In this paper, we address the problem of observer design for a class of nonlinear discrete-time systems in the presence of delays and unknown inputs. The nonlinearities studied in this work satisfy the one-sided Lipschitz and quadratically inner-bounded conditions which are more general than the traditional Lipschitz conditions. Both H∞ observer design and asymptotic observer design with reduced-order are considered. The designs are novel compared to other relevant nonlinear observer designs subject to time delays and disturbances in the literature. In order to deal with the time-delay issue as well as the bilinear terms which usually appear in the problem of designing observers for discrete-time systems, several mathematical techniques are utilized to deduce observer synthesis conditions in the linear matrix inequalities form. A numerical example is given to demonstrate the effectiveness and high performance of our results.
MINH CUONG NGUYEN AND HIEU TRINHis of both theoretical and practical importance. UIO designs can be employed in many applications, e.g., robust fault detection and isolation schemes [29]. In order to design UIOs, the H ∞ filtering approach [3,13,14,24,37] provides a robust solution when the UIO matching condition [8,16] is not satisfied.Since the pioneering works by Thau [27] and Rajamani [23], observer designs for Lipschitz nonlinear systems have been widely studied. In recent years, designing observers for Lipschitz systems subject to delays or disturbances has received considerable attention from researchers. In [28], Trinh, Aldeen, and Nahavandi considered a class of uncertain nonlinear time-delay systems where the delayed portion was decomposed into matched and mismatched parts. In [11], Ha and Trinh estimated simultaneously the states and inputs of Lipschitz nonlinear systems. In [18], Ibrir, Xie, and Su designed observers for a class of Lipschitz discrete-time systems. Later on, the authors extended their work by adding delays into the nonlinear discrete-time systems in [19]. In [34], Zemouche and Boutayeb presented both full-and reduced-order observer designs for a class of Lipschitz nonlinear discrete-time systems. The same authors introduced an observer design for a class of Lipschitz discrete-time delayed systems with extension to H ∞ performance analysis in [35]. In [24], Sayyaddelshad and Gustafsson presented a robust H ∞ observer for a class of uncertain nonlinear discrete time-delay systems with multiobjective optimization, i.e., maximizing the Lipschitz constant and minimizing the disturbance attenuation level.Nevertheless, nonlinear observer designs based on the traditional Lipschitz conditions expose a major limitation, which is the inability to deal with large-Lipschitzconstant systems. One solution to overcome this drawback is to reformulate the conventional Lipschitz conditions so that all the properties of the nonlinearities can be taken into account [14,36]. Another solution is to use more general conditions, which include the well-known Lipschitz condition as a special...