Note: Descriptions are shown in the official language in which they were submitted.
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Compact Actuator Arrangement
Field of the Disclosure
This disclosure relates to an actuator and a robotic arm incorporating such an
actuator.
Background
Many machines, and in particular robots, rely on compact, lightweight
actuators for
moving arms and other members. The movement often requires high levels of
precision and high torque, which can be difficult to achieve without
significantly
increasing the size or weight of the actuator.
Known actuators often comprise a motor and gear box arrangement for increasing
the amount of torque delivered from an electric motor in order to be usable,
and
reducing the rotational speed at output relative to that at the drive shaft of
the
motor. It is common that the gear box is designed and built separately from
the
motor and this can have the drawback that the axial extent of the overall
actuator
arrangement is disadvantageously large.
Known actuators may also suffer from low efficiency or from high levels of
inertia,
which may reduce the precision of the movement.
The present disclosure seeks to address at least some of the above problems.
Summary of the Invention
According to a first aspect of the invention, there is provided an actuator
comprising
a motor comprising a housing and a drive shaft, the motor arranged to rotate
the
drive shaft relative to the housing about a drive shaft axis; a first torque
transfer
device comprising a first drive member on the drive shaft and a first driven
member
on a second shaft, the first drive member being arranged to transfer torque to
the
first driven member, such that the first torque transfer device is arranged to
transfer torque from the drive shaft to the second shaft, the second shaft
being
rotatable about a second shaft axis spaced from the drive shaft axis; and an
output
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torque transfer device comprising an output drive member on the second shaft
and
an output driven member, the output drive member being arranged to transfer
torque to the output driven member, such that the output driven member rotates
about an output axis; wherein a torque ratio of the output torque transfer
device is
smaller than or equal to a torque ratio of the first torque transfer device.
With such an arrangement, given design constraints such as minimum shaft
diameters, there is provided a compact actuator arrangement.
Depending on which components of the actuators are considered fixed, the
actuator
may be considered to operate by the housing of the motor being arranged to
rotate
about the drive shaft axis, or by the second shaft being arranged to orbit the
drive
shaft axis. These two concepts may be substantially similar, but may have
alternative portions fixed relative to external components and alternative
portions
used as output members. Hence, whether the second shaft axis, and accordingly
the second shaft, orbits the motor or whether the motor housing may rotate may
be a question of reference frame. In either case, rotation of the motor
housing
relative to the position of the second shaft axis may be created.
Torque transfer devices as described herein may be, but are not limited to,
pulleys
and gears and a torque transfer device may comprise two gears on different
shafts,
the gears having teeth interlocking with each other or with an intermediate
gear,
or two pulleys on different shafts connected by at least one cable or band.
Torque
may be transferred keeping a speed of rotation and level of torque constant or
may
involve a level of mechanical advantage, by using gears having different
numbers
of teeth or pulleys having different radii. In certain embodiments, toothless
rollers
may be used in a similar manner to toothed gears, where differing radii or
differing
diameters of rollers are used to create a desired ratio of input torque and
speed to
output torque and speed.
The torque ratio of a torque transfer device or combination of torque transfer
devices may alternatively be referred to as a speed ratio, reduction ratio or
mechanical advantage. The torque ratio of a torque conversion device is the
ratio
by which a torque output by the device is greater than a torque input to the
device.
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The torque ratio between the output of the motor and the output at the output
member may be at least 12:1. Put another way, the output member may be
arranged to rotate at one twelfth of the speed of the drive shaft of the motor
for
less.
It will be understood that a motor having a larger volume may be expected to
have
a higher power. However, this must be balanced by the space available within
the
actuator and outside the motor for torque conversion devices. The present
inventors have realised the below design constraints for providing compact
actuators having sufficient output torque and precision.
The first and second torque transfer devices and the motor may be arranged
within
a cylindrical volume, the cylindrical volume having a diameter less than three
times
the diameter of the motor housing. The motor housing may have a diameter of at
least 40mm. The cylindrical volume may have a diameter of 100mm or less. The
cylindrical volume may have an axial extent less than three times the axial
extent
of the motor housing.
The actuator may comprise a housing, the housing defining the cylindrical
volume,
and the motor, the first torque transfer device and the output torque transfer
device
may be within the housing. With such an arrangement, delicate parts of the
arrangement may be protected by the housing. The housing may be substantially
cylindrical.
