U.S. patent application number 11/572371 was filed with the patent office on 2007-09-20 for actuator.
Invention is credited to Timothy John Jones.
Application Number | 20070219031 11/572371 |
Document ID | / |
Family ID | 32893862 |
Filed Date | 2007-09-20 |
United States Patent
Application |
20070219031 |
Kind Code |
A1 |
Jones; Timothy John |
September 20, 2007 |
Actuator
Abstract
This invention relates to an actuator and has particular
reference to linear and rotary actuators, particularly those for
use in the control of robotic arms. The invention provides a
relatively low cost means of achieving substantially backlashfree
rotary and/or linear motion. There is provided an actuator (10)
comprising: a first drive pulley (28); a second drive pulley (29),
the first drive pulley and the second drive pulley being
interconnected to rotate together; a first driven pulley (52; 70);
a second driven pulley (53; 72); an endless drive belt (40)
engaging the first and second drive pulleys and the first and
second driven pulleys; a motor (19) connected to drive the first
and second drive pulleys to rotate and drive the endless drive
belt; a driven member (50; 88; 71, 73) carrying at least one of the
driven pulleys; the first and second drive pulleys being arranged
so that upon rotation thereof the circumferential speed of the
first drive pulley is different from the circumferential speed of
the second drive pulley, the endless drive belt being looped around
the drive pulleys and the driven pulleys so that the difference
between the circumferential speed of the first drive pulley and the
circumferential speed of the second drive pulley causes movement of
the driven member.
Inventors: |
Jones; Timothy John;
(Wakefield, GB) |
Correspondence
Address: |
MOORE & VAN ALLEN PLLC
P.O. BOX 13706
Research Triangle Park
NC
27709
US
|
Family ID: |
32893862 |
Appl. No.: |
11/572371 |
Filed: |
July 18, 2005 |
PCT Filed: |
July 18, 2005 |
PCT NO: |
PCT/GB05/02834 |
371 Date: |
May 1, 2007 |
Current U.S.
Class: |
474/148 |
Current CPC
Class: |
Y10T 74/18832 20150115;
F16H 19/06 20130101; Y10T 74/18848 20150115; Y10T 74/1836 20150115;
F16H 2019/0609 20130101 |
Class at
Publication: |
474/148 |
International
Class: |
F16H 19/06 20060101
F16H019/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 20, 2004 |
GB |
0416186.5 |
Claims
1. An actuator (10) comprising: a first drive pulley (28); a second
drive pulley (29), the first drive pulley and the second drive
pulley being interconnected to rotate together; a first driven
pulley (52; 70); a second driven pulley (53; 72); an endless drive
belt (40) engaging the first and second drive pulleys and the first
and second driven pulleys; a motor (19) connected to drive the
first and second drive pulleys to rotate and drive the endless
drive belt a driven member (50; 88; 71, 73) carrying at least one
of the driven pulleys; the first and second drive pulleys being
arranged so that upon rotation thereof the circumferential speed of
the first drive pulley is different from the circumferential speed
of the second drive pulley, the endless drive belt being looped
around the drive pulleys and the driven pulleys so that the
difference between the circumferential speed of the first drive
pulley and the circumferential speed of the second drive pulley
causes movement of the driven member.
2. An actuator according to claim 1 in which the first and second
drive pulleys are arranged so that upon rotation thereof the
circumferential speeds differ because of: {i} the first drive
pulley having a different circumferential length than the second
drive pulley; {ii} the first drive pulley having a different rate
of rotation to the second drive pulley; {iii} the first drive
pulley having a different circumferential length than the second
drive pulley and a different rate of rotation to the second drive
pulley.
3. An actuator according to claim 1 in which there are two driven
members (71, 73), the driven members being interconnected together
by a second drive belt (74), the second drive belt engaging a
pulley (81) mounted for rotary movement.
4. An actuator according to claim 1 in which the driven member is a
carriage (50; 88) mounted for linear movement.
5. An actuator according to claim 4 in which the first driven
pulley and the second driven pulley are both mounted upon the
carriage (50; 88), the actuator further including: a first guide
pulley (48) and a second guide pulley (49) the endless drive belt
also engaging the first guide pulley and the second guide
pulley.
6. An actuator according to claim 5 in which the driven pulleys
(52, 53) are located between the drive pulleys (28, 29) and the
guide pulleys (48, 49).
7. An actuator according to claim 6 in which the endless drive belt
passes from the first drive pulley (28) to the first driven pulley
(52), from the first driven pulley to the second drive pulley (29),
from the second drive pulley to the first guide pulley (48), from
the first guide pulley to the second driven pulley (53), from the
second driven pulley to the second guide pulley (49), and from the
second guide pulley back to the first drive pulley, the endless
drive belt defining a first loop between the first driven pulley
and the drive pulleys, and a second loop between the second driven
pulley and the guide pulleys, rotation of the drive pulleys in a
first direction causing an increase in the length of the first loop
and a corresponding decrease in the length of the second loop.
