Data storage device on-line adapting disturbance observer filter

Description

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of U.S. Provisional Application No. 61/933,079, filed on Jan. 29, 2014, which is hereby incorporated by reference in its entirety.

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, 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.

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.

FIG. 2B shows control circuitry according to an embodiment for actuating the head over a disk using a disturbance observer filter.

FIG. 2C is a flow diagram according to an embodiment wherein the disturbance observer filter is adjusted based on a plurality of band-pass filters for filtering a disturbance compensation signal (DCS).

FIG. 3A shows a magnitude spectrum for each of a plurality of the band-pass filters, wherein each of the band-pass filters comprises a gain based on a torque sensitivity function of the control circuitry.

FIG. 3B shows the output of each of the band-pass filters.

FIG. 3C shows a magnitude spectrum of a PES when the disk drive is subject to a disturbance.

FIG. 4 shows an embodiment wherein at least one of a center frequency, a gain, and a bandwidth of the disturbance observer filter is adjusted based on the band-pass filters.

FIG. 5 shows an embodiment wherein an output of the disturbance observer filter is further filtered to compensates for the effect of a delay when a disturbance d(k) affecting the disk drive comprises a high frequency.

FIG. 6 shows an embodiment wherein the band-pass filters comprise two banks, and each bank filters every other sample of the DCS.

FIG. 7A shows a data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk.

FIG. 7B shows control circuitry according to an embodiment for actuating the head over a disk using a disturbance observer filter.

FIG. 7C is a flow diagram according to an embodiment wherein an actuator control signal (ACS) is adjusted based on an output of the disturbance observer filter while concurrently adjusting at least one of a center frequency, a gain, and a bandwidth of the disturbance observer filter.

FIG. 8 shows control circuitry according to an embodiment wherein the adjusted ACS is filtered with a loop shaping filter (LSF).

DETAILED DESCRIPTION

FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a head 16 and a disk 18 comprising tracks 20 defined by servo sectors 22₀-22_N. The disk drive further comprises control circuitry 24 configured to execute the flow diagram of FIG. 2C, wherein a location of the head over the disk is measured based on the servo sectors (block 26), and a position error signal (PES) is generated representing a difference between a target location for the head and the measured location for the head (block 28). An actuator control signal (ACS) is generated in response to the PES (block 30), a disturbance compensation signal (DCS) is generated based on the PES and the ACS (block 32), and the DCS is filtered with a disturbance observer filter (block 34). The ACS is adjusted based on an output of the disturbance observer filter (block 36), and the head is actuated over the disk based on the adjusted ACS (block 38). The DCS is filtered with a plurality of band-pass filters (block 40), and the disturbance observer filter is adjusted based on the plurality of band-pass filters (block 42).

In the embodiment of FIG. 2A, the control circuitry 24 processes a read signal 44 emanating from the head 16 to demodulate the servo sectors 22₀-22_N 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 circuitry 24 filters the PES using a suitable compensation filter to generate a control signal 46 applied to a voice coil motor (VCM) 48 which rotates an actuator arm 50 about a pivot in order to actuate the head 16 radially over the disk 18 in a direction that reduces the PES. The servo sectors 22₀-22_N may 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).

FIG. 2B shows an example servo control system implemented by the control circuitry 24, wherein the output of a head actuator 52 represents the measured location of the head 16 over the disk 18 based on the servo sectors 22₀-22_N. The measured location 54 is subtracted from a target location 56 to generate the PES 58. A compensator 60 processes the PES 58 to generate the ACS 62. A disturbance affecting the disk drive (e.g., a vibration) is modeled in FIG. 2B as a disturbance d(k) 64 added into the control signal 66 applied to the head actuator 52. To compensate for this disturbance d(k) 64, the servo control system comprises a disturbance compensator comprising a disturbance observer including a filter P⁻¹ 68 having an inverse transfer function of the head actuator 52 which generates an inverse signal 70 based on the output 54 of the head actuator 52 (the measured head location). The inverse signal 70 therefore represents the input 66 to the head actuator 52, including the effect of the disturbance d(k) 64. The ACS 58 is delayed 72 by m sample periods T_s (z^−m) to account for the phase delay of the head actuator 52, and the delayed signal 74 is subtracted from the inverse signal 70. The resulting disturbance compensation signal (DCS) 76 is filtered by a disturbance observer filter 78 (e.g., a band-pass filter) having a target center frequency ω₀, bandwidth, and gain corresponding to the expected frequency of the disturbance d(k) 64. The output 80 of the disturbance observer filter 78 represents an estimate of the disturbance d_est(k−m) 64 which is subtracted from the disturbance d(k) 64 at adder 82, thereby adjusting the ACS 62 to generate the adjusted ACS 84 which effectively cancels the effect of the disturbance d(k) 64 on the head actuator 52.

