Disk drive detecting crack in microactuator

This application claims priority to provisional U.S. Patent Application Ser. No. 61/806,798, filed on Mar. 29, 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 embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it moves from track to track.

FIG. 1 shows a prior art disk format 1 as comprising a number of servo tracks 3 defined by servo sectors 5 0 - 5 N recorded around the circumference of each servo track. Each servo sector 5 i comprises a preamble 7 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 9 for storing a special pattern used to symbol synchronize to a servo data field 11 . The servo data field 11 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track while the head is moving. Each servo sector 5 i further comprises groups of servo bursts 13 , which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 13 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.

As the density of the data tracks increases, a microactuator may be employed in combination with the VCM to improve the tracking performance of the servo system. Any suitable microactuator may be employed such as a suitable piezoelectric (PZT) actuator. It may be desirable to test the microactuators so that defective microactuators can be replaced or disabled.

FIG. 1 shows a prior art disk format comprising a plurality of servo sectors that define a plurality of servo tracks.

FIG. 2 is a flow diagram according to an example embodiment of the present inventive concept wherein a servo loop is disabled while exciting the microactuator to accentuate a crack in the microactuator.

FIG. 3A shows a disk drive according to an example embodiment of the present inventive concept comprising a head actuated over a disk surface by a voice coil motor (VCM) and a microactuator.

FIG. 3B shows an example embodiment of the present inventive concept wherein the disk drive comprises a plurality of disk surfaces and a plurality of heads actuated by the VCM and respective microactuators.

FIG. 3C shows control circuitry according to an example embodiment of the present inventive concept for servoing the head over the disk surface using the VCM and the microactuator.

FIG. 4 is a flow diagram according to an example embodiment of the present inventive concept wherein the microactuator is excited with a test frequency other than a resonant frequency of a load beam.

FIG. 5A is a flow diagram according to an example embodiment of the present inventive concept wherein after exciting the microactuator, the microactuator is evaluated in response to a position error signal (PES) generated while servoing the head in response to the servo sectors.

FIG. 5B shows an example embodiment of the present inventive concept wherein the PES indicates the microactuator is not defective.

FIGS. 5C-5D show example embodiments of the present inventive concept wherein the PES indicates the microactuator is defective.

FIG. 5E shows a graph of VCM wear leveling during PZT burn-in according to an example embodiment of the present inventive concept.

FIG. 5F illustrates a comparison of various waveforms according to an example embodiment of the present inventive concept.

FIG. 6A is a flow diagram according to an example embodiment of the present inventive concept wherein a PES level is measured before exciting the microactuator, and after exciting the microactuator, wherein a difference between the PES measurements is used to evaluate the microactuator.

FIG. 6B shows an example embodiment of the present inventive concept wherein the difference between the PES measurements indicates the microactuator is not defective.

FIG. 6C shows an example embodiment of the present inventive concept wherein the difference between the PES measurements indicates the microactuator is defective.

FIG. 6D shows an example embodiment of the present inventive concept wherein a slope of a metric is used to evaluate the microactuator.

FIG. 7 shows a flow diagram for the control circuitry to determine performance degradation according to an example embodiment of the present inventive concept.

FIG. 8 shows a flow diagram for the control circuitry to alternate the HSA between the inner diameter and the outer diameter according to an example embodiment of the present inventive concept.

FIG. 9 shows a flow diagram for the control circuitry to alternate the HSA between the inner diameter and the outer diameter according to an example embodiment of the present inventive concept.

FIG. 10 shows a flow diagram for the control circuitry to adjust the waveform applied to the microactuator according to an example embodiment of the present inventive concept.

FIG. 11 shows a flow diagram for the control circuitry to perform signal conditioning to the applied waveform according to an example embodiment of the present inventive concept.