8. An actuator according to claim 6 having a third driven pulley
(84) and a fourth driven pulley (85), the drive pulleys (28, 29)
and all four of the driven pulleys (52, 53, 84, 85) being mounted
upon the driven member (88), in which the endless drive belt passes
from the first drive pulley (28) to the first driven pulley (52),
from the first driven pulley to the first guide pulley (48), from
the first guide pulley to the third driven pulley (84), from the
third driven pulley to the second drive pulley (29), from the
second drive pulley to the fourth driven pulley (85), from the
fourth driven pulley to the second guide pulley (49), from the
second guide pulley to the second driven pulley (53), and from the
second driven pulley back to the first drive pulley, the endless
drive belt defining a first loop between the first guide pulley and
the first and third driven pulleys, and a second loop between the
second guide pulley and the second and fourth driven pulleys,
rotation of the drive pulleys in a first direction causing an
increase in the length of the first loop and a corresponding
decrease in the length of the second loop.
9. An actuator according to claim 1 having two driven members (71,
73), each driven member mounting a respective driven pulley (70,
72), in which the endless drive belt passes from the first drive
pulley (28) to the first driven pulley (70), from the first driven
pulley to the second drive pulley (29), from the second drive
pulley to the second driven pulley (72), and from the second driven
pulley back to the first drive pulley, the endless drive belt
defining a first loop between the first driven pulley and the drive
pulleys, and a second loop between the second driven pulley and the
drive pulleys, rotation of the drive pulleys in a first direction
causing an increase in the length of the first loop and a
corresponding decrease in the length of the second loop.
10. An actuator according to claim 1 having two driven members (71,
73), each driven member mounting a respective driven pulley (70,
72), a first guide pulley (48) and a second guide pulley (49), in
which the endless drive belt passes from the first drive pulley
(28) to the first guide pulley (48), from the first guide pulley to
the first driven pulley (70), from the first driven pulley to the
second drive pulley (29), from the second drive pulley to the
second driven pulley (72), from the second driven pulley to the
second guide pulley (49), and from the second guide pulley back to
the first drive pulley, the endless drive belt defining a first
loop between the first driven pulley and the second drive and first
guide pulleys, and a second loop between the second driven pulley
and the second drive and second guide pulleys, rotation of the
drive pulleys in a first direction causing an increase in the
length of the first loop and a corresponding decrease in the length
of the second loop.
11. An actuator according to claim 1 in which a control line (55;
55a, 55b) is connected to the driven member, movement of the driven
member causing corresponding movement of the control line.
12. An actuator according to claim 1 in which the first drive
pulley (28) and the second drive pulley (29) are interconnected to
rotate in opposite directions.
13. An actuator according to claim 1 in which the first drive
pulley and the second drive pulley each have an axis of rotation,
the axis of rotation of the first drive pulley being parallel to
the axis of rotation of the second drive pulley, and in which the
first driven pulley and the second driven pulley also each have an
axis of rotation, the axis of rotation of the first driven pulley,
and the axis of rotation of the second driven pulley, being
non-parallel with the axis of rotation of the first drive pulley
and the axis of rotation of the second drive pulley.
14. An actuator according to claim 13 in which the drive pulleys
and the driven pulleys are arranged so that the drive belt
undergoes twisting movement on the path between these pulleys, but
undergoes substantially no lateral movement.
Description
[0001] This invention relates to an actuator and has particular
reference to linear and rotary actuators, particularly those for
use in the control of robotic arms.
[0002] With the increasing use of robotic arms in manufacturing
processes and for general inspection and maintenance functions,
there has been an increase in the number of control functions
associated with each arm and it is not uncommon for a
sophisticated, multifunctional arm to require upwards of thirty
separate control elements. Each of these elements needs to be
individually controlled by a separate actuator.
[0003] It thus becomes necessary to concentrate a large number of
control functions in a relatively small space. Many robotic arms
are controlled or manipulated by control wires. As the arms become
more sophisticated and smaller in size and hence able to undertake
more intricate operations, those same intricate operations require
more control functions and the demand, increasingly, is for more
control functions in ever reducing space.
[0004] Traditionally, actuators for the control wires of robotic
arms have been of the capstan type. These comprise a motor driving
a windup capstan around which the control wire is wrapped.
