Note: Descriptions are shown in the official language in which they were submitted.
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APPARAI'US AND METHOD FOR DAMPING HEAD
POSITIONERS FOR HIGH PERFORMANCE DISC DRIVES
Technical Field
This invention relates generally to the control of
magnetic head movemen~ in high per~ormance magnetic disk
drives. More particularly, the inven~ion is directed to the
stabilization and control of mechanical resonances which
occur in head positioners for such drives.
Background Art
A disk drive is a rotating magnetic storage device
capable of storing data which may by accessed at high
speeds. This device includes a rotatirg, flat magnetic
surface onto which data can ba recorded and later retrieved,
usually in milliseconds because of the high rotational rate
of the disk and the fast radial accessing capability of the
head positioner. The head positioner of the disk drive
assembly functions to locate and maintain the magnetic
read/write heads at a commanded track position above the
disk, and this head positioner also provides precision sup-
port and stability which is necessary for the heads to
function properly. Most disk driv designs are based upon
the moving head principle as opposed to the faster, but more
costly, "head per track" concept. Usually more than one
disk or disk surface is used per drive, and in this case one
or two seperate head~ are used for accessing each disc
surface. The heads for each surface are mounted on aligned
arms of the positioner such that they all move in unison.
The resulting head positioner structure looks like a comb
and is often re~erred to the " head comb". The head comb is
typically aligned and moved by a support and motor structure
which is operative with either linear or rotary motion.
When a head positioner in a disk drive performs a very
fast seek, many resonant or ringing modes of vibration can
be excited and unles~ properly damped, will tend to ring for
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long periods of time, typically on the order of 100 mili-
seconds or more. Often the amplitude of these modes of
vibration at the magnetic head position is a large percen-
tage of the width o~ a data track on the magnetic disk. If
this is the case, data rea~ing or writing will be disabled
until the ringing amplitude drops to a value suf~iciently
low to permit accurate reading or writing.
Disk drives having a high track density typically
employ positioning servo schemes in order to ensure
accurate and ast track locating by the magnetic heads.
~owever, the accuracy and speed of such servo systems are
highly dependant upon the mechanical system resonances. A
servo system is a negative feedback control system in which
part of the system is mechanical. An ideal control loop for
such a servo system feeds the output signal of the system
back to the input o~ the system and out o~ phase with the
input thereto to force the system to exhibit a unity gain
transfer function. When the servo system mechanics are
introduced, mechanical resonances produce output signals
greater than the input signals as well as cause shifted
output signal phase relationships relative to the input
signal. Some compensation is customarily introduced into
the servo control loop to provide improved stability with
the mechanical system. But in spite of this compensation,
mechanical resonances of the above type produced by head
positioner vibrational resonances often are large enough and
provide a phase shift o~ suf~icient magnitude to create a
positive feedback for the system at greater than unity gain.
When this happens, the servo loop becomes unstable at that
resonance frequency.
In the low performance disc drive industry, one prac-
tice has been to employ rotational dampers on stepper motors
which are used to drive head positioners for the disc
drives. These dampers are used to increase the stability of
the stepper motor per se. However, as presently known,
these damping techniques o~ the prior art are not directed
to the damping of vibrational resonances o~ an entire head
positioner used in high performance track following se~o
disc drive actuators, as in the case with the present inven-
tion.
Disclosure of Invention
The general purpose of this invention is to provide a
new and improved solution to th~ above problems resulting
from vibrational resonances in head positioners for disk
drives~ This solution involves reducing the amplitude of
resonance modes of vibration in the head positioner .by
providing a chosen movable ~loating mass at a selected
location on the head positioner, and this location exhibits
large motions of the vibrating resonant modes in the head
positioner. This mass is positioned in a liquid film of
known viscosity, and the mass and film are located within a
housing securely fixed to the head positioner at the selec-
ted location thereon. As the head positioner traverses its
seeking path and abruptly comes toward a rest position,
vibrational mo~e energy is couple~ from the head positioner
to the mass to thus caus¢ the mass to move across the liquid
film, thereby creating shear forces at the mass-film inter-
face. These shear ~orces and the distance over which they
act represent damper or energy loss that is removed from the
vibration mode energy of the head positioner.
In a preferred embodiment of invention, the mass is
centered via a soft-spring between opposite end walls of the
housing, and vibrational energy received by the housing is
viscously coupled to the mass at frequencies close or equal
to the vibrational resonant frequencies of the head posi-
tioner. The spring only serve to keep the mass away from
the end walls of the ma s-housin~, so that the damper mass
remains functional regardless o~ its orientation relative to
gravity.
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Accordingly, an object of an aspect of the present
invention is to provide a novel damper apparatus which is
easy to construct and which is particularly and uniquely
adapted and well-suited for mounting on a head positioner
for a disc drive.
