Abstract:
A robotic arm arrangement including an arm having control cables extending from the base of the arm, and coupling members for coupling the cables to actuators for linear actuation of the cables. The coupling members extend radially outwardly around the arm to enable quick release and replacement of the arm.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of pending International patent application PCT/GB2009/001656 filed on Jul. 2, 2009 which designates the United States and claims priority from United Kingdom patent application 0812053.7 filed on Jul. 2, 2008, the content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to robotic arms, and in particular to a release mechanism to allow for interchangeable arms. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the field of robotics there has been considerable development of robotic arms having a tip following capability. Such arms can carry a workload or tool and can be used for inspection and repair in confined spaces, for instance within a jet engine or the human body. 
         [0004]    A major advance in tip following technology for robotic arms is described in our co-pending Patent Application No. WO 0216995. This application discloses a robotic arm comprising a plurality of longitudinal segments, each of which comprises one or more passive links and a control link. Control ropes or cables are provided that terminate at the control link at the end of each segment, so that by varying the length of the ropes, the arm can be caused to bend and adopt various planar or spatial shapes and configurations. This may be done for example by winding each control rope on or off a spindle using a rotary actuator or pulling the rope directly using a linear actuator. The actuators are located at the proximal end of the arm and are controlled for example by a computer control system. The ropes are generally located in the guide holes disposed towards the outer circumference of the arm. 
         [0005]    These arms are suitable for a number of operations which may require the work head at the distal end of the arm to be at adapted or changed for the intended purpose, possibly also with appropriate changes to the control means to provide the desired operation of the arm and the work head. Interchangeable work heads are widely used. However these typically involve location of rigid elements and feed through of services, for instance power and data. Certain work heads may also require additional control ropes for motion control of the work head, which ropes must also pass through the arm. This makes exchanging a work head more complicated, such that it would be advantageous to exchange both the arm and the work head together, whilst retaining the same actuators and control and power systems. The exchangeable component at the distal end (ie the arm and work head) tends to be lower cost than the proximal system end. Furthermore other factors such as wear, or sterilisation requirements, or a change in task, may make exchange of the entire arm preferable to exchange of the work head only. 
         [0006]    In order to do this, the exchange or release interface must enable mechanical and electrical power and electronic signals to be transferred. Furthermore the exchange process should not be time consuming, and should be straightforward in comparison with the task to be conducted by the arm. In order to provide an arm which is capable of ‘tip following’ along a predefined path in space in which there is little room for variance or deviation from the defined path, it is necessary to maintain the appropriate length of and tension in each control rope. In practice, due to build variance and operational effects, rope length and tension cannot be assumed. It is therefore essential that, when exchanging an arm, the control rope length and tension are managed. 
         [0007]    Furthermore the exchange may be required mid-procedure. It will be appreciated that in such circumstances the replacement arm of the same or different design as the original arm should operate in an equivalent manner. When exchanging an arm the operating algorithm may need to be changed. An arm may have different operating characteristics or may be identical except for some specific calibration parameters that are unique to a particular arm. 
         [0008]    Furthermore during the exchange process some actions are common to all control ropes, e.g. disengagement, and some actions of the specific to individual control ropes, e.g. tension control. It is advantageous to simplify the release and connect mechanisms, and where possible to use single mechanisms to achieve an action for multiple axes. 
         [0009]    There is, therefore, a need for a robotic arm in which the arm is interchangeable upon a given actuator and motor assembly, where differences in hardware or function may be managed with minimal intervention from the operator, and the coordinated exchange of a number of control ropes may ensure that control rope tension and position are maintained or re-established during the process. 
         [0010]    U.S. Pat. No. 6,866,671, U.S. Pat. No. 6,331,181 and U.S. Pat. No. 6,491,701 (Tierney) each describe a releasable tool attachment mechanism for a surgical robot. The tool uses wire ropes to transfer mechanical power from actuators through the interface to the various joints located at the tool tip. The interface also allows data to be exchanged bi-laterally. Within the tool four ropes are wound around four capstans and terminated. The releasable coupling is made between a rotating capstan and a rotating motor. 
         [0011]    The capstan is on the tool side of the exchange, which increases the cost and bulk of the tool. Also the rope is required to wrap around the capstan which leads to increased wear and reduced control of rope length due to unequal rope tension during wrap and unwrap. Furthermore the capstans are free to rotate. This means that the mechanism can be back driven and that stored energy in the rope may cause the capstan to unwind. 
         [0012]    U.S. Pat. No. 7,331,967 (Lee) describes a variation of Tierney in which three rope to rope connections are managed across three rotary couplings. This solution is complex with multiple rope pulleys and complex mechanisms to manage the coupling process. 
