Dynamics And Control Of Tether-net Capture System

Tethered system is an important technology of astronautics, which is a new domain of space technology in these days. Since the middle of 1970, the theory of tethered system has shown its widely potential application with several decades development. The technology of tethered system has been one of the most interesting fields of space research in America and Europe. Based on this background, the control and application of tethered system is researched.Tethered system is a special system which just has a soft tether and 2 spacecrafts and it has strong non-linearity. Tether-net capture system is the sample of the tethered system: the capture terminal is the component star. The purpose of this paper is to control this system which has strong non-linearity and make it release the terminal to the target location, retrieve the terminal stably.Firstly the intrinsic property of the system is presented, such as gravity gradient property, conservation of angular momentum and non-Keplerian orbit property. Then, the system model with oscillation suppression segment S is derived, using the Newton Second Law.Secondly, based on the mode, the stability analysis and its simulation are done, and capture program is also presented. From the analysis: the tethered system is stable when it releases the terminal and divergent when it retrieves the terminal, And its simulation of releasing and retrieving terminal proves it correct. About the problem of up or down release, the force analysis of it shows that it is reasonable for up or down release.Lastly, based on state feedback pole placement and model reference tracking theory, tether net control problem is researched. The retrieving period is divided into two periods: one for tension control, other for S segment control, according to dynamics and coupling of the tether net system. In retrieving the tether length and its rate are little coupling with in-plane and out-of-plane angles when length is long and much coupling with the angles when length is short. So in the first period the tension control is used; in the second period, in order to restrain the divergent effectively and enhance system's security, freedom of suspension center is increased, and S segment control is used with the state feedback pole placement method. The simulation of S segment control shows that: the S segments are very useful for restraining the retrieval divergent of the angles when the tether is short.