論文

The future prospects of satellite communications by the year 2010 in Japan, is discussed. The prospects of providing services of broadband Internet services to passengers in airplane, long-haul trains, or sea-going vessel is under process. After estimating the demand, a next-generation of broadband and scalable mobile satellite communication systems is proposed that could offer these services at affordable price. It is stated that to realize such a system key technologies must be developed to meet the technical challenges posed by the communication system and the problem related to the payload of the satellite.

Cables are very promising structural elements for space applications since they can be packaged easily in a small volume and function as truss members when in a stretched state. However, an exact analysis of cable structures is difficult because of cable slackening and large deformations. In this paper, a nonlinear cable structure analysis method is derived based on Hamilton's principle and a general purpose cable structure analysis code (CASA) is developed. Higher-order strains are included in the method by taking positions, not displacements, as variables, and cable slackenings are given by a nonlinear, axial stiffness of cable. Nonlinear static and linear dynamic analyses of structures composed of cables and/or trusses can be treated by the present method.

The tension truss antenna is a deployable mesh surface antenna featuring a cable network, called a tension truss, that gives a parabolic shape to the reflective mesh. Cables in the tension truss are rigid enough to prevent their elongation due to the tension, and the number of nodes and cables satisfies the statistically determinant condition. In this paper, surface sensitivity to magnitude and direction errors of the applied force, which gives the tensile force to the tension truss, is evaluated by Monte Carlo simulation. Results show that the surface deformation of a tension truss antenna is smaller than that of an antenna whose cable network is made of flexible cables. It is concluded that the tension truss antenna is a promising candidate for a large space antenna.

This paper describes how an on-board satellite large reflector structure was designed based on the thermal distortion predicted by structural analysis in coordination with thermal estimation. As the reflector consists of many small similar modules, it is possible to simplify the analytical model and the fabrication process. Module size and number are decided by estimating the surface accuracy for a 13 m aperture reflector. The maximum thermal distortion is found to be occurred during eclipses. As the module number increases, the thermal distortion increases. When the antenna system is used in 2.5. G Hz (S frequency bands), total surface accuracy needs to be 2.4 mm RMS (specified value). This requirement is satisfied by a reflector composed of 14 modules; maximum error is 1.6 mm RMS by slight repositioning the reflector.

A multibody dynamics formulation based on a nonlinear finite element method is presented, particularly targeting beams connected with hinges. A nonlinear kinematics, called direct strain measure, is employed to extract the strains of beams subjected to large displacement, including finite rotations but small strain. Hinges are mathematically expressed in terms of the relation between connecting nodes, rather than Lagrange multipliers. The resultant dynamic equation is a differential equation of minimum set of degree of freedom, while the sparsity of mass and stiffness matrices are preserved. Computational examples prove the validity of the proposed method. © 1997 Civil-Comp Ltd and Elsevier Science Ltd.

This paper presents a method to handle a cable element in the flexible multibody dynamics simulation. The formulation for the cable element successfully separates the material and geometric stiffness matrices irrespective to the choice of rotational parameter. The proposed formulation has been implemented in the previously developed flexible multibody dynamics computer program. An experiment on a simple deployable truss structure whose deployment is controlled by a cable reveals that the proposed method can yield results that are accurate enough for industrial applications.

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. © 1997 Elsevier Science Ltd.

An unconventional coordinate partitioning method for flexible multibody dynamics based on the corotational finite element method is proposed. The method establishes the relationship between the nodal degrees of freedom and independent degrees of freedom hinge by hinge, and results in a set of differential equations as the governing equation. Computational examples show that the proposed method can give a physically valid solution for flexible motion during mechanical motion.

A nonlinear kinematics called the direct strain measure is proposed for large deformation and finite rotation analyses of a two-node Timoshenko beam. The kinematics extracts pure elastic deformations without deriving any virtual frame to describe rigid body motion. Internal force and stiffness matrices are derived to constitute a Newton's method for finding the solution. Computational results show that the present method is valid for Timoshenko beams whose relative angular displacement in an element is less than 0.5 rad.

