Data storage device detecting fly height instability of head during load operation based on microactuator response

Description

BACKGROUND

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.

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 and/or a microactuator) in order to actuate the head radially over the disk in a direction that reduces the PES.

When the disk drive is powered off or enters an idle mode, the head is unloaded onto a ramp mounted over an outer edge of the disk before spinning down the disk. When the disk is powered on or exits the idle mode, the disk is spun up to an operating speed and the head is launched from the ramp over the spinning disk surface during a load operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by 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 surface by a voice coil motor (VCM) and a microactuator.

FIG. 2B is a flow diagram according to an embodiment wherein the microactuator is configured into a sensor and monitored during a load operation in order to detect fly height instability of the head.

FIG. 2C is a waveform of the microactuator sensor signal illustrating fly height instability during a load operation.

FIG. 3A is a waveform of the microactuator sensor signal illustrating fly height stability during a load operation.

FIG. 3B shows an embodiment wherein the microactuator sensor signal is compared to positive and negative thresholds during a sense window in order to detect fly height instability.

FIG. 4A shows an embodiment wherein the data storage device comprises a plurality of disk surfaces and at least one head actuated over each disk surface by respective microactuators.

FIG. 4B shows an embodiment wherein the plurality of microactuator sensor signals are wire-ORed and the resulting combined sensor signal compared to positive and negative thresholds to detect fly height instability.

FIG. 4C shows an embodiment wherein the microactuator sensor signals are evaluated individually in order to identify the head that is exhibiting fly height instability during the load operations.

FIG. 4D shows an embodiment wherein each microactuator sensor signal is evaluated over a respective sense window during the load operation.

FIG. 5A shows an embodiment wherein the current applied to the VCM during the load operation may be evaluated to verify a fly height instability detected based on the microactuator sensor signal.

FIG. 5B shows an embodiment wherein a spindle speed error during the load operation may be evaluated to verify a fly height instability detected based on the microactuator sensor signal.

FIG. 6A shows an embodiment wherein each head may comprise a suitable proximity sensor (e.g., fly height sensor, touchdown sensor, etc.).

FIG. 6B is a flow diagram according to an embodiment wherein the microactuator sensor signals are first evaluated to detect fly height instability of any one of the heads (global detection), and when fly height instability is detected, the proximity sensor signal of a selected head is evaluated during subsequent load operations to detect which head is exhibiting the fly height instability (local detection).

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a first disk surface 16₁, a first head 18₁, and a first microactuator 20₁ configured to actuate the first head 18₁ over the first disk surface 16₁. The disk drive further comprises control circuitry 22 configured to execute the flow diagram of FIG. 2B, wherein the first microactuator 20₁ is configured into a first sensor (block 24), and a first load operation is executed to load the first head over the first disk surface (block 26). A fly height instability of the first head is detected during the first load operation based on a first sensor signal generated by the first microactuator (block 28).

In the embodiment of FIG. 2A, the disk drive comprises a ramp 30 mounted over the outer edge of the first disk surface 16₁ wherein the first head 18₁ is unloaded onto the ramp 30 when the first disk surface 16₁ is spun down (e.g., at power off or idle mode). When the disk drive is powered on or exiting an idle mode, the first disk surface 16₁ is spun up to an operating speed, and the first head 18₁ is launched from the ramp 30 over the spinning disk surface during a load operation. The control circuitry 22 processes a read signal 32 emanating from the first head 18₁ in order to synchronize to servo sectors 34₀-34_N. Once synchronized to the servo sectors 34₀-34_N, the control circuitry 22 generates a control signal 36 applied to a voice coil motor (VCM) 38 which rotates an actuator arm 40A about a pivot in order to position the first head 18₁ over a target data track. Once the head 18₁ reaches the target data track, the control circuitry 22 controls both the VCM 38 and the first microactuator 20₁ (over line 42₁) in order to maintain the first head 18₁ over the target data track during an access operation (e.g., write/read operation).

In one embodiment, the first head 18₁ comprises at least one write and read element fabricated on a slider that is coupled to a suspension 44A using a suitable head gimbal assembly (HGA). The suspension 44A, which is coupled to the actuator arm 40A, comprises a flexible material that biases the head toward the disk surface. When the disk surface is spinning at a high speed, an air bearing forms such that the head is said to fly just above the disk surface.