FIG. 2 is a flow diagram according to an example embodiment of the present inventive concept wherein a servo loop is disabled while exciting the microactuator to accentuate a crack in the microactuator. In an embodiment, the flow diagram of FIG. 2 is executed by control circuitry of a disk drive, such as that shown in FIG. 3A . FIG. 3A shows a disk drive according to an example embodiment of the present inventive concept comprising a first disk surface 4 1 , a first head 2 1 coupled to a distal end of an actuator arm 6 1 , and a voice coil motor (VCM) 8 and a first microactuator 10 1 operable to actuate the first head 2 1 over the first disk surface 4 1 . The disk drive further comprises control circuitry 12 for executing various processes illustrated in the flow diagrams in this disclosure. Further details of the flow diagrams that can be implemented by the control circuitry are described below. The example flow diagram of FIG. 2 is implemented to measure first performance characteristics of a microactuator ( 700 ), control the head stack assembly to move in an alternating manner ( 701 ), measure second performance characteristics ( 702 ), compare the first performance characteristics to the second performance characteristics ( 703 ) and determine performance degradation ( 704 ). Additional details related to the flow will be described in conjunction with FIG. 7 .

Terms such as “first”, “second”, “third”, etc. are used for labeling purposes and are not meant to be limiting to any particular order. For example, in another example implementation by the control circuitry 12 , the control circuitry can implement move the HSA in an alternating manner across the disk media, while actuating the microactuator, measure one or more first performance characteristics of the microactuator; and detect failure based on one or more first performance characteristics. In another example implementation by the control circuitry 12 , the control circuitry measures one or more second performance characteristics of the microactuator before moving the HSA in an alternating manner across the disk media, and detects failure by comparing the one or more first performance characteristics to the one or more second performance characteristics to determine performance degradation of the microactuator.

In the example embodiment of FIG. 3A , each disk surface (e.g., disk surface 4 1 ) comprises a plurality of tracks 22 defined by a plurality of servo sectors 24 1 - 24 n . As the head 2 1 passes over a servo sector, the control circuitry 12 demodulates the read signal 26 into a position error signal (PES). The PES is filtered by a suitable compensation filter to generate a control signal 28 applied to the VCM 8 . The VCM 8 rotates an actuator arm 6 1 about a pivot in order to position the head 2 1 radially over the disk 4 1 in a direction that reduces the PES. The control circuitry 12 may also generate a control signal 30 applied to the microactuator 10 1 in response to the PES to further increase the bandwidth of the servo system.

Also, in the example embodiment of FIG. 3A , the disk drive comprises a ramp 32 located at an outer edge of the disk for unloading the actuator arm 6 1 while the disk drive is idle. In an example embodiment described in greater detail below, the control circuitry 12 may evaluate the microactuator 10 1 using the ramp 32 , for example, by measuring a voltage induced by the microactuator 10 1 when the load beam 34 1 is deflected as the actuator arm 6 1 strikes the ramp 32 .

FIG. 3B shows an example embodiment of the present inventive concept wherein the VCM 8 rotates a plurality of actuator arms 6 1 - 6 n about a pivot in order to coarsely position a plurality of heads 2 1 - 2 n over respective disk surfaces 4 1 - 4 n . A plurality of microactuators 10 1 - 10 n are also provided for fine positioning each head, wherein in one example embodiment disclosed below, the microactuators 10 1 - 10 n are simultaneously excited at the test frequency and then evaluated individually.

FIG. 3C shows control circuitry according to an example embodiment of the present inventive concept for servoing the head 2 1 over the disk surface 4 1 using the VCM 8 and the microactuator 10 1 . During normal operation, a read/write channel 36 processes the read signal 26 to demodulate the servo sectors 24 1 - 24 n into an estimated position 38 of the head 2 1 over the disk surface 4 1 . The estimated position 38 is subtracted 40 from a reference position 42 to generate the PES 44 . The PES 44 is processed by a VCM compensator 46 to generate a control signal 28 applied to the VCM 8 and processed by a microactuator compensator 48 to generate a control signal 30 applied to the microactuator 10 1 .

In another example embodiment, as illustrated in the flow diagram of FIG. 8 and described in further detail below, the control circuitry may servo the head to an inner diameter of the disk and then press the actuator arm against an inner diameter crash stop prior to exciting the microactuator 10 1 . This example embodiment ensures that data recorded on the disk is not corrupted if the head contacts the disk surface while exciting the microactuator 10 1 .