Operation of the motor drives the capstan and causes or allows
corresponding movement of the wire. These capstan drives are
relatively bulky, and furthermore tend to produce stretching and
slippage of the wire about the capstan thus rendering precision of
control more difficult. In order to overcome this problem, it is
frequently the case that the control mechanism and motor drives for
each control are spaced from the base or datum of the robotic arm
thus resulting in a relatively large and bulky control assembly for
the arm.
[0005] In an attempt to overcome this, linear actuators have been
produced having a movable carriage to which the control wire is
attached and the whole is mounted upon a screw or worm so the
rotation of the screw or worm results in movement of the carriage.
A disadvantage of this arrangement is the relative weight of the
assembly and the inevitable backlash in the worm or screw
arrangement. Furthermore, such arrangements tend to have relatively
high frictional forces and the mechanical advantage achievable is
effectively that of an inclined plane defined by the pitch of the
screw.
[0006] An alternative is to use a ball screw arrangement, which
could minimise backlash. However, such an arrangement is expensive
and only suitable for short stroke arrangements, as the weight of
the screw increases with length, and the inertia of long-stroke
ball screws is not generally acceptable. In addition, connecting a
control wire to a ball screw arrangement is typically difficult in
practice.
[0007] There is, therefore, a need for a linear actuator capable of
providing high loading to a control wire with low frictional loss
and yet of a compact construction so that a number may be clustered
in a relatively small space for control of a robotic arm or like
device.
[0008] Many actuators used in industry for general positioning (as
compared to those for pulling wires) are electrically driven and
can be subdivided into several speed categories and several
precision categories. The speed categories are slow (say 0-25 mm
per second), medium (say 25-500 mm per second) and high (>500 mm
per second). The precision categories can be defined as zero
precision (absolute positioning not required and backlash
unimportant), low precision (positioning required but low accuracy
and some backlash acceptable say 0.25-3 mm) and high precision
(where accurate positioning and zero/low backlash is required--say
0.001-0.25 mm). Such actuators are often powered by rotary electric
motors; although direct drive linear actuators are available, they
are costly to build and control. Where the linear actuator is
powered by a rotary electric motor, several design options are
available to produce the linear motion, these include belt drives,
lead screws, ball screws, and rack and pinion mechanisms. In most
cases for low and medium speed applications, a gearbox will be
required between the motor and the linear motion converter in order
to match the motor speed (typically 3000-6000 rpm) to the required
pulley/pinion/ball-screw speed. The gearbox adds considerably to
the cost, particularly if high precision is required as in a
zero/low backlash environment as the gearbox must be at least as
accurate as the motor and the linear motion converter.
[0009] The present invention relates to an actuator having a drive
belt to generate motion using a harmonic principle, which reduces
or eliminates the need for a gearbox between the motor and the
drive pulley for low and medium speed applications. As an
additional benefit, such a drive is substantially backlash free
thereby allowing a high precision drive at relatively low cost.
[0010] According to the invention, there is provided an actuator
comprising:
a first drive pulley;
a second drive pulley, the first drive pulley and the second drive
pulley being interconnected to rotate together;
a first driven pulley;
a second driven pulley;
an endless drive belt engaging the first and second drive pulleys
and the first and second driven pulleys;
a motor connected to drive the first and second drive pulleys to
rotate and drive the endless drive belt;
a driven member carrying at least one of the driven pulleys;
[0011] the first and second drive pulleys being arranged so that
upon rotation thereof the circumferential speed of the first drive
pulley is different from the circumferential speed of the second
drive pulley, the endless drive belt being looped around the drive
pulleys and the driven pulleys so that the difference between the
circumferential speed of the first drive pulley and the
circumferential speed of the second drive pulley causes movement of
the driven member.
[0012] In the following specific description, the embodiments
described utilise a drive belt. The term "drive belt" is also used
in the claims and preamble, but unless otherwise indicated the term
"belt" should be understood to encompass any suitable continuous or
endless drive member such as a chain, cable, wire or the like.
Similarly, the term "pulley" is used in the following specific
description throughout, as it is common to use pulleys with a drive
belt, but the term "pulley" should be understood to encompass gears
and sprockets as well as toothed or untoothed wheels and rollers,
or the like.
[0013] The circumferential speeds can differ because of the drive
pulleys having different circumferential lengths, having different
rates of rotation, or both.
[0014] The drive belt may be toothed, and in such embodiments at
least the drive pulleys are preferably toothed to engage with the
belt.
[0015] The actuator can be a rotary actuator, preferably having two
driven members which are interconnected together by a second drive
belt, the second drive belt engaging a pulley mounted for
(controlled) rotary movement. Alternatively the actuator can be a
linear actuator, preferably having a carriage mounted for linear
movement.
