MEASUREMENT AND BIO-DYNAMIC MODEL DEVELOPMENT OF SEATED HUMAN SUBJECTS EXPOSED TO LOW FREQUENCY VIBRATION ENVIRONMENT

Abstract

While travelling by vehicles: cars, buses, trains, ships and airplanes, there are many factors that cause discomfort, such as pressure at seat interface, sitting posture, vibration, noise, visual effects, humidity, temperature, etc. Among this, exposure to whole body vibration causes a complex distribution of vibration within the body with the unpleasant sensations giving rise to discomfort or annoyance which result in impaired performance and health risk. This distribution of vibration depends on extrinsic variables (those used to express the state or environment of the• dynamics system), intrinsic variables (those which refer to the human subject natural behavior, characteristic and condition) and interfaces between the body and the vibration environment. In this study, the effect of variations in inter-subject variability, posture, vibration magnitude and frequency on seat-to-head (STH) and back support-to-head (BTH) transfer functions has been studied under vertical sinusoidal vibration. Twelve healthy male subjects participated in the experimental work to measure vertical vibration transmitted to the occupants head in three representative postures (erect, vertical back on and forward lean on table) under three magnitudes of vibration (0.4, 0.8 and 1.2 m/s2 r.m.s.) in frequency range of 1 to 20 Hz. From collected data sets, the effect of inter-subject variability, vibration magnitude, vibration frequency and postures on STH and BTH transmissibilities magnitude and phase has been studied over the prescribed frequency range. Within frequency rangel-10 Hz and for vertical vibration magnitude of 1.2 m/s2 r.m.s., two nine degree-of-freedom (two postures) and ten degree of freedom lumped parameter models and one eleven degree of freedom multi-body model were proposed for which the parameters were estimated to satisfy both STH and BTH transfer function characteristics. The parameter identification technique involves optimization by minimizing the square error sum of seat to head (STH) and back to head (BTH) transmissibility and phase difference functions. The experimental studies reveal that STH transmissibility peak value frequency occurs in a frequency range 4-5.5 Hz for seated subject in all prescribed postures. The result showed that inclusion of all possible variables has great advantage in optimal design of vehicle seat, suspension and comfort analysis. Moreover the comparison of experimental and model response reveal that both models matched with mean experimental data sets most closely and the models provide best description about biodynamic response study of seated human subjects under vertical whole body vibration.