Any suitable frequency may be selected as the test frequency to excite the microactuator. For example, the microactuator can be excited with a control signal at a predetermined test frequency qualified through a procedure other than the load beam resonant frequency. In an example embodiment shown in FIG. 4 the test frequency is substantially different from a resonant frequency of the load beam 34 1 (block 56 ) so as not to damage the load beam 34 1 or the interconnect of the load beam 34 1 to the actuator arm 6 1 . In one example embodiment, an amplitude of the test frequency 50 is substantially higher than an amplitude of the control signal 30 applied during normal operation of the disk drive so as to minimize the time required to accentuate potential cracks in the microactuator 10 1 . Using a higher amplitude control signal to excite the microactuator 10 1 may induce vibrations that may cause the head to contact the disk surface. However, pressing the actuator arm 6 1 against the inner diameter crash stop ensures the head cannot corrupt user data recorded on the disk. In another example embodiment, the test frequency may excite the microactuator 10 1 after unloading the actuator arm 6 1 onto the ramp 32 ; however, in this example embodiment the vibrations of the load beam 34 1 may result in undesirable wear of the ramp 32 .

After exciting the microactuator 10 1 at the predetermined test frequency, the microactuator 10 1 evaluated to detect a crack or other defect in the microactuator 10 1 (block 20 ). Any suitable technique may be employed to evaluate the microactuator 10 1 , wherein in one example embodiment shown in the flow diagram of FIG. 5A , after the microactuator is excited with a control signal at a test frequency (block 16 ), the head is positioned over a servo track (block 58 ) by reading the servo sectors, and then a control signal applied to the microactuator 10 1 to offset the head (block 60 ). The servo sectors are read to generate a PES (block 62 ), and the PES evaluated to detect a crack or other defect in the microactuator (block 64 ). In an example embodiment, a sinusoidal control signal is applied to the microactuator 10 1 in order to offset the head from the servo track to generate the PES, wherein any suitable frequency of the control signal may be employed. Other example embodiments described in greater detail below may evaluate the microactuator 10 1 by selecting the output of the microactuator compensator 48 as the control signal 30 applied to the microactuator 10 1 .

FIG. 5B illustrates a sinusoidal PES as a result of applying the sinusoidal control signal 66 to the microactuator 10 1 . In one example embodiment, the PES is compared to thresholds Th+ and Th− representing an offset from each side of the target servo track, wherein the microactuator 10 1 is considered non-defective if the amplitude of the PES exceeds the thresholds. FIG. 5C shows an example of a defective microactuator 10 1 wherein the PES falls below both thresholds (both sides of the servo track), and FIG. 5D shows an example of a defective microactuator 10 1 wherein the PES falls below one of the thresholds (one side of the servo track).

FIG. 5E shows a graph of VCM wear leveling during PZT burn-in according to an example embodiment of the present inventive concept. In an example embodiment, the control circuitry executes servo fast interrupts (FIQ) and proportional-integral-derivative (PID) control to move the HSA. For example, the HSA can be controlled to move towards the inner diameter at a velocity determined by PID control, and can then change direction when within some distance of a target track. The dual stage actuator (DSA) exercise mechanism can involve a single frequency sine wave that uses output from the servo FIQ.

FIG. 5F illustrates a comparison of various waveforms according to an example embodiment of the present inventive concept. In an example embodiment, the waveform can be made smoother and more continuous by employing a low pass filter (LPF), as shown in the upper graph. As indicated by the lower graph, the waveform before the application of the LPF is not smooth and is step-like, whereas the waveform after the application of the LPF is smoother and takes the form of a sine wave.

FIG. 6A is a flow diagram according to an example embodiment of the present inventive concept for evaluating the microactuator 10 1 , wherein the head is positioned over a servo track (block 66 ) by reading the servo sectors, the head offset from the servo track by applying a control signal to the microactuator (block 68 ), and the servo sectors read to generate a first PES (block 70 ). The burn-in procedure is performed (block 72 ), wherein the microactuator can be excited at the test frequency to accentuate cracks. After exciting the microactuator, the head is repositioned over the servo track (block 74 ), the head offset from the servo track by applying a control signal to the microactuator (block 76 ), and the servo sectors read to generate a second PES (block 78 ). The microactuator is then evaluated by comparing the second PES to the first PES (block 80 ). This example embodiment is illustrated in FIGS. 6B and 6C wherein if the difference between the first and second PES is greater than a threshold, the microactuator is considered defective. Accordingly, in this example embodiment the microactuator is evaluated relative to a change in performance after exciting the microactuator as opposed to evaluating the absolute performance relative to an expected level.

