U.S. patent number RE29,411 [Application Number 05/648,693] was granted by the patent office on 1977-09-20 for harmonic drive for digital step motor.
This patent grant is currently assigned to Mesur-Matic Electronics Corporation. Invention is credited to Harold R. Newell.
United States Patent |
RE29,411 |
Newell |
September 20, 1977 |
Harmonic drive for digital step motor
Abstract
A digital step motor having a wobble plate which rotates about a
shaft, to produce mating contact between two sets of associated
gear teeth, under the influence of a stepping electromagnetic
drive. The wobble plate is coupled to the shaft about which it
rotates by a universal joint free to move axially but restrained
radially of the shaft.
Inventors: |
Newell; Harold R. (South
Newbury, NH) |
Assignee: |
Mesur-Matic Electronics
Corporation (Salem, MA)
|
Family
ID: |
27095445 |
Appl.
No.: |
05/648,693 |
Filed: |
January 13, 1976 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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627410 |
Mar 31, 1967 |
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Reissue of: |
664331 |
Aug 30, 1967 |
03644764 |
Feb 22, 1972 |
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Current U.S.
Class: |
310/49.47;
310/83; 310/82 |
Current CPC
Class: |
H02K
41/065 (20130101) |
Current International
Class: |
H02K
41/00 (20060101); H02K 007/06 () |
Field of
Search: |
;310/49,82,83 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Budd; Mark O.
Attorney, Agent or Firm: Pollock, Vande Sande &
Priddy
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of my copending
application for United States Letters Patent, bearing Ser. No.
627,410, entitled "Harmonic Drive for Digital Step Motors," filed
Mar. 31, 1967, and commonly assigned herewith.
Claims
I claim:
1. A motion transmitting system, comprising a pair of cooperating
gears having confronting frictional surfaces projecting from
normally spaced generally parallel planes, electromagnetic means
for selectively deflecting sequential points of one of said gears
toward and away from the other to produce sequential engagement of
said frictional surfaces at instantaneously limited regions of said
gears, said gears having mutually different configurations adapted
to provide continued digital rotations of said one gear responsive
to said deflections, a shaft, and universal joint means coupling
one of said gears to said shaft, said universal joint means being
arranged for wholly free axial and pivotal movement .Iadd.over at
least a predetermined range of movement .Iaddend.without radial
movement relative to said axis, wherein each of said frictional
surfaces comprise teeth, and wherein said frictional contact is
achieved by engagement of said teeth, said different configurations
being produced by different numbers of teeth on said gears, wherein
said cooperating gears are annular, lying along a common axis, the
teeth thereof projecting from normally uncurved confronting
surfaces thereof, said electromagnetic means including a
magnetically permeable armature coupled to said one gear, and
electromagnetic actuators disposed in circular array about said
common axis, each of said actuators effective when energized to
produce an electromagnetic field exerting deflecting forces on said
armature and therefore on said one gear coupled thereto, wherein
said armature comprises a rigid circular plate, said ring gear
fastened to a planar surface thereof adjacent the periphery of said
plate, a shaft extending along said axis and through said plate,
normally perpendicular thereto, said plate retained on said shaft
to permit relative rotation and limited angular orientation
therebetween from the normally perpendicular relationship.
2. The invention according to claim 1 wherein said armature is
responsive to sequential energization of said actuators in
predetermined switching format to wobble about said shaft, thereby
producing substantially single-point engagement between said gears,
and wherein is provided a second pair of gears coupled respectively
to said armature and said shaft to produce rotation of said shaft
at slow speed relative to the speed of rotation of armature wobble
about said shaft.
3. A motion transmitting system, comprising a pair of cooperating
gears having confronting frictional surfaces projecting from
normally spaced generally parallel planes, electromagnetic means
for selectively deflecting sequential points of one of said gears
toward and away from the other to produce sequential engagement of
said frictional surfaces at instantaneously limited regions of said
gears, said gears having mutually different configurations adapted
to provide continued digital rotation of said one gear responsive
to said deflections, a shaft, and universal joint means coupling
one of said gears to said shaft said universal joint means being
arranged for wholly free axial and pivotal movement .Iadd.over at
least a predetermined range of movement .Iaddend.without radial
movement relative to said axis, wherein each of said frictional
surfaces comprise teeth, and wherein said frictional contact is
achieved by engagement of said teeth, said different configurations
being produced by different numbers of teeth on said gears, wherein
said cooperating gears are annular, lying along a common axis, the
teeth thereof projecting from normally uncurved confronting
surfaces thereof; said electromagnetic means including a
magnetically permeable armature coupled to said one gear, and
electromagnetic actuators disposed in circular array about said
common axis, each of said actuators effective when energized to
produce an electromagnetic field exerting deflecting forces on said
armature and therefore on said one gear coupled thereto, further
including a shaft disposed along the axis of said armature, said
shaft and said armature arranged for relative rotation, and a
further pair of gears, one coupled to said armature and the other
coupled to the shaft in cooperating relationship with said one of
said further pair of gears, to produce rotation of said shaft in
response to deflection and consequent rotation of said
armature.
