referring to fig1 & amp ; 2 , the arm as shown comprises three segments 10 , each segment 10 comprising a plurality of link elements 11 . each segment 10 is connected to its neighbour by means of an articulated joint 12 . the distal end 13 of the arm carries a work head 14 incorporating a tool 15 . each plurality of link elements may comprise an end cap 11 ′ to engage with the first and last links of the segment and with the spring to contain rotation of the links with respect to the spring . each individual link 11 , is capable of limited movement with respect to its neighbour by virtue of an articulated joint 21 ( see fig2 ) and the segment 10 as a whole is controlled by means of control wires and 22 and 23 which extended through each of the intermediate links 11 and is fixedly attached to the end element 25 of segment 10 . each of the control wires extends to actuator means indicated generally at 24 in fig2 , which actuator means can be controlled to provide varying tensions to each of the control wires . in normal operation , each of the control wires 22 and 23 are maintained under slight tension so as to apply a slight compressive force to the articulated link joint 21 . in order to cause the segment to move the appropriate actuator means applies increased tension to the upper control wire 22 , and increased force is applied to the upper attachment point 26 of the end element 25 which causes the end element to move upwardly and hence the arm to bend upwards relative to its datum position . it will be appreciated by the person skilled in the art that by providing three control wires it will be possible to obtain movement of the arm in any of the three dimensions provided there is suitable freedom of movement by the articulated link joints 21 . turning now to the present invention , which is shown in a simple version shown in fig3 to 6 , the individual links 11 are surrounded by a single continuous wire helix 30 with the control wires 23 and 22 passing through the link as before . each link 11 is in the form of a disk 31 having a generally convex upper surface 32 , a similar convex lower surface 33 , and a generally cylindrical edge 34 incorporating a helical groove 35 . as will be apparent from fig6 a , disk 31 is provided with circumferentially spaced , axially aligned holes 36 each adapted to accommodate a control wire . these holes are fluted towards the upper and lower ends so that each hole increased slightly in diameter towards the exit . in the specific example illustrated there are 12 holes 36 allowing for the control wires for a four - segment arm . as can be seen from fig4 , the convex surfaces of 32 and 33 are adapted to abut the corresponding surface of the next adjacent link and constitute bearing surfaces to maintain the link spacing . such a bearing arrangement between links has no torsional constraint and in this case increasing tension in wires 23 , 24 will provide increased torsional stiffness . the external helical groove 35 is adapted to accommodate helical spring 37 which may optionally be fixedly attached to the link 11 by means of pins , adhesive or other appropriate means . the attachment of the helical spring 37 to the external surface of the disk 11 may be effected in such a way as to provide , in the rest position of the arm with no tension applied to the control wires , a slight spacing between adjacent discs 11 so that when tension is applied to the control wires 22 , 23 , the external helical spring 37 seeks to bias apart adjacent links 11 . in an alternative embodiment , the links may be attached to the helical spring 37 such that the spring , in the rest position , is already under tension to urge the links 11 together and to maintain them under a compressive force . in the particular embodiment of fig3 to 6 , there is no constraint on shear applied to the links within a segment except the constraint applied by the tension in the control wires 22 , 23 so that the links can be shifted laterally in relation to each other with no ready means of recovering that shear offset . furthermore , the provision of a central bore is not practicable , since the entire convex surface of the disk is used as a bearing . a modified link is shown in fig7 . this is similar to that of the fig4 to 6 except that it is provided with a projecting ball portion 40 in upper convex surface 32 and a socket 41 in the underside of surface 33 adapted to receive the ball portion 40 of the next adjacent link element 11 a . in this case , each link element 11 is provided with a central bore 42 extending from the base of the socket 41 up to the extremity of the ball 40 . as will be apparent from the fig8 b , the bore 42 of adjacent link 11 within the segment will provide a continuous conduit or duct for the provision of services along the length of the arm . the size of the ball and socket arrangement and hence the size of the central conduit 42 will be determined to a large extent by the desired degree of relative movement between one link and its neighbour and the number of wires to be provided for controlling the length of the arm . again , the bearing between the links has no torsional constraint and this is another instance where an increase in the wire tension will increase the torsional stiffness of the device . compared with the example of fig5 above , the problem of shear constraint has been overcome by the lateral constraint provided by the ball and socket . where an arm of relatively large diameter is to be considered , then the frictional forces of such a ball and socket arrangement tend to increase and this reduces the tendency of the segments to assume a truly arcuate form in use . it should be noted that holes 36 accommodating the actuator wires or ropes 23 , 24 are fluted at 43 towards each end to allow for the ease of passage of the wires within the bores when the links 11 are inclined at an angle one with respect to the other . a further modification of each link element 11 is shown in fig9 and 10 in which each link element is provided with a central bore 42 as previously described and the lower convex surface 33 is provided with a pair of diametrically spaced balls 50 . the upper convex surface 32 is provided with a pair of corresponding depressions 51 disposed on a diameter substantially at right angles to the diameter containing the balls 50 on the lower convex surface 33 . when assembled as shown in fig1 b , the balls 50 on one link 11 engage with the depressions 51 on the upper surface of the adjacent link . as before , the central bores 42 will align or correspond to provide a central conduit . in this case , however , the relative angles of the ball and depression bearings will allow movements or bending motion between the adjacent links in only one plane in the manner of a hinge . the next link 11 a in