Abstract:
A ring load cell configured to measure force, moment, and temperature components in an orthogonal three-axis coordinate system. The ring-load-cell structure consists of two annular flanges having a common central axis through which a shaft may be disposed. The annular flanges are interconnected by a plurality of filleted posts configured to enhance strain distribution and which are disposed parallel to the common central axis, equally spaced about the circumference of the annular flanges. A plurality of strain gauges is disposed either on the posts or on fillets, and a plurality of temperature sensors is disposed either on the posts or on the inner surfaces of the annular flanges. Signals from the strain gauges and temperature sensors are processed to provide thermally compensated strain measurements of the applied forces and moments on the ring-load-cell structure in the orthogonal three-axis coordinate system.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to load cells of the type used to measure forces and moments, and more particularly, to a ring load cell configured for placement around an axially disposed shaft to provide thermally compensated force and moment measurements in an orthogonal three-axis coordinate system. 
     It is frequently necessary to measure applied forces and moments between a motor or gear drive and a shaft driven component. Usually, the forces and moments acting upon such a mechanical system are expressed in terms of a three-dimensional coordinate system, and may be completely expressed in terms of six components, including three linear force components along each of three mutually orthogonal axes and three moments about the same set of axes. 
     A conventional approach to developing force sensors for measuring these six components is to use mechanical structures including various hinges, pivots, or other mechanisms to decouple the force components and to permit them to be measured independently from one another. Such mechanical structures are complex to manufacture, and require precise accuracy in dimensions and component elasticities to accurately decouple the individual force components. Accordingly, such structures are costly to manufacture and maintain. 
     An alternative approach which has been developed to overcome some of the problems associated with mechanically decoupling the various forces and moments to be measured is to use strain-based force sensors as is exemplified by U.S. Pat. No. 4,094,192 to Watson, et al. The system disclosed in the U.S. Pat. No. 4,094,192 patent employs shear strain gages and extensional gages mounted on beams disposed between a pair of annular rings to measure forces and moments acting on the ring. Signals from the individual sensors are processed by an analog or digital processor to derive the desired force and moment components in an orthogonal three-axis coordinate system without the need to know the dimensions or elasticities of the structure upon which the sensors are mounted. However, the design shown in the U.S. Pat. No. 4,094,192 patent results in a trade off of gage sensitivity between the shear strain gages and the extensional gages, which are mounted to the same beams. 
     There is an ongoing need to provide an improved compact multi-axis load cell particularly suited for use about a shaft disposed between a driving component and a driven component, and which can be selectively configured for sensitivity to forces or moments depending upon the particular application in which it is employed. 
     BRIEF SUMMARY OF THE INVENTION 
     Briefly stated, the present invention is a ring load cell configured to measure three force components and three moment components in an orthogonal three-axis coordinate system, together with one or more temperature components. The ring-load-cell structure consists of two annular flanges having a common central axis through which a shaft may be disposed. The annular flanges are interconnected by a plurality of filleted posts configured to enhance strain distribution and which are disposed parallel to the common central axis, equally spaced about the circumference of the annular flanges. A plurality of strain gauges is disposed either on the posts or on the fillets, and a plurality of temperature sensors are disposed either on the posts or on the inner surfaces of the annular flanges. Signals from the strain gauges and temperature sensors are processed to provide thermally compensated strain measurements of the applied forces and moments on the ring-load-cell structure in the orthogonal three-axis coordinate system. 
