Abstract:
A surgical robotic component comprising an articulated terminal portion, the terminal portion comprising: a distal segment having an attachment connected thereto, an intermediate segment, and a basal segment whereby the terminal portion is attached to the remainder of the surgical robotic component. The terminal portion further comprises a first articulation between the distal segment and the intermediate segment, the first articulation permitting relative rotation of the distal segment and the intermediate segment about a first axis, and a second articulation between the intermediate segment and the basal segment, the second articulation permitting relative rotation of the intermediate segment and the basal segment about a second axis. The intermediate segment comprises: a third articulation permitting relative rotation of the distal segment and the basal segment about third and fourth axes, a first torque sensor configured to sense torque about the third axis, and a second torque sensor configured to sense torque about the fourth axis. The first, second and third articulations are arranged such that in at least one configuration of the third articulation the first and second axes are parallel and the third and fourth axes are transverse to the first axis.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit under 35 U.S.C. §119 of United Kingdom Patent Application No. 1508260.5 filed on May 14, 2015 which is hereby incorporated herein by reference in its entirety for all purposes. 
       BACKGROUND 
       [0002]    This invention relates to robots for performing surgical tasks. 
         [0003]    Various designs of robot have been proposed for performing or assisting in surgery. However, many robot designs suffer from problems that make them unsuitable for performing a wide range of surgical procedures. A common reason for this is that in order for a surgical robot to work well in a wide range of surgical situations it must successfully balance a set of demands that are particular to the surgical environment. 
         [0004]    Normally a surgical robot has a robot arm, with a surgical instrument attached to the distal end of the robot arm. 
         [0005]    A common demand on a surgical robot is that its robot arm should offer sufficient mechanical flexibility to be able to position the surgical instrument in a wide range of locations and orientations so that the working tip of the surgical instrument (the end effector) can reach a range of desired surgical sites. This demand alone could easily be met by a conventional fully flexible robot arm with six degrees of freedom, as illustrated in  FIG. 1 . However, secondly, a surgical robot must also be capable of positioning its arm such that the end effector of the instrument is positioned very accurately without the robot being excessively large or heavy. This requirement arises because unlike the large-scale robots that are used for many other tasks, (a) surgical robots need to work safely in close proximity to humans: not just the patient, but typically also surgical staff such as anaesthetists and surgical assistants, and (b) in order to perform many laparoscopic procedures it is necessary to bring multiple end effectors together in close proximity, so it is desirable for surgical robot arms to be small enough that they can fit closely together. Another problem with the robot of  FIG. 1  is that in some surgical environments there is not sufficient space to be able to locate the base of the robot in a convenient location near the operating site. 
         [0006]    Many robots have a wrist (i.e. the terminal articulated structure of the arm) which comprises two joints that permit rotation about an axis generally along the arm (“roll joints”) and between them one joint that permits rotation about an axis generally transverse to the arm (a “pitch joint”). Such a wrist is shown in  FIG. 2 , where the roll joints are indicated as  1  and  3  and the pitch joint is indicated as  2 . With the wrist in the configuration shown in  FIG. 2  the axes of the joints  1  to  3  are indicated as  4  to  6  respectively. This wrist gives an instrument  7  the freedom of movement to occupy a hemisphere whose base is centred on axis  4 . However, this wrist is not well suited for use in a surgical robot. One reason for this is that when the pitch joint  2  is offset by just a small angle from the straight position shown in  FIG. 2  a large rotation of joint  1  is needed to produce some relatively small lateral movements of the tip of the instrument. In this condition, when the pitch joint is almost straight, in order to move the end effector smoothly in a reasonable period of time the drive to joint  1  must be capable of very fast operation. This requirement is not readily compatible with making the arm small and lightweight because it calls for a relatively large drive motor and a sufficiently stiff arm that the motor can react against it without jolting the position of the arm. 
         [0007]    Another common demand on a surgical robot is that it should be designed such that forces which are applied to the surgical instrument are measurable. Because the surgeon is not directly in contact with the surgical instrument during robotic surgery, tactile feedback is lost compared to manual surgery. This lack of tactile sensation means that the surgeon does not know how much force is being applied when using the surgical instrument. This affects the surgeon&#39;s dexterity. Additionally, too much exerted force can cause internal damage to the patient at the surgical site, and can also damage the surgical instrument and the robot arm. By measuring the forces applied to the surgical instrument, these can be implemented in a force feedback mechanism to provide force feedback to the surgeon. For example, haptic technology can be used to convert the measured forces into physical sensations in the input devices that the surgeon interacts with, thereby providing a replacement for the tactile sensation of manual surgery. 
