Two-axis sensor body for a load transducer

In one aspect, a transducer body includes a support having clevis halves. The sensor body includes a generally rigid peripheral member disposed about a spaced-apart central hub joined to each of the clevis halves. At least three flexure components couple the peripheral member to the hub. The flexure components are spaced-apart from each other at generally equal angle intervals about the hub; the sensor body further including a flexure assembly for some flexure components joining the flexure component to at least one of the hub and the peripheral member, the flexure assembly being compliant for forces in a radial direction from the hub to the peripheral member. Each flexure assembly is configured such that forces transferred concentrate strain at a midpoint along the length of each corresponding flexure component.

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 but not limited to 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 arranged to receive a model of a vehicle, a vehicle, or other actual or modeled test specimen.

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 gauges, 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.

SUMMARY

An aspect of the invention provides a transducer body, comprising a support comprising a pair of clevis halves; and a sensor body coupled to each of the clevis halves, wherein the sensor body is disposed between the clevis halves and configured to deflect with forces along two orthogonal axes, wherein the sensor body includes a generally rigid peripheral member disposed about a spaced-apart central hub, the central hub being joined to each of the clevis halves with the peripheral member spaced apart from each clevis half, wherein at least three flexure components couple the peripheral member to the central hub, and wherein the flexure components are spaced-apart from each other at generally equal angle intervals about the central hub; the sensor body further including a flexure assembly for each of said at least some flexure components joining the flexure component to at least one of the central hub and the peripheral member, the flexure assembly being compliant for forces in a radial direction from the central hub through the flexure component and to the peripheral member, wherein each flexure assembly is configured such that forces transferred between central hub and the peripheral member concentrate strain at a midpoint along the length of each corresponding flexure component.

Another aspect of the invention provides a transducer body, comprising a support comprising a pair of clevis halves; a sensor body coupled to each of the clevis halves, wherein the sensor body is disposed between the clevis halves and includes a generally rigid peripheral member disposed about a spaced-apart central hub, the central hub being joined to each of the clevis halves with the peripheral member spaced apart from each clevis half, wherein at least three flexure components couple the peripheral member to the central hub, and wherein the flexure components are spaced-apart from each other at generally equal angle intervals about the central hub; and a biasing assembly connected between the support and the sensor body and configured to provide a bias force between the sensor body and the support.

Another aspect of the invention provides a transducer body, comprising a support comprising a pair of clevis halves; a sensor body coupled to each of the clevis halves, wherein the sensor body is disposed between the clevis halves and includes a generally rigid peripheral member disposed about a spaced-apart central hub, the central hub being joined to each of the clevis halves with the peripheral member spaced apart from each clevis half, wherein at least three flexure components couple the peripheral member to the central hub, and wherein the flexure components are spaced-apart from each other at generally equal angle intervals about the central hub; and a lockup assembly configured to selectively inhibit movement of the sensor body relative to the clevis halves.

Additional aspects of the invention may be combined with any of the above aspects and with each other. Such additional aspects include the following:

An aspect wherein each flexure assembly is configured such that forces transferred between central hub and the peripheral member cause a first force at the connection of the flexure component to the central hub to be equal and opposite to a second force at the connection of the flexure component to the peripheral member, wherein the first and second force are tangential to the radial direction of each corresponding flexure component.

An aspect wherein said at least some of the flexure components are configured to concentrate strain in shear.

An aspect wherein said at least some of the flexure components are configured to concentrate strain in bending.

An aspect wherein said at least some of the flexure components are configured with a pair of beams.

An aspect wherein the pair of beams of each flexure component of at least some of the flexure components is formed by an aperture.

An aspect wherein the biasing assembly comprises a bias connector configured to operate in tension to provide the bias force.

An aspect wherein the bias connecter comprises an elongated strap having a width of the strap greater than a thickness of the strap.

An aspect wherein the biasing assembly comprises a pair of straps provided on opposite portions of the transducer body that are symmetrical.

An aspect wherein the bias connector comprises a flexible member fixedly connected to one of the sensor body or the support.

An aspect wherein the flexible member is integrally formed from a single unitary body of one of the sensor body or the support.

An aspect wherein the biasing assembly comprises a pair of biasing connectors wherein a biasing connector is provided each of opposite portions of the transducer body that are symmetrical.

An aspect wherein the biasing assembly comprises flexible members, a flexible member being fixedly connected to one of the sensor body or the support.

An aspect wherein the flexible members are integrally formed from a single unitary body of one of the sensor body or the support.

