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THESIS

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This is to certify that the
thesis entitled

TWO-DIMWNSIONAL KINETIC MODF‘LING
OF HUMAN POSTUR“ IN AUTOMOTIVE? SEATS

presented by

Davivaletcher Vkern

has been accepted towards fulfillment
of the requirements for

M.S. d . Mechanics
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TWO-DIMENSIONAL KINETIC MODELING OF HUMAN POSTURE
IN AUTOMOTIVE SEATS

By

David Fletcher Ekern

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Material Science and Mechanics

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ABSTRACT

TWO-DIMENSIONAL KINETIC MODELING OF HUMAN POSTURE
IN AUTOMOTIVE SEATS

By

David Fletcher Ekern

To assist automotive seat development, methods for predicting the
posture of seated occupants have been developed which utilize two-dimensional
computer models constructed using commercial kinetic modeling software. The
first model is a two-dimensional representation of the Society of Automotive
Engineers (SAE) three-dimensional testing manikin. This model was used to
simulate experimental data collected with the SAE three-dimensional testing
manikin regarding the stiffness and support force distribution of an automotive
seat cushion.

The second model developed was the torso geometry and articulation of a
mid-size male model known as JOHN. The kinetic two-dimensional JOHN model
was used in a comparative seat study to determine the effect of different seat
geometries on the final posture of the model.

This thesis describes the development, use and application of these

models for improving automotive seat development.

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DEDICATION

I would like to dedicate this work to my wife, Christie, who has given me
undying love and support through this whole process; thank you and I love you.

To my mom, who has encouraged me to learn and discover and who has
given me the support to do so, and to my dad, who was not able to help in body
and mind, but instead has given me encouragement in spirit.

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ACKNOWLEDGMENTS

This work would not have been possible without the support and
assistance of many people:

The crew in the Biomechanical Design Laboratory; Melissa Sloan, Akram
Ali, Richard Setyabudhy, Barry Frost, Bruce Liu, Cliff Beckett, and Frank Mills.

A big thanks goes Chris Gedraitis, for letting me bounce ideas off him and
making sure I did things right.

Thank you to Tammy and Neil Bush, for supporting and encouraging me
and for helping me keep focused.

To my graduate committee, Dr. Averill and Dr. Cloud, thank you for your
guidance and for taking the time to review my work.

To my graduate advisor, Dr. Robert P. Hubbard, thank you for the
opportunity to work with you over the past two years. You gave a young
graduate student the chance to learn and explore in your lab, and I want to tell
you how thankful I am for that chance. Working with you has shaped my
thinking and given me the tools to continue'on in life, much better for the

experience of working with you. Thank you.

[.00

TABLE OF CONTENTS

LIST OF TABLES ................................................................................................. vi
LIST OF FIGURES ............................................................................................. vii
J_. INTRODUCTION ............................................................................................. 1
1._1 Background .............................................................................................. 1
_‘l_.2_ Objectives .............................................................................................. 12

_2_. 2-D SAE KINETIC COMPUTER MODEL DEVELOPMENT ........................... 13
_2_._1_ Methods and Materials .......................................................................... 13
2L1 2-D SAE computer model construction .......................................... 13

_2_.1_.2_ Locating the COM's for the torso and thigh-pelvis segments. ....... 15

24; Experimental data collection .......................................................... 20

M Modeling of experimental data ....................................................... 26

2._2 Results and Discussion ......................................................................... 35

_3_. 2-D JOHN COMPUTER MODEL DEVELOPMENT ....................................... 42
'3_.‘l_ Methods and Materials .......................................................................... 42
3.1—.1 2-D JOHN kinetic computer model construction ............................ 42

3;; Locating the COM’s for the 2-D JOHN body segments ................. 52

§._1_,_3_ Seat model construction ................................................................ 61
3.1.3.1 BAC model development and construction ............................ 63

3.1.3.2 Chrysler LH seat model development and construction ........ 65

§_.1_.fi Seat evaluation protocol ................................................................ 74

§._2_ Results and Discussion ......................................................................... 83
3_._2_.1_ Chrysler LH results ....................................................................... 83

3‘22 BAC results ................................................................................... 95

32.; Upper thorax support location results ......................................... 106

1;: Observations ........................................................................................ 113

:1. CONCLUSION ............................................................................................. 115
BIBLIOGRAPHY ............................................................................................... 118

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LIST OF TABLES

Table 1 - Experimental Force Results ................................................................ 22
Table 2 - Experimental force and deflection results from Hubbard, et. al. [9] ..... 26
Table 3 - Piecewise spring constants calculated from experimental data .......... 36
Table 4 - 2-D SAE model force and deflection results ........................................ 37
Table 5 - Experimental vs. model results for force at H-pt .................................. 38
Table 6 - Experimental vs. model results for force at knee ................................. 38
Table 7 - Model force results for supports at H-pt and thigh .............................. 39

Table 8 - Model piecewise spring constants for supports at H-pt and thigh ....... 39
Table 9 - Comparison of H-pt piecewise spring constants for support at knee vs.

support at thigh ............................................................................................. 40
Table 10 - 2-D JOHN body segment masses ..................................................... 55
Table 11 - Simulation matrix for automotive seat study ...................................... 76
Table 12 - Simulation matrix for BAC study ........................................................ 77
Table 13 - Simulation results for automotive seat study ..................................... 83
Table 14 - Simulation results for BAC study ....................................................... 95
Table 15 - Simulation results for upper thorax support location study .............. 107

vi

 

 

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Figure 1:
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Figure 5:
Figure 6:
Figure 7:
Figure 8:
Figure 9:

Figure 10:
Figure 11:
Figure 12:
Figure 13:
Figure 14:
Figure 15:
Figure 16:

LIST OF FIGURES

SAE 2—D drafting template .................................................................... 2
SAE 3-D testing manikin ....................................................................... 2
2-D JOHN model .................................................................................. 5
2-D JOHN model at 0° TLC and 40° TLC ............................................. 6
3-D JOHN model in oblique view .......................................................... 8
2-D JOHN physical articulating template .............................................. 9
2-D SAE kinetic computer model ........................................................ 11
-D JOHN kinetic computer model ..................................................... 11
2-D SAE model torso and thigh-pelvis segments ............................... 14
2-D SAE model segments with joint centers ..................................... 14
Assembled 2-D SAE kinetic computer model ................................... 16
Picture of thigh-pelvis segment COM calculation ............................. 17
Picture of torso segment COM calculation ....................................... 19
2-D SAE computer model with COM positions ................................. 21
Force vs. deflection graph for seat pan response under H-pt .......... 24

Force vs. deflection graph for seat pan response under knee ......... 25

Figure 17: 2-D SAE model with spring supports at knee and H-pt .................... 27
Figure 18: 2-D SAE model with sliders controlling motion ................................. 28
Figure 19: 2-D SAE model with mass circles on body segments ....................... 30
Figure 20: 2-D SAE model with input and output boxes .................................... 31
Figure 21: Superimposed thigh orientations for first five cases ......................... 33
Figure 22: 2-D SAE model with spring supports at thigh and H-pt .................... 34
Figure 23: 2-D JOHN thorax, lumbar and pelvis segments ............................... 44
Figure 24: 2-D JOHN torso segments with points at joint centers ..................... 44
Figure 25: 2-D JOHN legs with redesigned thigh .............................................. 46
Figure 26: 2-D JOHN torso segments with gear circles and joint centers ......... 46
Figure 27: Gear circles pinned to thorax and pelvis segments .......................... 47
Figure 28: Lumbar segment joined to thorax and pelvis segments .................... 48
Figure 29: Gear linkage added to 2-D JOHN model .......................................... 49
Figure 30: 2-D JOHN torso showing segments with joint center orientations... 51
Figure 31: Output box showing TLC and TRA ................................................... 53
Figure 32: Rope added to 2-D JOHN torso ....................................................... 54
Figure 33: COM positions on all segments except thorax ................................. 56
Figure 34: Original orientation of head, neck, arms and thorax with COM
locations ........................................................................................................ 58
Figure 35: New orientation of head, neck, arms and thorax for ......................... 60

