Abstract:
An elongate robotic arm comprising articulated segments and a channel that extends along the longitudinal axis of the arm and contains a stiffening member which includes a sensor for measuring the shape of the arm. Using the central channel for this purpose improves the ease and accuracy of shape measurement.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of pending International patent application PCT/GB2009/001643 filed on Jun. 30, 2009 which designates the United States and claims priority from United Kingdom patent application 0811971.1 filed on Jun. 30, 2008, the content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to robotic arms and in particular to elongate arms comprising a plurality of sequentially arranged articulated links. 
     BACKGROUND OF THE INVENTION 
     Such articulated robotic arms are used particularly in situations wherein the arm is required to follow a path to a worksite with the body of the arm remaining on the path. This is known as “tip following”, with the arm advancing along the path taken by the tip, in a snake-like manner. The tip may for example carry a work tool for carrying out a task. 
     A known type of device is described in our co-pending application WO 0216995. In such an arm, the links are grouped into segments each comprising a plurality of links, and the shape of the arm is controlled by control cables passing through the peripheries of the links. A set of cables evenly spaced around the links, for example 3, terminates at the most distal link of each segment, such that each set of cables may be used to control the orientation of that link, and thus the shape of the segment. The links each have a central aperture, which apertures together form a channel or lumen for carrying services usually associated with the work tool, for example cables controlling the tool or for communicating with the tool. 
     As the cables are located at the periphery of each link, the central channel or lumen may be relatively large. Since it is coaxial with the arm, the service cables will have a minimum effect on the bending and torsional stiffness of the arm. 
     In some types of arm, such as that described in our co-pending application WO 070774, a helical spring is provided around the central lumen, which provides stiffness to the arm so that it tends towards a straight configuration. The arm may be constructed by engaging the links one-by-one with the helical spring and screwing them down onto the adjacent link. The control and service cables may then be threaded through the apertures in the links. 
     With such arms, the shape of the arm can be detected or measured in various ways, as described in our copending application no WO 0613682. This usually involves mounting measuring devices around the periphery of the links to measure the space between adjacent links. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an elongate robotic arm comprising a plurality of sequentially arranged articulated segments, a channel extending along the longitudinal axis of the arm, and an elongate stiffening member carried by the channel, the stiffening member comprising a sensor for measuring the shape of the arm. 
     This arrangement uses the arm axis to measure the arm shape, which simplifies the measurement. This is particularly advantageous for small diameter arms, where there is little peripheral space for placing sensors. 
     Furthermore, it has the advantage that the arm may be constructed by sliding links onto the stiffening member sequentially. Thus the links may engage with each other in a mechanically precise manner. The links may for example engage with each other along a hinge line, which ensure that the arm is torsionally stiff. Also, the control cables may be fed through individual or groups of links one at a time, rather than needing to be fed through the complete arm after it has been constructed, as is the case for an arm engaging with a helical spring. 
     The stiffening member is preferably flexible and resilient. It may be hollow or have an internal bore. This may be used to house the sensor for measuring the shape of the member, and thus the arm. The sensor may be an elongate sensor member such as an optical fibre, a plurality of optical fibres, or a single fibre with multiple cores extruded within it. The fibre may have Bragg gratings etched within each core for different strain sensing. By measuring the shape of the sensor, the shape of the arm may be calculated. It is advantageous to constrain the sensor at at least one location on each segment. This allows for the location of the ends of each segment to be sensed. 
     The sensor may fit tightly within the flexible member, and the stiffening member may fit tightly within the apertures in the links. Thus friction between the elements alleviates sliding movement of the elements relative to each other. In some examples, for instance where the links are long or where the links articulate through a large angle, the stiffening member may be secured to the links at predetermined locations in the arm. 
     Preferably, the locations at which the stiffening member is fixed are chosen to achieve a small change in the length of the member to alleviate buckling or stretch of the member and damage to the sensor within the stiffening member, and in order to optimise the shape measuring accuracy by ensuring that the number of points of inflexion in the sensor fibre between the fixed points in minimised. It has been found that fixing the stiffening member at the joints between the links helps to maintain the fixed length of the member, but concentrates the bend at one point. At the other extreme, fixing the member at the midpoints between adjacent links gives the greatest variation in length which must be accommodated by the stiffening member bending. Hence, the stiffening member is preferably fixed to the links at points between the midpoint and the joint. 
