Disk drive performing spiral scan of disk surface to detect residual data

This application claims priority to provisional U.S. Patent Application Ser. No. 61/822,232, filed on May 10, 2013, which is hereby incorporated by reference in its entirety.

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 actuator arm as it seeks from track to track.

FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 6 0 - 6 N recorded around the circumference of each servo track. Each servo sector 6 i comprises 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 servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6 , further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.

FIG. 2A shows a disk drive according to an embodiment comprising a head actuated over a disk and control circuitry.

FIG. 2B is a flow diagram according to an embodiment wherein the control circuitry controls a VCM to move the head over substantially the entire radius of the disk surface in response to a BEMF voltage while processing a read signal from the head to detect residual data recorded on the disk surface after erasing the disk surface.

FIG. 2C shows a disk drive according to an embodiment comprising a spindle motor operable to rotate the disk and generate a BEMF voltage used to synchronize a disk locked clock.

FIG. 3 is a flow diagram according to an embodiment wherein the head is scanned across the disk surface multiple times and the read signal samples filtered to detect residual data recorded on the disk surface.

FIG. 4A shows control circuitry according to an embodiment for controlling the VCM based on its BEMF while scanning the disk surface for residual data.

FIG. 4B shows a velocity profile for controlling the velocity of the head according to an embodiment.

FIGS. 5A-5C illustrate an embodiment wherein the residual data detected after erasing the disk surface may comprise part of a spiral servo track and/or part of a concentric servo sector.

FIG. 2A shows a disk drive according to an embodiment comprising a disk surface 16 A comprising a radius, a head 18 A operable to generate a read signal 20 , and a voice coil motor (VCM) 22 operable to actuate the head 18 A over the disk surface 16 A. The disk drive further comprises control circuitry 24 operable to execute the flow diagram of FIG. 2B , wherein a back electromotive force (BEMF) voltage 26 generated by the VCM is measured (block 28 ). The VCM is controlled to move the head over substantially the entire radius of the disk surface in response to the BEMF voltage (block 30 ), and while moving the head the read signal from the head is processed to detect residual data recorded on the disk surface after erasing the disk surface (block 32 ).

In the embodiment of FIG. 2A , the disk surface 16 A may be erased prior to writing servo data (e.g., servo sectors) on the disk surface 16 A. Before performing the servo write operation, in one embodiment the control circuitry 24 scans the disk surface 16 A to verify that the disk surface 16 A has been sufficiently erased. If the control circuitry 24 detects residual data recorded on the disk surface 16 A after erasing the disk surface 16 A, the disk may be discarded or reprocessed in order to erase the residual data. In one embodiment, the residual data may be erased using an external device (e.g., a magnet), or the head 18 A internal to the disk drive may be used to erase the residual data.

In one embodiment, a servo write operation for servo writing the disk surface 16 A may be interrupted, for example, if a servo error tolerance is exceed. Alternatively, after the servo writing operation the disk surface 16 A may fail a verification procedure which verifies whether the written product servo tracks satisfy certain limits, such as a track squeeze limit. When the servo write operation is interrupted, or the disk surface 16 A fails the verification procedure, it may be desirable to erase the disk surface 16 A prior to retrying the servo write operation. However, the erase operation may not completely erase the disk surface 16 A, thereby leaving residual data on the disk surface 16 A which may include part of a seed servo track (e.g., a spiral servo track) and/or part of a product servo track (e.g., a concentric servo sector). Accordingly, in one embodiment the control circuitry 24 scans the disk surface 16 A to determine whether there is any residual data prior to retrying the servo write operation.

FIG. 2C shows an embodiment wherein a spindle motor 34 rotates the disk surface 16 A while the control circuitry 24 moves the head 18 A radially over the disk surface 16 A to scan for residual data. Accordingly, in this embodiment the head 18 A will traverse the disk surface 16 A in a spiral trajectory having a slope that depends on the rotation speed of the disk surface 16 A and the radial velocity of the head 18 A. In one embodiment, the control circuitry 24 synchronizes a disk locked clock (DLC) to the rotation frequency of the disk 16 A, and uses the DLC to sample the read signal 20 during the scan operation and detect the residual data by processing the signal samples. In one embodiment, the DLC may be synchronized to the rotation frequency of the disk surface 16 A by synchronizing the DLC to a frequency of zero crossings in a BEMF voltage 36 generated by the spindle motor 34 . In one embodiment, the control circuitry 24 processes the DLC in order to determine the circumferential location of the head 18 A relative to the disk surface 16 A, as well as calibrate the radial velocity of the head 18 A for the scan operation.

This embodiment is understood with reference to the flow diagram of FIG. 3 wherein the disk surface 16 A is rotated by the spindle motor (block 38 ) and the DLC is synchronized to a rotation frequency of the spindle motor (block 40 ). The BEMF of the VCM is measured (block 42 ), and the VCM is controlled to move the head across the radius of the disk surface 16 A based on the BEMF (block 44 ). In one embodiment, the head 18 A may first be positioned near an inner diameter (ID) of the disk, for example, by controlling the VCM 22 until the actuator arm 19 ( FIG. 2A ) engages an ID crash stop. The VCM 22 may then be controlled to move the head 18 A toward the outer diameter (OD) of the disk surface 16 A until the head 18 A engages a ramp 21 located near an OD of the disk. While moving the head 18 A toward the OD, the number of DLC cycles is counted, and when the head 18 A engages the ramp 21 , the total number of DLC cycles is compared to a target number of cycles, where the difference (error) may be used to calibrate a gain of a BEMF detector (block 46 ). In one embodiment, blocks 42 - 46 of FIG. 3 may be repeated until the gain error falls below a threshold.

After calibrating the gain of the BEMF detector, a target circumferential phase for the head is initialized (block 48 ). The DLC is then evaluated to determine when the head reaches the target circumferential phase as the disk surface rotates (block 50 ). When the head reaches the target circumferential phase, the VCM is controlled to begin moving the head toward the OD of the disk surface (block 52 ) while sampling the read signal to generate signal samples based on the frequency of the DLC (block 54 ). The signal samples are filtered based on a frequency corresponding to an expected frequency of the residual data, and the filtered signal samples processed to detect the residual data (block 56 ). The residual data may be detected in any suitable manner, such as by comparing an average amplitude of the filtered signal samples to a threshold. In one embodiment, when residual data is detected, the radial and circumferential location of the residual data is logged. The logged location information may be used to evaluate and modify the erase operation, or in another embodiment, the logged location information may be used by the control circuitry 24 to erase the detected residual data.

The head is moved radially over the disk and the signal samples processed to detect the residual data at substantially any point during the moving. When the head engages the ramp (block 58 ), the VCM is controlled to stop moving the head, and then return the head to the ID of the disk surface. The target circumferential phase for the head is then adjusted (block 60 ) such as by incrementing the target circumferential phase. The flow diagram of FIG. 3 is then repeated starting from block 50 in order to perform multiple scans across the radius of the disk surface at different phase offsets, thereby covering a significant portion of the disk surface (block 62 ).

FIG. 4A shows control circuitry according to an embodiment for controlling the VCM 22 based on the BEMF 26 while scanning the disk surface for residual data. A BEMF detector 64 subtracts from the BEMF 26 a voltage component 66 representing the contribution due to the resistant R and inductance L 68 of the VCM 22 to generate a voltage component 70 that more accurately reflects the contribution due to the velocity of the VCM 22 . The resistance R and the inductance L 68 of the VCM 22 may be calibrated using any suitable technique, such as by injecting a small step input signal into the VCM 22 after pressing the actuator arm 19 against the ID crash stop, and measuring the resulting RL voltage drop across the VCM 22 . The voltage component 70 due to the velocity of the VCM 22 is sampled 72 , and the resulting sample values 74 are scaled by a gain 76 . The scaled sample values 80 representing the velocity of the VCM are processed by a control error generator 82 which may, for example, compute a velocity error 84 as a difference between the measured velocity of the head and a target velocity generated by a reference generator 86 based on a velocity profile, an example of which is shown in FIG. 4B . A servo compensator 88 processes the velocity error 84 to generate a digital control signal 90 . A digital to analog converter (DAC) 92 converts the digital control signal 90 into an analog control signal 94 applied to the VCM 22 so that the measured velocity 80 substantially follows the target velocity 86 .

