Patent Description:
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 invention seeks to address at least some of the above problems.

<CIT> discloses a modular gear train mechanism with an internal motor and a hollow casing with an opening, a flywheel, a motor, a fixing gear, a rotating gear and a top cap sealing the opening.

<CIT> discloses a "power multiplier" using the recoil force of an electric motor.

<CIT> discloses a device comprising a motor, the 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. The device further comprises a first torque transfer device arranged to transfer torque from the drive shaft to a second shaft, the second shaft being rotatable about a second shaft axis, and an output torque transfer device.

According to a first aspect of the invention, as defined in claim <NUM>, there is provided an actuator comprising: a motor having 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 arranged to transfer torque from the drive shaft to a second shaft, the second shaft being rotatable about a second shaft axis spaced from the drive shaft axis; and an output torque transfer device arranged to transfer torque from the second shaft to the housing of the motor; wherein, upon rotation of the drive shaft relative to the motor housing, the housing of the motor is arranged to rotate relative to the position of the second shaft axis.

Advantageous embodiments of the invention are laid down in the dependent claims.

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 and/or the housing of the motor. 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. The motor housing is generally the part to which static parts of the motor are mounted, such as any integrated control electronics and in particular the stator of the motor.

With the actuator arrangement according to the first aspect, there may be provided a more compact actuator. In particular the volume taken up by a motor may be accommodated within a torque transfer device, reducing the overall volume of the actuator.

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 actuator may further comprise a second shaft mount, which is optionally arranged to be fixed to a component external to the actuator, which is rotatably coupled to the motor housing, and arranged to support the second shaft. The second shaft mount may thereby allow the second shaft to move, such that the second shaft axis translates in a circular motion relative to the motor housing and allows an output to be taken as movement of the motor housing relative to the orbit or translation of the second shaft. This may provide a high level of mechanical advantage relative to the motor.

The second shaft mount may be arranged to support the second shaft at two axially spaced support locations. This may allow the second shaft to be supported more stably.

The output torque transfer device may be coupled to the second shaft between the two support locations. Since the output torque transfer device is likely to have a higher torque than the first torque transfer device, and thus a higher tensile force between the two components of the torque transfer device, it is preferable that it is not supported on a cantilevered shaft, which may induce undesirable bending stresses in the shaft. A pulley or gear of the output torque transfer device may therefore be mounted on the second shaft between the two support locations.

At least one of the first and second torque transfer devices may comprise a pulley system. Pulley systems may have lower moments of inertia about their shafts than gear arrangements and may thereby allow improved control of the actuator position.

The first torque transfer device may be arranged such that the torque provided to the second shaft is greater than the torque provided to the drive shaft, and/or the output torque transfer device may be arranged such that the torque provided to the motor housing is greater than the torque provided to the second shaft. Since electric motors often operate with a higher rotational speed than is desirable for robotic actuators, but with a lower torque, this may improve the suitability of the actuator for use in a robot arm.

The actuator may further comprise an outer housing fixed to the motor housing, the outer housing enclosing at least one of the second shaft, the motor and the first and second torque transfer devices, and the outer housing may optionally enclose all of these components. The outer housing may thereby provide a self-contained actuator, and may provide protection to the internal components.

The outer housing may be arranged to be fixed to a second component external to the actuator. This may allow the outer housing to be used as an output from the actuator or may allow secure mounting of the actuator.

A torque ratio, or mechanical advantage, of the first and final torque transfer devices may be substantially the same. This may allow more efficient drive and may provide a more efficient use of space, as the sizes of the torque transfer devices may be similar.

The second shaft and the motor housing may have a relative rotational or orbital range of less than <NUM> degrees, optionally less than <NUM> degrees. By not requiring that the actuator can complete a complete rotation, cross members may be implemented in the second shaft support housing or in an outer housing, substantially parallel to the second shaft, and cable-based pulleys i.e. pulley arrangements having at least one cable fixed to at least one pulley (as opposed to belt-based pulleys) may be used. This may allow improved precision of movement and greater stability for the actuator.

