Abstract:
A robot comprising an arm extending between a base and an attachment for an end effector, the arm comprising: a first arm part; a second arm part distal of the first arm part; and a joint whereby the first and second arm parts are coupled together, the joint permitting the first and second arm parts to rotate relative to each other about at least two mutually offset axes; a control rod attached to the second part of the arm at a location spaced from the first and second axes, the control rod extending distally of that location along the first arm part; and a drive mechanism for driving the control rod to move relative to the first arm part and thereby alter the attitude of the second arm part relative to the first arm part.

Description:
CROSS-REFERENCE 
       [0001]    This is a United States national phase application of PCT/GB2015/050021 filed Jan. 8, 2015 entitled “Articulation,” which claims priority from United Kingdom Application No. 1400569.8 filed Jan. 14, 2014 entitled “Articulation” and United Kingdom Application No. 1418254.7 filed Oct. 15, 2014 entitled “Articulation,” the entire disclosures of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to articulations, for example for surgical robots. 
       BACKGROUND 
       [0003]    A typical robot arm comprises a series of rigid links, each of which is connected to the next by a respective articulation. Each articulation is designed to have appropriate characteristics of strength, range of motion, size, etc. for the purpose the arm is to perform. 
         [0004]    One particular application of robots is for performing or assisting in surgery.  FIG. 1  illustrates a typical surgical robot arm. A patient  1  is lying on a bed  2 . The robot arm  3  extends from a base  4  towards the patient. The arm has a series of rigid links  5 ,  6 ,  7 , which are connected to each other and to the base by articulations  8 ,  9 ,  10 . The articulations provide a sufficient range of motion that the arm can approach the patient in different ways so as to perform a range of surgical procedures. The links can be made to move about the articulations by motors  11  which are under the control of a surgeon. The final link  7  of the arm terminates in a wrist articulation  12  to which an end effector  13  is attached. The end effector is designed for insertion into the patient and, for example, could be an endoscope or could terminate in a cutting or pinching tool. 
         [0005]    It is desirable for the wrist articulation  12  to be highly mobile, so that the end effector can be placed in a wide range of orientations relative to the final link of the arm. That assists in allowing the arm to perform a wide range of surgical procedures, and in allowing a surgeon to place multiple arms close to a surgical site. It is also desirable for the wrist joint to be kinematically well-functioning, without there being any attitudes in the core of its range of motion that are difficult to reach or where there could be poor control over the motion of the end effector. 
         [0006]    U.S. Pat. No. 4,257,243 describes a constant velocity joint for coupling a tractor drive shaft to an agricultural machine. U.S. Pat. No. 3,470,712 describes a similar arrangement for serving as a constant velocity coupling. 
       SUMMARY 
       [0007]    According to the present invention there is provided a robot, robot arm or articulation as set out in the accompanying claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: 
           [0009]      FIG. 1  shows a surgical robot arm. 
           [0010]      FIG. 2  shows various views of a first wrist joint for a surgical robot arm. 
           [0011]      FIG. 3  shows various views of a second wrist joint for a surgical robot arm. 
           [0012]      FIG. 4  shows various views of the second wrist joint for a surgical robot arm. 
           [0013]      FIG. 5  shows various views of a third wrist joint for a surgical robot arm. 
           [0014]      FIG. 6  shows a fourth wrist joint for a surgical robot arm. 
           [0015]      FIG. 7  shows a fifth wrist joint for a surgical robot arm. 
           [0016]      FIG. 8  shows example slaving arrangements for two links of a surgical robot arm. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 2  shows the terminal part of a surgical robot arm. The arm itself is generally of the type shown in  FIG. 1 , with a base and a number of inter-articulated rigid links. The end-most part of the terminal link of the arm is shown at  20  in  FIG. 2 . The terminal link of the arm ends in a wrist joint  21  which carries an attachment  22  to which a surgical end effector can be attached. The joint  21  articulates the attachment  22  relative to the terminal link  20  of the arm. 
         [0018]    The terminal link of the arm is a rigid shaft, defined by a stiff outer tube  23 . At the distal end of the tube is a spherical joint  21 . Spherical joint  21  comprises a part-ball  24  which is captive in a part-cup  25 . The part-cup is fast with the terminal link  20  of the arm. The part-ball is fast with the attachment  22 . The spherical joint allows the attachment to move with three degrees of rotational freedom, but no translational freedom, relative to the terminal link of the arm. 
