Patent Publication Number: US-2020281799-A1

Title: Lower limb of an exoskeleton with low power consumption

Description:
FIELD OF THE INVENTION 
     The invention relates to the field of exoskeletons and more particularly to the lower limbs of an ambulatory exoskeleton. 
     BACKGROUND OF THE INVENTION 
     Traditionally, a lower limb of an ambulatory exoskeleton comprises a pelvis segment, on which a first end of a leg segment is articulated about a hip, and a foot segment, which is articulated, by way of an ankle, on a second end of the leg segment. Control of the lower limb requires at least one actuator for driving the flexion of the leg segment during walking and for taking up the forces applied to the exoskeleton. The forces applied to the exoskeleton may be from several sources: a load applied to the exoskeleton (load-bearing exoskeleton), a partial substitution of the movements of the user (rehabilitation exoskeleton), and, generally, most of the actual weight of the exoskeleton. 
     Since the leg actuator takes up all of the load itself, it consumes a lot of power, including in static phases. The exoskeleton therefore has to comprise suitably dimensioned means of storing power (hydraulic, thermal or electric), said means impacting on the weight of the exoskeleton and affecting its autonomy and its inertia. Finally, in the event of a power supply failure, the entire mass of the exoskeleton and its possible load will weigh on the user, which may be dangerous to the latter. 
     OBJECT OF THE INVENTION 
     An object of the invention is to reduce the power consumption of an ambulatory exoskeleton and to increase safety in the event of a power failure or another software or hardware problem. 
     SUMMARY OF THE INVENTION 
     To this end, a lower limb of an ambulatory exoskeleton is provided, comprising at least a pelvis segment, a leg segment and a foot segment, the leg segment being articulated at its first end on the pelvis segment and at its second end on the foot segment, in which the leg segment comprises a spring element for exerting a force that opposes a movement of the ends of the leg segment toward each other, and means for varying the distance separating the ends of the leg segment in order to apply a force counter to that of the spring element, the means for varying said distance being carried by the pelvis segment. The means for varying the distance also comprise a device for pretensioning the spring element. 
     The spring element takes up the vertical forces applied to the exoskeleton without power consumption, particularly in the static phase when the pretensioning device makes it possible, without power consumption, to maintain tension in the spring element. The variation of the distance separating the ends of the leg segment allows the user to walk with such an exoskeleton without the spring element applying force to the user&#39;s foot during the swing phases of the foot. 
     The invention thus makes it possible to take up a load of unknown mass without the need to parameterize the lower limb, provided that this mass is less than the pretensioning force. 
     A particularly simple embodiment is obtained when the means for varying the distance separating the ends of the leg segment comprise a cable, which will advantageously be able to extend between the pelvis segment and the foot segment. The configuration is further simplified when the spring element is a leaf spring, it being possible for the leaf spring to perform the dual function of a spring element and of a structural component. The leaf spring can be positioned behind or in front of the leg of a user wearing the exoskeleton. 
     Advantageously, the means for varying the distance separating the ends of the leg segment comprise a geared motor and a winding pulley for winding and unwinding the cable. 
     According to a particular embodiment, the means for varying the distance separating the ends of the leg segment are controlled by a foot position sensor situated on the foot segment of the exoskeleton. According to an advantageous alternative, the sensor for detecting an intention to walk is situated on the ankle articulation located between the leg segment and the foot segment. 
     The variation of the distance separating the ends of the leg segment can then be directly controlled on the basis of the measurement output by this sensor, thus ensuring that the foot follows the leg of the user in the swing phase. 
