U.S. patent application number 08/992598 was filed with the patent office on 2001-11-15 for method and apparatus for positioning a read/write head of a hard disk drive.
Invention is credited to HEATON, MARK W., MASTEN, MICHAEL K..
Application Number | 20010040754 08/992598 |
Document ID | / |
Family ID | 25538512 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040754 |
Kind Code |
A1 |
HEATON, MARK W. ; et
al. |
November 15, 2001 |
METHOD AND APPARATUS FOR POSITIONING A READ/WRITE HEAD OF A HARD
DISK DRIVE
Abstract
a hard disk drive system (10) includes a rotating magnetic disk
(16), and a support arm (22) which is supported for movement
relative to the disk under control of a voice coil motor (21). a
microactuator (26) supports a read/write head (27) on the support
arm for movement relative thereto. a control arrangement (13)
controls the voice coil motor and the microactuator in response to
position information (31), which is read by the read/write head
from the disk and which indicates the position of the read/write
head relative to the disk. The system is free of a sensor for
detecting the actual position of the support arm relative to the
read/write head or the disk.
Inventors: |
HEATON, MARK W.; (DALLAS,
TX) ; MASTEN, MICHAEL K.; (COLLIN, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
25538512 |
Appl. No.: |
08/992598 |
Filed: |
December 17, 1997 |
Current U.S.
Class: |
360/78.04 ;
G9B/5.194 |
Current CPC
Class: |
G11B 5/5552 20130101;
G11B 5/596 20130101 |
Class at
Publication: |
360/78.04 |
International
Class: |
G11B 005/596 |
Claims
What is claimed is:
1. a control apparatus for controlling a system which includes a
first part, a movable member, a first actuator supporting a second
part on the member for movement relative to the member, and a
second actuator operative to effect movement of the member, the
first and second actuators each effecting relative movement of the
first and second parts, wherein said control apparatus comprises: a
first control portion having an input which receives an input
signal that specifies a target position of the second part relative
to the first part, said first control portion being operative to
output a first actuator control signal for causing the first
actuator to move the second part toward the target position with
respect to the first part; and a second control portion having an
input coupled to the first actuator control signal, said second
control portion being operative to output a second actuator control
signal to cause the second actuator to move the member in a manner
so that the second part moves toward the target position with
respect to the first part; wherein said apparatus is free of a
sensor for sensing a position of said member.
2. An apparatus according to claim 1, wherein the first actuator is
a microactuator which effects relative movement of the first and
second parts substantially faster than the second actuator effects
relative movement of the first and second parts.
3. An apparatus according to claim 1, wherein the first actuator is
a microactuator; and wherein a range of relative movement of the
first and second parts effected by the second actuator is
substantially greater than a range of relative movement of the
first and second parts effected by the microactuator.
4. An apparatus according to claim 1, including a position
detection arrangement operative to detect an actual position of the
second part relative to the first part; said first control portion
being responsive to the position detection arrangement and being
operative to generate the first actuator control signal as a
function of the input signal and the actual position detected by
the position detection arrangement.
5. An apparatus according to claim 1, wherein the first actuator is
a microactuator which has an initial state in which the second part
is in a predetermined position with respect to the member; and
wherein the microactuator is responsive to the first actuator
control signal for moving the second part away from the
predetermined position by a distance which is proportional to a
magnitude of the first actuator control signal.
6. An apparatus according to claim 1, wherein the first actuator is
a microactuator which has an initial state in which the second part
is in a predetermined position with respect to the member; wherein
the microactuator is responsive to the first actuator control
signal for moving the second part away from the predetermined
position by a distance which is proportional to a magnitude of the
first actuator control signal; and including a third control
portion which is responsive to the first actuator control signal
and which outputs a control signal obtained by subjecting the first
actuator control signal to integration and a gain, said input of
said second control portion receiving the control signal from said
third control portion.
7. An apparatus according to claim 1, wherein the first actuator is
a microactuator which has an initial state in which the second part
is in a predetermined position with respect to the member; wherein
the microactuator is operative to move the second part away from
the predetermined position in either of first and second directions
which are opposite in response to the first actuator control signal
respectively having positive and negative polarities; and wherein
the microactuator includes a resilient arrangement responsive to
movement of the second part away from the predetermined position in
either of the first and second directions for urging movement of
the second part toward the predetermined position.
8. An apparatus according to claim 7, wherein a force with which
the resilient arrangement urges the second part toward the
predetermined position increases as a distance of the second part
from the predetermined position increases.
9. An apparatus according to claim 1, wherein the first actuator is
a microactuator which includes a resilient arrangement responsive
to movement of the second part away from a predetermined position
in either of two opposite directions relative to the member for
urging movement of the second part toward the predetermined
position, said first and second control portions respectively
outputting first and second signals that respectively represent a
position of the second part and a position of the member; and
including a third control portion responsive to the first and
second signals for generating a signal representing a force which
the resilient arrangement is expected to be exerting between the
second part and the member; said first and second control portions
each being responsive to the signal from said third control
portion.
10. a disk drive apparatus, comprising: a disk supported for
rotation about an axis, and having thereon a magnetic surface; a
member supported for movement relative to said disk; an actuator
which is operative to effect movement of said member; a
microactuator provided on said member; a read/write head supported
by said microactuator for movement relative to said member, wherein
movement of said member by said actuator corresponds to movement of
said read/write head adjacent to and in a direction approximately
radially of said surface of said disk, and wherein movement of said
read/write head by said microactuator corresponds to movement of
said read/write head adjacent to and in a direction approximately
radially of said surface of said disk; a position detecting
arrangement operative to generate a position signal representative
of a position of said read/write head relative to said disk, said
apparatus being free of an arrangement for sensing an actual
position of said member; and a control system responsive to the
position signal, and responsive to an input signal specifying a
target position of said read/write head relative to said disk, said
control system being operative to control said actuator and said
microactuator so as to position said read/write head at the target
position with respect to said disk.