Any suitable metric may be evaluated to determine whether the microactuator is defective. FIG. 6D illustrates an example embodiment of the present inventive concept wherein a rate of change of a metric (e.g., absolute measurement or difference measurement) is evaluated to determine whether the microactuator 10 1 is defective. For example, the microactuator 10 1 may be considered defective if the slope (positive or negative) of the metric exceeds a threshold. In this example embodiment, the microactuator 10 1 may be excited multiple times and the metric measured after each excitation in order to measure the rate of change of the metric. The rate of change of the metric may be an indication of future failure even if the current value of the metric would otherwise pass the evaluation. In another example embodiment, prior to exciting the microactuator at the test frequency, a movement of the load beam is induced in order to induce a movement in the microactuator. A first induced voltage is detected in response to the induced movement, wherein the microactuator is excited at the test frequency. A movement of the load beam is again induced in order to induce a movement in the microactuator, and a second induced voltage is detected in response to the induced movement. A difference between the first and second induced voltages is evaluated in order to detect a defect in the microactuator. Other characteristics can include, for example, the frequency response of the microactuator. In one example embodiment, the absolute magnitude of the frequency response may be evaluated, and in another example embodiment, a difference between the magnitude before and after exciting the microactuator may be evaluated.

There may be potential head-disk interaction (HDI) concerns as the microactuator is excited at some test frequencies to accentuate cracks. Certain waveforms may potentially cause HDI events, which may result in head/media damage. Further, exciting the microactuator at a fixed track location for an extended time will increase the risk of HDI events, resulting failures such as lube moguling, lube depletion and slider air bearing surface (DLC layer) wear.

In another example embodiment of the present inventive concept, such HDI concerns are addressed by various configurations of the microactuator. Control circuitry is configured to select excitation waveforms and frequencies to reduce the HDI risks, as well as to configure the movement of the head stack assembly (HSA) to move between an inner diameter and an outer diameter of the media at a relatively low speed during excitation (e.g., less than 10 inches per second, about three inches per second or less, etc.), to avoid the situation where the heads fly on the media at the same location for an extended period of time.

As the head is moved back and forth between the inner diameter and the outer diameter, the speed and the path of the head can be adjusted based on the desired implementation, such that the head does not stay at a fixed location for a prolonged period of time. While the head is moving, the microactuator is excited at the test frequency to accentuate cracks or other defects. The movement and excitation can be repeated several times as well as during various conditions (e.g., in room temperature, at a hot temperature, etc.), depending on the desired implementation. The parameters of the microactuator can be examined before and after the excitation process for screening of failures.

FIG. 7 is a flow diagram for control circuitry according to an example embodiment of the present inventive concept for testing a microactuator in a disk drive. The flow diagram is previously shown in FIG. 2 and a more detailed description of the flow diagram is provided below. At block 700 , the control circuitry measures one or more first performance characteristics of the microactuator. At block 701 , the control circuitry controls the head stack assembly (HSA) to move in an alternating manner (e.g., inner diameter to outer diameter and back, etc.) across a rotating disk, while actuating the microactuator between a first actuation condition and a second actuation condition (e.g., between positive/negative voltage at full level, at high frequency). The movement of the HSA may be performed in a slow motion. At block 702 , the control circuitry measures one or more second performance characteristics of the microactuator. At block 703 , the control circuitry compares the one or more first performance characteristics to the one or more second performance characteristics. At block 704 , the control circuitry determines performance degradation from the comparison. The first and second performance characteristics can be any of the performance metrics as described above, for example, in FIGS. 6A-6D .

In one example embodiment, the velocity of the movement of the HSA can be configured through a timer mechanism, a velocity control mechanism, a servo control mechanism, or a combination of these mechanisms. In some embodiments, a particular mechanism may be applied to control the velocity when moving in one direction (e.g., using velocity control going from outer diameter to inner diameter) and another mechanism may be applied to control the velocity when moving in another direction (e.g., using timer when going from outer diameter to inner diameter). In one example embodiment, a default velocity can be used to move toward the inner diameter and a change of direction can be triggered when the servo control detects reaching a target track, or within a certain distance of a target track, on the disk. Each velocity control configuration may be coupled with a different DSA excitation waveform (e.g., single-frequency sine wave from a timer interrupt, single-frequency sine wave using some multiplier of the rate output of the servo control mechanism, etc.). The control circuitry can be configured to move the HSA in an alternating manner across the disk media based on the velocity and to detect a motion boundary based on at least one of the timer and an inner diameter crash stop. The velocity and timer can be configured depending on the layout of the disk drive and the desired implementation.