An aspect wherein each flexible member comprises a cantilevered beam with one of the biasing connectors connected to one of the flexible members.

An aspect wherein the flexible member is provided on the support.

An aspect wherein a flexible member is provided on each clevis half and a bridging block connects the flexible members together, the bridging block being spaced apart from the sensor body.

An aspect wherein the flexible member is provided on the sensor body.

An aspect wherein the biasing assembly comprises a removable biasing actuator configured to be connected between the sensor body and the support.

An aspect wherein the lockup assembly is configured to inhibit movement of the peripheral member relative to the clevis halves.

An aspect wherein the lockup assembly inhibits movement of the peripheral member by frictional contact.

An aspect wherein the lockup assembly is configured to selectively move portions having engaging surfaces for frictional contact to contact opposed surfaces of the peripheral member, the engaging surfaces and the opposed surfaces being spaced apart from each other to allow forces to be transferred by the flexure components when the lockup assembly is not engaged.

An aspect wherein the lockup assembly comprises a first plate member jointed to a first clevis half and a second plate member joined to the second clevis half, wherein the engaging surfaces are disposed on the plate members.

An aspect wherein a portion of each plate member is space apart from the associated clevis half.

An aspect wherein when the engaging surface engage the opposed surfaces, the portion of each plate member frictionally engages the associated clevis half.

An aspect and further comprising an actuator configured to selectively bring the engaging surfaces in contact with the opposed surfaces and also bring the portions of each plate member into contact with each associated clevis half.

An aspect wherein major surfaces of the portions of the plate members engage major surfaces of the associated clevis halves.

An aspect wherein the engaging surfaces are on the plate members, and wherein each plate member comprises a hinges and a link portion between the hinges, the link portion connecting portions of the plate members having the engaging surfaces with portions of the plate members having the major surfaces.

An aspect wherein the actuator is operably mounted to the portions of the plate members having the engaging surfaces, and wherein the actuator includes a pull rod to selectively pull the pull rod so as to bring the engaging surfaces in contact with the opposed surfaces.

An aspect wherein the pull rod extends through a bore opening to one of the opposed surfaces, the pull rod being spaced apart from inner surfaces of the bore at least when the actuator is not pulling on the pull rod to bring the engaging surfaces in contact with the opposed surfaces.

A platform balance may be provided in another aspect with transducer bodies and aspects as shown and described.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

Referring toFIG. 1, a transducer assembly is illustrated at10. The transducer assembly10includes a sensor body12and a clevis assembly14. The clevis assembly14includes a first clevis half16and a second clevis half18. The clevis halves16and18are joined together at one end with a connecting member17. The sensor body12is disposed between the clevis halves16and18, where the sensor body12and clevis halves16and18are joined together with a suitable fastener assembly. In the embodiment illustrated, the sensor body12includes a plurality of apertures15(FIG. 2) through which bolts or threaded rods can extend therethrough so as to secure each of the clevis halves16and18(having similar aligned apertures) to opposite sides of the sensor body12. In another embodiment, a bolt or threaded rod can extend through aligned bores19,21, and23(FIG. 1) in each of the clevis halves16and18and sensor body12. A nut (not shown) can be provided on one end of the rod and a super nut can be threaded upon an opposite end. A plurality of set screws extends though apertures in the super nut to engage in end of one of the clevis halves16or18. This fastening technique is described in U.S. Pat. No. 7,788,984, which is incorporated herein by reference in its entirety.

It should be noted that although portions of the clevis16and18will engage or contact a center portion of the sensor body12, gaps are provided between each of the clevis halves16and18and the sensor body12so as to allow portions of the sensor body12to move relative to the clevis halves16and18. In the embodiment illustrated, projecting center portions15A and15B provided on each side of the sensor body12ensure contact of the clevis halves16,18only with the center portions15A and15B (FIGS. 3 and 4), thereby maintaining the gaps as described above.

Referring toFIGS. 2-6, the sensor body12is preferably integral, being formed of a signal unitary block of material. The sensor body12includes a rigid central hub20upon which the surfaces15A and15B reside, and a rigid perimeter body22that is concentric with or disposed about central hub20. A plurality of flexure structures24join the central hub20to the perimeter body22. In the embodiment illustrated, the plurality of flexure structures24comprise four components31,32,33and34. Each of the components31-34extend radially from the central hub20along corresponding longitudinal axis31A,32A,33A and34A. Preferably, axis31A is aligned with axis33A, while axis32A is aligned with axis34A. In addition, axes31A and33A are perpendicular to axes32A and34A. Although illustrated wherein the plurality of flexure components equals four, it should be understood that any number of components three or more can be used to join the central hub20to perimeter body22. Preferably, the flexure components31-34are spaced at equal angular intervals about a central axis indicated at35.

