Abstract:
A circuit board mounting system in one example comprises an improved mounting system for a circuit board disposed between housing elements, wherein the improvement comprises a plurality of slots formed in the circuit board and a plurality of bushings disposed within the slots, such that the housing elements rest on opposed ends of the bushings, and the circuit board moves in a plane substantially parallel to its mounting surfaces in response to changes in temperature, thus reducing thermal stress.

Description:
BACKGROUND  
       [0001]     This application is directed generally to temperature stress reduction in mechanical assemblies and in particular to improved mounting for a circuit board used in conjunction with a plurality of interconnected assemblies, and is more particularly directed toward improved circuit board mounting for temperature stress reduction in a vibratory rotation sensor.  
         [0002]     Complex electro-mechanical systems are often designed and implemented in a modular fashion. In other words, there may be a module (or subassembly) that contains most or all of the electromechanical components, as well as electrical drivers and sensors. Another module or subassembly, interconnected with the first, may then include electronic circuitry to provide necessary drive signals, amplify and/or filter sensor outputs, and provide computational or signal processing resources. The separation of system components into modules, as described above, may often be dictated by manufacturing concerns, efficient testing of manufactured assemblies, or proper interoperability of system components.  
         [0003]     Of course, the modules or subassemblies must then be assembled into an integrated product. This often means that an electromechanical subassembly must be interconnected with an electrical connection header, for example, as well as one or more circuit boards containing electronic components. Particularly where size of the finished product is a concern, this generally means that the modules or subassemblies will be in close proximity to one another, making at least mechanical contact with one another, and often both mechanical and electrical contact. There is also generally a need to provide a housing around the modules or sub-assemblies, and the housing also is generally in at least mechanical contact with one or more of the system modules.  
         [0004]     Many modem electro-mechanical systems must be designed to operate in harsh environments, including extremes of temperature. Since the modules and sub-assemblies of the integrated product often have different coefficients of thermal expansion (CTEs), as do the mechanical housing components of such a system, thermal stress related to these differing CTEs, over a wide operating temperature range, is of significant concern to system designers.  
       SUMMARY  
       [0005]     The invention in one implementation encompasses a mounting system. The system comprises an improved mounting system for a circuit board disposed between housing elements, wherein the improvement comprises a plurality of slots formed in the circuit board and a plurality of bushings disposed within the slots, such that the housing elements rest on opposed ends of the bushings, and the circuit board moves in a plane substantially parallel to its mounting surfaces in response to changes in temperature, thus reducing thermal stress.  
         [0006]     Another implementation of the invention encompasses a method. The method comprising the steps of providing a circuit board disposed within first and second housing elements, providing a plurality of slots in the circuit board, disposing a plurality of bushings within the slots, and resting the housing elements on opposed ends of the bushings, such that the circuit board moves in a plane substantially parallel to its mounting surfaces in response to changes in temperature, thus reducing thermal stress. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0007]     Features of exemplary implementations of the invention will become apparent from the description, the claims, and the accompanying drawings in which:  
         [0008]      FIG. 1  is a representation of a vibratory rotation sensor of the prior art.  
         [0009]      FIG. 2  shows the vibratory rotation sensor of  FIG. 1  in conjunction with a header assembly, as known in the art.  
         [0010]      FIG. 3  is a section view of a complete HRG device.  
         [0011]      FIG. 4  illustrates a circuit board for use with the HRG device of  FIG. 3 .  
         [0012]      FIG. 5  depicts a modified circuit board in accordance with an exemplary implementation of the invention.  
         [0013]      FIG. 6  is a representation of a bushing.  
         [0014]      FIG. 7  is a detail view of a portion of the circuit board of  FIG. 5 .  
         [0015]      FIG. 8  is a detail view of a portion of the circuit board of  FIG. 5 , illustrating bushing placement.  
         [0016]      FIG. 9  is a section view of a complete HRG device in accordance with an exemplary implementation of the invention.  
         [0017]      FIG. 10  is a detail view of a portion of  FIG. 9 . 
     
    
     DETAILED DESCRIPTION  
       [0018]     A vibratory rotation sensor is a type of complex electromechanical assembly that is often subjected to environmental extremes during normal operation. These environmental extremes often include a very broad operating temperature range, so the vibratory rotation sensor must be designed to operate accurately and reliably over extremes of temperature.  
         [0019]      FIG. 1  is a simplified view of a portion of a vibratory rotation sensor  10  as known in the art. The vibratory rotation sensor  10  includes an outer support structure  12 , a resonator  14  of generally hemispherical shape, and an inner support structure  16 . Both support structures  12 ,  16 , as well as the resonator  14  itself, are preferably formed from quartz. The vibratory rotation sensor  10  is often termed a “hemispherical resonator gyro” (HRG) because it utilizes changes in vibration patterns on the thin-walled hemispherical quartz resonator  14  to detect when it is moved. The thin-walled hemispherical quartz resonator  14  is excited by an electrical field to induce a pattern of mechanical vibration. This pattern is electrically detected and used to determine changes in the HRG&#39;s subtle mechanical vibration. The mechanical disturbance in the resonator  14  is so small that there is virtually no mechanical stress or fatigue induced in the resonator  14 , and therefore the device itself is a high-reliability electro-mechanical system.  
