Abstract:
The present invention discloses locomotion simulator comprising a base having a surface movable along a base axis, a post mounted to the base a pelvic structure and a hip-thigh mechanism wherein coordinated displacement of the pelvic structure and pivoting of the thigh segment assembly simulates patterns of locomotion. The pelvic structure includes a first support movably mounted to the post, the first support allowing a displacement of the pelvic structure along a first pelvic axis generally perpendicular to the base axis and a second support movably mounted to the first support, the second support allowing a displacement of the pelvic structure along a second pelvic axis generally parallel to the base axis. As for the hip-thigh mechanism, it is mounted to the second support and includes a hip joint having a pivot axis generally perpendicular to the displacement of the second support and a thigh segment assembly pivotally so connected to the hip joint as to pivot in a plan defined by the first and second pelvic axes.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application claims the benefits of U.S. provisional patent application No. 60/832,138 filed Jul. 21, 2006, which is hereby incorporated by reference. 

   TECHNICAL FIELD 
   The present invention relates to simulators. More specifically, the present invention is concerned with a human locomotion simulator. 
   BACKGROUND 
   Over the years, many kinds of leg prostheses have been devised in effort to replace the leg or legs that amputees have lost. All these leg prostheses have the difficult task of giving to these amputees a gait as normal as possible. The complexity of human locomotion, however, is such that conventional leg prostheses have until now only been using passive mechanisms where the “computerized” passive leg prosthesis are considered on the market as the most sophisticated available devices. Conventional leg prostheses are very limited compared to a real human leg and some needs were thus not entirely fulfilled by them. 
   According to amputees, specific conditions of use of conventional leg prostheses, such as repetitive movements, continuous loading and assisted mobility from the amputee, typically entail problems such as increases in metabolic energy expenditures, increases of socket pressure, limitations of locomotion speeds, discrepancies in the locomotion movements, disruptions of postural balance, disruptions of the pelvis-spinal column alignment, and increases in the use of postural clinical rehabilitation programs. 
   Another problem is that during the amputees&#39; locomotion, energy used for moving the prosthesis mainly originates from the amputees themselves because conventional leg prostheses do not have self-propulsion capabilities. This has considerable short and long-term negative side effects. Recent developments in the field of energy-saving prosthetic components have partially contributed to improve the energy transfer between the amputees and their prosthesis. Nevertheless, the problem of energy expenditure is still not fully resolved and remains a major concern in the field of prosthesis and orthosis. 
   The difficulty related to the development of such complex leg prostheses design is compounded by the lack of testing equipment that realistically simulate human locomotion. The use of such testing equipment would allow the designers to perfect the leg prosthesis at early design stages. As well, a human locomotion simulator would permit, throughout the development, to test efficiently in controlled conditions the performance of prosthesis in various conditions such as walking, running, ascending or descending stairs, for example. Moreover, the use of such simulator means that the whole development and the perfecting of leg prosthesis is carried out without clinical trials with humans; which is benefic in terms of security. Furthermore, without limiting to this specific application, such testing equipment could be used also to test footwear to simulate more realistic environment of use. 
   Considering this background, it clearly appears that there was a need to develop a human locomotion simulator for the simulation of various types of gaits. 
   SUMMARY 
   In accordance with an illustrative embodiment of the present invention, there is provided a locomotion simulator comprising:
         a base having a surface movable along a base axis;   a post mounted to the base;   a pelvic structure including:   a first support movably mounted to the post, the first support allowing a displacement of the pelvic structure along a first pelvic axis generally perpendicular to the base axis;   a second support movably mounted to the first support, the second support allowing a displacement of the pelvic structure along a second pelvic axis generally parallel to the base axis;   a hip-thigh mechanism mounted to the second support, the hip-thigh mechanism including:
           a hip joint having a pivot axis generally perpendicular to the displacement of the second support;   a thigh segment assembly pivotally so connected to the hip joint as to pivot in a plan defined by the first and second pelvic axes;   
           wherein coordinated displacement of the pelvic structure and pivoting of the thigh segment assembly simulates patterns of locomotion.       