11. An apparatus according to claim 10, wherein said microactuator
effects relative movement of said read/write head and said disk at
a speed which is substantially faster than a speed at which said
actuator effects relative movement of said read/write head and said
disk.
12. An apparatus according to claim 10, wherein said actuator
effects a range of relative movement of said read/write head and
said disk which is substantially greater than a range of relative
movement thereof effected by said microactuator.
13. An apparatus according to claim 10, wherein said control system
includes a first portion responsive to the input signal for
generating a microactuator control signal which is coupled to said
microactuator, and includes a second portion responsive to the
microactuator control signal for generating an actuator control
signal which is coupled to said actuator.
14. An apparatus according to claim 10, wherein said microactuator
has an initial state in which said read/write head is in a
predetermined position with respect to the member; and wherein said
microactuator is responsive to the microactuator control signal for
moving said read/write head away from the predetermined position by
a distance which is proportional to a magnitude of the
microactuator control signal.
15. An apparatus according to claim 10, wherein said control system
includes a first portion responsive to the input signal for
generating a microactuator control signal which is coupled to said
microactuator, and includes a second portion that has an input and
generates an actuator control signal which is coupled to said
actuator, said microactuator having an initial state in which said
read/write head is in a predetermined position with respect to said
member, and being responsive to the microactuator control signal
for moving said read/write head away from the predetermined
position by a distance which is proportional to a magnitude of the
microactuator control signal; and including a third control portion
which is responsive to the microactuator control signal and which
outputs a control signal obtained by subjecting the microactuator
control signal to integration and a gain, said input of said second
control portion being coupled to the control signal from said third
control portion.
16. An apparatus according to claim 10, wherein said microactuator
has an initial state in which said second part is in a
predetermined position with respect to said member, said
microactuator being operative to move said second part away from
the predetermined position in either of first and second directions
which are opposite in response to the microactuator control signal
respectively having positive and negative polarities; and wherein
said microactuator includes a resilient portion responsive to
movement of said second part away from the predetermined position
in either of the first and second directions for urging movement of
said second part toward the predetermined position.
17. a method for controlling a system which includes a first part,
a movable member, a first actuator supporting a second part on the
member for movement relative to the member, and a second actuator
operative to effect movement of the member, the first and second
actuators each effecting relative movement of the first and second
parts, comprising the steps of: receiving an input signal that
specifies a target position of the second part relative to the
first part; generating a first actuator control signal as a
function of the input signal and without sensing an actual position
of the member, the first actuator control signal causing the first
actuator to move the second part toward the target position with
respect to the first part; and generating a second actuator control
signal as a function of the first actuator control signal and
without sensing an actual position of the member, the second
actuator control signal causing the second actuator to move the
member in a manner so that the second part moves toward the target
position with respect to the first part.
18. a method according to claim 17, including the step of detecting
an actual position of the second part relative to the first part;
and wherein said step of generating the first actuator control
signal is carried out by generating the first actuator control
signal as a function of the input signal and the detected position
of the second part relative to the first part, without sensing an
actual position of the member.
19. a method according to claim 17, including the step of causing
the first actuator to respond to the first actuator control signal
by moving the second part away from an initial position relative to
the first part by a displacement which is proportional to the
magnitude of the first actuator control signal.
20. a method according to claim 17, including the step of causing
the first actuator to respond to the first actuator control signal
by moving the second part away from an initial position relative to
the first part by a displacement which is proportional to the
magnitude of the first actuator control signal and in a direction
which corresponds to the polarity of the first actuator control
signal; and wherein said step of generating the second actuator
control signal includes the steps of subjecting the first actuator
control signal to integration and a gain in order to obtain a
modified signal, and then effecting feedback control of the
position of the member by using the modified signal as a position
error signal.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates in general to dual actuator systems
for positioning one part relative to another and, more
particularly, to a method and apparatus for positioning a
read/write head relative to a hard disk using a voice coil motor
and a microactuator.
BACKGROUND OF THE INVENTION
[0002] A hard disk drive typically includes a rotating magnetic
disk and a read/write head supported adjacent one side of the disk
for approximately radial movement relative to the disk. Data on the
disk is organized in the form of a plurality of concentric tracks,
each track being subdivided into a plurality of arcuate sectors
that are circumferentially distributed. Each track also includes
servo information which can be read by the read/write head, which
identifies the particular track, and which also indicates the
extent to which the read/write head is or is not accurately
radially aligned with that track.
[0003] The read/write head is typically supported on a movable
support arm, and an actuator such as a voice coil motor is provided
in order to effect movement of the support arm. When the support
arm is moved, the read/write head thereon is moved in a direction
approximately radially of the disk. A control system is responsive
to the servo information read from the disk by the read/write head
for controlling the voice coil motor so as to position the support
arm in a manner that radially aligns the read/write head with a
selected track on the disk.
[0004] The capacity of hard disk drives is progressively
increasing, due in part to a progressive increase in the number of
concentric tracks provided on a given hard disk. Of course, the
radial widths of the tracks decrease as the number of tracks is
increased. As a result, there has been an increase in the precision
and resolution needed for controlling the radial position of the
read/write head in order to keep it aligned with a particular
track. A further consideration is that, as central processing units
become progressively faster, there is an associated increase in the
need for hard disk drives with faster seek and access times.
[0005] One proposed approach for achieving greater precision and
resolution while reducing seek and access times involves the use of
a microactuator to movably support the read/write head on the
support arm. Microactuators are miniature actuators or motors,
which may be fabricated on silicon using semiconductor fabrication
techniques, and which are sometimes referred to as
microelectromechanical systems (MEMS). A microactuator is capable
of effecting rapid and accurate movement of the read/write head
relative to the support arm, in a direction approximately radially
of the disk, but within a relatively small range of movement. The
voice coil motor is thus used to move the support arm to effect
coarse positioning of the read/write head, and the microactuator is
used to effect fine positioning of the read/write head.
