Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title: A BRU.S~ S DIRECT CURRENT
- MOTOR WITH INVERTED MAGNET
C~IP
Inventors: hichard A. Wilkinson, Jr.
William C. Hunt
BACKCROUND OF THE INYENrION
This invention relates to a brushless direct current motor, and more
particularly to a brushless direct current motor designed for use
with rotating dis~ systems. Even more particularly, the iDvention
relates to a brushless direct current motor wherein all of the
rotating components can all be balanced as a unit in one balancing
operation.
The use of brushless direct current motors in electronic
equipment with rotating oomponents, such as a magnet1c disk storage
system, has become common. There are two reason~ for this: (1) the
speed of rotat$on of such devices must be precisely controlled, and
the brushless direct current motor, with its electronic control
system, allows accurate speed control to be easily accomplished; and
(2) the brushless direct current motor can be made an integral part
of ths ~ech~nical support structure, thereby saving both space and
cost.
In some uses of the brushle~s direct current motor~ the rotating
components of the motor must be balanced to an acceptable level. If
the correct leYel of balanoe is not aohieved, the electronic
equipment using the motor may not function as desired, or the life
of the motor may be shortened because of the stresses placed on the
bearings from the wobble intnoduced by the imb~lance.
~2~ 5
Balancing of brushless direct current motors is accomplished in
the prior art by individually balancing aach rotating component of
the motor. The components are then assembled and the assembled unit
1s checked for the correct balance. Quite often, additional
balancing is required.
If the motor should ever have to be ~ls~ss~mbled, e.g., because
of maintenance of the motor or the overall system in which the motor
is used, additional balancing may be required when the motor is
reassembled. This is bec~l~e the individual components of the motor
will probably not re~ssP~hle exactly as they were before
disassembly. Two approachas have been used in the prior art to
avoid this additional b~l~n~ing step: (1) the individual motor
components may be fabricated to very tight tolerances, thereby
ensuring that proper balance wlll be obtained regardless Or how the
components are reassembled; or (2) keying pins and matching
alignment holes may be selectively placed in the co~ponents, thereby
maint~;~ln~ a fixed relationship between the components each time
reassembly occurs. While the use of tight tolerances and keying
pins and Al~L -nt holes helps ensure that a motor can be
re~q~ ~led without needing rebalancing, these approache3 also
disadvantageously add significantly to the cost of the motor. What
i9 needed therefore is a low co~t, simple brushle^~s direct current
motor design wherein maint~inln~ the desired balance of the rotating
compon~nts ls no longer a problem.
SUMMARY OF THE INVENTION
It is an objective of the present invention to provide a brushless
direct current motor whose rotating components can be balanced prior
to assembly into the motor and which do not need additional
balancing after motor assemblyO
It is another objective or the present in~ention to provide such
a motor that can be disassembled, if necessary, and reassembled
without the need of balancing after re~ssp~hly~
The invention meets these and other objectives through a
brushless direct current motor design wherein all the rotating
components can be assembled as a subassembly. This subassembly is
then b~l~nced and assembled into the stationary components of the
motor. Consequently, the rotating components, being in balance
prior to assembly into the stationary components, and having no
additicnal parts added, require no further balancing after assembly
with the stationary components.
Further, because the rotating components of the motor are
designed as a balanced subassembly, there is never a need to
dlsassemble the subassembly. Thuq, whenever dis~q~ bly of the
motor i3 required, the balanced subassembly can be readily removed
from, and re-installed into, the stationary components of the motor,
yet the balanoed subassembly never requires rebalancing.
The rotating components included in the balanced subassembly of
the present invention comprise a shaft, a spindle hub (on which the
rotating di3k or disks may be mounted), and a cup assembly (wherein
permanent magnetics are mounted). Unlike the prior art, wherein
traditional design practice teaches that the cup assembly must be
mounted at one end of the shaft using a cup adapter, and the spindle
hub must be mounted at the other end of the shaft, the present
invention provides a unique subassembly wherein the cup assembly and
spindle hub may be advantageously mounted to the same end of the
shaft Once assembled and balanced, therefore, thi~ subassembly
need never be disassembled.