         [0013]    Grid type end to end connections are shown in U.S. Pat. No. 6,858,005 (Ohline) which present a different technical problem. It would be expected that the actuator associated with each rope will be of larger diameter than the rope. Hence there must be means of fanning out interface connections to the actuators. One method of achieving this is to use pulleys. These pulleys must be located some distance from the release mechanism unless the release junction is able to ride around the pulleys. Wrapping ropes around pulleys will also lead to more rapid rope wear. 
       SUMMARY OF THE INVENTION 
       [0014]    According to one aspect of the present invention there is provided a robotic arm arrangement comprising an elongate arm having a plurality of longitudinally extending control cables for controlling the position of the arm, a corresponding plurality of actuators for actuating the control cables, and a releasable coupling arrangement between each cable and the associated actuator, the coupling arrangement comprising a coupling member, in which each coupling member extends radially outwardly of the arm, and is arranged such that the actuators produce axial movement of the cable. 
         [0015]    Thus the actuators may be positioned radially around the arm, and the ropes or cables may extend axially out of the end of the arm for linear motion. With this arrangement the cables may stay substantially within the arm diameter, so that the arm may be replaced more simply. 
         [0016]    Each control cable may be equipped with an engagement member, for instance a swaged spherical ferrule, for engagement with a cooperating part associated within one end of the coupling members. Each coupling member may for example be equipped with a cup that can engage with the ferrule. The linear actuation of the control cable may thus be achieved by axial motion of the one end of the coupling members, and disengagement or reengagement may be achieved by radial motion of the one end of the coupling members. 
         [0017]    With such an arrangement, the actuators may be provided as an ‘actuator pack’, in which the actuators are arranged around an aperture for receiving the proximal end of the arm. For example the arm may have an ‘end of arm’ plate (i.e. at the end of the working part of the arm), with the cables extending axially out of the proximal end of the end of arm plate, to form a ‘cylinder’ of cables where the cables are exposed, terminating at an arm base plate. The ferrules may be provided mid way along the length of the cylinder between the end of arm plate and the arm base plate. The arrangement is preferably such that when all of the ferrules are aligned at the mid-point, the arm will be straight. The distance between the plates therefore will define the ‘stroke’ of each cable. This length may be varied depending upon the stroke required. 
         [0018]    The cables may be held in a co-linear position by a spring or elastic element. This also allows for preloading of the cables. For example the elastic element may be a cord which passes over a pulley in the arm base plate and returns to the end of arm plate internally of the cable. The cord may for example be about 1.5 times the length of the required stroke. 
         [0019]    This arrangement allows for each actuator with its releasable coupling to the respective cable ferrule to be located parallel, external and radially offset with respect to each cable. In this configuration, rope bending and wear occur only within the arm. Thus cables and/or the actuators may be arranged in a concentric ring, or in a plurality of eccentric rings. 
         [0020]    The end of arm plate and the arm base plate may be provided with locking members to engage with the structure of the actuator pack to enable precise location and transmission of forces. The arm base place may also be equipped with pins that enable electrical connection between the arm and the actuator pack. This enables bi-lateral communication of data. The control ropes may also be secured to the end of arm plate by means of a spring to ensure that each rope is under tension when disconnected. 
         [0021]    Such arms may be hollow or include a number of hollow bores for receiving tools or services. The bores may extend from the end of arm plate to the base plate and may be accessible at the base of the device. This may allow tools and services to be simply and swiftly inserted or exchanged within the hollow bores. 
         [0022]    Furthermore, a locking mechanism may be provided to hold the ropes in tension and to avoid the ropes becoming slack when the coupling is released. This may be particularly advantageous where a large pre-load is required. A suitable locking mechanism may secure all ropes equally; for example a drum brake with radially moving components that press against the ring of ropes. Alternatively, a sliding locking plate may be engaged and disengaged normal or perpendicular to the actuator spindles. 
         [0023]    Furthermore sensors may be incorporated to indicate to the control means that a rope has become slack, such that the control system can make an appropriate adjustment to one or more inputs. In one example, the control system may release the tension in all the ropes and apply the appropriate tension to the segment ropes sequentially starting with the most proximal segment, to ensure correct rope tension. 
         [0024]    A motor may be provided to disengage and reengage the coupling members with the actuators. This ‘quick release’ may be achieved in a number of ways. For example, the coupling members may comprise a series of pantographs operated by the motors. Alternatively, a passive spring load coupling may be provided. Decoupling may be achieved by driving the spring load couplings into the end of the arm plate, which allows the arm to be withdrawn axially. 
