Disk drive adjusting estimated servo state to compensate for transient when crossing a servo zone boundary

Abstract

A disk drive is disclosed comprising a disk comprising a plurality of servo sectors defining a plurality of servo tracks. The servo tracks form a plurality of servo zones, where a servo data rate of servo sectors in a first servo zone is different than a servo data rate of servo sectors in a second servo zone. A servo control system servos a head over the disk. An estimated servo state of a servo control system is generated, and when the head crosses from a first servo zone to a second servo zone, the estimated servo state is adjusted to compensate for a transient in a circumferential distance between a servo sector in the first servo zone and a servo sector in the second servo zone.

Claims

What is claimed is: 1. A disk drive comprising: a disk comprising a plurality of servo sectors defining a plurality of servo tracks, wherein: the servo tracks form a plurality of servo zones; and a servo data rate of servo sectors in a first servo zone is different than a servo data rate of servo sectors in a second servo zone; a head actuated radially over the disk; and control circuitry comprising a plurality of read channels where each read channel comprises an equalizer configured to equalize signal samples of the second read signal to generate equalized samples, the control circuitry operable to: receive a first read signal from the head while reading servo data from the servo sectors; program each read channel with a unique configuration to process the first read signal in parallel, wherein each configuration corresponds to a servo data rate of a respective servo zone; receive a second read signal from the head while reading servo data from the disk; and program the equalizer of each read channel with a unique configuration to process the second read signal in parallel, wherein each configuration has a different propensity to recover the servo data from the equalized samples. 2. The disk drive as recited in claim 1 , wherein: each read channel comprises a timing recovery circuit; each timing recovery circuit generates a respective disk locked clock to a servo data rate; the control circuitry is further operable to program the timing recovery circuit of each read channel with the unique configuration to process the first read signal in parallel; and each configuration attempts to synchronize the respective disk locked clocks to the respective servo data rates of the servo zones. 3. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to detect the servo zone the head is over in response to the read channel that successfully synchronizes the respective disk locked clock to the servo data rate of the servo zone the head is over. 4. A method of operating a disk drive, the disk drive comprising a head actuated over a disk comprising a plurality of servo sectors defining a plurality of servo tracks, and a plurality of read channels, wherein each read channel comprises an equalizer configured to equalize signal samples of the second read signal to generate equalized samples, the servo tracks form a plurality of servo zones, and a servo data rate of servo sectors in a first servo zone is different than a servo data rate of servo sectors in a second servo zone, the method comprising: receiving a first read signal from the head while reading servo data from the servo sectors; programming each read channel with a unique configuration to process the first read signal in parallel, wherein each configuration corresponds to a servo data rate of a respective servo zone; receiving a second read signal from the head while reading servo data from the disk; and programming the equalizer of each read channel with a unique configuration to process the second read signal in parallel, wherein each configuration has a different propensity to recover the servo data from the equalized samples. 5. The method as recited in claim 4 , wherein: each read channel comprises a timing recovery circuit; each timing recovery circuit generates a respective disk locked clock to a servo data rate; the method further comprises programming the timing recovery circuit of each read channel with the unique configuration to process the first read signal in parallel; and each configuration attempts to synchronize the respective disk locked clocks to the respective servo data rates of the servo zones. 6. The method as recited in claim 5 , further comprising detecting the servo zone the head is over in response to the read channel that successfully synchronizes the respective disk locked clock to the servo data rate of the servo zone the head is over.