The first driven member may overlap the output driven member in a direction
viewed along the drive shaft axis and/or the second shaft axis. The sum of the
radii
of the output driven member and the first driven member may exceed a distance
between the drive shaft axis and the second shaft axis. This may provide a
high
torque conversion ratio and a more compact actuator.
The second shaft may extend from the first driven member to the output drive
member in a first direction and the drive shaft may extend from the motor
housing
to the first drive member in a second direction opposite to the first
direction. This
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may be seen as the torque conversion device "doubling back" on itself and may
thereby reduce the axial extent of the actuator.
At least one of the first torque conversion device and output torque
conversion
device may be a pulley-based system. Optionally, both may be pulley-based.
Pulley-and-cable and pulley-and-band torque conversion systems have an
advantage over geared system in the present case, which the inventors have
realised. Specifically, two pulleys of pulley-based systems do not need to be
in
contact, unlike geared systems. This allows the smaller of the two pulleys in
such
a system to be smaller than a geared system would allow in certain situations
with
particular constraints on the positions of axes of rotation. Thus, a more
compact
arrangement with improved torque conversion may be provided.
The motor housing may be arranged at least partially inside the output driven
member. This may allow a more compact arrangement.
The output drive member may be arranged within a cylindrical sector volume,
the
cylindrical sector volume being defined by a cylindrical sector subtended at
the
drive shaft axis and having an internal angle at the drive shaft axis of less
than
100 , optionally less than 60 . This may allow the second shaft and members
fixed
to the second shaft to lie within a member which the actuator is arranged to
move.
Therefore, the actuator may be integrated compactly within a robotic arm.
The cylindrical sector may be viewed as a sector which is extruded along the
drive
shaft axis. The radius of the sector may be disregarded, as the internal angle
subtended at the drive shaft axis determines how the second shaft and the
output
drive member may fit within a robotic arm member proximate a joint.
According to a second aspect of the invention, there is provided an actuator
comprising: a motor comprising a housing and a drive shaft, the motor arranged
to
rotate the drive shaft relative to the housing about a drive shaft axis; a
first torque
transfer device comprising a first drive member on the drive shaft and a first
driven
member on a second shaft, the first drive member being arranged to transfer
torque
to the first driven member, such that the first torque transfer device is
arranged to
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transfer torque from the drive shaft to the second shaft, the second shaft
being
rotatable about a second shaft axis spaced from the drive shaft axis; an
output
torque transfer device comprising an output drive member on the second shaft
and
an output driven member, the output drive member being arranged to transfer
5 torque to the output driven member, such that the output driven member
rotates
about an output axis; a first connection flange fixed to the motor housing at
an
axial end of the motor housing opposite to the end of the motor housing where
the
drive shaft extends; and a second connection flange arranged such that the
drive
shaft is between the second connection flange and the motor housing, the
second
connection flange being fixed to the motor housing via a cross member.
The cross member may be shaped as an extruded arc. This may also be considered
as a curved sheet. This may provide good stiffness to the actuator while
avoiding
contact between the cross member and other components.
The cross member may have an arcuate extent of less than 1800. This may allow
orbital movement of the second shaft without the cross member impeding the
first
torque transfer device.
According to a third aspect of the invention, there is provided a robotic
joint
comprising: a first member; a second member pivotably coupled to the first
member; and an actuator according to the first or second aspect; wherein the
actuator is arranged to rotate the second member relative to the first member.
According to a fourth aspect of the invention, there is provided a robotic
joint
comprising: a first member; a second member pivotably coupled to the first
member; and an actuator comprising: a motor comprising a housing and a drive
shaft, the motor arranged to rotate the drive shaft relative to the housing
about a
drive shaft axis; a first torque transfer device comprising a first drive
member on
the drive shaft and a first driven member on a second shaft, the first drive
member
being arranged to transfer torque to the first driven member, such that the
first
torque transfer device is arranged to transfer torque from the drive shaft to
the
second shaft, the second shaft being rotatable about a second shaft axis
spaced
from the drive shaft axis; and an output torque transfer device comprising an
output
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drive member on the second shaft and an output driven member, the output drive
member being arranged to transfer torque to the output driven member, such
that
the output driven member rotates about an output axis;wherein the actuator is
arranged to rotate the first member relative to the second member, and wherein
the second shaft is supported by the first member.
By using the first member to support the second shaft, the distance between
the
second shaft and the drive shaft may be increased, allowing a larger motor and
a
larger torque ratio to be provided.
The robotic joint may further comprise a third member, arranged to rotate with
the
first member relative to the second member, the first and third members being
arranged to support the second shaft.
