THERMAL DAMPER FOR INFRARED DETECTOR

Abstract: Method and apparatus for reducing temperature variation in an infrared detector. The apparatus comprises a coldfinger (18) for receiving thermal energy from an infrared detector (12). A thermal damper (22) is also provided for conducting thermal energy from said detector to the coldfinger by one or more thermally conductive paths (24, 26, 28, 30). A detector mount is used for combining the thermal energy flowing through the paths, thereby reducing the temperature variation in the detector.

Description: THERMAL DAMPER FOR ΩIFRARED DETECTOR BaCKGROCND OF IHE INVENTION 1. Field of the Invention This invention relates to the field of infrared detection, and more particularly to a thermal damper for ιru_ι_imizing temperature variation in an infrared detector. 2. Description of Related Art Infrared detectors are often used in conjunction with missiles and night vision systems to sense the presence of electro¬ magnetic radiation having wavelengths of 1-15 ym. Because they are often most sensitive when operating at low temperatures, detectors such as those fabricated frcm irerσjry-caά 'um-telluride generally require a crycengine assembly to produce and maintain the necessary low operating temperature. Such crycengine assemblies are typically used in conjunction with an evacuated dewar in which an infrared detector is placed. The dewar is evacuated to remove gases which would otherwise occupy the region suΩXfunding the detector so that heat loss through convection and conduction is it j-imized. The detector is typically cooled by placing an indented region ("col well") of the dewar in contact with an expansion c-__mber ("coldfinger") of the crycengine assembly. Alternatively, the coldfinger of the crycengine assembly is used as the coldwell of the dewar to enable the detector to be mounted on the cryoengine coldfinger. The cryoengine assembly produces cooling by sequential ccrnpressicn of a working fluid such as helium, removal of the heat of compression of the fluid, and subsequent expansion of the working fluid in the coldfinger. Because the detector is in thermal cxarinunication with the coldfinger, the expansion of the working fluid causes heat to be withdrawn frcm the detector. Although the necessary operating temperatures can be achieved by the devices generally described above, the cyclical nature of the expansion of the working fluid would often produce a cyclical variation in the detector operating temperature. Because infrared detectors are often -temperature sensitive, this cyclical variation in operating temperature would produce a corresponding variation in the output signal of the detector, Thermal masses or resistances located between the detector and the coldfinger were often employed to minimize this temperature variation. While such solutions were εxirewhat effective in reducing temperature variation, they would often increase the time required to initially cool the detector frcm ambient to the necessary operating temperature. In addition, the use of a thermal resistor would often hinder the flew of thermal energy between the coldfinger and the detector, which would in turn generally require the use of a cryoengine assembly having a greater cooling capacity than would otherwise be necessary. SOWIARY OF THE INVENTION A method and apparatus for reducing temperature variation in an infrared detector is disclosed. The apparatus includes a coldfinger for receiving thermal energy frcm a detector and a thermal damper for conducting -thermal energy frcm the -detector to the coldfinger by way of one or irore thermally conductive paths. The paths are preferably solid studs whose lengths and materials are chosen so as to m_Lnimize temperature variation in the detector. BRIEF PESO-OPTION OF THE DRAWINGS Various advantages of the present invention will beccme apparent to one skilled in the art upon reading the following specification and reference to the drawings in which: FIG. 1 is a cross-sectional view of an infrared detector assembly using the thermal d__πper according to the present invention; FIG. 2 is a cross-sectional view of the infrared detector assembly taken along line 2-2 of FIG. 1; and FIG. 3 is an alternative eirbodiment of the thermal damper according to the present invention. DESCRIPTION OF THE PREFERRED BOOIMEiTIS Referring to FIG. 1, to detect incαrάng infrared signals, an infrared detector assembly 10 is provided having an infrared detector 12. The infrared detector 12 is mounted in a dewar 14 which is evacuated to remove gases which may otherwise increase the flew of thermal energy frcm the environment to the detector 12. Tto support the infrared detector 12, a detector irount 16 is