vii

Figure 36: 2-D JOHN model with COM positions for all body segments ........... 62
Figure 37: Picture of the BAC prototype ............................................................ 64
Figure 38: BAC computer model ........................................................................ 66
Figure 39: LH model seat frame ........................................................................ 68
Figure 40: LH model with undeflected seat cushion .......................................... 68
Figure 41: LH model with undeflected seat back ............................................... 69
Figure 42: LH model with deflected seat cushion .............................................. 69
Figure 43: LH model with segmented seat cushion ........................................... 71
Figure 44: LH model with four seat back contact regions displayed .................. 71
Figure 45: Completed LH seat model ................................................................ 73
Figure 46: 2-D JOHN set in place over LH seat ................................................ 80
Figure 47: 2-D JOHN set in place over BAC ..................................................... 81
Figure 48: LH seat with 23° frame angle, 0 mm lumbar prominence ................. 84
Figure 49: LH seat with 23° frame angle, 12.5 mm lumbar prominence ............ 85
Figure 50: LH seat with 23° frame angle, 25 mm lumbar prominence ............... 86
Figure 51: LH seat with 28° frame angle, 0 mm lumbar prominence ................. 87
Figure 52: LH seat with 28° frame angle, 12.5 mm lumbar prominence ............ 88
Figure 53: LH seat with 28° frame angle, 25 mm lumbar prominence ............... 89
Figure 54: LH seat with 33° frame angle, 0 mm lumbar prominence ................. 90
Figure 55: LH seat with 33° frame angle, 12.5 mm lumbar prominence ............ 91
Figure 56: LH seat with 33° frame angle, 25 mm lumbar prominence ............... 92
Figure 57: Graph plotting TRA vs. TLC at constant frame angle for LH
simulations .................................................................................................... 93
Figure 58: BAC set for 23° TRA, 0° TLC ........................................................... 96
Figure 59: BAC set for 23° TRA, 20° TLC ......................................................... 97
Figure 60: BAC set for 23° TRA, 40° TLC ......................................................... 98
Figure 61: BAC set for 28° TRA, 0° TLC ........................................................... 99
Figure 62: BAC set for 28° TRA, 20° TLC ....................................................... 100
Figure 63: BAC set for 28° TRA, 40° TLC ....................................................... 101
Figure 64: BAC set for 33° TRA, 0° TLC ......................................................... 102
Figure 65: BAC set for 33° TRA, 20° TLC ....................................................... 103
Figure 66: BAC set for 33° TRA, 40° TLC ....................................................... 104
Figure 67: Graph plotting TRA vs. TLC at constant recline bar angle for BAC
' simulations .................................................................................................. 105
Figure 68: LH seat with upper thorax support at 0 mm displacement .............. 108
Figure 69: LH seat with upper thorax support at 10 mm displacement ............ 109
Figure 70: LH seat with upper thorax support at 20 mm displacement ............ 110
Figure 71: LH seat with upper thorax support at 30 mm displacement ............ 111
Figure 72: Upper thorax location results with LH seat results ......................... 112

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INTRODUCTION

_1_._1_ BACKGROUND

Locating the position of seated occupants within the automotive interior
space has been an important design goal in the automotive industry for over 40
years. Automobile and seat manufacturers rely on occupant positioning data for
a number of reasons; among them are vision, controls reach, and restraint
position. One of the most important issues is comfort. Comfort design for
seating relies very heavily on accurate knowledge of the location of the
occupants in an auto seat. This information is used to determine seat back
contour, lumbar support location and many other factors that affect the posture
and comfort of the occupant.

Since the early 1960’s, the Society of Automotive Engineers (SAE) two-
dimensional (2-D) drafting template and three-dimensional (3—D) testing manikin
specified in SAE standard J826 [1] have been the main tools used in the design
and development of interior packaging and automotive seating (Figures 1 and
2). Though originally designed only for placing an occupant within an
automotive interior, the SAE 2-D drafting template and 3-D manikin have been
extensively used to design seat shapes and thereby affect the posture and

comfort of seated occupants for the past three decades [2].

 

 

 

Figure 2: SAE 3-D testing manikin

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The SAE drafting template and testing manikin are comprised of the same
body segments. They have a foot, a shank or lower leg segment, a combined
thigh-pelvis segment, and a combined lumbar-thorax or torso segment; the
manikin has two feet and two shanks These segments can rotate about the hip
joint, which is referred to as the H-point, the knee joint and ankle joint. However,
the major drawback of the SAE J826 tools are that the torso and thigh-pelvis
sections only articulate at the H-point so that, unlike human motion, the SAE
tools do not have the capability to represent a change in lumbar curvature. In
addition, the posture that the torso segment represents is a slumped posture,
with a flat back in the lumbar region. Due to these limitations, the manikin is
unable to fit in many automotive seats with large amounts lumbar prominence. If
the 3-D testing manikin is placed in an automotive seat and the thigh-pelvis
segment placed firmly against the seat back, the torso is unable to move from a
fully forward position to rotate about the H—point and come into contact with the
seat back without contacting the lumbar area. If the top of the manikin torso is
then forced back against the seat, the torso segment pivots about the lumbar
area and the thigh-pelvis segment shifts forward in the seat. In sum, the posture
and articulations of the SAE tools are not representative of how the torsos of
people actually move with variations in lumbar curvature

In recent years, biomechanical research at Michigan State University
(MSU) has been directed at developing better human body models for

automotive seat design and evaluation [3]. The main thrust of the work at MSU

 

 

 

 

 

 

 

 

 

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has been to provide greater comfort for the seated occupant by improving the
representation of human torso shape and posture with the introduction of lumbar
curvature as a movement factor. With this representation of lumbar curvature,
the human body models developed at MSU also represent more body segments
than the SAE tools. Unlike the combined thigh-pelvis segment and torso
segment of the SAE tools, the MSU models separately represent the thigh,
pelvic, lumbar and thoracic segments. The movement and posture
representation of the MSU models are more accurately representative of the
human body.

Haas [4] developed a model of human torso motion for an average size
male based on the relative movement between the thorax and pelvis (Figure 3).
This model was named JOHN to recognize the support of the Automotive
Systems Group of Johnson Controls, Inc. Haas selected a one-to-one
relationship of motion between the pelvis and thorax to represent human torso
articulation from a slumped to erect postures. For example, a pelvis rotation of
5° corresponds to a thorax counter-rotation of 5°, for a total change in posture of
10°. This rotation of the thorax relative to the pelvis was defined as Total
Lumbar Curvature (TLC). The zero reference (0°) for TLC is a straight lumbar
spine, with the angular orientation of the pelvis and thorax based on the seated
occupant posture defined in a study by the University of Michigan Transportation
Research Institute (UMTRI) [5,6]. Figure 4 shows the JOHN model moving from

a TLC of 0° to 40°. The other measures defining the position of JOHN are Torso

 

 

 

 

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Figure 4: 2-D JOHN model at 0° TLC and 40° TLC

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Recline Angle (TRA) and the Hip Joint Center (HJC). Torso Recline Angle is
defined as the angle between vertical and a line passing through the top and
bottom lumbar joint centers. These two lumbar joint centers are the twelfth
thoracic/first lumbar vertebrae joint (T12IL1) and fifth lumbar/first sacral joint
(L5/S1), respectively. The HJC is the location of the femur insertion into the
pelvis.

Boughner [7] expanded on the work of Haas and developed a three-
dimensional solid model of the human skeletal and muscle geometry for the
averaged sized male (JOHN), seen in Figure 5. In addition, 3-D models were
created to represent a small female, called JANE, and a large male, named
JERRY [3]. From Boughner's work, Bush [8] developed a 2-D articulating
template which simplified the JOHN model lumbar motion. By placing gears of
equal size at the top and bottom lumbar joints and connecting them by a chain
loop with a twist, the relative motion between the pelvis and thorax was
physically represented in template form (Figure 6). Bush also developed back
contours for the average man by adding flesh thickness to Boughner’s 3-D
JOHN model of the skeletal and muscle geometry.

The previous work at MSU concentrated on modeling just the shape and
kinematics, or geometry and motion, of the human body in seated postures. For
this thesis, two computer models were developed that were kinetic, predicting
both the motion and forces acting on the body, while continuing to represent the

posture descriptors developed for JOHN.

