Patent Publication Number: US-2005120808-A1

Title: Platform balance

Description:
REFERENCE TO CO-PENDING APPLICATION  
      This patent application claims priority to co-pending U.S. application for patent filed on Dec. 4, 2003, having Ser. No. 60/526,954, and entitled “Platform Balance,” which is incorporated by reference in its entirety into this disclosure. 
    
    
     BACKGROUND OF THE INVENTION  
      The present disclosure relates to devices that transmit and measure linear forces along and moments about three orthogonal axes. More particularly, the present disclosure relates to devices that are particularly well suited to measure forces and moments upon a test specimen in a test environment, such as in a wind tunnel.  
      The measurement of loads, both forces and moments, with accuracy and precision is important to many applications. A common use, where several moments and forces need to be measured, is in the testing of specimens in a wind tunnel. Test specimens can be placed on a platform balance located in a pit of the wind tunnel. The platform balance can be adapted to receive a vehicle or other large test specimen, rather than merely a scale model of the vehicle. Actual vehicles, rather than scale models of the vehicles, allows the designer to determine actual measurements of prototypes, rather than merely inferential measurements. If the test specimen is a vehicle with wheels, the platform balance can be equipped with a rolling belt to rotate the wheels, which can make a significant improvement in measurement accuracy.  
      Six components of force and moment act on a test specimen on the platform balance in the wind tunnel. These six components are known as lift force, drag force, side force, pitching moment, yawing moment, and rolling moment. The moments and forces that act on the test specimen are usually resolved into three components of force and three components of moment with transducers that are sensitive to the components. Each of the transducers carries sensors, such as strain gages, that are connected in combinations that form Wheatstone bridge circuits. By appropriately connecting the sensors, resulting Wheatstone bridge circuit unbalances can be resolved into readings of the three components of force and three components of moment.  
      Platform balances have a tendency to be susceptible to various physical properties of the test environment that can lead to inaccurate measurements without additional compensation. For example, temperature transients in the wind tunnel can result in thermal expansion of the platform balance that can adversely affect the transducers. In addition, large test specimens are prone to create large thrust loads on the transducers that can cause inaccurate measurements. Accordingly, there is a continuing need to develop a platform balance suitable for use with large test specimens.  
     SUMMARY OF THE INVENTION  
      The present disclosure is directed to a platform balance that is suitable for transmitting forces and moments in a plurality of directions. The platform balance is adapted to support a test specimen, such as a large vehicle, in a test environment such as a wind tunnel. The platform balance includes a frame support and at least three spaced-apart transducers coupled to the frame support. Each of the transducers is sensitive about two orthogonal sensed axes. The transducers cooperate to provide signals indicative of forces and moments with respect to at least two orthogonal axes. In one example, the frame support includes a first perimeter frame and a second perimeter frame. The platform balance of this example includes four spaced-apart transducers coupling the first perimeter frame to the second perimeter frame. Transducers sensitive about two orthogonal sensed axes do not suffer from the effects of thermal expansion of the frame support and reject the large thrust loads present in transducers sensitive about three orthogonal sensed axes.  
      The present disclosure is also directed to a transducer body having a support coupled to a sensor body along an axis of compliance. The sensor body is adapted to deflect about the two orthogonal sensed axes where the sensed axes are mutually orthogonal to the axis of compliance. In one aspect, the support includes a pair of clevis halves disposed on opposite sides of the sensor body along the axis of compliance. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  is a plan view of a platform balance constructed in accordance with the present disclosure.  
       FIG. 2  is an elevation view of the platform balance of  FIG. 1  having additional features and is suitable for receiving a test specimen.  
       FIG. 3  is an elevation view of the platform balance of  FIG. 2 , and having an exemplary test specimen.  
       FIG. 4  is a top view of a transducer constructed in accordance with the present disclosure and included in the platform balance of  FIG. 1 .  
       FIG. 5  is a front view of the transducer of  FIG. 4 .  
       FIG. 6  is a side view of the transducer of  FIG. 4 .  
       FIG. 7  is a detailed view of a portion of the transducer of  FIG. 4 .  
       FIG. 8  is a side view of another transducer constructed in accordance with the present disclosure.  