     Other features and advantages of the invention will become clear from reading the following description of particular non-limiting embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will be made to the accompanying figures, in which: 
         FIG. 1  is a schematic profile view of a user wearing a first embodiment of the lower-limb exoskeleton according to the invention; 
         FIG. 2  is a schematic perspective view showing details of the exoskeleton according to the invention; 
         FIG. 3  is a schematic perspective view showing details of the first embodiment of the exoskeleton according to the invention; 
         FIG. 4  is a schematic profile view of a user wearing a second embodiment of the lower-limb exoskeleton according to the invention; 
         FIG. 5  is a schematic perspective view of a user wearing a third embodiment of the lower-limb exoskeleton according to the invention; 
         FIG. 6  is a schematic perspective view of a user wearing a fourth embodiment of the lower-limb exoskeleton according to the invention; 
         FIG. 7  is a schematic perspective view showing details of the fourth embodiment of the invention; 
         FIG. 8  is a schematic profile view of a user wearing a fifth embodiment of the lower-limb exoskeleton according to the invention; 
         FIG. 9  is a schematic profile view of a user wearing a sixth embodiment of the lower-limb exoskeleton according to the invention; 
         FIG. 10  is a schematic view of details of a seventh embodiment of the exoskeleton according to the invention; 
         FIG. 11  is a schematic view of a detail from  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIGS. 1 and 2 , the lower limb of an ambulatory exoskeleton, generally designated  1 , is worn by a user  100 . The lower limb  1  comprises a pelvis segment  10  on which is articulated the first end  21  of a leg segment  20  comprising a thigh segment  22 , articulated on a tibia segment  23 , and a foot segment  30 . The foot segment  30  is articulated on the tibia segment  22  at a second end  24  of the leg segment  20 . The pelvis segment  10  and the foot segment  30  are connected respectively to the pelvis  110  and the foot  130  of the user  100  by straps. Optionally, the thigh segment  22  and tibia segment  23  can be connected respectively to the thigh  122  and the tibia  123  of the leg  120  of the user  100  by straps. 
     The leg segment  20  comprises a leaf spring  25 , which extends behind the leg segment  20  and of which a first end  25 . 1  is articulated on the pelvis segment  10  and a second end  25 . 2  is articulated on the foot segment  30 . The pelvis segment  10  carries an electric geared motor  40 , which is powered by accumulators  41  and of which the output shaft  42  carries a winding pulley  43 . A cable  44  extends between the foot segment  30  and the winding pulley  43 . More precisely, a first end  45  of the cable  44  is connected to the second end  25 . 2  of the leaf spring  25 . 
     As can be seen in  FIG. 2 , the foot segment  30  is coincident here with the sole  131  of the shoe  132  of the user  100  and comprises a first element  31  of foot segment  30  that is rigidly connected to the foot  130  of the user and that is articulated by a hinge  32  on a second element  33  of foot segment  30 . The second element  33  is substantially in the shape of a horseshoe and extends around the heel of the user. The second element  33  is itself articulated on the end  25 . 2  of the leaf spring  25 . A rotary encoder  34 , here a rotary potentiometer, measures the relative angular position of the second element  33  of foot segment  30  and the leaf spring  25 . From this angular position it is possible to deduce the attitude (estimation of the length) of the leg  122 , hence the distance separating the pelvis segment  10  from the foot segment  30 . Controlling the distance d on the basis of this angular position makes it possible to follow the foot of the user  100  when the leg  122  is in the swing phase (lifted from the ground). 
     With reference to  FIG. 3 , the pulley  43  comprises a stop  46  for abutting the end  11  of a screw  12 , which is engaged in a thread  13  formed in a bracket  14  rigidly connected to the pelvis segment  10 . 
     The geared motor  40  is connected to a command and control unit  50  carried by the pelvis segment  10 . As can be seen in  FIG. 1 , the cable  44  is located in a plane comprising the axis of articulation of the leg segment on the pelvis segment  10 , so as not to create a parasitic torque on the hip segment  10 . However, it may be of interest to have the cable run behind the point of rotation of the hip if it is desired to create a return torque for the back (especially in order to assist the bending of the user&#39;s back and thus to protect the lumbar vertebrae). It is possible to completely eliminate the parasitic torques in abduction by running the cable  44  through the center of rotation of the articulation of the leg segment  20  on the pelvis segment  10 . Such an embodiment is shown in  FIG. 3 . The first end  25 . 1  of the leaf spring  25  comprises a first clevis  26  having two first lugs  26 . 1  and  26 . 1 , which cooperate respectively in rotation with two second lugs  15 . 1  and  15 . 2  of a second clevis  15  rigidly connected to the pelvis segment  10 . The first lugs  26 . 1  and  26 . 2  bear against inner faces of the second lugs  15 . 1  and  15 . 2 , and the cable  44  extends between the two first clevises  26 . 1  and  26 . 2 . 
