Abstract:
A force gauge assembly used to measure forces or spring rate of an object utilizing a diaphragm strain gauge for mechanically compensating for loads not being centrally applied to the gauge. The construction of the gauge provides readings that will be substantially the same as if the load were applied in perfect alignment. The gauge utilizes internal components that remain the same even though the force gauge is adaptable for measuring different objects.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority of U.S. Provisional Patent Application No. 61/616,788 entitled “FORCE GAUGE FOR PLIABLE MATERIAL,” filed Mar. 28, 2012, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a device that is used to find the force necessary to measure the compression of pliable materials such as foam rubber or solid rubber or spherical materials such as fruit, soles for athletic shoes, or balls used in sports such as a golf ball or tennis ball or measure to trailer tongue weight or other application. 
     BACKGROUND OF THE INVENTION 
     Devices having strain gauges for converting an applied force or load into electrical signals are known the art. Such a device is structured so that the applied force or load deforms one more strain gauges. The strain gauges convert the deformation (i.e., strain) into electrical signals. The output is typically processed using an algorithm to calculate the force or load being applied to the device. 
     A force or multiple forces applied to an object to be measured may be converted into a compression scale. For the purposes of this discussion, “Compression” or “Compression Scale” will be defined as force per unit deflection that can be expressed as a “Spring Rate” or function of a spring rate that may be available via a look up table or mathematical formula. An example of a well-known compression scale is one used to designate golf ball hardness. 
     One problem with known load gauges is that an application of force that is not centered on the strain gauge, or that is delivered at an angle to the surface of the strain gauge produces errors in the measurement provided by the strain gauge. As an example,  FIG. 1  graphically shows how an accurate reading may only be made at a force delivering angle of 0, i.e., normal, when any departure from normal, results in a degradation of the force measured. 
     SUMMARY OF THE INVENTION 
     The strain gauge assembly of the invention is used to accurately measure the force of a flat or non-flat surface that may be used to derive a spring rate of an object. An example device that could employ the strain gauge of the invention is described in U.S. publication 2012/0166106, incorporated herein by reference. 
     The rings of the diaphragm strain gauge of the invention are designed to mechanically compensate for loads not being applied exactly in the center of the gauge. While a toggle foot may be used to ensure that the measured surface is in full contact with the gauge, if the force being applied is off center by a small amount, the reading will be the same as if it were in perfect alignment by using the diaphragm strain gauge. 
     The force gauge utilizes internal components that will remain substantially the same even though the force gauge is adapted for measuring different objects. The gauge may be adapted by varying the thickness of the bottom of cup to accommodate a selected range of forces and by affixing a variety of interface members or energy directors to impart force on the bottom surface of cup. 
     The strain gauge assembly of the invention includes a housing having a wall area and a lower portion, wherein the wall area and lower portion defining a cavity or cup-like structure. The lower portion of the housing has an internal surface that defines a bottom of the cavity. In one embodiment, the cavity of the invention retains same size and shape for all configurations of the gauge. The housing further defines a flange and a keying mechanism for preventing rotation of the housing within a suitable mounting structure. 
     A stem extends from an external surface of the lower portion. The stem provides a uniform load to a center of the lower portion of the housing to provide strain to the lower portion. 
     An interface member is affixed to a distal end of the stem. The interface member may be integrally formed with the stem, may be rigidly affixed to the stem, or may be hingedly or otherwise flexibly attached, as discussed below. The interface member is provided for contacting an object to be measured, such as a ball or fruit or other material to be tested. A width of the interface member may vary in size and concavity to accommodate different shapes of the objects to be measured. In one embodiment, the interface member is mounted on a toggle foot for tracking a surface area position of the object, wherein the toggle foot and strain gauge together combine to accurately read force even when there is misalignment or off-center stress of the gauge&#39;s center on the compressed object. 
     A support member is received within the cavity. The support member has an upper surface and a lower surface. 
     A strain gauge element is affixed to the internal surface of the lower portion of the housing, preferably with an adhesive, to allow the strain gauge element to measure strain imparted to the lower portion. The strain gauge element has a circle portion and a surrounding zig-zag portion. The circle portion defines ring segments that substantially form complete circles, e.g., wherein the ring segment traverse greater than 350 degrees. A diameter of the circle portion preferably is 60% to 70% of the diameter of the strain gauge element, more preferably the diameter of the circle portion comprises 60% of the diameter of the strain gauge element. 
     An interface circuit is provided on the upper surface of the support member for electronic interface with the strain gauge element. Data processing components are located on the upper surface of the support member. The data processing components are in electronic communication with the interface circuit for processing data collected from the strain gauge element. The data processing components are placed in close proximity to the strain gauge element to minimize electrical noise and interference; 
     An output conduit in communication with the data processing components is provided for for transmitting information from the data processing components. A potting compound is received within the cavity for protecting and securing the support member and attached strain gauge element therein, so that the strain gauge element is potted into the bottom of a cup or cavity. 
     The strain gauge element is located on an interior surface of a lower portion of the housing. An appropriately sized interface element is selected for securing to a stem extending from an external surface lower portion of the housing. An object to be measured is positioned adjacent to the interface element. The object is then compressed so that the interface element and attached stem bend the lower portion of the housing. When a force for compressing the object is centered on the interface element, the zig-zag portion of the strain gauge element is bent the same on opposite sides of the strain gauge element and the center portion is deflected uniformly. However, when a force for compressing the object is off-center, the lower portion is distorted wherein the strain gauge element experiences higher stress on a first side and lower stress on a second side. In particular, the circle portion experiences the different stresses so that the stresses are canceled out to “mechanically” average the deformation of strain gauge circles about a center of the gauge. 
     An example force range for the gauge is 0 to 500 pounds, which can be measured in accuracies of grams or milligrams. All operations of assembly and manufacturing are substantially identical. Cost will, therefore, be similar for all forms of gauges manufactured. 
     One object of this invention is to provide an electronic gauge that can be used under different configurations to find a force required to measure the “spring rate” of a wide variety of objects of various materials, such as balls used in sports or of fruits and vegetables, with a very high degree of accuracy and repeatability. 
     A further object of this invention is to provide a device that will assist in the measurement of “spring rate” to a very high degree of accuracy and repeatability, wherein the device has no deflection or minimal deflection such that the contribution to the measurement is so small that further consideration in not required. 
     A further object of the invention is to provide a gauge that can be traced to absolute standards, such as traceable scale, e.g. to measure commodities or shipping weight. 
     A further object of this invention is to provide a device that will assist in the creation of defined scales that relate the “spring rate” of an object of various materials, such as a ball, or fruit, to another scale that is normally used to describe such things to members of a particular industry. 
     A further object of this invention is to provide a gauge that will assist in the creation of compact and light-weight measurement devices that can be carried with little effort. 
     A further object of this invention is to provide a gauge that will not damage or mark the surface of an object to be measured, such as a ball or fruit, through the use of custom shapes for contact surfaces of an interface member. 
     A further object of this invention is to provide a gauge that uses largely the same set of internal components for all configuration of the gauge. 
     For the purpose of the disclosure, the conversion of “spring rate” to “compression scale” may vary from one ball type to another or from one fruit or vegetable to another. For example, the spring rate for a golf ball wherein most golf balls vary from 1100 pounds per inch to 3000 pounds per inch and can be expressed to one familiar to the game of golf as a “compression scale” of 30 to 120 respectfully. 
     A further example is that the “compression scale” of a tomato that may vary from ½ pounds per inch for a ripe tomato to three pounds per inch for a tomato that needs to stay on the vine to further ripen. As an example, the “compression scale” for a tomato could be set by those familiar to growth and sale of tomatoes as a range from one to ten. 
     As a further application, the device of the invention may be useful to help identify concussions. Concussions present concerns related to player safety in football and many other sports, as well as with military personnel. While the spring rate in a golf ball is measured in terms of pounds force, the gauge of the invention can measure much smaller forces, i.e., deflection change based upon change in milligrams or grams of force. It is believed that the gauge of the invention may be used to measure pressure changes within a human skull, i.e., may be used to measure intra-cranial pressure. The pressure measurements may then be used to identify concussions when measured against an “at-rest” baseline. Further, measurement may be taken across two time points immediately after a hard hit was sustained, e.g., during an athletic competition. Examples of time points may be 1 or 5 minute interval measurements taken post trauma to detect swelling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a graphical representation of a force gauge measurement at different angles of force application. 
         FIG. 2  shows an isometric exploded view of the gauge assembly of the invention; 
         FIG. 3  shows an enlarged plan view of the diaphragm gauge of  FIG. 2 ; 
         FIG. 4  shows an assembled isometric view of the gauge assembly of  FIG. 2 , wherein the gauge assembly is provided with a small interface; 
         FIG. 5  shows a plan view of the gauge assembly of  FIG. 4 ; 
         FIG. 6  shows a cross sectional view of the gauge assembly of  FIG. 5  taken along lines  6 - 6  of  FIG. 5 ; 
         FIG. 7  shows an isometric view of the gauge assembly of  FIG. 2 , wherein the gauge assembly is provided with a mid-sized interface; 
         FIG. 8  shows an isometric view of the gauge assembly of  FIG. 2 , wherein the gauge assembly is provided with a large-sized interface; 
         FIG. 9  shows a cross-sectional plan view of the gauge assembly of  FIG. 2 ,  7  or  8 ; 
         FIG. 10  shows a cross-sectional plan view of the gauge assembly of  FIG. 2 ,  7  or  8 , wherein the gauge assembly is subjected to a force F comprising a center load; 
         FIG. 11  shows a cross-sectional plan view of the gauge assembly of  FIG. 2 ,  7  or  8 , wherein the gauge assembly is subjected to a force F comprising an off-center load; 
         FIG. 12  shows an isometric view of an embodiment of the gauge assembly of the invention having toggle foot; 
         FIG. 13  shows an elevation view of the gauge assembly of  FIG. 12 ; 
         FIG. 14  shows a cross sectional elevation view of the gauge assembly of  FIG. 13 , taken along lines  14 - 14  of  FIG. 13 ; 
         FIG. 15  shows a graphical representation of a force reading provided by the gauge assembly of  FIGS. 12-14  over a range of force vectors angular offsets from vertical; 
         FIG. 16  shows a graphical representation of a force reading provided by the gauge assembly of  FIG. 11  over a range of force vectors horizontal offsets from center. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Strain gauge assembly  10  of the invention includes a housing  12  having a wall area  14  and a lower portion  16  that defines a cavity  18 . Lower portion  16  defines an internal surface  20  wherein internal surface  20  defines a bottom of cavity  18 . In a preferred embodiment, cavity  18  remains the same size and shape for any of the various configurations of gauge assembly  10 . The shape of housing  12  is preferably constructed such that wall area  14  is very rigid, which forces the bottom of the cup, or lower portion  16 , to take all of the strain when force is applied to external surface  28  of lower portion  16 . With all of the strain being experienced by lower portion  16 , the only part that needs to change shape to make different force ranges for strain gauge assembly  10  is the thickness of the bottom of the cup, i.e., the thickness of lower portion  16 . This thickness can be easily varied for multiple applications. For example, a 0.050 inch thickness of lower portion  16  is desirable for use in a gauge for measuring 0 to 50 pounds, when a lower portion  16  of 0.1″ could measure 0 to 500. 
     Wall area  14  of housing  12  further defines a flange  22  and a keying mechanism  24  for preventing rotation of housing  12  within a suitable mounting structure. Stem  26  protrudes from an external surface  28  of lower portion  16 . External surface  28  defines an annular recess  27  from which stem  26  protrudes. Annular recess  27  functions as an energy director in the center face lower portion  16  of stress gauge assembly  10 . Annular recess  27  preferably comprises a depth of between 25 and 30 percent of the thickness of lower portion  16 . Annular recess  27  causes strain to occur in the center of lower portion  16  of strain gauge element  50  that is affixed to internal surface  20  of the bottom of the cup, i.e., of lower portion  16 . 
     Stem  26  provides a uniform load to a center of lower portion  16  of housing  12  to provide strain to lower portion  16 . 
     Interface member  32  is affixed to a distal end of stem  26 . Interface member  32  is provided for contacting an object, such as a ball or fruit or other material to be tested. Width  34  of interface member  32  may vary in size to accommodate different shapes of objects to be measured. For example, interface member  32  may be in the form of small member  36  ( FIGS. 4-6 ), e.g. 1.5 to 1.7 inches, medium member  38  ( FIG. 7 ), e.g., 2.5 to 3 inches, or large member  40  ( FIG. 8 ), e.g., 9 to 10 inches. Additionally, a concavity of interface members  32  may be varied to accommodate different shapes of the objects to be measured. 
