Oscillatory failure case detection for aircraft using an adaptive sliding mode differentiator scheme

This paper proposes a nonlinear scheme for estimating an Oscillatory Failure Case (OFC) using a nonlinear model of the hydraulic actuator which requires an estimate of rod speed. The estimation of the rod speed is provided by a supertwist differentiator. The supertwist gains are allowed to adapt to maintain sliding during high frequency and high amplitude OFCs. Ideally low gains in the differentiator are required because of the noisy environment associated with the physical system. A Lyapunov based analysis for the novel adaptive supertwist scheme is also presented. Simulations on both liquid and solid OFCs have been carried out on the full nonlinear aircraft model with highly detailed models of the actuators which include sensor and process noise. Good estimation of both actuator rod speed and the OFC is obtained.