Abstract:
Provided for an embodiment is a support member for a cryogenic cooler&#39;s expander. The support member provides stiffness to the expander to reduce movements at the expander&#39;s distal end and may increase the natural frequency of the expander. The support member may increase the natural frequency of the expander at least about two times in the bending and/or twisting sense. The bending natural frequency of the expander and support sub-assembly may be at least about two times greater or lower than the natural frequency of the electrical wires that connect an infrared sensor to a control processing unit to reduce the maximum stress applied to the electrical wires during use. In another embodiment, additional or redundant electrical pathways are provided for connections between the infrared sensor and the CPU. Furthermore, shock absorber and/or shock diverters are provided on rigid pins that connect the electrical wires to the CPU.

Description:
FIELD OF THE INVENTION 
     This invention generally relates to integrated detector cooler assemblies (IDCA) and more specifically to IDCA modified to withstand external loads applied to the IDCA or the cryogenic coolers/infrared sensors that incorporate the inventive IDCA. 
     BACKGROUND OF THE INVENTION 
     Infrared cameras play an important role in the surveillance, law enforcement and military applications. Infrared cameras provide thermal images of objects in a thermal scene without illumination. Infrared cameras contain infrared (IR) sensors or focal plane array (FPA), which should be cooled to function properly. In some applications, infrared sensors are cooled to cryogenic temperature range. For these applications, a Stirling Cycle Rotary Cooling Engines are used to provide the cooling power. 
     Conventional Stirling engines generally have a compressor and an expander connected to a crank mechanism driven by an electrical motor. The compressor, also known as a pressure wave generator, is attached to the warm end of the expander and delivers acoustic power (compressor PV work) into the expander warm end inlet. Compressor PV work is the integration of the pressure-volume curve over one thermodynamic cycle or one complete revolution of the crank shaft. Compressor PV work has a unit of energy, and when derived over time, it is defined as acoustic power. The expander recovers this work at the cold end by causing the gas to expand and thus absorb heat from external power source such as an IR sensor. The gas expansion is achieved mechanically by placing the expander piston and compression piston at 90° mechanical phase to each other relative to the crank shaft. A working fluid, typically a noble gas, is compressed at the warm end and is expanded at the cold end. At the distal tip of the expander coldwell, when the expander piston is being pulled backward to iso-thermally expand the working gas, heat is absorbed from the load and very low temperatures are achieved due to efficient thermal isolation between the warm and cold end of the expander unit. Temperature can reach down to the cryogenic range, e.g., about 77° K. The IR sensor or FPA is attached to the coldwell to be cooled. A conventional Stirling engine is described in U.S. Pat. Nos. 7,555,908 and 7,587,896 and references cited therein, which are incorporated herein by reference in their entireties. 
     In the field, infrared cameras are often mounted on law enforcement and military vehicles, including helicopter, drones, naval vessels, military all-terrain vehicles, etc. These vehicles often experience shocks, such as recoils from a discharging weapon. For example, recoils from cannon fires can send damaging vibrations to equipments installed in the vicinity. These vibrations can damage the expander in the Stirling engine, and more specifically the weld between the cold finger and the Dewar body, which can cause the loss of vacuum within the Dewar. This load can also apply a bending stress on the cold finger, which can cause a displacement of the IR sensor or the FPA, which is mounted on the distal end of the cold finger. The vibrations can also break the wires that connect the IR sensor to the central processing unit. 
     Hence, there remains a need for rugged integrated IR detector and cooler assembly that can withstand damaging vibrations encountered in the field. 
     SUMMARY OF THE INVENTION 
     Hence, the invention is directed to a ruggedized integrated detector cooler assembly where the expander or expander cylinder is supported to reduce the effects of a load being applied to the integrated assembly during use. The assembly is incorporated into a cooler, preferably a cryogenic cooler and preferably a Stirling engine. The cooler is used to keep an infrared sensor cooled. 
     In one embodiment, the expander is supported by a support member, preferably a support tube. The support member has a larger proximal end and a smaller distal end. In a preferred embodiment, the support member is connected to the expander at the smaller distal end and is connected to the cooler at the larger proximal end. The support member provides additional stiffness to the expander to reduce movements at the distal end of the expander. The support member also increases the natural frequency of the expander. In one embodiment, the support member increases the natural frequency of the expander at least about two (2) times in the bending and/or twisting sense. 
     In another embodiment, the bending natural frequency of the expander and support sub-assembly is at least about two times greater or lower than the natural frequency of the electrical wires that connect the infrared sensor to a control processing unit to reduce the maximum stress applied to the electrical wires. 
     In another embodiment, additional or redundant electrical pathways are provided for the connection between the infrared sensor and the CPU. Furthermore, shock absorber and/or shock diverters are provided on the rigid pins that connect the electrical wires to the CPU. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings, which form a part of the specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views: 