The robotic joint may further comprise a second cross member, arranged to
couple
the first and third members to one another to provide further support to the
second
shaft. The second cross member may be arranged such that separation of the
first
and the second members in a direction parallel to the drive axis is prevented.
The actuator within the robotic joint of the fourth aspect may be an actuator
in
accordance with the first or the second aspect.
In the third or fourth aspects, the first member may be arranged to pivot
relative
to the second member about the drive shaft axis.
Brief Description of the Drawings
Embodiments of the invention will now be described, by way of example only,
with
reference to the accompanying drawings, in which:
Figure 1 shows a schematic drawing of an actuator according to the present
disclosure;
Figure 2 shows a perspective view of two torque transfer devices for use in an
actuator according to the disclosure;
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Figure 3 shows a cross section of an actuator according to the disclosure;
Figure 4 shows a cross section of an actuator according to the disclosure;
Figure 5 shows a perspective view of an actuator according to the disclosure;
Figure 6 shows an electrical cable for use in an actuator according to the
disclosure;
Figure 7 shows a schematic view of a torque transfer device for use in an
alternative
actuator according to the disclosure;
Figure 8 shows a robot arm according to the disclosure;
Figure 9 shows a general view of selected parts of an alternative actuator
assembly;
Figure 10 shows a general view of a robotic joint incorporating the
alternative
actuator assembly;
Figure 11 shows a robotic arm incorporating the alternative actuator assembly;
Figure 12 shows a pulley connected to a cable arrangement;
Figure 13 shows a pulley and cable arrangement, with one cable and connector
removed;
Figure 14 shows a sectional view of a connector; and
Figure 15 shows a plan view of a connector.
Detailed Description
Figure 1 shows a schematic diagram of an actuator 10 according to the present
disclosure. The actuator has a motor 100, having a stator 104 and a rotor 106,
the
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stator 104 and rotor 106 being arranged to generate a torque between them. The
stator 104 is fixed to a motor housing 102, which encloses the motor 100. The
rotor
106 is fixed to a drive shaft 108, which extends out of the housing 102 and
the
rotor 106 and drive shaft 108 are arranged to rotate about a drive shaft axis
Al
relative to the stator 104 and motor housing 102, due to the torque generated
within the motor 100.
The drive shaft 108 is coupled to a first torque transfer device 200, and in
particular
to a first pulley 202 of the torque transfer device 200, which may also be
referred
to as a first drive member. The first pulley 202 is coupled to a second pulley
204,
which may be referred to as a first driven member, via at least one cable 206
and
the cable 206 may be fixed to the first and/or second pulley 204 and/or may
pass
radially through at least one of the pulleys 202, 204. The second pulley 204
is fixed
to a second shaft 208, which is rotatable about a second shaft axis A2, and
supported by a second shaft mount 210. The second shaft mount 210 is rotatably
mounted to the motor housing 102 and/or the drive shaft 108 so as to be about
the drive shaft axis Al relative to the motor housing 102 and therefore
supports
the second shaft 208, such that the second shaft 208 may orbit the motor 100,
the
second shaft axis A2 orbiting the motor 100 with the second shaft 208.
The actuator 10 also comprises a second, output torque transfer device 300,
the
second torque transfer device 300 having a third pulley 302, which may be
referred
to as an output drive member, is fixed to the second shaft 208 and a fourth
pulley
304, which may be referred to as an output driven member, which is fixed to
the
motor housing 102 and may be an integral part of the motor housing 102. The
fourth pulley 304 may thereby surround the motor housing 102 and the stator
104.
The third and fourth pulleys 302, 304 are coupled via a cable 306, which may
also
be two or four cables.
By this arrangement, the torque generated by the motor 100 is arranged to
rotate
the second shaft mount 210 when the motor housing 102 is held stationary, such
that the second shaft axis A2 and the second shaft 208 orbit the motor 100.
Alternatively, where the second shaft mount 210 is held stationary, the motor
housing 102 and fourth pulley 304 may rotate.
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In order to fix a part of the actuator 10 to be stationary and/or to take an
output
torque from the actuator 10, there is provided a fixed plate 218, which is
fixed to
the second shaft mount 210 and there is provided a motor output member 110,
.. fixed to the motor housing 102. Both of the fixed plate 218 and motor
output
member 110 may be fixed to external components and the relative rotation of
the
motor housing 102 or the second shaft mount 210 may be a question simply of
reference frame and determined by which external components are considered to
be fixed and which are considered to be movable. It will be understood that
the
.. actuator 10 as a whole may be fixed to or form part of a moveable member or
vehicle and may therefore not be fixed in an absolute sense.