     In one embodiment, shown in  FIGS. 12-14 , interface member  32  is mounted on toggle foot  42  that is pivotally mounted on stem  26  for tracking a location of an object through the center of the object. Toggle foot  42  minimizes stress on gauge assembly  10  that may result from misalignment of the center of lower portion  16  and a center of the object to be measured. Toggle foot  42  allows for measurement of off-center or angled forces with accuracy. 
     As discussed above, the components received within cavity  18  may be the same regardless of the selected interface member  32  and configuration that is selected. The components received within cavity  18  include support member  44  ( FIG. 2 ). Support member  44  has an upper surface  46  and a lower surface  48 . 
     A strain gauge element  50 , shown in greater detail in  FIG. 3 , is affixed to internal surface  20  of lower portion  16  of housing  12  within an adhesive. Strain gauge element  50  measures strain imparted to lower portion  16 . Strain gauge element  50  is preferably a diaphragm strain gauge having a circle portion  52  and is surrounding outer portion  54 . Preferably, the diaphragm strain gauge  10  of the invention has a configuration of traces, or ring segments  52 , in the center of the strain gauge element  50  that resemble a set of concentric circle. The concentric circles in the center of strain gauge element  50  are what allow strain gauge assembly  10  to mechanically average off-center forces, as will be discussed below. 
     Outer portion  54  of strain gauge element  50  is preferably comprised of a zig-zag pattern. Circle portion  52  defines a plurality of ring segments  56  that form substantially complete circles. For example, in a preferred embodiment, ring segments  56  preferably traverse greater than 350 degrees. Ring segments  56  may be comprised of spirals, a double-back spiral pattern shown in  FIG. 3 , may overlap or may be arranged in other patterns that form at least substantially complete circles. In a preferred embodiment, a diameter of circle portion  52  comprises 60% to 70% of the diameter of strain gauge element  50 . More preferably, a diameter of circle portion  52  comprises 60% of a diameter of strain gauge element  50 . 
     Referring back to  FIG. 2 , interface circuit  58  is preferably affixed to upper surface  46  of support member  44  for electronic interface with strain gauge element  50 . The electronics that amplify the extremely small amount of output of strain gauge element  50  are positioned inside strain gauge assembly  10 , i.e., inside of cavity  18  defined by housing  12 , thereby protecting the signal received from strain gauge element  50  from interference by other electronic devices. 
     Data processing components  60  are preferably provided on upper surface  46  of support member  44 . Data processing components  60  are in electronic communication with interface circuit  58  for processing data collected from strain gauge element  50 . Data processing components  60  are placed in close proximity to strain gauge element  50  to minimize electrical noise and interference. 
     Output conduit  62  is provided in communication with data processing component  60  for transmitting information from data processing component  60  to, for example, from data processing component  60  to an electronic output, such as a microprocessor or display of a device. 
     Potting compound  64  is received within cavity  18  for protecting and securing support member  44  and attached strain gauge element  50  therein. 
     Referring now to  FIG. 9 , in use, strain gauge element  50  of strain gauge assembly  10  is located on internal surface  20  of lower portion  16  of housing  12 . Strain gauge element  50  has a circular portion  52  and surrounding outer portion  54 . An appropriately sized interface member  32  is provided that is either integrated with stem  26  or is secured to stem  26  that extends from external surface  28  of lower portion  16  of housing  12 . The object to be measured is located adjacent to interface member  32 . The object may then be compressed so that interface member  32  and attached stem  26  apply a bending force to lower portion  16  of housing  12 . 
     As shown in  FIG. 10 , when a force for compressing the object is centered on interface member  32 , outer portion  54  of strain gauge element  50  is bent the same amount on opposite sides of strain gauge element  50  and circle portion  52  of strain gauge element  50  is deflected uniformly. 
     However, as shown in  FIG. 11 , when a force for compressing the object to be measured is delivered off center to interface member  32 , lower portion  16  is distorted, wherein strain gauge element  50  experiences higher stress on a first side and lower stress on a second side. Due to the circular construction of strain gauge element  50 , discussed above, the measured stresses effectively cancel one another to result in a mechanical averaging of deformation of ring segments  56  about the center of strain gauge element  50 . 
