U.S. patent application number 16/935494 was filed with the patent office on 2021-09-09 for magnetic disk device.
The applicant listed for this patent is Kabushiki Kaisha Toshiba, Toshiba Electronic Devices & Storage Corporation. Invention is credited to Takuji Matsuzawa.
Application Number | 20210280210 16/935494 |
Document ID | / |
Family ID | 1000005795495 |
Filed Date | 2021-09-09 |
United States Patent
Application |
20210280210 |
Kind Code |
A1 |
Matsuzawa; Takuji |
September 9, 2021 |
MAGNETIC DISK DEVICE
Abstract
According to one embodiment, a magnetic disk device includes a
magnetic disk, a magnetic head, a first actuator that moves the
magnetic head to a predetermined position on the magnetic disk, a
second actuator that is provided in the first actuator and adjusts
a position of the magnetic head, a control unit that controls
operations of the first actuator and the second actuator, and a
storing unit that stores a coefficient of an approximation
polynomial calculated based on an approximation formula for
approximating voltage dependency of a gain of the second actuator.
When controlling the operation of the second actuator, the control
unit calculates the gain amplitude of the second actuator from the
approximation polynomial in which the coefficient is used and
amplitude of a voltage input to the second actuator.
Inventors: |
Matsuzawa; Takuji; (Kashiwa
Chiba, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kabushiki Kaisha Toshiba
Toshiba Electronic Devices & Storage Corporation |
Tokyo
Tokyo |
|
JP
JP |
|
|
Family ID: |
1000005795495 |
Appl. No.: |
16/935494 |
Filed: |
July 22, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G11B 5/5573 20130101;
G11B 5/012 20130101; G11B 5/82 20130101 |
International
Class: |
G11B 5/55 20060101
G11B005/55; G11B 5/012 20060101 G11B005/012; G11B 5/82 20060101
G11B005/82 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 5, 2020 |
JP |
2020-037688 |
Claims
1. (canceled)
2. A magnetic disk device comprising: a magnetic disk; a magnetic
head that reads data from and writes data in the magnetic disk; a
first actuator that moves the magnetic head to a predetermined
position on the magnetic disk; a second actuator that is provided
in the first actuator and adjusts a position of the magnetic head;
a control unit that controls operations of the first actuator and
the second actuator, and a storing unit that stores a first
coefficient of an approximation polynomial calculated based on an
approximation formula for approximating voltage dependency of a
first gain of the second actuator, wherein when controlling the
operation of the second actuator, the control unit calculates the
first gain of the second actuator from the approximation polynomial
in which the first coefficient stored in the storing unit is used,
a second coefficient of an odd number times angle of a sine wave
when an output of the second actuator calculated from an input sine
wave voltage is represented as a sum of odd numbers times angle of
a fundamental frequency of the sine wave, and a desired
displacement of the second actuator.
3. The magnetic disk device according to claim 2, wherein the
control unit represents the output of the second actuator as a sum
of odd numbers times angle of a fundamental frequency of the sine
wave voltage, and when the odd numbers are represented as 2k+1,
where k is 0, 1, 2, . . . , calculates the first gain by adding up
all the second coefficients of the odd numbers times angle when k
is an even number or subtracting all the second coefficients when k
is an odd number.
4. The magnetic disk device according to claim 2, wherein the first
coefficient is calculated from a plurality of second gains by
measuring the first gain of the second actuator by the discrete
Fourier transform at a number of measurement points greater than a
degree of the approximation polynomial and the second coefficient
of a sine wave fundamental frequency of the output of the second
actuator as a sum of odd numbers times angle of a fundamental
frequency of the sine wave voltage.
5. (canceled)
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from Japanese Patent Application No. 2020-037688, filed
Mar. 5, 2020, the entire contents of which are incorporated herein
by reference.
FIELD
[0002] Embodiments described herein relate generally to a magnetic
disk device.
BACKGROUND
[0003] There has been known a magnetic disk device including a
first actuator that locates a magnetic head on a magnetic disk and
a second actuator that finely adjusts the position of the magnetic
head on the magnetic disk. The second actuator is called, for
example, micro actuator. Since the magnetic disk device includes
the second actuator, the magnetic disk device can more accurately
perform positioning of the magnetic head. It is known that a gain
of input and output of the second actuator used in this type of the
magnetic disk device have voltage dependency.
