Patent Publication Number: US-11382705-B2

Title: Independent tensioning and aligning mechanisms for a surgical instrument

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/374,150, filed Dec. 9, 2016, which claims priority to British Patent Application No. GB 1521806.8, filed on Dec. 10, 2015, both of are hereby incorporated herein by reference. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit under 35 USC 119 of United Kingdom Application No. 1521806.8, filed Dec. 10, 2015. The contents of this application is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     It is known to use robots for assisting and performing surgery.  FIG. 1  illustrates a typical surgical robot  100  which consists of a base  108 , an arm  102 , and an instrument  105 . The base supports the robot, and is itself attached rigidly to, for example, the operating theatre floor, the operating theatre ceiling or a trolley. The arm extends between the base and the instrument. The arm is articulated by means of multiple flexible joints  103  along its length, which are used to locate the surgical instrument in a desired location relative to the patient. The surgical instrument is attached to the distal end  104  of the robot arm. The surgical instrument penetrates the body of the patient  101  at a port  107  so as to access the surgical site. At its distal end, the instrument comprises an end effector  106  for engaging in a medical procedure. 
       FIG. 2  illustrates a typical surgical instrument  200  for performing robotic laparoscopic surgery. The surgical instrument comprises a base  201  by means of which the surgical instrument connects to the robot arm. A shaft  202  extends between base  201  and articulation  203 . Articulation  203  terminates in an end effector  204 . In  FIG. 2 , a pair of serrated jaws are illustrated as the end effector  204 . The articulation  203  permits the end effector  204  to move relative to the shaft  202 . It is desirable for at least two degrees of freedom to be provided to the motion of the end effector  204  by means of the articulation. 
       FIG. 3  illustrates an example of a known surgical instrument  300  in which end effector  204  is permitted to move relative to shaft  202  by means of pitch joint  301  and two yaw joints  302 . Joint  301  enables the end effector  204  to rotate about pitch axis  303 . Joints  302  enable each jaw of the end effector  204  to rotate about yaw axis  304 . The joints are driven by cables  306 ,  307  and  308 . Pulley  305  is used to direct cables  307  and  308  from their passage over the pitch joint to the yaw joints. Pulley  305  is offset from the central axis of the articulation  203 . The external diameter of the shaft is 8 mm in order to accommodate the number, size and location of the internal elements of the articulated portion. 
     It is desirable to reduce the external diameter of the instrument in order to minimise the size of the incision through the skin of the patient and minimise disruption inside the patient&#39;s body. It is also desirable to minimise the weight of the surgical instrument so as to minimise the size and weight of the robot base and arm required to support the instrument, thereby enabling the robot as a whole to be more compact and hence occupy a smaller space in the operating theatre and be more moveable within the operating theatre. 
     In a typical laparoscopy operation, a surgeon utilises many instruments, and hence exchanges one instrument for another many times. It is therefore desirable to minimise the time taken and maximise the ease with which one instrument is detached from a robot arm and a different instrument is attached. Additionally, it is desirable to minimise the time taken in setting up the instrument ready for use once it has been attached to the robot arm. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the invention, there is provided a robotic surgical instrument as set out in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: 
         FIG. 1  illustrates a surgical robot performing a surgical procedure; 
         FIG. 2  illustrates a known surgical instrument; 
         FIG. 3  illustrates a known arrangement of an articulated end effector of a surgical instrument; 
         FIG. 4  illustrates a surgical robot; 
         FIGS. 5A and 5B  illustrate a distal end of a surgical instrument; 
         FIGS. 6A and 6B  illustrate a pulley arrangement of the distal end of the surgical instrument of  FIGS. 5A and 5B  in a straight configuration; 
         FIG. 7A-7E  illustrate a pulley arrangement of the distal end of the surgical instrument of  FIGS. 5A and 5B  in a variety of non-straight configurations; 
         FIG. 8  illustrates the offset pulleys of the pulley arrangement shown in  5 A and  5 B; 
         FIGS. 9A-9D  illustrate non-straight configurations of the distal end of a surgical instrument; 
         FIG. 10  illustrates a supporting body and redirecting pulleys of the articulation of a surgical instrument; 
         FIGS. 11A and 11B  illustrate a different view of the supporting body and redirecting pulleys of  FIG. 10 ; 
         FIGS. 12 and 13  illustrate the supporting body of the articulation of  FIGS. 10 and 11  in isolation; 
         FIG. 14  illustrates a spindle mounted redirecting pulley mounted to the supporting body of  FIGS. 12 and 13 ; 
         FIG. 15  illustrates a bevelled surface and groove of the supporting body of  FIGS. 12 and 13 ; 
         FIGS. 16A and 16B  illustrate arrangements of driving elements in an instrument shaft; 
         FIGS. 17A and 17B  illustrate two views of a surgical instrument; 
         FIG. 18  illustrates spokes in an instrument shaft; 
         FIGS. 19A, 19B and 19C  illustrate three views of an instrument interface; 
         FIGS. 20A and 20B  illustrate tensioning mechanisms; 
         FIGS. 21A, 21B and 21C  illustrate three views of a drive assembly interface of a robot arm; 
         FIGS. 22A, 22B and 22C  illustrate configurations of an instrument interface element; 
         FIG. 23  illustrates a configuration of a drive assembly interface element; 
         FIG. 24  illustrates an instrument interface engaged in a drive assembly interface; and 
         FIG. 25  illustrates a further view of an instrument interface. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 4  illustrates a surgical robot having an arm  400  which extends from a base  401 . The arm comprises a number of rigid limbs  402 . The limbs are coupled by revolute joints  403 . The most proximal limb  402   a  is coupled to the base by joint  403   a . It and the other limbs are coupled in series by further ones of the joints  403 . Suitably, a wrist  404  is made up of four individual revolute joints. The wrist  404  couples one limb ( 402   b ) to the most distal limb ( 402   c ) of the arm. The most distal limb  402   c  carries an attachment  405  for a surgical instrument  406 . Each joint  403  of the arm has one or more motors  407  which can be operated to cause rotational motion at the respective joint, and one or more position and/or torque sensors  408  which provide information regarding the current configuration and/or load at that joint. Suitably, the motors are arranged proximally of the joints whose motion they drive, so as to improve weight distribution. For clarity, only some of the motors and sensors are shown in  FIG. 4 . The arm may be generally as described in our co-pending patent application PCT/GB2014/053523. 
     The arm terminates in an attachment  405  for interfacing with the instrument  406 . Suitably, the instrument  406  takes the form described with respect to  FIG. 2 . The instrument has a diameter less than 8 mm. Suitably, the instrument has a 5 mm diameter. The instrument may have a diameter which is less than 5 mm. The instrument diameter may be the diameter of the shaft. The instrument diameter may be the diameter of the profile of the articulation. Suitably, the diameter of the profile of the articulation matches or is narrower than the diameter of the shaft. The attachment  405  comprises a drive assembly for driving articulation of the instrument. Movable interface elements of the drive assembly interface mechanically engage corresponding movable interface elements of the instrument interface in order to transfer drive from the robot arm to the instrument. One instrument is exchanged for another several times during a typical operation. Thus, the instrument is attachable and detachable from the robot arm during the operation. Features of the drive assembly interface and the instrument interface aid their alignment when brought into engagement with each other, so as to reduce the accuracy with which they need to be aligned by the user. 
     The instrument  406  comprises an end effector for performing an operation. The end effector may take any suitable form. For example, the end effector may be smooth jaws, serrated jaws, a gripper, a pair of shears, a needle for suturing, a camera, a laser, a knife, a stapler, a cauteriser, a suctioner. As described with respect to  FIG. 2 , the instrument comprises an articulation between the instrument shaft and the end effector. The articulation comprises several joints which permit the end effector to move relative to the shaft of the instrument. The joints in the articulation are actuated by driving elements, such as cables. These driving elements are secured at the other end of the instrument shaft to the interface elements of the instrument interface. Thus, the robot arm transfers drive to the end effector as follows: movement of a drive assembly interface element moves an instrument interface element which moves a driving element which moves a joint of the articulation which moves the end effector. 
     Controllers for the motors, torque sensors and encoders are distributed with the robot arm. The controllers are connected via a communication bus to control unit  409 . A control unit  409  comprises a processor  410  and a memory  411 . Memory  411  stores in a non-transient way software that is executable by the processor to control the operation of the motors  407  to cause the arm  400  to operate in the manner described herein. In particular, the software can control the processor  410  to cause the motors (for example via distributed controllers) to drive in dependence on inputs from the sensors  408  and from a surgeon command interface  412 . The control unit  409  is coupled to the motors  407  for driving them in accordance with outputs generated by execution of the software. The control unit  409  is coupled to the sensors  408  for receiving sensed input from the sensors, and to the command interface  412  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  412  comprises one or more input devices whereby a user can request motion of the end effector 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  411  is configured to respond to those inputs and cause the joints of the arm and instrument to move accordingly, in compliance with a pre-determined control strategy. The control strategy may include safety features which moderate the motion of the arm and instrument in response to command inputs. Thus, in summary, a surgeon at the command interface  412  can control the instrument  406  to move in such a way as to perform a desired surgical procedure. The control unit  409  and/or the command interface  412  may be remote from the arm  400 . 
       FIGS. 5A and 5B  illustrate opposing views of the distal end of a surgical instrument. In  FIGS. 5A and 5B , the end effector  501  comprises a pair of end effector elements  502 ,  503 , which in  FIGS. 5A and 5B  are depicted as a pair of opposing serrated jaws. It will be understood that this is for illustrative purposes only. The end effector may take any suitably form, such as those described above. The end effector  501  is connected to the shaft  504  by articulation  505 . Articulation  505  comprises joints which permit the end effector  501  to move relative to the shaft  504 . A first joint  506  permits the end effector  501  to rotate about a first axis  510 . The first axis  510  is transverse to the longitudinal axis of the shaft  511 . A second joint  507  permits the first end effector element  502  to rotate about a second axis  512 . The second axis  512  is transverse to the first axis  510 . A third joint  513  permits the second end effector element  503  to rotate about the second axis  512 . Suitably, the first end effector element  502  and the second end effector element  503  are independently rotatable about the second axis  512  by the second and third joints. The end effector elements may be rotated in the same direction or different directions by the second and third joints. The first end effector element  502  may be rotated about the second axis, whilst the second end effector element  503  is not rotated about the second axis. The second end effector element  503  may be rotated about the second axis, whilst the first end effector element  502  not rotated about the second axis. 
     Figures depict a straight configuration of the surgical instrument in which the end effector is aligned with the shaft. In this orientation, the longitudinal axis of the shaft  511  is coincident with the longitudinal axis of the articulation and the longitudinal axis of the end effector. Articulation of the first, second and third joints enables the end effector to take a range of attitudes relative to the shaft.  FIGS. 9A-9D  illustrate some of the configurations of the distal end of the instrument in which articulation about all the first, second and third joints has been driven relative to the straight configuration of  FIGS. 5A and 5B . 
     Returning to  FIGS. 5A and 5B , the shaft terminates at its distal end in the first joint  506 . The articulation  505  comprises a supporting body  509 . At one end, the supporting body  509  is connected to the shaft  504  by the first joint  506 . At its other end, the supporting body  509  is connected to the end effector  501  by second joint  507  and third joint  513 . Thus, first joint  506  permits the supporting body  509  to rotate relative to the shaft  504  about the first axis  510 ; and the second joint  507  and third joint  513  permit the end effector elements  502 ,  503  to rotate relative to the supporting body  509  about the second axis  512 . 