Referring to flexure component31by way of example, an intermediate member41is integral with, being formed from the unitary block of material, or otherwise connected to flexure component31at an end opposite central hub20. Intermediate member41is preferably symmetric with respect to flexure component31or longitudinal axis31A having side portions41A and41B on opposite sides of flexure component31or longitudinal axis31A. Each side portion41A,41B is connected to perimeter body22through a flexure assembly51A,51B, respectively. Referring to flexure assembly51A by way of example, each of the flexure assemblies51A and51B, in the embodiment illustrated, include a rigid connecting member55. The connecting member53is connected or joined to one of the side portions41A,41B through a thin flexible web55. At an end opposite the intermediate member41, the connecting member55is connected to perimeter body22through a thin web57. It should be noted that the webs55and57are relatively wide being, for example, similar to the width or thickness of the perimeter body22; however, each of the webs55and57are thin in a direction normal to the width of the perimeter body22. The orientation of each of the webs55and57connecting the intermediate member41to the perimeter body22are oriented perpendicular to the flexure component associated with each intermediate number41. In other words, each of the connecting webs55and57are relatively wide in a direction parallel to the central axis35, but thin in a cross-section perpendicular to axis35. In contrast, each of the flexure components31-34are thin in a direction parallel to the central axis35and relatively wide in a cross-section perpendicular to the axis35. Given this construction, the connecting webs55and57are compliant for forces along the longitudinal axis of the flexure component to which it is connected, but stiff for an axis orthogonal to the axis of the flexure component to which it is associated with, and the axis orthogonal to the foregoing axes (or the axis parallel to the central axis35).

In the exemplary embodiment comprising four orthogonal flexure components31-34, the flexure components31-34operate in pairs for forces along an axis that is orthogonal to the longitudinal axes of each pair of flexure components (31,33and32,34) and orthogonal to the central axis35. In particular, flexure components31and33transfer forces between the central body20and the perimeter body22for forces along an axis61(wherein connecting webs55and57associated flexure components32and34are compliant in this direction), while flexure components32and34transfer forces between the central body20and the perimeter body22for forces along an axis63(wherein connecting webs55and57associated flexure components31and33are compliant in this direction).

It should be noted that the flexure assemblies51A and51B (herein by example connecting member53and connecting webs55,57) associated with each flexure component31-34(on opposite sides of the flexure component) are disposed so as to coincide at least approximately with a midpoint along the length of the corresponding flexure component. Referring to the enlarged view ofFIG. 5a midpoint of the longitudinal length of the flexure component31is indicated at66. The flexure assemblies51A,51B on opposite sides of the flexure component31(herein comprising connecting webs55and57), are orthogonal to the associated flexure component in a planar sense, but are configured or disposed so as to be approximately inline with the midpoint66as represented by dashed line68, or substantially proximate to the midpoint66. In other words the web(s) of the flexure assemblies51A,51B on each side of the flexure component they are associated with can be defined by corresponding planes, the planes of which are orthogonal to a plane representing the flexure component. Orientation of the connecting web(s) of the flexure assemblies51A,51B relative to the associated flexure component at the midpoint66causes forces to be transferred through the center of the length of the flexure component which allows the component to be very stiff with most deflection due to strain deflection and not bending. Since the flexure component is very stiff it has a good frequency response with excellent resolution. Since each of the flexure components31-34and associated flexure assemblies51A,51B are connected in the manner described above about center axis35, the sensor body12includes flexure elements (flexure components31-34) that can be used to sense forces with respect to two orthogonal axes61,63that can carry high loads with high resolution.

In one embodiment, each flexure assembly is configured such that forces transferred between central hub20and the peripheral member22cause a first force at the connection of the flexure component to the central hub20to be equal and opposite to a second force at the connection of the flexure component to the peripheral member22, wherein the first and second force are tangential to the radial direction of each corresponding flexure component.

It should be noted one aspect of the invention is use of the flexure assemblies being configured such that on each side of the flexure component they are connected to provide compliance in a direction of the longitudinal length of the flexure component from the hub to the outer perimeter. The flexible elements of the flexure assemblies are defined by aligned corresponding planes, the planes of which are orthogonal to the direction of compliance and coincide at least approximately with a midpoint along the length of the corresponding flexure component. Although various embodiments of flexure components such as components31-34have and will be described, these specific structures should not be considered the only components that can be used, but rather other flexure components can be used.

