Disk drive measuring radial offset between heads by detecting a difference between ramp contact

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional U.S. Patent Application Ser. No. 61/877,399, filed on Sep. 13, 2013, which is hereby incorporated by reference in its entirety.

BACKGROUND

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 6₀-6_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_i 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIGS. 2A and 2B show a disk drive according to an embodiment comprising a plurality of disk surfaces wherein a head is actuated over each disk surface.

FIG. 2C shows an embodiment wherein a first head is offset radially from a second head.

FIG. 2D is a flow diagram according to an embodiment wherein the radially offset between the first and second heads is measured based on a measured interval for each head to contact a ramp.

FIG. 3A shows an embodiment wherein the heads are moved from an inner diameter of the disk to the crash stop.

FIG. 3B shows an embodiment wherein the heads are moved from a radial location identified by a reference track to the crash stop.

FIG. 4 is a flow diagram according to an embodiment wherein an interval for each head to contact the ramp is measured after calibrating a repeatable seek trajectory.

FIG. 5 illustrates a difference between measured ramp contact intervals for first and second heads which represents the radial offset between the first and second heads.

FIG. 6A shows an embodiment wherein a first spiral track is written to a first disk surface.

FIG. 6B shows an embodiment wherein the first spiral track on the first disk surface is read while moving the heads toward the ramp when measuring the corresponding intervals.

FIG. 6C shows an embodiment wherein a second spiral track is bank written to the first disk surface (and other disk surfaces) by servoing on a first spiral track written on the first disk surface.

DETAILED DESCRIPTION

FIGS. 2A and 2B show a disk drive according to an embodiment comprising a plurality of disk surfaces 16₁-16₄ including a first disk surface 16₁ and a second disk surface 16₂. The disk drive further comprises a plurality of heads 18₁-18₄ including a first head 18₁ actuated over the first disk surface 16₁, and a second head 18₂ actuated over the second disk surface 16₂, as well as a ramp 20 proximate an outer diameter of the disk surfaces. The disk drive further comprises control circuitry 22 configured to execute the flow diagram of FIG. 2D, wherein a first interval is measured while moving the first head toward the ramp until the first head contacts the ramp (block 24), and a second interval is measured while moving the second head toward the ramp until the second head contacts the ramp (block 26).

When executing the flow diagram of FIG. 2D, the disk surfaces 16₁-16₄ may be blank, partially written with servo data, or fully written with servo data such as the servo sectors shown in FIG. 1. In an embodiment described below, the flow diagram of FIG. 2D may be executed prior to servo writing the disk surfaces 16₁-16₄ with servo data (e.g., spiral servo tracks and/or servo sectors). After the disk surfaces 16₁-16₄ have been written with servo data, in one embodiment the control circuitry 22 processes a read signal 28 emanating from the head 18 to demodulate the servo data and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 22 filters the PES using a suitable compensation filter to generate a control signal 30 applied to a voice coil motor (VCM) 32 which rotates an actuator arm 34 about a pivot in order to actuate the head 18 radially over the disk 16 in a direction that reduces the PES. The servo data may comprise any suitable head position information, such as a spiral servo track, or a servo sector comprising a 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, there is a radial offset between the heads 18₁-18₄ due, for example, to a manufacturing tolerance of the disk drive. FIG. 2C illustrates an example of a radial offset between the first head 18₁ and the second head 18₂ such that when unloading the heads onto the ramp 20 the second head 18₂ will contact the ramp 20 before the first head 18₁ contacts the ramp 20. In one embodiment, the control circuitry 22 measures the relative radial offset between all of the heads 18₁-18₄ by moving each head toward the ramp 20 and measuring a corresponding interval until each head contacts the ramp 20.

FIG. 3A illustrates an embodiment wherein the control circuitry 22 rotates the head stack assembly shown in FIG. 2B until it presses against an inner diameter crash stop so that the heads are positioned at a starting reference position near the inner diameter of the disk surfaces. The control circuitry 22 then rotates the head stack assembly in the opposite direction so that the heads moves toward the ramp 20. While moving the heads toward the ramp 20, a first interval is measured until the first head 18₁ contacts the ramp 20. The control circuitry 22 then rotates the head stack assembly back until it again presses against the inner diameter crash stop, and then performs the same operation in order to measure the second interval for the second head, and so on for each head. After measuring the interval for each head, the control circuitry 22 may evaluate the intervals in order to measure the relative radial offset between the heads (e.g., as shown in FIG. 20).

The interval required for each head to contact the ramp 20 may be measured relative to any suitable reference point. FIG. 3B illustrates an embodiment wherein the heads may be moved starting from a radial location on one of the disk surfaces (e.g., the first disk surface 16₁) which may be defined by suitable reference servo data, such as a reference servo track 36 defined by servo sectors. In one embodiment, the reference servo track 36 may be written to the first disk surface 16₁ prior to servo writing the disk surface with, for example, servo sectors that define concentric servo tracks as shown in FIG. 1. Alternatively, the reference servo track 36 may comprise one of the concentric servo tracks after having servo written the first disk surface 16₁. In one embodiment, the control circuitry 22 positions all of the heads at the reference position by servoing the first head 18₁ over the first disk surface 16₁ until the first head 18₁ is positioned over the reference servo track 36. The control circuitry 22 then moves all the heads toward the ramp 20 while evaluating a suitable signal generated for one of the heads that indicates when the head has contacted the ramp 20.