[0004] Therefore, in order to more accurately control the second
actuator, it is requested to consider the voltage dependency of the
gain of the second actuator. For example, sine waves are input to
the second actuator at a plurality of voltage amplitudes. A gain at
each of the voltage amplitudes is calculated from ratios of
amplitudes obtained from DFT (discrete Fourier transform) to input
and output waveforms of the sine waves (hereinafter, DFT ratio MA
gain). The voltage dependency is polynomially approximated from the
gains. However, in this polynomial approximation, distortion of an
output (displacement) of the second actuator due to the voltage
dependency cannot be considered. An accurate gain cannot be
calculated.
[0005] A problem to be solved by the present invention is to
calculate an MA gain considering distortion of an output
(displacement) of the second actuator (hereinafter, amplitude ratio
MA gain). An object of the present invention is to provide a
magnetic disk device that can accurately control the second
actuator using the amplitude ratio MA gain.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a block diagram illustrating an example of a
configuration of a magnetic disk device according to a first
embodiment.
[0007] FIG. 2 is a diagram illustrating an example of a control
system according to the embodiment.
[0008] FIG. 3 is a diagram illustrating an example of an input
output relation of the second actuator according to the
embodiment.
[0009] FIG. 4 is a diagram illustrating an example of voltage
dependency of the second actuator according to the embodiment.
[0010] FIG. 5 is a diagram illustrating an example of an input
output relation of a sine wave according to the embodiment.
[0011] FIG. 6 is a diagram illustrating an example of a processing
result (an input) of DFT according to the embodiment.
[0012] FIG. 7 is a diagram illustrating an example of a processing
result (an output) of the DFT according to the embodiment.
[0013] FIG. 8 is a flowchart illustrating an example of processing
for storing a coefficient value according to the embodiment.
[0014] FIG. 9 is a flowchart illustrating an example of calculation
processing for an MA gain according to the embodiment.
[0015] FIG. 10 is a diagram illustrating an example of a hysteresis
characteristic according to a second embodiment.
DETAILED DESCRIPTION
[0016] In general, according to one embodiment, a magnetic disk
device includes: a magnetic disk; a magnetic head that reads data
from and writes data in the magnetic disk; a first actuator that
moves the magnetic head to a predetermined position on the magnetic
disk; a second actuator that is provided in the first actuator and
adjusts a position of the magnetic head; a control unit that
controls operations of the first actuator and the second actuator;
and a storing unit that stores a coefficient of an approximation
polynomial calculated based on an approximation formula for
approximating voltage dependency of a gain of the second actuator.
When controlling the operation of the second actuator, the control
unit calculates the gain of the second actuator from the
approximation polynomial in which the coefficient stored in the
storing unit is used and amplitude of a voltage input to the second
actuator.
[0017] Embodiments are explained below with reference to the
drawings. Note that disclosure is only an example. The invention is
not limited by content described in the following embodiments.
Modifications easily conceived by those skilled in the art are
naturally included in the scope of the present disclosure. To
further clarify the explanation, sizes, shapes, and the like of
portions are sometimes changed from those in actual implementation
forms and schematically shown in the drawings. In a plurality of
drawings, the same reference numbers are sometimes added to
elements corresponding thereto and detailed explanation of the
portions is omitted.
First Embodiment
[0018] FIG. 1 is a block diagram illustrating an example of a
configuration of a magnetic disk device 1 according to a first
embodiment.
[0019] The magnetic disk device 1 is configured from a head-disk
assembly (HDA) 10, a head amplifier integrated circuit
(hereinafter, head amplifier IC) 18, and a system on chip (SOC)
20.
[0020] The HDA 10 includes a magnetic disk 11, a spindle motor
(SPM) 12, an arm 13, and a voice coil motor (VCM) 14, which is a
first actuator. The magnetic disk 11 is rotated by the SPM 12. A
load beam 15 is attached to the distal end of an arm 13. A magnetic
head 16 is attached to the distal end of the load beam 15. The arm
13 controls to move the magnetic head 16 to a designated position
on the magnetic disk 11 with driving of the VCM 14.