In the embodiment of FIG. 2B, the DCS 76 is filtered by a bank of band-pass filters 86 which are then used to adjust the disturbance observer filter 78. For example, in one embodiment illustrated in the magnitude responses of FIG. 3A, the pass-band of each band-pass filters 86 covers a particular frequency range, wherein collectively the pass bands cover a range of disturbance frequencies that may be affecting the disk drive. FIG. 3C illustrates an example magnitude response of the PES 58 while a disturbance d(k) 64 is affecting the disk drive, wherein the magnitude response of the PES comprises two peaks at two different frequencies, and each peak comprises a particular magnitude and a particular bandwidth. FIG. 3B illustrates a corresponding output of each band-pass filter 86 while the disturbance d(k) 64 is affecting the disk drive. Accordingly, in one embodiment the outputs of the band-pass filters 86 (such as shown in FIG. 3B) are evaluated to determine a desired center frequency, gain, and bandwidth of the disturbance observer filter 78. In one embodiment, the disturbance observer filter 78 comprises multiple band-pass filters which are each adjusted to compensate for multiple peak frequencies of the disturbance d(k) 64. In the example of FIG. 3C, the disturbance observer filter 78 may comprise two band-pass filters each adjusted to comprise a center frequency, gain, and bandwidth corresponding to the two peak frequencies of the disturbance d(k) 64 as identified using the band-pass filters 86. In one embodiment illustrated in FIG. 3A, each of the band-pass filters 86 comprises a gain based on a torque sensitivity function of the control circuitry shown in FIG. 2B so that the outputs of the band-pass filters 86 more accurately represent the potential disturbance spectrum.

In one embodiment, the disturbance observer filter 78 in the embodiment of FIG. 2B comprises:
gQ₀(e^jωi)
where g is a gain and Q₀(e^jωi) is a band-pass filter. In one embodiment, the band-pass filter Q₀(e^jωi) comprises a lattice-based filter:

Q 0 ⁡ ( z ) = a 0 ⁡ ( z 2 - 1 ) z 2 + b 1 ⁢ z + b 2
where the above filter coefficients may be calculated according to:

a 0 = 1 - k 2 2 , b 1 = k 1 ⁡ ( 1 - k 2 ) , b 2 = k 2
in which k₁ and k₂ are determined by the center frequency F_c and the bandwidth B as follows:

k 1 = - cos ⁢ ⁢ ω c , k 2 = 1 - tan ⁡ ( Ω B / 2 ) 1 + tan ⁡ ( Ω B / 2 )
where ω_c=2πF_cT_s and Ω_B=2πBT_s.

Any suitable algorithm may be employed to adjust the disturbance observer filter 78 based on the outputs of the band-pass filters 86. In one embodiment, the control circuitry 24 of FIG. 2A is configured to adjust the center frequency of the disturbance observer filter 78 based on a peak output of the band-pass filters 86 that also exceeds a threshold, and configured to adjust the bandwidth of the disturbance observer filter 78 based on the number of adjacent outputs of the band-pass filters 86 that also exceed the threshold. In one embodiment, the control circuitry 24 of FIG. 2A is configured to adjust the gain of the disturbance observer filter according to:

min ( 1 , max ( G min , ( 1 - G min ) ⁢ mag pk - MLMG MLFG - MLMG + G min ) )
where G_min represents a minimum gain, mag_pk represents a magnitude of an output of one of the band-pass filters (corresponding to a peak in the spectrum of the DCS 76), MLMG represents a magnitude level of minimum gain, and MLFG represents a magnitude level of full gain.

FIG. 4 shows control circuitry according to an embodiment wherein the bank of band-pass filters 86 also processes the PES 58 to verify that the DCS 76 represents a disturbance 64 of significant magnitude that is affecting the disk drive. For example, if the magnitude of the PES 58 does not exceed a threshold, the outputs of the band-pass filters 86 may be ignored so that the disturbance observer filter 78 is not adjusted. In one embodiment, the disturbance compensation may be disabled (e.g., by opening switch 88 in FIG. 2A) while evaluating the DCS 76 (and optionally the PES 58) in order to adjust the disturbance observer filter 78. In another embodiment described below, the disturbance observer filter 78 may be adjusted based on the outputs of the band-pass filters 86 while the disturbance compensation is enabled (e.g., while switch 88 in FIG. 2A is closed).