Any suitable microactuator may be employed to actuate the head over the disk surface, such as a piezoelectric actuator. In addition, the microactuator may actuate the head over the disk surface in any suitable manner, such as by actuating the suspension relative to the actuator arm, or by actuating the HGA that couples the slider relative to the suspension.

In one embodiment, manufacturing defects/tolerances in the slider and/or defects/tolerances in the HGA and/or suspension and/or defects/tolerances of the disk and/or clamping of the disk may cause fly height instability of the head during a load operation. That is, when the head is launched off the ramp over the spinning disk surface, manufacturing defects of one or more components, as well as the inherent transient of the load operation may cause the head to exhibit an instable fly height which can damage the head and/or the disk surface due to head/disk contact (touchdown). Accordingly, in one embodiment fly height instability of a head is detected during a load operation so that any suitable corrective action may be taken, such as modifying the state trajectory of the load operation, improving the manufacturing processes, replacing defective components, or disabling a head in a multi-surface disk drive (depopulating a head).

In one embodiment, the control circuitry 22 detects fly height instability of the first head 18₁ over the first disk surface 16₁ during a load operation by configuring the first microactuator 20₁ into a sensor, and monitoring a sensor signal (over line 42₁) generated by the first microactuator 20₁ during the load operation. FIG. 2C illustrates an example fly height instability and resulting sensor signal which in this example is a voltage signal that oscillates between a positive and negative value (only the envelope of the oscillations is shown). That is, the fly height oscillations that occur during the fly height instability translate into an oscillating sensor signal, and therefore the instability in the fly height may be detected in one embodiment by evaluating the effect on the sensor signal.

FIGS. 3A and 3B show an embodiment wherein the control circuitry detects the fly height instability of the head based on a magnitude of the sensor signal over a sense window 46 during the load operation, wherein the sense window 46 may correspond to the time that fly height instability will typically occur. FIG. 3A shows an example load operation wherein the fly height is stable and therefore the magnitude of the sensor signal over the sense window 46 does not exceed positive and negative thresholds. FIG. 3B shows an example load operation wherein the fly height is instable (e.g., oscillating) and the resulting oscillations in the sensor signal exceed the positive and negative thresholds. Also in the embodiments of FIGS. 3A and 3B, the sense window 46 is opened when the current flowing through the VCM remains below a threshold for an interval. That is, the sense window 46 may be opened when the VCM current becomes relatively stable such that perturbations in the sensor signal are caused mainly by fly height instability rather than transients in the VCM servo system.

FIG. 4A shows an embodiment of a disk drive comprising a plurality of disk surfaces 16₁-16_N with at least one head 18₁-18_N actuated over each disk surface by a respective microactuator 20₁-20_N. FIG. 4B shows an embodiment wherein during a load operation the microactuator sensor signals 42₁-42₄ are wire-ORed 48, and the combined sensor signal 50 compared to positive and negative thresholds at comparators 52A and 52B. The outputs of the comparators are ORed at OR gate 54 such that the resulting signal 56 indicates fly height instability. In one embodiment, the control circuitry 22 configures the microactuator sensor signals 42₁-42₄ into the wire-ORed configuration shown in FIG. 4B during an initial number of load operations while testing for fly height instability. If fly height instability is detected during the initial load operations, the control circuitry 22 may execute at least one additional load operation to evaluate each of the microactuator sensor signals 42₁-42₄ individually in order to identify which of the heads 18₁-18_N is exhibiting fly height instability.

FIG. 4C shows an example of this embodiment wherein the control circuitry 22 may configure a multiplexer 58 to select one of the microactuator sensor signals 42₁-42₄ during a subsequent load operation. The control circuitry 22 may then evaluate the selected sensor signal over the entire sense window 46 shown in FIGS. 3A and 3B. A number of load operations may be executed, wherein a different sensor signal may be selected during each load operation in order to determine which head is exhibiting fly height instability. In one embodiment, after detecting fly height instability when evaluating the combined sensor signal 50 in FIG. 4B, the control circuitry 22 may execute a number of load operations for each head while evaluating the respective sensor signal in order to verify whether a head is exhibiting fly height instability.