Thus, in accordance with one broad aspect of the inven-
tion, there is provided an improved, low cost, compact brushless
direct current motor for use in a disk drive device comprising:
a housing having motor windings selectively spaced around
an outer surface thereof, and means for mounting said housing to
a disk drive device;
a shaft rotatably mounted in an opening extending through
said housing, said rotatable mounting being realized with at
least two non-resilient friction reducing bearings respectively
placed at each end of said shaft and at corresponding ends of
said opening in said housing;
a spindle hub affixed to a first end of said shaft, an
upper side of said spindle hub being adapted for engagement with
the center of the disk to be driven by said motor; and
a magnet cup attached to said spindle hub, said magnet cup
including at least one permanent magnet, said magnet cup bei.ng
positioned within the periphery of said spindle hub such that
said magnet maintains a selected juxtaposed, non-touching,
relationship with an outer surface of said motor windings.
In accordance with another broad aspect of the inven-
tion there is provided a method of manufacturing a compact, low
cost brushless direct current motor for use in a disk drive de-
vice, said motor including a housing, stationary windings
affixed to an outer surface of said housing so as to lie sub-
stantially in a plane orthogonal to a central longitudinal axis
of said housing, a shaft, a spindle hub, and a magnet cup having
an inside diameter greater than the diameter of said housing and
windings, said method comprising the steps of:
(a) selectively spacing a plurality or permanent magnets
around the inner periphery of said magnet cup;
~! - ~a"
P~ 5
~ b) attaching said spindle hub to a first end of said
shaft;
(c) attaching said magnetic cup to the shaft side of said
spindle hub, thereby forming a first assembly comprised of said
shaft, hub, and magnet cup;
(d) dynamically balancing said first assembly about a
central longitudinal axis of said shaft; and
(e~ rotatably and non-elastically mounting said first
assembly in said housing such that the longitudinal axes of said
housing and shaft coincide and maintain such relationship during
operation of said motorr whereby the magnets of said magnet cup
may rotate substantially in the same plane as said motor windings
without coming in physical contact with said windings.
` 3b
BRIEF DESCRIPTION OF THE DRA~INGS
The above and other object$ves, advantages and fea~ures of the
present invention will become more apparent from the ~ollowing
description of the preferred embodiment, which is described with
reference to the following drawings, wherein:
FIGURE l is a cross-sectional drawing of a brushless direct
current motor of the prior art, showing the rotating components that
may require balancing and how they are assembled with the stationary
components of the motor; an~
FIGURE 2 is a cross-sectional drawing of a brushless direct
current motor desi~ned according to of the present invention,
showing the same detail as FIGURE l.
DESCRIPTION OF THE PR~FERRED EMBODIMENT
The following is a description oP the be~t presently
contemplated mode of carrying out the present invention. This
description is given only ~or the purpose of describing the general
prlnciples of the present invention and should not be taken in a
limiting sense. The true scope of the invention can be ascertained
by referring to the appended claims.
In order to bettér appreciate the features and advantages of the
present invention1 it will first be instructive to describe in more
detail a conventional brushless direct currert ~otor 2S used in the
prior art ~or disk drive applications. Such a motor is shown in the
cros~-sectional view of FIGURE l. The rotating components of the
~otor consist of the shaft lO, spindle hub ll, cup adapter 12, and
rotating magnet cup 13. Mounted on the inner edge o~ the rotating
~2~
ma~net cup 13 are a plurallty of permanent magnets, shown in the
fieure as solid rectangles 14 and 15.
The stationary components of the motor consist of the spindle
housing l6 with a plurality of motor magnets, ~hown in the figure as
rectangles 17 and 18, arranged around the periphery. These magnets
re usually electro-magnets that are excited by applying a direct
current of a desired polarity to coil win~ing~ associated wlth the
magnets.
