Predicting the Response of a Seat-Occupant Model by Using Incremental Harmonic Balance
Abstract
Vehicle occupants are exposed to low frequency vibrations which can adversely affect the ride comfort. Exposure to vibrations can also lead to problems ranging from fatigue and lower back pain to injuries to the spine. The transmission of vibration to seated occupants can be controlled by appropriately designing car seats, which requires a deep understanding of the seat-occupant system behavior. A seat-occupant system is composed of two main components: the seat and the occupant. A key element in the seat, which is a challenge to model, is the flexible polyurethane foam in the seat cushion, which is a nonlinear and viscoelastic material exhibiting behavior on multiple time-scales. The occupant, which is modeled by a multi-body model, is also geometrically nonlinear. In this paper, a combined seat-occupant model is described that incorporates the nonlinear viscoelastic model of the car seat, the multi-body occupant model, and realistic profiles of the seat and the occupant. A robust computation technique is required to analyze the dynamics of the combined seat-occupant system. Traditional numerical techniques like direct time-integration of the equations to determine the steady state response to harmonic excitation is inefficient, restricting exploration of the model, e.g., to determine how foam properties affect the response. To accelerate calculation of the frequency response, a modified incremental harmonic balance technique is developed which is much faster than time integration techniques. Sample seat-occupant system frequency responses and mode shapes are presented. Finally, the accuracy and computation speed of the new technique is compared to that of the time integration method.
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