Servoing on concentric servo sectors of a first disk surface to write a spiral servo track to a second disk surface

[0001] When manufacturing a disk drive, concentric servo sectors 2₀-2_N are written to a disk 4 which define a plurality of radially-spaced, concentric data tracks 6 as shown in the prior art disk format of FIG. 1. Each data track 6 is partitioned into a plurality of data sectors wherein the concentric servo sectors 2₀-2_N are considered “embedded” in the data sectors. Each servo sector (e.g., servo sector 2₄) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a data field 12 comprising coarse head positioning information such as a track number, and servo bursts 14 which provide fine head positioning information. The coarse head position information is processed to position a head over a target track during a seek operation, and the servo bursts 14 are processed to maintain the head over a centerline of the target track while writing or reading data during a tracking operation.

[0002] In the past, external servo writers have been used to write the concentric servo sectors 2₀-2_N to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the concentric servo sectors 2₀-2_N are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the head disk assembly (HDA) without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process.

[0003] The prior art has suggested various “self-servo” writing methods wherein the internal electronics of the disk drive are used to write the concentric servo sectors independent of an external servo writer. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral servo tracks to the disk which are then processed to write the concentric servo sectors along a circular path. Each spiral servo track is written to the disk as a high frequency signal (with missing bits), wherein the position error signal (PES) for tracking is generated relative to time shifts in the detected location of the spiral servo tracks. The read signal is rectified and low pass filtered to generate a triangular envelope signal representing a spiral servo track crossing, wherein the location of the spiral servo track is detected by detecting a peak in the triangular envelope signal relative to a clock synchronized to the rotation of the disk.

[0004] The spiral servo tracks in the '679 patent are written by the control circuitry within each disk drive by controlling the velocity of the head open loop (no feedback during the constant velocity segment of the velocity profile). However, controlling the velocity of the actuator arm open loop means that the actual velocity of the head relative to the disk will vary while writing each spiral servo track, as well as vary between each spiral servo track. The velocity errors when writing each spiral servo track leads to tracking and timing errors when writing the concentric servo sectors.

[0019] FIG. 2 is a flow diagram according to an embodiment of the present invention. Concentric servo sectors are read across substantially the entire radius of a first disk surface using a first head to generate a read signal (step 16). The read signal is processed to generate a position error signal (PES) representing a radial location of the first head relative to the first disk surface (step 18). The PES is processed to generate a control signal applied to the VCM to move a second head radially over a second disk surface (step 20) while writing a spiral servo track on the second disk surface (step 22).

[0020] FIG. 3A illustrates an embodiment of the present invention wherein a first disk surface 24A comprises a plurality of concentric servo sectors 26₀-26_N forming wedges from the inner to outer diameter of the disk. The concentric servo sectors 26₀-26_N are read in order to servo a first head radially over the first disk surface at a predetermined velocity while a second head writes a spiral servo track 28_i to a second disk surface 24B. This process is repeated in order to write a plurality of spiral servo tracks 28₀-28_N to the second disk surface 24B as shown in FIG. 3B.

[0021] The concentric servo sectors 26₀-26_N may be recorded on the first disk surface 24A using any suitable technique. In one embodiment, the disk is installed into a head disk assembly (HDA) of the disk drive and then the concentric servo sectors 26₀-26_N are written to the first disk surface 24A using an external servo writer. In another embodiment, a media writer writes the concentric servo sectors 26₀-26_N to the first disk surface 24A, and then the disk is installed into the HDA of a disk drive. In yet another embodiment, the concentric servo sectors 26₀-26_N may be recorded to the first disk surface 24A using a magnetic printing technique.

[0022] In one embodiment, after writing the spiral servo tracks 28₀-28_N to the second disk surface 24B, a plurality of concentric servo sectors 30₀-30_N are written to the second disk surface 24B (FIG. 3C) in response to the spiral servo tracks 28₀-28_N. Further details of how the concentric servo sectors 30₀-30_N are written in response to the spiral servo tracks 28₀-28_N are provided below with reference to FIG. 9.

[0023] FIG. 4 shows a flow diagram according to an embodiment of the present invention which is an extension of the flow diagram of FIG. 2, wherein spiral servo tracks are written to a first disk surface (step 32). The spiral servo tracks may be written using any suitable technique, such as with an external spiral servo writer, media writer, or magnetic printing technique. The spiral servo tracks are then read from the first disk surface (step 34) in order to write concentric servo sectors to the first disk surface (step 35). The concentric servo sectors on the first disk surface are then processed to write spiral servo tracks to a second disk surface (steps 16-22).

[0024] FIG. 5A illustrates an embodiment of the present invention wherein a plurality of spiral servo tracks are written to a first disk surface 24A which are then processed in order to write a plurality of concentric servo sectors to the first disk surface (FIG. 5B). Thereafter, the concentric servo sectors on the first disk surface 24A are processed to write a plurality of spiral servo tracks to a second disk surface (FIG. 5C). The spiral servo tracks on the second disk surface 24B are then processed to write a plurality of concentric servo sectors on the second disk surface (FIG. 5D).