     The foregoing and other objects, features, and advantages of the invention as well as presently preferred embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the accompanying drawings which form part of the specification: 
     FIG. 1 is a simplified perspective illustration of a ring load cell, illustrating the relationship of force and moment components in an orthogonal three-axis coordinate system; 
     FIG. 2 is a perspective view of a front annular flange in a four-post embodiment of the ring load cell of the present invention; 
     FIG. 3 is a perspective view of a rear annular flange in the four-post embodiment of the ring load cell shown in FIG. 2; 
     FIG. 4 is an enlarged perspective illustration of a single post interconnecting the front and rear annular flanges of FIGS. 2 and 3; 
     FIGS. 5A-5C illustrate the front view, side sectional view, and rear view of a six-post embodiment of the ring load cell of the present invention; 
     FIGS. 6A-6C illustrate the front view, side sectional view, and rear view of a twelve-post embodiment of the ring load cell of the present invention; 
     FIG. 7 is a cross-sectional view of a post interconnecting the front and rear annular flanges, illustrating placement locations for strain gages on the uniform radius fillets; 
     FIG. 8 is a cross-sectional view of the annular structure of a multi-axis ring load cell, illustrating the placement of annular ridges on the exterior surfaces to increase bending strain on each post; 
     FIG. 9 is a cross-sectional view of the annular structure of a multi-axis ring load cell, illustrating the radial centerline placement of a post  24  in relation to the radial centerline of the front and rear annular flanges; and 
     FIG. 10 is a simplified illustration of a multi-axis ring load cell of the present invention affixed about the drive shaft of a drive component. 
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following detailed description illustrates the invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the invention, describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what is presently believed to be the best mode of carrying out the invention. 
     Turning to FIG. 1, a simplified representation of a multi-axis ring load cell (RLC) of the present invention is indicated generally at  10 . The multi-axis RLC  10  comprises an annular structure  12  having a large central opening  14  disposed along a central axis CA through which a shaft or other cylindrical member can pass. Further illustrated in FIG. 1 is a three-axis orthogonal coordinate system having an origin O and three principal axes, designated X, Y, and Z, orientated as shown relative to the multi-axis RLC  10 . As will be described below in further detail, the multi-axis RLC  10  is configured to measure radial forces in the X-direction, designated Fx, radial forces in the Y-direction, designated Fy, and thrust forces in the Z-direction, designated Fz. In addition to the force measurements, the multi-axis RLC  10  is further configured to measure the tilting moment in the X-direction, designated Mx, the tilting moment in the Y-direction, designated My, and the reaction torque in the Z-direction, designated Mz. 
     The multi-axis RLC  10  comprises a front annular flange  20  as shown in FIG. 2, and a rear annular flange  22  as shown in FIG. 3, interconnected by a number of equally spaced posts  24 . The specific number of equally spaced posts  24  may vary depending upon the particular application for which the multi-axis RLC  10  is configured. In the embodiment shown in FIGS. 2 and 3, four posts  24  are employed, equidistantly spaced on 90-degree centers about the central axis CA at a uniform radius therefrom. Alternate configurations of the multi-axis RLC  10  may employ six posts  24  centered every 60 degrees, as shown in FIGS. 5A-5C, or twelve posts  24  centered every 30 degrees, as shown in FIGS. 6A-6C, which are described below in further detail. Those of ordinary skill in the art will recognize that greater or fewer posts  24 , with varied positions, may be employed with the present invention. Posts  24  equidistantly spaced about the central axis CA and at a uniform radius therefrom, are preferred. 
     The front annular flange  20 , the rear annular flange  22 , and the interconnecting posts  24  are preferably integrally formed from the single annular structure  12 . Alternatively, those of ordinary skill in the art will recognize that the front annular flange  20  and the rear annular flange  22  may be formed from individual annular members, and then interconnected by affixing posts  24  therebetween at the desired circumferential locations and radial distance. Included in the front annular flange  20  and the rear annular flange  22  are a plurality of attachment points such as bores  26  or slots  28 , through which bolts or other suitable fixtures (not shown) may be passed to secure the multi-axis RLC  10  to a fixed surface. Those of ordinary skill in the art will recognize that the exact number and placement of the bores  26  and slots  28  may be varied depending upon the particular application for which the multi-axis RLC  10  is to be employed. 
     In the embodiment of the multi-axis RLC  10  shown in FIGS. 2 and 3, four posts  24  are positioned between the front annular flange  20  and the rear annular flange  22 , each centered at 90 degree intervals about the central axis CA. FIG. 4 provides an enlarged perspective illustration of one post  24  at a circumferential location designated as 0 degrees about the central axis CA. As can be see in FIG. 4, the post  24  is formed as a cylindrical section, having an inner radial surface  27  and an outer radial surface  29 , each of which are concentric with the central axis CA of the multi-axis RLC  10 . The radial thickness of the post  24  shown in FIG. 4 is substantially less than the radial thickness of the front annular flange  20  and the rear annular flange  22 . Preferably, each post  24  is positioned between the inner radial dimension R i  and outer radial dimension R o , of the annular flanges  20 ,  22  such that a first portion of the flanges  20 ,  22  extends radially inward from the posts  24 , and a second portion extends radially outward from the posts  24 , as seen in FIG.  4 . Each post  24  is joined to the front and rear flanges  20 ,  22  with a uniform radius fillet  30  to provide strength and flexibility at the interconnection regions. 
     In alternate embodiments of the present invention, such as is shown in FIGS. 5A-5C and  6 A- 6 C, the placement and configuration of the posts  24  may be varied. Posts  24  may be configured with cylindrical surfaces  27 ,  29  having greater width than radial depth, as seen in FIG. 4, to provide for larger surface areas across which uniform strain is distributed. Alternatively, posts  24  having reduced cylindrical surfaces  27 ,  29 , i.e. posts  24  with a smaller width than radial depth, as seen in FIGS. 5A-5C and  6 A- 6 C, are utilized to provide for a smaller surface area across which a concentrated strain is distributed. 
     To provide measurements of the force components Fx, Fy, and Fz, as well as the moment components Mx, My, and Mz applied to the multi-axis RLC  10 , a plurality of extension gages  100  and shear strain gages  200  are affixed to the structure of the multi-axis RLC  10 . The design of the multi-axis RLC  10  is such that forces and moments applied to the multi-axis RLC  10  are conveyed between the front flange  20  and the rear flange  22  through the plurality of posts  24 . In the preferred embodiment, the extension gages  100  and shear strain gages  200  are centrally affixed to any surface of the posts  24 , or alternatively, are affixed to the surfaces of the uniform radius fillets  30  on the radius centerlines, as shown in FIG.  7 . Placement of the extension gages  100  and the shear strain gages  200  is selected such that gages are affixed to locations, and at orientations, in which one form of strain is dominant in the structure of the multi-axis RLC  10 , i.e. bending, shear, or extensional strain. 
     Each extension gage  100  and shear strain gage  200  provides a measurement of the forces acting on the post  24  or fillet  30  to which it is affixed. Using conventional mathematical procedures, the measurements from multiple gages  100 ,  200  placed at known locations about the multi-axis RLC  10 , are utilized to calculate the force components Fx, Fy, and Fz, as well as the moment components Mx, My, and Mz. 
     In a preferred configuration of the multi-axis RLC  10  utilizing six posts  24 , an extension gage  100  is affixed to each of a first set of posts  24 , with each post displaced by 120 degrees about the central axis CA. For example, if a first extension gage  102  is affixed to the post  24  designated as the 0 degree post, the second extension gage  104  is affixed to the post  24  designated as the 120 degree post, and the third extension gage  106  is affixed to the post designated as the 240 degree post. Correspondingly, a shear strain gage  200  is affixed to each of a second set of posts  24 , with each post displaced by 120 degrees about the central axis CA and circumferentially by 60 degrees from the posts  24  which comprise the first set. 
     For example, as seen in FIGS. 5A-5C, if the extension gages  102 ,  104 , and  106  are affixed to the posts  24  designated as the 0 degree, 120 degree, and 240 degree posts, then a first shear strain gage  202  is affixed to the post  24  designated as the 60 degree post, the second shear strain gage  204  is affixed to the post  24  designated as the 180 degree post, and the third shear strain gage  206  is affixed to the post designated as the 300 degree post. The placement of the extension gages and the shear strain gages for a multi-axis RLC  10  configured with twelve posts  24  is preferably identical to the placement for a multi-axis RLC  10  configured with six posts  24 , leaving a number of posts  24  without a gage affixed thereto. It will be recognized that additional extension gages and shear strain gages may be affixed to alternate sets of posts having the above-described circumferential spacing to provide redundant force measurement capabilities. 
