Deign of a Portable Emittance Meaurement Sytem for Spacecraft Thermal Deign and Quality Control H. Yamana 1, S. Katuki 2, A. Ohnihi 3, 5 and Y. Nagaaka 4 1 School of Integrated Deign Engineering, Keio Univerity, 3-14-1, Hiyohi, Yokohama, 223-8522, Japan. 2 Specialty Chemical & Product, Aeropace Material, UBE Indutrie, LTD., 1978-10, Koguhi, Ube, Yamaguchi, 755-8633, Japan. 3 Intitute of Space and Atronautical Science (ISAS), Japan Aeropace and Exploration Agency (JAXA), 3-1-1, Yohinodai, Sagamihara, Kanagawa, 229-8510, Japan. 4 Department of Sytem Deign Engineering, Keio Univerity, 3-14-1, Hiyohi, Yokohama, 223-8522, Japan. 5 To whom correpondence hould be addreed. E-mail: ohnihi@ia.jaxa.jp
ABSTRACT Thi paper report the development of a portable hemipherical total emittance ε H meaurement ytem for pacecraft thermal deign and quality control. The meaurement principle i the reflective method, and the meaurement ytem conit of a blackbody furnace, an integrating phere, a thermopile and o on. A prototype ε H meaurement ytem wa contructed and evaluated by comparing the meaurement data with thoe obtained by the calorimetric method. From the evaluation reult, it wa confirmed that the meaurement accuracy of thi ytem i ±0.05. The ytematic error caued by the deviance from the retriction of Kirchhoff law and by the characteritic of the integrating phere were etimated and reduced or compenated. The prototype ε H meaurement ytem wa improved for a portable ytem with higher meaurement accuracy and higher meaurement peed. To evaluate the validity of the portable ytem, the ε H of variable thermal control material were meaured. The reult by the portable ytem agreed well with thoe obtained by the calorimetric method. KEY WORDS: hemipherical total emittance, on-ite meaurement, portable ytem, quality control, reflective method, thermal control material, pacecraft thermal deign. 1
1. INTRODUCTION The accurate data of hemipherical total emittance ε H of thermal control material are required for thermal deign of pacecraft. For more than two decade, our laboratory ha developed everal emittance meaurement ytem [1] which can be ued in the laboratory for variou purpoe. Generally, for pacecraft thermal deign, the ε H of thermal control material are referred to the value of the databae [2-3] which wa built by uing thee emittance meaurement ytem, and thee data are conidered to be contant before launch. However, there are poibilitie that the value of ε H of material actually ued for thermal deign differ from thoe of the databae, and that the qualitie of thermal control material change. The quality difference and change of the pacecraft material caue critical problem in pace flight miion, but on-ite meaurement before launch by uing conventional meaurement ytem i difficult becaue of their large ize or low accuracie. We have propoed to develop a portable ε H meaurement ytem with high accuracy, high meaurement peed, light weight, and uitable for the following purpoe. 1. On-ite meaurement of ε H change during aembling and preerving the pacecraft on the ground. 2. Quality control in material fabricating for material reearch and development. A a firt tep of thi tudy, a prototype ε H meaurement ytem ha been developed and evaluated. Thi paper decribe the detail of prototype ytem and meaurement reult of emittance for variou pacecraft material. In addition, a portable ε H meaurement ytem for which the prototype ytem wa improved and evaluation reult are dicued. 2. PRINCIPLE The meaurement principle i the reflective method. From the law of energy conervation, the relation between the directional pectral aborptance α λ and the directional/hemipherical pectral reflectance ρ λ for an opaque body i expreed a 2
θ, φ, T ) + ρ θ, φ, T ) = 1 ample λ ample α λ, (1) where λ i the wavelength of the incident light, θ i the incident angle, φ i the azimuth angle, and T ample i the temperature of the ample. Kirchhoff law for hemipherical total propertie are expreed a H ( T ) α ( T ) ε =, (2) ample and be applied to give H H ample ( T ) ρ ( T ) = 1 ε. (3) ample + HH ample Eq. (3) i atified when either of the following retriction i applied [4]. 1. The incident radiation i independent of angle and ha a pectral ditribution proportional to that of a blackbody at T ample. 2. The incident radiation i independent of angle, and the ample ha directional-gray urface. 3. The incident radiation from each direction ha pectral ditribution proportional to that of a blackbody at T ample, and the ample ha diffue-pectral urface. 