 

 

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Figure 6: 2-D JOHN physical articulating template

 

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The first model, a 2-D representation of the SAE 3-D testing manikin
(Figure 7), simulates the forces and displacements of the SAE 3-D testing
manikin in an automotive seat; the second, a 2-D version of the 3-D JOHN
human body model (Figure 8), is used to simulate the possible postures of an
occupant interacting with a seat. The 2-D SAE and 2-D JOHN computer models
were developed with the aid of Working Model”, a commercial software
product, that allows rapid prototyping and design of mechanical systems. It is
possible to constrain the model bodies with elements such as pin and rigid
joints, gears, and springs, to help simulate mechanisms.

The development of these models represents a step forward in
understanding and modeling the force-posture relationship of seated humans in
automotive seats. A major design and development goal of seat manufacturers
has been to position the seated occupant in the seat and vehicle according to
the vehicle manufacturer specifications. This positioning has been done since
the 1960’s through the use of the SAE J826 tools and will be done in the future
with the next generation manikin, currently being developed by MSU and UMTRI
in a project titled Automobile Seat Package Evaluation and Comparison Tool
(ASPECT). The manufacturers position the occupants and design the seat
using the SAE 2-D drafting template. After design, they build a prototype and
test the seat using the SAE 3-D testing manikin to determine if the prototype
places the manikin in the design position. Other tests are performed with human

subjects for subjective response to the comfort of the seat. This begins an

10

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Figure 8: 2-D JOHN kinetic computer model

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iterative cycle of re-design and further testing until the seat is declared ready for
production, as much as two years and numerous prototypes later. The
development of the kinetic 2-D SAE and JOHN computer models provide a way
to relate the measurable data, obtained with the manikin and people through
product evaluation, to the design tools. These models will allow the designers to
predict the interaction between the person and manikin and the seat in the

design stage, before prototyping, saving both time and money.

1_._2_ OBJECTIVES

The two objectives of this study were to:

1) Develop the 2-D SAE kinetic computer model and simulate
experimental data collected using the SAE 3-D testing manikin to further
understand the effect of the seat cushion system on the distribution of forces
supporting the manikin.

2) Develop the 2-D JOHN kinetic computer model and undertake a
comparative study between a standard automotive seat and an articulating

prototype seat using the 2-D JOHN model in order to study the effect of
I different seat designs on the final posture of the model.

This thesis is organized to describe the methods, results and discussion

for these two objectives. The final section is a conclusion that summarizes the

findings and presents recommendations for future work.

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2-D SAE KINETIC COMPUTER MODEL DEVELOPMENT

a METHODS AND MATERIALS
2.1.1 2-D SAE computer model const_r_uction

The 2-D SAE computer model was developed and used to simulate
experimental data collected by Hubbard et. al. [9] on automotive seat cushions
using the SAE 3-D testing manikin torso and thigh-pelvis segments.

To construct the 2-D SAE computer model, existing CAD computer files of
the 2-D drafting template were obtained from the Automotive Systems Group of
Johnson Controls, Inc. The main body segments, the torso and thigh-pelvis
segments, were imported into the Working ModelTM software (Figure 9).
Because the Hubbard experiment did not utilize the legs, they were not imported
into the software.

After importing the SAE body segments into the simulation, the next step
was to assemble the model. The body segment joint locations were found with
the aid of measurements taken from an SAE 3—D testing manikin, a 2-D drafting
template, and the CAD computer files of the 2-D drafting template. ln Working
Model”, points were placed on the torso segment at the H-point and on the

thigh-pelvis segment at the H-point and knee joint (see Figure 10). Once the

points had been located on the segments, the two segments were then

13

     

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2.1.2 Locating the centers of mass for the thorax and thigh-pelvis segments

 

The next step in developing the 2-D SAE computer model was to assign
the mass and center of mass (COM) locations to the torso and thigh-pelvis
segments. The SAE 3-D testing manikin was used to determine the location of
the COM for each segment.

The torso segment of the SAE 3-D testing manikin was separated from
the thigh-pelvis segment at the H-pt axis. Each segment was then attached to a
calibrated load cell and hung from the ceiling. The mass of each segment was
calculated from the load cell readings and assigned to the appropriate segment
in the model. The mass of the thigh-pelvis segment was 10.07 kg and the mass
of the torso segment was 9.09 kg.

To find the COM of the torso and thigh-pelvis segments, it was necessary
to locate the intersection of two lines extended from balancing points of the
segments. To determine the first line, a thin rod was attached to the floor and
then each segment was placed on top of the rod and balanced. A vertical line
was marked upward on each segment from the balance rod. To find an
intersecting line, each segment was suspended from the ceiling by a strap. The
thigh-pelvis segment hung from the ceiling by a strap that was attached to the
knee joint bar. Two pieces of string were tied to the outer ends of a thin metal

rod and then onto the knee joint bar (Figure 12). The strings and rod

15

Figure 11: Assembled 2-D SAE kinetic computer model

15

 

 

Figure 12: Picture of thigh-pelvis segment COM calculation

17

 

 

 

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combination acted as a plumb bob, and the intersection point of the string with
the line previously marked on the segment defined the COM for the thigh-pelvis
segment. The COM location was measured from the H-pt along an axis system
defined by the thigh line (a line from the H-pt to the knee joint) and a line in the
mid-sagittal plane perpendicular to the thigh line. The COM of the thigh and
buttocks segment was 152 millimeters (mm) forward of the H-pt (toward the
knee) and 10 mm above the thigh line. It was assumed that the COM was in the
mid-line plane of symmetry.

To determine the COM of the torso segment, a strap was attached to the
H-point axis and the segment was suspended from the ceiling by the strap. A
piece of string was tied to each end of the H-point axis and then to a thin metal
rod. This string and rod system acted as a plumb bob (Figure 13) and the
intersection of the plumb bob string line with the line previously marked on the
torso was defined as the COM. To measure the location of the COM, an axes
system for the torso segment was defined as the torso line, (a line through H-pt
that is parallel to the straight lumbar region) and a line in the mid-sagittal plane
perpendicular to the torso line. Using the H-pt as the origin, the distance to the
COM along those axes was 235 mm above the H-pt and 29 mm toward the back
of the thorax. Once again, mid-line plane symmetry was assumed for the COM.

The axes described above for the torso and thigh-pelvis segments were

then re-created on the 2-D SAE computer model. In Working Model”, a point

was placed as a marker on the torso and thigh-pelvis body segments where the

18

 

 

Figure 13: Picture of torso segment COM calculation

19

 

 

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positions of the physical COM were located. The COM of each segment was

then modified until it was positioned at the physical COM (Figure 14).

2.1.3 Experimental data collection

The 2-D SAE computer model was developed to simulate the test
methods used by Hubbard, et. al. [9], in an experiment to characterize the
stiffness of an automotive seat cushion. Their work involved developing a seat
cushion support model and an experiment which used the SAE 3—D testing
manikin to collect data. The seat cushion was modeled as two springs, one
supporting the H-pt and the other supporting the knee joint. The data collection
procedure determined the effective forces under the knee and H-pt of the SAE 3-
D testing manikin for various loading conditions, and then used the SAE 3—D
testing manikin to determine the deflections at the knee and H-pt for the same
loading conditions. The methods and results of the experiment are described in
detail below.

In the experiment, the distribution of weight at the knee axis and H-pt axis
was determined by suspending the 3-D manikin shell (torso and thigh-pelvis
segments only) from the ceiling with two load cells, one connected to the knee
axis and the other to the H-pt axis. The torso angle of the 3-D manikin was set
and kept at 0°, relative to vertical, while the manikin was loaded. The
distribution of the weight between the two load cells was recorded. Then two

torso masses (circular masses with holes in the center of 3.92 kg each) were set

20

Figure 14: 2-D SAE computer model with COM positions

21

 

in the manikin shell on the H-pt (the masses for the 3-D testing manikin are
shown in the torso and thigh-pelvis segments in Figure 2). The measurements
from the load cells were recorded. Three more sets of torso masses were
placed into the torso shell and the data recorded after each set until four (4)
pairs of torso masses were placed in the manikin, eight (8) weights total. Next,
the thigh masses (metal cylinders with holes at one end of the cylinder of 3.42 kg F
each) were placed in the manikin on the thigh pins. These thigh pins are small
pins set inside the thigh-pelvis segment about halfway between the knee and H-

pt. The load cell measurements were recorded again. Then another pair of

 

torso masses were placed directly in front of the thigh masses between the end
of the shell and the thigh weights, and the load cell readings recorded. Finally,
the two torso masses placed on the thigh were removed and placed over the
ends of the knee joint bar, and the load cell readings recorded. The loading

conditions and support forces at the knee and H-pt, in Nevvtons, are listed in

 

 

 

 

 

 

 

 

 

 

 

Table 1.
Table 1 - Experimental Force Results
' Loadinggondition Force at H-pt (N) Force at knee (N)
2-D template 0 0
3-D manikin shell 154 32
1 set of masses at H-pt 229 34
2 sets of masses at H-pt 305 35
3 sets of masses at H-pt 381 37
4 sets of masses at H-pt 458 36
1 set of masses on thigh . 485 78
2 sets of masses on thigh 505 125
1 set of masses at knee 485 158

 

 

 

 

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These loading conditions were intended to impose loads between the
SAE 3-D testing manikin and the seat which ranged from less to more than the
typical loading that occurs in manikin use. These conditions provide results for
understanding seat response to variations in loading.

