Data storage device servoing heads based on virtual servo tracks

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 6 0 - 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 , 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 data 12 and 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.

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 surface.

FIG. 2B shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a plurality of disk surfaces.

FIG. 2C is a flow diagram according to an embodiment wherein a servo sector on a first disk surface is read to generate a physical position measurement which is converted into a virtual position measurement used to servo the head.

FIG. 2D shows a servo control system according to an embodiment comprising a conversion operation for converting a physical position measurement into a virtual position measurement.

FIG. 3A shows an embodiment wherein the conversion from the physical position measurement of each disk surface is represented by a respective linear function that accounts for a difference between the actual radial density of servo tracks on each disk surface and a nominal radial density.

FIG. 3B shows an embodiment wherein servo zone boundaries are defined at a target virtual position (common radial location across the disk surfaces).

FIG. 4 shows an embodiment wherein the servo tracks on a disk surface are grouped to form servo zones, wherein a servo data rate varies across the servo zones.

FIGS. 2A and 2B show a data storage device in the form of a disk drive according to an embodiment comprising a first disk surface 16 1 comprising servo tracks 18 defined by servo sectors 20 0 - 20 N and recorded at a first radial density, and a first head 22 1 actuated over the first disk surface 16 1 . The disk drive further comprises control circuitry 24 , which in an embodiment is configured to execute the flow diagram of FIG. 2C , wherein a servo sector on the first disk surface is read to generate a first physical position measurement (block 26 ) that is converted into a first virtual position measurement (block 28 ) corresponding to a nominal radial density of servo tracks different from the first radial density. The first head is servoed over the first disk surface based on the first virtual position measurement (block 30 ).

FIG. 2D shows a servo control system according to an embodiment wherein a read signal 32 emanating from the first head 22 1 when reading a servo sector is demodulated into a physical position measurement 36 at block 34 . The physical position measurement may be represented as a track address and/or by a fraction of a servo track generated based on reading servo bursts in the servo sector. The physical position measurement 36 is converted at block 38 into a virtual position measurement 40 which is subtracted from a target position 42 to generate a position error signal (PES) 44 . A servo controller 46 processes the PES 44 using any suitable servo compensator to generate a control signal 48 applied to a voice coil motor (VCM) 50 . The VCM 50 rotates an actuator arm 52 about a pivot in order to move the head 22 1 radially over the disk surface 16 1 in a direction that reduces the PES.

In one embodiment, converting the physical position measurement generated by reading a servo sector into a virtual position measurement representing a nominal radial density of servo tracks reduces the complexity of the servo control system while enabling the disk surface to be servo written to achieve any desirable radial density of servo tracks. For example, certain aspects of the head (e.g., the geometry of the write and/or read element, and/or the sensitivity of read element) may dictate an optimal radial density of servo tracks for a particular head/disk surface combination. Rather than design the servo control system to compensate for a different radial density of servo tracks across different disk surfaces, in one embodiment the servo control system is designed to operate based on virtual servo tracks that are represented by a nominal radial density.

In one embodiment, a nominal radial density of the virtual servo tracks may be defined for a family of disk drives such that the servo control system may operate the same across the family of disk drives. For example, in one embodiment each disk drive may comprise a single disk surface having servo tracks defined by servo sectors and recorded at a particular radial density. However, the servo control system of each disk drive may operate the same across the disk drives after each disk drive converts the physical position measurement into a virtual position measurement. In another embodiment as shown in FIG. 2B , each disk drive may comprise a plurality of disk surfaces (e.g., four disk surfaces 16 1 - 16 4 ) with a head actuated over each disk surface. The servo tracks of each disk surface may be recorded at a different radial density such that when the head is at a particular radial location common to each disk surface, the resulting physical position measurement may be different across the disk surfaces due to the different radial density of servo tracks. However, after converting the physical position measurement into a virtual position measurement for each disk surface, the servo control system may operate the same across the disk surfaces.