     In the figures, the second joint  507  and third joint  513  both permit rotation about the same axis  512 . However, the second and third joints may alternatively permit rotation of the end effector elements about different axes. The axis of rotation of one of the end effector elements may be offset in the longitudinal direction of the shaft  504  from the axis of rotation of the other end effector element. The axis of rotation of one of the end effector elements may be offset in a direction transverse to the longitudinal direction of the shaft  504  from the axis of rotation of the other end effector element. The axis of rotation of one of the end effector elements may not be parallel to the axis of rotation of the other end effector element. The axes of rotation of the end effector elements  502 ,  503  may be offset in the longitudinal direction of the shaft and/or offset in a direction perpendicular to the longitudinal direction of the shaft and/or angled with respect to each other. This may be desirable as a result of the end effector elements being asymmetric. For example, in an electrosurgical element, a first end effector element may be powered and a second end effector element not powered and insulated from the first end effector element. To aid this, the axes of rotation of the two end effector elements may be offset in the direction perpendicular to the longitudinal direction of the shaft. In another example, a first end effector element may be a blade and a second end effector element a flat cutting surface. To aid use of the blade, the axes of rotation of the two end effector elements may be angled to one another. 
     The joints of the articulation are driven by driving elements. The driving elements are elongate elements which extend from the joints in the articulation through the shaft to the instrument interface. Suitably, each driving element can be flexed laterally to its main extent at least in those regions where it engages the internal components of the articulation and instrument interface. In other words, each driving element can be flexed transverse to its longitudinal axis in the specified regions. This flexibility enables the driving elements to wrap around the internal structure of the instrument, such as the joints and pulleys. The driving elements may be wholly flexible transverse to their longitudinal axes. The driving elements are not flexible along their main extents. The driving elements resist compression and tension forces applied along their length. In other words, the driving elements resist compression and tension forces acting in the direction of their longitudinal axes. The driving elements have a high modulus. The driving elements remain taut in operation. They are not permitted to become slack. Thus, the driving elements are able to transfer drive from the instrument interface to the joints. The driving elements may be cables. 
     Suitably, each joint is driven by a pair of driving elements. Referring to  FIGS. 5A and 5B , the first joint  506  is driven by a first pair of driving elements A 1 ,A 2 . The second joint  507  is driven by a second pair of driving elements B 1 ,B 2 . The third joint is driven by a third pair of driving elements C 1 ,C 2 . Suitably, each joint is driven by its own pair of driving elements. In other words, each joint is driven by a dedicated pair of driving elements. Suitably, the joints are independently driven. A pair of driving elements may be constructed as a single piece as shown for the third pair of driving elements in  FIGS. 5A and 5B . In this case, the single piece is secured to the joint at one point. For example, the third pair of driving elements C 1 ,C 2  comprises a ball feature  520  which is secured to the third joint  513 . This ensures that when the pair of driving elements is driven, the drive is transferred to motion of the joint about its axis. Alternatively, a pair of driving elements may be constructed as two pieces. In this case, each separate piece is secured to the joint. 
     The surgical instrument of  FIGS. 5A and 5B  further comprises a pulley arrangement around which the second and third pairs of driving elements are constrained to move. The pulley arrangement is better illustrated in  FIGS. 6A and 6B . The supporting body  509  is not shown in  FIGS. 6A and 6B  in order to more clearly illustrate the pulley arrangement. The pulley arrangement comprises a first set of pulleys  601 . The first set of pulleys  601  is rotatable about the first axis  510 . Thus, the first set of pulleys  601  rotate about the same axis as the first joint  506 . The pulley arrangement further comprises a second set of pulleys  602 . The pulley arrangement further comprises a pair of redirecting pulleys  603 . 
     The pulley arrangement is more clearly illustrated in  FIGS. 7A-7E . The supporting body, the first joint and the first pair of driving elements have all been omitted from  FIGS. 7A-7E  in order to more clearly illustrate the pulley arrangement. The second set of pulleys comprises a first pulley  701  and a second pulley  702 . The first pulley  701  is rotatable about a third axis  703  which is parallel to the first axis  510 . The third axis  703  is offset from the first axis  510  both in the longitudinal direction of the shaft and also transverse to the longitudinal direction of the shaft. The second pulley  702  is rotatable about a fourth axis  704  which is parallel to the first axis  510 . The fourth axis  704  is offset from the first axis  510  both in the longitudinal direction of the shaft and also transverse to the longitudinal direction of the shaft. The third and fourth axes are parallel but offset from each other. The third axis  703  and fourth axis  704  are in the same plane perpendicular to the longitudinal direction of the shaft.  FIG. 8  illustrates the distal end of the surgical instrument from a different view which more clearly shows the offset axes of the first pulley  701  and the second pulley  702  of the second set of pulleys. By offsetting the first pulley  701  and the second pulley  702 , the driving element wrapped around each pulley is able to extend down the shaft after having wrapped around the pulley. As shown in  FIG. 6A , the first pulley  701  and second pulley  702  of the second set of pulleys  602  are located on opposing sides of the first joint  506  in a longitudinal direction of the shaft  504 . The first pulley  701  and second pulley  702  are located on opposing sides of the first pair of driving elements A 1 ,A 2 . 
     The second set of pulleys is located between the first set of pulleys and the instrument interface end of the shaft. Suitably, the second set of pulleys is located within the shaft as shown in the figures. Alternatively, the second set of pulleys may be located within the articulation between the first joint  506  and the second joint  507 . However, by locating the second set of pulleys at the distal end of the shaft  508 , the distance between the first and second joints is reduced compared to the alternative arrangement in which the second set of pulleys are located in the articulation, thereby reducing the stiffness of the supporting body  509  required to maintain accurate positioning of the end effector  501 . 
     The first set of pulleys  601  comprises a first pulley  705  and a second pulley  706 . Both the first pulley  705  and the second pulley  706  rotate about the first axis  510 . The first pulley  705  and the second pulley  706  of the first set of pulleys are located on opposing sides of the first joint  506  in a longitudinal direction of the shaft  504 . The first pulley  705  and the second pulley  06  are located on opposing ends of the first axis  510 . The first pulley  705  and the second pulley  706  are located on opposing sides of the first pair of driving elements A 1 ,A 2 . 
     The second pair of driving elements B 1 ,B 2  is constrained to move around opposing sides of the first pulley  705  and the second pulley  706  of the first set of pulleys  601 . The second pair of driving elements B 1 ,B 2  is constrained to move around opposing sides of the first pulley  701  and the second pulley  702  of the second set of pulleys  601 . The second pair of driving elements is constrained to move around opposing sides of the first pulley  705  of the first set of pulleys  601  and the first pulley  701  of the second set of pulleys  602 . The second pair of driving elements is constrained to move around opposing sides of the second pulley  706  of the first set of pulleys  601  and the second pulley  702  of the second set of pulleys  602 . 
     The third pair of driving elements C 1 ,C 2  is constrained to move around opposing sides of the first pulley  705  and the second pulley  706  of the first set of pulleys  601 . The third pair of driving elements C 1 ,C 2  is constrained to move around opposing sides of the first pulley  701  and the second pulley  702  of the second set of pulleys  601 . The third pair of driving elements is constrained to move around opposing sides of the first pulley  705  of the first set of pulleys  601  and the first pulley  701  of the second set of pulleys  602 . The third pair of driving elements is constrained to move around opposing sides of the second pulley  706  of the first set of pulleys  601  and the second pulley  702  of the second set of pulleys  602 . 
     The second and third pairs of driving elements are each constrained to extend over the first joint  506  in order to reach the second and third joints respectively. Thus, the first one of the second pair of driving elements B 1  passes over one side of the first pulley  705  of the first set of pulleys on the first joint axis  510 , and the second one of the second pair of driving elements B 2  passes over an opposing side of the second pulley  706  of the first set of pulleys on the first joint axis  510 , so that whatever rotation there is of the supporting body  509  about the first joint  506 , the length of the second pair of driving elements B 1 ,B 2  is maintained the same. Similarly, the first one of the third pair of driving elements C 1  passes over one side of the second pulley  706  of the first set of pulleys on the first joint axis  510 , and the second one of the third pair of driving elements C 2  passes over an opposing side of the first pulley  705  of the first set of pulleys on the first joint axis  510 , so that whatever rotation there is of the supporting body  509  about the first joint  506 , the length of the third pair of driving elements C 1 ,C 2  is maintained the same. If the arrangement of the instrument interface is symmetric for both the second pair of driving elements B 1 ,B 2  and the third pair of driving elements C 1 ,C 2 , then the length of the second pair of driving elements is the same as the length of the third pair of driving elements for all rotation angles of the supporting body  509  about the first joint  506 . In every configuration of the surgical instrument, the second pair of driving elements and the third pair of driving elements remain taut. They are never slack. Thus, there is no backlash when articulating any of the joints of the surgical instrument. Thus, full control of all three degrees of freedom of movement of the surgical instrument is achieved in every configuration of the surgical instrument. 
     Suitably, each pulley of the first set of pulleys  601  comprises a pair of pulley elements. The first pulley  705  comprises an inside pulley element  708  and an outside pulley element  709 . Inside pulley element  708  is located between the outside pulley element  709  and the first pair of driving elements A 1 ,A 2 . Suitably, inside pulley element  708  abuts outside pulley element  709 . The inside pulley element  708  may be fast with the outside pulley element  709 . The inside pulley element  708  may be integrally formed with the outside pulley element  709 . The second pulley  706  comprises an inside pulley element  710  and an outside pulley element  711 . Inside pulley element  710  is located between the outside pulley element  711  and the first pair of driving element A 1 ,A 2 . Suitably, inside pulley element  710  abuts outside pulley element  711 . The inside pulley element  710  may be fast with the outside pulley element  711 . The inside pulley element  710  may be integrally formed with the outside pulley element  711 . Each pulley element comprises a groove for seating a driving element. 
     Suitably, each pulley of the second set of pulleys  602  comprises a pair of pulley elements. The first pulley  701  comprises an inside pulley element  712  and an outside pulley element  713 . Inside pulley element  712  is located between the outside pulley element  713  and the first pair of driving elements A 1 ,A 2 . Suitably, inside pulley element  712  abuts outside pulley element  713 . The inside pulley element  712  may be fast with the outside pulley element  713 . The inside pulley element  712  may be integrally formed with the outside pulley element  713 . The second pulley  702  comprises an inside pulley element  714  and an outside pulley element  715 . Inside pulley element  714  is located between the outside pulley element  715  and the first pair of driving element A 1 ,A 2 . Suitably, inside pulley element  714  abuts outside pulley element  715 . The inside pulley element  714  may be fast with the outside pulley element  715 . The inside pulley element  714  may be integrally formed with the outside pulley element  715 . Each pulley element comprises a groove for seating a driving element. 
     The second pair of driving elements B 1 ,B 2  is constrained to move around the inside pulley element  712  of the first pulley of the second set of pulleys and the outside pulley element  715  of the second pulley of the second set of pulleys. The second pair of driving elements B 1 , B 2  is constrained to move around the inside pulley element  708  of the first pulley of the first set of pulleys and the outside pulley element  711  of the second pulley of the first set of pulleys. 
     The third pair of driving elements C 1 ,C 2  is constrained to move around the outside pulley element  713  of the first pulley of the second set of pulleys and the inside pulley element  714  of the second pulley of the second set of pulleys. The third pair of driving elements C 1 ,C 2  is constrained to move around the outside pulley element  709  of the first pulley of the first set of pulleys and the inside pulley element  710  of the second pulley of the first set of pulleys. 
     Thus, the second pair of driving elements B 1 ,B 2  has a symmetrically opposing path around the first and second sets of pulleys  601 ,  602  than the third pair of driving elements C 1 ,C 2 . In the straight configuration of the instrument in which the end effector is aligned with the shaft, the path of the second pair of driving elements B 1 ,B 2  about the pulley arrangement is rotationally symmetrical about the longitudinal axis of the shaft  511  to the path of the third pair of driving elements C 1 ,C 2  about the pulley arrangement. The second and third pairs of driving elements B 1 ,B 2  and C 1 ,C 2  emerge from the second set of pulleys  602  into the distal end of the shaft in a symmetrical arrangement. As can be more easily seen on  FIGS. 7A-7E , the driving elements B 1  and C 2  emerge adjacent to each other on one side of the shaft, and the driving elements C 1  and B 2  emerge adjacent to each other on an opposing side of the shaft. The arrangement of driving elements B 1  and C 2  in the shaft is rotationally symmetrical to the arrangement of driving elements C 1  and B 2  in the shaft, about the longitudinal axis of the shaft  511 . The second set of pulleys  602  redirects the second and third pairs of driving elements from the first set of pulleys  601  into the shaft in this manner. 
