Note: Descriptions are shown in the official language in which they were submitted.
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ROTARY ACTUATOR
TECHNICAL FIELD
[0001] Actuators.
BACKGROUND
[0002] A high pole count motor has many advantages such as the potential
for high
torque and light weight. It has been shown in WIPO published patent
application
W02017024409A1 that a solid stator can provide adequate performance in regard
to minimizing
eddy currents when speeds are relatively low such as when used in robotics.
For higher speed
applications the use of laminates is preferable to reduce eddy current losses.
The challenge is that
a high pole count axial motor has a very thin profile (if it is to take
advantage of the torque to
weight potential) and is therefore very difficult to build out of laminates.
For example, if a single
rotor and single stator construction is used, the forces pulling the stator
and rotor together across
the airgap would be expected to shear the glue-lines holding the laminated
structure together
such that the airgap would not be maintained.
SUMMARY
[0003] A rotary actuator solves this problem in a number of ways that
include using a
double rotor configuration where the stator is positioned between the two
rotors. An advantage
of this configuration is that the magnetic forces on the stator are reasonably
equal in both axial
directions on each of the posts at all times. This reduces the load on each of
the posts and reduces
the stress on each of the glue lines in the stator assembly. The tangential
forces on each of the
posts can also be very high when under full power, but these forces are also
balanced on each
posts such that the glue lines are not highly stressed at any time.
[0004] Therefore, in an embodiment, there is disclosed an electric machine
comprising
a stator disposed between rotors, the rotors being mounted on bearings for
rotation relative to the
stator about an axis of the electric machine, the rotors being separated from
the stator by
respective air gaps; the stator being formed of structural members, each
structural member being
formed of laminates, each laminate having a smallest dimension that extends
axially; each
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structural member having slots, and magnetic posts fixed within the slots for
support of the
magnetic posts by the structural member; and one or more electrical conductors
disposed about
the posts for generating a series of commutated electromagnetic poles.
BRIEF DESCRIPTION OF FIGURES
[0005] Embodiments of a rotary actuator will now be described by way of
example, with
reference to the figures, in which like reference characters denote like
elements, and in which:
[0006] Fig 1. is a section view of an embodiment of a high speed actuator
showing the
rotor with magnets, thrust bearings, four-point contact bearings, a stator
with laminated posts,
laminated structural member of the stator, the solid structural member and the
conductors.
[0007] Fig 2. is a view of an exemplary embodiment having laminated posts
installed
between the laminated structural member.
[0008] Fig 3. is a view of the laminated structural member of the stator
showing a
preferred stack layup of laminations. Where the radial cut is radially inward
or outward of the
stator post slot, and alternates for each adjacent layer.
[0009] Fig. 4 is a view showing the installation of the laminated stator
posts into the
laminated structural member of the stator with no solid structural member
present.
[0010] Fig. 5 is a view showing the installation of the laminated stator
posts into the
laminated structural member of the stator with a solid structural member
present which has
mounting features.
[0011] Fig. 6 shows how the Eddy current path is broken by the radial cuts
made; both
inward and outward of the stator post slot, in the laminates of the laminated
structural member.
[0012] Fig. 7 is a section view of the stator and rotor to show the
orientation of the
magnets in the rotor and the flux path across the laminated stator posts.
[0013] Fig. 8 is a view of the final lamination assembly with 2 laminated
structural
pieces and laminated stator posts.
[0014] Fig. 9 is a cutaway view of the stator with some posts and coils
removed.
DETAILED DESCRIPTION
[0015] A rotary actuator is disclosed that uses a double rotor
configuration where the
stator is positioned between the two rotors. An advantage of this
configuration is that the
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magnetic forces on the stator are reasonably equal in both axial directions on
each of the posts at
all times. This reduces the load on each of the posts and reduces the stress
on each of the glue
lines in the stator assembly. The tangential forces on each of the posts can
also be very high
when under full power, but these forces are also balanced on each posts such
that the glue lines
are not highly stressed at any time.
[0016] It is desirable to use a "backiron" in this configuration (which
does not actually
become part of the flux path as with a conventional single stator) with high
structural strength
and rigidity, as well as high thermal conductivity. Aluminum would be an
excellent choice in
terms of high strength to weight and high thermal conductivity, but aluminum
also has high
electrical conductivity so it would generate high eddy currents especially at
high operating
speeds.
[0017] To take advantage of the structural and thermal benefits of aluminum
for the
backiron, a rotary actuator is disclosed that uses a stack of two or more
aluminum disks with
slots in the disks to receive the posts, and additional slots, such as
radially outward or inward
from the slots, to eliminate an electrically conductive path around each of
the posts. A single
piece of aluminum may be used with radial slots to prevent eddy currents, but
it is believed by
the inventors that a laminated aluminum structure with eddy current slots that
alternate from
layer to layer from radially inward to radially outward, provide a stronger
and stiffer structure for
a given thickness. This is because the eddy current slots on one layer align
with a non-slotted
ring of material on the next aluminum layer such that the no two adjacent
layers have aligned
eddy current slots.
