(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

42nd AIAA/ASME/ASCE/AHS/ASC Structures,^ Structural Dynamics, and Materials Conference and

f\() I "2530 I Seattle,WA ' * 16-19April2001

AIAA-2001-1597SHAPE MEASUREMENT OF A FLAT STRETCHED LIGHTWEIGHT MEMBRANE

FOR PLANAR ANTENNA STRUCTURE

Hiroaki Tsunoda*and Yumi Senbokuyaf

NTT Network Innovation Laboratories1-1 Hikari-no-oka, Yokosuka-shi, Kanagawa, 239-0847, JAPAN

Telephone: +81-468-59-8054, Facsimile: +81-468-59-3351, e-mail: tsunoda@wslab.ntt.co.jp

ABSTRACT

The next generation of on-board antennas forgeo-stationary mobile communication satellites willneed to be at least 30 meters in diameter. For this reasonthere is strong interest in using very thin, membranetype planar surfaces together with inflatable supportstructures. Unfortunately, existing membrane materialswould yield excessively heavy 30 m class antennas.Another problem is that high levels of tension are neededto make the membrane material flat enough. Thisimposes significant requirements on the supportstructure. This paper proposes the use of a mesh typearamid membrane based on a triaxial woven fabric torealize extremely lightweight antennas. Fold lines areformed from a triaxial fabric woven from poly-phenylene benzobisoxazole fiber. A 1.5 m diameter testmembrane is constructed and measured when mountedon a highly rigid steel ring and two types of inflatabletorus. With 24 tensioning points and a surface tensionof 30 N, the measured surface accuracy is 0.1 mmRMSand the maximum distortion error is 0.2 mm to 0.3 mm.These values indicate that the membrane can be usedas the basic structure for large membrane antennas.

NOMENCLATURE

D = Diameter of membrane, mmF = Cable tension, NT = Membrane tension, N/mn = Number of connecting points

AC = Membrane: aramid fiber fabrics withchlorosulfonated polyethyleneAF = Reinforced plastic: triaxial fabric woven using195-denier aramid fiberPBO = Poly-phenylene benzobisoxazole

PE = Membrane: polyester-polyarylate fiber fabricswith ethylene vinyl alcohol copolymerPF = Reinforced plastic: triaxial fabric woven using100-denier PBO fiber

1. INTRODUCTION

Many large space structures, such as planarantennas and reflector antennas, are being studied forfuture space missions. These structures must befabricated as lightweight structures that offer highaccuracy and high stowability1"3. Reflector antennas areused in microwave systems, so their surface accuracydirectly influences their performance. Active phasedarray antennas are being studied rather than reflectorantennas due to the need to change beam direction tomeet new service requirements. Active phased arrayantennas can electrically change the beam direction tocompensate antenna distortion4. For example, a methodof changing the beam direction has been studied formicrowave power transmission5. The method uses abeacon from the receive antenna on the ground tocontrol the beam direction of the on-board antenna. Thismeans that changing the beam direction by the distortionor vibration of large antennas can be compensatedelectrically. Since pointing accuracy of the beamappears to be assured, the next key requirement for highantenna performance is adequate surface accuracy.

This paper describes construction of lightweightmembrane structures and measurement of the surfaceaccuracy of a 1.5 m diameter membrane structure toconfirm the feasibility of large planar antenna structures.Large deployable membranes have been reported forsynthetic aperture radar6"7 and microstrip reflectarrays8.These membrane antennas are made of polyimide films.Polyimide film is suitable for membrane antennasbecause radiation elements or strip lines can be easily

Copyright ©2001 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

* Senior Research Engineer, Space Structure Research Group, Wireless Systems Innovation Laboratory, SeniorMember AIAA.f Engineer, Space Structure Research Group, Wireless Systems Innovation Laboratory, Member AIAA.

1American Institute of Aeronautics and Astronautics

(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

attached to the membrane. However, we needmembranes with extremely low weight to realize thetwo-dimensional deployable phased array antennas. Thesurface imperfections inherent in such membranes mustbe suppressed by uniform tensioning against the supportstructure. Most antennas will be circular since thisequalizes the tensioning forces and simplifies thesupport structure.

This paper introduces the use of triaxially wovenfabrics of aramid fibers and poly-phenylenebenzobisoxazole (PBO) fibers in forming antennamembranes with isotropic properties. Combined withexisting resin fillers, these fabrics are used to constructa test membrane 1.5 meters in diameter. Its surfaceaccuracy is measured when mounted on a highly rigidsteel ring and two types of inflatable torus. The surfaceaccuracy and the maximum distortion of the membraneunder gravity are measured for various tensioning pointarrangements and tension levels with a steel ring. Themembrane stretching achieved with the inflatable toriis also measured and surface accuracy is evaluated.

