Abstract:
A load-sensing, multi-axis connector is disclosed which comprises a series of complimentary leaf springs, which are connected by load cells through spherical bearings. These leaf springs resist the axial displacement of their attached load cells, but are isolated from non-axial displacements and moments due to the spherical bearings. These leaf springs are attached to a first end connector and a second end connector, which are in turn attached to the two objects that are to be connected. Using the load-sensing, multi-axis connector, two objects can be connected, even where the two objects are undergoing high displacements, and the stresses caused by the relative displacement of the two objects can be monitored.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without payment of any royalties thereon or therefore. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     This invention relates to a connector having the capacity to sense loads caused by displacements in multiple axes. Specifically, this invention relates to a connector having the capacity to both connect objects undergoing large displacements, and to provide an accurate measure of the loads caused by these displacements. 
     2. Description Of The Related Art 
     In general, where two large objects are going to be flexibly connected, there is a need to understand the forces experienced at that connection point. This information is both useful in the design of those connections, as well as in monitoring the motions and displacements of the structure after installation. At the same time, there needs to be a way to actually join the objects, for that joint to be realistic in light of the conditions likely to be experienced by that joint, and for a method of measuring the loads experienced by the joint under high displacements. For testing purposes, such a connection needs model a flexible joint being subjected to the type of force normally experienced by objects undergoing high displacements. For instance, examples of connectors undergoing high displacements include connections between oil rigs and the ocean floor or between coupled barges and ships where the oil rig, barge, or ship is being subjected to a high sea state. In addition, such a connector could be useful in modeling flexible buildings foundations being subjected to a severe earthquake. 
     Outside of testing, there is often a need to continuously monitor the behavior of a flexible joint undergoing high displacements to assess the health of that joint, and to use this assessment to determine whether to take corrective action to maintain the health of the joined structures. For instance, such a flexible connector would be useful in providing early warning to an oil rig should the high sea state cause unsafe stress at the connection between the ocean floor and the oil rig structure, allowing for the oil rig operators to react properly. However, the prior art is devoid of a suitable invention that both connects objects undergoing large displacements and allows for measurement of the stresses these displacements cause that connector. 
     First, it is known in the art to connect two objects that are undergoing large dynamic loading. For example, U.S. Pat. No. 4,717,288 teaches the use of a building joint being that is resistant to severe earthquake displacements. This joint consists of interconnected cantilever beams that provide flexibility in multiple axes. However, the joint taught by U.S. Pat. No. 4,717,288 does not teach an interconnection of cantilever beams where specific sets of beams isolate the displacements along a specific axes, which makes the sensing of loads caused by this displacements impracticable. Similarly, in U.S. Pat. No. 5,509,238, the joint utilized a series of “C” shaped springs that allows the attached structures to move in multiple axes relative to one another. However, this arrangement also fails to isolate the displacements as is needed to sense the loading that the joint is experiencing. 
     In addition, it is known to sense the loading of connections between objects undergoing these large dynamic motions and displacements. However, prior solutions were restricted to either measuring small displacements, or to measuring inflexible connectors. For instance, U.S. Pat. No. 3,648,514 discloses a sensing joint that relies upon a series of hollow flexural elements, with each flexural element containing a stiff internal rod. This combination attaches two structures at discrete attachment points. By combining a flexural element and a rod, this combination gives a single mechanism for measuring loads caused by displacements in three orthogonal directions. However, due to the stiffness of the internal rod, this solution is not suited for situations where the connected members are undergoing large displacements. 
     Similarly, in Richard Lewis,  Mobile Test Design and Preparation,  Presentation at the ONR Mobile Offshore Base Technology Exchange Conference (Arlington, Va.)(Sep. 22, 1998), a connector is proposed that is both capable of measuring the loads experienced by connectors linking modules in the proposed Mobile Offshore Base, and is capable of acting as a connector between these modules. The proposed connector relies upon a series of load cells connecting these modules. These load cells are arranged in an orthogonal pattern, which allows each load cell to substantially resist the displacement along either a vertical, lateral, or longitudinal direction, depending on the orientation of that load cell. To isolate the axial loads taken by the load cells, this invention also uses ball joints to prevent the load cell from resisting non-axial displacements and moments. However, this load cell arrangement is fairly inflexible and would not be suitable for modeling connected members undergoing large displacements. Thus previous joints and connectors used to measure large dynamic displacements and motions do not allow for accurate modeling of flexibly connected members undergoing large-scale displacements. 