The 3-D testing manikin was then placed in a 1996 Chrysler Neon seat
and loaded, using the same test protocol described above and shown in Table 1.
Retro-reflective targets were attached to the manikin at the knee and H-pt joints
so that the positions of those joints could be recorded by a QualisysTM video-
based motion measurement system as the manikin was loaded. The raw
position data was tracked in the QualisysTM motion measurement software and
then transferred to a spreadsheet to be analyzed. To obtain an undeflected
reference position, the torso and thigh-pelvis segments of the SAE 2-D drafting
template were placed in the seat with the torso angle set at 0°. Retro-reflective
targets were placed on the 2-D drafting template at the knee and H-pt and the
locations of the targets recorded with the motion measurement system. These
position data were also analyzed and transferred to a spreadsheet.

The deflection and force values at each loading condition are listed in
Table 1. The results obtained by Hubbard, et. al. [9], show that the seat system
responded nearly linearly at the H-pt (Figure 15) but that the seat did become
incrementally softer as more load was placed on it. At the knee (Figure 16), the
seat system responded nearly linearly after the thigh was loaded. Previous to

that, the knee joint moved vertically upward as the H-pt was loaded. As the H-pt

23

 

 

 

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sunk into the seat cushion due to the load, the thigh pelvis section pivoted about
a point between the H-pt and knee joint. This point worked as a lever and as a

result the knee joint moved upward as the H-pt moved downward.

Table 2 - Experimental force and deflection results from Hubbard, et. al. [9]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Loading Force at H-pt Force at knee Deflection at Deflection at
Condition (N) (N) H-pt (mm) knee (mm)

2-D template 0 0 0 0

3-D manikin 154 32 —8 -9

1 set @ H-pt 229 34 -12 -9
2 sets @ H-pt 305 35 -16.5 -8.5

3 sets @ H-pt 381 37 -22.5 -5

4 sets @ H-pt 458 36 -30 -3.5

1 5w thigh 485 78 -31 -9.5
2 setsfi thkLh 505 125 -32.5 -19

1 set @nee 485 158 -33 -22.5

 

2.1.4 Modeling of experimental data

The 2-D SAE computer model was used to simulate the experimental
results obtained by Hubbard, et. al. To replicate the static support system, one
end of a spring was placed on the H-pt of the model and the other end anchored
to the background of the modeling space. A second spring was used to support
the knee. One end of the knee support spring was attached to the knee joint
and the other end attached to the background of the modeling space (Figure
17). To assist in keeping the torso angle at 0°, the torso and thigh-pelvis
segment were pinned to vertical sliders (Figure 18). This system allowed
frictionless vertical motion but no horizontal motion. To correctly model the

experiment, circular geometric bodies were pinned to the 2-D SAE computer

26

 

 

 

 

 

 

 

 

 

 

Figure 17: 2-D SAE model with spring supports at knee and H-pt

27

 

 

 

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model to represent the placement of the masses on the SAE 3—D testing manikin.
These circular bodies were placed at the H-pt, mid-thigh at the thigh pins, just in
front of the thigh pin, and at the knee joint (Figure 19). The masses of these
circular bodies were changed during the simulations to represent the different
loading conditions shown in Table 1. To aid with the simulations, adjustable
slider boxes were created in Working ModelTM to allow the user to easily make
changes of the mass values. The slider boxes allow the user adjust the mass of
an object by sliding a marker up and down a scale in the Working ModelTM
software. In addition, the H-pt mass slider was broken into increments to allow
easy modification of the number of mass pairs positioned at the H-pt. Input
value boxes were created to allow the user to easily change the spring constants
of the supporting springs. On-screen output boxes were also created to show the
deflections at the knee joint and H-pt, and to display the force acting on each
spring support (Figure 20).

The 2-D SAE computer model was adjusted in Working ModelTM until the
thigh angle of the model matched the thigh angle of the physical 2-D drafting
template when it was placed in the Chrysler Neon seat. Gravity was applied to
the simulation and then the simulations were started, in effect “dropping” the 2-D
SAE model onto the spring supports. The loading conditions performed in the
experimental testing and described in Table 1 were repeated for the simulations.
From the results of Hubbard, et. al. [9], piecewise spring constants were

computed for each loading step and are given in Table 3. These were used for

29

l

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 19: 2-D SAE model with mass circles on body segments

30

 

 

lat Thigh mass Thorax Mass 3.901;”; Load

m ---N

2nd Thigh ms 13: Knee Kass

7

.80

 

 
 

 

 

 

 

 

 

   

 

 

 

 

 

 

 

 

 

 

 

 

Figure 20: 2-D SAE model with input and output boxes

31

 

the H-pt and knee joint springs at each separate loading condition. The
deflections and forces under the H-pt and knee were recorded after each
simulation had stabilized.

After performing the simulations described above, the spring at the knees
was moved to a point on the thigh. This repositioning of the support was
performed to more closely simulate the loading of an automotive seat by the
manikin and to see if the same deflection results from the experiment could be
simulated with a different support system.

A geometric calculation was used to find an appropriate location for the
thigh support. From the experimental data collected with the SAE 3-D manikin, it
can be seen that the H-pt settled into the seat cushion due to the loading at the
H-pt, and as this occurred the knee rose vertically upward relative to its initial
deflected position. By plotting this data, a thigh support location was
determined. Figure 21 shows the thigh orientations for the first five loading
cases (shown in Table 1) superimposed over each other. The thigh support was
placed at the point on the thigh about which the thigh rotated as the H-pt was
loaded. A thigh support spring would behave in this way, with no deflection, if no
load were added to the thighs while load was only added to the H-pt. The
location of the rotation point was visually measured from Figure 21 and the
spring support relocated to this position on the 2-D SAE model (Figure 22).

The purpose of relocating the spring support to the thigh was to provide a

more realistic seat support system for the 2-D SAE model and determine the

32

 

Figure 21: Superimposed thigh orientations for first five cases

33

 

 

 

Deflection of. Knee J.

 

let Thigh mess Thorax Mess H-poinr. Load
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2nd Thigh Heee 18c Knee Nee:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 22: 2-D SAE model with spring supports at thigh and H-pt

34

support spring constants. To accomplish this, the deflections at the H-pt and
knee obtained in the experiment performed by Hubbard, et. al. [9], were
reproduced with the 2-D SAE computer model. To do so, the 2-D SAE model
was reloaded in the manner described in Table 1 after relocating the knee spring
support to the thigh in the model simulation. The support forces at the thigh and
H-pt spring supports were recorded. These support forces were different from
the previous simulation due to the different support locations. Using the new
support force data from the H-pt support and the original experimental deflection
data at the H-pt, from Hubbard, et. al. [9], the new piecewise spring constants
were calculated for the H-pt. Next, the piecewise spring constants for the thigh
were calculated. To accomplish this, the simulation was run again, this time
using the new H-pt spring constants. At each loading condition, the spring
constants for the thigh support were modified until the deflections at the knee
joint matched those from the experimental data obtained by Hubbard, et. al. [9].
The spring constant values for the thigh support were recorded for each loading

condition in the simulation.

2_.2_ RESULTS AND DISCUSSION

The 2-D drafting template model results, with the spring supports located
under the knee and H-pt, were similar to the experimental results obtained by
Hubbard et. al. [9]. Using this deflection and force data, the piecewise spring

constants for both the knee and H-pt were computed for each increase in

35

loading using a linear spring assumption (F =kx). The spring constant results are

listed in Table 3. These spring constants are the slope of the line between each

loading condition in the graphs shown in Figures 15 and 16.

Table 3 - Piecewise spring constants calculated from experimental data

 

 

 

 

 

 

 

 

 

 

 

 

 

Loading Condition Spring constant @ H-pt Spring constant @
(Nlm) knee (Nlm)

2-D template --- --—-

3-D manikin shell 19.25 3.55

1 set of masses at Hjut 19.08 3.78
2 sets of masses at H-pt 18.48 4.12
3 sets of masses at H-pt 16.93 7.40
4 sets of masses at H-pt 15.26 10.28
1 set of masses on thig:h 15.64 8.21
2 sets of masses at thigh 15.54 6.58
1 set of masses at knee 14.70 6.20

 

As it can be seen in Table 4, the measured and modeled results for the

forces at the H-pt were within 2% or less for each loading condition. Table 5

displays the results obtained for the knee supports and shows that the modeled

and measured results were within 7% or less at every loading condition. The

values of the simulated support force under the H—pt compared more closely with

the experimental data than the simulated support force values under the knee,

but the modeled and experimental results for both supports are very close. The

modeled values for the force supporting the H-pt were higher than those in the

experiment. The opposite held true for the modeled force under the knee, where

the modeled values were less than the experimental values. However, the AF

36

 

columns in Table 5 and Table 6 show that the magnitude of the change between

the experimental and modeled force values is approximately the same. This

indicates that there is need for refining the mass placements on the 2-D SAE

model by moving a mass toward the knee to correct the discrepancy with the

experimental results. The results from the 2-D SAE computer model do show

that the model can be used to simulate experimental data collected on an

automotive seat cushion by the SAE 3-D testing manikin with a high degree of

repeatability.

Table 4 - 2-D SAE model force and deflection results

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Loading Force at H-pt Force at knee Deflection at Deflection at
Condition (N) (N) H-pt (mm) Knee (mm)

2-D template 0 0 0 0

3-D manikin 153 33 -7.9 -9.3

1 set @ H-pt 231 33 -12.1 -8.7

2 sets @ H-pt 308 34 -16.7 -8.3

3 sets @ H-pt 385 35 -22.7 -4.7

4 sets @ H-pt 463 35 -30.3 -3.4

1 set @ thigh 489 75 -31.3 -9.1
2 sets @ thigh 514 126 -33.1 -19.1

1 set @knee 486 150 -33.1 -24.2

 

37

 

Table 5 - Experimental vs. model results for force at H-pt

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Loading Force at l-l-pt - Force at H-pt Percent AF (N)
Condition experimental - model (N) difference
(N)
2-D template 0 0 --- 0
3-D manikin 154 153 0.6 -1
1 set @ H-pt 229 231 0.9 +2
2 sets @ H-pt 305 308 1.0 +3
3 sets @ H-pt 381 385 1.0 +4
4 sets @ H-pt 458 463 ' 1.0 +5
1 set @ thigh 485 489 0.8 +4
2 sets @ thigh 505 514 1.7 +9
1 set @ knee 485 486 0.2 +1

 

Table 6 - Experimental vs. model results for force at knee

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Loading Force at knee Force at knee - Percent AF (N)
Condition - experimental - model (N) ‘ difference
IN) , ’ ’ '
2-D template 0 0 --- 0
3-D manikin 32 33 3.0 +1
1 set @ H-pt 34 33 3.0 -1
2 sets @ H-pt 35 34 2.8 -1
3 sets @ H-pt 37 35 5.4 -2
4 sets @ H-pt 36 35 2.8 -1
1 set @ thigh 78 75 3.8 -3
2 sets @ thigh 135 126 6.6 -9
1 set 9 knee 158 150 5.0 -8

 

The simulated support force results for the H-pt and thigh support system
are shown in Table 7. Recall that the deflections are the same as those
obtained in the experiment by Hubbard, et. al [9]. As can be expected, when the
leg support is moved in from the knee to the thigh, forces acting on the H-pt and

thigh supports become more evenly distributed than with the supports at the H-pt

38

 

 

(Table 5) and knee (Table 6). With the support moved from the knee to the
thigh, the support force at the H-pt decreased accordingly, and, to achieve the
same deflections in the model as those predicted by the experiment the
piecewise spring constants under the H-pt (Table 3) decreased, or became
softer. The piecewise spring constants for the H-pt and thigh support system are
shown in Table 8. As it can be seen, the seat was substansially stiffer under the
H-pt than under the thighs. Table 9 shows a comparison between the H-pt
spring constants when modeling the seat systems with a H-pt and thigh support
as compared to a H-pt and knee support. The support system using springs at
the thigh and H-pt is a more realistic system for simulating the response of the
manikin placed in an automotive seat, and, as Table 9 shows, after the support
system had been changed, the spring constants under the H-pt decreased a

substantial amount at the very light and very heavy loading conditions.

Table 7 - Model force results for supports at H-pt and thigh

 

 

 

 

 

 

 

 

 

 

 

LoadingCondition Force at H-pt (N) Force at thigh (N)
2-D template 0 0

3—D manikin shell 143 44

1 set of masses at H-pt 220 44
2 sets of masses at H-pt 298 45
3 sets of masses at H-pt 376 43
4 sets of masses at H-pt 454 40
1 set of masses at thigh 467 94
2 sets of masses at thigh 476 163
1 set of masses at knee 442 197

 

 

 

39

 

Table 8 - Model piecewise spring constants for supports at H-pt and thigh

 

 

 

 

 

 

 

 

 

 

 

 

 

Loading Condition Spring constant @ H-pt Spring constant @
(kg/s2) thigh (kg/s”)

2-D template --- ---
3—D manikin shell 17.9 5.1
1 set of masses at H-pt 18.3 4.5
2 sets of masses at H-pt 18.1 4.2
3 sets of masses at H-pt 16.7 4.3
4 sets of masses at H-pt 15.1 4.6
1 set masses at thigh 15.1 6.5
2 sets of masses at thi h 14.6 7.3
1 set of masses at knee 13.3 7.2

 

Table 9 - Comparison of H-pt piecewise spring constants for support at knee vs.

 

 

 

 

 

 

 

 

 

 

 

support at thigh

Loading Condition H-pt spring H-pt spring Ak

constant with thigh constant with knee (Nlm)
support (Nlm) support (Nlm)

2-D template --—- --—- ---
3-D manikin shell 17.9 19.25 -1.35
1 set of masses at H-pt 18.3 19.08 -1.22
2 sets of masses at H-pt 18.1 18.48 -0.38
3 sets of masses at H-pt 16.7 16.93 -0.23
4 sets of masses at H-pt 15.1 15.26 —0.16
1 set masses at thigh 15.1 15.64 -0.54
2 sets of masses at thigh 14.6 15.54 -0.96
13.3 14.70 -1.40

 

 

 

 

1 set of masses at knee

Using the 2-D SAE computer model to repeat experimental results

obtained with the SAE 3-D testing manikin is of great importance to a seat

designer. Currently, the seat manufacturers are able to design and construct an

automotive seat with the SAE J826 tools. The development of the experimental

seat testing protocol by Hubbard, et. al. [9], now enables the manufacturers to

40

 

 

measure the response of an automotive seat cushion with the SAE 3-D testing
manikin. With the development of the 2-D SAE computer model, the seat
manufactures are able to simulate the response of the 3-D testing manikin in an
automotive seat cushion with the 2-D SAE computer model. This allows the
designers to modify the variables of the seat cushion, such as stiffness, and
study the effect of a change with the 2-D SAE computer model. In addition,
because the SAE 3-D testing manikin simulates human hip and knee locations
and torso angle, and the 2-D SAE computer model simulates the response of the
3-D manikin in a seat, designers can now use the 2-D SAE computer model to
model human hip and knee locations for this particular automotive seat. In the
future, with further seat testing, it will be possible to model the
force/displacement response of the SAE 3—D testing manikin on the computer for
different seats. This modeling ability will save the seat manufacturers time and
money by allowing them to simulate different seat stiffness characteristics

without having to build as many expensive prototypes.