4. A step motor comprising a shaft,
at least one pair of cooperating substantially planar gears having
confronting spaced frictional surfaces and having a common axis
with said shaft, .Iadd.said gears of said at least one pair having
an unequal number of teeth, .Iaddend.
a wobble plate supporting one .Iadd.gear .Iaddend.of said .Iadd.at
least one .Iaddend.pair of gears, .Iadd.the other gear of said at
least one pair of gears being non-rotatively fixed to said shaft,
.Iaddend.
means pivotally coupling said wobble plate to said shaft for free
axial movement therealong .Iadd.over at least a predetermined range
of movement .Iaddend.and rotation thereabout while substantially
confining said wobble plate against radial motion with respect to
said shaft, and
means for selectively forcing said wobble plate to pivot on said
coupling means to sequentially force portions of said gears in
frictional contact against one another for producing relative
rotation of said gears .Iadd.and thereby rotation of said shaft.
.Iaddend. .[.5. The invention according to claim 4 wherein is
further provided means coupling at least one of said pair of gears
to said shaft to produce rotation of said shaft..]. .[.6. The
invention according to claim 5 wherein said means coupling at least
one of said pairs of gears to said shaft includes a further pair of
gears, said wobble plate supporting one of said further pair of
gears, and means securing the other of said further pair of
gears
to said shaft..]. 7. The invention according to claim 4 wherein
said wobble plate comprises magnetically permeable material, and
wherein said selective forcing means includes electromagnetic means
selectively energizable to produce sequential deflection of said
wobble plate about
said pivotal coupling means. 8. The invention according to claim 7
wherein said electromagnetic means comprises a pair of stators each
including a plurality of core-coil combinations corresponding to
switching phases for
said motor, said wobble plate disposed between said stators. 9. The
invention according to claim 8 wherein is provided a further pair
of cooperating gears coaxial with the first-mentioned pair of
gears, said wobble plate supporting one of said further pair of
gears on a surface opposite that on which said one of said
first-mentioned pair of gears is supported, said pair of stators
arranged for selective energization to deflect said wobble plate
for frictional contact between diametrically opposed portions of
respective gears of said first-mentioned and further
pairs of gears. 10. The invention according to claim 7 wherein said
electromagnetic means includes a plurality of coils, each coil
having a
magnetically permeable core. 11. The invention according to claim 7
wherein said electromagnetic means comprises a stator having a
plurality of core-coil combinations corresponding to switching
phases for said motor, said stator having a permanently magnetized
portion to provide a magnetic detent for maintaining said wobble
plate in the last-deflected
position thereof upon cessation of said selective energization. 12.
The invention according to claim 11 wherein said magnetized portion
comprises a member fastened to all of the stator cores and
permanently magnetized in successive opposite polarity zones at the
respective points of fastening of said cores.
Description
BACKGROUND OF THE INVENTION
In my aforementioned copending application Ser. No. 627,410,
hereinafter referred to as my copending application, it is observed
that prior art devices for harmonic drive or strain wave gearing
have received one or both of those names as a result of operation
wherein areas of mating relationship or engagement between the
teeth of two ring gears are peripherally propagated in the form of
a sinusoidal or substantially sinusoidal wave representative of a
wave deflection or strain wave in one of the gears. The concept of
harmonic drive is set forth in some detail in U.S. Pat. No.
2,906,143 to Musser, issued Sept. 29, 1959. Basically, the Musser
invention constitutes a motion transmitting device in which is
provided a rigid circular ring gear, a flexible ring gear of
different diameter from but coaxial with the rigid ring gear, and
some form of strain inducing device by which the flexible ring gear
is driven and at the same time deflected to force its teeth into
meshing relationship with the teeth of the rigid ring gear at a
plurality of circumferentially spaced points separated by areas of
noncontact therebetween. In this manner, rotational driving of the
flexible ring gear by the strain inducer results in the propagation
of a strain wave about the periphery of the flexible ring gear,
accompanied by relative rotation between the two gears.
According to the Musser patent referred to above, the circular gear
or ring gear is an annular ring provided with internal teeth, i.e.,
teeth projecting radially along the inner periphery. The flexible
gear or strain gear is also annular, being disposed within the
rigid ring gear, fabricated of a thin resilient material capable of
elastic deflection, and provided with external teeth projecting
radially about and from the outer periphery thereof. The pitch
diameters of the two gears differ as a consequence of the
difference in number of teeth between the gears, the strain gear
having fewer teeth than the ring gear by a number equal to or a
multiple of the number of positions of mating engagement between
the gears, in accordance with the predetermined distortion of the
strain gear by the strain inducer when the gears are disposed one
within the other. By virtue of this arrangement, at any given
instant of time a large percentage of the teeth of the two
cooperating gears are in contact, more than 50 percent of each.
As generally disclosed by Musser, the strain inducer is mounted on
a shaft with which the strain gear and ring gear are coaxial, and
has a configuration adapted to exert forces on the inner periphery
of the strain gear, when inserted into a position inside the
latter, so as to deflect or distort the wall of the strain gear to
produce the desired configuration of mating relationship between
the two gears at a plurality of circumferentially spaced positions.
