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Module composition and deployment method on deployable modular-mesh antenna structures
Pergamon PII:
Acta Astronautica Vol. 39. No. 7. pp. 497-505. 1996 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain SOO!W5765(96)001614 0094-5765/96 515.00+0.00
MODULE COMPOSITION AND DEPLOYMENT METHOD ON DEPLOYABLE MODULAR-MESH ANTENNA STRUCTURESt MITSUNOBU
WATANABE,
AKIRA MEGURO, JIN MITSUGI and HIROAKI TSUNODA N’IT Wireless Systems Laboratories, Satellite Communications Systems Laboratory, Take, Yokosuka-shi, Kanagawa 238-03, Japan (Received 29 February 1996; revised version received 13 August 1996)
AIrstrati-A deployable modular-mesh antenna is the concept behind a large space antenna. To ensure reliable deployment, a synchronously deployable truss structure forming a curved reflector surface has been developed. The proposed antenna’s main reflector formed by two types of modules using mesh and cable network maintains a sufficient level of rigidity at deployment and deploys with high reliability. Importance has also been placed on the numerical analyses of cables, the mesh, and the truss structures. The truss structure analysis is based on a non-linear finite element method, rather than on multi-body dynamics, so that elastic motions of all truss members during the deployment can easily be handled. 0 1997 Elsevier Science Ltd
1. DITRODUCTION In recent years, the role of satellite communication has changed from being a supplementary system,
such as in the setup of emergency circuits and communication among isolated islands, to being an advanced mobile communication system and a multicast type of communication. The communications satellite N-STAR will be launched in 1995 to conduct advanced satellite communications. While N-STAR is expected to achieve its primary goal of advanced satellite communication, it may lack the necessary functions for personal and anytime-anywhere satellite communications due to its limited mission capabilities and relatively high cost. This type of personal and anytime-anywhere satellite communication cannot be achieved without further improvements in current technology. One such improvement is creating large-scale onboard antennas for future mobile satellite communication systems. The construction of such antennas consists of deployable structures that offer a light weight and a high level of fairing efficiency. A large antenna reflector must be stowed inside the rocket fairing during launch. Thus, the research trends concerning large deployable antennas are in the metamorphosis of a solid into a plane, a plane into a line, and a line into a point. Regarding deployable antennas, there are many research reports including those with practical examples such as Paper IAF95-16.05 presented Astronautical Congress,
1995.
at the 46th InternationoI
Oslo, Norway,
2-6 October
Refs[l-51. The mass per unit volume decreases rapidly with each of these metamorphoses. The satellite-system candidate for a modular-mesh deployable antenna is shown in Fig. 1. This paper proposes a modular-mesh antenna concept for the construction of an antenna with an aperture of 10 to 15 m that can be operated in the S-band. 2. MODULES COMPRISING ANTENNA STRUCTURE
2.1. Basic module structure The basic module structure for a large deployable antenna requires only two types of basic modules to
construct reflectors with an aperture of 10 to 15 m. Six basic modules are designed and fabricated to create a partial model of the antenna structure. In Fig. 2, each module is 1 m in equivalent diameter, and six integrated modules construct a partial model of the antenna structure that is 3 m in diameter. The modules are independent of each other, so a larger reflector can be easily constructed by adding more modules. In addition, the modules can be independently fabricated and tested, and are easy to handle and adjust. Other antenna subsystems consist of a deployable tower and an adaptive truss structure. As shown in Fig. 3, the first type of basic module can be stowed as a thin-plate form (Type 0 module) and the other can be stowed as a slender-cylindrical form (Type 1 module). The basic truss module is a modified hexagonal truncated pyramid and consists of truss members connected by spherical hinges that can rotate freely. The connection points where the standoffs are affixed 497
498
M. Watanabe
Basic module (Deployed)
c’f cd
Figure 4 shows some details of the structures for each module[6]. Deployment force is obtained through coil springs set inside telescopic truss members (axis members). Deployment motion is controlled through the control cable set along the deployment planes. Each control cable is driven by a motor through pulleys set inside the truss members. Module connection mechanisms were also designed and fabricated. The truss members must go through elastic deformation when they are stowed. Thus, the analysis of deployment characteristics must consider this elastic deformation. 2.2. Rejector
Fig. I. Large deployable antenna
to the truss module
represent points that fall on an approximated sphere, which is the best fit to a parabolic surface.
Fig. 2al-(Caption
swfuce
Figure 5 shows the general construction of the basic deployable antenna module. Each module consists of a gold plated mesh surface, a spatiallyfixed (when deployed) cable network, and a deployable truss module functioning as the support structure. The cable network consists of three kinds
opposite).