         [0019]    On the interior of the part-ball  24  is a Hooke&#39;s or universal joint  35 . The Hooke&#39;s joint is offset from the centre of rotation of the spherical joint  21  and connects the part-ball  24  to a control rod  26 . The control rod runs through the interior of the tube  23  towards the proximal end of the terminal link  20  of the arm. The universal joint  35  connects the control rod to the tube so that it has two degrees of rotational freedom relative to the part-ball. 
         [0020]    A pantograph mechanism  27  couples the control rod  26  to the tube  23 . The pantograph comprises a pair of hinged two-part links  28 ,  29  which terminate in collars  30 ,  31  through which the control rod  26  runs. The pantograph permits the control rod to have three degrees of translational freedom relative to the tube  23 , and to rotate relative to the tube about its longitudinal axis by spinning in the collars, but prevents the control rod from yawing about its transverse axes. The control rod is preferably rigid. 
         [0021]    With this mechanism, when the control rod translates laterally relative to the tube, as indicated by axes  32  and  33  in  FIG. 2 , this causes the centre of the Hooke&#39;s joint  35  to move laterally. That in turn causes rotation of the spherical joint  21 , which alters the direction of the attachment  22  relative to the terminal link  20  of the arm. In this way, when a surgical tool is coupled to the attachment the attitude of the surgical tool can be altered. 
         [0022]    The motion of the control rod relative to the tube can be driven by any suitable means, for example electric motors or hydraulic or pneumatic rams. The control rod can be elongated so that it runs from the distal end of the terminal link  20  to near the proximal end of the terminal link, with the result that those drive means can be located near the proximal end of the terminal link. That is convenient because it reduces the weight that is suspended near the distal end of the terminal link, making the terminal link easier to control. 
         [0023]    A further advantage of the mechanism described above is that the spherical joint  21  is relatively compact, meaning that when the attachment  22  is deflected at a significant angle to the terminal link  20  the terminal end of the arm can be brought relatively close to a patient on whom the robot is operating. The compactness of the joint also allows multiple similar robot arms to work in close proximity. 
         [0024]    The pantograph mechanism for maintaining the direction of the control rod could be replaced with another mechanism for achieving the same purpose, for example a set of interlinked rockers running between the inner wall of the tube and the control rod and terminating in slip rings in which the control rod runs. Alternatively, the control rod could be permitted to yaw relative to the tube. For example the control rod could run through a spherical joint mid-way along the tube. 
         [0025]      FIG. 3  shows an alternative design of joint for a surgical robot arm. In  FIG. 3  the the end-most part of the terminal link of the arm is shown at  50 . The terminal link of the arm ends in a wrist joint  51  which carries an attachment  52  to which a surgical end effector can be attached. The joint  51  articulates the attachment  52  relative to the terminal link  50  of the arm. A control rod  53  runs inside the terminal link of the arm for controlling motion of the joint  51 . 
         [0026]    The joint  51  comprises a can  54 , which is shown in more detail in  FIG. 4 . An inner end of the can is attached to the distal end of the control rod  53 . The attachment  52  is provided at the outer end of the can. The can is mounted relative to the terminal end of the arm in a joint  55 . The joint  55  provides the can with freedom to rotate about axes orthogonal to the terminal link of the arm. In the example illustrated in the figures the spherical joint is provided by a gimbal ring, but it could be provided in other ways, for example it could be a spherical joint provided by a part-cup fast with the terminal end of the arm in which a part-ball formation of the can  54  is captive. 
         [0027]    Referring to  FIG. 4 , the can comprises an outer shell  60 . At each end of the shell is a spherical joint  61 ,  62  defined by a part-cup  63 ,  64  that is attached to the shell and a part-ball  65 ,  66  that is captive in the cup. The outer side of one part-ball  65  is coupled to the control rod  53 . The outer side of the other part-ball  66  is coupled to the attachment  52 . The inner side of each part-ball is provided with a universal joint  67 ,  68  whose centre is offset from the rotation centre of the respective part-ball. The universal joints  67 ,  68  are linked by a connecting rod  69 . The connecting rod  69  is equipped with a mechanism  70  whose purpose is to prevent the connecting rod from rotating about axes transverse to its length. In the example of  FIG. 4 , that mechanism is provided by a flat slipper washer  71  which is attached to and extends transversely to the connecting rod  69 . The slipper washer can slide snugly in an annular passageway  72  which also runs transversely to the connecting rod. The fact that the slipper washer is located in the annular passageway prevents the connecting rod from yawing. 
