Patent Description:
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.

The documents <CIT>, <CIT> and <CIT> present robot arms having such wrists.

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.

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.

According to the invention, there is provided a surgical robot arm comprising a first arm segment having a first longitudinal axis and a second arm segment having a second longitudinal axis, the first and second arm segments being coupled to each other by a first revolute joint having a first rotation axis which is perpendicular to the first longitudinal axis and a second revolute joint having a second rotation axis which is perpendicular to the second longitudinal axis and intersects and is non-parallel to the first rotation axis, and a joint mechanism for articulating the first arm segment relative to the second arm segment about the first and second rotation axes, the joint mechanism comprising: a first driven gear disposed about an axle coincident with the first rotation axis, the axle being fast with a first arm segment of the robot arm; a second driven gear disposed about the second rotation axis and fast with a second arm segment of the robot arm and fast with the first driven gear about the first rotation axis; a first gear disposed about a third rotation axis and configured to drive the first driven gear to rotate about the axle, the first drive gear being arranged to engage the first driven gear; a second drive gear for driving the second driven gear to rotate about the second rotation axis; and an intermediary gear arrangement arranged to engage the second drive gear and the second driven gear and being disposed about the first rotation axis, whereby rotation of the intermediary gear arrangement relative to the first arm segment about the first rotation axis can be driven, the intermediary gear arrangement comprising a first intermediate gear arranged to engage the second drive gear, and a second intermediate gear arranged to engage the second driven gear, wherein the third rotation axis and the first rotation axis are non- intersecting.

The first drive gear may be attached to a first drive shaft extending along the first arm segment.

The second drive gear may be attached to a second drive shaft extending along the first arm segment.

The first and second drive shafts may lie on one side of a plane on which the axle lies. The first driven gear may be a ring gear.

The first driven gear may be a hypoid gear.

The intermediary gear arrangement may be mounted on the axle, the axle being mounted at each of its ends to the first arm segment.

The first intermediate gear may be fast with the second intermediate gear.

At least one of the first driven gear and first intermediate gear may be a sector.

Only the first driven gear may be a sector.

The operative arc of the sector may be less than or equal to <NUM> degrees.

The first intermediate gear may be a ring gear.

The first intermediate gear may be a hypoid gear.

The first drive gear and first driven gear may form a first hypoid gear set, and the second drive gear and first intermediate gear form a second hypoid gear set, the first and second hypoid gear sets having an equal skew offset.

The second intermediate gear and the second driven gear may be bevel gears.

The second intermediate gear and the second driven gear may be mitre gears.

The joint mechanism may further comprise a carrier attached to the first arm segment by the first revolute joint and attached to the second arm segment by the second revolute joint, the carrier being fast with the first driven gear.

The first and second revolute joints may be wrist joints of the robotic arm.

The robot arm may comprise a tool or tool attachment fast with the second arm segment about the first and second rotation axes.

The robot arm may further comprise a third arm segment adjacent the second arm segment and located on the opposite side of the second arm segment to the joint mechanism, and a third revolute joint that attaches the third arm segment to the second arm segment, wherein the third arm segment comprises a tool or a tool attachment.

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 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.

<FIG> shows a surgical robot having an arm <NUM> which extends from a base <NUM>. The arm comprises a number of rigid arm segments, or limbs <NUM>. The limbs are coupled by revolute joints <NUM>. 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 <NUM>. A wrist <NUM> is made up of four individual revolute joints. The wrist <NUM> couples one limb (3b) to the most distal limb (3c) of the arm. The most distal limb 3c carries an attachment <NUM> for a surgical instrument or tool <NUM>. Limb 3b may be referred to as the 'forearm'. Each joint <NUM> of the arm has one or more motors <NUM> which can be operated to cause rotational motion at the respective joint, and one or more position and/or torque sensors <NUM> 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 <FIG>. The attachment point <NUM> 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. Controllers for the motors, torque sensors and encoders may be distributed with the arm. The controllers are connected via a communication bus to control unit <NUM>.

A control unit <NUM> comprises a processor <NUM> and a memory <NUM>. Memory <NUM> stores in a non-transient way software that is executable by the processor to control the operation of the motors <NUM> to cause the arm <NUM> to operate in the manner described herein. In particular, the software can control the processor <NUM> to cause the motors (for example via distributed controllers) to drive in dependence on inputs from the sensors <NUM> and from a surgeon command interface <NUM>. The control unit <NUM> is coupled to the motors <NUM> for driving them in accordance with outputs generated by execution of the software. The control unit <NUM> is coupled to the sensors <NUM> for receiving sensed input from the sensors, and to the command interface <NUM> 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 <NUM> 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 <NUM> is configured to respond to those inputs and cause the joints of the arm to move accordingly, in compliance with a predetermined 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 <NUM> can control the robot arm <NUM> to move in such a way as to perform a desired surgical procedure. The control unit <NUM> and/or the command interface <NUM> may be remote from the arm <NUM>.