4. The ample ha diffue-gray urface. However, it i difficult to fulfill thee retriction and to meaure the accurate hemipherical/hemipherical total reflectance ρ HH by a portable ytem. Thu, we ued an integrating phere to detect the directional/hemipherical reflective intenity from a ample. The advantage in uing the integrating phere i that the directional/hemipherical reflectance ρ DH can be treated equal to the hemipherical/hemipherical reflectance ρ HH [5]. In addition, when the light ource ha a pectral ditribution approximately proportional to that of a blackbody over wide wavelength range, and the temperature difference between the ample T ample and the light ource T light i mall, ε H of the ample can be obtained by Eq. (3). 3. PROTOTYPE MEASUREMENT SYSTEM 3.1. Sytem Component The chematic diagram of the prototype hemipherical total emittance ε H 3
Parabolic Mirror Blackbody Furnace Shutter Integrating Sphere Thermopile DC Power PC Multimeter Sample or Reference Amplifier Figure 1 Schematic diagram of prototype ε H meaurement ytem. meaurement ytem i hown in Figure 1. Thi ytem conit of a blackbody furnace, a parabolic mirror coated with gold, an integrating phere with a hutter, a thermopile (KRS-5 window) with an amplifier, and a data acquiition ytem. The incident light from the blackbody furnace irradiate a ample at an angle of 7 degree in order to minimize the pecular reflective energy lo [6]. The reflected light from the ample multiple reflect inide the integrating phere, and the multiple reflected light i detected by the thermopile over the wavelength region from 0.6 to 42.0 µm, which cover 95 % of the total emiive power of the ideal blackbody at 300 K. The relation between the input infrared power and the output voltage of the thermopile with the amplifier ha linearity, thu the output voltage i expreed a a linear function of the ε H of the ample (ee 3.2.). The pectral characteritic of the light ource i the mot important in the reflective method. The deign of the light ource require the following performance: 1. cover pectral ditribution proportional to that of a blackbody and ha high emittance over wide wavelength range 2. low power upply control 3. mall ize and light weight The blackbody furnace which atifie the above condition wa deigned and contructed. The core of the blackbody furnace i cylindrical tructure made of carbon, 4
whoe ize i 9 mm in diameter and 45 mm length. The inner urface of the core i coated with black paint, which ha pectral emittance of 0.94 in the wavelength region from 2 to 25 µm. Tantalum wire i coiled around the core, and the blackbody furnace i controlled at 450 K loading 8.4 W. When the temperature of the inner urface of the core i contant, the effective emittance ε e of the blackbody furnace i expreed a [7] ε [ 1+ ( 1 ε bw )( a A f )] ε ( 1 a A) + a A = [ 1 ( 2D r ) ] 2 and a A 1 [ 2( 1 + 2D r black )] bw ε e =, (4) bw f + 1 black =, where ε bw i the emittance of the inide urface, D i the depth and r black i the diameter of the aperture of the core. From Eq. (4), the ε e of the blackbody furnace ued in the preent work i more than 0.99. The integrating phere i made of aluminum alloy and the inide urface i coated with gold. The inide diameter i 30 mm and it ha three port, the firt-port (6 mm in diameter) i ued for the input of light from the ource, the econd-port (6 mm in diameter) i ued for etting the ample, and the third-port (4 mm in diameter) i ued for the output of the reflected light from the ample into the detector. 3.2. Meaurement Procedure The detected energy of the thermopile E 0 when the hutter i cloed can be expreed a E ( ) E ( ε ) 0 E, I ε + i, I =, (5) i where E,I (ε ), E i,i (ε i ) are emiive energy of the ample and the integrating phere. On the other hand, the detected energy of the thermopile E 1 when the hutter i opened can be expreed a E ( ) + E ( ε ) E ( ε ) 1 E, I ε i, I i +, R =, (6) where E,R (ε ) i reflective energy of the ample. E,R (ε ) i obtained by taking the difference between E 0 and E 1 : E 1 E0 =, R ( ε ) E. (7) 5
The intenity difference of voltage between the condition when the hutter i cloed and opened i repreented a a linear function of E,R (ε ). Therefore, by meauring temperature and reflective intenitie of two reference ample with known temperature dependence of low and high emittance, the calibration line i obtained. The emittance of the ample i obtained by meauring it reflective intenity: ( T ) { ε ( T ) ε ( T )} V + ε ( T ) V ε ( T ) l l h h h h l l l h ε =, (8) Vl Vh where ε i the hemipherical total emittance, T i the temperature, V i the intenity difference of voltage when the hutter i cloed and opened, and the ubcript are the ample () and the reference ample with low emittance (l) and with high emittance (h). V 3.3. Emittance Evaluation Reult The hemipherical total emittance of pacecraft material were meaured by the prototype ε H meaurement ytem and the calorimetric method. The meaurement accuracy of the calorimetric method i 2.0 %, and a detailed explanation of thi ytem i decribed in Ref. 1 and 3. To evaluate prototype ytem, aluminum coated polyimide (25 UPILEX-R/Al), Highly Oriented Graphite Sheet (HOGS) [8], and germanium coated 0.1 0 Shutter CLOSE OPEN Black Kapton 25 UPILEX-R/Al HOGS Al Intenity, V -0.1-0.2-0.3-0.4-0.5-10 -5 0 5 10 15 20 Time, Figure 2 The time fluctuation in the intenitie of the detector by prototype ytem. 6
