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An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
An effective milli kelvin thermal management strategy for infrared imaging
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An effective milli kelvin thermal management strategy for infrared imaging

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  • 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME356AN EFFECTIVE MILLI KELVIN THERMAL MANAGEMENTSTRATEGY FOR INFRARED IMAGING SPECTROMETERKunal S. Bhatt1, Rahul Dev2, A. R. Srinivas3, Dr. D. P. Vakharia41, 4(Department of Mechanical Engineering, Sardar Vallabhbhai National Institute ofTechnology, Surat-395007, India)2, 3(SAC, ISRO, Ahmedabad-380015, India)ABSTRACTThermal infrared (TIR) spectroscopy is the subset of infrared spectroscopy that dealswith radiation emitted in the infrared part of the electromagnetic spectrum. Thermal imagingspectrometer (TIS) is the system that detects the thermal radiation emitted from theenvironment.TIS aims to detect very small range of infrared region (7-14 µm). To achievethis target it is required to maintain temperature of spectrometer detector within few milliKelvin accuracy. Achieving precise control of temperature in environment where thermaldissipation is varying is very challenging aspect. Commercially these types of systems arecontrolled by cryocoolers which are very heavy, cumbersome and introduce huge processtime to realize. The work presented in the paper brings out a cost effective and light weightthermal control strategy to precisely control the detector to 2 to 3 mK. The strategy issimulated by FEM tools and validated by the experiments.Keywords: detector, isothermal shield, PID controller, spectrometer, Thermo electric cooler1. INTRODUCTIONThermal infrared spectroscopy measures the thermal infrared radiation emitted (asopposed to being transmitted or reflected) from a volume or surface. This method iscommonly used to identify the composition of surface by analyzing its spectrum andcomparing it to previously measured materials. Data acquired by the spectrometer isprocessed and analyzed to map surface composition and mineralogy on the planet. Thistypical spectrometer presented here uses the micro bolometer detector as shown in Fig.1 tocapture the thermal radiation in the spectral range of 7-14µm of infrared region. Detector isINTERNATIONAL JOURNAL OF MECHANICAL ENGINEERINGAND TECHNOLOGY (IJMET)ISSN 0976 – 6340 (Print)ISSN 0976 – 6359 (Online)Volume 4, Issue 2, March - April (2013), pp. 356-366© IAEME: www.iaeme.com/ijmet.aspJournal Impact Factor (2013): 5.7731 (Calculated by GISI)www.jifactor.comIJMET© I A E M E
  • 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, Marchthe device consisting of a photo voltaic layer on charged coupled devices (CCDs). That isplaced at focal plane of the imagelectromagnetic (light) and transforms it in to an electronic charge and finally in digitalformat, which is read by the detector electronics. The micro bolometer detector has a vacuumsealed anti reflection coated germanium (Gr) window to allow the IR radiation within 714µm band. The resolution of 580 nm is required with minimum 12 nos. of spectral bandsfrom 7 to 14 microns wavelength to distinguish the different minerals present on surface [1].According to Wien’s displacement law the relation between wavelength (waves and absolute temperature (T) for maximum emissive power is given by [2],Hence there is requirement of controlling the temperature of Germanium window of theorder of 10mK to achieve the desired radiometric performance.The detector along with its processing electronics, mechanical mounting and thermalcontrol system is known as Detector Head Assembly (DHA) of the imaging system (Fig.2).Temperature control of such a system is very complex and challenging task as it consists ofoptical interfaces, electrical interfaces, mechanical structure and detector electronics. Typicalmodel of thermal imaging spectrometer is shown in the Fig.3.Figure 3 typical configuration of thermal imaging spectrometerFigure 1 internal structure ofmicro bolometer detectorInternational Journal of Mechanical Engineering and Technology (IJMET), ISSN6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME357the device consisting of a photo voltaic layer on charged coupled devices (CCDs). That isplaced at focal plane of the imaging system and it receives useful signals in the form