Abstract:
A linkage ( 100, 200 ) having a plurality of small angle links ( 114, 116; 208, 212 ) with axes of rotation directed to a single point to allow movement of an recipient object without moving the point of action of an applied force (F).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is the U.S. national phase of International Application No. PCT/EP2014/056368 filed Mar. 28, 2014, which claims priority of British Application No. 1305984.5 filed Apr. 3, 2013, the entirety of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention concerns a mechanical linkage. More specifically, the present invention concerns a gimbal-type linkage for the transference of force between two points whilst permitting limited movement of a force recipient in selected degrees of freedom. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is often desirable to transfer a force between an actuator and a recipient in a mechanical system. In particular, it is often desirable to exert the force on a “virtual point” which may not be accessible in reality. 
         [0004]    For example in the field of rehabilitation robots for victims of neurological disorders, the actuator needs to be able to provide both actuating and supporting forces to move and support patient limbs. This may be done, for example, by using a rod attached at one end to an actuator, and at the other end to the limb of a subject, such as an arm or leg. 
         [0005]    The connection between the rod and the subject&#39;s limb may be achieved in several ways. For example, the rod may be rigidly attached to a strap or brace which secures around the limb. This type of connection does not allow any free movement between the rod and limb, and as such movement of the limb in all six degrees of freedom is dependent upon movement of the rod. 
         [0006]    This is problematic in rehabilitation. The aim is to progress the subject towards self supporting motion, and allowing some movement is beneficial as the feeling of freedom of movement inspires the subject to exercise control. 
         [0007]    A spherical or Cardan joint may be provided between the rod and strap, however this only provides freedom in two rotational degrees of freedom, about the centre of the physical joint. In the event that the force is applied to a joint, for example an ankle, this can be problematic, as once the ankle joint rotates about the spherical joint, the line of direction of force from the rod is no longer coincident with the centre of rotation of the ankle. This is not ideal, as it creates a moment on the ankle which resists movement back to the nominal, aligned position and therefore resists the subject&#39;s efforts to correct their gait. 
         [0008]    What is required is a connection which allows transmission of a force from an actuator to a recipient, but minimises constraint free rotation of the recipient. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an aim of the present invention to overcome, or at least mitigate the above referenced problems. 
         [0010]    According to a first aspect of the invention there is provided an assembly for transferring a force from a force provider to a recipient object, the assembly comprising:
       a force provider;   a recipient object;   a linkage configured to transfer a force from the force provider to the recipient object, the linkage comprising a first link and a second link;   the first link having a first rotational joint defining a first link axis, and a second rotational joint defining a second link axis, the first and second link axes being at a first link angle to each other, which first link is pivotably attached to the recipient object for rotation about the first axis; and,   the second link having a third rotational joint defining a third link axis, and a fourth rotational joint defining a fourth link axis, the third and fourth link axes being at a second link angle to each other, the second link being pivotably attached to the first link such that the second and third link axes coincide, the second link being pivotably mounted at the fourth link axis to transfer a force from the force provider,   in which the first, second, third and fourth axes intersect at a predetermined point such that the recipient object can rotate about the predetermined point by articulation of the linkage;   in which the first and second link angles are each less than 45 degrees.       
 
         [0018]    Advantageously, this gimbal-like mechanism allows forces to be applied by the force provider to the predetermined point. The recipient object can articulate the linkage to move about the predetermined point, but importantly the force remains acting through that point during articulation of the linkage. Therefore forces can be applied in a desired direction, and movement of the recipient object is permitted without distortion of the force, or production of undesirable torques. The use of small angle links (less than 45 degrees) allows for a reasonable range of movement whilst keeping the arrangement compact. Preferably the sum of the internal angles of the links should be less and 90 degrees for this reason. The angles may be selected to provide a limited range of motion as desired—for example to prevent over-articulation in the case of a rehabilitation robot. 
         [0019]    Preferably the force provider is a push-pull rod driven by an actuator, such as a linear actuator or preferably a rotational actuator with a crank. 
         [0020]    As mentioned, preferably the push-pull rod and the actuator are configured to exert a force through the predetermined point. 