Thus, as the strain inducer undergoes rotation the strain gear is
driven such that the teeth of the two gears enjoy complete
engagement at only a limited portion of each position of mating
relation and have varying degrees of engagement at either side of
each limited portion, being completely separated from one another
in areas approximately midway between positions of mating relation
in those cases where sufficient disparity exists between pitch
diameters (and tooth differential) of the gears. Accordingly, a
strain wave is propagated about the periphery of the strain gear,
one complete revolution of which is characterized by a tooth
movement equal to the tooth differential between the gears, the
gears undergoing relative rotation. Musser emphasizes that the
strain inducer need not be the driving element; rather that any of
the three elements (i.e., ring gear, strain gear or strain inducer)
may be the driving element and either of the remaining two the
driven element. The gear having the largest number of teeth per
radian moves in the same direction as the strain inducer when the
latter is the driving element.
Among the variations of strain inducer or wave generator mentioned
by Musser in his aforementioned patent are a pair of
electromagnetic embodiments, one involving polyphase energization
and the other single-phase energization. More recent patents of
related disclosure indicate a recognition that electromagnetically
energized strain wave gearing or harmonic drives were not actually
previously constructed, and proceed to disclose suitable forms
thereof. One of these patents, U.S. Pat. No. 3,169,201, entitled
"Electromagnetic Harmonic Device" issued Feb. 9, 1965, in the names
of Spring et al., contemplates elimination of the mechanical strain
inducer or wave generator cam and its bearing, and of the shaft
coupled thereto, as provided in the invention disclosed in the
aforementioned Musser patent, and utilization in its stead of an
electromagnetic drive system including a stator and a rotor. The
rotor of the Spring et al. invention comprises a plurality of thin
flat magnetically permeable plates of substantial nonretentivity
projecting radially from and lying in planes intersecting the
common axis of the output shaft and the gears, toward respective
lineal positions adjacent the surface of the strain gear (also
termed "flexspline") remote from the ring gear (also termed
"circular spline"), and arranged to pivot against the surface when
subjected to magnetizing force. The stator comprises an even number
of evenly spaced pairs of solenoid coils (with magnetic cores)
disposed in a circular array coaxial with the array of rotor plates
and spaced from portions of the plates protruding from the
flexspline. Progressive radial distortion or deflection of the
flexspline to produce a mating relation between the two splines
(gears) at a plurality of points is effected by energization of an
appropriate plurality of the coils in a desired sequence, thereby
sequentially forcing the magnetic plates (i.e., deforming the
armature) against the internal surface of the flexspline, resulting
in strain wave meshing of the splines as in the aforementioned
Musser patent. The Spring et al. patent discloses this
electromagnetic wave generator as an actuator for a digital
stepping motor, wherein diametrically opposed pairs of solenoid
coils are energized in sequence by a control circuit, also
disclosed, to produce radial deflection of the flexspline into an
elliptoidal shape, with progressive circumferential strain wave
deflection in discrete steps.
In U.S. Pat. No. 3,169,202, issued Feb. 9, 1965 in the names of
Proctor et al. still other types of electromagnetic actuators for
strain wave gearing or harmonic drives are disclosed, these
actuators having a continuously rotating field and differing one
from another primarily in respect to type of armature. The basic
configuration by which the strain wave deflection is propagated is,
however, entirely similar to that disclosed in the aforementioned
Musser and Spring et al. patents. The armatures described in the
Proctor et al. patent include an endless chain of magnetically
permeable rigid links, adjacent links pivotal relative to one
another, the chain disposed adjacent the surface of the flexspline
remote from the ring gear; a laminated core, the laminations being
in successive plates along the axis of the actuator and coaxial
therewith, the periphery of the successive laminations being
interrupted by equiangularly spaced slots in which magnetic powder
is disposed, adjacent the internal surface of the flexspline; and a
coiled magnetically permeable flat strip positioned adjacent the
internal surface of the flexspline. In the case of each of these
types of armature the continuously rotating field produced by
appropriate energization of an associated stator is effective to
distort the respective armature, thereupon subjecting the
flexspline to deflecting forces.
In my copending application, I disclose an improvement upon the
aforementioned prior art forms of harmonic drive system, wherein
the basic concept of the wobble plate type of electromagnetic motor
is employed together with sequential switching of the stator
windings to produce the desired stepped rotation of a wobble plate
or wobble disk rotor. Accordingly to an embodiment of that
invention, a pair of cooperating circular ring gears of the same
diameter are provided with teeth projecting from confronting
planes. At least one of the ring gears is rigid, fastened to the
internal surface of the larger diameter wall of a concentric double
cylindrical walled housing having a bridge joining the walls at a
common end thereof to form a "doughnut cup" shaped enclosure. This
enclosure or housing contains a laminated annular magnetic core
having a plurality of equiangularly spaced coils, corresponding to
the desired number of motor phases, wound thereon. A shaft extends
within the inner wall of the housing along the axis thereof and is
mounted for rotation in bearings retained at either end of the
space encompassed by the inner wall. An armature or rotor in the
form of a magnetically permeable circular plate is retained on the
shaft for relative rotation therewith and has adjacent its
periphery along a planar surface of the plate the second of the
aforementioned ring gears, one ring gear (preferably that on the
armature) having at least one less tooth than the other ring gear,
the teeth of the two gears normally spaced from one another. In the
preferred embodiment the armature is rigid, as is its ring gear.
