Amplitude Dependent Bode Diagrams via Scaled Relative Graphs

TL;DR

This paper introduces a novel method to compute amplitude-dependent Bode diagrams for nonlinear systems using scaled relative graphs, providing less conservative gain bounds over restricted input sets and generalizing classical Bode plots.

Contribution

It develops a new approach combining SRGs and Sobolev theory to produce three-dimensional gain plots that depend on input frequency and energy, extending traditional Bode diagrams to nonlinear systems.

Findings

Less conservative gain bounds over restricted input sets.

Recovery of classical LTI Bode diagram in zero-energy limit.

Demonstration on a PLL-like system example.

Abstract

Scaled Relative Graphs (SRGs) provide an intuitive graphical frequency-domain method for the analysis of Nonlinear (NL) systems, generalizing the Nyquist diagram. In this paper, we develop a method for computing $L_2$-gain bounds for Lur'e systems over bounded frequency and amplitude ranges. We do this by restricting the input space of the SRG both in frequency and energy content, and combining with methods from Sobolev theory. The resulting gain bounds over restricted sets of inputs are less conservative than bounds computed over all of $L_2$, and yield three-dimensional NL generalization of the Bode diagram, plotting $L_2$-gain as function of both input frequency and energy content. In the zero-energy limit, the Linear Time-Invariant (LTI) Bode diagram is recovered, and at the infinite-energy zero-frequency limit, we recover the $L_2$-gain. The effectiveness of our method is demonstrated on an example that resembles Phase-Locked Loop dynamics.