         [0021]    Preferably the rotational joints are defined by bores in the links having common shafts extending therethrough. This allows for simple assembly. 
         [0022]    Preferably each link comprises a body having a mid section, a first end section and a second opposite end section, the end sections being angled with respect to the mid section. The rotational joints of the links may then be defined normal to respective end sections such that the sections are angled to provide the link angles. 
         [0023]    Preferably the links are constructed from generally tubular bodies, e.g. extruded bodies. This makes them light. 
         [0024]    The linkage may comprises a third link having a fifth rotational joint defining a fifth link axis, and a sixth rotational joint defining a sixth link axis, the fifth and sixth link axes being at a third link angle to each other, the third link being pivotably attached to the second link such that the second and third link axes coincide, the third link being pivotably mounted at the sixth link axis to transfer a force from the force provider. The linkage may comprise more than three links. 
         [0025]    The invention also provides a rehabilitation apparatus comprising a linkage according to the first aspect, in which the recipient object is a support for a body part. For example for a leg, arm or torso. 
         [0026]    Preferably the support for a body part is configured to receive a body part proximate a joint to as to align an axis of rotation of the joint with the predetermined point of the linkage. This keeps the point of action of the rehabilitation force on the joint. 
         [0027]    More preferably the support for a body part is configured to receive a body part proximate a joint to as to align a centre of rotation of the joint with the predetermined point of the linkage. 
         [0028]    The force provider may be configured to exert a force in a first direction, in which the first link axis is oriented at 80 to 100 degrees to the exerted force. Preferably it is oriented at 90 degrees. 
         [0029]    Preferably the support is a foot support, and the force provider is configured to exert the force in a fore-aft direction of a subject in use. Preferably the first link is connected to the support at a lateral side of the subject&#39;s foot in use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING VIEWS 
         [0030]    An example linkage in accordance with the present invention will now be described with reference to the accompanying figures, in which: 
           [0031]      FIG. 1  is a perspective view of a part of a gait training apparatus comprising a first linkage in accordance with the present invention; 
           [0032]      FIG. 2  is a detail view of a component of the linkage of  FIG. 1 ; 
           [0033]      FIG. 3  is a perspective view of the linkage of  FIG. 1 ; 
           [0034]      FIG. 4  is a side view of the linkage of  FIG. 1 ; 
           [0035]      FIGS. 5   a  and  5   b  are schematic views of a second linkage in accordance with the present invention; 
           [0036]      FIGS. 6   a  and  6   b  are schematic views of the linkage of  FIGS. 5   a  and  5   b  in a second position; 
           [0037]      FIGS. 7   a  and  7   b  are schematic views of the linkage of  FIGS. 5   a  and  5   b  in a third position; 
           [0038]      FIG. 8  is a diagram of the workspace and range of motion of a gait rehabilitation robot for the ankle degree of freedom; 
           [0039]      FIG. 9  is a diagram of the workspace and range of motion of a gait rehabilitation robot for the torso degree of freedom; 
           [0040]      FIG. 10  is a schematic perspective view of a further part of a gait training apparatus comprising a third linkage in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0041]    Referring to  FIG. 1 , there is shown a gait training assembly  100  comprising a foot brace  102 , a push rod  104  and a linkage  106 . 
         [0042]    The foot brace  102  comprises a base plate  108  configured to support the underside of a human foot (not shown), and a side plate  110  extending perpendicularly and vertically therefrom to support the outer side of the foot. The side plate  110  comprises a support member  112  also extending vertically from the base plate  108  on the outer side of the side plate  110 . 
         [0043]    A global coordinate system is shown in  FIG. 1  having axes X 0 , Y 0 , Z 0 . The foot brace  102  is arranged to hold a human foot with the centre of the ankle joint (modelled as a spherical joint for simplicity) A at the centre of the global coordinate system. 
         [0044]    The pushrod  104  has a main portion  105  connected to an actuator (not shown) arranged to provide a force in direction F, which is generally parallel to X 0 . An end portion  107  of the pushrod  104  is at an angle α to the main portion  105  as shown in  FIG. 3 . 
