Abstract:
Disclosed are several examples of a ground reaction force sensor for an article having an upper force plate for contacting the article, a lower force plate for contacting the ground, a vertical load cell disposed between the plates for measuring the force acting on the cell in a direction that is substantially perpendicular to the surface, a horizontal load cell disposed between the plates for measuring the force acting on the cell in a direction that is substantially parallel to the surface, and with the load cells being mounted between the plates in a configuration that is substantially insensitive to off-axis forces imposed on them for improved load cell measurement accuracies. Various other features and benefits are provided.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT. 
       [0001]    This invention was made with government support under Contract No. DE-AC05-000822725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0002]    None. 
       THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    None. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present disclosure relates to force measurements and more specifically to a wearable sensor for measuring the reaction force of an article on a surface such as the ground. 
         [0006]    2. Description of the Related Art 
         [0007]    Gait analysis is the study of locomotion and is one method of analyzing the effects of various factors on ordinary movement. A subject&#39;s gait may be influenced by factors such as a stroke, spine misalignment, joint replacements, sports injuries, shoe fitment, and prosthetic limb fitment, among other things. With regard to prosthetic limb fitment, it&#39;s essential for a prosthetic limb to function properly once it&#39;s fitted to an amputee. In order for this to occur, the amputee&#39;s normal gait must be acquired and examined by a clinician, for use as a baseline. The normal gait cycle includes several components and an issue with one or more components may cause the amputee to compensate for improper fitment and this can increase stress on joints and tendons. The normal gait of an amputee can be determined by measuring the ground force reaction forces in the unaffected limb. 
         [0008]    Known gait analysis devices include potentiometers for measuring the flexion or extension angle of a prosthetic device, sensors for mounting outside a shoe, instrumented insoles, and pressure sensitive mats, which the subject walks on. 
         [0009]    Despite the teachings of the current art, a ground force reaction sensor having a low profile, low mass, and minimal influence on the normal gait of a subject is needed. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Disclosed are several examples of a ground force reaction sensor for use in a gait analysis of a subject. The ground may be any surface that can support the subject such as a tiled floor, a carpeted floor, a mat, a stair, or a stage for example, and the subject may be a human, an animal, or a machine (e.g., a robot). 
         [0011]    According to an example, a ground reaction force sensor for an article such as a shoe includes: an upper force plate for contacting the article; a lower force plate for contacting the ground; a vertical load cell disposed between the plates for measuring the force acting on the cell in a direction that is substantially perpendicular to the ground; a horizontal load cell disposed between the plates for measuring the force acting on the cell in a direction that is substantially parallel to the ground, and with the load cells being mounted between the plates in a configuration that is substantially insensitive to off-axis forces imposed on them for improved load cell measurement accuracies. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]    The present ground force reaction sensor may be better understood with reference to the following drawings and detailed description. The components in the drawings are not necessarily drawn to scale, emphasis instead being placed upon illustrating principles. In the drawings, like referenced numerals refer to like parts throughout the different drawings unless otherwise specified. 
           [0013]      FIG. 1  is a perspective view of ground reaction force sensors installed on an article in accordance with an example of the present invention; 
           [0014]      FIG. 2  is a top, perspective view of a forefoot ground reaction force sensor in accordance with the example illustrated in  FIG. 1 ; 
           [0015]      FIG. 3  is a top, perspective view of a heel ground reaction force sensor in accordance with the example illustrated in  FIG. 1 ; 
           [0016]      FIG. 4  is a partially exploded view of the forefoot ground reaction force sensor in accordance with the example illustrated in  FIG. 2 ; 
           [0017]      FIG. 5  is an assembled view and an exploded view of a vertical load cell and bearing assembly in accordance with an example of the present invention; 
           [0018]      FIG. 6  is an assembled view and an exploded view of another vertical load cell and bearing assembly in accordance with another example of the present invention; 
           [0019]      FIG. 7  is an assembled view and an exploded view of another vertical load cell and bearing assembly in accordance with yet another example of the present invention; 
           [0020]      FIG. 8  is a partial sectional view of a vertical load cell and bearing assembly, in a first condition, in accordance with another example of the present invention; 
           [0021]      FIG. 9  is a partial sectional view of a vertical load cell and bearing assembly, in a second condition, in accordance with an example of the present invention; 
           [0022]      FIG. 10  is a perspective view of a horizontal load cell in accordance with an example of the present invention; 
           [0023]      FIG. 11  is a sectional view of the horizontal load cell taken along line  11 - 11  of  FIG. 10 ; 
           [0024]      FIG. 12  is a schematic diagram of a Wheatstone bridge circuit in accordance with an example of the present invention; and 
           [0025]      FIG. 13  is a schematic diagram of an electronics module in accordance with an example of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Referring first to  FIG. 1 , an article  20  such as a foot covering or shoe (shown), a prosthetic device, an animal&#39;s hoof, or a robotic limb, for example, transfers loads to the a surface such as the ground  22 . The ground  22  extends parallel to a horizontal plane defined by an X-axis and a Y-axis. The ground  22  also extends perpendicular to first vertical plane defined by the X-axis and a Z-axis and a second vertical plane defined by the Y-axis and the Z-axis. 