         FIG. 1  is a perspective view of an integrated detector cooler assembly (IDCA) with the outer housing omitted for clarity in accordance with the present invention; 
         FIG. 2  is a cross-sectional view of the IDCA shown in  FIG. 1  with parts of the Dewar assembly shown; 
         FIG. 2A  is an expanded view of element  2 A shown  FIG. 2 ; 
         FIG. 3  is a schematic drawing showing an improved Dewar pin and improved electrical connections between the Dewar pin and the IR sensor; and 
         FIG. 3A  is a schematic drawing showing an alternative electrical connection. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of integrated detector cooler assembly (hereinafter “IDCA”)  1  present invention is illustrated in  FIGS. 1 ,  2  and  2 A. IDCA  1  comprises expander  10  and detector  12 . Expander  10  is supported by and is at least partially covered by expander support tube  14 . Support tube  14  increases the stiffness of expander  10  to reduce the effects of a shock and/or vibrations applied on expander  10  from a load caused, for example, by a firing of a weapon attached to the same platform as IDCA  1  or from an external source impacting expander  10 . An expansion piston and a regenerator matrix within the expansion piston for exchanging heat with the working fluid are provided for reciprocal motions within expander  10 . These two elements are omitted in the figures of the present patent application for clarity. An expansion space is provided between the distal end of the expansion piston and the distal end of expander  10  to provide cooling power to an IR sensor or a FPA (hereinafter “sensor”)  15  mounted within detector  12  of IDCA  1  as described further below. 
     IDCA  1  and a compressor are housed in a housing or outer shell. The compressor and expander  10  are parts of a cooler or a cryogenic cooler, preferably a Stirling engine. The space within the housing, which is shown in part in  FIG. 2 , is a vacuum or partial vacuum space. 
     Preferably, support tube  14  has a hollow truncated conical shape with larger proximal end  16  and smaller distal end  18 , and is connected at proximal end  16  to the interface between Dewar flange  20 , which forms a part of the housing, and expander  10 , as best shown in  FIG. 2 . Preferably, this connection is accomplished by welding, laser welding and preferably by electron beam welding. At distal end  18 , support tube  14  is connected to expander  10  via upper flange  22 . Preferably, upper flange  22  has a star-shaped configuration as best shown in  FIG. 2A . In this embodiment, the connection between upper flange  22  and expander  10  is preferably continuous and the connection between upper flange  22  and support tube  14  is preferably intermittent to minimize the conductive heat transfer through support tube  14 , which thermally connects the warm end of the Stirling engine at or proximate to Dewar flange  20  to expander  10  proximate to the cold expansion space. Such heat transfer is considered a heat leakage or loss and is minimized by the intermittent contacts between upper flange  22  and support tube  14  and by minimizing the thickness of support tube  14 . In an alternative embodiment, the intermittent contacts may be deployed at the interface of upper flange  22  and expander  10 . In yet another embodiment, the intermittent contacts can be deployed at proximal end  16  between support tube  14  and Dewar flange  20 . 
     Another advantage of upper flange  22  is that it is flexible to allow relative movement caused by differential thermal expansion or contraction between expander  10  and support tube  14 . Advantageously, upper flange  22  is positioned substantially orthogonal to expander  10  and to support tube  14  to allow optimal flexibility. Alternatively, upper flange  22  is allowed to slide relative to expander  10  while being connected to support tube  14  or vice versa to handle the thermal expansion or contraction between expander  10  and support tube  14 . 
     Preferably, support tube  14  is made from a strong and light weight material to support expander  10 . Additionally, support tube  14  has a relatively thin thickness in order to further reduce the heat leakage or loss described above. In one embodiment, support tube  14  is made from titanium (Ti) and has a thickness from about 1.5 to about 4 mils, preferably from about 2 mils to about 3 mils, and more preferably about 2.5 mils. Titanium is preferred due to its strength and relatively low conductive thermal conductivity. Other suitable materials also include, but are not limited to, Ti6AL-4V. 
     An advantage of support tube  14  is that it can increase the natural frequency of expander  10 . The natural frequency, f, of a simple mechanical system can be expressed as 
                     f   =       1     2   ⁢   π       ⁢       k   M           ,           (   1   )               
where k is the effective spring constant of the system and M is the mass of the system. Higher natural frequency of a mechanical system, also known as resonance frequency, is beneficial because the vibration caused by a load experienced by expander  10  in the field generally has a low frequency. Moving the natural frequency of expander  10  away from the load vibration reduces the effects or impacts that the load vibration would have on expander  10 . A modal analysis, e.g., finite element analysis (FEA) or finite difference (FD), shows that support tube  14  doubles the natural frequency of expander  10  in a bending mode as well as a twisting mode. A bending mode is a vibration tending to bend expander  10  along arrows A, and a twisting mode is a vibration twisting expander about its longitudinal axis along arrow B. The results of the modal analysis are shown below.
 
