Postcapture robust nonlinear control for tethered space robot with constraints on actuator and velocity of space tether

Stabilization control is an essential mission for the tethered space robot-target combination during the postcapture phase of tethered space robot (TSR). In this paper, the stabilization problem of such a tumbling combination is studied. With the consideration of the space tether and the attitude of the TSR's gripper, the dynamic model of the combination is first derived using Lagrange method. Then a robust nonlinear controller for the combination is proposed based on the backstepping control method. Considering the constraint on the velocity of the space tether, command filter method is utilized to guarantee the velocity of the space tether within a permitted range. A feedback term is designed to compensate the saturation of the thruster. Moreover, an adaptive law is designed to estimate the disturbance of parameter uncertainties and this disturbance is compensated in the proposed controller. Numerical simulations suggest that the proposed robust controller can realize the orbit and attitude stabilization of the combination; besides, the velocity of the space tether is effectively constrained and the parameter uncertainties of the combination can be compensated via the adaptive law.