Note: Descriptions are shown in the official language in which they were submitted.
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TITLE
Axial gap electric machine with permanent magnets arranged between posts
FIELD
[0002] Electric machines.
BACKGROUND
[0003] In the design of electric machines, it is known to select structural
parameters such
as slot number depending on the intended application and desired performance
characteristics of the machine. However, not all values of the structural
parameters are used
in practice. There is room for improved performance of electric machines,
particularly in
robotics.
[0004] Electric machines typically use electrically conductive wire turns
wrapped around
soft magnetic stator posts (teeth) to generate flux. The manufacturing process
for this type
of motor construction can be time consuming and expensive. As well, such
motors
typically have a torque to mass ratio that makes them relatively heavy for
mobile actuator
applications such as in robotics where the weight of a downstream actuator
must be
supported and accelerated by an upstream actuator.
[0005] Common permanent magnet direct drive motors can be difficult to
assembly
because of high permanent magnet forces between the rotor and stator. These
high
magnetic forces typically require complex fixtures for assembly to avoid
damage to parts
and injury to personnel as the rotor and stator are brought together.
[0006] Large diameter, low profile bearings that are used in many motion
control devices
such as robot arm joints, must typically be physically retained in the
housings to prevent
separation of the bearing assembly. Many low profile bearings also tend to be
relatively
low tolerance compared to larger profile, smaller diameter bearings. Moreover,
bearings
typically require an adjustable preload that is typically provided by a
threaded or other type
of member. This is difficult to fit into a low profile assembly and is
especially challenging
with thin section bearings.
[0007] In a common axial flux actuator, the bearings are located at the inner
diameter of
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the magnetic active section of the rotor. This setup is a common practice
because placing
a bearing at the outer diameter of the rotor induces more drag and the overall
bearing profile
increases as the bearing diameter increases. Bearings on the OD of the rotor
will also tend
to limit the rotational speed of the device.
[0008] To make a single inner bearing work with a single rotor/single stator,
either the
rotor and stator structures must be thickened to provide a stiffer structure
to reduce
deflection, or the air gap distance must be increased to accommodate the rotor
and stator
deflection. The first method results in a heavier device and larger envelope
which reduces
actuator acceleration and torque density. The latter method result in a
reduction of torque
due to the larger air gap distance.
SUMMARY
[0009] The inventor has proposed an electric machine with a novel range of
structural
parameters particularly suited for robotics, along with additional novel
features of an
electric machine.
[0010] In an embodiment there is provided an electric motor having a stator
having an
array of electromagnetic elements and a rotor mounted on bearings. The rotor
has an array
of rotor posts, each of the rotor posts having a length defining opposed ends
and the array
of rotor posts extending along the rotor in a direction perpendicular to the
length of each
of the rotor posts. The rotor has electromagnetic elements defining magnetic
poles placed
between the plurality of rotor posts. An airgap is formed between the rotor
and the stator
when the stator and the rotor are in an operational position. A plurality of
rotor flux
restrictors is formed on the rotor. Each of the plurality of rotor flux
restrictors each lying
adjacent to one of the opposed ends of the rotor posts.
[0011] In various embodiments, there may be included one or more of the
following or
other features. The bearings may further comprise a first bearing connecting
the rotor and
the stator and a second bearing connecting the rotor and the stator. The first
bearing and
second bearing are arranged to allow relative rotary motion of the rotor and
the stator. The
array of rotor posts and the plurality of rotor flux restrictors may lie on
the rotor between
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the first bearing and the second bearing. The plurality of rotor flux
restrictors may further
comprise a plurality of holes within the rotor. The plurality of rotor flux
restrictors may
further comprise a plurality of blind holes. The plurality of rotor flux
restrictors may further
comprise a plurality of through holes. The electric motor may be an axial
electric motor.
The first bearing may further comprise an inner thrust bearing. The second
bearing may
further comprise an outer thrust bearing. The electromagnetic elements of the
stator and
the electromagnetic elements of the rotor may be arranged radially inward of
the outer
thrust bearing and radially outward of the inner thrust bearing. The inner
thrust bearing and
the outer thrust bearing may be arranged to maintain the airgap against a
magnetic
attraction of the electromagnetic elements of the rotor and the
electromagnetic elements of
the stator. The plurality of rotor flux restrictors may further comprise a
plurality of outer
flux restrictors lying radially outward from the rotor posts and radially
inward from the
outer thrust bearings. The plurality of rotor flux restrictors may further
comprise a plurality
of inner flux restrictors lying radially inward from the rotor posts and
radially outward
from the inner thrust bearing. The plurality of rotor flux restrictors may
further comprise a
plurality of inner flux restrictors lying radially inward from the rotor posts
and radially
outward from the inner thrust bearing. Each of the inner and outer flux
restrictors may be
radially aligned in an alternating pattern relative to the rotor posts, so
that the inner and
outer flux restrictors are adjacent to every second rotor post. The inner and
outer flux
restrictors may be adjacent to alternate rotor posts so that each rotor post
is adjacent to only
one of the inner flux restrictors or one of the outer flux restrictors. Each
of the inner and
outer flux restrictors may be radially aligned with the rotor posts, and the
inner and outer
flux restrictors may be adjacent to each rotor post. For each rotor posts, the
rotor post may
be adjacent to two inner flux restrictors and two outer flux restrictors. The
plurality of inner
flux restrictors and the plurality of outer flux restrictors may each further
comprise a
plurality of holes having the same geometry. The plurality of holes having the
same
geometry may further comprise a plurality of holes having a circular cross-
section. The
circular cross-section of each of the plurality of holes has an equal size.
The stator may
further comprise stator posts that form the electromagnetic elements of the
stator, with slots
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between the stator posts, one or more electric conductors in each slot, each
of the stator
posts having a length defining opposed ends and the array of stator posts
extending around
the stator circularly in a direction perpendicular to the length of each of
the posts. The
stator may further comprise a plurality of stator flux restrictors being
formed on the stator,
each of the plurality of stator flux restrictors lying adjacent to one of the
opposed ends of
the stator posts. The electric motor may further comprise a linear electric
motor. The first
bearing may further comprise a first linear bearing. The second bearing may
further
comprise a second linear bearing. The electromagnetic elements of the stator
and the
electromagnetic elements of the rotor may be arranged axially between the
first linear
bearing and the second linear bearing. The first linear bearing and the second
radially
bearing may be arranged to maintain the airgap against a magnetic attraction
of the
electromagnetic elements of the rotor and the electromagnetic elements of the
stator. The
plurality of flux restrictors may further comprise a plurality of first flux
restrictors lying
between the rotor posts and the first linear bearings. The plurality of flux
restrictors may
further comprise a plurality of second flux restrictors lying between the
rotor posts and the
second linear bearing. Each of the first and second flux restrictors may be
aligned with the
length of the corresponding one of the rotor posts in an alternating pattern
relative to the
rotor posts, so that the inner and outer flux restrictors are adjacent to
every second rotor
post. Each of the first and second flux restrictors may be aligned with the
length of the
corresponding one of the rotor posts, and the inner and outer flux restrictors
are adjacent
to each rotor post. The plurality of first flux restrictors and the plurality
of second flux
restrictors may each further comprise a plurality of holes having the same
geometry.
[0012] An electric machine is also disclosed in which a first carrier (rotor
or stator) is
supported for rotation relative to a second carrier (stator or rotor) by
bearings and the
bearings include bearing races that are homogenous extensions of homogenous
plates that
form the first carrier and second carrier. That is, the bearing races of the
bearings are
integrated with the respective carriers that are supported by the bearings.
The integrated
bearings races may be used for inner or outer bearings or both, in either an
axial flux or
radial flux machine.
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[0013] These and other aspects of the device and method are set out in the
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] Reference will now be made to preferred embodiments of the invention,
by way of
example only, with reference to the following figures in which:
Fig. 1 is an isometric view of an exemplary actuator;
Fig. 2 is an exploded view of the exemplary actuator of Fig. 1;
Fig. 3 is an isometric view of a rotor of the exemplary actuator of Fig. 1;
Fig. 4 is an isometric view of a stator of the exemplary actuator of Fig. 1;
Fig. 5 is an isometric view of a section of the exemplary actuator of Fig. 1;
Fig. 6 is a view of the body of the exemplary actuator along the section A-A
in Fig.
1;
Fig. 7 is an enlarged detail view of an outer bearing and thermal interference
fit
showing the detail Cl in Fig. 6;
Fig. 8 is an enlarged detail view of an inner bearing and safety ring showing
the
detail El in Fig. 6;
Fig. 9 is an isometric view of a section of an exemplary actuator having an
alternative thermal interference fit;
Fig. 10 is an section view of the exemplary actuator in Fig. 9;
Fig. 11 is an enlarged detail view of an outer bearing and an thermal
interference
fit showing the detail C2 in Fig. 10;
Fig. 12 is an enlarged detail view of an inner bearing and safety ring showing
the
detail E2 in Fig. 10;
Fig. 13 is an isometric view of a section of an exemplary stator plate with
integrated
bearing races;
Fig. 14 is an isometric view of a section of an exemplary rotor plate with
integrated
bearing races;
Fig. 15 is an isometric view of a section of exemplary actuator with
integrated
bearing races;
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Fig. 16 is a section view of a rotor and stator including representations of
magnetic
flux and forces along the section B-B in Fig. 6;
Fig. 17 is a view of the body of an exemplary actuator with a safety ring;
Fig. 18 is a detail view of a safety ring with a plain bearing;
Fig. 19 is a detail view of a safety ring with a thrust bearing;
Fig. 20 is a close up view of a rotor during installation and removal of the
magnets;
Fig. 21 is a partial cross section of a rotor plate section;
Fig. 22A is a partial view of a rotor plate section having flux restricting
holes;
Fig. 22B is a partial view of a rotor plate section having another arrangement
of
flux restriction holes;
Fig. 23 is a FEMM simulation result on a rotor plate without flux restricting
holes;
Fig. 24 is a FEMM simulation result on rotor plate with flux restricting
holes;
Fig. 25 is a cross section of a stator plate section with uninterrupted path
between
ID bearing and OD bearing;
Fig. 26 is an exploded view of an exemplary actuator;
Fig. 27 is a cross section of an embodiment showing an exemplary actuator
connected to an upper and lower housing;
Fig. 28 is an exploded isometric view of the exemplary actuator in Fig. 27;
Fig. 29 is an isometric cut away view of the exemplary actuator in Fig. 27;
Fig. 30 is a cross-section through a segment of an axial flux concentrated
flux rotor
with tapered magnets and flux path restrictions;
Fig. 31 is a close-up section view of a portion of an axial flux concentrated
flux
rotor with extended length magnets;
Fig. 32 is a simplified exploded section view of an embodiment of an axial
flux
stator-rotor-stator configuration of a concentrated flux rotor with end iron;
Fig. 33 is a simplified exploded section view of an embodiment of an axial
flux
stator-rotor-stator configuration of a concentrated flux rotor with back iron,
end
iron and flux path restrictions;
Fig. 34 is a simplified exploded section view of an embodiment of an axial
flux
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rotor-stator-rotor configuration of a concentrated flux rotor with end irons
and flux
path restrictions;
Fig. 35 is a simplified exploded section view of an embodiment of an axial
flux
rotor-stator-rotor configuration of a concentrated flux rotor with end irons,
flux
path restrictions and back irons;
Fig. 36 is a simplified perspective view of a linear flux machine with back
irons
and flux restrictors;
Fig. 37 is a simplified perspective view of a linear flux machine without back
irons
and with flux restrictors;
Fig. 38 is a simplified perspective view of a linear flux machine with an
alternating
pattern of flux restrictors;
Fig. 39A shows a graph of torque at constant current density for a simulated
series
of motors differing in slot pitch and post height;
Fig. 39B shows the highest stator current density possible at a given
temperature
for a simulated series of motors differing in slot pitch and post height;
Fig. 39C shows constant temperature torque as a function of slot pitch and
post
height for a series of electric machines;
Fig. 39D shows the value of a weighting function for at the highest stator
current
density possible at a given temperature for a simulated series of motors
differing in
slot pitch and post height;
Fig. 39E shows Km" for a simulated series of motors differing in slot pitch
and post
height, for a fixed current density;
Fig. 39F shows KR" for a simulated series of motors differing in slot pitch
and post
height, for a fixed current density;
Fig. 40 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 200 mm size and a boundary line
for
KR" > 1.3;
Fig. 41 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 200 mm size and a boundary line
for
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KR" > 1.5;
Fig. 42 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 200 mm size and a boundary line
for
KR" > 1.8;
Fig. 43 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 100 mm size and a boundary line
for
KR" > 1.5;
Fig. 44 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 100 mm size and a boundary line
for
KR" > 1.7;
Fig. 45 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 100 mm size and a boundary line
for
KR" > 1.9;
Fig. 46 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 50 mm size and a boundary line
for
KR" > 2.2;
Fig. 47 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 50 mm size and a boundary line
for
KR" > 2.5;
Fig. 48 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 50 mm size and a boundary line
for
KR" > 2.9;
Fig. 49 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 25 mm size and a boundary line
for
KR" >3.3;
Fig. 50 shows the region of benefit for KR", with respect to the rest of the
geometries in the domain, for a machine with 25 mm size and a boundary line
for
KR" > 3.4;
Fig. 51 shows the region of benefit for KR", with respect to the rest of the
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geometries in the domain, for a machine with 25 mm size and a boundary line
for
KR" > 3.6;
Fig. 52 shows the joint of a robot arm using a frameless motor/actuator;
Fig. 53 displays a cross-sectional view of the frameless motor/actuator and
robot
arm;
Fig. 54 shows a close up of the section view of the frameless motor/actuator
stator,
rotor and housing assembly;
Fig. 55 shows an exploded view of the frameless motor/actuator robot arm
assembly;
Fig. 56 displays a section view through the housing to view the stator and tab
features on the rotor;
Fig. 57 shows a representation of an up, over and down assembly motion used
with
the tab features in Fig. 56 to secure the rotor;
Fig. 58 shows a close up of the section view displaying the tab feature used
to
secure the rotor; and
Fig. 59 shows a section view through the housing to display the tab features
used
on the stator to secure the stator.
DETAILED DESCRIPTION
[0015] Several terms to be used throughout the text will first be defined.
[0016] A carrier, as used here in the context of electric machines, may
comprise a stator
or a rotor when referring to rotary machines.
[0017] A rotor as used herein may be circular. A rotor may also refer the
armature or
reaction rail of a linear motor. A stator may be circular. It may also refer
to the armature
or reaction rail of a linear motor.
[0018] Teeth may be referred to as posts.
[0019] In an electric machine, either a stator or rotor may have a commutated
electromagnet array defined by coils wrapped around posts, while the other of
the stator or
rotor may have magnetic poles defined by permanent magnets or coils or both
coils and
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permanent magnets. An electric machine may be configured as a motor or
generator.
[0020] Permanent magnets may be used in combinations with electromagnets on
the rotor
and/or stator to add flux to the system.
[0021] PM means permanent magnet. EM means electromagnet. ID means inner
diameter.
OD means outer diameter.
