An effective milli kelvin thermal management strategy for infrared imaging

International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 –

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME

356

AN EFFECTIVE MILLI KELVIN THERMAL MANAGEMENT

STRATEGY FOR INFRARED IMAGING SPECTROMETER

Kunal S. Bhatt1, 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-395007, India) 2, 3

(SAC, ISRO, Ahmedabad-380015, 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

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ISSN 0976 – 6340 (Print)

ISSN 0976 – 6359 (Online)

Volume 4, Issue 2, March - April (2013), pp. 356-366

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International Journal of Mechanical Engineering and Technology (IJMET), ISSN

6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March

the device consisting of a photo voltaic layer on charged coupled devices (CCDs). That is

placed at focal plane of the imag

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 (

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 the

order of 10mK to achieve the desired radiometric performance.

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

Figure 1 internal structure of

micro bolometer detector


6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME

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placed at focal plane of the imaging system and it receives useful signals in the form of



flection 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 bands


According to Wien’s displacement law the relation between wavelength (λ) of radiation


λmax T = 2900 µm 0K


order of 10mK to achieve the desired radiometric performance.



such a system is very complex and challenging task as it consists of


model of thermal imaging spectrometer is shown in the Fig.3.

configuration of thermal imaging spectrometer

nternal structure of

micro bolometer detector

Figure 2 exploded view of

Detector Head Assembly


April (2013) © IAEME


ing system and it receives useful signals in the form of



flection 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 bands


According to Wien’s displacement law the relation between wavelength (λ) of radiation


(1)




such a system is very complex and challenging task as it consists of


xploded view of

Assembly



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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 0C during operation

with accuracy of 10mK.

� TIS Electro Opto Mechanical (EOM) module should be maintained at 20±50C 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 400C.

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 ±0.10C

[3]. Customized TECs may give the control of temperature up to ±0.010C 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



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Boundary conditions

� Initial temperature: 200C

� Emissivity of aluminum black anodized surface : 0.8

� Ambient temperature: 16 to 190C

� TEC temperature : 200C

� TEC Dissipation(including detector dissipation of 150mW) : 280 to 320 mW

Temperature profiles

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

(a) (b)

(c) (d)

Figure 6 temperature profile of (a) Gr window (b) package (c) detector mount

(d) DHA frame for existing TIS configuration



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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].

4.1.1 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 ± 1.50C 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 1

4.1.2 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 ± 10C

variation in ambient as shown in Fig.9.

Figure 8 TIS DHA configuration with isothermal shield encompassing whole PCB


6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 2, March

4.1.4 Strategy-3 (Designing mini i

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

Figure 10 TIS DHA configuration with i

The maximum achieved variation on Gr

as shown Fig.11. The temperature profiles

Figure 11 temperatures on Gr window, p

Figure 9 temperatures on G


6359(Online) Volume 4, Issue 2, March - April (2013) © IAEME

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3 (Designing mini isothermal shield extended to detector mount)

an isothermal shield encompassing only the detector package is

and it is controlled along with DHA frame (heat sink).

TIS DHA configuration with isothermal shield extended to detector mount

maximum 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 for

strategy 3

emperatures on Gr window, package and TEC sink vs. ambient

temperature for strategy 2


April (2013) © IAEME

sothermal shield extended to detector mount)

an isothermal shield encompassing only the detector package is

frame (heat sink).

sothermal shield extended to detector mount

C variation in ambient

ackage and TEC sink vs. ambient temperature for

ackage and TEC sink vs. ambient



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Temperature profiles

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 ± 0.010C

The maximum variation on Germanium window is 12mK for 20mK variation in

isothermal 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 shield

temperature



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5.2 Variation of DHA frame temperature by ± 0.010C

The maximum variation on Gr window is 10mK for 20mK variation in DHA frame

temperature as shown in Fig.14.

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.

Figure 14 temperatures on Gr window, package and TEC sink vs. DHA frame

temperature



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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 0C and ambient temperature

is varied from 31.10C to 27.6

0C 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 3.5 0C variation in ambient temperature

Figure 17 experimental readings of two extreme temperatures 31.1 0C and 27.6

0C



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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 3.

7. 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 1.2 0C causes a change of 3

mK 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 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 ± 0.01 0C.

• 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, 2007.

[2] S.P. Sukhatme, text book on heat transfer fourth edition (University Press (India) Pvt.

Ltd., Hyderabad, 2005).



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[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, 1998.

[4] David G. Gilmore, precision temperature control, Handbook of space craft thermal

control, 17 (the aero space press, California, 2002) 639-666.

[5] Document of thermal properties of spacecraft materials, thermal system group, ISRO

satellite centre, Bangalore, 1999.

[6] D.B. Chauhan, Thermal management Solutions for high heat dissipative spacecraft

subsystems, master diss., Nirma Institute of Technology, Ahmedabad, India, 2012.

[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. 471 - 482, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359.

An effective milli kelvin thermal management strategy for infrared imaging

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