         [0020]     As noted, an electrical excitation is required in order to induce an appropriate vibration of the resonator  14  such that standing waves may be established. To provide the excitation, a plurality of electrodes  22  are provided on an interior surface  20  of the outer support structure  12 . These electrodes  22  are in close proximity to the outer surface  32  of the resonator  14 , which is metallized. When an electrical signal is applied to selected electrodes  22 , mechanical vibration is induced in the resonator  14  with the desired standing wave pattern.  
         [0021]     When the HRG  10  rotates about its axis, the standing wave pattern established in the resonator  14  rotates in the opposite direction. Consequently, by measuring the angle of rotation of the standing wave pattern, the rotation angle of the HRG  10  can be determined. Output signals from the resonator  14  are obtained through capacitive coupling between a plurality of output electrodes  24  and the metallized interior surface  30  of the resonator  14 . The output electrodes  24  are disposed on the inner support structure  16 . The vibration mode of the resonator  14  causes changes in capacitance that are readily measured at the output electrodes  24 . This capacitance data is sufficient to enable sensing circuitry to establish the degree of rotation of the HRG  10 .  
         [0022]     The outer support member  12 , inner support member  16 , and resonator  14  are vacuum sealed to form an HRG subassembly which is depicted by the numeral  202  in  FIG. 2 . In order to provide electrical connections to external circuitry, a header assembly  206  is provided. A plurality of electrical contact pins  208  are provided in the header assembly  206  in order to make electrical contact with electrical contact pads  204  disposed on exterior surfaces of the HRG subassembly  202 . In order to provide appropriate mechanical isolation between the HRG subassembly  202  and the header assembly  206 , contact springs  210  may be utilized as part of the electrical pin  208  to contact pad  204  interconnection. The electrical contact pins  208  provide electrical connection to the HRG subassembly  202  for both input and output electrical signals.  
         [0023]      FIG. 3  is a section view of a complete HRG device in which the HRG subassembly  202  is placed into a mechanical housing that includes lower housing  304  and upper housing  306 . The HRG subassembly  202  is similar to the HRG subassembly  202  depicted in  FIG. 2 ; however, specific details of the HRG subassembly  202  are not necessary for an understanding of the present invention. Consequently, details of the HRG subassembly  202  are omitted from  FIG. 3 .  
         [0024]     A circuit board  302  is electrically connected to the header assembly  206  via electrical contact pins  208 . The circuit board  302 , sometimes designated a “printed wiring board” or PWB, includes electronic circuits that provide both excitation and sensing capabilities for use with the HRG assembly  202 . The upper housing  306  is secured to the header assembly  206  by cover screws  308 . The header assembly  206  is preferably hermetically sealed to the HRG assembly  202 . In the mounting arrangement illustrated in  FIG. 3 , the circuit board  302  is effectively “sandwiched” between the upper housing  306  and the header assembly  206 , in order to provide a secure mechanical mounting for the circuit board  302 .  
         [0025]     In the illustrated implementation, the circuit board  302  is a polyclad polyimide circuit board manufactured in accordance with IPC 4101/40 or /41. As is well-known, the IPC was originally the Institute for Printed Circuits, then changed its name to Institute for Interconnecting and Packaging Electronic Circuits. IPC is now the formal name of the organization, which, among other things, establishes standards for printed circuit boards that have been widely adopted throughout the industry. Of course, other printed circuit constructions, such as G-10 or FR4, for example, may also be suitable in this context, depending upon the ultimate application of the HRG device.  
         [0026]     In environments where there are extreme excursions in operating temperature, the configuration illustrated in  FIG. 3  may result in unacceptable temperature stresses due to the “sandwich” style mounting of the circuit board  302  between the upper housing  306  and the header assembly  206 . The upper housing  306  may be termed a first housing element, while the header assembly  206  may be thought of as a second housing element, since the header assembly  206  contributes to the mechanical package integrity of the device. Since the interconnected subassemblies  306 ,  206 ,  302  generally have different coefficients of thermal expansion, or CTEs, temperature stress may occur to the circuit board  302  in particular, perhaps resulting in degraded operation of the completed device, or even premature failure.  