[0025] FIG. 6A shows an embodiment of the present invention wherein an external spiral servo writer 36 is used to write a plurality of spiral servo tracks 28₀-28_N to a first disk surface 24A of a disk drive 38, wherein each spiral servo track 28_i comprises a high frequency signal 40 interrupted by a sync mark 42 at a sync mark interval (FIG. 8B). The disk drive 38 comprises control circuitry 44 and a head disk assembly (HDA) 46 comprising the disk, an actuator arm 48, a head 50 coupled to a distal end of the actuator arm 48, and a voice coil motor 52 for rotating the actuator arm 48 about a pivot to position the head 50 radially over the disk. In one embodiment, a write clock is synchronized to the rotation of the disk, and the plurality of spiral servo tracks 28₀-28_N are written on the disk at a predetermined circular location determined from the write clock.

[0026] The external spiral servo writer 36 comprises a head positioner 54 for actuating a head positioning pin 56 using sensitive positioning circuitry, such as a laser interferometer. While the head positioner 56 moves the head 50 at a predetermined velocity over the stroke of the actuator arm 48, pattern circuitry 58 generates the data sequence written to the disk for a spiral servo track 28. The external spiral servo writer 36 inserts a clock head 60 into the HDA 46 for writing a clock track 62 (FIG. 6B) at an outer diameter of the disk. The clock head 60 then reads the clock track 62 to generate a clock signal 64 processed by timing recovery circuitry 66 to synchronize the write clock 68 for writing the spiral servo tracks 28₀-28_N to the disk. The timing recovery circuitry 66 enables the pattern circuitry 58 at the appropriate time relative to the write clock 68 so that the spiral servo tracks 28₀-28_N are written at the appropriate circular location. The timing recovery circuitry 66 also enables the pattern circuitry 58 relative to the write clock 68 to write the sync marks 42 (FIG. 8B) within the spiral servo tracks 28₀-28_N at the same circular location from the outer diameter to the inner diameter of the disk. As described below with reference to FIG. 9, the constant interval between sync marks 42 (independent of the radial location of the head 50) enables the servo write clock to maintain synchronization while writing the concentric servo sectors to the disk.

[0027] In the embodiment of FIG. 6B, each spiral servo track 28_i is written over a partial revolution of the disk. In an alternative embodiment, each spiral servo track 28_i is written over one or more revolutions of the disk. FIG. 7A shows an embodiment wherein each spiral servo track 28_i is written over multiple revolutions of the disk. The length of each spiral servo track 28_i (and number of revolutions spanned) is determined by the velocity of the actuator arm 48 as controlled by the VCM 52 in response to a velocity profile an example of which is shown in FIG. 7B. A velocity error signal (derivative of the PES) relative to the velocity profile is generated in response to the concentric servo sectors written on the first disk surface 24A.

[0028] Referring again to the embodiment of FIG. 6A, after the external spiral servo writer 36 writes the spiral servo tracks 28₀-28_N to the first disk surface 24A, the head positioning pin 56 and clock head 60 are removed from the HDA 46 and the concentric servo sectors are written to the first disk surface 24A during a “fill operation”. In one embodiment, the control circuitry 44 within the disk drive 38 is used to process the spiral servo tracks 28₀-28_N in order to write the concentric servo sectors. In an alternative embodiment, an external concentric servo writer is used to process the spiral servo tracks 28₀-28_N in order to write the concentric servo sectors to the first disk surface 24A.

[0029] FIG. 8B illustrates an envelope of the read signal as the head 50 passes over a spiral servo track 28_i. The read signal representing the spiral servo track comprises high frequency transitions 40 interrupted by sync marks 42. When the head 50 moves in the radial direction, the envelope will shift (left or right) while the sync marks 42 remain fixed. The shift in the envelope (detected from the high frequency signal 40) relative to the sync marks 42 provides the off-track information (position error signal or PES) for servoing the head 50.

[0030] FIG. 8A shows an embodiment of the present invention wherein a saw-tooth waveform 70 is generated by clocking a modulo-N counter with the servo write clock, wherein the frequency of the servo write clock is adjusted until the sync marks 42 in the spiral servo tracks 28₀-28_N are detected at a target modulo-N count value. The servo write clock may be generated using any suitable circuitry, such as a phase locked loop (PLL). As each sync mark 42 in the spiral servo tracks 28₀-28_N is detected, the value of the modulo-N counter represents the phase error for adjusting the PLL. In one embodiment, the PLL is updated when any one of the sync marks 42 within the envelope is detected. In this manner the multiple sync marks 42 in each envelope (each spiral servo track crossing) provides redundancy so that the PLL is still updated if one or more of the sync marks 42 are missed due to noise in the read signal. Once the sync marks 42 are detected at the target modulo-N counter values, the servo write clock is coarsely locked to the desired frequency for writing concentric servo sectors to the disk.