   
     BRIEF DESCRIPTION OF THE FIGURES 
     Embodiments of the invention will be described by way of example only with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic side elevational representation of the mechanical components of a human locomotion simulator according to an illustrative embodiment of the present invention, the stimulator being illustrated with a prosthesis leg attached; 
       FIG. 2  is a schematic side elevational representation of a portion of the pelvic structure, the hip joint, the thigh segment, the knee joint mechanical components and the leg prosthesis similar to  FIG. 1  and illustrating the various variables used in calculation; 
       FIG. 3  is a perspective view of a hip-thigh mechanism of the human locomotion simulator of  FIG. 1 ; 
       FIG. 4  is an exploded perspective view of a hip joint motor assembly of the hip-thigh mechanism of  FIG. 3 ; 
       FIG. 5  is an exploded perspective view of a hip joint ball-nut assembly of the hip-thigh mechanism of  FIG. 3 ; 
       FIG. 6  is an exploded perspective view of a thigh segment assembly of the hip-thigh mechanism of  FIG. 3 ; 
       FIG. 7  is a perspective view of a hip joint position sensor assembly of the thigh segment assembly of  FIG. 6 ; 
       FIG. 8  is a side elevational view of a pelvic structure and the hip-thigh mechanism portion of the human locomotion simulator of  FIG. 1 , illustrating the various bumper structures of the simulator; 
       FIG. 9  is a perspective view of the vertical and horizontal axis movement generators of the pelvic structure of  FIG. 8 ; 
       FIG. 10  is a top plan view of the vertical axis movement generator of the pelvic structure of  FIG. 8 ; 
       FIG. 11  is a perspective view of a portion of the vertical axis movement generator illustrating the mounting of the force sensors; 
       FIG. 12  is a sectional side elevation view of the vertical axis movement generator of  FIG. 10 , illustrating the magnetic sensor thereof; 
       FIG. 13  is a sectional side elevation view of the vertical axis movement generator of  FIG. 10 , illustrating the photo sensor thereof; 
       FIG. 14  is a perspective view of the hip-thigh mechanism of the human locomotion simulator of  FIG. 1 , illustrating the thigh bumpers; 
       FIG. 15  is a graph showing the displacement as a factor of time in an example of a modified trajectory; and 
       FIG. 16  is a graph showing the exerted vertical force as a factor of time in an example of a modified trajectory. 
   