         [0027]      FIG. 4  provides a detail view of the circuit board  302 , illustrating the electrical contacts  402  that are designed to mate with pins  208  extending from the header assembly  206 . There are also RF (radio frequency) type connectors  404  illustrated in  FIG. 4 . These RF connectors  404  are designed to mate with similar RF connectors extending from the header assembly  206 , although details of this interconnection are not illustrated in the drawings. In this implementation, the circuit board is approximately 0.075 inch thick, although physical dimensions are, of course, largely dictated by the specific application.  
         [0028]     To improve performance of the HRG device over extremes of operating temperature, a modified circuit board  502  is illustrated in  FIG. 5 . A plurality of slots  504  are provided with the longitudinal axes of the slots  504  arranged radially around the circuit board  502 . In the illustrated implementation, the open ends of the slots actually penetrate the circumference of the circuit board, which is generally circular, although, in the alternative, one or more of the slots  504  could be completely surrounded by circuit board material. Disposed within at least some of the slots are a plurality of bushings  506 . In the illustrated implementation, the bushings  506  are formed from Kovar, which is an alloy of iron, nickel, and cobalt, although other formulations may also perform adequately depending upon environmental factors.  
         [0029]      FIG. 6  depicts the physical structure of the bushing  506 . The bushing  506  includes a body portion  602  that is preferably about 0.080 inch in length, with the body portion  602  being defined by opposed flanges  604  integrally formed at opposing ends of the body  602 . In the illustrated implementation, each of the flanges  604  has a thickness of about 0.010 inch, resulting in an overall length for the bushing  506  of about 0.100 inch. An opening of about 0.070 inch diameter is provided through the bushing  506  to accommodate mounting screws, studs, or other mounting features, as appropriate. The outside diameter of the illustrated bushing  506 , measured around the flange  604 , is about 0.140 inch.  
         [0030]      FIG. 7  is a detail view of a portion of the circuit board  502 , illustrating the dimensions of the slots in one implementation. Each slot  504  is about 0.110 in width, and has a radius at its inner extremity of about 0.055 inch. Thus, the overall length for the illustrated slot  504  is about 0.228 inch. As noted above, the illustrated slot  504  actually has an open end at the outer periphery of the circuit board  502 , but it is not necessary that the slot actually penetrate the perimeter of the circuit board  502 . In fact, although the illustrated circuit board  502  is generally circular in layout, the mounting technique described herein would work equally well for circuit boards of other general shapes, such as rectangular or square.  
         [0031]      FIG. 8  illustrates a bushing  506  disposed within a slot in circuit board  502 . In the illustrated embodiment, a small amount of a compliant epoxy is applied between the bushing  506  and the slot. The epoxy compound may be Unistake tacking epoxy manufactured by Aptek, for example, although other formulations may also perform adequately. Also, it should be noted that the addition of tacking epoxy may not be necessary in all implementations, and its use may be dictated by particular circumstances.  
         [0032]      FIG. 9  is a section view of a completed HRG device illustrating the “floating” mounting system provided for circuit board  502 . Rather than creating a sandwich structure around the circuit board, the mounting detail illustrated in  FIG. 9  shows that the upper housing  306  and the header assembly  206  actually bear on the flanges of the bushings  506  rather than directly on the circuit board material. Attachment screws  308  hold the upper housing  306  and the header assembly  206  together, allowing the circuit board  502  to float. This ensures that the circuit board  502  can move at least slightly in a radial direction as the housing elements  306  and  206  expand and contract with temperature. Of course, the circuit board  502  also expands and contracts with temperature, generally at yet a different rate than the housing elements  306  and  206 . The radial motion of the floating circuit board  502  may be thought of as movement in a plane generally parallel to the mounting surfaces of the circuit board  502 . And, of course, where the circuit board  502  has a rectangular or square form-factor, movement of the circuit board  502  should be thought of as planar with respect to the mounting surfaces rather than radial with respect to the center of a generally circular circuit board. The flanges of the bushings  506  generally restrict movement of the circuit board  502  in an axial direction; that is, generally normal to the mounting surfaces of the circuit board  502 .  
         [0033]      FIG. 10  is a detail view of a portion of  FIG. 9 , illustrating the floating mounting of the circuit board  502  in greater detail. As can be appreciated from an examination of  FIG. 10 , the circuit board  502  effectively floats within the limits established by the exterior dimensions of the bushings  506 . The header assembly  206 , for example, is in close mechanical contact with the bushings  506  rather than the circuit board  502 . The same condition applies to the mechanical contact of the upper housing  306 . In this case as well, the upper housing  306  is in close mechanical contact with the bushings  506  rather than the circuit board  502 .  
         [0034]     The steps or operations described herein are just exemplary. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified.  
         [0035]     Although exemplary implementations of the invention have been depicted and described in detail herein, it will be apparent to those skilled in the relevant art that various modifications, additions, substitutions, and the like can be made without departing from the spirit of the invention and these are therefore considered to be within the scope of the invention as defined in the following claims.