[0021] Further, a pair of piezoelectric elements (for example,
Pb(Zr,Ti)O3) 17 is disposed near an attachment part of the load
beam 15 at the distal end portion of the arm 13. When a voltage is
applied to the pair of piezoelectric elements 17, the piezoelectric
elements on the left and the right expand and contract respectively
in opposite phases. By displacing the magnetic head 16 at the
distal end of the load beam 15 in a radial direction (a cross track
direction) on the magnetic disk 11, the position of the magnetic
head 16 is finely adjusted in the radial direction of the magnetic
disk 11 on the magnetic disk 11. A two-stage actuator in which the
piezoelectric element-driven load beam 15 is added to the distal
end of the arm 13 driven by the VCM in this way is realized. In the
following explanation, the piezoelectric elements 17 are referred
to as micro actuator (a second actuator; hereinafter simply
referred to as "MA" as well) 17. Note that, in the following
explanation in this embodiment, the micro actuator 17 is provided
near the attachment part of the load beam 15. However, not only
this, but the micro actuator 17 may be provided in the magnetic
head 16.
[0022] The magnetic head 16 has structure in which a read head
element and a write head element are separated and mounted on one
slider. The read head element reads out data recorded in the
magnetic disk 11. The write head element writes data in the
magnetic disk 11.
[0023] The head amplifier IC 18 includes a read amplifier and a
write driver. The read amplifier amplifies a read signal read by
the read head element and transmits the read signal to a read/write
(R/W) channel 22. On the other hand, the write driver transmits, to
the write head element, a write current corresponding to write data
output from the R/W channel 22.
[0024] The SOC 20 includes a CPU (microprocessor) 21, the R/W
channel 22, a disk controller 23, and a positioning controller 24.
The CPU 21 is a main controller for drive and executes servo
control for performing positioning of the magnetic head 16 via the
positioning controller 24 and executes read/write control for data
via the head amplifier IC 18.
[0025] The R/W channel 22 includes a read channel for executing
signal processing for read data and a write channel for executing
signal processing for write data. The disk controller 23 executes
interface control for controlling data transfer between a host
system (not illustrated) and the R/W channel 22. Note that the
positioning controller 24 may be realized as hardware or may be
realized as software (firmware).
[0026] A memory 25 includes a volatile memory and a nonvolatile
memory. For example, the memory 25 includes a buffer memory formed
by a DRAM and a flash memory. In the nonvolatile memory of the
memory 25, in addition to a region (not illustrated) for storing
programs and the like necessary for processing of the CPU 21, a
region 26 for storing a coefficient value (hereinafter referred to
as "coefficient-value storing unit") is provided. The
coefficient-value storing unit 26 stores a coefficient value of a
function for calculating an amplitude ratio of the micro actuator
17. Specific content of the coefficient value is explained below.
Note that, since the micro actuator 17 is provided for each of
magnetic heads 16, coefficient values corresponding to the number
of the magnetic heads 16 are stored in association with the
magnetic heads 16. In the following explanation in this embodiment,
the coefficient-value storing unit 26 is provided in the memory 25.
However, the coefficient-value storing unit 26 may be provided in
the SOC 20.
[0027] As illustrated in FIG. 1, in the magnetic disk device 1
mounted with the micro actuator 17, in order to perform positioning
control for the magnetic head 16 with the positioning controller
24, it is necessary to grasp, for each of the magnetic heads 16, a
gain (hereinafter referred to as "MA gain"), which is an input
output ratio of the micro actuator 17. This is because the CPU 21
can accurately position the magnetic head 16 in a desired position
on the magnetic disk 11 by operating the micro actuator 17
considering this MA gain.
[0028] A control system that operates the voice coil motor 14 and
the micro actuator 17 is explained.
[0029] FIG. 2 is a diagram illustrating an example of a control
system 30 that operates the voice coil motor 14 and the micro
actuator 17. Note that, in this embodiment, it is assumed that a
VCM controller 31, an MA controller 32, an MA-gain correcting unit
33, and an MA model unit 34 illustrated in FIG. 2 are provided in
the positioning controller 24.
[0030] In FIG. 2, a target signal is input to an adder 41. After a
signal from an adder 43 explained below is added to the target
signal in the adder 41, an output of the adder 41 is input to an
adder 42 and the MA controller 32. The signal output from the MA
controller 32 is input to the MA-gain correcting unit 33 and the MA
model unit 34. The MA model unit 34 is a control unit that reflects
the operation of the micro actuator on the operation of the voice
coil motor 14.
