Summary

This study explored a thin-film body-seat interface pressure measurement system (Tekscan Inc.) for the characterization of the biodynamic responses of human subjects seated on elastic seats and exposed to vertical vibration. The study included a rigid seat and three elastic seats: a seat with a flat 8 cm thick polyurethane (PUF) block (seat A); a soft and contoured automotive seat (seat B); and an inflatable air-bubble cushion (seat C). The validity of the measurement system was initially examined with 11 subjects seated with (WB) and without (NB) a back support in the absence of vibration. The results showed that the seat pressure measurement system can accurately measure the static body weight supported by the seat. The peak error was in the order of 4% for the flat (seat A), and 6% for both the contoured (seat B) and the air (seat C) seats.

The validity of the measurement system was subsequently assessed under vertical vibration. For this purpose, the rigid seat was installed on a single-axis force plate that was mounted on the whole-body vibration simulator (WBVS), while the seat mat was placed on the seat pan for the measurement of the body-seat interface force. The dynamic force measured by the force plate served as a reference for comparing the seat mat data. The WBVS was programmed to generate three levels of random vibration with nearly constant acceleration power spectral density (PSD) in the 0.5 to 20 Hz frequency range (overall rms acceleration = 0.25, 0.50 and 0.75 m/s²). The experiments were performed with different passive loads and human subjects. The force signals from the two measurement systems (seat mat and force plate) together with the acceleration signal were analyzed to derive the apparent mass (APMS) response. The results showed substantially lower APMS estimated by the seat mat at frequencies above 3 Hz compared to that from the force plate, irrespective of the seat load and excitation magnitude. These were attributed to the poor dynamic range and the lack of a scalable gain of the pressure measurement system. A correction function, ratio of APMS magnitude given by the force plate to that by the seat mat, was derived to account for the limitations of the pressure measurement system, for each load and vibration magnitude combination. The application of the correction functions resulted in comparable responses of both measurement systems.

Three series of experiments were subsequently undertaken to characterize the biodynamic responses of subjects seated on rigid and elastic seats, and to further examine the validity of the measurement system. The first two series, conducted simultaneously, involved the measurements of biodynamic responses of subjects seated on a rigid seat using the force plate and the seat mat, respectively. The results obtained from the first series served as a reference for the verification of the measurement system used during the second series of experiments. The third series involved the characterization of the APMS responses of subjects seated on three elastic seats, where the biodynamic force was measured using the seat mat. Owing to vibration attenuation properties of the visco-elastic seats, this final series of experiments involved the synthesis of identical levels of vibration at the seat surface. A methodology was developed for the synthesis of the desired vibration spectrum on the elastic seat using two micro-accelerometers installed in the vicinity of the ischial tuberosities of the subjects, which served as feedback for the WBVS vibration controller. Analyses of vibration levels measured at the seat and at the base revealed notable vibration attenuation by the elastic seats.

A total of 58 subjects (31 male and 27 female) participated in the experiments with a standing mass ranging from 45.5 to 106 kg. The experiments were performed with each subject sitting without (NB) and with (WB) a vertical back support on a rigid and on three elastic seats, and exposed to three different levels of broad-band vibration in the 0.5 to 20 Hz range. Selected anthropometric dimensions of the subjects such as stature, body fat, lean body mass, sitting height, C7 height, hip circumference and body-seat contact area were also recorded. The results obtained from the first series were analyzed to identify gender effect, and correlations with the anthropometric factors. The results showed strongly coupled effects of gender, body mass and anthropometric factors. The measured data were thus grouped within narrow ranges of body mass and anthropometric values to identify correlations between APMS responses and selected anthropometric factors. Comparisons of male and female subject responses clearly showed a strong gender effect coupled with anthropometric factors in a complex manner. Female subject responses revealed a distinct high magnitude second resonance peak at frequencies above 10 Hz, which was either not evident or less clear in the male subject responses. Male subjects invariably showed higher primary resonance frequency compared to female subjects of comparable body mass. The peak APMS magnitude increased with an increase in body mass and in most of the anthropometric parameters considered in this study.

The APMS responses derived from the seat mat (series 2) in conjunction with the correction functions agreed reasonably well with those obtained from the force plate. The peak difference between the responses obtained from the two methods was in the order of 6% under 0.75 m/s² excitation, and higher under 0.25 m/s² excitation, which was attributed to the poor dynamic range of the seat mat. It was thus concluded that the correction functions can adequately account for the frequency response of the measurement system, and hypothesized that these functions could be applied to the elastic seats. The APMS responses obtained with the elastic seats (series 3) were compared with those with the rigid seat for: (i) each individual subject; (ii) the mean responses of the subjects within each mass group; and (iii) the mean responses of all subjects. Examination of low frequency (near 1 Hz) APMS magnitude of each subject-seat combination revealed considerably lower body mass supported by the seat for some of the subjects. The deviation between the measured and expected values (75 to 80% of the standing body mass) exceeded 15% for some of the subjects, particularly under the lower excitation of 0.25 m/s². The datasets showing deviations in excess of 15% were excluded from the subsequent analyses. The remaining datasets for each seat were grouped under different mass groups of the two genders. The mean responses were analyzed to study gender, body mass, back support and vibration magnitude effects on the APMS of the subjects seated on the elastic seats.

The results showed that elastic seats tend to shift the primary resonance towards a lower frequency, while reducing the resonance peak. The results suggested strong influences of visco-elastic properties of seats in addition to gender and body mass-related factors. The mean peak APMS magnitudes of male and female subjects of similar body mass were comparable, while the primary resonance frequencies of female subjects were lower. The air-cushion seat (seat C) resulted in relatively higher peak APMS magnitudes for both genders, which was attributed to low damping of the seat. The flat PUF seat (seat A) with enhanced damping showed the lowest peak response magnitudes, irrespective of sitting and excitation conditions. It is thus concluded that the biodynamic responses of human subjects seated on elastic seats and exposed to vertical vibration differ significantly from those obtained with the rigid seat. Measured responses are considered to serve as important target values for developments in anthropodynamic manikins and seat design.