Abstract:
A birdcage-shaped harness assembly adaptable for electrically connecting a bulkhead support structure with a rotatable gimbal support structure. In order to avoid excessive torsional forces from acting on the harness, a plurality of conductor portions are bundled together and pre-compressed in length between the support structures. When assembled, the conductor portions of the harness assembly form a birdcage-shaped dynamic bundle wherein the middle of the conductor portions bow away from conductor portions on the opposite side of the bundle. This pre-compression creates a birdcage-shaped configuration that allows the bundle to bend as the conductor portions straighten. This occurs when the gimbal rotates relative to the bulkhead. The bending motion creates relatively little damage to the conductor strands as compared the excessive torsion forces affecting conventional loop-shaped wire harness assemblies.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a harness assemblies. More specifically, the present invention relates to harness assemblies for rotating gimbals. 
     2. Description of the Related Art 
     In a gimbaled system, one or more gimbals are typically supported for movement relative to a stationary support member. In order to electrically connect components mounted on the gimbal with the stationary support member, a number of conductors are bundled together in what is conventionally referred to a harness assembly. 
     Conventional harness assemblies often employ a torsion-loop design. These designs exercise, stretch, bend or twist the conductors, resulting in the generation of both torsion and bending stress. The tighter the bend radius and the larger the dynamic range, the greater the stress on the conductors when the harness cycles through its range of motion. To ensure proper movement of the gimbal and harness, the packaging of the harness loop is made relatively loose. 
     However, the packaging for conventional torsion-loop designed harnesses usually does not allow for axial displacement. This can result in a moment that multiplies the restrictive force caused by the harness. This force translates against the motor driving the mechanism. As a result, the performance of conventional torsion-loop harnesses tends to degrade sharply with the addition of each conductor and shield added to the bundle. 
     Of major concern in any electrical circuit is Electro-Magnetic-Interference (EMI) which is commonly referred to as “noise”. When least offensive, EMI still can serve to reduce the effectiveness of any electromechanical assembly. EMI can render such an assembly useless. 
     Another drawback of conventional harness systems resides in the fact that such systems function as miniature antennas by intercepting and introducing electro-magnetic waves into the assembly. Designers of conventional harness systems, in an attempt to reduce the effect of harness noise, first measure the input in magnitude and frequency and then incorporate filters into the design to compensate for the noise. This approach tends to be somewhat successful for stationary harnesses and antennas. However, the problem can not be solved so easily when the harness is dynamic as with cross-gimbal systems. In cross-gimbal harness assemblies, the harness not only picks up and amplifies existing EMI, but also actually creates EMI. 
     As is known in the art, when an electrical current passes through a wire, and that wire is moving through space, an electromagnetic field is created (Faraday&#39;s motor principle). This phenomenon is referred to as capacitive interference. 
     Torsion-loop harnesses also tend to be very sensitive to G force inputs. When employed in a missile seeker assembly and the entire seeker is put under heavy G forces as occur during flight, the loop harness assembles are susceptible to “flopping ” around and possibly interfering with one another or with the gimbal components. In addition, because torsion-loop harnesses require tight bend radii, the conductor strands and shielding may undergo some plastic deformation which, in turn, increases the force required to overcome or extend the harness. Plastic deformation also introduces work hardening of the conductor material in the harness. Work hardening occurs as the bend radius is extended and retracted in order to accommodate gimbal motion. In addition, over time the metal material in the bundle of conductors begins to “creep” or cold flow at the location of the tight bend radius. Cold flow causes the loop harness assembly to develop a memory. This is undesirable in closed loop control systems. In effect, material work hardening and material creep adversely effect the performance of the loop harness over cycle time. When the performance of the loop is degraded, the performance of the gimbal will also be degraded. 
     Thus, there is a need in the art for a harness assembly that is easy to assemble, robust in nature, and exhibits performance that is exactly repeatable. 
