Data storage device calibrating write parameter by pressing actuator against crash stop

A data storage device is disclosed comprising a head actuated over a disk surface by an actuator arm. The actuator is controlled to press the actuator arm against a crash stop in order to write a plurality of bursts on the disk surface each with a different write parameter setting. Each burst is read in order to measure a quality metric for each burst, and an operating setting for the write parameter is configured based on the measured quality metrics.

BACKGROUND

A disk drive typically comprises a plurality of disks each having a top and bottom surface accessed by a respective head. That is, the VCM typically rotates a number of actuator arms about a pivot in order to simultaneously position a number of heads over respective disk surfaces based on servo data recorded on each disk surface.FIG. 1shows a prior art disk format2as comprising a number of servo tracks4defined by servo sectors60-6Nrecorded around the circumference of each servo track. Each servo sector6icomprises a preamble8for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark10for storing a special pattern used to symbol synchronize to a servo data field12. The servo data field12stores 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 sector6ifurther comprises groups of servo bursts14(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 bursts14provide 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 bursts14, 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.

Data is typically written to the disk by modulating a write current in an inductive coil (write coil) to record magnetic transitions onto the disk surface in a process referred to as saturation recording. During read-back, the magnetic transitions are sensed by a read element (e.g., a magneto-resistive element) and the resulting read signal demodulated by a suitable read channel. Heat assisted magnetic recording (HAMR) is a recent development that improves the quality of written data by heating the disk surface during write operations in order to decrease the coercivity of the magnetic medium, thereby enabling the magnetic field generated by the write coil to more readily magnetize the disk surface. Any suitable technique may be employed to heat the surface of the disk in HAMR recording, such as by fabricating a laser diode and a near field transducer (NFT) with other write components of the head. Microwave assisted magnetic recording (MAMR) is also a recent development that improves the quality of written data by using a spin torque oscillator (STO) to apply a high frequency auxiliary magnetic field to the media close to the resonant frequency of the magnetic grains, thereby enabling the magnetic field generated by the write coil to more readily magnetize the disk surface.

DETAILED DESCRIPTION

FIG. 2Ashows a data storage device in the form of a disk drive according to an embodiment comprising a disk surface16, an actuator18, an actuator arm20actuated by the actuator18, and a head22actuated over the disk surface16by the actuator arm20. The disk drive further comprises control circuitry24configured to execute the flow diagram ofFIG. 2B, wherein the actuator arm is pressed against a crash stop (block26) in order to write a plurality of bursts on the disk surface each with a different write parameter setting (block28), and read each burst in order to measure a quality metric for each burst (block30). An operating setting is configured for the write parameter based on the measured quality metrics (block32).

In the embodiment ofFIG. 2A, the disk surface16comprises a plurality of servo sectors341-34Nthat define a plurality of servo tracks, wherein data tracks36are defined relative to the servo tracks at the same or different radial density. The control circuitry24processes a read signal38emanating from the head to demodulate the servo sectors 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. A servo control system in the control circuitry24filters the PES using a suitable compensation filter to generate a control signal40applied to a coarse actuator18(e.g., VCM) which rotates an actuator arm20about a pivot in order to actuate the head radially over the disk in a direction that reduces the PES. The head may also be servoed using a fine actuator, such as a piezoelectric (PZT) actuator, configured to actuate a suspension relative to the actuator arm20, and/or configured to actuate the head relative to the suspension. The servo sectors341-34Nmay comprise any suitable head position information, such as 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).

The head22may be fabricated with any suitable components, whereinFIG. 2Cshows an embodiment with the head22comprising a suitable read element42(e.g., a magnetoresistive element), a write assist element in the form of a laser and NFT42, and a write element44(e.g., a write coil). Other embodiments may employ a different type of write assist element, such as a STO in MAMR, or a material stack, including conductive materials, used in energy assisted recording. The head22may also comprise a suitable fly height actuator (not shown) that is biased to achieve a target fly height of the head22over the disk surface16. Any suitable fly height actuator may be employed, such as a suitable thermal actuator that adjusts the fly height through thermal expansion, or a suitable mechanical actuator such as a suitable piezoelectric actuator that adjusts the fly height through mechanical deflection. In the embodiments described herein, an operating setting for any suitable write parameter may be configured by writing/reading bursts while the actuator is pressed against the crash stop, such as a current amplitude applied to the write coil, a bias applied to a write assist element, a bias applied to a fly height actuator, etc. In one embodiment, an operating setting may be configured for multiple write parameters by adjusting the write parameters individually or concurrently in any number of permutations when writing the bursts.