Modular-mesh antenna
Fig. 2a
Fig. 2b. Fig. 2. Modular-mesh antenna structure. (a) Partial model of antenna structure. (b) Main-reflector model (deployed).
499
M. Watanabe et al.
500
using multi-layer insulators is widely used to suppress thermal deformation. However in this case, the complicated shape and motion of the truss prevents the use of such insulators. Therefore, we propose actively controlling the mesh surface by means of actuating the transverse position of each standoff, the mechanism for which is shown in Fig. 6. control
(a)
4
Deployment control cable
3. CONSTRUCTING
A LARGE ANTENNA
REFLECTOR
3.1. Rejector construction The composition of the main reflector is examined under the following assumed conditions.
Deployme/nt ’ control motor
(b)
(1) The effective diameter of the main reflector is 10m or 15m. (2) The size on the earth panel of the satellite bus is 2m x 3m. (3) The size of the stationary part of the deployable tower on the earth panel is 0.5 m x 0.5 m. When that the Type 0 modules types.
the composition is examined, it is revealed reflector is attached to the satellite bus with modules. The method for affixing these can be classified into the following three
Configuration A (Conf. A): Fixation Method using three Type 0 modules. Configuration B (Conf. B): Fixation Method using two Type 0 modules. Configuration C (Conf. C): Fixation Method using one Type 0 module.
Deployment /
Fig. 3. Folding/unfolding motion of basic modules. (a) Thin-plate form (Stow as plane: Type 0). (b) Slender-cylindrical form (Stow as line: Type I).
of cables, that is, surface, tie and back cables. Surface cables link surface nodes that are distributed uniformly on the parabolic surface. Adjusting the tie cable length displaces the surface cable nodes. In our cable network structure, the adjustment screws attached to the tie cables can mechanically adjust cable length to control the reflector shape. Surface error caused by thermal deformation in orbit is one of the most serious problems for maintaining high-level RF performance. Thermal
The maximum module size while still fulfilling condition (2) is 2 m for Conf. A, 2.3 m for Conf. B, and 4 m for Conf. C. The relationships between module structure and the number of modules are shown in Table 1 based on these fixed configurations. In Conf. A and Conf. B, it is necessary to avoid interfering with the antenna fixture and the tower in the rocket fairing for 15 m main-reflector. As for the rigidity when attaching to the satellite bus, Conf. A is the most suitable. Moreover, the rigidity decreases when reconfiguring into Conf. B or Conf. C. Stowability of the reflector depends on its thickness after it is folded. This thickness depends on the diameter of the member and the size of the hinge. Therefore, we must carefully examine the structure design. If the size of these modules is increased, the number of required modules will decrease, resulting in a reduction in the weight of the reflector. The best method for constructing the modules should be considered in order to obtain the required aperture diameter. Therefore, it is necessary to solve the problems concerning rigidity, size, mass, deployment reliability, and attaching to the satellite bus. 3.2. Controlling the beam direction The mesh reflector deforms in orbit due to two reasons: the high temperatures resulting from the
Modular-mesh antenna
501
LowarmAbar (6) \
Dqabymenteontrd cable (a) Type 0 module Upper rsdial
Lower member (6)
(b) Type 1 module Fig. 4. Basic structure of modules. (a) Type 0 module. (b) Type 1 mlodule.
sun’s incident heat which changes with time and the seasons, and the low temperature caused by deep space radiation. Moreover, a large-scale reflector decreases the eigenfrequency. A low eigenfrequency causes structural vibration and as a result, beampointing error increases due to this vibration. Using a mechanical control and an electrical control are considered as a compensation method for this transformation and the deformation of allocated multi-beams. The required reflector accuracy is 2.4-mm RMS or less, i.e., l/50 of the wavelength of the S-band. Thus, the compensation accuracy must be l- or 2-mm RMS to control the reflector shape. In a mechanical control, there is a method for controlling the position of the sub-reflector using an Antenna Pointing Mechanism (APM) and the attitude of the tower using an adaptive structure. The adaptive structure under the antenna tower module is shown in Fig. 2a. The mesh reflector is transformed
by the thermal deformation of the support structure and the cable network. In particular, the transformation of the standoff position and deformation of the cable network are large. Therefore, it is most effective to drive the standoff directly using the actuator or the motor that supports points on the mesh surface and the cable network. Figure 6 in Section 2.2 shows the compensation by driving standoffs using the actuator. Another method to compensate for the deformation of the cable network is driving the tie cables with a small motor. In an electrical control, there is a method for beam compensation using a function of the primary radiator. For instance, each beam deformation and transformation can be compensated for independently because the shape and the allocation of beams are controlled using a beam forming network (BFN). Estimating the deformed reflector shape is indispensable in compensating for beam deformation and
M. Watanabe er al.
502
Enlarging the size of the module as much as possible within the permitted range and decreasing the number of modules as described in Section 3.1 are effective in decreasing the weight. However, selecting a method for affixing modules to the satellite bus is dependent on the size of the satellite bus. The width of the module cannot be enlarged more than 2 m for Conf. A. So, it is considered that several members and hinges are arranged on the connecting side of the module. It is effective to collect two or more modules and make a group.