The Ahaft 10 of the rotating components is supported in the
spindle housing 16 of the stationary component3 by the ball-bearings
20-27. The ball-bearings can be considered to be part of either the
rotating components, the stationary components, or part of neither
since typically the inner races ~supports for the bearings) 26-27
rotate with the ~haft 10, the outer race3 24-25 are stationary
against She spindle housing 16, and the ball-bearings 20-23 rotate
as necessary between the inner and outer races to 3upport the load
of the rotatin~ components. While ball-bearings are ~hown in the
figure, lt is not uncom~on ~or any appropriate rotational bearing
mech~n1~ , e.g., roller bearing, ball-bearings, tapered pin
bearings, etc., to be used.
The splndle ho~sing 16 of the stationary components is attached
to the mount 30 of the mechanical structure o~ the equipment in
which the motor is being used. The manner of mounting is not
important to the present invention.
It i~ noted that ln a traditional direct current motor, (not a
brushless motor), the magnets are part of the stationary portion o~
the motor. These magnets may be permanent or electro-magnets which
are excited by the application of the proper direct current through
coils built into the motor housing. The motor w1ndinEs are part of
the rotating portion of the motor. Dir~ct current is supplied to
the windings tbrough a commutator, i.e., slotted, insulated segments
of copper (or other conductive material), which is made part of the
rotating shaft, through carbon brushes which rub against the
commutator. Pairs of segments of the commutator are electrically
connected to individual pairs of motor windings. Electrical
currents ln a pair of motor windings create a magnetic field which
oppose~ the magnetic f`ield oP the fixed magnets in the stationary
part of the motor, thus creatin~ tor~ional ~orces that cause the
shaft to rotate. A~ the shaft, and therefore the commutator,
rotate~ a different pair of motor w;n~ines receive the current from
the commutator, thereby allowing the torsional forces, and hence
shaft rotation to continue. Traditional direct current motors are
heavy and bulky, and precise speed control i~ difficult to achieve.
~ n contrast, the motor windin~.q of a brushless dirsct current
motor are part o~ the stationary component of the motor and the
permanent magnet are part of` the rotating components. In order to
develop the most torque, the permanent magnets are typically spaced
radlally ~rom the center line of the shaft as far a~q possible.
Hence, a magnet cup assembly 13, as shown in FIGURF 1, i employed
to place the magnets at a r-~il radial spacing, rather than merely
attaching the magnets to the side of the shaft 10. Thus in FIGURE
1, the permanert magnets 14-15 are placed a r~ distance from
the centerline of the shaft 10 and opposite the motor w1n~ngq
17-18. Direct current is electronically switched to the appropriate
pair of motor win~in~ to create a magnetic force which causes the
permanent magnet~, and therefore all the rotating components of the
motor, to rotate.
Advantageously, the rotational speed of the brushless motor can
easily be measured. For example, a slotted plate could be made part
of the rotating portion of the motor. As the slots of such a plate
pas~ between a light source and a photosensitive diode, a pulse
train whose frequency is proportional to the rotational speed of the
motor is created. This pulse train can be used to control the
switching of current to the motor windings and thus accurately
control the speed of the motor.
Therefore, the ability to precisely control the speed of a
bru3hleqs direct current motor allow~ such motors to be used ln many
applicatlons where a conventional direct current motor would not be
acceptable, such as in a disk drive device. Such appllcations also
require, however, a precise balance oP the rotating components. As
explained earlier, while it i9 possible to precisely balance a motor
such as that shown in FIG~RE 1, this balance will be lost if the
motor i~ disassembled. To explain further, the shaft 10 and spindle
hub 11 (see FIGURE 1) are fabricated from separate pieces of metal.
The spindle hub 11 is then attached permanently, e.g~, by means of
an interference fit, to the shaft 10. The resultant sl'h~q~embly is
then dynamically b~l~nced. This balancing is typically done by
spinning the object at the desired speed, and measuring the amount
of imbal~nce, for example with a ~trobe light. The object iY
stopped and an amount of material, estimated to be equal to the
imbalance, is removed from the proper area of the object, usually by
machinln~ or drilling the surface. The object is again spun and any
imbalance is again determined. This process is repeated until the
amount of r~m~ining imbalance i3 within a predetermined limit.
