Abstract:
A constant length strut includes an axially elongated body having a fixed attachment point at one end and an attachment point carried at its opposite end and supported for axial movement toward and away from the fixed attachment point. A quartz reference rod is connected at one end to one attachment point and extends internally for substantially the length of the strut. The free end of the reference rod moves axially lengthwise relative to the other attachment point as the strut changes length due to thermal expansion and contraction or tensile and compressive loading. A positional displacement transducer senses this relative movement. An electrical signal directly proportional to the displacement excites a linear actuator connected between the telescopic attachment point and the strut body to provide a corrective displacement to maintain the strut at a constant length.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to structural elements used in forming structural assemblies and frameworks and deals more specifically with a constant length strut for use in constructing such an assembly or framework. 
     It is known that structural assemblies and frameworks change their geometric shape due to the effects of thermal expansion and contraction and changes in mechanical loading on structural elements or struts used for attaching and interconnecting the various structural components of such structural assemblies. Although such changes are generally very small, they may have a significant affect on the accuracy of precision equipment and apparatus, such as, for example, optical measuring systems and the like supported by the structural assembly. It is desirable therefore, to provide a strut that maintains a constant length between attachment points by actively controlling deflections in the strut due to thermal expansion and contraction and due to changes in tensile and compressive loading so that the geometric shape of an assembly constructed with the struts is precisely controlled. 
     It is an object therefore, of the present invention to provide a strut having a constant length between attachment points. 
     It is another object of the present invention to provide a constant length strut having active compensation for deflections due to thermal expansion and contraction and changes in tensile and compressive loading. 
     It is a further object of the present invention to provide a strut that operates as an element of a closed loop servocontrol system to maintain a desired constant length between attachment points. 
     Other objects and advantages of the invention will be readily apparent from the following description and claims taken in conjunction with the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     The present invention resides in a constant length strut comprising an axially elongated body having a fixed termination at one end of the body for attachment to a first external structure with which the strut is used and a second termination carried at the opposite end of the body, and movable lengthwise of the body, for attachment to a second external structure. An axially elongated reference element extends lengthwise of the body and has a substantially fixed length. One end of the reference element is fixedly attached to the one termination and moves axially with that termination toward and away from the other termination. The opposite, unattached free end of the reference element is coupled to a measuring system for sensing relative displacement of that end of the reference element toward and away from the other termination as the reference element moves with the one termination. This measuring system generates an electrical displacement signal having a magnitude related to the relative displacement of the two terminations. A linear actuator is coupled between the second termination means and the strut body and drives the second termination toward and away from the first termination in response to the relative displacement signal to provide a corrective displacement to maintain a constant length between the first and second termination. 
     In a preferred embodiment, the reference element is an axially elongated rod which has a near zero coefficient of thermal expansion. The linear actuator is responsive to the displacement signal. The measuring system includes an eddy current positional displacement transducer comprising a conductive target fixed to the free end of the quartz rod and disposed between a first and second electric coil arranged equidistant from the target and at opposite sides of the target. The circuit includes means for producing an electric signal representative of and related to the displacement of the target and accordingly a deflection in the length of the strut. The displacement electric signal is used to control a voltage source for exciting the linear actuator which actuator exerts a force on the second termination to move the second termination with respect to the first termination to apply a corrective displacement to maintain the strut at a constant length. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a constant length strut embodying the present invention. 
     FIG. 2 is an axial cross-sectional view of the strut of FIG. 1. 
     FIG. 3 is a somewhat exploded view showing in greater detail the eddy current positional transducer and fixed termination and attachment point of the strut of FIG. 2 
     FIG. 4 is a somewhat schematic, cross-sectional view of the constant length strut of FIG. 1 showing the strut attached at its respective termination ends to associated external structures. The strut is shown as an element of a servocontrol system for maintaining a constant displacement between the termination ends. 