An object of an aspect of the invention is to
provide a new and improved damping method and apparatus
which allows faster disk drive track locating times and
wider bandwidth and higher performance of servo position-
ing systems associated therewith.
An object of an aspect of the invention is to
provide a damping method and apparatus of the type des-
scribed which dissipates energy from modes of head positioner
vibration to thereby reduce settling time after a seek
movement. Thus subsequent earlier reading and writing
operations are achieved.
An object of an aspect of the invention is to
provide a new and improved method and apparatus of the type
described which improves associated servo control loop
stability.
An object of an aspect of the invention is to
provide a new and improved method and apparatus of the type
described operative to improve servo and seek performance
without the need to increase resonant frequencies which,
in some cases, may not be possible.
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Various aspects of the invention are as follows:
In a head positioner apparatus for a magnetic disc
drive wherein one or more track accessing arms are rotatably
mounted on a chosen axis and having a point of maximum
vibrational resonance amplitude a first predetermined
distance from said chosen axis, the improvement comprising:
a floating mass damper mounted on said track accessing arms
a second predetermined distance from said central axis and
at a location for receiving significant vibrational
resonance amplitude from said track accessing arms; said
floating mass slidably mounted on a viscous damping film.
A method for damping resonant modes in a
positioning member for magnetic heads or the like,
comprising:
(a) providing a liquid film of known viscosity at
a selected location on said member and between said
member and a chosen movable mass, and
(b) coupling vibrational mode energy from said
member to said mass to thereby produce shear forces at
the mass-film interface, whereby said ~orces and the
distance over which they act provide damped energy loss
which is removed from said vibrational mode energy.
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5b
A method for damping resonant modes of vibration in
a head positionar for a rotating magnetic storage device
including:
(a) providing a chosen mass at a selected location
on a head positioner, which location exhibits large
motions of the vibrating resonant modes of said
positioner,
(b) providing a liquid film of chosen viscosity in
contact with said mass and in a ~ixed locakion with
respect to the loca~ion of maximum resonant modes of
said head positioner,
(c) coupling vibrational mode motion from said
head positioner to said mass, and thereby
(d) moving said positioner relatiYe to said liquid
film, whereby shear forces at the mass-film interface
and the distance over which they act represent damped
energy loss which is removed from the vibrational mode
energy of said head positioner.
In a head positioner apparatus for a magnetic
disc drive wherein one or more track accessing arms are
rotatably mounted on a chosen axis and have a poink of
maximum vibrational resonance amplitude at a certain
distance ~rom said chosen axis, the i~provement5 comprising:
a damper housing having interior walls
r ceivin~ a viscous film and including a mass located on
said viscous film, with said viscous film located
between said interior walIs and said mas~ for enabling
movement of said mass on said viscous film regardless o~
the orientation of said head positioner apparatus with
respect to gravity, said damper hou~ing mounted on said
track accessing arm~ a predetermined distance from said
chosen axis and at a location ~or receiving a
significant vibrational resonance amplitude from said
track accessing arms.
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5c
A method for damping resonant modes in a head
positioning mechanism for transducer heads comprising
the steps of:
viscously coupling vibrational mode energy
from said head positioning member to a movable mass,
moving said mass relative to said head
positioning member and effecting an energy dissipation
in said viscous coupling,
causing a concurrent reduction in vibrational
mode energy in said head positioning member, and
producing a reduction in magnitude of
vibrational modes of said head positioning member.
These and other objects and features of this
invention will become more readily apparent in the
following description of the accompanying drawing.
Brief D~escri~tion of Drawin~
Figure 1 is a isometric view of a head positioner
and damping apparatus according to the invention.
Figure 2 i~ a cros~-section view of the damping
apparatus of Figure 1.
Figure 3 is a graph plotting re~onant mode
amplitude versus resonant frequency for the damped (D)
and undamped (U) head positioner to illustrate the
reduction in resonant
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mode amplitude produced by the present invention.
Best Mode For Carryinq Out The Invention
Referring now to Figure 1, there is shown a rotary head
positioner including a motor and arm suppor~ structure 12
with front and back openings 14 and 16 respectively. The
support structure 12 serves as a housing for a magnetic head
accessing arm structure 18 which includes a plurality of
accessing head arms 22, 23, 24, 25, 26 and 27 extending as
shown out of the front opening 14 and having a plurality of
magnetic hsads 28, 29, 30, 31, 32 and 33 located on the ends
of the accessing arms. The accessing head arm structure 18
is pivotally mounted within the support structure 12 on a
vertical axis 34 about which it is driven with rotary motion
at high speeds during a fast seek operation. Magnetic discs
(not shown) are positioned in the openings between the
magnetic heads 28, 29, 30, 31, 32 and 33, and these heads
are driven to preselected positions on certain tracks of
these discs during a fast seek operation.
