Patent Publication Number: US-2007095156-A1

Title: Flexure system for strain-based instruments

Description:
RELATED APPLICATIONS  
      This application is a continuation of non-provisional patent application Ser. No. 10/849,429, filed May 19, 2004; which application claims the benefit of provisional application No. 60/472,757, filed May 22, 2003. The contents of both application Ser. Nos. 10/849,429 and 60/472,757 are incorporated herein in their entirety. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to force, torque, and deflection measuring devices, and more particularly to improved constructions for such devices. More particularly, the present invention is directed to compact, high-stiffness strain element systems having small length-to-height ratios to provide high metric strain levels for small deflections.  
     BACKGROUND OF THE INVENTION  
      Strain-based force, torque, and displacement instruments have conventionally included strain elements, in the form of flexures, columns, diaphragms, or shear panels. These elements react to stresses caused by tension, compression, bending, torsion, or shear. One objective of such devices has been to produce the required levels of strain in the element to actuate a strain-measuring transducer, such as a bonded resistance strain gage, while maintaining a minimum of strain and deflection in the overall structure; i.e., a high level of sensitivity to load. A further objective has been to provide compact and potentially miniature structures that incorporate adequate strain elements. Devices of the shear-panel element type have been constructed with round or rectangular blind cavities located transversely in a beam structure, leaving a thin shear panel to react to stresses as the beam is stressed.  FIGS. 1 through 4  are illustrative of the prior art constructions currently in use. While these instruments have been both successful and economical, they have been limited in compactness and stiffness.  
      Thus, there exists a need for a compact strain-based measurement device that provides high sensitivity (high surface and strain levels) at very low levels of actuating deflection.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to a compact, high-stiffness, high-sensitivity device for measuring forces or deflections acting on a sensor structure in at least one axis. An object of the embodiments of the present invention is that the structure of the device can be constructed to be much smaller, or shorter, in the length direction than known conventional sensors intended for similar use.  
      The specification and drawings of U.S. Provisional Application No. 60/472,757, filed May 22, 2003, are hereby incorporated by reference in their entirety.  
      In one embodiment, the device comprises a frame and an array of recesses formed in the frame, wherein the array of recesses has a length-to-height ratio significantly less than 1.0. This configuration allows the overall length of the element portion of the device to be significantly less than in a conventional single-element shear-panel design. The recesses are formed as blind cavities from each side to provide thin shear surfaces. A strain gage or similar transducer is mounted in at least one of the recesses, wherein when the element is incorporated in a sensor body, the device is sufficiently rigid to measure high strain levels under low deflection loads. Such devices are most suited to applications where a high level of strain corresponding to relatively small displacements is required.  
      In another embodiment, a compact, high-stiffness, high-sensitivity device is provided for measuring forces in frame members having a primary axis. This device comprises a clevis having a base and a pair of substantially parallel arms. An array of recesses is formed in each one of the pair of arms, each array of recesses having a length-to-height ratio of less than 1.0. At least one strain gage or similar transducer is mounted in at least one of the recesses in each array. When mounted to an axially-loaded member, the device is sufficiently rigid to measure high strain levels with low deflections.  
      In yet another embodiment, a compact, high-stiffness, high sensitivity device is provided for measuring forces of deflection in frame members. This device comprises a frame having a base and multiple spaced-apart arms integrally formed with and extending outwardly from the base. An array of recesses is formed in at least one of the arms, the array of recesses having a length-to-height ratio of less than 1.0. Depending upon the specific application, the arm in which the array is formed may extend outwardly at a perpendicular or oblique angle as defined by the vertical axis of the base and the axis formed by the lower surface of the arm. When mounted to a frame assembly, the device is sufficiently rigid to measure high strain levels with low deflections.  
      In a fourth embodiment, a high-stiffness, high sensitivity device is provided for measuring torque in axial members, such as pump shafts. This device comprises a wheel having inner and outer concentric rings, the inner and outer rings interconnected by a plurality of radially extending spokes. A recess is formed in at least two of the spokes, each recess having a length-to-height ratio of less than 1.0. At least one shear strain element is mounted in at least two of the recesses. As used to measure torque, the inner ring is mounted about the axial member to be measured and the outer ring is mounted to a load source.  