41

2-D JOHN COMPUTER MODEL DEVELOPMENT

3.1 METHODS AND MATERIALS

3.1.1 2-D JOHN kinetic computer model construction

Once the 2-D SAE computer model had been developed and shown to
accurately simulate a physical experiment in an automotive seat, the next step
was to develop a representation of JOHN in Working ModelTM. The goal of the
2-D kinetic JOHN model development was to provide a representation of human
torso motion which could be used to predict seated human position and the
forces between the model and seat.

To construct the 2-D JOHN model torso, numerous contour points were
selected from pre-existing 2-D CAD drawings of the pelvic, lumbar, and thoracic
body segments of JOHN [8], and the coordinates of these points were entered
into the Working Modelml software as the vertices of polygons (Figure 23). The
bedy segment contours represented the skeletal geometry of the three segments
surrounded by soft tissue. The body segment contours were adapted by
Hubbard et. al. [3] based on data obtained from the University of Michigan
Transportation Research Institute (UMTRI) study on seated occupant
anthropometry [5,6] and a corrected pelvis location that had been used in

developing the JOHN model in which the lumbar spine was lengthened by 30

42

mm. In addition, this anthropometric data was used to locate the joint centers on
the thorax, lumbar and pelvis body segments in Working Model”. Points were
placed on the pelvis body segment at the HJC and L5/S1 joint center, and on the
thorax segment at the T12lL1 joint center. Two corresponding points for the
T12IL1 and L5/S1 joint centers were placed on the lumbar segment (Figure 24).
After the torso had been completed, the legs for the 2-D JOHN model
were constructed. The average male legs of the SAE 2-D drafting template
(composed of the thigh-pelvis, shank and foot segments) [1] were imported into
Working ModelTM from a CAD computer file obtained from the Automotive
Systems Group of Johnson Controls, Inc. Unlike the typical SAE practice of
using the 95th percentile male leg lengths with the 50th percentile body, the SAE
average male legs were used for the development of the 2-D JOHN model
because the model was to be used only with seat related issues, and not
packaging issues; therefore a model with consistent anthropometry was
preferred. Because the 2-D JOHN model included a pelvis segment, the thigh-
pelvis segment from the SAE 2-D drafting template was reshaped to represent
only a thigh (Figure 25). The CAD file of the SAE 2-D drafting template
contained the placement of the hip joint center and knee joint on the thigh, the
knee joint on the shank, and the ankle joint on shank and foot. The location of
these joint centers were confirmed with measurements from the SAE 2-D drafting
template and the physical SAE 3-D testing manikin. In Working Model”, points

were placed on the thigh, shank and foot segments to represent the location

43

 

 

Pelvrs Thorax Lumbar

Figure 23: 2-D JOHN thorax, lumbar and pelvis segments

T12/L1 'oint
cenfir, \

   

   
 

    

T12/L1
joint center

 

Hi '0' t center
p] L5/Sl1 joint

COMET o /

Thorax Lumbar

 

 

Pelvis

Figure 24: 2-D JOHN torso segments with points at joint centers

44

of the joint centers.

With all the body segments sized appropriately and the joint center
locations correct, the JOHN model segments were assembled. To simulate the
one-to-one thorax to pelvis motion of JOHN, a linkage using gears was
developed. Two gear circles, with diameters of 75 mm, were created. Points
were placed at the centers of the two gear circles (Figure 26). The gear circles
were adjusted until points at the centers were aligned over the lumbar joint
center points on the pelvis segment at the T12/L1 joint and on the thorax
segment at the L5/S1 joint center. The two gear circles were rigidly pinned to
the thorax and pelvis segments (Figure 27). The lumbar segment was then
joined to the two gear circles at the gear circle points, creating the L5/S1 and
T121L1 joints and a completed torso (Figure 28). Although in the 2-D forms the
thorax, lumbar, and pelvis segments and gear circle bodies overlap, they do not
collide with each other because they are layered. Any two segments that are
pinned together are assumed to not collide in the Working ModelTM software.
Finally, a gear linkage was placed between the two circles, changing the circle
bodies into gears (Figure 29). Because the gear circles were of the same
diameter, they had a 1:1 gear ratio, thereby establishing the one-to-one counter-
rotation of the thorax and pelvis. In effect, the mechanism consisted of one gear
resting upon the other with the teeth of the two gears intermeshed, and with the

gears each pinned to a body and held a fixed distance apart.

45

Figure 25: 2—D JOHN legs with redesigned thigh

 

 

 

Figure 26: 2-D JOHN torso segments with gear circles and joint centers

46

 

 

 

 

 

Figure 27: Gear circles pinned to thorax and pelvis segments

47

 

 

 

 

 

Figure 28: Lumbar segment joined to thorax and pelvis segments

48

 

 

 

 

 

 

 

Figure 29: Gear linkage added to 2-D JOHN model

49

Once the 2-D JOHN model was fully assembled, a nominal posture for the
model was defined in Working Modelm, using previous data on the orientation of
JOHN [4,7,8]. The positioning of JOHN was described in joint center
orientations because JOHN had been constructed as an accurate skeletal model
of the human body. In order to orient the thorax, lumbar and pelvis body
segments of the 2-D JOHN model in Working ModelTM the same as the 2-D
JOHN model described in Hubbard et. al. [3], the model was placed in a 30°
TRA (Torso Recline Angle) posture, which corresponded to a line between the
T12/L1 and LSIS1 joint centers being 30° right of vertical. To achieve a 0° TLC
posture at 30° TRA, the thorax segment was oriented so that a line from the
T121L1 joint center to the C7/T1 joint center was at 0° or vertical. This measure
was determined from the 2-D articulating JOHN template developed by Bush [8].
The pelvis segment was oriented so that a line from the HJC to the L5/S1 joint
center was 64° right of vertical (Figure 30).

After the 2-D JOHN model body segments had been aligned, output
boxes were created to continuously update the values of TLC and TRA on-
screen during a simulation. As described previously, the measure of TRA was
the angle, from vertical, of a line between the L5/S1 and T12/L1 joint centers.
This measure corresponded to the rotational orientation of the lumbar segment
in Working Model”, so that when the lumbar segment was at 45° right of
vertical, the TRA of the JOHN model was also 45° right of vertical. The TLC was

measured by determining the relative rotation of the thorax and pelvis to each

50

C7/T1

 

 

 

Joint .
00
1
0° Joi
6 0*1 Joint
Hip Joint

Figure 30: 2-D JOHN torso showing segments with joint center orientations

51

other, starting with the 0° TLC value described above. The values displayed
were the state of each variable at each time step in the simulation run (Figure
31).

After initial trial simulations had been run with the 2-D JOHN model it
became apparent that another constraint was needed because the model would
occasionally slump so much that it would achieve a posture beyond the range of
human movement. To alleviate this problem, a rope was added between the
lower rear of the thorax and upper rear of the pelvis to act as a constraint (Figure
32). This rope restricts the model from movement to a TLC less than -10°, the
assumption being that people will not slump past that point due to passive

muscle and ligament constraints.

3.1.2 Locating the COM’s for the 2-D JOHN model body segments
When the 2-D JOHN model had been fully constructed, the mass and

center of mass (COM) were assigned to each body segment. For ease in
modeling a symmetrical posture was assumed in the sagittal plane, so the
masses of the 2-D JOHN model thigh, shank and foot are the combined mass of
the right and left body segments. In addition, because this study was focused on
the effects of seat geometry, not the vehicle interior, it was decided to eliminate
the steering wheel and its possible effect on the posture of the model due to the
moment placed about the back when the arms were held out to grasp the wheel.

An equivalent COM was calculated for the thorax, combining the masses of both

52

 

Body Measures

 
 

 

0 4’6"

9)

Figure 31: Output box showing TLC and TRA

53

f 3
i

"

I -1 0° TLC, 0° TRA 40° TLC, 0° TRA

I

 

 

 

\\A

 

 

 

Figure 32: Rope added to 2-D JOHN torso

M -

arms, thorax, head, and neck and orienting the arms straight down by the sides
of the body.

The COM’s for the foot, shank, thigh, pelvis and lumbar segments were
found utilizing a full size drawing of the average sized male developed in the
seated UMTRI study [5,6]. In order to locate the COM’s, a coordinate system
was established by placing a line between the known joint locations for the foot,
shank, thigh, pelvis and lumbar body segments on the drawing. Each line
formed one axis and a line perpendicular to it formed the other axis. For each
body segment, the distance to the center of mass was measured along those
axes from one joint center, which was the origin. The same axes were
duplicated on the respective body segments of the 2-D JOHN model and a point
was placed on each body segment at its COM. The COM for each 2-D JOHN
model body segment was then positioned according to the location defined by
the UMTRI data [5,6] (Figure 33). Table 10, below, is listing of the body

segment masses.

