Measurement and modelling of seating dynamics to predict seat transmissibility

The transmissibility of a seat depends on the dynamics of both the seat and the human body. Previous studies show that the apparent mass of the body, to which much attention has been paid, has a large influence on the vibration transmissibility of a seat. The influence of the seat dynamics on the seat transmissibility has received less systematic attention. The principal objective of this study was to develop a systematic methodology using finite element methods to model the dynamic interaction between a seat and the human body so as to predict the seat transmissibility. The purpose was to understand how the foam material, the seat structure, and the seat occupant influence the vibration transmitted through seats. The effect of the foam thickness at the seat cushion and the backrest on the transmissibility was investigated experimentally in the laboratory with a SAE J826 manikin and with 12 subjects during exposure to 60-s periods of fore-and-aft and vertical vibration, respectively, in the frequency range 0.5 to 20 Hz at 0.8 ms-2 r.m.s.. Increasing the thickness of the foam at the seat cushion decreased the resonance frequency of both the vertical vibration transmitted to the seat cushion and the fore-and-aft vibration transmitted to the backrest, while there was little effect of the foam thickness at the backrest. It appears that the foam at the seat cushion had a predominant effect on the transmission of the vibration. Load-deflection curves were measured at various points across the lateral and fore-and-aft centrelines of a car seat with three different loading rates: 0.5, 1.0 and 2.0 mm/s. The dynamic stiffness of the seat cushion and backrest was measured with 120-s broadband random vibration (1.5 to 15 Hz) with three static preloads and with three vibration magnitudes (0.25, 0.5, and 1.0 ms-2 r.m.s.). With the same deformation, the reaction force was greater during loading than during unloading, showing evidence of hysteresis. The stiffness increased with increasing preload force and tended to decrease with increasing magnitude of vibration, indicating the seat components were nonlinear. The dynamic stiffness was also found to be greater when the seat cushion was constrained with a leather cover than without a leather cover. The transmission of vibration from the seat base to six different positions on a car seat was investigated experimentally in the laboratory with a SAE J826 manikin and with 12 subjects exposed to 120-s periods of random vibration (0.5 to 40 Hz) at three magnitudes (0.4, 0.8, and 1.2 ms-2 r.m.s.) in the fore-and-aft and vertical directions, respectively. The transmissibility from the seat base to the seat cushion surface and frame, to the backrest surface and frame, and to the headrest surface and frame exhibited a peak around 4-5 Hz in the fore-and-aft and vertical directions, respectively. The principal resonance frequency in the transmissibilities to all locations decreased with increasing magnitude of vibration, indicating nonlinearity in the seat-occupant system. There was little effect of the seat track position on the measured seat transmissibilities. The transmissibilities with subjects and with the manikin were different. Based on the experimental studies, models of the seat cushion and the backrest assemblies were built up and calibrated separately using the measured load-deflection curves and dynamic stiffnesses. They were joined to form a complete seat model and integrated with the model of a manikin for further calibration with measured seat transmissibility. The calibrated seat model was combined with a re-calibrated existing human body model to predict the transmissibility of the seat. It was found that by combining a calibrated seat model with a calibrated human body model, and defining appropriate contacts between the two models, the vertical vibration transmissibility of a seat with an occupant can be predicted. The developed seat-occupant model could be further improved to predict fore-and-aft seat transmissibility to the backrest and the dynamic pressure distributions at the interfaces between the human body and the seat.