The Singular Angle of Nonlinear Systems

In this paper, we introduce an angle notion called the singular angle for nonlinear systems from an input-output perspective. The proposed system singular angle, based on the angle between $L_2$-signals, describes an upper bound for the ''rotating effect'' from system input to output signals. It quantifies passivity and serves as a counterpart to system $L_2$-gain. It also provides an alternative to a recently defined notion of system phase which adopts complexification of real-valued signals via the Hilbert transform. A nonlinear small angle theorem is established for feedback stability analysis, which involves a comparison of the loop system angle with $\pi$. The theorem generalizes the classical passivity theorem via a tradeoff between the singular angles of open-loop systems.