# A Systematic Literature Review of Various Control Techniques for Active Seat Suspension Systems
## 8 Control Methods Used in the Active Seating Suspension System
### Sliding Mode Controller  
Siding mode control (SMC) or variable structure control (SVC) has been widely used in the control research field due to its robustness against external disturbances and system uncertainty [88]. It is completely insensitive to the parametric uncertainty during the sliding surface. The control strategy was proposed in the 1950s in the Soviet Union by a research group consisting of [PERSON], [PERSON] and [PERSON]. The concept behind the slide mode control is to switch the state feedback control law from one continuous structure to another using high frequency switching control [89]. Hence, the system motion slides along a surface called sliding surface with certain boundaries called slide mode. Generally speaking, the feedback controller impulses the system to reach the sliding surface within a finite time and make it toggle between two phases either on and off or reverse and forward. When the system is finally at the sliding surface, the control input is sustained. Figure 14 shows the system trajectory with sliding surface. The main advantage of sliding mode controller is its robustness against the disturbances. However, the sliding mode controller has a drawback of chattering problems that is always associated with many design applications.  
Sliding mode controller has been implemented in the active seat suspension in [91] where the author used a modified Terminal Side Mode Controller (TSMC). The TSMC has advantages over the conventional sliding mode controller in terms of the fast convergence time. The disturbance observer was designed to reduce the chattering by reducing the switching gain. The state observer was used to estimate the absolute velocity, and a complementary filter was implemented to fuse the data to ensure accurate estimation results.  
The model simplification from a 6 DOF to 1 DOF active seat model was applied to design the TSMC with a remarkable consideration for disturbances generated from the simplified model. The active seat suspension performance was evaluated using the acceleration PSD and ISO 2631-1 under random vibration. The experimental results showed that the proposed controller has reduced RMS acceleration by 54.6% compared with the vibration reduction in a well-tune passive seat suspension.  
In addition, a non-singular siding mode controller was implemented to reduce the roll vibration and lateral acceleration in the top layer of active seat suspension [49]. A SMC based disturbance observer (DO) is used to estimate the unmeasurable states [92]. The proposed controller was robust to the uncertainties generated from the suspension mechanisms, road profile disturbances and driver weight differences. A cascade control method was proposed for active seating system suspension where the SMC was implemented for force and stiffness tracking of the active seating suspension system, and a proportional valve controller was used to regulate the actuator pneumatic positions [48]. The Lyapunov's method was used to evaluate the SMC stability. The experimental results showed a 20% reduction in the vibration acceleration amplitude of the active seating suspension system in the frequency range from 0.5 to 1 Hz.