         [0027]    Exemplary ground reaction force sensors  24   a,    24   b  may be attached to forefoot  26  and heal  28  regions at a bottom surface  30  of the article  20  by attachment means  32  such as tabs and fasteners (shown), bindings, straps, adhesives, and hook and loop fasteners, for example. In other examples, the sensors  24   a,    24   b  are formed integrally with the article  20  during its manufacture. In yet other examples, the article  20  is modified, after its manufacture, by removing a vertical slice to compensate for the vertical thickness of the sensors  24   a,    24   b . Please note that the sensors  24   a,    24   b  have a very slim vertical profile in comparison to the article  20 . The forefoot sensor  24   a  may also be slightly curved to conform to the shape of the forefoot portion  26 , thus allowing for a more natural gait by the subject during analysis. 
         [0028]    With reference to  FIGS. 2-4 , further details of the exemplary sensors  24   a,    24   b , which were designed and built at the Oak Ridge National Laboratory, will now be described in much greater detail. An upper force plate  34  includes an upper contact surface  36  for contacting the bottom surface  30  of the article  20 . Upper pockets  38  receive vertical load cells  40  and upper clevises  42  receive horizontal load cells  44 , which are rotationally affixed via vertically-oriented cylindrical pins  46 . The pins  46  are retained by C-clips, cotter pins or other pin retention means. Please note that the upper force plate  34  includes only one upper clevis  42  for each horizontal load cell  44 . In this example, three horizontal load cells were used  44 -X,  44 -Y 1 , and  44 -Y 2 . 
         [0029]    A lower force plate  48  includes a lower contact surface  50  for contacting the ground  22 . Lower pockets  52  receive the vertical load cells  40  and lower clevises  54  receive the horizontal load cells  44 , which are rotationally affixed via cylindrical pins  46  positioned vertically. Please note that the lower force plate  48  includes only one lower clevis  54  for each horizontal load cell  44 . 
         [0030]    The upper and lower force plates  34 ,  48  were formed using an additive manufacturing process that selectively solidifies metallic powder with an electron beam to form layers from a computer generated file, such as an STL file. In this example, the force plates  34 ,  48  were formed of a light-weight and high-strength Titanium Alloy using a system manufactured by Arcam AB of Gothenburg, Sweden. The force plates  34 ,  48  could also be formed of other light-weight and high-strength, metallic or nonmetallic, materials by stamping, forming, machining, molding, casting, or other known methods. 
         [0031]    With the force plates  34 ,  48  assembled together, a horizontal load cell  44 -X 1  is affixed between the upper and lower clevises  42 ,  54  by pins  46  in a direction that is parallel to the X-axis and in the horizontal plane defined by the X-axis and the Y-axis. Additionally, two horizontal load cells  44 -Y 1 , and  44 -Y 2  are affixed between the upper and lower clevises  42 ,  54  by pins  46  in a direction that is parallel to the Y-axis and in the horizontal plane defined by the X-axis and the Y-axis. The pins  46  assure that only substantially axial forces are transferred to the horizontal load cells  44 -X,  44 -Y 1 , and  44 -Y 2 . By including at least three horizontal load cells  44 -X,  44 -Y 1 , and  44 -Y 2 , the upper and lower force plates  34 ,  48  are inhibited from twisting and/or racking with respect to one another. Three horizontal load cells  44 -X,  44 -Y 1 , and  44 -Y 2 , are also necessary in order to measure Fx, Fy and Mz (moment about a vertical Z-axis). 
         [0032]    Protruding fingers  56  on each of the force plates  34 ,  48  retain elastomer bands  58 , which secure the plates  34 ,  48  together and impose a slight compressive load on the vertical load cells  40 . The elastomer bands  58  have a relatively low spring rate in comparison to the spring rate of the vertical load cells  40 . This compressive load counteracts any potential tension loads that might occur as the upper force plate  34  is raised. The slight compressive load is simply zeroed out while processing the actual load data that is collected during the gait analysis on a computing device. An additional advantage of the elastomer bands  58  is their ability to provide unencumbered cleaning, inspection, service, and replacement of the various components of the sensors  24   a,    24   b.    