[0006] The servo information read from the disk by the read/write
head identifies only the position of the read/write head relative
to the disk. In a typical hard disk drive system without a
microactuator, the read/write head is fixedly supported on the
support arm, and thus the position of the support arm is directly
related to the position of the read/write head. On the other hand,
when a microactuator is provided between the support arm and the
read/write head, the microactuator facilitates movement of the
read/write head relative to the support arm. Thus, knowledge of the
actual position of the read/write head based on the servo
information read from the disk provides no information at all
regarding the actual position of the support arm.
[0007] Accordingly, it has been considered necessary to supplement
the position information from the read/write head with a sensor
that determines the actual position of the support arm, either by
directly sensing the position of the support arm, or by sensing the
amount of relative movement effected by the microactuator between
the support arm and read/write head. However, the need to provide
such a sensor decreases the reliability of the system, while
increasing its costs. In this regard, as storage capacity increases
and the number of tracks increases, the actual position of the
support arm must be determined with progressively increasing
resolution and precision, which in turn involves increased cost and
complexity for the sensor and associated circuitry that are
provided to detect the actual position of the support arm.
Consequently, while existing hard disk drives which use
microactuators have been generally adequate for their intended
purposes, they have not been satisfactory in all respects, due in
part to the need to provide a sensor and supplementary
circuitry.
SUMMARY OF THE INVENTION
[0008] From the foregoing, it may be appreciated that a need has
arisen for a method and apparatus for controlling a dual actuator
system with just a single source of position information. According
to the present invention, a method and apparatus are provided to
address this need, and involve: providing a first actuator to move
a second part relative to a member; providing a second actuator to
effect movement of the member relative to a first part, the first
and second actuators each effecting relative movement of the first
and second parts; receiving an input signal that specifies a target
position of the second part relative to the first part; generating
a first actuator control signal as a function of the input signal
and without sensing an actual position of the member, the first
actuator control signal causing the first actuator to move the
second part toward the target position with respect to the first
part; and generating a second actuator control signal as a function
of the first actuator control signal and without sensing an actual
position of the member, the second actuator control signal causing
the second actuator to move the member in a manner so that the
second part moves toward the target position with respect to the
first part.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A more complete understanding of the present invention will
be realized from the detailed description which follows, taken in
conjunction with the accompanying drawings, in which:
[0010] FIG. 1 is a block diagram of a hard disk drive system which
embodies the present invention;
[0011] FIG. 2 is a diagrammatic perspective view of a microactuator
which is a component of the hard disk drive system of FIG. 1;
[0012] FIGS. 3 and 4 are graphs showing operational characteristics
of the hard disk drive system of FIG. 1;
[0013] FIG. 5 is a block diagram of the hard disk drive system of
FIG. 1, showing in more detail a control system which is part of
the hard disk drive system;
[0014] FIGS. 6A and 6B, which are collectively referred to
hereinafter as FIG. 6, are respective portions of a block diagram
showing in detail the control system of FIG. 5; and
[0015] FIG. 7 is a block diagram of an alternative embodiment of a
microactuator spring effect block that is a component of the
control system of FIG. 6.
DETAILED DESCRIPTION OF THE INVENTION
[0016] FIG. 1 is a diagrammatic view of a hard disk drive system 10
which embodies the present invention. FIG. 1 depicts only the
portions of the system 10 which are pertinent to an understanding
of the present invention. The system 10 includes a disk/head
assembly 12, and a control circuit 13.
[0017] The disk/head assembly 12 includes a plurality of spaced and
parallel disks 16, which are each fixedly supported on a spindle
17. The spindle 17 and the disks 16 together define a stack 18. The
spindle 17 and the disks 16 thereon are rotatably driven by a
not-illustrated spindle motor. The disks 16 each have on both sides
thereof a magnetic coating, which stores information. The stored
information on each side of each disk is organized in the form of a
plurality of concentric tracks, which are not illustrated. Each
track is broken into a plurality of arcuate and circumferentially
distributed sectors. Each sector of each track includes servo
information. The servo information provides position information,
so that a read/write head may be properly positioned relative to
the particular track on the particular disk 16.
[0018] The disk/head assembly 12 further includes an actuator which
is a voice coil motor (VCM) 21, and includes a plurality of support
arms 22. The support arms 22 are pivotally supported on a
stationary axle 23 that is parallel to the spindle 17. The voice
coil motor 21 urges simultaneous pivotal movement of the arms 22
about the axle 23. Each of the arms 22 has at the end thereof
remote from the axle 23 a microactuator, one of which is shown at
26. The microactuator 26 is described in more detail later. Each
microactuator supports a respective read/write head, one such
read/write head being shown at 27.
[0019] Each read/write head is disposed adjacent a respective side
of a respective disk 16 of the stack 18. When the voice coil motor
21 pivots the axle 23 and all of the support arms 22, the
read/write heads 27 each move approximately radially with respect
to the adjacent disk 16 in the stack 18. In addition, each
microactuator 26 can effect a small amount of movement of the
read/write head 27 thereon relative to the associated support arm
22, in a direction which causes the read/write head 27 to move
approximately radially with respect to the adjacent disk 16 in the
stack 18. Each read/write head 27 can read data from or write data
to the associated disk 16, and can read the servo information from
the disk 16. In general, just one read/write head 27 is active to
read or write information at any given point in time.
[0020] Servo information read from a disk 16 by a read/write head
27 is supplied at 31 as an analog servo information signal to a
servo channel circuit 32, which is a part of the control circuit
13. The servo channel circuit 32 processes the analog servo
information signal so as to generate an analog position signal,
which is supplied at 33 to an analog-to-digital (A/D) converter
circuit 34. The A/D converter circuit 34 converts the analog
position signal 33 to a digital position signal, and supplies it at
35 to a digital signal processor (DSP) 36. The DSP 36 is
operatively coupled to a memory 38, which stores program
instructions and data for the DSP 36. The DSP 36 receives at 41 a
digital signal identifying a desired or target track, or in other
words a track on one of the disks 16 with which the associated
read/write head 27 is to be radially aligned. The desired or target
track signal 41 may originate from a location external to the hard
disk drive system 10, for example from a computer to which the hard
disk drive system 10 is operationally coupled.
