By using Fourier transform, the signal can be converted to a series of trigonometric functions in the frequency domain. In real life, the vibration mechanisms are more complex and it is impossible to define the signal using one frequency and one amplitude. In a vibrating system with only one frequency, the experienced discomfort can simply be measured by multiplying the acceleration magnitude with the corresponding sensitivity weighting factor for that frequency. In ISO-2631, the human filter is presented as a transfer function with various sensitivities for different frequencies. The horizontal sensation function is used for both lateral and longitudinal accelerations. The frequency weight is offered in ISO-2631 and divides the sensitivity into two directions, horizontal and vertical. 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 human body is sensitive to vibrations and the feeling of the vibration is different depending on the frequency and the direction of the vibrations. However, in order to lower the vertical vibration acceleration felt by the human body, a seat suspension system is required. It is fully identified that vertical vibration at lower frequencies is the most dominant vibration to the driver’s body which influences the ride comfort and safety. Most passive suspensions in normal seats are not able to isolate these low-frequency vibrations. Based on the conducted studies by Paddan, the vibration at low excitation frequencies (0.5–4 Hz) are the principal hazard causes for lumbago or a backache, which harmfully affect the mental and physical well-being of operators and travellers and decrease their working performance. There is a direct link between vibration discomfort frequencies for heavy-duty vehicles and eigenfrequencies of the operator’s body. The results for operators are the lack of concentration, tiredness and reduction of the effectiveness of the work being conducted. The vibration felt in a typical earth-moving machine vehicle usually happens at frequencies from 0 to 20 Hz. Truck drivers and operators of any earth-moving machine during their operation will experience a wide range of vibrations, most often originating from road surface unevenness. According to the research, active suspension control systems are the best choice to reduce the transmitted vibration to the drivers’ body and provide the best ride comfort in comparison with passive and semi-active systems. In order to improve riding comfort, the operator’s body displacement and acceleration must be reduced. Active, passive and semi-active suspension systems are employed in vehicle seats to alleviate the harmful and damaging effects due to the transmitted vibration to the human body. Considering that exposure to such vibration in long term has some damaging effects on driver’s health, many comprehensive investigations have been carried out and researchers have proposed several measures for estimating discomfort and the suitability of various vehicles’ seats such as those of trucks, cars and agricultural vehicles in operating condition. Operators of either on-road or off-road vehicles are regularly subjected to an extended variety of various vibration levels, especially at low frequencies. This review will focus on the necessity for developing seat vibration control systems as a part of manufacturers’ investigation into finding innovative methods to increase the comfort and safety of the vehicles’ drivers.
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