         [0008]    The wrist shown in  FIG. 2  enables some force measurement. Specifically, forces which are exerted vertically on the surgical instrument (i.e. perpendicular to axis  5 ) illustrated by arrow  8 , can be measured using a torque sensor applied to the pitch joint  2 . However, forces which are applied to the surgical instrument from other directions cannot be sensed using torque sensors applied to the wrist. Other techniques are possible to detect forces applied from other directions, for example by using strain gauges. However, these other techniques require more sensors to be applied to the robot, and hence more electronics are required to route and process the data, which increases the weight and power requirements of the robot. 
       SUMMARY OF THE INVENTION 
       [0009]    According to one aspect of the invention, there is provided a surgical robotic component comprising an articulated terminal portion, the terminal portion comprising: a distal segment having an attachment connected thereto; an intermediate segment; a basal segment whereby the terminal portion is attached to the remainder of the surgical robotic component; a first articulation between the distal segment and the intermediate segment, the first articulation permitting relative rotation of the distal segment and the intermediate segment about a first axis; a second articulation between the intermediate segment and the basal segment, the second articulation permitting relative rotation of the intermediate segment and the basal segment about a second axis; wherein: the intermediate segment comprises: a third articulation permitting relative rotation of the distal segment and the basal segment about third and fourth axes; a first torque sensor for sensing torque about the third axis; and a second torque sensor for sensing torque about the fourth axis; and the first, second and third articulations are arranged such that in at least one configuration of the third articulation the first and second axes are parallel and the third and fourth axes are transverse to the first axis. 
         [0010]    The terminal portion may further comprise a third torque sensor for sensing torque about the first axis. The terminal portion may further comprise a fourth torque sensor for sensing torque about the second axis. 
         [0011]    Suitably, in the said configuration, the third and fourth axes are transverse to each other. Suitably, in the said configuration, the third and fourth axes are perpendicular to each other. In the said configuration the third and fourth axes may be perpendicular to the first axis. In the said configuration, the first and second axes may be collinear. 
         [0012]    The third and fourth axes may intersect each other. 
         [0013]    The first articulation may be a revolute joint. The second articulation may be a revolute joint. The third articulation may be a spherical joint or a pair of revolute joints. The third articulation may be a universal joint. 
         [0014]    Suitably, the only means of articulating the attachment relative to the basal segment are the first, second and third articulations. 
         [0015]    The attachment may be located on the first axis. 
         [0016]    Suitably, the surgical robotic component is located at the end of a surgical robot arm, and a surgical instrument is attached to the attachment. The surgical instrument may extend in a direction substantially along the first axis. 
         [0017]    Alternatively, the surgical robotic component is located at the end of a surgical instrument, and a surgical end effector is attached to the attachment. The surgical end effector may extend in a direction substantially along the first axis. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: 
           [0019]      FIG. 1  shows a prior art robot arm; 
           [0020]      FIG. 2  shows a prior art robot wrist; 
           [0021]      FIG. 3  shows a robot arm according to an embodiment of the present invention having a surgical instrument attached thereto; 
           [0022]      FIG. 4  shows a surgical wrist. The wrist is illustrated in three different configurations at  FIGS. 4 a , 4 b    and  4   c;    
           [0023]      FIG. 5  shows a surgical instrument; 
           [0024]      FIG. 6  shows a further surgical instrument; and 
           [0025]      FIG. 7  shows an alternative design of a robot arm. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The surgical robot arm of  FIG. 3  has a wrist in which two joints that permit rotation about axes generally transverse to the distal portion of the arm are located between two joints that permit rotation about axes generally parallel to the distal portion of the arm. This arrangement permits the instrument to move in a hemispherical space whose base is centred on the distal part of the arm, but without requiring high-speed motion of one of the joints in order to move the end effector smoothly, and without requiring motion of any of the other parts of the arm. 
         [0027]    In more detail,  FIG. 3  shows a robot arm (indicated generally at  10 ) having a surgical instrument  11  attached thereto. The robot arm extends from a base  12 . The base could be mounted to the floor of an operating theatre, or to a fixed plinth, could be part of a mobile trolley or cart, could be mounted to a bed or could be mounted to the ceiling of an operating room. The base is fixed in place relative to the patient&#39;s bed or chair when an operation is being carried out. The robot arm comprises a wrist portion shown generally at  13  and a main portion shown generally at  14 . The main portion makes up the majority of the extent of the arm and terminates at its distal end in its attachment to the wrist portion. The proximal end of the main portion is attached to the base. The wrist portion makes up the distal part of the arm and is attached to the distal end of the main portion. 
         [0028]    The main portion of the arm comprises four joints  15 ,  16 ,  17 ,  18  and three shaft sections  19 ,  20 ,  21 . The joints are revolute joints. The shaft sections are rigid, with the exception of joints  15  and  17  which are set into shaft sections  19  and  20  respectively. Each shaft section may have substantial length, and serve to provide the arm with reach and the ability to offset the wrist laterally and/or vertically from the base. The first shaft section could be truncated relative to the second and third shaft sections if the base is located in a suitable place; particularly if the base is elevated from the floor. 