     The pelvis segment  10  comprises an interface  60  for carrying a payload  61 . 
     During operation and before the exoskeleton  1  is fitted in place on the user  100 , the stop  46  and/or the length of the cable  44  are/is adjusted such that the distance d separating the pelvis segment  10  from the foot segment  30  is substantially equal to the distance separating the pelvis  110  from the foot  130  of the user  100 . The leaf spring  25  is then pretensioned, even in the absence of a torque exerted by the geared motor  40  on the pulley  43 . Once the exoskeleton  1  is connected to the user  100 , a load  61  is fixed on the interface  60 . The leaf spring  25  is dimensioned so as not to bend under the weight of the exoskeleton  1  plus the weight of the load  61 . Depending on the characteristics of the leaf spring  25  (material, cross section), the load supported by the user  100 , and taken up here by his leg  120 , can vary from 0% (total assistance) to 100% (no assistance) of the total load, which comprises the inherent weight P E  of the exoskeleton  1  and the weight of the load  61 . 
     With the user standing upright, the leaf spring  25  takes up vertical forces to which the exoskeleton  1  is subjected. A minimum amount of power is consumed by the exoskeleton  1  in this configuration since, with the pulley  43  being against its abutment, the geared motor  40  does not supply a torque and consumes little power (or none at all). When the user  100  wishes to walk, he transfers part of the load exerted on his foot  130  to his other foot and starts to lift his heel. 
     The encoder  34  measures a change in the relative angular position of the exoskeleton foot with respect to the leaf spring  25  and transmits this information to the control unit  50 . The unit  50  analyzes this measurement and then commands a rotation of the geared motor  40 , which acts on the cable  44  in order to adapt the length of deployed cable to the movement of the foot segment  30 . Thus, the control unit  50  dictates the distance d on the basis of the measurement of the attitude of the leg  122  and ensures a variation of the distance d while the leg  120  of the user is in the swing phase (lifted from the ground). For example, the torque reference value can correspond to a tensile load of about three hundred Newton applied by the geared motor  40  to the cable  44 , which corresponds to a tension of the leaf spring  25  making it possible to support the weight of the exoskeleton and an additional load of twenty kilograms when the leg  22  of the exoskeleton  1  is lifted. During the swing phase of the foot  130  of the user, the control unit  50  dictates the distance d as a function of the position of the foot  130  of the user  100  in such a way that no force is applied by the leaf spring  25  to the foot  130  of the user  100  during its movement remains lower. 
     Thus, a lower-limb exoskeleton  1  is obtained which consumes a tiny amount of power, if indeed any, in the stance phase, and of which the power consumption in the walking phase is reduced, since the geared motor  40  consumes power exclusively when the leg  120  is not straight (that is to say only in a part of the walking phase during which the leg  120  is in the air). 
     The elements that are identical or similar to those described above will bear an identical reference number in the following description of the second, third, fourth and fifth embodiments of the invention. 
     With reference to  FIG. 4  and according to a second embodiment of the invention, the foot segment  30  comprises a highly sensitive linear potentiometric sensor on the foot segment  30  of the exoskeleton  1  and connected to the unit  50 . 
     This sensor  62  measures the distance that separates the first element  31  from the second element  32  of foot segment  30 . From this distance it is possible to deduce the attitude (angular position and length) of the leg  122 , hence the distance separating the pelvis segment  10  from the foot segment  30 . The intention of the user  100  to leave the stance phase in order to walk is detected when the sensor  62  measures a movement. This measurement is transmitted to the unit  50 , which then controls the geared motor  40 . 
     This sensor  62  can also be composed of strain gauges in such a way as to measure the strain between the foot  30  and the ankle  25 . 2  (the distance d is then controlled on the basis of the strain measured, with a zero setpoint or with a slight offset). 