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT  
      This disclosure relates to devices and structures that transmit and measure linear forces along and moments about three orthogonal axes. The disclosure, including the figures, describes a platform balance and included transducers with reference to a several illustrative examples. For example, the disclosure proceeds with respect to frame supports attached to multi-part transducer assemblies described below. However, it should be noted that the present invention could be implemented in other devices or structures and transducers, as well. The present invention is described with respect to the frame supports and transducer assembly for illustrative purposes only. Other examples are contemplated and are mentioned below or are otherwise imaginable to someone skilled in the art. The scope of the invention is not limited to the few examples, i.e., the described embodiments of the invention. Rather, the scope of the invention is defined by reference to the appended claims. Changes can be made to the examples, including alternative designs not disclosed, and still be within the scope of the claims.  
      An exemplary embodiment of a platform balance  10  of the present disclosure is illustrated in  FIGS. 1-3 . In the embodiment illustrated, the platform balance  10  can include a first frame support  12  and a second frame support  14 . A plurality of transducer assemblies  16 , herein four although any number three or more can be used, couple the first frame support  12  to the second frame support  14 . The platform balance  10  can be used to measure forces and moments applied to a test specimen of nominally large weight or mass such as a vehicle, engine, plane, etc. The frame supports  12  and  14  are nominally unstressed reaction frames, wherein each of the transducers comprises a two-axis force transducer. Various levels of flexure isolation can be provided in the platform balance  10  to provide increased sensitivity, while nominally supporting large masses.  
      Referring to  FIGS. 4-6 , one of the transducer assemblies is illustrated at  40 , wherein each of the transducer assemblies  16  is preferably similarly constructed. The transducer assembly  40  includes a sensor body  42  and a clevis assembly  44 . The clevis assembly  44  includes a first clevis half  46  and a second clevis half  48 . The sensor body  42  is disposed between the clevis halves  46  and  48  and joined together with a suitable fastener. In the embodiment illustrated, the fastener comprises a bolt or threaded rod  50  extending through apertures  48 A,  42 A and  46 A of the clevis half  48 , sensor body  42  and clevis half  46 , respectively. A nut  51  is provided on an end  53  of rod  50  and a super nut  52  is threaded upon an end  54  of the threaded rod  50 . A plurality of set screws  56  extends through the apertures in the nut  52  to engage an end of the clevis half  46 . Tightening of the set screws  56  allows high clamping pressures to be achieved efficiently and at reduced torque values on each of the set screws  56  rather than through the use of a nut  52  by itself. It should be noted that although center portions of the devises  46  and  48  will engage or contact the center portion of the sensor body  42  about the apertures  46 A,  42 A and  48 A, gaps are otherwise provided between each of the clevis halves  46  and  48  and the sensor body  42  so as to allow the sensor body  42  to move relative to the clevis halves  46  and  48 .  
      The sensor body  42  is preferably integral, being formed of a single unitary block of material. The sensor body  42  includes a ridged central hub  60 , herein including the aperture  42 A, and a ridged perimeter body  62  that is concentric with, or disposed about, the central hub  60 . A plurality of flexure structures  64  (herein flexure beams  64  although other forms could be used) join the central hub  60  to the perimeter body  62 . In the embodiment illustrated, the plurality of flexure beams  64  comprises four straps  71 ,  72 ,  73  and  74 . Each of the straps  71 - 74  extend radially from the central hub  60  to the perimeter body  62  along corresponding longitudinal axes  71 A,  72 A,  73 A and  74 A. Preferably, axis  71 A is aligned on axis  73 A, while axis  72 A is aligned with axis  74 A. In addition, axes  71 A and  73 A are perpendicular to axes  72 A and  74 A. Although illustrated wherein the plurality of flexure beams  64  equals four, it should be understood that any number of straps three or more can be used to join the central hub  60  to the perimeter body  62 . Preferably, the flexure beams  64  are spaced at equal angular intervals about a. central axis indicated at  85 .  
      Flexure members  81 ,  82 ,  83  and  84  join an end of each flexure beam  71 - 74 , respectively, to the perimeter body  62 . The flexure members  81 - 84  are compliant with displacements of each corresponding flexure beam  71 - 74  along the corresponding longitudinal axes  71 A- 74 A. In the embodiment illustrated, the flexure members  81 - 84  are identical and include integrally formed flexure straps  86  and  88 . The flexure straps  86 and  88  are located on opposite sides of each longitudinal axes  71 A- 74 A and joined to corresponding flexure beam  71 - 74  and to the perimeter body  62 .  