[0021] The DSP 36 outputs at 46 a digital voice coil motor control
signal, which is received by a digital-to-analog (D/A) converter
circuit 47. The D/A circuit 47 converts the digital signal 46 to an
analog signal, which is supplied at 48 to a voice coil motor power
amplifier 51, which amplifies the analog voice coil motor control
signal. The amplified signal from the output of the amplifier 51 is
supplied at 52 to the voice coil motor 21. The voice coil motor 21
is responsive to the signal 52 to urge pivotal movement of the arms
22 about axle 23.
[0022] The DSP 36 outputs at 56 a digital microactuator control
signal, which is received by a further digital-to-analog (D/A)
converter circuit 57. The D/A converter circuit 57 converts the
digital microactuator control signal 56 to an analog signal, which
is supplied at 58 to a microactuator power amplifier 61. The analog
microactuator control signal is amplified by the amplifier 61, and
then supplied to each of the microactuators 26, as shown
diagrammatically at 62. Although the D/A converter circuit 57 and
the amplifier 61 control all of the microactuators in the disclosed
embodiment, it will be recognized that it would be possible to
provide a separate D/A converter and amplifier for each
microactuator, so that the DSP 41 could control the microactuators
individually.
[0023] The microactuator 26 will be briefly described in order to
facilitate a better understanding of the present invention. The
microactuator 26 is a small actuator or motor fabricated in silicon
for the purpose of moving a load through a small range of travel.
FIG. 2 is a diagrammatic perspective view of the microactuator 26.
The microactuator 26 includes a base portion 71, and a member or
platform 72 which is capable of a limited amount of movement
relative to the base portion 71, in directions parallel to the
arrows 73. The microactuator 26 has spring portions 76 and 77,
which are disposed on opposite sides of the platform 72 and which
urge movement of the platform 72 toward a central or equilibrium
position. In the equilibrium position, the spring portions 76 and
77 do not exert any forces on the platform 72. If the platform 72
moves away from the equilibrium position in one direction parallel
to arrows 73, two spring portions 76 are resiliently compressed and
the two spring portions 77 are resiliently expanded, whereas if the
platform 72 is moved away from the equilibrium position in the
opposite direction, the two spring portions 77 are resiliently
compressed and the two spring portions 76 are resiliently
expanded.
[0024] The microactuator 26 further includes two permanent magnets
78 and 79, which are fixedly mounted on the base portion 71 on
opposite sides of the platform 72. The magnets 78 and 79 are
oriented to have inverse polarities. Although permanent magnets 78
and 79 are used in the disclosed embodiment, it will be recognized
that small coils could alternatively be used to generate
electromagnetic fields. A coil 80 is fixedly mounted on the
platform 72, so that opposite sides of the coil are disposed
beneath the magnets 78 and 79. When a current is passed through the
coil 80, a small electromagnetic field is generated and urges the
platform 72 to move away from its equilibrium position in a
direction determined by the polarity of the current. Since the
magnets 78 and 79 are oriented with inverse polarities, and since
the portions of the coil 80 adjacent the magnets have respective
current flows which are opposite, the platform will be urged in the
same direction in the region of both magnets 78 and 79. The force
exerted on the platform 72 in response to the coil current is a
positioning force, and moves the platform 72 against the urging of
the spring portions 76 and 77.
[0025] In microactuator 26, the distance which the platform 72
moves away from the equilibrium position is directionally
proportional to the magnitude of the current supplied to the coil
80. Because of the small size of the microactuator 26, and the
small range of movement of the platform 72 relative to base portion
71, the speed with which the platform 72 can move relative to the
base portion 71 is substantially faster than the speed with which
the voice coil motor 21 (FIG. 1) can pivot the arms 22.
[0026] The base portion 71 of the microactuator 26 is fixedly
secured on a support arm 22, with an orientation so that the
direction indicated by arrows 73 is oriented approximately radially
of the disks 16 in the platter stack 18. The associated read/write
head 27 is fixedly supported on the platform 72. Thus, the
read/write head 27 is moved approximately radially of the adjacent
disk 16 in response to pivotal movement of the arms 22, or in
response to movement of the associated actuator platform 72 in the
direction of arrows 73. The spring portions 76 and 77 not only
resist movement of the platform 72 and the read/write head 27
thereon away from the center or equilibrium position, but also
provide support and alignment for the read/write head 27. In the
disclosed embodiment, the range of movement of the platform 72 in
either direction away from its equilibrium position relative to the
base portion 71 corresponds to movement of the associated
read/write head 27 by approximately four or five tracks in either
direction away from a track with which the read/write head is
currently aligned. Within this range of movement, the microactuator
26 can effect movement of the platform 72 relative to base portion
71 much faster than the voice coil motor 21 can effect an equal
amount of movement of the read/write head 27 by pivoting the arms
22. According, primary control for positioning the read/write head
27 is directed to the microactuator 26, and secondary control is
directed to the voice coil motor 21.
[0027] In general, this means that a necessary positioning movement
of the read/write head is first effected by using the microactuator
26 to move the read/write head 27 toward the new position, while
directing the voice coil motor 21 to move the arms 22 until the
platform 72 of the microactuator 26 has returned to its equilibrium
position with the read/write head 27 aligned with a new track. For
example, if the read/write head 27 is being maintained in radial
alignment with a particular concentric track on the associated disk
16, the arms 22 will ideally be positioned so that there is no
current flowing through the coil 80 of the microactuator, and thus
the platform 72 will be in its equilibrium position. If the
read/write head 27 shifts slightly radially relative to the track,
a small amount of current will be supplied to the coil 80 in order
to rapidly move the platform 72 of the microactuator 26 until the
read/write head 27 is again in radial alignment with that track.
