## 2 Passive Vibration Isolation Designs  
Most of the seating suspension systems use 1- Degrees of freedom(DOF) vibration isolators that only reduce vibration in the vertical direction. However, the driver and passenger receive the vibrations transmitted from multiple directions. Therefore, a seating suspension system capable of attenuating vibration from multiple degrees of freedom is desired. In order to solve these issues, many research studies have been conducted on multi-degree-of-freedom vibration isolation, especially on 6-DOF vibration isolation. Two most significant aspects for the design of vibration isolators are highly effective vibration isolation and large loading capacity. In conventional passive isolation design, the mechanism supporting the preload has linear stiffness, a trade-off that is unavoidable between the vibration isolation efficiency and loading capacity [37]. The deflection in the resilient support structure of the isolator can generate a force to support the preload. Thus, a large stiffness is required to avoid excessive deflection. In contrast, low stiffness coupling helps to reduce the vibration transmissibility and the resonant frequency of the isolation system. However, mechanical coupling with a low stiffness will decrease the loading capacity of the system, which is disadvantageous in the seating suspension systems that require high loading capacity to support the weight of the driver and passenger. The high static stiffness and low dynamic stiffness are ideal for a vibration isolation system, which is a typical nonlinear stiffness characteristic. The property of the high static and low dynamic stiffness is called quasi-zero-stiffness (QZS) [38; 39]. However, strong nonlinearity could induce the bifurcation phenomenon (e.g., jumping) in the existing QZS systems. Increasing the damping of the system could effectively reduce the jumping phenomenon. [PERSON] and [PERSON] [40] proposed a negative-stiffness structure (NSS) for low-frequency passive vibration isolation. The possibility of using air springs and shock absorbers to improve the vibration isolation performance of the seating suspension system was studied by [PERSON] et al. [41]. A seating suspension system has been designed by [PERSON] for typical articulated or rigid frame dump trucks, which have air springs and auxiliary volumes [42]. In addition, the passive vibration isolation of QZS devices and structures has been studied [43; 44]. [PERSON] et al. [45] developed a novel 6-DOF isolator in 2015, using magnetic levitation to provide the QZS vibration isolation in the vertical direction and zero stiffness in the other five directions. [PERSON] et al. [46] proposed a 6-DOF isolator in 2016 based on the Stewart platform configuration but used the novel biological inspiration X-shaped [47] structure as legs, rather than the active actuator/damper in the traditional Stewart platform. [PERSON] et al. [48] in 2017 presented a procedure or method for creating a 6-DOF QZS vibration  
Figure 3: Vibration transmission directions through a vehicle seat occupant [33]. isolation platform by using QZS struts, which is the latest development work of the 6-DOF vibration isolator proposed. [PERSON] et al. [49] proposed a low-frequency vibration isolation system with an NSS for a driver's seat where the NSS can be used to reduce the dynamic response of the isolated equipment while the load-carrying capacity is maintained. A vibration isolator with the QZS property in three different directions was proposed by [PERSON] et al. [50]. [PERSON] [51] proposed vibration isolators of the QZS characteristics. The proposed isolator consists of two adjustment mechanisms, which are the vertical adjustment and the lateral adjustment. The lateral adjustment is designed to match the geometric ratio with the stiffness ratio so that zero stiffness can be achieved under imperfect conditions. Instead of using coil springs, the isolator adopts specially designed planar springs. A planar spring can be easily redesigned to achieve an arbitrary stiffness.