[0022] Electromagnetic elements may comprise permanent magnets, posts, slots
defined
by magnetic posts, which may be soft magnetic posts, and electrical
conductors. In any
embodiment where one carrier has slots and posts, the other may have permanent
magnets
for the electromagnetic elements, and for any such embodiment, the term
electromagnetic
element may be replaced by the term permanent magnet. Magnetic poles in some
cases,
for example in a concentrated flux rotor embodiment, may be defined by
permanent
magnets in conjunction with adjacent posts in which a magnetic field is
established by the
permanent magnets.
10023] Unless otherwise specified, flux refers to magnetic flux. Soft Magnetic
Material is
a material that is magnetically susceptible and that can be temporarily
magnetised such as
but not limited to iron or steel or a cobalt or nickel alloy.
[0024] A fractional slot motor is a motor with a fractional number of slots
per pole per
phase. If the number of slots is divided by the number of magnets, and divided
again by
the number of phases and the result is not an integer, then the motor is a
fractional slot
motor.
[0025] Thrust bearings include any bearing arranged to support a substantial
axial thrust,
including angular contact bearings and four-point contact bearings as well as
pure thrust
bearings. A radially locating bearing is a bearing that, in use, prevents
relative
displacement of the axes of the elements connected by the bearing.
[0026] A bearing can be radial and thrust locating (such as a cross roller
bearing) or it can
be j us t radial or j us t thrust locating.
[0027] A carrier may be supported for motion relative to another carrier by a
frame or
bearings, and the bearings may be sliding, roller, fluid, air or magnetic
bearings.
[0028] An axial electric machine is an electric machine in which magnetic flux
linkage
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occurs across an axial airgap, and the carriers are in the form of discs
mounted coaxially
side by side. A first carrier can be arranged to move relative to another
carrier by either
carrier being supported by a frame, housing or other element, while the other
carrier moves
relative the first carrier.
[0029] A radial electric machine is an electric machine where the airgap is
oriented such
that magnetic flux is radially oriented, and the carriers are mounted
concentrically, one
outside the other.
[0030] A linear actuator is comparable in construction to a section of an
axial flux or radial
flux rotary motor where the direction of motion is a straight line rather than
a curved path.
[0031] A trapezoidal electric machine is an electric machine that is a
combination of both
an axial and radial flux machines, where the plane of the airgap lies at an
angle partway
between the planes formed by the airgaps in the axial and radial
configurations.
[0032] The airgap diameter for a rotary machine is defined as the diameter
perpendicular
to the axis of rotation at the centre of the airgap surface. In radial flux
motors, all of the
airgap resides at the same diameter. If the airgap surface is a disc-shaped
slice as in axial
flux motors, the average airgap diameter is the average of the inner and outer
diameter. For
other airgap surfaces such as a diagonal or curved surfaces, the average
airgap diameter
can be found as the average airgap diameter of the cross-sectional airgap
view.
[0033] For a radial flux motor, the airgap diameter refers to the average of
the rotor inner
diameter and stator outer diameter for an outer rotor radial flux motor or the
average of the
rotor airgap outer diameter and stator airgap inner diameter for an inner
rotor radial flux
motor. Analogues of the airgap diameter of a radial flux motor may be used for
other types
of rotary motors. For an axial flux machine, the airgap diameter is defined as
the average
of the PM inner diameter and PM outer diameter and EM inner diameter and EM
outer
diameter.
[0034] The back surface of the stator is defined as the surface on the
opposite side of the
stator to the surface which is at the magnetically active airgap. hi a radial
flux motor, this
would correspond to either the inner surface of the stator for an outer rotor
configuration,
or the outer diameter surface of the stator for an inner rotor configuration.
In an axial flux
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motor, the back surface of the stator is the axially outer surface of the
stator.
[0035] For distributed windings, the number of slots will be N x the number of
poles where
N is a multiple of the number of phases. So for a 3 phase machine N could be
3, 6, 9, 12,
etc. For concentrated windings, the number of slots can vary but must be a
multiple of the
number of phases. It does not depend on the number of poles, except that
certain
combinations of slots and poles will yield higher torque and better noise-
reduction or
cogging-reduction characteristics. The minimum number of slots for a given
number of
poles should not be below 50% to obtain adequate torque.
[0036] Conductor volume may be used to refer to the slot area per length of a
single stator.
The slot area is the area of a cross-section of a slot in the plane which is
orthogonal to the
teeth but not parallel to the plane of relative motion of the carriers. In an
axial motor, this
plane would be perpendicular to a radius passing through the slot. The slot
area effectively
defines the maximum conductor volume that can be incorporated into a stator
design, and
it is usually a goal of motor designers to have as high a fill factor as
possible to utilize all
the available space for conductors.
[0037] Since maximum conductor volume in a stator is defined in terms of slot
area, any
stator referred to as having a maximum conductor volume or slot area must have
slots and
teeth to define the slots. This parameter is defined for rotary motors as:
NsAs
Slot area per length = ____________ = slot density = As
TruAG
where As is the cross-sectional area of a single slot, or the average area of
a single slot for
stator designs that have varying slot areas.
[0038] As a relatively accurate approximation, As may be calculated as the
height of the
tooth, ht, multiplied by the average width of the slot, Ws, such that the
equation above
becomes:
Ilshtws
Slot area per length = ____________ = slot density = htws
TIDAo
[0039] Slot depth or post height may also be used as a proxy for the conductor
volume.
The post height, also known as the tooth height or slot depth, is a proxy for
the amount of
cross-sectional area in a slot available for conductors to occupy. Although
the slots may
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have a variety of shapes such as curved or tapered profiles, the slot height
is based upon
the closest rectangular approximation which best represents the total area of
the slot which
may be occupied by conductors. This dimension does not include features such
as pole
shoes which add to the height of the tooth without adding substantially to the
slot area. For
transverse flux motors, the post height is defined as the portion of the post
which is directly
adjacent to the conductor coil, perpendicular to the direction of the coil
windings.
[0040] A concentrated winding comprises individually wound posts or any
winding
configuration that results in the alternating polarity of adjacent posts when
energized. It is
understood that not all posts will be the opposite polarity of both adjacent
posts at all times.
However, a concentrated winding configuration will result in the majority of
the posts
being the opposite polarity to one or both adjacent posts for the majority of
the time when
the motor is energized. A concentrated winding is a form of fractional slot
winding where
the ratio of slots per poles per phase is less than one.
[0041] The terms one-piece, unitary, homogenous, solid, isotropic and
monolithic are used
interchangeably when referencing a stator or rotor herein. Each of the terms
excludes
laminates and powdered materials that include significant electrical
insulative materials.
However, small insulating particles may be present that do not significantly
interfere with
the electrically conducting properties of the material, for example where the
bulk isotropic
resistivity of the material does not exceed 200 microohm-cm. A one-piece,
unitary,
homogenous, solid, isotropic or monolithic material may comprise iron,
including ductile
iron, metal alloys including steel, and may comprise metal alloys formed of
electrically
conducting atoms in solid solution, either single phase or multi-phase, or
alloys formed of
mixtures of metals with other materials that improve the strength or
conductivity of the
material, for example where the bulk isotropic resistivity of the material
does not exceed
200 microohm-cm.
[0042] Embodiments of the present device use an integrated bearing race that
is preferably
machined into the stator and/or rotor where the bearing races and at least the
axial surfaces
of the stator and rotor posts can be machined in the same set-up. This can
provide for very
high tolerance manufacturing of the critical geometry relationship between the
bearing race
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axial and radial positions relative to the stator and rotor posts. Consistency
of these
geometric relationships is important for consistent cogging and other
performance
characteristics of the device.
[0043] Embodiments of the present device can allow for streamlined
manufacturing with
a rotor configuration that allows the permanent magnets to be installed into
the rotor
individually after the stator and rotor have been assembled.
[0044] Embodiments of the device can provide high torque density, ease of
manufacturability, ease of assembly and serviceability due to a very simple
assembly with
a minimal number of components, and excellent operational safety as a result
of high
torque-to-inertia which allows very fast emergency stopping.
[0045] As shown in Fig. 1, a non-limiting exemplary embodiment of an axial
flux motor
110 is housed in an upper arm member 100 and a lower arm member 200. The upper
and
lower arm members 100, 200 rotate around a rotational axis 300.
[0046] A non-limiting exemplary embodiment of the device in a robotic arm
assembly is
shown in Figure 2. The upper arm member 100 includes a support housing 101.
The lower
arm member 200 includes an arm housing 201. The support housing 101 and the
arm
housing 201 are preferably made of a light weight material such as, but not
limited to,
aluminum, magnesium or carbon fiber composite.
[0047] As shown in Figure 2 to Figure 5 the stator 102 is attached to the
upper arm 100
such as with bolts and/or adhesive and/or thermal fit or by being formed
integrally with the
arm. In Fig. 2, the stator 102 is connected to the upper arm 100 using a press
fit with a ring
101A. An outer bearing 302 and an inner bearing 301 allow relative rotation of
the stator
102 and rotor 202 and provide precise relative axial location of the stator
102 and rotor 202
to maintain an airgap between stator posts 105 (Fig. 4) and rotor posts 205
(Fig. 3). As
shown in Fig. 3, the rotor may have flux restriction holes 206 and permanent
magnets 204.
The permanent magnets are seated in slots 208.
[0048] The placement of the inner bearing 301 inside the ID of the airgap and
the outer
bearing 302 outside the OD of the airgap distributes the attractive forces
between the stator
102 and rotor 202 between two bearings 301, 302 for longer service life and/or
lighter
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bearings. The use of ID and OD bearings also reduces the mechanical stress on
the stator
102 and rotor 202 to allow a thinner cross section and lighter weight, for
example as is
possible with the high pole count of embodiments of the device.
[0049] As shown in Figures 3 and 4, the rotor 202 includes a rotor plate 203
(Fig. 3) and
the stator 102 includes a stator plate 103 (Fig. 4). The stator plate 103, as
shown in Figure
4, and the rotor plate 203, as shown in Figure 3, can be made of ductile iron.
The permanent
magnets 204 can be Neodymium - N52H. Many other materials can be used for the
various
components. These materials are given by way of example.
[0050] The rotor 202 is housed in the lower arm 200 and attached such as with
bolts and/or
adhesive and/or thermal fit or by being formed integrally with the arm. As
shown in Fig.
2, the rotor 202 is connected to the lower arm 200 using a press fit with a
ring 201A. The
axial magnetic attraction between the stator 102 and rotor 202 which results
from the
permanent magnet flux in the rotor 202 provides axial preload on the bearings
301 and
302. It has been shown by analysis and experimentation that with high strength
magnets
such as but not limited to neodymium N52 magnets, that this axial force is
adequate to
keep the bearings 301, 302 preloaded in the stator 102 and rotor 202 and to
provide
adequate axial force to allow the lower arm 200 to support useful loads in all
directions.
This load may be a combination of the arm weight and acceleration forces and
payload in
any direction.
[0051] The use of the magnetic forces to provide the bearing seating force and
axial
preload on the bearings allows for the use of thrust load and/or angular
contact bearings
which can be preloaded by the magnetic attraction of the stator and rotor to
remove bearing
play in the axial direction. By using a combination of bearings that are
radially and axially
locating, it is possible to preload the bearings with magnetic force, in
radial and axial
directions and to eliminate the need for additional mechanical retention of
the bearing races
to prevent movement of the races in the opposite direction of the magnetic
force. This
preload can significantly reduce bearing play and increases bearing rigidity
such that the
assembly becomes very precise in its movement. This may have advantages for
precision
applications such as robotics. It also may have the advantage of reducing the
inconsistent
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cogging effect that could result from radial displacement of the rotor. This
may be
especially important when the device has a high number of very small cogging
steps such
as with embodiments of the device.
[0052] A non-limiting example of axially preloaded races with no need for
mechanical
retention of both races on both bearings is shown in Figures 5 to 8. The
stator includes a
stator plate 103. The stator plate 103 includes an inner bearing race 111 that
defines an
inner bearing groove and an outer bearing race 112 that defines an outer
bearing groove.
The rotor includes a rotor plate 203. The rotor plate 203 includes an inner
bearing race 211
that defines an inner bearing groove and an outer bearing race 212 that
defines an outer
bearing groove. The rotor plate 203 may be connected to a rotor housing 201
using a press
fit between cooperating pieces 231 and 232. Similarly, the stator plate 103
may be
connected to a stator housing 101 using a press fit between cooperating pieces
131 and
132. An outer bearing element 322 (in this non-limiting example, a cross
roller bearing) is
sandwiched between the two outer bearing grooves 112, 212 such that the axial
magnetic
attraction between the stator 102 and rotor 202 eliminates axial and radial
play in the
bearing 301. An inner bearing element 321 (Fig. 6) is sandwiched between the
two inner
bearing grooves 111, 211. The bearing 301 is, in this non-limiting exemplary
embodiment,
a cross roller bearing with axial and radial locating stiffness. As a result,
the axial
preloading of the rotor and stator provided by the magnets 204 in the rotor
202 results in a
precise relative location of the stator 102 and rotor 202 in the axial and
radial directions.
This precise location is accomplished without the need for mechanical or
adhesive bearing
race retention in the opposite axial direction of the magnetic attraction
force between the
stator and rotor.
100531 Referring to Fig. 6, the axial flux motor 110 may have the design
shown. An outer
bearing 302 and an inner bearing 301 allow relative rotation of the stator and
rotor and
provide precise relative axial location of the stator and rotor to maintain
the desired airgap
between the stator posts 105 and the rotor posts that hold magnets 204 and
that provide a
flux path for the magnetic fields provided by the magnets. The rotor may have
flux
restriction holes 206 and magnets 204. The use of a bearing inside the ID of
the airgap and
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a second bearing outside the OD of the airgap distributes the attractive
forces between the
stator and rotor between two bearings for longer service life and/or lighter
bearings. The
use of ID and OD bearings can reduce the mechanical stress on the stator and
rotor to allow
a thinner cross section and lighter weight, for example as is possible with
the high pole
count of embodiments of the device.
[0054] The outer bearing 302 can also be a bushing, with for example one of
the rotor or
that stator incorporating bronze or another bushing material, with the other
of the rotor or
the stator being steel or other suitable material. With bronze, some of the
metal of the
material may migrate over to the other bushing part and lubricates it. Other
materials such
as nickel or copper may also be used instead of bronze. The bushing surface
could also be
another suitable material, such as Teflon Tm or other low friction material.
[0055] In the non-limiting exemplary embodiment shown in Figure 3 and Figure
4, there
are 96 stator posts (corresponding to 96 slots ) and 92 rotor posts with three
phase wiring
and each phase on the stator being divided into 4 equally array sections of
eight posts each.
The number of rotor posts in this example is 92 resulting in four equally
arrayed angular
positions where the rotor and stator posts are aligned. This, in-turn, results
in a peak axial
attraction force between the stator and rotor in four positions.
[0056] Note that many other combinations of stator post numbers and rotor post
numbers
may be used. Other numbers of phases may also be used. The examples here have
been
found to provide beneficial performance but do not limit the various
construction principles
to these exemplary geometries. For example features of embodiments of the
device such
as, but not limited to, the magnetically preloaded bearings or the wiring
constructions can
be used with rotors and stators with much lower or much higher numbers of
poles.
