Ruggedized dewar unit for integrated Dewar detector assembly

Abstract

An Integrated Dewar Detector Assembly (IDDA) is presented. The IDDA comprises: a cold finger base; an elongated Dewar envelope having a proximal end associated with the cold finger base and a distal end comprising an optical window; an elongated tubular cold finger located inside said elongated Dewar envelope and having a proximal end at the cold finger base and a distal end for carrying a detector so as to expose the detector to incoming radiation through said optical window; an internal front support member extending from an inner surface of the Dewar envelope at its distal end to the distal end of the cold finger; and at least one wideband dynamic vibration absorber assembly located outside the Dewar envelope and attached to at least one location on an exterior surface of the Dewar envelope, said dynamic vibration absorber thereby attenuating vibration of the cold finger and the detector.

Claims

1. An Integrated Dewar Detector Assembly (IDDA), comprising: a cold finger base; an elongated Dewar envelope having a proximal end associated with the cold finger base and a distal end comprising an optical window; an elongated tubular cold finger located inside said elongated Dewar envelope and having a proximal end at the cold finger base and a distal end for carrying a detector so as to expose the detector to incoming radiation through said optical window; an internal front support member extending from an inner surface of the elongated Dewar envelope at the distal end thereof to the distal end of the cold finger, said internal front support member being configured and operable for mechanically coupling between said cold finger and said elongated Dewar envelope; and at least one wideband dynamic vibration absorber assembly located outside the elongated Dewar envelope and attached to at least one location on an exterior surface of the elongated Dewar envelope such that said wideband dynamic vibration absorber assembly is configured and operable to be mechanically coupled to said elongated Dewar envelope, said at least one dynamic vibration absorber assembly thereby attenuating vibration of the elongated tubular cold finger and the detector.

2. The IDDA according to claim 1, wherein the at least one wideband dynamic absorber is configured as a damped mass-spring mechanical system.

3. The IDDA according to claim 1, wherein the at least one wideband dynamic absorber includes a grommet that is fastened to a mounting stud and is attached to the exterior surface of the elongated Dewar envelope by said mounting stud, and a tubular inertial member that is attached to the grommet and coaxially envelops the grommet.

4. The IDDA according to claim 3, wherein said tubular inertial member is made of a metal material composition.

5. The IDDA according to claim 3, wherein the grommet is configured as a low profile viscoelastic element being made of a damped elastomer or a wire-mesh bushing.

6. The IDDA according to claim 4, wherein the grommet is configured as a low profile viscoelastic element being made of a damped elastomer or a wire-mesh bushing.

7. The IDDA according to claim 2, wherein the wideband dynamic absorber includes a grommet that is fastened to a mounting stud and is attached to the exterior surface of the elongated Dewar envelope by said mounting stud, and a tubular inertial member that is attached to the grommet and coaxially envelops the grommet.

8. The IDDA according to claim 7, wherein said tubular inertial member is made of a metal material composition.

9. The IDDA according to claim 8, wherein the grommet is configured as a low profile viscoelastic element being made of a damped elastomer or a wire-mesh bushing.

10. The IDDA according to claim 3, wherein the tubular inertial member has a tooth feature that tightly fits a slot feature of said grommet, and a central portion of the grommet is squeezed between two washers using a fastening assembly that is attached to said mounting stud.

11. The IDDA according to claim 4, wherein the tubular inertial member has a tooth feature that tightly fits a slot feature of said grommet, and a central portion of the grommet is squeezed between two washers using a fastening assembly that is attached to said mounting stud.

12. The IDDA according to claim 5, wherein the tubular inertial member has a tooth feature that tightly fits a slot feature of said grommet, and a central portion of the grommet is squeezed between two washers using a fastening assembly that is attached to said mounting stud.

13. The IDDA according to claim 3, wherein elastic and damping properties of the at least one wideband dynamic absorber are tunable by varying a mass of the tubular inertial member and manipulating attachment of the grommet to the mounting stud.