         [0045]    The linkage  106  comprises a first link  114  and a second link  116 . The links  114 ,  116  in this embodiment are similar, and as such only the link  114  will be described here with reference to  FIG. 2 . 
         [0046]    The link  114  comprises a tubular, prismatic body  115  having a hollow square cross-section. The link defines a first end  124  and a second end  132 . The body  115  comprises a first end portion  118 , a second end portion  120  and a middle part  122  joining the first and second end portions  118 ,  120 . A first pair of bores  128 ,  130  extend through the first end portion proximate the first end  124  of the body  115 . The bores  128 ,  130  define a first axis  126  normal to the surfaces of the body  115  in which the bores  128 ,  130  are defined. Similarly a second pair of bores  136 ,  138  extend through the second end portion proximate the second end  132  of the body  115 . The bores  136 ,  138  define a second axis  134  normal to the surfaces of the body  115  in which the bores  138 ,  136  are defined. The first and second end portions  118 ,  120  are angled to the middle part  122  at an angle Θ/2 such that the axes  126 ,  134  are at an included angle of Θ to each other. 
         [0047]    Referring to  FIG. 3 , the first link  114  is connected to the support member  126  via a first rotational joint  138 , which comprises a shaft engaged in the aligned bores  128 ,  130  of the link  114 . The axis  126  of the first link  114  is aligned with the global axis Z 0  such that the first link  114  can rotate relative to the foot brace  102 . 
         [0048]    The second link  116  is connected to the second end portion  120  of the first link  114  by a second rotational joint  140  which allows the first and second links  114 ,  116  to rotate relative to one another about the second axis  134  of the first link  114 , and a first axis  126 ′ of the second link  116 , which are aligned. 
         [0049]    The end portion  107  of the pushrod  104  is connected to the second link  116  by a third rotational joint  142  for rotation about a second axis  134 ′ of the second link  116 . 
         [0050]    In the configuration shown in  FIGS. 3 and 4  (i.e. with the links in-line), the links  114 ,  116  span an angle of 2Θ relative to the axis  126 . 
         [0051]    Referring to  FIGS. 5   a  and  5   b , a schematic representation of a second assembly  200 , which works on the same principle as the first assembly  100 . The assembly  200  comprises a foot brace  202  holding a human foot  2 . The ankle joint of the human foot  2  is coincident with the global coordinate system X 0 , Y 0 , Z 0  having an origin O. 
         [0052]    A linkage  204  comprises a first joint connected to the foot brace  202  defining a first axis  206  (aligned with Z 0  in  FIGS. 5   a  and  5   b ). A first link  208  is connected to rotate about the first axis  206 , and is connected to a second joint defining a second axis  210 . A second link  212  is connected to rotate about the second axis  208  relative to the first link  208 , and is connected to a third joint defining a third axis  214 . The second link is thereby rotationally connected to a pushrod  216 . 
         [0053]    The axes  206 ,  210  and  214  intersect at a common point which is coincident with the origin O of the ankle. The links  208 ,  212  therefore form a gimbal-like mechanism permitting movement of the foot  2  in a controlled manner. 
         [0054]    The foot  2  in  FIGS. 5   a  and  5   b  is in a neutral, or static position, and as can be seen, force F when applied to the pushrod  216  in a direction generally aligned with X 0  will urge the foot forwards via the brace  202 . The pushrod  216  is configured such that the force F acts through the centre of rotation O of the ankle. Therefore the subject does not feel any torque on the ankle. 
         [0055]    Turning to  FIGS. 6   a  and  6   b , the foot  2  has undergone an endorotation of β degrees about the vertical axis Y 0 , thus providing new foot axes X 0 ′, Z 0 ′ each of which are rotated by β degrees from the global axes X 0 , Z 0 . 
