Disk drive opening spiral crossing window based on DC and AC spiral track error

A disk drive is disclosed comprising a head actuated over a disk comprising a plurality of spiral tracks including a first spiral track. A DC spiral track error common to the spiral tracks is measured relative to an ideal slope for the spiral tracks, and an AC spiral track error is measured for each spiral track relative to the ideal slope. As the head approaches the first spiral track, a spiral crossing window is opened based on the measured DC spiral track error and the measured AC spiral track error for the first spiral track, and the first spiral track is detected after opening the spiral crossing window.

BACKGROUND

When manufacturing a disk drive, concentric servo sectors20-2Nare written to a disk4which define a plurality of radially-spaced, concentric servo tracks6as shown in the prior art disk format ofFIG. 1. A plurality of concentric data tracks are defined relative to the servo tracks6, wherein the data tracks may have the same or a different radial density (tracks per inch (TPI)) than the servo tracks6. Each servo sector (e.g., servo sector24) comprises a preamble8for synchronizing gain control and timing recovery, a sync mark10for synchronizing to a data field12comprising coarse head positioning information such as a track number, and servo bursts14which provide fine head positioning information. The coarse head position information is processed to position a head over a target data track during a seek operation, and the servo bursts14are processed to maintain the head over a centerline of the target data track while writing or reading data during a tracking operation.

In the past, external servo writers have been used to write the concentric servo sectors20-2Nto the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the concentric servo sectors20-2Nare 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.

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.

DETAILED DESCRIPTION

FIG. 9Ashows a disk drive according to an embodiment comprising a disk16comprising a plurality of spiral tracks including a first spiral track180, and a head20actuated over the disk16. The disk drive further comprises control circuitry24operable to execute the flow diagram ofFIG. 9B, wherein a DC spiral track error common to the spiral tracks is measured relative to an ideal slope for the spiral tracks (block68), and an AC spiral track error is measured for each spiral track relative to the ideal slope (block70). As the head approaches the first spiral track, a spiral crossing window is opened based on the measured DC spiral track error and the measured AC spiral track error for the first spiral track (block72), and the first spiral track is detected after opening the spiral crossing window (block74).

FIG. 2Ashows a disk drive according to an embodiment wherein the control circuitry24is operable to execute the flow diagram ofFIG. 2B, wherein a slope of a spiral track is measured at a plurality of radial locations across the disk (block26), and a spiral track error at each radial location is generated based on a difference between the measured slope and a target slope (block28). The head is actuated over the disk based on the spiral track and the spiral track error (block30).

In one embodiment, the control circuitry24processes a read signal32emanating from the head20to measure the slope of the spiral track180in order to generate the spiral track error. The control circuitry24then generates a control signal34based on the spiral track180and the spiral track error, wherein the control signal34is applied to the actuator22in order to rotate an actuator arm36about a pivot to thereby actuate the head20radially over the disk16.FIG. 2Cillustrates a trajectory of the spiral track180written on the disk16relative to an ideal spiral track38. The trajectory of the ideal spiral track38changes at a constant slope over time as the disk16rotates at a constant speed, whereas the trajectory of the spiral track180changes with a varying slope due to errors when writing the spiral track180. Accordingly, in one embodiment a spiral track error is generated based on a difference between a measured slope of the spiral track180and a target slope of the ideal spiral track38, and then the spiral track error is compensated when servoing the head20over the disk16based on the spiral track180.

FIG. 3Aillustrates an embodiment wherein the spiral track error is generated at a plurality of different radial locations across the disk16from an outer diameter (OD) to an inner diameter (ID). In one embodiment, when measuring a radial position of the head20based on the written spiral track180, the measured position is adjusted by the spiral track error (position error) so that the adjusted position corresponds to the radial location of the ideal spiral track38.