     FIG. 5 is a schematic representation of an eddy current positional displacement transducer and instrumentation bridge circuit comprising a portion of the displacement measuring circuit of FIG. 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings and considering FIG. 1 in particular, a somewhat schematic view of a constant length structural element or strut is shown and designated 10. The strut 10 includes an axially elongated, tubular body 12 made of stainless steel or other suitable material well known to those skilled in the art. The strut 10 has a termination 14 fixedly connected to the body 12 at one end 16 of the body. The termination 14 is arranged with a precision ball end 18 which extends axially beyond the end of the body 12 and is used to connect one end 16 of the strut 10 to an external structure with which the strut is used. The strut 10 also includes a moving termination 20 telescopically received by the opposite end 22 of the body 12. The moving termination 20 is also arranged with a precision ball end 24 used to connect the end 22 to an external structure with which the strut is used. The moving termination 20 is arranged for linear movement within and relative to the body 12 in the direction of arrow 26 to provide corrective displacement maintaining a fixed length L between the precision ball ends 18 and 24. Such corrective displacement compensates for deflections in the strut 10 due to thermal contraction and expansion or changes in tensile or compressive loading of the strut as explained in further detail below. 
     Turning now to FIG. 2, a cross-sectional view is shown revealing the major component elements comprising an eddy current positional displacement transducer 58, a reference length element 52 and a linear actuator such as, for example, the piezoelectric motor 42 of the constant length strut 10 of FIG. 1. The linear actuator may be any suitable type well known in the art. A lock ring 28 is threadably screwed to the inner diameter of the body 12 at end 22 and includes an aperture 30 through which the moving termination 20 moves in the direction of arrow 26. The moving termination 20 includes an annular flange 32 having a circumference larger than the aperture 30 which prevents the moving termination 20 from moving out of the strut body 12. The inner portion of the moving termination 20 includes a recessed surface 38 arranged for complementary engagement with one end 40 of the piezoelectric motor 42. A load spring 34 in the form of a wave washer is located between an inner surface 31 of the lock ring 28 and a shoulder surface 36 of the flange 32. The load spring 34 urges the moving termination 20 axially toward the end 16 of the body 12 and into engagement with the one end 40 of the piezoelectric motor 42. A disc shaped stop 44 is welded to the inner portion 46 of the body 12 and includes an annular shoulder 48 arranged for complementary engagement with the other end 50 of the piezoelectric motor 42. 
     The reference length element 52 is an elongated rod and is fixedly coupled at one end 54 to the moving termination 20 and moves axially in the direction of arrow 26 as the termination 20 moves. The opposite end 56 of the rod 52 is free and coupled to the positional displacement transducer 58 as explained in greater detail in conjunction with the discussion of FIGS. 3 and 5. The reference rod 52 is made of a material having a very low coefficient of thermal expansion and preferably is quartz. Also, as illustrated in FIG. 2, the reference rod 52 is coaxial with the body 12 and is surrounded by a gap which provides a &#34;thermos bottle&#34; effect to minimize heat transfer from the body 12 to the rod 52. Consequently, the effects of thermal expansion and contraction of the reference rod 52 and accordingly movement of the positional displacement transducer 58 is due to a deflection of the strut 10 rather than thermal expansion and contraction of the reference rod 52. 
     A disc shaped stop 60 is welded to the inner surface 46 of the body 12 near the end 16 and at the side of the transducer 58 nearest the moving termination 20. A preload spring 62 in the form of a wave washer is located between one surface 64 of the stop 60 and one surface 66 of the transducer 58 to urge the components of the transducer body 58 into complementary engagement with each other as shown in detail and explained in conjunction with FIG. 3. The transducer body 58 has an axial bore through which the free end 56 of the reference rod 52 passes. A tension spring 68 is coupled to the free end 56 of the rod 52 to impart a slight tension to the reference rod 52. 