One end of the magnetic core in the housing confronts the armature
and as the phases (field windings) of the motor are energized in
the desired switching format, the armature is successively pulled
toward each energized coil. Accordingly, the teeth of the two ring
gears are forced into mating engagement, i.e., intermesh, at only
one limited region of each gear at any given instant of time. As
the coil switching progresses the armature wobbles about the shaft,
the position at which its ring gear meshes with the stationary ring
gear fastened to the housing propagating sinusoidally along that
gear in accordance with the wobble motion. This constitutes a
substantially sinusoidal wave motion, the armature constituting a
mass rotating at an extremely low rotatory rate which depends upon
tooth differential, number of motor phases, and switching format
for the phases. A relative rotation occurs between the two ring
gears, and if the armature has the fewer teeth it rotates that
number of fewer teeth for each revolution of the wobble (i.e., each
revolution of the intermeshed position of the gears), and in a
reverse direction to the direction of rotation of the wobble. A
second pair of ring gears is provided by which the armature is
coupled to the shaft to drive the latter in accordance with
armature rotation so that the shaft undergoes discrete (stepped)
rotational motion in accordance with the switching format phase
energization of the motor. This second pair of gears operates to
transmit torque to the shaft in a positive and reliable manner, yet
with a minimum of frictional drag or loss of power due to wear. In
the harmonic drive system of my copending application, it will be
noted that the teeth of the cooperating ring gears are engaged at
only one portion of the overall ring, so that complete or partial
meshing occurs between only a small number of teeth at any given
instant of time. Consequently, the load or force on the shaft is
not truly balanced, a condition which is somewhat disadvantageous
when compared with harmonic drive systems of the prior art.
However, the invention disclosed in my aforementioned application
enjoys several advantages over prior art harmonic drives, resulting
in a decided overall improvement. For example, no power need be
applied to the "strain inducer" unless actual stepped rotation is
desired. In contrast, the prior art systems require application of
power to the strain inducer in order to deform the flexspline even
when the system is in a condition undergoing no rotational movement
but in a state of preparedness to do so. Over a lengthly period of
continuous use this can result in a substantial power saving in
favor of the apparatus of my previous invention.
Moreover, the prior art flexsplines have been found to be resonant
at a number of different frequencies, a condition which results in
loss of smooth and efficient performance. This cannot readily occur
in the drive system of my earlier invention, because unlike the
prior art strain gears, there is no thin elastic tubular flexspline
structure.
Wobble plate motors per se are, of course, known insofar as basic
concepts are concerned. The invention disclosed in my copending
application, however, combines the broad wobble plate or wobble
gear concept with two pairs of mating gears, one gear of each pair
disposed on a wobble plate of magnetically permeable material, and
the other gear of one pair fixed by attachment to the housing,
while the other gear of the other pair is coupled in driving
relation to the shaft; along with a driving or energizing circuit
by which the motor windings are sequentially switched to produce
the stepped rotation with rotor "wobble around."
In only one prior art electromechanical wobble gear actuator of
which I am aware is there an arrangement utilizing at least two
pairs of gear teeth, one row of teeth of each pair on the wobble
plate and the other row of teeth of each pair affixed to a frame
member and to a shaft, respectively. In that prior art actuator,
however, the wobble gear, i.e., the rotor, has a web portion formed
of a high coercive force permanent magnet material radially
magnetized with a center of one polarity and a periphery of
opposite polarity. Stator poles are disposed about the periphery of
the frame with windings thereon, which when energized, drive the
polarized armature. A major problem resides in the maintenance of
perfect mating contact between the rigid gear surfaces. This
ordinarily requires the use of precise construction measures by
which all gear teeth are cut to provide complete meshing between
each pair of associated gears. It will be appreciated that such a
requirement is accompanied by high cost of production and is
essentially ruled out from a practical standpoint. A further
requirement is the establishment of sufficient force on gears to
insure complete and continuous meshing of the teeth as the wobble
plate rotates. Although the desired mating contact may be assured
in a new set of gears by the aforementioned exacting and expensive
methods of cutting the teeth, nevertheless gear teeth wear rather
rapidly when subjected to continuous or lengthly periods of
operation. A related problem, then, is the maintenance of desired
mating contact between teeth of associated gears despite wear, and
more particularly, uneveness of teeth in the initial gear sets.
With conventional wobble plate coupling arrangements, imperfections
in associated gear teeth and wear of the teeth as operation
continues are accompanied by play, i.e., excess and usually
nonuniform freedom, between the gears themselves, which is
transmitted also to members to which the gears are fastened, such
as input or output shaft. This situation creates nonuniformly of
rotation of the gears, with intervals of speedup and slowdown
during each period of rotation, compounding the uneveness of
wear.