       FIGS. 7A-7E  illustrate the distal end of the surgical instrument in five different configurations. Configuration (c) is the straight configuration previously mentioned, in which the end effector is aligned with the instrument shaft. In configurations (a), (b), (d) and (e), rotation about the first joint has occurred relative to configuration (c). In configurations (a), (b), (d) and (e), no rotation about either the second or third joint has occurred relative to configuration (c). Starting from configuration (c), the driving element A 2  (not shown) is pulled in order to cause the rotation about the first axis  510  leading to the arrangement of configuration (b). The driving element A 2  is further pulled to cause further rotation about the first axis  510  to lead to the arrangement of configuration (a). Starting from configuration (c), the driving element A 1  (not shown) is pulled in order to cause rotation about the first axis  510  in an opposing direction to that in configurations (a) and (b), thereby leading to the arrangement of configuration (d). The driving element A 1  is further pulled to cause further rotation about the first axis  510  to lead to the arrangement of configuration (e). 
     Rotation of the end effector  501  about the first axis  510  is bounded by the maximum travel of the first pair of driving elements A 1 ,A 2  about the first joint  506 . Configuration (a) shows the end effector  501  at maximum rotation about the first axis  510  in one direction, and configuration (e) shows the end effector  501  at maximum rotation about the first axis  510  in the opposing direction. The maximum rotation angle relative to the longitudinal axis of the shaft  511  in both configurations is the angle cp. The second set of pulleys  602  are located relative to the first set of pulleys  601  so as to ensure that the second and third pairs of driving elements are retained in contact with both the first set of pulleys  601  and the second set of pulleys  602  even at the maximum rotation angle cp. For all rotation angles of the end effector  501  about the first axis  510 , the end effector  501  always lies within the cone defined by the tangential line connecting the first pulley  701  of the second set of pulleys and the first pulley  705  of the first set of pulleys. That tangential line is the path taken by the driving element. The end effector  501  lies in this cone when the second and third joints are retained in the straight configurations of  FIGS. 5A  and  5 B, as shown in all the configurations of  FIGS. 7A-7E . As can be seen from  FIGS. 7A-7E , without the second set of pulleys  602 , the driving elements B 2  and C 1  would lose contact with the first set of pulleys  601  in configuration (a). Without the second set of pulleys  602 , the driving elements B 1  and C 2  would lose contact with the first set of pulleys  601  in configuration (e). 
     The second and third pairs of driving elements are retained in contact with the first and second sets of pulleys for all rotation angles of the end effector relative to the longitudinal axis of the shaft. Thus, regardless of the rotation about the first joint  506 , the length of the second pair of driving elements B 1 ,B 2  will be maintained the same. Also, regardless of the rotation about the first joint  506 , the length of the third pair of driving elements C 1 ,C 2  will be maintained the same. Thus, the second set of pulleys enable tension to be retained in the second and third driving elements regardless of how the first joint  506  is driven about the first axis  510 . Thus, control of the second and third driving elements is retained regardless of how the first joint  506  is driven about the first axis  510 . 
     The pulley arrangement further comprises a pair of redirecting pulleys  716 , 717 . These redirecting pulleys are in the articulation  505  between the first joint  506  and the second and third joints  507 ,  513 . The redirecting pulleys are positioned so as to redirect the second pair of driving elements B 1 ,B 2  from the first set of pulleys  601  to the second joint  507  and to redirect the third pair of driving elements C 1 ,C 2  from the first set of pulleys  601  to the third joint  513 . 
     The second pair of driving elements B 1 ,B 2  is constrained to move around the first redirecting pulley  716 . The first redirecting pulley  716  rotates about a first redirecting pulley axis  718 . The first redirecting pulley axis  718  is at an angle ϑ to the first axis  510 . The angle ϑ is such that the first one of the second pair of driving elements B 1  is redirected from a take-off point of the first pulley  705  of the first set of pulleys  601  to a pick-up point  721  on the second joint  507 . Suitably, the first redirecting pulley  716  comprises a groove which seats the driving element B. The third pair of driving elements C 1 ,C 2  is not constrained to move around the first redirecting pulley  716 . However, the second one of the third pair of driving elements C 2  does pass by the first redirecting pulley  716  between its take-off point of the third joint  513  and its pick-up point on the first pulley  705  of the first set of pulleys  601 . The driving element C 2  may be partially enclosed by the first redirecting pulley  716 . For example, the driving element C 2  may partially pass between the wings of the groove of the first redirecting pulley  716 , but the driving element C 2  is not seated in the groove of the first redirecting pulley  716 . 
     The third pair of driving elements C 1 ,C 2  is constrained to move around the second redirecting pulley  717 . The second redirecting pulley  717  rotates about a second redirecting pulley axis  719 . The second redirecting pulley axis  719  is at an angle ψ to the first axis  510 . The angle ψ is such that the first one of the third pair of driving elements C 1  is redirected from a take-off point  720  of the second pulley  706  of the first set of pulleys  601  to a pick-up point on the third joint  513 . Suitably, the second redirecting pulley  717  comprises a groove which seats the driving element C 1 . The second pair of driving elements B 1 ,B 2  is not constrained to move around the second redirecting pulley  717 . However, the second one of the second pair of driving elements B 2  does pass by the second redirecting pulley  717  between its take-off point  720  of the second joint  507  and its pick-up point on the second pulley  706  of the first set of pulleys  601 . The driving element B 2  may be partially enclosed by the second redirecting pulley  717 . For example, the driving element B 2  may partially pass between the wings of the groove of the second redirecting pulley  717 , but the driving element B 2  is not seated in the groove of the second redirecting pulley  717 . 
     A take-off point is the point at which a driving element loses contact with a pulley. A pick-up point is the point at which a driving element first contacts a pulley. For a driving element passing directly from a first pulley to a second pulley, the take-off point of the driving element from the first pulley and the pick-up point of the driving element on the second pulley are points on a line which is tangential to both the first pulley and the second pulley, the take-off point being where that tangential line intersects the first pulley, and the pick-up point being where that tangential line intersects the second pulley. This is for the purposes of explanation only and treats as negligible the thickness of the driving element. Thus, in reality, the tangential line has a thickness equal to the thickness of the driving element, with the take-off point being where one side of the tangential line meets the first pulley, and the pick-up point being where the other side of the tangential line meets the second pulley. 
     The redirecting pulley  716  causes the driving element B 1  to wrap more fully around the second joint  507  than would happen if the redirecting pulley  716  was not there, thereby increasing the length of engagement between the driving element B 1  and the second joint  507 . Thus, the driving element B 1  has a greater travel around the second joint  507 , and is hence able to cause a larger rotation of the end effector element  502  about the second axis  512  than would be possible without the redirecting pulley  716 . The redirecting pulley  716  causes the pick-up point of the driving element B 1  on the second joint  507  to change relative to where it would have been without the redirecting pulley  716 . 
     The redirecting pulley  717  causes the driving element C 1  to wrap more fully around the third joint  513  than would happen if the redirecting pulley  717  was not there, thereby increasing the length of engagement between the driving element C 1  and the third joint  513 . Thus, the driving element C 1  has a greater travel around the third joint  513 , and is hence able to cause a larger rotation of the end effector element  503  about the second axis  512  than would be possible without the redirecting pulley  717 . The redirecting pulley  717  causes the pick-up point of the driving element C 1  on the third joint  513  to change relative to where it would have been without the redirecting pulley  717 . 
     The redirecting pulleys are each located towards the outside edge of the articulation, on opposing sides of the articulation. This is more easily seen on  FIG. 5A . As seen in  FIG. 6A , each redirecting pulley is located between the longitudinal axis of the articulation and the external profile of the articulation, on opposing sides of the articulation. Suitably, the diameter of each redirecting pulley is maximised for the space available. In other words, the redirecting pulley is as large as possible, whilst enabling one driving element to engage the pulley on one side of the pulley and another driving element to pass next to the pulley on the opposing side of the pulley without snagging, the pulley and the two driving elements being encapsulated within the profile of the articulation. 
     The first redirecting pulley  716  is located in a plane defined by the following three points: (i) the desired take-off point of driving element B 1  from the first pulley  705  of the first set of pulleys  601 , (ii) the desired pick-up point of driving element B 1  on the second joint  507 , and (iii) a point on the boundary of the articulation, the point being such that the first redirecting pulley  716  is encapsulated within the boundary of the articulation when located in the plane. Suitably, the first redirecting pulley  716  is as large as possible whilst still being located in this plane, encapsulated within the profile of the articulation, not impeding the path of the driving element C 2 , and enabling driving element B 1  to freely move around it. 
     The second redirecting pulley  717  is located in a plane defined by the following three points: (i) the desired take-off point of driving element C 1  from the second pulley  706  of the first set of pulleys  601 , (ii) the desired pick-up point of driving element C 1  on the third joint  513 , and (iii) a point on the boundary of the articulation, the point being such that the second redirecting pulley  717  is encapsulated within the boundary of the articulation when located in the plane. Suitably, the second redirecting pulley  717  is as large as possible whilst still being located in this plane, encapsulated within the profile of the articulation, not impeding the path of the driving element B 2 , and enabling driving element C 1  to freely move around it. 
     The desired take-off points and pick-up points are determined so as to allow the desired travel of the driving elements around the second and third joints so as to allow the desired maximum rotation of the end effector elements about the second axis. 
     The first and second redirecting pulleys are located in different planes. As can be seen in  FIG. 6A , those planes may be symmetrical about a plane which is perpendicular to the first axis  510 . Those planes may be rotationally symmetrical about a plane which is perpendicular to the first axis  510 . Specifically, those planes may be rotationally symmetrical about a line in a plane which is perpendicular to the first axis  510 . When the instrument is in the straight configuration illustrated in  FIG. 6A , those planes are rotationally symmetrical about the longitudinal axis of the shaft  511 . This is second order rotational symmetry. Those planes may be a reflection of each other in the plane which is perpendicular to the first axis  510 . In the example illustrated, the end effector elements  502  and  503  are rotationally symmetrical, and the paths of the driving elements about the joints  507  and  513  are rotationally symmetrical. Alternatively, the axes of the end effector elements  502  and  503  may be rotationally asymmetrical and/or the paths of the driving elements about the joints  507  and  513  may be asymmetrical. The paths of the driving elements about the joints  507  and  513  may be asymmetrical as a result of the joints having different diameters (to enable different tension ratios) and/or being positioned at different offsets from the centreline of the supporting body  509 . In any of these alternative examples, the first and second redirecting pulleys  716  and  717  would not be rotationally symmetric. They would have different sizes and/or different positions in order to cause the driving elements to have the desired take-off points and pick-up points as previously described. 
     Suitably, the whole pulley arrangement comprising the first set of pulleys, the second set of pulleys and the redirecting pulleys is symmetrical about a plane which is perpendicular to the first axis  510 . Specifically, a first partial arrangement comprising the first pulley of the first set of pulleys  705 , the first pulley of the second set of pulleys  701 , and the first redirecting pulley  716  is rotationally symmetrical to a second partial arrangement comprising the second pulley of the first set of pulleys  706 , the second pulley of the second set of pulleys  702 , and the second redirecting pulley  717  about a plane which is perpendicular to the first axis  510 . Suitably, the first partial arrangement is a reflection of the second partial arrangement in the mentioned plane which is perpendicular to the first axis  510 . The second pair of driving elements B 1 ,B 2  is constrained to move around the pulley arrangement in a rotationally symmetrically opposing manner to that in which the third pair of driving elements C 1 ,C 2  is constrained to move around the pulley arrangement. Since the pulley arrangement has the described symmetry, the second and third driving elements that are constrained to move symmetrically around the pulley arrangement also have the same symmetry. Thus, the path of the second pair of driving elements B 1 ,B 2  about the pulley arrangement is rotationally symmetrical to the path of the third pair of driving elements C 1 ,C 2  about the pulley arrangement. 