         [0033]    Referring now to  FIGS. 5-8 , further details of the vertical load cells  40  will be described. The vertical load cells  40  used in the exemplary sensors  24   a,    24   b  are subminiature load buttons having a 250 lb (113 kg) compression load capacity, model LLB250, and sold by FUTEK Advanced Sensor Technology, Inc., City of Irvine, Calif., USA, for example. The vertical load cells  40  include a crowned surface  60  to approximate a point loading condition. This allows for slight flexing of the force plates  34 ,  48  with respect to each other without transmitting moment loads to the vertical load cells  40 . In some examples, at least three vertical load cells  40  are used, in other examples, at least four vertical load cells  40  are used and in yet other examples, at least six vertical load cells  40  are used. 
         [0034]    It is to be noted again that the upper force plate  34  and the lower force plate  48  are not rigidly attached to one another and that slight relative motion is necessary to measure the horizontal forces. The single axis, vertical load cells  40  are not sensitive to this off-axis loading. To ensure that the vertical load cells  40  only measure forces that are substantially perpendicular to the ground  22 , a bearing assembly  62  is disposed between the vertical load cells  40  and a force plate  34 ,  48 . 
         [0035]    In the bearing assembly  62  example of  FIG. 5 , a pair of roller-type bearings  64   a ,  64   b  each include a series of individual rollers  66  confined in a cage  68  having a number of through slots  70  that are sized to accept the rollers  66 . The slots  70  in the example were formed by wire EDM; however, punching, stamping, laser cutting, water jet, or other forming techniques could similarly be used. Note that all of the rollers  66  in roller-type bearing  64   a  are aligned in a first direction that differs from a second direction of the rollers  66  in the roller-type bearing  64   b.  In this specific embodiment, the rollers  66  in roller-type bearing  64   a  are aligned in a direction that is perpendicular to the rollers  66  of roller-type bearing  64   b . This perpendicular alignment ensures that the vertical load cells  40  are substantially insulated from all lateral and fore to aft loads. A hardened bearing plate  72  (e.g., stainless steel) is disposed between the two roller-type bearings  64   a,    64   b.  In some examples, hardened bearing plates  72  are disposed on each side of the roller-type bearings  64   a,    64   b  (shown). Roller-type bearings  64   a  and  64   b  are available from The Timken Company, 1835 Dueber Ave., S.W. Canton, Ohio 4470-2790, USA, for example. 
         [0036]    The two roller-type bearings  64   a,    64   b  and the hardened bearing plates  72  may each include a clocking feature  74  that interacts with a centering element  76  made of a resilient material (e.g., 40 durometer polyurethane elastomer). In this example, a square clocking feature  74  was used; however, other clocking features (e.g., asymmetric shape, spline, slot, offset pin, etc . . . ) could also be used. The centering element  76  permits: a slight amount of unimpeded relative motion between the two roller-type bearings  64   a  and  64   b  and the bearing plates  72 ; permits a slight lateral movement between force plates  43  and  48 ; assures the two bearings  64   a,    64   b  are orthogonal relative to each other; and assures the bearing plates  72  are concentric with one another and with the roller cages  68  after each loading cycle. The centering element  76  includes an aperture  78  for accepting a protruding pin  80  that is affixed in a pocket  38  or  52  of a force plate  34 ,  48 . In one example, the pin  80  is affixed to the lower force plate  48  and the vertical load cell  40  contacts the upper force plate  34  (shown). In another example, the pin  80  is affixed to the upper force plate  34  and the vertical load cell  40  contacts the lower force plate  48 . In another example, one of the roller-type bearings,  64   a  or  64   b,  is disposed above a vertical load cell  40  and the other of the roller-type bearings,  64   a  or  64   b,  is disposed below the vertical load cell  40 . 
         [0037]    In the bearing assembly  62  example of  FIG. 6 , a ball-type bearing  82  includes a series of hardened steel balls  84  confined in a steel cage  86  designed for axial loading. These bearings are also known as thrust bearings. A hardened bearing plate  72  is disposed on each side of the ball-type bearing  82 . The bearing plates  72  each include an aperture  88  that cooperates with a centering element  76 , made of a resilient material (e.g., 40 durometer polyurethane elastomer), to ensure proper alignment of the bearing assembly  62 . Please note that a clocking feature  74  is not shown in this example, because the balls  84  are free to rotate in any direction within the horizontal plane defined by the X-axis and Y-axis. Ball-type bearings  82  are available from McMaster-Carr, 200 Aurora Industrial Parkway, Aurora, Ohio 44202-8087, USA, for example. 
         [0038]    While each type of bearing assembly  62  will work in this application, the roller-type bearings  64   a,    64   b  provide a superior load handling capability for their size and offer a relatively low vertical profile, which enhances the function of the sensors  24   a,    24   b  and ensures nearly unencumbered motion during gait analysis. 