In one embodiment, when moving all the heads relative to a reference point on the first disk surface 16₁, the first disk surface 16₁ may comprise any suitable servo data disbursed at any suitable frequency on the first disk surface 16₁. For example, the first disk surface 16₁ may comprise multiple reference servo tracks (such as reference servo track 36 in FIG. 3B) that are spaced radially across the disk surface. When the first head 18₁ is moved toward the ramp 20, the periodic servo data written on the first disk surface 16₁ may be read by the first head 18₁ in order to adjust a seek trajectory for the heads during the seek toward the ramp 20. In this embodiment, the interval for each head to contact the ramp 20 may be measured relative to the last reference point read from the first disk surface 16₁. In one embodiment, the servo data on the first disk surface 16₁ may comprise a spiral track as described in more detail below, wherein a reference point may be generated each time the head crosses the spiral track. In yet another embodiment, the servo data written on the first disk surface 16₁ may comprise a full set of servo written concentric servo tracks such as shown in FIG. 1, wherein the reference point for measuring the interval may be the last concentric servo track detected on the first disk surface 16₁ before each head contacts the ramp 20.

Any suitable signal may be generated in order to detect when one of the heads contacts the ramp 20. For example, in one embodiment the read signal emanating from the head may indicate when the head contacts the ramp 20. In another embodiment, each head may be fabricated with a suitable fly height sensor or touchdown sensor (e.g., a capacitive or magnetoresistive element) for generating a signal that may indicate when each head contacts the ramp 20. In yet another embodiment, the disk drive may employ a suitable microactuator (e.g., a piezoelectric actuator) for actuating each head over the respective disk surface in fine movements, wherein the microactuator may actuate the head in any suitable manner, such as by actuating a suspension relative to the actuator arm, or actuating the head relative to the suspension. In one embodiment, the control circuitry 22 may be configured to sense a signal generated by the microactuator which may indicate when the head contacts the ramp 20.

In one embodiment, the control circuitry 22 may evaluate the ramp contact signals generated by all of the heads concurrently while moving the head stack assembly toward the ramp 20 in a single pass. In alternative embodiment, the control circuitry 22 may evaluate the ramp contact signal generated by a single head which requires the head stack assembly to be moved toward the ramp in multiple passes (one pass for each head). This embodiment is understood with reference to the flow diagram of FIG. 4 wherein first a repeatable seek trajectory is calibrated for moving the heads from the reference point toward the ramp (block 38). Any suitable technique may be employed to calibrate the repeatable seek trajectory, such as by performing multiple seeks from the reference point to the ramp 20 and adjusting the seek trajectory (e.g., acceleration, constant velocity, and deceleration segments) until the seek time becomes substantially constant. In one embodiment, when calibrating the repeatable seek trajectory, the seek time is evaluated for a single head, such as the first head 18₁ in the embodiments of FIG. 3A or FIG. 3B, wherein the seek time may be measured as the interval between the beginning of the seek until the first head 18₁ contacts the ramp 20. In another embodiment, the first disk surface 16₁ may comprise reference servo data, such as one or more concentric servo tracks or a spiral track as described below. When calibrating the repeatable seek trajectory, the seek trajectory may be adjusted each time the first head 18₁ crosses over the reference servo data. In this embodiment, the interval for each head to contact the ramp 20 may be measured relative to the last reference point on the first disk surface 16₁ during the seek before the heads contact the ramp 20.

After calibrating the repeatable seek trajectory at block 38, an index i is initialized to the first head (block 40) to select the first head to measure the first interval required to move the head from the reference point until contacting the ramp. The selected head is then moved to the reference point (block 42) and then moved toward the ramp (block 44) while measuring the i^th interval until the i^th head contacts the ramp (block 46). The index i is incremented (block 48) and the flow diagram repeated for the next head until an interval has been measured for each head. The relative radial offset between the heads is then measured based on the measured intervals (block 50).

FIG. 5 illustrates a first interval (t1) measured for the first head 18₁ and a second interval (t2) measured for the second head 18₂. Referring to the example of FIG. 2C, the second head 18₂ is radially offset from the first head 18₁ such that the second head 18₂ will contact the ramp sooner, and therefore the second interval (t2) is shorter than the first interval (t1). The difference between the first interval (t1) and the second interval (t2) represents the radial offset between the first head 18₁ and the second head 18₂. In one embodiment, the difference between the intervals may be converted into a physical distance based on the seek trajectory used to move the heads toward the ramp 20.

