Disk drive characterizing microactuator by injecting sinusoidal disturbance and evaluating feed-forward compensation values

Description

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 velocity of the actuator arm as it seeks from track to track.

FIG. 1 sows a prior art disk format 2 comprising a plurality of data tracks 4 defined by a number of servo sectors 6₀-6_N recorded around the circumference of each data track. Each servo sector 6, comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6, further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 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 characterize the microactuator in order to calibrate a gain for the compensator, or disable the microactuator altogether if it is found defective.

BRIEF DESCRIPTION OF THE DRAWINGS

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

FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a head actuated over a disk by a VCM and a microactuator.

FIG. 2B is a flow diagram according to an embodiment of the present invention wherein a sinusoidal disturbance is applied to the microactuator, and the resulting feed-forward compensation value is processed to characterize the microactuator.

FIG. 3A shows an embodiment of the present invention wherein a microactuator compensator is disabled while applying the sinusoidal disturbance to the microactuator, and the feed-forward compensation value comprises a coefficient of a sinusoid.

FIGS. 3B-3C illustrate how the coefficients of a feed-forward sinusoid adapt for different microactuator effective gain values while applying the sinusoidal disturbance to the microactuator according to an embodiment of the present invention.

FIG. 3D is a flow diagram according to an embodiment of the present invention wherein if a magnitude of the coefficients is less than a threshold, the microactuator compensator remains disabled.

FIG. 4 shows an embodiment of the present invention wherein the algorithm for generating the feed-forward sinusoid is also used to generate the sinusoidal disturbance applied to the microactuator.

FIG. 5 is a flow diagram according to an embodiment of the present invention for calibrating a characterization threshold used to enable/disable the microactuator.

FIGS. 6A-6C shows an embodiment of the present invention wherein the feed-forward compensation value comprises multiple sinusoids generated at different frequencies.

FIG. 6D is a flow diagram according to an embodiment of the present invention wherein if the magnitude of coefficients of a feed-forward sinusoid at a frequency other than that of the sinusoidal disturbance are greater than a threshold, the microactuator compensator remains disabled.

FIG. 7A shows an embodiment of the present invention wherein feed-forward compensation values are generated for the microactuator.

FIG. 7B shows a flow diagram according to an embodiment of the present invention wherein while applying the sinusoidal disturbance the microactuator feed-forward compensation values are evaluated to characterize the microactuator.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 2A shows a disk drive according to an embodiment of the present invention comprising a disk 16 comprising a plurality of tracks 17, a head 18, and a voice coil motor (VCM) 20 and a microactuator 22 for actuating the head 18 over the disk 16 in response to a feed-forward compensation value. The disk drive further comprises control circuitry 24 operable to execute the flow diagram of FIG. 2B. A sinusoidal disturbance is applied to the microactuator (step 26), and the resulting feed-forward compensation value is processed to characterize the microactuator (step 28).

In the embodiment of FIG. 2A, the disk 16 comprises a plurality of servo sectors 30₀-30_N that define the plurality of tracks 17. The control circuitry 24 processes the read signal 34 to demodulate the servo sectors 30₀-30_N into a position error signal (PES). The PES is filtered with a suitable compensation filter to generate a control signal 36 applied to a voice coil motor (VCM) 20 which rotates an actuator arm 38 about a pivot in order to position the head 18 radially over the disk 16 in a direction that reduces the PES. The servo sectors 30₀-30_N may comprise any suitable position information, such as a track address for coarse positioning and servo bursts for fine positioning.

Any suitable microactuator 22 may be employed in the embodiments of the present invention, such as a piezoelectric (PZT) actuator which transduces electrical energy into a mechanical displacement. In the embodiment of FIG. 2A, the microactuator 22 is integrated with and actuates the suspension 39 that couples the head 18 to the actuator arm 38. However, the microactuator may be integrated at any suitable location, such as with a slider to which the head 18 is mounted. In addition, the microactuator 22 may comprise multiple actuators (e.g., multiple PZTs) that may operate to move the head 18 in different radial directions.