An output of the actuator may be provided at a member arranged to rotate about the drive shaft axis i.e. having a rotational axis co-linear with the drive shaft axis. By having the output of the actuator centred about the same axis as the motor housing, the actuator may be more well-balanced.

The second shaft may overlap the motor in a direction along the drive shaft access. Put another way, the second shaft may run alongside and parallel to the motor and in particular alongside the motor housing. This may reduce the axial extent of the actuator and may thereby provide a more compact actuator.

A component of the first torque transfer device, which may be a pulley or gear, fixed to the second shaft may overlap the motor housing in a radial direction. Put another way, the component of the first torque transfer device may be arranged parallel to an axial end face of the motor or motor housing and may be alongside it at some point. This may reduce a radial extent of the actuator and may thereby provide a more compact actuator.

The output torque transfer device may comprise a third shaft, a second torque transfer device arranged to transfer torque from the second shaft to the third shaft, and a third torque transfer device arranged to transfer torque from the third shaft to the motor housing. By providing three torque transfer devices, the mechanical advantage of the actuator over the motor may be improved. Fourth or fifth torque transfer devices may also be employed.

The actuator may further comprise a plurality of encoders, each encoder arranged to measure a rotational position of the drive shaft, second shaft or motor housing relative to the motor housing or second shaft mount as appropriate. Such encoders may provide data indicative of wear of the actuator, position of the actuator, and speed of the actuator. The encoders may also be interpreted to determine strain within the torque transfer devices and may thereby be used to determine torque delivered from the motor and/or by the actuator.

The actuator may further comprise a control system arranged: to determine a strain within at least one of the torque transfer devices from data provided by the encoders, and to determine an output torque provided by the actuator based on the strain. In particular, where the torque transfer devices are pulleys, strain in the cables or bands between the pulleys may be determined from the actuators and may be used to determine the torque provided by the actuator. This may allow improved control of the actuator and may provide an indication of wear of the actuator, allowing pre-emptive maintenance of the actuator to be carried out before failure.

The actuator may further comprise an electrical cable arranged to provide power to the motor from a location outside the actuator, the electrical cable extending at least partially around the drive shaft and/or the motor housing. Optionally, the electrical cable may have a <NUM> degree bend, so that it extends at least partially around the drive shaft and/or the motor housing in a first direction before the bend and extends at least partially around the drive shaft and/or the motor housing in a second direction after the bend. With such an arrangement, where a motor housing may be movable or rotatable relative to an adjacent member, from which power is supplied, power may still be supplied to the motor by allowing movement of the electrical cable. The electrical cable may extend through the actuator in order to provide power and/or information to and/or from distal parts of a mechanism, downstream of the actuator.

The electrical cable may have a cross section which is elongated in a direction parallel to the drive shaft axis. This may allow the cable to be thinner in a direction perpendicular to the drive shaft axis and thereby allow increased flexibility of the cable for extending at least partially around the drive shaft and/or the motor housing.

According to a second aspect of the invention, there is provided a robotic arm comprising a first member, a second member and a joint where the first and second members join, wherein an actuator according to any of the claims <NUM>-<NUM> is situated at the joint and configured to actuate the joint to move the first member relative to the second member.

The second member may be an upper arm, the first member may be a forearm, and the joint may be an elbow joint.

The position of the second shaft axis may be fixed relative to the upper arm or to the forearm, and the motor housing may be fixed relative to the forearm or the upper arm respectively.

<FIG> shows a schematic diagram of an actuator <NUM> according to the present invention.

The actuator has a motor <NUM>, having a stator <NUM> and a rotor <NUM>, the stator <NUM> and rotor <NUM> being arranged to generate a torque between them. The stator <NUM> is fixed to a motor housing <NUM>, which encloses the motor <NUM>. The rotor <NUM> is fixed to a drive shaft <NUM>, which extends out of the housing <NUM> and the rotor <NUM> and drive shaft <NUM> are arranged to rotate about a drive shaft axis A1 relative to the stator <NUM> and motor housing <NUM>, due to the torque generated within the motor <NUM>.

