Abstract:
A manipulator ( 110 ) for e.g. gait training is constructed from an Evans mechanism with an additional degree of freedom to provide a two dimensional workspace.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is the U.S. national phase of International Application No. PCT/EP2014/055651 filed Mar. 20, 2014, which claims priority of British Application No. 1305989.4 filed Apr. 3, 2013, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is concerned with a manipulator. More specifically the present invention is concerned with a 2 degree of freedom manipulator comprising an end effector which can be moved in an uncoupled sense in two substantially perpendicular linear directions in a planar workspace. 
         [0003]    Two degree of freedom manipulators have many uses. For example, they may be used in the manipulation of an end effector such as a robot arm or machine tool in order to pick and place and object, or perform a manufacturing operation on a component. 
       BACKGROUND OF THE INVENTION 
       [0004]    Another application for two degree of freedom manipulators is in the use of rehabilitation robots in order to provide support and/or assistive forces to a patient undergoing rehabilitation. Such manipulators are attached to a body part of the subject and can be used to provide assistive forces and support during a rehabilitation exercise such as gait training 
         [0005]    For example, a 2DOF manipulator may be connected to a subject&#39;s pelvis to support their weight and provide predetermined gait cues to assist in walking One such manipulator is shown in US2007/0016116. In this document, a pair of pneumatically driven manipulators apply forces to the subject&#39;s pelvis with an arrangement of cylinders. In particular, fore-aft movement of the subject is provided by pneumatic cylinders mounted in the fore-aft direction, and lateral movement is provided by laterally orientated pneumatic cylinders. A problem with this arrangement is that the lateral cylinders mean that the manipulator is quite wide. This makes it more difficult to install, and the laterally extending pneumatic cylinders may clash with the subject&#39;s arms during normal gait motion. Also, the workspace is quite small compared to the size of the manipulator. 
         [0006]    A different, known, 2DOF manipulator comprises a 2D Cartesian slideway arrangement in which a carriage is slidable on a first rail in a first direction, which first rail is slideable between two further parallel rails in a second direction, perpendicular to the first. Such systems have certain disadvantages. 
         [0007]    One disadvantage is that there is a significant amount of equipment surrounding and within the workspace. This is generally undesirable in many applications, as the manipulator and the workpiece or subject may clash, and in the event that the manipulator is used for gait training, the rails may clash with the subject&#39;s arms. 
         [0008]    Another disadvantage with such systems is that sliding joints between components are generally undesirable because they are prone to contamination and wear. 
         [0009]    Also, in such a system a motor is provided to move the carriage on the first rail. The provision of a motor attached to the first rail, and arranged to move the carriage, means that the first rail has a high inertia, which is undesirable when being moved on the parallel rails. 
       SUMMARY OF THE INVENTION 
       [0010]    It is an object of the present invention to overcome or at least mitigate the above referenced problems. 
         [0011]    According to the first aspect of the invention, there is provided a manipulator comprising:
       a frame;   a first link;   a second link; and,   a first coupler;   arranged to form an Evans straight-line mechanism such that a point on the first coupler describes a substantially straight line in a first direction for a part of its locus;   wherein the second link is attached to the frame via a crank, such that actuation of the crank moves the point on the first coupler in a substantially straight line in a second direction, perpendicular to the first direction.       
 
         [0018]    The invention provides an Evans mechanism in which an additional degree of freedom is provided at the mounted end of one of the driver arms. The actuation of the additional crank provides motion in the degree of freedom perpendicular to the normal linear degree of freedom of the Evans mechanism. This arrangement has many advantages. Firstly, the majority of the mechanism is placed outside of the workspace and rearwardly thereof. Secondly the arrangement only uses rotational joints, which do not suffer the disadvantages of linear joints per the prior art. 
         [0019]    Preferably:
       L 1  is the distance along the second link ( 122 ;  222 ) between an axis of rotation with the coupler ( 118 ;  218 ) and an axis of rotation with the crank ( 144 ;  248 );   L 2  is the distance along the coupler ( 118 ;  218 ) between an axis of rotation with the first link ( 114 ;  214 );   L 3  is the distance along the coupler ( 118 ;  218 ) between the axis of rotation with the coupler ( 118 ;  218 ) and the end point ( 128 ;  228 );   in which the manipulator is configured such that L 2  is within 10% of the value determined by L2 2 =L1*L3.
 
This provides a good approximation to a straight line throughout a significant portion of the travel of the end point.
       
 
         [0024]    Preferably the mechanism comprises a first actuation assembly having a first motor configured to articulate the manipulator to move the point on the first coupler in the first direction. Preferably the first actuation assembly comprises a third link, driven by a first actuation assembly crank, which is driven by the first motor, in which the third link is arranged to drive the first coupler. 