Any suitable feed-forward compensation value may be evaluated in order to characterize the microactuator 22. In an embodiment shown in FIG. 3A, the feed-forward compensation value comprises coefficients (a,b) of a sinusoid 40, wherein k represents a servo sector 30, out of N servo sectors 30_N. A VCM feed-forward signal 42 is combined 44 with a VCM control signal 46 generated by a VCM compensator 48. A microactuator compensator 50 generates a microactuator control signal 52 applied to the microactuator 22. When the head 18 reads a servo sector 30, a read channel 54 demodulates the read signal 34 into an estimated position 56 of the head 18. The estimated position 56 is compared 58 to a reference position 60 to generate a PES 62. The PES 62 is processed to adapt the coefficients (a,b) of the feed-forward sinusoid 40, processed by the VCM compensator 48 to generate the VCM control signal 46, and processed by the microactuator compensator 50 to generate the microactuator control signal 52.

In the embodiment of FIG. 3A, when characterizing the microactuator 22 a sinusoidal disturbance 64 is applied to the microactuator 22 (through adder 66) while disabling the microactuator compensator 50 by opening switch 68. The affect of the sinusoidal disturbance 64 on the feed-forward compensation value is then evaluated to characterize the microactuator 22. For example, a magnitude of the coefficients (a,b) of the feed-forward sinusoid 40 may be evaluated to determine an effective gain of the microactuator 22. FIG. 3B illustrates different magnitudes of the coefficients (a,b) for corresponding effective gain values of the microactuator 22 after adapting the coefficients (a,b) to a steady state value over multiple disk revolutions. In one embodiment, in order to expedite the characterization procedure the coefficients (a,b) are evaluated after a single disk revolution as illustrated in FIG. 3C. That is, the coefficients (a,b) may adapt sufficiently to characterize the microactuator after a single disk revolution. In one embodiment, the change in the magnitude of the coefficients is evaluated since the coefficients (a,b) may have a non-zero value prior to applying the sinusoidal disturbance 64. The change in the magnitude of the coefficients reflects an effective gain of the microactuator since there are multiple components along the controlled plant that may affect the gain of the plant.

FIG. 3D shows a flow diagram according to an embodiment of the present invention wherein after disabling the microactuator compensator (step 70) the head is servoed over a target track (step 71). A sinusoidal disturbance is applied to the microactuator (step 72), and the coefficients of the VCM feed-forward sinusoid are adapted to compensate for the resulting disturbance in the PES (step 73). After the coefficients have adapted for a predetermined interval, the coefficients of the VCM feed-forward sinusoid are evaluated (step 74) wherein if a magnitude of the coefficients exceeds a threshold (step 76), the microactuator is considered operating normally and therefore the microactuator compensator is enabled (step 78). If the magnitude of the coefficients does not exceed the threshold (step 76), then the microactuator is considered defective and therefore the microactuator compensator remains disabled so that the microactuator is not used during normal operation.

In an alternative embodiment, the magnitude of the coefficients may be used to adjust a gain of the microactuator compensator. For example, if the coefficients of the VCM feed-forward sinusoid indicate the effective gain of the microactuator is low, the gain of the microactuator compensator may be increased rather than disable the microactuator.

The sinusoidal disturbance 64 may be generated in any suitable manner. In an embodiment illustrated in FIG. 4, the sinusoidal disturbance 64 is generated using the same algorithm for generating the feed-forward sinusoid 40. Both sinusoids may be generated in any suitable manner, such as with a lookup table or a mathematical function. In one embodiment, the frequency of the sinusoidal disturbance 64 is generated at the same frequency as the feed-forward sinusoid 40. In yet another embodiment, the coefficients are selected such that the phase of the sinusoidal disturbance 64 substantially matches the initial phase of the feed-forward sinusoid 40. In one embodiment, this is implemented by duplicating the coefficients (a,b) of the feed-forward sinusoid 40 (prior to applying the sinusoidal disturbance to the microactuator) and then increasing the coefficients for generating the sinusoidal disturbance. While the magnitude and phase of the feed-forward signal 42 will change as the coefficients (a,b) adapt, the magnitude and phase of the sinusoidal disturbance 64 will remain constant during the characterization process.

