# Review on Seat Suspension System Technology Development
## 3 Semi Active Vibration Control Systems  
The vibration transmissibility of passive vibration isolation systems is low in some frequencies, especially in the low-frequency range. The main objective of implementation of the semi-active seat suspension systems is to find a way to reduce vibration transmissibility in the low-frequencies, in addition to the high performance at high frequencies. Semi-active suspension measures the stiffness  
Figure 4: The main parts of the passive seat suspension [6]. Figure 3: Single-degree-of-freedom system [56]. and damping characteristics of the isolation system and makes it possible to control the vibration in a broad vibration frequency range [61; 62]. Furthermore, if the stiffness matrix of the system is positive definite, then the semi-active control systems are unconditionally stable. Besides this, semi-active systems are more efficient because of their lower power consumption in comparison with other active control systems. Adjustable damping and stiffness features in semi-active suspensions have resulted in this system having been applied in broad industrial fields such as vibration control of high-precision machinery [63], automotive suspensions [64], helicopters [65], engine mounts [66] and earthquake-response suppression [67]. Vibration isolation by semi-active control can be enhanced by employing controllable stiffness and damping systems. Semi-active devices manipulate the stiffness of the structure very quickly to turn uncontrollable stiffness systems into governable systems, and this feature makes it possible to decrease the vibration transmissibilities. With the advancement of magnetorheological (MR) or electrorheological (ER) dampers, semi-active control of seat suspension has been developed to enable variable damping force and has less power consumption than active control systems [14; 68]. Oil and ferrous particles (20-50 microns in diameter) are the main components of Magnetorheological fluids [68], as shown in Figure 5. When a magnetic field is applied on the MR fluids, it affects their main characteristics, and their properties could be immediately changed or rearranged within a very short time (Millisecond). The main benefit of applying MRF Dampers is their quick response to the force variation (over 100 times per second faster response than the passive system). Furthermore, they are active and fully adjustable without consuming external power. Besides this, there are no moving parts in this system. The main disadvantage of the MRF dampers is the high cost of construction. In 2005, [PERSON] and [PERSON] [69], by implementing a series of experiments on an ER semi-active damping system, showed that using controllable damping devices is a smart way to provide low-transmissibility systems. Based on their obtained results, the minimum damping of the system is the most important parameter that regulates the acceleration transmissibility, and the controllable damping and stiffness provides more efficient vibration insulation. The development of the magnetorheological fluid (MRF) dampers--with MRF being very sensitive the magnetic field and having low power consumption--motivates researchers to use them in order to make semi-active seat suspension systems [70]. There are many numerical and experimental investigations about semi-active seat suspensions by using MR dampers. [PERSON] and [PERSON] [71] applied MR dampers to reduce vertical seat vibration. Another study was done by [PERSON] and [PERSON] [72], where they examined the application of MR dampers that were implemented on helicopter crew seats and they reported the advantages of using a semi-active suspension system in comparison with passive systems. In order to decrease the effect of ground vehicle crashes, [PERSON] and [PERSON] [73] studied semi-active magnetorheological energy absorbers of MR seat suspension in the crash. They found that the performance of semi-active seat suspension control systems partially depends on the damping force performance of MR dampers.