Further, optional components are also shown in Figure 1. These include
bearings.
The bearings shown are exemplary and certain bearings may be omitted or moved.
.. There may be a motor housing bearing 112 disposed about the motor housing
102
and between the motor housing 102 and the second shaft mount 210. There may
also be a first drive shaft bearing 114 and a second drive shaft bearing 118
between
the drive shaft and the second shaft mount 210. The second shaft 208 may be
supported on the second shaft mount 210 by first bearings 212, second bearings
.. 214 and third bearings 216. The bearings may be arranged such that the
second
and third bearings 214, 216 are disposed either side of the third pulley 302
so that
the second shaft 208 is supported on both sides of the third pulley 302.
The actuator 10 may also comprise a plurality of encoders arranged to
determine
.. relative rotational positions of components. A first encoder 402 may
measure a
relative angular position of the drive shaft 108 relative to the second shaft
mount
210, a second encoder 404 may measure a relative position of the second pulley
204 and thereby a relative position of the second shaft 208 relative to the
second
shaft mount 210 (or may measure a position of the second shaft 208 relative to
the
second shaft mount 210 directly), and a third encoder 406 may measure a
position
of the motor housing 102 relative to the second shaft mount 210.
By measuring relative rotational positions of the drive shaft, the motor
housing and
the second shaft 108, 208, 102, strain in the torque transfer devices can be
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determined. In particular, strain in the cables 206, 306 between pulleys of
the
torque transfer devices 200, 300 can be determined (it may be assumed that
strain
within the solid, fixed components of the torque transfer devices i.e. the
pulleys
and the shafts, may be negligible) and thereby torque in the actuator 10 may
be
5 determined. Observation of the encoder readings over time may also give
an
indication of creep in the cables, and may indicate wear.
In some arrangements, only two encoders may be used, for example the first and
second encoders 402, 404, as the strain in the first cable 206 alone may be
10 sufficient to determine the torque delivered by the actuator 10.
The cables used may be formed of Kevlar, as Kevlar may have particularly
easily
determined tensile stress, for a given strain and may have sufficient strength
to
provide the required torque from the actuator. The cables 206a, b in the first
torque
transfer device 200 may carry less tension than the cables 306a, b of the
second
torque transfer device 300. The cables 206a, b of the first torque transfer
device
200 may therefore be thinner, or have a smaller diameter or cross section,
than
the cables 306a, b of the second torque transfer device 300.
Figure 2 shows the first and second torque transfer devices 200, 300 of the
actuator
10, with the other parts removed for clarity. It will be understood that the
motor
100 may be inside the fourth pulley 304 and that the drive shaft 108 extends
from
the motor 100. It can also be seen that the first pulley 208 may be formed
intrinsically with the drive shaft 108, and may comprise a helical groove
formed on
an outer surface of the drive shaft 108. Two cables 206a, 206b are wound
around
the first pulley 202, lying within the helical groove and may be wound so
that, when
the first pulley 202 is rotated about the drive shaft axis Al, a first cable
206a is
wound further onto the first pulley 202 and a second cable 206b is wound from
the
first pulley 202. The two cables 206a, 206b may be wound so as to end at
opposite
ends of the first pulley 202 or may extend through the first pulley 202
radially and
meet inside the first pulley 202. The cables 206a, 206b are also wound around
a
second pulley 204 and may lie in helical grooves of the second pulley 204. The
two
cables 206a, 206b may be separate portions of a single cable, joined at or
inside
the first or second pulley 202, 204.
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The two cables 206a, b may be parallel in the region where they extend between
the first pulley 202 and the second pulley 204. This may reduce the prospect
of the
two cables 206a, b touching or interfering with each other during operation of
the
actuator 10. With such an arrangement, the first and second pulleys 202, 204
will
be arranged to rotate in the same direction.
The second pulley 204 is connected to a second shaft 208, which is also
connected
to a third pulley 302, the third pulley 302 forming part of the second torque
transfer
device 300. The second pulley 204 and third pulley 302 may have substantially
different diameters, and their diameters may have a ratio in line with the
mechanical advantage of each individual torque transfer device 200, 300.
Specifically, the second pulley 204 may have a radius approximately five times
that
of the first pulley 202 and the fourth pulley 304 may have a radius
approximately
five times that of the third pulley 302. Therefore, in order to allow a
compact
arrangement, the second pulley 204 may have a radius approximately five times
that of the third pulley 302. Numbers other than five may be used, with the
principle
of the ratios of radii between the first-to-second pulleys 202, 204 and third-
to-
fourth pulleys 302, 304 being substantially the same remaining.
