Reconfiguration control laws for fault-tolerant hydraulic actuators of fly-by-wire helicopters

This paper deals with the development of a reconfiguration strategy of the closed-loop control of a fault-tolerant hydraulic actuator for fly-by-wire helicopters, capable of compensating both failure effects and system nonlinearities. Actually, a basic concern about the control reconfiguration of systems with active redundancies is that a fault is typically compensated by increasing the demand to the remaining active lanes, but the resulting signal amplification can bring on system nonlinearities (e.g. voltage or current saturation) and unexpected dynamic behaviours. The study, focused on the actuator performance analysis in case of servovalve coil faults, points out that a standard reconfiguration based on the current demand amplification can imply “jump resonance” phenomena, and that these effects can occur in the frequency range of interest for helicopter flight controls. The proposed reconfiguration strategy, essentially based on the reduction of the current demand signal when saturation conditions are achieved, allows to eliminate the problem. The result is obtained by identifying the relationship between the jump resonance frequency and the system dynamic characteristics, and by inserting in the servovalve closed-loop control a dynamic notch filter, located at the estimated jump resonance. The effectiveness of the proposed approach is assessed via simulation, by using an experimentally-validated model of the reference actuator developed by the authors.