[0016] Certain embodiments of linear and rotary actuators also
include one or more guide pulleys which are also engaged by the
endless drive belt and serve to guide and/or redirect the drive
belt between the drive and driven pulleys. Alternatively, the first
drive pulley and the second drive pulley are interconnected to
rotate in opposite directions, which can avoid the requirement for
guide pulleys.
[0017] A control line can be connected to the driven member,
movement of the driven member causing corresponding movement of the
control line. The control line can be connected to a robotic arm,
for example.
[0018] The motor may be an electric motor. In a particular
embodiment of the present invention, the electric motor may drive
the drive pulleys via a worm and wheel assembly thus providing
additional mechanical advantage. In these embodiments, it is
sometimes preferred that the worm and wheel have a pitch selected
to prevent back drive. This has the advantage that no braking
mechanism is necessary.
[0019] It can be arranged that the drive belt is always flexed in
one direction around the various pulleys, i.e. preferably towards
the teeth in embodiments of belt having teeth. Alternatively, the
drive belt can be flexed in both directions, i.e. both towards the
teeth and away from the teeth as it passes around the various
pulleys, so that the belt teeth face outwards as they pass around
some of the pulleys. The flexing of the belt in both directions is
described herein as contraflexure drive, and contraflexure drive
can be used with both linear and rotary actuators.
[0020] In all embodiments of the invention where a drive belt is
specifically used (i.e. as opposed to a chain or cable for example)
the path of the drive belt between the drive pulleys, the driven
pulleys and the guide pulleys (if present) respectively, is
orientated by means of an angled pulley. The size and angle of the
pulley is chosen so that the central axis of the drive belt fibres
is substantially co-axial between the respective pulleys. This
means that the drive belt is twisted along this axis with little or
no translation or lateral movement, thus minimising stress
variation across the belt fibres.
[0021] Linear actuators in accordance with the present invention
have been found to be particularly useful in the control of robotic
arms of the type described in international application no.
WO2002/016995, the disclosure of which is incorporated herein by
reference.
[0022] There follows a description of several exemplary embodiments
of actuator in accordance with the present invention, with
reference to the accompanying drawings, in which:
[0023] FIG. 1 is a perspective view of a first embodiment of linear
actuator in accordance with the present invention,
[0024] FIG. 2 is a schematic representation, in end view, showing
how the driven pulleys are positioned with respect to the drive
pulleys,
[0025] FIG. 3 is a top view of the actuator of FIG. 1,
[0026] FIG. 4 is a cut-away side view of the actuator of FIG.
1,
[0027] FIG. 5 is a sectional view on the line B-B of FIG. 3,
[0028] FIG. 6 is a part section on the line E-E of the carriage of
FIG. 3,
[0029] FIG. 7 is a part section on the line C-C of FIG. 4,
[0030] FIG. 8 is a section through the drive pulleys on the line
A-A of FIG. 4,
[0031] FIG. 9 is a detailed view of the area labelled D in FIG.
4,
[0032] FIG. 10 is an exploded view of the drive assembly of the
actuator of FIG. 1,
[0033] FIG. 11 is a perspective view of the actuator of FIG. 1 with
the housing removed,
[0034] FIG. 12 is a perspective view of a second embodiment of
linear actuator according to the invention, with part of the
carriage removed,
[0035] FIG. 13 is a perspective view of the drive belt
configuration in the actuator of FIG. 12,
[0036] FIG. 14 is a sectional view along line G-G of the FIG.
15,
[0037] FIG. 15 is a top view of the actuator of FIG. 12, with the
carriage complete,
[0038] FIG. 16 is a side view of the actuator of FIG. 15,
[0039] FIG. 17 is a sectional view along line F-F of FIG. 15,
[0040] FIGS. 18-20 are diagrammatic views of a first embodiment of
a linear and/or rotary actuator in accordance with the present
invention,
[0041] FIG. 21 is a variant of the embodiment of FIGS. 18-20,
[0042] FIG. 22 is a perspective view of an embodiment of rotary
actuator according to the invention,
[0043] FIG. 23 is a sectional view along the line H-H of FIG.
25,
[0044] FIG. 24 is a perspective view of the drive belt
configuration in the embodiment of FIG. 22,
[0045] FIG. 25 is a top view of the actuator of FIG. 22, and
[0046] FIG. 26 is a sectional view along the lines L-L of FIG.
25.
[0047] In the following description the same reference numerals are
used for similar components in the various embodiments.
[0048] Referring first to FIG. 1, the actuator illustrated
generally at 10 comprises a longitudinal, channel shaped housing 11
having, at a first end 12, a motor and control assembly indicated
generally at 14, and having at a second end 13, a control wire
guide 15 (see FIG. 7)
[0049] The motor and control assembly 14 comprises a worm block 16
the base 17 of which is secured to the base of the channel shaped
housing 11 at first end 12. The rear face 9 of worm block 16
carries an electric motor 19, which through coupling 7 (FIG. 5)
turns the drive shaft 20 which projects through said rear face 9
and terminates in a drive worm 21 (see FIGS. 5 and 10). The front
face of the worm block 16 is cut away to provide a front opening 31
(FIG. 10).