[0057] It has been shown by simulation and experimentation that the total
axial preload
between the stator and rotor, for embodiments of the device, remains
relatively constant
such as within 10% in a multiphase wiring configuration such as, but not
limited to, a three
phase configuration, regardless of the current supplied to the windings and
the torque
developed by the motor. This is because the electromagnetic forces are
reasonably equally
in repelling and attraction. But although the total axial force on the stator
and rotor remains
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reasonably constant, the axial attraction force on an individual post on the
stator or rotor
will vary quite a bit more (such as 14% or more). For this reason, in some
embodiments, it
is beneficial to distribute the number of phase sections into more than two
sections per
phase so the peak axial load from the permanent magnets occurs at more than
one angular
position (for example, at four equally arrayed angular positions). This can be
beneficial to
provide a more consistent axial preload on the bearings around the
circumference of
especially the OD bearing so any cantilevered external loads that would pull
the stator from
the rotor (such as a cantilevered load on a SCARA arm that is pulling the
stator and rotor
apart primarily at one angular position) are opposed by one or more peak axial
force areas
at all times, regardless of angular position of the arm. The greater the
number of sections
per phase, the greater the manufacturing complexity, in some respects, so four
peak axial
force positions (as a result of four sections per phase) is considered a good
balance of
manufacturability and peak axial force consistency. Four peak axial force
positions can be
accomplished with many different numbers of stator and rotor posts with the
important
characteristics being that there is a four post difference between the number
of posts on the
rotor and the number of posts on the stator.
[0058] Furthermore, it is beneficial for embodiments of the device that use
embodiments
of the wiring configuration shown that the number of posts on the stator be a
multiple of
three sections such as 3, 6, 9, 12, 14, 16 etc with each section having an
even number of
posts such as 2, 4, 6, 8, 10, 12 etc on the stator.
[0059] Another consideration when deciding how many peak axial force positions
to
choose in the design of an embodiment of the device, is the number of cogging
steps that
will result. A high number of cogging steps is beneficial to reduce cogging
(because a
higher number of steps generally results in a lower force variation between
the maximum
and minimum torque of each cogging step) so a two post difference
(corresponding with
two sections per phase) between the stator and rotor would seem to be
preferable to reduce
cogging because, in a non-limiting exemplary embodiment of 96 stator posts and
94 rotor
posts, the number of cogging steps is 4512, which is a very high, resulting in
a theoretical
cogging torque that is very low. However, a two magnet difference between
stator and
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rotor results in only two peak axial attraction force position at any given
time resulting in
a less stable support of a cantilevered load on the output of the actuator
such as in a SCARA
arm configuration when lifting a payload. For this reason, a rotor/stator post
difference of
four is considered to be a good choice in terms of payload lifting stability
even though it
has a lower number of cogging steps and theoretically higher cogging forces. A
96 stator-
post to 92 rotor-post configuration results in only 2208 cogging steps which
would be
expected to result in about two times greater cogging force variation. A post
difference of
four, would, therefore, not seem to be beneficial in terms of cogging
reduction because the
cogging steps would be fewer and, as a result, larger in magnitude. However,
there can be
another benefit of fewer cogging steps (which results from a larger post
number difference
between the stator and the rotor - such as, for example, a four post
difference as shown in
Figure 3 and Figure 4 of four, as opposed to a one or two post difference).
This advantage
is related to a correlation between the size of the cogging steps and the
required accuracy
of the stator and rotor axis alignment during manufacturing/assembly and in
operation
under various loads. Specifically, if the cogging steps are smaller (measured
circumferentially at the average airgap diameter) than the radial displacement
of the rotor
axis relative to the stator axis (due to lack of manufacturing accuracy) the
stator and rotor
will not be aligned sufficiently to achieve consistent cogging steps. This
will result in
inconsistent cogging forces during rotation. Any radial displacement of the
rotor relative
to the stator will have a misaligning effect, in the same radial direction, on
posts that are
diametrically opposed, resulting in less than ideal cogging cancellation. Some
combinations of rotor/stator axis misalignment and relative angular position
of the stator
with very high cogging step embodiments (such as with a two post difference
between
stator and rotor) may even result in greater cogging force variation in some
conditions than
if a larger rotor/stator post difference is used (assuming similar radial
misalignment in each
exemplary case).
[0060] The effect of bearing stiffness in the radial direction can be an
important
consideration because the cogging forces or the payload forces will, at times,
displace the
rotor more, radially, relative to the stator, with bearings which are less
radially stiff. If this
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radial displacement is a significant percentage of a cogging step size
(cogging step size
measured tangentially at the average airgap diameter and at 90 degrees to the
direction of
radial displacement) then the cogging steps and cogging force will be
inconsistent and
cogging force could possibly, in some cases and angular positions, be larger
in magnitude
("magnitude" here, referring to the difference between max and min torque
during a
cogging step) than if the stator/rotor post difference is smaller and the
number of cogging
steps is higher (resulting in theoretically lower cogging torque if stator and
rotor ). It is
believed possible that very high cogging step numbers on a ¨10" OD actuator
will result
in inconsistent cogging if there is a radial displacement of the rotor
relative to the stator (as
a result of manufacturing inaccuracy or radial displacement due to loading of
the actuator
in service) of ¨.001" to .002". This is considered to be high tolerance for
high volume
manufacturing of stator, rotor and bearings. Achieving these high tolerances
during
manufacturing is time consuming and expensive, so it has been determined that
more
consistent cogging torque (and possibly even lower peak cogging torque) can be
achieved
if more than a one or two post difference is used. A four (4) post difference
between the
rotor and stator has the advantage of providing at least two peak axial
attraction force
positions on the load side of the actuator in a robotics application (such as
when supporting
a cantilevered load) at all times. The allowable radial displacement of the
rotor relative to
the stator can be higher because the cogging steps are larger. This is
expected to allow for
consistent cogging torque to be achieved with lower manufacturing tolerances
and bearing
stiffness than if a higher number of cogging steps is used. Where there are N
posts on a
stator and M poles on a rotor, the number of each of N and M may be selected
so that N
and M have the property that the greatest common divisor of N and M is four or
more.
[00611 It is common with many typical three phase motors to have wires from
two or three
phases in a single slot. Embodiments of the present device use a wiring
configuration where
two or more adjacent slots in a row contain conductors from only one phase.
Many different
winding methods may be used with this device but the advantages of a winding
configuration 104 as shown in Figs. 4 and 5 includes the ability to use
axially aligned
(circumferentially layered in each slot) non-overlapping flat wire
(overlapping the wire ¨
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as is typically done in three phase distributed winding machines, is
problematic with flat
wire). To take advantage of the simplicity of assembly of this winding
configuration and
method, it can be beneficial to have as few sections per phase as possible
(such as one
section per phase EG: 32 slots per phase for a 96 slot stator, or two sections
per phase EG:
16 slots per phase for a 96 slot stator). The number of rotor posts for this
winding
configuration is preferably equal to the number of stator slots plus or minus
the number of
sections per phase EG 94 or 98 rotor posts for a 96 stator slots having two
equally arrayed
sections per phase.
[0062] Figure 5 to Figure 8 shows the exemplary actuator with a safety ring
121 that
attached to the stator housing 101 and Figure 9 to Figure 12 shows an
alternative exemplary
actuator with the safety ring 121 attached to stator plate 103.
[0063] The safety ring 121 is installed on the stator 102 to keep the stator
and rotor from
separating in the case of a force being applied to the end of an arm attached
to the rotor,
along the rotational axis of the actuator, which is greater than the axial
attracting force from
the PM magnetic attraction across the airgap. A section of the actuator in
Figure 8 shows
that the safety ring 121 is located at the inner diameter of the stator.
Section view in Figure
8 shows that a lip (first shoulder) 122A of the safety ring overlaps the lip
(second shoulder
122B) of the arm housing. Between the lip 122A and the arm housing 200, a thin
plain
bearing ring 124 is in place to provide low resistance gliding contact in
event of rotor and
stator separation. The first shoulder 122A protrudes in a first radial
direction, the second
shoulder protrudes 122B in a second radial direction opposed to the first
radial direction,
and the first shoulder 122A is configured to cooperate with the second
shoulder 122B to
prevent separation of the rotor and the stator beyond a pre-determined
distance. The safety
ring 121 is attached to the stator housing 101 using a press fit between
cooperating pieces
123A and 123B.
[0064] In both exemplary actuators, as shown in Figure 8 and Figure 12, an
overlapping
feature forming a first shoulder 122A, which has a larger OD than the ID of
the rotor
housing, in this example, is located around the ID of the stator and rotor.
The safety ring
and first shoulder 122A do not need to contact the rotor during normal
operation, and serve
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to prevent complete separation of the rotor and stator in the axial direction
if the separating
load on the rotor 202 and stator 102 exceeds the axial preload on the bearings
provided by
the permanent magnets. As shown in Fig. 8, a counter bearing or bushing 124 is
attached
to the stator or rotor or other member, after assembly of the stator and
rotor. In the case
where axial overload causes a separating force and displacement on the
bearings, there will
be contact between the first shoulder 122A and the bushing 124 and the rotor,
so the
material combination of the first shoulder 122A and the bushing 124 and the
rotor is
preferably suitable for sliding contact.
[0065] A plain bushing material 124 can also be used between these two
surfaces as shown
in Figure 18. In Figure 19, a thin section thrust bearing 124 is used to allow
rotation without
damage if the magnetic preload is exceeded during actuator rotation. The first
shoulder can
also be used on the OD of the actuator with similar effects. If a rolling
element bearing is
used as a counter bearing, and if it is desired to have a small amount of
separation of the
stator and rotor in case of an emergency to reduce the force of the robot arm
on an
unintended object, it may be desirable to use a preload spring to keep the
counter bearing
lightly preloaded in order to prevent the bearing balls from spinning. A wave
washer could,
as a non-limiting example, be used for this purpose.
[0066] As shown in Fig. 11, the rotor housing 201 and the rotor plate 203
(Fig. 10) are
connected by a press fit using cooperating pieces 231B and 232B. The stator
housing 101
and stator plate 103 (Fig. 10) are connected by a press fit using cooperating
pieces 131B
and 132B.
[0067] A monolithic material from post to post on the stator and/or rotor can
used to
provide a housing structure. The rotor and/or stator have the structural
rigidity to eliminate
the need for an additional housing on one or both members. Integrating the
stator and rotor
as a homogenous plate may reduce weight, as well as manufacturing cost and
complexity.
An integrated bearing race that is formed as part of each homogenous plate may
allow the
structural load path from the stator posts to the bearing race in contact with
the rolling
elements to be formed from a single piece of magnetic metal such as shown in
Figures 13
to 15. ID and OD bearings are used to reduce rotor and stator material stress
with axially
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thin components and to maintain a small airgap. An uninterrupted soft magnetic
homogeneous material may be used such as, but not limited to iron or steel
alloy between
two or more of: a stator or rotor post and a bearing, a stator or rotor post
and an adjacent
post, a stator or rotor post and an OD bearing or bearing seat, a stator or
rotor post and an
ID bearing or bearing seat, and a stator or rotor post and a structural member
in the load
path between the post and a bearing.
[0068] For example, the homogenous material for the stator and/or rotor could
include
ductile iron or other type of iron construction. The homogenous material for
the stator
and/or rotor could also include from one of iron, ductile iron and steel alloy
and may also
include an electrical conductivity inhibitor, such as silicone.
[0069] Referring to Figures 13 to 15, the stator plate 103 has a bearing
groove 111B at the
inner diameter and a bearing groove 112B at the outer diameter. The stator may
be formed
as a homogenous plate having both the inner bearing groove 111B and the outer
bearing
groove 112B as homogenous extensions of the homogenous plate. Referring to
Figure 14,
the rotor plate 203 also has a groove 211B at the inner diameter and a groove
212B at the
outer diameter. The rotor may be formed from a homogenous plate having both
the inner
bearing groove 211B and the outer bearing groove 212B as homogenous extensions
of the
homogenous plate.
[0070] As shown in
Figure 15, these grooves are for the steel balls 304 and the steel
rollers 303. Materials of the rotor plate and the stator plate may be of many
materials such
as but not limited to a high strength metallic material that has soft magnetic
properties to
provide electromagnetic functionality as well as high enough structural
strength to provide
the strength to maintain a small and consistent airgap between the stator and
rotor posts,
and high enough mechanical hardness to provide bearing race functionality.
Ductile iron
has been found to posses these and other qualities for certain applications
and especially
when combined with the claimed range. The bearing grooves 111B, 112B, 211B,
and 212B
may also be hardened for increased load capacity and service life. The
combined cost of
purchasing the steel balls and rollers and machining and hardening the grooves
is expected
to be lower than the cost of purchasing separate modular bearings with races
and installing
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them into the actuator. In mass production, this integrated bearing design may
have
advantages over the use of pre-manufactured bearing including lower cost and
potentially
higher precision because there is a reduced stack-up of tolerances that
results from a
reduced number of components.
[0071] Integrating the bearing races into the stator 102 and/or rotor 202 as
shown, for
example, with bearing races 111B, 211B, 112B, and 212B in Figure 15 is made
practical
by the use of a solid stator and/or rotor material such as but not limited to
steel or cast iron.
Ductile iron, such as but not limited to 60-45-15 or 100-70-03 can be
hardened, if necessary
such as by nitriding or other method, to provide adequate bearing race
hardness for higher
load configurations of the present device. For lower load configurations or
lower service
life configurations, it is believed to be possible to use unhardened ductile
iron in some
embodiments. For high service life it is possible to use two or more rows of
bearings on
the ID and/or OD bearings to reduce the hertzian stress between the balls or
rollers and
races, thereby allowing a softer bearing race than is used for typical bearing
races (which
are typically made of hardened steel). Ductile iron or other cast iron
products are not
usually used for bearing races, but iron is used in railway car wheels and
rails so it is
expected that this integrated bearing can be configured for adequate service
life for robotics
and other motion control applications if made of ductile iron or other
suitable materials
preferably with high magnetic saturation density so the bearing races on the
stator can be
of the same monolithic material as the stator posts (on the stator) and the
rotor bearing
races can be of the same monolithic material as the rotor posts (on the
rotor). The
advantages of an integrated bearing race may include lower cost, and the
potential for
increased precision due to the elimination of tolerance stack up of the
bearing races and
bearing race seats in the rotor and/or stator. The use of an integrated
bearing race can also
reduce the volume and mass of the stator and rotor because the bearing race
becomes an
integral part of the load bearing structure, thus eliminating the need for
additional material
to support separate component bearing races.
[0072[ The use of ductile iron for the stator and/or rotor allows a
combination of
characteristics that may be uniquely suited to the unusual requirements of
embodiments of
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the device. Some of the features of ductile iron that may be beneficial in
some embodiments
include poor electrical conductivity due to the high carbon content which
results in reduced
eddy current losses, excellent machinability for low cost manufacturing,
excellent
castibility for net or near-net shape casting of stator and/or rotor, high
fatigue strength for
long service life, self lubricating properties which may allow an integrated
bearing to
operate with minimal or no additional lubricant, excellent wear properties
between certain
seal materials in the dry condition to provide bearing and actuator sealing
with no need for
lubricant in some applications, and good damping qualities to reduce noise and
vibration
from cogging and other high frequency effects
[0073] As mentioned above, embodiments of the device include a set of bearing
elements
at or near the inner diameter (ID) and a set of bearing elements at or near
the outer diameter
(OD). This combination of bearings provide axial and radial support between
the rotor and
stator when combined with the claimed geometry range may allow the rotor and
stator to
be light weight. The ID and OD bearings also maintain a fixed air gap
distance.