located within the assembly 10 and is positioned to allow infrared signals entering the dewar 14 to be received by the detector 12. While the detector mount 16 may be fabricated frcm copper, it is to be understood that other suitable materials may be used. Receiving thermal energy frcm the dewar 14 and the infrared detector 12 is a coldfinger 18, which is located within the coldwell 20 of the dewar 14. Thermal energy is drawn frcm the detector 12 by the expansion of a working fluid inside the coldfinger 18. By cooling the detector 12 in this manner, the detector 12 is able to operate at a temperature where it is most sensitive. While a coldfinger 18 is vised to receive thermal energy frcm the detector 12, it is to be understood that other means for receiving thermal energy frcm the detector 12 may be used. To reduce the temperature variations in the infrared detector 12 due to the cyclical operation of the expander, a thermal damper 22 is provided which allcws thermal energy to flow between the coldfinger 18 and the detector 12. The thermal damper 22 includes two studs 24 and 26, though it is to be understood that a different number of studs may be used as discussed subsequently. The studs 24 and 26 are disposed between the detector mount 16 and two bosses 28 and 30 on the cold tip of the coldfinger 18. The bosses 28 and 30 are used to cαtplete the paths of thermal energy flowing frcm the detector 12 through the studs 24 and 26 to the coldfinger 18. While the studs 24 and 26 may be composed of stainless steel or titanium, it is to be understood that other suitable materials may be used. The tarperature of the studs 24 and 26 may be shewn to vary approximately according to the following equation: -£ ττfCp/k COS(2τrfτ-J-ι / τrfCp/k) Where: T = temperature variation at the end of the stud adjacent to the detector mount T. = temperature variation at the end of the stud adjacent to the coldfinger I = length of stud f = expander cyclical frequency (Hz) Cp = specific heat of stud k = thermal conductivity of stud τ = time Accordingly, the construction of the studs 24 and 26 may be chosen to optimize the above equation. The lengths and composition of the studs 24 and 26 are selected to achieve the necessary detector operating temperature and optimum temperature variation. By appropriate selection of these parameters, the phase angles of the teπperature waves flowing through the studs 24 and 26 may be shifted with respect to each other. By shifting the phase angle of the temperature wave through stud 26 such that it becomes out of phase with respect to the wave flowing through stud 24, the fluctuations in temperature of the studs 24 and 26 effectively offset each other when the thermal energy flowing through the studs 24 and 26 is ccmbined at the detector mount 16. The phase lag required to miiiimize temperature variation in the detector 12 is somewhat less than 180° due to the damping factor -j-/τrfCto/k e ^ in the equation, which makes the amplitude of the temperature wave in the longer of two studs smaller than the other. A 180° degree phase shift will continue to be used in the discussion in the interest of simplicity. The operation of the thermal damper 22 may be explained by means of a non-liirdting exairple. Assuming that the temperature of the cold tip of the coldfinger 18 has a fluctuation of ±1° K, the thermal damper 22 can be designed so that the airplitude of the temperature wave flcwing through the stud 26 is at its iraximum (+1° K) while the airplitude of the temperature wave flowing through stud 24 is at its ir mum (-1° K) when the waves act upon the detector mount 16. If the materials for both of the studs are the same, then this 180° phase shift can be acccπplished by making stud 24 one-half wavelength longer than stud 26. Assuming the studs 24 and 26 are made frcm grade 304 stainless steel, one-half wavelength corresponds to a stud length of £= ιτk/fCp or .044 inches. Because it is substantially independent of the teirperature variation of the detector 12, the cross-sectional areas of the studs 24 and 26 are selected to meet the minimum requirements for structural integrity, conductivity, and other factors which depend on the particular application and which do not directly influence temperature variation. In one particular configuration, for exarrple, the stud 24 is 0.20 inches long, 0.10 inches in diameter and constructed of 304 stainless steel, whereas the stud 26 is .244 inches long, 0.10 inches in diameter and is also constructed of 304 stainless steel. In an alternative preferred e bod-iirent of the present invention as shown in FIG. 3, the cold tip of the coldfinger 32 has a planar surface 36 and the opposing surface of the detector