   DETAILED DESCRIPTION 
   Generally stated the present invention is concerned with the simulation of human locomotion.  FIG. 1  schematically illustrates the mechanical components of a human locomotion simulator  20  according to an illustrative embodiment of the present invention. The human locomotion simulator  20  is mainly concerned with locomotion patterns of the human body by the fully coordinated simulation of the pelvic structure, the hip joint and the thigh segments with longitudinal displacement of the ground including 3-D mobility of the ground. This mechanical framework is completed by the connection of an above knee leg prosthesis equipped with at least a motorized knee joint and a motorized or a passive ankle joint in order to complete the simulation of the locomotion movements with the knee joint and the ankle joint motions. Of course other uses of the human locomotion simulator described herein are possible, such as, for example, the testing of footwear. 
   It is to be understood that in the foregoing the words “vertical” and “horizontal” are to be construed broadly. For example, generally orthogonal orientations would be encompassed thereby. 
   Mechanical Design 
   The human locomotion simulator  20  consists of a five degrees of freedom (DOF) system which are actively controlled by a controller or a computer network running a control software; the vertical and the horizontal linear axes of the pelvic structure, the hip-thigh mechanism (hip joint and the thigh segment) of the simulator itself, the knee joint of the motorized leg prosthesis and longitudinal displacement of the ground. Optionally, the human locomotion simulator  20  could also include the four vertical displacement pistons of the treadmill to allow for the 3-D variable positioning of the ground and a controlled ankle joint in the case where the leg prosthesis includes a active ankle joint. 
   Referring to  FIG. 1 , the mechanical components of the human locomotion simulator  20  include a base  22  onto which is mounted a conventional treadmill  24 , a vertical post  26  mounted to the base  22 , a pelvic structure  29  composed of a vertically movable support  28  mounted to the vertical post  26  as to produce the vertical displacement of the pelvic structure  29  and a horizontally movable support  30  so mounted to the vertically movable support  28  as to move the pelvic structure  29  horizontally, a hip-thigh mechanism  33  including a hip joint  40  represented by a pivot pin and a thigh segment assembly  38  (schematically illustrated in  FIG. 1 ) mounted on the horizontally movable support  30  of the pelvic structure  29  providing the rotational mobility at the hip joint  40  of the thigh segment assembly  38 .  FIG. 1  also illustrates a schematic prosthesis leg  32  provided with a knee joint, a shank segment, a ankle joint and a foot mounted to the thigh segment assembly  38 . 
   The hip-thigh mechanism  33  is illustrated in  FIG. 3 . It is designed to allow easy installation and maintenance of all it&#39;s components. The unit can be completely assembled before attaching to the rest of the system. And all it&#39;s sub-assemblies can be assembled or disassembled individually. 
   Calculations have been done to ensure that the hip-thigh mechanism  33  can provide the required level of torque and speed with the torque and speed range of the motor. The calculation (Equation 1) is also used in the control software to translate the hip angle into linear displacement along the motor axis. The variables used in Equation 1 are shown in  FIG. 2 .
 
ρ=√{square root over (( L   2   +L   4 ·Sin θ) 2 +( L   4 ·Cos θ− L   1 ) 2 )}{square root over (( L   2   +L   4 ·Sin θ) 2 +( L   4 ·Cos θ− L   1 ) 2 )}  Equation 1
 