The use of special biasing arrangements has been suggested, for
example the employment of springs arranged to force the gears and
gear teeth together. However, if the spring force is excessive a
substantial amount of energy must be expended to overcome that
force and to cause the gears to mate, thereby reducing efficiency;
and if insufficient spring force is available the gears will
intermittently separate under conditions of loading.
SUMMARY OF THE INVENTION
According to the present invention, a wobble plate step motor is
provided in which the advantageous motor construction disclosed in
my copending application is generally retained, with the further
provision of improvements in the manner of coupling of the two sets
of gears. The wobble plate or disk is driven by a stepped magnetic
field to rotate on tracks formed by the two sets of gears. The
coupling point, in the form of a universal joint between wobble
plate and shaft on which the plates rotates, is free to move
longitudinally along the axis of rotation, but a sufficient tight
fit is maintained to prevent radial movement of the coupling point
during rotation. The force exerted by the driving magnetic field is
thereby divided between the two sets of gears such that both sets
are held in full mesh at all times with virtually none of the
looseness or play found in prior art wobble motors.
Accordingly, it is a principal object of the present invention to
provide improvements in harmonic drive for digital step motors.
It is a more specific object of my invention to provide wobble
plate or wobble disk step motors having improved coupling
arrangements whereby to assure perfect mating contact between gears
throughout motor operation, despite lack of precise cutting and
matching of gear teeth and wear or uneveness of wear of the
teeth.
Another object of the present invention is to provide a wobble
plate step motor in which the wobble path is coupled to a shaft,
forming an axis of rotation therefor, by a universal joint free to
move axially but restrained against radial motion on the shaft.
Still another object of the present invention resides in the
provision of a magnetic detent, in step motors of the
abovementioned type, whereby to maintain the wobble plate held in a
tipped position without the need for applied power.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and still further objects, features and attendant
advantages of the present invention will become apparent from a
consideration of the following detailed description of certain
preferred embodiments thereof, especially when taken in conjunction
with the accompanying drawings, in which:
FIG. 1 is a top or end view, looking toward the armature end, of a
step motor in accordance with an embodiment of the present
invention;
FIG. 2 is a sectional view taken along the lines 2--2 of FIG.
1;
FIG. 3 is a fragmentary perspective view of engaged teeth of a set
of cooperating gears in the motor of FIGS. 1 and 2;
FIG. 4 is a sectional view showing a dual stator arrangement in a
step motor of the general type shown in FIG. 1;
FIG. 5 is an end view of a radial stator arrangement;
FIG. 6 is a fragmentary sectional view taken along the lines 6--6
of FIG. 5, and
FIGS. 7 and 8 are, respectively, side and end views of a
premagnetized stator structure for provision of magnetic detent in
the digital step motor.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, an embodiment of my wobble plate
step motor includes a doughnut-shaped housing 10 comprising a pair
of coaxial cylindrical walls 12, 13 bridged at a common end 15 by a
wall 17. Housing 10 is composed of any suitable nonmagnetic
material and is adapted to partially enclose and retain the field
windings 20 and magnetic core 21 of the stator portion 25 of the
digital step motor.
Core 21 preferably comprises a strip of magnetically permeable
sheet material wound in successive layers in an annular or toroidal
configuration. The core is preferably provided with angularly
spaced slots 30 to accommodate field windings 20 wound on the
successive laminations of magnetic sheet material of which the core
is comprised. The field winding end terminations or leads are
brought out through the housing at any convenient point or points
(not shown) to permit connection to an energizing circuit (not
shown). The winding or phase energizing circuit is preferably of
the step driver type, such as that disclosed in my copending
application Ser. No. 581,334, although other types may be used, as
desired.
Pole faces 32 of magnetic core 21 are milled off at a slight angle,
1.degree. for example, (as shown at 33) to the plane perpendicular
to the axis of the core, for reasons which will be discussed
presently.
A shaft 35 is disposed along the axis of the symmetrical motor
structure thus far described, mounted for rotation in bearings 37
and 38 at the bridged and free ends, respectively, of cylindrical
wall 12 of housing 10. The shaft is provided with a larger diameter
section 39, or with suitable collars, to insure its longitudinal
retention within the bearings, the latter being preferably designed
to accept axial as well as radial loads. Pinned or otherwise
suitably fastened to shaft 35 at a point adjacent bearing 38 is a
circular ring gear 40, hereinafter referred to also as "inner" or
"internal" ring gear 40 to prevent confusion with other ring gears
of greater diameter. It will be apparent then that ring gear 40
follows the rotation of the shaft, or vice versa.