FIG. 3A illustrates a relationship between the physical position measurements (vertical axis) generated by reading the servo sectors of each disk surface relative to the corresponding virtual position measurement (horizontal axis) after the conversion, according to an embodiment. In this embodiment, the conversion between physical position measurement to virtual position measurement is represented by the following linear function:

y=m −1 ( x−b )+ c

where y represents the virtual position measurement, x represents the physical position measurement, m represents the slope of the line, and b and c are arbitrary offsets. In the example of FIG. 3A , the slope m 0 represents the nominal radial density of servo tracks, the slope m 1 represents the radial density of servo tracks on a first disk surface, and the slope m 2 represents the radial density of servo tracks on a second disk surface. In this example embodiment the radial density of servo tracks on the first disk surface is higher than the nominal radial density, and the radial density of servo tracks on the second disk surface is lower than the nominal radial density. Therefore when the first and second heads are at the same radial location represented by virtual position y i , the physical position measurement x 1 for the first disk surface will be higher than the corresponding nominal physical position measurement x 0 , and the physical position measurement x 2 for the second disk surface will be lower than the corresponding nominal physical position measurement x 0 . However, when the physical position measurements are converted, for example, using a conversion function that compensates for the different radial densities, the resulting virtual position measurement y i for both the first and second heads will be the same.

In one embodiment illustrated in FIG. 3A , when the first and second heads are at a radial location represented by offset c, the physical position measurements for each disk surface will equal the nominal physical position measurement (i.e., at the intersection of the lines defined by the slopes m 0 , m 1 and m 2 ). In one embodiment, this radial location may identify a boundary servo track, such as the first servo track (servo track zero). However, in another embodiment this radial location may represent any virtual position measurement and any corresponding servo track number. In one embodiment, at least one parameter used to convert the physical position measurements into the virtual position measurements may be stored on at least one disk surface at the radial location represented by offset c. In this manner, the servo control system may seek the heads to this radial location without needing to perform a conversion from physical to virtual since the physical and virtual position measurements are equal at this radial location. In other words, when the disk drive is initialized (e.g., at power on) the servo control system may seek the heads to the radial location represented by offset c and read the conversion parameters from at least one disk surface regardless as to the radial density of servo tracks on each disk surface. Once the conversion parameters have been successfully recovered, the servo control system may seek to any radial position (represented by the virtual position measurement) for any disk surface using the conversion operation.

In the embodiment of FIG. 3A , the conversion from physical to virtual position measurement is represented by a linear function. However, in other embodiments the conversion may be represented in any suitable manner, such as with a non-linear function (e.g., a polynomial function), or a piecewise linear function. For example, in one embodiment the radial density of data tracks may vary across each disk surface (e.g., decrease near the outer diameter) and therefore require a more sophisticated function to represent the relationship between the physical position measurements and corresponding virtual position measurement. In one embodiment, the conversion from physical to virtual position measurement may be implemented using a suitable lookup table generated based on a conversion function. In one embodiment, the lookup table may represent discrete data points of a conversion function where interpolation and/or extrapolation may be used to fill in missing data points as part of the conversion process during normal operation.

In one embodiment, the servo tracks of the first and second disk surfaces are grouped to define servo zones, and a servo data rate of each servo zone varies across the servo zones. FIG. 4 illustrates an example of this embodiment wherein the servo tracks 18 of a disk surface 16 i are grouped to define three servo zones (Z 1 to Z 3 ), wherein the servo data rate of the servo sectors remains substantially constant within the servo zone, but changes across the servo zones (e.g., increases toward the outer diameter zones to achieve a more constant linear bit density). FIG. 3B illustrates an embodiment wherein the boundaries of the servo zones may align radially across the disk surfaces (e.g., across first and second disk surfaces) so that a servo zone transition occurs at known radial locations (virtual position measurements) when performing a head switch. In one embodiment, the offset c in FIG. 3B may represent the boundary of servo zone Z 1 (e.g., servo track zero), and in other embodiments the boundary of servo zone Z 1 may be defined at a different radial location and corresponding virtual position measurement.

In one embodiment, the radial locations that define the boundaries for the servo zones (e.g., SEAM 1 and SEAM 2 ) in FIG. 3B may be determined prior to recording the servo sectors onto each disk surface, and in one embodiment, may be the same radial location across a family of disk drives. Accordingly, in one embodiment the boundaries for the servo zones represented as virtual position measurements may be converted into corresponding physical position measurements for each disk surface so that when servo writing each disk surface, the servo data rate of the servo sectors may be adjusted based on the physical position measurement that corresponds to a servo zone boundary. The servo sectors may be servo written on each disk surface in any suitable manner, including the use of an external media writer prior to inserting each disk into a production disk drive. In another embodiment, the control circuitry 24 internal to each production disk drive may self-servo write the servo sectors, including to adjust the servo data rate when reaching the boundary of a servo zone.

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.

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.