14. The IDDA according to claim 4, wherein elastic and damping properties of the at least one wideband dynamic absorber are tunable by varying a mass of the tubular inertial member and manipulating attachment of the grommet to the mounting stud.

15. The IDDA according to claim 5, wherein elastic and damping properties of the at least one wideband dynamic absorber are tunable by varying a mass of the tubular inertial member and manipulating attachment of the grommet to the mounting stud.

16. The IDDA according to claim 1, wherein the at least one wideband dynamic absorber includes a highly damped ring-like elastomer coaxially enveloping at least part of a circumferential portion of an outer surface of the elongated Dewar envelope, an inertial ring-like member enveloping the outer surface of said highly damped ring-like elastomer, and a fastening assembly for attaching to the outer surface of the elongated Dewar envelope.

17. The IDDA according to claim 16, wherein the inertial ring-like member is formed of two parts configured with a slot therebetween, inside which said highly damped ring-like elastomer is located.

18. The IDDA according to claim 16, wherein elastic and damping properties of the at least one wideband dynamic absorber are tunable by varying a mass of the highly damped ring-like elastomer and operating the fastening assembly.

19. The IDDA according to claim 1, wherein the at least one wideband dynamic absorber includes a proximal electronics unit supported by a mechanical holder that is mounted on an outer surface of the elongated Dewar envelope using a damped elastomer or wire mesh ring coaxially enveloping at least part of a circumferential portion of the outer surface of the elongated Dewar envelope.

20. A Dewar assembly for use in an Integrated Dewar Detector Assembly (IDDA), the Dewar assembly comprising: an elongated Dewar envelope having a proximal end associated with a cold finger base of the IDDA and a distal end comprising an optical window; an elongated cold finger located inside the elongated Dewar envelope and having a proximal end associated with the cold finger base of the IDDA and a distal end configured for carrying a detector for exposing the detector to incoming radiation through said optical window; a front support member extending from an inner surface of the Dewar envelope at the distal end thereof and supporting a distal portion of the cold finger; and at least one wideband dynamic vibration absorber assembly located outside the elongated Dewar envelope and attached to at least one location on an exterior surface of the elongated Dewar envelope, said at least one dynamic vibration absorber thereby suppressing vibration of the elongated cold finger and the associated detector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In order to better understand the subject matter that is disclosed herein and to exemplify how it may be carried out in practice, embodiments will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which:

(2) FIG. 1 is a schematic diagram of the state of the art IDDA with an unsupported cold finger.

(3) FIG. 2 is a schematic diagram of the state of the art IDDA with a supported cold finger.

(4) FIGS. 3A to 3C exemplify typical dynamic responses of the IDDA of FIG. 1 featuring an unsupported cold finger.

(5) FIGS. 4A to 4C exemplify typical dynamic responses of the IDDA of FIG. 2 featuring a supported cold finger.

(6) FIGS. 5A and 5B schematically illustrate an IDDA of an embodiment of the present invention, in which a wideband dynamic absorber is mounted upon the distal end of the evacuated Dewar envelope.

(7) FIGS. 6A and 6B show a specific example of the support assembly of an embodiment of the present invention;

(8) FIGS. 7 and 8 show two more possible embodiments of the configuration of the wideband dynamic absorber.

(9) FIG. 9 illustrates an experimental setup of actual IDDA with mounted wideband dynamic absorber configured as the example of FIGS. 6A-6B;

(10) FIGS. 10A to 10F show the experimental results for the typical modification of the absolute transmissibility of the FPA as a result of mounting wideband dynamic absorber, where FIGS. 10A and 10B compare the absolute transmissibilities with and without the optimized wideband dynamic absorber mounted on the Dewar envelope, and FIGS. 10C-10D and 10E-10F compare PSD of the acceleration and relative deflection of the FPA and Dewar envelope before and after the mounting of the dynamic absorber.

(11) FIGS. 11A and 11B compares the typical time histories of the FPA acceleration (FIG. 11A) and relative deflection (FIG. 11B) subjected to a half-sine shock, with and without wideband dynamic absorber.

