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BACKGROUND, FEATURES OF INVENTION
This invention relates to electromagnetic actuator assemblies and in
particular to novel improved linear voice-coil assemblies adapted for
reciprocating magnetic transducer means relative to magnetic recording
surfaces.
Workers know that computers today commonly employ magnetic disk files for
recording and storing data. Disk files have the advantage of facilitating
data transfer at randomly selected address locations (tracks) and without
need for a "serial seek" as with magnetic tape. Such transducers must be
reciprocated very rapidly between selected address locations (tracks) with
high precision. This will be recognized as depending on how fast the
system can move a transducer between locations and do so with high
positional accuracy between closely-spaced track addresses. This
constraint becomes very tricky as track density increases.
Disk file systems commonly mount a transducer head on an arm carried by a
block that is supported by a carriage. This carriage is usually mounted on
tracks for reciprocation by an associated transducer actuator. This
invention is concerned with improving the efficiency of such actuators and
particularly with improving linear "voice-coil" positioners.
Known Positioners
The actuators commonly used with magnetic disk files are subject to some
exacting requirements; for instance, these systems typically involve a
stack of several magnetic disks, each with many hundreds of concentric
recording tracks spanning a radius of about 7 inches; and a head-carrying
arm is typically provided to access each pair of opposing disk surfaces.
This arm will typically carry two to ten heads, each to be reciprocated
over a radial excursion of several inches to position its heads adjacent a
selected track. Thus, it will be appreciated that such applications
require a high positioning accuracy together with very fast translation to
minimize access time--a significant portion of which is used for head
positioning.
Disk heads must commonly be reciprocated very rapidly between selected
address locations (tracks) with high precision. Thus it is critically
important for an actuator system to move a transducer very rapidly between
data locations and to do so with high positional accuracy between
closely-spaced track addresses. This constraint becomes ever more
burdensome as track density increases--as is presently the case.
That is, such a positioner must move its transducer heads very rapidly so
that the associated computer can process data as fast as
possible--computer time being so expensive that any significant delay over
an extended period can inflate costs enormously. That is, the "transition
time", during which heads are moved from track to track, is "dead time"
insofar as data processing is concerned.
Now, the present trend is toward ever higher track density with increased
storage capacity and decreased access time. Of course, as track density
rises, closer control over the actuator mechanism is necessary to position
transducer heads accurately over any selected track, lest signals be
recorded, or read, with too much distortion, and without proper amplitude
control, etc.
Thus, computer manufacturers typically set specifications that call for
such inter-track movements within no more than a few milliseconds. Such
high speed translation is most demanding on actuators, it postulates a
powerful motor of relatively low mass (including carriage weight) and low
translational friction. Another requirement for such head positioners is
that they exhibit a relatively long stroke, (several inches) in order to
minimize the number of heads required per disk.
The prior art disclosed many such positioner devices, including some
intended for use in magnetic disk memory systems: e.g. see U.S. Pat. Nos.
3,135,880; 3,314,057; 3,619,673; 3,922,720; 4,001,889; 3,544,980;
3,646,536; 3,665,433; 3,666,977; 3,827,081; and 3,922,718 among others.
Voice Coil Motors:
Workers in the art are familiar with prior art magnetic actuators,
especially those adapted for reciprocating magnetic transducers relative
to magnetic disk surfaces or the like. Such an actuator is the well known
voice coil motor VCM or moving coil actuator arrangement shown in FIGS. 1A
and 1B. This structure will be recognized as comprising as E-shaped
magnetic structure M including a central core M.sub.c along which a moving
coil C is adapted to be movably mounted. Thus, working flux (see phantom
magnetic flux line F) circulating through magnet M traverses the indicated
gap g.sub.p, between the pole pieces P and the core M.sub.c, and is
intercepted by coil C. When the coil is energized with a prescribed
electric current and cuts a certain flux (prescribed flux density B and
current i in coil C of length L yields certain force F (F=BLI), it will be
induced to move as indicated by the arrow. The direction of motion will
depend on the polarity of the current relative to the flux, as known in
the art.
The voice coil motor (VCM) comprises a solenoid like those used to drive an
audio speaker. In disk drives, magnetic read/write heads are commonly
carried by a carriage driven by a VC motor including a mobile electric
coil positioned in a magnetic field and fed by a current of selected
intensity and polarity. This magnetic field is typically established by
permanent magnetic means disposed about the movable coil.
