Vibration isolators are essential in applications like high precision manufacturing, aerospace, automobiles, and sensitive scientific equipment, where vibration can significantly degrade performance and reduce operational lifespan. Conventional vibration isolators apply springs and dampers between the load and the vibration source to absorb vibrations. However, these isolators are linear and face an inherent trade-off: achieving low-frequency vibration suppression requires a low static stiffness, that is, a very soft spring, but this compromises the system’s capability for supporting static loads.
This trade-off has prompted researchers to explore non-linear isolators with high static and low-dynamic stiffness properties, known as quasi-zero stiffness (QZS) isolators. QZS isolators can decouple static and dynamic behaviors by implementing a positive stiffness element, like a linear spring, that bears the static load combined with a parallel negative stiffness element that enables effective isolation of low-frequency vibrations. Despite their advantages, QZS isolators have two critical limitations. Their performance is dependent on precise parameter tuning for a specific payload, meaning that any change in payload can lead to a significant degradation or even complete loss of isolation performance. Second, these systems do not eliminate the residual resonant peak, which can result in large-amplitude oscillations and, under certain conditions, even chaotic motion.
To address these limitations, a research team led by Professor Seunghun Baek from the School of Mechanical Engineering at Pusan National University in South Korea has developed an innovative hybrid control strategy for a QZS isolator. “ We have developed a controllable QZS isolator that utilizes motor actuation to address both payload variations and vibrations associated with the residual resonant peak, ” explains Prof. Baek. Their study was made available online on June 10, 2026, and published in Volume 257 of Mechanical Systems and Signal Processing on August 1, 2026.
The rhombus-shaped QZS system investigated in the study consists of four identical links, two fixed vertical springs, and a horizontal spring. In its passive configuration, the system is highly sensitive to payload variations. To solve this, the researchers modified the structure by integrating electric actuators at the joints to which the horizontal spring is connected. The actuators are employed to modulate the horizontal spring’s pretension, changing its effective initial length, which serves as the control parameter.
To achieve a zero-stiffness condition with this system, the researchers employed a hybrid control strategy with two control laws. The first control law is designed to help the system adapt to payload variations by changing the effective length of the horizontal spring. This shifts the system’s equilibrium point to maintain the QZS condition, despite any change in the payload. The second control law enables the same actuators to continuously adjust the horizontal spring in real time based on the system's current state. This generates a counteracting force that stabilizes the payload at its equilibrium position, even under vibrations transmitted from the base, effectively eliminating the residual resonant peak.
Experiments using a prototype validated the strategy. The first control law allowed the system to adapt to payloads ranging from 1.01 kg to 1.21 kg, successfully maintaining the low-frequency isolation characteristics, where passive systems would have failed. The second law demonstrated complete elimination of residual ultra-low-frequency resonance under a 1.11 kg payload.
“ Our hybrid control strategy effectively addresses the static payload-matching problem and the dynamic resonance problem as two coupled aspects of a single control challenge, ” remarks Prof. Baek. “ By expanding the capabilities of QZS isolators, our ‘ smart cushion’ could inspire isolators that automatically sense a change in weight and re-tune themselves in seconds. This will be crucial for fields like chip manufacturing where precision is paramount, and even for robots carrying fragile goods.”
Overall, this innovative strategy brings QZS isolators a step closer to becoming adaptive, intelligent systems, creating machines that respond smartly to their physical environment.
Reference
Title of original paper: Active equilibrium control of a rhombus QZS isolator: A hybrid strategy for payload compensation and resonance elimination
Journal: Mechanical Systems and Signal Processing
DOI: https://doi.org/10.1016/j.ymssp.2026.114561
About Pusan National University
Pusan National University, located in Busan, South Korea, was founded in 1946 and is now the No. 1 national university of South Korea in research and educational competency. The multi-campus university also has other smaller campuses in Yangsan, Miryang, and Ami. The university prides itself on the principles of truth, freedom, and service and has approximately 30,000 students, 1,200 professors, and 750 faculty members. The university comprises 14 colleges (schools) and one independent division, with 103 departments in all.
Website: https://www.pusan.ac.kr/eng/Main.do
About the author
Prof. Seunghun Baek is a Professor in the School of Mechanical Engineering at Pusan National University, South Korea. He studies how machines and structures vibrate, and how to stop unwanted vibrations from causing problems in precision equipment. His lab designs smart mechanical systems that can sense changes in their environment and automatically adjust themselves to stay stable. Prof. Baek has a particular interest in quasi-zero-stiffness isolators — a clever type of spring-based cushion — and has led research to make them smarter, more adaptable, and ready for real-world use.
Lab: https://sites.google.com/view/sysdyn
ORCID Id: 0000-0001-9020-6579
Mechanical Systems and Signal Processing
Experimental study
Not applicable
Active equilibrium control of a rhombus QZS isolator: A hybrid strategy for payload compensation and resonance elimination
1-Aug-2026
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.