Electrodynamic Tether Included In The Atmospheric Drag System Stability Analysis And Control

The electro-dynamic tethered (EDT) system is widely used in astronautics and aeronautics, such as, it can be mainly used to retain the orbit of space station using the electro-dynamic tether to generate power which is higher effcient than the fuel cell; provide the micro-gravity environment freely; recycle the useful loads of the space station and remove the growing space debris effectively; change the orbit of spacecraft based on the principle of momentum exchange; do experiments in the microgravity field and so on. To make full use of the role of the geomagnetic field, the EDT system generally runs in near-Earth orbit. This kind of system is perturbed by the atmosphere significantly during flying. An intensive study of the analysis and control of the electro-dynamic tethered satellite system considering the atmospheric drag is done in this thesis. The contents of research work are as follows,(1) The orbital elements and operating principle of the EDT are introduced, and coordinate frames, which are used to establish the dynamic model, and the transformation of their relations are presented.(2) The various influence factors of atmospheric drag, for instance, the atmospheric drag coefficient, the atmospheric density, the height of the satellite's orbit and so on, are analyzed in detail. It is particularly mentioned that the atmospheric density is a complex variable, which are influenced by the geocentric latitude, the orbit altitude, the seasonal variation and the diurnal variation and etc. The atmospheric drags under the influence of various factors are obtained by numerical simulation. The analysis results show that all influence factors of the atmospheric drag couple with each other.(3) The two-dimension and three-dimension governing equations of the electro-dynamic tethered satellite system are derived using the Lagrangian method, and the stability states of the two equations are studied. The generalized forces include the gravity of the Earth, the electromagnetic force and the atmosphere drag. The calculation results of two-dimensional EDT system show that the self-balancing system will lose its stability because of the atmospheric drag. The analysis of three-dimensional model shows that there is a coupling between the in-plane motion and the out-of-plane motion. The system disturbed in the plane will not produce the out-of-plane motion. However, when the system is disturbed out of the plane, the in-plane motion will be caused. The atmospheric drag mainly influences on the in-plane motion, but not the out-of-plane motion of the system, because it is reverse with the orbital speed of the system. As the length of the tether increases, the swing angles of both the in-plane motion and the out-of-plane motion of the system are obviously weakened due to the disturbance both the atmospheric drag and electromagnetic force. The divergent trend of the in-plane motion of the system is substantially reduced because of the increasing of satellite orbit height because the atmospheric drag is decreased.(4) The current feedback control method is introduced to the two-dimensional model of the EDT system. The control results show that this control method is effective.