Full-stroke constant-zero stiffness vibration isolation enabled by a stiffness-programmable dual-bar spring module

Quasi-zero stiffness (QZS) isolators have received extensive attention due to their advantageous performance in terms of low-frequency vibration isolation. However, the inherent stiffness nonlinearity of conventional QZS designs can amplify the jump phenomenon and introduce stability concerns, in particular when subject to high excitation levels. To address this issue, this paper presents a dual-bar spring vibration isolator that enables a full-stroke constant-zero stiffness (CZS). A dual-bar spring mechanism is formulated as a stiffness-programmable module capable of exhibiting constant positive stiffness (CPS), constant negative stiffness (CNS), and QZS characteristics through parametric design. Based on the programmed stiffness modes, the corresponding stiffness components are combined in parallel to synthesize full-stroke constant-zero stiffness. An analytical model of the equivalent restoring force and stiffness characteristics of the stiffness-programmable dual-bar spring module is established based on a static mechanical analysis. The dynamic isolation performance of the synthesized full-stroke CZS configuration is subsequently investigated to evaluate its low-frequency vibration isolation capability. A functional prototype that exhibits full-stroke CZS characteristics is manufactured, and static and dynamic experiments are conducted for performance validation. Experimental results demonstrate force equilibrium at multiple positions and effective vibration isolation at frequencies as low as 2 Hz under harmonic, frequency-sweep, and stochastic excitations. The results indicate that the proposed isolator provides an effective passive solution for ultralow-frequency vibration isolation, and its full-stroke CZS characteristics further support stable operation under high excitation levels.