A study of force-motion and vibration transmission properties of seated body under vertical vibration and effects of sitting posture
This dissertation research concerns with characterization of the biodynamic responses of seated human body to vibration in terms of force-motion and vibration transmission properties. The force-motion biodynamic responses were measured considering the primary driving-point (seat-buttock interface) u...
| Summary: | This dissertation research concerns with characterization of the biodynamic responses of seated human body to vibration in terms of force-motion and vibration transmission properties. The force-motion biodynamic responses were measured considering the primary driving-point (seat-buttock interface) using 13 male and 14 female subjects under different postural and excitation conditions. The force-motion data were also analyzed to derive the power absorbed within the vibration-exposed body, apart from the apparent mass (APMS). The measured data are interpreted to demonstrate the significant effects of sitting postures, involving variations in backrest support, hands position and seat geometry, on the biodynamic responses. The results revealed most significant effect of the body mass. The results attained from ANCOVA further show that the primary resonant frequency and bandwidth of the biodynamic responses are strongly influenced by the combined effects of hands position and back support condition, while the peak magnitude is further affected by the seat height. Owing to the limitations of the single-driving point force-motion relationships, experiments were undertaken to measure both the force-motion and transmission of seat vibration to the head simultaneously. A secondary driving-point, formed by the backrest and upper body, is also incorporated for fully characterizing the force-motion biodynamics of the body seated with a back support. An adjustable head-strap comprising a three-axes acceleration measurement system was developed to measure the vibration transmitted to the seated subjects' head. These experiments were performed with 12 adult male subjects and the data were analyzed to derive the biodynamic responses in terms of seat-to-head transmissibility (STHT), total apparent mass measured at the seat pan, cross-axis apparent mass of the upper body reflected at the back support. The results attained were used to characterize the roles of various contributing factors, such as back support condition, hands position and excitation magnitude. The results reveal the non-linearities in the APMS and STHT responses. The results of the ANOVA further show the strong influences of the three back support conditions on both vertical and fore-and-aft STHT responses over the entire frequency ranges. The vertical APMS magnitudes in the vicinity of the secondary resonance tend to be higher for the back supported postures. The mechanical equivalent models of the seated body are further attempted on the basis of observed biodynamic responses. Owing to the significant effects of the back support conditions, one-and two-dimensional models are formulated to simulate the APMS and STHT responses for all three back support conditions. The target datasets, however, are limited to those representing mean body mass of 75.58 kg in the excitation of 1m/s 2 rms acceleration (0.5-15 Hz). A 4-DOF one-dimensional model is developed using simultaneously measured vertical APMS and STHT response. The model parameters are identified for the three back support conditions respectively to emphasize the significance of back support conditions. The model parameter analysis suggest that both the force-motion and motion-motion measures need to be satisfied in order to obtain a more reliable model parameter set. The two-dimensional 5-DOF model allows for the consideration of the upper body interactions with the inclined backrest support. The identified models show good agreements with the measured target responses in APMS measured at the seat pan and the backrest, and vertical STHT. The model validity is further demonstrated in terms of the absorbed power property of the seated body. Considering that the physical responses of the tissues are more directly related to localized responses, alternate methods that can predict the distributed absorbed power property in body segment are realized in both one-dimensional and two-dimensional models |
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