     In an exemplary implementation, the first and second redirecting pulleys are mounted on the supporting body  509 .  FIG. 10  illustrates the supporting body  509  and the redirecting pulleys in isolation. Each redirecting pulley is mounted to a surface of the supporting body  509  by a spindle. The spindle  1001  secures the first redirecting pulley  716  to the supporting body  509 . A spindle  1102  also secures the second redirecting pulley  717  to the supporting body  509 . As is more clearly illustrated in the view shown in  FIGS. 11A and 11B , the supporting body  509  has a bevelled surface  1101  onto which the first redirecting pulley  716  is mounted. The first redirecting pulley  716  has a mounting surface  1104  which faces the bevelled surface  1101  of the supporting body  509 . The mounting surface  1104  is flush with the bevelled surface  1101 . The first redirecting pulley has an opposing surface  1105  which opposes the mounting surface  1104 . The opposing surface  1105  is parallel to the mounting surface  1104 . The supporting body  509  has a further bevelled surface onto which the second redirecting pulley  717  is mounted by spindle  1103 . The second redirecting pulley  717  has a mounting surface  1106  which faces the bevelled surface  1102  of the supporting body  509 . The mounting surface  1106  is flush with the bevelled surface  1102 . The second redirecting pulley has an opposing surface  1107  which opposes the mounting surface  1102 . The opposing surface  1107  is parallel to the mounting surface  1102 . 
     The bevelled surfaces of the supporting body  509  are not parallel to the longitudinal axis of the supporting body. The bevelled surface  1101  of the supporting body  509  is located in a plane  1108  parallel to the plane  1109  in which the first redirecting pulley  716  is located. In other words, the bevelled surface  1101  is located in a plane  1108  parallel to the plane  1109  defined by the following three points: (i) the desired take-off point of driving element B 1  from the first pulley  705  of the first set of pulleys  601 , (ii) the desired pick-up point of driving element B 1  on the second joint  507 , and (iii) a point on the boundary of the articulation, the point being such that the first redirecting pulley  716  is encapsulated within the boundary of the articulation when located in the plane  1109 . The plane  1108  of the bevelled surface  1101  is offset from the plane  1109  defined by these points by half the width of the first redirecting pulley  716 , illustrated as d 1  in  FIGS. 11A and 11B . 
     The bevelled surface  1102  of the supporting body  509  is located in a plane  1110  parallel to the plane  1111  in which the second redirecting pulley  717  is located. In other words, the bevelled surface  1102  is located in a plane  1110  parallel to the plane  1111  defined by the following three points: (i) the desired take-off point of driving element C 1  from the second pulley  706  of the first set of pulleys  601 , (ii) the desired pick-up point of driving element C 1  on the third joint  513 , and (iii) a point on the boundary of the articulation, the point being such that the second redirecting pulley  717  is encapsulated within the boundary of the articulation when located in the plane  1111 . The plane  1110  of the bevelled surface  1101  is offset from the plane  1111  defined by these points by half the width of the second redirecting pulley  717 , illustrated as d 2  in  FIGS. 11A and 11B . Suitably, the first redirecting pulley  716  and the second redirecting pulley  717  are the same shape and size. In this case, d 1 =d 2 . 
     As discussed above, each redirecting pulley is mounted to the corresponding bevelled surface of the supporting body by a spindle. The spindle comprises a spindle body and a spindle head. The spindle body passes through a central aperture of the redirecting pulley. The central aperture is a through-hole which extends perpendicularly between the mounting surface and the opposing surface of the redirecting pulley. The spindle body passes through the central aperture of the redirecting pulley into a bore of the supporting body.  FIGS. 12 and 13  illustrate the supporting body  509  in isolation. An exemplary implementation of the bore of the supporting body is depicted in these figures. The bore is a recess in the supporting body which tapers to a point. The spindle body passes through the initial opening of the bore and securely lodges in the tapered section. The spindle head is bigger than the central aperture of the redirecting pulley, and thus is unable to pass through the central aperture of the redirecting pulley. Consequently, the spindle head secures the redirecting pulley flush against the bevelled surface of the supporting body. The spindle head contacts at least a portion of the opposing surface of the redirecting pulley, through which contact the redirecting pulley is retained against the bevelled surface. 
     Referring to  FIG. 14 , the diameter of the bore  1401  through the supporting body is greater than the diameter of the spindle body  1403 . The diameter of the central aperture  1402  of the redirecting pulley may be greater than the diameter of the spindle body  1403 . Thus, the spindle body may loosely fit through the central aperture of the redirecting pulley. The spindle body loosely fits through the bore of the supporting body except for in the region in which it is secured to the bore, in the example above the tapered section. Typically, the bore through the supporting body is created by drilling through the bevelled surface of the supporting body during manufacture. Since the opening of the bore in the bevelled surface has a greater diameter than the spindle body, the precision of the angle at which the supporting body is drilled to create the bore is not critical. The spindle sits in the bore at the correct angle for the redirecting pulley to sit flush with the bevelled surface. It would have been critical that the angle of the bore was drilled very accurately if the fit of the spindle body through the bore was a tight fit and was the means by which the redirecting pulley was caused to rotate about the redirecting pulley axis  718 , 719 . In that case the pulley would have been mounted tight on to the spindle which would have been mounted tight into the bore to stop the pulley from precessing. Thus, there would have been no room for manufacturing deviations in the angle at which the hole was drilled through the supporting body. In the described implementation, however, the redirecting pulley is caused to rotate about the redirecting pulley axis  718 , 719  as a result of the redirecting pulley being held flush against a bevelled surface of the supporting body  509  which is perpendicular to the redirecting pulley axis  718 , 719 . Thus, greater manufacturing variation in the accuracy of the angle at which the bore is drilled through the supporting body is acceptable in the described implementation. 
     The bevelled surfaces  1101  and  1102  of the supporting body are not parallel to each other. The bevelled surfaces may be symmetrical about a plane which is perpendicular to the first axis  510 . The bevelled surfaces may be rotationally symmetrical about a plane which is perpendicular to the first axis  510 . Specifically, the bevelled surfaces may be rotationally symmetrical about a line in a plane which is perpendicular to the first axis  510 . When the instrument is in the straight configuration illustrated in  FIG. 5A , the bevelled surfaces are rotationally symmetrical about the longitudinal axis of the shaft  511 . This is second order rotational symmetry. The bevelled surfaces may be a reflection of each other in the plane which is perpendicular to the first axis  510 . 
     Referring to  FIG. 15 , in one example, the supporting body  509  comprises grooves adjacent to each bevelled surface for seating a driving element. The supporting body comprises a groove  1501  adjacent the bevelled surface  1101  for seating the second one C 2  of the third pair of driving elements. The driving element C 2  is seated in the groove  1501  and partially enclosed by the first redirecting pulley  716 . The supporting body comprises a groove  1502  adjacent the bevelled surface  1102  for seating the second one B 2  of the second pair of driving elements. Groove  1502  is shown in  FIGS. 11A and 11B . The driving element B 2  is seated in the groove  1502  and partially enclosed by the second redirecting pulley  717 . 
     The first, second and third pairs of driving elements extend through the instrument shaft from the distal end of the shaft connected to the articulation to the proximal end of the shaft connected to a drive mechanism of the instrument interface.  FIGS. 17A and 17B  illustrate two views of the first, second and third pairs of driving elements extending from the described articulation to an exemplary instrument interface  1701 . In an exemplary implementation, the second and third pairs of driving elements overlap in the shaft so as to emerge from the proximal end of the shaft in a different arrangement to that at which they are in at the distal end of the shaft.  FIGS. 16A and 16B  illustrate cross-sections of the shaft depicting the positions of the driving elements. 
     Configuration (a) of  FIGS. 16A and 16B  show a cross-section of the shaft at the distal end of the shaft. In other words, configuration (a) shows the positions of the driving elements just as they have left the second set of pulleys  602 . The driving elements A 1  and A 2  are at opposing sides of the shaft after having left the first joint  506 . The driving elements C 1  and B 2  are adjacent each other on an opposing side of the shaft to the driving elements B 1  and C 2  which are also adjacent each other. The driving elements C 1  and B 2  are offset from the driving elements B 1  and C 2  about an axis  1601  which is transverse to the axis  1602  connecting driving elements A 1  and A 2 . This is a result of the offset axes of the two pulleys of the second set of pulleys. 
     Configuration (b) of  FIGS. 16A and 16B  show a cross-section of the shaft at the proximal end of the shaft. In other words, configuration (b) shows the positions of the driving elements as they are about to exit the shaft into the instrument interface. The first pair of driving elements A 1  and A 2  are on opposing sides of the shaft in a similar arrangement to their arrangement in configuration (a). The first pair of driving elements may be closer together, by virtue of them having moved slightly towards each other over the course of their extent through the shaft. In configuration (b), driving element B 1  is located on an opposing side of the shaft to its location in configuration (a). In configuration (b), driving element C 1  is located on an opposing side of the shaft to its location in configuration (a). To achieve this, driving element B 1  and driving element C 1  have not extended down the shaft parallel to the longitudinal axis of the shaft  511 . Instead, driving element B 1  and driving element C 1  have overlapped each other during their extent in the shaft. This overlapping occurs without the driving elements B 1  and C 1  clashing because of their offset positions in configuration (a) owing to the pulleys of the second set of pulleys  602  having offset axes. Driving element B 2  has moved a little in the shaft, but remained on the same side of the shaft as in configuration (a), so as to emerge at the proximal end of the shaft adjacent to driving element B 1 . Driving element C 2  has moved a little in the shaft, but remained on the same side of the shaft as in configuration (a), so as to emerge at the proximal end of the shaft adjacent to driving element C 1 . 
     The instrument interface comprises a further pulley arrangement around which the first, second and third pairs of driving elements are constrained to move. The driving elements A 1 , A 2 , B 1 , B 2 , C 1  and C 2  emerge at the proximal end of the shaft in a configuration which enables them to engage directly with components of the instrument interface. In one implementation, the driving elements emerge at the proximal end of the shaft as shown in configuration (b) in order to engage directly with the further pulley arrangement of the instrument interface described herein. Suitably, the first, second and third driving elements extend from the pulley arrangement at the distal end of the shaft to the instrument interface without wrapping around any intervening pulleys. Suitably, there are no intervening pulleys in the shaft around which the first, second and/or third pairs of driving elements are constrained to move. 
     As can be seen in  FIGS. 17A and 17B , the instrument interface is relatively flat. The instrument interface extends mostly in a central plane viewed head on in  FIG. 17A . Suitably, the instrument shaft  504  is rigidly attached to the instrument interface  1701 . The instrument shaft  504  does not rotate or otherwise move relative to the instrument interface  1701 . Suitably, the second axis  512  about which the end effector elements  502 ,  503  rotate is perpendicular to the central plane of the instrument interface. This is the case in the straight configuration of the instrument shown in  FIGS. 17A and 17B . Thus, in the straight configuration of the instrument, the jaws of the end effector are moveable in the central plane of the instrument interface. 
     A driving element may be a uniform component having the same shape and size along its length and constructed of the same material along its length. Alternatively, the driving element may be composed of different portions. In one example, the portion of the driving element which engages components of the instrument interface (such as pulleys and interface elements) is flexible. Similarly, the portion of the driving element which engages components of the distal end of the surgical instrument (such as the pulleys and joints in the articulation) is flexible. Between these two flexible portions are spokes  1702  illustrated in  FIGS. 17A and 17B . Thus, each pair of driving elements comprises two spokes and two flexible portions. Each pair of driving elements forms a loop. The loop comprises alternating spokes and flexible portions. The two spokes are predominantly or wholly enclosed in the instrument shaft. A distal flexible portion terminates at one end in the distal end of one of the spokes, and at the other end in the distal end of the other spoke. The distal flexible portion engages components of the articulation. A proximal flexible portion terminates at one end in the proximal end of one of the spokes, and at the other end in the proximal end of the other spoke. The proximal flexible portion engages components of the instrument interface. The spokes are stiffer than the flexible portions. Suitably, the spokes are rigid. The spokes may be hollow. Typically, the spokes have a larger diameter than the flexible portions. Thus, the flexible portions may be cables, and the spokes hollow tubes. The flexible portions may terminate where they meet the spokes. Alternatively, the spokes may encapsulate the material of the flexible portions. For example, the spokes may be rigid sheaths which cover flexible cables. 