         [0028]      FIG. 5  shows a similar can to that of  FIG. 4 . Like parts are designated the same in  FIG. 5  as in  FIG. 4 . In  FIG. 5  the mechanism for preventing the connecting rod from yawing is a pantograph having two links  73 ,  74  which are hinged relative to each other. One of the links,  73 , is also hinged relative to the interior of the can. The other of the links,  74 , carries a slip ring in which the connecting rod runs snugly. 
         [0029]    As the part-balls  65 ,  66  rotate relative to the can the distance between the universal joints  67 ,  68  will change. To accommodate that the connecting rod could be made in two parts, one sliding snugly over the other. Alternatively, the attachment  52  and the control rod  53  could run slidably through the part-balls  65 ,  66  and terminate within the can in the universal joints. Then the connecting rod could be of fixed length. 
         [0030]    When part-ball  65 , which is connected to the control rod  53 , is rotated relative to the can about an axis other than the can&#39;s longitudinal axis, that rotation causes the connecting rod  69  to translate laterally within the can. That in turn causes the part-ball  66  to rotate relative to the can in a way that mirrors the rotation of the part-ball  65 . 
         [0031]    Referring again to  FIG. 3 , the can is mounted in a spherical joint  55  relative to the terminal link of the arm. The control rod runs through a guide tube  75  that is mounted in a spherical joint  76  in the mid-part of the terminal link of the arm. That arrangement permits the control rod to rotate about that spherical joint and also to slide along its axis relative to that joint. When the control rod is moved so that its distal end moves transverse to the terminal link of the arm, that motion is transmitted to the inner part-ball  65  of the can. The can reacts against the spherical joint  55 , resulting in rotation of the inner part-ball  65  relative to the can about an axis transverse to the terminal link of the arm and also in rotation of the can relative to the arm about an axis transverse to the terminal link of the arm. The action of the connecting rod  69  means that the rotation of the inner part-ball is transmitted to the outer part-ball  66 , causing it also to rotate relative to the can about an axis transverse to the terminal link of the arm. 
         [0032]    The terminal link of the arm of  FIG. 3  is a rigid shaft, defined by a stiff outer tube  53 . At the distal end of the tube is a spherical joint  21 . Spherical joint  21  comprises a part-ball  24  which is captive in a part-cup  25 . The part-cup is fast with the terminal link  20  of the arm. The part-ball is fast with the attachment  22 . The spherical joint allows the attachment to move with three degrees of rotational freedom, but no translational freedom, relative to the terminal link of the arm. 
         [0033]    As can be seen in  FIG. 3 , this arrangement allows the attachment  52  for the end effector to be deflected to relatively large angles relative to the terminal link of the arm. In a typical embodiment it may be expected that the attachment can be deflected through a cone approaching 180°. 
         [0034]    It can also be seen from  FIG. 3  that the joint  51  at the terminal end of the arm is relatively compact, meaning that when the attachment  52  is deflected at a significant angle to the terminal link  50  the terminal end of the arm can be brought relatively close to a patient on whom the robot is operating. This is illustrated at  70 . The compactness of the joint also allows multiple similar robot arms to work in close proximity. 
         [0035]    A further advantage of the joint of  FIGS. 3 to 5  is that rotation of the control rod  53  about its longitudinal axis can be conveyed to the end effector with constant velocity. This may be useful if, for example, the end effector is a drill. It may also simplify the strategy needed to manage the motion of the control rod. To permit this behavior it is preferable that the joint  55  in which the can  54  is mounted relative to the terminal link of the arm does not permit rotation of the can about the longitudinal axis of the terminal link of the arm. The joint  55  could be a gimbal joint. 
         [0036]    The motion of the control rod  53  relative to the tube can be driven by any suitable means, for example electric motors  71  or hydraulic or pneumatic rams. The control rod can be elongated so that it runs from the distal end of the terminal link  50  to near the proximal end of the terminal link, with the result that those drive means can be located near the proximal end of the terminal link. That is convenient because it reduces the weight that is suspended near the distal end of the terminal link, making the terminal link easier to control. 