     With reference to  FIG. 5  and according to a third embodiment of the invention, the pelvis segment  10  comprises a ball-type slideway  70  oriented substantially vertically when the exoskeleton  1  rests on a horizontal ground surface. A first carriage  71  is mounted freely, or with a weight compensation spring, to slide on the slideway  70 . The first carriage  71  carries the interface  60 , to which the load  61  is connected, and also the geared motor  40 , the pulley  43 , its stop  46 , the screw  12  and the bracket  14 . Other elements of the pelvis segment  10  can also be carried by the first carriage  71 , for example the command and control unit  50  and/or the electric accumulators  41 . 
     The functioning of the exoskeleton  1  is identical to what has been described above. The set-up of the geared motor  40  and of the interface  60  for carrying the load  61  permits a free flexion travel of the legs (making it possible, for example, to crouch down) free, during which it is not necessary to adjust the geared motor  40 , which again makes it possible to limit the power consumption of the exoskeleton  1 . This can prove particularly useful in the case of an exoskeleton for assisting in tasks that require brief and frequent bending of the legs, for example for picking things up from the ground (items of rubbish, crops) or for raking. 
     An upper stop  72  can also be provided in order to limit the flexion travel; this travel can also be managed with the aid of the encoder  34  and the control unit  50 . 
     With reference to  FIGS. 6 and 7  and according to a fourth embodiment of the invention, the pelvis segment  10  comprises a ball-type slideway  73  oriented substantially vertically when the exoskeleton rests on a horizontal ground surface. A second carriage  74  is mounted to slide on the slideway  73 . An electric actuator  75 , connected electrically to the unit  50 , extends between the pelvis segment  10  and the second carriage  74  in such a way that a retraction of the actuator  75  causes a translation of the second carriage  74 , which moves the latter away from the first end  21  of the leg segment  20 , which end  21  is articulated on the pelvis segment  10 . As can be seen in  FIG. 6 , the first end  45  of the cable  44  is connected to the second end  25 . 2  of the leaf spring  25  and, as can be seen in  FIG. 7 , the second end  46  of the cable  44  is coupled to the pelvis segment  10 , here to a point of the slideway  73 . The second carriage  74  carries an idler pulley  76  about which the cable  44  winds. Thus, a movement of the second carriage  74  makes it possible to act on the deployed length of cable  44  and therefore on the distance d separating the pelvis segment  10  from the foot segment  30 . As can be seen in  FIG. 7 , a lower stop  16 , fixed to the pelvis  10 , makes it possible to limit the downward travel of the second carriage  74  and thereby limit the maximum length of the distance d, while placing the actuator  75  in a rest position (no force taken up by the actuator  75 , hence no power consumption). 
     When the user  100  wishes to walk, he transfers some of the load exerted on his foot  130  to his other foot and begins to lift his heel. 
     The encoder  34  measures a change in the relative angular position of the foot of the exoskeleton with respect to the leaf spring  25  and transmits this information to the control unit  50 . The unit  50  analyzes this measurement and then commands a retraction of the actuator  75  which, moving the second carriage  74 , pulls on the cable  44  in order to adapt the length of deployed cable  44  to the movement of the foot segment  30 . Thus, the control unit  50  dictates the distance (d) on the basis of the measurement of the attitude of the leg  122  and ensures a variation of the distance d while it is in the swing phase (lifted from the ground). 
     The third and fourth embodiments can be combined, and the first carriage  71 , which is mounted freely, can receive the second carriage  74 , which is controlled. 
       FIG. 8  shows a fifth embodiment, in which the leaf spring  25  is positioned in front of the leg  22  of the exoskeleton  1 . The curvature of the leaf spring  25  is here oriented forward, although it can also be oriented rearward. 
     In a sixth particular embodiment, shown in  FIG. 9 , the cable  44  winds on an upper portion of a first idler deflection pulley  17  and thereafter winds on a lower portion of a second idler deflection pulley  18 . The cable  44  then winds about the winding pulley  43 . The first and second idler deflection pulleys  17  and  18  make it possible to move the point of traction of the cable  44  on the pelvis segment  10  in such a way as to control the torque caused by the traction of the cable  44  on the pelvis segment  10  and on the user  100 . The first and second idler deflection pulleys  17  and  18  will also be able to be mounted elastically so as to automatically take up the slack in the cable  44  in the manner of a preloaded tensioning roller. This type of device is particularly useful in situations where the user  100  wishes to crouch down and compresses the leaf spring  25 , causing a slackening of the cable  44  that it is necessary to compensate. 
