6340(Print), ISSN (Online) Volume 4, Issue 2, March - April (2013) IAEME AND TECHNOLOGY (IJMET)

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1 INTERNATIONAL International Journal of Mechanical JOURNAL Engineering OF MECHANICAL and Technology (IJMET), ENGINEERING ISSN 0976 AND TECHNOLOGY (IJMET) ISSN (Print) ISSN (Online) Volume 4, Issue 2, March - April (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJMET I A E M E AN EFFECTIVE MILLI KELVIN THERMAL MANAGEMENT STRATEGY FOR INFRARED IMAGING SPECTROMETER Kunal S. Bhatt 1, Rahul Dev 2, A. R. Srinivas 3, Dr. D. P. Vakharia 4 1, 4 (Department of Mechanical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat , India) 2, 3 (SAC, ISRO, Ahmedabad , India) ABSTRACT Thermal infrared (TIR) spectroscopy is the subset of infrared spectroscopy that deals with radiation emitted in the infrared part of the electromagnetic spectrum. Thermal imaging spectrometer (TIS) is the system that detects the thermal radiation emitted from the environment.tis aims to detect very small range of infrared region (7-14 µm). To achieve this target it is required to maintain temperature of spectrometer detector within few milli Kelvin accuracy. Achieving precise control of temperature in environment where thermal dissipation is varying is very challenging aspect. Commercially these types of systems are controlled by cryocoolers which are very heavy, cumbersome and introduce huge process time to realize. The work presented in the paper brings out a cost effective and light weight thermal control strategy to precisely control the detector to 2 to 3 mk. The strategy is simulated by FEM tools and validated by the experiments. Keywords: detector, isothermal shield, PID controller, spectrometer, Thermo electric cooler 1. INTRODUCTION Thermal infrared spectroscopy measures the thermal infrared radiation emitted (as opposed to being transmitted or reflected) from a volume or surface. This method is commonly used to identify the composition of surface by analyzing its spectrum and comparing it to previously measured materials. Data acquired by the spectrometer is processed and analyzed to map surface composition and mineralogy on the planet. This typical spectrometer presented here uses the micro bolometer detector as shown in Fig.1 to capture the thermal radiation in the spectral range of 7-14µm of infrared region. Detector is 356

2 6340(Print), ISSN (Online) Volume 4, Issue 2, March - April (2013) IAEME the device consisting of a photoo voltaic layer on charged coupled devices (CCDs). That is placed at focal plane of the imaging system and it receives useful signals in the form of electromagnetic (light) and transforms it in to an electronic charge and finally in digital format, which is read by the detector electronics. The micro bolometer detector has a vacuum sealed anti reflection coated germanium (Gr) window to allow the IR radiation within 7-14µm band. The resolution of 580 nm is required with minimum 12 nos. of spectral bands from 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 radiation waves and absolute temperature (T) for maximum emissive power is given by [2], λ max T = 2900 µm 0 K (1) Hence there is requirement of controlling the temperature of Germanium window of the order of 10mK to achieve the desired radiometric performance. Figure 1 internal structure of micro bolometer detector Figure 2 exploded view of Detector Head Assembly The detector along with its processing electronics, mechanical mounting and thermal control 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 of optical interfaces, electrical interfaces, mechanical structure and detector electronics. Typical model of thermal imaging spectrometer is shown in the Fig.3. Figure 3 typical configuration of thermal imaging spectrometer 357

3 2. THERMAL REQUIREMENTS OF SPECTROMETER TIS uses bolometer detector which dissipates 150mW heat and detector card dissipates 80mW in operation; the detector is required to be maintained at 20±5 0 C during operation with accuracy of 10mK. TIS Electro Opto Mechanical (EOM) module should be maintained at 20±5 0 C during operation. The electronic card components dissipate 1.2 watts of heat the design should ensure proper heat transfer from pcb components to heat sink and should not allow the temperature to rise more than 40 0 C. Conventionally Thermo electric coolers (TECs) are used to control the temperatures of such detector. Commercially available TECs with PID controller have accuracy of the order of ±0.1 0 C [3]. Customized TECs may give the control of temperature up to ± C but that again increase the cost of the system. 3. PRESENT TIS DHA CONFIGURATION Figure 4 present TIS-DHA configuration As shown in the Fig.2&4 the detector package seats on the detector mount. The detector mount, package and electronic card are fixed to the DHA frame (heat sink). Thermal design ensures that there are no hot spots in the vicinity of detector. Thermal radiations pass through various 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 the ambient temperature on germanium (Gr) window. Results and temperature profiles of the same are shown in Fig.5 and Fig.6 respectively. Figure 5 temperatures on Gr window, package and TEC sink vs. ambient temperature for present configuration 358

4 Boundary conditions Initial temperature: 20 0 C Emissivity of aluminum black anodized surface : 0.8 Ambient temperature: 16 to 19 0 C TEC temperature : 20 0 C TEC Dissipation(including detector dissipation of 150mW) : 280 to 320 mw Temperature profiles (a) (b) (c) (d) Figure 6 temperature profile of (a) Gr window (b) package (c) detector mount (d) DHA frame for existing TIS configuration 4. THERMAL DESIGN OPTIMIZATION The design strategies shown in table1are analyzed to resolve thermal control. In all of the strategies the germanium window temperature and sink temperature which form background for the detector are monitored and controlled. Table 1 Various thermal design strategies 359

5 4.1 Thermal simulation of TIS detector Here the results of thermal analysis performed for different strategies are shown. Standard properties are taken for the thermal simulations [5] Strategy-1 In this strategy entire structure is controlled at higher temperature without any design modifications in present configuration. The maximum achieved variation on Gr window is 2mK for ± C variation in ambient as 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 for strategy Strategy-2 (Designing isothermal shield encompassing whole PCB) In this strategy a shield encompassing the whole PCB along with detector is designed as shown in Fig.8. The maximum achieved variation on Gr window is 4mK for ± 1 0 C variation in ambient as shown in Fig.9. Figure 8 TIS DHA configuration with isothermal shield encompassing whole PCB 360

