Data storage device estimating servo zone to reduce size of track address

The present application claims priority from provisional U.S. Patent Application Ser. No. 61/970,727, filed on Mar. 26, 2014, the specification of which is incorporated herein by reference.

Data storage devices such as 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.

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. 1 , 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).

The prior art disk format of FIG. 1 also comprises a number of servo sectors 6 0 - 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 at the same or different radial density. Each servo sector 6 , 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. A position error signal (PES) is generated by reading the servo bursts 14 , wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES. In the prior art disk format of FIG. 1 , the data rate of the servo sectors 6 0 - 6 N changes in each physical zone similar to the data sectors in order to improve format efficiency.

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors, wherein data tracks are defined relative to the servo sectors.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk comprising a plurality of servo zones.

FIG. 2B is a flow diagram according to an embodiment wherein a full track address is generated based on a detected servo zone the head is over and a partial track address detected in a servo sector of the servo zone.

FIG. 2C shows an embodiment wherein the detected servo zone represents at least one high order bit of the full track address and the partial track address represents a plurality of low order bits of the full track address.

FIG. 3 shows an embodiment wherein each servo zone comprises a unique sync mark recorded in the servo sectors, and the current servo zone the head is over is detected based on the detected sync mark.

FIG. 4A shows an embodiment wherein unique sync marks are used to identify sub-zones within each servo zone, and each servo zone comprises servo sectors recorded at a unique data rate.

FIG. 4B shows an embodiment wherein the detected servo zone represents at least one high order bit of the full track address, the detected sync mark (sub-zone) represents at least one middle order bit of the full track address, and the detected partial track address represents a plurality of lower order bits of the full track address.

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 actuated over a disk 18 comprising a plurality of servo sectors 20 0 - 20 N defining a plurality of servo tracks 22 . The servo tracks 22 form a plurality of servo zones (e.g., servo zone Z0, Z1, Z2), and at least one servo sector of a servo track comprises a partial track address. The disk drive further comprises control circuitry 24 configured to execute the flow diagram of FIG. 2B , wherein a read signal generated by the head 16 is processed to detect a current servo zone for the head (block 26 ), and the read signal is processed to detect the partial track address in one of the servo sectors of the current servo zone (block 28 ). A full track address is generated based on the detected servo zone and the detected partial track address (block 30 ). In the embodiment of FIG. 2A , the control circuitry 24 processes a read signal 32 emanating from the head 16 to demodulate the servo sectors 20 0 - 20 N that define the servo tracks 22 . A position error signal (PES) is generated representing an error between the actual position of the head and a target position relative to a target track. A servo control system in the control circuitry 24 filters the PES using a suitable compensation filter to generate a control signal 34 applied to a voice coil motor (VCM) 36 which rotates an actuator arm 38 about a pivot in order to actuate the head 16 radially over the disk 18 in a direction that reduces the PES. The servo sectors 20 0 - 20 N may comprise any suitable head position information, including a partial track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern ( FIG. 1 ).

In one embodiment in order to improve the format efficiency of the disk 18 , the size of each servo sector 20 , is reduced by recording only a partial track address therein, wherein in an embodiment shown in FIG. 2C , the partial track address represents a plurality of low order bits of the full track address. At least one of the high order bits of the full track address is generated based on the current servo zone the head 16 is over. For example, if the disk 18 comprises eight servo zones then the detected servo zone may represent the two high order bits of the full track address. This improves the format efficiency since it reduces the size of each servo sector by these high order bits (e.g., by two bits in the above example).

The control circuitry 24 may detect the current servo zone the head 16 is over in any suitable manner. In one embodiment illustrated in FIG. 2A , 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, and the control circuitry 24 is configured to detect the current servo zone for the head 16 based on a detected data rate of the servo sectors. For example, in one embodiment the data rate of the servo sectors may be increased from the inner diameter servo zone toward the outer diameter servo zone in order to achieve a more constant linear bit density. Within each servo zone the data rate of the servo sectors remains constant, and therefore the servo sectors form servo wedges within each servo zone as shown in FIG. 2A . The data rate of the servo sectors may be detected in any suitable manner, such as by searching for the servo sync mark using a suitable timing recovery circuit, and then evaluating a control signal in the timing recovery circuit (e.g., a phase-locked loop control signal) to estimate the data rate of the current servo zone. In another embodiment, after detecting the servo sync mark the data rate of the servo preamble may be detected using a suitable frequency detector operating on the signal samples that precede the detected servo sync mark.

FIG. 3 shows an embodiment wherein at least one servo sector in each servo zone comprises a sync mark (a pattern of bits) that is unique to the servo zone. The control circuitry 24 may then detect the current servo zone the head is over based on the detected sync mark. That is, the control circuitry 24 may search for the sync mark based on the current radial location of the head 16 using any suitable timing recovery circuitry as described above. The signal samples may be processed with a bank of sync mark detectors (e.g., correlators) each configured to detect a sync mark in a corresponding servo zone. When one of the sync mark detectors successfully detects a sync mark, the control circuitry 24 may preliminarily detect the corresponding servo zone. The control circuitry 24 may then confirm the accuracy of the detected sync mark by configuring a servo gate window and detecting the sync mark in a following servo sector. If the sync mark is not detected, it is assumed the previous sync mark detection was a false positive and the search for the sync mark may be restarted.

FIG. 4A shows an embodiment wherein the servo sectors in each servo zone are recorded with a unique data rate as described above with reference to FIG. 2A , and each servo zone is further divided into sub zones identified by a unique sync mark. FIG. 4A shows a single servo zone (SERVO_ZONE — 0) comprising N+1 sub zones, where each sub zone comprises a plurality of the servo tracks, and at least one servo sector in each sub zone comprises a unique sync mark. In one embodiment shown in FIG. 4B , the partial track address represents a plurality of low order bits of the full track address, the detected sync mark represents at least one middle order bit of the full track address, and the detected servo zone represents at least one high order bit of the full track address. This embodiment further improves the format efficiency by further decreasing the number of bits stored in the track address field of each servo sector. For example, if the disk 18 comprises eight servo zones and each servo zone comprises eight sub zones, the size of each servo sector may be reduced by four bits (by reducing the number of bits in the partial track address).

Techniques other than the above described embodiments may be employed to detect the current servo zone (and/or sub zone) the head is over. For example, in one embodiment each servo zone may comprise a unique servo burst pattern or different type of servo burst that uniquely identifies the servo zone. After detecting the sync mark the current servo zone may be detected by evaluating the following servo bursts in the servo sector. For example, the down-track location and/or the frequency of the servo bursts may be used to identify the servo zone. In another embodiment, at least one servo sector in each servo zone may comprise a dedicated field for recording a servo zone identifier that may be evaluated after detecting the sync mark in the servo sector.

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.

While the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.

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.