ofelectromagnetic (light) and transforms it in to an electronic charge and finally in digitalformat, which is read by the detector electronics. The micro bolometer detector has a vacuumflection coated germanium (Gr) window to allow the IR radiation within 7m band. The resolution of 580 nm is required with minimum 12 nos. of spectral bandsfrom 7 to 14 microns wavelength to distinguish the different minerals present on surface [1].According to Wien’s displacement law the relation between wavelength (λ) of radiationwaves and absolute temperature (T) for maximum emissive power is given by [2],λmax T = 2900 µm 0KHence there is requirement of controlling the temperature of Germanium window of theorder of 10mK to achieve the desired radiometric performance.The detector along with its processing electronics, mechanical mounting and thermalcontrol system is known as Detector Head Assembly (DHA) of the imaging system (Fig.2).such a system is very complex and challenging task as it consists ofoptical interfaces, electrical interfaces, mechanical structure and detector electronics. Typicalmodel of thermal imaging spectrometer is shown in the Fig.3.configuration of thermal imaging spectrometernternal structure ofmicro bolometer detectorFigure 2 exploded view ofDetector Head AssemblyInternational Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –April (2013) © IAEMEthe device consisting of a photo voltaic layer on charged coupled devices (CCDs). That ising system and it receives useful signals in the form ofelectromagnetic (light) and transforms it in to an electronic charge and finally in digitalformat, which is read by the detector electronics. The micro bolometer detector has a vacuumflection coated germanium (Gr) window to allow the IR radiation within 7-m band. The resolution of 580 nm is required with minimum 12 nos. of spectral bandsfrom 7 to 14 microns wavelength to distinguish the different minerals present on surface [1].According to Wien’s displacement law the relation between wavelength (λ) of radiationwaves and absolute temperature (T) for maximum emissive power is given by [2],(1)Hence there is requirement of controlling the temperature of Germanium window of theThe detector along with its processing electronics, mechanical mounting and thermalcontrol system is known as Detector Head Assembly (DHA) of the imaging system (Fig.2).such a system is very complex and challenging task as it consists ofoptical interfaces, electrical interfaces, mechanical structure and detector electronics. Typicalxploded view ofAssembly
  • 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME3582. THERMAL REQUIREMENTS OF SPECTROMETERTIS uses bolometer detector which dissipates 150mW heat and detector card dissipates80mW in operation; the detector is required to be maintained at 20±5 0C during operationwith accuracy of 10mK.TIS Electro Opto Mechanical (EOM) module should be maintained at 20±50C duringoperation.The electronic card components dissipate 1.2 watts of heat the design should ensureproper heat transfer from pcb components to heat sink and should not allow thetemperature to rise more than 400C.Conventionally Thermo electric coolers (TECs) are used to control the temperatures of suchdetector. Commercially available TECs with PID controller have accuracy of the order of ±0.10C[3]. Customized TECs may give the control of temperature up to ±0.010C but that again increasethe cost of the system.3. PRESENT TIS DHA CONFIGURATIONFigure 4 present TIS-DHA configurationAs shown in the Fig.2&4 the detector package seats on the detector mount. The detectormount, package and electronic card are fixed to the DHA frame (heat sink). Thermal designensures that there are no hot spots in the vicinity of detector. Thermal radiations pass throughvarious optical components and they are focused on germanium window by focusing optics.Detector collects dispersed wavelengths allowed by germanium window in 7-14µm range.The simulations show that the present design allows 130mk over one degree variation in theambient temperature on germanium (Gr) window. Results and temperature profiles of the sameare shown in Fig.5 and Fig.6 respectively.Figure 5 temperatures on Gr window, package and TEC sink vs. ambient temperature for presentconfiguration