      These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the described embodiments when considered in conjunction with the drawings. It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  illustrates a prior art rectangular beam shear element as conventionally used in force and torque measurements;  
       FIG. 2  illustrates a prior art shear element formed by cylindrical bores in the axis of loading;  
       FIG. 3  illustrates a prior art beam shear element with a panel formed by blind cylindrical bores transverse to the load axis as conventionally used in force and torque measurements;  
       FIG. 4  illustrates a prior art flanged beam shear element with a panel formed by blind cylindrical bores transverse to the load axis as conventionally used in force and torque measurements;  
       FIG. 5  illustrates one embodiment of the present invention configured for single-axis measurement with high sensitivity and stiffness and low deflection in the primary load axis;  
       FIG. 6  depicts a second embodiment of the present invention configured for high sensitivity and stiffness and low deflection in a clevis assembly intended for weighing applications or force measurements in axially loaded members;  
       FIG. 7  depicts a third embodiment of the present invention configured for high sensitivity and stiffness in a frame assembly intended for application as a deflectometer, dilatometer, or diametric extensometer;  
       FIG. 8  depicts a fourth embodiment of the present invention configured for measuring torque about a shaft;  
       FIG. 8A  is a sectional view of the device of  FIG. 8  taken along Line  8 A- 8 A; and  
       FIG. 9  illustrates an analogy of the strain behavior of a parallel system of active shear panels to the motion produced by a system of parallel links. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
      The present invention is directed to a high-stiffness, high-sensitivity device for measuring forces or deflections imposed on a sensor body. While several possible embodiments of the present invention are described herein, those skilled in the art will appreciate, upon reviewing the aspects of the disclosed constructions, that there are numerous constructions and configurations of the device which may be devised according to the principles described herein. That is, devices for other applications may be developed having the high-stiffness and high-sensitivity dimensions disclosed herein.  
      As shown in  FIG. 5 , one embodiment of a device having features of the present invention, shown generally as  100 , comprises a beam  110 , an array  120  including recesses  120   a ,  120   b ,  120   c  or blind cavities, formed in the beam, and at least one strain gage or similar strain transducer  130   a ,  130   b , mounted in at least one recess  120   a ,  120   b ,  120   c . The beam  110  is formed to increase the effective height of the device in bending, and to increase the effective radius about the lengthwise horizontal axis of the panel in torsion. The through hole  120   b  is used to further increase the height of the device without adding stiffness. As shown in the figure, the height of the array of recesses  120  is designated as ‘h’ and the length of the recess  120  is designated as ‘l’. To provide high stiffness and sensitivity, the length ‘l’ to height ‘h’ ratio of the array of recesses is equal to or less than 1.0. This construction is configured for single axis measurement with high deflection sensitivity and stiffness in the primary load axis. Each recess  120   a ,  120   c  and strain transducer  130  combination is also known as an active shear panel. Desirably, the active shear panels have a length-to-height ratio of about 1.0 and, as shown in  FIG. 5 , may be arranged in vertical arrays wherein the shear panels are in parallel relation to one another. In a second embodiment of the present invention shown in  FIG. 6 , a high-stiffness, high-sensitivity device  200 , having features of the present invention, may find applications, among others, in weighing applications, or force measurement in axially loaded members  280 . The device  200  comprises a clevis assembly  210  having a base, shown in  FIG. 6  as a pair of symmetrical portions  210   a ,  210   b , and a pair of substantially parallel arms  212   a  and  212   b  extending outward/downward from the base. Preferably, though not necessarily, the base  210   a ,  210   b  and the arms  212   a ,  212   b  are integrally formed. An array  220  of recesses  220   a ,  220   b , and  220   c  is formed in each of the symmetrical base portions  210   a ,  210   b ; however, the number of recesses  220   a ,  220   b ,  220   c  formed in each of the symmetrical base portions  210   a ,  210   b  is not a limiting feature of this embodiment. Rather, the array  220  recesses are so formed in the base portions  210   a ,  210   b , that each array has a length ‘l’ to height ‘h’ ratio of less than 1.0. At least one strain gage  230  is mounted within at least one recess in each base portion  210   a ,  210   b . As in the embodiment of  FIG. 5 , where multiple strain gages  230  are mounted within the same array, the active shear panels are desirably in parallel relation to one another, and each has desirably (but not necessarily) an individual length-to-height ratio of 1.0.  
