Three Dimensional Analysis of a Seat–Occupant Model to Investigate the Effects of Vertical and Horizontal Vibrations

Abstract

A detailed 3-D model of a seat–occupant system was developed to investigate the behavior of different body parts due to vertical and horizontal vibration. The human body was modeled as a system of rigid bodies, including head, neck, upper torso, pelvis, upper legs, lower legs and feet, connected at body joints modeled as torsional spring- damper systems. The seat was considered to be consisted of backrest and the seat cushion, each modeled as a rigid body with spring-damper units distributed over them. The vibration excitation was applied to the seat base through a translational actuator. The multi body dynamics method and Rung-Kutta numerical technique were employed to derive and solve the governing equations of the model, respectively. Results indicated that the main resonance frequency of the whole body and body parts were in the range of 4.5-5.5 Hz. For both vertical and horizontal vibrations, the maximum acceleration ratio occurred at head and the earliest and latest resonances were observed at pelvis and legs, respectively. With the increase of the vibration magnitude, the acceleration ratios decreased for both directions. When the hanging feet were fixed, the acceleration ratio increased in upper parts of body but decreased in lower parts. Removing the headrest had almost no effect on the resonance frequency of the upper part of the body, however, it increased the resonance frequency of pelvis. With stiffening the seat springs, the resonance frequency increased for all body parts. However, while the acceleration ratio decreased in upper parts of the body it increased in lower parts.