         [0025]    Preferably the third link is attached to the first coupler between the first link and the second link. 
         [0026]    Preferably the manipulator comprises:
       a second coupler connected to the first coupler and configured to move therewith; and,   an end effector connecting the first and second couplers. This provides a more stable mechanism, and allows an end effector to be used which transfers torques as well as point forces. Preferably the first and second couplers are connected by a coupler connector spaced from the end effector. Preferably the first and second couplers are connected by the end effector and coupler connector so as to form two parallel sides of a parallel linkage.       
 
         [0029]    The manipulator may comprise a further second link connected to the second coupler, wherein the further second link is attached to the frame via a further crank, such that actuation of the crank moves a point on the second coupler in a substantially straight line in the second direction. 
         [0030]    Preferably the crank and the further crank are arranged for synchronised motion. The crank and the further crank may be driven by a common actuation assembly, for example the first motor may drive the first crank and the further crank via a common pushrod. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0031]    An example manipulator in accordance with the present invention will now be described with reference to the following figures. 
           [0032]      FIG. 1  is a side schematic view of a known Evans mechanism. 
           [0033]      FIG. 2  is a side schematic view of a first mechanism in accordance with the present invention. 
           [0034]      FIGS. 3   a  to  3   e  are schematic views of the range of motion of the mechanism of  FIG. 2 . 
           [0035]      FIG. 4  is a side schematic view of a second mechanism in accordance with the present invention. 
           [0036]      FIG. 5  is a side schematic view of an application of the mechanism of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    Turning to  FIG. 1 , a known Evans mechanism  10  is shown schematically. The Evans mechanism  10  comprises a frame  12  which is fixed in use. The different areas of the frame  12  in  FIG. 1  are rigidly attached to each other. 
         [0038]    A first link  14  is provided, and pivotably connected to the frame  12  at a first joint  16  positioned at a first end of the first link  14 . 
         [0039]    A coupler  18  is provided which is pivotably connected via a second rotational joint  20  to the first link  14  at a second end of the first link  14  and a first end of the coupler  18 . 
         [0040]    A second link  22  is pivotably connected to the frame  12  via a third rotational joint  24  at a first end thereof. A second end of the second link  22  is pivotably connected to the coupler  18  via a fourth rotational joint  26 . 
         [0041]    In  FIG. 1 , L 1  is the distance between the third and fourth joints  24 ,  26  on the second link  22 . L 2  is the distance from the second joint  20  to the fourth joint  26  on the coupler  18  and L 3  is the distance between the fourth joint  26  and the end point  28  (i.e. the point which is to be manipulated). The mechanism is configured such that L2 2 =L1*L3, which provides the most accurate straight line motion for the end point  28 . 
         [0042]    The first rotational joint  16  and the third rotational joint  24  connecting the links  14 ,  22  with the frame  12  are spaced apart. It will also be noted that the rotational joints  16 ,  20 ,  24 ,  26  are positioned such that the first and second links  14 ,  22  are approximately 90 degrees to each other. 
         [0043]    The frame  12 , first and second links  14 ,  22  and the coupler  18  form a four bar link mechanism known in the art as an Evans mechanism. The coupler  18  extends from the first rotational joint  20  past the fourth rotational joint  26  to an end point  28 . When the first link  14  rotates clockwise about the first rotational joint  14 , and the second link  22  rotates about the third rotational joint  24 , linear motion of the end point  28  in direction D 1  results. 
         [0044]    The geometry of the mechanism (as described by L2 2 =L1*L3) dictates that for a significant part of the locus of the endpoint  28  during actuation, a substantially, or an approximation of linear motion is observed. Should the mechanism be actuated far beyond the position shown in  FIG. 1 , then the path of the end point  28  will deviate away from linear direction D 1  and become curved, however for a significant proportion of the movement of the mechanism, the path is linear. As such, the Evans mechanism is also known as a “straight line mechanism”. 
         [0045]    The Evans mechanism may be actuated in several ways. In the embodiment shown in  FIG. 1 , the Evans mechanism  10  is actuated by a separate actuation assembly  30 . 
         [0046]    The actuation assembly  30  comprises a first motor  32  which is mounted to the frame  12 . The motor  32  forms a fifth rotational joint  36 , about which a crank  34  is driven. A third link  38  is attached to a free end of the crank  34  via a sixth rotational joint  40  and to the coupler at a seventh rotational joint  42 . The seventh rotational joint  42  is positioned between the second rotational joint  20  and the fourth rotational joint  26  on the coupler  18 . Using the actuation assembly  30  the motor  32  can drive the crank  34  which in turn will push or pull the coupler  18  via the third link  38  to actuate the Evans mechanism and drive the end point  28  along in direction D 1 . 
