BACKGROUND

This invention relates to drive arrangements for robot joints, with particular relevance to robot wrists.

Robots that are required to manipulate objects, which may for example be industrial or surgical robots, frequently have an arm composed of rigid elements which are linked together in series by a number of flexible joints. The joints could be of any type but are typically revolute joints, or a combination of revolute and prismatic joints. The arm extends from a base, whose location might be fixed or moveable, and terminates in a tool or an attachment for a tool. The tool could, for example be a gripping, cutting, illuminating, irradiating or imaging tool. The final joint in the arm may be termed the wrist. The wrist may permit motion about only a single axis, or it may be a complex or compound articulation, which permits rotation about multiple axes. As disclosed in our co-pending patent application PCT/GB2014/053523, the wrist may provide two roll joints whose axes are generally longitudinal to the arm, separated by two pitch/yaw joints, whose axes are generally transverse to the arm.

In the case of a surgical robot there are a number of important criteria that influence the design of the distal joint(s) of the arm.

1 . It is desirable for the arm, and particularly its distal portion where the wrist is located, to be small in size. That allows multiple such robot arms to work in close proximity and hence opens up a wider range of surgical procedures that the arm can perform.

2. It is desirable for the outer profile of the distal portion of the arm to be circularly symmetrical about the length of the arm. This allows the distal portion to be rotated longitudinally without having to be repositioned if it is close to another robot, to some other equipment or to the patient.

3. It is desirably for the joints to be capable of delivering a high torque, so that they can carry heavier tools and deliver high acceleration to the tool tip.

4. It is desirable for the joints to be stiff, with little or no backlash or elasticity, so that when a tool tip has been positioned it will be fixed in position. A conventional approach

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to minimising backlash is to designate one or more gear elements as sacrificial, but this requires a high level of maintenance, and can result in worn gear particles being liberated within the arm.

5. It is desirable for all articulations to have position and force/torque sensors, so that the control mechanism can take data from those sensors.

6. It is desirable for the distal portion of the robot arm to be as light as possible, to reduce the force that must be exerted by more proximal joints of the robot arm.

7. A typical robot arm carries cables that provide power to its drive motors and perhaps to a tool, and carry signals back from sensors such as position, torque and imaging sensors. It is desirable for the arm to include a path for such cables to pass in the interior of the arm.

8. It is desirable for there to be a method of cooling for the motors driving the distal joints of the robot arm and payload or tool.

The number of important criteria makes it difficult to design an arm that best balances all the requirements.

One particular problem is how to fit the motors and gearing into the wrist of a robot arm. The arrangement should be compact but also allow for high stiffness and torque transfer. Many existing designs compromise one of these criteria.

There is a need for an improved drive arrangement for a joint of a robot arm.

SUMMARY

According to the present invention there is provided a robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising: an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis; a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier; a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs; a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon; and an intermediate gear train borne by the carrier and coupling the second shaft gear to the second drive gear.

The intermediate gear train may comprise a first intermediate gear disposed about the first rotation axis, the first intermediate gear being arranged to engage the second shaft gear. The first intermediate gear may be rotatable about the first rotation axis.

The robot arm may further comprise a control unit arranged to respond to command signals commanding motion of the robot arm by driving the first and second drive shafts to rotate. The control unit may be configured to, when the robot arm is commanded to articulate about the first axis without articulating about the second axis, drive the first shaft to rotate to cause articulation about the first axis and also drive the second shaft to rotate in such a way as to negate parasitic articulation about the second axis. The control unit may be configured to perform that action automatically.

The intermediate gear train may comprise a plurality of interlinked gears arranged to rotate about axes parallel with the first rotation axis.

The intermediate gear train may comprise an intermediate shaft arranged to rotate about an axis parallel with the first rotation axis. The intermediate shaft may have a third shaft gear thereon, the third shaft gear being arranged to engage the second drive gear.

The interlinked gears are on one side of a plane perpendicular to the first axis and containing the teeth of the first drive gear, and at least part of the third shaft gear is on the other side of that plane.

The third shaft gear may be a worm gear: i.e. a gear whose tooth/teeth follow a helical path. One or both of the first and second shaft gears may be worm gears.

One or both of the first drive gears may be bevel gear(s): i.e. gears whose pitch surface is a straight-sided or curved cone and/or whose teeth are arranged on such a cone. The tooth lines may be straight or curved. One or both of the first drive gears may be skew axis gear(s).

The first drive gear may be a part-circular gear. At least part of the second drive gear may intersect a circle about the first axis that is coincident with the radially outermost part of the first drive gear. At least part of the intermediate shaft may intersect a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

According to a second aspect of the present invention there is provided a robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising: an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis; a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier; a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs; a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs on a first side of a plane containing the second rotation axis and extending through that plane to the second side of that plane; and an intermediate linkage that meshes with the second drive shaft on the second side of the plane and that couples the second shaft gear to the second drive gear.

The second shaft may comprise a flexible element. The flexible element is located on the first rotation axis. The flexible element may be a universal joint.

The second shaft is coupled to the carrier by a revolute joint on the second side of the said plane.

The second drive shaft may have a second shaft gear on the second side of the said plane. The intermediate linkage may comprise an intermediate shaft having a first intermediate gear that meshes with the second shaft gear and a second intermediate gear that meshes with the second drive gear.

The second drive shaft may be arranged to rotate about an axis perpendicular to the second rotation axis.

The second intermediate gear may be a worm gear. The first shaft gear may be a worm gear.

One or both of the first drive gears may be bevel gear(s). One or both of the first drive gears may be skew axis gear(s).

The first drive gear may be a part-circular gear. At least part of the second drive gear may intersect a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

According to a third aspect of the present invention there is provided a robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising: an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis; a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier; a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs; a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon, the second shaft gear being arranged to engage the second drive

gear; the second drive shaft comprising a prismatic joint whereby the length of the shaft can vary in response to motion of the carrier about the first axis.

The prismatic joint may be a sliding splined coupling.