     Lastly, there are other solutions to multi-axis force connector measurement available. However, some, such as U.S. Pat. Nos. 5,490,427 and 5,063,788, while capable of measuring loads experienced by a connection in multiple axes, are unsuitable to also be used as a connector between objects experiencing large displacements. Still other solutions exists such as that suggested by U.S. Pat. No. 5,129,265, which utilizes flexible mechanisms to allow for large displacements. However, this invention does not allow for loads experienced by more than two axes and is not be suitable for use as a connector between two large objects that is experiencing displacements in more than two axis. Additionally, U.S. Pat. No. 4,981,552 discloses a highly sensitive multi-axis measuring devices using leaf springs that allow for high displacement motions. However, these leaf springs are used as multi-axis measuring devices, and would not be suitable for adaptation as a multi-axis connector. 
     As such, there exists no suitable connector that both connects two objects undergoing large displacements and is capable of measuring the loads caused by these large displacements. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is the object of this invention to provide a connector that allows for large displacements between flexibly connected objects. 
     It is a further object of this invention to provide a connector that can also measure the loads caused by these large displacements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an orthogonal view of the load-sensing, multi-axis connector according to the present invention. 
     FIG. 2 is an exploded orthogonal view of the load-sensing, multi-axis connector according to the present invention. 
     FIG. 3 is a side view showing an example of a load cell being attached to prongs through spherical bearings where that load cell is undergoing non-axial displacement according to the present invention. 
     FIG. 4 a  is a top view of the load-sensing, multi-axis connector undergoing single axis displacement along the Y axis showing the interaction of the longitudinal and lateral leaf springs in resisting these displacements according to the present invention. 
     FIG. 4 b  is a side view of the load-sensing, multi-axis connector undergoing single axis displacement along the Z axis showing the interaction of the longitudinal and vertical leaf springs in resisting these displacements according to the present invention. 
     FIG. 4 c  is a side view of the load-sensing, multi-axis connector undergoing single axis displacement along the X axis showing the interaction of the longitudinal and vertical leaf springs in resisting these displacements according to the present invention. 
     FIG. 4 d  is a side view of the load-sensing, multi-axis connector undergoing flexural motion about the Y axis showing the interaction of the longitudinal and vertical leaf springs in resisting these displacements according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 1 and 2, in a preferred embodiment, the load-sensing, multi-axis connector  100  comprises four load cells connecting a pair of end units: a vertical load cell  170 , a lateral load cell  190 , an upper longitudinal load cell  180 , and a lower longitudinal load cell  182 . These four load cells connect opposable pairs of leaf springs which substantially resist displacement along a particular axis as defined in FIG.  1 : a first vertical leaf spring  140  and a complimentary second vertical leaf spring  220  substantially resisting displacement along the z axis; a first lateral leaf spring  150  and a complimentary second lateral leaf spring  240  substantially resisting displacement along the y axis; and a first longitudinal leaf spring  160  and a complimentary second longitudinal leaf spring  260  resisting displacement along the X axis. 
     As shown in FIGS. 1 and 2, the vertical load cell  170  is connected near its ends to a first vertical leaf spring  140  and a second vertical leaf spring  220  by spherical bearings  200  (not shown). The lateral load cell  190  is connected near its ends to first lateral leaf spring  150  and second lateral leaf spring  240  by spherical bearings  200  (not shown). Next, the upper longitudinal load cell  180  and the lower longitudinal load cell  182  are connected near their ends to a first longitudinal leaf spring  160  and a second longitudinal leaf spring  260  by spherical bearings  200  (not shown). The first longitudinal leaf spring  160  has an upper prong  162  and a lower prong  164 . The second longitudinal leaf spring  260  also has an upper prong  270  and a lower prong  280 . The upper longitudinal load cell  180  connects the upper prongs  162  and  270 , while the lower longitudinal load cell  182  connects the lower prongs  164  and  280 . 
     As is more clearly shown in FIG. 2, the first vertical leaf spring  140  is mounted by fasteners  210  (not shown) to the bottom  112  (not shown) of first end unit  110 , with prong  142  extending toward the second end unit  120 . The second vertical leaf spring  220  is mounted by fasteners  210  (not shown) to the top  122  of second end unit  120 , with prong  230  extending toward said first end unit  110 . 