Such a VC linear positioner can exhibit certain disadvantages--for example:
undesirably large mass and associated excess power requirements; and drive
and control circuitry which is unduly-complicated. That is, such actuators
typically involve a relatively heavy carriage; accordingly a lot of
inertia must be overcome each time the carriage is accelerated from rest.
This acceleration must be maximized to minimize access time. Thus, a great
burden is placed upon the power requirements to the voice coil to provide
the necessary high acceleration. Such VC actuators are not particularly
efficient in converting electrical power either; also they typically
require relatively complicated drive and control circuitry to effect the
requisite precise positioning despite high acceleration. Further, a VC
motor is not sufficiently "linear", i.e., its coil impedance commonly
varies with position and thus its force/excursion curve is relatively
non-linear. This invention is intended to improve the efficiency and
performance of such VC positioners, making them more linear in a dual coil
array.
FIG. 1 represents a conventional moving coil magnetic actuator (VCM) very
schematically shown (see also Fujitsu Scientific and Technical Journal
June 1972, page 60 and following). Here a moving coil (armature) C will be
understood as mounted upon a movable bobbin adapted to reciprocate along
the core portion M.sub.c of an E-shaped magnetic circuit M, the circuit
also including opposing poles P connected by yoke section Y. Such
reciprocation will be responsive to electric current through coil C as is
well known in the art.
Here the permanent magnetic source of magnetic flux will be recognized as a
cylindrical, or semi-cylindrical, shell P with its inner core M.sub.c to
be encircled by the moving solenoid coil C. Coil C will be recognized as
conventionally translated along core M.sub.c when energized with current
(due to inductive interaction with the magnetic flux--see arrows emanating
between core M.sub.c and peripheral magnetic parts). Force arrows T.sub.F,
T.sub.R indicate the resultant reciprocal translation forces so developed
(forward, reverse)--the force direction being determined by direction of
current through coil C, as well known in the art.
The magnetic flux field set-up by coil current will flow mainly through the
"path of least reluctance" (as indicated by flux loops F through the
magnetic).
Workers are aware that, since the flux return path traverses the
cross-section of core M.sub.c, then in certain instances actuator
efficiency and the upper limit of operation will be affected by "flux
saturation" at this relatively narrow section--whereby an increment in
coil current fails to produce a proportionate significant increase in
actuator force. One might even say that such incremental current and flux
is wasted. Flux may also be deemed wasted insofar as the flux return path
traverses yoke portion Y (an "open loop" flux) rather than moving through
the "working gap" between coil C and (the inner facing surfaces of ) poles
P (in a "closed-loop").
In accordance with one salient aspect of the present invention, such a
transducer positioner is formed to include a second moving coil. According
to this feature, this second moving coil (and associated magnet means) is
intended to provide a low leakage "balanced" flux path, one which is more
efficient than the prior art ("single coil/single magnet" configurations,
which tend to describe a conventional "unbalanced", high leakage flux
path). That is, this second coil is provided, along with a companion pole
piece, and is so-wound and so-excited as to complete the overall flux gap
in a "closed loop" mode. Such "dual coil" actuators advantageously use the
second coil, etc., as a "working return" for flux, also this facilitates
reducing the magnet mass needed and enhance efficiency.
Workers in the art will be given to understand that such a "closed loop",
"balanced" flux path is considerably more efficient than prior art
"unbalanced" flux modes, especially as regards leakage; a "balanced" flux
also tends to improve "linearity" (i.e., make coil impedance more constant
as a function of coil position, giving a more linear force/distance curve
for the overall device). Such an improved actuator assembly will be seen
as better balanced magnetically and, because of its inherently improved
linearity, will no longer critically depend upon magnet thickness (except
for achieving a higher flux).
As seen hereafter, it will be readily apparent to workers how such a
dual-coil armature provides a moving coil structure of improved linearity.
Such an improved armature will be seen to give superior performance, e.g.,
as a disk head positioner with "balanced" flux as compared with the
conventional VC positioner.
In accordance with another salient feature, such dual coil positioners are
taught in operative combination with a disk drive arrangement.