The accessing head arm structure 18 is driven by a
motor coil 36 which is mounted inside the housing 12 and on
the head arm structure 18. This motor coil 36 is positioned
as shown at the rear center of the accessing head arm struc-
ture 18. The motor coil 36 is also centered between upper
and lower motor magnets 38 and 40, and includes a pair of
end flanges 42 and 44 between which the motor coil is wound.
A floating mass damper 46 is mounted as shown between these
flanges 42 and 44.
Referring now to Figure 2, the floating mass damper 46
is shown in more detail in a cross section view thereof and
includes a cylindrical housing 48, typically of magnesium,
having an end wall 50 and another removable wall or cap 52
which permits the loading therein of a floating damper mass
54. The floating damper mass 54 is typically non-magnetic
stainless steel and has a central passage 56 along the
longitudinal center line thereof and is centered in the
housing ~8 with a pair of soft centering springs 58 and
60. These springs extend between the opposite end walls 50
and 52 of the housing 48 and recessed or cavity regions 62
and 64 in the respective ends o~ the ~loating damper mass
54.
A viscous damping film 66 is provided as shown between
the outer surface of the floating damper mass 54 and the
interior surface of the housing 48, and this viscous damping
film 66 will typically consist o~ silicone oil.
During a fast seek operation, the ends of the accessing
head arms 22, 24 and 26 will typically undergo a resonant
mode vibrational amplitude (greatly exaggerated) as
indicated by the distance A in Figure 1. When undergoing
such a resonant mode vibration, the rotary head positioner
cannot become electrically useable until the value o~ A
is reduced to near zero as the head arm vibration ceases.
In accordance with the present invention, the floating mass
damper 46 provides an elegantly simple solution to reducing
the time re~uired ~or the value of A to be reduced. In
accordance with this in~ention, the floating mass damper 46
is mounted at the raar of the accessing head arm structure
18 and in a vertical plane with the dimension B. This
dimension B corresponds to the maximum resonant mode vibra-
tion which the rear of the accessing head arm structure 18
will undergo during a fast seek operation and corresponding
to the displac~ment A o~ the accessing head arms 22, 24 and
26. During the resonant mode vibration which produces the
displacement B as indicated, the hou~ing 48 is rapidly
driven laterally back and forth acros~ the viscous damping
film 66. : Thi~ movement o~ the housing 48 in cooperation
with less movement of ~he ~loating damper ma~s 54 produces
lateral shear ~orces which serve to substantially dissipate
vibrational mode enerqy in the rotating accessing head arm
structure 18. This energy dissipation thus quickly reduces
the value of dimension B and maintains the value of B at a
level much lower than it would otherwise have been in the
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absence of the floating mass damper 46.
Referring now to Figure 3, there is shown a graph of
the vibrational magni~ude of the accessing head arm struc-
ture 18 as a function of vibration frequency. Line U of the
curve in Figure 3 represents the vibrational magnitude M
without damping, whereas line D indicates the variation of M
with frequency after damping. As shown in Figure 3, the
maximum point on line D curve will be slightly shifted
(lowered) in frequency by a certain amount as a result of
the damping action. Thus, this reduction in the magni-
fication factor M as a result of employing the floating mass
damper 46 substantially reduces the magnitude of the
resonance values at A and B in Figure 1 to more stably
locate the magnetic heads 28, 30 and 32 on preselected
tracks of the magnetic discs previously described.
Thus, there has been described a new and improved
method and appartus which is easy to construct and which is
reliable and durable in operation and adds a relatively
small increase in cost to the rotary head positioner 10.
In contrast to the vastly more complex viscous damping
systems of the prior art, the floating mass damper 46 of the
present invention employs only a ~inyle floating damper mass
54 which is spring centered using only a pair of springs 58
and 60 and generates substantial shear forces as the
housing 48 moves over the viscous damping film 66. It has
been determined that the reduction in settling time of the
damped head positioner has been 5 to 10 times that of the
undamped head positioner.
Various modifications may be made in the above des
cribed embodiment of the invention without departing from
the scope of the app~nded claims. For example, for reasons
of ease of assembly, it is often preferable to use a 3-piece
construction for the accesRing head arm structure 18. Thus,
one piece may consist of the arms 22, 23, 2~, 25, 26 and 27
integrally joined at one end; a second or center inter-
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locking piece may be rotatably mounted at the axis 34 andmechanically interlocked with the accessing arms 22, 23, 24,
25, 26 and 27; and an end or motor coil support piece 41 may
be mounted at the rear of center interlocking piece to
facilitate bringing the motor coil 36 within the housing 12
and between the upper and lower motor magnets 38 and 40,
respectively.