   Returning to  FIG. 3 , the hip-thigh mechanism  33  includes a hip frame assembly  34 , a hip joint motor assembly  36  and a thigh segment assembly  38  with a connector or attachment member  39  for mounting the prosthesis leg  32 . 
   The hip frame assembly  34  is configured and sized to be mounted to the horizontally movable support  30  of the pelvic structure  29  as will be described hereinbelow. 
   The hip joint motor assembly  36  shown in  FIG. 4  is pivotally mounted to the hip frame assembly  34 . The hip joint motor assembly  36  and the thigh segment assembly  38  are interconnected by a hip joint ball-nut assembly  42  shown in  FIG. 5 . Similarly, the thigh segment assembly  38  is pivotally mounted to the hip frame assembly  34  via a hip pivot pin  40  ( FIG. 3 ) that simulates the biomechanical axis of the human locomotion structure at the hip. 
     FIG. 4  illustrates the hip joint motor assembly  36  in an exploded view. The hip joint motor assembly  36  includes a hip motor  44 , a hip joint ball-screw holder  46  and a ball screw  48 . The hip motor  44  is fixedly mounted to the hip joint ball-screw holder  46  that is itself pivotally mounted to the hip frame assembly  34  of  FIG. 3  via bearings  54  and a fastener  52  (see  FIG. 3 ). The ball screw  48  is mounted to the hip motor  44  to rotate therewith, passing through the angular-contact bearings set  55 . 
   The hip joint ball-nut assembly  42  is shown in an exploded view in  FIG. 5 . It includes a body  50  that is pivotally mounted to the thigh segment assembly  38  via bearings  57  and a threaded element  52  fixedly mounted to the body  50 . The threaded element  52  is so internally threaded as to receive the externally threaded ball screw  48 . 
   The hip joint motor assembly  36  provides a linear motion to the hip joint ball-nut assembly  42 , which induces a rotational movement to thigh segment assembly  38  around the hip pivot pin  40 . The ball-screw  48  is inserted into the hip joint ball screw holder  46  with angular-contact bearings set  55  in a back-to-back arrangement (see  FIG. 4 ). Because this arrangement provides a stiff linkage between the ball screw  48  and the hip joint ball-nut assembly  42 , it is necessary to have an accurate alignment between the ball screw  48  and the hip joint ball-nut assembly  42 . 
   The thigh segment assembly  38  is illustrated in an exploded perspective view in  FIG. 6 . The thigh segment assembly  38  makes the link between the prosthesis leg  32  ( FIG. 1 ) and the horizontally movable support  30 . 
   The thigh segment assembly  38  includes two parallel plates  56  and  58  interconnected by a spacer  60  and a bracket  62  configured and sized to mount the prosthesis leg thereto. Two toller bearings  64  are provided to pivotally mount the thigh segment assembly  38  to the hip frame assembly  34 . A hip joint position sensor assembly  66  is located between the two plates  56  and  58 . 
   The position measurement of the thigh segment assembly  38  is achieved via the hip joint position sensor assembly  66  illustrated in a perspective view in  FIG. 7 . Angular position measurement of the thigh segment assembly  38  is supplied by rotational optical sensor disk  68  installed on the hip pivot pin  40  and read by an encoder module  70 . The hip joint axis sleeve  72  and hip joint sensor hub  74  receive the hip pivot pin  40  that pivotally mount the thigh segment assembly  38  to the hip frame assembly  34 . Without limiting the present description, it has been found that the sensor model HEDS-9040-T00 E3-2048-1000-IHUB made by US Digital is adequate to be used as the hip position sensor assembly  66 . 
   Turning now to  FIGS. 8 to 11  of the appended drawings, the pelvic structure  29 , its vertically movable support  28 , its horizontally movable support  30 , its attached vertical and horizontal axis movement generator assemblies and the bumper structure will be described. 
   As can be seen from  FIG. 9 , the vertically movable support  28  includes a generally triangular body defined by two triangular plates  76  and  78 , maintained at a predetermined spacing by spacers  80  (only one shown), and both a vertical plate  82  and a horizontal plate  84 . 
   A vertical axis movement generator  86  is mounted to the vertical plate  82  and a horizontal axis movement generator  88  is mounted to the horizontal plate  84 . The vertical and horizontal axis movement generators  86  and  88  are identical. Accordingly, for concision purposes, only the vertical axis movement generator  86  will be described hereinbelow with respect to  FIG. 10 . 
   The vertical axis movement generator  86  includes a motor  94  to which is associated a ball screw  92 . A pair of linear slides  95  are mounted to the fixed portion of the motor  94 . A mobile unit  96  is slidably mounted to the pair of slides  95  via linear bearings  97 . 
   The mobile unit  96  includes a carriage portion  98  and secondary portions  100 . Both portions  98  and  100  being slidably mounted to the slides  95  via the linear bearings  97 . 
   A ball nut  102  is mounted to the carriage portion  98  of the mobile unit  96  and is engaged by the ball screw  92 . Accordingly, rotation of the ball screw  92  by the motor  94  causes a linear movement of the mobile unit  96  on the slides  95 . 
   Four springs  104  are provided between the carriage portion  98  and the secondary portions  100  of the mobile unit  96 . These springs  104  are used as a suspension between the carriage portion  98  and the secondary portions  100 . This suspension is interesting in the simulation of human locomotion because this type of mechanism provides the expected damping effects of the mobility of the vertical movable support  28  of the pelvic structure  29 , as will easily be understood by one skilled in the art. The four springs  104  are part of the Series Elastic Actuators (SEA) that are used to control the force applied on the corresponding vertical and horizontal movable supports  28  or  30 . These springs  104  allow the simulation of various persons weight and to separate the inertia of the actuator from the inertia of the vertical and horizontal movable supports  28  and  30 . 
   In other words, the linear slides  95  and linear bearings  97  guide the movement and the actuation is provided by a combination of motor  94 , ball-screw  92  and ball-nut  102 . The vertical and horizontal axis movement generators  86  and  88  are controlled in position and force and use a special mechanism and sensors to perform this task as will be described hereinbelow. 
   The position control loop utilizes position sensors  99  to get position feedback on both vertical and horizontal axes. Without limiting the present disclosure, Table 1 presents the technical information on linear optical sensors that have been found suitable to be used as position sensors  99 . 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Horizontal and vertical position feedback sensors 
             