     SUMMARY OF THE INVENTION 
     The need in the art is met by the unique, birdcage, torsion harness assembly of the present invention. The present invention provides a torsion harness assembly that avoids the undesirable characteristics associated with known torsion-loop harness assemblies. The birdcage harness assembly consists of a plurality of separate conductors, many twisted and shielded, that are capable of transmitting signals and power from a fixed position bulkhead support mechanism and nearby circuit cards to and across an outer gimbal to various gimbal components. The birdcage-shaped harness combines three separate harness designs extending to various components into one compressed bundle of continuous conductor strands that extend between the bulkhead and the outer gimbal. Preferably, the birdcage-shaped portion of the harness extends from the bulkhead along the axis of rotation of the gimbal into connection with block mounted on the gimbal from which a number of separate harness bundles extend to the gimbal supported components. By following the gimbal axis of rotation, the birdcage harness eliminates a moment arm multiplier and is thereby less sensitive to conductor/shield material on the bundle. In comparison, a conventional loop harness usually locates the bending stress away from the axis of rotation, creating a torque arm and increasing the force acting on the harness system. 
     When assembled, a first portion of the bundle of conductor strands are attached, preferably by gluing into a top potting block clamped to the rotating gimbal body. Individual conductor/shields exit the bottom of the clamp and are formed in equal lengths. While the actual number of conductors forming the bird cage portion of the harness assembly is considered a design choice, it has been found that up to approximately 35 conductors and/or shields can be bundled together in the present invention. Each of the conductors preferably extends substantially one (1) inch from the top potting block until being received in an opening formed in a lower potting block. The actual length of the various strands is considered a design choice, however, the lengths of the portions of the strands extending between the upper and lower potting blocks should be substantially the same. While the preferred embodiment of the invention employs continuous strands extending from the bulkhead to the gimbal components, it is considered within the scope of the invention to employ separate strands for each portion, which strands are connected to form a continuous electrical connection. A lower potting clamp mounted on the bulkhead is employed to frictionally compress the lower potting block into fixed attachment with the plurality of conductors forming the birdcage portion of the harness, thereby preventing the birdcage conductors from separating from the bulkhead. A plurality of separate bundles of conductors extend from the lower potting block to various components mounted on the bulkhead support mechanism, thereby forming electrical connections between the gimbal mounted components and bulkhead mounted components. 
     The lower potting block is mounted on the bulkhead or sensor support at a set distance from the upper potting block carefully determined such that the plurality of conductor strands extending from the gimbal to the bulkhead form a substantially bird cage configuration. In particular, the bundle of conductor strands are each bowed in a generally outwardly direction from the axis of the bundle which coincides with the axis of rotation of the gimbal member. The predetermined amount of “birdcaging” or bowing of the various individual conductor strands may easily be controlled by controlling the axial separation of the lower and upper potting blocks. 
     At first inspection, the birdcage-shaped torsion harness of the present invention appears to move or navigate with the gimbal in pure torsion. While it is true that the bowed conductors do twist as the gimbal moves though an angle of rotation, it becomes clear that the individual conductor strands primarily move in bending. Because the individual strands are pre-compressed into their initial bowed or birdcage-shaped positions, movement of the gimbal has the effect of providing slack that is taken up in the strands as the gimbal rotates relative to the fixed position sensor. In effect, the conductor strands bend or stretch from their respective bowed configurations until they reach substantially linear shape. The conductor strands then bend back into their original bowed shapes as the gimbal rotates back to its original neutral position. 
     An advantage of the present invention resides in the effective reduction of the size of the harness package as compared to conventional loop harnesses. In particular, loop harnesses extending between the support and gimbal are required to be substantially longer than the preferred approximate one (1) inch length of the birdcage conductors. The loop conductors must be longer because they require a relatively large “swing” or range of motion per angle of rotation of the gimbal. Besides reducing the package size of the harness assembly, the birdcage design eliminates the high stress normally inherent in harness assemblies facing large angles of rotation of the gimbal. In addition, the birdcage harness is not G sensitive, and dramatically reduces spring torque and friction input to gimbaled system components caused by conventional loop harness assemblies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a prior art loop harness assembly. 
     FIG. 2 is a perspective view of a birdcage harness assembly formed in accordance with the present invention. 
     FIG. 3 is an exploded view of the birdcage harness assembly of FIG.  2 . 
     FIG. 4 is view of a portion of the birdcage harness assembly of FIG.  2 . 
     FIG. 5 illustrates the movement of a conductor employed in the birdcage harness assembly of the present invention. 
    
    
     DESCRIPTION OF THE INVENTION 
     Illustrative embodiments and exemplary applications are described below with reference to the accompanying drawings in order to disclose the advantageous teachings of the present invention. 