         [0047]    It will be noted that other types of actuation assembly are possible, for example, rotation of the first or second links  14 ,  22  can be achieved by providing motors at the first or third rotational joints  16  or  24 . Provision of a motor at the joint  16  may be problematic depending on the range of motion used- at a position where the coupler  18  and the second link  22  are parallel, rotation of the joint  16  would not be possible via a torque about the centre of rotation of the joint. 
         [0048]    The Evans mechanism of  FIG. 1  can be used as a one degree of freedom manipulator. The present invention provides a  2  degree of freedom mechanism. This is achieved by the arrangement shown in  FIG. 2 . The reference numerals shown in  FIG. 2  are similar to those shown in  FIG. 1  for common features, albeit incremented by 100. 
         [0049]    A two degree of freedom mechanism  110  in accordance with the present invention comprises a frame  112 , a first link  114  connected to the frame  112  via a first rotational joint  116  and connected to a coupler  118  via a second rotational joint  120 . A second link  122  is provided being connected to the coupler  118  via a fourth rotational joint  126 . An actuation assembly  130  is provided, being substantially similar to the actuation assembly  30 , having a first motor  132  defining a fifth rotational joint  136 , a first crank  134  driven by the motor and a third link  138  connected between a sixth rotational joint  140  on the crank and a seventh rotational joint  142  on the coupler  118 . 
         [0050]    Instead of being directly attached to the frame  112 , the second rotational link  122  is connected to a second crank  144  at a third rotational joint  124 , which crank in turn is driven by a second motor  146  which is mounted on the frame  112 , the second motor forming an eighth rotational joint  125 . 
         [0051]    With the second crank  144  in a stationary position, the mechanism  110  acts in substantially the same manner as the Evans mechanism of  FIG. 1 . The end point  128  of the coupler  118  moves in direction D 1  when the mechanism is actuated by the first motor  132 . 
         [0052]    Per  FIG. 1 , L 1  is the distance between the third and fourth joints  124 ,  126  on the second link  122 . L 2  is the distance from the second joint  120  to the fourth joint  126  on the coupler  118  and L 3  is the distance between the fourth joint  126  and the end point  128  (i.e. the point which is to be manipulated). The mechanism is configured such that L2 2 =L1*L3, which provides the most accurate straight line motion for the end point  128 . 
         [0053]    However, it will be noted that the second crank  144  can also be driven in order to move the end point  128  of the coupler  118  in a direction D 2 , which is substantially perpendicular to the direction D 1 . As such a two-dimensional workspace W is formed in which the end point  128  is moved linearly in two, normal, directions. 
         [0054]    It will be noted that for the range of movement around the position shown in  FIGS. 2 , D 1  and D 2  are substantially straight and perpendicular. Movement out of the workspace W will result in progressively less rectilinear behaviour. 
         [0055]    Turning to  FIG. 3   a , a mechanism similar to that of  FIG. 2  is shown with its range of movement through a finite number of angles of both the degrees of freedom of the first and second cranks  134 ,  144 . It will be noted in  FIG. 3   a  that the first crank  134  is attached to the frame at the same point as the first link  114 , but this does not significantly affect the kinematics of the mechanism. 
         [0056]    The angle φ 1  represents the angle of the first crank  134  from its central position shown in  FIG. 3   a , and the angle φ 2  represents the angle of the second crank  144  about its central position shown in  FIG. 3   a.    
         [0057]      FIG. 3   a  shows the mechanism  110  at a position within the workspace W. Turning to  FIG. 3   b , the mechanism  110  is shown at a first corner of the workspace W, beyond which point the motion of the end point  128  becomes less linear. At the position of  FIG. 3   b , φ 1  is at −30 degrees and φ 2  at 40 degrees. Similarly, in  FIGS. 3   c  (φ 1 =−30, φ 2 =−40),  3   d  (φ 1 =30, φ 2 =−40) and  3   e  (φ 1 =30, φ 2 =−40) the mechanism  110  is shown at the extreme of movement after which its motions becomes significantly less linear i.e. outside of the defined workspace W. 
         [0058]    As can be seen by the gridlines in each of  FIGS. 3   a  to  3   e , motion of the end point  128  is relatively rectilinear and provides a good approximation to a 2 degree of freedom manipulator, such as the Cartesian slide manipulator mentioned earlier. 
         [0059]    In the embodiments of  FIGS. 2 and 3   a  to  3   e , it may be desirable to attach a pushrod to the endpoint  128  in direction D 1  (away from the mechanism  110 ). For example in a gait rehabilitation robot, a pushrod can be attached to the lower back of the subject. As mentioned, such robots need to guide the subject, and as such must resist forces from the subject to the mechanism. 
