Attitude tracking of Crew Exploration Vehicles (CEVs) driven by nonlinear on -off actuators

This work investigates the attitude control problem of Crew Exploration Vehicles (CEVs) driven by on-off actuators. A smooth Variable Structure Control (VSC) method is proposed to control the firing sequence of the Reaction Control System (RCS) so that CEVs attitude is adjusted automatically. Due to the nonlinear relationship between the RCS command and the firing sequences, it is impractical to directly use the well-known inverse function method for control design. In this work we design the control rate (instead of the control action itself) directly so that the real control command is obtained by a simple integration. As a result, there is no need to search for inverse function, thus the design procedure is simplified and the computation burden is reduced. Furthermore, the integration smoothens the control action and removes the chattering problem in traditional VSC. It is shown that this method is computationally inexpensive and is able to ensure high precision CEV attitude tracking in the presence of system parameter uncertainties and varying operation conditions, as confirmed by both theoretical proof and computer simulations.