The third pulley 302 may be coupled to the fourth pulley 304 via four cables,
of
which only two 306a, 306b are shown. Two parallel cables may be wound onto the
third pulley 302 in interleaved helices as a single cable would have a
significantly
larger diameter, thereby requiring greater bending stresses to be imparted to
the
cable.
The four cables between the third and fourth pulleys 302, 304 may be parallel
in
the region where they extend between the third pulley 302 and the fourth
pulley
304. This may reduce the prospect of the cables touching or interfering with
each
other during operation of the actuator 10. With such an arrangement, the third
and
fourth pulleys 302, 304 will be arranged to rotate in the same direction.
Figure 3 shows a detailed cross section of an actuator 10, showing the shape
of the
second shaft mount 210. The second shaft mount 210 may be substantially
annular
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or semi-annular and may comprise a cross-member, disposed approximately
parallel to the drive shaft axis Al and the second shaft axis A2 and
approximately
diametrically opposite the second shaft axis A2, so that the drive shaft axis
Al is
between the cross member and the second shaft axis A2.
An electrical cable 500, which may extend through the actuator 10 can also be
seen
in Figure 3. A first end 502 of the electrical cable 500 is at an axial end of
the
actuator 10 adjacent the motor 100, and a second end 506 of the electrical
cable
500 is adjacent the fixed plate 218. An intermediate end 504 of the electrical
cable
500 may be an end of the electrical cable 500 within the actuator 10, which
may
carry electrical power to the motor 100 and/or may carry information from the
encoders out of the actuator 10.
The electrical cable 500 may also have a loop portion 508, which may extend
about
the motor housing 102, and the loop (shown in full in Figure 6) may move as
the
actuator rotates, moving the first end 502 relative to the second end 506. The
thin
cross section of the cable 500 can also be seen in Figure 3, where the cross
section
is elongated in a direction along the drive shaft axis Al. This may help to
form the
loop portion 508. The cross section of the cable 500 is therefore elongated in
a
direction along the actuator axis Al, having a thinner cross section in a
direction
perpendicular to the actuator axis Al (i.e. a radial direction).
Figure 4 shows the actuator 10 with an outer housing 600 included. It can be
seen
that the outer housing 600 is coupled to the motor housing 102 via motor
housing
output members 110. The outer housing 600 also has a central hole at an
opposite
end to the motor 100, via which the fixing plate 218 coupled to the second
shaft
mount can be accessed. It can also be seen that the electrical cable 500
extends
out of the outer housing 600 in both axial directions.
The outer housing 600 is substantially cylindrical, having a curved side
surface and
two axial end surfaces, and thereby encloses substantially all other
components of
the actuator 10, including the drive shaft 108, motor 100, first torque
transfer
device 200 and second torque transfer device 300 and all components thereof.
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From Figure 5, it can be seen that the outer housing 600 is formed from a
first part
600a and a second part 600b, the first and second parts 600a, b each being
substantially cylindrical and being axially separable. By forming the outer
housing
600 from two such parts, the housing may be more easily constructed around the
actuator 10.
The housing 600 may be 100mm long and 100mm in diameter. Preferably, a longest
dimension of the housing 600 is less than 150mm. The housing 600 encloses the
motor and the torque transfer devices. By providing a compact and cylindrical
housing 600, the actuator may overall provide a package which is suited to use
within humanoid robots.
There may be sufficient space inside the housing 600 adjacent the motor 100
for
providing a brake (not shown). The brake may be arranged to provide a braking
force to the drive shaft 108 and may therefore be positioned about the drive
shaft
108 on an opposite side of the motor 100 from the first pulley 202. For this
reason,
the drive shaft 108 may extend through the motor 100 and may protrude from the
motor 100 in two opposite directions.
Below are disclosed dimensions of three specific examples of actuator
arrangements, each having a housing with a diameter of 100mm. The smaller of
the pulleys in each torque transfer device has a diameter of 10mm.
In a first example, the motor has a diameter of 40mm and both torque transfer
devices have torque ratios of 4:1.
In a second example, the motor has a diameter of 50mm, the first torque
transfer
device has a torque ratio of 3:1, and the output torque transfer device has a
torque
ratio of 5:1.
In a third example, the motor has a diameter of 60mm, the first torque
transfer
device has a torque ratio of 2:1, and the output torque transfer device has a
torque
ratio of 6:1.
