# Review on Seat Suspension System Technology Development
## 4 Active Vibration Control Systems
There are two main sections in this seat suspension: a top layer including two separately controlled rotational DOFs and a bottom layer to decrease the heave vertical motions. One of the main characteristics of this model is its ability to control the vibration of the driver's body in five DOFs, except for the yaw vibration, which interestingly has the smallest effect on  
Figure 15: Active seat suspension configuration. humans. The top-layer suspension is examined then its rotational stiffness and friction are recognized. In Figure 17, a constructed model of a multiple-DOF seat suspension is shown. Some experimental tests have been done in order to evaluate the interaction of the top layer and the bottom layer, and according to the obtained results, this interaction is small so it is obvious that by using this technique, the whole body vibration experienced a substantial reduction. According to the obtained results by [PERSON] et al. [27], when the fully controlled two-layer seat suspension is employed, the whole body vibration for roll and pitch direction decreases by 41.4% and 44.3% respectively. This multiple-DOF seat suspension will improve the driver's work condition and is effective in application. The roll vibration will be fully transmitted to the driver's body in conventional single-DOF seat isolation systems and only the affected vibration in the vertical direction will be damped. A two-layer (integrated seat system) multiple-degrees-of-freedom seat suspension system was proposed by [PERSON] et al. [53]. The efficacy of the multiple-DOF seat suspension system was examined by doing a series of both simulations and experimental test. The seat was constructed in two layers: the layer which is located on top of the seat is considered to be affected by vertical vibration in the z-axis, and the lower layer is designed for the roll vibration. A multiple-DOF seat suspension is shown in Figure 18, developed by [PERSON] et al., presenting the bottom part of the multiple-DOF seat suspension as a mechanism for controlling the vertical vibration. As shown in Figure 19, a U-joint is connected to two actuators in the top layer. In the structure of the U-joint, there are four springs in the four corners to provide rotational stiffness, which can be equal to two torsional springs in the universal joint. An active multiple-DOF seat suspension was compared with a single-DOF active seat suspension and a conventional passive one to evaluate the impacts of transmitted vibration on health and ride comfort. Based on the collected measurement results, the total vibration of the active multiple-DOF active seat surface experienced a 29.8% vibration reduction, in comparison with the single-DOF active  
Figure 16: Schematic diagram of a two-layer multiple-degrees-of-freedom (multiple-DOF) seat suspension [53]. Figure 17: Multiple-DOF seat suspension prototype [53]. one. The multiple-DOF seat suspension system could efficiently reduce the whole body vibration (WBV) of heavy-duty vehicle operators. Another active force control system suggested by [PERSON] et al. [100] is the neuro active force control system. They believed this system was simple, effective and accurate. This system was designed and employed for an active seat suspension of heavy-duty vehicles likes trucks. They proved the efficiency of the neuro active force control system by doing a series of experimental tests and numerical simulations. According to the results, the neuro active force control mechanism as an intelligent control approach reduced the unwanted transmitted vibration in the driver's body. Sliding mode control is a control scheme that changes the dynamics of a system by use of an intermittent signal. Sliding mode control is an efficient method to control nonlinear uncertainties in a system and are employed in suspension control systems. A sliding mode controller including a state observer for semi-active seat suspension was proposed by [PERSON] and [PERSON] [49]. The terminal sliding mode controller gives some improved characteristics.