The spiral track error may be generated in any suitable manner, wherein one embodiment is described in view ofFIG. 3Bwhich shows a magnified view of the written spiral track180relative to the ideal spiral track38near the OD of the disk16. The head is first positioned at radial location R0based on the written spiral track180. That is, the written spiral track180is read to generate a radial position measurement, and the head is servoed radially over the disk16until the measured position corresponds to R0. If the slope of the written spiral track180matched the slope of the ideal spiral track38, then the measured R0would correspond to the target radial location tR0. However, because the slope of the written spiral track180is steeper than the slope of the ideal spiral track38, the measured radial location corresponding to R0will differ from the target radial location tR0based on the ideal spiral track38by a position error ΔR0. In one embodiment, this initial position error ΔR0may be estimated according to:
ΔR0=r−r·s0/st
where r represents a radial increment between the OD and the first radial location corresponding to tR0, s0represents the measured slope of the spiral track180at R0, and strepresents the target slope (corresponding to the ideal spiral track38). The head is then servoed over the disk16based on the written spiral track180until the measured position corresponds to R1. Due to the error in the slope of the written spiral track180, the resulting position error ΔR1may be estimated according to:

R^1=R^0+r·(s1+s0)/2st;Δ⁢⁢R1=(2·r)-R^1
where {circumflex over (R)}0=tR0−ΔR0. In general, the position error ΔRiat each incremental radial location tRimay be estimated based on:

FIG. 3Billustrates how the position error ΔRichanges in magnitude as the slope of the written spiral track180changes, and in the example shown the polarity of the position error ΔR8changes sign at R8due to the slope of the written spiral track180being less than the slope of the ideal spiral track38.

The slope of the written spiral track180may be measured at each radial location RiinFIG. 3Busing in any suitable technique.FIGS. 4A and 4Billustrate an embodiment wherein a width of the written spiral track180varies proportionally with the slope. In one embodiment, the width of the written spiral track180may be measured by evaluating the read signal that is generated as the head crosses over the written spiral track180at a constant velocity. For example, the read signal may be integrated and the width of the written spiral track180estimated based on the width of the resulting ramp signal.FIG. 4Billustrate how the width of the spiral track crossing signal decreases as the slope of the written spiral track180increases as compared to the spiral track crossing shown inFIG. 4A.

The written spiral track180shown inFIG. 2Amay be used to servo the head20over the disk16during any suitable operation of the disk drive. In one embodiment, the written spiral track180may represent a product servo track used to servo the head20over the disk16during normal write/read operations. In one embodiment, there may be a number of spiral servo tracks180-18Nwritten to the disk16and processed to servo the head20during normal write/read operations. As described below with reference toFIG. 7, in one embodiment the measured radial position of the head20when servoing on the spiral servo tracks180-18Nmay be adjusted by the corresponding position error ΔR described above so that the head20is servoed over the disk16based on ideal spiral servo tracks.

In another embodiment illustrated inFIGS. 5A and 5B, a number of spiral tracks180-18Nmay be written on the disk16and then processed to write concentric servo sectors400-40Non the disk16, wherein the concentric servo sectors400-40Nare used to servo the head20over the disk16during normal write/read operations. In the example embodiment shown inFIGS. 5A and 5B, each spiral track18kis written over a partial disk revolution, whereas in other embodiments each spiral track18kmay be written over multiple disk revolutions as shown inFIG. 2A.

FIGS. 6A and 6Billustrate another embodiment wherein the spiral track180may be used as a bootstrap spiral track that is processed to write one or more spiral servo tracks440. In one embodiment, the bootstrap spiral track180may be written while moving the head20from the ID of the disk16toward the OD until the head20contacts a ramp42. In one embodiment, a number of bootstrap spiral tracks may be written on the disk by starting the writing at the ID at a different circumferential position (different rotation angle).FIG. 6Bshows an embodiment wherein a spiral servo track440is written to the disk16while servoing the head from the OD toward the ID of the disk16. When the head20crosses over the bootstrap spiral track180, a radial position is measured and used to adjust the servoing of the head20. When the radial position is measured, it is adjusted by the position error ΔR as described above so that the spiral servo track440is written based on an ideal bootstrap spiral track. In one embodiment, a number of spiral servo tracks440-44Nare written to the disk16which may be used as final servo tracks, or used to write concentric servo sectors as described above with reference toFIGS. 5A and 5B.