FIG. 8 shows a flow diagram for the control circuitry to alternate the HSA between the inner diameter and the outer diameter according to an example embodiment of the present inventive concept. At block 800 , the HSA is moved towards the inner diameter under velocity control. At block 801 , the HSA changes directions towards the outer diameter. The change in direction can be initiated when the inner diameter crash stop is detected, or can also be based on timer and velocity control. At block 802 , the HSA changes directions to travel towards the outer diameter based both the time and velocity of the HSA under velocity control. At block 803 , the HSA changes directions towards the inner diameter after the timer has elapsed.

FIG. 9 shows a flow diagram for the control circuitry to alternate the HSA between the inner diameter and the outer diameter according to an example embodiment of the present inventive concept. At block 900 , a first track is selected within a first range of an inner diameter and a second track is selected within a second range of an outer diameter. This can be implemented, for example, by controlling a trajectory of the VCM. At block 902 , the HSA is controlled to change directions when the HSA is within a third range of the selected track. The control circuitry can utilize servo metadata to determine the position of the HSA. The first, second and third ranges can be utilized to compensate for variation in HSA movement, and/or to provide a safety margin such that the HSA does not fly off the disk media or crash against the inner diameter crash stop, depending on the desired implementation. The first and second tracks can also be selected based on the position of the HSA.

To further reduce the potential HDI risks, an additional waveform and/or frequency selection may be conducted for each head/media combination prior to the excitation. For example, a smooth sine wave with a certain frequency may be selected to avoid interaction to structural resonance in off-track and vertical directions. The waveform may be smoothed to avoid high-frequency harmonics which may excite the load beam motion in a vertical direction at a high-frequency range.

The selection of the waveform may be performed by recording the waveform from the control circuitry and utilizing the recorded waveform for exciting the microactuators while measuring motion of the load beam by using a vibrometer. The recorded waveform can be a predetermined waveform having a shape, a frequency, and an amplitude that is set based on the disk drive implementation. For example, one or more predetermined waveforms can be preset based on an impact to at least one component of the disk drive (e.g., the slider, the head, the disk media, the gimbal, etc.), and thereby selected based on such an impact and the desired implementation of the disk drive. Such impact can involve, for example, a frequency response on the components of the disk drive, and the waveform can be selected by selecting at least one of the shape, the frequency and the amplitude to apply to the desired implementation of the disk drive. The waveform may then be adjusted by the control circuitry to eliminate certain harmonics (e.g., higher) and/or undesired transients, based on the measurements from the vibrometer. The frequency selection may be repeated until the motion for the load beam is smoothed to a threshold from the measurements of the vibrometer, wherein the microactuators can be excited for an extended period of time to check for microactuator damage. The control circuitry can be configured to actuate the microactuator by generating a waveform for an actuation signal.

FIG. 10 shows a flow diagram for the control circuitry to adjust the waveform applied to the microactuator according to an example embodiment of the present inventive concept. At block 1000 , the control circuitry applies a voltage waveform to the microactuator. The control circuitry can generate and apply the waveform based on a predetermined waveform having a shape, a frequency, and an amplitude. At block 1001 , the control circuitry measures vertical slider motion of the HSA and compares the vertical slider motion before and during the waveform injection. At block 1002 , the control circuitry adjusts the waveform based on the measured slider motion. The waveform can be generated based on the predetermined waveform by using a digital-analog conversion of the predetermined waveform as described in FIG. 11 below, and signal conditioning can be performed on the generated waveform to substantially match the generated waveform to the converted predetermined waveform. A low pass filter can be utilized to perform signal conditioning on the generated waveform.

FIG. 11 shows a flow diagram for the control circuitry to perform signal conditioning to the applied waveform according to an example embodiment of the present inventive concept. At block 1100 , a digital pulse train is generated based on frequency/waveform selection. At block 1101 , digital to analog conversion of the selected digital waveform is conducted. At block 1102 , signal conditioning is performed on the applied voltage waveform to match the converted selected waveform.

Any suitable control circuitry may be employed to implement the flow diagrams in the example embodiments of the present inventive concept, 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 actions described above may be performed by a read channel and others by a disk controller. In one example embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative example 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 an SOC.

In one example embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the actions of the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one example 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 example 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 example embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.