         [0029]    The first shaft section  19  is attached to the base  12 . In practice the first shaft section can conveniently extend in a generally upright direction from the base but it could extend at a significant incline to vertical, or even horizontally. 
         [0030]    Joint  15  is located in the first shaft section. Joint  15  permits relative rotation of the proximal part of the first shaft section, which is fixed to the base, and the remainder of the arm about an axis  22 . Conveniently, axis  22  is parallel with or substantially parallel with the main extent of the first shaft section in forming the arm, which runs from the base towards joint  16 . Thus, conveniently the angle of axis  22  to the main extent of the first shaft section in forming the arm could be less than 30°, less than 20° or less than 10°. Axis  22  could be vertical or substantially vertical. Axis  22  could extend between the base and joint  16 . 
         [0031]    Joint  16  is located at the distal end of the first shaft section  19 . Joint  16  permits relative rotation of the first shaft section  19  and the second shaft section  20 , which is attached to the distal end of joint  16 , about an axis  23  which is transverse to the first shaft section  19  and/or the second shaft section  20 . Conveniently axis  23  is perpendicular or substantially perpendicular to either or both of the first and second shaft sections. Thus, conveniently the angle of axis  23  to the main extents of either or both of the first and second shaft sections could be greater than 60°, greater than 70° or greater than 80°. Conveniently axis  23  is perpendicular or substantially perpendicular to axis  22  and/or to the axis  24  to be described below. 
         [0032]    Joint  17  is located in the second shaft section. Joint  17  permits relative rotation of the proximal part of the second shaft section and the remainder of the arm about an axis  24 . Conveniently, axis  24  is parallel with or substantially parallel with the main extent of the second shaft section. Thus, conveniently the angle of axis  24  to the main extent of the second shaft section could be less than 30°, less than 20° or less than 10°. Axis  24  could intersect or substantially intersect (e.g. within 50 mm of) axis  23  and the axis  25  that will be described below. In  FIG. 3  joint  17  is shown located closer to the distal end of the second shaft section than the proximal end. This is advantageous because it reduces the mass that needs to be rotated at joint  17 , but joint  17  could be located at any point on the second shaft section. The second shaft section is conveniently longer than the first shaft section. 
         [0033]    Joint  18  is located at the distal end of the second shaft section  20 . Joint  18  permits relative rotation of the second shaft section and the third shaft section  21 , which is attached to the distal end of joint  18 , about an axis  25  which is transverse to the second shaft section  20  and/or the third shaft section  21 . Conveniently axis  25  is perpendicular or substantially perpendicular to either or both of the second and third shaft sections. Thus, conveniently the angle of axis  25  to the main extents of either or both of the second and third shaft sections could be greater than 60°, greater than 70° or greater than 80°. Conveniently axis  25  is perpendicular or substantially perpendicular to axis  24  and/or to the axis  29  to be described below. 
         [0034]    In summary, then, in one example the main portion of the arm can be composed as follows, in order from the base to the distal end of the main portion: 
         [0000]    1. a first shaft section  19  having substantial or insubstantial length and containing a joint  15  that permits rotation e.g. about an axis generally along the extent (if any) of the first shaft section in forming the arm (a “roll joint”);
 
2. a joint  16  permitting rotation transverse to the first shaft section and/or to the axis of the preceding joint (joint  15 ) and/or to the axis of the succeeding joint (joint  17 ) (a “pitch joint”);
 
3. a second shaft section  20  having substantial length and containing a joint  17  that permits rotation about an axis generally along the extent of the second shaft section and/or to the axis of the preceding joint (joint  16 ) and/or to the succeeding joint (joint  18 ) (a roll joint);
 
4. a joint  18  permitting rotation transverse to the second shaft section and or to the preceding joint (joint  17 ) and/or to the succeeding joint (joint  28 ) (a pitch joint); and
 
5. a third shaft section  21  having substantial length.
 
         [0035]    The wrist portion  13  is attached to the distal end of the third shaft section.  FIG. 4  illustrates a surgical wrist, such as the wrist portion  13  of  FIG. 1 , in more detail. In  FIG. 4, 4   a  shows the wrist in a straight configuration,  4   b  shows the wrist in a bent configuration from movement at a joint  26  and  4   c  shows the wrist in a bent configuration from movement at joints  26 ,  27  and  28 . The straight configuration represents the mid-point of the motions of the transverse joints ( 26 ,  27 ) of the wrist. 
