Smooth variable structure attitude control of crew exploration vehicles driven by nonlinear actuators

Most existing attitude control methods are based on linear actuators. This work investigates the attitude control problem of CEV driven by nonlinear actuators. Nonlinear function inverse approach is often used to address such problem, which demands intensive calculation at each control step, and in general it is nontrivial to find such inverse analytically. In this work we investigate a new method that does not involve nonlinear function inverse. The fundamental idea behind this method is to directly design the control rate, instead of the control action itself, using VSC. This approach avoids using inverse of nonlinear function, thus reducing the computation burden. Furthermore, the chattering problem in most existing VSC control methods is removed because the integration of the control rate smoothens the control action. It is worth mentioning that the proposed method can also be applied to flight vehicle with reaction rocket engines whose propulsion directions are adjustable. The potential advantage of using such adjustable gimbal-like rocket engine is that the number of rocket engines can be significantly reduced for a given flight vehicle, which could make the overall system structurally simpler and operationally more effective.