  • 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME359Boundary conditionsInitial temperature: 200CEmissivity of aluminum black anodized surface : 0.8Ambient temperature: 16 to 190CTEC temperature : 200CTEC Dissipation(including detector dissipation of 150mW) : 280 to 320 mWTemperature profiles4. THERMAL DESIGN OPTIMIZATIONThe design strategies shown in table1are analyzed to resolve thermal control. In all ofthe strategies the germanium window temperature and sink temperature which formbackground for the detector are monitored and controlled.Table 1 Various thermal design strategies(a) (b)(c) (d)Figure 6 temperature profile of (a) Gr window (b) package (c) detector mount(d) DHA frame for existing TIS configuration
  • 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME3604.1 Thermal simulation of TIS detectorHere the results of thermal analysis performed for different strategies are shown.Standard properties are taken for the thermal simulations [5].4.1.1 Strategy-1In this strategy entire structure is controlled at higher temperature without any designmodifications in present configuration.The maximum achieved variation on Gr window is 2mK for ± 1.50C variation in ambientas shown in Fig.7. The temperature profiles are shown in the Fig.8.Figure 7 temperatures on Gr window, package and TEC sink vs. ambient temperature forstrategy 14.1.2 Strategy-2 (Designing isothermal shield encompassing whole PCB)In this strategy a shield encompassing the whole PCB along with detector is designedas shown in Fig.8. The maximum achieved variation on Gr window is 4mK for ± 10Cvariation in ambient as shown in Fig.9.Figure 8 TIS DHA configuration with isothermal shield encompassing whole PCB
  • 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March4.1.4 Strategy-3 (Designing mini iIn this strategy an isothermal shield encompassing only the detector package isdesigned as shown in Fig.10 and it is controlled along with DHAFigure 10 TIS DHA configuration with iThe maximum achieved variation on Gras shown Fig.11. The temperature profilesFigure 11 temperatures on Gr window, pFigure 9 temperatures on GInternational Journal of Mechanical Engineering and Technology (IJMET), ISSN6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME3613 (Designing mini isothermal shield extended to detector mount)an isothermal shield encompassing only the detector package isand it is controlled along with DHA frame (heat sink).TIS DHA configuration with isothermal shield extended to detector mountmaximum achieved variation on Gr window is 3.9 mK for ± 1.50C variation in ambient. The temperature profiles are shown in the Fig.12.window, package and TEC sink vs. ambient temperature forstrategy 3emperatures on Gr window, package and TEC sink vs. ambienttemperature for strategy 2International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –April (2013) © IAEMEsothermal shield extended to detector mount)an isothermal shield encompassing only the detector package isframe (heat sink).sothermal shield extended to detector mountC variation in ambientackage and TEC sink vs. ambient temperature forackage and TEC sink vs. ambient
  • 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME362Temperature profilesFigure 12 Temperature profiles of (a) Gr window (b) package (c) DHA frame-sink (d) DPEcard for strategy 35. SENSITIVITY ANALYSISSensitivity analysis is performed to observe the effect of variation of controllingtemperatures on germanium window.5.1 Variation of isothermal shield temperature by ± 0.010CThe maximum variation on Germanium window is 12mK for 20mK variation inisothermal shield temperature. Results are shown in the Fig. 13.(a) (b)(c) (d)Figure 13 temperatures on Gr window, package and TEC sink vs. isothermal shieldtemperature
  • 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME3635.2 Variation of DHA frame temperature by ± 0.010CThe maximum variation on Gr window is 10mK for 20mK variation in DHA frametemperature as shown in Fig.14.6 EXPERIMENTAL VALIDATIONSThe Fig.15 shows the experimental test set up of the spectrometer system.Fig.15 experimental test set up for TIS-DHA thermal controlThe TIS structure along with all mounted components is taken for experimentation.The heat is supplied to the DHA frame and isothermal shield with thermo foil heaters. Thetemperature on the shield and DHA frame is controlled by the PID controller which maintainsthe temperature on the system by controlling the heat supply from the heaters. The othersingle foil heater is mounted on the PCB to simulate the heat dissipated by electroniccomponents. This heater is given the power supply of 1.2 watts with the help of variac. ThePT 100 temperature sensors are mounted on the spectrometer system at required locations.The temperatures of various subsystems are monitored, recorded and plotted through thesoftware interface of temperature data acquisition system [6]. The environmental temperatureof spectrometer system is varied by varying environmental chamber temperature. Fig.16shows the realized hardware mounted with sensor & heaters and entire system is wrappedwith MLI blankets.Figure 14 temperatures on Gr window, package and TEC sink vs. DHA frametemperature
