Fixed-Time Sliding Mode Control for a Reusable Launch Vehicle in Attitude Adjustment Phase Including Actuator Dynamics

For the attitude control problem of VTVL RLV in the attitude adjustment phase, a novel fixed-time non-singular fast terminal sliding mode controller is proposed. First, the attitude model is established using quaternions to prevent the singularity phenomenon. Combined with the properties of quaternions, a fixed-time nonsingular fast terminal sliding mode surface is designed, which has fast convergence performances. Meanwhile, a fixed-time disturbance observer (FxTDO) is developed to estimate uncertainties and disturbances. Furthermore, a model predictive control (MPC) method is presented to address the problems of response delay and control signal mutation caused by the dynamics of actuators. Compared to the traditional method, the proposed controller achieves lower overshoots, smaller attitude tracking errors, shorter convergence time and chattering-free performance. Simulation results demonstrate the scheme’s effectiveness and applicability.