FIG. 5 is a flow diagram according to an embodiment of the present invention for calibrating a characterization threshold for the microactuator, such as a gain threshold for disabling the microactuator as described above. In one embodiment, a nominal threshold is calibrated using a disk drive with a normal, functioning microactuator. The nominal threshold is then copied to a family of production disk drives during manufacturing. During the calibration procedure, the gain of the microactuator is decreased (step 80) and a sinusoidal disturbance applied to the microactuator (step 82). The coefficients (a,b) of the feed-forward sinusoid are then evaluated to set the characterization threshold. For example, in one embodiment the gain is decreased until a three sigma for the track miss-registration (TMR) reaches a point where the VCM alone is better than if it where augmented by the microactuator, wherein the magnitude of the coefficients (a,b) at the corresponding gain is selected as the characterization threshold (disabling threshold).

FIG. 6A shows an embodiment of the present invention wherein the VCM feed-forward signal 42 comprises a plurality of feed-forward sinusoids 40₁-40_M generated at different frequencies (as determined from n₁, n₂, etc) and magnitudes (as determined from a₁,b₁, a₂,b₂, etc.) in order to compensate for a number of repeatable disturbances (e.g., 1×, 2×, etc). During normal operation, the coefficients for each feed-forward sinusoid 40, are adapted using a suitable learning algorithm, such as shown in the example of FIGS. 6B and 6C. When characterizing the microactuator, the coefficients are adapted in response to the sinusoidal disturbance applied to the microactuator. That is, if the microactuator is functioning properly, the disturbance in the PES due to the sinusoidal disturbance applied to the microactuator will cause a corresponding change in the coefficients of the feed-forward sinusoid at the frequency of the sinusoidal disturbance. In one embodiment, the sinusoidal disturbance may be applied at several different frequencies and the corresponding feed-forward sinusoids evaluated (e.g., averaged) in order to characterize the microactuator.

In one embodiment, the magnitude of the feed-forward sinusoids at frequencies other than the frequency of the sinusoidal disturbance are evaluated to determine the linearity and stability of the microactuator servo loop. This embodiment is illustrated in the flow diagram of FIG. 6D which extends on the flow diagram of FIG. 3D. If the magnitude of the coefficients (e.g., a₂,b₂) of a feed-forward signal at a frequency different from the sinusoidal disturbance is greater than a threshold (step 86), it may indicate the microactuator servo loop is exhibiting a non-linear or unstable behavior and therefore the microactuator is not enabled.

FIG. 7A shows control circuitry according to an embodiment of the present invention comprising a microactuator feed-forward compensation block 88 for generated a microactuator feed-forward signal 90. In the flow diagram of FIG. 7B, when characterizing the microactuator the microactuator compensator 50 is disabled (step 92) and a sinusoidal disturbance applied to the microactuator (step 94). A microactuator feed-forward compensation value (e.g., a coefficient for generating feed-forward signal 90) is evaluated in order to characterize the microactuator (step 96). That is, if the microactuator is operating properly, or if the gain needs adjusting, it will be reflected in the microactuator feed-forward compensation value. In one embodiment, both a VCM feed-forward compensation value and a microactuator feed-forward compensation value may be evaluated in order to characterize the microactuator.

The microactuator may be characterized at any suitable time, such as once during manufacturing of the disk drive and/or while the disk drive is deployed in-the-field. In one embodiment, the microactuator is characterized (and optionally adjusted) each time the disk drive is powered on, and in another embodiment, the microactuator may be characterized at a predetermined interval (e.g., once every hour). In the embodiment described above with reference to FIG. 3C, the microactuator is characterized after the coefficients of the feed-forward sinusoid(s) have been adapted over a single revolution of the disk. In other embodiments, the coefficients may be allowed to adapt longer before characterizing the microactuator (e.g., after multiple disk revolutions).

Any suitable control circuitry may be employed to implement the flow diagrams in the embodiments of the present invention, 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 steps 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 an SOC.

In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the steps of 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.