         [0037]    The joints described above can be used in other applications. For example, the joints could be used for joints in robots other than surgical robots; and for joints other than wrist joints, whether in surgical robots or for other purposes. The joints could be used in non-robotic applications, for example in vehicles (e.g. in drive shafts or steering columns) or in other machinery. 
         [0038]    The end effector could be engaged in the attachment  22 ,  52  by any suitable mechanism, for example by a screw, bayonet or snap fitting. 
         [0039]    The can  54  need not enclose the connecting rod  69 . 
         [0040]      FIG. 6  shows a further way in which the can  54  could be controlled. In this mechanism three push rods  80 ,  81 ,  82  are attached to the inner end of the can, at locations spaced around the can. The push rods can be moved axially relative to the terminal link of the arm, e.g. by screw drives, to cause the can to adopt a desired location. The control rod  53  is mounted to a universal joint  83  within the terminal link of the arm, and is made in two parts, with one surrounding and being splined to the other in order to accommodate changes of distance between the universal joint  83  and its point of attachment to the inner part-ball  65 . This arrangement is convenient in that the control rod  53  can readily be rotated by way of the universal joint  83  independently of the mechanism for setting the attitude of the end effector. 
         [0041]      FIGS. 3 to 6  illustrate using a spherical joint to couple the control rod to the can and another spherical joint to couple the attachment to the can. Other joints may be used in these instances instead of a spherical joint. For example, a gimble joint may be used. As another example, a universal joint may be used.  FIG. 7  illustrates an example in which two universal joints  90  and  91  are used to couple control rod  53  to attachment  52  via intermediate shaft  96 . Control rod  53  terminates in U-joint  92  which rotates about axes A 1  and A 2 . Intermediate shaft  96  comprises U-joint  93  which is arranged perpendicular to U-joint  92  and is coupled to U-joint  92  via cross-piece  97 . U-joint  93  rotates about axes A 1  and A 2 . Attachment  52  terminates in U-joint  94  which rotates about axes A 3  and A 4 . Intermediate shaft  96  comprises U-joint  95  which is arranged perpendicular to U-joint  94  and perpendicular to U-joint  92  and is coupled to U-joint  94  via cross-piece  98 . U-joint  95  rotates about axes A 3  and A 4 . 
         [0042]    Intermediate shaft  96  may house the components interior to the can shown in  FIGS. 3 to 6 . In this case the attachment  52  is mechanically slaved to the control rod  53  via the mechanisms described with respect to  FIGS. 3 to 6  except that the universal joints  90  and  91  provide the articulation provided by the spherical joints in  FIGS. 3 to 6 . In other words, the rotation of universal joint  90  about axes A 1  and A 2  mirrors the rotation of universal joint  91  about axes A 3  and A 4 . In an alternative implementation, the rotation of universal joint  90  about axes A 1  and A 2  is asymmetric to the rotation of universal joint  91  about axes A 3  and A 4 . For example, the double universal joint may be constructed such that universal joint  90  has ˜±90° of travel about axis A 1  and ˜±30° of travel about axis A 2 , and universal joint  91  has ˜±30° of travel about axis A 4  and ˜±90° of travel about axis A 3 . 
         [0043]    In this alternative implementation, the slaving may be accomplished mechanically by driving both joints from a common drive but with different gear ratios in the joint mechanisms. In the example given, a common drive input causes universal joint  90  to rotate around axis A 1  and universal joint  91  to rotate around axis A 4 . However different gear ratios are used in the joint mechanisms, such that when driven, universal joint  90  rotates three times as far as universal joint  91 . This would lead to both joints reaching the limit of their range at the same time. Another common drive input causes universal joint  90  to rotate about axis A 2  and universal joint  91  to rotate about axis A 3 . Different gear ratios are used in the joint mechanisms, such that when driven, universal joint  91  rotates three times as far as universal joint  90 . This would lead to both joints reaching the limit of their range at the same time. Alternatively the joints may be slaved electronically. In this case, each axis is independently controlled and software implemented to ensure the correct relationship between all the joint movements. 
         [0044]    The attachment  52  and control rod  53  may be mechanically slaved together as illustrated in  FIGS. 3 to 6 . Alternatively, the attachment  52  and control rod  53  may be partially or fully electronically slaved to one another in order to provide the same range of motion described with respect to  FIGS. 3 to 6 . 