DETAILED DESCRIPTION OF EMBODIMENTS

(12) FIG. 1 shows the schematic diagram of the state of the art IDDA 1 mounted on a mechanical interface 2. The IDDA 1 includes a tubular cold finger 6 having a proximal (warm) end 6A and a distal (cold) end 6B, and extending from a cold finger base 3 to a cold finger cup 8, where an FPA 10 is mounted. The cold finger 6 with the cup 8 and FPA 10 are located inside a tubular evacuated Dewar envelope 4 extending from the cold finger base 3 wherein high vacuum environment is provided for the cryogenically cooled portions of the IDDA. Incident radiation (infrared radiation) arrives to the FPA 10 through an infrared transparent window 12 which is mounted upon a distal end of the tubular evacuated Dewar envelope.

(13) FIG. 2 shows the schematic diagram of the state of the art IDDA configured generally similar to the IDDA of FIG. 1, but further including a disk-like support member 14. The latter serves as the low heat conductive front support member with purpose of additional support of the cold finger distal (cold) end 6B from the evacuated Dewar envelope 4. The support member 14 has a central hole 14A which is tightly matched and bonded to the distal (cold) end 6B of the cold finger 6, and the circumferential surface 14B coaxial with the central hole which is tightly matched and bonded to the interior surface 4 of the tubular Dewar envelope 4.

(14) Reference is made to FIGS. 3A-3C and 4A-4C showing, for the reference, typical dynamic responses of the IDDA of FIG. 1 featuring unsupported cold finger 6 (FIGS. 3A-3C) and those of the IDDA of FIG. 2 featuring cold finger 6 supported by the support member 14, under typical random vibration test per MIL-STD 810F having uniform power spectral density (PSD) over the frequency range 10-2000 Hz, the overall level is 5 g rms.

(15) The function

(16) $X_0\left(j\right)={}_{-}^{}{x}_0\left(t\right)e^{-jt}\mathrm{dt}$
(FIGS. 1 and 2) is the complex Fourier transform of the base vibratory motion given by the time function x.sub.0(t), where the angular frequency is w and j={square root over (1)} is the complex unity.

(17) Similarly,

(18) $X_{1,2}\left(j,\right)={}_{-}^{}{x}_{1,2}\left(t\right)e^{-jt}\mathrm{dt}$
are the complex Fourier transforms of the cold finger tip and Dewar envelope vibratory motion, respectively. The appropriate power spectral densities of relative displacements an absolute accelerations are denoted as X0() X1(w) and X2(w) and A0(), A1() and A2(), respectively.

(19) In the experiments, the results of which are shown in FIGS. 3A-3C and 4A-4C, the IDDA with and without support member 14, was mounted in the fixture of vibration exciter, and accelerometers were used for monitoring the motion of the FPA 10 and Dewar envelope 4.

(20) In particular, in FIG. 3A, three graphs are shown, A0(w), A1(w) and A2(w) corresponding to acceleration PSD of, respectively, the base 3, FPA 10 and Dewar envelope 4. The dynamic response A1(w) of the cold finger 6 (and thus FPA 10 fixed thereon) shows a well pronounced resonant amplification at approximately 800 Hz, while graph A2(w) shows that the resonance of the Dewar envelope 4 occurs well above the 2 kHz margin.

(21) FIG. 3B shows the spectra of modules of absolute transmissibility of the FPA 10 and Dewar envelope 4, T1ref(w) and T2ref(w), respectively. The frequency range in this figure is extended to 3 kHz in order to demonstrate that the Dewar envelope resonance occurs at approximately 2100 Hz. As shown, the resonant amplification of the Dewar envelope is much higher, namely 165, as compared with 25 of the cold finger/FPA.