The second dive shaft may comprise a first flexible joint on one side of the prismatic joint and a second flexible joint on the other side of the prismatic joint.

The second drive shaft may be connected by a revolute joint to the carrier on the opposite side of the second flexible joint to the prismatic joint.

One or both of the first and second shaft gears may be worm gears.

One or both of the first drive gears may be bevel gear(s). One or both of the first drive gears may be skew axis gear(s). The first drive gear may be a part-circular gear.

At least part of the second drive gear may intersect a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

The first and second axes may be orthogonal. The first and second axes may intersect each other.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example with reference to the accompanying drawings.

In the drawings:

Figure 1 is a general representation of a surgical robot arm.

Figure 2 shows in more detail the rotation axes at the wrist of the arm of figure 1 .

Figure 3 shows part of a first wrist mechanism from distally and one side.

Figure 4 shows part of the first wrist mechanism from distally and the other side.

Figure 5 shows part of a second wrist mechanism from proximally and one side.

Figure 6 shows part of the second wrist mechanism from distally and one side.

Figure 7 shows a third wrist mechanism from distally and one side.

Figure 8 shows the third wrist mechanism from distally and the other side.

Figure 9 shows the third wrist mechanism in section on a central longitudinal plane viewed from one side.

Figure 10 shows the third wrist mechanism in section on a central longitudinal plane viewed from the other side.

Figure 1 1 illustrates communication paths in a robot arm.

Figure 12 shows a terminal module for a robot arm in longitudinal cross-section.

DETAILED DESCRIPTION

The wrist mechanisms to be described below have been found to provide compact and mechanically advantageous arrangements for at least some of the joints of a robot wrist, or for other applications.

Figure 1 shows a surgical robot having an arm 1 which extends from a base 2. The arm comprises a number of rigid limbs 3. The limbs are coupled by revolute joints 4. The most proximal limb 3a is coupled to the base by joint 4a. It and the other limbs are coupled in series by further ones of the joints 4. A wrist 5 is made up of four individual revolute joints. The wrist 5 couples one limb (3b) to the most distal limb (3c) of the arm. The most distal limb 3c carries an attachment 8 for a surgical instrument or tool 9. Each joint 4 of the arm has one or more motors 6 which can be operated to cause rotational motion at the respective joint, and one or more position and/or torque sensors 7 which provide information regarding the current configuration and/or load at that joint. For clarity, only some of the motors and sensors are shown in figure 1 . The arm may be generally as described in our co-pending patent application PCT/GB2014/053523. The attachment point 8 for a tool can suitably comprise any one or more of: (i) a formation permitting a tool to be mechanically attached to the arm, (ii) an interface for communicating electrical and/or optical power and/or data to and/or from the tool, and (iii) a mechanical drive for driving motion of a part of a tool. In general it is preferred that the motors are arranged proximally of the joints whose motion they drive, so as to improve weight distribution. As discussed below, controllers for the motors, torque sensors and encoders are distributed with the arm. The controllers are connected via a communication bus to control unit 10.

A control unit 10 comprises a processor 1 1 and a memory 12. Memory 12 stores in a non-transient way software that is executable by the processor to control the operation of the motors 6 to cause the arm 1 to operate in the manner described herein. In particular, the software can control the processor 1 1 to cause the motors (for example via distributed controllers) to drive in dependence on inputs from the sensors 7 and from a surgeon command interface 13. The control unit 10 is coupled to the motors 6 for driving them in accordance with outputs generated by execution of the software. The control unit 10 is coupled to the sensors 7 for receiving sensed input from the sensors, and to the command interface 13 for receiving input from it. The respective couplings may, for example, each be electrical or optical cables, or may be provided by a wireless connection. The command interface 13 comprises one or more input devices whereby a user can request motion of the arm in a desired way. The input devices could, for example, be manually operable mechanical input devices such as control handles or joysticks, or contactless input devices such as optical gesture sensors. The software stored in memory 12 is configured to respond to those inputs and cause the joints of the arm to move accordingly, in compliance with a pre-determined control strategy. The control strategy may include safety features which moderate the motion of the arm in response to command inputs. Thus, in summary, a surgeon at the command interface 13 can control the robot arm 1 to move in such a way as to perform a desired surgical procedure. The control unit 10 and/or the command interface 13 may be remote from the arm 1 .

Figure 2 shows the wrist 5 of the robot in more detail. The wrist comprises four revolute joints 300, 301 , 302, 303. The joints are arranged in series, with a rigid part of the arm extending from each joint to the next. The most proximal joint 300 of the wrist joins arm part 4b to arm part 310. Joint 300 has a "roll" rotation axis 304, which is directed generally along the extent of the limb 4b of the arm that is immediately proximal of the articulations of the wrist. The next most distal joint 301 of the wrist joins arm part 310 to arm part 31 1 . Joint 301 has a "pitch" rotation axis 305 which is perpendicular to axis 304 in all configurations of joints 300 and 301 . The next most distal joint 302 of the wrist joins arm part 310 to arm part 31 1 . Joint 302 has a "yaw" rotation axis 306 which is perpendicular to axis 305 in all configurations of joints 301 and 302. In some configurations of the wrist, axis 306 is also perpendicular to axis 304. The next most distal joint of the wrist 303 joins arm part 31 1 to arm part 4c. Joint 303 has a "roll" rotation axis 307 which is perpendicular to axis 306 in all configurations of joints 302 and 303. In some configurations of the wrist, axis 307 is also perpendicular to axis 305 and parallel with (and preferably collinear with) axis 304. It is preferable for axes 305 and 306 to intersect each other, since this gives a particularly compact configuration. Joints 300 and 303 may be positioned so that axes 304 and 307 can pass through the intersection of axes 305, 306 for some configurations of the wrist.

This design of wrist is advantageous in that it allows a wide range of movement from a tool attached to the attachment point 8 at the distal end of arm part 4c, but with the wrist being capable of being assembled in a relatively compact form and without there being singularities at certain parts of the range of motion that could demand excessively high rates of motion at individual joints.