Table 10 - 2-D JOHN body segment masses

 

 

 

 

 

 

 

 

 

Body Segment Mass (kg)
Thorax (combined thorax, head, neck 36.447
and both arms (including hands))
Lumbar 2.365
Pelvis 1 1 .414
Thigh 1 7.228
Shank 7.174
Foot 1.962

 

55

 

 

é. ..

Figure 33: COM positions on all segments except thorax

56

The equivalent COM for the 2-D JOHN thorax was determined by utilizing
the UMTRI drawing of the average male seated occupant with the arms straight
down from the shoulders (vertical) along the sides of the body (Figure 34). The
head and neck were oriented as described in the UMTRI data [5,6] on seated
anthropometry. The UMTRI data was used because this was the only reference
available showing the human body with the centers of mass for the body
segments in a typical automotive seated position.

To calculate this equivalent COM, a coordinate system was set up on an
full size drawing of the average male developed by UMTRI. A coordinate system
was established which placed the origin at the glenohumeral joint of the
shoulder. The x and y axes of the coordinate system corresponded to the x and
y axes of the UMTRI drawing. The length of the upper arm, from the shoulder
joint to the elbow joint, and the length of the lower arm, were determined from
the UMTRI drawing. The upper arm and lower arm (which included the hand)
were redrawn in a new orientation, with the arm joint centers in a line straight
down from the glenohumeral joint, oriented parallel to the y axis.

The COM for the upper arm and lower arm body segments were
measured using a segmental coordinate system. The upper arm COM was
measured from the shoulder joint using a line between the shoulder joint and
elbow joint as one axis, and a line perpendicular to that as the second axis. The
same method was applied to the lower arm, using instead the elbow joint and

wrist joint as one axis, a line perpendicular to that as the second axis and the

57

 

Figure 34: Original orientation of head, neck, arms and thorax showing COM’s

58

elbow joint as the origin. The upper and lower arm COM’s were relocated on the
full sized UMTRI drawing using the new upper and lower arm segment
orientations (see Figure 35).

With all the segment centers of mass in place and the segments oriented
correctly, the equivalent COM for the vertical arm placement was calculated.
The x-direction distance of the equivalent COM from the origin, 3?, equals the
sum of all the body segment moments in the x-direction, divided by the total
mass. The sum of the moments in the x-direction is the mass of each segment
multiplied by the distance from the origin, positive or negative, in the x-direction.
The same approach was used for calculating the y-direction location of the

equivalent COM, y [10]. To calculate the moments for y, the masses of the

head, neck, thorax, upper arm and lower arm segments were all multiplied by the
distance, in the y-direction, each segment was from the shoulder joint. The
moments of all the segments were summed and divided by the total mass of all
the segments in order to find the location of the equivalent COM in the y-
direction. When the calculations had been completed, the equivalent COM for
the thorax, which included the masses of the arms, neck, head and thorax, was
found to be 8.4 mm forward (x-direction) and 111.3 mm below (y-direction) the
Shoulder joint. The equivalent thorax COM was 2.6 mm rearward (x—direction)
and 1 4.7 mm above (y-direction) the normal COM of the thorax. This point was

Iocated and marked on the thorax segment of the average male UMTRI drawing.

59

60

_/ Thorax COM

Upper arm COM

 

Figure 35: New orientation of head, neck, arms and thorax for
2-D JOHN thorax equivalent COM calculation

60

In order to apply the equivalent center of mass to the thorax segment of
the 2-D JOHN model, the location of the equivalent center of mass was
measured from the UMTRI drawing relative to the C7/T1 (seventh cervical/first
thoracic vertebrae) joint center along a body segment axis system that consisted
of a line from the C7/T1 joint center to the T12/L1 joint center and another line
perpendicular to it. When the location had been determined, the body segment
axes were duplicated on the thorax of the 2-D JOHN model, a point was placed
at the location of the equivalent COM of the thorax, and the model COM
positioned to coincide with the calculated COM (Figure 36).

Combining the masses and locations of the arms, head, and neck, with
the thorax mass and COM effectively fixes the orientation of those body
segments with respect to the thorax. This may have an effect on the final
position of the 2-D JOHN model. However, since the distance between the
equivalent COM and the normal thorax COM is only 2.6 mm in the x-direction
and 14.7 mm in the y-direction, fixing the orientation of the head, neck and arms
segments with respect to the thorax will likely have only a minimal effect of the

final posture of the 2-D JOHN model.

3.1.3 Seat model construction
After the 2-D JOHN model had been completed, it was used in a seat
geometry comparison. The purpose of this study was to determine if the model

could be used to differentiate between seat designs based on the postural

61

O
°e’
\a’ ’

Figure 36: 2-D JOHN model with COM positions for all body segments

62

response of the model to different seat geometries. The two seats compared in
this study were an automotive seat, a Chrysler LH, and a prototype articulating

seat called the Biomechanically Articulating Chair (BAC) [11].

3.1.3.1 BAC model development and construction
The BAC was designed to support the body in a wide variety of postures

by matching the chair motions to the motions of the human body segments,
defined through the JOHN model. There are four major parts of the BAC; 1. a
pelvis support that cradles the back and bottom of the pelvis and rotates under
the pelvis near the ischial tuberosities, 2. a thorax support that pivots in the mid-
back and allows rotation of the thorax with spinal flexion and extension, 3. a
recline bar that connects the pivots of the pelvis and thoracic supports, and 4. a
thigh support that pivots on the front of the pelvis support (Figure 37). Like the
JOHN model, the thorax and pelvis supports move together in a one—to-one
motion where a rotation of the pelvis support results in an equivalent counter
rotation of the thorax support. Because the chair moves like the JOHN model,
the position of the BAC can be described in terms of TLC and TRA. The
coupled rotation of the thorax and pelvis supports result in a change of TLC. A
seated person’s TRA can be changed by adjusting the angle of the recline bar.
To develop the BAC in the Working ModelTM simulation, dimensions of the
pelvis support, thorax support and recline bar were measured from a full size

working prototype. The dimensions were entered into the simulation to construct

63

 

Picture of the BAC prototype

Figure 37

four rectangles and the seat assembled with pin and rigid joints. The thigh
support was not simulated because the focus of this study was on torso posture.
To determine the starting relationship between the pelvis and thorax
supports of the BAC, as well as the initial recline bar angle, the full size physical
JOHN 2-D articulating template, developed by Bush [8], was set into the BAC
prototype. The TLC of the template was set to 40° and the pelvis and thorax
supports were adjusted to fit the shape of the template and support it at 40° TLC.
The angle of the recline bar was then adjusted until the TRA of the JOHN
template was 28°. The angles of the pelvis support, thorax support and recline
bar on the BAC prototype were measured relative to the vertical. The reasoning
behind the 40° TLC and 28° TRA is described later in Section 2.4. The BAC seat
computer model was adjusted to fit the support structure angles measured from

the BAC prototype (Figure 38).

3.1.3.2 Chgsler LH seat model development and construction

To develop the model of a Chrysler LH automobile seat, a design drawing
for the seat was obtained from the manufacturer, Johnson Controls, Inc. This
full sized drawing was used to determine the seat geometry. First, the
coordinates of the seat frame were measured from the design drawing. Fifteen
points on the seat frame were measured, using the design drawing H-pt position
as the origin of a coordinate system, and the x and y axis of the drawing as

horizontal (forward) and vertical (upward), respectively. The measured points

65

I Thorax support

- -%.. _-

 

V Pelvis support

Figure 38: BAC computer model

66

were entered into the program as vertices of a polygon body that represented
the seat frame. The seat frame body was then anchored at the seat frame
recliner location with a pin joint (Figure 39).

