Fast smooth second order sliding mode control design for near-space hypersonic vehicle's re-entry attitude

This paper deals with the attitude control problem for a near-space hypersonic vehicle (NHV) subject to strong parameter uncertainties and external disturbances. To solve the issue that the slow-loop virtual control signal should be smooth enough so as to be well tracked in the fast loop, which is usually neglected in sliding mode control design with two-loop structure for NHV, a fast smooth second order sliding mode (SSOSM) algorithm is proposed and its finite time convergence is proved based on Lyapunov theorem. Then it is utilized to deign the slow-loop control system to generate smooth angular rate command as virtual control signal so as to be tracked by the fast loop. Further in the fast loop, a modified super twisting algorithm with enhanced robustness is used to construct the robust fast-loop control system. Moreover, to achieve finite time convergence for tracking errors in the slow and fast loop respectively, terminal sliding mode approach is integrated into the two-loop control scheme. Finally, simulation results are presented to show the effectiveness of the proposed approach.