The drive shaft <NUM> is coupled to a first torque transfer device <NUM>, and in particular to a first pulley <NUM> of the torque transfer device <NUM>. The first pulley <NUM> is coupled to a second pulley <NUM> via at least one cable <NUM> and the cable <NUM> may be fixed to the first and/or second pulley <NUM> and/or may pass radially through at least one of the pulleys <NUM>, <NUM>. The second pulley <NUM> is fixed to a second shaft <NUM>, which is rotatable about a second shaft axis A2, and supported by a second shaft mount <NUM>. The second shaft mount <NUM> is rotatably mounted to the motor housing <NUM> and/or the drive shaft <NUM> so as to be about the drive shaft axis A1 relative to the motor housing <NUM> and therefore supports the second shaft <NUM>, such that the second shaft <NUM> may orbit the motor <NUM>, the second shaft axis A2 orbiting the motor <NUM> with the second shaft <NUM>.

The actuator <NUM> also comprises a second torque transfer device <NUM> the second torque transfer device <NUM> having a third pulley <NUM>, which is fixed to the second shaft <NUM> and a fourth pulley <NUM>, which is fixed to the motor housing <NUM> and may be an integral part of the motor housing <NUM>. The fourth pulley <NUM> may thereby surround the motor housing <NUM> and the stator <NUM>. The third and fourth pulleys <NUM>, <NUM> are coupled via a cable <NUM>, which may also be two or four cables.

By this arrangement, the torque generated by the motor <NUM> is arranged to rotate the second shaft mount <NUM> when the motor housing <NUM> is held stationary, such that the second shaft axis A2 and the second shaft <NUM> orbit the motor <NUM>. Alternatively, where the second shaft mount <NUM> is held stationary, the motor housing <NUM> and fourth pulley <NUM> may rotate.

In order to fix a part of the actuator <NUM> to be stationary and/or to take an output torque from the actuator <NUM>, there is provided a fixed plate <NUM>, which is fixed to the second shaft mount <NUM> and there is provided a motor output member <NUM>, fixed to the motor housing <NUM>. Both of the fixed plate <NUM> and motor output member <NUM> may be fixed to external components and the relative rotation of the motor housing <NUM> or the second shaft mount <NUM> 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 <NUM> 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 <FIG>. These include bearings. The bearings shown are exemplary and certain bearings may be omitted or moved. There may be a motor housing bearing <NUM> disposed about the motor housing <NUM> and between the motor housing <NUM> and the second shaft mount <NUM>. There may also be a first drive shaft bearing <NUM> and a second drive shaft bearing <NUM> between the drive shaft and the second shaft mount <NUM>. The second shaft <NUM> may be supported on the second shaft mount <NUM> by first bearings <NUM>, second bearings <NUM> and third bearings <NUM>. The bearings may be arranged such that the second and third bearings <NUM>, <NUM> are disposed either side of the third pulley <NUM> so that the second shaft <NUM> is supported on both sides of the third pulley <NUM>.

The actuator <NUM> may also comprise a plurality of encoders arranged to determine relative rotational positions of components. A first encoder <NUM> may measure a relative angular position of the drive shaft <NUM> relative to the second shaft mount <NUM>, a second encoder <NUM> may measure a relative position of the second pulley <NUM> and thereby a relative position of the second shaft <NUM> relative to the second shaft mount <NUM> (or may measure a position of the second shaft <NUM> relative to the second shaft mount <NUM> directly), and a third encoder <NUM> may measure a position of the motor housing <NUM> relative to the second shaft mount <NUM>.

By measuring relative rotational positions of the drive shaft, the motor housing and the second shaft <NUM>, <NUM>, <NUM>, strain in the torque transfer devices can be determined. In particular, strain in the cables <NUM>, <NUM> between pulleys of the torque transfer devices <NUM>, <NUM> 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 <NUM> may be 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 <NUM>, <NUM>, as the strain in the first cable <NUM> alone may be sufficient to determine the torque delivered by the actuator <NUM>.

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 <NUM> may carry less tension than the cables 306a, b of the second torque transfer device <NUM>. The cables 206a, b of the first torque transfer device <NUM> may therefore be thinner, or have a smaller diameter or cross-section, than the cables 306a, b of the second torque transfer device <NUM>.