A finite-element-method-based formulation for flexible multibody dynamics, particularly targeting flexible link mechanisms, is proposed. The formulation is composed of two techniques: kinematics of largely displaced Euler beam element and direct coordinate partitioning. The kinematics is basically a corotational method. Direct coordinate partitioning, which is effective only in finite-element-based multibody dynamics, partitions the independent and dependent degrees of freedom hinge by hinge. A computational example on a slider crank with an assembly error shows the validity of the formulation and the developed program, SPADE.

This paper describes a concept and key technologies for a modular mesh deployable antenna. A modular concept is suggested. The advantage of the modular concept and the influence of alignment errors on the surface accuracy are estimated quantitatively. The antenna reflector consists of a mesh surface, a cable network, and deployable truss modules. The cable network comprises three kinds of cables - surface, tie, and back cables. Adjustment of tie cable lengths improves the surface accuracy. Synchronous deployment truss structures are considered as a supporting structure. Their design method, bread board models, and deployment analysis are explained.

A nonlinear static analysis method for cable structures, particularly emphasizing cable networks, is presented. Cable strains are measured from the current geometry and compressed cables are analytically disassembled with the construction of the equilibrium equation and the stiffness matrix. Finite rotations of cable intersections, referred to as nodes, and cable elements passing through more than two nodes are included in the formulation. An integrated analysis with a linear finite element method is also presented to account for the elastic deformations of supporting structures for the cable networks. The formulation is programmed for a CRAY2 supercomputer using parallel processing to solve the linear equations. Applications of the method used for mesh antenna development are also presented. © 1994.

Nonlinear analyses of cable network structures using Newton's iteration often encounter difficulties in solving for a singular stiffness matrix stemming from the tension-stabilizing nature and slackening of cables. This paper presents a solution method, called null space projection, to traverse such a singular linear system in Newton's iteration. In the method, the current unbalance of internal and external forces is projected on the null space of the stiffness matrix and the contributions of null space basis for the variable updating are derived such that the total potential energy is reduced. Numerical example shows that the method successfully traverses singular systems in Newton's iteration process and consequently gives the equilibrium state of the structure.

A pillow distortion analysis method for modular mesh antennas composed of mesh and cable networks has been developed. The mesh is modeled with equivalent cables. The amount of distortion in the mesh's shape and the degradation in the antenna pattern caused by pillowing was calculated with the method for two values of the tension ratio. It is shown that the ratio should be above 0.5m to minimize antenna pattern degradation.

The deployable modular mesh antenna is composed of independently fabricated and tested antenna modules to reduce difficulties in ground testing. Each module comprises a metallic mesh shaped by a cable network and a deployable truss structure. This paper states the basic constitution of each module, especially of the cable network, and results of surface shape adjustment simulations and experiments. Employing the present method, reflector distortions after the fabrication can be adjusted by changing cable lengths without any sensitivity analysis. Numerical simulations and experiments clarify that the attainable surface accuracy of a module is determined mainly by the accuracy of the truss structure.

This paper describes a precise verification procedure for antenna-deployment mechanisms using ground-deployment test data. Ground-deployment tests are carried out for a communication antenna reflector for the Engineering Test Satellite VI (ETS-VI). The antenna-deployment mechanism studied consisted of a bearing, a helical spring, a chassis, and a latch mechanism. The characteristics of individual parts were measured under space and ground conditions. Then the initial characteristic of a deployment mechanism is statistically predicted. Ground verification of a deployment mechanism is usually difficult because of atmospheric and gravitational effects especially at the system level. Therefore, a new verification procedure is employed. Two kinds of suspending system are developed to compensate for gravity effects. The characteristics of a deployment mechanism can be estimated by identifying system parameters for the equation of deployment motion. This estimation procedure covers all development stages, so that a characteristic history of a deployment mechanism can be easily obtained. Atmospheric and gravitational effects on deployment motion are taken into account as system parameters. It is clarified that all mechanical parts retain their initial characteristics during assembly and environmental testing.