Study On Multi-source Errors And Shape Stability For Space Mesh Antennas

Space mesh antenna has become one of the key equipment worked in space engineering projects including satellite communication,environment exploration,manned spaceflight,moon probe and so on.With the development of space technology,all kinds of space tasks have put forward some very stick requirements for the surface accuracy of space mesh antenna.However,there are always some unavoidable errors appearing in the engineering practice which will make the surface accuracy reduced and reflected electromagnetic signal scattered.These errors will eventually worsen antennas' electrical performance indicators like pointing accuracy,gain,radiation efficiency and cross-polarization index,and become a technical bottleneck of the development of space mesh antennas towards to large size and high accuracy.Therefore,it is of great significance to investigate how to establish mathematical model for multiple structural errors and explore their influence on antennas' accuracy and electrical performance.In this paper,several errors caused by patch fitting,pillow,pretension design,manufacturing,thermal deformation and cable mechanical relaxation are concerned.The main contributions of this paper can be summarized as follows:1.Space mesh antennas belong to the family of flexible tension structures characterized by strong geometric nonlinearities.Their main components,cable net and wire mesh,are lack of compressive and bending stiffness,so pretensions must be applied on cables and membranes to obtain their initial stiffness and shapes.In order to improve the computational efficiency,an equivalent-force density method was firstly developed in this paper.In this part,equivalent axial force density and equivalent transversal force density were identified to establish the nonlinear force density equations for cable-membrane structure,and a solution strategy based on fixed point method was proposed to solve the nonlinear equations rapidly.In order to decrease the pretension design error of space mesh antennas,a shape and pretension combined design method was further investigated.Eventually,several numerical simulations including planar cable-membrane structure,catenoid and umbrella/tent-like cable-membrane structures,and a hoop truss antenna reflector were conducted to confirm the efficiency and robustness of the proposed method.2.Since the cable is also lack of compressive and bending stiffness,when the wire mesh attachment is performed continuously along the cable network,the pillow distortion caused by mesh bulging opposite to the curvature of the network occurs.So,an alternative approach for the pillow distortion analysis of space mesh antennas was developed in this paper.First,the triangular and quadrilateral cable-membrane elements were selected as conventional base units for the space mesh antenna.Second,the equations and solving method were derived to investigate the pillow distortion of triangular elements for the space mesh reflector antenna based on the differential equations of the membranes and cables,and then extended to adopt other polygonal elements.To verify the validity and feasibility of the proposed analytical method,several triangular or quadrangular cable-membrane elements and a space mesh reflector antenna were taken as numerical examples in which the surface error distribution,root mean square error,and the far field electrical performance of the antenna influenced by the pillow distortion were conducted.3.Errors of member lengths,cable tensions and coefficients of thermal expansions are unavoidable in the manufacturing process.For this problem,the stochastic finite element method was applied to derive the computational formulas of fabrication imperfection and random thermal strains,by which the sensitivity of surface accuracy to component imperfection could also be revealed.In addition,a stochastic wave scattering method was investigated for dynamic response analysis of frameworks with random errors based on wave scattering method and random factor method.With the uncertain physical,geometric and loading properties in consideration,the stochastic wave scattering equation and stochastic wave translation matrix were derived for frame structures,and the methodology to extract the generalized displacements and forces from stochastic wave modes was proposed.Both the proposed methods were verified by Monte Carlo simulation method.4.Due to the load-elongation characteristics of viscoelastic fiber cables are time-dependent,space mesh antennas show a mechanical relaxation behavior.Therefore,the creep and recovery constitutive model for the mechanical relaxation behavior analysis of space mesh antennas was established based on Schapery's nonlinear viscoelastic theory and force density method.To decrease the influence of the errors aroused by this behavior on antennas,a whole process for the compensation design of cable net reflector antennas with creep and recovery behaviors in consideration was summarized,and both the force density and nonlinear finite element methods were extended together to establish and solve the nonlinear optimization model for compensating design.5.The design goal of space mesh antenna is to realize an excellent electric performance.Therefore,this paper would like to investigate an analysis method for the rapid radiation pattern analysis of distorted reflector antennas.First,the time-dependent far-field radiated pattern was established by physical optics method for mesh reflector antennas with creep and recovery behavior in consideration.Next,to improve the calculation efficiency and avoid repetitive computation of physical optics radiation integrals,the exponential error terms were expanded by Taylor series at the mean values of phase errors in each small region,and the time factor was separated from the integral for some special case.Furthermore,to give a more efficient serve for the antennas' design,fabrication,adjustment,control and/or compensation,a database-based method was put forward based on Zernike polynomials of which the coefficients were separated from the physical optics radiation integrals so that the repetitive computation of physical optics radiation integrals can be avoided.Eventually,a detailed procedure to calculate the radiation patterns of distorted reflector antennas using the database was introduced,and several distorted reflector antennas were taken as numerical examples to show the practicability,validity and robustness of the proposed method.