG. 1schematically illustrates a portion of a storage disk100and the paths followed by a head107as servo wedges are written to a first data track101and a second data track102. Low-frequency fluctuations in the position of head107during the process of writing servo wedges111-115cause head107to vary from ideal track positions101A,102A, thereby producing track squeeze between first data track101and second data track102. Track squeeze occurs when track-to-track spacing is inadequate to ensure the data integrity of adjacent data tracks. When writing the servo wedges for first data track101, head107follows a write head path110that varies from ideal track position101A at a low frequency. As a result, the servo information140for servo wedges111-115for first data track101is located along write head path110, rather than along ideal track position101A. Similarly, when writing the servo wedges for second data track102, head107follows a write head path120that varies from ideal track position102A at a low frequency, and servo information150for servo wedges111-115for second data track102is located along write head path120, rather than along ideal track position102A. Consequently, first data track101can vary from ideal track position101A and second data track102can vary from ideal track position102A in such a way that first data track101and second data track102partially or completely overlap. As a result, data stored in one track can overwrite data stored in an adjacent track, which is highly undesirable.

Furthermore, the low-frequency track squeeze illustrated inFIG. 1is known to be the exacerbated by position error produced by preamble phase shift. In hard disk drives using null-pattern demodulation, the preamble field for each servo wedge provides a timing reference for the servo wedge, and preamble phase shift can occur due to the non-ideal shape of the magnetic flux transitions making up the preamble field of servo patterns written to servo wedges111-115. Ideally, the preamble fields of adjacent data tracks are made up of linear magnetic flux transitions that are connected, or “stitched,” together at a preamble stitch line between the adjacent data tracks to form a radially continuous line from track-to-track. In practice, such magnetic flux transitions are often written with unwanted curvature and/or tilt, producing significant discontinuity at the preamble stitch lines between data tracks. Such discontinuity can affect timing reference accuracy provided by the preamble field when a write head is positioned over the preamble stitch line. Thus, when head107is positioned over a preamble stitch line between two data tracks during normal operation, inaccuracy of the timing reference near the preamble stitch line can produce significant additional head position error. Such head position error can exaggerate any track squeeze already present between first data track101and second data track102.

In light of the above, there is a need in the art for a system and method for preventing track squeeze in hard disk drives that use null-pattern demodulation schemes.

SUMMARY

One or more embodiments of the present invention provide systems and methods for writing a preamble field on a disk drive to reduce track squeeze. Preamble fields for a particular data storage track are written over multiple revolutions of a storage disk to eliminate low-frequency variations of the preamble stitch line from an ideal position of the preamble stitch line. By writing the preamble fields for one data storage track over multiple revolutions, and by writing the preamble fields in each revolution to non-consecutive servo wedges, low-frequency variations of the preamble stitch line from its ideal position can be converted to high-frequency variations of the preamble stitch line that do not produce low-frequency track squeeze.

A method of writing servo information on a magnetic storage medium while the magnetic storage medium is rotated, according to one embodiment of the present invention, includes the steps of, during a first revolution of the magnetic storage medium, writing first and second timing references on the magnetic storage medium at a common radial position, and, during a second revolution of the magnetic storage medium, writing a third timing reference on the magnetic storage medium at the common radial position and between the first and second timing references.

A method of writing servo information on a magnetic storage medium while the magnetic storage medium is rotated, according to another embodiment of the present invention, includes the steps of, during a first revolution of the magnetic storage medium, writing a timing reference on a first portion of the magnetic storage medium and a first portion of a servo burst on the magnetic storage medium, and, during a second revolution of the magnetic storage medium, writing a timing reference on a second portion of the magnetic storage medium and a second portion of the servo burst on the magnetic storage medium.

DETAILED DESCRIPTION

FIG. 2is a schematic view of an exemplary disk drive200, according to an embodiment of the invention. For clarity, disk drive200is illustrated without a top cover. Disk drive200includes at least one storage disk210that is rotated by a spindle motor214. Spindle motor214is mounted on a base plate216. An actuator arm assembly220is also mounted on base plate216, and has a slider221mounted on a flexure arm222with a read/write head227. Flexure arm222is attached to an actuator arm224that rotates about a bearing assembly226. Voice coil motor228moves slider221relative to storage disk210, thereby positioning read/write head227over the desired concentric data storage track disposed on the surface212of storage disk210. Spindle motor214, read/write head227, and voice coil motor228are coupled to electronic circuits230, which are mounted on a printed circuit board232. The electronic circuits230include a read channel, a microprocessor-based controller, random access memory (RAM) and/or a flash memory device. For clarity of description, disk drive200is illustrated with a single storage disk210and a single actuator arm assembly220. Disk drive200may also include multiple storage disks and multiple actuator arm assemblies. In addition, each side of storage disk210may have an associated read/write head coupled to a flexure arm.