           
        
         
             
                 
               Position Feedback Sensor 
                 
             
           
        
         
             
                 
               Axis 
               Type 
               Model 
               Resolution 
             
             
                 
                 
             
             
                 
               Vertical 
               Linear optic 
               US Digital 
               1/250 inch 
             
             
                 
                 
                 
               EMI-0-250 
               (0.1 mm) 
             
             
                 
                 
                 
               LIN-250-16- 
             
             
                 
                 
                 
               S2037 
             
             
                 
               Horizontal 
               Linear optic 
               US Digital 
               1/250 inch 
             
             
                 
                 
                 
               EMI-0-250 
               (0.1 mm) 
             
             
                 
                 
                 
               LIN-250-16- 
             
             
                 
                 
                 
               S2037 
             
             
                 
                 
             
           
        
       
     
   
   Force sensors are used to measure the force levels applied on the vertical and horizontal axes. Those sensors measure the displacement between the carriage portion  98  and the secondary portions  100  of the mobile unit  96  for each axe. The secondary portions  100  being linked to the carriage portion  98  with springs  104 , the applied force is a function of the displacement between the two portions ( 98 ,  100 ) and of the known strength of the springs  104 . Force sensors advantageously require fine position measurement accuracy. Therefore, magnetic stripe technology was selected. Without limiting the present disclosure, Table 2 presents the technical information on linear magnetic sensors that have been found adequate for this application. Along with the linear magnetic sensors, an index sensor is used to determine the reference position. 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Horizontal and vertical force feedback sensors 
             
           
        
         
             
                 
               Force Feedback Sensor 
                 
             
           
        
         
             
                 
               Axis 
               Type 
               Model 
               Resolution 
             
             
                 
                 
             
             
                 
               Vertical 
               Linear magnetic 
               SIKO 
               4 μm 
             
             
                 
                 
                 
               MSK200/1 
             
             
                 
                 
                 
               MB200 
             
             
                 
               Horizontal 
               Linear magnetic 
               SIKO 
               4 μm 
             
             
                 
                 
                 
               MSK200/1 
             
             
                 
                 
                 
               MB200 
             
             
                 
                 
             
           
        
       
     