         [0025]    Alternatively, the coupling members may comprise a simple offset plate equipped with a mechanical quick release arrangement. In this embodiment the control cable actuators may be mounted in line with the straight arm with a radial offset. The coupling member between the actuator and cable may be an offset plate. Each cable may be equipped with a ferrule and each offset plate may have a corresponding component which can engage with the ferrule. The engagement may be arranged such that by driving all of the actuators and offset plates to an end stop, the engagement means releases the rope ferrule. 
         [0026]    It will be appreciated that creating a sterile boundary between the arm and the environment (e.g. the human body) may be important. The most proximal arm link may be securely fixed to the end of arm plate, and may be designed and locate with the actuator mechanism housing in a way to provide a sterile boundary. However it will be appreciated that the control ropes must pass through the link creating a need for a sterile boundary between a sliding wire rope and guide hole. The link may be equipped with a series of brushes or air jets. Furthermore the cable guide holes through the end of arm plate may be equipped with reservoirs that may be filled with lubricant for acting as a barrier to ingress the egress of material. If the link is made adequately long compared with the stroke of each rope then each rope may be coated with a wax or grease which will tend to remain within the link. This fluid barrier may remain in place for the life of the arm or may be replenished at intervals. 
         [0027]    The arm may also be equipped with a protective skin or sleeve such that the complete arm remains within the sterile boundary. This sleeve may engage with just the end link or may engage with both end link and the actuator pack or just the actuator pack. It will be appreciated that the same concerns arise for an interchangeable rope controlled end of arm work head, and similar structures may be applied there. 
         [0028]    In a particular aspect of the invention, the arm may also incorporate data relating to the nature and state of the arm, its prior use, the way in which it is to be controlled, and the intended purpose of the arm and the manipulation of work piece at the distal end thereof. The calibration and the control algorithms for the arm, therefore, may be incorporated in the arm and may be read from the arm by the control program. For example, different lengths of arm may be applied each with different motor control characteristics. The motor drive means may have a large plurality of motors incorporated therein not all of which may be arranged for connection to an appropriate control rope. In this way, therefore, a drive assembly generally arranged for a five segment arm, say, can equally well be employed to drive a three segment arm. 
         [0029]    In another aspect, the invention provides a robotic arm arrangement comprising an elongate arm having a plurality of longitudinally extending control cables for controlling the position of the arm, a corresponding plurality of movable actuators for actuating the control cables, a locking mechanism for locking the actuators against movement, a corresponding plurality of drive elements for driving movement of the actuators, and a releasable coupling arrangement between each actuator and each drive element. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0031]      FIG. 1  is a diagrammatic perspective view of a robotic arm arrangement according to one embodiment of the present invention; 
           [0032]      FIG. 2  is a diagrammatic perspective view of the drive arrangement of the arm of  FIG. 1 ; 
           [0033]      FIG. 3  is a diagram of the actuator arrangement of  FIG. 1 ; 
           [0034]      FIG. 4  is a diagram of a rotary coupling of the arrangement of  FIG. 1 ; 
           [0035]      FIG. 5  is a diagram of a locking mechanism for the actuators in arrangement of  FIG. 1 ; 
           [0036]      FIGS. 6   a  to  6   c  are a sequence of diagrams showing the locking mechanism action; 
           [0037]      FIG. 7  is a diagrammatic view of an arm arrangement according to another embodiment of the present invention; 
           [0038]      FIG. 8  is a diagram of a drive arrangement of the arm of  FIG. 7 ; 
           [0039]      FIG. 9  is a diagram of an actuator arrangement for the arm of  FIG. 7 ; 
           [0040]      FIG. 10  is a diagram of a rotary coupling for the arm of  FIG. 7 ; 
           [0041]      FIG. 11  is a diagrammatic perspective view of a coupling arrangement for an arm according to a further embodiment of the present invention; 
           [0042]      FIG. 12  is a further diagrammatic view of the coupling of  FIG. 11 ; 
           [0043]      FIG. 13  is a diagrammatic perspective view of a coupling arrangement according to an alternative embodiment of the present invention; 
           [0044]      FIG. 14  is a further diagram showing the coupling of  FIG. 13 ; 
           [0045]      FIG. 15  is a more detailed view of the coupling of  FIG. 13 ; and 
           [0046]      FIGS. 16  ,  17  and  18  show the proximal end of the arm for use with the couplings shown in  FIGS. 11 to 15 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0047]    Referring to  FIG. 1 , a base plate  10  of a robot arm is attached via an arm mount  11  to an arm actuator pack  12 . The robot arm (not shown) is of the type comprising a plurality of articulated links and control cables or ropes extending from the base plate to terminate at various control links for controlling the shape of the arm. Each control rope  13  is wrapped around a spindle  14  of each actuator and passes through a Bowden Sleeve to the arm mount. (Only one control rope is shown for ease of description.) A drive pack  15  comprises thirty motors that drive the thirty spindles through quick release couplings, as described in more detail below. 