Figures 3 and 4 show one example of a mechanism suitable for implementing part of the wrist 5 of the arm 1 of figure 1 . Figures 3 and 4 concentrate (as to figures 5 to 10) on the mechanism associated with the joints designated 301 and 302 in figure 2.

In the region of the wrist 5 the rigid arm parts 310, 31 1 have hollow outer shells or casings 310', 310", 31 1 '. The shells define the majority of the exterior surface of the arm, and include a void which is partly or fully encircled by the exterior wall of the respective shell and within which the motors, sensors, cables and other components of the arm can be housed. The shells could be formed of a metal, for example an aluminium alloy or steel, or from a composite, for example a fibre-reinforced resin composite such as carbon fibre reinforced resin. The shells constitute part of the rigid structure of the arm parts that attaches between the respective joints. The shells may contain a structural framework as shown later in relation to the embodiment of figure 7.

In figures 3 and 4, for clarity the shell of arm part 310 is shown in two parts: 310' and 310", both of which are drawn in outline and exploded from each other. The shells of arm parts 4b and 4c are omitted, as is the mechanism associated with joints 300 and 303. The shell of arm part 31 1 is shown in part, the majority extending from spur 31 1 '.

The shell of arm part 310 (constituted by shell parts 310' and 310") and the shell of arm part 31 1 (which extends from spur 31 1 ') are movable with respect to each other about two rotation axes, shown at 20 and 21 . These correspond to axes 305, 306 of figure 2. Axes 20 and 21 are orthogonal. Axes 20 and 21 intersect. A central coupler 28 is mounted to arm part 310 by bearings 29, 30. The coupler extends between the bearings 29, 30. The bearings 29, 30 hold the coupler fast with arm part 310 except that they permit relative rotation of the coupler and that arm part about axis 20, thus defining a revolute joint corresponding to joint 301 of figure 2. A further bearing 31 attaches the distal shell connector spur 31 1 ' to the coupler 28. Bearing 31 holds the distal shell connector spur 31 1 ' fast with the coupler 28 except for permitting relative motion of the spur and the coupler about axis 21 , thus defining a revolute joint corresponding to joint 302 of figure 2.

Thus coupler 28 is fast with the arm part 310 about axis 21. Coupler 28 is also fast with the arm part 31 1 about axis 20. That is, the mechanism is arranged so that coupler 28 and arm part 310 cannot undergo relative rotation or motion about axis 21 ; and coupler 28 and arm part 31 1 cannot undergo relative rotation or motion about axis 20.

Two electric motors 24, 25 (see figure 4) are mounted in arm part 310. The motors drive respective drive shafts 26, 27 which extend into the midst of the wrist mechanism.

Shaft 26 drives rotation about axis 20. Shaft 27 drives rotation about axis 21 . Drive shaft 26 terminates at its distal end in a worm gear 32. The worm gear 32 engages a bevel gear 33 which is fast with the coupler 28. Drive shaft 27 terminates at its distal end in a worm gear 34. The worm gear 34 engages a gear train shown generally at 35 which terminates in a further worm gear 36. Worm-form pinion gear 36 engages a hypoid-toothed bevel gear 37 which is fast with the distal shell connector 31 1 '.

In this example, the gear 33 is directly attached to the coupler 28. That is, the coupler 28 abuts the gear 33. Gear 33 is therefore mounted to the coupler 28. The distal shell connector spur 31 1 ' is also directly attached to the gear 37. Thus the gear 37 may abut the connector spur 31 1 '.

Shafts 26 and 27 are parallel. They both extend along the arm part 310. In particular, shafts 26 and 27 extend in a direction substantially parallel to the longitudinal direction of the arm part 310. The shafts could be parallel to the longitudinal direction of the arm part 310, or they could be mounted at an angle to the general longitudinal direction of the arm part 310. For example, the arm part 310 may taper in the direction from the proximal end towards the distal end, and the shafts 26 and 27 may extend in a direction that is parallel to the taper angle of the arm part.

Worms 32 and 34 are attached to the drive shafts 26 and 27 respectively and so may be referred to as shaft gears. Rotation of gear 33 drives rotation of arm part 31 1 relative to arm part 310 about axis 20, and thus gear 33 may be referred to as a drive gear. Similarly, rotation of gear 37 drives rotation of arm part 31 1 relative to arm part 310 about axis 21 , and thus gear 37 may also be referred to as a drive gear.

Gear 33 is formed as a sector gear: that is its operative arc (defined in the example of figures 3 and 4 by the arc of its teeth) is less than 360°.

The gear train 35 is borne by the coupler 28. The gear train comprises an input gear 38 which engages the worm 34. Input gear 38 is located with its rotation axis relative to the coupler 28 being coincident with axis 20. That means that the input gear can continue to engage the worm 34 irrespective of the configuration of the coupler 28 relative to arm part 310 about axis 20. A series of further gears whose axes are

parallel with axis 20 transfer drive from the input gear 38 to an output gear 39 on a shaft 40 whose rotation axis relative to the carrier 28 is parallel with but offset from axis 20. Shaft 40 terminates in the worm 36. Shaft 40 extends parallel to axis 20. The gears of gear train 35, together with shaft 40, are borne by the coupler 28.

The operation of the wrist mechanism will now be described. For motion about axis 20, motor 24 is operated to drive shaft 26 to rotate relative to arm part 310. This drives the bevel gear 33 and hence coupler 28 and distal shell spur 31 1 ' to rotate about axis 20 relative to arm part 310. For motion about axis 21 , motor 25 is operated to drive shaft 27 to rotate relative to arm part 310. This drives the bevel gear 37 and hence distal shell connector 31 1 ' to rotate about axis 21 relative to arm part 310. It will be observed that if drive shaft 26 is rotated, driving the coupler 28 to rotate, whilst drive shaft 27 remains stationary then gear 38 will also rotate relative to the coupler 28, causing parasitic motion of the distal shell connector spur 31 1 ' about axis 21 . To prevent this, the control system 10 of the arm is configured so that when required there is compensatory motion of drive shaft 27 in tandem with motion of drive shaft 26 so as to isolate motion about axis 21 from motion about axis 20. For example, if it is required to cause relative motion of shells 310, 31 1 about only axis 20 then motor 24 is operated to cause that motion whilst motor 25 is simultaneously operated in such a way as to prevent input gear 38 from rotating relative to carrier 28.