[0018] The aluminum in the backiron laminates may be coated but they are
preferably
anodized such as with a hard anodized finish. Anodizing is essentially a
ceramic coating which
provides high dielectric strength and reasonably good thermal conductivity.
[0019] An electric motor/actuator may comprise of a stator which utilizes
ferromagnetic
material laminates for the electromagnetic posts to reduce the Eddy Current
losses. And a high
thermal conducting material is preferred to be used in the stator structure to
get heat out of the
device. The rotor may be made of a ferrous material that performs as required.
[0020] ID Ref. # Description
[0021] 20 Stator Coil
[0022] 22 Stator Post Laminate
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[0023] 24 Stator Non-Ferrous Structural Laminate
[0024] 26 Stator back Bone
[0025] 28 Outer Rotor Housing
[0026] 30 Rotor Magnet
[0027] 32 Thrust Bearing
[0028] 34 Ball Bearing
[0029] 36 Stator Post Laminated Assembly
[0030] 38 "M" Non-Ferrous Stator Structural Laminate
[0031] 40 "W" Non-Ferrous Stator Structural Laminate
[0032] 42 Discontinuous Eddy Current Loop Path
[0033] 44 Internal Stator Cooling Chamber
[0034] 46 Radial Cut
[0035] 48 Stator Post and structural laminate
[0036] 50 Rotor Side 1
[0037] 52 Rotor Side 2
[0038] 54 Rotor pole
[0039] 56 Structural laminate Assembly
[0040] 58 Air gap
[0041] 60 Slots
[0042] 62 Ridges on the stator backbone
[0043] 64 Channel around inside of stator structural members
[0044] 66 Chambers between the structural members and between the posts
[0045] As shown in Fig. 1, an electric machine comprises a stator, with a
backbone 26
and structural laminate assembly 56 disposed between rotors 50 and 52, the
rotors 50, 52 being
mounted on bearings 32, 34 for rotation relative to the stator about an axis
of the electric
machine. Approximate location of the axis is identified as A in Fig. 4. The
rotors 50, 52 are
separated from the stator by respective air gaps 58. As shown in Fig. 2, the
stator structural
laminate assembly 56 may comprise structural members 24, each structural
member being
formed as shown in Fig. 2 of annular laminates 38, 40 each laminate 38, 40
having a smallest
dimension that extends axially. Each structural member 24 and the
corresponding laminates
have openings or slots 60, and (as shown in Fig. 4) magnetic posts 36 fixed
within the slots 60
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for support of the magnetic posts by the structural member 24. The slots 60
may have a longest
dimension that extends radially, an intermediate dimension that extends
circumferentially and a
depth that extends axially. As shown in Fig. 2, one or more electrical
conductors 20 are disposed .
about the posts 36 for generating a series of commutated electromagnetic
poles. There may be
M poles and N posts and the greatest common factor of N and M is three or
more.
[0046] As shown in Fig. 5, the backbone 26 comprises an outer backbone 68
and inner
backbone 70, with the structural members 24 being secured on either side of
ridges 62 that
extend respectively inward of the outer backbone and outward of the inner
backbone. The
structural members 24 may be secured to the ridges 62 by any suitable means
such as glue.
[0047] The rotors 50, 52 are mirror images of each other and are secured to
each other
for example with bolts or screws (not shown) at their outside peripheries. As
shown in Fig. lthe
rotors 50, 52 are mounted for rotation relative to the stator on radial
bearings 34 at the inside of
the stator and on thrust or axial bearings 32 at the outside of the stator.
Bearing races are formed
on the backbone 26 of the stator and in the rotors 50, 52. The stator backbone
26 may be secured
to a fixed structure at the inner periphery of the backbone 26 by any suitable
means. The
outward periphery 28 of the rotors 50, 52 may then be used as the output.
Power for the
windings 20 may be supplied through the inner part of the backbone 26 through
channels (not
shown). As shown in Fig. 2 the radial length of the stator posts 22 between
the structural
members 24 may be less than the distance between the ridges 62 of the stator
backbone 26 to
form a channel 64 around the stator that may be used for flow of a cooling
fluid. Channels (not
shown) in the inner part of the stator backbone 26 may be used for flowing a
cooling fluid in and
out of the channel 64.
[0048] An exemplary embodiment may use an Iron alloy for the stator posts
laminations
and an Aluminium alloy for the structural laminates. The stator of an electric
machine is formed
of structural laminates 24 that have slots that posts 22 are fixed within. The
structural laminates
24 have a thinnest dimension in the axial direction, and in the radial
direction are annular.
