A unidirectional convergence sliding mode control with bounded control is proposed. For second-order system,
the unidirectional convergence means that the initial state and the reaching position on sliding mode are in the same quadrant
of phase plane. The cause of non directional motion is revealed by analyzing the dynamic motion of the reaching phase. The
unidirectional convergence condition is proposed, and the unidirectional convergence domain is obtained. The above control
method is used in rendezvous and docking control system which needs unidirectional convergence, and the results show the
effectiveness of the proposed control method.