(22) FIG. 3C shows superimposed PSD curves of relative deflection for the FPA and for the Dewar envelope, X1(w) and X2(w), respectively, which were evaluated indirectly using complex form of the absolute transmissibility:

(23) $X1\left(\right)=\frac{{.Math.T1\left(\right)-1.Math.}^2}{{}^4}A\left(\right).$

(24) It is evident from FIGS. 3A-3C that the cold finger 6 with FPA 10 and the Dewar envelope 4 behave very similarly to a single degree-of-freedom (DOF) systems having very low damping. From single DOF curve-fitting, the modal frequencies and damping ratios are respectively 800 Hz and 2% for the cold finger and 2150 Hz and 0.3% for the Dewar envelope. From the experimental test results, the overall (rms) acceleration and relative deflection levels are, respectively, 20 g rms and 7.8 m rms for the cold finger tip, and 11 g rms and 0.6 m rms for the Dewar envelope.

(25) FIGS. 4A-4C show similar graphs for the IDDA of FIG. 2. More specifically, FIG. 4A shows superimposed acceleration PSDs, A0(), A1() and A2(). FIG. 4B shows the superimposed moduli of absolute transmissibility for the cold finger tip and for the Dewar envelope, T1ref(w) and T2ref(w), respectively; and FIG. 4C shows superimposed PSDs of relative deflection for the FPA and the Dewar envelope, X1(w) and X2(w), respectively.

(26) FIGS. 4A-4C show that the combined system of FIG. 2, formed by the cold finger 6 with FPA 10 and the front support member 14 and Dewar envelope 4, behaves as a lightly damped two DOF dynamic system. The first observed resonant frequency is essentially higher than that of the unsupported FPA, because of the added stiffness; the penalty, however, is the higher amplification at resonance. This is because adding stiffness without affecting damping results in a reduction of the effective damping ratio, which manifests itself in the form of elevated resonant amplification. The overall acceleration and relative deflection of the FPA are now 84 g rms and 7 m rms, respectively.

(27) Comparing FIGS. 3A-3C and FIG. 4A-4C, the resonant frequency of the combined system of FIG. 2 is still within the range of the vibration profile and the resonant amplification is extremely high, because there is insufficient stiffness and a lack of damping in the Dewar envelope and support member. The combination of these unfavorable factors results in only a minor attenuation of 12% in the relative deflection of the FPA, and a massive 4-fold amplification of the acceleration response. An additional penalty is a 40 mW increase in the Dewar heat load at 77K@23 C.

(28) Reference is now made to FIGS. 5A and 5B illustrating an IDDA, generally designated 10, configured according to at least one embodiment of the invention. To facilitate understanding, the same reference numbers are used to identify components common in all the figures. Thus, as shown in FIG. 5A, the IDDA 10 includes a tubular evacuated Dewar 11 including a Dewar envelope 4 extending from a cold finger base 3 and ending with an infrared transparent window 12, and a tubular cold finger 6 inside the Dewar envelope 4. The cold finger also extends from the cold finger base 3, and may have a cold finger cup 8 on its distal (cold) end 6B where an FPA 10 is mounted. The cold end 6B is supported by a front support member 14 with a central hole 14A tightly matched and bonded to the cold finger 6, and a circumferential surface 14B surrounding the central hole and bonded to an inner surface 4 of the Dewar envelope 4.

(29) According to at least one embodiment of the invention, the IDDA 10 includes a wideband dynamic absorber assembly 18 located outside the Dewar envelope 4 and coupled to an external surface 4 of the envelope 4. As illustrated in FIG. 5B, the wideband dynamic absorber assembly 18 may be represented schematically as a heavily damped single degree of freedom system having properties M, K, B (mass, spring rate and damping, respectively).

(30) In this configuration, the support member 14 additionally serves as a mechanical coupler which couples the cold finger 6 to the Dewar envelope, and the dynamic vibration absorber assembly 18, in turn, which is coupled to the exterior of the Dewar envelope, operates to suppress vibration of the cold finger. This provides improved image quality and increased durability of the cold finger.

