Ellipsotopes: Uniting Ellipsoids and Zonotopes for Reachability Analysis and Fault Detection

Ellipsoids are a common representation for reachability analysis, because they can be transformed efficiently under affine maps, and they allow conservative approximation of Minkowski sums, which let one incorporate uncertainty and linearization error in a dynamical system by expanding the size of the reachable set. Zonotopes, a type of symmetric, convex polytope, are similarly frequently used, because they allow efficient numerical implementations of affine maps and exact Minkowski sums. Both of these representations also enable efficient, convex collision detection for fault detection or formal verification tasks, wherein one checks if the reachable set of a system collides (i.e., intersects) with an unsafe set. However, both representations often result in conservative representations for reachable sets of arbitrary systems, and neither is closed under intersection. Recently, representations, such as constrained zonotopes and constrained polynomial zonotopes, have been shown to overcome some of these conservativeness challenges, and are closed under intersection. However, constrained zonotopes cannot represent shapes with smooth boundaries, such as ellipsoids, and constrained polynomial zonotopes can require solving a nonconvex program for collision checking or fault detection. This article introduces ellipsotopes, a set representation that is closed under affine maps, Minkowski sums, and intersections. Ellipsotopes combine the advantages of ellipsoids and zonotopes while ensuring convex collision checking. The utility of this representation is demonstrated on several examples.