Disk drive employing repeatable disturbance compensation for fly height control

[0001] 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 seeks from track to track.

[0002] An air bearing forms between the head and the disk due to the disk rotating at high speeds. Since the quality of the write/read signal depends on the fly height of the head, conventional heads (e.g., a magnetoresistive heads) may comprise an actuator for controlling the fly height. Any suitable fly height actuator may be employed, such as a heater which controls fly height through thermal expansion, or a piezoelectric (PZT) actuator. It is desirable to determine the appropriate fly height actuator control signal (e.g., appropriate current applied to a heater) that achieves the target fly height for the head.

[0009] FIG. 1A shows a disk drive according to an embodiment of the present invention comprising a disk 2, a head 4 actuated over the disk 2, a fly height controller 22 for controlling a fly height of the head over the disk 2, and control circuitry 6 operable to generate a fly height control signal (FHC) 8, including the control circuitry shown in FIG. 1B. A fly height of the head 4 is measured using a suitable fly height estimator (FHE) 10, and the measured fly height (MFH) 12 is combined 14 with a target fly height (TFH) 16 and a first feedforward signal 18, wherein the first feedforward signal is generated according to:
a₁·cos(2πn₁k/N)+b₁·sin(2πn₁k/N)
where a₁ and b₁ are coefficients and 2πn₁k/N represents a frequency of a first repeatable disturbance in the fly height of the head 4. In an embodiment disclosed below, the coefficients a₁ and b₁ are initialized to nominal values (e.g., determined for a family of disk drives) and then adapted within each disk drive toward optimal values.

[0010] In the embodiment of FIG. 1B, the measured fly height 12 is subtracted from the target fly height 16 to generate an error signal e(k) 20 that is adjusted by the first feedforward signal 18 to compensate for the first repeatable disturbance in the fly height. The error signal e(k) 20 is processed by the fly height controller 22 which may include suitable compensation filtering to generate the fly height control signal 8. Also in the embodiment of FIG. 1B, the head 4 comprises a suitable actuator (not shown) for actuating the fly height, such as a suitable heater actuator or a suitable PZT actuator. The fly height estimator (FHE) 10 may employ any suitable technique to measure the fly height, such as by processing the read signal 36 emanating from the head, by processing a gain control signal for adjusting a gain of the read signal 36, or by measuring a capacitance between a surface of the head 4 and the surface of the disk 2.

[0011] In the embodiment of FIG. 1A, the disk 2 comprises a number of embedded servo sectors 24₀-24_j recorded around the circumference of the disk 2 which define a plurality of tracks 26. FIG. 1C shows an embodiment wherein each servo sector 24_i comprises a preamble 28 for storing a periodic pattern that enables proper gain adjustment and timing synchronization of the read signal, and a sync mark 30 for storing a special pattern used to symbol synchronize to a servo data field 32. The servo data field 32 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 24_i further comprises groups of servo bursts 34 (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 34 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.

[0012] In the embodiment of FIG. 1A, the control circuitry 6 processes the read signal 36 emanating from the head 4 to demodulate the embedded servo sectors 24₀-24_j and generate a position error signal (PES) representing a radial offset of the head 4 from a target track 26. The control circuitry 6 processes the PES with a suitable servo compensator to generate a control signal 38 applied to a voice coil motor (VCM) 40. The VCM 40 rotates an actuator arm 42 about a pivot in order to actuate the head 4 radially over the disk 2 in a direction that decreases the PES.

[0013] In one embodiment, N in feedforward equation 44 of FIG. 1B for generating the feedforward signal 18 represents a number of discrete time intervals over a rotation of the disk 2, and k represents the current time interval for updating the feedforward signal 18. The number of discrete time intervals N may be any suitable number. In one embodiment, N represents the number of servo sectors 24₀-24_j recorded around the circumference of the disk in each track 26, such that the feedforward signal 18 is updated at each servo sector 24_i. The number of discrete time intervals N may be more or less than the number of servo sectors, and the feedforward signal 18 may be generated at any suitable time, such as when the control circuitry 6 is processing a servo sector 24_i or a data sector during a write or read operation.

[0014] In one embodiment, the scalar n₁ in the feedforward equation 44 of FIG. 1B is a multiple of a once around frequency of the disk 2. For example, in one embodiment n₁=1 such that the feedforward signal 18 compensates for the once around or fundamental frequency of the repeatable disturbance in the fly height. However, the feedforward signal 18 may compensate for any frequency of the repeatable disturbance, and in an embodiment described below, multiple feedforward signals are generated to compensate for multiple repeatable disturbance frequencies.

[0015] In an embodiment shown in FIG. 2A, the control circuitry 6 adapts the coefficients a₁ and b₁ of the feedforward equation 44 according to:
a₁(k+1)=a₁(k)−g₁·e(k)·cos(2πn₁k/N)
b₁(k+1)=b₁(k)−g₁·e(k)·sin(2πn₁k/N)
where g₁ is a gain and e(k) comprises a difference between the measured fly height and the target fly height. Any suitable gain g₁ may be employed in the embodiments of the present invention to achieve a desired bandwidth while maintaining stability. In one embodiment, a nominal gain is determined for a family of disk drives which may be tuned or adapted for each individual disk drive.

[0016] In one embodiment, the control circuitry 6 adapts the coefficients a₁ and b₁ during a calibration procedure which may be executed one time, such as during manufacturing, or periodically such as every time the disk drive is powered on. In another embodiment, the control circuitry 6 may adapt the coefficients a₁ and b₁ continuously while the disk drive is operating normally which may help compensate for changes in environmental conditions, such as ambient temperature. In an embodiment shown in FIG. 2B, the control circuitry 6 adapts the coefficients a₁ and b₁ of the feedforward equation 44 according to:
a₁(k+1)=a₁(k)−g₁·e(k)·cos(2πn₁k/N+φ₁)
b₁(k+1)=b₁(k)−g₁·e(k)·sin(2πn₁k/N+φ₁)
where φ₁ represents a phase of the first repeatable disturbance.

[0017] FIG. 3A shows an embodiment of the present invention wherein multiple feedforward signals 18₁-18_M are generated using a number of equations 44₁-44_M corresponding to a plurality of repeatable disturbance frequencies in the fly height of the head. For example, in one embodiment the control circuitry generates a second feedforward signal according to:
a₂·cos(2πn₂k/N)+b₂·sin(2πn₂k/N)
where a₂ and b₂ are coefficients and 2πn₂k/N represents a frequency of a second repeatable disturbance in the fly height of the head. In one embodiment, the control circuitry adapts the coefficients a₂ and b₂ as shown in FIG. 3B and FIG. 3C in a similar manner described above.

[0018] Any suitable repeatable disturbance frequency may be attenuated by the feedforward signals. In one embodiment, n₁ is a first multiple of a once around frequency of the disk, and n₂ is a second multiple of the once around frequency of the disk, where n₂ is greater than n₁. For example, in one embodiment n₁=1 and n₂=2 such that the fundamental frequency and the next harmonic of the repeatable disturbance are attenuated. However, as illustrated in FIG. 3A any suitable number of repeatable disturbance frequencies may be attenuated.

[0019] In one embodiment, the coefficients a and b of the feedforward equation (s) are generated for each head/disk combination. In one embodiment, a single set of coefficients a and b may be generated for an entire disk surface, and in another embodiment, multiple sets of coefficients a and b are generated corresponding to multiple radial locations of the disk, such as over multiple zones of the disk surface. As the head transitions into a new zone, the corresponding coefficients a and b are loaded into the feedforward equation (s).

[0020] 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.

[0021] 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.