Study On The Dynamics And Control Of The Tethered Satellite

The tethered satellite has important applications in both near earth space utility and deep space exploration. The dynamics and control of the tethered satellite are investigated. And the problems of the deployment and retrieval of the tethered satellite, the non-Keplerian orbit and orbit maneuver characters of the tethered satellite and the stability and the tether design of the lunar tethered satellite are studied.Firstly the dynamic model of the tethered satellite is built. The differential equation of the system is established . And the tension control law of the deployment and retrieval of the tether is designed by the KTC theorems and the optimal algorithm. According to the stability judgment of the KTC theorems, the effects on the stability of the system of the elastic force, the gyro force and the damping force are analyzed. Based on the analysis, different control schemes are designed and numerical simulations are carried out. The results of the simulation show the control law is valid. Another control law is designed by the optimal algorithm that is combined with particle swarm optimization and pattern search algorithm. This control law's advantage is that it has the ability to control the terminal state as that when deployed to the equilibrium the velocity of the sub-satellite is zero, and that when deployed to the equilibrium the velocity of the sub-satellite is rather large, and that when deployed to the non-equilibrium position the velocity of the sub-satellite is zero. The scenarios above are all useful for tethered satellites' special applications. So the control law by the optimal algorithm makes the utility of tethered satellite expanded.Under the effect of the tether tension, the orbit of the tethered satellite is a non-keplerian orbit. The two dimensional orbit of the tethered satellite that in one plane is studied and the analytic solution is given. The orbit equation of the sub-satellite is found in the case that the in-plane angle is zero. It is proved that as long as the main-satellite of the tethered satellite is in a circle orbit, the sub-satellite is also in a circle orbit.And the angular velocity of the sub-satellite and the main-satellite is of the same value. Then the hung-up orbit is analyzed. The orbit maneuver of the sub-satellite after deployment and release is studied. The results of static release in a circle orbit and in a ellipse orbit and dynamic release in a circle orbit are gained. The changes of the length of tethers, orbit altitude and the main satellite's orbital eccentricity that have effects on the results of the orbit maneuver are analyzed. Finally, the deployment control and the dynamic release are considered together.The lunar tethered satellite is an application of the tethered satellite system in deep space exploration. Through the analysis of satellite under restrictive three-body problem that considered the gravity of the earth and the moon, differential equations of the system are established. And it is proved the positions at L1 and L2 are the equilibrium positions and the system potential energy is the local minimal value at L1 and L2 so the lunar tethered satellite system which is through the L1, L2 is stable. Finally, the strength and material of the tethers are analyzed. It is showed that only the carbon nanotube meets the requirements.