2. CONFIGURATION OF LIGHTWEIGHTMEMBRANE

2.1 Concept of Membrane

Lightweight membranes were constructed usingfiber-reinforced plastics based on triaxial woven fabricsusing aramid fiber or PBO fiber. Conventional two-directional woven fabrics tend to deform under tensiondue to their anisotropic characteristics. A commonsolution is to cross-laminate several layers whichincreases membrane weight. However, triaxial wovenfabrics have quasi-isotropic characteristics, even if onlyone ply is used. Triaxial woven fabrics encourage thedevelopment of th£ ultimate lightweight membrane.

The materials, triaxial woven fabrics with epoxyresin, examined in this paper are shown in Table 1.Aramid fiber has a long history of space use. PBO fiberhas higher tensile strength (1.6 times) and tensilemodulus (2.1 times) than aramid fiber. AF type material,based on 195-denier (mass (g) per 9000 m length)aramid fiber, was selected as the main membranecomponent. PF type using 100-denier PBO fiber wasused to connect the membrane panels and permitfolding.

2.2 Construction of Membrane

A circular membrane, diameter of 1500 mm ±1 mm, was built and tested. Dimensions of themembrane are shown in Fig. 1 and the specificationsare shown in Table 2. Seventy-two connecting pointswere placed around the circumference of the membraneat 5 degree intervals. The connecting points werereinforced using patches of polyimide film and stainlesssteel washers. This reinforcement method was designedto offer a safety factor of three with regard to tensileloading.

Connectingpoint (for 72points)

Reinforced plasticsusing triaxial wovenfabrics of PBO fiber

375 375

Connecting point(for 24 and 72points)

Reinforced plasticsusing triaxial woven

/ fabrics of aramid fiber

375 375

1500

Section A-A

Reinforced plasticsusing triaxial wovenfabrics of PBO fiber

Reinforced plasticsusing triaxial wovenfabrics of aramid fiber

Scale: mm

Fig. 1 Construction of membrane.

Table 1 Characteristics of triaxial woven fabrics.

Type

AF

PF

Resincontent %

29.1

28.6

Massg/m2

71

38

Thicknessmm

0.10

0.08

Densityg/cm3

0.72

0.51

Table 2 Characteristics of membrane.

Diameter 1500 mmMass 126 g (Surface density 71.3 g/m2)Membrane material Triaxial woven fabric (aramid fiber)

(Thickness 0.115 mm)Overlap material Triaxial woven fabrics of poly-phenylene benzobisoxazole

(PBO) fiber (Thickness 0.07 mmX2)

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Large membranes must be stowed for launch andso must be folded. Moreover, these fabrics can not befabricated in one piece which is large enough to form acomplete antenna. Accordingly, the test membrane wasformed from four panels. The fold lines consist of PFmembranes as shown in Fig. 1. The width of overlapregions and connection space were set at 10 mm. Thethicknesses of the AF membrane, AF and two PFmembranes, and two PF type membranes were 0.115mm ± 0.02 mm, 0.198 ± 0.03 mm and 0.117 mm ±0.01 mm, respectively. The average mass and areadensity of the complete membrane were 126 g and 71.3g/m2, respectively.

The thickness of this new AF membrane posednew questions. Since all satellite antennas should betested on ground prior to launch, the membrane mustachieve adequate surface accuracy and antennaperformance under gravity conditions. To that end, asteel support ring was used for stretching the membranestructure to measure the surface accuracy of themembrane. The support ring was rotated and highlyaccurate surface measurements were achieved in thevertical and horizontal to the ground. Two types ofinflatable torus, representative of those to be used inspace, were used to support the membrane structure inthe horizontal state and surface accuracies weremeasured.

