This study is for the development of a biomechanical model on a seat with backrest support for evaluating the dynamic ride quality of vehicle, and for the seat design in point of both dynamic and static comfort.
To obtain the whole-body vibration response, the acceleration was measured at the floor and hip surface in z-axis, the back surface in x-axis, and the head in z-axis under excitation of 1m/s2 r.m.s. From this measurement, the three transmissibilities of each subject were obtained. The characteristics of whole-body vibration show the inter-subject and the intra-subject variability, and it is found that it is important to include the backrest support in biomechanical model as the backrest support increases the natural frequency.
Four kinds of biomechanical models are discussed to depict human motion. Single DOF model mainly describes z-axis motion of the hip while two and three DOF models describe z-axis motion of the hip and head. In this study a 9 DOF model having three rigid bodies is developed counting the seating posture with the backrest support. Nine DOF model has multiple outputs that include the major axes for evaluating the ride quality in vehicle such as z-axis of the floor, hip, and x-axis of the back, in addition to the z-axis of the head for describing the whole-body vibration.
To identify the biomechanical parameters, the measurable parameters were measured like this. The joint positions are measured by the device having pointer, the contact position between the human body and the seat are measured by body pressure distribution, and the parameters of seat and back cushion were measured by using dummy. While the immeasurable biomechanical parameters were obtained by matching the simulated to the experimental transmissibilities at the hip, back, and head. The comparison of the experimental data with those of 1,2,3,and 9 DOF model shows that 9 DOF model produces the more matching natural frequency as well as the transmisibilities than others.
From the estimated biomechanical model, the optimal seat parameters are determined to minimize the overall ride value at the floor, the hip, and the back on three different road - highway, Korean national road, and unpaved road. The optimal seat characteristic is found that the lower stiffness and higher damping transmits the lower vibration to human body. From the developed polyurethane foam, it is found that polyurethane foams has the constraint that the stiffness has a roughlly linear relation with the damping, when making a polyurethane form by changing the mixing ratio, the index, and molded weight. So the incorporation of physical constraint produces the optimal seat properties are stiffness 26.2kN/m and damping 242Ns/m for averaged road, and for the unpaved road the lower stiffness is required than highway and Korean national road. The evaluation of static comfort that is also important in seat design are studied by the subjective questionnaire, it is found that the seat of low stiffness gives more comfort for the normal and expert drivers.