When data is transferred to or from storage disk210, actuator arm assembly220sweeps an arc between an inner diameter (ID) and an outer diameter (OD) of storage disk210. Actuator arm assembly220accelerates in one angular direction when current is passed through the voice coil of voice coil motor228and accelerates in an opposite direction when the current is reversed, allowing for control of the position of actuator arm assembly220and attached read/write head227with respect to storage disk210. Voice coil motor228is coupled with a servo system known in the art that uses positioning data read from servo sectors that are embedded in each data track of storage disk210by read/write head227to determine the position of read/write head227over a data storage track. The servo system determines an appropriate current to drive through the voice coil of voice coil motor228, and drives said current using a current driver and associated circuitry.

FIG. 3illustrates storage disk210with data organized after servo wedges300have been written on storage disk210. Servo wedges300may be written on storage disk210by either a media writer or by disk drive200itself via a self servo-write (SSW) process. Servo wedges300are substantially radially aligned and are shown crossing data storage tracks320. Each servo wedge300includes a plurality of servo sectors350. Each servo sector350contains servo information that defines the radial position and track pitch, i.e., spacing, of data storage tracks320. Servo sectors350are described in greater detail below in conjunction withFIG. 4. In practice, servo wedges300may be somewhat curved, for example, servo wedges300may be configured in a spiral pattern that mirrors the path that would be followed by read/write head227if it were to move across the stroke while storage disk210is not spinning. Such a curved pattern advantageously results in the wedge-to-wedge timing being independent of the radial position of read/write head227. For simplicity, servo wedges300are depicted as substantially straight lines inFIG. 3.

Storage disk210also includes concentric data storage tracks320located in data sectors325for storing data. Data storage tracks320are positionally defined by the servo information written in servo sector350. Each servo sector350contains a reference signal that is read by read/write head227during read and write operations to position read/write head227above a desired data storage track320. Typically, the actual number of data storage tracks320and servo wedges300included on storage disk210is considerably larger than illustrated inFIG. 2. For example, storage disk210may include hundreds of thousands of concentric data storage tracks320and hundreds of servo wedges300.

FIG. 4illustrates a partial schematic diagram of a portion of a servo wedge300that is disposed on storage disk210and includes servo information for three adjacent data storage tracks321-323. The servo information for data storage tracks321-323is included in servo sectors351-353, respectively, and is written onto storage disk210by either a media writer or during a servo-self-write process for one data storage track at a time. Servo sectors351-353each include a preamble field410, a Gray code area420, and servo burst patterns430for tracks321-323, respectively. Preamble field410is used as a timing reference to synchronize the timing of the read channel immediately prior to reading Gray code area420and servo burst patterns430, and is also used as an amplitude reference to adjust signal amplitude. Gray code area420provides coarse position of the read/write head by indicating the current track number. Servo burst patterns430are null-pattern servo bursts, and are used to determine the fine position of read/write head227relative to a specific data storage track.

Preamble field410, Gray code area420, and servo burst patterns430are each formed by a plurality of magnetic flux transitions405. Ideally, magnetic flux transitions405are straight lines oriented perpendicular to data storage tracks321-323, and are stitched together, i.e., connected or slightly overlapped, at preamble stitch lines415, where preamble stitch lines415define the demarcation between adjacent preamble fields410. When magnetic flux transitions405are stitched together, they form substantially continuous lines, so that preamble fields410form a radially continuous preamble field. In this way, the timing reference provided by preamble field410during normal operation of disk drive200is uniform for servo wedge300regardless of the exact radial position of read/write head227. Given such a timing reference by preamble field410, a precise location of read/write head227relative to a specific data storage track can be determined by demodulating the servo-burst signal generated when read/write head227subsequently passes over servo burst pattern430.

In practice, when written on the surface of storage disk210, magnetic flux transitions405are not always perpendicular to data storage tracks321-323. Instead, near preamble stitch lines415, magnetic flux transitions405can be curved, and consequently are not stitched together at preamble stitch lines415to form substantially continuous lines that are perpendicular to storage tracks321-323. This is due to skew angle of read/write head227with respect to data storage tracks320, fringe field effects, side writing, and other factors. Because of the non-ideal shape of magnetic flux transitions405in some regions of servo wedge300, substantial phase shift of the timing reference provided by preamble field410can result when read/write head227is positioned near one of preamble stitch lines415. Thus, the timing reference provided by preamble field410can vary depending on the radial position of read/write head227, which results in unwanted phase variation of the preamble. Preamble phase variation is known to directly affect the demodulation of certain burst patterns, such as the null-pattern, resulting in a significant error in the measured position of read/write head227and exacerbating track squeeze. For example, the non-ideal shape of magnetic flux transitions405can produce a preamble phase shift of ±30° or 40°, translating to an error in position of read/write head227as great as 10% of track width458—a significant disturbance in the position of read/write head227relative to a desired ideal track position. Because such a disturbance is the product of permanently written features on the surface of the storage disk, it is considered “written-in” repeatable runout (RRO).