         [0039]    In another example of a bearing assembly  62 , as illustrated in  FIG. 7 , an additional example of a centering element  76  is shown. In this example, the centering element has a series of compliant arms that mate with clocking features  74  as in the earlier example. An aperture  78  in the centering element  76  accepts a protruding pin  80  that is affixed in a pocket  38  or  52  of a force plate  34 ,  48 . This centering element may be made of a resilient material (e.g., 40 durometer polyurethane elastomer); however, due to its compliant design, may also be made of a plastic or spring steel material for example. The centering element  76  permits: a slight amount of unimpeded relative motion between the two roller-type bearings  64   a  and  64   b  and the bearing plates  72 ; permits a slight lateral movement between force plates  43  and  48 ; assures the two bearings  64   a,    64   b  are orthogonal relative to each other; and assures the bearing plates  72  are concentric with one another and with the roller cages  68  after each loading cycle. 
         [0040]    Referring now to  FIGS. 8-9 , another example of a vertical load cell  40  is illustrated. In this example, the vertical load cell  40  is a series of individual strain gages  90  affixed to one of the upper or lower force plates  34 ,  48 . Here, the four individual strain gages  90  react to the deflection of a force plate  34 ,  48  as a force F is applied. In this example, the upper force plate  34  has a beam shaped cross sectional portion  92  that is approximately 0.020 inches (0.508 mm) thick. A crowned plate  94  sits atop a bearing assembly  62  and fits within a pocket  52 , as earlier described. When a force F is applied to the load plates  34 ,  48 , the beam portion  92  deflects slightly, as shown in the condition of  FIG. 9 , and the two inner strain gages  90  will be subjected to a tension load, while the two outer strain gages  90  will be subjected to a compression load. The sum of these four loads is indicative of the total vertical load on the load cell  40 . In this example, the strain gages  90  are affixed directly to a force plate  34 ,  48 , at the time of manufacture, instead of being a prefabricated component as in the earlier examples of  FIGS. 5-7 . 
         [0041]    Referring now to  FIGS. 10 and 11 , further details of the horizontal load cells  44  will now be discussed. The horizontal load cells  44  have an I-beam shaped body  98  with clevis attachments  100  at each end for engaging clevises  42 ,  54  on the force plates  34 ,  48 . In this example, the horizontal load cells  44  were machined from an aluminum alloy material, although other materials are also contemplated. A web portion  102  of the body  98  is approximately 0.020 inches (0.508 mm) thick and includes two strain gages  104  affixed on each side of the web portion  102 . One strain gage  104  on each side is aligned parallel to the X-axis and one strain gage on each side is aligned parallel to the Y-axis. Strain gages  104  and associated hardware are available from Omega Engineering, Inc., One Omega Drive P.O. Box 4047, Stamford, Conn. 06907-0047, USA, for example. 
         [0042]    The strain gages  90 ,  104  are wired in a full Wheatstone bridge circuit  106  as illustrated in  FIG. 12 , because of its ability to measure minute resistance changes in the strain gage  90 ,  104  wires. The full Wheatstone bridge has two fully active strain gages in the principal stress direction and two strain gages that will see the effect of Poisson&#39;s Ratio. The full bridge circuit  106  tends to cancel thermal and off-axis errors. The output voltage of the Wheatstone bridge is expressed in millivolts output per volt input. Wheatstone bridge circuits  106  are well known in the art of strain measurements and, although this specific circuit was illustrated in the example, other circuits may also be used. 
         [0043]    Referring finally to  FIG. 13 , a sensor electronics module  108  is shown. A printed circuit board (PCB)  110 , located in each sensor  24   a,    24   b,  acquires electronic signals from each of the vertical  40  and horizontal  44  load cells through directly wired or wireless connections as shown. The electronics module  108  also includes a power supply (e.g., battery)  112 , and access to a local  114  and/or remote  116  data storage device. A local storage device  114  may include a hard drive, a memory card, a memory stick or other device that stores electronic load signals in the electronics module  108 . A memory card slot  118  may be utilized with specialized cards and plug-in devices such as, for example, a wireless networking card, to expand the capabilities of functionality of the electronics module  108 . The electronics module  108  may include a communications device  120  such as an antenna to facilitate connectivity and transfer of electronic data to the remote storage device  116  via one or more communication protocols such as: WiFi (WLAN); Bluetooth or other personal area network (PAN) standard; cellular communications; an infrared (IR) for communication via the Infrared Data association (IrDA) standard and/or any other communication standard known or yet to be developed. Once the acquired load data is communicated to and stored on the local  114  or remote  116  data storage device, it may be reviewed, manipulated, and further analyzed by a gait clinician using commercially available or custom coded software using, for example, a personal computing device  122 . 
         [0044]    While this disclosure describes and enables several examples of a wearable ground reaction force foot sensor, other examples and applications are contemplated. Accordingly, the invention is intended to embrace those alternatives, modifications, equivalents, and variations as fall within the broad scope of the appended claims. The technology disclosed and claimed herein may be available for licensing in specific fields of use by the assignee of record.