[0031] The signal passing through the MA model unit 34 is input to
the adder 42. Consequently, after a signal on which the processing
of the MA model unit 34 is reflected is added to the signal from
the adder 41 by the adder 42, the signal from the adder 41 is input
to the VCM controller 31. The VCM controller 31 operates the voice
coil motor 14. In this way, the signal after operating the voice
coil motor 14 is input to the adder 43.
[0032] On the other hand, the MA-gain correcting unit 33 performs
calculation of an MA gain for the signal input from the MA
controller 32 such that the micro actuator 17 is accurately
positioned in a target position. The signal on which the
calculation of the MA gain is reflected in this way is input to the
micro actuator 17. Consequently, the micro actuator 17 is
accurately positioned in the target position. The signal input to
the micro actuator 17 is output to the adder 43. The adder 43 adds
up the signals from the voice coil motor 14 and the micro actuator
17 and inputs an added-up signal of the signals to the adder 41.
The operations of the voice coil motor 14 and the micro actuator 17
are controlled by the control system 30 explained above.
[0033] In general, in the magnetic disk device including the micro
actuator 17, a sensor for measuring displacement of the micro
actuator 17 is not provided. This is because manufacturing cost
increases. Accordingly, it is conceivable to perform processing
explained below. An MA gain is estimated from a transfer function
from an input to a position in a state in which the micro actuator
17 is positioned with being operated and a sensitivity function in
the state in which the micro actuator 17 is positioned without
being operated. The CPU 21 controls the MA-gain correcting unit 33
based on the estimated MA gain and operates the micro actuator 17.
By performing such processing, the micro actuator 17 is accurately
operated.
[0034] The MA gain has voltage dependency as explained above.
Therefore, the voltage dependency is explained with reference to
FIGS. 3 and 4.
[0035] FIG. 3 is a diagram illustrating an example of an input
output relation of the micro actuator 17. In FIG. 3, the vertical
axis represents displacement and the horizontal axis represents a
voltage. A relation in which the displacement increases as the
voltage increases is illustrated. FIG. 4 is a diagram illustrating
an example of voltage dependency of the micro actuator 17. In FIG.
4, the vertical axis represents a value of an approximate value and
the horizontal axis represents a voltage. A relation in which
deviation of the approximate value increases as the voltage
increases is illustrated. When FIG. 3 and FIG. 4 are compared, it
can be understood that there is a correlation between the input
output relation of the MA gain and the voltage dependency and there
is voltage dependency in which the deviation of the approximate
value increases as displacement of input and output increases.
[0036] However, in the case in which there is such voltage
dependency, when a voltage of a sine wave is input to the micro
actuator 17, an output waveform of the voltage is not the sine
wave. FIG. 5 is a diagram illustrating an example of an input
output relation at the time when the voltage of the sine wave is
input in the case in which there is the voltage dependency
illustrated in FIG. 3. In FIG. 5, the vertical axis represents
amplitude and the horizontal axis represents time. An input
waveform (Input), an output waveform (Output), a 3.times.input
waveform (Input), and an input output waveform (Output/Input) are
illustrated. Note that, in FIG. 5, an input output ratio is 3. When
the output waveform (Output) and the 3.times.input waveform (Input)
are compared, it is indicated that the output waveform (Output) and
the 3.times.input waveform (Input) do not coincide and the output
waveform is distorted and is not a sine wave.
[0037] Since the distortion occurs in the output waveform in this
way, an error occurs when a DFT ratio MA gain using DFT is
calculated. In FIG. 5 referred to above, the input output ratio is
"3" but the input output ratio is not 1:3 because of the influence
of the distortion. This is explained more in detail with reference
to FIGS. 6 and 7. FIG. 6 is a diagram illustrating an example of a
processing result (an input) of the DFT. FIG. 7 is a diagram
illustrating an example of a processing result (an output) of the
DFT.
[0038] In FIGS. 6 and 7, the horizontal axis represents a
frequency. When FIG. 6 and FIG. 7 are compared, whereas the input
near 100 Hz exceeds 1 (see FIG. 6), the output is lower than 3 (see
FIG. 7). That is, the input output ratio is not 3. The DFT ratio MA
gain not being 3 in this way is considered to be because the
influence of distortion appears near 300 Hz in FIG. 7 and the
output is dispersed near 300 Hz.
[0039] Processing for measuring DFT ratio MA gains of a plurality
of voltage values explained above, polynomially approximating
voltage dependency from a result of the measurement, and
calculating a DFT ratio MA gain is explained.