                                                               TABLE 1                   Expander 10   Expander 10   Shorter expander           without support   with support   10 with support       Mode of Vibration   tube 14   tube 14   tube 14                                Bending   770   Hz   1550   Hz   1720   Hz       Twisting   3400   Hz   &gt;6000   Hz   5797   Hz       Max displacement   0.0336   inch   0.0050   inch   0.0016   inch       of cold finger 10       at support 22                    
As shown, the natural frequency due to the bending moment can be reduced further by shortening the length of expander  10 . In one example, expander  10  can be shortened from about 0.25 inch to about 0.31 inch. Additionally, the maximum displacement of support plate  24 , where an IR sensor  15  is mounted, is less due to the lower bending natural frequency by a factor of about six (0.0336/0.0050) in the example shown above. This also lowers any load on sensor  15  applied.
 
     Displacement of expander  10  at sensor  15  can also place a load on the electrical wires that connect the sensor  15  mounted on support plate  24  to a control processor unit (CPU) that receives IR information from sensor  15 . As best shown in  FIG. 2 , sensor  15  is mounted on support plate  24  and a plurality of pins  26  is placed around sensor  15 /plate  24  outside the IR radiation shield of detector unit  12 . IR radiation shields are discussed in commonly owned U.S. Pat. No. 6,144,031, which is incorporated herein by reference in its entirety. Pins  26  extend from the Dewar vacuum space  30  inside IDCA  1  to outside of IDCA  1 . The portions of pins  26  within Dewar vacuum space  30  are designated as  26   vacuum . Electrical wires  28  connect each pin  26  to sensor  15 , as best shown in  FIG. 3 , to transmit the IR information received by sensor  15  through pins  26  to the CPU. The present invention further improves the connection of these wires to better withstand the vibrations encountered by IDCA  1  in the field. Such vibrations can sever the wires and cause IDCA  1  to fail to function, and can shear pins  26  which may cause a loss of vacuum inside IDCA  1 . 
     In accordance to another aspect of the present invention, the natural frequency(ies) of wires  28  are designed to be different than that of expander  10 , so that in the event that expander  10  is excited by vibrations proximate to its natural frequency, which would resulted in large physical loads on expander  10 , the impact on wires  28  is minimized. Preferably, the natural frequency of wires  28  is different than both the bending and twisting natural frequencies of expander  10 . More preferably, the natural frequency of wires  28  is at least about 2 times under or above both the bending and twisting natural frequencies of expander  10 . In one preferred embodiment the natural frequency of wires  28  is at least different than the bending natural frequency of expander  10 , or at least about 2 times under or above the bending natural frequency of expander  10 , since the bending natural frequency has a higher tendency to displace sensor  15  and wires  28 . 
     The natural frequency of electrical wires depends on the material, the diameter and the length of the wires. A modal analysis of electrical wires  28  made from a metal alloy, for example Manganin® alloys, which typically comprises 86% copper, 12% manganese, and 2% nickel, was conducted. The results are as follows. 
     
       
         
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                 Expander 
                   
                   
                   
               
               
                   
                   
                   
                 10 Max 
                 Wire 28 
                 Expander 10 
                 Max 
               
               
                   
                   
                   
                 RMS 
                 Natural 
                 Natural 
                 Wire 28 
               
               
                   
                 Wire 28 
                 Wire 28 
                 Movement* 
                 Frequency 
                 Frequency* 
                 Stress‡ 
               
               
                 Case† 
                 Diameter (inch) 
                 Length (inch) 
                 (inch) 
                 (Hz) 
                 (Hz) 
                 (psi) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 0.001 
                 0.25 
                 0.005 
                 805 
                 1570 
                 7010 
               
               
                 2 
                 0.002 
                 0.25 
                 0.005 
                 1650 
                 1570 
                 39,537 
               
               
                 3 
                 0.004 
                 0.25 
                 0.005 
                 1950 
                 1570 
                 12,233 
               
               
                 4 
                 0.001 
                 0.25 
                 0.033 
                 805 
                 780 
                 142,000 
               
               
                   
                 (conventional) 
               
               
                   
               
               
                 *Expander 10&#39;s movements and natural frequencies are similar to those from Table 1. 
               