[0074] It has been shown that, even though it is detrimental to torque, due to
the drag from
the bearing on the OD of the rotor, placing a bearing set on the OD of the
rotor in an axial
flux machine enables a more precisely controlled, and therefore smaller air
gap distance
between the rotor and stator with the benefit of generating more torque with
the device.
Air gap distance between the rotor and stator can be limited by machining
tolerance and
deflection of the rotor during operation due to permanent magnet (PM)
attraction. The rotor
in an axial machine will deflect due to magnetic flux in the air gap, so the
air gap needs to
be larger than the operational deflection of the rotor to avoid contact
between the stator
and rotor. Comparing the deflection between an actuator with only an ID
bearing with an
actuator with ID and OD bearings, the rotor and stator in an ID-only actuator
deflect
significantly more than the rotor in the ID/OD bearing actuator. The reduction
in deflection
in the ID/OD actuator may allow a smaller air gap distance to be maintained
which result
in greater torque for a given input power. It has been shown by analysis and
experimentation that the torque gained by air gap distance reduction may be
larger than the
drag induced by the OD bearing in some embodiments. It has also been shown
that the
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increase in torque-to-weight that results from the use of an OD bearing, due
to the reduction
of structural material needed to maintain the airgap, may be more significant
than the
weight of the additional bearing and material needed to support the additional
bearing.
[0075] In a non-limiting exemplary embodiment of the present device, the outer
diameter
of the stator is 200mm and the axial air gap is approximately .010".
[00761 A non-limiting exemplary embodiment of the device has one stator and
one rotor
as shown in Figures 7 to 9. The single stator / single rotor setup enables the
rotor to preload
the ID and OD bearings by constantly attracting the stator in the axial
direction. As
illustrated in Figure 16, permanent magnets 204 generate magnetic flux
represented by the
arrow 401. Meanwhile, an adjacent magnet also generates the same polarity
magnetic flux
402 into the pole 205. Both flux 401 and 402 travel through the rotor pole
205, pass through
the airgap 400, into stator post 105, and generate magnetic attractive forces
403 on both
the stator 102 and the rotor 202. The magnetic forces 403 are so strong that
they are able
to hold the stator and the rotor together during passive and active operation
under usable
operating conditions for many applications. The posts are connected to a back
iron 106.
[0077] Figures 17 to 19 shows an example of the operation of the safety ring
121. When
there is a dislocating force 404 exerted on the rotor and the force is higher
than the magnetic
attractive forces 403 (Fig. 16), the rotor including the rotor plate 203 will
start separating
from the stator including the stator plate 103. When the rotor begins to
detach from the
stator, the lip of the safety ring will contact the arm housing 201 of the
rotor and keep the
arm assembly 200 from separating. During normal operation, the bearing ring
124 (Fig.
18) will be free spinning in the gap between the lip 122A and arm housing 201
and does
not create drag and friction. When the coils are engaged or powered, the coils
generate
attracting and repelling forces which are very similar resulting in primarily
tangential
forces along the rotational plane. Any axial repelling force under power is,
therefore, very
small relative to the permanent magnet attractive forces, so the permanent
magnet
attractive forces are available at all times to prevent separation of the
stator and rotor and
to maintain adequate preload on the bearings under predetermined maximum load
conditions. In some embodiment, the bearing 124 may form a shoulder by being
or having
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some part of the bearing secured to or integral with the rotor plate 203.
[0078] This design provides room for the rotor joint to provide a limited
break-away effect
for a small displacement before the safety ring contacts. This break-away
effect would be
beneficial, for example, if a robot arm makes unwanted contact with a human,
pinning
them between the arm and an immovable object. In this case, the arm may have a
very
short stopping time, but there may still be a small amount of movement before
the actuator
comes to a full stop. The partial separation of the rotor from the stator of
one or more
actuators in the arm, before the safety ring comes into contact, can be used
to provide a
maximum axial load on one or more actuators in the arm which are loaded from
the impact,
in such a way as to cause the rotor and stator of these actuators to partially
separate. With
a small amount of separation and very fast acting and fast decelerating
actuators, such as
embodiments of the present device, this partial separation is believed to
provide a level of
increased safety by reducing the impact or pinning force of a robot arm.
[0079] For a 10" OD actuator of the present device, attractive forces have
been
demonstrated in the range of ¨2000 lbs. This force is high enough to make
assembly and
disassembly of the device extremely difficult for small devices, and
prohibitively difficult
and unsafe for larger versions of the device.
[0080] Assembly and disassembly safety concerns may be reduced with
embodiments of
the device, and the cost and complexity of assembly fixtures may be reduced.
[0081] The rotor plate shown in Figure 3 has no back iron immediately axially
outward
from the permanent magnets (corresponding to radially outward from the
permanent
magnets in a radial flux embodiment of the device etc.). As a result, magnet
slots 208 are
open on the back face of the rotor so magnets can be assembled into the slots
after the
stator and rotor are assembled. Figure 20 shows that the magnets 204 can be
accessed from
the back of the rotor which allows each of those magnets to be removed or
installed
individually without removing the rotor from the stator.
[0082] The magnets 204 may be installed into the slots as follows. Align the
magnet to the
slot with the same polarity magnetic flux contacting the rotor post as the
adjacent magnet
contacting the same post. Every second magnet will be in the same
circumferential polarity
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alignment. Every first magnet will be the opposite of every second magnet so
the posts are
alternating polarity. Slide the magnet into the slot until it is secured
against the tabs (if
parallel sided) or, if tapered magnets are used, until the tapered magnet
seats into the
tapered slot. Repeat the above steps until all the magnets are installed.
Apply bonding agent
(eg, wax, epoxy, glue) to fill the clearance gap. This step may not be
necessary in all cases,
such as with a precision tapered magnet in a precision tapered slot.
[0083] To remove the rotor and access the stator coils and ball bearings, the
rotor can be
easily demagnetized by removing the magnets individually.
[0084] As shown in Figure 16, each of the permanent magnets 204 in the rotor
generates
the same polarity flux as its immediately adjacent permanent magnet which
means every
magnet will be repelling the adjacent magnets on both sides of it. This would
cause the
magnets to repel each other, except it has been shown that certain geometries
are able to
prevent these repelling forces from causing the magnets to dislodge themselves
form the
slots. The smaller the airgap, for example, the stronger the force, in many
cases, which will
cause the magnets to lodge themselves into, instead of out of, the slots. The
use of tapered
magnets is also beneficial in this sense, because a tapered magnet, with the
large dimension
of the taper toward the back face of the rotor, will generally be more apt to
pull itself axially
toward the rotor posts and therefore toward the airgap.
[0085] As shown in Figure 21, a physical stop is used to stop the magnet from
moving into
the airgap. In this embodiment, the stops are tabs 210 on each side of the
slot generate
attractive forces as the magnet slides into the slot. Their combined force
pull the magnet
into the slot. Since the repelling forces partially or completely cancelled
out, the combined
force from the poles and tabs becomes the resultant force acting on the
magnet. The
magnets sit on the tabs and the magnetic attractive forces secure the magnets
to the poles.
When configured correctly, as described in an earlier disclosure, the net
force on the
magnets can be tailored to use the magnetic forces to magnetically retain the
magnets in
the slots. Adhesive or mechanical mechanism is not required in this case
except to prevent
side-to-side movement of a magnet in a slot.
[0086] A non-limiting exemplary embodiment of the actuator is shown in Figure
22A with
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flux restriction holes 206 placed between magnet slots 208, and along the
outside and
inside radius of the magnet slots 208 on the rotor to reduce flux leakage
between the
opposite polarity faces of a magnet and between adjacent rotor poles. Magnetic
simulation
was done to verify if those holes reduce flux leakage and it has been shown
that the flux
leakage between rotor poles can be substantially reduced while still
maintaining the
necessary structural strength and stiffness to achieve a small and consistent
airgap.
[0087] The flux restriction holes can, alternatively, be located between every
second post
on the OD and between every second post on the ID as shown in Figure 22B. As
shown in
Fig. 22B, the inner and outer flux restriction holes are staggered so that
each post is
adjacent to only one of the inner or outer flux restriction holes. This
provides an
unrestricted flux linkage between only the N posts around the OD and only the
S posts
around the ID as well as increased structural integrity for every first post
around the OD
and every second post around the ID. These holes can be thru-holes or blind
holes, as long
as they provide the necessary structural strength and stiffness as well as the
desired flux
path reluctance.
[0088] Figure 23 shows the flux path from the magnetic simulations without
flux
restriction holes and Figure 24 shows the flux path from the magnetic
simulations with
flux restriction holes. From the figures, it is shown that flux restriction
holes reduce flux
leakage between adjacent rotor poles. For example, when flux restriction holes
are used,
the flux density increased at the air gap surfaces of the rotor poles and more
flux is directed
to pass through the stator. As a result, electromagnetic force increases when
the coils are
engaged and torque generated by the stator and rotor increases.
[0089] MagNet simulations on the rotor plate with and without flux restriction
holes also
led to the same conclusions. More flux is directed from the posts into the
airgap.
[0090] In an embodiment shown in Figure 25, the stator is formed of unitary
material
(instead of a common laminated structure) and comprises a stator post 105, a
stator back
iron 106, inner bearing race 111B, and outer bearing race 112B. Looking at the
cross
section of the stator in Figure 25, there is no interruption along the stator
material path 500
between the tip of a stator post and the inner bearing race, the tip of said
stator post and the
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outer bearing race.
[0091] The stator plate, which is held inside the integrated housing, is
machined from a
solid piece of material. A typical stator is often made using laminated steel
layers. In an
exemplary embodiment, as shown in Figure 25, material path between the inner
bearing
race 111B, stator post 105 and outer bearing race 112B is uninterrupted and
comprises a
homogeneous material such as, but not limited to, ductile iron or magnetic
steel such as
M19. The stator core can be cast or machined from a solid piece of steel. The
benefit of
this construction may include lower cost and complexity due to a single part
rather than an
assembly of many small laminated parts, and much higher strength, stiffness
and creep
resistance because there is no adhesives in the load path as there would be in
typical
laminated stator constructions. This allows the use of much thinner stator
cross sections
which is beneficial for reduced weight.
[0092] The uninterrupted radial flux path corresponds to an uninterrupted
axial path in a
radial flux device. The flux path 500 in Fig. 25 terminates at the ID and OD
at an integrated
bearing race. The uninterrupted path may also terminate at a bearing race seat
if a separate
bearing race is used. It may also terminate at an intermediate component or
layer between
the stator and the bearing race seat. The uninterrupted radial path need not
extend purely
radially, but connects inner and outer diameters of the carrier (here, a
stator). That is, the
path follows a three dimensional path from ID to OD that is not interrupted.
Thus, holes
may be drilled in the cross section shown in Fig. 25, but there would still be
an
uninterrupted monolithic material path from ID to OD.
[0093] Referring to Fig. 26, an exploded view of exemplary rotor and stator is
shown that
is connected to a pair of robot arms using bolts. A first arm 700 is connected
to a rotor
housing 702 using bolts 718. The rotor housing 702 is connected to a rotor 708
using bolts
720. A first bearing element 706 connects between the rotor 708 and a stator
712 and is
connected by a press fit ring 704. A second bearing element 710 also connects
between the
rotor 708 and the stator 712 using bolts 722. The stator 712 is connected to a
stator housing
714 using bolts 724. A second arm 716 is connected to the stator housing 714
using bolts
726.
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[0094] Referring to Figs. 27 to 29, a rotor 606 is made from a ferrous
material, such as
Ductile Iron, and holds an equi-spaced array of magnets 605 that are polarised
in a
circumferential direction. The polarity of the magnets 605 is alternated in
order to generate
alternating north and south poles in the radial webs of the rotor 606. The
stator 609 is made
from a ferrous material, such as Ductile Iron, and includes an equi-spaced
array of axial
posts around which a set of stator windings 610 are wrapped. Applying
commutated power
to the stator windings 610 polarises the posts of the stator 609 such that
circumferential
attraction and repulsion forces are generated between the posts of the stator
609 and the
radial webs of the rotor 606, thereby generating torque. The stator windings
610 are
encapsulated by the stator potting compound 611, which serves to prevent
movement of
the wires and helps to transfer heat from the wires to the stator 609. As
shown in Fig. 28,
a stator cap 612 may be placed over the stator 609 and hold the wires 610 in
place.
[0095] The magnets 605 also cause attraction between the stator 609 and the
rotor 606.
The bearings 603 and 604 counteract the attraction force between the stator
609 and the
rotor 606 via the housings 601, 602, 607 & 608 and act to accurately control
the gap
between them. The axial attraction force between the stator 609 and the rotor
606 is
adequate, in most applications, to prevent the upper housing 601 from
separating from the
lower housing 602, thereby eliminating the need for additional retention
between them.
Diametral fits at the interfaces between the housings 601, 602, 607 & 608 and
the rotor
606 and the stator 609 carry radial loads between the two assemblies via the
inner 4-point
contact bearing 604. External moments applied to the assembly are carried
primarily
through the outer thrust bearing 603.
[0096] The flow of current through the stator windings 610 tends to increase
the
temperature of the stator 609 relative to the other components. Conduction of
the generated
heat to the adjacent housings helps to reduce the increase to its temperature.
The example
shown includes light alloy housings which have a higher coefficient of thermal
expansion
than the stator 609. To maintain an interference fit at the interface between
the outer
diameter of the stator 609 and the inner diameter of the lower housing 602 as
the
temperature increases the primary diametral location occurs at the inner
diameter of the
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locating hook of the stator 609.
[0097] In Fig. 29, removeable caps 614 and 616 sit in the arms which allow the
stator and
rotor to be inserted, and for the magnets to be inserted last.
[0098] It is also possible to provide force to retain the magnetics in the
rotor slots using a
combination of mechanical and magnetic force. Tapered magnets can provide a
structure
in which a significant percentage of magnetic flux goes through the airgap
while retaining
the magnets in the rotor slots.
[0099] Magnets which taper tangentially such that they are thinner toward the
air gap, can
provide high performance in a concentrated flux rotor configuration. Referring
to Figs. 30
to 35, there is shown a rotor 3300 in an axial flux configuration with magnets
3302 having
tapered ends 3316 and rotor posts 3304 with tapered ends 3318. The magnets and
rotor
posts taper in opposite directions to form an interlocking arrangement.
Permanent magnets
taper in the direction of the stator 3330 while rotor posts 3304 taper away
from the stator.
In this embodiment two substantially mirrored rotors 3300 can be assembled
between a
pair of stators, with tapered posts of each rotor meeting back to back and
tapered magnets
of each rotor meeting back to back. Tapering the magnets 3302 in this way,
allows for
greater rotor post width at the air gap. It also allows for greater magnet
width at the wide
end of the magnet taper to provide more flux to the rotor post 3304 away from
the air gap,
where if the sides were parallel the posts 3304 would tend to be less
saturated. In this way,
the active permanent magnet 3302 and soft magnetic materials are used more
effectively
to provide more flux at the airgap. The two rotors parts can be secured
together for example
by an adhesive, but in some preferred variations a mechanical feature such as
bolts (not
shown) or a securing ring (not shown) may be used.