Table 1 Meaurement reult of emittance by prototype ytem. Sample Meaurement reult of ε H Preent work Calorimetric method ε H 25 UPILEX-R/Al 0.54 0.57 [2] -0.03 HOGS 0.33 0.29 [8] +0.04 Ge/50RN/Al 0.64 0.69-0.05 aluminized polyimide (Ge/50RN/Al) were ued a tet ample. Aluminum depoited film (Al: ε H = 0.05 @ 293 K) of low emittance and electrically conductive black polyimide (Black Kapton: ε H = 0.80 @ 293 K) of high emittance were ued a reference ample. In the preent tudy, the reflective intenitie of the ample were meaured at room temperature and the emittance of the reference ample were obtained from data at 293 K. The time fluctuation in intenitie of voltage of detector i hown in Figure 2 and the meaurement reult of emittance i hown in Table 1. In all ample, the reult of the prototype ytem agreed within 0.05 with thoe of the calorimetric method. 4. ESTIMATION OF SYSTEMATIC ERRORS 4.1. Temperature Difference between Sample and Light Source In the reflective method, the incident light mut have a pectral ditribution proportional to that of a blackbody at ample temperature T ample becaue of the retriction of Kirchhoff law. However, in the preent prototype meaurement ytem, the blackbody furnace a a light ource i ued at 450 K, and T ample i at room temperature. Thi difference between light ource temperature T light and T ample ( T) caue ytematic error. Thi ytematic error α i the difference between hemipherical total emittance and the hemipherical total aborptance of the ample, and it can be expreed a α = α = ( Tample, Tlight ) α ( Tample, Tample ) { 1 ρ Tample )} ibλ Tlight ) d ibλ Tlight ) dλ λ { 1 ρ Tample )} ibλ ( ibλ Tample ) λ, T dλ ample ) dλ. (9) 7
0.05 0.3 α 0.04 0.03 0.02 (a) HOGS 25 UPILEX-R/Al α (MAX) 0.2 0.1 (b) 0.01 Black Kapton 0 0 50 100 150 T Al 0 0 50 100 Figure 3 The ytematic error α, which i the difference between hemipherical total emittance and the total hemipherical aborptance of the ample, veru the temperature difference between T light and T ample ( T) of Al, Black Kapton, 25 UPILEX-R/Al, HOGS (a) and imaginary aumed material which bring the error maximum (b) when ample temperature T ample i 300K. T 150 The value of α of Al, Black Kapton, 25 UPILEX-R/Al and HOGS were etimated when ample temperature T ample i 300K. The pectral reflectance ρ(λ,t ample ) of the ample were meaured by uing the Fourier Tranform Spectrocopy (Bio-Rad: FTS-60A/896) in the wavelength range from 1.6 to 100 µm, and the pectral emiive power of the blackbody i bλ (λ, T) wa calculated from Planck law. In addition, α of the imaginary aumed material which bring the error maximum wa etimated. The pectral reflectance of the imaginary aumed material ρ im (λ) can be expreed a ρ im ( λ) 1 = 0 [ i' Tlight ) > i' Tample )] [ i' T ) i' T light ample )], (10) where i (λ, T) are obtained by normalizing i bλ (λ, T). α veru T are hown in Figure 3. When light ource temperature T light i 450K, the ytematic error for pacecraft material are le than 0.05. We have worked to reduce thi error and were ucceful in conducting meaurement at T = 60K by a portable ytem hereinafter decribed (ee 5.). A a reult, thi ytematic error for thee material i le than 0.02. 8
4.2. Characteritic of Integrating Sphere The ideal meaurement relationhip i for the ratio of radiance produced inide the phere to be equal to the ratio of the reflectance for each material [9]: I I r ρ =. (11) ρ r Where I i the reflective intenity, ρ i the reflectance of the ample, and the ubcript are the ample () and the reference ample (r). However, the average reflectance of the phere change when the ample i ubtituted for the reference ample. The precie meaurement equation for a ubtitution phere i expreed a I I r ρ 1 ρ r =, (12) ρ 1 ρ r ( S S ) ρ w w p + ρ ps p ρ =, (13) S w where ρ i the average reflectance for the entire integrating phere, S i the area, and the ubcript are the phere urface (w) and the phere port (p). Figure 4 how the relationhip between the meaurement reflectance and the actual reflectance in the preent work by Eq. (11) and (12) when the phere urface and (Meaured-Actual) Reflectance of Sample 0.03 0.02 0.01 0-0.01-0.02-0.03 0 0.2 0.4 0.6 0.8 1 Actual Reflectance of Sample Figure 4 The difference between the meaurement reflectance and the actual reflectance in the preent work when the phere urface and ample are ideal diffuer. 9