[0050] The sidewalls 23 of worm block 16 each carry a circular
opening to provide a transverse bore 24, each of said openings
accommodating a respective bearing 25. Each bearing 25 is adapted
to receive for rotation therein an axle 26 having mounted thereon
and for rotation therewith a worm wheel 27, a first drive pulley 28
and a second drive pulley 29. In this embodiment, the worm wheel
27, first drive pulley 28 and second drive pulley 29 are formed as
a unit; it will be appreciated that the first drive pulley 28 and
the second drive pulley 29 may be formed as independent components
each of which may be keyed to worm wheel 27 for rotation therewith
by means well-known in the art, or as shown in this embodiment be
joined by pin 8. The worm wheel and pulleys assembly is provided
with shims and washers 34 for appropriate location of the worm
wheel 27 and its associated first and second drive pulleys 28 and
29 relative to the front opening 31.
[0051] Each of the first drive pulley 28 and the second drive
pulley 29 are provided with respective teeth 32. In this embodiment
the radius and therefore the circumferential length of the second
drive pulley 29 is greater than that of the first drive pulley 29,
so that the second drive pulley 29 is provided with one more tooth
than the first drive pulley 28 (in other embodiments the drive
pulleys can differ in circumferential length by more than one
tooth). Each of the drive pulleys 28 and 29 is adapted to receive a
wrap of an endless drive belt 40.
[0052] Drive belt 40 is an endless or continuous belt which in this
embodiment is of generally rectangular shaped cross-section having
teeth on the inner surface thereof. Each of said teeth is adapted
to engage corresponding teeth 32 on pulleys 28 and 29
respectively.
[0053] The fixings for worm block 16 also secure a rearward
extension 35 to housing 11 which extension 35 carries a printed
circuit board 36 through which electric motor 19 is controlled.
[0054] The second end 13 of housing 11 also carries a pulley
assembly indicated generally at 45 (see FIG. 3). The pulley
assembly 45 is associated with control wire guide indicated
generally at 15 (FIG. 7). The assembly 45 is supported by slots 46
(FIG. 1) in housing 11 which slots carry a transverse axle 47
(FIGS. 1 and 7) which in turn carries, for rotation thereon, guide
pulleys 48 and 49 respectively. Each of guide pulleys 48 and 49 is
adapted to accept a wrap of belt 40 and each of guide pulleys 48
and 49 is independently rotatable on transverse axle 47. The
position of the axle 47 is maintained in slots 46 by the tension in
drive belt 40. The drive belt tension is set during assembly by
virtue of ramps 6 acting on bearings 25 in housing 11 (see FIG. 4).
In an alternative embodiment, a support piece for axle 47 may be
releasably secured to the housing and means provided for biasing
the support piece away from the motor and control assembly 14 for
the purpose of tensioning the drive belt 40.
[0055] Driven member or carriage 50 comprises a longitudinal member
at 51 having a pair of longitudinally spaced driven pulleys 52 and
53 mounted for rotation with respect thereto. A forward extension
54 of longitudinal member 51 carries connecting means of generally
known type for connecting a control wire 55 to the carriage by
wrapping control wire 55 around helix 59 and securing with clamp 60
(see FIG. 9). In this particular embodiment, control wire 55
extends from the carriage 50 through guide hole 56 provided in the
cylindrical surface of axle 47 and about an idler wheel 57 to exit
the housing 11 by means of opening 58. The arrangement is such that
movement of carriage 50 generally along the longitudinal axis of
housing 11 results in corresponding movement of control wire
55.
[0056] The drive belt 40 is configured generally as shown in FIGS.
10 and 11. The drive belt passes from the first drive pulley 28 to
the first driven pulley 52, from the first driven pulley 52 to the
second drive pulley 29, from the second drive pulley 29 to the
first guide pulley 48, from the first guide pulley 48 to the second
driven pulley 53, from the second driven pulley 53 to the second
guide pulley 49, and from the second guide pulley 49 back to the
first drive pulley 28. The drive belt 40 defines a first loop
between the first driven pulley 52 and the drive pulleys 28,29, and
a second loop between the second driven pulley 53 and the guide
pulleys 48,49, rotation of the drive pulleys 28,29 in a first
direction causing an increase in the length of the first loop and a
corresponding decrease in the length of the second loop.