     Next, the first lateral leaf spring  150  is mounted by fasteners  210  (not shown) to a first side  114  of the first end unit  110 , with prong  152  extending toward said second end unit  120 . The second lateral leaf spring  240  is mounted by fasteners  210  (not shown) to a second side  124  (not shown) of the second end unit  120 , with prong  250  extending toward said first end unit  110 . 
     Lastly, the first longitudinal leaf spring  160  is mounted by fasteners  210  (not shown) to the first front  116  (not shown)of the first end unit  110 , with the prongs  162  and  164  facing inward towards the second side  124  of the second end unit  120 , but not extending past the first lateral leaf spring  150 . In addition, the lower prong  164  and the spring body  166  define a first gap through which the first vertical leaf spring  140  extends. 
     Similarly, the second longitudinal leaf spring  260  is mounted by fasteners  210  (not shown) to the second front  126  of the second end unit  120 , with the prongs  270  and  280  facing inward towards the second side  118  (not shown) of the first end unit  110 , but not extending past the second lateral leaf spring  240 . In addition, the upper prong  270  and the spring body  275  defines a second gap through which the second vertical leaf spring  220  extends. Pursuant to this arrangement, both the vertical load cell  170  and the lateral load cell  190  can be positioned between the first end unit  110  and the second end unit  120 . 
     It is understood that where more axes than that shown in FIGS. 1 and 2 are to be monitored or connected, additional load cells and opposable pairs of complimentary leaf springs will be employed. In addition, where required, prongs may be added to or removed from each opposable pair of complimentary leaf springs depending on the need. 
     An example of how spherical bearings  200  are mounted in prongs of an opposable pair of complimentary leaf springs according to a preferred embodiment is shown in FIG.  3 . In FIG. 3, the longitudinal load cell  180  is connected to prongs  162  and  270  by spherical bearings  200 . The spherical bearings  200  are mounted within prongs  162  and  270 , with the longitudinal load cell  180  secured to the spherical bearings  200  using a combination of threads  201 , coupling nuts  202 , and cap screws  204 . 
     FIG. 3 shows how such an arrangement reacts to non-axial displacements. As shown in FIG. 3, the spherical bearings  200  provide support along the common axis for the opposable leaf springs, which in FIG. 3 is the X axis. At the same time, this arrangement allows for minimal resistance to displacements perpendicular to the X axis, effectively isolating the longitudinal load cell  180  from displacements perpendicular to the X axis by allowing the load cell  180  to pivot in reaction to the non-axial load. 
     This mounting method is repeated for all connections between load cells and prongs as exists for the spherical bearings  200  mounted in prongs  142 ,  152 ,  162 ,  164 ,  230 ,  250 ,  270  and  280 . It is recognized that a spherical bearing  200  is only one type of axial motion isolation bearing. Other bearing types, ball joints, rubber flexible connectors, universal joints, or other similar connectors may be used so long as it substantially isolates the load passing through the axis of the load cell from other non-axial loads and bending moments. It is further understood, but not shown, that other mechanisms exists to attach load cells to spherical bearings  200 . 
     In the preferred embodiment, the load cells  170 ,  190 ,  180 ,  182  are commercial threaded rod load cells, which have their strain gages mounted internally. However, other load cells will work so long as the strain gages are attached to the connector, and that connector is capable of handling the desired loading. In addition, where no strain information is needed along a particular axis, simple bolts, rods, or other connectors would suffice for that connection. Thus it is recognized that where there is no need to monitor the loads in certain directions, load cells can be replaced with connectors, and additionally, where a pure connector embodiment is to be employed, no load cells will be used in the load-sensing, multi-axis connector  100 . 
     In combination, these various parts compliment one another, as shown in FIG.  1 . Essentially, the first combination of first end unit  110 , first vertical leaf spring  140 , first lateral leaf spring  150 , and first longitudinal leaf spring  160  faces the second combination of the second end unit  110 , second vertical leaf spring  140 , second lateral leaf spring  150 , and second longitudinal leaf spring  160 , only with the second end unit rotated 180° about the X axis as defined in FIG.  1 . 