         [0056]    Because of the constraint of the first joint defining axis  206  to the brace  202 , the axis  206  rotates with the foot axis Z 0 ′ by β degrees. Although this is the case, articulation of the links  208 ,  212  means that the pushrod  216  (and therefore the axis  214 ) remains in the same position. Therefore the force F can still be applied in the same direction, through the origin O of the ankle joint. This allows the foot to undergo exorotation and endorotation whilst the rehabilitation is taking place. During walking, the subject ankle undergoes a few degrees of endo-exorotation (this is normal). By allowing this rotation, gait is allowed to occur naturally without unnecessary restriction on this movement. There is the possibility to apply corrective forces on the ankle, e.g. increase step length or to place the foot more outward, without applying endo exorotation. Exo- and endo-rotation does not result in any forces being applied which are not coincident with the origin O of the ankle. In particular, no forces which oppose movement of the ankle back to the neutral position shown in  FIGS. 5   a  and  5   b  are applied. 
         [0057]    It will be noted that the range of movement of the mechanism is limited by the included angles Θ of the links  208 ,  212 . 
         [0058]    Turning to  7   a  and  7   b,  the foot  2  has undergone an inversion of γ degrees about the horizontal axis X 0 , thus providing new foot axes Y 0 ′, Z 0 ′ each of which are rotated by γ degrees from the global axes Y 0 , Z 0 . 
         [0059]    Because of the constraint of the first joint defining axis  206  to the brace  202 , the axis  206  rotates with the foot axis Z 0 ′ by γ degrees. Although this is the case, articulation of the links  208 ,  212  means that the pushrod  216  (and therefore the axis  214 ) remains in the same position. Therefore the force F can still be applied in the same direction, through the origin O of the ankle joint. This allows the foot to undergo inversion and eversion whilst the rehabilitation is taking place. Inversion and eversion does not result in any forces being applied which are not coincident with the origin O of the ankle. In particular, no forces which oppose movement of the ankle back to the neutral position shown in  FIGS. 5   a  and  5   b  are applied. 
         [0060]    Again, it will be noted that the range of movement of the mechanism is limited by the included angles Θ of the links  208 ,  212 . 
         [0061]    As seen in the Figures, the point on at which the first link  208  is attached to the brace  202 , is on the side of the foot (as opposed to the force F, which is applied from the rear of the foot towards the front). As such, the axis  206  of the attachment of the linkage  204  to the brace  202  is at 90 degrees to the direction of the applied force F. Therefore the included angle Θ is less than 45 degrees (and is 17 degrees in this example). This provides a reasonable degree of movement of the foot  2 , whilst ensuring that the linkage  204  is compact. 
         [0062]    During rehabilitation, it is desirable to provide 25 degrees of dorsiflexion, and 35 degrees of plantarflexion (rotation about Z 0 ). Due to this significant range of motion required, the first axis (which is highly unconstrained) is aligned to the ankle flexion axis (i.e. the first link is positioned at the side of the foot). 
         [0063]    Turning to  FIG. 8 , the required range of motion for an ankle support is shown (for inversion/eversion and endo/exo rotation). Endo/exo rotation is shown on the horizontal axis, and inversion/eversion on the vertical axis. 
         [0064]    The circular area  300  represents the range of motion provided by a linkage having two links each with Θ=12 degrees. The circle has a diameter D of 24 degrees—i.e. 2Θ. This means that a potential range of 24 degrees in either degree of freedom is possible. 
         [0065]    10 degrees of ankle inversion and eversion (i.e. rotation about a horizontal fore-aft axis) are also desirable. It is desirable to provide a hard limit for inversion to avoid injury (the most common type of ankle sprains arise from inversion). 
         [0066]    10 degrees of endorotation and 20 degrees of exorotation are also required for normal gait. 
         [0067]    The required workspace  302  is also shown (as defined by the range of motion above). It is rectangular because the range of angular motion is specified by the design and geometry of the linkage. The workspace is a two-dimensional area because it is possible to undergo two types of motion simultaneously (because the ankle is effectively a ball joint). 
         [0068]    By configuring the linkage appropriately, a centre  304  of the circular workspace  300  can be moved such that it is at 5 degrees exorotation (i.e. the mid-point of the exo-endo rotation range), and 10 degrees inversion. Usefully, this positions almost all of the required workspace  302  within the range of motion of the linkage, and also provides a hard limit of around −10 degrees inversion (although in reality this is dependent upon simultaneous endo/exo rotation). 
