Purpose <p>Low-frequency vibration transmission into vehicle cabins remains a significant challenge in ride comfort enhancement. The design of vibration isolators typically requires precise knowledge of the system mass; however, in practical scenarios, passenger weight varies under different operating conditions. To address this issue, this paper proposes an active hydraulic vibration isolator aimed at improving vehicle ride comfort. The system incorporates a nonlinear negative-stiffness mechanism in parallel with a conventional suspension, effectively reducing the natural frequency and broadening the vibration isolation bandwidth.</p> Methods <p>To accommodate variations in driver weight and maintain optimal performance, a hydraulic jack is integrated to compensate for deviations from the nominal design mass. A proportional hydraulic valve, together with a PID controller, is employed to regulate the hydraulic actuator and minimize transmitted forces. In addition, a fuzzy logic supervisory controller dynamically adjusts the PID parameters to ensure robust performance under varying conditions.</p> Results <p>The proposed design equations are validated through simulations conducted in ADAMS software, showing excellent agreement with the theoretical model. Comparative analysis between the proposed active isolator and a conventional passive system demonstrates the superior vibration reduction capability of the quasi-zero-stiffness isolator. Furthermore, experimental results closely align with theoretical predictions, confirming the validity of the proposed model.</p> Conclusion <p>Overall, the proposed approach offers an effective solution for vibration isolation in systems with variable mass, such as automobiles, buses, and trains.</p>