         [0045]      FIG. 8  illustrates some exemplary slaving arrangements for a first joint J 1  which is the terminal joint of control rod  53  and a second joint J 2  which is the terminal joint of attachment  52 . Joints J 1  and J 2  may be spherical joints, universal joints, gimble joints or any other joints which enable the same articulation between the control rod  53  and the intermediate shaft  96 /can  54  and the intermediate shaft  96 /can  54  and the attachment  52  as described above. 
         [0046]    In one implementation of  FIG. 8( a ) , J 1  and J 2  are wholly electronically slaved together. In this case, control shaft  101  driven by motor  103  controls part of the motion of J 1  and J 2 . The other part of the motion of J 1  and J 2  is controlled by control shaft  102  driven by motor  104 . Control shaft  101  is coupled to J 1  and terminates at J 2 . Control shaft  102  is coupled to J 1  and terminates at J 2 . Motor  103  is located either in control rod  53  or further towards the base of the robot arm. Motor  104  is located either in control rod  53  or further towards the base of the robot arm. In the case of a double universal joint as shown in  FIG. 7 , rotation of the universal joint  90  about axis A 1  is controlled by motor  103  via control shaft  101 . Similarly, rotation of the universal joint  91  about axis A 4  is controlled by motor  103  via control shaft  101 . Rotation of the universal joint  90  about axis A 2  is controlled by motor  104  via control shaft  102 . Rotation of the universal joint  91  about axis A 3  is controlled by motor  104  via control shaft  102 . Motors  103  and  104  drive their respective control shafts to cause J 1  and J 2  to articulate in the same manner as if J 1  and J 2  were mechanically slaved together as described above. 
         [0047]    In an alternative implementation of  FIG. 8( a ) , J 1  and J 2  are mechanically slaved together by intermediate shaft  96 /can  54 , for example as discussed above with reference to  FIGS. 3 to 7 . Control shaft  101  driven by motor  103  terminates at J 2 . Motor  103  is located either in control rod  53  or further towards the base of the robot arm. Control shaft  102  driven by motor  104  also terminates at J 2 . Motor  104  is located either in control rod  53  or further towards the base of the robot arm. In the case of a double universal joint as shown in  FIG. 7 , rotation of the universal joint  91  about one axis A 3  or A 4  is controlled by motor  103  via control shaft  101 . Similarly, rotation of the universal joint  91  about the other axis A 3  or A 4  is controlled by motor  104  via control shaft  102 . J 1  is mechanically slaved to J 2 , thus when J 2  is driven by motors  103  and  104 , J 1  also moves in a manner determined by the manner in which J 1  and J 2  are mechanically slaved. In  FIG. 8( a )  the joint J 2  which is the most distal of joints J 1  and J 2  from the control rod  53  is driven by motors  103  and  104 . Alternatively, the control shafts  101  and  102  may be attached to and drive joint J 1 , and joint J 2  moves in a manner determined by the mechanical slaving between J 1  and J 2 . 
         [0048]    In one implementation of  FIG. 8( b ) , J 1  and J 2  are wholly electronically slaved together. In this case, control shaft  105  driven by motor  106  controls part of the motion of J 1  and J 2 . The other part of the motion of J 1  and J 2  is controlled by control shaft  107  driven by motor  108 . Control shaft  105  is coupled to J 1  and terminates at J 2 . Control shaft  107  driven by motor  108  terminates at one end at J 1  and at the other end at J 2 . Motor  108  is located in intermediate shaft  96  between J 1  and J 2 . Motor  106  is located either in control rod  53  or further towards the base of the robot arm. In the case of a double universal joint as shown in  FIG. 7 , rotation of the universal joint  90  about axis A 1  is controlled by motor  106  via control shaft  105 . Similarly, rotation of the universal joint  91  about axis A 4  is controlled by motor  106  via control shaft  105 . Rotation of the universal joint  90  about axis A 2  is controlled by motor  108  via control shaft  107 . Rotation of the universal joint  91  about axis A 3  is controlled by motor  108  via control shaft  107 . Motors  103  and  104  drive their respective control shafts to cause J 1  and J 2  to articulate in the same manner as if J 1  and J 2  were mechanically slaved together as described above. 