<FIG> shows the first and second torque transfer devices <NUM>, <NUM> of the actuator <NUM>, with the other parts removed for clarity. It will be understood that the motor <NUM> may be inside the fourth pulley <NUM> and that the drive shaft <NUM> extends from the motor <NUM>. It can also be seen that the first pulley <NUM> may be formed intrinsically with the drive shaft <NUM>, and may comprise a helical groove formed on an outer surface of the drive shaft <NUM>. Two cables 206a, 206b are wound around the first pulley <NUM>, lying within the helical groove and may be wound so that, when the first pulley <NUM> is rotated about the drive shaft axis A1, a first cable 206a is wound further onto the first pulley <NUM> and a second cable 206b is wound from the first pulley <NUM>. The two cables 206a, 206b may be wound so as to end at opposite ends of the first pulley <NUM> or may extend through the first pulley <NUM> radially and meet inside the first pulley <NUM>. The cables 206a, 206b are also wound around a second pulley <NUM> and may lie in helical grooves of the second pulley <NUM>. The two cables 206a, 206b may be separate portions of a single cable, joined at or inside the first or second pulley <NUM>, <NUM>.

The two cables 206a, b may be parallel in the region where they extend between the first pulley <NUM> and the second pulley <NUM>. This may reduce the prospect of the two cables 206a, b touching or interfering with each other during operation of the actuator <NUM>. With such an arrangement, the first and second pulleys <NUM>, <NUM> will be arranged to rotate in the same direction.

The second pulley <NUM> is connected to a second shaft <NUM>, which is also connected to a third pulley <NUM>, the third pulley <NUM> forming part of the second torque transfer device <NUM>. The second pulley <NUM> and third pulley <NUM> may have substantially different diameters, and their diameters may have a ratio in line with the mechanical advantage of each individual torque transfer device <NUM>, <NUM>. Specifically, the second pulley <NUM> may have a radius approximately five times that of the first pulley <NUM> and the fourth pulley <NUM> may have a radius approximately five times that of the third pulley <NUM>. Therefore, in order to allow a compact arrangement, the second pulley <NUM> may have a radius approximately five times that of the third pulley <NUM>. Numbers other than five may be used, with the principle of the ratios of radii between the first-to-second pulleys <NUM>, <NUM> and third-to-fourth pulleys <NUM>, <NUM> being substantially the same remaining.

The third pulley <NUM> may be coupled to the fourth pulley <NUM> via four cables, of which only two 306a, 306b are shown. Two parallel cables may be wound onto the third pulley <NUM> 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 <NUM>, <NUM> may be parallel in the region where they extend between the third pulley <NUM> and the fourth pulley <NUM>. This may reduce the prospect of the cables touching or interfering with each other during operation of the actuator <NUM>. With such an arrangement, the third and fourth pulleys <NUM>, <NUM> will be arranged to rotate in the same direction.

<FIG> shows a detailed cross section of an actuator <NUM>, showing the shape of the second shaft mount <NUM>. The second shaft mount <NUM> may be substantially annular or semi-annular and may comprise a cross-member, disposed approximately parallel to the drive shaft axis A1 and the second shaft axis A2 and approximately diametrically opposite the second shaft axis A2, so that the drive shaft axis A1 is between the cross member and the second shaft axis A2.

An electrical cable <NUM>, which may extend through the actuator <NUM> can also be seen in <FIG>. A first end <NUM> of the electrical cable <NUM> is at an axial end of the actuator <NUM> adjacent the motor <NUM>, and a second end <NUM> of the electrical cable <NUM> is adjacent the fixed plate <NUM>. An intermediate end <NUM> of the electrical cable <NUM> may be an end of the electrical cable <NUM> within the actuator <NUM>, which may carry electrical power to the motor <NUM> and/or may carry information from the encoders out of the actuator <NUM>.