<FIG> shows the wrist <NUM> of the robot in more detail. The wrist comprises four revolute wrist joints <NUM>, <NUM>, <NUM>, <NUM> and arm segments <NUM> and <NUM>. The joints are arranged in series, with a rigid part of the arm extending from each joint to the next. The most proximal joint <NUM> of the wrist joins arm segment 3b to arm segment <NUM>. Joint <NUM> has a "roll" rotation axis <NUM>, which is directed generally along the extent of the limb 3b of the arm that is immediately proximal of the articulations of the wrist. The next most distal joint <NUM> of the wrist joins arm segment <NUM> to arm segment <NUM>. Joint <NUM> has a "pitch" rotation axis <NUM> which is perpendicular to axis <NUM> in all configurations of joints <NUM> and <NUM>. The next most distal joint <NUM> of the wrist joins arm part <NUM> to arm part <NUM>. Joint <NUM> has a "yaw" rotation axis <NUM>. Axes <NUM> and <NUM> are in general non-parallel rotation axes in all configurations of the joints <NUM> and <NUM>. In this example, axes <NUM> and <NUM> are perpendicular to each other in all configurations of joints <NUM> and <NUM>. In some configurations of the wrist, axis <NUM> is also perpendicular to axis <NUM>. The next most distal joint of the wrist <NUM> joins arm segment <NUM> to arm segment 3c. Arm segment 3c is therefore located on the opposite side of the arm segment <NUM> than the joints <NUM> and <NUM>. Joint <NUM> has a "roll" rotation axis <NUM> which is perpendicular to axis <NUM> in all configurations of joints <NUM> and <NUM>. In some configurations of the wrist, axis <NUM> is also perpendicular to axis <NUM> and parallel with (and preferably collinear with) axis <NUM>. There may be no other joints connecting arm segment 3c to arm segment <NUM>. Thus arm segment 3c (and hence the tool attachment point <NUM>) may be fast with the arm segment <NUM> about the axes <NUM> and <NUM> such that there is no relative motion between arm segment <NUM> and attachment point <NUM> about these axes.

Axes <NUM> and <NUM> intersect each other, what gives a particularly compact configuration. Joints <NUM> and <NUM> may be positioned so that axes <NUM> and <NUM> can pass through the intersection of axes <NUM>, <NUM> 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 <NUM> at the distal end of arm segment 3c, 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.

Various views of an example joint mechanism for implementing wrist <NUM> are shown in <FIG> focus on the mechanism associated with joints <NUM> and <NUM> in <FIG>. <FIG> shows the joint mechanism with respect to the first arm segment <NUM> with the second arm segment <NUM> omitted for clarity. <FIG> shows an enlarged view of the joint mechanism with the arm segments <NUM> and <NUM> omitted for clarity. <FIG> shows various two-dimensional views of the joint mechanism, and <FIG> shows a cross section view of the distal end of arm segment <NUM> in a plane transverse to the length of the segment <NUM>.

Referring first to <FIG> and <FIG>, the joint mechanism is indicated generally at <NUM>. The joint mechanism <NUM> is arranged to enable arm segment <NUM> to be articulated with respect arm segment <NUM> (omitted in these figures for clarity) about the non-parallel rotation axes <NUM> and <NUM>. The arm segment <NUM> has an outer casing <NUM>. The outer casing may be rigid, and defines an outer surface of the arm segment <NUM>. The outer casing <NUM> defines an interior volume in which components of the robot arm, such as motors, sensors (e.g. position and/or torque sensors), cables and other components may be housed. Arm segment <NUM> also comprises an outer casing similarly defining an interior volume for housing components of the arm, but this has been omitted from <FIG> for clarity.

The arm segments <NUM> and <NUM> (including their associated outer casings) are moveable with respect to each other about the axes <NUM> and <NUM>. Axes <NUM> and <NUM> are in general non-parallel axes. In this example, axes <NUM> and <NUM> are perpendicular to each other and additionally intersect.

The joint mechanism <NUM> comprises a first driven gear <NUM>, a second driven gear <NUM>, a first drive gear <NUM> and a second drive gear <NUM>. The first and second drive gears <NUM> and <NUM> may alternatively be referred to as input gears, and the first and second driven gears <NUM> and <NUM> may alternatively be referred to as output gears. The joint mechanism additionally comprises an intermediary gear arrangement - indicated generally at <NUM> - that couples the second drive gear <NUM> to the second driven gear <NUM>.

Referring to <FIG>, it can be seen that the first driven gear is coupled to the first arm segment <NUM> via a carrier <NUM>. The carrier <NUM> is disposed about, or mounted on, an axle <NUM>. Axle <NUM> is coincident with the rotation axis <NUM>. Axle <NUM> is mounted to, and thus fast with, the arm segment <NUM>. The carrier <NUM> is mounted to the arm segment <NUM> via laterally spaced bearings <NUM> and <NUM> that permit relative rotation of the carrier <NUM> relative to the arm segment <NUM> about the axle <NUM> (and thus axis <NUM>), thus defining revolute joint <NUM>. The first driven gear <NUM> is fast with the carrier <NUM>. That is to say, the gear <NUM> and carrier <NUM> are fixed with respect to each other. The gear <NUM> is fixedly mounted to the carrier <NUM>. The gear <NUM> may abut the carrier <NUM>.

That is, the carrier <NUM> may touch, or interface with, the gear <NUM>. In other examples the carrier <NUM> may be fast with gear <NUM> but physically separated by one or more intermediary components such as spacers, washers etc. In this particular example the gear <NUM> is coupled to the carrier <NUM> via a torque sensor <NUM> that measures an applied torque about axis <NUM>.

Carrier <NUM> also supports the second driven gear <NUM>. Carrier <NUM> is coupled to the second driven gear <NUM> via bearings <NUM> to permit relative rotation of the carrier <NUM> and gear <NUM> about the rotation axis <NUM>. Gear <NUM> is otherwise fast with carrier <NUM>, and in particular fast about the rotation axis <NUM> such that there is no relative rotation or motion between gear <NUM> and carrier <NUM> about axis <NUM>. The coupling of carrier <NUM> to the second driven gear <NUM> via bearings <NUM> defines the revolute joint <NUM>. Gear <NUM> is fast with the arm segment <NUM>.

Due to the function of the carrier <NUM> supporting the first driven gear and second driven gear, carrier <NUM> may alternatively be referred to as a chassis, or hub.

The arm may (as shown) further comprise torque sensors <NUM> and <NUM> for measuring torque about the joints <NUM> and <NUM>. Torque sensor <NUM> measures the torque about the rotation axis <NUM> and torque sensor <NUM> measures the torque about the rotation axis <NUM>. The sensors are housed within the interior volume defined by the outer casing <NUM>. Both sensors comprise tubular components in the form of a sleeve. Torque sensor <NUM> sits around the axle <NUM> and torque sensor <NUM> sits around, i.e. is disposed about, axis <NUM>. Both sensors <NUM> and <NUM> may be coupled to the control unit <NUM> and be configured to provide sensed torque readings to the control unit.

Referring back now to <FIG>, the structure of the joint mechanism will be described in more detail.

The first driven gear <NUM> is a ring gear, or crown wheel. It is additionally formed as a sector; that is to say the arc defined by its teeth is less than <NUM> degrees. The sector has an operative arc, that is, a range of rotational motion through which it can be driven, that is less than <NUM> degrees. The arc of teeth of the sector may be greater than its operative arc. This is because the pinion <NUM> is not in point contact with gear <NUM> but instead engages a subset of its teeth in order to drive it. Thus the pinion <NUM> cannot drive the gear <NUM> through a range of motion equal to the arc of its teeth.

The gear <NUM> is disposed, or arranged, about the rotation axis <NUM>. The gear <NUM> therefore has as its rotation axis the axis <NUM>. Specifically, the first driven gear <NUM> is rotatably mounted on axle <NUM> (more clearly seen in <FIG> and <FIG>) such that it can rotate with respect to the arm segment <NUM> about axis <NUM>.

Driven gear <NUM> is engaged by the first drive gear <NUM>, which in this example is a pinion. The term 'engage', as used herein when applied to a pair of gears, means that the teeth of one gear mesh, or interface, with the teeth of the other gear.

Pinion <NUM> is affixed to a first drive shaft <NUM> which is driven by a first electric motor mounted in the arm segment <NUM> (not shown). The drive shaft <NUM> is mounted in the arm segment <NUM> by bearings <NUM> to permit the shaft to rotate about its longitudinal axis <NUM> relative to the arm segment <NUM> (shown most clearly in <FIG>). The drive shaft <NUM> extends along the arm segment <NUM> and is attached at its (distal) terminal end to the pinion <NUM>. The pinion <NUM> may be fast with the drive shaft <NUM>. The drive shaft <NUM> extends in a direction perpendicular to the rotation axis <NUM> in all configurations of the joints <NUM> and <NUM>. In other words, the longitudinal axis <NUM> of the drive shaft is perpendicular to the rotation axis <NUM> in all configurations of the joints <NUM> and <NUM>.

Drive gear <NUM> drives rotation of the driven gear <NUM> about axis <NUM>, as will be described in more detail below.

