# Review on Seat Suspension System Technology Development
## 4 Active Vibration Control Systems  
Active suspensions are able to utilize external energy to power the system to reduce undesired oscillation through continuous control. The main goal of using the active suspensions is to reduce the effects of road conditions on the suspension travel, and bumpy road profiles do not change the efficiency of the suspension system [90]. The active suspension system is a more efficient vibro-isolating system but because of its high costs of construction and complex structure, they are applied infrequently. There are various methods for the implementation of active control systems to reduce vibration, such as using conventional linear actuators like hydraulic cylinders or linear motors, pneumatic actuators, and rotary and linear electro motors. A system employing active vibration control has the opportunity to add and eliminate energy from the system through the use of actuators that apply force or torque to the structure. The advantage is that they are more robust and usually provide better performance. Many researches have been performed to enhance the ride comfort of passengers. The active system can generate force through the use of electric or hydraulic actuators according to the state of the system. Consequently, an external energy source is required, which results in active systems being larger, more energy-consuming and more costly than both passive and semi-active control systems. In addition to the increased complexity of the system, another disadvantage is the stability of the system in case of system failure due to faulty sensors. Time delay may occur in active suspension systems because of delay in data processing, sensors or some mechanical faults. In order to control the actuator time delay, various controllers such as Proportional Integral Derivative [91], Fuzzy [92] and Adaptive Neuro-Fuzzy Inference System (ANFIS) [93] are applied and tested to optimise the active suspension systems. In 2017, [PERSON] et al. [94] designed the mentioned controllers in MATLAB for a half car active suspension model of 4 Degrees of Freedom (4-DOF) on the road of the ISO 8608 [95] random road profile. According to their obtained results, among the different controllers, the ANFIS-based controller presents the best performance in terms of'settling time' and 'peak overshoot' in comparison with other controllers. In 2012, [PERSON] et al. [96] designed an integrated seat suspension system by applying a controller in order to enhance suspension performance and achieve better ride comfort. Their model consists of a quarter-car suspension, a seat suspension, and 4-DOF driver body and a feedback controller as shown in Figure 13. By employing a static output feedback controller, the driver head acceleration in bumpy road conditions experienced a substantial decrease, and because of the reduction in head acceleration, the integrated seat suspension system could provide better performance in terms of ride comfort in comparison with other seat suspension systems. Negative-stiffness elements have been known as novel mechanisms for improving vibration damping. This method of isolation employs a revolutionary concept in low-frequency vibration isolation. [PERSON] and [PERSON] in 2014 [12] analyzed this mechanism comprehensively and incorporated an active pneumatic system to isolate the vibrations by applying negative stiffness for a vehicle seat in low frequencies, defining it as an active system with negative stiffness structures. Negative stiffness structure is used to decrease the absorbed vertical vibration energy by the seat. In the structure, the mass is displaced by some random amount from the primary position because of the action of the force that opposes the displacement. When the force has influence on the seat structure, it leads to  
Figure 12: (**A**) Semi-active regenerative seat suspension schematic. (**B**) Seat suspension prototype [89]. that two horizontal springs will be compressed and two vertical restoring moments act on the mass.