Thus, one object of this invention is to provide the mentioned and other
features and advantages. Another object is to teach the use of such dual
coil VC actuators in transducer assemblies, especially as adapted for
positioning heads in a disk drive. Another object is to provide head
actuators for disk drives exhibiting better linearity and yet a further
object is to teach the advantageous use of such transducer actuators in
disk drive assemblies.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing, and other related objects, features and advantages of the
present invention will be better appreciated by workers as they become
familiar with the following detailed description of presently preferred
embodiments, these being considered in conjunction with the accompanying
drawings, wherein like reference indicia denote like elements:
FIG. 1A depicts a prior art voice coil actuator in array schematic,
partly-sectional side view, with selected portions thereof shown in the
fragmentary perspective of FIG. 1B;
FIG. 2 depicts, in fragmentary perspective, a multi-head carriage for use
with such an actuator;
FIG. 4 shows, in section, a like carriage-actuator embodiment modified
according to the invention; with magnet coil portions thereof shown in the
schematic fragmentary perspective of FIG. 3 and FIG. 5; while FIG. 6 shows
these portions partly sectioned, and
FIGS. 4A, 4B show the coils in "FORWARD" and "REVERSE" position; and
FIG. 7 depicts an alternate embodiment after the manner of FIG. 4A.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 2 discloses, in a very simplified functional perspective view,
portions of a transducer linear actuator assembly 20 especially adapted
for a disk drive. Here and elsewhere, it will be understood that all
elements are conventionally constructed and operated except as otherwise
specified. It will be understood that a magnetic recording disk 15 is
provided as part of a magnetic disk pack (not shown), together with a
carriage assembly 18 adapted to support and to controllably reciprocate
read/write heads on support arms, translating them across recording
portions of the disks for the reading and writing of information
impressions thereon as well known in the art. Here, only head 14 on arm 31
(for disk 15) is shown.
Workers will recognize that actuation of the carriage assembly 18 may be
conventionally provided by a linear motor comprised essentially of a
stationary magnetic core SLM and a movable coil assembly, or bobbin B.
Bobbin B is mounted from carriage 18 and arranged so that, with proper
energization of its coil (through electrical leads, not shown), carriage
18 can be induced to move toward and away from the magnet core,
reciprocating along the ways 22, 24. This motion will be understood as
controllably scanning the head assemblies across the disk recording
surface, etc., as well known in the art.
The fixed parallel ways, or rails, 22, 24 support the carriage for such
reciprocation and carry a support structure 30 in which are provided a
plurality of slots 33 and associated mounting means. Each slot
accommodating a respective head support--one for each inter-disk gap (only
one arm 31 indicated here, with associated head assembly 14, for
simplicity's sake).
Construction and operation of such linear motors is well known and is
summarized relative to FIGS. 1A, 1B above.
Such translation of carriage 18 will, of course, shift the read/write head
assembly 14 along the recording surfaces of rotating disk 15 to precisely
position it at any of the various concentric data tracks thereon. The
accuracy of head positioning will depend on the accuracy with which
carriage 18 is translated. The present invention is directed towards
improving linear motors driving such carriage assemblies.
FIGS. 3-6 will be understood as indicating various views of certain
portions of a voice coil motor embodiment improved according to the
invention and including a supplemental coil and associated magnetic means
according to the invention. More particularly, FIG. 4 shows, in simplified
side sectional view, carriage 18 and the bobbin B plus the head mount
structure 30 carried thereby (the latter indicated in phantom and
fragmentarily). Bobbin B is relatively conventional and serves to mount a
pair of solenoid coils CL-a, CL-b (outside the distal end thereof), these
coils being of relative conventional construction and operation as known
in the art. FIG. 3 also shows poles P-1, P-2 and coils CL-a, CL-b on
bobbin B.
Shown in operative inductive relation with coils cL, is the magnetic
support structure SLM, including a central core portion CR disposed to be
surrounded by the coils cL and to define the path along which they are
reciprocated by carriage 18. SLM also includes a pair of outboard arms
mounting the permanent annular magnets P-1, P-2, and a narrow backplate
BP. Plate BP may preferably be made unusually narrow (light) since
relatively little magnet flux need be passed thereby according to such a
"dual coil" design. Each magnet P is arranged and positioned to be in
potential flux-cutting relation with a respective coil cL. FIGS. 4A and 4B
illustrate the full-retract (or Reverse) and full-extended (or Forward)
positions respectively of coils cL, with a schematic indication of the
"balanced" ("closed") flux flow therewith. Coil CL-b thus is swept across
pole P-1, and its current polarity selected accordingly; likewise for coil
CL-a to be swept across pole P-2.