         [0036]    The wrist is designated  13  in  FIG. 4 .  FIG. 4  illustrates an example in which the wrist is implemented in the robot arm of  FIG. 3 . In this implementation, the distal part of the third shaft section is designated  21   a  in  FIG. 4 . The wrist is attached to the distal end of the third shaft section by a joint  28 . Joint  28  is a revolute joint which permits the wrist to rotate relative to the distal end of the arm about an axis  29 . Conveniently, axis  29  is parallel with or substantially parallel with the main extent of the third shaft section. Thus, conveniently the angle of axis  29  to the main extent of the third shaft section could be less than 30°, less than 20° or less than 10°. Axis  29  could intersect or substantially intersect (e.g. within 50 mm) axis  25 . Axis  29  is conveniently transverse to axis  25 . 
         [0037]    In an alternative implementation, the wrist  13  is implemented in a surgical instrument. The wrist is implemented in the distal end of the surgical instrument. The proximal end of the surgical instrument is attached to the robot arm. In this implementation, the wrist is attached to the shaft of the surgical instrument by joint  28 . As above, joint  28  is a revolute joint which permits the wrist to rotate relative to the instrument shaft about an axis  29 . Conveniently, axis  29  is parallel with or substantially parallel with the main extent of the instrument shaft. Thus, conveniently the angle of axis  29  to the main extent of the instrument shaft could be less than 30°, less than 20° or less than 10°. 
         [0038]    The proximal end of the wrist is constituted by a wrist base block  30 . The wrist base block  30  is attached to joint  28 . Wrist base block  30  abuts the distal end of the third shaft section  21  in the case that the wrist is attached to the robot arm of  FIG. 3 . Alternatively, the wrist base block  30  abuts the distal end of the instrument shaft in the case that the wrist is attached to the surgical instrument. The wrist base block is rigid and comprises a base  33 , by which it is attached to joint  28 . The wrist base block also comprises a pair of spaced apart arms  31 ,  32  which extend from the base  33  of the wrist base block in a direction away from the third shaft section  21   a  or instrument shaft to which the wrist base block is attached. An intermediate member  34  is pivotally suspended between the arms  31 ,  32  in such a way that it can rotate relative to the arms  31 ,  32  about an axis  35 . This constitutes a revolute joint  27  of the wrist. The intermediate member  34  is conveniently in the form of a rigid block which may be of cruciform shape. A wrist head block  36  is attached to the intermediate member  34 . The wrist head block is rigid and comprises a head  37  by which it is attached to a joint  38  to be described below, and a pair of spaced apart arms  39 ,  40  which extend from the head  37  towards the intermediate member  34 . The arms  39 ,  40  embrace the intermediate member  34  and are attached pivotally to it in such a way that the wrist head block can rotate relative to the intermediate member about an axis  41 . This provides revolute joint  26  of the wrist. Axes  35  and  41  are offset from each other at a substantial angle. Axes  35  and  41  are conveniently transverse to each other, and most conveniently orthogonal to each other. Axes  35  and  41  can conveniently intersect or substantially intersect (e.g. within 50 mm). However, the intermediate member could have some extent so that those axes are offset longitudinally. Axes  35  and  29  are conveniently transverse to each other, and most conveniently orthogonal to each other. Axes  35  and  29  can conveniently intersect or substantially intersect (e.g. within 50 mm). Axes  35  and  29  can conveniently intersect axis  41  at a single point, or the three axes may substantially intersect at a single point (e.g. by all intersecting a sphere of radius 50 mm). 
         [0039]    In this way the wrist base block, intermediate member and wrist head block together form a universal joint. The universal joint permits the wrist head block to face any direction in a hemisphere whose base is perpendicular to the axis  29  of joint  28 . The linkage between the wrist base block and the wrist head block could be constituted by other types of mechanical linkage, for example by a ball joint or a constant velocity joint. Preferably that linkage acts generally as a spherical joint, although it need not permit relative axial rotation of the wrist base block and the wrist head block since such motion is accommodated by joints  28  and  38 . Alternatively, joints  26 ,  27  and  28  could be considered collectively to form a spherical joint. That spherical joint could be provided as a ball joint. 
         [0040]    A terminal unit  42  is attached to the head  37  of the wrist head block by revolute joint  38 . Joint  38  permits the terminal unit to rotate relative to the head block about an axis  43 . Axes  43  and  41  are conveniently transverse to each other, and most conveniently orthogonal to each other. Axes  35  and  29  can conveniently intersect or substantially intersect (e.g. within 50 mm). Axes  35  and  29  can conveniently intersect axis  41  at a single point, or the three axes may substantially intersect at a single point (e.g. by all intersecting a sphere of radius 50 mm). 
