Further results on the set invariance approach to stability analysis of linear systems under saturated linear feedback

Ellipsoids, as level sets of quadratic Lyapunov functions, and the convex hull of ellipsoids, as a level set of a certain composite quadratic Lyapunov function, have both been extensively used as estimates of the domain of attraction of a linear system under saturated linear feedback. By expressing the saturated linear feedback law on the convex hull of a group of linear feedback laws, conditions have been established under which an ellipsoid or the convex hull of a group of ellipsoids are contractively invariant sets and are thus estimates of the domain of attraction. These conditions are usually less conservative for single input systems than for multiple input systems. In this paper, we consider multiple input systems and establish conditions for contractive invariance of both ellipsoids and convex hull of ellipsoids that are less conservative than the existing conditions.