Various aspects of the mechanism shown in figures 3 and 4 are advantageous in helping to make the mechanism particularly compact.

1 . It is convenient for bevel gear 33 to be of part-circular form: i.e. its teeth do not encompass a full circle. For example, gear 33 may encompass less than 270° or less than 180° or less than 90°. This allows at least part of the other bevel gear 37 to be located in such a way that it intersects a circle coincident with gear 33, about the axis of gear 33 and having the same radius as the outermost part of gear 33. Whilst this feature can be of assistance in reducing the size of a range of compound joints, it is of particular significance in a wrist of the type shown in figure 2, comprising a pair of roll joints with a pair of pitch/yaw joints between them, since in a joint of that type there is a degree of redundancy among the pitch/yaw joints and hence a wide range of positions of the distal end of the arm can be reached even if motion about axis 20 is restricted.

2. It is convenient if the part gear 33 serves rotation about the axis 20 by which the carrier 28 is pivoted to the next-most-proximal arm part 310, as opposed to rotation about axis 21 , since the part gear can also be cut away to accommodate shaft 40 intersecting the said circle. That saves space by permitting the worm 36 to be located on the opposite side of bevel gear 33 to the gear train 35. However, in other designs the part gear could serve rotation about axis 21 , so gear 37 could be of part-circular form.

3. It is convenient if the worms 32, 34 are located on the opposite side of axis 20 to bevel gear 37: i.e. that there is a plane containing axis 20 on one side of which are the worms 32, 34 and on the other side of which is the bevel gear 37. This helps to provide a compact packaging arrangement. Thus, both worms 32 and 34 are located on a single side of a plane containing axis 20 that is parallel to the longitudinal direction of the arm part 310.

4. It is convenient if the worm 34 is located on the opposite side of bevel gear 33 from worm 36 and/or that the gear train 35 is located exclusively on the opposite side of bevel gear 33 from worm 36. This again helps to provide a compact packaging arrangement. That is, the gear train 35 (including all its interlinked gears such as gears 38 and 39) may be located on one side of the gear 33. Put another way, gear train 35 (including its interlinked gears) and the gear 33 are located on opposite sides of a plane parallel to both axis 21 and the longitudinal direction of arm part 310. That plane may contain the axis 21 .

5. The gears 33 and/or 37 are conveniently provided as bevel gears since that permits them to be driven from worms located within the plan of their respective external radii. However, they could be externally toothed gears engaged on their outer surfaces by the worms 32, 34 or by radially toothed gears.

6. The bevel gear 33 is conveniently located so as to be interposed between worms 32 and 34. This helps the packaging of the motors 24, 25.

7. The bevel gears and the worm gears that mate with them can conveniently be of hypoid or skew axis, e.g. Spiroid®, form. These gears allow for relatively high torque capacity in a relatively compact form.

Figures 5 and 6 show a second form of wrist mechanism suitable for providing joints 301 , 302 in a wrist of the type shown in figure 2.

As shown in figure 5 the wrist comprises a pair of rigid external shells 310', 31 1 ' which define the exterior surfaces of arm parts 310, 31 1 respectively of figure 2. 310' is the more proximal of the shells. The arm parts formed of the shells 310', 31 1 ' can pivot relative to each other about axes 62, 63, which correspond respectively to axes 305, 306 of figure 2. Axes 62, 63 are orthogonal. Axes 62, 63 intersect. The shells 310', 31 1 ' define the exterior of the arm in the region of the wrist and are hollow, to accommodate a rotation mechanism and space for passing cables etc., as will be described in more detail below. The shells could be formed of a metal, for example an aluminium alloy or steel, or from a composite, for example a fibre-reinforced resin composite such as carbon fibre. The shells constitute the principal rigid structure of the arm parts that attaches between the respective joints.

Figure 6 shows the same mechanism from distally and one side, with the shell 31 1 ' removed for clarity.

Shell 310' is coupled to shell 31 1 ' by a cruciform coupler 64. The coupler has a central tube 65 which defines a duct through its centre, running generally along the length of the arm. Extending from the tube are first arms 66, 67 and second arms 68, 69. Each of the shells 310', 31 1 ' is attached to the coupler 64 by a revolute joint: i.e. in such a way that it is confined to be able to move relative to the coupler only by rotation about a single axis. The first arms 66, 67 attach to shell 310' by bearings 70, 71 which permit rotation between those first arms and the shell 310' about axis 62. The second arms 68, 69 attach to shell 31 1 ' by bearings 72, 73 which permit rotation between those second arms and the shell 31 1 ' about axis 63. A first bevel gear 74 is concentric with the first arms 66, 67. The first bevel gear is fast with the coupler 64 and rotationally free with respect to the proximal one of the two shells 310'. A second bevel gear 75 is concentric with the second arms 68, 69. The second bevel gear is fast with the distal one of the two shells 31 1 ' and rotationally free with respect to the coupler 64.

The pair of arms 66, 67 of the coupler are perpendicular to the pair of arms 68, 69. Arms 66 and 67 lie on the rotation axis 62; and arms 68 and 69 lie on the rotation axis 63. The coupler 64 is directly attached to the gear 74. Thus the coupler 64 abuts gear 74. Coupler 64 (and hence gear 74) can rotate relative to the arm part 310 about axis 62. However, the coupler 64 and gear 74 are fast with the arm part 310 about the axis

63 such that there can be no relative motion or rotation between the coupler 64 and arm part 310 about axis 63.

The bevel gear 75 may be mounted directly to the arm part 31 1 . The bevel gear 75 can rotate with respect to the coupler 64 (and hence arm part 310) about axis 63. However, the gear 75 is fast with the coupler 64 about axis 62. That is, there can be no relative rotation or motion between coupler 64 and gear 75 about axis 62.