         [0060]    For provision of such a pushrod, it may be desirable to lock the rotation of the endpoint or to place the effective point outside the mechanism. For this purpose, referring to  FIG. 4 , there is shown a mechanism  210  which is better suited to reacting the forces from the subject via a pushrod by locking the rotation of an end effector. The mechanism has some components in common with the mechanism  110  as shown in  FIG. 2 . These will be numbered  100  greater. 
         [0061]    The mechanism  210  comprises a frame  212 , to which a first link  214  is pivotably attached via a first rotational joint  216  at a first end and pivotably attached via a second rotational joint  220  to a first coupler  218  at the second end. 
         [0062]    A second link  222  is connected to a first rocker  248  (to be described in more detail below) via a third rotational joint  224  and to the first coupler  218  via a fourth rotational joint  226 . As with the mechanism  110  an actuation assembly  230  comprising a first motor  232  defining a fifth rotational joint  236 , a first crank  234  driven by the first motor  232  and a third link  238  connected to the crank  234  via a sixth rotational joint  240  and to the coupler  218  via a seventh rotational joint  242 . 
         [0063]    A second actuation assembly  252  is provided comprising a motor  254  connected to the frame  212  and defining an eighth rotational joint  225 . The assembly  252  comprises a crank  256  and a push rod  258  connected to the crank  256  via a ninth rotational joint  259 . 
         [0064]    The first rocker  248  is a member mounted for rotation to the frame  212  via a tenth rotational joint  250 . The rocker  248  is driven in rotation about the tenth rotational joint  250  by the push rod  258  which is connected to the rocker  248  via an eleventh rotational joint  260 . Each of the joints  224 ,  250 ,  260  on the first rocker  248  are spaced apart so as to define the vertices of a triangle. 
         [0065]    A second rocker  262  is provided, identical to the first rocker but spaced therefrom, being attached to the frame  212  via a twelfth rotational joint  264 . The push rod  258  extends beyond the first rocker  248  to drive the second rocker  262  at a thirteenth rotational joint  265 . The second rocker also comprises a fourteenth rotational joint  276  as will be described below. 
         [0066]    A second coupler  266  is provided, being generally offset and parallel to the first coupler  218 . The second coupler  266  is connected to the first coupler  218  via a first intermediate link  268  and an end effector  284  (i.e. a pushrod), so as to form a parallel linkage (i.e. the opposing members are always parallel). The first intermediate link  268  is joined to the first rocker  218  via an fifteenth rotational joint  270 , proximate the joint  220  and to the second rocker  262  via a sixteenth rotational joint  272 . The second coupler  266  is driven by a fourth link  274  which attaches to the second rocker  262  via the fourteenth rotational joint  276 , and to the second coupler  266  via a seventeenth rotational joint  278 . 
         [0067]    The end effector  284  is connected to the first coupler  218  via an eighteenth rotational joint  228  and to the second coupler  266  via a nineteenth rotational joint  280 . 
         [0068]    In use the mechanism  210  can be actuated in much the same way as the mechanism of  FIG. 2 . With the second motor  254  stationary, the first motor  232  drives the end effector  284  per a normal Evans mechanism i.e. in linear direction D 1 . It will be noted that the two couplers  218 ,  266  remain parallel throughout the range of motion as they are constrained by the first intermediate member  268  and the end effector  284 . 
         [0069]    Motion in direction D 2  is provided by the motor  254  which drives the rockers  248 ,  262  to provide a vertical force through the second and fourth links  222 ,  274 . A benefit of this particular arrangement is that the end effector  284  remains horizontal and parallel to direction D 1 , so that it can resist any rotational motion as required. In other words the mechanism  210  is capable of applying forces to all and any point on the end effector  284 . It can be made any suitable shape to provide the desired location of the point of actuation. The system is also inherently stiffer, which is advantageous. 
         [0070]    It will be noted that as an alternative to the rockers  248 ,  262 , a pair of synchronised motor/crank assemblies could be used. 
         [0071]    Turning to  FIG. 5 , the mechanism  210  is shown connected to a harness  300  for a rehabilitation patient. The mechanism  210  is arranged in the horizontal plane as shown, such that the height of the subject or patient is in a direction perpendicular to the page. As such D 1  is in a fore aft direction of the subject and D 2  is in a left right direction. It will be noted that the frame  212  can be provided in a stationary fashion with the subject walking on a treadmill, or alternatively can be moveable, in order to provide the ability to the patient or subject to walk. In particular during gait rehabilitation, the systems can be used to support the patient and/or provide input forces as required. In particular, the mechanism of the present invention is particularly well-suited to use with systems which utilise admittance control, so that it can be configured to be effectively “transparent” or provide restorative or input forces as required.