Then, the arms 22 would be pivoted slightly while decreasing the
current flowing through the coil 80 to zero, so that the read/write
head 27 remains in radial alignment with the track as the platform
72 moves to its equilibrium position. As another example,
essentially the same approach would be used where the read/write
head 27 is to be moved to a different track which is less than four
or five tracks away from the current track, or in other words
within the range of movement of the platform 72 of the
microactuator 26.
[0028] Still another example is a situation where the read/write
head 27 is to be moved into radial alignment with a different track
which is more than four or five tracks away from the current track,
or in other words beyond the range of movement of the platform 72
relative to base portion 71. For example, the target track might be
ten tracks away from the current track. In this situation, the
primary control would attempt to use the microactuator 26 to
rapidly position the read/write head 27 at the target track, but
the platform 72 would reach the end of its range of travel after
the read/write head moved four or five tracks and before the
read/write head reached the target track. Further movement of the
read/write head 27 toward the target track would then be effected
through pivotal movement of the arms 22 by the voice coil motor 21.
When the read/write head 27 reached the target track, the current
through the coil 80 would be progressively decreased as the arms 22
slowed to a stop, until the platform 72 reached its equilibrium
position with the arms 22 positioned so that the read/write head 27
was in radial alignment with the target track.
[0029] In this particular type of situation, a system without a
mircroactuator may limit the speed of pivotal movement of the arms
22 in order to avoid or minimize overshoot of the arms past their
target position, because excessive overshoot and the resulting need
for a corrective return movement could result in a longer seek time
than simply moving the arms at a lower velocity. On the other hand,
through the provision of the microactuator 26, the arms 22 can be
pivoted at a higher rate of speed than in a system without a
microactuator, and can be allowed to overshoot their target
position so long as the overshoot is less than four or five tracks.
In particular, if the read/write head 27 is within four or five
tracks of the target track, the microactuator 26 can keep the
read/write head 27 in alignment with the target track while the
arms 22 are carrying out the overshoot and the necessary corrective
return.
[0030] More specifically, as the read/write head 27 reached the
target track in this situation, the current through the coil 80 of
the microactuator 26 would be decreased to zero as the arms 22
moved to their target position, and then would be progressively
increased with a reversed polarity as the arms 22 overshot their
target position, so as to keep the read/write head in alignment
with the target track. Thereafter, the reversed polarity current
would be progressively decreased to zero during the corrective
return of the arms 22 to their target position. If the arms 22
carried out a small amount of damped oscillation around their
target position, the polarity of the current through the coil 80 of
the microactuator might be changed several times in order to keep
the read/write head 27 in accurate alignment with the target track
during the oscillation.
[0031] FIG. 3 is a graph showing a situation where the read/write
head 27 is moved to a new track which is only two tracks away from
the current track, where the vertical axis represents tracks and
the horizontal axis represents time. More specifically, the
displacement of the microactuator platform 72 is shown at 86. It
will be noted that there is an initial spike at 87 representing an
initial displacement of the microactuator platform that effects
rapid movement of the read/write head by a distance of
approximately 1.75 tracks, which is most of the two-track
displacement required for the read/write head to reach its new
position. The movement of the outer end of the support arm 22 is
indicated at 88. By the time the microactuator 26 has moved the
read/write head through a distance of 1.75 tracks, the support arm
22 is just starting to move. The position of the read/write head 27
is represented by the curve 89. Since the movement of the
read/write head 27 is the sum of the movements caused by the
microactuator and the actuator arms 22, the curve 89 is the sum of
the curves 86 and 88.
[0032] Following the initial spike 87, the displacement of the
microactuator platform 72 is gradually decreased until the
microactuator platform is back in its equilibrium position, while
the support arm 22 moves toward a new position in which it is
displaced by a distance of two tracks from its original position.
It will be noted that it takes the support arm between 0.004 and
0.005 seconds to reach its new position. Thus, if the microactuator
was not present, it could take this long before the read/write head
was aligned with the new track and could read or write information.
In contrast, because of the provision of the microactuator, the
read/write head reaches a position of steady alignment with the new
track in less than 0.001 seconds, or in other words at least five
times faster than in a system without a microactuator.
[0033] FIG. 4 is a graph depicting a situation where the read/write
head is being moved to a target track which is ten tracks away from
its current track. The displacement of the platform 72 of the
microactuator is shown at 96, and includes an initial spike 97 that
effects rapid movement of the read/write head 27 through a
displacement of five tracks toward the new track. Since the range
of movement of the microactuator platform is limited to about five
tracks, further movement of the read/write head 27 toward the new
track is effected by pivotal movement of the support arm 22. The
movement of the read/write head 27 is the sum of the displacements
of the microactuator platform and the support arm, and thus the
curve 99 representing this movement is a sum of the curves 96 and
98.
[0034] After the initial spike at 97, the current through the coil
80 is progressively reduced, so that the microactuator platform 72
is moved back toward its equilibrium position as the support arm 22
moves through the last five tracks of a ten-track displacement. As
the support arm 22 reaches its target position, the microactuator
platform 72 reaches its equilibrium position at 103. However, the
support arm 22 overshoots its target position at 101, and the
microactuator is controlled after 103 so as to displace the
platform 72 in a direction opposite its original displacement and
by an amount sufficient to compensate for the overshoot of the
support arm 22.
[0035] As evident from FIG. 4, the corrective movement of the
support arm 22 does not bring the support arm 22 back to its target
position until more than 0.01 seconds have elapsed from the start
of movement. Nevertheless, because of the provision of the
microactuator, the read/write head reaches its target position in
approximately 0.0006 seconds after the start of movement, and is
thereafter maintained in accurate alignment with the target track
through appropriate control of the microactuator so as to
compensate for the overshoot of the support arm 22.
