Apparent mass of seated man -: First determination with a soft seat and dynamic seat pressure distributions

Data of the impedance and/or the apparent mass of the sitting human body during the exposure to whole-body vibration in z-direction using rigid seats were standardized in the ISO 5982. These data are available as target functions for model developments. Models developed on this data basis should also apply to driver seats with a soft seat and backrest cushion, although the qualitative different contact conditions were neglected. Due to insufficient technical prerequisites, the determination of forces at the interface between subject and soft seat was impossible until very recently. Results of studies during static conditions showed clear differences in the pressure distributions between the rigid and the soft contact areas. In this experimental study pressure distributions on a seat cushion were measured during whole-body vibration in z-direction (random signal in the frequency range between 0.3 and 20 Hz, vibration magnitudes 0.25, 0.8, and 1.6 in s(-2) unweighted root mean square measured at the seat base) with a sampling rate of 32 in s. The apparent masses were calculated by the forces derived from the pressure distributions and accelerations measured at one point of the seat cushion near the buttocks. The moduli of the apparent masses derived for the soft seat were clearly lower than those determined for a rigid seat. These apparent masses showed a similar dependence on the vibration magnitude as the apparent mass functions derived in the usual way for rigid seats. Factors that could explain differences between the apparent mass functions derived for the soft and rigid seat were discussed and evaluated. The data of this study indicate the possibility and necessity to consider the contact conditions at the interface when deriving target functions for the model development. Recommendations for technical improvements and further experimental studies with driver seats were derived. (c) 2006 Elsevier Ltd. All rights reserved.