In one embodiment each of the flexure components31-34includes sensor elements to measure shear deflection or strain therein. The sensing elements can take any number of forms known to those skilled in the art, including electrically and optically based sensor elements to name just a few. In the embodiment illustrated, strain gauges are connected in a Wheatstone bridge with strain gauge elements placed on both sides of the flexure component on the principle stress axis. Referring to the enlarged view ofFIG. 5and the circuit diagram ofFIG. 7, the Wheatstone bridge70includes sensor elements71and72, on one side of the flexure component, while on a side opposite of the flexure component that is shown inFIG. 5, sensor elements and73and74(shown with dashed lines) are affixed to the flexure component.

It should be noted in the embodiment illustrated, each of the flexure components31-34are relatively thin in a direction parallel to central axis35. However, it should be noted, that the component is not thin in this direction in order to necessarily provide compliance but rather, the thickness of the flexure components are minimized in order to obtain a high output signal (maximize deflection) and a higher signal to noise ratio. In yet an alternative embodiment illustrated inFIG. 8, the flexure component31includes a sensing portion80upon where the sensor elements71and72are disposed (sensor elements73and74being on the opposite side of sensing portion80) and portions82A and82B that are on opposite sides of sensor portion80and are of greater thickness in order to provide greater stiffness in the direction parallel to the central axis35, while still maintaining required sensitivity in the axis of measurement.

Another sensor body is indicated at102atFIGS. 9-13, which can be used in place of the sensor body12, described above, in one exemplary embodiment. The sensor body102has elements similar in function to that described above with respect to sensor body12and has such similar components are identified with the same reference numbers. As illustrated, the sensor body102includes flexure components31and33, intermediate members41, connecting members53and connecting webs55-57. The flexure components31and33measure forces between the central body20and the perimeter body22for forces in a direction parallel to axis61. Sensor body102however includes flexure structures102and104to transfer forces between the central body20and the perimeter body22along axis63. The flexure structures103and104are designed to be substantially stiffer then the flexure components31and33so as to transfer substantially larger forces between the central body20and the perimeter body22.

Referring to flexure structure103by way of example, each of the flexure structures103and104include two flexure components112A and112B extending from the central body20to an intermediate member111. As illustrated, the flexure components112A,112B each have a longitudinal axis indicated at113A and113B wherein an acute angle116is formed between the axes113A,113B. In the embodiment illustrated, the flexure components112A,112B are oriented so as to converge in a direction toward the intermediate member111; however, in an alternative embodiment, if desired, an acute angle can be formed between the flexure components with convergence toward the central body20rather than the intermediate member111.

The intermediate member111is connected to the perimeter body22with flexure assemblies115A and115B (herein by example each comprising a connecting web117) on opposite sides of the intermediate member111. The flexure assemblies115A and115B are substantially stiff for forces along axis63, but significantly more compliant for forces along axis61such that these forces are transferred between the central body20and the perimeter body22through the flexure components31and33.

In one embodiment each of the pairs of the flexure components112A,112B for flexure structures103and104includes sensor elements to measure component deflection or strain therein. The sensor elements can take any number of forms known to those skilled in the art, including electrically and optically based sensor elements to name just a few. In the embodiment illustrated, strain gauges are connected in a Wheatstone bridge with strain gauge elements placed on opposite sides of each flexure components112A,112B. Referring toFIG. 9and the circuit diagram ofFIG. 13, a Wheatstone bridge130includes sensor elements131and132on opposite sides of flexure component112A, while sensor elements133and134are on opposite sides of flexure component112B.FIG. 12illustrates location of the sensor elements131and133on the sides of each of the flexure components112A and112B (i.e. parallel to the sides of the sensor body102, rather than between the sides of the sensor body102).

Another sensor body is indicated at202atFIGS. 14-15, which can be used in place of the sensor body12, described above, in one exemplary embodiment. The sensor body202has elements similar in function to that described above with respect to sensor body12and sensor body102and as such similar elements are identified with the same reference numbers. As illustrated, the sensor body202includes flexure components31and33, intermediate members41, connecting members53and connecting webs55-57. The flexure components31and33measure forces between the central body20and the perimeter body22for forces in a direction parallel to axis61. Sensor body202however includes flexure structures203and204to transfer forces between the central body20and the perimeter body22along axis63. The flexure structures203and204are designed to be substantially stiffer then the flexure components31and33so as to transfer substantially larger forces between the central body20and the perimeter body22.

