I. Introduction
QZS vibration isolator is a typical nonlinear isolator that can achieve dynamic characteristics of quasi-zero or lower stiffness. A single degree of freedom (1DOF) QZS isolator typically consists of two side springs and one vertical spring. As a positive stiffness mechanism, vertical springs can support loads; The springs on both sides are negative stiffness mechanisms, which are combined with positive stiffness mechanisms to obtain the quasi-zero stiffness characteristics of the system. QZS vibration isolators have been widely used in engineering [1]–[5], which has led researchers to continuously conduct in-depth research on them to further improve their performance.