Abstract:
A controlled collapse joint includes a piezoelectric coupler that frictionally couples two bodies. The coupler includes concentric inner and outer members mounted for relative motion on cooperating bearing surfaces. The two surfaces engage each other with an interference fit that resists relative motion and frictionally couples the members. One of the surfaces is formed on a piezoelectric component that moves towards or away the other surface when a voltage difference is applied to the piezoelectric component. The coupling of the two members is adjustable by varying the applied voltage difference to increase or decrease the interference fit between the piezoelectric component and the other member.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a piezoelectric coupler for selectively coupling two bodies and a joint incorporating the coupler. 
     BACKGROUND OF THE INVENTION 
     Piezoelectric material has a crystalline structure that axially expands or contracts when a voltage difference is applied along the axis. Whether the material expands or contracts depends on the polarity of the voltage difference. For example, a positive voltage difference may cause the material to expand and a negative voltage difference may cause the material to contract. The greater the voltage difference, the greater the deformation. When the voltage difference is removed the structure returns to its initial undeformed state. 
     Devices that utilize the ability of piezoelectric materials to selectively expand or contract are known. Such devices may form couplers or joints that selectively couple or join two bodies. The device is attached to one body. The piezoelectric material expands and extends to engage the other body and couple the two bodies together. The material contracts to uncouple the two bodies. The expansion or contraction of the piezoelectric material can be finely controlled by varying the applied voltage difference. 
     In one known type of piezoelectric device a number of piezoelectric elements are stacked together to form an element stack having an initial stack height. The elements are each connected to electrodes that apply a voltage difference from a voltage source to opposite sides of the element. The voltage difference increases the height of each element and so the overall stack height increases. The stack height is controlled by varying the voltage difference. The stack extends to engage the end of the stack against the other member and apply a coupling force to the other member. 
     Piezoelectric stacks have a number of disadvantages that often render them unsuitable for joining or coupling two bodies. The large number of electrodes requires complicated and expensive wiring. The element stack must be supported in a housing, with the electrodes and wiring contained in the housing. Such an assembly is heavy and takes up a large amount of space. The stack force is applied over a small area and often a number of stacks must operate simultaneously to apply sufficient coupling force. 
     Thus, there is a need for an improved piezoelectric coupler for selectively coupling two bodies. The improved coupler should be compact and lightweight. The coupler should apply a coupling force over a large area, and should be simple to install and not require extensive wiring. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved coupler for coupling two bodies. The coupling force is applied over a relatively large area. The coupler is compact, lightweight and does not require extensive wiring. Structural elements and actuator elements of the coupler are incorporated in a self contained, integrated unit. 
     A coupler having features of the present invention includes an outer member mounted about an inner member for relative motion on mutually engaged concentric bearing surfaces. The two members are assembled together such that the surfaces engage each other with an interference fit that resists relative motion and frictionally couples the members. 
     One member includes a piezoelectric component having inner and outer walls. The surface of the one member is located on an inner or outer wall of the piezoelectric component. The surface on the piezoelectric component moves towards or away from the other surface when a voltage difference is applied across the inner and outer walls of the piezoelectric component and increases or decreases the interference fit. The frictional coupling of the two members is selectively controlled by varying the voltage difference applied across the walls of the piezoelectric component to increase or decrease the interference fit between the two surfaces. 
     The coupling force is generated over a relatively large surface area and efficiently couples the members. Only one piezoelectric component is needed to couple the two bodies and so the coupler is lightweight, compact and easily wired to a voltage source. 
     In possible embodiments of the present invention, only two electrodes are required to wire the piezoelectric component to a voltage source. One electrode is electrically connected to the component outer wall and the other electrode is electrically connected to the component inner wall. The need to wire a large number of piezoelectric bodies is eliminated and the weight of the coupling is reduced. 
     In particularly advantageous embodiments, the other member is formed from an electrically conductive material, such as steel. The conductive material forms part of the electrical connection between the wall of the piezoelectric component engaging the conductive member and a terminal of a voltage source. This allows an electrode to be easily attached to an exposed surface of the conductive member. 
     In preferred embodiments of the present invention the piezoelectric component forms a portion of the inner member. The outer member presses inwardly and compresses the piezoelectric component. Conventional piezoelectric materials are stronger in compression than in tension, and so it is mechanically advantageous to have the piezoelectric component inside the outer member. However, if the piezoelectric material is sufficiently strong in tension, the piezoelectric component could form the outer member. In such embodiments the interference fit could tension the piezoelectric component. 