The axis of rotation of the first drive gear <NUM> (axis <NUM>) and the first driven gear <NUM> (axis <NUM>) are non-parallel (in particular, they are perpendicular to each other) and non-intersecting, as shown most clearly in <FIG> and <FIG>. The first driven gear <NUM> is therefore a skew gear. In this particular example, the first driven gear <NUM> is a hypoid gear and drive gear <NUM> a hypoid pinion. Together they may be referred to as a hypoid gear set. The skew offset, that is the perpendicular distance between axes <NUM> and <NUM>, is denoted d.

The second driven gear <NUM> is a bevel gear arranged about axis <NUM>. It is disposed about the rotation axis <NUM> and arranged to undergo rotation about this axis relative to the first arm segment <NUM> and the first driven gear <NUM>. The second driven gear <NUM> is fast with the arm segment <NUM>. The second driven gear <NUM> may be mounted to the arm segment <NUM>. The driven gear <NUM> and arm segment <NUM> may be arranged so that the gear <NUM> abuts the arm segment <NUM>. The second driven gear <NUM> is also fast with the first driven gear <NUM> about the rotation axis <NUM>. That is, gears <NUM> and <NUM> cannot undergo relative rotation or motion about axis <NUM>. Put another way, gears <NUM> and <NUM> may be fast with each other except that gear <NUM> can rotate relative to gear <NUM> about axis <NUM>. This means that a rotation of the first driven gear <NUM> about axis <NUM> causes a corresponding rotation of the second driven gear <NUM> about the axis <NUM>.

The second driven gear <NUM> is fast with the first driven gear <NUM> about the rotation axis <NUM> by virtue of those gears' couplings to the carrier <NUM> (shown in <FIG>). This is because: i) the first driven gear <NUM> is fast with the carrier <NUM>; and ii) the second driven gear <NUM> is mounted to the carrier <NUM> via bearings <NUM> to permit relative rotation of the gear <NUM> and carrier about the rotation axis <NUM> but is otherwise fast with the carrier. The second driven gear <NUM> is therefore fast with the first driven gear <NUM> about the rotation axis <NUM>.

In an alternative arrangement, the second driven gear <NUM> may be secured, or mounted, to the first driven gear <NUM> via bearings to permit relative rotation of the two driven gears about rotation axis <NUM>. Such an arrangement would enable the two driven gears to be fast with each other about axis <NUM> without the second driven gear <NUM> being mounted to the carrier <NUM>.

Because the first driven gear <NUM> is of part-circular form, the second driven gear <NUM> may be located in such a way that it intersects a circle coincident with gear <NUM> about axis <NUM> and having a radius equal to the outermost part of gear <NUM>. This is most clearly shown in <FIG> and <FIG>. This arrangement is particularly useful for reducing the size of the joint mechanism.

The second driven gear <NUM> is driven by the second driving gear <NUM> to rotate about the axis <NUM>. In this example the second driving gear <NUM> is a pinion. The pinion is fast with a second drive shaft <NUM> which is driven by a second electric motor mounted in the arm segment <NUM> (not shown). The second drive shaft <NUM> extends along the length of the arm segment <NUM> and is attached at its distal terminal end to the driving gear <NUM>. Drive shaft <NUM> is mounted in the arm segment <NUM> by bearings <NUM> that permit the shaft to rotate about its longitudinal axis <NUM> relative to the arm segment <NUM>.

The drive shaft <NUM>, and hence its longitudinal axis <NUM>, extends in a direction perpendicular to the rotation axis <NUM> in all configurations of the joints <NUM> and <NUM>. Drive shaft <NUM> is therefore parallel to drive shaft <NUM>. Drive shaft <NUM> is also symmetrical with drive shaft <NUM> about a first plane that contains the rotation axis <NUM> and is perpendicular to axis <NUM> (most clearly seen in <FIG>). The first plane therefore contains the rotation axis <NUM> and the centreline of arm segment <NUM>. It can further be seen from <FIG> that both drive shafts <NUM> and <NUM> lie on the same side of a second plane containing the axle <NUM> (and rotation axis <NUM>). That second plane is parallel to the longitudinal direction, or axis, of the arm part <NUM>. Thus, the second plane is parallel to the longitudinal axes <NUM> and <NUM> of the drive shafts <NUM> and <NUM> respectively. The second plane is perpendicular to the axis <NUM> in at least one configuration of the joints <NUM> and <NUM>. In the arrangement shown, that configuration is when the longitudinal axis of arm part <NUM> is parallel to the longitudinal axis of arm part <NUM> (i.e. when arm part <NUM> is in not in pitch or yaw relative to arm part <NUM>).