Two shafts 76, 77 operate the motion of the compound joint. The shafts extend into the central region of the joint from within the proximal one of the shells 310'. Each shaft is attached at its proximal end to the shaft of a respective electric motor (not shown), the housings of the motors being fixed to the interior of the proximal shell 310'. In this way the shafts 76, 77 can be driven by the motors to rotate with respect to the proximal shell 310'.

Shaft 76 and its associated motor operate motion about axis 62. Shaft 76 terminates at its distal end in a worm gear 78 which engages bevel gear 74. Rotation of shaft 76 causes rotation of the bevel gear 74 relative to shell 310' about axis 62. Bevel gear 74 is fast with the coupler 64, which in turn carries the distal shell 31 1 '. Thus rotation of shaft 76 causes relative rotation of the shells 310', 31 1 ' about axis 62.

Shaft 77 and its associated motor operate motion about axis 63. In order to do that it has ultimately to drive bevel gear 75 by means of a worm gear 79 carried by the coupler 64. Rotation of that worm gear can cause relative rotation of the coupler and the distal shell 31 1 '. To achieve this, drive is transmitted from the shaft 77 through a pair of gears 80, 81 borne by the carrier 64 to a shaft bearing the worm gear 79. Shaft 77 approaches the carrier 64 from the proximal side. The gears 80, 81 are located on the distal side of the coupler. Gears 80, 81 and 79 are thus fast with the coupler 64 about axes 62 and 63. The shaft 77 passes through the duct defined by tube 65 in the centre of the coupler. To accommodate motion of the coupler 64 relative to the first shell 310' the shaft 77 has a universal or Hooke's joint 82 along its length. The universal joint 82 lies on axis 62. Instead of a Hooke's joint the shaft could have another form of flexible coupling, for example an elastic coupling (which could be integral with the shaft) or a form of constant velocity joint.

Worm 78 is attached to drive shaft 76 and so may be referred to as a shaft gear. Rotation of gear 74 drives rotation of the arm part 31 1 relative to the arm part 310 about axis 62, and thus gear 74 may be referred to as a drive gear. Similarly, rotation of gear 75 drives rotation of arm part 31 1 relative to arm part 310 about axis 63, and so gear 75 may also be referred to as a drive gear.

Shaft 77 traverses a plane that contains rotation axis 63. The plane additionally contains rotation axis 62. Thus the shaft 77 comprises a proximal portion that is proximal of that plane, and a distal portion that is distal of that plane. The proximal portion of the shaft 77 is attached or otherwise coupled to the motor. The distal portion of the shaft attaches to the gear 80. Gear 80 is therefore located on the distal side of that plane. Gear 80 may also be referred to as a shaft gear. The proximal and distal portions of the shaft 77 may be separated by the Hooke's joint 82. The Hooke's joint permits the proximal and distal portions of the shaft 77 to rotate with each other such that rotation of the proximal portion is coupled to the distal portion. Since the distal portion of shaft 77 is attached to gear 80, it follows that gear 80 is rotationally fast with shaft 77.

Gear 80 engages, or meshes with, gear 81 . In this example gears 80 and 81 are spur gears. Gears 80 and 81 have parallel but offset rotation axes. The rotation axis of gear 80 is collinear with the rotation axis of the distal portion of shaft 77. Worm 79 is arranged to rotate in response to a rotation of gear 81 . Worm 79 may be rotationally fast with gear 81 such that a rotation of gear 81 causes a corresponding rotation of worm 79. Worm 79 may have a rotation axis that is collinear with the rotation axis of gear 81 . Thus gears 80 and 81 operate to couple rotation of shaft 77 to rotation of gear 79 about a rotation axis parallel to the rotation axis of the distal portion of shaft 77.

The rotation axis of worm 79 is not parallel and does not intersect the rotation axis of gear 75 (axis 63). Gear 75 is therefore a skew-axis gear. Similarly, the rotation axes of worm 78 and gear 74 are non-parallel and non-intersecting. Thus gear 74 is also a skew-axis gear.

It is observed that rotation of the shaft 76, which causes the coupler 64 to rotate about axis 62, may cause gears 80 and 81 (and thus worm gear 79) to rotate when the shaft 77 is held stationary, causing parasitic motion of the distal shell 31 1 ' relative to the shell 310' about the rotation axis 63. This is because the rotation of the coupler 64 about axis 62 driven by the rotation of the shaft 77 needs to be accommodated by the Hooke's joint 82, and that rotation of the coupler 64 may cause a parasitic rotation of the Hooke's joint about the longitudinal axis of the shaft 77. Any such parasitic rotation of the Hooke's joint may cause a consequent rotation of gears 80 and 81 , and thus rotation of the bevel gear 75. Such parasitic motion may be prevalent if the hinge axes of the Hooke's joint are not perpendicular to each other, and/or if one of the hinge axes is not parallel and coincident with the rotation axis 62.

To prevent this parasitic motion, the control system 10 may be configured to drive compensatory motion of the shaft 77 in tandem with motion of the shaft 76 so as to isolate motion about axis 62 from motion about axis 63. Thus the control system 10 may be arranged to operate the motor to drive rotation of shaft 76 to cause rotation of arm part 31 1 ' relative to arm part 310' about axis 63 whilst simultaneously operating the motor to drive shaft 77 to rotate in such a way as to prevent parasitic rotation about axis 63. The control system 10 may be configured to operate in this manner when the robot arm is commanded to articulate about axis 62 without articulating about axis 63.

The control system 10 may also be configured to drive rotation of shafts 76 and/or 77 in such a way as to reduce irregularities (i.e. increase smoothness) in the rotation of the Hooke's joint 82. The Hooke's joint may experience irregularities in its rotation when it is off axis, i.e. when arm part 31 1 ' is pitched relative to arm part 310' about axis 62. Thus when the arm part 31 1 ' is commanded to articulate relative to arm part 310' about axis 63 when arm part 31 1 ' is pitched relative to arm part 310' about axis 62, the control system may operate to drive the rotation of shaft 77 in such a way as to maintain a smooth or consistent rotation speed of the Hooke's joint 82. This may help to provide a smooth and/or consistent rotation about axis 63.