[0036] With reference to FIG. 2, it will be recognized that, if the
support arm 22 and the microactuator base portion 71 thereon are
moved, the inertia of the platform 72 and read/write head on the
platform will tend to urge the platform 72 to move relative to the
base portion 71. The spring portions 76 and 77 will, of course,
damp any such relative movement due to inertia. Similarly, if a
current is passed through the coil 80 in order to urge movement of
the platform 72, inertia of the read/write head 27 and the platform
72 will initially cause the base portion 71 and the support arm 22
to be urged in a direction opposite the direction of movement of
the platform 72. Again, the spring portions 76 and 77 will damp out
this inertial effect.
[0037] In order to effect proper control of the position of the
read/write head 27, it is important to know the position of the
support arm 22, which differs from the position of the read/write
head 27 by an amount equal to the displacement of the actuator
platform 72 relative to the base portion 71. It would be possible
to determine the actual position of the support arm 22 through the
provision of a sensor, which directly sensed the position of the
support arm 22, or which sensed the displacement of the actuator
platform 72 relative to the base portion 71. However, the disclosed
embodiment avoids the need to provide such a sensor, through the
use of microactuator 26 which, as mentioned above, has a
displacement that is proportional to the magnitude of the current
supplied to the coil 80. That is, the direction and magnitude of
the displacement of the platform 72 corresponds to the polarity and
magnitude of the microactuator current.
[0038] FIG. 5 includes a block diagram of a control system 106
which is implemented in the DSP 36 of FIG. 1 in order to effect
appropriate control of the microactuator 26 and the voice coil
motor 21. Components in FIG. 5 which also appear in FIG. 1 are
designated in FIG. 5 with the same reference numerals as in FIG.
1.
[0039] The movement of the support arms 22 by the voice coil motor
21 is shown diagrammatically at 109 in FIG. 5. The forces which the
spring portions 76 and 77 can exert on the read/write head 27 are
shown diagrammatically at 111 and 112, and the positioning forces
exerted on the read/write head 27 in response to a microactuator
current through coil 80 are shown diagrammatically at 114.
[0040] The control system 106 of FIG. 5 includes a microactuator
control loop 121, a voice coil motor control loop 122, a
microactuator control technique 123, and a microactuator spring
effect adjustment block 124. The microactuator control loop 121 is
responsive to the desired or target position signal 41, and the
digital position signal 35 from the A/D converter circuit 34. The
microactuator control loop 121 is also responsive to the output of
the microactuator spring effect adjustment block 124. The
microactuator control loop 121 generates the digital microactuator
control signal 56, which is supplied to the D/A converter circuit
57.
[0041] As previously mentioned, the disclosed embodiment positions
the read/write head 27 using the microactuator 26 for primary
control and the voice coil motor 21 for secondary control. Stated
differently, the voice coil motor 21 is controlled primarily as a
slave or follower to the microactuator 26. Thus, in the disclosed
embodiment, the desired or target position signal 41 is supplied to
the microactuator control loop 121, but not to the voice coil motor
control loop 122. Instead, an output signal 128 from the
microactuator control loop 121 is suppled to the microactuator
control technique 123, which in turn outputs a signal 129 to the
voice coil motor control loop 122. The microactuator control loop
121 thus effects the primary response to the desired or target
position signal 41 through appropriate control of the microactuator
26, whereas the voice coil motor control loop 122 carries out a
slave or follower function.
[0042] The microactuator spring effect adjustment block 124 is
responsive to signals 136 and 137 from the microactuator control
loop 121 and the voice coil motor control loop 122, respectively.
The signals 136 and 137 are indicative of the control implemented
by the control loops 121 and 122. The microactuator spring effect
adjustment block 124 outputs a signal 138, which is supplied to
each of the control loops 121 and 122, and which is representative
of at least one characteristic of the spring portions 76 and 77 of
the microactuator 26.
[0043] The control system 106 of FIG. 5 is shown in more detail in
FIG. 6. With reference to FIG. 6, the microactuator control loop
121 includes a proportional gain element 151 which receives and
scales the desired or target position signal 41 by a constant
K1.sub.ma. The output of the gain element 151 is coupled to a
positive input of a junction 152, the output of which is coupled to
an input of an amplifier 153 with a gain of K.sub.dma. The output
of the amplifier 153 is coupled to a limit block 156 which applies
a limit to the output signal from amplifier 153. The blocks 153 and
156 correspond functionally to the microactuator power amplifier
61, which operates with a five volt supply and thus cannot produce
an output signal in excess of five volts.
[0044] The output of the limit block 156 is coupled to a positive
input of a junction 157, the output of which is coupled to the
positive input of a junction 158. The output of the junction 158 is
coupled to the input of a gain element 161. The gain of the element
161 is 1/Lma, where Lma represents an inductance which corresponds
functionally to the inductance of the coil 80 of the microactuator
26. The output of the element 161 is coupled to a positive input of
a junction 162, the output of which is coupled to an integrator
163. The integration function is designated symbolically by the
LaPlace operator 1/s, which is normally associated with analog
control loops, but it will be recognized that the integration
function it represents may be implemented in the DSP 36 using an
appropriate digital technique.
[0045] The output of the integrator 163 is coupled to the input of
a gain element 166, gain element 166 having a gain K.sub.tma that
represents a motor force constant. The output of the gain element
166 is coupled to a positive input of a junction 167, the output of
which is coupled to an input of a gain element 168 having a gain of
1/J.sub.ma. The term J.sub.ma represents the combined mass of the
microactuator platform 72 and the read/write head 27. The input to
the gain element 168 is a force, and the output of the gain element
168 represents an acceleration. The output of element 168 is
coupled to a positive input of a junction 171, the output of which
is coupled to a further integrator 172. Since the input of
integrator 172 is an acceleration, the output of integrator 172
represents velocity or speed. The output of integrator 172 is
coupled to a positive input of a junction 173, the output of which
is coupled to another integrator 176. Since the input to integrator
176 is a velocity or speed, the output of integrator 176 represents
position, and in particular the estimated position of the platform
72 and thus the estimated position of the read/write head 27.
[0046] The output of integrator 176 is coupled to a negative input
of a junction 177. The actual position signal 35 from the A/D
converter 34 is supplied to a positive input of the junction 177.