In the arrangement shown, drive gears <NUM> and <NUM> also lie on the same side of a third plane that contains axis <NUM> and is perpendicular to the longitudinal axis of the arm part <NUM> (most clearly shown in <FIG>). The third plane contains both axes <NUM> and <NUM> in at least one configuration of the joints <NUM> and <NUM>. That configuration is when the longitudinal axis of arm part <NUM> is parallel to the longitudinal axis of arm part <NUM> (i.e. when arm part <NUM> is in not in pitch or yaw relative to arm part <NUM>).

The second drive gear <NUM> and second driven gear <NUM> are coupled by the intermediary gear arrangement <NUM> to transfer rotation of the drive gear <NUM> to the driven gear <NUM>. Specifically, the intermediary gear arrangement <NUM> engages both the drive gear <NUM> and the driven gear <NUM>. The intermediary gear arrangement in this example comprises a first intermediate gear <NUM> that engages the drive gear <NUM>, and a second intermediate gear <NUM> that engages the driven gear <NUM>. The first and second intermediate gears are fast with each other. As is more clearly shown in <FIG> and <FIG>, the intermediate gears <NUM> and <NUM> are fixed, or mounted, to each other. In particular, gear <NUM> abuts gear <NUM>.

The first intermediate gear <NUM> is a ring gear, or crown wheel. The second intermediate gear <NUM> is a bevel gear. Both gears are rotatably mounted on the same axle <NUM> as the first driven gear <NUM> and so are arranged around axis <NUM>. Thus the intermediary gear arrangement <NUM> is disposed about the axis <NUM>; i.e. the intermediary gear arrangement has as its rotation axis the axis <NUM>. The Intermediary gear arrangement <NUM> is rotatable about axle <NUM> such that it can undergo relative rotation with respect to the arm segment <NUM> about axis <NUM>. As can be seen in <FIG>, the intermediary gear arrangement <NUM> and the first driven gear <NUM> are located on the axle <NUM> such that they are on opposite sides of the rotation axis <NUM>. The intermediary gear arrangement <NUM> and first driven gear <NUM> are therefore on opposite sides of the plane that contains axis <NUM> and is perpendicular to axis <NUM>. Thus the intermediary gear arrangement is located on a single side of the first driven gear <NUM> in a direction along the rotation axis <NUM>.

The axis of rotation of the second drive gear <NUM> (axis <NUM>) and the axis of rotation of intermediate gear <NUM> (axis <NUM>) are non-parallel and non-intersecting. Thus the first intermediate gear <NUM>, like the first driven gear <NUM>, is a skew gear. In this example, the first intermediate gear <NUM> is a hypoid gear, and the drive gear <NUM> a hypoid pinion. The drive gear <NUM> and first intermediate gear <NUM> therefore form a hypoid gear set.

The first intermediate gear <NUM> and first driven gear <NUM> have the same radius and tooth density (i.e. number of teeth per unit circumference). They may also have the same tooth profile. Though in this arrangement gears <NUM> and <NUM> have different widths when viewed in cross-section (as shown in <FIG> and <FIG>), in other arrangements gears <NUM> and <NUM> may be identical with the exception that gear <NUM> is a sector. The first intermediate gear <NUM> and first driven gear <NUM> also have an equal skew offset; i.e. the offset d between the axis of rotation <NUM> of drive gear <NUM> and axis of rotation <NUM> of driven gear <NUM> is equal to the offset d between the axis of rotation <NUM> of drive gear <NUM> and axis of rotation <NUM> of gear <NUM>. This is most clearly shown in <FIG> and <FIG>. Gears <NUM> and <NUM> are therefore identical hypoid gears, with the exception that gear <NUM> is a sector. Similarly, drive gear <NUM> is identical to drive gear <NUM>.

The second intermediate gear <NUM> and second driven gear <NUM> have equal numbers of teeth and a drive ratio of <NUM>:<NUM>. Gears <NUM> and <NUM> are therefore mitre gears. The mitre gears could be straight toothed or spiral toothed.

The operation of the joint mechanism <NUM> will now be described.

To drive motion about axis <NUM>, the first motor is operated to drive the drive shaft <NUM> to rotate relative to the arm segment <NUM> about its longitudinal axis <NUM>. Rotation of the drive shaft <NUM> causes the driven gear <NUM> to rotate about the axis <NUM>. Because the teeth of the drive gear <NUM> are meshed with the teeth of the driven gear <NUM>, rotation of the drive gear <NUM> drives the driven gear <NUM> to rotate about its axis of rotation <NUM> relative to the arm segment <NUM>. This drives a rotation of the second driven gear <NUM> (and hence the arm segment <NUM>) about the axis <NUM> relative to the arm segment <NUM> via joint <NUM>. Referring briefly to <FIG>, it can be seen how rotation of the first driven gear <NUM> about axis <NUM> causes rotation of carrier <NUM> (which is fast with gear <NUM>) about that axis. This in turn causes the second driven gear <NUM> (which is fast with the carrier <NUM> about axis <NUM>) to also rotate about axis <NUM>.