The electrical cable <NUM> may also have a loop portion <NUM>, which may extend about the motor housing <NUM>, and the loop (shown in full in <FIG>) may move as the actuator rotates, moving the first end <NUM> relative to the second end <NUM>. The thin cross section of the cable <NUM> can also be seen in <FIG>, where the cross section is elongated in a direction along the drive shaft axis A1. This may help to form the loop portion <NUM>. The cross section of the cable <NUM> is therefore elongated in a direction along the actuator axis A1, having a thinner cross section in a direction perpendicular to the actuator axis A1 (i.e. a radial direction).

<FIG> shows the actuator <NUM> with an outer housing <NUM> included. It can be seen that the outer housing <NUM> is coupled to the motor housing <NUM> via motor housing output members <NUM>. The outer housing <NUM> also has a central hole at an opposite end to the motor <NUM>, via which the fixing plate <NUM> coupled to the second shaft mount can be accessed. It can also be seen that the electrical cable <NUM> extends out of the outer housing <NUM> in both axial directions.

The outer housing <NUM> is substantially cylindrical, having a curved side surface and two axial end surfaces, and thereby encloses substantially all other components of the actuator <NUM>, including the drive shaft <NUM>, motor <NUM>, first torque transfer device <NUM> and second torque transfer device <NUM> and all components thereof.

From <FIG>, it can be seen that the outer housing <NUM> 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 <NUM> from two such parts, the housing may be more easily constructed around the actuator <NUM>.

The housing <NUM> may be <NUM> long and <NUM> in diameter. Preferably, a longest dimension of the housing <NUM> is less than <NUM>. The housing <NUM> preferably encloses the motor and the torque transfer devices. By providing a compact and cylindrical housing <NUM>, the actuator may overall provide a package which is suited to use within humanoid robots.

There may be sufficient space inside the housing <NUM> adjacent the motor <NUM> for providing a brake (not shown). The brake may be arranged to provide a braking force to the drive shaft <NUM> and may therefore be positioned about the drive shaft <NUM> on an opposite side of the motor <NUM> from the first pulley <NUM>. For this reason, the drive shaft <NUM> may extend through the motor <NUM> and may protrude from the motor <NUM> in two opposite directions.

Below are disclosed dimensions of three specific examples of actuator arrangements, each having a housing with a diameter of <NUM>. The smaller of the pulleys in each torque transfer device has a diameter of <NUM>.

In a first example, the motor has a diameter of <NUM> and both torque transfer devices have torque ratios of <NUM>:<NUM>.

In a second example, the motor has a diameter of <NUM>, the first torque transfer device has a torque ratio of <NUM>:<NUM>, and the output torque transfer device has a torque ratio of <NUM>:<NUM>.

In a third example, the motor has a diameter of <NUM>, the first torque transfer device has a torque ratio of <NUM>:<NUM>, and the output torque transfer device has a torque ratio of <NUM>:<NUM>.

<FIG> shows the full extent of the electrical cable <NUM> arranged to extend through the actuator <NUM>, including the <NUM> degree loop portion <NUM>, which may be movable as the actuator <NUM> rotates, in order to avoid stretching and potentially damaging the cable.

<FIG> shows an alternative pulley arrangement <NUM>. The arrangement is powered by a motor <NUM>, only an end view of which is visible, and which is arranged to rotate a drive shaft and pulley <NUM>. The drive shaft and first pulley <NUM> are coupled via a first cable or band <NUM> to a second pulley <NUM>, which is on a shaft with a third pulley <NUM>, the third pulley <NUM> being coupled to a fourth pulley <NUM> via a second cable or band <NUM>. The fourth pulley <NUM> is coupled via a shaft to a fifth pulley <NUM>, which is coupled via a cable or band <NUM> to a sixth pulley <NUM>, which is coupled via a shaft to a seventh pulley <NUM>, which is coupled by a further cable or band <NUM> to the motor housing, incorporating a final pulley <NUM>.

The arrangement shown in <FIG> 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.

<FIG> shows a robot arm <NUM>. The robot arm <NUM> has a vertical axis actuator <NUM> fixed to a horizontal base B, and a shoulder joint <NUM> coupled to the vertical axis actuator <NUM> and arranged to move a first member <NUM>, which may also be referred to as an upper arm <NUM>. The upper arm <NUM> is coupled to a second member <NUM>, which may also be referred to as a lower arm <NUM> or a forearm <NUM>. The upper arm <NUM> is coupled to the forearm <NUM> at an elbow joint <NUM> and there is, at an end of the forearm <NUM> opposite the elbow joint <NUM>, an end effector <NUM>.

