Data storage device employing staggered servo wedges to increase capacity

A data storage device is disclosed comprising a voice coil motor (VCM) having a resonance frequency, a first disk surface comprising a first set of servo sectors written at a frequency less than twice the VCM resonance frequency, and a second disk surface comprising a second set of servo sectors circumferentially offset from the first set of servo sectors and written at a frequency less than twice the VCM resonance frequency. An access of the first disk surface is performed by reading at least one of the first set of servo sectors to generate a first position error signal (PES), reading at least one of the second set of servo sectors to generate a second PES, and controlling the VCM based on the first PES and the second PES to position a first head over the first disk surface while accessing the first disk surface.

BACKGROUND

A disk drive typically comprises a plurality of disks each having a top and bottom surface accessed by a respective head. That is, the VCM typically rotates a number of actuator arms about a pivot in order to simultaneously position a number of heads over respective disk surfaces based on servo data recorded on each disk surface.FIG. 1shows a prior art disk format2as comprising a number of servo tracks4defined by servo sectors60-6Nrecorded around the circumference of each servo track. Each servo sector6icomprises a preamble8for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark10for storing a special pattern used to symbol synchronize to a servo data field12. The servo data field12stores 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 sector6ifurther comprises groups of servo bursts14(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 bursts14provide 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 bursts14, 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.

DETAILED DESCRIPTION

FIGS. 2A and 2Bshow a data storage device in the form of a disk drive according to an embodiment comprising a first head161actuated over a first disk surface181, and a second head162actuated over a second disk surface182by a voice coil motor (VCM)20having a resonance frequency. The first disk surface181comprises a first set of servo sectors221-22Nwritten at a frequency less than two times the VCM resonance frequency. The second disk surface182comprises a second set of servo sectors241-24Ncircumferentially offset from the first set of servo sectors221-22Nand written at a frequency less than two times the VCM resonance frequency. The disk drive further comprises control circuitry26configured to execute the flow diagram ofFIG. 2Cwherein during an access operation to the first disk surface (block28), at least one of the first set of servo sectors is read to generate a first position error signal (PES) (block30), and at least one of the second set of servo sectors is read to generate a second PES (block32). The VCM is controlled based on the first PES and the second PES to position the first head over the first disk surface (block34) while accessing the first disk surface (block36).

In the embodiment ofFIGS. 2A and 2B, the first disk surface181is the top surface of a disk38and the second disk surface182is the bottom surface of the disk38. The first set of servo sectors221-22Nare written to the top disk surface181and the second set of servo sectors241-24Nare written to the bottom disk surface182circumferentially offset from the first set of servo sectors221-22N. The control circuitry26processes the read signals emanating from the heads to demodulate the servo sectors 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. A servo control system in the control circuitry26filters the PES using a suitable compensation filter to generate a control signal40applied to the VCM20which rotates an actuator arm42about a pivot in order to actuate the head radially over the disk in a direction that reduces the PES. The servo sectors 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 (FIG. 1). In one embodiment, the first set of servo sectors221-22Nand the second set of servo sectors241-24Nare written to the disk surfaces during a servo writing procedure. The servo writing procedure may be executed by an external servo writer wherein the servo written disks are installed into production disk drives, and in another embodiment the servo writing procedure may be executed by the control circuitry26of each disk drive (i.e., the servo sectors may be self-servo written).

Conventionally the frequency of the servo sectors written to any single disk surface (and the corresponding number of servo sectors per track) is configured to achieve adequate servo performance for a target data density (tracks per inch (TPI) and linear bits per inch (BPI)). That is, as the target data density increases it results in a corresponding increase in the servo sector frequency in order to maintain adequate servo performance. The tradeoff of a higher servo sector frequency is a reduction in overall capacity due to a reduction in the recording area available for storing user data. In one embodiment, the number of servo sectors written to any one disk surface may be reduced by processing the servo sectors from multiple disk surfaces during an access operation.