         [0041]    Wrist  13  also comprises a series of torque sensors, S 1 , S 2 , S 3  and S 4 . Each torque sensor is associated with a joint, and senses applied torque about the joint&#39;s rotation axis. S 1  is associated with joint  27  and senses torque about axis  35 . S 2  is associated with joint  26  and senses torque about axis  41 . S 3  is associated with joint  28  and senses torque about axis  29 . S 4  is associated with joint  38  and senses torque about axis  43 . The outputs of the torque sensors are passed to a control unit (not shown) where they form the inputs to a processor (not shown). The processor may also receive inputs from the motors driving the sensed joints. The processor thereby resolves the torque applied to the joints due to motion driven by the motors and the torque applied to the joints due to external forces. All of the torque sensors S 1 , S 2 , S 3  and S 4  may be applied to the wrist  13 . Alternatively, S 3  and/or S 4  may be omitted. 
         [0042]    In the implementation in which the wrist is at the distal end of the robot arm of  FIG. 3 , the terminal unit  42  has a connector such as a socket or clip to which surgical instrument  11  can be attached. This surgical instrument is shown in more detail in  FIG. 5 . The instrument comprises in instrument base  50 , an elongate instrument shaft  51 , optionally one or more joints  52  and an end effector  53 . The end effector could, for example, be a gripper, a pair of shears, a camera, a laser or a knife. The instrument base and the connector of the terminal unit  42  are designed cooperatively so that the instrument base can be releasably attached to the connector with the shaft extending away from the instrument base. Conveniently the shaft extends away from the instrument base in a direction that is transverse to the axis of joint  26  and/or parallel or substantially parallel and/or coaxial or substantially coaxial with axis  43  of joint  38 . This means that the end effector has substantial range of movement by virtue of the joints of the wrist, and that the joints of the wrist can be used conveniently to position the end effector. For example, with the elongation of the instrument shaft running along axis  43 , joint  38  can be used purely to orientate the end effector without moving part or all of the instrument shaft  51  with a lateral component in a way that could result in disruption to the tissue of a patient through which the shaft has been inserted to reach an operation site. The fact that the elongation of the instrument shaft extends away from the wrist as described above means that the wrist has a degree of articulation that is similar to the wrist of a human surgeon. One result of that is that many surgical techniques practised by humans can readily be translated to motions of this robot arm. This can help reduce the need to devise robot-specific versions of known surgical procedures. The shaft is conveniently formed as a substantially linear, rigid rod. 
         [0043]    In the description above, the length of the wrist base block  30  is less than that of the final shaft section  21  of the robot arm. This is advantageous because it reduces the mass that needs to be rotated at joint  28 . However, joint  28  could be located closer to joint  25  than to joints  26  and  27 . 
         [0044]    In the implementation in which the wrist is at the distal end of the surgical instrument, the terminal unit  42  is connected to an end effector  53 . This surgical instrument is shown in more detail in  FIG. 6 . The instrument comprises an instrument base  50 , an elongate instrument shaft  51 , the wrist  13  and an end effector  53 . The end effector may be any of the forms mentioned above. The end effector  53  and the terminal unit  42  are designed such that motion is transferable through the terminal unit  42  to the end effector  53 . The joints of the end effector may be driven by motive means in the robot arm to which the instrument is connected. The motion may be transmitted to the joints through cables or linkages in the robot arm and instrument. 
         [0045]    It will be appreciated that the wrist of  FIG. 4  has a kinematic redundancy. The end effector  53  could be placed in a wide range of locations in a hemisphere about axis  29  merely by motion of joints  28  and  27 . However, it has been found that the addition of joint  26  greatly improves the operation of the robot for surgical purposes by eliminating the kinematic singularity that results from joint pair  28 ,  27  alone. The addition of joint  26  also simplifies the mechanism of moving the end effector within the patient so that multiple robot arms can work more closely with each other, as will be described in more detail below. 
         [0046]    Referring to  FIG. 4 a    in the case that the wrist  13  is at the distal end of a robot arm, when a force  8  is applied vertically to the surgical instrument (i.e. perpendicular to axis  41 ), this causes rotation of the wrist about axis  41 . More specifically, the resolved component of a force applied in the direction  8  causes rotation of the wrist about axis  41 . Torque sensor S 2  senses this torque, and outputs the sensed torque to the processor. When a force  9  is applied laterally to the surgical instrument (i.e. perpendicular to axis  35 ), this causes rotation of the wrist about axis  35 . More specifically, the resolved component of a force applied in a direction  9  causes rotation of the wrist about axis  35 . Torque sensor S 1  senses this torque, and outputs the sensed torque to the processor. Thus, loads applied to the surgical instrument are measured by the wrist. Typically, a force applied to the surgical instrument results in a torque about multiple joints. Any force applied to the surgical instrument parallel to the plane of the axes  35  and  41  can be sensed by torque sensors S 1  and S 2 . The processor resolves the outputs of the torque sensors S 1  and S 2  to determine the direction and magnitude of the torque applied to the surgical instrument. 