3. SURFACE ACCURACY MEASUREMENTWITH STEEL RING

3.1 Stretching the Membrane

The membrane will be wrinkled due tomanufacturing constraints and distorted due to gravity.Both effects decrease the surface accuracy of themembrane. The number of connecting points and themembrane tension are the main factors which increasethe surface flatness of the membrane. At first, themembrane was held in a steel ring (50 mm X 100 mm)instead of the inflatable torus that might be used in space

because we wanted to measure just the membrane'scharacteristics. The inner diameter of the steel ring was2000 mm. The 72 connecting points on the steel ringwere spaced at 5-degree intervals. An overview of themeasurement setup is shown in Fig. 2. Each membraneconnecting point was attached by a short fixed lengthof braided PBO cord (diameter of 0.32 mm with massof 0.106 g/m) to a coil spring and thence to the steelring by a steel cable (diameter of 0.54 mm with mass of1.17 g/m) whose length was adjusted to secure the cabletension desired. Adjustment accuracy was within ± 1N of the desired cable tension. Twenty four of theconnecting points (equi-spaced) on the steel ring wereequipped with a load cell to measure the tension applied.

3.2 Measurement method

Surface distortion was measured using a CCDlaser displacement sensor attached to a scanning table.A block diagram of the measurement system is shownin Fig. 3. The out-of-plane displacement at 76 pointson the membrane were measured by the displacementsensor; scanning speed was 80 mm/sec. The output ofdisplacement sensor was converted into voltage dataand processed by a personal computer. The resolutionof the CCD laser displacement sensor was 3 // m.

Surface distortion was measured while changingthe number of connecting points and cable tension.Measurements were also conducted in the horizontaland vertical state to evaluate the influence of gravity.The membrane tension 7, that is the tension per unitlength on the circumference of the membrane, is definedas Fn/TtD, Measurements were not conducted at largevalues of T at n = 6 or 12 because the circumference of

Membrane (test article)

Scanning unitsKYOWA USS-22A

Data loggerKYOWA UCAM-20PC

X-Y scanning table

ITCCD laser

displacement sensor

KEYENCE LK-080

Control box

KEYENCE LK-2100KEYENCE RJ-800

Data loggerKEYENCE NR-2000

RS232C PC card

Personal computer (Windows95)

Fig. 2 Surface accuracy measurement of membranestructure with steel ring.

Fig. 3 Surface accuracy measurement system formembrane structure with steel ring.

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the membrane became greatly deformed. Measurementswere conducted at 25 degrees Celsius and at least fivedata points were acquired and averaged to yield eachmeasurement point.

The measurement data was collectedcontinuously. The baseline of the measurement data wastaken as the front surface of the steel ring. The surfacestandard position was calculated by averaging the 76point data. The surface accuracy was calculated as theroot-mean-square of the deviation of the 76 point data.Maximum distortion value was taken as the differentialof maximum and minimum values in the collected 76point data. The data collected at holes in the triaxialwoven fabrics were ignored. Maximum distortionoccurred at the center of the membrane due to gravity.

3.3 Experiment Results

deviation and surface accuracy versus the distance ofconnecting points. The membrane (horizontal state) wastensioned at 49 N/m and 98 N/m. This figure showsthat at least 12 tensioning points (i.e. no more than 200mm separation) are needed. Therefore, subsequentmeasurements were made using 24 (196 mm separation)and 72 connecting points (65.3 mm separation).

Figure 5 shows the maximum distortion andsurface accuracy of the membrane in the horizontal state.This figure shows that surface tension must exceed 130N/m to hold the surface accuracy under 0.1 mmRMSwith 24 connecting points. This figure also shows thatsurface tension must exceed 110 N/m to hold the sameaccuracy with 72 connecting points. Figure 6 shows themaximum distortion and surface accuracy of themembrane in the vertical state with 24 connecting points.This figure shows that surface accuracy is constant for

Figure 4(a) shows surface deviation and surfaceaccuracy versus the number of connecting points (n =6, 12, 24 and 72). Figure 4(b) also shows surface

25

co20DC

E E

I"8 8

I 5O)

0

Average (mm)

Difference of maximumvalue and minimumvalue (mm)

Surface accuracy(mmRMS)

0 8020 40 60Number of connecting points, n

(a) Surface accuracy vs. number of connecting points

25

co 20DC

E EE E 15

.> o 10(D O

"O Cti(D <DS 311 5V) CO

Average (mm)

Difference of maximumvalue and minimumvalue (mm)

Surface accuracy(mmRMS)

0 200 400 600 800 1000

Distance of connecting points, mm

(b) Surface accuracy vs. distance of connecting points.Fig. 4 Estimation of number of connecting points.

CO

50 100 150

Surface tension, N/m

200

Fig. 5 Surface accuracy in horizontal configuration.

CO

DC

I•

1.4

1.2

1.0

0.8

0.6

0.4

0.2

£i3co 0

n = 24

Surface maximumdeviation

Surface accuracy

50 100 150

Surface tension, N/m

200

Fig. 6 Surface accuracy in vertical configuration.