     The spokes are stiffer than the flexible portions. Thus, by forming a pair of driving elements from spokes as well as flexible portions, the likelihood of the driving element stretching is reduced. For this reason, the proportion of each driving element which is a spoke is preferably maximised whilst ensuring that the spoke does not come into contact with components of the articulation or the instrument interface, and also that adjacent driving elements do not collide. The spokes are stronger than the flexible portions, and hence more resilient to compression and tension forces applied in any direction than the flexible portions. Thus, by incorporating the spokes, the driving element as a whole is stiffer and less likely to stretch. Thus, the lifetime of the driving element before it needs re-tensioning or replacing is extended. 
     In  FIG. 18  the spokes of the driving elements A 1 , A 2 , B 1  and C 1  are visible, labelled as A 1   s , A 2   s , B 1   s  and C 1   s  respectively.  FIG. 18  depicts a straight configuration of the surgical instrument in which the end effector  501  is aligned with the shaft  504 . As can be seen in  FIG. 18 , the distal flexible portion of driving element A 1  terminates in the spoke A 1   s  at a point  1801  along the longitudinal direction x of the shaft. The longitudinal direction x is the direction of the longitudinal axis  511  of the shaft. The distal flexible portion of driving element A 2  terminates in the spoke A 2   s  at a point  1802  along the longitudinal direction x of the shaft. The distal flexible portion of driving element B 1  terminates in the spoke B 1   s  at a point  1803  along the longitudinal direction x of the shaft. The distal flexible portion of driving element C 1  terminates in the spoke C 1   s  at a point  1804  along the longitudinal direction x of the shaft. The distal flexible portions of driving elements B 2  and C 2  terminate in their respective spokes further towards the proximal end of the shaft, as can be seen in  FIG. 17A . 
     As can be seen in  FIG. 18 , the longitudinal positions  1801 ,  1802 ,  1803  and  1804  at which the distal flexible portions of the driving elements terminate in the distal ends of the spokes are not coincident when the instrument is in the straight configuration depicted. Instead, the longitudinal positions  1801 ,  1802 ,  1803  and  1804  are offset from each other. In other words, the distal ends of the spokes of the driving elements are offset along the longitudinal direction of the shaft when the instrument is in the straight configuration. Specifically, the distal ends of adjacent spokes are offset along the longitudinal direction of the shaft in the straight configuration. The distal ends of spokes which are not adjacent to each other may be coincident along the longitudinal direction of the shaft in the straight configuration. For example, in  FIG. 18 , the non-adjacent spokes A 1   s  and A 2   s  both terminate at the same point  1801 , 1802  along the longitudinal direction of the shaft. Suitably, the distal ends of the spokes of the driving elements are offset along the longitudinal direction of the shaft in every configuration of the surgical instrument. Specifically, suitably, the distal ends of adjacent spokes are offset along the longitudinal direction of the shaft in every configuration of the surgical instrument. 
     As previously discussed in relation to  FIGS. 16A and 16B , the driving elements do not all extend parallel to each other in the shaft in an implementation in which driving elements pass directly from the pulley arrangement at the distal end of the shaft to the pulley arrangement in the instrument interface without moving around any intervening pulleys. The first one of the first pair of driving elements A 1  extends substantially parallel to the second one of the first pair of driving elements A 2  in the shaft. Driving elements A 1  and A 2  may move slightly closer to each other over the course of the length of the shaft in the direction from the articulation to the instrument interface. The first one of the second pair of driving elements B 1  extends at an angle to the second one of the second pair of driving elements B 2  in the shaft. Driving element B 1  also extends at an angle to driving elements A 1 , A 2 , C 1  and C 2  down the shaft. Driving element B 2  extends at an angle to driving elements A 1 , A 2 , C 1  and C 2  down the shaft. The first one of the third pair of driving elements C 1  extends at an angle to the second one of the third pair of driving elements C 2  in the shaft. Driving element C 1  also extends at an angle to driving elements A 1 , A 2 , B 1  and B 2  down the shaft. Driving element C 2  extends at an angle to driving elements A 1 , A 2 , B 1  and B 2  down the shaft. 
     The longitudinal positions of the distal ends of the spokes are selected such that the spokes do not collide when the instrument is being articulated. Since the spokes have a larger diameter than the flexible portions, although the flexible portions can extend down the length of the shaft without colliding the spokes may not be able to. Suitably, the longitudinal positions of the distal ends of the spokes in the straight configuration of the instrument are such that for any configuration of the end effector, no portion of any driving element contacts a portion of another driving element. Suitably, the positions of the proximal and distal ends of the spokes in the straight configuration are selected so as to maximise the spoke length whilst satisfying the condition that the driving elements will not contact. The spokes are stiffer than the flexible portions. Thus, this maximises the stiffness of the driving elements whilst enabling them to wrap around components in the articulation and instrument interface. This maximises the strength of the driving elements whilst enabling them to wrap around components in the articulation and instrument interface. 
     In  FIG. 19A  the spokes of the driving elements A 1 , A 2 , B 1  and C 1  are visible, labelled as A 1   s , A 2   s , B is and C is respectively.  FIG. 19A  depicts a non-straight configuration of the surgical instrument in which the end effector  501  is not aligned with the shaft  504 . As can be seen in  FIG. 19A , the proximal flexible portion of driving element A 1  terminates in the spoke A 1   s  at a point  1901  along the longitudinal direction x of the shaft. The longitudinal direction x is the direction of the longitudinal axis  511  of the shaft. The proximal flexible portion of driving element A 2  terminates in the spoke A 2   s  at a point  1904  along the longitudinal direction x of the shaft. The proximal flexible portion of driving element B 1  terminates in the spoke B 1   s  at a point  1902  along the longitudinal direction x of the shaft. The proximal flexible portion of driving element C 1  terminates in the spoke Cis at a point  1903  along the longitudinal direction x of the shaft. The proximal flexible portions of driving elements B 2  and C 2  terminate in their respective spokes further towards the distal end of the shaft, as can be seen in  FIG. 17A . The spokes may terminate in the proximal flexible portions inside the shaft, as is the case in the example shown for driving elements B 2  and C 2 . The spokes may terminate in the proximal flexible portions inside the instrument interface, as is the case in the example shown for driving elements A 1 , A 2 , B 1  and C 1 . Some spokes may terminate in the proximal flexible portions inside the shaft and some spokes may terminate in the proximal flexible portions inside the instrument interface. In the design of the instrument interface depicted in  FIG. 19A , the driving elements B 2  and C 2  engage pulleys as they enter the instrument interface from the shaft, thus the spokes of driving elements B 2  and C 2  terminated in their proximal flexible portions in the shaft (not shown) to enable the proximal flexible portions to engage the pulleys. Driving elements A 1 , A 2 , B 1  and C 1  all extend some distance into the instrument interface before engaging with components of the instrument interface, thus the spokes of driving elements A 1 , A 2 , B 1  and C 1  are able to extend into the instrument interface. 
     As can be seen in  FIG. 19A , the longitudinal positions  1901 ,  1902 ,  1903  and  1904  at which the proximal flexible portions of the driving elements terminate in the proximal ends of the spokes are not coincident. Instead, the longitudinal positions  1901 ,  1902 ,  1903  and  1904  are offset from each other. In other words, the proximal ends of the spokes of the driving elements are offset along the longitudinal direction of the shaft when the instrument is in the configuration shown. Suitably, the proximal ends of the spokes of the driving elements are offset along the longitudinal direction of the shaft for the straight configuration of the instrument. Specifically, the proximal ends of adjacent spokes are offset along the longitudinal direction of the shaft in the straight configuration. The proximal ends of spokes which are not adjacent to each other may be coincident along the longitudinal direction of the shaft in the straight configuration. For example, in  FIG. 19A , the non-adjacent spokes B 1   s  and C 1   s  both terminate at the same point  1902 , 1903  along the longitudinal direction of the shaft. Suitably, the proximal ends of the spokes of the driving elements are offset along the longitudinal direction of the shaft in every configuration of the surgical instrument. Specifically, suitably, the distal ends of adjacent spokes are offset along the longitudinal direction of the shaft in every configuration of the surgical instrument. 
     The longitudinal positions of the proximal ends of the spokes are selected such that the spokes do not collide when the instrument is being articulated. Suitably, the longitudinal positions of the proximal ends of the spokes in the straight configuration of the instrument are such that for any configuration of the end effector, no portion of any driving element contacts a portion of another driving element. 
     Each pair of driving elements engages a single instrument interface element in the instrument interface. Each driving element engages an instrument interface element in the instrument interface. In the example illustrated in  FIGS. 19A, 19B and 19C , each driving element engages its own instrument interface elements. A single instrument interface element drives a pair of driving elements. Each driving element is driven independently by a single instrument interface. 
     In alternative arrangements, there may be a compound driving motion in which more than one instrument interface element drives a single driving element, a single instrument interface element drives more than one pair of driving elements, or a plurality of instrument interface elements collectively drive a plurality of driving elements. 
       FIG. 19B  illustrates a first instrument interface element  1905  which engages the first pair of driving elements A 1 ,A 2 . A second instrument interface element  1906  engages the second pair of driving elements B 1 ,B 2 . A third instrument interface element  1907  engages the third pair of driving elements C 1 ,C 2 . Suitably, each driving element is secured to its associated instrument interface element. In other words, each driving element is fast with its associated instrument interface element. 
     The instrument interface  1701  has a significantly larger profile than the instrument shaft  504 . Typically, the instrument shaft has a circular cross-section having a diameter of less than or the same as 5 mm, whereas a corresponding cross-section through the instrument interface may be larger than this. The instrument interface comprises an internal portion and an external portion. The internal portion is bounded by the dotted line  1950  (shown in  FIGS. 19A and 19B ). The external portion is the remainder of the instrument interface which is not in the internal portion. The internal portion is within the projected profile of the shaft. In other words, the internal portion is the part of the instrument interface that would have been encompassed had the profile of the shaft continued within the instrument interface. The external portion is outside of the projected profile of the shaft. In the example illustrated, the shaft has a constant circular cross-section, and hence the internal portion is a cylinder having the same circular cross-section as the shaft, and having the same longitudinal axis  511  as the shaft. In other words, the internal portion is an extrapolation of the constant cross-section of the shaft in the instrument interface. The internal portion  1950  is shown from the side in  FIG. 19A  and from the top in  FIG. 19B . 
     The instrument interface elements  1905 ,  1906  and  1907  are dispersed across the width of the instrument interface as shown in  FIG. 19B . In the arrangement depicted in  FIG. 19B , one instrument interface element  1905  is within the internal portion  1950  of the instrument interface. Specifically, the part of the instrument interface element  1905  which engages the driving element is within the internal portion  1950  of the instrument interface. The instrument interface element  1905  as a whole may be substantially within the internal portion  1950  of the instrument interface, as shown in  FIG. 19B . The instrument interface element  1905  as a whole may be wholly within the internal portion  1950  of the instrument interface. Suitably, the instrument interface element  1905  is aligned with the longitudinal axis  511  of the shaft  504 . In an exemplary arrangement, only one instrument interface element is located within the internal portion of the instrument interface. The remainder of the instrument interface elements  1906 ,  1907  are within the external portion of the instrument interface. These other instrument interface elements  1906 ,  1907  are located on either side of the aligned instrument interface element  1905 . Specifically, the other instrument interface elements  1906 ,  1907  are located on either side of the aligned instrument interface element  1905  in a direction perpendicular to the longitudinal axis of the shaft  511 . The instrument interface elements  1906  and  1907  are not aligned with the longitudinal axis  511  of the shaft  504 . The instrument interface elements  1906  and  1907  are constrained to move parallel to the longitudinal axis  511  of the shaft  504 , since they are constrained to move along rails  1929  and  1930  respectively. 