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Figure 6 shows the full extent of the electrical cable 500 arranged to extend
through
the actuator 10, including the 180 degree loop portion 508, which may be
movable
as the actuator 10 rotates, in order to avoid stretching and potentially
damaging
the cable.
Figure 7 shows an alternative pulley arrangement 700. The arrangement is
powered
by a motor 702, only an end view of which is visible, and which is arranged to
rotate
a drive shaft and pulley 704. The drive shaft and first pulley 704 are coupled
via a
first cable or band 706 to a second pulley 708, which is on a shaft with a
third pulley
710, the third pulley 710 being coupled to a fourth pulley 714 via a second
cable
or band 712. The fourth pulley 714 is coupled via a shaft to a fifth pulley
716, which
is coupled via a cable or band 718 to a sixth pulley 720, which is coupled via
a shaft
to a seventh pulley 722, which is coupled by a further cable or band 724 to
the
motor housing, incorporating a final pulley 726.
The arrangement shown in Figure 7 may be incorporated within an actuator
substantially similar to that described above, but with further pulleys and
shafts
incorporated so as to increase the mechanical advantage obtainable.
It would be understood that, although arrangements having one second shaft and
three second shafts as shown, arrangements having two shafts disposed away
from
the motor or four shafts disposed away from the motor may also be used.
Figure 8 shows a robot arm 800. The robot arm 800 has a vertical axis actuator
802 fixed to a horizontal base B, and a shoulder joint 804 coupled to the
vertical
axis actuator 802 and arranged to move a first member 806, which may also be
referred to as an upper arm 806. The upper arm 806 is coupled to a second
member
810, which may also be referred to as a lower arm 810 or a forearm 810. The
upper
arm 806 is coupled to the forearm 810 at an elbow joint 808 and there is, at
an
end of the forearm 810 opposite the elbow joint 808, an end effector 812.
An actuator as described above may be placed at the elbow joint 808 or at the
shoulder joint 804 and the second shaft mount 210 and fixation plate 218 may
be
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fixed to any of the vertical actuator 802, upper arm 806 or lower arm 810 as
appropriate.
Figures 9 shows an alternative actuator 850. The actuator 850 comprises a
motor
5 852 having a housing 853 and a drive shaft 854 extending from the housing
853.
The motor 852 is arranged to rotate the drive shaft 854 relative to the
housing 853
about a drive shaft axis A3. The drive shaft 854 may also act as a pulley or
may be
fixed to a pulley and thus may drive a cable or band in order to rotate a
second
pulley 856.
The second pulley 856 is supported on a second shaft 853, which is arranged to
rotate about a second shaft axis A4, spaced apart from the drive shaft axis
A3. The
second shaft 853 also has a third pulley 857, which is arranged to drive a
band or
cable fixed or otherwise coupled to a fourth pulley 860. The motor housing 853
is
within, or may be fixed to or integral with the fourth pulley 860.
It will be understood that the above-described aspects of the alternative
actuator
850 are substantially similar to corresponding aspects of the actuator 10
described
previously. The alternative actuator 850 may therefore also share other
features of
the actuator 10 not explicitly described in conjunction with the alternative
actuator
850.
Figure 10 shows how the alternative actuator 850 may be incorporated within a
robotic joint 880. The robotic joint 880 is arranged to pivot two members
relative
to each other about the drive shaft axis A3.
As shown in figure 10, the second shaft 853 is arranged to be supported by
external
members 884a, b. For this reason, the second shaft 853 has circular ends 872a,
b,
which may further comprise bearings, arranged to be received in corresponding
holes in the external members 884a, b.
The external members 884a, b are coupled to face flanges 868a, b, which may
also
be referred to as connection flanges and which each have bearing surfaces
867a,
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b, which are centred on the drive shaft axis A3 and the external members 884a,
b
are therefore arranged to rotate about the drive shaft axis A3.
The face flanges 868a, b have bolt holes to allow them to be coupled to
further
external parts. The face flanges 868a, b also act to provide resilient
supports for
respective bearing surfaces 867a, b.
The first face flange 868a is coupled to the motor housing 853 via a cross
member
870. The cross member 870 has the shape of a sector of an annulus or of an
extruded arc and extends partially around the drive shaft 854. By having this
shape,
the cross member 870 may provide good strength to the actuator 850. The
portion
of the actuator coupled to the cross member 870 at an end opposite to the
motor
housing 853 may also support the drive shaft 854 and may thereby improve the
stiffness of the driveshaft 854.