         [0069]    The centre of the circle in  FIG. 8  is the position of the links at which the axes  206  and  214  coincide. This can be determined by appropriate mechanical set up of the link axes during manufacture and assembly. 
         [0070]    A similar example is shown in  FIG. 9  for the human torso. In this instance, the linkage is attached with its virtual centre on the hip joint. Two linkages may be used—one centred on each hip joint to provide full freedom of movement (this is described in more detail with reference to  FIG. 10  below). The present invention is particularly advantageous because forces should be applied into the hip joint without producing torques on the pelvis. 
         [0071]    In  FIG. 9 , the range of motion of the linkage is shown as area  400 . Pelvis axial rotation is represented on the horizontal axis, and pelvis sagittal rotation on the vertical axis. With two links having Θ=17 degrees, the diameter D′ is 34 degrees. 
         [0072]    The desired degrees of freedom required by the pelvis during gait are as follows:
       (i) rotation of the pelvis in the frontal plane about a fore-aft axis (frontal rotation);   (ii) rotation of the pelvis in the horizontal, transverse plane about a vertical axis (axial rotation); and   (iii) rotation of the pelvis in the sagittal plane about a left-right axis (sagittal rotation).       
 
         [0076]    The linkage is positioned behind the patient—at the side of the patient there is no available space, since in gait the subject&#39;s arms must be able to swing. 
         [0077]    The range of motion required by the pelvis is as follows:
       Frontal rotation=±10 degrees;   Axial rotation=±15 degrees;   Sagittal rotation=−26.9 to 22.6 degrees.
 
The above range provides a required workspace  402 .
       
 
         [0081]    The linkage is assembled such that the axis equivalent to Z 0  is aligned with the frontal axis of rotation (i.e. in a fore-aft direction), and positioned at the rear of the subject. In effect, the linkage does not therefore limit pure frontal rotation. This is not problematic, as during gait excessive frontal rotation is not a problem (it would be very difficult to achieve). 
         [0082]    As mentioned above, ±15 degrees axial rotation is required, and as such the minimum link angle (for two links) Θ=15/2=7.5 degrees. 
         [0083]    One minor drawback of the linkage is that the centre point (the origin of the circle in  FIG. 9 ) creates a slightly distracting sensation for the subject, and therefore it would not be desirable to have this in the “normal” range of motion. As such, it is offset by 10 degrees in axial rotation. 
         [0084]    From the circle centre  404 , the angle Θ is selected so that the circle  400  covers the required workspace  400 . In this case Θ=17 degrees. 
         [0085]    Turning to  FIG. 10 , there is shown a gait training assembly  500  comprising a waist band  502 , a first push rod  504 , a first linkage  506 , a second push rod  508  and a second linkage  510 . 
         [0086]    The waist band is configured to surround the abdomen of a subject to affix the linkages  506 ,  510  at the rear of the respective left and right hips. 
         [0087]    The first linkage  506  comprises a first link  512  pivotably attached to the first push rod  504  about an axis  513  and a second link  514  pivotably attached to the first link  512  for rotation about an axis  515 . The second link  514  is also pivotably attached to a bracket  516  downwardly depending from the waist band  502  for rotation about an axis  518 . 
         [0088]    Each link  512 ,  514  has an included angle of Θ between its respective axes of rotation, as with previous embodiments. All the axes  513 ,  515 ,  518  intersect at a hip joint centre of rotation H, and the assembly  500  is configured such that this imaginary intersection point is aligned with the subject&#39;s hip. 
         [0089]    The second linkage  510  is configured in substantially the same way as the first linkage  506 . 
         [0090]    Variations fall within the scope of the present invention. 
         [0091]    For example, more than two links may be provided for extra range of movement. The eventual number of links must be chosen for the specific application, as although an increased number of links would provide a more fluid motion, the stiffness of the linkage is decreased. 
         [0092]    Another variation is a simple arrangement that points to the centre of mass of the pelvis—between the hips (instead of at each hip joint). With this setup you can apply forces to the centre mass without applying distracting torques on the subject. 
         [0093]    The present invention can be used to apply forces to other joints, providing the axes are aligned to intersect at the centre of rotation of the subject&#39;s joint.