Each of the flexure structures203and204include a flexure component212that is rectangular (preferably square) in cross-section along the length thereof, but at least two sides, preferably opposite to each other, are tapered along the length of the flexure component212such that one end portion of the flexure component212is smaller in cross-section than the other end portion, herein end portion212A connected to intermediate member111is smaller in cross-section (before connection to intermediate member111). In the illustrated embodiment all sides are tapered along the length of the flexure component212, i.e. being frusto-pyramidal in a center section. This construction allows the strain field in the center of the flexure component212to be approximately 80% (although this value is adjustable based on the shape of the flexure component212) of the strain in the connecting fillets at the ends of the flexure component212. Each of the sides of the flexure component212can include a sensor element such as those described above connected in a conventional Wheatstone bridge (not shown). Strain gauges231and232are illustrated by way of example.

Yet another sensor body is indicated at242atFIGS. 16-21, which can be used in place of the sensor body12, described above, in one exemplary embodiment. The sensor body242has elements similar in broad function to that described above with respect to sensor body12and as such similar elements are identified with the same reference numbers. As illustrated, the sensor body242includes flexure components251and253, intermediate members41, connecting members53and connecting webs55-57. The flexure components251and253measure forces between the central body20and the perimeter body22for forces in a direction parallel to axis61. In this embodiment, flexure components252and254, intermediate members41, connecting members53and connecting webs55-57measure forces between the central body20and the perimeter body22for forces in a direction parallel to axis63and are also substantially the same as the flexure structures for measuring forces in a direction parallel to axis61. However, this is not a requirement as demonstrated by the previous embodiments. Hence, any of the other flexure structures can be used, typically in pairs, but otherwise without limitation, of any of the previous embodiments for either measuring forces in a direction parallel to axis61or to axis63.

In the embodiment ofFIGS. 16-21, the flexure components251-254are very similar to flexure components31-34; however, flexure components251-254include corresponding apertures251A,252A,253A and254A. The strain gauges on the flexure components251-254are configured to measure strain in bending (as parallel double cantilever bending beams255A and255B illustrated inFIG. 20) rather than to measure strain in shear as flexure components31-34operate. Each of the flexure components251-254includes sensor elements to measure bending deflection or strain therein. The sensing elements can take any number of forms known to those skilled in the art, including electrically and optically based sensor elements to name just a few. For instance, resistive strain gauges connected in a suitable Wheatstone bridge can be secured to each of the beams255A and255B of each flexure component251-254. In one embodiment, the strain gauges are secured to the inwardly facing surface254B of each beam255A,255B formed by each aperture251A-254A, although the strain gauges could also be secured to the outwardly facing surfaces254C, which face in opposite directions. Like the flexure components31-34, the sensing gauges for sensing deflection of each of the beams255A and255B are located approximately at the midpoint of each beam255A,255B of each flexure component251-254and where the flexible elements (connecting members53and connecting webs55-57) of the flexure assemblies are defined by aligned corresponding planes, the planes of which are orthogonal to the direction of compliance and coincide at least approximately with a midpoint along the length of each beam255A,255B of the corresponding flexure component251-254, or stated another way bisect each of the apertures251A-254A. The structure of the flexure components251-254provides high stiffness with very good resolution and low cross-talk. Although apertures251-254are illustrated as round holes, it should be understood that the apertures could be of any suitable shape, such as but not limited to square apertures with rounded corners, or the like, without departing from the scope of the disclosure.

An exemplary embodiment of any of the foregoing transducer bodies with suitable sensing elements to form a transducer assembly can be incorporated in a platform balance300an example of which is illustrated inFIGS. 22-23. In the embodiment illustrated, the platform balance300can include a first frame support302and a second frame support304. A plurality of transducer assemblies340A-D, herein four although any number three or more can be used, couple the first frame support302to the second frame support304. The platform balance300can be used to measure forces and moments applied to a test specimen of nominally large weight or mass such as a vehicle, plane, etc. or models thereof. The frame supports302and304are nominally unstressed reaction frames, wherein each of the transducers comprises a two-axis force transducer as described above. Various levels of flexure isolation can be provided in the platform balance300to provide increased sensitivity, while nominally supporting large masses.