mount 34 has a nonplanar surface 38. The nonplanar surface 38 serves to eliminate the need for the bosses 28 and 30 of FIGS. 1 and 2. The surfaces 36 and 38 are adapted to locate two studs 40 and 42 having the requisite length and fabricated from appropriate materials so as to create an offsetting phase shift in the temperature waves 44 and 46 flowing therethrough. The temperature wave 44 flowing through the stud 40 therefore combines with the temperature wave 46 flowing through the stud 42 in the detector mount 34 thereby minimizing the temperature variation in the detector 12. In practicing the method of the present invention, a source of thermal energy such as coldfinger 18 is provided. The studs 24 and 26 are located between the coldfinger 18 and the detector mount 16. The studs 24 and 26 divide the flow of thermal energy propagating between the detector mount 16 and the coldfinger 18 into two paths having two corresponding temperature waves 44 and 46. The phase shift between the temperature waves 44 and 46 is produced by the appropriate selection of the lengths and compositions of the studs 24 and 26 as discussed above. The temperature waves 44 and 46 are then recσmbined at the detector mount 16 causing the temperature waves 44 and 46 to offset one another. By offsetting the temperature waves 44 and 46 in this manner, the temperature variation of the detector 12 is reduced. It will be apparent f cm the foregoing that more than two studs can be used in the thermal damper of the present invention. By increasing the number of studs, the temperature waves of the thermal energy flowing between the detector mount 16 and the coldfinger 18 can be offset more effectively. In addition, it is also apparent that a single stud may be used in which the length, specific heat, and thermal conductivity of the stud are so chosen as to produce the requisite temperature variation by minimizing the e~ A * π ^ ( - term in the above equation. In designing the thermal damper with a single stud, the heat produced by the detector 12 (T-£) and expander cylical frequency (f) are generally set parameters, whereas the length (Jl), specific heat (C p ) and thermal conductivity (k) of the stud are variables. Assuming that one chooses a material such as 304 stainless steel (thereby defining C and k), and assuming an expander frequency of 15 Hz at a nominal operating temperature of 80° K, the following performance figures can be calculated: Length Damper Factor Temperature Wave (inches) Material β -^ntCp/k Reduction Factor (T^T ) 0.10 304 CRES 8.59 x 10-4 1.164 x 103 0.20 304 CRES 7.38 x 10~ 7 1.355 x lθ6 It should be understood that while the invention was described in connection with a particular example thereof, other modifications will become apparent to those skilled in the art after a study of the specification, drawings and following claims.

Claims: WHAT IS CLAIMED IS 1. A method for reducing temperature variation in an infrared detector, said method comprising the steps of: providing a s.ource of thermal energy operable to cool said detector, the temperature of .said source of thermal energy varying according ' to a pre-_ete_-rd_ned frequency; dividing the flew of thermal energy between said source and said detector into at least two paths each having a temperature wave; producing a phase shift in the temperature wave of the thermal energy flowing through at least one of said paths; and r__x-ctτbining the flew of -thermal energy through each of said paths to reduce the temperature variation in said detector. 2. The method of Claim 1, wherein at least one of said paths is through a material operable to cause a phase shift in the temperature wave of the thermal energy flowing through said path. 3. The method of Claim 1, wherein the flow of thermal energy between said source and said detector is divided by placing at least two studs of different lengths between said source and said detector. 4. The method of Claim 3, wherein at least one stud is designed to obtain a predetermined reduction in the temperature variation of said detector by selecting the length and material from which said one stud is composed according to the following equation: where T = temperature variation at the end of said stud closest to said detector T. = temperature variation at the end of said stud closest to said source of thermal energy £ = length of stud 4. The method of Claim 3, wherein at least one stud is designed to obtain a predetermined reduction in the temperature variation of said detector by selecting the length and material frcm which said one stud is composed according to the following equation: - irfCpΛ COS ( 2πfτ-J-/τrfCpA) where: T 0 = temperature variation at the end of said stud closet to said detector Ti = temperature variation at the end of said stud closet to said sink of thermal energy I = length of stud f = predetermined frequency (Hz) Cp = specific heat of said stud k = thermal conductivity of said stud τ = time 5. The method of Claim 1, wherein: said sink of thermal energy is a coldfinger connected to an expander operating at a given frequency. 