     Instrument interface element  1905  engages a first pair of driving elements A 1 , A 2 . As can be seen in  FIG. 19A , between the proximal end of the shaft and the instrument interface element  1905 , the pair of driving elements A 1 , A 2  lie wholly within the internal portion  1950 . Between the proximal end of the shaft and the instrument interface element  1905 , the pair of driving elements A 1 , A 2  lie wholly parallel to the longitudinal axis of the shaft  511 . Suitably, there are no intervening pulleys or other structures in the instrument interface around which the pair of driving elements A 1 , A 2  is constrained to move between the proximal end of the shaft and the instrument interface element  1905 . Suitably, only instrument interface element  1905  engages its pair of driving elements A 1 , A 2  in the internal portion  1950  of the instrument interface. 
     Instrument interface element  1906  engages a second pair of driving elements B 1 , B 2 . The instrument interface element  1906  engages the second pair of driving elements B 1 , B 2  in the external portion of the instrument interface. 
     Instrument interface element  1907  engages a third pair of driving elements C 1 , C 2 . The instrument interface element  1907  engages the third pair of driving elements C 1 , C 2  in the external portion of the instrument interface. 
     A pulley arrangement is used to shift the driving elements over to engage with the instrument interface elements which are in the external portion. Each pair of driving elements engages a first pair of pulleys to shift it over from the proximal end of the shaft  504  to its respective instrument interface element, and a second pair of pulleys to shift it back from alignment with the instrument interface element to alignment with the shaft  504 . 
     In the arrangement shown, the second pair of driving elements B 1 ,B 2  emerges from the proximal end of the shaft in a direction aligned with the shaft. The driving elements B 1 ,B 2  do not run exactly parallel to the longitudinal axis  511  of the shaft  504  as a result of the direction changes described with respect to  FIGS. 16A and 16B . The second pair of driving elements B 1 , B 2  is then constrained to move around pulley pair  1908  and  1909  to shift it from where it emerges from the shaft  504  to engagement with the second instrument interface element  1906 . The second pair of driving elements B 1 , B 2  emerges from the pulley pair  1908  and  1909  in a direction parallel to and offset from the direction that the second pair of driving elements B 1 , B 2  emerges from the proximal end of the shaft. The second pair of driving elements B 1 ,B 2  is constrained to move around pulley pair  1910  and  1911  to shift it from alignment with the second instrument interface element  1906  to alignment with the shaft  504 . 
     In the arrangement shown, the third pair of driving elements C 1 , C 2  emerges from the proximal end of the shaft in a direction aligned with the shaft. The driving elements C 1 ,C 2  do not run exactly parallel to the longitudinal axis  511  of the shaft  504  as a result of the direction changes described with respect to  FIGS. 16A and 16B . The third pair of driving elements C 1 ,C 2  is then constrained to move around pulley pair  1912  and  1913  to shift it from where it emerges from the shaft  504  to engagement with the third instrument interface element  1907 . The third pair of driving elements C 1 , C 2  emerges from the pulley pair  1912  and  1913  in a direction parallel to and offset from the direction that the third pair of driving elements C 1 , C 2  emerges from the proximal end of the shaft. The third pair of driving elements C 1 ,C 2  is constrained to move around pulley pair  1914  and  1915  to shift it from alignment with the third instrument interface element  1907  to alignment with the shaft  504 . 
     In the arrangement shown in  FIGS. 19A, 19B and 19C , pair of driving elements A 1 , A 2  engage with the first instrument interface element  1905  which is within the internal portion. Pair of driving elements A 1 , A 2  drive rotation of the articulation, and hence the end effector, about the first axis  510  (see  FIG. 5A ). There is a smaller range of motion about first joint  506  than there is about second joint  507  and third joint  513 . As described below, the first instrument interface element  1905  is linearly displaceable through a maximum distance d 1  minus the length of the first instrument interface element  1905 , which is smaller than the maximum displacement of the second instrument interface element  1906  d 2  minus the length of the second instrument interface element  1906 , and smaller than the maximum displacement of the third instrument interface element  1907  d 3  minus the length of the third instrument interface element  1907 . Since motion about the first joint  506  is controlled via a shorter range of travel of the instrument interface element than the second and third joints, greater sensitivity of that motion is preferred. Cables may slip and/or stretch over pulleys. Thus, the simplest cabling scheme is preferably utilised for transferring motion between the first instrument interface element and the first joint  506 . By locating the first instrument interface element  1905  within the internal portion on the longitudinal axis of the shaft  511 , the first pair of driving elements is not constrained to pass around any intervening pulleys between the first joint  506  and the first instrument interface element  1905 . 
     Each instrument interface element is displaceable within the instrument interface. Since each instrument interface element is secured to a corresponding pair of driving elements, a displacement of the instrument interface element is transferred to a displacement of the pair of driving elements. Suitably, each instrument interface element is displaceable along the same line as the line of the pair of driving elements that it is secured to. Each instrument interface element engages with a corresponding drive assembly interface element of the robot arm. Thus, displacement of the instrument interface element is driven by the robot arm. In this way, the robot arm drives the pairs of driving elements. 
     Each pair of driving elements engages with an instrument interface element in the instrument interface. The pair of driving elements also engages with a tensioning mechanism and an alignment mechanism. When manufacturing the instrument, the tensioning mechanism is used to achieve a desired tension in the pair of driving elements. The alignment mechanism is used to set the instrument interface elements to a predetermined alignment position in the longitudinal direction of the shaft when the end effector has a predetermined configuration. Each instrument interface element has a displacement range over which it is displaceable. The predetermined alignment position may be the midpoint of the displacement range for each instrument interface element. The predetermined configuration of the end effector may be the straight configuration, in which the end effector elements are closed together (for example the jaws are closed), and the longitudinal axis of the articulation and the longitudinal axis of the end effector are aligned with the longitudinal axis of the shaft  511 . By setting the instrument interface elements to a predetermined alignment position when the end effector has a predetermined configuration, when changing instruments during an operation, the time taken to set up the new instrument ready for use may be reduced. In practice, when an instrument is removed from the robot arm, the robot arm assembly may be configured to go to an arrangement in which it is ready to receive the instrument interface elements in the predetermined alignment position. For example, the robot arm assembly interface elements may go to a default position in which they are arranged to receive each of the instrument interface elements at the midpoint of their displacement range. Then, the instrument is manually put in the predetermined configuration and then slotted into the robot arm. For example, the technician moves the articulation and end effector into the straight configuration and then slots the instrument into the robot arm. Because it is known that the instrument interface elements have the predetermined alignment position when the instrument is in the predetermined configuration, the instrument interface elements engage directly with the robot arm assembly interface elements. The control system does not need to perform an additional calibration or software setup procedure in order to map the position and orientation of the end effector, because it is known that the end effector is in the predetermined configuration. 
     The following describes tensioning and alignment mechanisms which are independent of each other. By isolating the tensioning mechanism from the alignment mechanism the process by which the desired tension and desired alignment are achieved is simplified. Thus, the time taken to achieve the desired tension and desired alignment during manufacture is reduced. 
       FIGS. 19A, 19B and 19C  illustrate a tensioning mechanism utilising pulleys. Each pair of driving elements is independently tensioned. Each pair of driving elements is constrained to move around a pulley which is displaceable.  FIGS. 19A, 19B and 19C  depict two different exemplary pulley arrangements for tensioning the pairs of driving elements. In both examples, the pulley is linearly displaceable. 
     Referring firstly to the tensioning mechanism shown for the pairs of driving elements B 1 ,B 2  and C 1 ,C 2 . Taking pair of driving elements B 1 ,B 2  first, pulley  1911  is used to tension B 1 ,B 2 . Pulley  1911  is linearly displaceable along a displacement axis  1920  which is parallel to the longitudinal axis  511  of the shaft. The displacement axis  1920  is offset from the longitudinal axis  511  of the shaft. Displacement axis  1920  is shown in  FIG. 19B . The tensioning pulley  1911  is mounted to a block  1918  which is slideable along a rail  1919 . Sliding the block  1918  along the rail  1919  causes the pulley  1911  to displace along the displacement axis  1920 . When the block  1918  is moved away from the shaft, the tension of the second pair of driving elements B 1 ,B 2  increases. When the block  1918  is moved towards the shaft, the tension of the second pair of driving elements B 1 ,B 2  decreases. Any suitable mechanism may be used to move the block. For example, a screw adjustment mechanism may be used.  FIGS. 19A, 19B and 19C  show a screw adjustment mechanism in which screw  1921  is threaded into block  1918 . This is most clearly seen on  FIG. 19A . The screw  1921  is constrained by portion  1922  of the instrument interface such that it is able to rotate but not able to be displaced linearly. Thus, when the screw is rotated, the screw thread engages with the thread inside the block  1918  causing the block and hence the pulley  1911  to displace linearly. When the screw  1921  is tightened, the pulley  1911  moves in one linear direction. When the screw  1921  is loosened, the pulley  1911  moves in the opposing linear direction. The tensioning mechanism for driving elements C 1 ,C 2  depicted in  FIGS. 19A, 19B and 19C  works in a corresponding manner to that described with relation to driving elements B 1 ,B 2 . 
     Referring now to the tensioning mechanism shown for the first pair of driving elements A 1 ,A 2  in  FIG. 19A . Pulley  1923  is used to tension A 1 ,A 2 . Pulley  1923  is linearly displaceable along a displacement axis  1924 . Displacement axis  1924  is at an angle to the longitudinal axis  511  of the shaft. Suitably, the displacement axis  1924  may be at a 45° angle to the longitudinal axis  511  of the shaft. The tensioning pulley  1923  is mounted to a block  1925  which is captive in a socket  1926  of the instrument interface. The block  1925  and tensioning pulley  1923  are able to slide through the socket  1926 . Sliding the block  1925  through the socket  1926  causes the pulley to displace along the displacement axis  1924 . When the block  1925  is slid further into the socket, the tension of the first pair of driving elements A 1 ,A 2  increases. When the block  1925  is slid out of the socket, the tension of the first pair of driving elements A 1 ,A 2  decreases. Any suitable mechanism may be used to move the block  1925 . For example, a screw adjustment mechanism as described above with respect to block  1918  may be used. Since the first pair of driving elements A 1 ,A 2  wrap almost fully around tensioning pulley  1923  such that they run almost parallel to each other, a greater tension is applied per unit displacement of the tensioning pulley compared to the tensioning mechanism described with respect to the second and third pairs of driving elements. 
     Although  FIGS. 19A, 19B and 19C  show the first pair of driving elements using the angled tensioning mechanism, and the second and third pairs of driving elements using the linear tensioning mechanism, any pair of driving elements may be tensioned using any suitable mechanism as long as that mechanism packages into the instrument interface. 
       FIG. 20A  illustrates an alternative tensioning mechanism. Each pair of driving elements terminates in a lug  1927  of an instrument interface element (see  FIG. 19C ).  FIG. 20A  illustrates an alternative arrangement of lug  1927  to that shown in  FIG. 19C . In the arrangement of  FIG. 20A , the lug is utilised in the tensioning mechanism. The lug of  FIG. 20A  has a pair of lug elements  2001  and  2002 . One driving element of a pair of driving elements terminates in one lug element, and the other driving element of the pair of driving elements terminates in the other lug element. A first one of the third pair of driving elements C 1  has been depicted as terminating in lug element  2001 , and the second one of the third pair of driving elements C 2  has been depicted as terminating in lug element  2002 . The pair of lug elements are coupled so as to be linearly displaceable with respect to each other. Suitably, the pair of lug elements are linearly displaceable along the direction y of the pair of driving elements to which they are attached. Suitably, the pair of lug elements are displaceable along a displacement axis that is parallel to and offset from the longitudinal axis  511  of the shaft. The lug elements  2001  and  2002  may be coupled by any suitable mechanism which is able to move the lug elements relative to each other along the displacement axis. For example, the lug elements may be coupled together by a screw  2003 . The screw  2003  is captive in a first lug element  2002  and constrained by the first lug element  2002  so as to prevent the screw  2003  from displacing linearly with respect to the first lug element  2002 . For example, as shown in  FIG. 20A , the screw may pass through a hole in the first lug element in which it is able to rotate, and be constrained from linearly displacing through the first lug element by two portions  2004  and  2005  which have a larger diameter than the hole through the first lug element. The screw  2003  is threaded through the second lug element  2001 . Thus, the lug elements  2001  and  2002  displace linearly towards each other when the screw is tightened, and displace linearly away from each other when the screw is loosened. 