   
   Turning now to  FIGS. 11 to 13 , the linear magnetic sensors will be described.  FIG. 11  illustrates, in a perspective view, the mobile unit  96  of the vertical axis movement generator  86  without the slides  95  and the motor  94 . A magnetic stripe  106  is attached to the carriage portion  98  and a linear magnetic sensor  108  is in close proximity to the magnetic stripe  106  and is designed to be rigidly connected to the secondary portions  100 . More specifically, as can be better seen from  FIG. 12 , the linear magnetic sensor  108  is secured to a bracket  101  itself mounted to a plate  103  that connects to the secondary portions  100 . Therefore, relative displacement of the carriage portion  98  with respect to the secondary portions  100  is detected and measured by the linear magnetic sensor  108 . 
   An optical index sensor  110  is also mounted to the carriage portion  98 . The optical index sensor  110  serves as a means to determine the absolute home position of the linear magnetic sensor  108 . As can be better seen from  FIG. 13 , the index sensor  110  includes a thin opaque mask  111  attached to the carriage  98  that moves between the emitter and the receptor of a photo sensor  113  attached to the plate  103  of the mobile unit  96 . For example, and without limiting the present disclosure, it has been found that a transmissive photomicrosensor made by Omron under model number EE-SX1042 has been found suitable for the present application. 
   The actuators used to move the mobile units of the vertical and horizontal axis movement generator  86  and  88  are Series Elastics Actuators (SEA). These actuators are mechanisms that allow to control position and force while eliminating undesired inertia of the drive system. Since SEA actuators are believed well known in the art they will only be briefly discussed herein. 
   Referring to  FIG. 10 , the present implementation of the SEA consists of a motor  94  and a motor drive transmission (ball-screw  92  and ball-nut  102 ) connected at the output of the motor  94 . An elastic element, in the form of the four springs  104 , is connected in series with the motor drive transmission, and this elastic element is positioned to alone support the full weight of any load connected at an output of the actuator. Referring to  FIG. 11 , a position sensor, in the form of the linear magnetic sensor  108  positioned between the carriage  98  and the mobile unit  96  generates a signal proportional to the deflection of the elastic element and indicates the force applied by the elastic element to the output of the actuator. 
   Referring now to  FIGS. 8 and 14  of the appended drawings, the bumper structure will be described. The bumper structure is so designed that each axis is completely independent. The bumper structure includes an upper vertical bumper assembly  112 , a lower vertical bumper assembly  114 , a front horizontal bumper assembly  116 , a back horizontal bumper assembly  118 , a back thigh bumper bracket  120  and a front thigh bumper bracket  122 . 
   It is to be noted that even though only one of each bumper assembly  112 ,  114 ,  116  and  118  is illustrated in  FIG. 8 , two of each of these assemblies are present, one for each side of the simulator. 
   The upper vertical bumper assembly  112  includes a bumper  112 A mounted to the triangle plate  76  of the vertically movable support  28  and a stop bracket  112 B, mounted to the vertical post  26  (see  FIG. 1 ) and vertically aligned with the bumper  112 A to upwardly stop the course of the vertically movable support  28 . Similarly, the lower vertical bumper assembly  114  includes a bumper  114 A mounted to the triangle plate  76  of the vertically movable support  28  and a stop bracket  114 B, mounted to the vertical post  26  and vertically aligned with the bumper  114 A to stop the course of the vertically movable support  28  at the lowers desired position. 
   The front horizontal bumper assembly  116  includes a bumper  116 A and the back horizontal bumper assembly  118  includes a bumper  118 A where the both bumper  116 A and  118 A are positioned on the mobile unit of the horizontal axis movement generator  88 . The front and back horizontal bumpers assembly  116 ,  118  share a common stop bracket screwed on the triangle plate  76  of the vertically movable support  28  providing the front stop bracket  116 B and the stop bracket  118 B. Front and rear movement of the hip-thigh mechanism  33  is stopped by the contact of the bumpers  116 A,  118 A with the stop bracket portions  116 B and  118 B, respectively. 
   Referring to  FIG. 14 , the back thigh bumper bracket  120  includes a bumper  124  and a bracket  126  positioned to the hip frame assembly  34 . Similarly, front thigh bumper bracket  122  includes a bumper  128  and a bracket  130  positioned to the hip frame assembly  34 . The thigh bumper brackets  120  and  122  limit the movement of the thigh segment assembly  38 . 
   The bumpers were selected such that the system&#39;s kinetic energy can be absorbed by the bumpers. All bumpers are the same, simply for standardization. The worst case condition that produces the highest kinetic energy level is when the system stands at the highest point and is let down in free-fall. The motor of the vertical axis movement generator  86  could also add to the total energy, but its contribution is negligible compared to the free-fall. Both bumper  114 A of the lower vertical bumper assembly  114 B and the bumper  128  of the front thigh bumper bracket  122  shall be able to sustain the free-fall drop. The condition where the bumper  128  can be solicited is when the foot enters in contact with the floor before the said bumper  114 A hits its respective stop bracket  114 B. The total energy is calculated as follow:
 
 E=F*d;  
 
 F= 9.8 m/s 2 *70 kg=686  N;  
 
d=0.28 m; and
 
 E =686 N*0.28 m=192 N.m=1700 lb.in.
 
