## 2 Passive Vibration Isolation Designs
The flat cross-section of the spring makes the assembled isolator very compact. The vertical adjustment is used to maintain the equilibrium position so that the isolation applies to different loads. Recently, many vibration isolators of the QZS characteristics have been proposed to overcome the drawbacks of traditional passive linear isolators [51; 52; 53; 54]. Specifically, a QZS isolator has two advantages. One is being able to attenuate the transmission of the low-frequency vibrations, and the other is possessing high static stiffness to prevent large static deflections [55]. The NSS is the vital component in the QZS isolator [56]. There exist several formulas to realize a negative-stiffness structure mechanism. The oblique-spring mechanism was the earliest form [57] and was often utilized as a theoretical model for performance evaluation of the QZS isolator. Moreover, an oblique-spring could be bulky. Changing it into a planar or oblique link-connecting horizontal spring could avoid the possible bulk. Other ways for achieving the negative stiffness include the scissor-like structures [50], cam-roller spring mechanism (CRSM) [58], magnetic springs, and bi-stable structures [59]. All these methods outlined above indicate that the QZS isolator could be an effective system to isolate the low-frequency vibration. There are different approaches proposed to achieve QZS characteristics. A summary of different types of isolators is listed in Table 1 below. As shown in Table 1, [PERSON] [47] presented a 6-DOF passive vibration isolator with the Stewart platform and X-shaped support legs. It is a simple design due to its passive vibration isolation approach. This is a very flexible and versatile scissor-like structure, which can be independent of or combined with other vibration isolators, to provide damping and stiffness as a passive or semi-active isolator. Furthermore, a prototype, as shown in Figure 4, does not contain any active or semi-active actuator devices. Flexible structure size is applicable for different application situations and can also provide good vibration isolation performance. The peak amplitude result of the displacement transmissibility is 4 at 0.35 Hz. However, there are some limitations, such as the load capacity of the system, which is primarily dependent on the ratio of the height of the system over the length of the support legs H/h and dependent on the stiffness of the spring, as the seat suspension system will have to support the load of the driver and seat weight. One of the essential parameters to be considered is the load capacity of the system that was not discussed clearly in the paper [46]. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Type of Structure** & **Geometry** & **Key Features and Novelity** & **Advantage** & **Disadvantage** & **Performance Criterion** & **Performance** \\ \hline \multirow{4}{*}{(1) Three layers:} & (1) A very flexible and versatile & & & & & The peak values of the \\  & (2) Each layer consists of connecting & (2) In ten layers independent of & & & & \\  & \(\alpha\)s and corresponding reading joints, & combined with other vibration & & & & \\  & (3) Support of the left between  
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline
**Type of Structure** & **Countory** & **Key Features and Novelty** & **Advantage** & **Disadvantage** & **Performance Citation** & **Performance** \\ \hline \multirow{6}{*}{(D-DO)} & \multirow{6}{*}{(D-DO)} & (1) Then shown that it can proceed to the base through two horizontal springs and a vertical spring & (1) Good looking performance in the frequency vibration and mixing vibration (2) by reduction in the thickness  
There is a new system that has been presented by [PERSON] (2018) where a unique human body-inspired passive vibration isolation system was developed with a special design of the coupled nonlinear inertia. This isolator is inspired by the anti-vibration structures of the nonlinear inertia, including a compact X-shaped structure.