Disk drive writing spiral tracks on a slave surface using repeatable runout compensation for a master 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.

[0016] FIG. 2A shows a method of servo writing a disk drive according to an embodiment of the present invention, wherein the disk drive comprises a first head actuated over a first disk surface, and a second head actuated over a second disk surface. A first repeatable runout (RRO) of the first disk surface is measured (step 16), and a seek is performed to seek the first head over the first disk surface in response to the measured first RRO (step 18) while writing a first spiral track to the second disk surface using the second head (step 20), wherein the first spiral track comprises a high frequency signal interrupted by sync marks, and the first spiral track comprises a second RRO that substantially matches the first RRO of the first disk surface.

[0017] FIG. 2B shows a first RRO 22A measured for a first disk surface 24A as comprising a sinusoid having a period equal to one revolution of the disk. The RRO is typically caused by a non-centric alignment of the disk with respect to the center of a spindle motor hub that rotates the disk. When multiple disks are clamped to the spindle motor, each disk center may have a different non-centric alignment with the center of the spindle motor hub. In one embodiment, a plurality of spiral tracks are written to a second disk surface while servoing on the first disk surface, wherein the spiral tracks are used to servo write concentric servo sectors on the second disk surface. In order to align the concentric servo sectors of both the first and second surfaces, the first RRO 22A of the first disk surface 24A shown in FIG. 2B is used to write a first spiral track 28₀ to the second disk surface 24B shown in FIG. 2C. As a result, the first spiral track 28₀ comprises a second RRO 22B that substantially matches the first RRO 22A of the first disk surface 24A, even though the RRO of the second disk surface may be different than the RRO of the first disk surface.

[0018] In an embodiment illustrated in FIG. 3A, a media writer is used to write a plurality of concentric servo sectors 26₀-26_N to the first disk surface 24A of a first disk. The first disk and at least one other blank disk are then clamped to the spindle motor hub of a disk drive, wherein each disk may exhibit a different RRO based on the non-centric alignment with the spindle motor hub. Control circuitry within the disk drive servos a first head over the first disk surface 24A by reading the concentric servo sectors 26₀-26_N, and measures the first RRO of the first disk surface 22A by measuring the amplitude and phase of a sinusoidal disturbance in the position error signal (PES) used to servo the head in response to the servo sectors 26₀-26_N. In an embodiment described below, the first RRO is learned by adjusting the amplitude and phase of sinusoidal feedforward compensation until the PES falls below a threshold. The resulting sinusoidal feedforward compensation represent the first RRO of the first disk surface.

[0019] After measuring the first RRO of the first disk surface, the control circuitry seeks the first head over the first disk surface at a substantially constant velocity using the sinusoidal feedforward compensation while writing a spiral track 28, to the second disk surface 24B using the second head. The control circuitry performs multiple seeks in order to write a plurality of spiral tracks 28₀-28_N to the second disk surface 24B as illustrated in FIG. 3B. Each spiral track 28, is written over a partial disk revolution in the embodiment of FIG. 3B, but may be written over multiple disk revolutions as shown in the embodiment of FIG. 2C. The resulting spiral tracks 28₀-28_N will comprise an RRO that substantially matches the RRO of the first disk surface.

[0020] After the spiral tracks 28₀-28_N are written to the second disk surface, the control circuitry servos on the spiral tracks while writing concentric servo sectors 30₀-30_N to the second disk surface 24B as illustrated in FIG. 3C. Since the RRO of the spiral tracks 28₀-28_N substantially matches the RRO of the first disk surface, the same sinusoidal feedforward compensation values used to write the spiral tracks 28₀-28_N can be used to write the concentric servo sectors 30₀-30_N on the second disk surface 24B. As a result, the concentric servo sectors 30₀-30_N on the second disk surface 24B will align radially with the concentric servo sectors 26₀-26_N on the first disk surface 24A. In this manner, the servo tracks (and data tracks) of the second disk surface 24B will align radially with the servo tracks (and data tracks) of the first disk surface 24A.

[0021] FIGS. 4A-4D illustrate an alternative embodiment of the present invention wherein a media writer is used to write a plurality of spiral tracks to the first disk surface 24A of a first disk, and then the first disk together with at least one blank disk are clamped to a spindle motor hub of a disk drive. The control circuitry then servos on the spiral tracks of the first disk surface 24A in order to write a plurality of concentric servo sectors to the first disk surface 24A as shown in FIG. 4B. The first RRO of the first disk surface 24A is then measured in response to the concentric servo sectors written on the first disk surface 24A, and used to write spiral tracks to the second disk surface 24B as shown in FIG. 4C. The control circuitry then servos on the spiral tracks of the second disk surface 24B (together with the sinusoidal feedforward compensation values used to write the spiral tracks on the second disk surface 24B) in order to write a plurality of concentric servo sectors to the second disk surface 24B as shown in FIG. 4D.

[0022] FIG. 5A shows a disk drive according to an embodiment of the present invention comprising the first head 32A actuated over the first disk surface 24A by a voice coil motor (VCM) 34 that rotates an actuator arm 36A about a pivot. The disk drive comprises control circuitry, including a read/write channel 38 for processing a read signal 40 emanating from the first head 32A as it reads the concentric servo sectors 26₀-26_N. The read/write channel 38 generates an estimated position 40 representing an estimated radial location of the first head 32A over the first disk surface 24A. The estimated position 40 is subtracted 42 from a reference position 44 (e.g., a target radial location) to generate a position error signal (PES) 46. The PES 46 is filtered with a suitable VCM compensator 48 to generate a control signal 50 used to servo the first head 32A over the first disk surface 24A.