         [0048]    Referring to  FIG. 2  the drive pack  15  is shown without the actuator pack. The drive elements each have a drive coupling  18  comprising a disc having a protruding ridge  16 , which are shown aligned horizontally. 
         [0049]    Referring to  FIG. 3  the actuator pack contains thirty spindles  14  onto which the control cables  13  are wound. The rotational position of each spindle controls the deployed length of each cable; the length of each cable controls the arm shape. Each spindle has an actuator coupling  19  comprising a disc having a plurality of depressions  21  which are sized to receive the ridges  16  of the drive couplings  18 . The actuator couplings  19  are attached to the spindles and angularly fixed thereto by means of a key and keyway. Each actuator coupling  19  is shaped so as to interface to the drive couplings  18  on the drive pack to link the angular position of each spindle to the angular position of the associated drive element or motor. 
         [0050]    The spindle box assembly includes a locking mechanism including plates  22  which allow the angular rotation of each spindle to be locked in a specific position. This enables the removal of the actuator pack and arm from the drive pack without loss of control or knowledge of the angular rotational position of each spindle. Thus, an arm may be detached from the drive pack and later re-attached to the drive pack without the need for re-initialisation of the arm. 
         [0051]    A useful feature of this arrangement of the quick release mechanism is that it allows the spindle box and, by association, the arm, to be attached to the actuator pack in one of two possible orientations. These orientations would be termed left- and right-handed configurations in common robotics parlance. This property derives from the existence of an axis of revolute symmetry (at the quick release interface), which is located in the centre of the spindle box and in the centre of the actuator pack. 
         [0052]      FIG. 4  shows in more detail the drive, actuator and coupling assembly for a single cable. The assembly comprises a controller  31 , an encoder  32 , a brake  33 , a motor  34 , a gearbox  35 , a mounting plate  36 , an initialisation sensor  37  and rotary flag  28 , a drive coupling  29 , an actuator coupling  30 , a ratchet  31 , bearings  32 ,  33 , a control rope  13  and a control rope grip  34 . 
         [0053]    The drive coupling part is mounted on the gearbox output shaft. A toothed sensor flag is supported on the drive coupling part. The mounting plate includes location features for a reflective opto-switch. This switch, in combination with the sensor flag, is used to enable the controller to sense when the gearbox output shaft is suitably oriented to enable the drive coupling part to engage with the actuator coupling part mounted on the spindle assembly. 
         [0054]    The spindle is fitted with an actuator coupling part (which interfaces to the drive coupling part of the assembly). The actuator coupling part is slotted in a number of positions, in this case, 6 slots each at 30 degrees to the next, allowing 12 positions of coupling with respect to the drive coupling part, or 30 degrees between possible coupling orientations. The angle between adjacent slots must be small enough so that the spindle can be rotated to the nearest available slot without excessively deforming the arm. In this case, it was determined that 30 degrees between slots was adequate, given the size and flexibility of the arm and the elasticity of the system elements, most particularly the rope itself. 
         [0055]    The number of ratchet teeth is the same as the number of slots. The ratchet forms part of the locking mechanism (described in more detail below), the intention being that, regardless of which ratchet tooth is held in the locking mechanism, the front face of the actuator coupling appears to offer the same interface features to the drive coupling part. 
         [0056]    Referring to  FIG. 6 , the locking mechanism for a set of spindles is controlled by movement of a slide plate  40 . In  FIG. 6   a  the ratchets  31  are locked by engagement of the ratchet teeth with latches  42  provided on the end of pawls  38 . When the plate  40  moves upwardly, as shown in  FIGS. 6   b  and  6   c , the tail  39  of each pawl  38  is pushed up by the lower surface of each corresponding indent  41  in the plate  40  which receives the tail. Thus the latches  42  move downwardly releasing the ratchets  31  and allowing the spindles to rotate. 
         [0057]    In this example, the slide plate is one of two slide plates which are, in turn, operated by a crank. Thus, all 30 spindles may be locked by one operating member (i.e. the crank). The symmetrical arrangement of two slide plates may also enable automatic operation of the slide plates by a single electric actuator. The symmetry matches the symmetry of the spindle pack as a whole, enabling “right-handed” and “left-handed” installations of the spindle box on the actuator pack. The arrangement allows the state of the locking mechanism, which comprises a set of spindle locks, to be determined by two switches. 
