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

[0001] 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.

[0002] 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 (Z0) to the outer diameter of the disk (Z2).

[0003] The prior art disk format of FIG. 1A also comprises a number of servo sectors 6₀-6_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.

[0004] To facilitate demodulating the servo sectors 6₀-6_N, a timing control loop generates a disk locked clock synchronized to the data rate of the servo sectors 6₀-6_N. The disk locked clock generates suitable timing information, such as a servo gate that times the circumferential location of the servo sectors 6₀-6_N, and a sync window that times the circumferential location of the sync marks 10 within the servo sectors 6₀-6_N as shown in FIG. 1B.

[0005] In the embodiment of FIG. 1A, the data rate of the servo sectors 6₀-6_N changes in each physical zone similar to the data sectors in order to improve format efficiency. However, a zoned servo sector format may be problematic when a seek operation crosses a servo zone boundary. For example, if the location of a servo zone boundary is estimated incorrectly (too early or too late), the disk drive can lose synchronization to the servo sectors due to incorrect adjustments to servo control parameters, such as an incorrect adjustment to the servo gate, or an incorrect adjustment to the sync window, or an incorrect adjustment to the timing control loop that synchronizes the disk locked clock to the data rate of the servo sectors.

[0022] FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a disk 16 comprising a plurality of servo sectors 18_i0-18_iN (i represents a servo zone) defining a plurality of servo tracks 20, wherein the servo tracks form a plurality of servo zones (e.g., Z0-Z2), 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 disk drive further comprises a head 22 actuated radially over the disk 16, and control circuitry 24 operable to execute the flow diagram of FIG. 2B. A disk locked clock is synchronized to the data rate of the servo sectors in the first servo zone (block 26), wherein the disk locked clock for generating a timing window relative to the servo sectors. A seek operation is executed to seek the head over the disk (block 28), and when the head crosses from the first servo zone to the second servo zone (block 30), at least one of the disk locked clock and the timing window is adjusted to compensate for a timing transient (block 32).

[0023] In the embodiment of FIG. 2A the control circuitry 24 processes a read signal 34 emanating from the head 22 to demodulate the servo sectors 18_i0-18_iN 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 24 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 22 radially over the disk 16 in a direction that reduces the PES. The servo sectors 18_i0-18_iN 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.

[0024] Varying the servo data rate across the servo zones (by increasing the servo data rate toward the outer diameter servo zones) increases the capacity of the disk 16 by reducing the area consumed by the servo sectors. To facilitate a servo zone crossing during a seek operation, at least one of the disk locked clock and the timing window is adjusted to compensate for a timing transient. FIG. 2C shows an embodiment wherein the timing window comprises a servo gate that times a circumferential location of the servo sectors 18_i0-18_iN, and a sync window that times a circumferential location of a sync mark 10 within the servo sectors 18_i0-18₁N.

[0025] Any suitable timing control loop may be employed in the embodiments of the present invention for generating the disk locked clock and the timing windows. FIG. 3 shows an example timing control loop comprising a phase locked loop (PLL) for synchronizing a sampling device 42 to the data rate of the read signal 34. An expected sample generator 44 generates expected samples 46 corresponding to a target response (e.g., a suitable partial response) relative to the read signal samples 48. A phase error detector 50 generates a phase error 52 in response to the expected samples 46 and the read signal samples 48. The phase error 52 is filtered with a suitable compensation filter 54 to generate a control signal 56. The control signal 56 is added 58 to a center frequency 60 to generate a frequency control signal 62 for a voltage controlled oscillator (VCO) 64. The output of the VCO 64 is a disk locked clock 66 that adjusts the frequency of the sampling device 42 until the read signal 34 is sampled synchronously. The disk locked clock 66 is also used to clock a signal generator 68 that generates the servo gate 70 and sync window 72. In another embodiment, the timing control loop may comprise a fixed rate sampling clock with an interpolator for generating the synchronous sample values and for timing the servo gate and sync window. When the head 22 crosses a servo zone boundary during a seek operation, a control signal 74 configures the signal generator 68 in order to adjust the servo gate 70 and/or the sync window 72.

