Dynamic inversion-based adaptive/reconfigurable control of the X-33 on ascent

A quaternion-based attitude control system is developed for the X-33 in the ascent flight phase. A nonlinear control law commands body-axis rotation rates that align the angular velocity vector with an Euler axis defining the axis of rotation that will rotate the body-axis system into a desired-axis system. The magnitudes of the commanded body rates are determined by the magnitude of the rotation error. The commanded body rates form the input to a dynamic inversion-based adaptive/reconfigurable control law. The indirect adaptive control portion of the control law uses online system identification to estimate the current control effectiveness matrix to update a control allocation module. The control allocation nominally operates in a minimum deflection mode; however, if a fault is detected, it can operate in a null-space injection mode that excites and decorrelates the effectors without degrading the vehicle response to enable online system identification. The overall system is designed to provide fault and damage tolerance for the X-33 on ascent. The baseline control law is based on a full envelope design philosophy and eliminates trajectory-dependent gain scheduling that is typically found on this type of vehicle. Results are shown to demonstrate the feasibility of the approach.