         [0047]    In the case that the wrist  13  is at the distal end of a surgical instrument, then forces applied to the end effector  53  are detectable as described with respect to the surgical instrument in the preceding paragraph. 
         [0048]    Each joint of the robot arm of  FIG. 3  can be driven independently of the other joints by one or more motive devices such as electric motors or hydraulic pistons. The motive device(s) could be located locally at the respective joint, or it/they could be located closer to the base of the robot and coupled to the joints by couplings such as cables or linkages. The motive devices are controllable by a user of the robot. The user could control the motive devices in real time by one or more artificial input devices, such as joysticks, or by inputs derived from sensors acting on a master arm that is moved by the user. Alternatively, the motive devices could be controlled automatically by a computer that has been pre-programmed to perform a surgical procedure. The computer could be capable of reading a computer-readable memory that stores a non-volatile program executable by the computer to cause the robot arm to perform one or more surgical procedures. 
         [0049]      FIG. 7  shows an alternative design of surgical arm. The arm of  FIG. 7  comprises a base  112 , four joints  115 ,  116 ,  117 ,  118 , three shaft sections  119 ,  120 ,  121  and a wrist unit  113 . The joints are revolute joints. The shaft sections are rigid, with the exception of joints  115  and  117 . A surgical instrument  111  is attached to the terminal part of the wrist unit. 
         [0050]    The first shaft section  119  extends from the base  112  and comprises joint  115 . The first shaft section  119  is attached to the second shaft section  120  by joint  116 . The second shaft section  120  comprises joint  117 . The second shaft section is attached to the third shaft section  121  by joint  118 . The third shaft section  121  terminates in a revolute joint  128  whereby it is attached to the wrist unit  113 . The wrist unit comprises an intermediate pair of revolute joints  126 ,  127 , which together constitute a universal joint, and a terminal revolute joint  138 . 
         [0051]    As with the analogous joints in the robot arm of  FIG. 3 , the axes of each of the following pairs of joints may independently be transverse to each other, substantially orthogonal to each other (e.g. within any of 30°, 20° or 10° of being orthogonal) or orthogonal to each other:  115  and  116 ,  116  and  117 ,  117  and  118 ,  118  and  128 ,  128  and  126 ,  128  and  127 ,  126  and  138 ,  127  and  138 ,  126  and  127 . As with the analogous joints in the robot arm of  FIG. 3 , the axes of the following joints may independently be aligned with (e.g. within any of 30°, 20° or 10° of) or parallel with the principal axis of elongation of the shaft in or on which they are set: joint  117  (with shaft section  120 ), joint  128  (with shaft section  121 ). As with the analogous joints in the robot arm of  FIG. 3 , the wrist may be configured such that the axes of joints  128  and  138  can in one or more configurations of the arm be aligned. Conveniently that alignment may happen when the wrist is in the mid-range of its side-to-side movement. As with the analogous joints in the robot arm of  FIG. 3 , conveniently the axis of elongation of the instrument  111  may be aligned with (e.g. within any of 30°, 20° or 10° of) or parallel with the axis of joint  138 . The axis of elongation of the instrument may be coincident with the axis of joint  138 . 
         [0052]    The robot arm of  FIG. 7  differs from that of  FIG. 3  in that the arm sections  119 ,  120 ,  121  are configured so that the axis of joint  116  has a substantial lateral offset from the axes of joints  115  and  117  and so that the axis of joint  118  has a substantial lateral offset from the axes of joints  116  and  118 . Each of those offsets may independently be, for example, greater than 50 mm, 80 mm or 100 mm. This arrangement is advantageous in that it increases the mobility of the arm without increasing the swept volume close to the tip of the instrument. 
         [0053]    In the robot arm of  FIG. 7  the axis of the revolute joint closest to the base (joint  115 ) is fixed at a substantial offset from vertical, e.g. by at least 30°. This may be achieved by fixing the base in an appropriate orientation. If the arm is set up so that the axis of joint  115  is directed generally away from the end effector, as illustrated in  FIG. 7 , this reduces the chance of a kinematic singularity between joint  115  and joint  117  during an operation. 
         [0054]    Thus the arm of  FIG. 7  has a number of general properties that can be advantageous in a surgical robot arm.