[0040] For example, when approximation is performed by a quadratic
polynomial indicated by the following Expression (1),
f(x)=a.sub.0+a.sub.1x+a.sub.2x.sup.2 (1)
[0041] G.sub.D(X.sub.1), G.sub.D(X.sub.2), and G.sub.D(X.sub.3),
which are DFT ratio MA gains, are measured in three voltage
amplitudes X.sub.1, X.sub.2, and X.sub.3 and calculation is
performed as indicated by the following Expressions (2) and
(3).
{ a 0 = G D .function. ( x 1 ) + x 1 .times. x 2 .times.
.beta..gamma. - ax 1 a 1 = .alpha. - ( x 1 + x 2 ) .times.
.beta..gamma. a 2 = .beta..gamma. ( 2 ) .alpha. := G D .function. (
x 2 ) - G D .function. ( x 1 ) x 2 - x 1 , .times. .beta. := G D
.function. ( x 3 ) - G D .function. ( x 1 ) x 3 - x 1 - .alpha. ,
.times. .gamma. := 1 x 3 - x 2 ( 3 ) ##EQU00001##
[0042] In polynomial approximation for calculating coefficients of
the quadratic polynomial, an error based on the distortion
explained with reference to FIGS. 3 to 7 occurs. Therefore, the CPU
21 cannot perform accurate control for the micro actuator 17.
[0043] That is, it is likely that the micro actuator 17 operates
larger or smaller than instructed by the CPU 21 because of the
error. The magnetic head 16 cannot be accurately positioned in a
desired position on the magnetic disk 11 by influence due to the
error.
[0044] Therefore, in this embodiment, the coefficients of the
quadratic polynomial are theoretically calculated by executing the
following calculation without using Expressions (2) and (3)
described above.
[0045] First, voltage dependency of the amplitude ratio MA gain is
approximated by the following Expression (4).
f .function. ( x ) = k = 0 n .times. a k .times. x k ( 4 )
##EQU00002##
[0046] When an input to the micro actuator 17 during DFT ratio MA
gain measurement is represented as u=u.sub.0 sin .theta.,
displacement y of MA at this time is as indicated by the following
Expression (5).
y = f .function. ( u ) .times. u = u 0 .times. k = 0 n .times. a k
.function. ( u 0 .times. sin .times. .times. .theta. ) k .times.
sin .times. .times. .theta. ( 5 ) ##EQU00003##
[0047] An odd power of sine can be represented by a sum of odd
number times angles of sin .theta. (a sine wave fundamental
frequency) as indicated by the following Expression (6).
sin 2 .times. m + 1 .times. .theta. = k = 0 m .times. g ( m , k )
.times. sin .function. ( 2 .times. k + 1 ) .times. .theta. ( 6 )
##EQU00004##
[0048] A coefficient g.sub.(m,k) can be strictly calculated without
using approximation.
[0049] From Expression (6) and the following Expression (7),
sin .times. .times. .theta. = 2 .pi. - 4 .pi. .times. m = 1 .infin.
.times. 1 4 .times. m 2 - 1 .times. cos .times. .times. 2 .times.
.times. m .times. .times. .theta. ( 7 ) ##EQU00005##
the displacement y of the micro actuator 17 is as indicated by the
following Expression (8). Only the odd number times angle of sin
.theta. appears.
y = k = 0 n 2 .times. 1 = 0 k .times. a 2 .times. k .times. u 0 2
.times. k + 1 .times. g ( k , 1 ) .times. sin .function. ( 2
.times. l + 1 ) .times. .theta. + k = 0 n - 1 2 .times. l = 0 k
.times. 2 .times. a 2 .times. k + 1 .times. u 0 2 .times. ( k + 1 )
.pi. .times. g ( k , l ) .times. sin .function. ( 2 .times. l + 1 )
.times. .theta. - 2 .pi. .times. k = 0 n - 1 2 .times. m = 1
.infin. .times. l = 0 k .times. a 2 .times. k + 1 .times. g ( k , l
) .times. u 0 2 .times. ( k + 1 ) 4 .times. m 2 - 1 .times. { sin
.function. [ 2 .times. ( m + 1 ) + 1 ] .times. .theta. - sin
.function. [ 2 .times. ( m - 1 ) - 1 ] .times. .theta. } ( 8 )
##EQU00006##
[0050] A gain obtained by dividing, by the coefficient of sin
.theta., a value obtained by adding up together all coefficients of
an odd number times angle sin (2k+1).theta. (k: natural number)
when k is an even number or subtracting all the coefficients from
one another when k is an odd number is an amplitude ratio MA gain
desired to be calculated. A coefficient .alpha..sub.k of the
approximation polynomial can be calculated from DFT ratio MA gains
measured at different input voltage amplitudes at n+1 points.