CROSS REFERENCE TO RELATED APPLICATIONS This application is a divisional of U.S. patent application Ser. No. 13/529,958 filed on Jun. 21, 2012, which is hereby incorporated by reference in its entirety. BACKGROUND 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 velocity of the actuator arm as it seeks from track to track. Because the disk is rotated at a constant angular velocity, the user data rate is typically increased toward the outer diameter tracks (where the surface of the disk is spinning faster) in order to achieve a more constant linear bit density across the radius of the disk. To simplify design considerations, the data tracks are typically banded together into a number of physical zones, wherein the user data rate is constant across a zone, and increased from the inner diameter zones to the outer diameter zones. This is illustrated in FIG. 1A , which shows a prior art disk format 2 comprising a number of data tracks 4 , wherein the data tracks are banded together in this example to form three physical zones from the inner diameter of the disk (Z 0 ) to the outer diameter of the disk (Z 2 ). The prior art disk format of FIG. 1A also comprises a number of servo sectors 6 0 - 6 N recorded around the circumference of the disk 2 that define a plurality of servo tracks, wherein the data tracks 4 are defined relative to the servo tracks. Each servo sector 6 i may comprise 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 track address, used to position the head over a target data track during a seek operation. Each servo sector 6 i may further comprise groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. To facilitate demodulating the servo sectors 6 0 - 6 N , a timing control loop generates a disk locked clock synchronized to the data rate of the servo sectors 6 0 - 6 N . The disk locked clock generates suitable timing information, such as a servo gate that times the circumferential location of the servo sectors 6 0 - 6 N , and a sync window that times the circumferential location of the sync marks 10 within the servo sectors 6 0 - 6 N as shown in FIG. 1B . In the embodiment of FIG. 1A , the data rate of the servo sectors 6 0 - 6 N changes in each physical zone similar to the data sectors in order to improve format efficiency. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A shows a prior art disk format comprising a plurality of servo zones defined by servo sectors recorded at varying data rates. FIG. 1B shows prior art servo timing signals, including a servo gate and a sync window. FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head actuated over a disk comprising zoned servo sectors. FIG. 2B is a flow diagram according to an embodiment of the present invention wherein during a servo zone crossing an estimated state of a servo control system is adjusted to compensate for a transient in a circumferential distance between a servo sector in the first servo zone and a servo sector in the second servo zone. FIG. 2C illustrates an embodiment of the present invention wherein an estimated radial position of the head is increased during a servo zone crossing to compensate for an increase in the interval between consecutive servo sectors. FIG. 3 illustrates an embodiment of the present invention wherein an estimated radial position of the head is decreased during a servo zone crossing to compensate for a decrease in the interval between consecutive servo sectors. FIG. 4 shows an embodiment of the present invention wherein during a servo zone crossing the estimated radial position of the head is adjusted based on an estimated radial velocity of the head. FIG. 5A shows an embodiment of the present invention wherein the disk drive comprises a plurality of read channels programmed with unique configurations. FIG. 5B shows a read channel comprising programmable components according to an embodiment of the present invention. FIG. 6 is a flow diagram according to an embodiment of the present invention wherein the read channels are programmed relative to each servo zone when detecting the servo zone the head is over, or programmed with different data detection propensities when reading servo data during an access operation. DETAILED DESCRIPTION FIG. 2A shows a disk drive comprising a disk 16 comprising a plurality of servo sectors 18 0 - 18 N defining a plurality of servo tracks 20 . The servo tracks 20 form a plurality of servo zones (Z 0 -Z 2 ), where a servo data rate of servo sectors in a first servo zone is different than a servo data rate of servo sectors in a second servo zone. The disk drive further comprises control circuitry 22 comprising a servo control system operable to servo a head 24 over the disk 16 . The control circuitry 22 is operable to execute the flow diagram of FIG. 2B wherein an estimated servo state of the servo control system is generated (block 26 ) while seeking the head over the disk (block 28 ). When the head crosses from the first servo zone to the second servo zone (block 30 ), the estimated servo state is adjusted (block 32 ) to compensate for a transient in a circumferential distance between a servo sector in the first servo zone and a servo sector in the second servo zone. In the embodiment of FIG. 2A the control circuitry 22 processes a read signal 34 emanating from the head 24 to demodulate the servo sectors 18 0 - 18 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 suitable compensation filters to generate a control signal 36 applied to a voice coil motor (VCM) 38 which rotates an actuator arm 40 about a pivot, thereby actuating the head 24 radially over the disk 16 in a direction that reduces the PES. The servo sectors 18 0 - 18 N may comprise any suitable position information, such as a track and wedge address for coarse positioning and servo bursts for fine positioning as described above with reference to FIG. 1A . The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern as shown in FIG. 1A , or a suitable phase based servo pattern. In the embodiment of FIG. 2A , a boundary of a first servo zone (e.g., Z 0 ) overlaps with a boundary of a second servo zone (e.g., Z 1 ) over a transition zone, and the servo sectors of the first servo zone are