     As will be explained below, the present invention is unique in its ability to reduce the EMI problem described above as compared with existing harness assembles by minimizing the movement of the wire forming the bundles. Known “twist cap” wire arrangements require relatively long wires and incorporate a geometry which requires the wires to fold and slide over one another as the gimbal navigates across a specified angle. This movement is quantified as D≅2πd(Ø/360), where d is the diameter of a twist cap and Ø is the gimbal angle navigated. An average diameter for a twist cap housing including 30-40 conductors is about 2 inches. For a twist cap navigating 180 degrees D (movement of the wires) equates to about 6 inches. By comparison, the present invention employs conductors undergoing movement of between 0.050 inches and 0.100 inches. As can be readily understood, the capacitive EMI created by a harness constructed in accordance with the present teachings will be substantially less than that of the existing twist cap harness design. 
     Reference is made now to the drawings wherein like reference numerals designate like elements throughout. 
     FIG. 1 is a perspective view of a prior art loop harness assembly. The conventional, prior art loop harness assembly is shown at  10 . The gimbal harness design of FIG. 1 includes a fixed support or bulkhead member  12  and a rotatable gimbal assembly  14  spaced therefrom. Gimbal assembly  14  includes, among other components, an IG resolver  16  and a gyro  18 . A plurality of separate wire harness assemblies  20 ,  22  and  24  extend between support  12  and gimbal assembly  14 . As noted, each harness is doubled up at  21 ,  23  and  25  respectively, and forced to bend or loop to provide dynamic service slack as gimbal assembly  14  rotates relative to support  12 . Because harness assemblies  20 ,  22  and  24  are positioned away from the axis of rotation  26  of gimbal assembly  14 , a torque arm is created which increases the force acting on gimbal assembly  14 . 
     Attention is directed to FIG. 2 which shows a birdcage torsion harness assembly  110  formed in accordance with the present invention. Harness assembly  110  is mounted between two spaced-apart mechanisms  112  and  114  capable of relative movement. One mechanism comprises a fixed support member or bulkhead identified at  112 . A second mechanism spaced therefrom consists of an outer, movable gimbal body  114 . Gimbal body  114  preferably supports a number of separate components. 
     FIG. 3 is an exploded view of the birdcage harness assembly of FIG.  2 . An outer gimbal  114  is shown aligned above bulkhead  112  in the exploded view. Also shown in exploded view are various components making up birdcage torsion harness assembly  110 . These components include a lower potting block  132 , a lower potting clamp  134 , an attachment plate  135 , a top potting block  136  and a top potting clamp  138 . The single bundle of conductor strands is shown at  140 . 
     FIG. 4 shows separate harness assembles  142  and  144 , supporting separate gimbal functions, extend within openings in top potting block  136  and are preferably glued together with epoxy to prevent separation. These end portions of harness assemblies  142  and  144  pass through upper potting block  136  and form the end portions of conductors  141 . A dynamic bundle  140  is formed by the plurality of conductors  141 . The conductors extend toward the lower potting block  132 . A dynamic electrical connection is formed from IG connector  116  and Gyro connector  118 , through top potting block  136 , and through conductors  141 . As shown in FIG. 4, each of the conductors  141  extends into an opening  133  in lower potting block  132  only to exit on the other side of potting block  132  as three separate harnesses  146 ,  148  and  150 , respectively. Each of the harness bundles  146 ,  148  and  150  joins with a component, not shown, mounted on bulkhead  112 . This assembly of conductors creates a harness assembly that, at the end portions, takes the configuration of a plurality of separate harnesses and in middle portion acts as a dynamic bundle  140 . The result is a continuous electrical connection between bulkhead support components and gimbal mounted components. 
     The number of components mounted on bulkhead  112  and gimbal  116  are considered to be design choices. It is important that harness assembly  110  establish continuous electrical connections between the various components and that the electrical connections be maintained even as gimbal  114  rotates relative to support bulkhead  112 . Whether each electrical connection between bulkhead  112  and gimbal  114  is, preferably, formed by a single, continuous stand of conductor material or is formed by a plurality of conductors joined end-to-end is also considered a design choice. 
     When forming harness assembly  110 , top potting block  136  is preferably poured into top clamp  138 , with the assembled members then joined to attachment plate  135  which is itself attached to gimbal  114 . At its side facing lower potting block  132 , top potting block  136  has a single opening for receiving the bundle of conductors  140 . 