The first driven gear <NUM> is capable of rotating about axis <NUM> in either angular direction. The direction of rotation about axis <NUM> is dependent on the direction of rotation of the drive gear <NUM>. That is, rotation of drive gear <NUM> in a first direction about the longitudinal axis <NUM> of the drive shaft <NUM> causes rotation of the arm segment <NUM> relative to the arm segment <NUM> in a first direction about axis <NUM>; and rotation of drive gear <NUM> about axis <NUM> in a second direction opposite the first direction causes rotation of the arm segment <NUM> relative to the arm segment <NUM> in a second direction about axis <NUM> opposite the first direction.

It will be noted that rotation of the second driven gear <NUM> about axis <NUM> whilst the drive shaft <NUM> remains stationary will also cause the second driven gear <NUM> to rotate relative to the first driven gear <NUM> (and hence also the carrier <NUM> and arm segment <NUM>) about axis <NUM>, causing parasitic motion of the arm segment <NUM> relative to arm segment <NUM> about axis <NUM>. When isolated rotation about axis <NUM> is desired, control system <NUM> may operate to drive the drive shaft <NUM> in tandem with drive shaft <NUM> to isolate rotation about axis <NUM> from rotation about axis <NUM>. In the arrangement shown, drive shaft <NUM> is driven in the same angular direction as drive shaft <NUM> to isolate rotation about the axis <NUM>. The control system <NUM> may therefore be configured to cause drive shaft <NUM> to be driven when drive shaft <NUM> is driven in such a way as to prevent gear <NUM> from rotating relative to gear <NUM> about the rotation axis <NUM>.

To drive motion about axis <NUM>, the second motor is operated to drive the drive shaft <NUM> to rotate about its longitudinal axis <NUM> relative to the arm segment <NUM>. Rotation of the drive shaft <NUM> causes the affixed second drive gear <NUM> to also rotate about axis <NUM> relative to the arm segment <NUM>. The teeth of the second drive gear <NUM> mesh with the teeth of the first intermediate gear <NUM> and so rotation of the second drive gear <NUM> drives the rotation of the first intermediate gear about its axis of rotation <NUM>. The first intermediate gear <NUM> is fast with the second intermediate gear <NUM> and so rotation of the second drive gear <NUM> also causes rotation of the second intermediate gear <NUM> about its axis of rotation <NUM>. The second intermediate gear <NUM> engages the second driven gear <NUM> and so rotation of gear <NUM> drives rotation of the gear <NUM> about its axis of rotation <NUM>. Thus the second intermediate gear <NUM> and the second driven <NUM> gear (which in this example form a pair of mitre gears) operate to transfer rotation of the intermediary gear arrangement about its axis of rotation <NUM> to rotation of the second driven gear about axis <NUM>. The intermediary gear arrangement <NUM> thus operates to transfer rotation of the second drive gear <NUM> about the longitudinal axis <NUM> of its affixed drive shaft <NUM> to rotation about the non-intersecting axis <NUM>.

The intermediary gear arrangement <NUM> is capable of rotating about axis <NUM> in either angular direction, and the second driven gear <NUM> is capable of rotating about axis <NUM> in either angular direction. The direction of rotation of the intermediary gear arrangement <NUM> about axis <NUM> - and thus the direction of rotation of the second driven gear <NUM> about axis <NUM> - is dependent on the direction of rotation of the second drive gear <NUM> about the longitudinal axis <NUM> of the drive shaft <NUM>. That is, rotation of the second drive gear <NUM> in a first direction about axis <NUM> causes rotation of the arm segment <NUM> relative to arm segment <NUM> in a first direction about axis <NUM>; and rotation of the second drive gear <NUM> in a second direction about the axis <NUM> opposite the first direction causes rotation of the arm segment <NUM> relative to the arm segment <NUM> in a second direction about axis <NUM> opposite the first direction.

It is noted that the intermediary gear arrangement <NUM> is mounted on the same axle <NUM> as the first driven gear <NUM> but is not fast with it. For example, it can be seen in <FIG> and <FIG> that the intermediary gear arrangement <NUM> and first driven gear <NUM> are laterally spaced on axle <NUM>. Intermediary gear arrangement <NUM> and driven gear <NUM> are therefore capable of rotating with respect to each other about axis <NUM>. This means that rotation of the drive shaft <NUM> (which causes the second driven gear <NUM> to rotate about axis <NUM>) does not cause parasitic motion of gear <NUM> about axis <NUM>. Thus the joint mechanism <NUM> permits isolated rotation of the second driven gear <NUM> (and hence arm segment <NUM>) about axis <NUM> with respect to arm segment <NUM> without the need for compensatory motion of drive gear <NUM>.