Further along the shaft there is disposed a rotor or armature 43
which in the preferred embodiment is a magnetically permeable rigid
circular plate. A bearing cup 45 is fastened within a centrally
located hole in 43 to hold the rotor to a bearing 46 which includes
a ball member 47, having an axial hole therein of sufficient
diameter to accept the shaft in relatively tight fit but slidable
relationship, and on which the rotor is permitted to pivot. Hence,
rotor or armature 43 can undergo rotation relative to the shaft and
is capable of assuming a cocked position at a slight angle to the
shaft. A retaining ring 48 is press-fitted on shaft 35 to prevent
the rotor from lifting too far. The central or axial hole in ball
member 47 permits the entire ball or universal joint to move
axially along the shaft to take up any play or looseness which
might otherwise be manifested in the meshing of the gears, as the
result of wear or unevenness of teeth, for example, and yet
prevents any significant radial motion of the ball joint.
Accordingly, the wobble plate 43 is similarly capable of free axial
movement but is restrained radially.
Armature (i.e., rotor or wobble plate) 43 is provided with a pair
of ring gears 49, 50 fastened to or cut on that one of its surfaces
confronting the pole faces 32 of magnetic core 21, ring gear 49
being identical or substantially identical to ring gear 40 in
respect to diameter and number or character of teeth, and adapted
to mate therewith. The outer or external ring gear 50 is disposed
at the underside of armature 43 adjacent its outer periphery, and
opposite a stationary ring gear 52 of substantially equal diameter
fastened to the inner surface of the free end of cylindrical wall
13 of housing 10.
The relative positions of the several ring gears of the motor
configuration are such that confronting teeth on cooperating outer
gears 50 and 52 and cooperating inner gears 49 and 40 can mesh only
in a limited area of each respective pair of gears along a common
sector from the axis of the overall structure at any given instant.
The diameters of the outer cooperating gears 50 and 52 are
substantially equal, but the number of teeth, and hence tooth size,
differ. In the exemplary embodiment shown, armature ring gear 50
has 359 teeth and stationary ring gear 52 has 360 teeth. Each of
the inner gears 40 and 49 is provided with 60 teeth, for example,
all teeth cut on, say, a 30.degree. angle with respect to a radial
plane.
Housing 10 is closed by a cuplike cover 60 (FIG. 2) which, like the
housing, is composed of nonmagnetic material. Preferably, the
stator, including winding 20 and core 21, is potted within housing
10 up to or slightly beyond the level of the field windings, using
an epoxy resin for example. While eight field windings (phases) are
shown, a smaller or greater number of phases may be utilized, as
desired.
In operation, the field windings are selectively energized by the
switching or energizing circuit (not shown) in accordance with the
preselected switching format, for example. A+B, B+C, C+D, D+E, E+F,
F+G, G+H, H+A, et cetera, the armature being pulled toward the
energized windings in the recited sequence. Thus, the two outer
gears 50 and 52 mesh along a sector region, and this meshed region
propagates with the "phase switchings" which produce the wobble of
armature disk 43 about shaft 35. The slight angle at which the pole
faces are milled with respect to a plane perpendicular to the shaft
permits this rotor wobble to occur without interference between
members. Because of the differing number of teeth on the rotor and
stator outer gears 50 and 52, respectively, a relative rotation
between these elements takes place as the sector of meshing rotates
one full revolution, i.e., for each 360.degree. of wobble-around of
the rotor. In particular, for the above-stated number of teeth, the
armature (rotor) rotates 1.degree., which coincides with one tooth,
for each 360.degree. of wobble, and in a reverse direction to the
direction of wobble. If the number of teeth on the rotor ring gear
50 were greater than that on the stator ring gear 52, the relative
rotation would be in the same direction as the wobble.
As wobble plate 43 rotates, the teeth of the inner gears 40 and 49
also mesh in a corresponding sector to that of gears 50 and 52,
thereby driving shaft 35. For the stated number of teeth, the shaft
undergoes 1.degree. of rotation per full revolution of wobble. With
either field windings (phases), each revolution of wobble requires
eight-phase switchings. Accordingly, each 360.degree. rotation of
the shaft requires 8.times.360=2,880 steps or phase switchings.
Meshing of the two sets of gears in the aforementioned manner is
maintained irrespective of slight imperfections in or uneveness of
the teeth of the two sets of gears, and despite gear wear occurring
with continued use of the motor, as a consequence of the axial
freedom of the ball joint and wobble plate. The magnetic field
generated by the stator in accordance with the switching format
produces a force which is thereby divided between the two sets of
gears and assures substantially equal meshing pressure on the gears
at the sector corresponding to the actuated stator coils. The
absence of radial motion of the ball joint, and hence of the wobble
plate, further assures accurate rotation of the output shaft 35
without looseness or play between gears.
The force exerted on the gears in the aforementioned limited sector
(which, of course, rotates with the phase switchings) by the
magnetic field accompanying the energization of field windings
produces cocking or tipping of disk 43 despite the axial freedom of
the ball joint. In this respect, it will be noted that the
described operation creates an unbalance of forces, with a strong
downward pressure exerted on the left-hand side of plate 43, as
viewed in FIG. 2, and practically zero force on the diametrically
opposite region of the plate, for a particular instant of
operation. The wobble plate pivots about the ball or universal
joint, moving it slightly upward on the shaft 35, so that the plate
is cocked. The consequent position of the ball joint longitudinally
along the shaft is self-aligning or self-orienting according to the
wear on the gears and the uneveness of the teeth, as previously
discussed.
