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Research Papers

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

[+] Author and Article Information
Yousof Azizi

Ray W. Herrick Laboratories,
School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47907
e-mail: yazizi@purdue.edu

Tarun Puri, Anil K. Bajaj, Patricia Davies

Ray W. Herrick Laboratories,
School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47907

Contributed by the Design Engineering Division of ASME for publication in the JOURNAL OF COMPUTATIONAL AND NONLINEAR DYNAMICS. Manuscript received May 1, 2014; final manuscript received November 3, 2014; published online February 11, 2015. Assoc. Editor: Rudranarayan Mukherjee.

J. Comput. Nonlinear Dynam 10(3), 031011 (May 01, 2015) (10 pages) Paper No: CND-14-1115; doi: 10.1115/1.4029047 History: Received May 01, 2014; Revised November 03, 2014; Online February 11, 2015

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.

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Figures

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Fig. 1

Measured and predicted stress in foam during a quasi-static compression test (compression rate equals to 0.0088 1/s). Blue: measured response and red: predicted response.

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Fig. 2

Seat–occupant model. (a) Occupant consists of three bodies while seat is replaced by nonlinear springs, viscoelastic elements, and viscous dampers. (b) The seat–occupant model’s geometric parameters.

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Fig. 3

A section of the seat–occupant model showing the arrangement of springs at the seat back and the seat bottom. p is fixed and q is the depth of the foam.

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Fig. 4

(a) The horizontal location (ξ) and (b) the vertical location (ζ) of the H-Point, when the number of elements at the seat bottom is varied between 10 and 40 (light gray to black) and the number of elements at the seat back was kept constant at 15. The coefficient of friction μ is 0.25.

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Fig. 5

(a) The horizontal location (ξ) and (b) vertical location (ζ) of the H-Point when the number of elements at the seat back is varied between 10 and 40 (light gray to black) and the number of springs at the seat bottom was kept constant at 15. The coefficient of friction μ is 0.25.

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Fig. 6

The predicted seat–occupant interface pressure between the occupant and the seat bottom as a function of the distance along the seat bottom. Twenty elements were used to model the seat bottom and the seat back. The coefficient of friction μ is 0.25.

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Fig. 7

(a) The horizontal location of the H-Point ξ and (b) the vertical location of the H-Point ζ, when the coefficient varies between 0.20 and 0.45 (light black to dark black). The number of springs in the seat back and the seat bottom is equal to 20.

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Fig. 8

The occupant positions at three instants, t = 0, 10, 25 s, for μ = 0.25

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Fig. 9

The pressure distribution at the occupant and the seat bottom interface as a function of the distance along the seat bottom. Red: experimental result, blue: analytical result when the coefficient of friction μ is equal to 0.45.

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Fig. 10

Schematic of the experimental setup for measuring force distribution at the seat–occupant interface

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