[0057] As is more clearly seen in FIG. 2, the axis of rotation of
the first driven pulley 52 is angled with respect to the (common)
axis of rotation of the drive pulleys 28 and 29. This allows the
path of the drive belt 40 to be aligned with both of the drive
pulley 28 and the drive pulley 29 without requiring lateral
movement or deformation of the belt.
[0058] As is also seen in FIG. 2, the driven pulley 52 is
positioned and sized such that the central axis 90 of the drive
belt 40, i.e. the central axis of the longitudinal fibres of the
drive belt, at the point at which the drive belt joins and leaves
the driven pulley 52, is precisely aligned with the central axis 90
of the drive belt 40 at the point at which the belt leaves or joins
the drive pulleys 28 and 29 respectively. It will be understood
that the belt 40 is caused to twist as it moves between a drive
pulley 28,29 and the driven pulley 52, but that the twist is
effected about the central axis 90 of the belt fibres. It is well
recognised that a drive belt of this type is less likely to suffer
damage or deterioration if it is twisted about its central axis,
than would be the case if it was twisted about some other axis,
and/or is required to move or deform laterally.
[0059] Clearly, it is not necessary that the driven pulley aligns
the path of the drive belt precisely with the drive pulleys, nor
that the central axis of the drive belt as it leaves or joins the
driven pulley is precisely aligned with the central axis of the
belt as it joins or leaves a drive pulley, but the greater the
misalignment the greater the likelihood of damage and deterioration
of the belt, so that substantial alignment is preferred and precise
alignment is ideal.
[0060] The relationship between the driven pulley 53 and the guide
pulleys 48, 49 is the same as that shown in FIG. 2 so that the
drive belt 40 only undergoes twisting movement, about its central
axis, during its whole path of movement, regardless of the position
of the carriage 50. Whilst this ideal relationship between the
pulleys is specifically described for the embodiment of FIGS. 1-11,
the relationship between the pulleys in the other embodiments shown
in the Figures is similarly ideal, though in those embodiments also
the pulleys could be substantially aligned rather than precisely
aligned, if desired.
[0061] In embodiments using a chain, cable or the like instead of
the drive belt 40, the substantial or precise alignment of the
pulleys could be maintained, though the location of the axis about
which the chain, cable or the like should ideally twist may depend
upon the structure of that component.
[0062] It will be appreciated that by tensioning the drive belt 40,
the carriage 50 can be effectively suspended between the pulley
assembly 45 and the motor and control assembly 14. This has the
advantage of reducing the frictional forces within the actuator
which would otherwise be occasioned by having the carriage travel
along a track. In an alternative embodiment of the present
invention, where tension in the continuous drive means cannot be
maintained, or vibration of the belt needs to be controlled, track
means may be provided for guiding and/or supporting the
carriage.
[0063] Operation of the motor 19 causes rotation of the drive worm
gear 21. Drive worm gear 21 engages worm wheel 27 and transmits
rotation to each of drive pulleys 28 and 29. Since drive pulley 29
is provided with one more tooth then drive pulley 28, rotation of
each pulley by one complete revolution will ensure that the part of
the drive belt 40 which is wrapped around pulley 29 will advance
further than the part of the drive belt 40 which is wrapped around
pulley 28, by the pitch of one tooth. This will result in an
increase in the length of the first loop of the belt (between the
drive pulleys and the driven pulley 52) by the pitch of the teeth
and a corresponding increase in the distance between the drive
pulleys 28,29 and the carriage 50 by half the pitch of the teeth.
Correspondingly, the feed of the drive belt 40 from guide pulley 48
will advance more quickly due to the take-up of drive pulley 29,
with a result that there will be a corresponding reduction in the
length of the second loop (between guide pulleys 48,49 and the
driven pulley 53), and as a result the carriage 50 will be drawn
towards the second end 13 of the housing 11, thus effectively
extending the length of the control wire 55 outside the housing
11.
[0064] Reversing the direction of operation of the motor 19
reverses the effect on the size of the two loops described above
with a result that the carriage will then move along the housing
towards the motor 19 at the same time applying tension to and/or
withdrawing control wire 55 into the housing.
[0065] In a conventional belt drive actuator utilising, for
example, a belt having teeth with a pitch of five mm and a 20-tooth
driving pulley, in order to achieve a 250 mm per second linear
carriage speed would require a rotational rate of 150 rpm at the
driving pulley. To achieve this rotational rate with a motor
operating at a typical 6000 rpm would require a 40:1 gearbox. If
the gearbox has one degree of backlash, this results in linear lost
motion by linear backlash of 0.28 mm. In addition, any gaps between
the pulley teeth and the drive belt will also become apparent as
linear backlash on drive reversal.