interleaved with the servo sectors of the second servo zone within the transition zone as illustrated in FIG. 2C . The overlapped servo sectors in the transition zone facilitates switching between the first servo zone and the second servo zone during a seek operation. In the example shown in FIG. 2C , the head 24 is seeking across the disk 16 from servo zone Z 0 to servo zone Z 1 . When the head 24 reaches the transition zone, the servo timing is adjusted to delay the servo gate so as to read the servo sector in servo zone Z 1 (instead of the servo sector in servo zone Z 0 ). The adjustment increases the interval between consecutive servo sectors which introduces a delta in the estimated state of the servo control system, such as an estimated radial position of the head. This is illustrated in FIG. 2C wherein a state estimator may estimate the radial position of the head at the next servo sector i+1 based on the servo sector 42 in servo zone Z 0 . However, since the servo timing is adjusted to read the servo sector 44 in servo zone Z 1 , there is a corresponding radial position delta (+Δ) that is added to the estimated radial position of the head in order to compensate for the transient. FIG. 3 shows an example wherein when seeking the head from servo zone Z 1 to servo zone Z 0 , the servo sector 42 in servo zone Z 0 is reached sooner than the servo sector 44 in servo zone Z 1 . There is therefore a corresponding radial position delta (−Δ) that is subtracted from the estimated radial position of the head in order to compensate for the transient. In one embodiment, the estimated position of the head is adjusting during a servo zone crossing based on an estimated radial velocity of the head. This is because the magnitude of the radial position delta (A) in FIG. 2C and FIG. 3 is proportional to the radial velocity of the head; that is, the higher the radial velocity the more significant the adjustment to the estimated radial position. FIG. 4 illustrates this embodiment wherein a control signal 46 selects a time delta +δ or −δ 48 depending on the direction of the servo zone crossing, where the time delta δ represents the time transient (positive or negative) due to the transient in the interval between a servo sector in the first servo zone and a servo sector in the second servo zone as described above. The selected time delta is scaled by a scalar 50 having a magnitude based on the estimated radial velocity 52 of the head. The resulting scaled radial position delta +Δ or −Δ is enabled by control signal 54 through gate 56 during the transition to the next servo zone. The scaled radial position delta +Δ or −Δ is added to an estimated radial position 58 generated by a state estimator 60 to generate an adjusted estimated radial position 62 . FIG. 4 also illustrates an embodiment of the present invention wherein a measured position 64 of the head is generated from the position information derived from reading a servo sector. The adjusted estimated radial location 62 is compared 66 to the measured radial location 64 , and the result 68 used to control a multiplexer 70 . If the difference between the adjusted estimated radial location 62 and the measured radial location 64 is less than a threshold, then the servo control system uses the measured radial location 64 to servo the head over the disk. If the difference is greater than the threshold, then the measured radial location 64 is deemed unreliable (e.g., due to reading a defective servo sector) and therefore the adjusted estimated radial location 62 is used to servo the head over the disk. Any suitable estimated servo state may be adjusted in the embodiments of the present invention when transitioning between servo zones (instead of, or in addition to the estimated position of the head). The adjustment to the estimated servo state(s) may be generalized by considering the equations that define the servo control system. Consider a simple continuous-time rigid-body state-space model 1 s 2 represented in continuous-time by: {dot over (x)}=Ax+Bu y=Cx+Du where: [ A | B C | D ] = [ 0 1 | 0 0 0 | γ 1 0 | 0 ] with the radial position and velocity of the head denoted by x 1 and x 2 respectively. The above continuous-time equation can be transformed into discrete-time assuming a nominal sampling period of T and a 4× multi-rate zero-order-hold (ZOH) control system with values denoted by u 1 , u 2 , u 3 , and u 4 for the respective time intervals given by: [ t 0 , t 0 + T 4 ) , [ t 0 + T 4 , t 0 + T 2 ) , [ t 0 + T 2 , t 0 + 3 ⁢ ⁢ T 4 ) , [ t 0 + 3 ⁢ ⁢ T 4 , t 0 + T + δ ) ⁢ ⁢ then x ~ ⁡ ( t 0 + T + δ ) = [ 1 T + δ T 0 1 ] A ~ d ⁢ x ~ ⁡ ( t 0 ) + γ ⁢ [ 7 32 + δ 4 ⁢ ⁢ T 5 32 + δ 4 ⁢ ⁢ T 3 32 + δ 4 ⁢ ⁢ T ( T + 4 ⁢ δ ) 2 32 ⁢ ⁢ T 2 1 4 1 4 1 4 1 4 + δ T ] B ~ d ⁢ [ u ~ 1 u ~ 2 u ~ 3 u ~ 4 ] u ~ ⁢ ⁢ where S x = [ r ⁢ ⁢ α 0 0 rT ⁢ ⁢ α ] , S u = [ r ⁢ ⁢ T 2 ⁢ α ] , x ~ = S x ⁢ x , u ~ = S u ⁢ u r represents the VCM 38 actuator arm 40 length (e.g., inches) from the pivot point to the head 24 , and α represents radial track density units (e.g., tracks per inch), thereby normalizing the position and velocity states {tilde over (x)} to respective units of servo tracks and servo tracks/sample. In the above equations, γ represents the gain of the servo actuator (e.g., VCM). The above equations demonstrate how the time delta δ can be used to adjust the estimated servo states during a servo zone crossing (i.e., adjust the estimated radial position and the estimated radial velocity in this embodiment). In one embodiment, the data tracks are also banded together to define data zones, wherein the user data rate is increased toward the outer diameter zones in order to improve the format efficiency. The number of data zones may equal the number of servo zones, or the number of data zones may be less than or greater than the number of servo zones. The boundaries of the data zones may align with the boundary of a servo zone, or the data zone boundaries may be located at radial locations different from the boundaries of the servo zones. In the embodiment shown in FIG. 2A the servo sectors are offset circumferentially between the servo zones to facilitate overlapping the servo sectors. In another embodiment, the