The output of the junction 177 thus represents a position error
between the actual position of the read/write head, which is
determined from the servo information read by the read/write head
from the spinning disk, and the estimated position that the control
loop 121 calculates the read/write head is theoretically expected
to have in response to the control signals being output from the
control system 106. The output of the junction 177 is coupled to
the inputs of three gain elements 181-183, which have outputs
respectively coupled to positive inputs of the junctions 162, 171
and 173, respectively. The gain elements 181-183 have respective
gains of Lm3, Lm2 and Lm1, which are estimation gains that cause
the elements 181-183 to function as state adjustors. That is, the
elements 181-183 generate state adjustment values based on the
position error from junction 177, and inject these values into the
control loop through junctions 162, 171 and 173.
[0047] The outputs of the elements 176, 172 and 168, which
respectively represent position, velocity and acceleration, are
coupled to inputs of respective gain elements 186-188, which have
respective proportional gains of K1ma, K2ma and K3ma. The outputs
of the gain elements 186-188 are coupled to respective negative
inputs of the junction 152, and the elements 186-188 thus define
respective feedback paths. The output of the element 172 is
similarly coupled to the input of a further gain element 191, which
has a gain Kbma representing the back emf of the coil 80 of the
microactuator. The output of the element 191 is coupled to a
negative input of the junction 157, thus defining a further
feedback path. The output of the element 163 is coupled to the
input of a gain element 164 having a gain Rma, which is
representative of a resistance of the coil 80 of the microactuator.
The output of the element 164 is coupled to a negative input of the
junction 158, and the element 164 is thus part of another feedback
path.
[0048] The output of the junction 152 serves as the microactuator
control signal 56, which is supplied through D/A converter circuit
57 to the microactuator power amplifier 61. The output of the limit
element 156 serves as the signal 128 to the microactuator control
technique 123. The signal 128 is representative of the direction
and magnitude of the displacement of the platform 72 of the
microactuator 26. Since a goal in controlling the voice coil motor
21 is to cause it to position the arm 22 so that the platform 72 is
at its equilibrium position, or in other words has a displacement
of zero, the signal 128 may be viewed as an error signal for
purposes of controlling the voice coil motor 21. Accordingly, the
input signal 129 to the voice coil motor control loop 122 is
derived from the signal 128 through the control technique 123, for
purposes of causing the control loop 122 to appropriately control
the positioning arms 22.
[0049] More specifically, the microactuator control technique 123
includes an integrator 196 which receives and integrates the signal
128, the output of the integrator 196 being coupled to the input of
a gain element 197. The gain element 197 has a gain Kct which is a
constant. The output of the gain element 197 serves as the signal
129 supplied to the input of the voice coil motor control loop
122.
[0050] The voice coil motor control loop 122 includes a gain
element 201 which receives the signal 129, scales it by a
proportional gain K1, and supplies the result to a positive input
of a junction 202. The output of the junction 202 is coupled to an
amplifier 203 having a gain of K.sub.drvr, and the output of the
amplifier 203 is coupled to a limit element 206. The elements 203
and 206 together correspond functionally to the voice coil motor
power amplifier 51, which works with a 12 volt supply and cannot
produce an output signal in excess of 12 volts. Thus, the limit
element 206 limits the magnitude of the output signal from the
amplifier 203 to an appropriate range.
[0051] The output of the limit element 206 is coupled to a positive
input of a junction 207, the output of the junction 207 being
coupled to the positive input of a further junction 208. The output
of junction 208 is coupled to the input of a gain element 211 which
has a gain 1/Lm, where Lm is an inductance of a coil of the voice
coil motor 21. The output of the gain element 211 is coupled to a
positive input of a junction 212, the output of which is coupled to
the input of an integrator 213. The output of the integrator 213 is
coupled to the input of a gain element 216 having a gain Kt, where
Kt is a torque constant for the coil of the voice coil motor 21.
The output of the gain element 216 is coupled to the positive input
of a junction 217, the output of which is coupled to the input of a
gain element 218. The gain element 218 has a gain 1/Jm, where Jm
represents the mass of the parts moved by the voice coil motor
21.
[0052] The input to the element 218 represents a force, and the
output represents acceleration. The output of the element 218 is
coupled to the positive input of a junction 221, the output of
which is coupled to the input of an integrator 222. Since the input
to the integrator 222 represents acceleration, the output of the
integrator 222 represents velocity, and is coupled to the positive
input of a junction 223. The output of the junction 223 is coupled
to the input of a further integrator 226, the output of which
represents position. The output of the integrator 226 is coupled to
the input of a gain element 227. The gain element 227 has a gain R,
which represents the radial distance from the axle 23 (FIG. 1) to
the read/write head 27. The output of the gain element 227
represents position, and in particular the position of the support
arms 22 rather than the position of the read/write head.
[0053] The output of the gain element 227 is coupled to a negative
input of a junction 228, the positive input to which is the actual
position signal from line 35. The output of the junction 228 is
thus an error signal representing the difference between the actual
position of the read/write head indicated by the position signal
35, and the position which the control loop 122 calculates that the
read/write head is theoretically expected to have in response to
the control signals output from the control system 106.
[0054] The error signal from junction 228 is supplied to the inputs
of three gain elements 231-233. The gain elements 231-233 have
respective gains of Lv3, Lv2 and Lv1, which are estimation gains
that cause the gain elements to function as estimators. The outputs
of the gain elements 231-233 are each coupled to a positive input
of a respective one of the junctions 212, 221 and 223, in order to
inject into the control loop respective estimator values developed
from the error signal output by the junction 228.
[0055] As explained above, the outputs of the elements 226, 222,
and 218 respectively represent position, velocity and acceleration.
The outputs of the elements 226, 222, and 218 are coupled to the
inputs of respective proportional gain elements 236-238, which have
respective gains K1, K2 and K3. The outputs of the elements 236-238
are each coupled to a respective negative input of the junction
202, and the gain elements 236-238 are thus parts of respective
feedback paths.