Joint mechanism <NUM> can also drive a compounded motion of arm segment <NUM> relative to arm segment <NUM> about both axes <NUM> and <NUM>. One way to drive rotation about both axes concurrently is to drive the drive shaft <NUM> whilst keeping drive shaft <NUM> fixed, as described above. In certain circumstances the resultant rotation of the second driven gear <NUM> about axis <NUM> may be desirable and not parasitic. In other circumstances compound articulation of both joints <NUM> and <NUM> may be effected by driving both driving shafts <NUM> and <NUM> in tandem. The control unit <NUM> may independently control the rotation speed and direction of each drive shaft <NUM> and <NUM> by controlling their associated electric motors. In this way the control unit <NUM> can control the direction of rotation about each of axes <NUM> and <NUM> respectively and the ratio of the rotation speeds about axis <NUM> to <NUM> (or vice versa). The ratio of the rotation speeds about axes <NUM> and <NUM> may be dependent on the ratio of the rotation speeds of drive gears <NUM> and <NUM> respectively. For example, a higher rotation speed of the first drive gear <NUM> relative to the second drive gear <NUM> causes relatively larger amounts of rotation about axis <NUM> relative to axis <NUM> compared to a slower rotation speed of drive gear <NUM> relative to drive gear <NUM>.

Various aspects of the example joint mechanism <NUM> described above may contribute to providing an advantageous mechanism for articulating one robot arm segment relative to another.

The mechanism <NUM> enables one robot arm segment to be articulated relative to another robot arm segment by a pair of joints having two non-parallel intersecting axes of rotation without requiring a universal joint in either arm segment to effect the motion. This enables the length of the arm segments to be reduced as they don't need to accommodate the universal joints or space for their motion during articulations. The avoidance of universal joints to effect motion may also lead to a smoother motion of the joints.

The mechanism can conveniently be arranged so that both drive gears <NUM> and <NUM> and their associated drive shafts <NUM> and <NUM> lie on one side of a plane containing axis <NUM> (as shown in <FIG>). This provides an efficient packing arrangement within the outer casing <NUM> of the arm segment <NUM> by freeing up space within the interior volume of the outer casing to house other components of the robot arm, such as electronic cables, sensors etc..

The design of the intermediary gear arrangement <NUM> and the fact it has a single axis of rotation <NUM> enables it to be mounted on the same axle <NUM> as the first driven gear <NUM>. The use of a single axle that is secured at both its terminal ends to support both the intermediary gear arrangement and first driven gear provides a stiff arrangement with potentially less bending of the axle. Reducing the bending of the axle may be helpful for improving the accuracy of the readings from the torque sensor <NUM> that measures torque about that axle.

Mounting both the intermediary gear arrangement <NUM> and first driven gear <NUM> on common axle <NUM> may also simplify the installation and/or maintenance of the torque sensor <NUM> by enabling the sensor to be slid into place from the outside of the joint.

The arrangement of the joint mechanism <NUM> enables a large number of similar components to be used. For example, the first driven gear <NUM> and first intermediate gear <NUM> may have an identical radius and tooth profile, meaning they could be manufactured using common equipment. This in turn means that drive gear <NUM> may have an equal tooth profile to drive gear <NUM>, meaning they too could be manufactured using common equipment. The associated drive shafts <NUM> and <NUM> may also be identical. Additionally, the second intermediate gear <NUM> and second driven gear <NUM> may form a pair of mitre gears having an equal number of teeth and equal pitch and pressure angles. If the mitre gears are additionally straight-toothed, the gears <NUM> and <NUM> may be identical. The reduced number of mechanically distinct parts may reduce the design and manufacturing effort and cost of the mechanism.

Though the above disclosure has been made with reference to the specific example illustrated in <FIG>, it will be appreciated that various modifications to the joint mechanism may be made.

For example, in the above-described embodiments, the first driven gear <NUM> is a sector, but in other examples the first intermediate gear <NUM> may be a sector and the first driven gear <NUM> not a sector (that is, the first driven gear <NUM> could have an operative arc of <NUM> degrees). Alternatively, both the first driven gear <NUM> and first intermediate gear <NUM> may be sectors. Having both gears as sectors may be permissible in certain implementations where only reduced operating angles of the joints are required, and enables the joint mechanism to be of reduced weight and potentially of reduced size.