A second of the bearing surfaces 867b is coupled to the motor housing 853 at
an
end opposite to the drive shaft 854. The second bearing surface 867b may be
directly fixed to the motor housing 853.
The external members 884a, b are connected via a second cross member 886,
which may improve the structural stiffness of the joint arrangement 880. The
cross
member 886 may also prevent separation of the external members 884a, b. The
external members may be part of or may be fixed to or integral with a member
of
a robotic arm, such as the robotic arm 800 shown in figure 8.
An adjacent member of the robotic arm may be fixed to or integral with two
further
external members 882a, 882b. A first of the further external members 882a is
fixed
to the cross member 870 or between the cross member 870 and the first face
flange
868a. A second of the further external members 882b is fixed to the motor
housing
853.
As shown in figure 10, the second shaft 853 may be supported within a member
of
a robotic arm within which the actuator 850 is located. By increasing the
separation
distance between the drive shaft axis A3 and the second shaft axis A4, larger
pulleys
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and/or a larger motor to be used and may therefore improvement of the torque
supplied by the actuator 850 may be achieved without an increase in the size
of
the joint.
In a first example of the alternative actuator, the motor has a diameter of
60mm,
the first torque transfer device has a torque ratio of 4:1, and the output
torque
transfer device has a torque ratio of 6:1. The smaller of the pulleys in each
torque
transfer device has a diameter of 10mm.
In a second example of the alternative actuator, the motor has a diameter of
90mm,
the first torque transfer device has a torque ratio of 4:1, and the output
torque
transfer device has a torque ratio of 9:1. The smaller of the pulleys in each
torque
transfer device has a diameter of 10mm.
The improved torque conversion and motor size may be enabled by the spacing
between the drive shaft axis and the second shaft axis allowed by the second
shaft
axis being supported by an external member.
Figure 11 shows a robotic arm 890 incorporating the alternative actuator 850
and
the robotic joint 880. The robotic arm has a first member 892, which is a
lower arm
and a second member 894 which is an upper arm. From figure 11, it can be seen
that the envelope in which the actuator 850 lies sits comfortably within an
elbow
joint and that the external members 882, 884 may lie along the first and
second
members 892, 894.
Figure 12 shows a reverse view of the fourth pulley 304, showing how the cable
portions may be connected to the second pulley 304 via connectors 900.
From Figure 12, it can be seen that the connectors 900 are substantially
curved,
having a similar curvature to that of the outer surface of the fourth pulley
304 and
have cable portions 912, 914 extending away from the connectors, the cable
portions 912, 914 being between a body of the connector 900 and the second
pulley
304, and the cable portions 912, 914 are connected to the connectors 900 at
fixation points 902, 906, which are located on the connectors 900 at an
opposite
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end from that at which the cable portions 912, 914 extend away from the
connectors 500.
It will be understood that the cable portions 912 may be the same cable
portions
.. as the first and second cable portions 306A, B shown in figure 2.
Figure 13 shows the fourth pulley 304 with one of the connectors 900 removed,
exposing a receiving portion 1000 for receiving the connector 900. The
receiving
portion 1000 has two helical grooves 1004, 1006 for receiving the first and
second
.. cable portions 912, 914 and receiving teeth 1002 for engaging with
respective teeth
of the connector 900.
Figure 14 shows a section view of a connector 900, with a portion removed. It
can
therefore be seen that the cable portion 912 extends along the length of the
connector from a first fixation point 902. The connector 900 also comprises a
body
920, which is a substantially flat, curved portion radially outside the cable
portion
912, and which has a toothed portion 922 on a radially inner side, adjacent
the
cable portions 912.
.. The toothed portion 922 comprises teeth 924, each tooth having an
engagement
surface 926, facing a first direction away from the first fixation point 902
and
substantially perpendicular to the body portion 920, and facing towards a
second
fixation point 904. Each tooth also has an angled surface 928, whose structure
supports the engagement surface 926, and subtends an angle of between 20 and
60 degrees with the body portion 920 and a curved surface 930 joining the
engagement surface 926 and the angled surface 928. Each tooth 924 may be solid
and defined by the engagement surfaces 926, angled surface 928 and curved
surface 930 and may extend away from the body portion 920.
.. Figure 15 gives a plan view of the connector 900, showing two parallel
cable
portions 912, 914 and the toothed potion 922 lying between the cables. The
second
cable portion 914 is also fixed to the connector 900 at two fixation points
906, 908,
and the toothed portion 922 lies between the fixation points 902, 904, 906,
908.