(31) FIGS. 6A and 6B show a specific but not limiting example of the configuration of Dewar unit of at least one embodiment of the invention with the wideband dynamic absorber assembly 18 mounted on the evacuated Dewar envelope 4. In particular, the wideband dynamic absorber assembly 18 includes a low profile viscoelastic grommet 20, which is coaxially enveloped by the inertial heavy metal tubular member 22. The circular tooth feature of the tubular member 22 tightly fits a circular slot feature of the grommet 20, the central portion of which is squeezed between two flat washers 24A and 24B using a fastening assembly, which in this example is formed by a nut 26 and a threaded stud 28 protruding the central hole of the grommet 20, wherein a free end of the threaded stud 28 is attached externally to the Dewar envelope 4 using a mounting stud 29. The low profile grommet 20 is made of highly damped elastomer (like ISODAMP or VersaDamp produced of E-A-R Specialty Composites) or wire-mesh (like ShockTech wire-mesh bushings) having persistent mechanical properties over the wide range of temperatures and time. FIG. 6B exemplifies the preferable mounting position of the wideband dynamic absorber assembly 18. It should however be noted that the wideband absorber 18 may be mounted in any convenient and available position. The tuning of the wideband dynamic absorber 18 is possible by varying the mass of the ring 22 and squeezing the grommet 20 resulting in changing its elastic and damping properties. After final tuning, the nut 26 is secured from loosening.

(32) FIG. 7 shows another specific but not limiting example of the configuration of the Dewar unit of at least one embodiment of the invention. The wideband dynamic absorber assembly 18 includes a highly damped elastomer (like ISODAMP or VersaDamp produced of E-A-R Specialty Composites) or wire mesh ring (like this produced by Kinetic Structures) 20 coaxially enveloping the circumferential portion of the outer surface 4 of the Dewar envelope 4, and a heavy metal tubular member (inertial ring) 22 enveloping the outer surface of the ring 20. In this example, the tubular member 22 is formed of two parts 22A and 22B configured with a slot 30 between them, inside which the compliant ring 20 is located. Tuning of the wideband dynamic absorber 18 is possible by varying the mass of the ring 20 and operating a fastening assembly, i.e. tightening screws 32A and 32B, resulting in changing the elastic and damping properties of the ring 20. After final tuning, the screws 32A and 32B are secured from loosening.

(33) FIG. 8 illustrates yet further not limiting example of the Dewar unit 11 of at least one embodiment of the invention. In this example, the wideband dynamic absorber assembly 18 includes a proximal electronics unit 34 supported by a mechanical holder 22 which is mounted upon the outer surface 4 of the evacuated Dewar envelope 4 using the highly damped elastomer (like ISODAMP or VersaDamp produced of E-A-R Specialty Composites) or wire mesh ring (like this produced by Kinetic Structures) 20 coaxially enveloping the circumferential portion of the Dewar envelope surface 4. Fine squeezing of the rings provides for optimal tuning of the wideband dynamic absorber assembly 18.

(34) FIG. 9 portrays the experimental setup in which the wideband dynamic absorber assembly configured according to the example of FIGS. 6A-6B is mounted upon the actual IDDA, which is mounted upon the shaker table and thus subjected to the vibration testing, using the above mentioned random vibration profile 5 g rms with uniform PSD over the frequency range 10-2000 Hz. The mass of the dynamic absorber assembly is 25 gm, the tubular member is made of Tungsten for compactness; optimized resonant frequency and damping ratio are 1600 Hz and 15%, respectively. Such a dynamic absorber adds only 2% to the weight of the entire IDDA.