       It comprises a series of revolute joints along its length that include a series of four joints in order along the arm that alternate between (a) having an axis that runs generally towards the next joint in the sequence (a “roll joint”) and (b) having an axis that runs generally transverse to the axis of the next joint in the sequence (a “pitch joint”). Thus the arm of  FIG. 7  includes roll joints  115  and  117  and pitch joints  116  and  118 . This series of joints can provide the arm with a high degree of mobility without the arm needing to be heavy or bulky or to comprise an excessive number of joints. As indicated above, alternate joints can usefully be offset laterally from each other.   It comprises a wrist section commencing at a roll joint ( 128 ) and having two pitch joints ( 126 ,  127 ). The pitch joints of the wrist can be co-located such that their axes intersect in all configurations of the arm. This series of joints can provide the end effector with a high degree of mobility about the end of the arm.   A wrist section that terminates in a revolute joint ( 138 ) whose axis is coincident with the axis of extension of the surgical instrument. Most conveniently the instrument shaft is elongated linearly and the axis of that joint is aligned with the shaft. This arrangement can permit the end effector to be readily rotated to a desired orientation through motion at a single joint without excessively disrupting a wound channel in the patient. Without this arrangement, the arm joints would have to be controlled to move together to achieve rotation of the instrument shaft about its own axis. This would require the workspace of an elbow joint of the arm to be increased to enable it to swing across its range of motion to achieve the desired motion of the instrument. Large joint workspaces are undesirable for the operating theatre staff who work around the robot arm. This arrangement can also reduce the need for a joint performing an equivalent function in the instrument itself.   A proximal revolute joint ( 115 ) that is substantially offset from vertical and most conveniently directed away from the site of the operation in the patient. This reduces the chance of that joint having a kinematic singularity with another joint of the arm.       
 
         [0059]    As discussed above, the proximal series of joints in the arms of  FIGS. 3 and 7  in order towards the distal end of the arm are, using the terms defined above, roll, pitch, roll and pitch joints. This series of joints may be denoted RPRP, where “R” denotes a roll joint, “P” denotes a pitch joint and the joints are listed in series from the proximal towards the distal end of the arm. Using the same terminology, other convenient joint sequences for surgical arms include the following: 
         [0000]    1. PRPRP: i.e. the joint sequence of the robot arm of  FIGS. 3 and 7  but with an additional pitch joint between the RPRP joint sequence and the base.
 
2. RPRPR: i.e. the joint sequence of the robot arm of  FIGS. 3 and 7  but with an additional roll joint between the RPRP joint sequence and the wrist.
 
3. RPRPRP: i.e. a series of three RP pairs in succession, akin to the joint sequence of the robot arm of  FIGS. 3 and 7  but with an additional pitch joint between the RPRP joint sequence and the base and an additional roll joint between the RPRP joint sequence and the wrist.
 
         [0060]    Further joints could be added to the arm. 
         [0061]    Each of these arms could have a wrist of the type shown in  FIG. 3 or 7 . One of the joints  28 ,  38  could be omitted from the wrist. 
         [0062]    As indicated above, the surgical instrument of  FIG. 5  may have one or more joints  52  near its tip. If the robot arm is of the type described herein then the surgical instrument may conveniently include only two joints. They can conveniently be revolute joints whose axes run transversely to the instrument shaft  51 . The axes of those joints could intersect, forming a universal joint, or could be offset in the direction of elongation of the instrument shaft. The joints of the instrument could be driven by motive means in the arm, and the motion transmitted to the joints through cables or linkages in the instrument. The connector in the terminal part of the wrist unit and the instrument base  50  may be configured to provide for transmitting such motion into the instrument. Conveniently the joints on the instrument do not include a revolute joint whose axis is aligned with the shaft of the instrument. The motion that would be provided by such a joint can conveniently be served by the joint  38  on the wrist of the robot arm. In many surgical procedures such motion is not needed. The instruments are often intended to be disposable; therefore cost can be reduced by omitting such a joint from the instrument. Omitting such a joint also simplifies the mechanical interaction needed between the instrument and the arm since motion for that joint need not be transmitted into the instrument. 
         [0063]    The torque sensors may be sensors that are in direct contact with the wrist. For example, a resistance-based strain gauge may be used as a torque sensor. This is connected directly to the portion of the wrist being rotated. As torque is applied about the axis of rotation, the strain gauge deforms changing its resistance. This resistance change is measured, for example by a bridge circuit and output to the processor of the control unit. The processor determines the torque applied about the axis to be a function of the resistance change. 
         [0064]    As another example, a piezoelectric strain gauge may be used as a torque sensor. Again, this is connected directly to the portion of the wrist being rotated. The gauge is made of a piezoelectric material which generates a voltage across it when strained. As torque is applied about the axis of rotation, the piezoelectric strain gauge is deformed, generating a voltage across it which is measured and output to the processor of the control unit. The processor determines the torque applied about the axis to be a function of the voltage change. 
         [0065]    The torque sensors may be sensors which are not in direct contact with the wrist. For example, a magnetostrictive-based gauge may be used as a torque sensor. Magnetostrictive material is deposited on the portion of the wrist being rotated. As torque is applied about the axis of rotation, external flux is generated by the magnetostrictive material being stressed. Sensors positioned in close proximity to the magnetostrictive material, but not in contact with the joint which is moving, pick up the current generated and output it to the processor of the control unit. The processor determines the torque applied about the axis to be a function of the generated current. 