High-frequency disturbances from an ideal track location caused by written-in RRO can be removed or minimized by associating a correction factor with each servo sector on a particular data storage track during the initial calibration and set-up of the disk drive. In contrast, low-frequency disturbances from the ideal track location is difficult to compensate using such correction factors, and can produce unwanted track squeeze. According to embodiments of the invention, a method of writing servo information for a data storage track320on storage disk210substantially eliminates such low-frequency disturbances in the position of the servo information embedded in the data storage track, thereby reducing track squeeze.

FIG. 5schematically illustrates a portion of a data storage track500on storage disc210that is spanned by servo wedge locations501-515and has preamble fields501A-515A written for each of servo wedge locations501-515in a fashion known in the art. Preamble fields501A-515A are substantially similar to preamble fields410inFIG. 4, and are written in servo wedge locations501-515, respectively, during a single revolution of storage disk210. Preamble fields501A-515A may also include other servo sector information that has been written in the same revolution. Such servo sector information may include position information about the radial location of read/write head227, such as Gray code areas420and/or portions of servo burst patterns430.

As shown, preamble fields501A-515A, which are made up of multiple magnetic flux transitions, are each written near but displaced from an ideal track line550for data storage track500by a displacement520. Displacements520of preamble fields501A-515A from ideal track line550correspond to fluctuations in the position of a servo writer while preamble fields501A-515A are being written. For reference, a servo writer path560indicates the path followed by the servo writer as preamble fields501A-515A are written during the revolution of storage disk210. The position of the servo writer includes high- and low-frequency fluctuations from ideal track line550while writing preamble fields501A-515A. Consequently, displacements520of preamble fields501A-515A each include a low-frequency component521and a high-frequency component522. These high- and low-frequency fluctuations are illustrated by servo writer path560inFIG. 5. For clarity, displacement520, low-frequency component521, and high-frequency component522are only illustrated for preamble field503A inFIG. 5. Because preamble fields501A-515A are written during a single revolution of storage disk210, the preamble stitch line between data storage track500and adjacent data storage tracks closely follows servo writer path560.

As is known in the art, position correction factors can be determined and applied for each of servo sectors on data storage track500to remove high-frequency components522of displacements520. In this way, data storage track500follows a relatively smooth path proximate ideal track line550that is more easily followed by read/write head227during normal operation of disk drive200, and that avoids high-frequency track squeeze. With the use of such correction factors, high-frequency components522can be substantially eliminated, but low-frequency components521are difficult to reduce. Consequently, low-frequency track squeeze can still occur along portions of data storage track500in which preamble fields501A-515A have significant low-frequency displacement components, since such components cause low-frequency variations of the preamble stitch line from its ideal position. By way of illustration, preamble stitch line570indicates an approximate location of the preamble stitch line for data storage track500. As shown, preamble stitch line570has a portion571with a significant low-frequency displacement572that can produce low-frequency track squeeze.

According to embodiments of the invention, preamble fields for a particular data storage track are written over multiple revolutions of a storage disk to eliminate low-frequency variations of the preamble stitch line from an ideal position of the preamble stitch line. By writing the preamble fields for one data storage track over multiple revolutions, and by writing the preamble fields in each revolution to non-consecutive servo wedges, low-frequency variations of the preamble stitch line from its ideal position can be converted to high-frequency variations of the preamble stitch line that do not produce low-frequency track squeeze.

FIG. 6Aschematically illustrates a portion of a data storage track600that is spanned by servo wedge locations601-615and has preamble fields written for a portion of servo wedge locations601-615, according to an embodiment of the invention. The preamble fields illustrated inFIG. 6are written to non-consecutive servo wedge locations on data storage track600as a servo writer follows a path620during a first revolution of storage disk210. Specifically, preamble fields601A,603A,605A,607A,609A,611A,613A, and615A are written for data storage track600at servo wedge locations601,603,605,607,609,611,613, and615, respectively. For reference, a hypothetical preamble stitch line670is depicted inFIG. 6indicating the location of the preamble stitch line for data storage track600if only servo information at servo wedge locations601,603,605,605,609,611,613, and615were used to control the radial position of read/write head227in operation. As shown, hypothetical preamble stitch line670includes a region671, in which a significant low-frequency displacement672from ideal track line650is present that can produce low-frequency track squeeze.