[01001 The interlocking arrangement of tapered posts 3304 and magnets 3302
operate as
stops that prevent the permanent magnets from dislodging, which reduces the
need for
magnetic force to retain the magnets in the rotor, and therefore reduces the
need for
magnetic flux to leak through the end iron 3314.
[0101[ In some embodiments an array of flux path restrictions 3328 can be
formed in the
end iron 3314, for example, as holes in the end iron 3314 at the base of each
rotor posts
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3304 where they connect with the end iron 3314. These flux path restrictions
3328 reduce
the available flux path between rotors posts 3304 and end iron 3314.
[0102] Fig. 30 shows an axial flux configuration of a tapered slot rotor, but
the tapered slot
rotor can be equivalently constructed in a radial flux configuration. Tapered
magnets may
narrow towards or away from the opposing carrier.
[0103] A second effect of tapering the magnets in this way is to bias a high
percentage of
the flux from a permanent magnet toward the air gap. This is beneficial in at
least two
ways. A first is that the tapered permanent magnet will be drawn toward the
air gap where
they will close the airgap between the permanent and the rotor slot wall for
lower
reluctance flux linkage and where they will be mechanically prevented from
further
movement and therefore securely retained by the tapered rotor posts. Secondly,
the
narrower rotor posts at the back surface results in a greater distance from
post to post along
the center plane of the rotor. This reduces the amount of leakage through the
air from post
to post along the center plane of the rotor. By assembling two substantially
mirrored rotor
halves with tapered posts and tapered magnets back-to-back a large percentage
of the flux
from the permanent magnets can be forced to link across the air gap.
[0104] In this way, very high flux density can be achieved in the air gap
while magnetically
and mechanically retaining the magnets. A cost effective way to manufacture a
tapered
rotor post rotor is to use two symmetrical rotors 3300 back to back. This
construction does
not allow for the use of a back iron to stiffen the rotor, so a soft magnetic
end iron 3314 is
used instead. The end iron 3314 has sections that are preferably as thin as
possible to create
a high reluctance flux path between rotor posts through the end iron, and as
thick as
necessary to provide the mechanical strength and rigidity to maintain a small
and consistent
air gap.
[0105] To compensate for the loss of flux from post to adjacent post through
the end iron
connection, an embodiment uses permanent magnets 3302 that are longer than the
soft
magnetic stator posts 3332 at the air gap. This is shown in Fig. 31 where the
permanent
magnet 3302 are longer than rotor posts 3304 which would have the same or
nearly the
same length as the stator posts 3332. As shown in Fig. 32, a winding
configuration 3334
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extends around the stator post 3332. By increasing the permanent magnet depth
compared
to the stator radial length, the permanent magnets 3302 will be adequate to
saturate the end
iron 3314 while still maintaining high flux density in the rotor posts at the
airgap. As shown
in Fig. 31, there are two flux restrictors 3328 adjacent to each end of each
rotor post 3304.
The rotor posts 3304 have a larger width at the axial outer end of the rotor.
The flux
restrictors 3328 are larger adjacent to the outer end of the rotor posts and
smaller at the
inner end of the rotor posts.
[0106] The flux restriction holes described for example in the embodiments
disclosed in
Fig. 3, Fig. 14, Fig. 22A, Fig. 22B, and Figs. 34-38 are designed to meet an
acceptable
trade-off between power and structural strength. The cross-sectional area
above the
magnets provides the strength to maintain the airgap and the flux restrictors
prevent flux
from excessively extending between the magnets. The flux restrictors can be
placed with
holes adjacent to every second post, rather than adjacent to every post, which
will provide
for a stronger structure but does not have a significant impact on the flux.
The flux
restrictors could be blind or through-holes, so long as there is a cross-
sectional area
reduction in the flux path and the structural load path. In a preferred
embodiment, the flux
restrictors will lie on either end of the posts, between the array of posts
and each set of
bearings. The flux restrictors will preferably lie parallel with the length of
each post. The
flux restrictors can be designed so that there is a greater cross-sectional
area in a structural
load path than in a magnetic flux path. The flux restrictors could also be
used in a radial
flux machine in an equivalent manner as those described for the axial and
linear flux
machines described herein. An embodiment of the machines described herein with
flux
restrictors may have a solid material made for example with ductile iron which
is strong
enough to support magnetic forces, but thin enough to be lightweight. The flux
restrictors
may be placed adjacent to every post on the rotor or stator or adjacent to
every second post
on the rotor or stator. The flux restrictors will generally be placed on both
ends of each
post, or each second post. The flux restrictors may be placed adjacent to
every post on one
end of each post and adjacent to every second post on the other end of each
post. The flux
restrictors may be placed in an alternating pattern so that each post is
adjacent to only one
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flux restrictor, and for each adjacent post, the corresponding flux restrictor
is adjacent to
an opposite end of the adjacent posts. The flux restrictors may have different
sizes while
maintaining the same geometry. The cross-sectional flux path may be consistent
between
every second post, but the cross-sectional flux path may be selected so that
it alternates
between adjacent posts so that each post has a different cross-section flux
path than the
post directly adjacent to it. Where the flux restrictors are placed in an
alternating pattern
so that each second post is adjacent to flux restrictors, then the cross-
section of each post
that is adjacent to the flux restrictors may be smaller than the cross-section
of each post
that is not adjacent to the flux restrictors. In such an embodiment, every
second post will
have a larger cross-section than each of the adjacent posts that are adjacent
to the flux
restrictors. Although flux restrictors will generally be more effective to
reduce cogging
when placed on the rotor, rather than the stator, the flux restrictors can be
placed on both
rotor and stator, or only on the rotor. As shown in Fig. 31, there may be
multiple flux
restrictors adjacent to each end of the posts.
[0107] Manufacturing methods for the rotor can include casting or forming or
powdered
metal construction, additive manufacturing, machining etc. Manufacturing of
the magnets
can be done by forming or additive or subtractive manufacturing. Magnets can
also be
magnetised after insertion into slots. It may be possible with present or
future processes to
press powdered hard magnetic material into the rotor slots and then
magnetizing the PM
material after pressing, or a slurry of PM magnet material in an epoxy or
other polymer
can be used to fill the slots and then magnetized after hardening. Magnetizing
of the hard
magnetic material can be done by applying very high flux density to two or
more posts at
a time.
[0108] Back irons, side irons and end irons serve as retaining elements and
form a rigid
connection with the rotor posts. Features of one embodiment may be combined
with
features of other embodiments.
[0109] Referring to Fig. 32, there is shown a stator-rotor-stator
configuration with an end
iron 3314. The end iron 3314 and rotor posts 3304 can be formed from a single
piece of
isometric soft metallic material, with a single array of permanent magnets
3302 fitting
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between rotor posts 3304. End iron 3314 is formed at both ends of the rotors
3300. In this
embodiment, flux path restrictions 3328 can be included as shown in Fig. 33.
[0110] Fig. 33 shows an embodiment of a stator-rotor-stator configuration with
a back iron
3310, end iron 3314 and flux path restrictions 3328. In this embodiment the
two array of
permanent magnets 3302 are separated by back iron 3310. Flux path restrictions
3328 are
formed as bores at the ends of the permanent magnets 3302 to reduce the flux
leakage in
the end iron 3314.
401111 Fig. 34 shows an embodiment of a rotor-stator-rotor configuration. Two
concentrated flux rotors 3300 engage a central stator 3330. The rotors 3300
each include
end iron 3314 and flux path restriction 3328. In many applications end iron
only or back
iron only will be sufficient to provide adequate rigidity to the concentrated
flux rotor 3300.
[0112] Fig. 35 shows an embodiment of a rotor-stator-rotor configuration. The
embodiment is essentially the same as that shown in Fig. 34 with the addition
of a think
back iron 3310 on each rotor 3300.
[0113] Fig. 36 shows an embodiment of a rotor-stator-rotor configuration of a
linear flux
machine. The stator 3330 has an array of posts 3332. The rotor surrounds the
stator and is
made of one or more pieces of material, for example, a soft magnetic isotropic
material.
Receiving slots for the permanent magnets 3302 on the internal structure of
the rotor 3300
act as rotor posts 3304, rotor back iron 3310 and rotor end iron 3314. Many
constructions
of a linear motor are contemplated herein. The side section of the rotor, for
example, may
be of a different material than the upper and lower rotor portions. Fig. 37
shows an
embodiment of the rotor-stator-rotor configuration of a linear flux machine
without a back
iron 3310 on the rotor 3300 and having a number of flux restrictors 3306
adjacent to each
of the permanent magnets 3302 on either side of the slots. Fig. 38 shows a
rotor-stator-
rotor configuration with an alternating pattern of flux restrictors 3306 that
are adjacent to
every second permanent magnet.
[0114] An embodiment of an electric machine will now be described the
configuration of
which may utilize the design elements disclosed in this patent document, for
example the
inner and outer bearing configuration.
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[0115] Any of the disclosed structures may be used with an electric machine
that has
electromagnetic elements including posts and slots between the posts, where
the posts are
wound to create poles, at least on either of a stator or rotor, where the pole
density is within
a range of pole density defined by the equations specified in this patent
document and the
post height is within a range of post height defined by the equations
specified in this patent
document. These equations each define a bounded area. The bounded areas are
dependent
on the size of the electric machine, where the size is defined by the radius
of the machine.
The bounded areas together define a bounded surface in a space defined by pole
density,
post height and size of machine. This bounded region is disclosed in copending
W02017024409 published February 16, 2017, and repeated here.
[0116] Based on modelling studies and FEMM analysis, it is believed that the
following
conclusions follow: at least beyond a specific pole density and for a
specified conductor
volume or post height for a given diameter of motor: 1) an electric machine
having pole
density and conductor volume or post height as disclosed has increased heat
production
(and thus lower efficiency) for a given torque or force as compared with an
otherwise
equivalent machine having lower pole density and/or higher conductor volume
but has
corresponding effective heat dissipation; and 2) the increased pole density
and lower
conductor volume or post height also has the effect of decreasing mass as
compared with
an otherwise equivalent machine having lower pole density and/or higher
conductor
volume, with an overall increased torque to mass ratio (torque density).
[0117] An electric machine with increased torque to mass ratio is particularly
useful when
several of the electric machines are spaced along an arm, such as a robotic
arm, since
efficiency is less important relative to the need for one electric machine to
lift or accelerate
one or more other electric machines. It is believed that improved performance
of an electric
machine having pole density and conductor volume or post height as disclosed
results at
least in part from 1) a narrower slot having a shorter heat flow path from the
hottest
conductor to a post and 2) a shorter heat flow path from the top of a post to
a heat
dissipation surface.
[0118] For example, each electric machine embodiment disclosed is shown as
having a
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pole density and post height that is within the definition of pole density and
post height
that is believed to provide a benefit in terms of KR.
[0119] With a pole density in the range of 0.5 and higher, for example, and
considering
that it is not unusual for a slot to be about as wide as a tooth, tooth width
can be in the order
of 1 mm for a 25 mm wide machine. Narrower teeth can be used. An advantage of
thinner
teeth is that solid materials such as, but not limited to steel or iron or a
magnetic metal
alloy, may can be used with minimal eddy currents due to the teeth being
closer to the
thickness of normal motor laminations. A common motor lamination for this size
of motor
can be in the range of 0.015" to 0.025". The proposed pole density and tooth
geometry
(many short posts) also helps avoid eddy currents in the first carrier
(stator). For example,
for an electric machine with 144 slots, eddy current loss was found to be only
7% of the
total resistive losses in the windings at 200 rpm and 70 A/mm2. Use of solid
(non-
laminated) materials may provide advantages in strength, stiffness and
reliability.
[0120] Embodiments of the disclosed machines may use fractional windings. Some
embodiments may use distributed windings; others may use concentrated
windings.
Distributed windings are heavier due to more copper in the end turns and lower
power
(requiring a bigger motor). They also require thicker backiron because the
flux has to travel
at least three posts, rather than to the next post as with a fractional
winding. Distributed
windings produce more heat because of the longer conductors (the result of
longer distance
the end tums have to connect between).
[0121] An embodiment of an electric machine with the proposed pole density may
have
any suitable number of posts. A minimum number of posts may be 100 posts. A
high
number of posts allows fewer windings per post. In anon-limiting exemplary
embodiment,
the windings on each posts are only one layer thick (measured
circumferentially, outward
from the post). This reduces the number of airgaps and/or potting compound
gaps and/or
wire insulation layers that heat from the conductors conduct through for the
conductors to
dissipate heat conductively to the stator posts. This has benefits for heat
capacity (for
momentary high current events) and for continuous operation cooling. When
direct cooling
of the coils by means of gas or liquid coolant in direct contact with the
conductors, a low
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number of circumferential layers, and for example a single circumferential
layer of wire
on a post, combined with high pole density, results in a very high surface
area of the
conductors (relative to the volume of the conductors) exposed to the cooling
fluid. This is
beneficial for cooling the conductors and is one of many exemplary ways to
take advantage
of the low conductor volume as disclosed. A single row (or low number of rows)
of coils
per posts also reduces manufacturing complexity allowing for lower cost
production. In
another embodiment, the windings of each post are two layers thick.
[0122] For a 175 mm or more average airgap electric machine, the number of
slots may be
60 or more, or 100 or more for an axial flux electric machine, for example 108
slots in an
exemplary 175 mm diameter embodiment. In addition, for such an electric
machine, the
average radial length-to-circumferential width of the posts may be above 4:1,
such as about
8:1 but may go to 10:1 and higher. For the exemplary 108 slot embodiment, the
ratio is
about 8:1. With such a configuration, the heat dissipation is improved. A
lower aspect ratio
would be a lot of material for very little torque, so the aspect ratio helps
achieve torque
useful for high KR and robotics while at the same time taking advantage of the
heat
dissipation effects.
[0123] In some embodiments there is a reduced rigidity requirement by coating
the airgap
with a low friction surface that maintains the airgap. In an embodiment of a
linear motor a
low friction surface is applied in the airgap which maintains a 0.008" airgap.
Coatings,
such as DLC (diamond-like coating), can be deposited at 0.0025" on both the
rotor and the
stator and the gap will be maintained.
[0124] Ranges of pole pitch (or density) and conductor volume have been found
which
give a significant benefit either in terms of KR, or in terms of a weighting
function
combining torque, torque-to-weight, and Km (as described further). The amount
of benefit
in terms of the weighting function is dependent on the amount of cooling and
other factors,
but the equations define novel structures of electric machines that provide
benefits as
indicated. Equations are given which define bounded regions determined by the
ranges of
pole density and conductor volume which yield these benefits.
[0125] In an embodiment, advantages are obtained by operating within a region
of a phase
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space defined by machine size, pole density and post height. A series of
graphs shown in
Fig. 39A to Fig. 39F, show torque density (z axis) v slot density (x axis) and
post height (y
axis) for an exemplary series of linear motor section geometries, created and
analysed
using FEMM software using an automated solver generated in OCTAVETm(which is a
program for solving numerical computations). Slot density was used in this
example
because it is the same as pole density.
[0126] The following rules and assumptions were applied to all of the motors
in the series.