A fragmentary perspective of the coil-supporting end of bobbin B, with
segments of coils cL wound thereon, is shown in FIG. 5.
According to a related feature, coils cL will be understood as arranged to
be oppositely energized and magnets P to be likewise oppositely-poled,
thus balancing flux flow symmetrically and allowing "return flux" to be
put to work.
The support sections of magnet structure SLM may comprise any suitable
magnetic (low reluctance) magnet material, while the permanent magnet
sections P-1, P-2, preferably comprise annular ceramic ferromagnets or the
like of suitable strength. The dimensions of magnets P, their materials
and the coil' length, number of turns, current, etc., will be a matter of
choice, as well known in the art. However, according to a feature of
advantage, the thickness of magnets P and of backplate BP may now be
ignored, essentially, except where increased flux is to be provided--thus
advantageously reducing the necessary thickness and weight of magnets and
reducing the weight, cost, etc., of the overall structure and rendering it
somewhat more compact. Directing return flux from one magnet/coil unit
through the second magnet/coil unit will obviously divert it from
backplate BP.
FIG. 6A shows a fragmentary somewhat simplified perspective view of magnet
structure SLM, with a section of magnets P and associated coils cL
indicated only diagrammatically and for purposes of illustrating
operational characteristics. Here coils cL are depicted in "REVERSE", or
full-retracted position (full-line coils) as in FIG. 4B; while the
opposite "FORWARD", or full-advanced position is indicated in dottedline
fashion (as in FIG. 4A).
Results
Workers in the art will recognize that such improved dual coil actuators
with a balanced (closed loop) flux configuration are considerably more
efficient than prior art unbalanced (open loop) structures, with little or
none of the leakage inherent therein. Dual coil actuators also exhibit
superior, more constant coil impedance as a function of coil position and
a surprisingly more linear Force/Displacement characteristic. Thus, such
an actuator assembly will be viewed by workers as better balanced and
because of its inherently improved linearity and more efficient flux-use,
the thickness and mass of its magnets may be considerably reduced without
sacrificing performance.
Variations
Workers will recognize that such a dual coil actuator design may be
modified in various ways within the spirit of the overall concept. Thus,
the working length of the actuator may, in certain cases, be extended to
include two or more pairs of opposite-polarity coils (e.g., c.sub.a,
c.sub.b, c.sub.c and c.sub.d in FIG. 7), each coil operatively associated
with a respective magnet of matching polarity (e.g., poles P.sub.a,
P.sub.b, P.sub.c, P.sub.d) suspended from low-reluctance return CAP in
FIG. 7, each coil shown in "FORWARD" position as in FIG. 4A and assumed
mounted on a reciprocating bobbin with current input means, etc., none
shown here). The resultant magnetic flux is indicated in dotted line
fashion. It will be recognized that for optimized flux use the magnet
poles are placed in abutment, and fashioned so that their length (L.sub.p)
approximates the sum of the inter-coil separation (D.sub.c) and coil width
(w.sub.c): L.sub.p =D.sub.c +w.sub.c.
Conclusion
Workers will appreciate how aptly such dual coil (paired coil) actuators
are combined to drive transducer assemblies for disk drive apparatus and
the like. In particular it will be appreciated that such actuators can be
used to improve the efficiency, power and the cost effectiveness of a
transducer actuator and to increase its speed (acceleration)
accordingly--something workers in the are will applaud. Workers will also
appreciate that such actuators may be used to reciprocate other similar
loads in related environments.
It will be understood that the preferred embodiments described herein are
only exemplary, and that the invention is capable of many modifications
and variations in construction, arrangement and use without departing from
the spirit of the invention.
Further modifications of the invention are also possible. For example, the
means and methods disclosed herein are also applicable to positioning
other transducers and related loads in similar systems and environments.
For instance, related embodiments may be employed to position transducers
for other forms of recording/reproduced systems, such as those in which
data is recorded and reproduced optically.
The above examples of possible variations of the present invention are
merely illustrative. Accordingly, the present invention is to be
considered as including all possible modifications and variations coming
within the scope of the invention as defined by the appended claims.
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