Limited Adaptive Authority Flight Control for the Crew Launch Vehicle

Abstract

The control system of the NASA Crew Launch Vehicle is designed to meet performance and robustness requirements during its ascent flight phase. However, the controller bandwidth and the attainable level of robust performance is limited by the degree of flexibility inherent in the long and slender design that has been adopted for this vehicle. Since there remains a substantial degree of uncertainty regarding the structural dynamics of this vehicle, the degree of risk associated with flight control is reduced by permitting a greater level of robust performance to be attained by augmenting the existing flight control system design with an adaptive element to recover nominal performance in the event of severe performance degradation. Attitude stabilization and flexible mode suppression of the model can be achieved by using a model reference adaptive controller designed to maintain the design level of tracking performance in the presence of disturbances, parametric uncertainties and unmodeled dynamics. The control law developed in this paper employs a limited authority adaptive element based on a new Virtual Hedging concept that augments an existing linear control law. The adaptation process is separate from the realization of the adaptive control decision. This guarantees a stable adaption process by employing a state emulator. It allows the system uncertainty to be learned online without applying any additional control. This is critical for implementing an adaptive control law as an emergency backup control law since the adaptive element will be engaged when the system performance is already poor. Initial errors between the system and the reference model, once the adaptive control is engaged, decays as the free response of the reference model (to within the ultimate bound). The adaptive design is carried out in both state feedback and output feedback form. Simulation examples illustrate the viability of the method.