Failure sensitivity and robustness in reconfigurable flight control systems

This paper is concerned with the design of reconfigurable flight control systems furnished with aerodynamic redundancy. Our focus will be on dealing with the aircraft surface impairment, such as locked, float, or missing surfaces. Failures of such nature display weak signatures and require considerable effort for determining their presence, whereabouts and severity. Despite the employment of the control reconfiguration feature, the fault tolerance capability of the overall flight control system cannot be improved unless the coverage for such failures is made sufficiently high. One difficulty for achieving a much enhanced failure coverage is due to the fact that a controller capable of providing performance robustness also tends to mask the effect of failures, for the failure signals are acquired from within a closed control system loop. In this paper, we propose a control design criterion that facilitates the detection of failures without compromising the required performance robustness. Details are carried out for the design of the pitch axis controller of an experimental highly maneuverable aircraft, where redundancy in the control authority is provided by both the elevons and the canards. The goal here is to explain our design procedure and to demonstrate enhanced overall system fault tolerance. The design effort is focused on the selection of controllers that can differentiate their effects on failures that require a control reconfiguration from the effects on other uncertainties that do not require a control reconfiguration. The key idea for achieving such differentiation is to examine the FDI performance index of controllers from a parameterized set that attains a certain control performance index. The scenario with a loss of surface effectiveness is simulated for the aircraft model. The deviation in the variance of the residual signal is shown to be most significantly accentuated in the presence of failures when the controller selected under our criterion is applied.