Development of a Multibody Model to Predict the Settling Point and Interfacial Pressure Distribution in a Seat–Occupant System

The location of the hip-joint (H-Point) of a seat occupant is an important design specification which directly affects the seat static comfort. Most car seats are made of polyurethane foam and so the location of the H-Point is dependent on the quasi-static behavior of foam. In this research, a previously developed model of the seat–occupant system is refined by incorporating an improved foam model which is used to study seat and occupant interactions and the location of occupant’s H-Point. The seat is represented by a series of discrete nonlinear viscoelastic elements that characterize the seating foam behavior. The nonlinear elastic behavior of these elements is expressed by a higher order polynomial while their viscoelastic behavior is described by a hereditary type model with parameters that are functions of the compression rate. The nonlinear elastic and viscoelastic model parameters were estimated previously using data obtained from a series of quasi-static compression tests on a car seat foam sample. The occupant behavior is described by a constrained two-dimensional multibody model with five degrees of freedom. A Lagrangian formulation is used to derive the governing equations for the seat–occupant model. These differential equations are solved numerically to obtain the H-Point location. These results are then used to calculate the force distribution at the seat and occupant interfaces. The force distribution at the seat–occupant interface is also investigated experimentally and is found to match qualitatively with the results obtained using the seat–occupant model.