servo sectors may be aligned across the servo zones without overlapping the servo sectors similar to the format shown in FIG. 1A . FIG. 5A shows an embodiment of the present invention wherein the control circuitry 22 of the disk drive comprises a plurality of read channels 72 1 - 72 N . The control circuitry 22 is further operable to execute the flow diagram shown in FIG. 6 , wherein a first read signal is received from the head while reading servo data from the servo sectors (block 74 ), and each read channel is programmed with a unique configuration to process the first read signal in parallel (block 76 ), wherein each configuration corresponds to a servo data rate of a respective servo zone so that the current servo zone the head is over can be determined (block 78 ). A second read signal is received from the head while reading servo data from the disk (block 80 ), and each read channel is programmed with a unique configuration (block 82 ) to process the second read signal in parallel, wherein each configuration has a different propensity to recover the servo data. In the embodiment of FIG. 6 , the read channel having the highest quality output is selected for use by the servo control system (block 84 ). Accordingly, in the embodiment of FIG. 6 the first read signal is generated when detecting the current servo zone the head is over, and the second read signal is generated when attempting to read the servo data in the current servo zone. Each read channel 72 i in the embodiment of FIG. 5A may comprise any suitable components, where FIG. 5B shows an example read channel comprising suitable gain control and timing recovery circuitry. The gain control circuitry comprises a preamp 86 and a variable gain amplifier (VGA) 88 that amplifies the read signal 34 to generate an amplified read signal 90 . The amplified read signal 90 is filtered with a continuous time filter (CTF) 92 in order to equalize the amplified read signal 90 according to a target response (e.g., a target partial response such as PR4, EPR4, etc.). The output 94 of the CTF 92 is sampled by a sampling device 96 to generate discrete-time read signal samples 98 . An equalizer filter 100 equalizes the read signal samples 98 according to the target response to generate equalized samples 102 . A detector 104 (e.g., a Viterbi detector) processes the equalized samples 102 to generate a data sequence 106 representing the detected servo data. A sample estimator 108 (e.g., a slicer) estimates a target sample value 110 from an equalized sample value 102 . The target sample value 110 and equalized sample value 102 are processed by timing recovery 112 to synchronize a disk locked clock 114 to the data rate of the servo data, and processed by gain control 116 to generate a VGA gain setting 118 for adjusting the gain of the VGA 88 . Other embodiments may employ interpolated timing recovery wherein synchronous read signal samples are generated by interpolating asynchronous read signal samples. When processing the read signal 34 to determine the current servo zone the head is over, the control circuitry 22 programs the components of the read channel shown in FIG. 5B according to different possible servo zones. For example, the control circuitry 22 may program the equalizer filter 100 with different coefficients that may provide optimal equalization depending on the servo zone (the coefficients may be pre-calibrated). In another embodiment, the control circuitry 22 may program the timing recovery 112 in order to synchronize the respective disk locked clocks to the respective servo data rates of the servo zones. For example, the control circuitry 22 may program a center frequency of a phased-locked loop (PLL) with a value corresponding to the servo data rates of the servo zones. In one embodiment, the current servo zone the head is over is determined by the read channel that successfully synchronizes the respective disk locked clock to the servo data rate of the servo zone the head is over (for example, successfully detects the sync mark in the servo sectors after synchronizing to the preamble). Once the current servo zone has been determined, the read signal 34 may be processed in parallel by the read channels 72 1 - 72 N in order to reliably detect the servo data in the servo sectors. The components in each read channel 72 i may be programmed into different configurations each having a different propensity to recover the servo data. For example, each equalizer filter 100 may be programmed with different coefficients in order to equalize the read signal samples 98 into different responses, wherein one of the responses will likely be closest to the target response. Similarly, the timing recovery 112 and/or gain control 116 may be programmed into different configurations to provide a range of performance propensities. When processing the read signal 34 in parallel, a suitable quality metric is measured to determine which read channel is providing the best performance (and therefore the most reliable output). Any suitable quality metric may be measured, such as measuring a mean squared error between the equalized samples 102 and target samples of the target response. Other quality metrics may include an accumulated timing recovery error, an accumulated gain control error, or a metric generated by the detector 104 . In one embodiment, one or more of the read channels 72 1 - 72 N may be programmed with suboptimal values in order to detune the read channel. The output of the detuned read channel(s) may then be evaluated to help verify the reliability of the read channel selected to output the detected servo data to the servo control system. For example, errors in the detected servo data may be identified by comparing the outputs of the read channels (where a mismatch represents an error). If the number of errors exceeds a threshold, the output of the selected read channel may be considered unreliable and the selected servo data discarded. Any suitable control circuitry may be employed to implement the flow diagrams in the embodiments of the present invention, 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.

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