   Therefore, each bumper should be able to sustain about 1700 lb.in. Miner&#39;s GBA-5 bumpers or one GBA-9 meet this requirement. One skilled in the art will understand that the range of motion of the vertically movable support  28  can be adjusted by changing the position of respective stop brackets  112 B,  114 B,  116 B,  118 B,  120  and  122  or their corresponding bumpers  112 A,  114 A,  116 A and  118 A. 
   From the kinetics standpoint, all joints provide enough force/torque to simulate the locomotion activities characterizing a human subject, which mass is corresponding to the mechanical simulator lower-limb linkage (i.e., about 72.5 kg in the illustrated embodiment) by adequately mobilizing the vertically and horizontally movable supports  28  and  30  of the pelvic structure  29 , the hip-thigh mechanism  33  and the thigh segment assembly  38 . 
   Another aspect of the present invention is concerned with the simulation of human locomotion in stairs. In order to simplify the simulation approach, limit the number of subsystems required, minimize modifications to the actual platform design, and facilitate integration with the actual level-walking simulation capabilities of the platform, the implementation of a complete stance phase simulation with a modified swing phase using the treadmill was proposed over the use of an approach requiring the use of a stepmill-like device. In the proposed approach, the treadmill moving surface is used to simulate the step tread as well as the velocity corresponding to the horizontal progression speed of a normal human subject climbing or descending stairs. 
   This approach allows to correctly simulate the pelvic, the hip and the knee joint mobility during both stairs ascent and descent tasks stance phase, while the swing phase needs to be modified to account for the limited motion range available on the platform and in order to generate coherent stance initial conditions. The swing phase trajectories modifications mostly affect the vertical and horizontal degrees-of-freedom and do not harm the overall simulation validity in a significant manner of this type of locomotion and more specifically the respective stance phase. 
   The range of motion provided by the vertical, horizontal displacement of the pelvic structure  29  and the rotational displacement of the thigh segment assembly  38 , combined with the constant treadmill  24  movement, allows the simulation of the desired tasks: level walking, ascending and descending stairs. The trajectory of the vertical axis of the pelvic structure  29  has been modified (as can be seen in  FIGS. 15 and 16 ) for the stair ascent and descent to address the fact that the simulator&#39;s  20  flat treadmill  24  approach doesn&#39;t allow natural kinematics during those tasks. For example, at the end of the stance phase of a step during stair ascent, the pelvic structure  29  would normally continue going up until the next step, but due to the limited vertical freedom of movement on the simulator  20 , the body of the simulator  20  will go down during the swing phase and ensure that the foot is placed properly on the treadmill  24  for the next step. 
     FIGS. 15 and 16  show an example of how the vertical axis motion of the pelvic structure  29  is modified for the stairs ascent. The graph of  FIG. 15  displays the modified displacement  142  during the stair ascent simulation whereas the  FIG. 16  displays the force level  144  required on the vertical axis to follow the desired trajectory. The trajectory displacement  142  and the exerted vertical force  144  represent the kinematics and the kinetics respectively of the vertical mobility of the pelvic bone of the human body. 
   It is to be noted that the forces displayed in  FIG. 16  represent the vertical forces to be applied in order to precisely follow the given trajectory, with the assumption that there is no ground contact. This assumption provides force levels that are at least as high as with ground contact condition. It is obvious that the highest force peaks originate from the modification of the trajectory instead of the original gait motion itself (high acceleration level at the end of the modified trajectory). 
   All three axes of the simulator  20  are driven by drive systems that allow following their respective trajectories while providing the required level of forces and accelerations. The range of motion was established directly from the trajectory to follow, and the required motor forces are computed from acceleration levels to reach and from the masses/inertias of the moving bodies. To select the different components of a drive system (electric drive/motor/screw), the motor torques and speeds are computed and compared with the capacity chart of the drive system. 
   Dimensions and Specifications of a Simulator 
   Without limiting the present disclosure, we present here below an example of dimensions and specifications that could be used to build the simulator  20 . 
   Referring to  FIG. 2 , the following dimensions have been used: 
   L 1 =80.0 mm; 
   L 2 =210.0 mm; 
   L 4 =103.08 mm 
   The distance between the hip joint  40  and the prosthesis knee axis was selected as 403.34 mm. 
   Without limiting the present inventions, Table 3 specifies the axes characteristics: 
   