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membrane tensions over 50 N. 4.1 Inflatable Torus

3.4 Comparison of results in Horizontal and VerticalStates

An interesting conclusion of the above results isthat better surface accuracy is achieved in the horizontalstate compare to the surface accuracy of the verticalstate. Increasing the tension improves the surfaceflatness. In the horizontal state, the center of membraneis distorted symmetry by gravity. The membrane canbe well flatness by increasing the tension. In the verticalstate, the gravity-induced distortion is not symmetryand so increasing the tension does not significantlyimprove the surface accuracy. The deviation in cabletension was calculated using the membrane mass. Theestimated deviation in cable tension was ± 13 % at themembrane tension of 49 N/m and ± 3 % at themembrane tension of 196 N/m. At these membranetension values, the change in actual cable tension value,which includes adjustment error, was slightly larger at± 17 % and ± 3 %, respectively. The maximumsurface distortion and surface accuracy were maintainedalmost constant at 0.9 mm and 0.15 mmRMS,respectively, even if the surface tension was changed.Since in orbit the antenna does not experiencesignificant gravity deformation, it appears to be betterto test large antennas in the horizontal state.

4. SURFACE ACCURACY MEASURED WITHINFLATABLE TORUS

Outerilorosulfonated

polyethylene(0.2 mm)Aramid fiber fabrics(0.1 mm)

Chloroprene type adhesive (< 0.05 mm)

Total Thickness: 0.5 mmSurface density: 666 g/m2

(a) Type AC.

Outer

Polyurethane (0.02 mm)

Polyester-polyarylate fiberfabrics (0.12 mm)•Ethylene VinylAlcohol Copolymer(0.012mm)

Total Thickness: 0.18 mmSurface density: 150 g/m2

(b) Type PE.Fig. 7 Membranes for inflatable structure.

Two types of inflatable torus were fabricated tosupport the lightweight membrane structure. Type ACand Type PE membranes, shown in Fig. 7, were usedto construct the inflatable tori. Each consisted of 24straight tubes. The outer diameter was 2100 mm, tubediameter was about 150 mm, and tube volume was about0.108 m3. Figure 8 shows the deployment of thelightweight membrane structure with inflatable torus.This structure folds into a cylindrical shape with heightof about 380 mm and diameter of about 300 mm. It isdifficult to achieve high surface accuracy if the thinmembrane is sewn or glued directly to construct theinflatable support structure. Instead, tabs were attachedto the inflatable torus to yield a complanate plane. Inthese tests, 24 interface tabs were glued to eachinflatable torus; their mounting positions weredetermined by inflating the torus to its baseline pressure,placing it on a flat plate, and marking the torus so as toachieve a complanate plane.

Type AC membrane used aramid fiber fabric

Memorane antennastructure

Inflatable torus Deployed

Fig. 8 Deployment of membrane structure withinflatable torus.

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with chlorosulfonated polyethylene as the gas barriermaterial. This material coated both sides of the aramidfiber fabrics and its thickness was about 0.2 mm oneach side. The total thickness of the membrane was 0.5mm so the mass of the inflatable torus using type ACmembrane was 3790 g. This mass could be reduced byabout 60% by using only one gas barrier membraneand halving its thickness. The type PE membrane usedpolyester-polyarylate fiber fabrics with ethylene vinylalcohol copolymer film as the gas barrier material. Thetotal thickness of the PE type membrane was 0.18 mmand the mass of the inflatable torus using PE typemembrane was 850 g.

4.2 Measurement Method

The tension method is decided from the resultsdescribed in the previous chapter. The 24 interfacepoints on the membrane structure and tabs on theinflatable torus were connected using braided PBOcords with miniature turnbuckles. The surface accuracyof the membrane structures was measured using thesame surface accuracy measurement system asdescribed in section 3.2. However, the CCD laserdisplacement sensor was replaced with a long range unit;the resolution of the CCD laser displacement sensorwas 10 {Ji m. Experiment view is shown in Fig. 9.

The baseline pressures of the two inflatable toridiffered because of their membrane materials andthickness as shown in Fig. 7. The baseline pressures ofthe AC and PE inflatable tori were 35 kPa and 9.8 kPa,respectively. The surface accuracy of the membranestructure was measured at 4.9, 9.8,14.7, 24.5, 35.0 kPafor the AC type torus and at 4.9, 9.8, 14.7 kPa for thePE type torus. The length of the PBO cord connectingthe membrane structure to the torus was kept constantat 127 (126) mm for the AC (PE) type torus.