[0049] For the structural laminate 24, as shown in Fig. 3, it is preferred
to have radial .
cuts 46 made from the post slot to the edge of the material to remove the Eddy
current loop path
42 around the stator post, as shown in Fig. 6. Slots may also be between the
posts such as
circumferentially between every second post. The preferred embodiments have
opposing radial
cuts per layer, as seen in Fig. 3, these may be referred to as the "M" 38 and
"W" 40 laminates.
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This is to remove the Eddy current loop path 42 on all the layers of the
structural laminate while
still maintaining adequate strength and rigidity in the aluminum layers by
virtue of the
overlapping sections on one or both sides of each slot on another layer. In an
embodiment shown
in Fig. 2, it is shown to have but not limited to five layers in each
laminated assembly 24, the
quantity of the layers is driven by the design scope. This then creates a
thicker assembly that has
the strength requirement and will reduce the loss from the Eddy Currents by
virtue of the
interrupted eddy current path on each layer, and the electrical insulation,
such as an anodized
surface, between each layer.
[0050] The stator post laminates 36, which are preferred to be mounted
perpendicular to
the structural laminate 24, are then to be mounted between two structural
laminates to create the
stator, this can be seen in Fig. 4 where an embodiment is mechanically fixed
between the
structural laminates by a tab at the inner and outer radial position. This
assembly may be
preferred to have interference and be pressed together to create a solid
assembly 48 as seen in
Fig, 8. It may be preferred to then coat this sub-assembly in a potting
compound to add another
material to help the heat get from the stator posts to the structural
laminate. The magnetic posts
may have an enlarged central section that defines respective shoulders that
form the tabs and the
respective shoulders engage the structural members to resist axial movement of
the magnetic
posts within the structural members. The posts and structural members together
define chambers
66.
[0051] In this preferred configuration a post lamination 36 is used for two
stator posts,
and acts as a single magnetic dipole. This requires the rotor to have the
magnets 30 on side 52 to
form poles 54 rotated by one pitch relative to side 50. So that a North Pole
is across from a South
Pole on the other side of the rotor, seen in Fig. 7 so that axially opposed
magnets have opposite
polarity.
[0052] The chambers 66 and channel 64 together create a chamber 44 between
the two
structural laminates as seen in Fig. 2 and Fig, 9, which may extend throughout
the space between
the stator backbone and rotors that is not occupied by the structural members
24 or the posts.
This chamber may be filled with a fluid or gas to remove heat from the stator
and stator coils.
This is preferred as the fluid or gas will be in direct contact with the
center of stator post and
structural laminated member which will allow effective heat transfer. This may
be preferable as
this allows the device to run at higher currents while maintaining a stable,
desired temperature.
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The fluid or gas in this chamber is preferred to flow through the chamber due
to a pressure
differential between an inlet and an outlet (not shown, but may be in the
inner backbone). The
fluid or gas may also remain static, or if air cooling is preferred, ambient
air may also flow
through by natural convection.
[0053] To manufacture the device, it may be necessary or helpful to insert
a spacer
between the two laminated structural members when the posts and aluminum disks
are
assembled. Then after the coils are added and the stator is potted, the spacer
prevents the potting
compound from filling the space between the laminated aluminum disks. This
spacer is
preferably made of a dissolvable material or a meltable material such as wax,
which can be
removed by dissolving or melting after potting is complete.
[0054] To attach the laminated stator assembly to another entity it may be
required to
insert a solid member in-between the laminates during the assembly process.
This is shown in
Fig. 5 where an exemplary member is inserted between the structural
laminations. This
exemplary member allows bearing on the ID and OD to be used and a bolt hole
pattern on the ID
flange 72 of stator backbone 26.
[0055] A single set of coils could be used between the two structural
members with
shorter posts, instead of the coils 20 shown, that only just protrude from the
structural members.
This would not have the cooling benefits but would be a lower profile
assembly.
[0056] With a rotor on each side of the stator, there are balanced axial
forces on the stator
poles that results from the rotor poles acting with equal force on both axial
ends of each post.
This tends to eliminate the shear force on the stator post laminates, which
reduces the strain on
the glue layers between the laminates. The mechanical securing of the stator
post laminates
between the two aluminum layered disks (with the wider section of the posts
between the
aluminum layered disks) resists movement of the laminates even if the glue
fails. The design
reduces eddy currents in the laminates of the structural members as a result
of the alternating ID
- OD slots in each layer. Alternating from ID to OD with each successive layer
provides a non-
interrupted surface on at least one side of each eddy current prevention slot
on an adjacent layer.
[0057] The use of aluminum for the structural members results in a
lighter weight
structure with excellent heat dissipation characteristics. Anodizing these
layers before assembly
provides electrical insulation with minimal thermal insulation between layers.
The space
between the aluminum layered disks can also be used for internal fluid
cooling.
