# Review on Seat Suspension System Technology Development
## 3 Semi Active Vibration Control Systems
The schematic of the MR damper is shown in Figure 6[74]. Figure 5: The mechanism of magnetorheological (MR) fluid [68]. As previously mentioned, the main disadvantage of the MR dampers is the high cost of construction because of the price of the MR fluid [15; 75]. In linear MR dampers, large amounts of magnetorheological fluids are needed to fill the MR fluid tank, but in a rotary damper, the required amount of MR fluid is much smaller and the cost of construction will be decreased substantially. The less MR fluid there is, the lower the cost of construction becomes [76]. One of the restrictions of using linear dampers is sealing the system [77]. As the working mechanism of the linear MR damper fully depends on the pressure difference between the two sides of the piston, it means the pressure of at least one side is high and therefore needs good sealing. On the other hand, in a rotary MR damper, the need for sealing is eliminated since the pressure in rotary dampers is quite low; thus, it is much simpler to solve the sealing problem [78]. The other problems of linear MR dampers such as sedimentation and hard adjustment are solved in rotary dampers. Compared to a linear MR damper, the rotary MR damper generates a significant reduction displacement and device compactness and requires less space for the configuration [79]. A rotary MR damper is shown in Figure 7.  
A novel seat suspension structure by employing a rotary MR damper was proposed by [PERSON] et al. [80]. The structure of the proposed seat suspension system is shown in Figure 8. Based on their study, the MR rotary dampers have many advantages compared with linear MR dampers. Figure 6: Schematic of the magnetorheological (MR) damper [74]. Figure 7: The structure of a MR rotary damper [79]. Based on the implemented experimental tests by [PERSON] et al., they considered transmissibility as a parameter, which is the ratio of affected response acceleration to excitation acceleration. The transmissibility ratio shows whether the suspension system is efficient or not. They offered a comparison between passive and semi-active seat system acceleration and explained that the acceleration amplitude in the semi-active case is considerably decreased in comparison with passive ones. According to the obtained results in the semi-active state, the seat acceleration had the minimum value, which verified that the semi-active rotary MR damper-based seat suspension was much more efficient in decreasing the vibrations. The experimental results, as given in Figure 9, proved that the semi-active system by utilizing MR damper can efficiently decrease the transmissibility and presents the best performance over a wide frequency range. Both ER and MR fluid are useful just for a small frequency range, and out of that range they will lose their efficiencies. To expand the efficient frequency range, in a study by [PERSON] et al. [24], they suggested MR elastomers (MREs). Conversely, from MR/ER fluids, magnetorheological elastomers (MREs) have controllable damping capabilities [81]. MRE is a kind of composite which is formed by magnetized particles scattered with the alliance in a solid polymer such as rubber. The elastic behaviour of an MRE grows monotonically based on the magnetic field. Simultaneous to the removal of the magnetic field, the MRE quickly returns to its primary status [82]. These unique characteristics of MREs have pointed to a variety of uses such as tuned vibration absorbers [83; 84]. This novel vibration isolator has potential in utilisation where the reinforced vibration properties are wanted. Study of MRE isolators in vibration attenuation of structural systems has been done in [85; 86; 87], and a model of an MRE isolator has been constructed and experimentally examined in [88]. The efficiency of the MRE isolator depends on some significant parameters, like the relative MR effect, the material's strength and its life span. Figure 10 represents a schematic diagram of the mentioned MRE seat isolator using the fabricated MRE.