[0031] In one embodiment, the servo write clock is further synchronized by generating a timing recovery measurement from the high frequency signal 40 between the sync marks 42 in the spiral servo tracks 28₀-28_N. Synchronizing the servo write clock to the high frequency signal 40 helps maintain proper radial alignment (phase coherency) of the Gray coded track addresses in the concentric servo sectors. The timing recovery measurement may be generated in any suitable manner. In one embodiment, the servo write clock is used to sample the high frequency signal 40 and the signal sample values are processed to generate the timing recovery measurement. The timing recovery measurement adjusts the phase of the servo write clock (PLL) so that the high frequency signal 40 is sampled synchronously. In this manner, the sync marks 42 provide a coarse timing recovery measurement and the high frequency signal 40 provides a fine timing recovery measurement for maintaining synchronization of the servo write clock.

[0032] FIG. 9 illustrates how concentric servo sectors 72₀-72_N are written to a disk surface after synchronizing the servo write clock in response to the high frequency signal 40 and the sync marks 42 in the spiral servo tracks 28₀-28_N. In the embodiment of FIG. 9, the dashed lines represent the centerlines of the data tracks. The sync marks in the spiral servo tracks 28₀-28_N may be written so that there is a shift of two sync marks 42 in the envelope (FIG. 8B) between data tracks. In an alternative embodiment, the sync marks 42 in the spiral servo tracks 28₀-28_N are written so that there is a shift of N sync marks in the envelope between data tracks. In the embodiment of FIG. 9, the data tracks are narrower than the spiral servo tracks 28, however, in an alternative embodiment the data tracks are wider than or proximate the width of the spiral servo tracks 28_i.

[0033] Once the head 50 is tracking on a servo track, the concentric servo sectors 72₀-72_N are written to the disk surface using the servo write clock. Write circuitry is enabled when the modulo-N counter reaches a predetermined value, wherein the servo write clock clocks the write circuitry to write the concentric servo sector 72 to the disk surface. The spiral servo tracks 28₀-28_N on the disk are processed in an interleaved manner to account for the concentric servo sectors 72₀-72_N overwriting a spiral servo track. For example, when writing the concentric servo sectors 72₁ to the disk, spiral servo track 28₂ is processed initially to generate the PES tracking error and the timing recovery measurement. When the concentric servo sectors 72₁ begin to overwrite spiral servo track 28₂, spiral servo track 28₃ is processed to generate the PES tracking error and the timing recovery measurement. In the embodiment of FIG. 9, the spiral servo tracks 28 are written as pairs to facilitate the interleave processing; however, the spiral servo tracks may be written using any suitable spacing (e.g., equal spacing) while still implementing the interleaving aspect. In other embodiments, the disk comprises only one set of spiral servo tracks 28₀-28_N all of which are processed to servo the head (except the spiral servo track being overwritten).

[0034] FIG. 10A illustrates an embodiment of the present invention wherein the disk drive comprises a plurality of disk surfaces 24A-24F, and a plurality of actuator arms 48A-48C having heads for accessing each disk surface 24A-24F. In one embodiment, one of the disk surfaces corresponding to an actuator arm is written with concentric servo sectors used to write spiral servo tracks to the other disk surface corresponding to the actuator arm. For example in FIG. 10B, disk surface 24A is written with concentric servo sectors that are used to write spiral servo tracks to disk surface 24B. The spiral servo tracks are then processed to write concentric servo sectors to disk surface 24B. In FIG. 10C, disk surface 24F is written with concentric servo sectors that are used to write spiral servo tracks to disk surface 24E. The spiral servo tracks are then processed to write concentric servo sectors to disk surface 24E.

[0035] FIG. 11 shows a flow diagram according to another embodiment of the present invention wherein the spiral servo tracks are propagated in a daisy chain fashion. Concentric servo sectors written to a first disk surface are read (step 74) in order to servo the heads across the disk surfaces (step 76) while writing spiral servo tracks to a second disk surface (step 78). The spiral servo tracks are processed to write concentric servo sectors to the second disk surface (step 80). The concentric servo sectors are read from the second disk surface (step 82) in order to servo the heads across the disk surfaces (step 84) while writing spiral servo tracks to a third disk surface (step 86). The spiral servo tracks are processed to write concentric servo sectors to the third disk surface (step 88). This daisy chain process repeats for any suitable number of disk surfaces, and in one embodiment, there may be multiple daisy chains corresponding to different sequences of disk surfaces.

[0036] The heads may be servoed in response to concentric servo sectors written on one of the disk surfaces in any suitable manner when writing spiral servo tracks to another disk surface. In one embodiment, the servo algorithm for seeking the heads across the disk surfaces during normal operation is used to servo the heads while writing a spiral servo track. For example, the normal seek algorithm may control the velocity of the VCM relative to a velocity profile such as shown in FIG. 7B.

[0037] 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 steps 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.

[0038] In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the steps of 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.