FIG. 6A illustrates an embodiment wherein a spiral track 52 may be written to the first disk surface 16₁ while moving the first head 18₁ at a substantially constant velocity across the first disk surface 16₁ until the first head 18₁ contacts the ramp 20. The seek trajectory for writing the spiral track 52 may be calibrated, for example, by performing multiple seeks and adjusting the seek trajectory until the seek time from the ID crash stop to the ramp 20 is substantially constant. After writing the spiral track 52, the repeatable seek trajectory may be calibrated at block 38 of FIG. 4 by reading the reference points 54A-54C on the first disk surface 16₁ at each spiral track crossing as illustrated in FIG. 6B. As each reference point is read, the seek trajectory for measuring the intervals at block 46 of FIG. 4 is adjusted. Once the seek trajectory has been calibrated, the heads are moved across the radius of the disk based on the calibrated seek trajectory and the reference points 54A-54C are read from the first disk surface 16₁ using the first head 18₁. When the last reference point 54C is reached, an interval is measured from the last reference point 54C until each head contacts the ramp 20 (i.e., the intervals shown in FIG. 5 in this embodiment are relative to the last reference point 54C on the first disk surface 16₁ as shown in FIG. 6B). In the embodiment of FIG. 4 where a seek is performed to measure the ramp contact interval for each head, each seek is started from the same radial location and from the same angular phase on the first disk surface 16₁ so that all the ramp contact intervals are measured from the same reference point 54C on the spiral track 52 during each seek. In one embodiment, the seek trajectory when measuring the ramp contact intervals causes the first head 18₁ to move faster over the first disk surface 16₁ as compared to the seek trajectory used to write the spiral track 52. The faster seek trajectory when measuring the ramp contact intervals causes the first head 18₁ to cross the spiral track 52 multiple times during the seek as shown in FIG. 6B.

In one embodiment, the disk surfaces in the disk drive may initially be devoid of any servo data (i.e., blank) prior to writing the spiral track 52 to the first disk surface 16₁. The embodiment of FIGS. 6A and 6B therefore enables measuring the ramp contact interval for each head and the relative radial offsets starting from blank, unservowritten disks. Further, servoing on the spiral track 52 written on the first disk surface 16₁ helps improve the accuracy of the measured ramp contact intervals since the spiral track 52 helps calibrate a repeatable seek trajectory at block 38 of FIG. 4. In addition, when the first head 18₁ reaches the last reference point 54C on the spiral track 52, the first head 18₁ is moving at a substantially constant velocity which improves the accuracy of the measured ramp contact intervals (as compared to starting from zero velocity at reference point 54C).

The radial offset measured relative to each head may be used for any suitable purpose. In one embodiment, the measured radial offsets may be used as a screening criterion during manufacturing in order to identify disk drives that need to be reworked or discarded. In another embodiment, the measured radial offsets may be used as feedback to improve the manufacturing process of the head stack assembly, for example, to reduce the radial offset between the heads. In yet another embodiment, the measured radial offsets may be used to limit the stroke of the heads for any suitable reason, such as to facilitate a self servo writing operation wherein the control circuitry writes servo data to each disk surface, such as spiral tracks and/or concentric servo sectors. For example, a boundary for the written servo data may be defined at the outer diameter of the disk based on the outer most diameter head that first contacts the ramp 20 so that when bank servo writing multiple disk surfaces concurrently, the servo data may extend up to the ramp 20 just before the outer most diameter head contacts the ramp 20.

An example of this embodiment is understood with reference to FIG. 6C where a first spiral track 56 is first written to the first disk surface 16₁ while seeking the first head 18₁ from the inner diameter toward the outer diameter, and then while servoing on the first spiral track 56 a second spiral track 58 is bank written to all of the disk surfaces including the first disk surface 16₁ as shown in FIG. 6C (where the second spiral track 58 is written by moving the heads from the outer diameter toward the inner diameter of each disk surface). The first spiral track 56 may be the same as the spiral track 52 shown in FIG. 6A, or it may be an intermediate spiral track that is written to the first disk surface 16₁ while servoing on spiral track 52. During the bank writing of the second spiral track 58 to all of the disk surfaces, the stroke of the heads is limited based on the radial offset between the heads to ensure the second spiral track 58 may be written to all of the disk surfaces, including to the disk surface with the outer most diameter head. That is, limiting the stroke of the heads ensures that all of the heads are over their respective disk surface (and not on the ramp 20) when bank writing the second spiral track 58 near the outer diameter of the disk surfaces.

In another embodiment, the servo data (e.g., second spiral track 58) may be written to each disk surface serially (rather than bank servo written) while servoing on the first spiral track 56 written on the first disk surface 16₁. For example, the second spiral track 58 may be written to the first disk surface 16₁ while servoing on the first spiral track 56, and then a second spiral track 58 may be written to the second disk surface 16₂ while servoing on the first spiral track 56. In this embodiment, it may still be desirable to limit the stroke of all the heads based on the outer most diameter head. For example, in one embodiment it may be desirable to servo write the same number of concentric servo tracks on all of the disk surfaces, and therefore the stroke of all the heads when writing the second spiral track 58 (from which the concentric servo tracks are written) may be limited based on the stroke of the outer most diameter head.

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 configured 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.