         [0058]    The capacity to sense the state of the locking mechanism allows an automatic controller (e.g. a computer and software) to control the process of locking or unlocking the spindles. Thus, an operator may request that the controller locks or unlocks the spindles and the controller can detect whether the process is complete or still under way. 
         [0059]    Referring to  FIG. 7  another embodiment is shown in which an actuator pack  51  and drive pack  50  are connected together within a housing  49 . The arm  52  is stowed within the actuator pack and is advanced out of the actuator pack or retracted into the actuator pack by means of the drive elements located in the drive pack. 
         [0060]    Referring to  FIG. 8 , an array of drive couplings  53  is shown, along with a spring pin  54  for making electrical connections between the actuator pack and drive pack. Referring to  FIG. 9  an array of actuator couplings  55  of the actuator pack is shown with a corresponding electrical connector  56 . The actuator couplings are mounted on springs such that they may only engage with the drive coupling when the drive coupling is correctly aligned. The alignment method is to rotate each motor by a maximum of one half turn. 
         [0061]    Referring to  FIG. 10  the arrangement for a single cable is shown in more detail. The arrangement comprises an encoder  101 , a motor  102 , a gearbox  103 , a drive coupling  104 , an actuator coupling  105 , a spring  106 , a gear  107 , a spindle  108 , and a pulley  109 . The control rope is not shown. The control rope is secured to the spindle and follows a path to the pulley block shown in  FIG. 9  at the base of the arm before passing up through the arm. 
         [0062]    Referring to  FIG. 11  another embodiment is shown in which the actuation is by linear movement of the control ropes. A ferrule  302  is attached to each control rope  205  near the proximal end of the control rope. The proximal end of the control rope is secured to an arm base plate  300  through a spring  301 , as shown in  FIG. 16 . The spring is used to ensure that the ropes do not become slack in any arm configuration. The spring also holds the section of rope between the base plate  300  and the base of the arm straight on a known line. 
         [0063]    The arm is controlled by many control ropes  205 . The actuator pack comprises a number of linear actuators one for each control rope. Each linear actuator  206  is connected to a pantograph mechanism comprising two pantographs. Each pantograph has two inputs  201 , 202  and one output  203 . The outputs from each pantograph are connected to a cup  209  which engages with the ferrule  302  secured to the control rope. Each linear actuator is shown connected to the central inputs  201  of the pantograph. The other inputs  202  are connected to the quick release column. The linear actuator  206  is used to pull the control rope  205 . The input  202  is connected to a quick release column. For simplification only one actuator is shown. 
         [0064]    By moving the quick release column down the output will move the cup down and allow the cup to disengage from the ferrule. Engagement is achieved by reversing this process. 
         [0065]    It will be appreciated that a single pantograph can be used although a different motion will be achieved. 
         [0066]      FIG. 16  shows the arm base plate  300  and the end of arm link  305 . The end plate may have an engagement feature  307  for locating with an engagement feature in the actuator pack. The end of arm link may also have an engagement feature  312  for locating within a socket in the actuator pack housing. A skin or sleeve  308  secured to the arm may engage with the actuator pack housing at the arm socket providing a sterile boundary. 
         [0067]    Referring to  FIG. 13 , in an alternative coupling arrangement, a linear rope actuator  400  is mounted on a linear slide  401 . A radially extending offset plate  402  is attached to the actuator and is equipped with a rope engagement plate  403 . The engagement plate for engaging a ferrule  413  on the rope  414  includes a spring  405  which biases the engagement plate inwardly towards the rope. Only three actuators, offset plates and engagement plates are shown for clarity. 
         [0068]    When the actuator is driven to its full extent, as shown in  FIG. 14  and  FIG. 15 , contact with an end stop plate  404  causes the engagement plate  403  to pivot and retract to release the control rope ferrule  413 . The end stop may also be used as a physical datum for initialising the actuators. 
         [0069]    Referring to  FIGS. 16 and 18  tension is maintained in the control ropes by means of the spring  301 , which avoids wires becoming slack and ensures that the ropes are aligned with the engagement member of the actuator. The arm is equipped with an end of arm link  305  which engages with the end stop plate  404 . A sterile boundary may be created by securing the sleeve  308  to the end stop plate. The end of arm plate  300  may be equipped with electrical connectors  309  for power and data and electronics to store information within the arm. 
         [0070]    Referring to  FIG. 17  the end of arm link  305  may be long enough such that the holes  500  through the end of arm link can be made long enough so that when filled with a viscous fluid the fluid will tend to remain within the hole in order to provide a seal. The holes may include features  510  to act as reservoirs for the viscous fluid.