ample are ideal diffuer. The reflectance of the phere urface ρ w which wa meaured by uing the FT-IR in the wavelength range from 1.6 to 100 µm i 0.93, and the reflectance of the port for input light and detector are 0. From thi calculation reult, the ytematic error caued by the characteritic of the integrating phere i le than ±0.03. Thi error i compenated in the portable ytem hereinafter decribed (ee 5.) and can be reduced by increaing calibration point, decreaing the area of the phere port and improving the reflective characteritic of the phere urface. 5. PORTABLE SYSTEM We miniaturized and divided the preent prototype meaurement ytem into a meaurement unit and an operating unit. The appearance and chematic diagram of a portable ytem are hown in Figure 5 and 6. The meaurement unit conit of a blackbody furnace, a parabolic mirror, an integrating phere with a hutter, a thermopile, three thermo-enor and temperature control unit. The thermo-enoor1, 2 and 3 meaure blackbody furnace temperature T light, room temperature T room, and ample temperature T ample repectively. By employing the temperature control unit and the thermo-enor1 and 2, T light can be controlled at deired temperature (T room ~ 363K). By introducing the thermo-enor3, the accurate ε H of reference ample from known Operating Unit Meaurement Unit Figure 5 The appearance of the portable hemipherical total emittance meaurement ytem. 10
T light Thermo-Senor 1 T room Operation Button Temperature Control Unit Thermo-Senor 2 Blackbody Furnace LCD Control Unit Shutter Parabolic Mirror Thermopile Integrating Sphere T ample Power Supply Unit Preamplifier Thermo-Senor 3 Operating Unit Meaurement Unit Figure 6 The chematic diagram of the portable ε H meaurement ytem. temperature dependence data, and accurate calibration line could be obtained. The operating unit ha a control unit with memorie for data calculation and ave, a power upply unit and a LCD diplay. The dimenion of the meaurement unit are 88 mm by 134 mm by 78 mm, and the total weight i le than 2 kg. The meaurement i completed within one to two minute. The hemipherical total emittance of variou pacecraft thermal control material were meaured by uing the portable ytem at room temperature. In thi meaurement, temperature of the blackbody furnace wa controlled at 363K. Figure 7 how the emittance reult meaured by the portable ytem veru by the calorimetric method. Only for 7 UPILEX-S/Al and 12 UPILEX-S/Al, the meaurement reult were compared with the calculation value in Ref. 10. The meaurement reult difference between the portable ytem and the calorimetric method were +0.03 and +0.07 for 25 UPILEX-R/Al and HOGS repectively, while thee difference between the prototype ytem decribed in chapter 3 and the calorimetric method were -0.03 and +0.04. Although a light deviation can be een for low emiive material, the over all reult of the preent portable ytem agreed well with the calorimetric method. 11
Emittance Meaured by Portable Sytem 1 0.8 0.6 0.4 0.2 Au Mirror HOGS 7 UPILEX-S/Al 12 UPILEX-S/Al 25 UPILEX-R/Al 50 UPILEX-R/Al 75 UPILEX-R/Al 250 Teflon/Al Black Paint Z306 0 0 0.2 0.4 0.6 0.8 1 Emittance Meaured by Calorimetric Method Figure 7 The comparion of meaured ε H conducted uing the portable ytem and the calorimetric method for variou thermal control material. 6. CONCLUSIONS The development of a portable hemipherical total emittance ε H meaurement ytem wa preented in thi paper. The principle of thi ytem wa reflective method uing the integrating phere and the blackbody furnace. The ytematic error caued by the temperature difference between ample and light ource and by the characteritic of the integrating phere were etimated. To evaluate the portable ε H meaurement ytem, hemipherical total emittance for variou pacecraft thermal control material were meaured and were compared with thoe obtained by the calorimetric method. For almot all material, the reult obtained by the portable ytem agreed well with thoe obtained by the calorimetric method. ACKNOWLEDGMENTS We would like to thank S. Tachikawa of ISAS/JAXA, Ph.D. H. Nagano of Keio Univerity and J. Kimura of JST Corporation for their exciting dicuion. Profeor T. Makino of Kyoto Univerity i gratefully acknowledged for helpful uggetion. 12
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