A wide number of variations of construction may be effected within
the contemplation of the present invention. For example, the rotor
43 may be composed of flexible material incorporating rigid or
powdered magnetic material to permit flexing into contact with the
stator outer gear as the field windings are switched.
Moreover, the cooperating surfaces of the gears need not be
provided with teeth, but may be splined, ridged, knurled or
otherwise supplied with surfaces capable of frictional engagement
or contact. In fact, such frictional resilient material as natural
or synthetic rubber may be utilized for the cooperating surfaces of
the gears, or one surface may be rubber and the other rigid with
frictional contact portions such as ridges for enhancement of
mating. In such cases, the angle of the rotor requires that the
circumference of the contacting surface of the rotor be somewhat
greater than the circumference of the contacted surface of the
stator "gear." The smaller the angle of rotor tilt, the greater the
ratio of "wobble-arounds" versus shaft rotation, as may readily be
determined from trigonometry.
The magnetic field effecting the rotor wobble may be produced by
energizing separate phases in a DC switching format, or by two- or
three-phase AC energization.
Any of these modifications produces, as in the preferred
embodiment, extremely low rotational speeds with high positioning
accuracy and with simple construction and inexpensive
components.
In the preferred embodiment which has been described, references to
the single limited region or area of contact or engagement should
not be taken as necessarily implying a single point or position of
contact. In many instances, the single region of contact may, in
fact, extend over a substantial length, but unlike the prior art
there are no multiple areas of engagement separated by areas in
which the two gears are displaced substantially from one another.
In the case where the cooperating surfaces of the gears are rigid
triangular teeth, for example, it happens that actual contact
between the two toothed surfaces occurs at two spaced points at
which the sides of the mating teeth rest against one another and
between which the teeth are lifted slightly from each other (see
FIG. 3). For practical purposes, however, the entire area or region
between these contact points constitutes the area of frictional
contact or mating engagement, since true meshing, cooperation, or
engagement exists throughout such area.
The advantages of this cooperating frictional surface approach as
contrasted with prior art flexible spline arrangements include, for
example, a very low holding power requirement, since there is no
"springiness" to overcome; improved transient (start-stop) response
because of the lack of such "springiness" in the drive; greater
efficiency; economy in fabrication and production; and nonslip
operation.
Referring now to FIG. 4, there is shown a sectional view, similar
to FIG. 2, of a dual stator arrangement of a step motor otherwise
corresponding closely to that described above in connection with
FIGS. 1-3. In view of these similarities, the motor configuration
of FIG. 4 will be described only insofar as it differs
significantly from the previously described step motor.
The housing 110 is comprised of two matching portions, 111 and 112,
which may be secured together in any convenient and conventional
manner, as by a strap or band and suitable fasteners (not shown).
Housing portion 111 corresponds in almost every respect to housing
10 of FIGS. 1-3, except that it has no outer stationary ring gear
as at 52 of the earlier Figures. Nor does wobble plate (rotor or
armature) 143 have an outer gear. Instead, stationary gear 114 is
cut along or fastened to an annular region or frame 116 of housing
portion 112.
Mating ring gears 118 and 119 for stationary gear 114 and
shaft-pinned gear 140, respectively, are cut or otherwise provided,
as by fastening, on opposite sides of wobble plate 143. In this
embodiment, the same number of teeth are provided on both
stationary ring gear 114 and wobble disc ring gear 118, while a
tooth differential exists, as determined by considerations
previously noted, for ring gears 119 and 140.
Wobble disc 143 is coupled to the shaft 135 by the same type of
axially free, radially confined universal or ball joint 147 as
described earlier. Because of the dual stator configuration, in
which a second corresponding stator portion 127 is disposed
opposite the stator portion 125, the shaft 135 is longer than in
the earlier-described embodiments, and is maintained for rotation
in bearing 130 as well as bearings 131 and 132.
In operation of the dual stator embodiment, a coil or adjacent
coils of one of the stators (125 or 127) are energized
simultaneously with the energization of a coil or adjacent coils
diametrically opposite thereto on the other of the stators (127 or
125, respectively). For an eight-phase stator, for example, the
sequence for the lower stator, as viewed in FIG. 4, might be the
switching sequence 1 and 2 (this numbering referring to the
coil-core combinations, or phases), 2 and 3, 3 and 4, 4 and 5, ...,
7 and 8, 8 and 1, and so forth. During the respective time
intervals over which the preceding phase switchings occur, the
upper stator would be energized according to the sequence 5 and 6,
6 and 7, 7 and 8, 8 and 1, ..., 3 and 4, 4 and 5, and so forth.