FIG. 3Ashows an example embodiment wherein the second set of servo sectors241-24Nare circumferentially offset from the first set of servo sectors221-22Nby a phase offset substantially equal to 180 degrees. The first and second set of servo sectors are written to their respective disk surfaces at a frequency that is less than twice the VCM resonance frequency, wherein processing the first and second set of servo sectors in an interleaved manner effectively doubles the servo sample rate such that the combined servo sample frequency is greater than twice the VCM resonance frequency. This embodiment enables a reduction in the number of servo sectors per disk surface and a corresponding increase in the overall capacity as compared to a disk drive that processes the servo sectors written on a single disk surface.

FIG. 3Bshows control circuitry according to an embodiment for controlling the VCM20based on the first and second set of servo sectors while writing data to at least the first disk surface. In this embodiment, the read signal emanating from each head while reading a respective servo sector is demodulated (blocks441and442) into estimated positions461and462which are subtracted from reference positions to generate respective position error signals PES1and PES2. The first PES1and second PES2are processed by a servo compensator48to generate a control signal40applied to the VCM20in order to position at least the first head over the first disk surface during the access operation. In one embodiment, the VCM20controls the position of both the first and second heads in order to concurrently access both disk surfaces, such as concurrent write or read operations. In this embodiment, the servo compensator48is able to compensate for the VCM resonance frequency since the combined servo sample frequency of the first and second set of servo sectors is greater than twice the VCM resonance frequency.

FIG. 4Ashows an embodiment wherein the second set of servo sectors241-24Nare circumferentially offset from the first set of servo sectors221-22Nby a phase offset unequal to 180 degrees (where a phase offset of 180 degrees occurs at location50). In one embodiment, the second set of servo sectors241-24Nare written to the second disk surface so as to achieve a predetermined phase offset unequal to 180 degrees. For example, in one embodiment the second set of servo sectors241-24Nmay be written to the second disk surface with a phase offset that improves the detection of an off-track condition so that a write operation may be aborted before overwriting data in adjacent data tracks. Referring to the example ofFIG. 4A, writing the second set servo sector241at a phase offset less than 180 degrees means that the second set servo sector241is processed at a reduced interval after processing the first set servo sector221, wherein the reduced interval may result in the earlier detection of an off-track condition.

In one embodiment, a vibration affecting the disk drive may more significantly impact the servoing of the heads over the disk surfaces that are closest to the enclosure of the disk drive. That is, in a disk drive having multiple disks clamped to the spindle motor, the top disk surface of the top disk and the bottom disk surface of the bottom disk are typically more significantly impacted by a vibration affecting the enclosure. Accordingly in one embodiment, the first set of servo sectors221-22Nmay be written to the top surface of a top disk and the second, staggered set of servo sectors241-24Nmay be written to the bottom surface of a bottom disk. In one embodiment, the second set of servo sectors241-24Nmay be written at a phase offset less than 180 degrees as described above to improve the off-track detection of a vibration affecting the top surface of the top disk and the bottom surface of the bottom disk.

In one embodiment shown inFIG. 4B, the servo compensator48of the control circuitry may process only the first PES1generated from reading the first set of servo sectors in order to control the VCM20to position of the first head over the first disk surface while accessing the first disk surface. In this embodiment, an off-track detector52may detect an off-track condition based on the first PES1or the second PES2, for example, if either PES exceeds a predetermined threshold, wherein a write operation is aborted when the off-track condition is detected. In the embodiment ofFIG. 4A, the servo sector frequency of the first set of servo sectors221-22Nis twice the VCM resonance frequency so that the servo compensator48may compensate for disturbances that may excite the VCM resonance frequency.

FIG. 5Ashows a data storage device in the form of a disk drive according to an embodiment wherein the second set of servo sectors241-24Nare circumferentially offset from the first set of servo sectors221-22Nby a phase offset unequal to 180 degrees.FIG. 5Bis a flow diagram according to an embodiment executed by the control circuitry26in order to write data to at least the first disk surface during an access operation (block54).

The first set of servo sectors are read to generate a first PES (block56), and the second set of servo sectors are read to generate a second PES (block58). The second PES is phase shifted by a delta phase to generate a phase shifted PES substantially aligned with a 180 degree phase offset between the first set of servo sectors and the second set of servo sectors (block60). The VCM is controlled based on the first PES and the phase shifted PES to position at least the first head over the first disk surface (block62) while accessing the first disk surface (block64).