In one embodiment, the bursts are written at a target write angle around the circumference of the disk surface while the actuator is pressed against the crash stop.FIG. 3Ashows an example of this embodiment wherein the actuator arm20is pressed against an inner diameter crash stop such that the head22is positioned near the inner diameter of the disk surface16. As the disk surface16rotates, a burst is written by the head22when the disk surface reaches a target write angle (rotation angle). For example, the write coil44writes burst46at its corresponding write angle, writes burst48at its corresponding write angle, and so on. Prior to writing each burst, the control circuitry24adjusts the write parameter such that each burst is written with a different setting as represented by each burst having a different shade inFIG. 3A. Each servo burst may comprise any suitable pattern of magnetic transitions, such as a pattern having a single frequency of magnetic transitions, or multiple frequencies of magnetic transitions.FIG. 3Bshows an embodiment wherein a first set of bursts50may be written each with a different setting for a first write parameter (e.g., write current applied to write coil), and a second set of bursts52written each with a different setting for a second write parameter (e.g., bias applied to a laser). In one embodiment, each set of burst may be written using a combination of write parameters, including any suitable permutation of write parameters. That is, in one embodiment changing the write parameter setting prior to writing each burst may comprise changing the setting(s) of one or more write parameters, or changing any suitable permutation of write parameters.

FIG. 4is a flow diagram according to an embodiment wherein after pressing the actuator arm against the crash stop (block54), a write parameter setting is initialized at block56(e.g., to a low value) and a write angle is initialized (block58). During a first revolution of the disk surface, a burst is written at the write angle (block60), and during a second revolution of the disk surface, the burst is read in order to measure a quality metric (block62). If not done at block64, the write parameter setting is adjusted at block66(e.g., incremented) and the write angle is adjusted at bock68(e.g., incremented). The flow diagram is then repeated from block60in order to write/read the next burst using the adjusted write parameter setting. In one embodiment, the flow diagram ofFIG. 4terminates at block64when the quality metric measured at block62reaches a target value. Writing/reading the burst over multiple revolutions in this embodiment may help prevent the write parameter setting from overshooting so as to prevent damaging the corresponding write component. For example, overshooting a bias setting applied to a laser may overheat the laser and/or damage the NFT due to thermal expansion causing contact with the disk surface. Accordingly in this embodiment, the write parameter setting may be gradually incremented while writing the bursts over multiple revolutions and the quality metric measured until it reaches a target value.

In another embodiment, overshooting the write parameter setting may not be a concern, and so the bursts may be written with different write parameter settings over a single revolution of the disk surface. An example of this embodiment is shown in the flow diagram ofFIG. 5, wherein after pressing the actuator arm against the crash stop (block70), a write parameter setting is initialized at block72(e.g., to a low value) and a write angle is initialized (block74). During a first revolution of the disk surface, a burst is written at the write angle (block76), and if there are more write parameter settings at block78, the write parameter setting is adjusted at block80(e.g., incremented) and the write angle is adjusted at block82(e.g., incremented). Another burst is written at the adjusted write angle using the adjusted write parameter setting (block76), and this process continues until a number of bursts have been written over the range of parameter settings to be tested. During a second revolution of the disk, the write angle is initialized (block84) and the corresponding burst at the write angle is read to generate the corresponding quality metric (block86). The write angle is adjusted (block90) and the flow diagram repeated until, for example, the measured quality metric achieves a target value at block88.