         [0049]    In one implementation of  FIG. 8( d ) , J 1  and J 2  are wholly electronically slaved together. In this case, control shaft  117  driven by motor  118  controls part of the motion of J 1  and J 2 . Control shaft  117  is coupled to J 1  and terminates at J 2 . The other part of the motion of J 1  is controlled by control shaft  119  driven by motor  120 . The other part of the motion of J 2  is controlled by control shaft  121  driven by motor  122 . Motor  122  is located in intermediate shaft  96  between J 1  and J 2 . Motor  118  is located either in control rod  53  or further towards the base of the robot arm. Motor  120  is located either in control rod  53  or further towards the base of the robot arm. In the case of a double universal joint as shown in  FIG. 7 , rotation of the universal joint  90  about axis A 1  is controlled by motor  118  via control shaft  117 . Similarly, rotation of the universal joint  91  about axis A 4  is controlled by motor  118  via control shaft  117 . Rotation of the universal joint  90  about axis A 2  is controlled by motor  120  via control shaft  119 . Rotation of the universal joint  91  about axis A 3  is controlled by motor  122  via control shaft  121 . Motors  118 ,  120  and  122  drive their respective control shafts to cause J 1  and J 2  to articulate in the same manner as if J 1  and J 2  were mechanically slaved together as described above. 
         [0050]      FIG. 8( c )  illustrates an arrangement in which J 1  and J 2  are wholly electronically slaved together. Control shaft  109  driven by motor  110  terminates at J 1 . Motor  110  is located either in control rod  53  or further towards the base of the robot arm. Control shaft  111  driven by motor  112  terminates at J 1 . Motor  112  is located either in control rod  53  or further towards the base of the robot arm. Control shaft  113  driven by motor  114  terminates at one end in intermediate shaft  96  between J 1  and J 2  and at the other end at J 2 . Motor  114  is located in intermediate shaft  96  between J 1  and J 2 . Control shaft  115  driven by motor  116  terminates at one end in intermediate shaft  96  between J 1  and J 2  and at the other end at J 2 . Motor  116  is located in intermediate shaft  96 . Motor  110  drives J 1  to articulate about one of its axes. Motor  112  drives J 1  to articulate about the other of its axes. Motor  114  drives J 2  to articulate about one of its axes. Motor  116  drives J 2  to articulate about the other of its axes. Motors  110 ,  112 ,  114  and  116  drive their respective control shafts to cause J 1  and J 2  to articulate in the same manner as if J 1  and J 2  were mechanically slaved together as described above. 
         [0051]      FIG. 8( e )  illustrates an arrangement in which J 1  and J 2  are wholly electronically slaved together. Control shaft  123  driven by motor  124  terminates at J 1 . Motor  124  is located either in control rod  53  or further towards the base of the robot arm. Control shaft  125  driven by motor  126  terminates at J 1 . Motor  126  is located either in control rod  53  or further towards the base of the robot arm. Control shaft  127  driven by motor  128  terminates at one end in attachment  52  and at the other end at J 2 . Motor  128  is located in attachment  52 . Control shaft  129  driven by motor  130  terminates at one end in attachment  52  at the other end at J 2 . Motor  130  is located in attachment  52 . Motor  124  drives J 1  to articulate about one of its axes. Motor  126  drives J 1  to articulate about the other of its axes. Motor  128  drives J 2  to articulate about one of its axes. Motor  130  drives J 2  to articulate about the other of its axes. Motors  128 ,  130 ,  124  and  126  drive their respective control shafts to cause J 1  and J 2  to articulate in the same manner as if J 1  and J 2  were mechanically slaved together as described above. 
         [0052]    The control shafts of  FIG. 8  may drive the respective joints about their axes using, for example, a worm and spur gear or a worm and face gear. The control shafts may be coaxial. For example, control shafts  117  and  119  in  FIG. 8( d )  may be coaxial shafts where the inner shaft  117  drives J 2  and the outer shaft  119  drives J 1 . Alternatively, the joints may be driven from an off-axis control shaft which drives the joints via a bevel gear, worm gear or offset hypoid gear. Suitably, the control shafts are hollow in order to allow for control cables to pass through them. 
         [0053]    Suitably, the motors and drive elements are located towards the base of the robot arm. This reduces the weight suspended near the distal end of the attachment, making the attachment easier to control. It also reduces the required strength of the other arm joints, enabling the arm to be lighter and hence easier to control. 
         [0054]    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.