FIG. 6Ashows an embodiment wherein the first set of servo sectors221-22Nand the second set of servo sectors241-24Nare processed in an interleave manner in order to control the VCM20to position at least the first head over the first disk surface during an access operation. In this embodiment, the second set of servo sectors241-24Nare circumferentially offset from the first set of servo sectors221-22Nby a phase offset unequal to 180 degree. The phase offset corresponding to 180 degrees occurs at location50, wherein there is a delta phase (Δθ) between the second servo sector241and the 180 degree location50.FIG. 6Bshows control circuitry according to an embodiment wherein the second PES2is filtered with an interpolation filter66in order to phase shift the second PES2by the delta phase (Δθ), thereby generating a phase shifted PES at a 180 degrees phase offset (i.e., aligned with location50inFIG. 6Ain order to achieve a uniform servo sample rate when interleave processing the first and second set of servo sectors). In one embodiment, the off-track detector52may detect an off-track condition during a write operation based on the first PES1, the second PES2, and/or the phase shifted PES output by the interpolation filter66as shown inFIG. 6B. For example, an off-track condition may be detected when any one of these inputs exceeds a threshold, or based on any suitable function of the inputs (e.g., using a suitable prediction algorithm).

FIG. 7shows control circuitry according to an embodiment wherein each head may be actuated by the VCM20together with a suitable secondary actuator68i, such as a secondary actuator configured to actuate a suspension relative to the actuator arm, or configured to actuate the head relative to the suspension. In this embodiment, the first PES1is processed by a first compensator701configured to generate a control signal applied to a first secondary actuator681, and the phase shifted PES output by the interpolation filter66is processed by a second compensator702to generate a control signal applied to a second secondary actuator682. In one embodiment, at least one of the first or second heads are servoed by the control circuitry inFIG. 7in order to access a respective disk surface, and in another embodiment both the first and second heads are servoed by the control circuitry in order to concurrently access both disk surfaces.

In one embodiment, the phase offset between the first and second set of servo sectors may be an intentional phase offset intended to improve the servo control system, such as improving the resolution of the off-track detector52as described above. In another embodiment, the phase offset of the second set of servo sectors241-24Nmay deviate from a target phase offset (e.g., 180 degrees) due to inaccuracies in writing the servo sectors. That is, the delta phase (Δθ) from a target 180 degree phase offset such as shown inFIG. 6Amay be a result of inaccuracies in the servo writing process. In one embodiment, after the servo sectors are written to the disk surfaces the control circuitry may measure the delta phase (Δθ) during a calibration procedure, for example, by reading the first and second set of servo sectors in an interleaved manner and measuring an interval between the servo sectors, wherein a non-uniform interval will correspond to the measured delta phase (Δθ).

In certain embodiments, servo sectors may be written in a staggered format to the top and bottom disk surfaces of a disk such as shown inFIG. 3A. In another embodiment shown inFIG. 8A, the disk drive may comprise multiple disks wherein servo sectors may be written in a staggered format to the disk surfaces of different disks. For example, servo sectors may be written to the top disk surface of disk381, and corresponding staggered servo sectors may be written to the top or bottom disk surface of disk382. In other embodiments, the servo sectors may be written in a staggered format across more than two disk surfaces. For example in the embodiment ofFIG. 8A, the servo sectors may be written to the four disk surfaces in a staggered format with varying phase offsets such as shown inFIG. 8BorFIG. 8C. In this embodiment, the number of servo sectors recorded on each disk surface may be one-fourth of a full set of servo sectors while still achieving the target servo sample rate.

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 data storage controller, or certain operations described above may be performed by a read channel and others by a data storage controller. In one embodiment, the read channel and data storage 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 power large scale integrated (PLSI) circuit implemented as a separate integrated circuit, integrated into the read channel or data storage 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 some embodiments, at least some of the flow diagram blocks may be implemented using analog circuitry (e.g., analog comparators, timers, etc.), and in other embodiments at least some of the blocks may be implemented using digital circuitry or a combination of analog/digital circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, a hybrid disk drive comprising non-volatile semiconductor memory, 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.