System Identification of Quasi-static Foam Behavior and its Application in H-point Prediction

Hip joint location (H-point) is an important design specification used by car seat manufacturers. Since most of the modern car seats are full-foam seats, the H-point location is primarily dependent on the quasi-static behavior of foam which is a highly nonlinear and viscoelastic material. In this work, a constitutive model is developed for flexible polyurethane foam. The stiffness characteristics of foam are described by a polynomial in displacement and the viscoelastic behavior is described by a convolution of the response with a kernel which is assumed to be a sum of exponentials. 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. Once identified, these models of foam are incorporated into two dimensional multi-degree-of-freedom seat occupant models to determine the static equilibrium position of an occupant in a seat from which the H-point location is determined. A graphical user interface is developed to facilitate study of the influence of changes in system parameters and also to visualize the settling of the occupant in the seat.