4.3 Experiment Results

0.5

0-4

£ £0.3

5 o0.2"D CO

8 8*t T:13 D ft 10)0)U ' '

o.—— ̂ A\\\\

n I

At baselinepressure(Type PE)

' ̂ l

Surface maximum deviation, mm

_^_ Type AC

•^ ~^~ TypePE

k\""^xx ____ ——— o

Surface accuracy, T/ mmRMS At baseline/ pressure1 (Type AC)

1^T0 10 20 30

Inflation pressure of inflatable torus, kPa40

Fig. 9 Surface accuracy measurement of membranestructure with inflatable torus.

Fig. 10 Surface deviation and accuracy of membranestructure with inflatable torus.

The surface accuracy and maximum deviationof the membrane structure on the inflatable tori areshown in Fig. 10. The surface accuracy and maximumdeviation of the AC type torus decreased as the inflationpressure fell under the baseline pressure because themembrane shape became deformed by gravity.However, the surface accuracy does not significantlydecrease and remained better than 0.1 mmRMS even ifthe inflation pressure was only one seventh of thebaseline pressure. The surface accuracy of themembrane structure with the PE type torus wereminimum at the baseline pressure. However, the surfaceaccuracy did not remarkably change and remained betterthan 0.06 mmRMS. The maximum deviation increasedas the pressure fell because the PE type torus started todeform. The tension of the interface cable could not bemeasured in this experiment. The predicted tension fromthe results of section 3.3 using the steel ring is about 30N.

5. CONCLUDING REMARKS

This report described the construction oflightweight membranes composed of plastic reinforcedtriaxial fabrics woven with aramid fiber and PBO fiber.This membrane is exceedingly thin, only 0.115 mm,and its surface density is 71.3 g/m2. Extensive surfaceaccuracy measurements were made in the vertical andhorizontal states using a rigid steel ring. The majorresults of the experiment are that the surface accuracyof 0.1 mmRMS and maximum distortion error of 0.2mm to 0.3 mm can be achieved with the cable tensionof 28.9 N (24 tensioning points) or under 10 N (72tensioning points). The membrane structure wasmounted on two inflatable tori and its surface accuracymeasured. Acceptable surface accuracy of better than0.1 mmRMS was also obtained in this experiment. Theresults confirm that this new membrane can offeradequate surface accuracy with low levels of tension.

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(c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

REFERENCES

^reeland, R. E, and Veal, G. R., "Significance of theInflatable Antenna Experiment Technology,"Proceedings of 39th AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and MaterialsConference, AIAA-98-2104, (1998), pp. 2789-2796,Long Beach, CA.

2Bernasconi, M. C., Pagana, E., and Reibaldi, G. G.,"Large Inflatable Space-Rigidized Antenna Reflectors:Land Mobile Services Development," XXXVIIICongress of the International Astronautical Federation,IAF Paper 87-315, (1987), Brighton, UK.

3Cadogan, D., Grahne, M., and Mikulas, M., "InflatableSpace Structures: A New Paradigm for Space StructureDesign," 49th International Astronautical Congress, IAFPaper 98-1.1.02, (1998), Melbourne, Australia.

4Urasaki, S., "Inclined GEO Satellite CommunicationSystem with Deploy able Phased Array Antenna," 48thInternational Astronautical Congress, InternationalAstronautical Federation, IAF Paper 97-M.3.06, (1997),Turin, Italy.

5Kaya, N., "New Retrodirective Antenna for SPS ofSandwich Type," 48th International AstronauticalCongress, International Astronautical Federation, IAFPaper 97-R.2.07, (1997), Turin, Italy.

6Lou, M. C., Feria, V. A., and Huang, J., "Developmentof an Inflatable Space Synthetic Aperture Radar,"Proceedings of 39th AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and MaterialsConference, AIAA-98-2103, (1998), pp. 2783-2788,Long Beach, CA.

7Feria, V. A., Lou, M. C., Huang, J., and Speer, S. E.,"Lightweight Deployable Space Radar Arrays,"Proceedings of 39th AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and MaterialsConference, AIAA-98-1933, (1998), pp. 1871-1875,Long Beach, CA.

8Cadogan, D. P., Lin, J. K., and Grahne, M. S., "TheDevelopment of Inflatable Space Radar Refectarrays,"Proceedings of 40th AIAA/ASME/ASCE/AHS/ASCStructures, Structural Dynamics, and MaterialsConference, AIAA-99-1517, (1999), pp. 2488-2494, St.Louis, MO.

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