[0066] In the embodiment of the invention described above, if the
first drive pulley 28 has nineteen teeth and the second drive
pulley 29 has twenty teeth, the teeth having a pitch of 5 mm, the
linear displacement of the carriage 50 per revolution of the common
axle 26 will be 2.5 mm. As the required linear speed is 250 mm per
second, this may be achieved by a direct drive on the axle 26 from
the motor at 100 revolutions per second or 6000 rpm (i.e. without
requiring the reduction provided by the worm gear 21).
[0067] It will be appreciated that due to the topology of the drive
belt 40 and driven pulleys 52,53, tension will be maintained for
all positions of the carriage 50, and given the quality of
commercially available drive belts and pulleys, an actuator
according to the present invention can be economically produced
that is substantially backlash free.
[0068] In the alternative contraflexure drive embodiment shown in
FIG. 12, the drive pulleys 28,29 (having a tooth difference of one)
are fixed to rotate with input drive shaft 26. These pulleys 28,29
and drive shaft 26 are mounted to a carriage 88 together with
driven pulleys 52,53,84 and 85, all of which are free to rotate on
respective bearings. Drive belt 40 passes from the first drive
pulley 28 to the first driven pulley 52, from the first driven
pulley 52 to the first guide pulley 48, from the first guide pulley
48 to the third driven pulley 84, from the third driven pulley to
the second drive pulley 29, from the second drive pulley to the
fourth driven pulley 85, from the fourth driven pulley 85 to the
second guide pulley 49, from the second guide pulley 49 to the
second driven pulley 53, and from the second driven pulley 53 back
to the first drive pulley 28.
[0069] Carriage 88 is supported by bearing blocks 87 which are
mounted to track 86 to allow linear motion along the track. The
drive belt configuration is shown in FIG. 13, with the drive belt
40 defining a first loop between the first guide pulley 48 and the
first and third driven pulleys 52,84, and a second loop between the
second guide pulley 49 and the second and fourth driven pulleys
53,85. Rotation of the drive shaft 26 and pulleys 28,29 in a first
direction causes an increase in the length of the first loop and a
corresponding decrease in the length of the second loop, causing
motion of the carriage 88 along the track 86. Reverse rotation of
drive shaft 26 reverses the direction of travel.
[0070] This embodiment has an advantage over the first embodiment
of linear actuator in that it has a longer travel for a given
length of drive belt, and is consequently stiffer.
[0071] This embodiment requires the drive motor (not shown) to be
mounted to the carriage 88 which is also an advantage in certain
applications.
[0072] The embodiment of FIGS. 12-17 also clarifies that the
invention is suitable for use in a linear actuator where forces on
the carriage 88 can be in either direction, and the position of the
carriage can be controlled substantially backlash free.
[0073] In the alternative embodiment shown in FIGS. 18-20, the
drive pulleys 28 and 29 are each separately rotatable on axles 61
and 62 respectively and have the same number of teeth (although in
other embodiments they could differ by one or more teeth). Each of
axles 61 and 62 carry gear wheels 63 and 64 respectively, the
arrangement being such that the number of teeth on each of gear
wheels 63 and 64 is different by one tooth (although in other
embodiments they could have the same number of teeth, or differ by
more than one tooth). The gears are enmeshed one with the other so
that rotation of axle 61 is transmitted to axle 62 in the opposite
sense. This also allows the drive belt 40 to pass around the drive
pulleys and driven pulleys without any guide pulleys being required
to reverse or redirect the path of the drive belt.
[0074] The drive belt 40 passes from the first drive pulley 28 to
the first driven pulley 70, from the first driven pulley 70 to the
second drive pulley 29, from the second drive pulley 29 to the
second driven pulley 72, and from the second driven pulley 72 back
to the first drive pulley 29. The drive belt 40 defines a first
loop between the first driven pulley 70 and the drive pulleys
28,29, and a second loop between the second driven pulley 72 and
the drive pulleys 28,29, rotation of the drive pulleys in a first
direction causing an increase in the length of the first loop and a
corresponding decrease in the length of the second loop.
[0075] Unlike in the earlier embodiments described, the
circumference of the respective pulleys 28,29 (and consequently the
number of teeth if these are toothed pulleys) is the same. However,
because of the difference in the number of gear teeth of gears 63
and 64, there is a corresponding difference in the relative
rotational speeds of pulleys 28 and 29 giving a different
circumferential speed for the pulleys 29, 29 and therefore a
similar result as in the earlier embodiments.
[0076] It should be noted that different circumferential speeds of
the pulleys 28, 29, and therefore different linear speeds of the
driven members or U-shaped elements 71,73, could be achieved for a
given input speed of shaft 61 by varying the difference in the
number of teeth of the gears 63 and 64, by varying the number of
teeth on the pulleys 28 and 29, or both.