     In greater detail, a standard strain gauge utilized by typical force gauge assemblies has a configuration that senses the strain of a round area of the strain gauge element changing shape when the strain gauge element is loaded in the center. Consequently, typical gauges must ensure that, during their use, energy is directed exactly to the center of the assembly. Typical gauges are, therefore, limited to a configuration that has a point on the front end of the gauge. 
     Force gauge assembly  10 , of the invention, uses a diaphragm strain gauge in the bottom of the cup, i.e., affixed to internal surface  20  of lower portion  16  of housing  12 . If forces applied lower portion  16  through the energy director, i.e., through annular recess  27 , is off center by a small amount, there will be a slight twisting action instead of a direct force through the center of the bottom of the cup, i.e., through the center of lower portion  16  of housing  12 . 
     The twisting action will generate an increased strain on one a first side of strain gauge element  50  and a lower strain on a second side of strain gauge element  50 . The average strain applied to the strain gauge element  50  and to the bottom of the cup, i.e., to lower portion  16 , will be the same as if the force were applied exactly in the center. 
     The mechanical action of averaging the strain will work to a certain degree well within the range of off-centeredness expected to be experienced a typical application, e.g., well within a user&#39;s expected ability to make sure a golf ball is centered within a measurement device employing strain gauge assembly  10  of the invention. In another example application, i.e., wherein strain gauge assembly  10  is used for measuring degradation of rubber used in shoes, the areas that get measured for spring rate may not be parallel. In this case, the force applied to strain gauge element  50  may not be directed through the center of strain gauge element  50 . However, the force is mechanically averaged by the twisting action against diaphragm style strain gauge element  50  located in the in the bottom of the cup, i.e., affixed to internal surface  20  of lower portion  16 . 
     As shown in the example of  FIG. 10 , force F is delivered to stem  26  at an angle offset from perpendicular to lower portion  16 . As a result of the stress canceling features described above, the resultant readings are accurate between a wide range of force application angles. In the graphical example shown in  FIG. 15 , it can be seen that the force data provided by strain gauge element  50  are consistent when the force vector is within approximately 25 degrees from normal. In the case of the application of an off centered force on a force gauge employing a typical strain gauge element, the force readings would only be accurate for the case where force is applied normal to the strain gauge element, wherein the measured force would decrease as the angle of application departed from normal. A force graph of such a device would resemble a sign wave, e.g.,  FIG. 1 , wherein an accurate reading would be reflected by the peak of the sign wave and all other readings would be erroneous. In contrast, Applicant&#39;s device provides accurate readings over a wide range of force application angles, as is shown by  FIG. 15 . 
     Referring to  FIG. 11 , it can be seen that a similar phenomenon is seen when a normal force is applied off center to the interface member  32  of the invention. Due to the stress cancelling features discussed above, the resultant readings are accurate over a range of force application distances, e.g., from approximately 0.035 units to the left of center to approximately 0.035 units to the right of center in the example shown in  FIG. 16 . 
     Advantages of the strain gauge assembly  10  of the invention include low cost of manufacturing due to the consistency of components located within cavity  18 . In particular, all assemblies are the same. The inside of the cup shape is always exactly the same. The configuration of the face of the gauge is whatever will be necessary to follow the shape of what will be measured. For example:
         a. The golf ball compression meter has a dimple negation shape on the tip of the stem.   b. A shoe spring rate measurement device will use a toggle foot, applied in a similar manner to the use of a c-clamp.   c. The shape of an interface element for use in tomato gauge will be a larger diameter, e.g., a 1.25 inch radius to match the average diameter of a ripe tomato typically available in the grocery store.   d. The shape of an interface element for use in a potato gauge may be a flat surface of about ½ inch in diameter. With the mechanical averaging of the force within the gauge, the potato spring rate could still be measured without finding a parallel surface press against.   e. In the case of the skull gauge, a typical human skull will have a large surface area and a relatively unknown shape. The surface of the interface element could have three feet on spherical mounting to allow the feet to track the surface of the patient&#39;s skull and apply a uniform pressure to make a measurement.   f. A separate configuration may be used to enable the gauge of the invention to measure force of flex on trailer hitches and fishing rods.   g. In all cases, a primary difference between the various configurations of gauges is the shape of the interface element mounted on the stem.       

     Thus, the present invention is well adapted to carry out the objectives and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those of ordinary skill in the art. Such changes and modifications are encompassed within the spirit of this invention as defined by the claims.