This mechanism has been found to be capable of providing a particularly compact, light and rigid drive arrangement for rotation about axes 62 and 63 without the

components of the mechanism unduly restricting motion of the shells. It permits both motors to be housed in the proximal shell which reduces distal weight.

Various aspects of the mechanism shown in figures 5 and 6 are advantageous in helping to make the mechanism particularly compact.

1 . It is convenient for bevel gear 74 to be of part-circular form: i.e. its teeth do not encompass a full circle. For example, gear 74 may encompass less than 270° or less than 180° or less than 90°. This allows at least part of the other bevel gear 75 to be located in such a way that it intersects a circle coincident with gear 74, about the axis of gear 74 and having the same radius as the outermost part of gear 74. Whilst this feature can be of assistance in reducing the size of a range of compound joints, it is of particular significance in a wrist of the type shown in figure 2, comprising a pair of roll joints with a pair of pitch/yaw joints between them, since in a joint of that type there is a degree of redundancy among the pitch/yaw joints and hence a wide range of positions of the distal end of the arm can be reached even if motion about axis 62 is restricted. As shown in figure 6, the bevel gear 74 is of reduced radius in the region not encompassed by its teeth. Part-circular bevel gears of the other embodiments may be formed in the same manner.

2. The gears 74 and/or 75 are conveniently provided as bevel gears since that permits them to be driven from worms located within the plan of their respective external radii.

However, they could be externally toothed gears engaged on their outer surfaces by the worms 76, 79, or by radially toothed gears.

4. The bevel gears and the worm gears that mate with them can conveniently be of skew axis, e.g. Spiroid®, form. These allow for relatively high torque capacity in a relatively compact form.

Figures 7 to 10 illustrate another form of wrist mechanism. In these figures the shells of arm parts 310, 31 1 are omitted, exposing the structure within the arm parts. Proximal arm part 310 has a structural framework 100, which is shown in outline in some of the figures. Distal arm part 31 1 has a structural framework 101 . Arm parts 310 and 31 1 are rotatable relative to each other about axes 102, 103, which correspond to axes 305, 306 respectively of figure 2. A carrier 104 couples the arm parts 310, 31 1 together. Carrier 104 is attached by bearings 105, 190 to arm part 310. Those bearings define a revolute joint about axis 102 between arm part 310 and the

carrier 104. Carrier 104 is attached by bearing 106 to arm part 31 1 . Those bearings define a revolute joint about axis 103 between arm part 31 1 and the carrier 104. A first bevel gear 107 about axis 102 is fast with the carrier 104. A second bevel gear 108 about axis 103 is fast with arm part 31 1 .

The carrier 104 can therefore rotate relative to the arm part 310 about axis 102. However, the carrier 104 is otherwise fast with the arm part 310 and in particular is fast about axis 103. Thus the carrier 104 is not permitted to undergo relative rotation with respect to arm part 310 about axis 103. The second bevel gear 108 can rotate relative to the carrier 104 about axis 103. The second bevel gear 108 (and hence the arm part 31 1 ) may be fast with the carrier about axis 102. Thus the second bevel gear 108 is permitted to undergo relative rotation with respect to the carrier 104 about axis 103 but is not permitted to undergo relative rotation with respect to the carrier about axis 102.

Axes 102 and 103 are in this example perpendicular, but in general are two non-parallel axes. They may be substantially orthogonal to each other. The axes are substantially transverse to the longitudinal direction of the arm part 310 in at least one configuration of the joints 301 and 302. In the arrangement shown, one such configuration is when arm part 31 1 is not articulated with respect to arm part 310. In the context of a Cartesian coordinate system, axis 102 may be considered as a "pitch" rotation axis and axis 103 as a "yaw" rotation axis.

claims.
CLAIMS

1 . A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:

an intermediate carrier attached to a first one of the limbs by a first revolute joint having a pitch rotation axis and to a second one of the limbs by a second revolute joint having a yaw rotation axis;

a first drive gear disposed about the pitch rotation axis, the first drive gear being fast with the carrier;

a second drive gear disposed about the yaw rotation axis, the second drive gear being fast with the second one of the limbs;

a first drive shaft for driving the first drive gear to rotate about the pitch rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the yaw rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon; and

an intermediate gear train borne by the carrier and coupling the second shaft gear to the second drive gear.

2. A robot arm as claimed in claim 1 , wherein the intermediate gear train comprises a first intermediate gear disposed about the pitch rotation axis, the first intermediate gear being arranged to engage the second shaft gear.

3. A robot arm as claimed in claim 2, further comprising a control unit arranged to respond to command signals commanding motion of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the pitch axis without articulating about the yaw axis, drive the first shaft to rotate to cause articulation about the pitch axis and also drive the second shaft to rotate in such a way as to negate parasitic articulation about the yaw axis.

4. A robot arm as claimed in any preceding claim, wherein the intermediate gear train comprises a plurality of interlinked gears arranged to rotate about axes parallel with the pitch rotation axis.

5. A robot arm as claimed in any preceding claim, wherein the intermediate gear train comprises an intermediate shaft arranged to rotate about an axis parallel with the pitch rotation axis, the intermediate shaft having a third shaft gear thereon, the third shaft gear being arranged to engage the second drive gear.

6. A robot arm as claimed in claim 5 as dependent on claim 4, wherein the interlinked gears are on one side of a plane perpendicular to the pitch axis and containing the teeth of the first drive gear, and at least part of the third shaft gear is on the other side of that plane.

7. A robot arm as claimed in claim 5 or 6, wherein the third shaft gear is a worm gear.

8. A robot arm as claimed in any preceding claim, wherein one or both of the first and second shaft gears is/are worm gears.

9. A robot arm as claimed in any preceding claim, wherein one or both of the first drive gears is/are bevel gear(s).

10. A robot arm as claimed in claim 9, wherein one or both of the first drive gears is/are skew axis gear(s).

1 1 . A robot arm as claimed in any preceding claim, wherein the first drive gear is a part-circular gear.

