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Sudden or incipient faults can lead to a degradation in plant performance, impairment of important functions and eventual loss of safety. Thus, the diagnosis and accommodation of incipient faults in dynamical systems is important to meet performance and reliability requirements. This paper outlines a fault tolerant control algorithm for a hydraulic actuator with a faulty piston seal. An experimental setup capable of simulating different kinds of faults observed in hydraulic systems is constructed. A detailed nonlinear model of the experimental setup is developed and validated. A robust nonlinear control strategy is developed for position control of the actuator. A nonlinear stochastic parameter estimation algorithm is also developed and its effectiveness in estimating leakage faults is demonstrated. The nonlinear control strategy is then improved upon by adapting based on the online fault estimate. The experimental results demonstrate the effectiveness of the adaptive fault tolerant control strategy by maintaining a position tracking error below .2mm compared to simple robust control for which the error increases to .5mm as the fault size increases.

In this paper we present a formulation of an aircraft systems model for prognosis-based control. An aircraft systems model with hydraulic actuators for control surfaces is developed in SIMULINK and is connected with a model for predicting degradation of an actuator. An LQR based feedback control is employed to compensate for changes in the plant dynamics and a feedforward controller is used to achieve better performance through set-point tracking, while taking into account the current actuator degradation rate. The advantage of the combination of feedback and feedforward control strategies is then demonstrated through reduction of degradation of the actuator while achieving a minimum loss of performance using constrained optimization.

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