An operating setting for any suitable write parameter may be configured in the above described embodiments, wherein in an embodiment shown inFIG. 6, an operating setting for a bias (e.g., voltage or current) applied to a laser may be configured by writing/reading the bursts. In this embodiment, the quality metric measured by reading each burst comprises a burst amplitude which may be generated, for example, by integrating the read signal while reading the burst. In this embodiment, the laser bias is initialized to a low value to write/read a burst and measure the corresponding burst amplitude. The laser bias is then incremented in order to write/read the next burst and measure the corresponding burst amplitude. In one embodiment, the operating setting for the laser bias is configured as the setting when the burst amplitude saturates as shown inFIG. 6. In one embodiment, the laser bias may be incremented initially by a coarse value and then incremented by a fine value as the burst amplitude measurement approaches saturation.

In one embodiment, the control circuitry24is configured to self-servo write the embedded servo sectors341-34Nthat define the data tracks36shown inFIG. 2A. The control circuitry24may self-servo write the servo sectors341-34Nin any suitable manner such as by propagating the servo bursts across the disk surface. In another embodiment, the control circuitry24may write an intermediate set of spiral tracks on the disk surface to facilitate servoing the head while writing the servo sectors341-34N. Regardless as to the self-servo writing technique, in one embodiment the control circuitry24configures an operating setting for one or more write parameters prior to executing the self-servo writing operation. However prior to executing the self-servo writing, there may not be any servo information recorded on the disk surface (i.e., the disk surface may be essentially blank). Accordingly in the embodiments described above, the actuator arm is pressed against a crash stop (e.g., inner or outer diameter crash stop) while writing/reading bursts so that servo information is not needed to control the position of the head in order to calibrate the write parameters. In one embodiment, a disk locked clock is synchronized to back electromotive force (BEMF) zero crossings of a spindle motor configured to rotate the disk, thereby synchronizing the disk locked clock to the rotation speed of the disk. The disk locked clock may then be used to identify the target write angles for each burst, as well as clock write/read circuitry when writing/reading the bursts.

In one embodiment, the control circuitry may execute a “scan read” when reading one or more bursts and measuring the corresponding quality metric. A scan read in this embodiment means the head is scanned radially across the burst over multiple revolutions of the disk surface by decrementing the bias applied to the actuator (e.g., VCM18inFIG. 2A). That is with each decrement of the actuator bias, the head moves a corresponding increment away from the crash stop position. The burst is read to measure the quality metric (e.g., burst amplitude) during a revolution of the disk surface, the actuator bias is again decremented to increment the head position, the burst metric is read again, and so on. This embodiment may help compensate for the head deviating from a circular trajectory while writing the bursts due to a “spongy” response when pressing the actuator arm against, for example, a rubber crash stop. That is executing a scan read of the bursts may, in one embodiment, ensure the head passes over the center of each burst when the center position is unknown due to a spongy response of the crash stop. In one embodiment, multiple scan reads of a burst may be executed to ensure the head passed over the center of the burst during at least one of the revolutions.

In one embodiment, a radial band of the disk surface that corresponds to the general position of the head while pressing the actuator arm against the crash stop may be erased before and/or after writing the bursts (e.g., the bursts shown inFIG. 3A). In one embodiment, the write parameter settings may be configured with nominal settings to help erase the radial band prior to writing/reading the bursts, and in another embodiment, the radial band may be erased after writing/reading the bursts using the calibrated settings for the write parameters. In one embodiment, a “scan erase” is executed similar to the “scan read” embodiment described above. That is, in one embodiment a scan erase is executed by decrementing the actuator bias over multiple revolutions of the disk surface so that the head scans across the radial band while configured into an “erase” mode (e.g., AC or DC erase). Similar to a scan read, in one embodiment a scan erase may be executed multiple times to ensure the entire radial band is erased similar to scanning an erasure over a white board.

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 data storage controller, or certain operations described above may be performed by a read channel and others by a data storage controller. In one embodiment, the read channel and data storage 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 data storage 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. In some embodiments, at least some of the flow diagram blocks may be implemented using analog circuitry (e.g., analog comparators, timers, etc.), and in other embodiments at least some of the blocks may be implemented using digital circuitry or a combination of analog/digital circuitry.

In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, a hybrid disk drive, 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.