12. A robot arm as claimed in claim 1 1 , wherein at least part of the second drive gear intersects a circle about the pitch axis that is coincident with the radially outermost part of the first drive gear.

13. A robot arm as claimed in claim 1 1 or 12 as dependent on claim 5, wherein at least part of the intermediate shaft intersects a circle about the pitch axis that is coincident with the radially outermost part of the first drive gear.

14. A robot arm as claimed in any preceding claim, wherein the pitch and yaw axes are orthogonal.

15. A robot arm as claimed in any preceding claim, wherein the pitch and yaw axes intersect each other.

16. A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:

an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis;

a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier;

a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs;

a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs on a first side of a plane containing the second rotation axis and extending through that plane to the second side of that plane; and

an intermediate linkage that meshes with the second drive shaft on the second side of the plane and that couples the second shaft gear to the second drive gear.

17. A robot arm as claimed in claim 16, wherein the second shaft comprises a flexible element.

18. A robot arm as claimed in claim 17, wherein the flexible element is located on the first rotation axis.

19. A robot arm as claimed in claim 17 or 18, wherein the flexible element is a universal joint.

20. A robot arm as claimed in any of claims 16 to 19, wherein the second shaft is coupled to the carrier by a revolute joint on the second side of the said plane.

21 . A robot arm as claimed in any of claims 16 to 19, wherein the second drive shaft has a second shaft gear on the second side of the said plane and the intermediate linkage comprises an intermediate shaft having a first intermediate gear that meshes with the second shaft gear and a second intermediate gear that meshes with the second drive gear.

22. A robot arm as claimed in claim 21 , wherein the second drive shaft is arranged to rotate about an axis perpendicular to the second rotation axis.

23. A robot arm as claimed in any of claims 16 to 22, wherein the second intermediate gear is a worm gear.

24. A robot arm as claimed in any of claims 16 to 23, wherein the first shaft gear is a worm gear.

25. A robot arm as claimed in of claims 16 to 24, wherein one or both of the first drive gears is/are bevel gear(s).

26. A robot arm as claimed in any of claims 16 to 25, wherein one or both of the first drive gears is/are skew axis gear(s).

27. A robot arm as claimed in any of claims 16 to 26, wherein the first drive gear is a part-circular gear.

28. A robot arm as claimed in claim 27, wherein at least part of the second drive gear intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

29. A robot arm as claimed in any of claims 16 to 28, wherein the first and second axes are orthogonal.

30. A robot arm as claimed in any of claims 16 to 29, wherein the first and second axes intersect each other.

31 . A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:

an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis;

a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier;

a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs;

a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon, the second shaft gear being arranged to engage the second drive gear;

the second drive shaft comprising a prismatic joint whereby the length of the shaft can vary in response to motion of the carrier about the first axis.

32. A robot arm as claimed in claim 31 , wherein the prismatic joint is a sliding splined coupling.

33. A robot arm as claimed in claim 31 or 32, wherein the second dive shaft comprises a first flexible joint on one side of the prismatic joint and a second flexible joint on the other side of the prismatic joint.

34. A robot arm as claimed in claim 33, wherein the second drive shaft is connected by a revolute joint to the carrier on the opposite side of the second flexible joint to the prismatic joint.

35. A robot arm as claimed in any of claims 31 to 34, wherein one or both of the first and second shaft gears is/are worm gears.

36. A robot arm as claimed in any of claims 31 to 35, wherein one or both of the first drive gears is/are bevel gear(s).

37. A robot arm as claimed in claim 36, wherein one or both of the first drive gears is/are skew axis gear(s).

38. A robot arm as claimed in any of claims 31 to 37, wherein the first drive gear is a part-circular gear.

39. A robot arm as claimed in claim 38, wherein at least part of the second drive gear intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

40. A robot arm as claimed in any of claims 35 to 39, wherein the first and second axes are orthogonal.

41 . A robot arm as claimed in any of claims 35 to 40, wherein the first and second axes intersect each other.

42. A robot arm as claimed in any of claims 16 to 41 , further comprising a control unit arranged to respond to command signals commanding rotation of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the first axis without articulating about the second axis, drive the first shaft to rotate to cause articulation about the first axis and also drive the second shaft to rotate in such a way as to negate parasitic articulation about the second axis.

43. A robot arm as claimed in any of claims 16 to 42, further comprising a control unit arranged to respond to command signals commanding rotation of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the second rotation axis when the second one of the limbs is articulated with respect to the first one of the limbs about the first rotation axis, drive the second drive shaft to rotate to cause articulation about the second axis in such a way as to provide smooth rotation about the second axis.

44. A robot arm substantially as herein described with reference to figures 1 to 12 of the accompanying drawings.

AMENDED CLAIMS

received by the International Bureau on 29 November 2016 (29.1 1 .2016)

1. A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:

an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis;

a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier;

a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs;

a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon;

an intermediate gear train borne by the carrier and coupling the second shaft gear to the second drive gear, the intermediate gear train comprising a first intermediate gear disposed about the first rotation axis, the first intermediate gear being arranged to engage the second shaft gear; and

a control unit arranged to respond to command signals commanding motion of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the first axis without articulating about the second axis, drive the first shaft to rotate to cause articulation about the first axis and also drive the second shaft to rotate in such a way as to negate parasitic articulation about the second axis.

2. A robot arm as claimed in any preceding claim, wherein the intermediate gear train comprises a plurality of interlinked gears arranged to rotate about axes parallel with the first rotation axis.

3. A robot arm as claimed in any preceding claim, wherein the intermediate gear train comprises an intermediate shaft arranged to rotate about an axis parallel with the first rotation axis, the intermediate shaft having a third shaft gear thereon, the third shaft gear being arranged to engage the second drive gear.

4. A robot arm as claimed in claim 3 as dependent on claim 2, wherein the interlinked gears are on one side of a plane perpendicular to the first axis and containing the teeth of the first drive gear, and at least part of the third shaft gear is on the other side of that plane.