Surface
Surface mesh \r
Sack cable
0
n
1
L L
L
4. DEPLOYMENT
L Standoff
\ Deployable buss
Fig. 5. Construction of basic module
transformation. For example, a reflector shape sensor is attached on the satellite and transformations of the reflector surface are measured directly. Moreover, a method is considered for calculating the difference between the designed and measured antenna patterns on the ground. 3.3. Mass qf rejector The mass allocated to the main reflector, when the total mass of the mission equipment is assumed to be approximately 600 kg, is lo&150 kg per reflector. The resulting mass distribution for one module is shown in Fig. 7 based on the three configurations in Table 1. Figure 7a shows the relationship between the number of modules necessary to form the main reflector and the mass of one module. Due to the fact that there are strict weight limitations for the hinges and members, construction of light-weight parts for the hinges are indispensable in decreasing the weight of the truss structure. Next, the relationship between the width and the mass of one module is shown in Fig. 7b. A partial model of the main reflector shown in Fig. 2 is used to confirm the deployment concept and basic characteristic test for deploying. Decreasing the weight of the reflector is a must and can be accomplished by considering light weight material, the most suitable method for constructing the reflector, and decreasing the hinge size in the future. The mass of the truss structure is disproportionally large compared to the mesh surface and the cable network. Therefore, it is necessary to optimize the truss structure composition of the main reflector in order to achieve a lightweight module.
METHOD
4.1. Deployment qf‘ modules The basic concept behind the deployable truss modules is the proposed truss structure with a synchronized deployment motion. Truss joints are capable of either revolving or translating the deployment motion, and each truss module has only one degree of freedom. Synchronized deployment motion was confirmed by ground deployment tests. For reliable deployment, the transformation must be accomplished smoothly without any elastic deformation. The reason for this is that the antenna modules were not designed for transient deployment but only for stowed and deployed configurations. 4.2. Deployment analysis A new design program was developed named Object-Oriented Coordinated Designer (OOCD)[7]. The OOCD program structure is similar to the actual hardware in order to allow the use of object-oriented languages. Structural and mechanical analysis models are created by OOCD and are updated using the results of ground or micro gravity performance testing of actual hardware. So, the analysis models are also updated automatically. A new analysis tool for deployable structures and mechanisms was also developed. We named this program the Simple Partitioning Algorithm based on Dynamics of finite Elements (SPADE)[S]. SPADE can analyze the dynamic behavior of flexible deployable structures such as a deployable truss with slender-cylindrical members by means of connecting finite element models. The biggest advantage of SPADE is that we can simply deal with flexible deployment structures. In addition, we do not need to consider redundant hinge constraints for close linkage structures. This means that we can make a simple analytical model corresponding to the actual hardware. We used Dynamic Analysis and Design System (DADS) to analyze deployment characteristics of deployable truss modules. DADS can deal with elastic deformation. However, it is not suitable for solving the deployment behavior of flexible structures whose members must be treated as flexible elements. During deployment, the truss members were assumed
Modular-mesh antenna
503
Fig. 6. Mesh-reflector shape controlling actuator.
As mentioned previously, the basic truss module forms a modified hexagonal truncated pyramid. This results in a great advantage in that only two types of
not to incur elastic deformation, and the necessary deployment force was determined from the reaction force of the surface cable network at the standoffs.
Table 1. Construction of main-reflector.
Aperture diameter of 10 m 0epl0yed Sat
stowed
Configuration and modulesize W L Conf
A
Aperture diameter of 15 m Deployed
stowed
M. Watanabe
504
Ed rrl.
4.3. Ground depIoyment tesfs[fl Ground deployment tests must be carried out to verify the deployment reliability and repeatability of the antenna modules and to improve the analytical model. Therefore, the tests were carried out to confirm the following basic characteristics.
Diameter 10 m
(1) Six combined modules can be unfolded with permissible structural deformation. (2) Six combined modules can be deployed in the ground suspended condition.
“0
10
20
30
40
50
60
70
The antenna module, more specifically the integrated modules, has an insufficient level of structural strength to overcome gravity. In addition, deployment characteristics are seriously influenced by gravity. Therefore, gravity compensation must be
80
Number of modules
(a)
(b)
01
1
2
3
4
I 5
Width of one module [ml Fig. 7. Mass of module. (a) Number
vs mass of module. Size vs mass of module.