      A sensing device measures displacement or deformation of portions of the sensor body  42 . In the body illustrated, a plurality of strain sensors  90  are mounted on the flexure beams  64  to sense strain therein. Although the plurality of sensors  90  can be located on the plurality of flexure beams  64  to provide an indicated of shear stresses, in the embodiment illustrated, the strain sensors are mounted conventionally to provide an output signal indicative of bending stresses in the flexure beams  64 . In the embodiment illustrated, eight strain sensors are provided on the sensor body  42  of each transducer  40  wherein two conventional Wheatstone bridges are formed. A first Wheatstone bridge or sensing circuit is conventionally formed from the strain sensors provided on flexure beam  71  and  73 , while a second Wheatstone bridge or second sensing circuit is formed from the strain sensors provided on flexure beams  72  and  74 . The plurality of sensors  90  can comprise resistive strain gauges. However, other forms of sensing devices such as optically based sensors or capacitivity based sensors can also be used to measure deformation or displacement of the flexure beams  64 , or other portions of the sensor body  42  such as each of straps  86  and  88  if desired.  
      Output signals from the sensing devices are indicative of force components transmitted between the central hub  60  and the perimeter body  62  in two degrees of freedom. For purposes of explanation, a coordinate system  97  can be defined wherein an X-axis  97 A is aligned with the longitudinal axes  71 A and  73 A; a Z-axis  97 B is aligned with the vertical axes  72 A and  74 A and a Y-axis  97 C is aligned with the axis  85 .  
      In the embodiment illustrated, each of the transducer assemblies  16  measures two forces. Specifically, a force along the X-axis is measured as bending stresses created in the flexure beams  72  and  74  since the flexure members  81  and  83  on the ends of the flexure beams  71  and  73  are compliant in this direction. Similarly, a force along the Z axis is measured as bending stresses in the flexure beams  71  and  73  since the flexure members  82  and  84  on the ends of the flexure beams  72  and  74  are compliant in this direction.  
      The transducer  40  is also compliant along the axis  85 , because of flexures provided on the clevis assembly  44 . In the. embodiment illustrated, the clevis assembly  44  is formed of substantially identical clevis halves  46  and  48 . In the illustrated embodiment, the sensor  42  is the “inner member” of the transducer body. Other embodiments are contemplated. For example, a single clevis half by itself could also be used. Still further, a single clevis half as an inner member connected to two sensors, which is described later with respect to  FIG. 8  could also be used.  
      In the embodiment illustrated, each clevis half  46  and  48  includes a central hub  102  through which, in the embodiment illustrated, apertures  46 A and  48 A are provided, and a rigid outer body  104 . A flexure mechanism couples the rigid central hub  102  with the outer body  104 . In the embodiment illustrated, a plurality of flexure straps  106  are provided with a first pair of flexure straps  111  and  112  extending from the central hub  102  to a first portion  104 A of the outer body  104  and a second pair of flexure straps  113  and  114  extending from the central hub  102  to a second portion  104 B of body  104 . However, it should be noted that other forms of flexure members or mechanism can be used between the rigid hub  102  and the outer body  104  to allow compliance along axis  85  if desired. Such forms can include other integral flexure mechanisms such as a diaphragm(s), or multi-component assemblies having flexible couplings such as slides or pivot connections.  
      Referring  FIGS. 1-3 , the sensor body  42  of each of the transducer assemblies  40  is joined to the frame support  12 , while each of the clevis halves  46  and  48  of each transducer assembly  40  is joined to a frame support  14 . In the embodiment illustrated, mounting plates  120  are used to couple the sensor bodies  42  to the frame support  12 , while mounting plates  122  are used to join the clevis halves  46  and  48  to the frame support  14 . In this manner, the frame support  12  provides an inner perimeter frame, while the frame support  14  provides an outer perimeter frame. Use of the mounting plates  120  and  122  allows the frame supports  12  and  14  to be nested thereby reducing an overall height of the platform balance  10 .  