       FIG. 20B  illustrates a further alternative tensioning mechanism.  FIG. 20B  illustrates a further alternative arrangement of lug  1927  to that shown in  FIG. 19C . In the arrangement of  FIG. 20B , the lug is utilised in the tensioning mechanism. The lug of  FIG. 20B  has a pair of lug elements  2006  and  2007 . One driving element of a pair of driving elements terminates in one lug element, and the other driving element of the pair of driving elements terminates in the other lug element. A first one of the third pair of driving elements C 1  has been depicted as terminating in lug element  2006 , and the second one of the third pair of driving elements C 2  has been depicted as terminating in lug element  2007 . The pair of lug elements are coupled so as to be linearly displaceable with respect to each other. Suitably, the pair of lug elements are linearly displaceable along the direction y of the pair of driving elements to which they are attached. Suitably, the pair of lug elements are displaceable along a displacement axis that is parallel to and offset from the longitudinal axis  511  of the shaft. The lug elements  2006  and  2007  may be coupled by any suitable mechanism which is able to move the lug elements relative to each other along the displacement axis. For example, the lug elements may be coupled together by a screw  2008 . The screw  2008  is captive in a first lug element  2007  and constrained by the first lug element  2007  so as to prevent the screw  2008  from displacing linearly with respect to the first lug element  2007 . For example, as shown in  FIG. 20B , the screw may pass through a hole in the first lug element in which it is able to rotate, and be constrained from linearly displacing through the first lug element by two portions  2009  and  2010  which have a larger diameter than the hole through the first lug element. The screw  2008  is threaded through the second lug element  2006 . Thus, the lug elements  2006  and  2007  displace linearly towards each other when the screw is tightened, and displace linearly away from each other when the screw is loosened. 
     Referring to  FIG. 19C , each instrument interface element  1905 ,  1906  and  1907  is linearly displaceable parallel to the longitudinal axis of the shaft  511 . The instrument interface element may be slideable along a linear rail. For example, first instrument interface element  1905  is slideable along rail  1928 , second instrument interface element  1906  is slideable along rail  1929 , and third instrument interface element  1907  is slideable along rail  1930 . Each instrument interface element can be displaced over a displacement range between a minimum displacement position and a maximum displacement position. For example, the minimum and maximum displacement positions may be determined by the ends of the rail along which the instrument interface element slides in the longitudinal direction x of the shaft. The minimum and maximum displacement positions are labelled  1931  and  1932  on  FIGS. 19B and 19C  for the second and third instrument interface elements  1906  and  1907 . The minimum and maximum displacement positions are labelled  1931  and  1943  on  FIG. 19B  for the first instrument interface element  1905 . The first instrument interface element is linearly displaceable through a maximum distance d 1  minus the length of the first instrument interface element in the direction x. The second instrument interface element is linearly displaceable through a maximum distance d 2  minus the length of the second instrument interface element in the direction x. The third instrument interface element is linearly displaceable through a maximum distance d 3  minus the length of the third instrument interface element in the direction x. Suitably d 1 &lt;d 2  and d 1 &lt;d 3 . Suitably, d 2 =d 3 . 
     Suitably, in the straight configuration of the instrument in which the end effector is aligned with the shaft, the first, second and third instrument interface elements  1905 ,  1906  and  1907  are all located in the same plane perpendicular to the longitudinal axis of the shaft. Alternatively, in the straight configuration of the instrument, the first instrument interface element  1905  may be centred in a different plane to the plane in which the second and third instrument interface elements  1906 ,  1907  are centred. This is because the midpoint of the travel of the first instrument interface element  1905  over d 1  is offset from the midpoint of the travel of the second and third instrument interface elements  1906 ,  1907  over d 2 ,d 3 . 
     Suitably, each instrument interface element comprises a body  1933 ,  1934 ,  1935  and a lug  1927 ,  1936 ,  1937 . The body  1933 ,  1934 ,  1935  is linearly displaceable between the minimum displacement position and the maximum displacement position of the instrument interface element. The pair of driving elements which engages the instrument interface element is secured to the lug of the instrument interface element. The lug is linearly displaceable within the body parallel to the direction along which the body is displaceable. Suitably, the lug is linearly displaceable along the longitudinal direction x of the shaft parallel to the longitudinal axis  511  of the shaft. The alignment mechanism adjusts the displacement position of the body without displacing the lug. For example, the alignment mechanism may comprise a screw adjustment mechanism coupled to the body and lug which enables the body to move without moving the lug.  FIG. 19C  depicts such a screw adjustment mechanism. The body  1933 ,  1935  comprises a slot  1938 ,  1939  aligned with the direction along which the body is displaceable. A screw  1940 ,  1941  is threaded into the lug through the slot  1938 ,  1939 . The screw  1940 ,  1941  is constrained to slide along the slot. For example, the screw head may be too large to pass through the slot and the screw body a loose fit through the slot. Thus, the when the screw is loose, the body is displaceable relative to the lug along the width of the slot. When the screw is tight, the body is held fast with the lug. Thus, the relative position of the body and the lug can be adjusted by the width of the slot. 
     The following describes steps to be carried out during manufacture following assembly of the instrument in order to set the tension of the driving elements and the alignment of the instrument interface elements. 
     Initially, the instrument interface is loosened from the driving elements. The instrument interface elements are set to the alignment position. For example, if the alignment position is with each instrument interface element at the mid-point of its travel over its displacement range, then the instrument interface elements are aligned to these positions. This initial step may be a rough alignment of the instrument interface elements to their alignment positions. Alternatively, this initial step may not be carried out. Next, the end effector is placed in the predetermined configuration. Next, the pairs of driving elements are tensioned. This may be done using any of the tensioning mechanisms described herein, for example by sliding a tensioning pulley along a rail or through a socket, or by displacing a pair of lug elements. Once tensioned, the displacement position of the instrument interface element is then set to the predetermined alignment position using the alignment mechanism. For example, in the implementation shown in  FIGS. 19A, 19B and 19C , the screw  1940 ,  1941  is loosened, and the body  1933 ,  1935  of the instrument interface element displaced along the rail  1930 ,  1929  relative to the lug  1938 ,  1937  until the body of the instrument interface element is in the predetermined alignment position. The end effector may be held in the predetermined configuration whilst the pairs of driving elements are tensioned. Alternatively, or additionally, the end effector may be returned to the predetermined configuration after the pairs of driving elements have been tensioned. The screw is then tightened. 
       FIGS. 21A, 21B and 21C  illustrates a drive assembly interface  2100 . The drive assembly interface is at the terminal end of the terminal link of the robot arm. That terminal link is connected to the link next to it by a roll joint. The roll joint permits the terminal link to rotate about a longitudinal axis  2104  of the terminal link. Drive assembly interface  2100  comprises drive assembly interface elements  2101 ,  2102  and  2103 . The drive assembly interface elements are configured to receive instrument interface elements  1905 ,  1906  and  1907 . First drive assembly interface element  2102  is configured to receive first instrument interface element  1905 . Second drive assembly interface element  2101  is configured to receive second instrument interface element  1906 . Third drive assembly interface element  2102  is configured to receive third instrument interface element  1907 . 
     Each drive assembly interface element is displaceable along a direction parallel to the longitudinal axis  2104  of the drive assembly. Each drive assembly interface element is displaceable over a displacement range. When the instrument interface is seated in the drive assembly, as shown in  FIG. 24 , each drive assembly interface element is displaceable in the same direction as the direction in which the instrument interface element that it engages with is displaceable in. 
     The first drive assembly interface element  2102  engages the first instrument interface element  1905  on the longitudinal axis  2104  of the drive assembly. Thus, the first drive assembly interface element  1905  drives the first instrument interface element  1905  along the longitudinal axis of the drive assembly, and hence along the longitudinal axis of the terminal link of the robot arm. Suitably, of all the drive assembly interface elements in the drive assembly, only the first drive assembly interface element  2102  is displaceable along the longitudinal axis  2104  of the terminal link. The first instrument interface element  1905  drives the first pair of driving elements A 1 , A 2  to drive rotation of the distal end of the instrument about the first axis  510  which is perpendicular to the instrument shaft axis  511 . When the instrument interface  1701  is seated in the drive assembly  2100 , the longitudinal axis  511  of the instrument shaft is parallel to the longitudinal axis  2104  of the terminal link. Suitably, the longitudinal axis  511  of the instrument shaft is coincident with the longitudinal axis  2104  of the terminal link. 
     The second drive assembly interface element  2101  engages the second instrument interface element  1906  on an axis parallel to but offset from the longitudinal axis  2104  of the drive assembly. The second drive assembly interface element  2101  is displaceable along this axis so as to drive the second instrument interface element  1906  along this axis. The second instrument interface element  1906  drives the second pair of driving elements B 1 , B 2  to drive rotation of an end effector element  502  about the second joint  507 . 
     The third drive assembly interface element  2103  engages the third instrument interface element  1907  on an axis parallel to but offset from the longitudinal axis  2104  of the drive assembly. The third drive assembly interface element  2103  is displaceable along this axis so as to drive the third instrument interface element  1907  along this axis. The third instrument interface element  1907  drives the third pair of driving elements C 1 , C 2  to drive rotation of the end effector element  503  about the third joint  513 . 
     Suitably, the drive assembly interface elements releasably engage the corresponding instrument interface elements. 
       FIGS. 21A, 21B and 21C  illustrate an exemplary mechanism for driving the linear displacement of the drive assembly interface elements within the drive assembly. Each drive assembly interface element  2101 ,  2102 ,  2103  is driven by a respective threaded driveshaft  2105 ,  2106 ,  2107 . Suitably, the first, second and third drive assembly interface elements are independently driven by the driveshafts. A guide structure constrains each drive assembly interface element, thereby preventing the drive assembly interface element from rotating as the corresponding driveshaft is rotated. For example, a guiderail constrains each drive assembly interface element, thereby preventing the drive assembly interface element from rotating as the corresponding driveshaft is rotated. The guiderails constrain the drive assembly interface elements such that the only motion that the drive assembly interface elements are permitted to do is to move linearly parallel to the guiderail. The drive assembly interface elements may, for example, slide along the guiderails. In the implementation illustrated, the first drive assembly interface element  2102  and the second drive assembly interface element  2101  are both constrained by the same guiderail  2108 . The third drive assembly interface element  2103  is constrained by a different guiderail  2109 . In an alternative arrangement, the first and third drive assembly interface elements  2102  and  2103  are both constrained by the same guiderail  2109 . The second drive assembly interface element  2101  is constrained by the guiderail  2108 . In yet a further alternative arrangement, each drive assembly interface element is constrained by its own guiderail. In another example, the guide structure is a guide slot, which constrains the motion of the drive assembly interface element such that it is only able to move linearly parallel to the guide slot. As with the guiderails, the guide slots are parallel to the longitudinal direction  2104  of the drive assembly interface. The first drive assembly interface element  2102  is linearly displaceable through a maximum distance s 1 . The second drive assembly interface element  2101  is linearly displaceable through a maximum distance s 2 . The third drive assembly interface element  2103  is linearly displaceable through a maximum distance s 3 . Suitably s 1 &lt;s 2  and s 1 &lt;s 3 . Suitably, s 2 =s 3 . 