     
       
             
           
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               Axes characteristics 
             
           
        
         
             
                 
                 
                 
                 
               Screw 
               Force/ 
             
             
               Axis 
               Range 
               Motor type 
               BUS 
               lead 
               Torque 
             
             
                 
             
             
               Vertical: 
               300 mm 
               Baldor 
               160 
               20 mm 
               −5691 N 
             
             
               Horizontal: 
               105 mm 
               Baldor 
               160 
               20 mm 
               −1709 N 
             
             
                 
                 
               BSM50N-333 
               VDC 
                 
               +1709 N 
             
             
               Hip: 
               −60° 
               Baldor 
               160 
               10 mm 
                −228 Nm 
             
             
                 
               +30° 
               BSM50N-333 
               VDC 
                 
                +228 Nm 
             
             
               Treadmill: 
                0.8 km/h 
               Drive and 
               N/A 
               N/A 
               N/A 
             
             
                 
                16 km/h 
               control from 
             
             
                 
                 
               Schwinn 
             
             
                 
             
           
        
       
     
   
   Table 4 indicates the characteristics of the position and force feedback sensors: 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               Position and force feedback sensors characteristics 
             
           
        
         
             
                 
               Force Feedback Sensor 
               Position Feedback Sensor 
             
           
        
         
             
               Axis 
               Type 
               Model 
               Resolution 
               Type 
               Model 
               Resolution 
             
             
                 
             
             
               Vertical 
               Linear 
               SIKO 
               4 μm 
               Linear 
               US Digital 
               1/250 inch 
             
             
                 
               magnetic 
               MSK200/1 
                 
               optic 
               EMI-0-250 
               (0.1 mm) 
             
             
                 
                 
               MB200 
                 
                 
               LIN-250-16- 
             
             
                 
                 
                 
                 
                 
               S2037 
             
             
               Horizontal 
               Linear 
               SIKO 
               4 μm 
               Linear 
               US Digital 
               1/250 inch 
             
             
                 
               magnetic 
               MSK200/1 
                 
               optic 
               EMI-0-250 
               (0.1 mm) 
             
             
                 
                 
               MB200 
                 
                 
               LIN-250-16- 
             
             
                 
                 
                 
                 
                 
               S2037 
             
             
               Hip 
               N/A 
               N/A 
               N/A 
               Rotational 
               US Digital 
               1/2048 turn 
             
             
                 
                 
                 
                 
               optic 
               HEDS-9040- 
               (0.18°) 
             
             
                 
                 
                 
                 
                 
               T00 E3- 
             
             
                 
                 
                 
                 
                 
               2048-1000- 
             
             
                 
                 
                 
                 
                 
               IHUB 
             
             
                 
             
           
        
       
     
   
   The vertical and horizontal axes of the pelvic structure  29  are controlled in position and force (see  FIG. 11 ). Position control is conventional, and relatively straightforward. Force control is utilized to eliminate the appearance of inertia induced by the drive system. In the case of the vertical axis of the pelvic structure  29 , the mass of the system is about 75 kg, but for the reason that the motor/ball screw system rotates when the mass moves vertically, the apparent mass when accelerations are induced would increase to about 85 kg (apparent inertia). The force control mechanism allows eliminating the additional apparent inertia of the drive system. This system also allows simulating weights different than the system&#39;s weight by requesting the desired level of force on the force control loop. 
   Although the present invention has been described by way of particular embodiments and examples thereof, it should be noted that it will be apparent to persons skilled in the art that modifications may be applied to the present particular embodiment without departing from the scope of the present invention.