     In an advantageous embodiment of the present invention, the coupler forms a portion of a controlled collapse joint that varies the coupling between two bodies in response to some triggering event. The outer member is connected to one body and the inner member is connected to the other body. The joint enables the controlled coupling of one body with respect to the other body in response to external events to advantageously regulate the coupling of the two bodies during the event. 
     The piezoelectric component is a piezoelectric cylinder that forms a portion of the inner member. The outer member is a steel cylinder mounted on the piezoelectric cylinder for rotation, translation or combined rotation and translation about or along the piezoelectric cylinder. The outer and inner walls of the piezoelectric cylinder are electrically connected to a variable voltage system that forms a portion of a control system that actively controls the coupling of the coupler. 
     The control system includes a computer that controls the output of the voltage source and a sensor that transmits data to the computer enabling the computer to resolve the triggering event. The computer varies the coupling of the two bodies in response to the triggering event. 
     Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying drawings illustrating the invention, of which there are four sheets of drawings and two embodiments. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a piezoelectric coupler made in accordance with the present invention; 
     FIG. 2 is a sectional view of the piezoelectric coupler taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a sectional view of the piezoelectric coupler taken along line  3 — 3  of FIG. 1; and 
     FIG. 4 illustrates a controlled collapse joint incorporating the piezoelectric coupler shown in FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1-3 illustrate a piezoelectric coupler  10  made in accordance with the present invention. The coupler  10  couples two bodies about an axis of rotation  12 . 
     The coupler  10  includes concentric outer and inner members  14  and  16  respectively that extend along the axis of rotation  12 . The outer member  14  includes a cylindrical body  18  that is assembled over a cylindrical body  20  of the inner member  16  for rotation about the axis  12 . The outer body  18  is electrically conductive and may be made of steel. The cylindrical bodies  18 ,  20  are assembled with an initial interference fit. The outer member  14  is nonrotably connected to a first body by an arm  22  located on one end of the joint  10  for conjoint rotation about the axis  12 . The inner member  16  is nonrotatably attached to a second body by a second arm  24  located on the other end of the joint  10 . 
     The inner cylindrical body  20  is a cylinder formed from piezoelectric material. The piezoelectric cylinder  20  has an inner cylinder wall  28  and an outer cylinder wall  30  separated by the radial thickness of the cylinder. In operation, the inner wall  28  is electrically connected to a positive terminal  32  of a voltage source by a first electrical connection  34 . The outer wall  30  is electrically connected to a ground terminal  36  of a voltage source by a second electrical connection  38 . 
     The piezoelectric cylinder  20  extends from one end of the outer cylindrical body  18  to a free end  40  adjacent the second arm  24 . The end  40  of the piezoelectric cylinder may include conventional structure (not shown) such as teeth or splines formed on the end of the piezoelectric cylinder  20  to connect the piezoelectric cylinder with the second arm. 
     The outer member  14  has the first arm  22  integrally formed with the cylindrical body  18 . The cylindrical body  18  has inner and outer cylindrical walls  44  and  46  respectively separated by the radial thickness of the body  18 . When unassembled, the diameter of the inner wall  44  is slightly less than the diameter of the outer wall  30  of the piezoelectric cylinder  20 . When assembled, the facing inner and outer walls  44 ,  30  engage each other to form an initial interference fit. The walls press against each other with a mutual contact pressure along a common cylindrical interface  48 . 
     The mutual contact pressure of the two cylindrical walls  44 ,  30  electrically connects the outer member  14  with the outer wall  30  of the piezoelectric cylinder  20  and forms a portion of the second electrical connection  38 . The electrical connection  38  includes an electrode  50  fastened to the outside of the outer member  14  for connection to the ground terminal  36 . 
     The first electrical connection  34  includes a thin cylindrical conductor or conductive film  52  bonded to the inner wall  28  of the piezoelectric cylinder  20  by an electrically conducting bonding agent. The bonding agent electrically connects the film  52  with the inner wall  28 . Second electrode  56  is attached to an exposed surface of the film  52  for connection to the positive terminal  32 . 
     Each of the mechanical connections may include electrical insulation that insulates the bodies attached to the members  14  and  16  from the electrodes  50 ,  56 . In this embodiment the body attached to the inner member  16  is insulated by insulation  58  at the end  40  of the piezoelectric cylinder  20 . 
     FIG. 4 illustrates a controlled collapse joint  60  incorporating the coupler  10 . The controlled collapse joint  60  acts to control the relative rotation of a movable body  62  about a stationary body  64 . The movable body  62  is nonrotatably attached to arm  22  of the outer member  14  for conjoint rotation about the axis  12 . The stationary body  64  is nonrotatably attached to arm  24  of the inner member  16 . 