(b)
(b) basic modules are required to construct a large modular antenna. However, as the form of the basic modules is not a completely regular hexagonal truncated pyramid, the basic modules undergo some elastic deformation during stowing. Thus analysis of the folding and unfolding of the basic modules is very important in designing structural members and driving control forces. Figure 8 shows the unfolding behavior of deployment truss module Type 0. Variation in strain energy during unfolding is indicated as a variation in truss member shading. Severe strain arises in synchronous members and axial members. Figure 9 shows bending and axial strain energy profiles of a synchronous member. Maximum axial strain energy, caused at the full stowed position, was calculated as 10,000 kgfmm2/s2. This exceeds the buckling load of the synchronous members. However, the unfolding motion of the truss structure is restricted by the physical contact of the truss members.
Fig.
8.
Unfolding behavior of three Deploying. (b) Deployed.
moduies[‘l].
(a)
505
Modular-mesh antenna
- 1oooo.
Axial strain in Synchronous member
N P
g
6000.
b 5
4oo0 .
8
2000.
.f
Axial strain in Axis member
Bending strain in Axis member
-1
50
100
200
150
250
300
escence balls were used as tracking markers. Measurement accuracy is within 1 mm. Figure 10 is a plot of the nodal position of the deployment truss module during deployment. It was confirmed that the combined modules smoothly folded and unfolded under the ground testing conditions. Some unique motion was found just before the deployed position. Nodal displacement surged just before full deployment. At this instance, the antenna modules jumped up slightly. This unique phenomenon was observed only for the combined module. This seems to be the same behavior as the latching-up motion of the offset hinge mechanisms.
Folding step
Fig. 9. Strain energy profile during folding motion[7].
considered. A simple suspension method was employed in the ground deployment tests for both single and multi-module configurations. The suspension equipment consists of pulleys, suspending wires and counterweights. Suspension wires are fixed at the top of each standoff and the wires are held taut by counterweight through pulleys. These pulleys are affixed to the fixture located approximately 4 m above the antenna modules. The deployment motion was measured by a three-dimensional video tracking system. The system consists of two CCD cameras, camera controllers, a video tracker, and a microcomputer. Small fluor-
5.
CONCLUSION
This paper has proposed a modular-mesh antenna concept for the construction of a 10 to 15 m aperture antenna that operates in the S-band RF. The newly designed deployable modular-mesh antenna structure and the advantages of the structures and mechanisms of the two types of deployment truss modules are described. The composition of the main reflector is examined under some assumed conditions. Stowability of the reflector is also discussed concerning the thickness of its folded shape. Feasibility of this design and an analysis method were confirmed through ground deployment tests. Acknowledgements We are deeply grateful to Dr Shuichi Samejima, the Executive Manager of the Satellite Communications Laboratory. REFERENCES
1400 1200 1000 I? 5 Em t 8soo LO i B z 200 0
0
-r--,
-a-----
m-m-_
-200 5
10
Deployment
15
20
time (s)
Fig. 10. Deployable profile obtained through ground test[A. AA 3917
B
I. A. Bernd, Deployable precision reflectors. Space Technology Ind. Commer. 9, 195-200 (1989). 2. G. G. Reibaldi, Antenna mechanical technologies within ESA. Proc. ESA Workshop on Antenna Technology, ESTEC, Noordwijk, The Netherlands, pp. 20-22 (May 1986). 3. R. A. Russell, T. G. Campbell and R. E. Freeland, NASA technology for large space antennas. AGARD Report 7 No. 676, pp. 2.1-2.27 (1980). 4. M. C. Bernasconi, D. Gloster and W. J. Rits, The L-band ISRS reflector for SAT-2: a summary of the initial system study. Proc. ESA Workshop on Anfenna Technology, ESTEC, Noodwijk, The Netherlands, pp. l-3 (Nov. 1989). 5. K. Miura and Y. Miyazaki, Concept of tension truss antenna. AIAA Jo&al 28, 1098-I iO4 (1990). 6. M. Watanabe. A. Meauro, H. Tanaka and J. Mitsutzi. Analytical aid exp&me~tal study on deploya& modular mesh antenna structures. Proceedings of 45th Congress of the International Astronautical Fe&ration, IAF-94-1.1.172, Jerusalem, Israel, 9-14 October (1994). 7. A. Meguro and J. Mitsugi, Design and analysis of deployable modular structures for a large space antenna. ESA Sixth European Space Mechanisms & Tribology Symposium, 4-6 October (1995) (to be published). 8. J. Mitsugi, Direct coordinate partitioning for multibody dynamics based on finite element method. AfAA/ ASME/ASCE/AHS/ACS 36th Structures, Structural Dynamics and Materials Conference, New Orleans, U.S.A., AIAA-95-1442~CP, pp. 2481-2487.