The operative arc of the sector may vary in dependence on the mechanical implementation of the joint mechanism <NUM>. The operative arc may for example be less than <NUM>°, or <NUM>° or <NUM>°, or <NUM>°. In the specific implementation of the joint mechanism <NUM> shown in <FIG> (i.e. within the wrist of the robot arm), it has been appreciated that the operative arc of the sector <NUM> may be reduced to <NUM>° so that the mechanism provides ±<NUM>° of rotation about the axis <NUM>. This is because the arrangement of the joints <NUM>, <NUM>, <NUM> and <NUM> (i.e. a pair of roll joints with a pitch and jaw joint between them) provides a degree of mechanical redundancy meaning a wide range of positions of the distal end of the robot arm can be achieved even with a restricted range of motion about axis <NUM>. Reducing the operative arc of the sector <NUM> to <NUM>° may lead to further reductions in the size and weight of the joint mechanism. For example, a rotational range of motion of ±<NUM>° may only require the sector to have teeth defining an arc spanning <NUM>°.

The first driven gear <NUM> and first intermediate gear <NUM> are described as hypoid gears in the above examples. The use of hypoid gears allows for a relatively high torque capacity in a relatively compact form. However, these gears may in general be any suitable type of ring gear. For example, one or both of the first driven gear <NUM> and first intermediate gear <NUM> may be another type of skew-axis gear, such as a Spiroid ® gear, or worm gear. It follows that one or both of the drive gears <NUM> and <NUM> may be Spiroid pinions or worms as appropriate. As another example, the first intermediate gear <NUM> may be a bevel gear such that the axis of rotation of the associated drive gear <NUM> intersects the axis of rotation <NUM>. The bevel gears may be straight-toothed or spiral-toothed.

The second intermediate gear <NUM> and the second driven gear <NUM> need not be mitre gears as in the above examples, but could in general be bevel gears. That is, the drive ratio of the gears <NUM> and <NUM> need not be <NUM>:<NUM> and the gears need not have equal numbers of teeth.

The intermediate gear arrangement <NUM> and the first driven gear have been described as being mounted on a common axle <NUM>. In an alternative arrangement the intermediate gear arrangement <NUM> and the first driven gear <NUM> could be mounted on respective cantilevered axles. Such an arrangement may be useful to provide a volume between the gear arrangement and driven gear that may be used to house other components of the robot arm, such as sensors, electronics etc..

In the illustrative examples described above, the joint mechanism <NUM> is arranged so that the two rotation axes <NUM> and <NUM> are orthogonal. However, the axes <NUM> and <NUM> need not be orthogonal and are in general two non-parallel intersecting axes. They may be substantially orthogonal to each other. The angle between axes <NUM> and <NUM> may be set by the arrangement of the second intermediate gear <NUM> and the second driven gear <NUM>. That is, the second driven gear <NUM> and second intermediate gear <NUM> may be arranged so that the axis of rotation of the second driven gear <NUM> (axis <NUM>) is non-perpendicular to the axis of rotation of the second intermediate gear <NUM> (axis <NUM>).

The intermediary gear arrangement <NUM> has been described as comprising two intermediate gears <NUM> and <NUM>. However, the intermediary gear arrangement may comprise three or more intermediate gears each disposed about the axis <NUM>. A first of the intermediate gears may engage the drive gear <NUM> and a second of the intermediate gears may engage the driven gear <NUM>. The remaining intermediate gears may be used to drive further components of the robot arm or form part of a gearbox transmission between the drive gear <NUM> and driven gear <NUM>.

Claim 1:
A surgical robot arm (<NUM>) comprising a first arm segment (<NUM>) having a first longitudinal axis (<NUM>) and a second arm segment (<NUM>) having a second longitudinal axis (<NUM>), the first and second arm segments being coupled to each other by a first revolute joint (<NUM>) having a first rotation axis (<NUM>) which is perpendicular to the first longitudinal axis and a second revolute joint (<NUM>) having a second rotation axis (<NUM>) which is perpendicular to the second longitudinal axis and intersects and is non-parallel to the first rotation axis, and a joint mechanism for articulating the first arm segment relative to the second arm segment about the first and second rotation axes, the joint mechanism comprising:
a first driven gear (<NUM>) disposed about an axle (<NUM>) coincident with the first rotation axis, the axle being fast with the first arm segment of the robot arm;
a second driven gear (<NUM>) disposed about the second rotation axis and fast with the second arm segment of the robot arm and fast with the first driven gear about the first rotation axis;
a first drive gear (<NUM>) disposed about a third rotation axis and configured to drive the first driven gear to rotate about the axle, the first drive gear being arranged to engage the first driven gear;
a second drive gear (<NUM>) for driving the second driven gear to rotate about the second rotation axis;
an intermediary gear arrangement arranged to engage the second drive gear and the second driven gear and being disposed about the first rotation axis, whereby rotation of the intermediary gear arrangement relative to the first arm segment about the first rotation axis can be driven, the intermediary gear arrangement comprising a first intermediate gear (<NUM>) arranged to engage the second drive gear, and a second intermediate gear (<NUM>) arranged to engage the second driven gear,
wherein the third rotation axis and the first rotation axis are non-intersecting.