FIG. 7shows a servo control system implemented within the control circuitry24ofFIG. 2A, wherein the read signal32emanating from the head20is processed at block46when crossing the spiral track180to generate a measured position48. The measured position48is adjusted at adder50by the corresponding position error ΔR generated at block52as described above to generate an adjusted position54that corresponds to the ideal spiral track38. A state error generator56processes the adjusted position54and a reference position58to generate an error signal60(e.g., a position error signal (PES)). The error signal60is processed by a servo compensator62to generate a digital control signal64that is converted by a digital-to-analog converter (DAC)66into an analog control signal34applied to the VCM22in order to actuate the head20over the disk16so as to reduce the error signal60. The servo control system ofFIG. 7may be used during normal write/read operations, or when writing concentric servo sectors to the disk as shown inFIG. 5B, or when writing spiral servo tracks to the disk as shown inFIG. 6B, or during any other suitable operation wherein the head20is servoed over the disk16based on a spiral track.

In one embodiment, the block52ofFIG. 7may generate an interpolated position error ΔR for adjusting the measured position48. For example, when the measured position48indicates the head20is between radial location R0and R1inFIG. 3B, block52may generate the corresponding position error ΔR by interpolating between position error ΔR0and position error ΔR1. In other words, in one embodiment the control circuitry24may interpolate the spiral track error (e.g., position error) in order to increase the resolution of the spiral track error.

The target slope of the ideal spiral track38shown inFIG. 3Amay be generated in any suitable manner. In one embodiment, the target slope is predetermined based on a known ideal spiral track that should be written to the disk. In another embodiment, the control circuitry24may generate the target slope by averaging the measured slopes of the spiral track180at the plurality of radial locations shown inFIG. 3B. For example, in one embodiment the control circuitry24may measure and save the slope of the spiral track180at the different radial locations shown inFIG. 3B. The saved slope measurements may then be processed to generate the target slope (e.g., as the average), as well as to generate the spiral track errors (e.g., position errors) for each radial location.

In one embodiment, there may be a number of spiral tracks180-18Nwritten to the disk16as illustrated inFIG. 8. The control circuitry24may generate the target slope and/or the spiral track errors for each individual spiral track, or in another embodiment, the control circuitry24may average the measurements to generate a target slope and/or spiral track errors used to process all of the spiral tracks180-18N. For example, in one embodiment the control circuitry24may average the spiral track errors (e.g., position errors) generated for each spiral track18kat each radial location Rito generate a spiral track error that is applied to all of the spiral tracks (e.g., at block52ofFIG. 7). In another embodiment, the control circuitry24may generate an independent spiral track error for each spiral track18kwhich is then generated at block52ofFIG. 7, for example, using a lookup table indexed by the spiral track number.

The spiral track180may comprise any suitable format, wherein in one embodiment, the spiral track180is written as a high frequency signal interrupted periodically by a sync mark. When the head20moves radially over the disk16, an eye pattern generated by reading the spiral track180will shift (left or right) while the sync marks remain fixed (ideally). The shift in the eye pattern (detected from the high frequency signal) relative to the sync marks provides the position information for servoing the head20.

Referring again to the embodiment ofFIG. 6B, the spiral servo track440may be written starting from an OD reference track76A to an ID reference track76B. As the head20approaches a bootstrap spiral track180, the control circuitry24opens a spiral crossing window to begin searching for the bootstrap spiral track180. Searching for the bootstrap spiral track180when the spiral crossing window is open improves the detection accuracy by avoiding false detections while the head is between bootstrap spiral tracks. In one embodiment, the spiral crossing window may also track the sequence of bootstrap spiral tracks180-18N. That is, even if one of the bootstrap spiral tracks18kis missed while the spiral crossing window is open, an index that tracks the sequence180-18Nof bootstrap spiral tracks may still be incremented so that the sequence is accurately tracked as the disk rotates. Referring again toFIG. 2C, in one embodiment the error in the trajectory of the bootstrap spiral track180relative to an ideal spiral track38is compensated when opening the spiral crossing window.

