This paper provides a systematic way to generalize the controller design for discrete-time nonlinear descriptor models. The controller synthesis is done using a Takagi-Sugeno representation of the descriptor models and linear matrix inequalities. We propose approaches to exploit the discrete-time nature of the problem by using delayed Lyapunov functions, which allow delayed controller gains. In addition, the well-known Finsler's Lemma is used to avoid the explicit substitution of the closed-loop dynamics. Examples are provided to show the effectiveness of the proposed results.

Stability analysis of nonlinear singular systems, arising from differential algebraic equations, is addressed in this report, exploiting the fact that it belongs to the class of Positvstellensatz problems. By means of the direct Lyapunov method and a variety of convex descriptor forms, sufficient conditions in terms of linear matrix inequalities are developed. Issues concerning proper initialization and simulation are also considered. Illustrative examples are provided.

This work proposes a proportional-integral fault estimation observer for nonlinear systems by means of convex structures. Unlike the traditional approaches that consider quadratic Lyapunov functions, this work employs nonquadratic ones. As a result, less conservative conditions in the form of linear matrix inequalities are obtained. Furthermore, an $H_{\mathrm{\infty }}$ performance is employed in order to ensure robustness against sensor noise and disturbances; thus, the designed observer estimates both the state and the actuator faults even in the presence of disturbances. The performance of the proposed methodology is illustrated through a numerical example and a three-tank hydraulic system.