[0023] When learning the first RRO of the first disk surface 24A, an RRO compensator 52 measures the corresponding sinusoidal disturbance in the PES 46. In one embodiment, the RRO compensator 52 adjusts the amplitude A and phase 0 of a sinusoidal feedforward compensation Ae^jθ 54 having an angular frequency of 2πk/N where k is a current servo sector out of N servo sectors. The sinusoidal feedforward compensation Ae^jθ 54 is added 56 to the control signal 50 to generate a compensated control signal 58 applied to the VCM 34. The amplitude and phase of the sinusoidal feedforward compensation Ae^jθ 54 is adjusted until the PES is substantially zero (except for the affect of non-repeatable disturbances or higher harmonic RRO). In one embodiment, a feedforward compensation value is generated for each of the concentric servo sectors 26₀-26_N and optionally for each track or zone of tracks.

[0024] In one embodiment, the control circuitry writes the spiral tracks to the second disk surface 24B by seeking the first head 32A over the first disk surface 24A using a velocity profile, an example of which is shown in FIG. 5B. The velocity profile comprises an acceleration segment, a constant velocity segment, and a deceleration segment. In the embodiment of FIG. 5B, the control circuitry writes a spiral track to the second disk surface 24B during the constant velocity segment of the velocity profile so that the spiral track comprises a substantially constant slope across the radius of the disk. When seeking the first head 32A over the first disk surface 24A, the RRO of the first disk surface 24A is a disturbance to the states of the servo system (e.g., position, velocity, etc.). In one embodiment, the sinusoidal feedforward compensation Ae^{j θ} 54 shown in FIG. 5A compensate for the RRO disturbance so that the first head 32A moves at a substantially constant velocity relative to the first disk surface 24A, thereby moving the second head 32B (FIG. 7A) over the second disk surface 24B at a substantially constant velocity relative to the first disk surface 24A (i.e., relative to the RRO of the first disk surface 24A).

[0025] FIG. 6 shows a flow diagram according to an embodiment of the present invention wherein the RRO of the first disk surface 24A changes relative to the radial location of the first head 32A. Accordingly, the RRO of the first disk surface is measured at a plurality of radial locations (step 60) and corresponding RRO compensation values stored in a semiconductor memory (step 62). The control circuitry then seeks the first head over the first disk surface (step 64) using a velocity profile (e.g., FIG. 5B). When the first head reaches a servo sector (step 66), the RRO compensation value corresponding to the servo sector and radial location of the head is read from the semiconductor memory (step 68) and used to adjust the seek (step 70) such as by adding a sinusoidal feedforward compensation Ae^jθ to the VCM control signal as described above.

[0026] FIG. 7A illustrates an embodiment of the present invention wherein the disk drive comprises a plurality of disk surfaces 24A-24D and respective heads 32A-32D actuated over each disk surface. In one embodiment, the control circuitry seeks the first head 32A over the first disk surface 24A while simultaneously writing a first spiral track to the second disk surface 24B and a second spiral track to the third disk surface 24C (and optionally writing a third spiral track to the fourth disk surface 24D). That is, the spiral tracks may be written to the disk surfaces using a bank write operation, whereas in an alternative embodiment, the spiral tracks may be written to each disk surface using separate write operations.

[0027] FIG. 7A illustrates another embodiment of the present invention wherein when the first head 32A reaches a target circumferential and radial location during the seek to write an index spiral track to each disk surface (bank write or individually write), a seam 72B-72D is written in the index spiral tracks on each disk surface. The seam 72B-72D may be written in any suitable manner, such as by writing a gap in each index spiral track. In an embodiment shown on FIG. 7B, the seam 72B-72D is written in each index spiral track by switching a sync mark in the spiral track. The seam is detectable by detecting the change in sync marks as the head moves radially over the disk (e.g., using correlators matched to each sync mark). In the embodiment of FIG. 7B, the seam is written in the index spiral tracks when the first head 32A is halfway between consecutive servo sectors N and N+1 during the seek. In this manner, the track where the seam is written can be estimated as track J+K/2, where track J corresponds to servo sector N and track J+K corresponds to servo sector N+1 as illustrated in FIG. 7B.

[0028] FIG. 7C illustrates an embodiment of the present invention wherein after writing spiral tracks to disk surfaces 24B-24D, including an index spiral track comprising a seam 72B-72D, concentric servo sectors are written to each disk surface by servoing on the spiral tracks. Prior to writing the concentric servo sectors to one of the disk surfaces, the seam in the index spiral track is located in order to initialize the radial and circumferential location of the head. The concentric servo sectors are then written so as to align radially and circumferentially with the other disk surfaces. This is illustrated in FIG. 7C where the servo sectors are aligned circumferentially across the disk surfaces, including an index servo sector SS0. In addition, the concentric servo sectors are written on each disk surface such that the track 74 on the first disk surface 24A corresponding to when the seam 72B-72D was written (track J+K/2 in FIG. 7B) is the same track across the disk surfaces (i.e., the concentric servo sectors and resulting servo tracks are aligned radially across the disk surfaces).

[0029] In one embodiment, aligning the concentric servo sectors radially and circumferentially across the disk surfaces improves performance in accessing the disk surfaces. For example, aligning the concentric servo sectors may enable a rotational position optimization (RPO) algorithm to estimate the time needed to access a target data sector of a target track on a second disk surface after completing an access command on a first disk surface. Alternatively, aligning the concentric servo sectors may improve performance when executing an access command (write or read) that spans multiple disk surfaces by minimizing the latency after the head switch.

[0030] 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.

[0031] 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.