6 6340(Print), ISSN (Online) Volume 4, Issue 2, March - April (2013) IAEME Figure 9 temperatures on Gr window, package and TEC sink vs. ambient temperature for strategy Strategy-3 (Designing mini isothermal shield extended to detector mount) In this strategy an isothermal shield encompassing only the detector package is designed as shown in Fig.10 and it is controlled along with DHA frame (heat sink). Figure 10 TIS DHA configuration with isothermal shield extended to detector mount The maximum achieved variation on Gr window is 3.9 mk for ± C variation in ambient as shown Fig.11. The temperature profiles are shown in the Fig.12. Figure 11 temperatures on Gr window, package and TEC sink vs. ambient temperature for strategy 3 361

7 Temperature profiles (a) (b) (c) (d) Figure 12 Temperature profiles of (a) Gr window (b) package (c) DHA frame-sink (d) DPE card for strategy 3 5. SENSITIVITY ANALYSIS Sensitivity analysis is performed to observe the effect of variation of controlling temperatures on germanium window. 5.1 Variation of isothermal shield temperature by ± C The maximum variation on Germanium window is 12mK for 20mK variation in isothermal shield temperature. Results are shown in the Fig. 13. Figure 13 temperatures on Gr window, package and TEC sink vs. isothermal shield temperature 362

8 5.2 Variation of DHA frame temperature by ± C The maximum variation on Gr window is 10mK for 20mK variation in DHA frame temperature as shown in Fig.14. Figure 14 temperatures on Gr window, package and TEC sink vs. DHA frame temperature 6 EXPERIMENTAL VALIDATIONS The Fig.15 shows the experimental test set up of the spectrometer system. Fig.15 experimental test set up for TIS-DHA thermal control The 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. The temperature on the shield and DHA frame is controlled by the PID controller which maintains the temperature on the system by controlling the heat supply from the heaters. The other single foil heater is mounted on the PCB to simulate the heat dissipated by electronic components. This heater is given the power supply of 1.2 watts with the help of variac. The PT 100 temperature sensors are mounted on the spectrometer system at required locations. The temperatures of various subsystems are monitored, recorded and plotted through the software interface of temperature data acquisition system [6]. The environmental temperature of spectrometer system is varied by varying environmental chamber temperature. Fig.16 shows the realized hardware mounted with sensor & heaters and entire system is wrapped with MLI blankets. 363

9 Fig.16 integrated thermal imaging-infrared imaging spectrometer system 6.2 Measurements of the temperatures for the spectrometer system with thermal control strategy Table 2 Temperatures on alumina package, detector mount and focusing optics lens for ambient temperature variation The table 2 shows the ambient temperatures, controlling temperatures, temperatures on detector mount and package. The control temperature is set to 32 0 C and ambient temperature is varied from C to C temperature. The Fig.17 shows the temperature measured and corresponding plots obtained by data logger system. The maximum achieved temperature variations on alumina package and detector mount are 22 mk and 26 mk for C variation in ambient temperature Figure 17 experimental readings of two extreme temperatures C and C 364

10 6.3 Measurements of sensitive parameters The change of the temperature on the alumina package is measured in relation to the change in controlling temperatures. The set temperature value is kept offset by ± 100 to 200 mk intentionally and maintained constant by the PID controller. Table 3 Temperatures on alumina package, detector mount and focusing optics lens for controlled temperature variation The maximum achieved variation on package is 134 mk for 203 mk variation in isothermal shield temperature and maximum achieved variation on detector mount is 113 mk for 222 mk variation in isothermal shield temperature. Results are shown in table CONCLUSIONS Thermal infrared imaging spectrometer system has been rigorously analyzed to yield milli Kelvin thermal control. The simulated results are validated by experimentation. The measurements performed on the developed hardware for the simulated strategies show the followings. For a change in the ambient temperature of the order of C causes a change of 3 mk on the package thus meeting the set target of 10 mk. Similarly a change of C in ambient causes 3 mk on detector mount, 15 mk on heat sink of detector and 6mK on the isothermal shield. In order to meet above target it is required that the control temperature targets of heat sink and isothermal shield are to be kept within a range of ± C. Mini isothermal shield extended up to detector mount (strategy-3) with temperature control on DHA frame and isothermal shield is a viable solution to be adopted for thermal control of TIS detector. REFERENCES [1] Michael R. Holt, Thermal management strategy for the hyper spectral imager for the coastal ocean, master diss., Utah state university, Logan, Utah, [2] S.P. Sukhatme, text book on heat transfer fourth edition (University Press (India) Pvt. Ltd., Hyderabad, 2005). 365

11 [3] G Johnson, Thermal management for CCD performance on the advanced camera for surveys (ACS), SPIE conference on space telescopes and instruments, Kona, Hawaii, volume 3356, [4] David G. Gilmore, precision temperature control, Handbook of space craft thermal control, 17 (the aero space press, California, 2002) [5] Document of thermal properties of spacecraft materials, thermal system group, ISRO satellite centre, Bangalore, [6] D.B. Chauhan, Thermal management Solutions for high heat dissipative spacecraft subsystems, master diss., Nirma Institute of Technology, Ahmedabad, India, [7] Ganni Gowtham, Ksitij Kumar, S.S Charan and K Manivannan, Experimental Analysis of Solar Powered Ventilation Coupled with Thermo Electric Generator on unroofed Parked Vehicles, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 3, 2012, pp , ISSN Print: , ISSN Online: