# A Systematic Literature Review of Various Control Techniques for Active Seat Suspension Systems
## 5 Type of Suspension Systems
### Multiple Degrees of Freedom Vibration  
In the last few decades, most of the studies have paid more attention to suppressing the vibration that essentially comes from the vertical direction only [41]. However, the human body can be exposed to the vibration from different directions such as lateral and rotational directions. Undoubtedly, the effects of the vibration, which comes from all the other directions are not as severe as those from the vertical direction. Plenty of researches have been conducted on the 1 DOF active seating suspension system in the vertical vibration, whereas other researches have been focused on multiple DOF seating suspension system in multiple axis directions, for example, in the vertical and horizontal vibrations [46] or vertical and longitudinal vibrations [47]. Different types of actuators with various mechanism structures are normally adopted in these multiple DOF seating suspension systems.  
Some multiple DOF active seating suspension systems have been reported. The force and stiffness control method was used to reduce the vertical and lateral vibration generated from the forest machinery [48]. The proposed seat was designed by [PERSON] as illustrated in Figure 4 where the seating suspension structure consists of four pneumatic cylinders. The four pneumatic cylinders can rotate around the lateral axes. The seating suspension structure provides a smoother movement and more reliable performance than the conventional design. The seat basically can be moved based on the opening valve of the cylinder for each pneumatic system, where the input of the servo valves is the control signal directed from the F-K controller.  
Another 2 DOF active seating suspension system was designed to eliminate the vibration in the vertical and roll axis directions [49]. The seating suspension structure system consists of two layers:  
Figure 4: CAD model of the seating suspension structure [43].  
Figure 3: Seat suspension system. (**a**) Passive seat suspension. (**b**) Active seat suspension [45].  
The bottom layer was used to control the vertical vibration, and the top layer was primarily designed to reduce the roll vibration. Figure 5 shows the structure of the active seat suspension.  
The seat suspension was designed for the military vehicle to reduce the effect of the vibration in the vertical and lateral directions as reported in [50]. A 2-DOF active seating suspension system was proposed to reduce the vibration in the vertical and pitch directions [51; 52]. The scissor structure mechanism was adopted on the base frame to reduce the vibration in the vertical direction. The attached tilting platform on the top of the seating suspension was proposed to control the pitch motion [51]. The experimental test results showed that the active seating suspension system is stable in the low frequency range of 0-5 Hz.  
Few recent studies have studied the effects of the WBV in the fore-aft and horizontal directions [53; 54; 55]. The worst frequency range which affects the human body in the horizontal direction is from 1-2 Hz [56]. These studies were carried out under three categories: vertical axis direction only (z), horizontal axis direction only (x) and combination of both the vertical and horizontal axis directions (z and x) with a driver mass variation from 60 kg to 90 kg. The results showed a minor impact on the backrest under low frequency range due to the cross coupling between the vertical and lateral axes [57]. Evidently, the majority of studies confirmed that the horizontal vibration is insignificant in influencing the WBV in many cases [58; 59].