FIG. 4D shows an embodiment wherein during a load operation, the sense window 46 shown in FIGS. 3A and 3B may be divided into a number of smaller sense windows 46₁-46_N, wherein during each sense window the control circuitry 22 may evaluate a respective one of the microactuator sensor signals 42₁-42₄. In one embodiment, the control circuitry 22 may execute a number of load operations, wherein during each load operation the control circuitry 22 may select a different microactuator sensor signal 42₁-42₄ to evaluate for each sense window so that the control circuitry 22 evaluates each sensor signal over the entire sense window 46. In one embodiment, the control circuitry 22 may employ the smaller sense windows shown in FIG. 4D after detecting a fly height instability while evaluating the combined sensor signal 50 shown in FIG. 4B over the entire sense window 46.

In one embodiment, the control circuitry 22 may evaluate each microactuator sensor signal during each load operation over an even smaller sense window, wherein the sense windows may be time-division multiplexed over the entire sense window 46. That is, during a single load operation the control circuitry 22 may time-division multiplex the microactuator sensor signals 42₁-42₄ over small slices of the sense window 46 such that each microactuator sensor signal 42₁-42₄ is evaluated multiple times over the sense window 46. In one embodiment, the time-division multiplexing technique may be employed after detecting fly height instability while evaluating the combined sensor signal 50 shown in FIG. 4B over the entire sense window 46.

In one embodiment, other signals generated within the data storage device may be evaluated in order to confirm whether fly height instability of a head is actually occurring. FIG. 5A shows an example of this embodiment wherein a current profile of a current flowing through the VCM during the load operations may be evaluated to verify a detected fly height instability. In this example, when the fly height is stable the VCM current may remain fairly constant during the load operation (after the initial transients), whereas when there is fly height instability, the VCM current may exhibit a ramp shape as illustrated in FIG. 5A. FIG. 5B illustrates another example of this embodiment wherein a speed error of a spindle motor (not shown) that rotates the disks during the load operations may be evaluated to verify a detected fly height instability. Other embodiments may evaluate different and/or additional signals to verify a detected fly height instability, such as evaluating a signal generated by an acoustic sensor or a fly height/touchdown sensor. For example, in one embodiment each head 18₁-18_N may be fabricated with a temperature sensitive proximity sensor (e.g., fly height sensor, touchdown sensor, etc.) that may be evaluated during load operations in addition to the microactuator sensor signals 42₁-42₄ in order to verify a detected fly height instability.

FIG. 6A shows an embodiment wherein each head 18_i may comprise a suitable write element (WE), a suitable read element (RE), and a suitable proximity sensor (PS) 60_i (e.g., fly height sensor, touchdown sensor, etc.) capable of detecting a proximity of the head 18_i to the disk surface 16_i. In one embodiment, the control circuitry 22 may be capable of evaluating the sensor signal generated by one of the proximity sensors from a selected one of the heads. For example, the control circuitry 22 may comprise a preamp circuit capable of selecting one of the proximity sensor signals in order to evaluate the proximity of the corresponding head to the corresponding disk surface. Accordingly, in one embodiment during a first load operation the microactuator sensor signals 42₁-42₄ may be evaluated as a combined wire-ORed signal 50 as shown in FIG. 4B in order to detect a fly height instability of any one of the heads (global detection). When the global fly height instability is detected, the control circuitry 22 may execute a number of subsequent load operations wherein each of the proximity sensor signals may be evaluated individually in order to determine which of the heads is exhibiting the fly instability (local detection).

This embodiment is understood with reference to the flow diagram of FIG. 6B wherein the plurality of microactuators are configured into sensors (block 62) and a first load operation is executed (block 64). The combined microactuator sensor signal is evaluated (block 65) during the first load operation to detect a fly height instability of at least one of the heads (global detection), and when fly height instability is detected (block 66), the control circuitry executes a subsequent load operation (block 68) and evaluates the proximity sensor signal generated for a selected head (block 70). When the proximity sensor signal indicates fly height instability (block 72), the selected head is flagged (block 74). Subsequent load operations may be executed (block 76) in order to evaluate the proximity sensor signal for each head individually, thereby detecting a local fly height instability for each head after detecting a fly height instability globally based on the combined microactuator sensor signal.

The threshold(s) used to detect the fly height instability, such as the thresholds shown in the embodiments of FIGS. 3A and 3B, may be configured to any suitable level. In one embodiment, the threshold(s) may be pre-set, and in another embodiment, the threshold(s) may be configured before a load operation while the head is parked on the ramp. For example, in one embodiment the microactuator sensor signal may be evaluated to obtain a measure of the noise level in real time, and the threshold(s) for the corresponding head configured accordingly. In one embodiment, the threshold(s) may also be adjusted relative to environmental condition(s), such as the operating temperature.

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.

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