[0056] The output of the element 222 is also coupled to the input
of a further gain element 241. The gain element 241 has a gain Kb
representing the back emf of the coil in the voice coil motor 21.
The output of the gain element 241 is coupled to a negative input
of the junction 207. The gain element 241 is thus part of a further
feedback path. The output of the element 213 is coupled to the
input of another gain element 242, which has a gain Rm representing
a resistance of a coil in the voice coil motor 21. The output of
the element 242 is coupled to a negative input of junction 208, and
the element 242 thus is part of yet another feedback path.
[0057] The microactuator spring effect adjustment block 124
includes a junction 246 with positive and negative inputs. The
output of element 176 serves as the signal 136 which is coupled to
the positive input of the junction 246, and the output of the
element 227 serves as the signal 137 which is coupled to the
negative input of the junction 246. The output of the junction 246
is coupled to an input of a gain element having a gain Kma, which
is representative of a spring constant for the two spring portions
76 and 77 of the microactuator. The output of the element 247 is
the signal 138, which is representative of the net force resulting
from the opposed forces of the microactuator spring portions 76 and
77. The signal 138 is coupled to a negative input of the junction
167 and, through a gain element 249, to a positive input of the
junction 217. The gain element 249 has a gain R, which is the same
as the gain R of the element 227. The output of the junction 202
serves as the voice coil motor control signal supplied at 46 to the
D/A converter 47.
[0058] In general terms, the microactuator control loop 121 takes
the desired or target position signal 41 received through the gain
element 151 and generates, with some feedback injected at the
junction 152, an appropriate control signal 156 for the
microactuator. The remaining elements of the microactuator control
loop 121 combine the control signal 56 with real world
characteristics of the electromechanical structure controlled by
the signal 56, in order to derive at the output of the element 176
an expected or theoretical position of the read/write head 27. This
theoretical or expected position is compared at 177 to the actual
position of the read/write head indicated by signal 35, in order to
develop an error signal that is fed back to the control loop
through the estimator gain elements 181-183.
[0059] The gain elements 186-188 respectively scale signals
corresponding to the position, velocity and acceleration of the
read/write head, and effect the feedback control through junction
152. The elements 164 and 191 provide feedback paths corresponding
to respective characteristics of the microactuator coil, namely
back emf and resistance. The effect of the microactuator springs,
represented by the signal 138 from the microactuator spring effect
adjustment block 124, is taken into account in the control loop 121
through the junction 167.
[0060] The voice coil motor control loop 122 operates in a
generally similar manner, except that the input signal 129 is based
on the magnitude and direction of the displacement of the actuator
platform from its equilibrium position. Thus, the control loop 122
controls the voice coil motor 21 as a slave or follower to the
microactuator 26. Since the operation of the control loop 122 is
generally similar to that of the control loop 121, a detailed
explanation of the operation of the control loop 122 is believed
unnecessary.
[0061] The microactuator spring effect block 124 of FIG. 6 models
the primary characteristic of the microactuator spring portions 76
and 77, namely the net resilient force which they exert between the
platform 72 and the base portion 71. However, there are secondary
characteristics of the microactuator and the spring portions which
may optionally be taken into account, including a damping
characteristic and a stroke limit.
[0062] More specifically, FIG. 7 shows a microactuator spring
effect adjustment block 256 which is an alternative embodiment of
and which may be substituted for the block 124 in FIG. 6. With
reference to FIG. 7, the block 256 includes a junction 246 and a
gain element 247, which are equivalent to those depicted in FIG. 6
and are therefore identified with the same reference numerals. The
output of the gain element 247 is coupled to a positive input of a
junction 257, the output of which is a signal 138' that is
functionally similar to the signal 138 of FIG. 6.
[0063] The output of the junction 246, which represents a position,
is also coupled to the input of a differentiating element 259.
Differentiation of a position yields a rate or velocity. The output
of the differentiating element 259 is coupled to the input of a
proportional gain element 261 which has a gain Dma, where Dma
represents a damping characteristic of the microactuator spring
portions 76 and 77. The output of the gain element 261 is coupled
to a positive input of the junction 257. The block 256 also
includes a stroke limit block 262 which receives the output signal
from the junction 246. The stroke limit block 262 limits the
magnitude of the signal from block 246 to a predefined range, in
order to reflect the fact that the platform 72 of the microactuator
26 has a range of movement relative to the base portion 71 which is
physically limited. The output of the stroke limit block 262 is
coupled to a proportional gain element 263 having a gain Kma1,
where Kma1 is a scaling factor for the output of the stroke limit
block 262. The output of the gain element 263 is coupled to a
positive input of the junction 257.
[0064] The present invention provides various technical advantages.
One such technical advantage is that a microactuator can be
utilized in a hard disk drive, while avoiding the need to provide a
position sensor to determine the actual position of the support
member on which the microactuator movably supports the read/write
head. This reduces the cost of the system, while achieving more
efficient control through use of the dual actuator arrangement. In
particular, seek times and thus access times are reduced. a further
technical advantage is increased reliability, due to elimination of
the need for a sensor and its associated support circuitry.
[0065] Although one embodiment has been illustrated and described
in detail, it should be understood that various changes,
substitutions and alterations can be made therein without departing
from the scope of the present invention. For example, the disclosed
embodiment utilizes a digital signal processor to control the
position of the read/write head, but it will be recognized that the
position of the read/write head could also be controlled by an
analog control circuit. Moreover, a suitable control loop for the
microactuator and a suitable control loop for the voice coil motor
have been disclosed, but it will be recognized that there are many
variations and modifications of these specific control loops which
lie within the scope of the present invention. In this regard, it
will be recognized that direct connections disclosed herein could
be altered, such that two disclosed components or elements are
coupled to one another through an intermediate device or devices
without being directly connected, while still realizing the present
invention. Other changes, substitutions and alterations are also
possible without departing from the spirit and scope of the present
invention as defined by the following claims.
* * * * *