The platform balance300is 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 balance300can include flexures315isolating the frame support302and304from the test specimen and a ground support mechanism. In the embodiment illustrated, four flexures315are provided between each of the transducer assemblies, being coupled to the plates320. Similarly, four flexures324are coupled to the mounting plates322. The flexures315,324thereby isolate the frame supports302and304. The flexures315,324are generally aligned with the sensor bodies of each corresponding transducer assembly.

The platform balance300is 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 belts332are supported by an intermediate frame334coupled to the flexure members315. The rolling roadway belts332support the vehicle tires. In some embodiments, a single roadway belt is used for all tires of the vehicle. The platform balance300and rolling roadway belt assemblies332are positioned in a pit and mounted to a turntable mechanism336so as to allow the test specimen, for example a vehicle, to be selectively turned with respect to the wind of the wind tunnel.

Each of the frame supports302and304comprise continuous hollow box components formed in a perimeter so as to provide corresponding stiff assemblies. The frame support302holds the sensor bodies in position with respect to each other, while the frame support304holds the clevis assemblies in position with respect to each other. Stiffening box frame members333can also be provided in the support frame 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 can be combined so as to sense or provide outputs indicative of forces and moments upon the platform balance in six degrees of freedom. A coordinate system for platform300is illustrated at331. Output signals from transducer assemblies340A and340C are used to measure forces along the X-axis, because transducer assemblies340B and340D are compliant in this direction. Likewise, output signals from transducer assemblies340B and340D are used to measure forces along the Y-axis, because transducer assemblies340A and340C are compliant in this direction. Outputs from all of the transducers340A-340D are used to measure forces along the Z-axis. The flexure components251-254are relatively stiff or rigid for lateral loads, that being in a direction parallel to axis62. Overturning moments about the X-axis are measured from the output signals from transducers340A and340C; while overturning moments about the Y-axis are measured from the output signals from transducers340B and340D; and while overturning moments about the Z-axis are measured from the output signals from transducers340A-340D. Processor380receives the output signals from the sensing circuits of the transducers to calculate forces and/or moments as desired, typically with respect to the orthogonal coordinate system331.

If desired a counter balance system or assembly can be 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 transducer assemblies 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 actuators330.

However, in a further aspect of the present invention, the counter balance system can be removed as explained below, which can be a very large cost savings. Referring back toFIGS. 16 and 18, the sensor body242includes a biasing structure402disposed on the sensor body242so as to develop a biasing offset force in a selected direction, herein by way of example along the axis63in the Z-direction. The biasing structure402comprises cantilevered beams404A and404B. In the embodiment illustrated, remote ends406A and406B extend in opposite directions where each of the beams404A and404B are mounted to the sensor body242by a base support408. It should be noted that use of a single base support408is not necessary in that the cantilevered beams404A and404B can each have a separate base support secured to sensor body242; however use of a single base support408is of a simpler construction. Likewise, although illustrated with the beams404A and404B extending in opposite directions alternative embodiments may have the beams extend toward each other. Finally, the biasing structure402need not be a cantilevered beam, but can be any structure that is configured to provide a biasing force for the purpose described below.

In the embodiment illustrated the biasing structure402can be formed integral with the sensor body242from a single unitary body; however, this should not be considered limiting in that individual components can be joined together and/or joined to the sensor body242to realize the same structure.

Referring also toFIGS. 24 and 25, biasing retaining elements418connect the biasing structure402(located between the clevis halves16and18) to the clevis halves16and18. In the embodiment illustrated, the biasing retaining elements418operate in tension and herein comprise elongated connectors420each joined at a first end420A to one of the remote ends406A or406B with a suitable fastener herein bolts422. A second end420B of the elongated connectors420is joined to both of the clevis halves16and18, herein by a bridging block424separately connected to each of the clevis halves16and18with a suitable fastener herein bolts426.

Biasing retaining elements418in one embodiment comprise straps or flexible members. As shown, straps418, under tension, are coupled at one end420A to a cantilevered beam at its remote end, and are coupled at the other end420B to bridging block424coupled to clevis halves16and18. Together, the biasing elements418, bridging block424, and fasteners such as422and426comprise a biasing assembly connected between the support (clevis halves16and18) and the sensor body12to provide a bias force between the sensor body12and the clevis halves. As shown, a width423of the straps418is greater than a thickness425of the straps418. A biasing assembly in one embodiment comprises a pair of straps provided on opposite portions of the transducer body that are symmetric in configuration, to allow for compliance in a direction orthogonal to the offset. For example only and not by way of limitation, the straps418may have a square cross-section, that is, an equal width423and thickness425, or cylindrical, with a constant diameter in every cross-section direction, or other symmetric configurations such as will be evident to those of skill in the art.