5. A robot arm as claimed in claim 3 or 4, wherein the third shaft gear is a worm gear.

6. A robot arm as claimed in any preceding claim, wherein one or both of the first and second shaft gears is/are worm gears.

7. A robot arm as claimed in any preceding claim, wherein one or both of the first drive gears is/are bevel gear(s).

8. A robot arm as claimed in claim 7, wherein one or both of the first drive gears is/are skew axis gear(s).

9. A robot arm as claimed in any preceding claim, wherein the first drive gear is a part-circular gear.

10. A robot arm as claimed in claim 9, wherein at least part of the second drive gear intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

11. A robot arm as claimed in claim 9 or 10 as dependent on claim 3, wherein at least part of the intermediate shaft intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

12. A robot arm as claimed in any preceding claim, wherein the first and second axes are orthogonal.

13. A robot arm as claimed in any preceding claim, wherein the first and second axes intersect each other.

14. A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:

an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis;

a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier;

a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs;

a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs on a first side of a plane containing the second rotation axis and extending through that plane to the second side of that plane; and

an intermediate linkage that meshes with the second drive shaft on the second side of the plane and that couples the second shaft gear to the second drive gear.

15. A robot arm as claimed in claim 14, wherein the second shaft comprises a flexible element.

16. A robot arm as claimed in claim 15, wherein the flexible element is located on the first rotation axis.

17. A robot arm as claimed in claim 15 or 16, wherein the flexible element is a universal joint.

18. A robot arm as claimed in any of claims 14 to 17, wherein the second shaft is coupled to the carrier by a revolute joint on the second side of the said plane.

19. A robot arm as claimed in any of claims 14 to 17, wherein the second drive shaft has a second shaft gear on the second side of the said plane and the intermediate linkage comprises an intermediate shaft having a first intermediate gear that meshes with the second shaft gear and a second intermediate gear that meshes with the second drive gear.

20. A robot arm as claimed in claim 19, wherein the second drive shaft is arranged to rotate about an axis perpendicular to the second rotation axis.

21. A robot arm as claimed in any of claims 14 to 20, wherein the second intermediate gear is a worm gear.

22. A robot arm as claimed in any of claims 14 to 21 , wherein the first shaft gear is a worm gear.

23. A robot arm as claimed in of claims 14 to 22, wherein one or both of the first drive gears is/are bevel gear(s).

24. A robot arm as claimed in any of claims 14 to 23, wherein one or both of the first drive gears is/are skew axis gear(s).

25. A robot arm as claimed in any of claims 14 to 24, wherein the first drive gear is a part-circular gear.

26. A robot arm as claimed in claim 25, wherein at least part of the second drive gear intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

27. A robot arm as claimed in any of claims 14 to 26, wherein the first and second axes are orthogonal.

28. A robot arm as claimed in any of claims 14 to 27, wherein the first and second axes intersect each other.

29. A robot arm comprising a joint mechanism for articulating one limb of the arm relative to another limb of the arm about two non-parallel rotation axes, the mechanism comprising:

an intermediate carrier attached to a first one of the limbs by a first revolute joint having a first rotation axis and to a second one of the limbs by a second revolute joint having a second rotation axis;

a first drive gear disposed about the first rotation axis, the first drive gear being fast with the carrier;

a second drive gear disposed about the second rotation axis, the second drive gear being fast with the second one of the limbs;

a first drive shaft for driving the first drive gear to rotate about the first rotation axis, the first drive shaft extending along the first one of the limbs and having a first shaft gear thereon, the first shaft gear being arranged to engage the first drive gear; a second drive shaft for driving the second drive gear to rotate about the second rotation axis, the second drive shaft extending along the first one of the limbs and having a second shaft gear thereon, the second shaft gear being arranged to engage the second drive gear;

the second drive shaft comprising a prismatic joint whereby the length of the shaft can vary in response to motion of the carrier about the first axis.

30. A robot arm as claimed in claim 29, wherein the prismatic joint is a sliding splined coupling:

31 . A robot arm as claimed in claim 29 or 30, wherein the second dive shaft comprises a first flexible joint on one side of the prismatic joint and a second flexible joint on the other side of the prismatic joint.

32. A robot arm as claimed in claim 31 , wherein the second drive shaft is connected by a revolute joint to the carrier on the opposite side of the second flexible joint to the prismatic joint.

33. A robot arm as claimed in any of claims 29 to 32, wherein one or both of the first and second shaft gears is/are worm gears.

34. A robot arm as claimed in any of claims 29 to 33, wherein one or both of the first drive gears is/are bevel gear(s).

35. A robot arm as claimed in claim 34, wherein one or both of the first drive gears is/are skew axis gear(s).

36. A robot arm as claimed in any of claims 29 to 35, wherein the first drive gear is a part-circular gear.

37. A robot arm as claimed in claim 36, wherein at least part of the second drive gear intersects a circle about the first axis that is coincident with the radially outermost part of the first drive gear.

38. A robot arm as claimed in any of claims 33 to 37, wherein the first and second axes are orthogonal.

39. A robot arm as claimed in any of claims 33 to 38, wherein the first and second axes intersect each other.

40. A robot arm as claimed in any of claims 14 to 39, further comprising a control unit arranged to respond to command signals commanding rotation of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the first axis without articulating about the second axis, drive the first shaft to rotate to cause articulation about the first axis and also drive the second shaft to rotate in such a way as to negate parasitic articulation about the second axis.

41. A robot arm as claimed in any of claims 14 to 40, further comprising a control unit arranged to respond to command signals commanding rotation of the robot arm by driving the first and second drive shafts to rotate, the control unit being configured to, when the robot arm is commanded to articulate about the second rotation axis when the second one of the limbs is articulated with respect to the first one of the limbs about the first rotation axis, drive the second drive shaft to rotate to cause articulation about the second axis in such a way as to provide smooth rotation about the second axis.

42. A robot arm substantially as herein described with reference to figures 1 to 12 of the accompanying drawings.