An actuator as described above may be placed at the elbow joint <NUM> or at the shoulder joint <NUM> and the second shaft mount <NUM> and fixation plate <NUM> may be fixed to any of the vertical actuator <NUM>, upper arm <NUM> or lower arm <NUM> as appropriate.

<FIG> shows an alternative actuator <NUM>. The actuator <NUM> comprises a motor <NUM> having a housing <NUM> and a drive shaft <NUM> extending from the housing <NUM>. The motor <NUM> is arranged to rotate the drive shaft <NUM> relative to the housing <NUM> about a drive shaft axis A3. The drive shaft <NUM> 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 <NUM>.

The second pulley <NUM> is supported on a second shaft <NUM>, which is arranged to rotate about a second shaft axis A4, spaced apart from the drive shaft axis A3. The second shaft <NUM> also has a third pulley <NUM>, which is arranged to drive a band or cable fixed or otherwise coupled to a fourth pulley <NUM>. The motor housing <NUM> is within, or may be fixed to or integral with the fourth pulley <NUM>.

It will be understood that the above-described aspects of the alternative actuator <NUM> are substantially similar to corresponding aspects of the actuator <NUM> described previously. The alternative actuator <NUM> may therefore also share other features of the actuator <NUM> not explicitly described in conjunction with the alternative actuator <NUM>.

<FIG> shows how the alternative actuator <NUM> may be incorporated within a robotic joint <NUM>. The robotic joint <NUM> is arranged to pivot two members relative to each other about the drive shaft axis A3.

As shown in <FIG>, the second shaft <NUM> is arranged to be supported by external members 884a, b. For this reason, the second shaft <NUM> 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 each have bearing surfaces 867a, 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 <NUM> via a cross member <NUM>. The cross member <NUM> has the shape of a sector of an annulus or of an extruded arc and extends partially around the drive shaft <NUM>. By having this shape, the cross member <NUM> may provide good strength to the actuator <NUM>. The portion of the actuator coupled to the cross member <NUM> at an end opposite to the motor housing <NUM> may also support the drive shaft <NUM> and may thereby improve the stiffness of the driveshaft <NUM>.

A second of the bearing surfaces 867b is coupled to the motor housing <NUM> at an end opposite to the drive shaft <NUM>. The second bearing surface 867b may be directly fixed to the motor housing <NUM>.

The external members 884a, b are connected via a second cross member <NUM>, which may improve the structural stiffness of the joint arrangement <NUM>. The cross member <NUM> 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 <NUM> shown in <FIG>.

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 <NUM> or between the cross member <NUM> and the first face flange 868a. A second of the further external members 882b is fixed to the motor housing <NUM>.

As shown in <FIG>, the second shaft <NUM> may be supported within a member of a robotic arm within which the actuator <NUM> is located. By increasing the separation distance between the drive shaft axis A3 and the second shaft axis A4, larger pulleys and/or a larger motor to be used and may therefore improvement of the torque supplied by the actuator <NUM> 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 <NUM>, the first torque transfer device has a torque ratio of <NUM>:<NUM>, and the output torque transfer device has a torque ratio of <NUM>:<NUM>. The smaller of the pulleys in each torque transfer device has a diameter of <NUM>.

In a second example of the alternative actuator, the motor has a diameter of <NUM>, the first torque transfer device has a torque ratio of <NUM>:<NUM>, and the output torque transfer device has a torque ratio of <NUM>:<NUM>. The smaller of the pulleys in each torque transfer device has a diameter of <NUM>.

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.

<FIG> shows a robotic arm <NUM> incorporating the alternative actuator <NUM> and the robotic joint <NUM>. The robotic arm has a first member <NUM>, which is a lower arm and a second member <NUM> which is an upper arm. From <FIG>, it can be seen that the envelope in which the actuator <NUM> lies sits comfortably within an elbow joint and that the external members <NUM>, <NUM> may lie along the first and second members <NUM>, <NUM>.