This assumes that the numbering of phases is identical for the two
stators, i.e., upper stator phase No. 1 is directly opposite lower
stator phase No. 1, and so forth. For reversal of wobble, the phase
switching sequence is reversed. Accordingly, an upward force, for
example, is exerted on the right-hand side of wobble disc 143, as
viewed in FIG. 4, while a downward force is exerted on the
left-hand side of the wobble disc, during a given time interval,
the plate wobbling about the "tracks" provided by the gears. It
will be observed, then, that the dual stator configuration doubles
the effective torque capability of the motor for the same size
disc, and thereby increases the speed of step response of the
motor. Moreover, since the ball joint 147 provides axial freedom
and radial confinement of the disc, both sets of gears bottom,
i.e., are forced into meshing engagement, despite any unevenness or
nonuniformity which may result from gear wear or production
tolerances.
Preferably, the wobble disc gear 118 and stationary or frame gear
114 have triangular teeth cut on a 30.degree. angle, and the disc
gear 119 and shaft gear 140 have triangular teeth cut on a
45.degree. angle. Because gears 114 and 118 have the same number of
teeth, disc or plate 143 does not rotate, but only wobbles about
the axis of the shaft on the tracks presented by the gears. This
reduces inherent motor inertia while concurrently enhancing the
external inertia handling capability of the motor. Ring gear 140,
and hence shaft 135 to which it is pinned or keyed, rotates slowly
either clockwise or counter-clockwise about the axis, depending
upon the difference in number of teeth between that gear and wobble
disc gear 119, as disc 143 wobbles.
By introducing a tooth differential between disc gear 118 and frame
gear 116, as well as retaining the tooth differential between disc
gear 119 and shaft gear 140, the ratio of wobble around to shaft
rotation may be considerably increased or decreased, as desired.
If, for example, the shaft gear 140 has 91 teeth, and mating disc
gear 119 has 92 teeth, shaft 135 will rotate at a rate of one
complete revolution per 91 "wobble arounds" of the disc. If, in
addition, disc gear 118 has 90 teeth and frame gear 116 has 89
teeth, the disc will rotate one complete revolution in the forward
direction (i.e., in the direction of the phase switchings) for each
90 wobble arounds. Under these conditions, shaft 135 will rotate in
a direction opposite that of the disc, and will make one complete
revolution (relative to the housing) for each (90).sup.2 wobble
arounds. For two 90:1 ratios in the same direction, an overall 45:1
ratio would result.
Referring again to the embodiment of FIGS. 1 and 2, it will be
noted that the force on the wobble plate, as exerted by the
magnetic field, acts between the two pairs of gears; and since
their horizontal point is a ball joint which is free to move
axially (i.e., longitudinally) along the shaft but is restrained in
the radial direction, the force is divided between the two gears in
such a manner that both pairs are held in mesh in the
aforementioned limited sector without any tendency toward
looseness. This cannot occur if the pivot point of the gears is
constrained to a fixed point along the axis, e.g., if it were
fastened to the shaft, unless the gears are precision cut for
perfect engagement and not subject to wear over long periods of
use. It is to be observed that the problem of maintaining a tight
mesh between the gears is aggravated by the existence of more than
one pair or set of gears, and is practically insoluble in the
absence of a "floating" pivot point in accordance with the present
invention.
Similar considerations exist for the dual stator embodiment of FIG.
4.
The use of springs or other resilient biasing means to compensate
for irregularities in one or more pairs of gears is not completely
effective and leads to further problems. Where the spring force is
too large to be easily overcome under normal motor driving force,
as would be the case if tight intermeshing of at least one pair of
gears is to be contemplated, a decrease in motor efficiency is
observed as a result of the energy required to overcome spring
tension. Moreover, the presence of resilient biasing arrangements
for the gears tends to set up spontaneous vibrations and mechanical
resonance, or "ringing," problems during the stepping motor
operation.
A modification of the stator poles in the embodiment of FIGS. 1 and
2 is shown in FIGS. 5 and 6. Instead of a circumferential placement
of the poles on a common or substantially common core, the magnetic
poles are radially positioned with each set of north and south
poles 190 and 191, respectively, associated with a different core
and coil, such as 194 and 195, respectively. By virtue of this
magnetic separation of neighboring poles, it is possible to provide
a faster buildup of magnetic lines of force or flux with each phase
switching.
Still another modification of the stator poles is shown in FIGS. 7
and 8. Here, a magnetic ring 200 is employed as the base of the
stator core, the ring being permanently magnetized in zones, as
shown, to provide alternate north and south poles matched with the
positions at which magnetically permeable pole pieces 202 are
mounted to the magnetic ring. The field coils (e.g., 204, 205) are
wound about the upstanding pole pieces in a manner corresponding to
that shown in FIGS. 1 and 2.
This embodiment provides a magnetic detent to maintain the
permeable rotor (wobble plate) tipped against the poles to which it
is most closely adjacent when energization of the phases is ceased,
since the attractive force exerted on the rotor varies inversely as
the square of the pole to rotor spacing. Accordingly, the rotor may
be held, or latched, without the need for applying power to the
motor, a desirable feature in such applications as incremental tape
drive where operation is intermittently halted pending the
recording of information.
While I have disclosed certain preferred embodiments of my
invention, it will be apparent to those skilled in the art to which
the invention pertains that variations in the specific details of
construction which have been illustrated and described may be
resorted to without departing from the spirit and scope of the
invention as defined by the appended claims.
* * * * *