Abstract:
A packaging system in one example comprises an improved mechanical packaging system for a circuit board disposed between housing elements, the improvement comprising an upper housing member having a centrally disposed opening provided therethrough, a unitary electromagnetic shield disposed on at least an upper surface of the circuit board and secured to the circuit board, the unitary electromagnetic shield secured to the upper housing member, such that undesired mechanical vibration modes are substantially reduced.

Description:
BACKGROUND  
       [0001]     This application is directed generally to housings designed for mechanical assemblies and in particular to improved packaging for a plurality of interconnected electro-mechanical assemblies, and is more particularly directed toward an improved package designed to minimize shock and vibration effects 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 electro-mechanical 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 electro-mechanical 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 environments that subject the system to extremes of shock and vibration. Since shock and vibration effects can cause unacceptable stress levels to system components and interconnections, shock and vibration performance is of significant concern to system designers.  
       SUMMARY  
       [0005]     The invention in one implementation encompasses a packaging system. The system comprises an improved mechanical packaging system for a circuit board disposed between housing elements, the improvement comprising an upper housing member having a centrally disposed opening provided therethrough, a unitary electromagnetic shield disposed on at least an upper surface of the circuit board and secured to the circuit board, the unitary electromagnetic shield secured to the upper housing member, such that undesired mechanical vibration modes are substantially reduced.  
         [0006]     Another implementation of the invention encompasses a method. The method comprising the steps of providing at least an upper housing member having a centrally disposed opening therethrough, disposing an electromagnetic shield on at least an upper surface of the circuit board, securing the electromagnetic shield to the upper housing member, such that undesired mechanical vibration modes are substantially reduced. 
     
    
     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  is a more detailed view of the HRG device of  FIG. 3 .  
         [0012]      FIG. 5  depicts a modified HRG device in accordance with an exemplary implementation of the invention.  
         [0013]      FIG. 6  is a partial exploded view of the HRG device of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0014]     A vibratory rotation sensor is a type of complex electro-mechanical assembly that is often subjected to environmental extremes during normal operation. These environmental extremes often include a broad range of shock and vibration, so the vibratory rotation sensor must be designed to operate accurately and reliably even under these extreme operating conditions.  
         [0015]      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.  
         [0016]     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.  
         [0017]     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 .  
         [0018]     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.  
         [0019]      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 .  
         [0020]     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 .  
         [0021]     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.  
         [0022]     In environments where there are extreme excursions in shock and vibration, the configuration illustrated in  FIG. 3  may result in unacceptable mechanical 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. Some of the mechanical stress also stems from the mass distribution on the circuit board  302 , as can be seen more clearly in the detail view of  FIG. 4 .  
         [0023]      FIG. 4  shows that there are a plurality of individual electromagnetic shields  406  disposed on both upper and lower surfaces of the circuit board  302 . These shields  406  are necessary to provide appropriate electromagnetic shielding for the driver, sensor, and signal conditioning circuitry disposed on the circuit board  302 . However, because of this mass distribution, and because the circuit board  302  is effectively mounted near its perimeter, unwanted vibration modes can occur under certain environmental conditions that may adversely affect operation of the HRG. When resonance occurs in these vibration modes, mechanical standing waves within the circuit board  302  can cause extreme excursions, which can rapidly lead to excess mechanical stress and device failure.  
         [0024]     Another feature of the complete HRG device illustrated in  FIG. 4  is a pinch tube  402 . As noted previously, the resonator portion of an HRG is maintained in a vacuum. In order to accomplish this, assembly of the HRG and the header  206  is generally conducted in a vacuum chamber, and the pinch tube  402  is crimped to provide a seal. The crimp tube structure  402  is inherently fragile, however. The tube  402  itself must be relatively thin so it can be crimped closed. The tube  402  must also be sealed to the header  206 , since it is not possible to integrally form the pinch tube  402  within the header  206 . This relatively long and thin pinch tube structure is susceptible to vibration modes that can weaken its attachment point to the header  206 , thus resulting in a vacuum failure and degradation of device operation.  
         [0025]      FIG. 4  further depicts a high voltage contact spring  404  used to provide necessary high voltage to the resonator. This contact spring  404  is basically a flat spring that is bent back on itself to provide both electrical and mechanical contact to the resonator assembly. Unfortunately, this flat spring contact  404  is also susceptible to unwanted vibration modes that can introduce excessive stress to the mechanical and electrical connections, thus degrading device performance and potentially leading to operational failure.  
         [0026]      FIG. 5  illustrates one implementation of an improved packaging system designed to minimize undesired vibration modes that may lead to device failure. First, the plurality of electromagnetic shield structures have been replaced by single, unitary electromagnetic shields  508  for both the upper and lower surfaces of the circuit board  502 . The flat spring  404  (shown in  FIG. 4 ) has been replaced by a plurality of coil springs  514  and a plurality of electrical contact pins  512 .  
         [0027]     In addition, the upper housing  504  now has a centrally disposed opening  506  therethrough. This new structure for the upper housing  504  allows the upper electromagnetic shield  508  to effectively become the cover for the completed assembly. Furthermore, instead of “sandwiching” the circuit board  502  between the upper housing  504  and the header, the circuit board  502  is now mounted to the upper housing assembly  504  by a plurality of mounting screws  510  that thread directly into the upper shield  508 . Since the shields  508  are securely fastened to the circuit board  502 , this mounting technique provides a secure mounting system for the circuit board and reduces overall parts count for the completed assembly. Since circuit board  502  mounting points have now been moved closer to the center of the circuit board  502 , and the mass distribution of the electromagnetic shields  508  is also more centralized, unwanted vibration modes of the circuit board  502  are effectively eliminated, this reducing mechanical stress on the completed assembly during shock and vibration occurrences.  
         [0028]     In addition to the improved mounting system, the pinch tube  404  (illustrated in  FIG. 4 ) has also been removed. Final assembly of the resonator and header is still conducted in a vacuum chamber, but an evacuation passage  516  is now provided in the lower housing. Instead of necessitating a crimp operation for pinch tube sealing, the evacuation passage  516  is simply laser sealed upon completion of assembly. The laser sealed passage  516  is not susceptible to vibration modes, and consequently provides a high-integrity sealing methodology for the completed assembly.  
         [0029]      FIG. 6  is an exploded view of the implementation shown in  FIG. 5 , more clearly illustrating the upper housing  504  and the opening  506  provided therein. Both upper and lower surfaces of the circuit board  502  are now provided with one-piece, unitary electromagnetic shield structures  508 . Rather than required a top cover for the completed assembly, the circuit board  502  is secured to the upper housing  506  by attaching directly to the electromagnetic shield  508  on the upper surface of the circuit board  502 . This technique provides a cover by virtue of the electromagnetic shield itself, while improving the mechanical mounting of the circuit board  502  by eliminating the previously-known sandwich structure and moving the mechanical mounting points toward the center of the circuit board.  
         [0030]     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.  
         [0031]     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.