FIG. 10illustrates an example of this embodiment which shows two consecutive bootstrap spiral tracks18kand18k+1relative to an ideal spiral track38. Each bootstrap spiral track comprises a DC trajectory78common to the bootstrap spiral tracks180-18N(e.g., an average spiral track error across the bootstrap spiral tracks), and an AC trajectory that is unique to each bootstrap spiral track18k. As the head20moves radially over the disk16along trajectory80, the control circuitry24opens the spiral crossing window when the head20nears one of the bootstrap spiral tracks18k. In one embodiment a DC spiral track error (due to the DC trajectory78) and an AC spiral track error (due to the AC trajectory) measured for each bootstrap spiral track18kis used to open the spiral crossing window at the correct time. In the example ofFIG. 10, the head20is shown at radial location ri82at current time ti. The control circuitry24may open the spiral crossing window based on:
Δti(k)=(ri−DCi−ACi(k)−rs)/L
pi(k)=Δti(k)−(ti−Tik)
where:k represents a bootstrap spiral track index;rsrepresents a radial location of reference track76B (FIG. 6B);Tikrepresents a phase of an ideal bootstrap spiral track k at the reference track;ri: represents a current radial location of the head;tirepresents a current time;ACi(k) represents the AC spiral track error for bootstrap spiral track k at radial location ri;

DCirepresents the DC spiral track error at radial location ri;Δti(k) represents a timing delta based on the reference track rsand current radial location ri;pi(k) represents a timing delta between the current time tiand an expected crossing time for bootstrap spiral track k at the current radial location ri; andL represents the ideal slope for the bootstrap spiral tracks.
As the head20moves radially over the disk16along trajectory80, and as the disk16rotates, the timing delta pi(k) will decrease as the head20moves toward the bootstrap spiral track18k. In one embodiment, the spiral crossing window is opened when the absolute value of the timing delta pi(k) is less than a threshold, and the spiral crossing window is closed when the absolute value of the timing delta pi(k) is greater than a threshold.

This embodiment is understood with reference to the flow diagram ofFIG. 11wherein the bootstrap spiral index k is initialized to zero (block84). The timing delta pi(k) is computed as described above (block86), and if the spiral crossing window is not open (block88), the spiral crossing window is opened (block92) if the absolute value of the timing delta pi(k) is less than a threshold (block90). In the embodiment ofFIG. 11, the threshold at block90equals half the maximum width W of the spiral crossing window. The flow diagram is then repeated starting from block86to update the timing delta pi(k). If the spiral crossing window is open at block88, the control circuitry24searches for the current bootstrap spiral track18k(block94). If the current bootstrap spiral track18kis detected at block94, the bootstrap spiral index k is incremented (block98) and the spiral crossing window is closed (block100). If the current bootstrap spiral track18kis not detected at block94, and the absolute value of the timing delta pi(k) exceeds the threshold W/2 (block96), it is assumed the current bootstrap spiral track18kwas missed and therefore the bootstrap spiral index is incremented (block98) and the spiral crossing window is closed (block100). In this embodiment, the bootstrap spiral index k tracks the correct sequence of bootstrap spiral track crossings even when one of the bootstrap spiral tracks is missed due, for example, to a defect on the disk16at the location where the head20crosses over the bootstrap spiral track.

The DC spiral track error and the AC spiral track error for any given radial location such as shown in the example ofFIG. 10may be generated in any suitable manner. In one embodiment, the DC spiral track error may be generated as the average spiral track error generated for the plurality of bootstrap spiral tracks180-18Nas described above with reference toFIG. 8. In one embodiment, the AC spiral track error for each bootstrap spiral track18kmay be generated at a given radial location based on the error signal60generated by the servo control system ofFIG. 7while servoing the head20at the given radial location. In one embodiment, the DC and AC spiral track errors may be measured at a plurality of discrete radial locations, and then the DC and AC spiral track errors generated for any given radial location by interpolating between the discrete measurements, or by using a polynomial curve fitted to the discrete measurements, or any other suitable technique.

In the embodiment ofFIG. 6B, the spiral servo track440is written by moving the head20in an opposite radial direction as when writing the bootstrap spiral track180. In the example ofFIG. 6B, the bootstrap spiral track180is written while moving the head from the ID toward the OD of the disk16, and the spiral servo track440is written while moving the head from the OD toward the ID of the disk16. In another embodiment, the writing direction for the spiral servo track440and the bootstrap spiral track180may be reversed, and in yet another embodiment, the spiral servo track440may be written in the same radial direction as the bootstrap spiral track180. In the latter embodiment, the spiral servo track440may be written while moving the head20at a radial velocity that is different (slower or faster) than the radial velocity used to write the bootstrap spiral track180to ensure the head20crosses the bootstrap spiral track180while writing the spiral servo track440.