# Review on Seat Suspension System Technology Development
###### Abstract
Nonetheless, the increased price and complexity of active systems only justify their application in cases when the performance is critical, while the semi-active system can produce notable performance gains for the small extra cost [21]. The passive and active seat suspension is shown in Figure 1.  
The human body is sensitive to vibrations and the feeling of the vibration is different depending on the frequency and the direction of the vibrations [9]. By using a frequency weight, the feeling can be translated from the vibration in space to how it is felt inside the driver's brain where discomfort is recognised. The frequency weight is offered in ISO-2631 [23] and divides the sensitivity into two directions, horizontal and vertical [24]. The horizontal sensation function is used for both lateral and longitudinal accelerations. In ISO-2631, the human filter is presented as a transfer function with various sensitivities for different frequencies. In a vibrating system with only one frequency, the experienced discomfort can simply be measured by multiplying the acceleration magnitude with  
Figure 1: Prototype of passive seat suspension (**A**) and active seat suspension (**B**) [22]. the corresponding sensitivity weighting factor for that frequency [17]. In real life, the vibration mechanisms are more complex and it is impossible to define the signal using one frequency and one amplitude. By using Fourier transform, the signal can be converted to a series of trigonometric functions in the frequency domain. These trigonometric functions can be filtered individually for each frequency and amplitude and then transformed back to the time domain where an RMS value can be estimated [25]. Considering the direct relationship between the vibration exposure levels and the driver's health, it is clear that the ride comfort is fully dependent on the frequency, amplitude and exposure time duration of the transmitted vibration [26]. According to the frequency-weighting curves for single-degree-of-freedom (DOF) system vibration described in ISO 2631-1, the driver's comfort is most seriously affected by 5-8 Hz translational vibration along the \(z\) axis (Heave motion), 1-1.25 Hz translational vibration along the \(x\) and \(y\) axes, and 0.8 Hz rotational vibrations around three axes [27]. Usually, the isolation of the vertical vibration, which is confirmed to have the greatest vibration amplitude and affects drivers' comfort in vehicles, is recognised in designing seat suspension [28]. The whole human body system is a complex dynamic system with sophisticated mechanical characteristics. During operation of any land vehicle, the severe vibration generated by bumpy roads and the operation of the machineries generate a load which is transferred to the seat; thus, the operators regularly feel Whole Body Vibration (WBV) [29; 30; 31]. To assess human vulnerability due to transmitted vibrations in vehicles, ISO 2631-1 [23] is always utilised, which has set different ways for the analysis of Whole Body Vibration and presents guidance on the possible impacts of vibration on health, comfort, attention, and motion illness. The objective of obtaining a biodynamic model for a seated body is to be able to predict the performance of seat design and support the suspension control system configuration [32]. Biodynamic modelling of the seated human body has attracted considerable attention in recent years, and many mathematical models ranging from 1-DOF to 11-DOF have been offered [33; 34]. These models are obtained from different modelling methods, such as lumped-mass models, finite element models, and multi-body models. The lumped-mass body model can be expressed in the frequency domain as a transfer function or in the time domain as a differential equation of motion. A linear 4-DOF model of a seated human body to investigate whole-body vibration was studied by [PERSON] [35]. In order to model the human body seating system, [PERSON] prepared a model for the human bodies which contained four mass segments: the head, upper trunk, lower trunk, and thighs.