               
                 †Cases 1-3 include support tube 14 and case 4 does not. 
               
               
                 ‡Measured at connections to pins 26. 
               
             
          
         
       
     
     The results show that in cases  1  and  4  by reducing the movement of expander  10  and by moving the natural frequency of expander  10  with the addition of support tube  14 , the maximum stress encountered by wires  28  reduces from about 142,000 psi to about 7,010 psi for a 20+ folds reduction. The results also show that increasing the diameter of wires may not be as beneficial. A two-fold increase (case 2) actually increases the wire&#39;s natural frequency and matching it with the expander&#39;s natural frequency. A four-fold increase (case 3) brings the wire&#39;s natural frequency above and beyond the expander&#39;s natural frequency and reduces the maximum stress encountered by wires  28 ; however, larger diameter wires would increase conduction heat loss from sensor  15 , which is kept at cryogenic temperatures, through wires  28  and pins  26 . Hence, in a preferred embodiment, the natural frequency of expander  10  is increased by using support tube  14 , while the natural frequency of wires  18  is maintained to be lower than that of expander  10 . 
     In accordance to another aspect of the present invention, the pin portion  26   vacuum , of pins  26  that is within Dewar vacuum space and connected wires  28  is modified as best shown in  FIG. 3 . The vibration load represented as elements  31  generally enters pins  26  where they are connected to base  32  of detector unit  12 . As vibration  31  travels through pins  26 , it may sever the connection with wire  28  at the top of portion  26   vacuum . In this embodiment, a number of shallow slots  34  are carved or formed on the shaft of portion  26   vacuum . In one example, pins  26  have a diameter of about 0.030 inch and slots  34  have depths of about 0.015 inch or less. Slots  34  are preferably filled with an electrically conductive gel or epoxy. Suitable conductive epoxies include, but are not limited to, EPO-TEK-E4110 conductive epoxy for vacuum vessels. 
     Filled slots  34  can block elastic wave front path along pins  26  and may direct these waves, which are caused by vibrations  31 , away from the top of pins  26   vacuum , where wires  28  are attached. Filled slots  34  can also provide vibration damping to absorb some of the vibration. Preferably, slots  34  are arranged in an overlapping fashion along pins  26   vacuum  as shown in  FIG. 3  to eliminate clear, direct path for the elastic wave front to reach the top of pins  26   vacuum . In other words, slots  34  divert at least some of the vibrations away from the connections to wires  28 . Although two slots  34  are shown in  FIG. 3 , any number of slots  34  can be provided. Vibration  31  also travels along pins  26  in the opposite direction, and if necessary or desired filled slots  34  can be provided on the portions of pins  26  that are outside of Dewar vacuum space  30 . 
     In accordance to another aspect of the present invention, looped wires  36  can be employed to provide additional electrical paths, as shown in  FIG. 3 . Unlike wires  28 , which are direct electrical connections between sensor  15  and pins  26 , looped wires  36  are generally longer and loop around pins  26   vacuum  before being attached at the top of pins  26   vacuum . Looped wires  36  provides two additional electrical paths: a path along the entire length of wires  36  shown by arrows C, and a path along part of wires  36  and through slot(s)  34  and along portions of pins  26   vacuum  shown by arrows D. In this embodiment, if wires  28  are severed, wires  36  can still provide electrical connection. Even if both wires  28  and  36  are severed from the top of pins  26   vacuum  electrical connection along arrows D is still available. 
     As As shown, looped wires  36  can wrap around slots  34 , and the latter can be anchors for looped wires  36 . Wires  36  may have at least one loop or one bend to absorb vibration or shock. The electrical conductive gel or epoxy assures an electrical connected at slots  34 . These anchors are located further from the tip of pins  26   vacuum , where the vibration is less and the pin movement is smaller. Furthermore, looped wires  36  are longer than wires  28  and preferably have an amount of slacks in the wires. These slacks can also absorb some of the vibrations without putting stress or strain on wires  36 . Additionally, slacks can be provided in wire  28  as shown as element  29  in  FIG. 3A  to absorb vibration. Preferably, slacks  29  comprise one or more coils as shown. 
     While it is apparent that the illustrative embodiments of the invention disclosed herein fulfill the objectives stated above, it is appreciated that numerous modifications and other embodiments may be devised by those skilled in the art. Therefore, it will be understood that the appended claims are intended to cover all such modifications and embodiments, which would come within the spirit and scope of the present invention.