FIG. 6Bschematically illustrates data storage track600with preamble fields written for the remaining portion of servo wedge locations601-615, according to an embodiment of the invention. The preamble fields written for the remaining portion of servo wedge locations601-615, i.e., servo wedge locations602,604,606,608,610,612, and614, are written as the servo writer follows a path630during a second revolution of storage disk210. Specifically, preamble fields602A,604A,606A,608A,610A,612A, and614A are written at servo wedge locations602,604,606,608,610,612, and614, respectively. A preamble stitch line690is depicted inFIG. 6indicating the location of the preamble stitch line for data storage track600after all preamble fields601A-615A have been written to storage disk210. As shown, the displacement of preamble stitch line690from ideal track line650has very little or no low-frequency components and instead includes primarily high-frequency components. Consequently, regions692of data storage track600in which the displacement of preamble stitch line690from ideal track line650is relatively large can be compensated for using position correction factors at each such servo-wedge location, thereby reducing track squeeze.

Thus, by writing preamble fields601A-615A over two or more revolutions of storage disk210, low-frequency components of the displacement between preamble stitch line690and ideal track line650are broken up into a large number of high-frequency displacements. In other words, preamble fields are written to a first group of non-consecutive servo sectors on a data storage track during one revolution of storage disk210, and preamble fields are written to a second group of non-consecutive servo sectors on the data storage track during a subsequent revolution of storage disk210. The preamble fields of the second group are interspersed between the preamble fields of the first group, i.e., a preamble field of the second group is written on the data storage track at an azimuth position between the azimuth positions of two preamble fields of the first group.

In the embodiment illustrated inFIGS. 6A,6B, preamble fields for all servo wedges of data storage track600are written over two revolutions of storage disk210. In other embodiments, the writing of preamble fields for a specific data storage track is distributed over three or more revolutions of storage disk210.

FIG. 7sets forth a flowchart of method steps for writing servo information on a magnetic storage medium while the magnetic storage medium is rotated, according to one embodiment of the present invention. Although the method steps are described in conjunction with the disk drive200inFIG. 2, persons skilled in the art will understand that any disk drive configured to perform the method steps, in any order, is within the scope of the invention.

As shown, method700begins at step701, during a first revolution of a magnetic storage medium. Storage disk210of disk drive200is one example of such a magnetic storage medium. In step701, a timing reference, such as preamble field410, is written on a first portion of storage disk210. The timing reference may be written by a media writer or by read/write head227of disk drive200during a servo-self write process. The first portion of storage disk210includes a group of non-consecutive servo sectors on a specific data storage track320, and the timing reference is written on each of the non-consecutive servo sectors. For example, in one embodiment, the timing reference is written to every third servo sector on the specific data storage track320.

In step702, a first portion of a servo burst, such as servo burst430, is also written to some or all of the servo sectors on the data track320of interest. For example, the first portion of the servo burst may be one or two of magnetic flux transitions405when servo burst430includes five to ten total flux transitions. In some embodiments, the first portion of the servo burst is written to each servo sector embedded in the data track of interest. It is noted that in some embodiments, step702takes place during the same revolution as step701, i.e., during the first revolution of the magnetic storage medium.

In step703, during a second revolution of storage disk210, the timing reference is written on a second portion of storage disk210. The second portion of storage disk210also includes a second group of non-consecutive servo sectors on a specific data storage track320, and the timing reference is written on each of the non-consecutive servo sectors in the second group. It is noted that the servo sectors of the second group are interspersed between the servo sectors of the first group. In this way, low-frequency track squeeze can be prevented in disk drive200by breaking up low-frequency displacements of the servo writer used to write the timing references to data storage track320.

In step704, a second portion of each servo burst is also written to some or all of the servo sectors on the data track320of interest. For example, the second portion of the servo burst may be one or two of magnetic flux transitions405when servo burst430includes five to ten total flux transitions. In such an embodiment, the use of multiple revolutions to write the preamble fields to all servo sectors on a specific data storage track320does not affect the time required to write servo information to the data storage track. This is because servo bursts for the data storage track can be written during the same revolutions that the preamble fields are being written for the data storage track. It is noted that in some embodiments, step704takes place during the same revolution as step703, i.e., during the second revolution of the magnetic storage medium.

While embodiments of the invention are described herein in terms of disk drive200during a servo-self-write process, embodiments of the present invention can be applied to any apparatus writing servo sector information on a storage disk, including servo writers.

In sum, embodiments of the invention provide systems and methods for writing servo information on a magnetic storage medium to reduce track squeeze. By writing the preamble fields for one data storage track over multiple revolutions, and by writing the preamble fields in each revolution to non-consecutive servo wedges, low-frequency variations of the preamble stitch line from an ideal position can be converted to high-frequency variations of the preamble stitch line that advantageously do not produce low-frequency track squeeze. Often servo bursts are also written in multiple steps, so if the number of preamble write steps is less than or equal to the number of burst write steps, then the technique this method does not increase the time required for self servo writing.