     The slack in the cable can also be taken up electro-mechanically, for example with the aid of a torque sensor  47  (not shown) positioned on the output of the geared motor  40 . The sensor  47  is connected to the control unit  50 , which controls the geared motor  40  in such a way that the latter applies a minimum torque Cmin to the winding pulley  43  and thereby ensures a minimum tension Tmin of the cable  44 . Thus, when the user  100  wishes to crouch down, his own weight exerts a pressure on the leaf spring  25 , the effect of which is to reduce the distance d and therefore the tension in the cable  44 , the torque exerted on the output shaft  42  of the geared motor  43  falls, and the control unit  50  commands a rotation of the geared motor  40  in order to wind the cable  44  until the torque applied to the output pulley  43  by the geared motor  40  reaches the value of the minimum torque Cmin, thus bringing the tension of the cable  44  to the value Tmin, avoiding slack in the cable  44 . 
     In the third embodiment of the invention, the device for taking up slack in the cable comprises a movement sensor  77  (here a potentiometric sensor) which detects movement of the carriage  71  and is connected to the control unit  50 , which then commands a rotation of the geared motor  40  in order to wind the cable  44 , the amplitude of the rotation of the geared motor  40  depending on the movement of the carriage  71  measured by the movement sensor  77 . The measurement of the movement of the carriage  71  makes it possible to ascertain the intention of the user  100  to crouch down. 
     According to a seventh embodiment of the invention and with reference to  FIG. 10 , the foot segment  30  is here coincident with the sole  131  of the shoe  132  of the user  100  and comprises a first element  31 , of foot segment  30 , that is rigidly connected to the foot  130  of the user and has substantially the shape of a horseshoe and extends around the heel of the shoe  132 . The first element  31  comprises a bracket  35 , of which the distal end  35 . 1  is connected by a hinge  36  to a cylindrical slide  37  through which the cable  44  passes. The slide  37  comprises a resistance rotary potentiometer  38 , of which the rotor  38 . 1  is provided with means for elastic return to its initial position, which corresponds to a resistance equal to zero, measured at its terminals. A wire  38 . 2  connects the second end  25 . 2  of the leaf spring  25  to the rotor  38 . 1 . The rotary potentiometer  38  is connected to the control unit  50 . Depending on the resistance measured at the terminals of the rotary potentiometer  38 , the control unit  50  estimates the relative angular position of the first element  31  of foot segment  30  and of the leaf spring  25 . From this angular position it is possible to deduce the attitude (estimation of the length) of the leg  122  and therefore the distance separating the pelvis segment  10  from the foot segment  30 . Controlling the distance d on the basis of this angular position makes it possible to follow the foot of the user  100  when the leg  122  is in the swing phase (lifted from the ground). 
     Of course, the invention is not limited to the described embodiments and instead encompasses any variant covered by the field of the invention as defined by the claims. 