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By providing such a symmetrical arrangement, stresses on the connector may be
more equally balanced and bending forces on the teeth may be reduced.
The connector 900 may be formed by a moulding process, optionally an injection
moulding process, and may be moulded around the cable portions 912, 914. The
cable portions 912, 914 may be placed in the mould and maintained in tension
as
plastic is introduced into the mould and the plastic may diffuse through the
fibres
of the cables. By moulding the connectors in this way, a more consistent
tension
may be formed along the cables. The connectors 900 may thereby be formed with
a level of residual stress, which manifests as a tensile stress in each of the
cable
portions 912, 914 and a compressive stress in the body portion 920.
Portions of excess cable may extend out of the mould in both directions (i.e.
in both
directions from the fixation point 902, 906) and these cable portions may be
used
to secure the connector 900 to the second pulley 304 and subsequently removed.
The excess cable portions (not shown) may extend away from the fixation points
902, 906 and may be held in tension in order to resiliently couple the
connector
900 to a pulley 304.
A further disclosure is set out in the following clauses:
A. A connector for coupling a cable to a pulley, comprising:
a toothed portion having a body and a plurality of teeth extending away
from the body, the teeth each having an engagement surface facing a first
direction,
and
a cable portion, extending along the toothed portion and fixed to the toothed
portion at a first fixation point, the cable portion extending away from the
first
fixation point and along the toothed portion in the first direction.
B. The connector of clause A, wherein the cable portion is fixed to the
toothed
portion at a second fixation point, the plurality of teeth being located
between the
first and the second fixation points.
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C. The connector of clause A or B, wherein the cable portion is a first cable
portion
and
wherein the connector further comprises a second cable portion fixed to the
toothed portion at a third fixation point, the cable portion extending away
from the
5 third fixation point and along the toothed portion in the first direction
substantially
parallel to the first cable portion.
D. The connector of clause C, wherein the second cable portion is fixed to the
toothed portion at a fourth fixation point, the plurality of teeth being
located
10 between the third and the fourth fixation points.
E. The connector of clause C or D, wherein the plurality of teeth are located
between
the first and second cable portions.
15 F. The connector of any preceding clause, wherein the first and/or
second cable
portion terminates at the first and or third fixation point respectively.
G. The connector of any preceding clause, wherein the toothed portion is
curved.
20 H. The connector of any preceding clause, wherein the toothed portion is
less
flexible than the cable portion.
I. The connector of any preceding clause, wherein each engagement face is
substantially perpendicular to the body.
J. The connector of any preceding clause, wherein the engagement faces of the
teeth each are normal to and lie along a circular arc.
K. The connector of any preceding clause, wherein the teeth are substantially
triangular, each tooth having an angled face extending between the engagement
face and the body.
L. The connector of clause K, wherein the angled faces meet the body at an
angle
of between 10 and 60 .
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M. The connector of clause K or L, wherein the teeth each have a curved
surface
where the angled face meet the engagement face.
N. A pulley, having a cylindrical surface, the cylindrical surface having a
receiving
portion, the receiving portion having a toothed recessed arranged to receive
the
connector of any preceding clause.
0. The pulley of clause N, further comprising a helical groove arranged to
receive
the cable portion.
P. A pulley and cable system, comprising the pulley of clause N or 0 and the
connector of any one of clauses A to M.
Q. A method of manufacturing a connector, the method comprising:
providing a mould for forming a connector portion;
inserting a cable portion through at least one wall of the mould; and
moulding a connector portion around the cable portion while the cable
portion is held in tension in the mould, such that the connector portion is
formed
around the cable with a tensile residual stress in the cable.
R. The method of clause Q, wherein the cable portion extends through the
connector
portion, such that the cable portion extends away from the connector portion
in two
directions.
S. The method of clause Q or R, wherein the material used to form the
connector
portion diffuses through the fibres of the cable portion.
T. The method of clause Q, R or S, wherein the method forms a connector
according
to any one of clauses A to M.
U. A method of constructing the pulley and a cable system of clause P.
comprising:
holding an excess cable portion extending from the first or third fixation
point away from the connector portion,
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moving the connector into engagement with the receiving portion while
exerting tension on the excess cable portion; and
removing the excess cable portion after the connector is engaged with the
receiving portion.
Although the invention has been described above with reference to one or more
preferred embodiments, it will be appreciated that various changes or
modifications
may be made without departing from the scope of the invention as defined in
the
appended claims.