     FIG. 4 is view of a portion of the birdcage harness assembly of FIG.  2 . As shown in FIG. 4, the opposite side of top potting block  136  includes a number of openings. Each opening receives one of the harness bundles  142  and  144  extending from IG Power Connector  116  or Gyro connector  118 , respectively. 
     The lower potting block  132  is formed in a tool separate from lower potting clamp  134 . When fixed to support bulkhead  112 , potting clamp  134  acts to compress lower potting block  132 , wherein potting block  132  frictionally engages the birdcage shaped bundle of conductor strands  141 , preventing the individual strands  141  from moving relative to bulkhead support  112 . 
     FIG. 4 shows in more detail the relationship between a bundle of birdcage-shaped conductor strands  141  and each of the potting blocks  132  and  136 , respectively. As shown in FIG. 4, gimbal-mounted components may include an IG Power Connector  116  and a gyro connector  118 . In place of the three separate wire harnesses  20 ,  22  and  24  employed in the prior art harness assembly  10  and extending between bulkhead  12  and gimbal  14 , the present invention utilizes a torsion harness assembly including a single bundle of conductors  140  extending between bulkhead  112  and  114 . The bundle  140  comprises a plurality of separate conductors  141  that, taken as a whole, form a birdcage-shaped configuration. Bundle  140  preferably extends along the rotational axis  126  of gimbal body  114  and is capable of electrically connecting bulkhead  112  with gimbal body  114 . 
     As shown, each of the plurality of individual conductors  141  forming bundle  140  bows slightly outwardly from oppositely disposed strands, thereby forming the distinctive birdcage configuration. 
     FIG. 5 illustrates the movement of a conductor employed in the birdcage harness assembly of the present invention. As shown in FIG. 5, the unique birdcage shape results from the middle portions  143  of each conductor strand  141  being spaced a greater distance from the rotational axis  126  of gimbal  114  than the respective opposite end portions  145  of each conductor  141 . The birdcage configuration of bundle  140  is considered unique to the present invention and allows the individual conductors  141  to undergo primarily bending motion as opposed to the excessive torsional stresses encountered by loop harness conductors as will be explained. 
     FIG. 5 shows various positions of a single conductor strand  141  when attached at its one end to lower potting block  132  and at an opposite end to top potting block  136 . In its normal position, achieved when bulkhead  112  and gimbal  114  are at rest, conductor strand  141  assumes the position shown at  141   a , wherein the bowed configuration of the strand is achieved by pre-compression of the ends of strand  141 . The amount of pre-compression is controlled by the separation of top and lower potting blocks  136  and  132 , respectively. When gimbal body  114  and its attached top potting block  136  undergo rotation through an angle Ø relative to bulkhead  112  and its lower potting block  132 , each conductor strand  141  assumes the position and shape shown at  141   b . The movement of the strand  141  from the position of  141   a  to  141   b  requires the strand to twist and bend. By bending, the initial compression of strand  141   a  is, in effect, taken up and strand  141   a  straightens out until it achieves the substantially straight configuration of  141   b.    
     When gimbal  114  proceeds to rotate in the opposite direction toward its neutral position, each extended strand  141   b  will again be compressed into its pre-compressed shape at  141   a . If the rotation carries gimbal  114  beyond its neutral or rest position, the strands  141   a  will bend in the opposite direction until they achieve the configuration of strand  141   b . During operation, the strands  141  move to position  141   a , each strand moves a distance of between 0.05 inches and 0.10 inches. This small distance significantly reduces the EMI created by the strands  141  as compared to existing harness assemblies. 
     The birdcage torsion harness assembly of the present invention substantially eliminates the need for three separate harnesses each routed in a free 180° loop, positioned off center from the rotation axis of outer gimbal  114 . Instead, the birdcage harness assembly  110  includes a single bundle  140  of conductors  141  preferably routed along the center axis of the movable gimbal and then separated into distinct harnesses to electrically connect with components mounted on gimbal  114 . 
     Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof. Although the invention has been shown as being applicable to a missile mounted seeker assembly, it is in no way limited to this application. Basically mechanisms that undergoes axial motion relative to another mechanism and requires a harness assembly to connect the mechanisms despite the relative movement should benefit from the present invention. Examples of systems that could benefit include, but are not limited to, radar, automated production machines and robots. 
     It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. 
     Accordingly,