     Where each segment comprising a plurality of links, the stiffening member is preferably attached to each link, while the sensor may be fixed to the stiffening member at greater intervals. 
     There may be plurality of stiffening members provided in the channel around the longitudinal axis. Thus it is possible to achieve a specific bending stiffness which may be symmetric or asymmetric. 
     The hinges between adjacent pairs of links may for example be located at approximately 60° offsets. This provides a compact layout, particularly in the case where each segment has three control cables. 
     Thus according to a further aspect of the invention, there is provided a robotic arm comprising a plurality of sequentially arranged articulated segments, in which the articulations between the segments allow bending substantially about a hinge line across the links, in which the hinge lines are offset from each other by about 60 degrees. 
     Therefore the hinges are in one of three offset positions. The hinges may be arranged such that adjacent hinges are at offset angles, and progress between the three possible positions in turn. However, it is also possible that adjacent hinges may be in the same location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which: 
         FIGS. 1 ,  2  and  3  are perspective, end cross-sectional, and side cross-sectional views of a part of an arm according to one embodiment of the invention; 
         FIGS. 4 ,  5  and  6  are similar views of an arm according to another embodiment of the invention, 
         FIGS. 7 ,  8  and  9  are schematic side views of an arm according to the present invention, and 
         FIG. 10  in a schematic side cross-sectional view of an arm according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a robotic arm  2  comprising a plurality of sequentially arranged articulated links  4 . The articulation are formed by hinges comprising a recess  6  extending along the diameter of one face of each link, and a corresponding projection  8  extending along the diameter of the opposite face of the adjacent link  4 . The links  4  also each have a central aperture  10 , extending along the axes thereof. 
     The arm  2  is constructed by threading the links  4  onto a stiffening member  12  through the central apertures  10 . The stiffening member  12  is elongate and is formed of a resilient flexible material such as spring, steel or nitinol. The member  12  includes a sensor for measuring the shape of the member, such as an optical fibre (not shown). 
     The central apertures  10  may be shaped such that the member  12  is a close fit towards the middle of the link, but the opening  11  of the aperture is wider than the diameter of the member  12 . 
     The links  4  each comprise a plurality of cable apertures  14  arranged around their circumference, for carrying control cables for controlling the position of each segment, and thus the shape of the arm. The links also include a plurality of further lumen  16 ,  18  which may be used for carrying services associated with work tools at the distal end of the arm. 
     It can be seen that, when the links fit together by the projections  8  locating in the recesses  6 , the cable apertures  14  and the service lumen  16 ,  18  are axially aligned. It is possible to thread services cables or control cables through the links  4  individually as the arm is constructed. 
     In the embodiments shown in  FIGS. 1 ,  2  and  3 , it can be seen that the projection  8  and recess  6  pairs, which act as hinges between the links  4 , are aligned, such that the arm may bend in one direction about the hinges. Referring to  FIGS. 4 ,  5  and  6 , a similar construction is shown. However, in this case, the recesses  20  and projections  22  which extend along the diameter of opposite faces of the links  24  are offset with respect to each other by 60°. Each group of links forming a segment of this arrangement may have three control cables which may thus each control bending about each hinge line to give a full range of movement. In this arranged, three or six service lumen  26  may be present. 
     Referring to  FIGS. 7 ,  8  and  9 , the stiffening member  30  may be constrained or fixed in relation to each segment or link  32 .  FIG. 9  shows the arm in a straight configuration. If the member  30  is fixed at the articulations or joints  34 , as shown in  FIG. 8 , then the bend is concentrated at the joint, which means the sensor must bend about a high radius of curvature, which is undesirable. If the member  30  is fixed at midpoints  36  between joints, as shown in  FIG. 7 , then the member  30  may bend over the distance between the points. Thus the sensor within the member  30  will not follow the arm shape so accurately, moving off the axis at the joints  34 , which is also undesirable. Therefore an optimal fixing point is between these two extremes. 
       FIG. 10  shows how the stiffening member  40  including the sensor may be fixed to a section of the arm  41 . Spacers  44  may be provided around the member  40  for attachment to selected links  42  at intervals along the arm.