Each section consisted of 144 electromagnets and 146 permanent magnets. The
rotor
comprised sections of NdFeB 52 magnets and M-19 silicon steel. Every permanent
magnet
was placed tangentially to the rotor and oriented so that its magnetic field
direction was
aligned tangentially to the rotor and are opposite to its adjacent permanent
magnets. M-19
silicon steel sections were placed between permanent magnets. The stator was
made from
M-19 silicon steel. The electromagnets used concentrated winding coils in a 3-
phase
configuration. A 75% fill factor of the coils was assumed, consisting of 75%
of the slot
area. The two variables that were investigated were the post height and slot
density. The
remainder of the geometry variables were scaled according to the following
relationships:
1.25 inches constant model thickness across all simulations, Rotor permanent
magnet
width is set at 50% of permanent magnet pitch, Rotor permanent magnet height
is set at
2.3 times of permanent magnet width, Stator slot width is 50% of stator
electromagnet
pitch (equal width of posts and slots), Stator back iron height is set at 50%
of stator post
width, Airgap axial height of 0.005 inches.
[0127] The bounded region which represents the unique geometry disclosed is
modeled
for the preferred embodiment, namely the embodiment which will yield the
highest torque-
to-weight and KR. Certain design choices have been made in this embodiment
such as the
selection of grade N52 NdFeB magnets in the rotor, a rotor pole to stator post
ratio of
146:144, and a flux concentrating rotor with back iron. It is believed that
this configuration
may provide one of the highest practical torque-to-weight configurations for
sizes of
actuators in the disclosed diameters while still retaining a reasonable level
of
manufacturability and structural stability. Many other configurations are
possible such as
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different rotor types (surface permanent magnet, buried permanent magnet,
etc), different
magnet materials and grades including but not limited to ceramic, samarium
cobalt, and
high-temperature NdFeB, different rotor pole to stator post ratios, different
stator winding
configurations, different stator materials, etc. In many cases, different
design choices for
these parameters will not have as great a KR benefit as compared to the
preferred
embodiment by either resulting in reduced torque or increased weight for the
same pole
pitch and post height as the preferred embodiment. However, for the majority
of designs,
there is a benefit to KR by using the pole pitch and post height of inside the
disclosed
region over geometry outside the disclosed region when all other design
variables and
geometrical relationships are held constant. This principle holds true for
both concentrated
and distributed winding designs, for linear motors, axial flux rotary motors,
radial flux
rotary motors, trapezoidal/toroidal rotary motors, and transverse flux linear
and rotary
motors.
101281 For each of those motor section geometries, magnetic simulation and
heat
simulation were performed. For every magnetic simulation, the program yielded
values for
mass, horizontal force, and power consumption. Geometrical extrapolations of
the coil
cross sections were used to find the mass and power consumption of the end
windings in
order to more accurately predict the mass and power consumption of the entire
system. For
calculating stall torque and torque at low speed, the square root of resistive
losses is the
dominant part of the power consumption, with a multiplier based on the slot
geometry to
account for the resistive losses of the end windings. These values were used
to calculate
the mass force density (force per unit mass) and the area-normalized force
(force per unit
area of the airgap) of each simulation. For every heat simulation, the program
yielded
values for coil temperature, rotor temperature and stator temperature. A set
cooling rate
was applied to the stator inner surface using water as the coolant and a
convection
coefficient of 700 W/m2K. The temperature of the water was set at 15 C and it
had a flow
rate between 6-20 mm/s. Steady state conditions were assumed.
[0129] For constant current density simulations, a fixed current density was
applied to the
conductor and the resulting force, mass, power consumption, and maximum stator
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temperature were calculated by the program.
[0130] For constant temperature, force per area, or force density simulations,
the current
density was adjusted at each geometry point until the parameter of interest
reached the
target value, and the other parameters were recorded at that point. The target
error for
constant temperature, force per area, and force density simulations are 1
degrees, 0.002
N/mm2, and 1 N/kg respectively. This data can be directly applied to any size
of rotary
motor by multiplying the area-normalized force by the circumferential area of
the airgap
in the rotary motor, and multiplying the force by the diameter to yield the
resulting torque.
There will be some small deviations due to the radius of curvature of the
motor, and the
errors associated with approximating a curved structure with a linear one,
however our
simulations have shown the rotary simulated torque typically to be within 10%
of that
predicted by the linear model.
[0131] High torque-to-weight is of benefit in some applications, but a minimum
level of
torque may be necessary for applications such as robotics where the arm, no
matter how
light it may be as a result of high torque-to-weight actuators, must still
have enough torque
to lift and move a payload. Electric machines having a pole density and
conductor volume
within the ranges disclosed in this patent document provide high torque and
torque-to-
weight at acceptable power consumption levels.
[0132] The force per area at a constant current density 2320 is plotted in
Fig. 39A as a
function of slot pitch and post height. The same current applied to all motors
in the virtual
series results in dramatically lower force per area in the disclosed ranges
2322 (indicated
schematically by the dashed lines). The dashed lines correspond to the middle
boundary
from each size (25 mm, 50 m, 100 mm and 200 mm as discussed in relation to the
equations
below) projected onto the 3D surface. The middle boundaries correspond to the
sets of
equations A2, B2, C2 and D2. In this graph, the force per area at constant
current density
2320 is shown for a series of motors that were analyzed in FEMM using a script
in
OCTAVE to find the highest torque rotary position for a given 3 phase input
power. These
motors are identical in every way apart from the conductor volume and slot
density, which
are varied as shown.
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[0133] The highest current density possible at a given temperature 2324 is
plotted in Fig.
39B as a function of slot pitch and post height. The exponentially higher heat
dissipation
characteristic in the disclosed ranges 2322 allows much higher current density
at a given
temperature. Low conductor volume tends to reduce the actuator weight, but low
conductor
volume also tends to reduce the actuator torque. When the conductor volume and
slot
density is in the disclosed ranges, however, there is a dramatic reduction in
the heat flow
resistance from the conductors to the back of the stator or to any other
surface where
cooling can be applied, thus allowing very high current densities to be
applied to the
conductors without overheating the actuator.
[0134] In Fig. 39B, the same series of motors is used as in Fig. 39A, but
instead of constant
current density applied to each motor, the current density was varied until
the steady state
temperature of the conductors was ¨70 C. A reasonable representation of a
typical water
cooling effect was applied to the outer axial surface of the stators at a
convection
coefficient of 700 W/m2K. The temperature of the water was set at 15 C.
Ambient
temperature was set at 15 C. No air convective cooling was applied to the
rotor for
simplicity because the water cooled surface was highly dominant in terms of
cooling and
because the rotor was not producing heat of its own. Steady state conditions
were assumed.
For each point on the 3D graph, the current density of the motor was increased
from zero
until the temperature of the coils reached ¨70 deg C.
[0135] Fig 39C is the same as Fig. 39D except that it has constant current at
6 Aimm2 as
apposed to constant temperature of 70 deg C. Thus demonstrating how the heat
dissipation
benefit of short posts give unexpected benefit disclosed range, Fig. 39C was
developed
using the following weighting convention, Torque ¨ weighting of 1, Torque-to-
weight ¨
weighting of 3, Power consumption ¨ weighting of 2. Torque-to-weight was the
most
highly weighted because the weight of the arm is determined by the weight of
the actuator
and because the weight of the arm will typically be significantly higher than
the weight of
the payload. Torque was weighted at 1 to include it as an important
consideration but
recognizing that the payload may be quite a bit lower than the weight of the
arm. Power
consumption was given a moderate weighting because it is an important
consideration, but
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power consumption is known to benefit from lower arm weight, as is
accomplished by a
higher weighting on torque-to-weight, so a higher weighting on power
consumption was
deemed to be potentially counter-productive.
[0136] By applying a constant current density to the series of motors, and
combining the
results with the above weighting, the surface 2328 in Fig. 39D shows a trend
toward lower
overall performance toward and continuing through the disclosed ranges 2322 of
slot (or
pole) density and conductor volume. Fig. 39D shows a benefit in the disclosed
range when
the constant temperature current density is applied from Fig. 39B.
[0137] An industry standard metric for motor capability is the KM which is
basically
torque-to-power consumption. KM assumes sufficient cooling for a given
electrical power.
It only considers the amount of power required to produce a certain level of
torque. The
Km" surface 2330 as a function of slot pitch and post height is plotted in
Fig. 39E.
[0138] The torque to weight to power consumption shows the most unexpected and
dramatic benefit in the disclosed ranges 2322 as seen from the graph of the
K'rzr surface
2332 as a function of slot pitch and post height in Fig. 39F. High KR may not
be of great
benefit in stationary applications, but in applications such as robotics, KR
indicates that
power consumption benefits can be achieved by reducing the weight of the
entire system.
[0139] A method of producing a graph showing how 'CR varies with pole density
and post
height is as follows. Consider a motor section with geometry A having low
conductor
volume (low post height) and low pole density. The motor section with geometry
A is
simulated; a set cooling rate is applied to the stator inner surface using
water as the coolant
and a convection coefficient of 700 W/m2K. The temperature of the water is set
at 15 C
and it has a flow rate between 6-20 mm/s. Steady state conditions are assumed.
The current
passing through the conductor of geometry A is then increased until the
maximum
temperature of the conductors reaches 70 C. The torque density of geometry A
at this
point is then recorded and plotted in the graph for the corresponding values
of post height
and pole density. The process is repeated for other geometries, obtained, by
example,
through varying the post height and pole density and scaling the remaining
parameters as
described above. For instance, a geometry B may be is obtained from geometry A
by
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increasing the post height, with all other parameters scaled as described
above. A geometry
C may have the same post height as geometry A but greater pole density. A
geometry D
may have increased post height and increased pole density as compared to
geometry A.
Plotting the torque densities results in a surface in a graph.
[0140] It is found that the torque density increases as pole density increases
and post height
decreases. No such increase in torque density is shown to occur with
geometries having
either a low post height or a high pole density; the benefit in torque density
is only observed
for geometries combining these two factors. Yet, in this region, efficiency is
decreasing.
While the graph was produced based on the assumptions indicated, it is
believed that, based
on the disclosed cooling effect and reduction of flux losses of increasing
pole density and
decreasing conductor volume or post height, that the same geometry will have a
benefit at
other values of the parameters that were used in the simulations. Changes in
motor design
elements which do not affect post height or pole density are not expected to
result in a loss
of the benefits. For instance, an electric machine comprising a rotor with
tangentially
oriented permanent magnets and an analogous electric machine comprising a
rotor with
surface-mounted permanent magnets may possess somewhat different Ki.'z
surfaces;
nonetheless, the principles described above will still apply and a benefit
would still be
predicted within the region of geometries of low post height and high pole
density
described previously. As currently understood, the principles apply only to
electric
machines with posts, such as axial flux and radial flux machines.
[0141] In the disclosed equations and graphs, the parameter KR" is size-
independent and
has been converted from a conventional KR to use force instead of torque, and
to be
independent of both circumferential length and axial length. Therefore, the
conventional
KR of any size motor can be found from the ICR' value. And for two motors of
identical size
(diameter at the airgap and axial length) but different geometry (i.e. pole
density and/or
post height), the multiplying factor will be the same, so the motor with
higher KR" Will have
a higher conventional KR.
[0142] KR" as a function of pole density and post height greatly resembles the
surface of a
graph showing conventional KR. However, this particular surface, corresponding
to the
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torque density, may change considerably when different temperatures are used
as the
constraint in the analysis. K, by contrast, does not change substantially
(provided the
current doesn't get sufficiently high for the motors in the series start to
saturate; then the
3D curve shape will change.) It is the K, therefore, that is used to define
the specific range
of pole density and post height which result in the previously-discussed
benefits.
[0143] The ranges of benefit disclosed depend on the resultant motor diameter
at the
airgap. Smaller motors are more constrained because the physical size of the
motor
prevents lower slot densities from being used. We have defined 4 discrete
motor diameter
ranges corresponding to 200 mm and above, 100 mm and above, 50 mm and above,
and
25 mm and above. For each diameter range, we describe three levels of Icr'Z.
The first
corresponds to where a small benefit to lqz begins, the second to a moderate K
benefit,
and the third to a high lq benefit for that specific diameter range. Higher
KR" values
generally correspond to lower overall torque values for that motor size range.
[0144] These motor sizes disclosed (25 mm and up to 200 mm diameter and above)
represent small to large motors. The airgap of 0.005 inches used in the
simulation is
believed to be the smallest reasonable airgap size for this range of motors.
Smaller airgaps
are not practical for this motor range due to manufacturing tolerances,
bearing precision,
component deflection, and thermal expansion.
[0145] The coefficients in the equations above were chosen in a manner to
bound the
region of interest and make the resulting relation nearly continuous.
[0146] A 50:50 ratio of post:slot width was chosen for these simulations, as
analysis had
shown that highest benefits are obtained when the ratio is between 40:60 and
60:40. A
50:50 ratio represents a typical best-case scenario; at fixed post height,
using a 10:90
slot:post width ratio will have a significantly degraded performance by
comparison.
Analysis shows that at constant post height, an embodiment exhibits the
maximum of
torque and torque density at a 50% slot width, and the maximum of Km and Kr at
40% slot
width. However, the maximum values of Km and Kr are within 5% of the values
given at
a 50:50 geometry; consequently a 50:50 ratio was viewed as a reasonable choice
of scaling
parameter for the simulations. Other ratios of post:slot width would give a
portion of the
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benefits disclosed.
[0147] Equations and graphs are discussed below which show the ranges of pole
density
and conductor volume which give a significant benefit either in terms of KR,
or in terms
of a weighting function combining torque, torque-to-weight, and Km, for
different
embodiments. As with the previously-described equations, the region of benefit
in terms
of the weighting function is dependent on the amount of cooling.
[0148] Size of an electric machine means the airgap diameter of an axial flux
machine or
radial flux machine as defined herein or the length in the direction of
translation of the
carriers of a linear machine.
[0149] The first bounded region corresponds to regions where a significant KR
benefit is
found with respect to the rest of the geometries in the domain. For a given
device size, KR
has a higher value in the disclosed range of geometry than anywhere outside of
the range,
indicating potential benefits to overall system efficiency for certain
applications using
devices of these geometries. The graph of lq is used to define the boundary by
placing a
horizontal plane through at a specified KR" value. Four values of K are used
to define areas
of benefit for four different actuator size ranges corresponding to sizes of
200mm and
larger, 100mm and larger, 50mm and larger, and 25mm and larger.
[0150] In the following tables, pole pitch is represented by the variable S,
in mm. Post
height is also represented in millimetres.
[0151] In a machine with 25 mm size, the boundary line for K'F'z > 3.3 is
defined by the
values shown in Table 1 and the corresponding graph is Fig. 49.
Table l Set Al
Post Height > Points
Pole Pitch Post Height
-1.070*S+2.002 for 0.572<S<1.189 0.572 1.390
1.175*S+-0.667 for 1.189<S<2.269 1.189 0.730
13.502*S-28.637 for 2.269<S<2. 500 2.269 1.999
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Post Height < 2.500 5.118
-5.898*S+19. 863 for 1.970<S<2.500 1.970 8.244
0.229*S+7.794 for 1.349<S<1.970 1.349 8.102
7.607*S-2.160 for 0.723<S<1.349 0.723 3.340
11.430*S-4.924 for 0.572<S<0.723 0.572 1.614
0.572 1.390
[0152] In a machine with 25 mm size, the boundary line for 1(11 > 3.4 is
defined by the
values shown in Table 2 and the corresponding graph is Fig. 50.