A biasing actuator432(illustrated schematically with dashed lines) preloads the biasing structure402and in particular bends the cantilevered beams404A and404B by pulling on the bridging block424upwardly with the biasing actuator432operably connected to standoffs434. Any form of actuator can be used such as but not limited to a hydraulic, electric, etc. In one embodiment the actuator432comprises a screw or bolt mechanically connecting the standoffs434with the bridging block424.

A biasing force can be provided as follows. With a loose connection of the bridging block424to the clevis halves16and18, each biasing actuator432on each side of the sensor body242is operated to obtain the desired preloading on the biasing structure402as a whole at which point the bridging blocks424are then securely fixed to the clevis halves16and18to retain the desired bias force. In one embodiment, the bias force from each cantilever404A and404B is iteratively increased until the desired bias force is obtained. The contribution of the bias force from each cantilever404A and404B should be the same so as to not induce a moment in the sensor body242, but rather provide a purely linear bias force in a direction parallel to axis63in the illustrated embodiment.

It should be noted that the biasing structure need not be provided on the sensor body242, or only on the sensor body242.FIGS. 26-28illustrate a transducer assembly500having many of the same components of the previous transducer assembly, which have been identified with the same reference numbers. In this embodiment though, additional biasing structures502have been formed on each of the clevis halves16and18. The biasing structure502is similar to biasing structure402discussed above; and thus, much if not all the discussion applicable to biasing structures402is applicable to biasing structures502. For example, in one embodiment, cantilevered beams504A and504B and a single base support508are integrally formed from a single unitary body; however, this is but one embodiment, where other structures as described above with respect to biasing structure402can also be used.

Referring toFIG. 28, biasing retaining elements518connect the biasing structures502to the to the sensor body242. In the embodiment illustrated, each of the biasing retaining elements518operate in tension and herein comprise elongated connectors520with a first end having a bridging block524at a first end520A connecting the remote ends506A or506B together with a suitable fastener herein bolts522. A second end520B of each of the elongated connectors520is joined to the sensor body242, with a suitable fastener herein bolts526. Biasing retaining elements518in one embodiment have the properties and characteristics described above with respect to biasing elements418.

In this embodiment, sensor body242also includes biasing structure402having similar components identified with the same reference numbers. A biasing actuator not shown but connectable in a manner similar to that described above and is in effect removably connected to each of the beams404A,404B so as to pull the beams404A,404B upwardly inFIG. 28at which point the retainer418is securely fixed to retain the bias force such as by securely fixing the bridging block424to the sensor body242. In a similar manner, a biasing actuator not shown but connectable in a manner similar to that described above and is in effect removably connected to each of the beams504A of each clevis16,18so as to pull the beams504A or504B in pairs on each side of the transducer500using corresponding bridging blocks524and suitable standoffs as needed. When the desired bias force is obtained in each pair of beams504A and504B, the associated end520B of the bias retainer518can be securely fixed to the sensor body242. As in the previous discussion of bias structure402, any form of actuator can be used such as but not limited to screws or bolts, hydraulic, electric, etc.

An overtravel stop can be provided to limit the bias force created by the pairs of beams504A and504B on the clevis halves16and18. Referring toFIGS. 29 and 30, a bolt550is secured to support408and extends through an aperture552provided in element17connecting the clevis halves16,18together. The head of the bolt550is of size to be larger than the aperture552. The bolt552is secured to the support408with the head of the bolt spaced apart from a surface of the connecting element a selected distance corresponding to a limit of bias force to be generated by beams504A and504B. Since loading of beams504A and504B causes the sensor body242to move upwardly inFIG. 29, contact of the head of the bolt552limits the bias force that can be generated.