Acta Astronautica Vol. 39. No. 7. pp. 497-505. 1996 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain SOO!W5765(96)001614 0094-5765/96 515.00+0.00
MODULE COMPOSITION AND DEPLOYMENT METHOD ON DEPLOYABLE MODULAR-MESH ANTENNA STRUCTURESt MITSUNOBU
WATANABE,
AKIRA MEGURO, JIN MITSUGI and HIROAKI TSUNODA N’IT Wireless Systems Laboratories, Satellite Communications Systems Laboratory, Take, Yokosuka-shi, Kanagawa 238-03, Japan (Received 29 February 1996; revised version received 13 August 1996)
AIrstrati-A deployable modular-mesh antenna is the concept behind a large space antenna. To ensure reliable deployment, a synchronously deployable truss structure forming a curved reflector surface has been developed. The proposed antenna’s main reflector formed by two types of modules using mesh and cable network maintains a sufficient level of rigidity at deployment and deploys with high reliability. Importance has also been placed on the numerical analyses of cables, the mesh, and the truss structures. The truss structure analysis is based on a non-linear finite element method, rather than on multi-body dynamics, so that elastic motions of all truss members during the deployment can easily be handled. 0 1997 Elsevier Science Ltd
1. DITRODUCTION In recent years, the role of satellite communication has changed from being a supplementary system,
such as in the setup of emergency circuits and communication among isolated islands, to being an advanced mobile communication system and a multicast type of communication. The communications satellite N-STAR will be launched in 1995 to conduct advanced satellite communications. While N-STAR is expected to achieve its primary goal of advanced satellite communication, it may lack the necessary functions for personal and anytime-anywhere satellite communications due to its limited mission capabilities and relatively high cost. This type of personal and anytime-anywhere satellite communication cannot be achieved without further improvements in current technology. One such improvement is creating large-scale onboard antennas for future mobile satellite communication systems. The construction of such antennas consists of deployable structures that offer a light weight and a high level of fairing efficiency. A large antenna reflector must be stowed inside the rocket fairing during launch. Thus, the research trends concerning large deployable antennas are in the metamorphosis of a solid into a plane, a plane into a line, and a line into a point. Regarding deployable antennas, there are many research reports including those with practical examples such as Paper IAF95-16.05 presented Astronautical Congress,
1995.
at the 46th InternationoI
Oslo, Norway,
2-6 October
Refs[l-51. The mass per unit volume decreases rapidly with each of these metamorphoses. The satellite-system candidate for a modular-mesh deployable antenna is shown in Fig. 1. This paper proposes a modular-mesh antenna concept for the construction of an antenna with an aperture of 10 to 15 m that can be operated in the S-band. 2. MODULES COMPRISING ANTENNA STRUCTURE
2.1. Basic module structure The basic module structure for a large deployable antenna requires only two types of basic modules to
construct reflectors with an aperture of 10 to 15 m. Six basic modules are designed and fabricated to create a partial model of the antenna structure. In Fig. 2, each module is 1 m in equivalent diameter, and six integrated modules construct a partial model of the antenna structure that is 3 m in diameter. The modules are independent of each other, so a larger reflector can be easily constructed by adding more modules. In addition, the modules can be independently fabricated and tested, and are easy to handle and adjust. Other antenna subsystems consist of a deployable tower and an adaptive truss structure. As shown in Fig. 3, the first type of basic module can be stowed as a thin-plate form (Type 0 module) and the other can be stowed as a slender-cylindrical form (Type 1 module). The basic truss module is a modified hexagonal truncated pyramid and consists of truss members connected by spherical hinges that can rotate freely. The connection points where the standoffs are affixed 497
498
M. Watanabe
Basic module (Deployed)
c’f cd
Figure 4 shows some details of the structures for each module[6]. Deployment force is obtained through coil springs set inside telescopic truss members (axis members). Deployment motion is controlled through the control cable set along the deployment planes. Each control cable is driven by a motor through pulleys set inside the truss members. Module connection mechanisms were also designed and fabricated. The truss members must go through elastic deformation when they are stowed. Thus, the analysis of deployment characteristics must consider this elastic deformation. 2.2. Rejector
Fig. I. Large deployable antenna
to the truss module
represent points that fall on an approximated sphere, which is the best fit to a parabolic surface.
Fig. 2al-(Caption
swfuce
Figure 5 shows the general construction of the basic deployable antenna module. Each module consists of a gold plated mesh surface, a spatiallyfixed (when deployed) cable network, and a deployable truss module functioning as the support structure. The cable network consists of three kinds
opposite).
Modular-mesh antenna
Fig. 2a
Fig. 2b. Fig. 2. Modular-mesh antenna structure. (a) Partial model of antenna structure. (b) Main-reflector model (deployed).