Table 2 Set A2
Post Height > Points
Pole Pitch Post Height
-1.340*S+2.305 for 0.619< S<1.120 0.619 1.475
1.100*S-0.429 for 1.120<S<2.074 1.120 0.803
3.830*S-6.082 for 2.074<S<2.269 2.074 1.852
Post Height < 2.269 2.598
-69.510*S+160.318 for 2.222<S<2.269 2.222 5.865
-3.430*S+13.492 for 1.667<S<2.222 1.667 7.770
2.830*S+3.056 for 1.133<S<1.667 1.133 6.260
8.650*S-3.545 for 0.619<S<1.133 0.619 1.812
0.619 1.475
[0153] In a machine with 25 mm size, the boundary line for lq > 3.6 is defined
by the
values shown in Table 3 and the corresponding graph is Fig. 51.
Table 3 Set A3
Post Height > Points
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Pole Pitch Post Height
-4.160*S+5.032 for 0.723<S<0.967 0.723 2.024
0.839*S+0.198 for 0.967<S<1.692 0.967 1.009
2.713*S-2.973 for 1.692<S<1.939 1.692 1.617
Post Height < 1.939 2.287
-53.233*S+105.506 for 1.879<S<1.939 1.879 5.481
-1.406*S+8.122 for 1.465<S<1.879 1.465 6.063
3.898*S+0.353 for 1.035<S<1.465 1.035 4.387
7.535*S-3.412 for 0.723<S<1.035 0.723 2.036
0.723 2.024
[0154] In a machine with 50 mm size, the boundary line for lqz > 2.2 is
defined by the
values in Table 4 and the corresponding graph is Fig. 46.
Table 4 Set B1
Post Height > Points
Pole Pitch Post Height
0.254*S+0.462 for 0.319<S<3.667 0.319 0.543
2.665*S+-8.380 for 3.667<S<5.000 3.667 1.394
5.000 4.947
Post Height < 4.500 14.088
-18.282*S+96.357 for 4.500<S<5.000 2.738 22.304
-4.663*S+35.071 for 2.738<S<4.500 1.447 18.967
2.585*S+15.227 for 1.447<S<2.738 0.319 0.904
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16.013*S-4.204 for 0.319<S<1.447 0.319 0.543
101551 In a machine with 50 mm size, the boundary line for KIF'z > 2.5 is
defined by the
values in Table 5, and the corresponding graph is Fig. 47.
Table 5 Set B2
Post Height > Points
Pole Pitch Post Height
0.269*S+0.456 for 0.380<S<3.016 0.380 0.558
3.051*S-7.936 for 3.016<S<4.167 3.016 1.267
Post Height < 4.167 4.779
-14.766*S+66.309 for 3.667<S<4.167 3.667 12.162
-3.952*S+26.654 for 2.315<S<3.667 2.315 17.505
3.108*S+10.310 for 1.278<S<2.315 1.278 14.282
14.542*S-4.303 for 0.389<S<1.278 0.389 1.354
88.444*S-33.051 for 0.380<S<0.389 0.380 0.558
[0156] In a machine with 50 mm size, the boundary line for lq > 2.9 is defined
by the
values in Table 6, and the corresponding graph is Fig. 48.
Table 6 Set B3
Post Height > Points
Pole Pitch Post Height
0.191*S+0.626 for 0.472<S<2.181 0.472 0.716
2.135*S-3.613 for 2.181<S<3.095
2.181 1.043
53.475*S-162.511 for 3.095<S<3.175
3.095 2.994
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Post Height < 3.175 7.272
-5.095*S+23.450 for 2.222<S<3.175
2.222 12.128
0.805*S+10.339 for 1.381<S<2.222
1.381 11.451
10.251*S-2.706 for 0.572<S<1.381
0.572 3.158
24.420*S-10.810 for 0.472<S<0.572
0.472 0.716
[0157] In a machine with 100 mm size, the boundary line for K> 1.5 is defined
by the
values in Table 7, and the corresponding graph is Fig. 43.
Table 7 Set Cl
Post Height > Points
Pole Pitch Post Height
0.322*S+0.359 for 0.233<S<6.667 0.233 .. 0.434
2.202*S-12.179 for 6.667<S<8.333 6.667 2.504
Post Height < 8.333 6.173
-25.555*S+219.122 for 7.778<S<8.333 7.778 20.356
-5.585*S+63.794 for 4.000<S<7.778 4.000 41.455
3.214*S+28.600 for 1.793<S<4.000 1.793 34.362
21.749*S-4.633 for 0.233<S<1.793 0.233 .. 0.434
[0158] In a machine with 100 mm size, the boundary line for K> 1.7 is defined
by the
values in Table 8, and the corresponding graph is Fig. 44.
Table 8 Set C2
Post Height > Points
Pole Pitch Post Height
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0.277*S+0.593 for 0.250<S<5.182 0.250 0.662
2.342*S-10.111 for 5.182<S<7.222 5.182 2.026
Post Height < 7.222 6.804
-13.149*S+101.763 for 6.111 < S < 7.222 6.111 21.412
-4.885*S+51.265 for 3.333<S<6.111 3.333 34.983
4.291*S+20.680 for 1.520<S<3.333 1.520 27.203
20.788*S-4.395 for 0.251<S<1.520 0.251 0.823
161.000*S-39.588 for 0.250<S<0.251 0.250 0.662
[0159] In a machine with 100 mm size, the boundary line for Kir'z > 1.9 is
defined by the
values in Table 9, and the corresponding graph is Fig. 45.
Table 9 Set C3
Post Height > Points
Pole Pitch Post Height
0.277*S+0.591 for 0.278 <S < 4.425 0.278
0.668
1.916*S-6.663 for 4.425 < S < 6.111 4.425
1.817
Post Height < 6.111 5.048
-21.337*S+135.438 for 5.556 < S < 6.111 5.556
16.890
-4.985*S+44.588 for 3.175 <S < 5.556 3.175
28.76
2.749*S+20.031 for 1.560 <S < 3.175 1.560
24.320
18.321*S-4.260 for 0.278 < S < 1.560 0.278
0.833
0.278 0.646
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[0160] In a machine with 200 mm size, the boundary line for lqz > 1.3 is
defined by the
values in Table 10, and the corresponding graph is Fig. 40.
Table 10 Set D1
Post Height > Points
Pole Pitch Post Height
0.257*S + 0.327 for 0.208 <S < 7.778 0.208 0.381
1.977*S + -13.044 for 7.778 <S < 9.444 7.778 2.330
Post Height < 9.444 5.623
-36.195*S + 347.445 for 8.889 < S < 9.444 8.889
25.711
-5.777*S +77.062 for 4.833 <S < 8.889 4.833 49.142
1.950*S + 39.718 for 2.222 < S < 4.833 2.222
44.051
20.301*S + -1.058 for 0.389 <S < 2.222 0.389 6.839
34.481*S + -6.574 0.208 <S < 0.389 0.208 0.598
0.208 0.381
[01611 In a machine with 200 mm size, the boundary line for K> 1.5 is defined
by the
values in Table 11, and the corresponding graph is Fig. 41.
Table 11 Set 02
Post Height > Points
Pole Pitch Post Height
0.322*S + 0.359 for 0.233 <S < 6.667 0.233 0.434
2.202*S + -12.179 for 6.667 <S < 8.333 6.667 2.504
Post Height < 8.333 6.173
-25.555*S + 219.122 for 7.778 <S < 8.333 7.778 20.356
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-5.585*S + 63.794 for 4.000 <S < 7.778 4.000 41.455
3.214*S + 28.600 for 1.793 <S < 4.000 1.793 34.362
21.749*S + -4.633 for 0.233 <S < 1.793 0.233 0.434
[0162] In a machine with 200 mm size, the boundary line for K'F'z > 1.8 is
defined by the
values in Table 12, and the corresponding graph is Fig. 42.
Table 12 Set D3
Post Height > Points
Pole Pitch Post Height
0.212 *S + 0.600 for 0.264 <S < 4.833 0.264
0.656
3.017 *S + -12.960 for 4.833 <S < 6.667 4.833 1.623
Post Height < 6.667 7.157
-12.356*S + 89.531 for 5.556 <S < 6.667 5.556
20.884
-4.551 *S + 46.170 for 3.175 < S < 5.556 3.175
31.72
3.850 *S+ 19.496 for 1.502 < S < 3.175 1.502
25.279
19.751*S + -4.387 for 0.264 < S < 1.502 0.264
0.827
0.264 0.656
[0163] At each machine size, each boundary line is defined for a given K"
value, such that
for each machine size there is a set of K" values and a corresponding set of
boundary lines.
Pairs of boundary lines can be chosen, in which one boundary line is chosen
from each of
two consecutive sizes of device, i.e. 25mm and 50mm, 50mm and 100mm, or 100mm
and
200mm. The boundary lines occupy a space or volume defined by size, pole pitch
and post
height. A boundary surface may be defined as the two-dimensional uninterrupted
surface
in the space that is the exterior surface of the union of all lines that
connect an arbitrary
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point in the first boundary line and an arbitrary point in the second boundary
line. The
boundary surface encloses a benefit space. For each pair of boundary lines,
the boundary
surface defines a benefit space. An electric machine with a size, pole pitch
and post height
that is within a given benefit space is considered to fall within the
embodiment defined by
the corresponding boundary lines for that size of machine.
[0164] For machine sizes greater than the largest calculated size, the
boundary lines
calculated for the largest calculated size are used. The benefit space beyond
the largest
calculated size is thus simply the surface defined by the calculated boundary
lines for that
size and the volume of points corresponding to greater size but with pole
pitch and post
height equal to a point on the surface.
[0165] The main components of an electric machine comprise a first carrier
(rotor, stator,
or part of linear machine) having an array of electromagnetic elements and a
second carrier
having electromagnetic elements defining magnetic poles, the second carrier
being
arranged to move relative to the first carrier for example by bearings, which
could be
magnetic bearings. The movement may be caused by interaction of magnetic flux
produced by electromagnetic elements of the first carrier and of the second
carrier (motor
embodiment) or by an external source, in which case the movement causes
electromotive
force to be produced in windings of the electric machine (generator
embodiment). An
airgap is provided between the first carrier and the second carrier. The
electromagnetic
elements of the first carrier include posts, with slots between the posts, one
or more electric
conductors in each slot, the posts of the first carrier having a post height
in mm. The first
carrier and the second carrier together define a size of the electric machine.
The magnetic
poles having a pole pitch in mm. The size of the motor, pole pitch and post
height are
selected to fall within a region in a space defined by size, pole pitch and
post height. The
region is defined by 1) a union of a) a first surface defined by a first set
of inequalities for
a first size of electric machine, b) a second surface defined by a second set
of inequalities
for a second size of electric machine; and c) a set defined as containing all
points lying on
line segments having a first end point on the first surface and a second end
point on the
second surface,or 2) a surface defined by a set of inequalities arid all
points corresponding
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to greater size but with pole pitch and post height corresponding to points on
the surface.
[0166] The first set of inequalities and the second set of inequalities are
respectively sets
of inequalities A and B, or B and C, or C and D where A is selected from the
group of sets
of inequalities consisting of the equations set forward in Tables 1, 2 and 3
(respectively
sets of equalities Al, A2 and A3), B is selected from the group of sets of
inequalities
consisting of the equations set forward in Tables 4, 5 and 6 (respectively
sets of equalities
B 1, B2 and B3), C is selected from the group of sets of inequalities
consisting of the
equations set forward in Tables 7, 8 and 9 (respectively sets of inequalities
CI, C2, C3)
and D is selected from the group of sets of inequalities consisting of the
inequalities set
forward in Tables 10, 11 and 12 (respectively sets of inequalities DI, D2 and
D3).
[0167] The space in which the electric machine is characterized may be formed
by any
pair of inequalities that are defined by sets of inequalities for adjacent
sizes, for example:
Al Bl, Al B2, Al B3, A2 Bl, A2 B2, A2 B3, A3 Bl, A3 B2, A3 B3, B1 Cl, B1 C2,
B1 C3, B2 Cl, B2 C2, B2 C3, B3 Cl, B3 C2, B3 C3, Cl D1, Cl D2, Cl D3, C2 D1,
C2 D2, C2 D3, C3 D1, C3 D2, C3 D3. It may also be formed by any set of
inequalities and
all points corresponding greater size but having post height and pole pitch
within the region
defined by the set of inequalities.
[0168] All of the devices described in this application may have sizes, pole
pitches and
post heights falling within the regions and spaces defined by these equations.
[0169] The range of geometry may provide unusually high torque-to-weight for a
given
electrical power input. This efficiency is independent of temperature. For
example, at a
given torque-to-weight, an actuator inside the disclosed range, may run
cooler, for a given
method of cooling, than a similar actuator outside of the disclosed range,
because device
device in the disclosed range will use less power.
[0170] The low conductor volume, in this case has the benefit of lower thermal
resistance
due to the shorter conductors. Within the disclosed range, the need to power
these
conductors at higher current densities is more than compensated for by the
heat dissipation
benefits of the device to achieve a given torque-to-weight. Within the
disclosed KR" range,
the reduction in weight (which results, in part, from the low conductor
volume) can exceed
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the extra power required (which results from the higher current densities)
such that net
benefit can be produced in terms of KR. The stated ranges of geometry in a
machine of
the given diameter provides a heat dissipation effect associated with feature
geometry
known for much smaller machines, but used according to the principles of the
present
device, in a large diameter machine.
[0171] For clarity, cooling is still needed to achieve the KR benefit, but it
is assumed for
the KR calculation that adequate cooling is used. For some motors and
applications,
radiative cooling is sufficient. For others a fan and cooling fins is needed.
For others at full
power, water cooling is needed.
[0172] For the disclosed electric machine, the KR is the same at low to high
power output
(until the stator saturates at which time the KR will be reduced) so different
levels of cooling
will be needed depending on the power output but the torque-to-weight-to-power
consumption remains reasonably constant. The disclosed range of pole density
and
conductor volume may provide unusually high torque-to-weight for a given rate
of heat
dissipation with a given method of cooling. The disclosed range of pole
density and
conductor volume may produce higher torque-to-weight for a given cooling
method
applied to the back surface of the stator and a given conductor temperature.
The primary
form of electrical conductor cooling for the disclosed range of pole density
and electrical
conductor volume is thermal conductive heat transfer from the electrical
conductors to the
back surface of the stator.
[0173] Heat can be extracted from the back surface of the stator though direct
contact with
a cooling fluid or through conduction to another member such as a housing, or
through
radiation, for example. Other surfaces of the stator or conductors can also be
cooled by
various means. Cooling the back surface of the stator is shown to be a cost
effective and
simple option for many motor types. A sample analysis (not shown here)
indicates that
geometry in the disclosed range which shows better heat dissipation from the
back surface
of the stator (as compared to motors outside of the disclosed range) will also
generally
show improved heat dissipation than motors outside of the disclosed range when
other
surfaces of the stator or conductors are cooled. The back surface of the
stator is, therefore,
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viewed as a useful cooling surface, as well as an indicator of the
effectiveness of each
motor in the series to the application of cooling to other surfaces of the
stator and
conductors. The back surface of the stator has been chosen for the main
cooling surface for
the motor series analysis which is used to identify the disclosed range.