      Each of the frame supports  12  and  14  comprise continuous hollow box beams formed in a perimeter so as to provide corresponding stiff assemblies. The frame support  12  holds the sensor bodies  42  in position with respect to each other, while the frame support  14  holds the clevis assemblies  44  in position with respect to each other. Stiffening box frame members  124  can also be provided in the support frame  12  as illustrated.  
      As appreciated by those skilled in the art, outputs from each of the two-axis sensing circuits from each of the transducer assemblies  16  can be combined so as to sense or provide outputs indicative of forces and moments upon the platform balance in six degrees of freedom. It should be noted that the flexure mechanisms of the clevis assembly  44  causes the transducers  16  to operate in a manner similar to how the flexure members  81 - 84  provide compliance in the sensor body  42 .  
      A coordinate system for platform  10  is illustrated at  131  in  FIGS. 1 and 2 . Output signals from transducer assemblies  40 A and  40 C are used to measure forces along the X-axis, because transducer assemblies  40 B and  40 D are compliant in this direction. Likewise, output signals from transducer assemblies  40 B and  40 D are used to measure forces along the Y-axis, because transducer assemblies  40 A and  40 C are compliant in this direction. Outputs from all of the transducers  40 A- 40 D are used to measure forces along the Z-axis. Overturning moments about the X-axis are measured from the output signals from transducers  40 A and  40 C; while overturning moments about the Y-axis are measured from the output signals from transducers  40 B and  40 D; and while overturning moments about the Z-axis are measured from the output signals from transducers  40 A- 40 D. Processor  180  receives the output signals from the sensing circuits of the transducers  40  to calculate forces and/or moments as desired, typically with respect to the orthogonal coordinate system  131 .  
      As described above, the platform can comprise four two-axis transducer assemblies. This particular design can have advantages over an embodiment having four three-axis (or more) transducer assemblies. In addition to the rejection of thermal expansion of the frames  12  and  14  relative to each other during lab or tunnel temperature transients, the platform  10  does not have to reject a relatively large thrust load on each of the four transducer assemblies (the clevis flexures are all very soft in thrust (along axis  86 ) thus shedding load to the two orthogonal two-axis transducer assemblies when an x or y side load is applied). This allows the platform  10  to be more optimally tuned for the four sensing flexure straps in each two-axis sensor body  42  than if the assembly was trying to react and measure thrust at the four transducer assembly positions about the platform as in three or more than three axis transducer assemblies. The design allows cross axis dimensions and I/c of orthogonal flexure beams to be changed independently to optimize sensitivity. For example, two can be thicker than the other two and can be thickness variable as well. If the transducer assemblies were three axis transducers and this occurred, two of the beams in line with each other would be stiffer and give different outputs from the orthogonal pair and thus make the sensor behave strangely with off axis or combined loadings. Lack of need to measure and react to thrust also allows higher stress and strain designs since there is no second bending stress tensor which would add bending in an additional axis at beam root connections to inner central hubs. Again higher sensitivity, higher resolution and higher signal to noise ratio with greater span on scalability both absolute and measured components relative to each other.  
      In a further embodiment, over travel stop mechanisms are provided in each of the transducer assemblies  16  so as to prevent damage to the sensor bodies  42  or flexure mechanisms of the clevis assemblies  44 . Referring back to  FIGS. 4-6 , one or more pins  140  are provided so as to limit displacement of the sensor body  42  relative to the clevis assembly  44 . In the embodiment illustrated, apertures  46 B,  48 B,  42 B are provided in the clevis halves  46  and  48  and the sensor body  42 , respectively. The pin  140  is secured, for example, to the sensor body  42  such as by a press fit so that extending portions of the pin  140  extend into the apertures  46 B and  48 B of the clevis halves  46  and  48  and are nominally spaced apart from inner walls thereof. If displacement of the displaceable portions of the sensor bodies  42  exceeds that desired relative to the bodies of the clevis halves  46  and  48 , extending portions of the pin  140  will contact the inner wall of the apertures  46 B and/or  48 B provided in the clevis half  46  and/or  48  thereby coupling the perimeter body  62  of the sensor body  42  with the outer bodies  104  of the clevis halves  46  and  48  to prevent damage to the flexure straps or mechanism. Note that the perimeter body  62  can be appropriately spaced from the clevis half (halves)  46  and/or  48  to provide overtravel protection. In particular, the perimeter body  62  can engage the clevis halves  46  and/or  48 , if displacement along axis  85  exceeds a selected distance.  