         [0066]    The output of the torque sensors may be passed to the control unit by signals passing through wires attached externally to the casing of the robot body. Alternatively, the wires may pass internally down the cores of shafts of the robot arm to the control unit. Alternatively, the torque sensors and the control unit may be capable of wireless communications. In this case, the outputs of the torque sensors are transmitted wirelessly by the torque sensors to the control unit. 
         [0067]    The control unit may implement a force feedback mechanism to convert the sensed torque about the joints of the wrist into force feedback to the surgeon. For example, haptic technology may be used to convert the sensed torques into physical sensations in the input devices that the surgeon interacts with. 
         [0068]    In operational use, the robot arm of  FIG. 3  could be covered by a sterile drape to keep the arm separated or sealed from the patient. This can avoid the need to sterilise the arm before surgery. In contrast, the instrument would be exposed on the patient&#39;s side of the drape: either as a result of it extending through a seal in the drape or as a result of the drape being sandwiched between the connector in the terminal part of the wrist unit and the instrument base  50 . Once the instrument has been attached to the arm it can be used to perform an operation. In performing an operation the arm can first be manipulated so that the axis of the instrument shaft  51  is aligned with the axis between a desired entry point on the exterior of the patient (e.g. an incision in the patient&#39;s skin) and the desired operation site. Then the robot arm can be manipulated to insert the instrument through the incision and onwards in a direction parallel to the axis of the instrument shaft until the end effector reaches the operation site. Other tools can be inserted in a similar way by other robot arms. Once the required tools are at the operation site the operation can be conducted, the tools can be withdrawn from the patient&#39;s body and the incision(s) can be closed, e.g. by suturing. If it is desired to move the end effector in a direction transverse to the axis of the instrument shaft when the instrument is located in the patient, such motion is preferably performed by rotating the instrument shaft about a centre of motion located at the incision through which the instrument is passing. This avoids making the incision bigger. 
         [0069]    A robot arm of the type described above can provide a range of advantages for performing surgical procedures. First, because it does not include an excessive number of joints whist still providing the range of motion needed to position the instrument as a whole and particularly the end effector of the instrument in a wide range of locations and orientations the robot arm can be relatively slim and light-weight. This can reduce the chance of a human being injured through undesirable motion of the arm, e.g. when nurses are working around the arm when an operating theatre is being set up to receive a patient. It can also improve the accessibility of multiple such arms to an operation site, especially a site for a procedure such as an ENT (ear, nose and throat) procedure where typically multiple instruments must access the operation site through a small opening. Similar considerations arise in, for example, abdominal procedures where it is common for multiple instruments to enter the patient from a region near the umbilicus and to extend internally of the ribcage into the abdomen of the patient; and in procedures in the pelvic area where the direction in which instruments can approach the operation site is limited by the need to avoid the pelvic bone and other internal structures. Similarly, an arm having improved range of motion can make it easier to position the bases of multiple robots around an operating site because surgical staff have more freedom over where to locate the robot bases. This can help to avoid the need to redesign existing operating room workflows to accommodate a robot. Second, the arm provides sufficient redundant motion that surgical staff have flexibility in positioning the base of the robot relative to the patient. This is important if multiple robots need to work at a small surgical site, if there is additional equipment in the operating theatre or if the patient is of an unusual dimension. Third, when the wrist section comprises a roll joint located proximally of a pair of crossed-axis pitch joints, as in  FIG. 4 , and particularly if in addition the arm and the instrument are configured so that the instrument shaft extends directly away from those pitch joints, then the motion of the wrist is close to that of a human, making it easier to translate conventional surgical procedures so that they can be performed by the robot. This relationship between the wrist and the instrument also assists in enabling multiple arms to closely approach each other near a surgical site since the terminal sections of the main arm members (e.g.  21  and  121 ) can be angled relative to the instrument shaft without compromising the freedom of motion of the instrument shaft. This is in part because when the end effector needs to be moved within the patient by rotation about a centre located at the external point of entry of the instrument shaft into the patient, that rotation can be provided exclusively by the wrist, without being hindered by kinematic singularities or complex interactions between multiple joints having spatially offset axes, whilst the remainder of the arm merely translates the wrist to the required location. When the robot is under computer control the program for the computer may be defined so as to cause the robot to translate the location of the end effector by rotation of the end effector about a point along the shaft of the instrument. That point may be coincident with or distal of the incision into the patient. The program may be such as to achieve the said translation of the end effector by commanding the motive driver(s) for the wrist to cause joints  26  and/or  27  to rotate the instrument about the point and by simultaneously commanding the motive driver(s) for the remainder of the arm to cause the wrist to translate. 
         [0070]    The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.