The cup adapter 12 and rotating magnet cup 13 must be balanced
in a similar manner. ~hen all the rotating components 10-15 are
balanced, the rotating components re assembled with the stationary
components, and a complete motor assembly is realized. Depending
upon the amount of imbalance allowed by the application 7 the
resulting oomplete motor assembly may require additional balancing.
Also, as explained previously, i~ the motor should ever have to
disassembled, a strong po~sibility exist~, dependent upon the care
and cost expended in ~abricating the rotating parts, that upon
reassembly balano$ng will again be required.
FIGURE 2 is a cross-sectional drawlng of a brushless direct
current motor configured acaording to the present invention. As can
b~ seen ~rom the figure, a rotating ~agnet cup 43~ along with the
permanent magnet~ 44~45, are attached to a ~pindle hub 11 instead of
to the opposite end of the shaft 10 (as taught in the prior art),
and the ~otor windings ~7~48 are located at the spindle hub end of a
spindle housing 46. The magnets 44-45 (which may be a single
doughnut shaped core piece that is selectively magnetized, or a
plurality of individual magnetized pieces) are mounted in the cup 43
so as to ma1ntain a selected juxtaposed relationship (depending upon
th~ rotational position of the shaft~ an outer sur~ace of the motor
windings 47-48. The shaft 10 rotates about an axis 50~ A disk 52,
typically having a center hole 54 therein, engages the spindle hub
11 when the motor is used for disk drive application
these ~eatures of the invention advantageously allow all the
rotating components o~ the motor, i.e., the shaft 10, the spindle
hub 11, rotating magnet cup l13, and permanent magnets 44-45 to be
manufactured using conventional manu~acturing practices. No
additional costs are incurred just because the components are going
to be balanced. The rotating components are then assembled into a
single compact subassembly. This subassembly is then balanced in
the manner described above. The b~l~nced subassembly can then be
installed into the stationary component~ of the ~otor.
a unique feature of the present invention i9 that once the
rotating components are jolned together in a 3ubassembly, there is
never a need to disassemble them. Thus~ once balanced, the rotating
suba~sembly can be installed, and removed and reinstalled., in the
motor without requiring additional balancing. This feature
signi~icantly reduces the cost of manu~acturing and maintaining the
motor assembly because only one subassembly need be balanced, and
t~at need only be ~ nced one time.
While seemingly a simple change, placing the rotating magnetic
cup 13, permanent magnets 14-159 and motor w~ndlng~ 17-18 at the
spindly hub end o~ the motor housing as shown in FIGURE 2 represents
a significant advance in the art. Prior art disk drive brushless
motors known ~o applicants have all placed the magnetic cup 13 at
the opposite end of the shaft from the spindle in order to more
evenly distribute the motor inert~a along the length of the shaft.
Such inertia distribution is neces~ry, as taught in the art, in
order to properly distribute the rotational shaft stresses along the
~ull length of the sha~t and in order to maintain a moment of
inertia that is more or less centered within the motor housing. The
inventors herein where the first to recogni~e that coupling most of
the motor inertia to one end of the ~haft as shown in FIGURE 2 does
not create a problem as the prior art teachings wo~ld suggest It
is believed that the reason ~or this is that achieving an~
maintaining a proper dynamic balance, in a simple and inexpensive
manner as is done with the configuration of the PreSent invention,
results in much less wear and strain of the type that would
otherwise make uneven motor inertia distribution ti.e., most of the
motor inertia coupled to one end of the sha~t) significant. In any
event, the result~ obtained to date by u~ing the motor configuraSion
shown in FIGU~E 2 have been surprisingly and unexpectedly good.
Accordlngly, because of the advantages it provides, the present
invention is planned for use in many of the dlsk drive products
manufactured by the inventors' employer.