[0026] FIG. 4 shows an embodiment of the present invention wherein the timing window (e.g., servo gate 70 and or sync window 72) are generated by clocking a counter 76 with the disk locked clock 66. When the counter 76 reaches a first target count, a signal 78 is toggled to indicate the start of the timing window. When the counter 76 reaches a second target count, the signal 78 is toggled to indicate the end of the window and the counter 76 is reloaded by signal 80 with an inter-window gap 82 in order to start the timing interval to the start of the next window. When the head 22 crosses a servo zone boundary during a seek operation, a transition offset 84 is added 86 to the inter-window gap 82 in order to compensate for the timing transient. In one embodiment, the timing transient is due to changing the center frequency 60 of the VCO 64 of the timing control loop shown in FIG. 3 which changes the frequency of the disk locked clock 66 that clocks the counter 76. The resulting timing transient may cause a positive or negative offset in the timing window, and therefore the transition offset 84 added to the inter-window gap 82 may be negative or positive, respectively. As described in greater detail below, the transition offset 84 may be calibrated by adjusting the transition offset 84 over multiple seek operations until a metric is optimized.

[0027] FIG. 5A shows an alternative embodiment for compensating for the timing transient when the head 22 crosses a servo zone boundary by generating a transient in the center frequency 60 of the VCO 64 that generates the disk locked clock 66. In the example of FIG. 5A, the center frequency is being adjusted from reading a lower data rate servo zone to a higher data rate servo zone. In this embodiment, there is a delay in the center frequency 60 reaching the target value, and a corresponding delay in the frequency of the disk locked clock 66. To compensate for the delay, an overshoot is added to the center frequency 60 for a predetermined interval before being reduced to the target value as shown in FIG. 5A. The overshoot is selected so that on average the number of clock cycles of the disk locked clock 66 over the inter-window gap is the same as the number of clock cycles when the disk locked clock 66 reaches steady state. The center frequency 60 may be adjusted by any suitable transient, wherein in an embodiment shown in FIG. 5B the transient may comprise an oscillation with a calibrated magnitude and frequency that may improve the timing compensation during a servo zone crossing.

[0028] FIG. 6A illustrates an embodiment of the present invention wherein the servo sectors 18_i0-18_iN are recorded so they overlap by a controlled amount at the servo zone boundaries as illustrated in FIG. 6B. In one embodiment, the overlapping servo sectors 18_i0-18_iN are interleaved within the transition zone such that the servo sectors of each servo zone pass under the head 22 in an alternating sequence. This is accomplished by recording the servo sectors 18_i0-18_iN with a circumferential offset between servo zones as illustrated in FIG. 6B. While the head 22 is within the transition zone during a seek operation, at least one of the disk locked clock and the timing window is adjusted in order to transition between the servo zones.

[0029] In one embodiment, user data is written in the data tracks outside the transition zones, whereas user data is not written in the data tracks within the transition zone. In this manner, the format complexity of accounting for twice the number of servo sectors within the transition zone is avoided. In one embodiment, the head 22 comprises a read element and a write element separated by a radial offset, and the transition zone shown in FIG. 6B is at least as wide as the radial offset between the read element and write element. This embodiment may help ensure both the read element and the write element are within the same servo zone when performing an access operation (write or read) of a data track.

[0030] 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.