fig1 b will hinge at right angles to the plane containing the hinge of the links 11 , lib . in this way , the full flexibility of the arm is preserved while the ball and socket arrangement provides torsional constraint so the torsional stiffness of this assembly is not affected by wire tension . it will be appreciated that the size of the central bore in this case is limited only by the separation between the balls 50 and the corresponding depressions 51 . furthermore , the balls 50 and their corresponding depressions or sockets 51 can be chosen to be as small as possible ( depending upon the loads they are required to accommodate ) which has the effect of minimising friction at the hinge positions . the joints between links in the assembly as illustrated have only one degree of freedom in that they are a hinge , so alternate joints are disposed at an angle to each other . this angular disposition need not necessarily be a right angle . by providing angular spacing in this way the segment can bend with several degrees of freedom . in an alternative embodiment of the invention , the hinge joints may be aligned to obtain a segment with only a single degree of freedom which would be suitable for planar applications only . the embodiment illustrated in fig1 is a modification of that described with respect to fig1 . in this embodiment , the cylindrical surface or edge 34 of disk 31 is provided with a plurality of circumferentially spaced lugs 60 each of which is configured to provide a shelf or engagement to define a path to receive helical spring 37 . the lugs 60 provide an upwardly facing shelf 61 juxtaposed but circumferentially spaced from a downwardly - facing shelf 62 , the surface of which shelf is adapted to bear against a surface of part of helical spring 37 . the arrangement of lugs and shelves is such to define a helical path about the external cylindrical surface 34 of disk 31 so that the spring 37 can snap between the lugs 60 and shelves 61 and 62 . the spring can , once again , be optionally fixedly secured to the disk 31 by means of pins , adhesive or any other suitable fastening means . the unloaded pitch 37 ′ of the spring may be slightly longer or shorter than the pitch 61 ′ of the engagement member on the links so that as the links are sequentially rotationally engaged with the spring , the arrangement is such that the spring will urge adiacent links apart or together . if they are urqed apart , this reduces any applied compressive load at the interface between adiacent links . if urged together , this helps to ensure that the links engage correctly during use . should excessive force cause the links to disengage , the spring will cause them to re - engage on removal of the force , such that the arm is resilient to damage . the spring may have a different pitch at different positions ( 60 ′, 60 ″, 60 ′″) along its length in order to vary the pre - tension or pre - compression in the spring . in this embodiment , the central bore 42 is of generally square cross - section the size of which is limited only by the separation between the balls and depressions , and as before the joints between adjacent links have only one degree of freedom . this arrangement allows for ease of moulding and enables components to be readily manufactured by injection moulding from plastics materials . the embodiment described with respect to fig1 is a variation on the foregoing in that the spring on this occasion is an internal spring 70 . to accommodate this , the central bore 42 is of generally circular cross - section and is provided with an internal groove 71 to receive and locate the spring 70 . the central bore 42 is additionally provided towards each end with a portion of wider diameter 72 at each end thereof to allow for movement or flexing of the spring as the links 11 bend one with respect to the other . the single internal groove 71 may be machined or produced by a complex casting or injection moulding . as before each link element 11 may be secured to the spring in the manner described above . the fig1 illustrates an embodiment in which multiple springs and may be employed . in this case the cylindrical edge of 34 of disk 31 is provided with multiple grooves or lands 75 each of which is adapted to receive the helix of a helical spring 37 . in the example shown , three identical helical springs 37 may be employed each fixed into the grooves 75 as described above . alternatively , the springs may be may have differing properties to impart different characteristics upon the segment . an aspect of the invention envisages an arm construction having a plurality of segments , each segment of which has properties different from its neighbour . turning now to the embodiment of fig1 , this construction has a combination of an internal spring 70 and a plurality of external helical springs 37 . the basic construction of each of the link elements 11 is identical , with a central bore 42 having a helical groove as generally described with respect to fig1 above . the advantage here is that the handing of the helical springs can be different and the combination of the left - handed and right - handed spring sets assist in maintaining the assembly during use , for example , if in operation , one of the spring sets starts to “ wind up ” this action will be opposed by the other spring ( s ). in the assembly of the device of fig1 , it is essential that the links are screwed firstly onto the internal spring 70 and the external spring or springs 37 is then subsequently applied . it will be appreciated by the person skilled in the art that the individual links within the each segment will be identical . by providing only one degree of freedom between adjacent links , no additional moving parts will be necessary and the necessary tension can be applied by a combination of spring ( s ) and tension in the operating wires or ropes . the construction of an arm in this way , therefore , becomes relatively simple and the extensive machining of components can be reduced to a minimum . furthermore , in operations where the link elements 11 can be of a plastic material , it is envisaged that this construction will permit the use of injection moulded components . the invention provides , therefore , for a significantly cheaper method of producing a “ snake arm ” than heretofore and furthermore , permits the production of arms of different properties which can be readily interchangeable for any given actuator array and control system .
a robotic arm comprising a plurality of segments , each comprising articulated links , and means for causing each segment to bend so the arm can follow a serpentine path . a helical spring is provided coaxially with the arm to urge the links to an initial datum position , and to distribute the bending over the links of each segment .