     As shown in schematic FIGS. 4 a ,  4   b ,  4   c , and  4   d , when in use, the load-sensing, multi-axis connector  100  is able to react to relative displacements along the X, Y, and Z axes, flexural motion about the X, Y, Z axes, or any combination thereof. As shown in FIG. 4 a , the lateral leaf springs  150  and  240  resist displacements along the common lateral axis, the Y axis. As shown in FIG. 4 b , the vertical leaf springs  140  and  220  substantially resist displacements along the common vertical axis, the Z axis. As shown in FIG. 4 c , the longitudinal leaf springs  160  and  260  substantially resist all displacements along the common longitudinal axis, the X axis. In addition, FIG. 4 b  shows the longitudinal load cells  180  and  182 , which are symmetrically arranged on the longitudinal leaf springs  160  and  260 , jointly resist displacements along the longitudinal axis, the X axis. 
     Where the load-sensing, multi-axis connector  100  is undergoing flexural motion, a combination of the longitudinal load cells  180  and  182 , lateral load cell  190 , and/or the vertical load cell  170  will resist this moment. As an example, FIG. 4 d  shows the reaction of the load-sensing, multi-axis connector  100  where this moment is about the Y axis. As shown in FIG. 4 d , the longitudinal load cells  180  and  182  in combination with the vertical load cell  170  provide resistance to this moment. 
     It is recognized that where multiple connectors are added or removed to an opposing pair of leaf springs, that these connectors be similarly symmetrically mounted to their complimentary leaf springs so as to balance the resistance to motion along their common axis. 
     The responses shown in FIGS. 4 a ,  4   b ,  4   c , and  4   d  are those experienced in relatively low displacements, where the load cells  170 ,  190 ,  180  and  182  are remain orthogonal and provide highly decoupled response. By decouple, this means that there is little interdependence between load cells  170 ,  190 ,  180  and  182 . Thus vertical load cell  170  resists the vast majority of the vertical displacement along the Z axis, the lateral load cell  190  resists the vast majority of the lateral displacement along the Y axis, and the longitudinal load cells  180  and  182  jointly resist the vast majority of the longitudinal displacement along the X axis. However, as the displacement increases, this orthogonality decreases and load cells  170 ,  190 ,  180  and  182  begin to couple, meaning that they become interdependent with each load cell resisting non-negligible amounts of displacements from each direction. 
     It is important to understand that by substantially resisting displacements along a particular axis, it is understood that, especially in high displacements, that all of the and load cells  170 ,  190 ,  180  and  182  will provide some resistance to all displacement. However, the vertical load cell  170  will still resist the majority of the vertical displacement along the Z axis, lateral load cell  190  will still resist the majority of the lateral displacement along the Y axis, and longitudinal load cells  180  and  182  will jointly still resist the majority of the longitudinal displacement along the X axis. Thus the load cells  170 ,  190 ,  180  and  182  substantially resist the displacement along their common axis, but still provide non-negligible to minimal resistance to non-axial displacements. 
     In addition, in order to interpret the load data in high displacement situations, different procedures will need to be followed to account for the coupling effect. In low displacement situations, straight readings from the load cells provide an accurate measurement of the loads experienced by the connector since the load cells  170 ,  190 ,  180  and  182  are decoupled. However, in these high displacement situations, the load components along the X, Y, and Z axis cannot be directly derived since the load cells  170 ,  190 ,  180  and  182  are coupled, but they may still be derived by utilizing an iterative decomposition procedure to derive these constituent forces and moments. 
     In a preferred embodiment, the end units  110  and  120  are made from 6061-T6 aluminum. In addition, the vertical leaf springs  140  and  220 , the lateral leaf springs  150  and  240 , and the longitudinal leaf springs  160  and  260  being made of ¾ inch thick 7075-T6 aluminum. In the preferred embodiment, this resulted in the vertical leaf springs  140  and  220  having a spring constant of 1021 lbs/in, the lateral leaf springs  150  and  240  having a spring constant of 517 lbs/in, and the longitudinal leaf springs  160  and  260  having a spring constant of 1105 lbs/in. However, it is recognized that other metals, plastics, rubber, other compliant materials may be used instead of aluminum for some or all of these parts. In addition, these spring constants may be altered by changing material, material thickness, or for existing leaf springs, attaching chocks to a leaf springs to increase its thickness. 
     What has been described is only one of many possible variations on the same invention and is not intended in a limiting sense. The claimed invention can be practiced using other variations not specifically described above.