# Review on Seat Suspension System Technology Development
## 4 Active Vibration Control Systems
The experimental results confirm that the isolation efficiency of the suggested system is better than that of the active system without negative-stiffness structures. In 2015, a project was performed by [PERSON] and [PERSON] [97] at the Department of Applied Mechanics at the Chalmers University of Technology in collaboration with Volvo Group Trucks Technology [98]. The main purpose of this research was to improve ride comfort by using active and semi-active suspensions, with a focus on the vertical dampers. They examined the effects of vertical dampers on the ride comfort feature of a commercial truck through the use of active and semi-active cab suspensions based on the principle of individual control of each damper separately. By this method, the restoring force mainly depends on the state of the suspension itself and is independent of the other dampers and the overall motion of the cab. The benefits of this method are that it is easy to implement and easy to overview. Figure 14 presents the truck coordinate system that was considered to study damping forces in the model coordinates, i.e., heave, roll and pitch. According to the obtained results by [PERSON] and [PERSON], the active control systems improve the ride comfort regardless of the road profiles while the semi-active control systems principally work well for the road profile that induces low levels of vibration. To apply a control suspension system, one of the main methods is the use of conventional linear actuators, e.g., hydraulic cylinders or linear electric motors to reduce the effects of vibration on the  
Figure 14: Pitch, roll and heave in Cab [97]. Figure 13: Integrated seat and suspension model [96]. human body [99]. However, one of the restrictions of this method is that they require to be fully maintained/powered and are usually expensive when large force outputs are needed. In order to overcome this limitation, [PERSON] et al. [99] designed a seat suspension system that was able directly utilise a rotary motor to provide a reliable and efficient seat suspension system without maintaining the basic structure of the seat suspension. The idea of using an electric servomotor, as noted by [PERSON] and [PERSON] [13], in order to reduce vibration in a truck seat was applied and they designed a ball screw mechanism as the actuator. Using an electric servomotor is simpler than implementing hydraulic or pneumatic actuators. Furthermore, its loading capacity and frequency span are appropriate for a seat vibration system. [PERSON] et al. [99] used an electrical rotary motor to achieve this goal. These motors are applied in many industrial fields. It worth noting that other advantages of the rotary motor in comparison with linear actuators are lower price and ease of maintenance and control. By using a gear reducer, it is possible to increase a motor's torque output. Thus, a low-rated power motor can be used for active vibration control. A rotary-motor-based active seat suspension model is shown in Figure 15.  
According to the obtained result from experimental evaluations on active seat suspension, which were done by [PERSON] et al. [99], this system is able to overcome the vibration acceleration especially around the critical value (4 Hz). Considering that the vibrations with the greatest effect on human health in heavy-duty vehicles occur in the range of 3 to 5 Hz, active seat suspension could significantly enhance ride comfort. As mentioned before, the idea of using an integrated seat suspension was investigated by [PERSON] et al. [96] and the results were explained. In 2018, [PERSON] et al. [27] developed the integrated seat suspension system and tried to overcome the whole body vibration of vehicles, especially in heavy-duty ones, by designing a novel two-layer multiple-degrees-of-freedom (multiple-DOF) seat. In Figure 16, the simplified plan of the aforementioned two-layer multiple-DOF seat suspension design is shown.