[0051] When measured voltages of the DFT ratio MA gains at the n+1
points are represented as x.sub.p, p=1, . . . , n+1, sin .theta. of
the MA displacement y is represented by the following Expression
(9). Therefore, all coefficients .alpha..sub.0, . . . ,
.alpha..sub.n of the approximation polynomial can be calculated
from Expression (9).
k = 0 n 2 .times. a 2 .times. k .times. x p 2 .times. k + 1 .times.
g ( k , 0 ) + k = 0 n - 1 2 .times. 2 .times. a 2 .times. k + 1
.times. x p 2 .times. ( k + 1 ) .pi. .times. g ( k , 0 ) - 8 .pi.
.times. k = 0 n - 1 2 .times. l = 0 k .times. a 2 .times. k + 1
.times. g ( k , l ) .times. x p 2 .times. ( k + 1 ) .function. ( 2
.times. l + 1 ) ( 4 .times. l + 1 ) .function. [ 4 .times. ( l + 1
) 2 - 1 ] ( 9 ) ##EQU00007##
[0052] All the coefficients of the approximation polynomial
calculated as explained above are stored as coefficient values in
the coefficient-value storing unit 26 of the memory 25 in
association with the magnetic heads 16. Note that the amplitude
ratio MA gain may be calculated every time a voltage value applied
to the micro actuator 17 changes or, since the approximation
polynomial is calculated, the amplitude ratio MA gain may be
calculated from the approximation polynomial.
[0053] Approximation of voltage dependency of an MA gain by the
quadratic polynomial of Expression (1) described above is
explained.
[0054] Displacement of MA at this time can be represented by the
following Expression (10) obtained by setting n=2 in Expression
(8).
y = ( f .times. u ) .times. u = ( a 0 + 2 .times. a 1 .times. u 0
.pi. + 3 .times. a 2 .times. u 0 2 4 ) .times. u 0 .times. sin
.times. .times. .theta. - a 2 .times. u 0 3 4 .times. sin .times.
.times. 3 .times. .theta. - 2 .times. a 1 .times. u 0 2 .pi.
.times. m = 1 .infin. .times. 1 4 .times. m 2 - 1 .function. [ sin
.function. ( 2 .times. m + 1 ) .times. .theta. - sin .function. ( 2
.times. m - 1 ) .times. .theta. ] ( 10 ) ##EQU00008##
[0055] A third term of the right side is sufficiently small if a
value of m is large.
[0056] The coefficients .alpha..sub.0, .alpha..sub.1, and
.alpha..sub.2 of the quadratic polynomial can be calculated from
DFT ratio MA gains measured at input amplitudes at three or more
points. It is assumed that G.sub.D(X.sub.1), G.sub.D(X.sub.2), and
G.sub.D(X.sub.3), which are DFT ratio MA gains, are measured at
three points X.sub.1, X.sub.2, and X.sub.3.
[0057] G.sub.D(X.sub.1), G.sub.D(X.sub.2), and G.sub.D(X.sub.3) can
be represented by the following Expression (11).
{ G D .function. ( x 1 ) = a 0 + 2 .times. a 1 .times. x 1 .pi. + 3
.times. a 2 .times. x 1 2 4 + 2 .times. a 1 .times. x 1 2 3 .times.
.pi. G D .function. ( x 2 ) = a 0 + 2 .times. a 1 .times. x 2 .pi.
+ 3 .times. a 2 .times. x 2 2 4 + 2 .times. a 1 .times. x 2 2 3
.times. .pi. G D .function. ( x 3 ) = a 0 + 2 .times. a 1 .times. x
3 .pi. + 3 .times. a 2 .times. x 3 2 4 + 2 .times. a 1 .times. x 3
2 3 .times. .pi. ( 11 ) ##EQU00009##
[0058] Expression (12) can be transformed into the following
Expression (13). Therefore, by solving an equation of Expression
(12), the coefficients .alpha..sub.0, .alpha..sub.1, and
.alpha..sub.2 of the quadratic polynomial can be calculated.