499
M. Watanabe et al.
500
using multi-layer insulators is widely used to suppress thermal deformation. However in this case, the complicated shape and motion of the truss prevents the use of such insulators. Therefore, we propose actively controlling the mesh surface by means of actuating the transverse position of each standoff, the mechanism for which is shown in Fig. 6. control
(a)
4
Deployment control cable
3. CONSTRUCTING
A LARGE ANTENNA
REFLECTOR
3.1. Rejector construction The composition of the main reflector is examined under the following assumed conditions.
Deployme/nt ’ control motor
(b)
(1) The effective diameter of the main reflector is 10m or 15m. (2) The size on the earth panel of the satellite bus is 2m x 3m. (3) The size of the stationary part of the deployable tower on the earth panel is 0.5 m x 0.5 m. When that the Type 0 modules types.
the composition is examined, it is revealed reflector is attached to the satellite bus with modules. The method for affixing these can be classified into the following three
Configuration A (Conf. A): Fixation Method using three Type 0 modules. Configuration B (Conf. B): Fixation Method using two Type 0 modules. Configuration C (Conf. C): Fixation Method using one Type 0 module.
Deployment /
Fig. 3. Folding/unfolding motion of basic modules. (a) Thin-plate form (Stow as plane: Type 0). (b) Slender-cylindrical form (Stow as line: Type I).
of cables, that is, surface, tie and back cables. Surface cables link surface nodes that are distributed uniformly on the parabolic surface. Adjusting the tie cable length displaces the surface cable nodes. In our cable network structure, the adjustment screws attached to the tie cables can mechanically adjust cable length to control the reflector shape. Surface error caused by thermal deformation in orbit is one of the most serious problems for maintaining high-level RF performance. Thermal
The maximum module size while still fulfilling condition (2) is 2 m for Conf. A, 2.3 m for Conf. B, and 4 m for Conf. C. The relationships between module structure and the number of modules are shown in Table 1 based on these fixed configurations. In Conf. A and Conf. B, it is necessary to avoid interfering with the antenna fixture and the tower in the rocket fairing for 15 m main-reflector. As for the rigidity when attaching to the satellite bus, Conf. A is the most suitable. Moreover, the rigidity decreases when reconfiguring into Conf. B or Conf. C. Stowability of the reflector depends on its thickness after it is folded. This thickness depends on the diameter of the member and the size of the hinge. Therefore, we must carefully examine the structure design. If the size of these modules is increased, the number of required modules will decrease, resulting in a reduction in the weight of the reflector. The best method for constructing the modules should be considered in order to obtain the required aperture diameter. Therefore, it is necessary to solve the problems concerning rigidity, size, mass, deployment reliability, and attaching to the satellite bus. 3.2. Controlling the beam direction The mesh reflector deforms in orbit due to two reasons: the high temperatures resulting from the
Modular-mesh antenna
501
LowarmAbar (6) \
Dqabymenteontrd cable (a) Type 0 module Upper rsdial
Lower member (6)
(b) Type 1 module Fig. 4. Basic structure of modules. (a) Type 0 module. (b) Type 1 mlodule.
sun’s incident heat which changes with time and the seasons, and the low temperature caused by deep space radiation. Moreover, a large-scale reflector decreases the eigenfrequency. A low eigenfrequency causes structural vibration and as a result, beampointing error increases due to this vibration. Using a mechanical control and an electrical control are considered as a compensation method for this transformation and the deformation of allocated multi-beams. The required reflector accuracy is 2.4-mm RMS or less, i.e., l/50 of the wavelength of the S-band. Thus, the compensation accuracy must be l- or 2-mm RMS to control the reflector shape. In a mechanical control, there is a method for controlling the position of the sub-reflector using an Antenna Pointing Mechanism (APM) and the attitude of the tower using an adaptive structure. The adaptive structure under the antenna tower module is shown in Fig. 2a. The mesh reflector is transformed
by the thermal deformation of the support structure and the cable network. In particular, the transformation of the standoff position and deformation of the cable network are large. Therefore, it is most effective to drive the standoff directly using the actuator or the motor that supports points on the mesh surface and the cable network. Figure 6 in Section 2.2 shows the compensation by driving standoffs using the actuator. Another method to compensate for the deformation of the cable network is driving the tie cables with a small motor. In an electrical control, there is a method for beam compensation using a function of the primary radiator. For instance, each beam deformation and transformation can be compensated for independently because the shape and the allocation of beams are controlled using a beam forming network (BFN). Estimating the deformed reflector shape is indispensable in compensating for beam deformation and
M. Watanabe er al.
502
Enlarging the size of the module as much as possible within the permitted range and decreasing the number of modules as described in Section 3.1 are effective in decreasing the weight. However, selecting a method for affixing modules to the satellite bus is dependent on the size of the satellite bus. The width of the module cannot be enlarged more than 2 m for Conf. A. So, it is considered that several members and hinges are arranged on the connecting side of the module. It is effective to collect two or more modules and make a group.