  • 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME364Fig.16 integrated thermal imaging-infrared imaging spectrometer system6.2 Measurements of the temperatures for the spectrometer system with thermal controlstrategyTable 2 Temperatures on alumina package, detector mount and focusing optics lens forambient temperature variationThe table 2 shows the ambient temperatures, controlling temperatures, temperatures ondetector mount and package. The control temperature is set to 32 0C and ambient temperatureis varied from 31.10C to 27.6 0C temperature. The Fig.17 shows the temperature measuredand corresponding plots obtained by data logger system.The maximum achieved temperature variations on alumina package and detector mountare 22 mK and 26 mK for 3.5 0C variation in ambient temperatureFigure 17 experimental readings of two extreme temperatures 31.1 0C and 27.6 0C
  • 10. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME3656.3 Measurements of sensitive parametersThe change of the temperature on the alumina package is measured in relation to thechange in controlling temperatures. The set temperature value is kept offset by ± 100 to 200mK intentionally and maintained constant by the PID controller.Table 3 Temperatures on alumina package, detector mount and focusing optics lens forcontrolled temperature variationThe maximum achieved variation on package is 134 mK for 203 mK variation in isothermalshield temperature and maximum achieved variation on detector mount is 113 mK for 222mK variation in isothermal shield temperature. Results are shown in table 3.7. CONCLUSIONSThermal infrared imaging spectrometer system has been rigorously analyzed to yieldmilli Kelvin thermal control. The simulated results are validated by experimentation. Themeasurements performed on the developed hardware for the simulated strategies show thefollowings.• For a change in the ambient temperature of the order of 1.2 0C causes a change of 3mK on the package thus meeting the set target of 10 mK.• Similarly a change of 1.2 0C in ambient causes 3 mK on detector mount, 15 mK onheat sink of detector and 6mK on the isothermal shield.• In order to meet above target it is required that the control temperature targets of heatsink and isothermal shield are to be kept within a range of ± 0.01 0C.• Mini isothermal shield extended up to detector mount (strategy-3) with temperaturecontrol on DHA frame and isothermal shield is a viable solution to be adopted forthermal control of TIS detector.REFERENCES[1] Michael R. Holt, Thermal management strategy for the hyper spectral imager for thecoastal ocean, master diss., Utah state university, Logan, Utah, 2007.[2] S.P. Sukhatme, text book on heat transfer fourth edition (University Press (India) Pvt.Ltd., Hyderabad, 2005).
  • 11. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME366[3] G Johnson, Thermal management for CCD performance on the advanced camera forsurveys (ACS), SPIE conference on space telescopes and instruments, Kona, Hawaii, volume3356, 1998.[4] David G. Gilmore, precision temperature control, Handbook of space craft thermalcontrol, 17 (the aero space press, California, 2002) 639-666.[5] Document of thermal properties of spacecraft materials, thermal system group, ISROsatellite centre, Bangalore, 1999.[6] D.B. Chauhan, Thermal management Solutions for high heat dissipative spacecraftsubsystems, master diss., Nirma Institute of Technology, Ahmedabad, India, 2012.[7] Ganni Gowtham, Ksitij Kumar, S.S Charan and K Manivannan, “ExperimentalAnalysis of Solar Powered Ventilation Coupled with Thermo Electric Generator on unroofedParked Vehicles”, International Journal of Mechanical Engineering & Technology (IJMET),Volume 3, Issue 3, 2012, pp. 471 - 482, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

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