Libration control of flexible tethers using electromagnetic forces and movable attachment

This paper proposes utilising the distributed Lorentz forces that are induced in an electromagnetic tether as a control actuator for controlling the tether motion. The control input governing the magnitude of the applied actuator force is the current being conducted within the tether. A wave-absorping controller is also proposed to suppress the unstable high order modes that tend to be initiated by electromagnetic forces. The absorption of travelling waves along the tether may be achieved by proper movement of the tether attachment point on the main satellite. A mission function control law is presented for controlling the tether length and in- and out-of-plane librations, derived from a model that treats the tether as an inextensible rigid rod. The control law is numerically simulated in a continuum model of the tether system. It is shown that the out-of-plane motion of the tether can be effectively damped by appropriate control of the tether current for inclined orbits. The effect of tether flexibility does not significantly alter the time history of the tether librations, but causes significant bowing that grows into instability of the lateral modes. It is shown that this instability can be effectively suppressed by applying the proposed waveabsorbing controller.