[ G D .function. ( x 1 ) G D .function. ( x 2 ) G D .function. ( x
3 ) ] = [ 1 2 .times. ( 3 + x 1 ) .times. x 1 3 .times. .pi. 3
.times. x 1 2 4 1 2 .times. ( 3 + x 2 ) .times. x 2 3 .times. .pi.
3 .times. x 2 2 4 1 2 .times. ( 3 + x 3 ) .times. x 3 3 .times.
.pi. 3 .times. x 3 2 4 ] .function. [ a 0 a 1 a 2 ] ( 12 )
##EQU00010##
[0059] When a quadratic polynomial f(x) can be calculated in this
way, amplitude ratio MA gain with respect to any input can be
calculated from f(x).
[0060] Processing for storing a coefficient value of an MA gain in
the coefficient-value storing unit 26 is explained. FIG. 8 is a
flowchart illustrating an example of processing for storing a
coefficient value executed by the CPU 21. The coefficient value is
a coefficient of a quadratic polynomial. When a plurality of
magnetic heads 16 are present, coefficient values are stored in
association with the magnetic heads 16.
[0061] As illustrated in FIG. 8, the CPU 21 measures a DFT ratio MA
gain (ST101). In this embodiment, in order to calculate the
coefficient of the quadratic polynomial, DFT ratio MA gains are
measured in positions at three or more points. When a coefficient
of a polynomial of degree n is calculated, DFT ratio MA gains are
measured in positions at n+1 or more points.
[0062] Subsequently, the CPU 21 derives the quadratic polynomial
from Expression (1) and Expression (10) using Expression (12)
(ST102). That is, the CPU 21 calculates the coefficient of the
quadratic polynomial. The CPU 21 stores a coefficient value in the
coefficient-value storing unit 26 (ST103). More specifically, the
CPU 21 stores the coefficient calculated in step ST102 in the
coefficient-value storing unit 26 as the coefficient value, for
example, in association with a head number of the magnetic head
16.
[0063] Processing for calculating an MA gain is explained. FIG. 9
is a flowchart illustrating an example of the processing for
calculating an MA gain executed by the CPU 21. This processing is
executed when the magnetic head 16 is adjusted to a desired
position on the magnetic disk 11 by the micro actuator 17.
[0064] The CPU 21 acquires positioning information (ST201) and
acquires a voltage value for operating the micro actuator 17
(ST202). The positioning information is information indicating a
position where data is read from and written in the magnetic disk
11. The CPU 21 acquires, based on the position information, a
voltage value for moving the arm 13 with the voice coil motor 14
for moving the magnetic head 16 from a present position to the
position indicated by the position information and a voltage value
for operating the micro actuator 17. Note that, in FIG. 9,
description about processing for moving the arm 13 is omitted.
[0065] Subsequently, the CPU 21 acquires a coefficient value
corresponding to the magnetic head 16 from the coefficient-value
storing unit 26 (ST203). In this embodiment, a coefficient of a
quadratic polynomial is acquired. The CPU 21 calculates an MA gain
of the micro actuator 17 from the quadratic polynomial acquired in
this way and the amplitude of a voltage input to the micro actuator
17 (ST204). Consequently, the CPU 21 can calculate, every time the
micro actuator 17 is operated and every time the voltage for
operating the micro actuator 17 is changed based on the positioning
information, the MA gain of the micro actuator 17 considering
distortion depending on the voltage dependency. Accordingly, the
magnetic disk device 1 can accurately control the micro actuator
17.
Second Embodiment
[0066] A case in which there is a hysteresis characteristic in an
amplitude ratio MA gain is explained.
[0067] FIG. 10 is a diagram showing an example of a case in which
there is a hysteresis characteristic in an input output relation of
an MA gain. In FIG. 10, the vertical axis represents an output and
the horizontal axis represents a voltage. A graph g1 shows an
output at the time when the voltage rises and graph g2 shows an
output at the time when the voltage drops.
[0068] When there is such a hysteresis characteristic, x of the
input output relation f(x) indicated by Expression (1) described
above may be doubled. A calculation formula is calculated by
translating a lower left point of f(x) obtained by doubling x in
this way to the origin. A hysteresis may be calculated using the
calculated calculation formula. Note that, f(x) may be calculated
by halving an input voltage without doubling x.
[0069] While certain 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. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
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