      Although the sensor body  42  and clevis halves  46  and  48  can be formed from any suitable material, in one embodiment, the sensor body  42  is formed from steel, while the clevis halves are formed from aluminum. Each of the pins  140  can be formed from hardened steel and if necessary, hardened bushings can be provided in the apertures  46 B,  48 B of the clevis halves  46  and  48  to engage the remote portions of the pin  140 . It should be noted that the extending portions of the pin  140  can be provided with a curved or spherical surface  151 , as illustrated in  FIG. 7 , relative to a shank portion  153  so as ensure distributed contact of the pin  140  with the inner wall of the apertures  46 B,  48 B formed in the clevis halves  46  and  48 .  
      It should also be noted that depending on the intended application the sensor body  42  and clevis half or halves can be formed a single unitary body.  
       FIG. 8  shows an alternative embodiment of the transducer, i.e., transducer  40 A and corresponding body. Like parts are indicated with like reference numerals. In this embodiment, one of the clevis halves  46  of  FIGS. 4-6  becomes the inner member. Two sensor members  42  from  FIGS. 4-6  become the clevis halves. In this example and unlike the previous examples, the inner member is not instrumented. Rather, the sensor member structures of the previous embodiment are instrumented with sensors, but in this embodiment function as clevis halves. Suitable sensors such as strain gauges  90  are still connected to the members  42 . The illustrated example includes twice as many sensors  90  as in the embodiment of  FIGS. 4-6 . In order to provide usable outputs, the sensor signals can be combined in each transducer such as by combining or summing the signals in Wheatstone bridges as is known in the art. The configuration of  FIG. 8  is stiffer in the y-direction (as indicated in the coordinate system) than the embodiment of  FIGS. 4-6 . The embodiment of  FIGS. 4-6 , however, is stiffer in a moment about the x-axis than the embodiment of  FIG. 8 .  
      The platform balance  10  is particularly well suited for measuring force and/or moments upon a large specimen such as a vehicle in an environment such as a wind tunnel. In this or similar applications, the platform balance  10  can include flexures  170  isolating the frame support  12  and  14  from the test specimen and a ground support mechanism. In the embodiment illustrated, four flexures  170  are provided between each of the transducer assemblies  40 , being coupled to the plates  120 . Similarly, four flexures  172  are coupled to the mounting plates  122 . The flexure  170 ,  172  thereby isolate the frame supports  12  and  14 . The flexures  170 ,  172  are generally aligned with the sensor bodies  42  of each corresponding transducer assembly  40 .  
      A counter balance system or assembly is generally provided to support the nominal static mass of the test specimen, other components of the operating environment such as roadways, simulators and components of the platform balance itself. The counter balance system can take any one of numerous forms such as airbags, hydraulic or pneumatic devices, or cables with pulleys and counter weights. An important characteristic of the counter balance system is that it is very compliant so as not to interfere with the sensitivity or measurement of the forces by the transducers assemblies  40  in order to measure all of the forces and moments upon the test specimen. In the embodiment illustrated, the counter balance system is schematically illustrated by actuators  190 .  
      The platform balance  10  is particularly well suited for use in measuring forces upon a vehicle or other large test specimen in a wind tunnel. In such an application, rolling roadway belts  182  are supported by an intermediate frame  184  coupled to the flexure members  170 . The rolling roadway belts  182  support the vehicle tires. In some embodiments, a single roadway belt is used for all tires of the vehicle. The platform balance  10  and rolling roadway belt assemblies  182  are positioned in a pit and mounted to a turntable mechanism  186  so as to allow the test specimen, for example a vehicle, to be selectively turned with respect to the wind of the wind tunnel.  
      The present invention has now been described with reference to several embodiments. The foregoing detailed description and examples have been given for clarity of understanding only. Those skilled in the art will recognize that many changes can be made in the described embodiments without departing from the scope and spirit of the invention. Thus, the scope of the present invention should not be limited to the exact details and structures described herein, but rather by the appended claims and equivalents