Surface
Surface mesh \r
Sack cable
0
n
1
L L
L
4. DEPLOYMENT
L Standoff
\ Deployable buss
Fig. 5. Construction of basic module
transformation. For example, a reflector shape sensor is attached on the satellite and transformations of the reflector surface are measured directly. Moreover, a method is considered for calculating the difference between the designed and measured antenna patterns on the ground. 3.3. Mass qf rejector The mass allocated to the main reflector, when the total mass of the mission equipment is assumed to be approximately 600 kg, is lo&150 kg per reflector. The resulting mass distribution for one module is shown in Fig. 7 based on the three configurations in Table 1. Figure 7a shows the relationship between the number of modules necessary to form the main reflector and the mass of one module. Due to the fact that there are strict weight limitations for the hinges and members, construction of light-weight parts for the hinges are indispensable in decreasing the weight of the truss structure. Next, the relationship between the width and the mass of one module is shown in Fig. 7b. A partial model of the main reflector shown in Fig. 2 is used to confirm the deployment concept and basic characteristic test for deploying. Decreasing the weight of the reflector is a must and can be accomplished by considering light weight material, the most suitable method for constructing the reflector, and decreasing the hinge size in the future. The mass of the truss structure is disproportionally large compared to the mesh surface and the cable network. Therefore, it is necessary to optimize the truss structure composition of the main reflector in order to achieve a lightweight module.
METHOD
4.1. Deployment qf‘ modules The basic concept behind the deployable truss modules is the proposed truss structure with a synchronized deployment motion. Truss joints are capable of either revolving or translating the deployment motion, and each truss module has only one degree of freedom. Synchronized deployment motion was confirmed by ground deployment tests. For reliable deployment, the transformation must be accomplished smoothly without any elastic deformation. The reason for this is that the antenna modules were not designed for transient deployment but only for stowed and deployed configurations. 4.2. Deployment analysis A new design program was developed named Object-Oriented Coordinated Designer (OOCD)[7]. The OOCD program structure is similar to the actual hardware in order to allow the use of object-oriented languages. Structural and mechanical analysis models are created by OOCD and are updated using the results of ground or micro gravity performance testing of actual hardware. So, the analysis models are also updated automatically. A new analysis tool for deployable structures and mechanisms was also developed. We named this program the Simple Partitioning Algorithm based on Dynamics of finite Elements (SPADE)[S]. SPADE can analyze the dynamic behavior of flexible deployable structures such as a deployable truss with slender-cylindrical members by means of connecting finite element models. The biggest advantage of SPADE is that we can simply deal with flexible deployment structures. In addition, we do not need to consider redundant hinge constraints for close linkage structures. This means that we can make a simple analytical model corresponding to the actual hardware. We used Dynamic Analysis and Design System (DADS) to analyze deployment characteristics of deployable truss modules. DADS can deal with elastic deformation. However, it is not suitable for solving the deployment behavior of flexible structures whose members must be treated as flexible elements. During deployment, the truss members were assumed
Modular-mesh antenna
503
Fig. 6. Mesh-reflector shape controlling actuator.
As mentioned previously, the basic truss module forms a modified hexagonal truncated pyramid. This results in a great advantage in that only two types of
not to incur elastic deformation, and the necessary deployment force was determined from the reaction force of the surface cable network at the standoffs.
Table 1. Construction of main-reflector.
Aperture diameter of 10 m 0epl0yed Sat
stowed
Configuration and modulesize W L Conf
A
Aperture diameter of 15 m Deployed
stowed
M. Watanabe
504
Ed rrl.
4.3. Ground depIoyment tesfs[fl Ground deployment tests must be carried out to verify the deployment reliability and repeatability of the antenna modules and to improve the analytical model. Therefore, the tests were carried out to confirm the following basic characteristics.
Diameter 10 m
(1) Six combined modules can be unfolded with permissible structural deformation. (2) Six combined modules can be deployed in the ground suspended condition.
“0
10
20
30
40
50
60
70
The antenna module, more specifically the integrated modules, has an insufficient level of structural strength to overcome gravity. In addition, deployment characteristics are seriously influenced by gravity. Therefore, gravity compensation must be
80
Number of modules
(a)
(b)
01
1
2
3
4
I 5
Width of one module [ml Fig. 7. Mass of module. (a) Number
vs mass of module. Size vs mass of module.