     The piezoelectric motor 42 is generally well known to those skilled in the art and functions as a linear actuator to drive the moving termination 20 and accordingly the precision ball end 24 in a direction toward and away from the fixed termination 14. The piezoelectric motor 42 is responsive to an applied electrical signal related to and derived from the output of transducer 58 and axially expands or contracts in accordance with the applied signal to make a corrective change in length to compensate for a deflection between the precision ball ends 18 and 24. The piezoelectric motor 42 may be of any suitable piezoelectric material such as, for example, lead zirconate titanate (PZT) ceramic. The piezoelectric motor 42 comprises a number of piezoelectric discs arranged in a continuous stack wherein each of the discs is fabricated from the PZT material. The discs have the property of changing their physical dimension in response to a voltage potential of the proper magnitude and polarity placed across the discs. The ends 40 and 50 of the piezoelectric motor are electrically insulated from the remaining portions of the strut 10. The piezoelectric discs 70,70 are in the shape of a washer so that the reference rod 52 passes axially through the opening formed in the stack of discs. 
     Considering FIG. 3, a fragmentary somewhat exploded view of the eddy current positional displacement transducer 58 is shown in cross-section revealing the component parts of the transducer. The transducer 58 comprises two halves for ease of assembly. One half includes a shoulder body portion 72 having an axial bore 74 through which the reference rod 52 passes. The body 72 has a diameter somewhat less than the diameter of an aperture 76 in the stop 60 so that the body 72 moves coaxially in the aperture 76. The body 72 includes a flange 78 arranged for complementary engagement with a recess 80 in another body portion 82 which is the other half of the transducer 58. The body 82 is disc shaped and includes a relieved portion 84 which forms a cavity in the area between one surface 86 of the body 72 and the relieved surface 84 when the body 82 is in complementary engagement with the body 72. The body 82 includes threads around its periphery and is arranged for threadable engagement with threads located along the inner surface of the strut body 12 at the end 16. The preload spring 62 is located between one surface of the stop 60 and the flange portion 78 of the body 72 and urges the body 72 into complementary engagement with the body 82. The body 82 also includes an axial bore 88 which is in registry with the axial bore 74 in the body 72 to permit the reference rod 52 to pass therethrough. 
     Electromagnetic coils or sensors 90 and 92 are mounted on surfaces 86 and the relieved portion 84, respectively and are arranged coaxially with the reference rod 52. A conductive target 94 is disc shaped and is mounted coaxially on the reference rod 52 for axial movement with the rod 52. The target 94 is located equidistant from the sensors 90 and 92. The eddy current positional displacement transducer 58 operates on the principle of impedance variation caused by eddy currents induced in a conductive target located within the range of each of the sensors 90 and 92. The positioning of the conductive target 94 is made equidistant from the sensors 90 and 92 when the strut 10 is at desired length that is to remain constant. 
     Turning now to FIG. 4, a somewhat schematic view of the constant length strut embodying the present invention is shown in a cross-sectional view wherein the precision ball end 18 is attached to an external structure 96 to couple one end of the strut 10 to the structure 96 and the precision ball 24 is shown connecting an external structure 98 to the moving termination coupling 20 of the strut 10. The sensors 90 and 92 are electrically connected to the input of a displacement measuring circuit 100 through electrical conductors 102 and 104, respectively. 
     The displacement measuring circuit 100 is sensitive to an electrical signal present on either of the inputs 102 and 104 and provides an output electrical signal on lead 106 which signal has a magnitude directly proportional to the displacement of the moving termination coupling 20 with respect to the fixed termination coupling 14. The output electric signal is coupled to a piezoelectric motor excitation voltage source 108 and is used to control the source 108. The piezoelectric motor excitation voltage source 108 produces a voltage on lead 110 which is coupled to the piezoelectric motor 42 and causes the piezoelectric motor to change dimensionally by expanding or contracting to provide a corrective displacement in a direction opposite from the displacement direction sensed by the transducer 58. As the deflection is compensated for, the displacement error sensed by the transducer 58 becomes smaller and smaller as the length of the strut approaches the desired constant length of the strut. 