     In particular,
         although the exoskeleton here has only a single lower limb, the invention also applies to an exoskeleton provided with two lower limbs;   although the pelvis, thigh, tibia and foot segments are here connected to the user by straps, the invention also applies to other means of fastening the segments of the exoskeleton to the user, for example rigid cylindrical elements or specific garments rigidly joined to the segments of the exoskeleton;   although the thigh and tibia segments are here connected to the user, the invention also applies to thigh or tibia segments that are independent of the user;   although the leg segment here comprises a thigh segment articulated on a tibia segment, the invention also applies to a leg segment which is without thigh and tibia segments and which would comprise only the leaf spring and the cable (the leaf spring then serving as a structural component);   although the leg segment here comprises a leaf spring, the invention also applies to other types of spring elements, for example a stack of Belleville washers, an elastomeric element, a gas spring, a coil spring (for example on the knee), a buffer spring, a deformable parallelogram structure, or a block of elastic cables;   although the lower limb here comprises a geared motor and a pulley providing the traction on a cable, the invention also applies to other means of varying the distance separating the ends of the leg segment, for example a block moved by a ball screw or a rack;   although an electrical actuator here causes the translation of the carriage relative to the pelvis segment, the invention also applies to other types of actuators for moving the carriage relative to the pelvis segment, for example a screw/nut assembly, one of the components of which is motorized, a toothed belt drive, a geared motor which is rigidly connected to the carriage and of which the output shaft engages with a rack rigidly connected to the slideway;   although the cable here is connected to the foot segment, the invention also applies to other points of connection of the cable to the exoskeleton that are able to bring about a variation of the distance separating the ends of the leg segment, for example a connection to the ankle, the tibia segment or the thigh segment;   although the actuation system is here fixed to the pelvis segment, the invention also applies to other points for fixing the actuation system that are able to bring about a variation of the distance separating the ends of the leg segment, for example fixing to the hip, the top of the leg, the tibia segment or the thigh segment;   although the pulley here comprises a stop cooperating with a nut, the invention also applies to other devices for pretensioning the spring element and to other locations on the chain of actuation;   although the foot segment here comprises a linear or rotary potentiometric sensor on the foot or ankle, the invention also applies to other types of sensor for detecting the intention to walk, for example a pressure sensor, a force sensor or a rotation sensor linked to an articulation of the ankle segment, or even contact-free measurement systems (for example: optical sensor, magnetic sensor, etc.);   although the leaf spring here is positioned to the rear of the leg of the user, the invention also applies to other set-ups of the leaf spring, for example a leaf spring positioned to the front of the leg of the user (curvature toward the front), or on the outer side of the legs of the user;   although the geared motor here comprises a pulley for winding a cable, the invention also applies to other means of varying the distance separating the ends of the leg segment, for example a wheel cooperating with a flexible force-transmitting element, for example a toothed belt or a chain;   although the articulation of the leg segment on the pelvis segment here comprises two first lugs of a clevis rigidly connected to the leaf spring, which cooperate in rotation with two second lugs of a clevis rigidly connected to the pelvis segment, the invention also applies to other means of articulation of the leg segment on the pelvis segment that allow the cable to pass through the center of the articulation, for example a perforated shaft;   although the pelvis, the motors, the batteries and the controller are here at the back of the user, the invention applies to the case where the exoskeleton is worn the other way round, that is to say with the pelvis in front of the user;   although the lower limb here comprises a geared motor and a pulley providing the traction on a cable, the invention also applies to other means of varying the distance separating the ends of the leg segment, for example an SPC (supercoiled polymer) actuator, in which the variation of the length of the cable is obtained by heating the latter through application of an electrical voltage to its terminals;   although the lower limb here comprises a potentiometric movement sensor detecting the movement of the carriage, the invention also applies to other types of sensors that detect crouching, for example an inductive, capacitive or optical movement sensor for detecting movement of a carriage, or, in the case of a lower-limb exoskeleton without carriage, an inertial unit;   although the foot segment here is coincident with the sole of the shoe of the user, the invention also applies to other configurations of the foot segment of the robot, for example a platform coupled by strapping, adhesive bonding or screwing to the foot (or the shoe) of the user;   although the means for taking up slack in the cable here comprise a torque sensor positioned on the output of the geared motor, the invention also applies to means for taking up slack in the cable that comprise a slackness sensor carrying out a cable slack measurement directly on the cable;   although the potentiometer here is a resistance rotary potentiometer, the invention also applies to other types of sensors for detecting the foot position, for example a linear potentiometer, an inductive sensor, ultrasound sensor or capacitive sensor.       

     The leaf spring can be articulated on the foot segment and/or the pelvis segment at points identical to or different from the points of articulation of the leg segment on the foot and/or hip segment. 
     Advantageously, the cable  44  can be lined by a second cable, which is active or inactive (i.e. does or does not take up some of the tension of the cable  44 ) and which contributes to the operating safety and prevents the consequences of accidental rupture of the cable  44 . 
     Not all the articulations of the segments of the exoskeleton have been described, and they can be adapted to the specific use of the exoskeleton. For example, it will be possible for the hip flexions to be either actuated or left free or else coupled, depending on the circumstances. 
     Similarly, depending on the circumstances, it will be possible for the hip abduction articulations to be actuated, left free, or stressed (bilateral or unilateral abutment, with or without spring). The same applies to the ankle articulations.