Another aspect of the present invention is a lock up assembly600that selectively secures the position of the sensor body12relative to the clevis halves16and18. Referring toFIGS. 31 and 32, the lock up assembly600includes friction plates602attached to each of the clevis halves. Each friction plate602is attached to its corresponding clevis half with a fastener604such as a plurality of fasteners in the center of the plate. However, it should be noted that only the center of the plate is in permanent contact with the corresponding clevis halves in that the extending ends606of the friction plates602are spaced apart from the outer surface of the corresponding clevis half. A spacer608is securely fixed to the sensor body and is disposed between ends606of the friction plates602on each side of the transducer. The length of the spacer608is slightly shorter than the distance between the inner surfaces of the friction plates such that a gap610is present between one or both of the end surfaces of the spacer608and corresponding inwardly facing surfaces612of the friction plates602. An actuator614is operably coupled to the friction plates602. The actuator614includes a pull rod616that extends through a bore618in the spacer. The bore618is of size to maintain a gap between the pull rod616and the spacer618for movements of the sensor body12or the clevis halves16and18relative to the sensor body12. When it is desired to inhibit movement of the sensor body12relative to the clevis halves16and18, the actuator614is operated so as to retract the pull rod616which pulls the ends606of the friction plates602together thereby eliminating the gap(s)610between the end surface(s) of the spacer608and the inwardly facing surface(s)612of the friction plates602as well as eliminating the gap(s)620between the inwardly facing surface(s)612of the friction plates602and the outwardly facing surface622of each corresponding clevis half. As such, when the actuator614is operated, a solid connection is formed between the spacer608and the friction plates602wherein the friction plates602frictionally engage the outer surfaces622of each corresponding clevis half.

The actuator614can be of any suitable form such as but not limited to an electric, hydraulic, or pneumatic actuator.

In the embodiment illustrated, each of the friction plates602includes areas of reduced thickness that form flexible hinges624. The flexible hinges624ensure that the ends606of the friction plates602will maximize contact of the end surfaces of the friction plates602with the clevis halves16and18rather than being slightly at an angle if the flexible hinges624were not present. In other words, the portion of the friction plates602that secure the friction plates602to the clevis halves by the fasteners indicated at604is maintained in a planar fashion to the corresponding clevis halves. Likewise, when the actuator614is operated, each of the end portions606of the friction plates602will contact the corresponding clevis half in a planar fashion. Any slight difference in width between the center sections of the friction plates602and the end portions of the friction plates602is accommodated by the middle sections between each of the flexible hinges624.

The embodiments pre-loading the transducer body with respect to the clevis plates, as shown and described above, allow for accurate full scale measurement even if the tare weight placed on the platform300is many times a full scale measure load weight. For example, a 20,000 pound upper frame is supportable with four transducer bodies while still allowing accurate measurement of loads in a full scale measure load of +/−2,000 pounds vertical, without frequency degradation of a dead-weight type tare system. Such embodiments are amenable to use with other load cells where tare mitigation is employed, without departing from the scope of the disclosure.

In such a pre-loading, thermal expansion differences can lead to thermal structural temperature equilibration between components of the transducer body and any sensing elements therein. Thermal expansion differences between, for example, parallel springs (e.g., the cantilevered beams) in series with straps and those in parallel with gauged beam assemblies, and the resulting disparate temperatures between elements, may result in thermal drift for a duration of a test.

FIG. 33shows a fluid enclosure700for a transducer assembly such as the various transducer assembly embodiments described herein. In one embodiment, fluid enclosure700is an oil enclosure or oil bath, such as a fluid recirculating bath assembly. Elements of the transducer, especially straps, cantilevered beams, gauged elements, sensor body, clevis plates, and bottom plates, are immersed in the fluid of the fluid enclosure700, and are held in one embodiment to a same temperature with a tolerance of about 0.1° F. for a testing cycle. In one embodiment, fluid enclosure700contains an oil702heated to a desired temperature at which the elements of the transducer are to be held. Oil702has high thermal mass and very good heat conduction and convection to and with the elements of the transducer assembly. This allows maintenance of thermal uniformity within a desired tolerance even when the transducer assembly is in an operating wind tunnel.

Oil enclosure700further comprises cross flow inlets704and outlets706, and a fluid circulating bath tank708mounted to a plate710. A gasket712seals tank708to plate710in one embodiment. Further gaskets714may be used to seal plate710to a bottom plate such as connecting member17of a transducer body (FIG. 1), for example by sealing each bolt between bottom flexures716and connecting member17, or by sealing with a larger gasket around a circumference of the bottom flexures716. Top flexures718may be coupled, for example with bolts, to the sensor body of the transducer assembly. Associated plumbing (not shown) provides oil702at the desired temperature for the components to the enclosure700via inlets704, recirculated to the plumbing and heater (not shown) via outlets706. A top cover720, shown inFIG. 34, may be used to provide protection against contaminants such as dirt or dust into the tank708. In one embodiment, oil is provided to fill tank708to fill line722, so that sensing elements that may be in the transducer body are completely submerged in the oil.