     The piezoelectric motor excitation voltage source 108 may be one of a number of circuits well known to those skilled in the art. Of necessity, the design of such a voltage source 108 must be capable of providing a voltage potential having a magnitude and polarity compatible with the piezoelectric material used in the piezoelectric motor. 
     The displacement measuring circuit 100 may be of a suitable electrical circuit design and sensitivity for providing a signal directly proportional to the movement of the conductive target 94 located between the sensors 90 and 92. The circuit 100 design is made to be sensitive to strut length deflection yet relatively insensitive to changes in length caused by vibration. Preferably, a differential measuring system such as one available from Kaman Measuring Systems of Colorado Springs, Colo. referred to as a differential impedence transducer and specifically identified as a KD-5100 Differential Measuring System is used to provide the desired output control signal representative of the deflection or relative movement of the conductive target from the initial position corresponding to the constant length of the strut. A schematic block diagram of a typical displacement measuring circuit is shown in FIG. 5 and briefly explained below. 
     Considering now FIG. 5, a schematic block diagram representative of the above-referenced KD-5100 system is shown. The eddy current positional displacement transducer 58 is shown schematically and includes sensors 90 and 92 and a conductive target 94 arranged for movement in the direction of arrow 112. The operation of the displacement measuring circuit 100 uses the principle of impedance variation caused by eddy currents induced in the conductive target 94 located within the range of the sensors 90 and 92. The strength of the electromagnetic coupling between sensors 90 and 92 and the conductive target 94 depends upon the gap 114 between the sensor 90 and the target 94 and the gap 116 between the sensor 92 and the target 94. Each sensor 90 and 92 is electrically connected via a conductor pair 118 and 120 respectively to opposite legs of an instrumentation bridge circuit designated generally 122. The specific design of the bridge circuit is such that the bridge is balanced and the differential output voltage signal from the bridge is zero when the sensors 90 and 92 are equidistant from the conductive target 94. The sensors are equidistant at the centered or null position of the conductive target 94 and which position corresponds to the length of the strut which is to be maintained constant. As the conductive target 94 moves from the null position away from one of the sensors 90 and 92 and toward the other of the sensors 90 and 92, the electromagnetic coupling between each sensor and the conductive target 94 is no longer equal and causes an impedance imbalance which is detected by the bridge circuit. The differential output voltage signal in the illustrated circuit is connected to a demodulator 124 which demodulates the bridge output signal produced by the well known operation of the combination of the oscillator 126 and the bridge circuit 122. The output signal from the demodulator 124 appears across the output conductor pair 128 and is a linear analog signal having a magnitude directly proportional to the conductive target position. An eddy current positional displacement transducer of the type used with the present invention provides a greater displacement sensitivity than a capacitive micrometer gauge used with piezoelectric stacks. The eddy current transducer described above provides a sensitivity in the range of +/- 2nM (nanometers) relative displacement. Consequently, the constant length strut of the present invention utilizing a feedback loop as described above and an elongated rod as a reference length element also as described above, said reference length element being subject to thermally-induced changes in length of a few nanometers, provides precise, linearly repeatable relative positioning to an accuracy of approximately +/- 5nM. 
     A constant length strut for precisely maintaining a fixed length between two ends has been described wherein corrective changes in the length of the strut are made to compensate for deflections caused by thermal expansion and contraction and tensile and compressive loading. Although the constant length strut of the present invention is disclosed with an eddy current transducer arranged to cooperate with a quartz reference rod having a near zero coeffient of thermal expansion to produce a signal representative of a deflection in the length of the strut, it will be understood that other distance measuring systems, such as, for example, an optical distance measuring system may be used to sense a deflection in the length of the strut. It will also be understood that numerous other modifications and substitutions may be implemented without departing from the spirit and scope of the invention and therefore the invention has been described by way of illustration rather than limitation.