     The joint  60  includes a control system  66  operatively connected to a variable voltage source  68  having terminals  32  and  36 . The terminals  32 ,  36  are wired to the electrodes  56  and  50  respectively. The control system  66  includes a computer  70  operatively connected to the voltage source  68 . The computer  70  selectively varies the coupling of coupler  10  in response to some triggering event. 
     In this embodiment, the control system  66  responds to an impact against the movable body  62  and includes a proximity sensor  72  operatively connected between the two members  14 ,  16  and the computer  70 . The sensor  72  transmits a data signal  74  to the computer  70  corresponding to the angular position of the outer member  14  with respect to the inner member  16 . The data signal  74  enables the computer  70  to detect relative motion of the movable body  62  from an impact and calculate the speed and position of the body during the impact. 
     Operation of the controlled collapse joint  60  will now be described. The coupling of the two bodies by the coupler  10  is varied by the control system by varying the interference fit between the members  14 ,  16 . The joint  60  enables controlled deceleration or braking of the movable body  62  and yet minimizes the force of the impact against the body. The computer  70  monitors the motion of the movable body  62  and brakes the body in a controlled manner to prevent the impact forces acting on the body from exceeding acceptable levels. 
     The control system  66  can respond to impacts in one of two operating modes. In the first mode, the coupling of the two bodies  62 ,  64  by coupler  10  is increased during the impact to increase resistance to motion. The increased coupling more quickly brakes the movable body  62  and can decelerate the body in a controlled manner. In the second mode, the coupling of the two bodies  62 ,  64  by coupler  10  is decreased during impact to reduce resistance to motion. The reduced coupling enables the movable body  62  to rotate more easily during impact and reduces the impact force acting on the body. 
     The coupling of the two bodies by the coupler  10  is initially set by the control system  66  to prevent rotation of the outer member  14  about the inner member  16  during normal operating conditions. The control system  66  directs the voltage source  68  to generate a predetermined voltage difference across terminals  32 ,  36 . The voltage difference is applied by the electrical connections  34  and  38  to the inner and outer cylinder walls of the piezoelectric cylinder  20 . The piezoelectric cylinder  20  attempts to radially expand in response to the applied voltage difference. Expansion of the piezoelectric cylinder  20  is opposed by the cylindrical body  18  of the outer member  14 . The interference between the two members at the interface  48  increases above the initial interference fit to an operating interference fit. The contact pressure at the interface  48  increases and friction between the members  14 ,  16  more tightly couples the members together. The operating fit is selected to ensure that the body  62  attached to the outer member  14  remains stationary during normal operating conditions. 
     An impact overcomes the frictional coupling between the two members  14 ,  16  and causes the body  62  to rotate with the outer member  14  about the axis  12 . The proximity sensor  72  transmits the data signal  74  to the computer  70  indicating the initial motion of the now rotating body  62 . The computer  70  detects the initial motion of the body  62  from the sensor data and determines that an impact has occurred. 
     When operating in the first mode, the computer  70  increases the voltage difference applied to the piezoelectric cylinder  20 . The cylinder  20  attempts to further expand, and the interference fit at the interface  48  increases. The increased friction force generated at the interface  48  further resists the motion of the movable body  62  and quickly brings the movable body  62  to rest. The computer monitors the deceleration of the movable body and adjusts the voltage difference as needed to ensure that the force bringing the movable body to rest is acceptable. 
     When operating in the second mode, the computer  70  decreases the voltage difference applied to the piezoelectric cylinder  20 . The piezoelectric cylinder radially contracts, with its outer wall  30  moving inwardly from the inner wall  44  of the cylindrical body  18 . The interference fit at the interface  48  decreases. The friction force resisting motion of the moving body decreases and the coupling of the two bodies decreases. The moving body can now rotate with less resistance and forces acting on the body are reduced. The computer  70  monitors the deceleration of the movable body  62  via proximity sensor  72  and adjusts the voltage difference as required to ensure that the force braking the movable body is not excessive. 
     In other embodiments, the control system can respond to other types of trigger events in addition to or instead of impacts. Other sensors or data signals, including manual controls, accelerometers, bar code readers and the like may be used for determining a trigger event to vary the coupling between the two bodies. 
     Although the coupler  10  includes concentric members that rotate with respect to each other, other embodiments of the present invention could have members that move axially or move with combined axial and rotational movement with respect to each other. 
     While I have illustrated and described a preferred embodiment of my invention, it is understood that this is capable of modification, and I therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall within the purview of the following claims.