FIG. 5 shows an embodiment wherein the disturbance compensation further comprises a filter F 90 which compensates for the effect of the delay 72 when the disturbance d(k) 64 comprises a high frequency. That is, when the disturbance d(k) 64 comprises a high frequency, the delay 72 may induce a mismatch between the target center frequency of the disturbance observer filter 78 and the actual center frequency of the disturbance observer filter 78. At high frequencies, the inverse filter P⁻¹(z⁻¹) 68 may be modeled as z^−mP_n⁻¹(z⁻¹) and the relationship between the PES 58 and the disturbance d(k) 64 may be represented as:
e(k)=−S₁(z⁻¹)P(z⁻¹)d(k)
where:
S₁(z⁻¹)=S₀(z⁻¹)(1−z^−mQ(z⁻¹)F(z⁻¹)).
For optimal disturbance cancellation at ω_i, the following equation is minimized:
min:|1−e^jmωiQ(e^jωi)F(e^jωi)|
In one embodiment, the filter F 90 comprises a finite impulse response (FIR) filter of the form:
F(z⁻¹)=c₀+c₁z⁻¹.

When the disturbance observer filter 78 comprises the above described lattice-based band-pass filter, the solution that will minimize the above equation is:

c 0 = sin ⁡ ( ( m + 1 ) ⁢ ω i ) sin ⁡ ( ω i ) , ⁢ c 1 = - sin ⁡ ( m ⁢ ⁢ ω 1 ) sin ⁡ ( ω i )
In an embodiment described below, the filter F 90 of FIG. 5 may facilitate adjusting the disturbance observer filter 78 based on the outputs of the band-pass filters 86 while the disturbance compensation is enabled (e.g., while switch 88 in FIG. 2A is closed) without rendering the servo control system unstable.

FIG. 6 shows an embodiment wherein the bank of band-pass filters 86 comprises a first bank comprising band-pass filters BP[1] to BP[m] and a second bank comprising band-pass filters BP[m+1] to BP[2m]. The DCS 76 is filtered with an anti-aliasing filter 92, and the resulting output samples are processed in an even an odd interleave. That is, multiplexer 94 is configured to transmit the even samples to the first bank of band-pass filters and configured to transmit the odd samples to the second bank of band-pass filters. This down-sampling technique may reduce the computational time and complexity of the band-pass filters 86 without a significant reduction in performance.

FIG. 7A shows a disk drive according to an embodiment comprising a head 16 and a disk 18 comprising tracks 20 defined by servo sectors 22₀-22_N. The disk drive further comprises control circuitry 96 configured to execute the flow diagram of FIG. 7C, wherein a location of the head over the disk is measured based on the servo sectors (block 98), and a position error signal (PES) is generated representing a difference between a target location for the head and the measured location for the head (block 100). An actuator control signal (ACS) is generated in response to the PES (block 102), and a disturbance compensation signal (DCS) is generated based on the PES and the ACS (block 104). The DCS is filtered with a disturbance observer filter (block 106), and the ACS is adjusted based on an output of the disturbance observer filter while concurrently adjusting at least one of a center frequency, a gain, and a bandwidth of the disturbance observer filter (block 108). The head is actuated over the disk based on the adjusted ACS (block 110).

FIG. 7B shows a servo control system implemented by the control circuitry 24 similar to the embodiment of FIG. 2B except that block 112 may implement any suitable algorithm to adjust at least one of a center frequency, a gain, and a bandwidth of the disturbance observer filter 78. For example, in one embodiment block 112 comprises a plurality of band-pass filters that filter the DCS 76 as described above in order to adjust the center frequency, the gain, and the bandwidth of the disturbance observer filter 78 based on a peak output of the band-pass filters. In one embodiment, block 112 operates to adjust the disturbance observer filter 78 while the disturbance observer is enabled (i.e., while the output 80 of the disturbance observer filter 78 adjust the ACS 62 to compensate for the disturbance d(k) 64).

In one embodiment, the disturbance observer filter 78 may comprise a lattice-based filter as described above which may help maintain the stability of the servo control system shown in FIG. 7B while adjusting at least one of the center frequency, the gain, and the bandwidth of the disturbance observer filter 78. In another embodiment, the servo control system shown in FIG. 7B may comprise the filter 90 shown in FIG. 5 which compensates for the effect of the delay 72 when the disturbance d(k) 64 comprises a high frequency as described above. The filter 90 may also improve the stability of the servo control system while adjusting at least one of the center frequency, the gain, and the bandwidth of the disturbance observer filter 78.

FIG. 8 shows another embodiment wherein the servo control system may comprise a loop shaping filter (LSF) 114 comprising a plurality of second-order filters:

∏ i ⁢ ⁢ A i ⁡ ( z - 1 ) B i ⁡ ( z - 1 ) .
The LSF 114 is configured to filter the adjusted ACS 84, for example, to attenuate a peak in the error rejection curve of the servo control system, thereby improving the stability of the servo control system while adjusting at least one of the center frequency, the gain, and the bandwidth of the disturbance observer filter 78.

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.

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.