(35) The absolute displacements of the modified system in selected locations 1 and 2 are determined by a superposition of the base induced motion and the response to the stimulus produced by the motion of the dynamic absorber relative to the point of its attachment to the Dewar envelope, the complex Fourier transform of which looks like (K+jB)[X.sub.2(j)X.sub.3(j)]. Equations of motion in complex frequency domain may be written as follows:
X.sub.1(j)=T.sub.1(j)A.sub.0(j)H.sub.21(j)(K+jB)[X.sub.2(j)X.sub.3(j)]
X.sub.2(j)=T.sub.2(j)A.sub.0(j)H.sub.22(j)(K+jB)[X.sub.2(j)X.sub.3(j)]
X.sub.3(j)=T(j)X.sub.2(j)
wherein, T.sub.1,2(j) are the absolute complex transmissibilities of the FPA and Dewar envelope, H.sub.22(j) is the local complex receptance of the Dewar envelope, H.sub.12(j) is the complex receptance from the Dewar envelope to FPA and

(36) $T\left(j\right)=\frac{K+jB}{-M{}^2+K+jB}$
is the single-mode approximation of the complex absolute transmissibility of the lumped wideband dynamic absorber expressed in terms of its mass M, spring constant K and damping factor B.

(37) The solution to the above equation produces a set of two complex transmissibilities for the combined system

(38) $T_2\left(j\right)=\frac{X_2\left(j\right)}{A_0\left(j\right)}=\frac{T_2\left(j\right)}{1-M{}^2{H}_{22}\left(j\right)T\left(j\right)};$$T_1\left(j\right)=\frac{X_1\left(j\right)}{A_0\left(j\right)}=T_1\left(j\right)+M{}^2{H}_{21}\left(j\right)T\left(j\right)H_{22}\left(j\right)$

(39) Calculation of the power spectral densities and the root mean square (rms) values of the relative deflection and acceleration of the FPA in such a combined system when subjected to a vibration profile given by the acceleration PSD A.sub.0() is as follows:

(40) ${\stackrel{\mathrm{.Math.}}{X}}_1\left(\right)={.Math.T_1\left(j\right).Math.}^2{A}_0\left(\right);X_1\left(\right)={.Math.T_1\left(j\right)-1.Math.}^2{A\left(\right)}_0{}^{-4};$${}_A=\sqrt{\frac{1}{2}{}_0^{}{\stackrel{\mathrm{.Math.}}{X}}_1\left(\right)d;}{}_X=\sqrt{\frac{1}{2}{}_0^{}{X}_1\left(\right)d}$

(41) Based on the above equation for the absolute displacements, the dynamic properties of the combined system depend on those of the reference system and the properties of the dynamic absorber, M, K and B. They may be modified as to minimize the rms displacement of FPA, .sub.X. The optimization procedure with M=0.025 kg yields the optimum resonant frequency /2={square root over (K/M)}=1600 Hz and damping ratio =B/2M=0.15. The calculation of the spring rate and damping factor provide: K=M.sup.2; B=2M.

(42) Reference is made to FIGS. 10A-10F showing graphically the experimental results, demonstrating the modifications of the dynamic responses of the Dewar envelope and FPA, as a result of mounting the wideband dynamic absorber assembly of FIGS. 6A-6B. In particular, FIGS. 10A and 10B compare the absolute transmissibilities with and without the optimized dynamic absorber mounted on the Dewar envelope. As shown in the figures, the presence of the optimally tuned dynamic absorber assembly yields massive suppression of the resonant phenomena over the entire frequency range. This results in massive attenuation of the acceleration and relative deflections. FIGS. 10C-10D and 10E-10F compare PSD of the acceleration and relative deflection of the FPA and Dewar envelope before and after the mounting of the dynamic absorber. The overall rms acceleration and deflection responses of the FPA are almost 3-fold attenuated from 86 g rms to 26 g rms and from 7.3 m rms to 2.4 m rms. It is noted that the acceleration response of the Dewar envelope is slightly increased from 7.4 g rms to 8.8 g rms, while the deflection rms response is slightly improved from 1.5 m rms to 0.9 m rms.

(43) FIGS. 11A and 11B compare the time histories of acceleration (FIG. 11A) and relative deflection (FIG. 11B) of the IDDA subjected to a half-sine shock 1000 g@1 ms per MIL-STD 810F, each for the cases with and without the wideband dynamic absorber assembly of at least one embodiment of the invention. It is evident from these figures that the dynamic absorber assembly drastically improves the settling time along with relieving the dynamic stresses during the entire transient process.

(44) 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.