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In this paper, we solve the problem of output tracking for linear uncertain systems in the presence of unknown actuator failures using discontinuous projection-based output feedback adaptive robust control (ARC). The faulty actuators are characterized as unknown inputs stuck at unknown values experiencing bounded disturbance and actuators losing effectiveness at unknown instants of time. Many existing techniques to solve this problem use model reference adaptive control (MRAC), which may not be well suited for handling various disturbances and modeling errors inherent to any realistic system model. Robust control-based fault-tolerant schemes have guaranteed transient performance and are capable of dealing with modeling errors to certain degrees. But, the steady-state tracking accuracy of robust controllers, e.g. sliding mode controller, is limited. In comparison, the backstepping-based output feedback adaptive robust fault-tolerant control (ARFTC) strategy presented here can effectively deal with such uncertainties and overcome the drawbacks of individual adaptive and robust controls. Comparative simulation studies are performed on a linearized Boeing 747 model, which shows the effectiveness of the proposed scheme.

This paper proposes the design of an observer to estimate the velocity of an electro-hydraulic system by using pressure measurements only. The difficulties involved in the design of an observer for such a system include the highly nonlinear system dynamics, severe parametric uncertainties such as large variation of inertial load and unmatched model uncertainties. In order to address these issues, a nonlinear model-based adaptive robust observer is designed to estimate the velocity. The contributions of the proposed work is twofold. First, it introduces a novel coordinate transformation to reconstruct the velocity estimate. And second, from a structural viewpoint, the design has two important features: (i) an underlying robust filter structure, which can attenuate the effect of uncertain nonlinearities such as friction and disturbances on the velocity estimation, and (ii) an adaptation mechanism to reduce the extent of parametric uncertainties. Experimental results on the swing motion control of an electro-hydraulic robot arm demonstrate the effectiveness of the proposed observer.AN ADAPTIVE ROBUST OBSERVER FOR VELOCITY ESTIMATION IN AN EH SYSTEM 1077 common because of the cheaper cost of installing pressure sensors. Position and velocity feedback sensors, however, are not typically used in such systems because of the prohibitive costs and the high likelihood of failure in harsh environments. Hence, design of observers, which estimate the velocity from available measurements, for example, the cylinder pressures, is essential for the implementation of the feedback control law [6].The theoretical challenges in designing a velocity observer for EHS arises from the inherent nonlinear nature of the system, for example, deadband and hysteresis present in the control valves, nonlinear pressure/flow relations, and variation in the fluid volumes due to the movement of the actuator. Apart from the nonlinearities, hydraulic systems have large extents of modeling uncertainties. The uncertainties can be classified into (i) parametric uncertainties and (ii) uncertain nonlinearities. Examples of parametric uncertainties include the large changes in load experienced by the system and the variations in hydraulic parameters due to change in temperature, pressure, and component wear [7,8]. Another class of uncertainties such as external disturbances, leakage, and friction cannot be modeled exactly, and the nonlinear functions that describe them are not known. These uncertainties are termed as uncertain nonlinearities [9,10]. Addressing both the issues simultaneously makes the observer design a fairly challenging problem.In the past, observer design was largely performed by linearizing the system and applying Luenberger observer design techniques to the linearized plant. Unfortunately, as discussed in the previous section, the hydraulic system is inherently nonlinear, and many of the nonlinearities are non-smooth and discontinuous like control input saturation, directional change of valve opening, friction, and valve deadband. Hence,...