[0174] Other methods of cooling may be applied to an electric machine with the
disclosed
range of pole density and conductor volume, but the heat flow path from
conductors to the
back of the stator will preferably always be used for cooling the motor
regardless of what
other types of cooling (EG: direct coil cooling) are used.
[0175] Stator back iron may have an axial depth that is 50% of the width
(circumferential
or tangential width) of the posts. The posts may each have a tangential width
and the stator
may comprise a backiron portion, the backiron portion having a thickness equal
to or less
than half of the tangential width of the posts, or may be less than the
tangential width of
the posts. Thicker back iron adds weight with minimal benefit. Thinner
backiron helps with
cooling but the effect of back iron thickness on cooling is not very
significant. The backiron
surface may be in physical contact with the housing to conduct heat physically
from the
stator to the housing, and/or the back surface of the stator can be exposed to
an actively
circulated cooling fluid and/or the back surface of the stator can be
configured for radiative
heat dissipation to the atmosphere or to the housing or other components,
and/or the back
surface of the stator can be configured for convective or passive cooling
through movement
of air or liquid over the surface of the stator and or housing. Gas or liquid
moving past the
back surface of the stator may be contained or not contained. The back surface
of the stator
may be sealed from the atmosphere or exposed to the atmosphere. The atmosphere
may be
air or water or other fluid surrounding the actuator. The environment may also
be a
vacuum, such as is necessary for some manufacturing processes or the vacuum of
space.
The back surface of the stator may be configured with cooling fins which
increase the
surface area. These cooling fins may be exposed to a cooling fluid and/or in
contact with a
heat sink such as the housing or other solid member. The cooling fins on a
stator may have
a height greater than 50% of the post width in the circumferential direction..
[0176] In addition to heat being dissipated from the back surface of the
stator, other heat
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dissipating surfaces may include the surface of a post which may be exposed to
a cooling
fluid such as air or liquid which is circulated through a slot such as between
a conductor
and the post.
[0177] Other methods of cooling the stator and/or the conductors may include
cooling
channels on or below the surface of the stator and/or on or below the surface
of the
conductors. These and other forms of cooling are seen as supplementary to the
primary
thermally conductive cooling from the conductors to the back surface of the
stator. In some
cases the supplementary cooling methods may even draw more heat away from the
stator
than the primary conductive cooling effect, but active cooling methods require
energy and
additional cost and complexity, so the conductive cooling path from the
conductors to the
back surface of the stator is disclosed here as the primary mode of cooling.
[01781 For a single actuator producing a fixed torque, the power consumption
rises in the
disclosed range, and becomes exponentially larger towards the smallest post
heights and
slot pitches inside the disclosed range. From simulations of the power
consumption
necessary to produce 100 N m of torque with a single 200mm average airgap
diameter
actuator with a radial tooth length of 32mm and rotor and windings, it can be
seen that the
lowest power consumption occurs outside of the disclosed range, and that the
power
consumption increases significantly inside the disclosed range. In order to
minimize power
consumption, a designer would be led toward larger slot pitch and larger
conductor volume
devices. Any actuators using the geometry of the present device will have
higher power
consumption than those outside of the disclosed range towards larger slot
pitch and
conductor volume values for this type of application.
[0179] With the disclosed structure, in which a pole carrier of the electric
machine includes
slots and posts, the slots having a slot or pole pitch s and the posts having
a height h, in
which s is related to h according to the disclosed equations, electric
excitation may be
applied to conductors in the slots with a current density of at least 70
A/mm2. Electric
excitations in excess of 70 A/mm2 are generally considered suitable for the
operation of the
disclosed device. The cooling effect of having the disclosed slot and
conductor structure
provides cooling to offset some or all of the heat generated by the current in
the conductors.
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Any remaining heat generated may be dissipated using one or more of the
disclosed cooling
structures or channels. Motors inside the disclosed range show a reduction of
the average
flux density in the magnetic flux path for a given electrical input power.
This is due, in
part, to the reduced flux path length of the shorter posts and reduced
distance from post to
adjacent post through the backiron, as well as the reduced flux leakage
between posts. The
result is the ability to run higher current density in motors in the disclosed
range without
reaching saturation. The combination of increased cooling capability and lower
flux
density at a given current density as compared to motors outside of the
disclosed range,
creates a combination of conditions where higher continuous torque-to-weight
can be
achieved for a given temperature at a given cooling rate, and where the peak
momentary
torque-to-weight of motors in the disclosed range can be significantly higher
due to
operating at a lower flux density for a given torque-to-weight in the
disclosed range.
[0180] One of the most significant challenges that must be overcome in order
to achieve
the performance and power consumption benefits of the disclosed geometry, is
to provide
a structure that can withstand the immense magnetic forces that exists between
the rotor
and stator. Embodiments of the disclosed rotor can achieve unusually high flux
density in
the airgap leading to high attraction forces on the stator posts. At the same
time, achieving
the high torque-to-weight of an embodiment of the disclosed electric machine
requires the
use of a backiron that has an axial thickness that, in an embodiment, is less
than the
circumferential thickness of the posts (and, in an embodiment, is about half
of the thickness
of the posts). Furthermore, the axial flux motor configuration disclosed and
the relatively
short stator posts of the disclosed range results in an inherently thin stator
structure. With
a radial flux motor, circular laminates with integrated posts can be used.
This has an
inherent rigidity and naturally provides a desirable flux path along the
circumferential and
radial orientation of the laminates. In contrast, the axial flux function of
an embodiment of
the present device requires an assembly of individual laminated parts. The
result is the need
to manufacture up to hundreds of post components for each actuator, which
increases
manufacturing complexity, time and cost. Furthermore, the relatively thin
backiron does
not provide an adequate surface area for many potting compounds or adhesives
to reliably
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fix the posts to the backiron, especially at the high frequency force
variation and elevated
temperatures that are common to electrical machines. As an example, a typical
aerospace
adhesive that might be used to fix a stator post into a receiving slot in the
stator, might have
a heat deflection temperature of under 80 deg C for a stress on the epoxy of
less than 300
psi.
[0181] The back-iron disk of an embodiment can be made of laminates, powdered
metal,
or solid metal. The use of laminates has certain advantages, including the
possibility of
stamped material construction; however; if laminates are used, they must be
attached
through means capable of withstanding the forces and temperatures of operation
of the
device. Common methods such as glue may not be sufficient for certain regimes
of
operation where the forces and/or temperatures are high. Nonetheless,
laminations may be
a good choice for other regimes, and are expected to work well for many high-
speed
applications.
101821 The use of powdered metal with electrical insulator coating on each
particle for the
back-iron of an embodiment has the advantage of reducing eddy currents. This
coating,
however, will typically reduce the magnetic force because it acts like
multiple tiny airgaps
in the flux path. This material is also typically less strong than solid steel
or iron with
significantly higher creep rate, especially at elevated temperatures
[0183] A stator manufactured of solid steel typically has high eddy current
losses.
However, geometric features of motors in the disclosed range have an eddy
current and
hysteresis reducing effect that, in some regimes of operation of embodiments
of the present
device, for instance when operating at speeds which are suitable for robotics,
the eddy
current losses may be sufficiently low to enable the use of a solid stator.
Using solid
material is advantageous for strength, rigidity, heat resistance, and fatigue
strength. Since
embodiments of the present device can often generate sufficient torque to be
used without
a gearbox in certain applications, the resulting operational speeds may be
sufficiently low
that the eddy current losses be acceptable even with a solid steel stator.
Solid cast iron has
been found to give sufficiently low eddy current losses to be practical with
some
configurations and regimes of operation.
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[0184] Stators may be constructed of either laminated stacks or a sintered
powdered metal.
An objective of these constructions, as compared to the use of solid
materials, is to reduce
the cross sectional area of electrically insulated soft magnetic material
perpendicular to the
flux path and thus reduce the generation of eddy currents. Eddy currents
reduce the
efficiency by requiring additional input power; they produce extra heat which
must be
dissipated by the system; and they reduce the output torque by creating a
damping effect
[0185] A single-piece stator fabricated from a solid electrically conductive
material may
be used with embodiments of the disclosed device, particularly within the
disclosed ranges
of pole density and post height. To avoid eddy current generation, the
application should
be sufficiently low speed, for example a duty cycle that consists of 50% (60%,
70%, 80%,
90%) of the operation at 200 rpm or less for a 175mm average airgap diameter
motor
having the disclosed range of geometry. By combining this relatively low speed
range with
the relatively small cross sectional geometry of the stator teeth in the
disclosed range, the
individual stator teeth act somewhat like laminations and reduce the
production of eddy
currents. Speeds of less than 200 rpm are generally considered suitable for
the operation
of the device. Speeds of less than 100 rpm, less than 50 rpm and less than 25
rpm are also
considered suitable for the operation of the device.
[0186] Additionally, the production of eddy currents is reduced by the
relatively short
tooth height in the disclosed range. Eddy current and hysteresis losses are
volumetric, so
the low volume of the present device contributes to the reduction of total
iron losses for a
given flux density and switching frequency.
[0187] The continuous flux path may be provided by a stator made of isotropic
materials
such as ductile iron, steel alloy such as cobalt or silicon steel, pressed or
sintered powdered
metal, for example. The metal may be isotropic from post to adjacent post and
non-
isotropic from a post to a bearing race or a post to a member or assembly that
connects to
a bearing, including variable material alloy from backiron to cooling fins
and/or to
bearings. This can be done by explosion welding or fused deposition additive
manufacturing, or stir welding or other forms of combining dissimilar
materials.
[0188] The stator may be one piece or unitary from a post to an adjacent post
and from a
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post to a bearing race seat (or bushing seat or contact). The stator may be
unitary from a
post to a post and from one of these posts to a member or assembly that is in
compression
so-as to pre-load a bearing or bushing. The stator may be unitary from a post
to a post and
from one of these posts to a member or assembly that is in compression so-as
to pre-load
a bearing or bushing and all or part of the compressive load is a result of
magnetic attraction
between the stator and a rotor. In cases of pre-loaded bearings, the housing
assembly may
be flexible enough to displace the bearing race seat in the direction of
bearing preload past
the bearing seat position if the bearing is present, by more than .002" if the
bearing is not
present. In cases of pre-loaded bearings, the housing assembly may be flexible
enough to
displace the bearing race seat in the direction of bearing preload, past the
bearing seat
position if the bearing is present, by more than .002" if the bearing is not
present and the
force exerted on the stator to cause this deformation of the housing is
provided at least in
part, by the magnetic attraction of a stator to a rotor.
10189] An embodiment of an electric machine with inner and outer bearings
supporting a
rotor will now be described.
[0190] Fig. 52 to Fig. 59 show an overview and simplified section views of an
exemplary
stator 3802 and rotor 3801 of a device within the disclosed range of pole
density and post
height inserted into a robot arm 3800 as a frameless motor/actuator. Note that
conductors
and wiring are not shown in these figures for simplicity. An outer bearing
3804 that is used
for the arm pivot support is also used to define an airgap 3809. This allows
the frameless
actuator to be used in the system without the mass and complexity of a
separate actuator
housing. An additional bearing 3808 may be used on the ID of the frameless
actuator
assembly in conjunction with a spacer ring 3803 to maintain the desired airgap
dimension
with a longer radial post length. Interlocking features 3812 (Fig. 59), allow
the attachment
of the stator 3802 to lower arm housings 3806 and 3807 by sliding the stator
tabs 3812
between the housing tabs 3816 and locking them into place according to an up-
and-over
path 3815 in Fig. 57. Similar tabs 3814 on the rotor secure the rotor 3801 to
upper arm
housing 3805, 3810 and the inner bearing spacer ring 3803. The mass of the
stator and
rotor is only increased by the additional securing features 3814, 3812 and the
weight of the
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bearing spacer ring 3803 and inner bearing 3808. The spacer element 3803 can
be made
of a low density materials such as aluminum or magnesium. This exemplary
embodiment
has a 175mm average airgap diameter and 25 mm radial post length. The
isotropic steel
alloy or iron alloy stator 3802 and isotropic steel alloy or iron alloy rotor
3801 with
backiron are sufficiently rigid to maintain a 0.005" airgap when supported at
the ID and
OD with a bearing.
[0191] In an embodiment, the magnetic attraction between the rotor 3801 and
stator 3802
can be used to provide preload on the bearings 3804, 3808 and may be used to
reduce or
eliminate the need for fasteners to keep the bearings seated in upper and
lower arm
housings 3805, 3806, respectively. This construction is considered to be
beneficial in terms
of simplicity and light weight to the point of allowing the entire arm
assembly to be lighter
than if it used a motor outside of the disclosed range.
[0192] Due to the axially inward magnetic attraction between the rotor 3801
and the stator
3802, they must both be secured to prevent movement toward each other at the
airgap 3809.
It is beneficial to achieve a light weight but stiff robot arm housing, so
this exemplary
embodiment provides a way to assemble the arm and magnetic components from the
airgap
axial end of the actuator. This is accomplished by the use of an array of tabs
3812, 3814
on the OD of the stator 3802 and rotor 3801 which allow the stator and rotor
to be inserted
in to the housings 3805, 3806 and then turned to engage with the matching
array of tabs
3816, 3813 on the housings 3805, 3806. Threaded engagements would be another
option.
[0193] Once the rotor 3801 and stator 3802 are assembled in their respective
arms, the
upper arm assembly with stator and lower arm assembly with rotor are brought
together.
The force between the rotor and stator will then preload the bearings 3804,
3808 and hold
the arm joint together with up to approximately 400 KG of axial force for a
device of this
size.
[0194] A 10 OD actuator of the present device can have a passive PM preload of
up to
1500 lbs or more between the stator or rotor. This makes it very challenging
and even
dangerous to assemble. Embodiments of the present device allow PM's to be
inserted after
the stator and rotor are assembled together. This allows precision and low
risk alignment
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of the stator and rotor and bearings and connections before any PM's and their
magnetic
force is added to the assembly.
[0195] The ability to add and remove the magnets individually may be very
helpful for
large motor/actuators to allow disassembly for servicing of bearings etc. The
only tools
needed for such a procedure would be a magnet removal tool. If the magnets
could not be
removed before removing the rotor, a large actuator could require 10,000 lbs
of force or
more to remove the rotor.
[0196] By using a bearing on the ID and OD of an axial or conical motor, a
reasonably
consistent axial preload can be achieved on the bearings. This has a number of
potential
advantages: (a) no bearing race retention may be needed in the opposite axial
direction, (b)
the preload of the bearing may remain reasonably constant despite bearing
seating, wear,
or thermal expansion because the preload is provided by magnetic attraction
which does
not vary significantly if the bearing race seats move relatively to each other
in the axial
direction by as much as would be expected in normal service and (c) this may
also have
the additional advantage of allowing lower axial manufacturing tolerance
[0197] Although the foregoing description has been made with respect to
preferred
embodiments of the present invention it will be understood by those skilled in
the art that
many variations and alterations are possible. Some of these variations have
been discussed
above and others will be apparent to those skilled in the art.
[0198] In the claims, the word "comprising" is used in its inclusive sense and
does not
exclude the possibility of other elements being present. The indefinite
article "a/an" before
a claim feature does not exclude more than one of the feature being present
unless it is
clear from the context that only a single element is intended.
