Study On Dynamics And Control Of Electrodynamic Tether System

Electrodynamic tether systems(EDT)have the potential to play an important role in future spacecraft systems.They can provide propulsion without any propellant,hence they can be widely applied in many space missions such as orbital maneuvering,space debris removal,and satellite formation,et al.The libration dynamics and control,orbital dynamics of EDT running in the orbit are studied in this dissertation.The main work and contributions can be summarized as follows:The libration dynamics of EDT for the notilted dipole model is studied.Based on the assumption that the tether of EDT is in tention,the dumbbell model is built and the libraiton dynamic equation is derived.The stationary point iteration method is employed to obtain the periodic solution of the nonlinear libration dynamic equation.Accroding to the Floquet theory,the stability of the peridioc solution is analysied.During the analysis,the precise integration method is used to calculate the monodromy matrix.Simulation results show that periodc solutions for the system are all unstable.Several examples are employed to show the variation of the instability of the periodic solution with the electrodynamic parameters,the inclination and the eccentricity.The mass of the main satellite,the mass of the sub-satellite,the mass of the tether and the current in the tether for the EDT are regarded as system parameters.The effects of these parameters on the libration dynamics are investigated.The libration dynamics of the EDT for 13 degree International Geomagnetic Reference Field(IGRF)is investigated.The generalized forces related to the in-plane and out-of-plane angles are considered as the sum of generalized forces for the nontilted dipole model and generalized forces for higher order geomagnetic model terms.Both of the components and the generalized forces for 13 degree IGRF are compared with that for the nontilted dipole model.In the analysis of the libration dynamic characteristics,the generalized forces for higher order geomagnetic model terms are regarded as perturbations to the dynamic equations for the nontilted dipole model.Examples for different electrodynamic parameters and orbit parameters are simulated.The influences of perturbations caused by higher order terms of 13 degree IGRF for different parameters are all obtained.After setting virtual control terms in the dynamic system,the Extended Time-Delayed Autosynchronization(ETDA)control method is employed to calculate the periodic solutions of the libration dynamic equation for 13 degree IGRF.Based on the Floquet theory,the stabilities of the periodic solutions are analyzed.The failure time is defined to measure the instability,and validate the results from the Floquet theory.In addition,the relationships between the instabilities of the periodic solutions and the free parameters are obtained.Libration control of EDT using Ampere force produced by current in the tether is studied.The control scheme of the Ampere force is based on the ETDA method.The theory of control domain and the data ranges of the control parameters are presented.The validity of the control scheme is confirmed by numerical simulations in cases of circular orbit,elliptical orbit,orbit in the equatorial plane and noises existing in measuring the states of libration.The optimal control model for the libration control of EDT is built.The Radau pseusespectral method used for solving the optimal control problem is introduced.An example that switching libration motions between periodic solutions for different current parameters is employed to validate the optimal control method.The scheme of the current in the tether and the maneuvering range of the sub-satellite during the contrl process are presented.The orbital dynamics of EDT considering the libration of the tether is studied.The orbital dynamic equation is given in the form of the equinox orbital elements.The orbital altitude of the perigee obviously decreases when the system running in orbit.To investigate effects of system parameters on the orbital dynamics,the deorbit time is used to measure the efficiency of EDT.Synthesizing the simulation results and considering the launch cost and other factors in actual running,suggestions about the design of every system parameter are given.The unstability of the libration of tether affects the deorbit of EDT.Therefore,a deorbit scheme which considers the libration control during the deorbit is put forward.A simulation example is used to validate this scheme.