[0031] FIG. 7A illustrates an embodiment of the present invention wherein the servo gate and the sync window are adjusted when transitioning between servo zones. In the example of FIG. 7A, the head 22 is seeking from servo zone Z0 to servo zone Z1 in FIG. 6A. When reading the servo sectors in servo zone Z0, the servo gate and sync window are opened relative to servo sectors 18₀₃ and 18₀₄. When a servo zone transition is executed, the servo gate and sync window are adjusted (increased) so as to time the circumferential location of the servo sector 18₁₄ in servo zone Z1. A positive transition offset 84 may be added to the inter-window gap 82 as described above with reference to FIG. 4 to compensate for the timing transient due to the servo zone crossing. Alternatively in the example shown in FIG. 7A, the center frequency 60 of the VCO 64 that generates the disk locked clock 66 may be adjusted using a suitable transient when increasing the frequency toward the servo data rate of servo zone Z1 as described above with reference to FIGS. 5A and 5B. FIG. 7B illustrates an example where the head 22 is seeking from servo zone Z1 to servo zone Z0 in FIG. 6A. In this example, a negative transition offset 84 may be added to the inter-window gap 82 as described above with reference to FIG. 4, and/or the center frequency 60 of the VCO 64 that generates the disk locked clock 66 may be adjusted using a suitable transient when decreasing the frequency toward the servo data rate of servo zone Z0.

[0032] FIG. 8A is a flow diagram according to an embodiment of the present invention for calibrating the adjustments to the disk locked clock and/or the timing window. After seeking the head across the disk (block 28) and across a servo zone boundary (block 30), at least one of the disk locked clock and the timing window is adjusted (block 32). A metric is measured and saved representing an efficacy of the adjustment (block 90). For example, the metric of the efficacy may be whether the sync mark in the following servo sector of the next servo zone is accurately detected. Other metrics of the efficacy may include an accumulated timing recovery error, an accumulated gain control error, or an accumulated sample error when acquiring the preamble of the following servo sector in the next servo zone. The flow diagram is repeated (block 92) starting from block 26 using different adjustments to the disk locked clock and/or timing window with a corresponding metric measured and saved. The adjustment that generates the optimal metric is then selected as the operating adjustment used during normal operation of the disk drive (block 94).

[0033] FIG. 8B is a flow diagram according to an embodiment of the present invention which expands on the flow diagram of FIG. 8A, wherein when the operating adjustment is selected corresponding to the optimal metric (block 94), the ambient temperature is also measured (block 96). The selected operating adjustment is then saved in an array indexed by the ambient temperature. During normal operation when the head 22 crosses a servo zone during normal operation (block 30) as shown in the flow diagram of FIG. 9, the ambient temperature is measured (block 100) and used to lookup of the corresponding timing adjustment from the array (block 102). This embodiment may help compensate for the effect ambient temperature may have on the operating characteristics of the timing recovery loop (e.g., on the VCO 64 that generates the disk locked clock 66).

[0034] The flow diagram of FIG. 9 also illustrates an embodiment wherein after making the timing adjustment during a servo zone crossing (block 32), the timing window (e.g., servo gate or sync window) may be opened early as the head approaches the following servo sector in the next servo zone (block 104). This embodiment may help compensate for the inaccuracy of the timing adjustment during a servo zone crossing by increasing the tolerance of the timing window(s).

[0035] The timing adjustment may be made at any suitable time during a servo zone crossing after reading the last servo sector of the current servo zone and prior to the head reaching the following servo sector in the next servo zone. FIG. 10A shows an embodiment of the present invention wherein the timing adjustment is made at the end of the servo gate window and prior to computing the PES for the last servo sector 18₁₃ in the current servo zone. In an alternative embodiment shown in FIG. 10B, the timing adjustment is made after computing the PES for the last servo sector 18₁₃ in the current servo zone. The latter embodiment reduces the transport delay of the servo control system by computing the PES first, and then making the timing adjustment. The latency associated with computing the PES in FIG. 10B is taken into account when calibrating the timing adjustment to the disk locked clock and/or the timing window. In one embodiment, the delay in making the timing adjustment is measured relative to the disk locked clock, and the delay (in clock cycles) is selected to be at least as long as the latency required to compute the PES in FIG. 10B.

[0036] 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 an SOC.

[0037] 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.