6. A method for reducing temperature variation in an infrared detector, said method comprising the steps of: providing a sink of thermal energy operable to cool said detector, the temperature of said sink of thermal energy varying according to a piredetermined frequency; locating a thermal conductor between said sink and said detector, said thermal conductor being designed to obtain a predeter¬ mined reduction in the temperature variation of said detector by selecting the length and material from which at least a portion of said thermal conductor is composed of according to the following equation: -A/πfCr k ^ ___ ^ T 0 =Tie COS(2πfτ-£/τrfCp/k) where: T 0 = temperature variation at the end of said thermal conductor closet to said detector T^ = temperature variation at the end of said thermal conductor closet to said sink of thermal energy I = length of said thermal conductor f = said predetermined frequency (Hz) C = specific heat of said thermal conductor k = thermal conductivity of said thermal conductor τ = time; and conducting thermal energy from said detector to said sink through said thermal conductor. 7. The method of Claim 6, wherein said thermal conductor comprises a single stud. 8. The method of Claim 6, wherein said cyclical sink of thermal energy is a coldfinger of a cryoengine assembly. 9. The method of Claim 8, wherein said thermal conductor comprises at least two studs thermally ccrrmunicating with both said coldfinger and said detector and creating at least two thermal paths between said detector and said coldfinger. 10. The method of Claim 9, wherein the thermal energy is conducted by said studs according to at least two generally sinusoidal temperature waves, at least two of said studs having lengths and composed frcm materials operable to shif the pihase of at least one of said temperature waves by a predetermined angle with respect to at least one other of said teπperature waves. 11. The method of claim 9, wherein said studs are composed of the same thermally conductive material and have different lengths. 12. An apparatus for reducing the temperature variation in an infrared detector, said apparatus comprising: means for receiving thermal energy frcm said detector; and means for defining at least two different -thermal paths frcm said source to said detector, each of said paths conducting thermal energy according to a generally sinusoidal temperature wave, said paths constructed of a material and of a length chosen such that the temperature wave through one of said paths is shifted with respect to the temperature wave in at least one other path whereby the teπperature variation at the detector is reduced. 13. The apparatus of Claim 12, wherein said means for defining at least two different thermal paths comprises at least two studs disposed between said means for receiving thermal energy and said detector. 14. The apparatus of Claim 13, wherein at least two of said studs having lengths and composed of materials operable to allow said two studs to conduct thermal energy in teπperature waves having different phase angles. 15. The apparatus of Claim 12, wherein said apparatus further includes means for ccmbining at least two teπperature waves, said means for combining thermally cc-Tiπunicating with said detector and operable to reduce temperature variation of said detector when at least two of .said temperature waves are offsetting. 16. The apparatus of Claim 13, wherein said means for receiving thermal energy is a coldfinger of a cryoengine assembly. 17. The -apparatus of Claim 16, wherein said means for defining at least two different thermal paths includes a plurality of bosses disposed between said coldfinger and said studs. 18. The apparatus of Claim 17, wherein said studs are comprised of materials selected frcm the group consisting of stainless steel and titaniim. 19. The apparatus of Claim 18, wherein said means for combining comprises a detector irount which is operable to provide mechanical support to said detector. 20. The apparatus of Claim 19, wherein said studs longitudinally extend between said bosses and said detector mount.

Inventors: BULLER JOSEPH STANLEY, BULLER, JOSEPH, STANLEY
Applicants: SANTA BARBARA RES CENTER, SANTA BARBARA RESEARCH CENTER
Application year: 1986
Main classification: F25B19
IPC codes: F25D3, F25D19, G01J1, G01J5