[0077] The displacement of the U-shaped element 73 for one
revolution of shaft 61 is equal to (t1/t2*T2/T1)/2, where t1 and t2
are the number of teeth on gears 63 and 64 respectively and T1 and
T2 are the number of teeth on pulleys 28 and 29.
[0078] The U-shaped elements 71,73 are interconnected by means of a
secondary belt loop 74, which passes about pulley 81. The principle
of operation for this embodiment is the same as described above,
but in this case the driven members are the first and second
U-shaped elements 71 and 73 respectively. A secondary advantage of
this arrangement is that the two driven members can provide
separate control means for two differing, but related, functions
since each driven member 71,73 moves in the opposite sense on
appropriate motion of the drive shaft 61. It should be noted that
this embodiment can be used to produce precise rotary motion of the
pulley 81 and the secondary belt loop 74.
[0079] For wire or cable driven robotic arms, where the control
wires or cables are used to control bending of sections, a single
actuator can be used for two opposing cables by removing the
secondary belt loop 74 and associated pulley and using control
wires in place of secondary belt loop 74 to provide the necessary
tension in the assembly. FIG. 21 shows such an arrangement in which
the secondary belt loop 74 is replaced by control wires 55a,
55b.
[0080] The arrangement of the drive and driven pulleys of FIGS.
18-21 can therefore be used in a rotary actuator (to provide
controlled rotation of the pulley 81), or in a linear actuator (to
provide controlled movement of the driven members 71,73 or elements
connected to the control wires 55a,55b.
[0081] The embodiment illustrated in FIGS. 22-26 is a variant of
that described in FIGS. 18-21, in which a central drive shaft 80 is
adapted to drive first drive pulley 28 and second drive pulley 29
(with in this embodiment a tooth difference of one). Drive belt 40
passes from the first drive pulley 28 to the first guide pulley 48,
from the first guide pulley 48 to the first driven pulley 70, from
the first driven pulley 70 to the second drive pulley 29, from the
second drive pulley 29 to the second driven pulley 72, from the
second driven pulley 72 to the second guide pulley 49, and from the
second guide pulley 49 back to the first drive pulley 28.
[0082] The drive belt 40 defines a first loop between the first
driven pulley 70 and the second drive 29 and first guide pulleys
48, and a second loop between the second driven pulley 72 and the
second drive pulley 29 and second guide pulleys 49, rotation of the
drive pulleys 28,29 in a first direction causing an increase in the
length of the first loop and a corresponding decrease in the length
of the second loop.
[0083] Driven members or U-shaped elements 71,73 provide attachment
to belt 74 by way of tensioning devices 82,83 to provide tension
around pulley 81 so that controlled rotation of the pulley 81 can
be achieved. This embodiment has the advantage of having minimal
belt twist and a low profile.
[0084] The embodiments of FIGS. 18-20 and FIGS. 22-26 also clarify
that the invention is suitable for use in a rotary actuator where
torque on the pulley 81 can be in either direction, and the angular
position of the pulley can be controlled substantially backlash
free.
[0085] The actuators described above further offer the possibility
of providing very considerable force on a control wire 55, or
carriage 88, or considerable torque on pulley 81, using a motor of
relatively small power. In the preferred embodiment, using a worm
gear such as 21, the mechanical advantage of the drive worm 21 and
worm wheel 27 is considerable. The mechanical advantage of the
pulley arrangement in accordance with the present invention is to
the effect that the carriage will move by half the pitch of one
tooth on a complete revolution of worm wheel 27. The mechanical
advantage here again is considerable.
[0086] It will be appreciated by the person skilled in the art that
the mechanical advantage can be altered by changing the pitch of
the teeth on the drive belt 40 and on the drive pulleys 28, 29
and/or in the alternative by simply changing the difference in the
number of teeth on the drive pulleys. Further changes can be
effected by providing an additional gearbox within the drive train
between motor 19 and output shaft 20, but as indicated above this
will add to the cost and is generally not preferred.
[0087] It will be understood that in some of the described
embodiments the drive belt 40 flexes around the drive pulleys,
driven pulleys and guide pulleys (if present) in the same
direction, i.e. towards the teeth of the belt. Those skilled in the
art will appreciate that this allows (in the applicable
embodiments) a reduction in the size of the pulleys, as compared to
embodiments having contraflexure drive.
[0088] The present applicant has found that actuators of the kind
described above result in excellent control for segmented robotic
arms of the type described, for example, in international patent
application WO2002/016995 and WO2002/100608. These actuators permit
the use of relatively low-power motors to exert considerable force
on the control wires for such robotic arms and their relative
compactness enables a cluster of actuators to be mounted in
juxtaposition with the arm with little additional routing of the
control wires 55.
* * * * *