(b)
(b) basic modules are required to construct a large modular antenna. However, as the form of the basic modules is not a completely regular hexagonal truncated pyramid, the basic modules undergo some elastic deformation during stowing. Thus analysis of the folding and unfolding of the basic modules is very important in designing structural members and driving control forces. Figure 8 shows the unfolding behavior of deployment truss module Type 0. Variation in strain energy during unfolding is indicated as a variation in truss member shading. Severe strain arises in synchronous members and axial members. Figure 9 shows bending and axial strain energy profiles of a synchronous member. Maximum axial strain energy, caused at the full stowed position, was calculated as 10,000 kgfmm2/s2. This exceeds the buckling load of the synchronous members. However, the unfolding motion of the truss structure is restricted by the physical contact of the truss members.
Fig.
8.
Unfolding behavior of three Deploying. (b) Deployed.
moduies[‘l].
(a)
505
Modular-mesh antenna
- 1oooo.
Axial strain in Synchronous member
N P
g
6000.
b 5
4oo0 .
8
2000.
.f
Axial strain in Axis member
Bending strain in Axis member
-1
50
100
200
150
250
300
escence balls were used as tracking markers. Measurement accuracy is within 1 mm. Figure 10 is a plot of the nodal position of the deployment truss module during deployment. It was confirmed that the combined modules smoothly folded and unfolded under the ground testing conditions. Some unique motion was found just before the deployed position. Nodal displacement surged just before full deployment. At this instance, the antenna modules jumped up slightly. This unique phenomenon was observed only for the combined module. This seems to be the same behavior as the latching-up motion of the offset hinge mechanisms.
Folding step
Fig. 9. Strain energy profile during folding motion[7].
considered. A simple suspension method was employed in the ground deployment tests for both single and multi-module configurations. The suspension equipment consists of pulleys, suspending wires and counterweights. Suspension wires are fixed at the top of each standoff and the wires are held taut by counterweight through pulleys. These pulleys are affixed to the fixture located approximately 4 m above the antenna modules. The deployment motion was measured by a three-dimensional video tracking system. The system consists of two CCD cameras, camera controllers, a video tracker, and a microcomputer. Small fluor-
5.
CONCLUSION
This paper has proposed a modular-mesh antenna concept for the construction of a 10 to 15 m aperture antenna that operates in the S-band RF. The newly designed deployable modular-mesh antenna structure and the advantages of the structures and mechanisms of the two types of deployment truss modules are described. The composition of the main reflector is examined under some assumed conditions. Stowability of the reflector is also discussed concerning the thickness of its folded shape. Feasibility of this design and an analysis method were confirmed through ground deployment tests. Acknowledgements We are deeply grateful to Dr Shuichi Samejima, the Executive Manager of the Satellite Communications Laboratory. REFERENCES
1400 1200 1000 I? 5 Em t 8soo LO i B z 200 0
0
-r--,
-a-----
m-m-_
-200 5
10
Deployment
15
20
time (s)
Fig. 10. Deployable profile obtained through ground test[A. AA 3917
B
I. A. Bernd, Deployable precision reflectors. Space Technology Ind. Commer. 9, 195-200 (1989). 2. G. G. Reibaldi, Antenna mechanical technologies within ESA. Proc. ESA Workshop on Antenna Technology, ESTEC, Noordwijk, The Netherlands, pp. 20-22 (May 1986). 3. R. A. Russell, T. G. Campbell and R. E. Freeland, NASA technology for large space antennas. AGARD Report 7 No. 676, pp. 2.1-2.27 (1980). 4. M. C. Bernasconi, D. Gloster and W. J. Rits, The L-band ISRS reflector for SAT-2: a summary of the initial system study. Proc. ESA Workshop on Anfenna Technology, ESTEC, Noodwijk, The Netherlands, pp. l-3 (Nov. 1989). 5. K. Miura and Y. Miyazaki, Concept of tension truss antenna. AIAA Jo&al 28, 1098-I iO4 (1990). 6. M. Watanabe. A. Meauro, H. Tanaka and J. Mitsutzi. Analytical aid exp&me~tal study on deploya& modular mesh antenna structures. Proceedings of 45th Congress of the International Astronautical Fe&ration, IAF-94-1.1.172, Jerusalem, Israel, 9-14 October (1994). 7. A. Meguro and J. Mitsugi, Design and analysis of deployable modular structures for a large space antenna. ESA Sixth European Space Mechanisms & Tribology Symposium, 4-6 October (1995) (to be published). 8. J. Mitsugi, Direct coordinate partitioning for multibody dynamics based on finite element method. AfAA/ ASME/ASCE/AHS/ACS 36th Structures, Structural Dynamics and Materials Conference, New Orleans, U.S.A., AIAA-95-1442~CP, pp. 2481-2487.