Nonlinear multi-body dynamic modeling of seat–occupant system with polyurethane seat and H-point prediction

Hip joint location (H-point) is an important design specification used by car seat manufacturers. Since most modern car seats are full-foam, the H-point location is primarily dependent on quasi-static behavior of foam which is a highly nonlinear and viscoelastic material. In this work, the seat–occupant dynamic model is developed in the form of a planar multi-degree of freedom system that also incorporates nonlinear viscoelastic behavior of flexible polyurethane foam. The foam force is modeled as an additive sum of nonlinear elastic and linear viscoelastic effects. The viscoelastic force is modeled as a convolution integral with a sum of exponentials kernel. The interface between the occupant and the seat at the seat-back and at the seat-bottom is modeled using Coulomb friction type laws. A system identification procedure, based on linear least squares fitting and ARMA modeling, is developed to identify the parameters from data collected in quasi-static foam experiments. The resulting nonlinear integro-differential algebraic model of the seat–occupant system is used to simulate the response of the system and determine the H-point for the occupant. A few parametric studies related to the effect on H-point of different foam types and inertia properties of the occupant are also reported. Such a model can be very useful for investigating the effects on H-point location of design changes in the seat foam. It can also be used to determine the forces at the occupant–seat interface and thus in studies related to occupant's comfort.