In the embodiment of FIG. 4A , a gain adjust block 98 adjusts the gain 76 of the BEMF detector 64 based on a measured number of DLC cycles before the head 18 A engages the ramp 21 . The measured number of DLC cycles is compared to a target number of DLC cycles, and the gain 76 is adjusted based on the difference. For example, if the measured number of DLC cycles is greater than the target number meaning that the scan velocity was too slow, the gain 76 may be decreased so as to increase the radial velocity of the head during the next move across the disk surface 16 A. In one embodiment, the gain 76 of the BEMF detector 64 may be calibrated prior to starting the scan operation to detect the residual data, and in another embodiment the gain 76 may be adjusted during the scan operation each time the head is moved across the radius of the disk surface 16 A.

In one embodiment, the gain of the read signal 20 emanating from the head 18 A during the scan operation may be calibrated prior to executing the scan operation. For example, in one embodiment the head 18 A may be positioned at the ID of the disk surface, such as by pressing the actuator arm 19 against the ID crash stop. In this embodiment, it is assumed that no data is recorded at the ID, and therefore the amplitude of the read signal will represent the signal noise. Accordingly, the gain of the read signal may be adjusted until the amplitude of the read signal reaches a target amplitude. During each seek across the radius of the disk surface, the residual data may be detected by comparing the average amplitude of the read signal to a detection threshold representing a margin above the noise floor of the read signal. In one embodiment, the detection threshold may be configured so as to maximize a probability of detecting true residual data while minimizing the probability of false detections.

FIG. 5A shows an embodiment wherein as part of a servo write operation, a plurality of spiral servo tracks may be written to the disk surface 16 A. The spiral servo tracks may be written using an external servo writer, or the spiral servo tracks may be written internally by the control circuitry 24 . In one embodiment, a plurality of bootstrap spiral tracks may be written to the disk surface 16 A to facilitate the internal writing of the spiral servo tracks. FIG. 5B shows an embodiment wherein the control circuitry 24 may process the spiral servo tracks in order to write a plurality of concentric servo sectors that define product servo tracks on the disk surface 16 A.

In one embodiment, the quality of the servo write operation may be monitored to verify that the product servo tracks are written with sufficient accuracy. For example, the consistency of the spiral servo tracks may be monitored during and/or after writing the spiral servo tracks. If the spiral servo tracks deviate from predetermined limits, the servo write operation may be aborted and retried. Similarly, the servo write operation may be aborted during and/or after writing the concentric servo sectors that define the product servo tracks. After the servo write operation is aborted, in one embodiment it is desirable to completely erase the disk surface so that the servo write operation can be restarted with a clean disk surface. If the disk surface is not erased completely, any residual data may interfere with the subsequent servo write operation.

FIG. 5C illustrates an example of residual data after erasing the disk surface, wherein in this example a concentric band of residual data remains on the disk surface after the erase operation. Also in this example, the erase operation was performed after writing spiral servo tracks to the disk surface, and after at least partially writing the concentric servo sectors to the disk surface, such that the residual data comprises part of a spiral servo track as well as part of a concentric servo sector.

In one embodiment, each spiral servo track may be written to the disk surface as a high frequency signal periodically interrupted by a sync mark. In addition, each spiral servo track may be written at a frequency that differs from a frequency of data written in the concentric servo sectors. In yet another embodiment, the concentric servo sectors may be written at a varying frequency based on the radial location of the head (zoned servo sectors). For example, the data rate of the concentric servo sectors may be increased toward the OD so as to achieve a more constant linear bit density. In one embodiment, the frequency of the data written in the spiral servo tracks and the concentric servo sectors may be based on a frequency of a DLC. Accordingly, during the scan operation the read signal may be sampled using the DLC synchronized to the frequency of the spindle motor so that the resulting signal samples may be filtered based on a frequency corresponding to an expected frequency of the residual data. In the embodiment where the frequency of the residual data may differ based on the type of data, the signal samples may be filtered at multiple frequencies in order to distinguish between the different types of residual data. In one embodiment, the type of residual data, together with the location of the detected residual data, may be logged and used to evaluate and modify the erase operation.

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, 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.

The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.

While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the embodiments disclosed herein.