      Turning now to  FIG. 7 , a further embodiment of a device having features of the present invention is directed to a high-stiffness, high-sensitivity device  300  for measuring forces of deflection in a frame assembly  380 . Such a device might be used for applications such as a deflectometer, dilatometer, or diametric extensometer. This device  300  comprises a frame  310  having a base  310   a  with a vertical axis, and arms  310   b  and  310   c  that are spread-apart from each other and extend outwardly from the base  310   a . Desirably, the arms  310   b ,  310   c  are integrally formed with the base, each arm  310   b ,  310   c  having an upper surface, a lower surface, and a free end. An array  320  of recesses  320   a ,  320   b , is formed in at least one of the arms. Again, the array of recesses has a length to height ratio of less than 1.0. A strain gage  230  is mounted in at least one of the recesses  320   a ,  320   b . As in the other embodiments of the present invention, when a strain gage  230  is mounted in more than one recess of an array, the active sheer panels in that array  320  are arranged in parallel relation. Depending upon the particular application, one of the arms  310   b ,  310   c  may extend outwardly from the base  310   a  at an obtuse angle to provide greater sensitivity to forces of deflection. The obtuse angle is defined by the vertical axis of the base and the axis formed by the lower surface of the arm. As shown in  FIG. 7 , arm  310   b  extends outwardly and upwardly from the base  310   a.    
      In another embodiment shown in  FIG. 8 , a high-stiffness, high sensitivity device  800  is shown for measuring torque in axial members, such as pump shafts and the like. This device  800  comprises a wheel having an inner ring  810 , an outer ring  820 , and a plurality of spokes  830  extending radially outward from the inner ring  810  to interconnect the inner ring  810  and outer ring  820 . In a preferred embodiment, a recess  832  is formed in each one of the spokes  830 . As formed, the plurality of recesses  832  are arranged in a circumferential array between the inner rings  810  and outer rings  820 . Each recess has a length-to-height ratio of about 1.0 At least one measuring element  834  is mounted in alternating recesses  832 . In operation, the device  800  is mounted so that the inner ring  810  is mounted about the shaft of the equipment being measured, such as a pump shaft. The outer ring is then mounted to a load source such as a motor, wherein the device is sufficiently rigid to measure torque when the shaft is loaded.  
      The principle of operation of the devices of the present invention is based on the fact that shear strain in a panel structure is angular displacement. The shear strain of a member is the change in an initially right angle on the panel, one side being aligned with the axis of load, and is thus a function of displacement of the loaded end of the structure and the effective length of the shear system.  
      Although it is widely known that an unmeasured load path within or in parallel with a strain element typically produces a statically indeterminate structure with low stability and poor precision, it can be easily seen by an analysis of the structures shown in  FIG. 5 , that high-stiffness shear structures can, within reason and good design practice, be constructed with multiple parallel stages without encountering these effects. Where a conventional shear beam cell, as in  FIGS. 3 and 4 , would become problematically flexible and subject to bending stresses as it was scaled down to reduce the length of the shear system, the parallel system of the disclosed structure can be of any height and stiffness without limitation on the shear system length other than the practical considerations of strain elements and wiring. The above can be modeled as a simple system of parallel links, as shown in  FIG. 9 . Shortening the links  940   a ,  940   b ,  940   c  (corresponding to active shear panels  920   a ,  920   b ,  920   c ) increases the angle at which they are displaced to produce a given vertical motion of the movable end. Likewise, shortening a shear structure produces a higher angular strain, and therefore a higher surface shear strain, for a given displacement. That is, angular shear strain γ is related to τ, the unit surface shear strain normally used as the metric property of a shear strain element; i.e., τ=γ/2, where γ is expressed in radians.  
      As an additional benefit, since each part of a shear structure experiences the same shear stress, provided displacements are small enough that bending is negligible, not all sections of the system must be instrumented. Additional panels or beams can be added to stiffen and control the structure without further complicating the strain gage circuit, so long as effective length-to-height ratios are kept low (less than 1.0) to avoid bending stresses.  
      These mechanisms can be combined into more complex structures, producing high-sensitivity, low-deflection designs for applications such as torsion instruments and multi-axis cells. Since high spring rates can be achieved without sacrificing sensitivity, this mechanism will also work well in summed multi-segment devices, such as a group of instrumented supports that carry the force delivered by a large linear actuator that is normally bolted in place by interposing a separate instrument between the actuator and structure.  
      Although the present invention has been described with exemplary embodiments, it is to be understood that modifications and variations may be utilized without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the appended claims and their equivalents.