     Suitably, in one configuration of the drive assembly, the first, second and third drive assembly interface elements  2101 ,  2102  and  2103  are all located in the same plane perpendicular to the longitudinal axis  2104  of the terminal link. This configuration is the one depicted in  FIGS. 21A, 21B and 21C . All the drive assembly interface elements are centred on a single cross section of the terminal link perpendicular to the longitudinal axis  2104 . Suitably, in this configuration, each drive assembly interface element is at the midpoint of its linear displacement over its displacement range Suitably, this configuration is the default configuration that the assembly interface adopts when an instrument has been removed from the robot arm. This configuration is arranged to receive the instrument interface elements in their predetermined alignment positions, described above. The predetermined alignment position may be the position at which each drive assembly interface element is at the midpoint of its travel. The predetermined alignment position may be the position at which the centre points of all the drive assembly interface elements lie on the same plane. 
     In all configurations of the drive assembly, the second and third drive assembly interface elements  2101  and  2103  are both centred on a second plane which is perpendicular to the plane that the first, second and third drive assembly interface elements  2102 ,  2102  and  2103  are all centred on in the configuration depicted in  FIGS. 21A, 21B and 21C . This second plane does not intersect the longitudinal axis  2104  of the terminal link. In all configurations, the first drive assembly interface element  2102  is centred on a third plane which is parallel to but offset from the second plane. 
     The drive assembly depicted in  FIGS. 21A, 21B and 21C  may drive the instrument interface depicted in  FIGS. 19A, 19B and 19C  which in turn drives the first, second and third joints depicted in  FIGS. 5A and 5B , such that the first drive assembly interface element  2102  drives the first joint  506 , the second drive assembly interface element  2101  drives the second joint  507 , and the third drive assembly interface element  2103  drives the third joint  513 . In an alternative arrangement, the drive assembly interface elements may drive different joints. For example, if the first pair of driving elements A 1 , A 2  are connected to the second instrument interface element  1906 , then the second drive assembly interface element  2101  drives the first joint  506 . If the second pair of driving elements B 1 , B 2  are connected to the first instrument interface element  1905 , then the first drive assembly interface element  2102  drives the second joint  507 . In this example, the third pair of driving elements C 1 , C 2  are connected to the third instrument interface element  1907 , so that the third drive assembly interface element  2103  drives the third joint  513 . In this example, the first drive assembly interface element  2102  is linearly displaceable through a maximum distance s 1  The second drive assembly interface element  2101  is linearly displaceable through a maximum distance s 2 . The third drive assembly interface element  2103  is linearly displaceable through a maximum distance s 3 . Suitably s 2 &lt;s 1  and s 2 &lt;s 3 . Suitably, s 1 =s 3 . 
     Each instrument interface element comprises a body which is receivable in a corresponding socket of the drive assembly interface element. The shapes of the body and socket correspond such that when the drive assembly interface element is displaced, this displacement is transferred to the instrument interface element without any slippage. Thus, the body fits snugly into the socket along at least one line in the displacement direction. Suitably, the instrument interface element is displaceable over the same displacement range as its corresponding drive assembly interface element. 
       FIG. 22A  illustrates an exemplary arrangement of the part of the body of the instrument interface element which is received in the socket of the drive assembly interface element. The body comprises lower sidewalls  2201  and  2202  which are separated by the length of the body in the displaceable direction x. The lower sidewalls are perpendicular to the displaceable direction x of the body. Upper sidewalls  2203  and  2204  taper to a point  2205  from the lower sidewalls. Suitably, the upper sidewalls taper symmetrically to a point. On engaging the instrument interface with the drive assembly interface, the point  2205  is inserted into the drive assembly interface element first, followed by the rest of the upper sidewalls  2203 ,  2204  and finally the lower sidewalls  2201 ,  2202 . The angle α at which the upper sidewalls meet is preferably less than or the same as 80°. By selecting α≤80°, the body will slide into the socket when force is applied in the direction F as long as the point is inside the socket, even if the body and socket are not fully aligned along the displaceable direction x. The direction F is perpendicular to the displaceable direction x. In other words, the direction F is perpendicular to the longitudinal axis of the shaft  504  and perpendicular to the longitudinal axis of the drive assembly. 
       FIG. 22B  illustrates another exemplary arrangement of the part of the body of the instrument interface which is received in the socket of the drive assembly of the drive assembly interface element. The body is the same as that described with reference to  FIG. 22A  except that it has a roller  2206  located on the point at which the two upper sidewalls meet. The roller  2206  is configured to rotate about an axis which is perpendicular to the displaceable direction x. The angle β at which the upper sidewalls meet may be greater than 80°. This is because the roller  2206  aids seating of the body in the socket even if the body and socket are not fully aligned along the displaceable direction x when force is applied in the direction F. 
       FIG. 22C  illustrates another exemplary arrangement of the part of the body of the instrument interface which is received in the socket of the drive assembly of the drive assembly interface element. The body is the same as that described with reference to  FIG. 22A  except that it has a roller  2207 ,  2208  located on each upper sidewall. The rollers  2207 ,  2208  are configured to rotate about axes which are perpendicular to the displaceable direction x. The angle γ at which the upper sidewalls meet may be greater than 80°. This is because the rollers  2207 ,  2208  aid seating of the body in the socket even if the body and socket are not fully aligned along the displaceable direction x when force is applied in the direction F. 
       FIG. 23  illustrates an exemplary arrangement of a socket of a drive assembly interface element. The body comprises sidewalls  2301  and  2302  which are separated by the length of the body in the displaceable direction x. The sidewalls are perpendicular to the displaceable direction x of the body. The sidewalls each terminate in a roller  2303 ,  2304 . The rollers  2303 ,  2304  are configured to rotate about axes which are perpendicular to the displaceable direction x. The rollers  2303 ,  2304  aid seating of the body in the socket even if the body and socket are not fully aligned along the displaceable direction x when the instrument interface is applied to the drive assembly interface. 
     In one example, the length A of the body in the displaceable direction x is greater than the maximum distance the body is able to travel over the displacement range in the displaceable direction x. Suitably, the length A of the body in the displaceable direction x is greater than the maximum distance the socket of the drive assembly interface element is able to travel over its displacement range in the displaceable direction. Thus, whatever displacement position the body has and whatever displacement position the socket has, when the instrument interface is brought into engagement with the drive assembly interface, the body seats into the socket. Thus, no pre-alignment of the instrument interface elements and the drive assembly interface elements is required to cause the instrument interface and the drive assembly interface to mate. Suitably, the maximum distance the body is able to travel over its displacement range is half the length of the body A/2. A is the length of the body. Suitably, the maximum distance the socket is able to travel over its displacement range is half the length of the body A/2. 
     In one example, the length B of the socket in the displaceable direction x is greater than the maximum distance the body is able to travel over the displacement range in the displaceable direction x. Suitably, the length B of the socket in the displaceable direction x is greater than the maximum distance the socket of the drive assembly interface element is able to travel over its displacement range in the displaceable direction. Thus, whatever displacement position the body has and whatever displacement position the socket has, when the instrument interface is brought into engagement with the drive assembly interface, the body seats into the socket. Thus, no pre-alignment of the instrument interface elements and the drive assembly interface elements is required to cause the instrument interface and the drive assembly interface to mate. Suitably, the maximum distance the body is able to travel over its displacement range is half the length of the socket B/2. B is the length of the socket. Suitably, the maximum distance the socket is able to travel over its displacement range is half the length of the socket B/2. 
     Suitably, the length A of the body in the displaceable direction is equal to the length B of the socket in the displaceable direction. 
     The instrument interface may have a guide bar to aid alignment and seating of the instrument interface into the drive assembly interface when they are being brought into engagement. The guide bar is located on the exterior face of the instrument interface which faces the drive assembly interface when they are brought into engagement. The guide bar is received in the drive assembly interface prior to the instrument interface elements. Suitably, the guide bar is the first part of the instrument interface to be received in the drive assembly interface as they are brought into contact with each other. Once the guide bar has been received in the drive assembly interface, it constrains the relative orientation with which the instrument interface and drive assembly interface are able to engage so as to align their longitudinal attitudes. Suitably, the guide bar only permits the instrument interface to seat fully into the drive assembly interface if the longitudinal axis  511  of the instrument shaft is aligned with the longitudinal axis  2104  of the terminal link of the robot arm. 
     Suitably, the guide bar is elongate, straight and parallel to the longitudinal axis  511  of the instrument shaft. The guide bar may extend wholly across the instrument interface. For example, the guide bar may extend in the longitudinal direction x from the end of the instrument interface which abuts the instrument shaft  504  to the opposing end of the instrument interface. Alternatively, the guide bar may extend in the longitudinal direction x only over the longitudinal range bounded by the minimum and maximum displacement positions of the  1931 ,  1932  of the instrument interface elements  1905 ,  1906 ,  1907 . If the drive assembly interface and instrument interface are arranged such that they both adopt their default predetermined alignment positions before being brought into engagement, then the guide bar may extend in the longitudinal direction x only over the longitudinal range bounded by the displacements of the instrument interface elements in the predetermined alignment positions. If in the predetermined alignment positions, the instrument interface elements are all aligned in the same plane perpendicular to the longitudinal axis  511  of the instrument shaft, then the guide bar may extend in the longitudinal direction x only over the instrument interface elements themselves. Suitably, the guide bar is narrower than the diameter of the instrument shaft  504 . The drive assembly receives the guide bar parallel to the longitudinal axis  2104  of the drive assembly. 
     The guide bar may be formed of a single part, such as guide bar  1960  shown in  FIG. 19A . This illustrated guide bar extends across the whole length of the instrument interface. The guide bar partially or wholly envelops driving elements A 1 , A 2 , B 1  and C 1 . In this way, the driving elements are not exposed at the exterior of the instrument interface as the instrument interface is brought into engagement with the drive assembly interface. The guide bar  1960  also stiffens the instrument interface. 
     Alternatively, the guide bar may comprise two or more parts.  FIG. 25  shows a view of the face of an instrument interface which first engages with the drive assembly interface. Here, two guide bar parts  2501 ,  2502  are shown. This guide bar extends in the longitudinal direction x across part of the longitudinal range bounded by the minimum and maximum displacement positions of the instrument interface elements. 
     Both the guide bar and the first instrument interface element  1905  are received in the first drive assembly interface element  2102 . The first instrument interface element comprises two body parts  1905   a  and  1905   b , one of which is located on one side of the guide bar, and the other of which is located on an opposing side of the guide bar. The first drive assembly interface element comprises two socket parts  2101   a  and  2101   b , located on either side of the longitudinal axis  2104  of the drive assembly. As the instrument interface and the drive assembly interface are brought into engagement, the guide bar is received in the first drive assembly interface element first. The guide bar seats along the longitudinal axis  2104  of the shaft between the two socket parts  2101   a  and  2101   b . As the guide bar is received in the first drive assembly interface element  2101 , it prevents the instrument interface from twisting as it is located in the drive assembly interface. The guide bar causes the attitudes of the drive assembly interface and the instrument interface to remain aligned as they engage. As the instrument interface is further lowered into the drive assembly interface, the first body part  1905   a  engages the first socket part  2101   a  on one side of the guide bar whilst the second body part  1905   b  engages the second socket part  2101   b  on the opposing side of the guide bar. 
     It will be appreciated that the drive assembly interfaces described herein could be modified to include further drive assembly interface elements to transfer drive to further instrument interface elements. The instrument interfaces described herein could be modified to include further instrument interface elements to transfer drive to further joints of the articulation at the distal end of the instrument shaft. The articulation itself could also be modified to include further joints. 
     It will also be appreciated that the end effector may only have one end effector element. In this case, the articulation does not include the third joint  513 , the instrument interface does not include an instrument interface element for driving the third joint, and the drive assembly does not include a drive assembly interface element for driving that instrument interface element. 
     The instrument could be used for non-surgical purposes. For example it could be used in a cosmetic procedure. 
     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.