     In the multi-axis RLC  10 , the “bending strain” is measurable in the deflection or bending of the posts  24  in response to forces applied through the front annular flange  20  and the rear annular flange  22 . In optional embodiments of the multi-axis RLC  10  employing strain gages to measure bending strain, one or more structural variations may be incorporated to alter the degree to which posts  24  bend or deflect under applied forces. 
     FIG. 8 illustrates a first optional structural variation designed to increase the bending strain on the posts  24  interconnecting the front annular flange  20  and the rear annular flange  22 . The inclusion of an annular ridge  50  on the outer surfaces  51 ,  52  of the respective annular flange  20 ,  22  increases the bending strain on the posts  24  which is proportional to the distance between the annular ridge  50  radial centerline and the radial centerline of the posts  24 . Additional inner and outer radial clearance ridges  61 ,  62  on the inner or outer radial surface of the annular flanges  20 ,  22  ensure that the radial motion of the annular flanges  20 ,  22  is not inhibited by other fixtures. 
     Correspondingly, the selection of the radial centerline on which posts  24  are positioned about the central axis CA alters the bending strain on the posts. As is seen in FIG. 9, placement of the posts  24  radially inward or radially outward from the annular flange radial centerline results provides a increase in the bending strain exerted on the posts  24  by forces applied to the outer surfaces  52  of the annular flanges  20 ,  22 . Those of ordinary skill in the art will recognize that the inclusion of an annular ridge  50 , the selection of the post radial centerline, or both structural variations may be utilized in the design to alter the bending strain on the posts  24 , depending upon the particular application for which the multi-axis RLC  10  is to be employed. 
     During use, the multi-axis RLC  10  of the present invention is configured for placement between a drive component  304 , such as an electric motor or transmission, and an application fixture  300  receiving a motive force from a driven component  302  and a drive shaft  306  passing through the mutli-axis RLC  10  on the central axis CA, as shown in FIG.  10 . The multi-axis RLC  10  is preferably secured in a fixed relationship to the application fixture  300  and the drive component  304  by means of removable bolts passed through a plurality of the bores  26  in the front annular flange  20  and slots  28  in the rear annular flange  22 . When affixed between the components  300  and  304 , forces acting on the separate components will be transferred through the posts  24 , which provide a fixed connection there between, and are measured by the individual gages  100  and  200 . Those of ordinary skill in the art will recognize that a variety of methods for fixed attachment of the multi-axis RLC  10  between the drive component  304  and application fixture  300  may be employed, such that forces acting on the components  300 ,  304  will be transferred through the multi-axis RLC  10 . 
     Measurements of the force and moment components acting on the multi-axis RLC  10  are obtained from the individual gages  100 ,  200  placed throughout the annular structure  12 . In one embodiment, one or more measurements of the temperature of annular structure  12  are obtained from the one or more thermal sensors  250  affixed to the multi-axis RLC  10 . Temperature measurements are utilized in subsequent processing to compensate for thermal effects on strain gages and to provide information related to the temperature of the multi-axis RLC  10  operating environment. 
     In an exemplary application, a drive motor or gear drive  304  is suspended from the application fixture  300  by a multi-axis RLC  10  secured there between. A drive shaft  306  from the drive motor or gear drive  304  passes through the multi-axis RLC  10  on the central axis CA, and provides a motive force to the driven component  302 . Measurements of the reaction torque Mz exerted on the multi-axis RLC  10  are equal to, and opposite in direction to, the shaft torque of the drive motor or gear drive  304  exerted on the driven component  302  through the drive shaft  306 , providing an inexpensive way to measure shaft torque Mz for a specific drive configuration. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.