<FIG> shows a reverse view of the fourth pulley <NUM>, showing how the cable portions may be connected to the second pulley <NUM> via connectors <NUM>.

From <FIG>, it can be seen that the connectors <NUM> are substantially curved, having a similar curvature to that of the outer surface of the fourth pulley <NUM> and have cable portions <NUM>, <NUM> extending away from the connectors, the cable portions <NUM>, <NUM> being between a body of the connector <NUM> and the second pulley <NUM>, and the cable portions <NUM>, <NUM> are connected to the connectors <NUM> at fixation points <NUM>, <NUM>, which are located on the connectors <NUM> at an opposite end from that at which the cable portions <NUM>, <NUM> extend away from the connectors <NUM>.

It will be understood that the cable portions <NUM> may be the same cable portions as the first and second cable portions 306A, B shown in <FIG>.

<FIG> shows the fourth pulley <NUM> with one of the connectors <NUM> removed, exposing a receiving portion <NUM> for receiving the connector <NUM>. The receiving portion <NUM> has two helical grooves <NUM>, <NUM> for receiving the first and second cable portions <NUM>, <NUM> and receiving teeth <NUM> for engaging with respective teeth of the connector <NUM>.

<FIG> shows a section view of a connector <NUM>, with a portion removed. It can therefore be seen that the cable portion <NUM> extends along the length of the connector from a first fixation point <NUM>. The connector <NUM> also comprises a body <NUM>, which is a substantially flat, curved portion radially outside the cable portion <NUM>, and which has a toothed portion <NUM> on a radially inner side, adjacent the cable portions <NUM>.

The toothed portion <NUM> comprises teeth <NUM>, each tooth having an engagement surface <NUM>, facing a first direction away from the first fixation point <NUM> and substantially perpendicular to the body portion <NUM>, and facing towards a second fixation point <NUM>. Each tooth also has an angled surface <NUM>, whose structure supports the engagement surface <NUM>, and subtends an angle of between <NUM> and <NUM> degrees with the body portion <NUM> and a curved surface <NUM> joining the engagement surface <NUM> and the angled surface <NUM>. Each tooth <NUM> may be solid and defined by the engagement surfaces <NUM>, angled surface <NUM> and curved surface <NUM> and may extend away from the body portion <NUM>.

<FIG> gives a plan view of the connector <NUM>, showing two parallel cable portions <NUM>, <NUM> and the toothed potion <NUM> lying between the cables. The second cable portion <NUM> is also fixed to the connector <NUM> at two fixation points <NUM>, <NUM>, and the toothed portion <NUM> lies between the fixation points <NUM>, <NUM>, <NUM>, <NUM>. 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 <NUM> may be formed by a moulding process, optionally an injection moulding process, and may be moulded around the cable portions <NUM>, <NUM>. The cable portions <NUM>, <NUM> 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 <NUM> may thereby be formed with a level of residual stress, which manifests as a tensile stress in each of the cable portions <NUM>, <NUM> and a compressive stress in the body portion <NUM>.

Portions of excess cable may extend out of the mould in both directions (i.e. in both directions from the fixation point <NUM>, <NUM>) and these cable portions may be used to secure the connector <NUM> to the second pulley <NUM> and subsequently removed. The excess cable portions (not shown) may extend away from the fixation points <NUM>, <NUM> and may be held in tension in order to resiliently couple the connector <NUM> to a pulley <NUM>.

Claim 1:
An actuator comprising:
a motor (<NUM>) having a housing (<NUM>) and a drive shaft (<NUM>), the motor arranged to rotate the drive shaft relative to the housing about a drive shaft axis (A1);
a first torque transfer device (<NUM>) arranged to transfer torque from the drive shaft to a second shaft (<NUM>), the second shaft being rotatable about a second shaft axis (A2) spaced from the drive shaft axis; and
an output torque transfer device (<NUM>) arranged to transfer torque from the second shaft to the housing of the motor;
wherein, upon rotation of the drive shaft relative to the motor housing, the housing of the motor is arranged to rotate relative to the position of the second shaft axis.