Retrofit Fault-Tolerant Flight Control Design Under Control Effector Damage

In this paper a theoretical framework for retrofit reconfigurable flight control is developed and applied to a large class of nonlinear models of aircraft dynamics. The proposed approach solves a difficult problem of retrofit control design that retains a baseline controller and, at the same time, accommodates severe structural damage so that the overall system is stable and the control objective is met This is accomplished using minimum prior information regarding the baseline controller. It is shown that the retrofit control design strongly depends on the properties of the nominal closed-loop system, which consists of the unperturbed system dynamics and the dynamics of the nominal controller. Using those properties in a judicious manner, it is shown that a retrofit control signal that compensates control effector damage and resulting state-dependent disturbances can be designed using the failure detection and identification subsystem and variable structure adaptation of suitably chosen signals within the proposed retrofit control architecture. In this paper, the focus is on the compensation of the control effector damage and resulting disturbances. It is shown that the proposed retrofit control design results in asymptotic convergence of the tracking error to zero. Its properties are illustrated through simulations of a high-performance aircraft dynamics. The end result of the paper is a retrofit reconfigurable control design framework that is well suited for a large class of problems encountered in reconfigurable flight control.