The second step involved importing the undeflected seat contours to act
as a visual reference. Eighteen points were measured along the undeflected
centerline contour of the seat cushion. The point coordinates were used as the
vertices on one side of a long, thin polygon. By doubling back over the same x-
coordinate points, but with a 1 mm offset in the negative y-direction, the other
side of the seat cushion polygon was created (Figure 40). The one mm
thickness was chosen so that the contour was thick enough to be seen on the
computer screen as a reference, but thin enough to not distract from the
simulations. The seat back cushion was constructed in the same manner;
sixteen points along the undeflected seat back contour line were measured from
the design drawing and the point coordinates were used to create the vertices of
a 1 mm thick polygon (Figure 41). Because they were made only for visual
reference, both the seat cushion and seat back objects were designated to not
collide with the rest of the model.

The third step involved representing the deflected contour surface of the
seat cushion. The contour shape used for the deflected seat cushion was a
reference line from the seat design drawing that represented the interface
between the deflected seat cushion and a fully loaded SAE 3-D testing manikin.

The construction of the deflected contour body in Working ModelTM followed the

67

 

 

Figure 39: LH model seat frame

\
\’\

 

Figure 40: LH model with undeflected seat cushion

68

 

Figure 41: LH model with undeflected seat back

 

Figure 42: LH model with deflected seat cushion

69

same approach as the undeflected contours. Seventeen points were taken from
the design drawing and these points were used to create a 1 mm thick polygon
to represent the deflected contour (Figure 42). During the initial simulation runs,
this contour interacted with the 2-D JOHN model. However, during the first few
simulations, the running speed of the simulation was very slow. The main
reason for the slow simulation speed was that the collision between the pelvis
body and the deflected seat cushion body was very complex. The computer
spent an excessive amount of time calculating the position of the pelvis as it
collided with the deflected seat cushion body. This situation was resolved when
the deflected seat cushion body was replaced by a several rectangular objects
that were smaller than the deflected contour (Figure 43). To develop the
multiple piece contour from the one piece contour, the one piece contour was set
in place, and seven small, thin rectangles were oriented on top of it, following
the contour as smoothly as possible. These rectangles were then anchored to
the background and were assigned to be the surfaces with which the 2-D JOHN
model collided.

When the simulations were run with the multiple piece contour, the
amount of time to run a simulation decreased noticeably. By replacing the one
piece seat cushion body with several rectangles, the computer was able to run
each simulation more quickly because it was easier for the program to predict a
collision between the pelvis body segment and a smaller body with simpler

geometry than a large body with very complex geometry.

70

 

 

 

\

Figure 43: LH model with segmented seat cushion

Top of frame

   
   
   

Mid-back wire
tie-down
region

 

 

 

umbar
support region

 

 

 

Rear pelvis wire
tie-down region

Figure 44: LH model with four seat back contact regions displayed

71

Representing the seat back contour was the fourth and final step in the
development of the automobile seat model. Based on recommendations from
the seat manufacturer, it was decided to represent the seat back contour as the
maximum deflection of the foam at four regions of known foam thickness. To
develop the contour, the four regions of known foam thickness, the top of the
seat frame, lumbar paddle, mid back wire tie down, and rear pelvis wire tie down,
were located on the design drawing (Figure 44). The distance between the
undeflected seat back contour and the back of the foam was measured from the
seat drawing at these regions The maximum deflection into the seat was
estimated as 65% of the foam thickness at these four regions. This estimation
was based on the experience of Johnson Controls Inc., Automotive Systems
Group. The deflections at the four regions were calculated and the new
deflected points placed on the design drawing. Four rectangle bodies were
created in Working ModelTM to represent the deflected cushion surface at each
of the regions. These rectangle bodies were then placed appropriately in the
simulation (Figure 45).

After the model geometry had been completed, the material properties for
all contact surfaces were specified. There was very little data on the coefficient
of friction between a clothed human and a car seat. A series of simulations were
run in Working Modelm using the 2—D JOHN model and the automotive seat to
determine a coefficient of friction values for static and kinetic friction (lulu) that

would resemble a real world situation. For all Working ModelTM simulations, Its-'-

72

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DIED. Three simulations were run with different values of l-l- These values were
0.3, 0.5, and 0.7. When I1 was set at 0.3 and 0.5, the 2-D JOHN model would
occasionally slip when in contact with the seat surfaces. At 0.7, this did not
happen, and as a result, 0.7 was chosen at the coefficient of friction for all
contact surfaces. It was reasoned that the model should represent a person
clothed in cotton pants sitting on a cloth trimmed seat rather than a person
wearing silk sitting on a leather covered seat, since the cotton/cloth combination
is more likely to occur in a normal driving scenario.

The other material property which had an effect on the simulations was
the coefficient of restitution, or coefficient of elasticity. It was reasoned that
human sitting into an automotive seat would have a very low coefficient of
restitution because of the nature of the two materials which come in contact.
Both human flesh and foam absorb much of the energy in a collision, therefore
the contact between those two surfaces would result in an inelastic collision. In
addition, a goal of the simulations was to approach a static position as soon a
possible in order to simulate static equilibrium. For these reasons, the

coefficient of restitution was set at a value of 0.02.

3.1.4 Seat evaluation protocol

With the 2-D JOHN model and the seat models completed, a simulation
protocol and study conditions were developed. In order to compare the two

seats, a matrix of seating possibilities was defined. To study the ability of the

74

seat models to support different postures, the effects of two seat factors on the
posture of the 2-D JOHN model were simulated. The seat factors chosen were;
1. amount of lumbar curvature promoted, defined for the 2—D JOHN model as
TLC, and 2. angle of the torso support, relative to vertical, defined for the 2-D
JOHN model as TRA. The two variables of the Chrysler LH automotive seat that
produced such posture changes were seat back recline angle and amount of
lumbar prominence. The seat back recline angle was the angle between the
seat frame and vertical, and the amount of lumbar prominence was defined as
the distance the lumbar support was forward of its fully retracted design position.
The corresponding variables for the BAC were the recline bar angle and the
Total Lumbar Curvature (TLC). The recline bar angle was the angle of the
recline bar with respect to vertical, and the TLC was defined as the relative
position of the thorax support to the pelvis support, as measured with the JOHN
articulating template. In order to study whether the Chrysler LH and BAC could
support a wide range of postures, LH seat frame angles and BAC recline bar
angles were chosen to support upright, reclined, and intermediate angles of
torso recline. In addition, values of LH lumbar support prominence and angular
orientation of the thorax support relative to the pelvis support of the BAC were
selected that would support slumped, erect and intermediate postures. The
variables affecting torso recline and lumbar curvature for each seat were placed
on different axes of a matrix and when the possibilities were combined, the 3x3

grid resulted in nine potential seating conditions.

75

The baseline measure for developing the matrix of seating possibilities
was obtained from the design drawing of the Chrysler LH seat. In design
position, the seat frame angle was at 28°, which corresponded to a torso angle
of 24°, and there was zero lumbar prominence. The torso angle is determined
with the SAE 3-D testing manikin The seat frame angle of 28° was included in
the matrix as an intermediate value, and an upright seat frame angle of 23° and
reclined seat frame angle of 33° were also added. The level of lumbar
prominence shown in the design drawing of the Chrysler LH seat was defined as
0 mm, corresponding to a slumped posture, and was included in the matrix. The
two other values used for the matrix were a maximum value of 25 mm, for an
erect posture, and an mid-range value of 12.5 mm for an intermediate posture.
The amount of lumbar prominence is defined as the distance the lumbar paddle
was forward of the zero position, along a line perpendicular to the seat frame.
The pre-simulation matrix for the automotive seat is shown in

Table 11.

Table 11 - Simulation matrix for automotive seat study

 

Lumbar
Prominence 0 mm 12.5 mm 25 mm

Seat frame
recline angle

 

23°

 

28°

 

 

33°

 

 

 

 

76

 

In order to compare the seating conditions of the Chrysler LH to the BAC
seat model, the measures of seat recline angle and amount of lumbar curvature
supported were re-defined for the BAC. The seat recline angle for the BAC was
the recline bar angle and the amount of lumbar curvature supported by the BAC
was defined as the TLC promoted by the chair. Values for these variables were
assigned which corresponded to upright, intermediate and reclined torso angles,
and erect, intermediate and slumped torso postures. As mentioned earlier, the
positions of these supports corresponding to values of TLC and TRA were
selected using the BAC prototype and the 2-D articulating template. To model
erect and reclined torso angles, the recline bar was set to produce 23° and 33°
TRA’s, as well as an intermediate TRA of 28°. To model from a slumped to an
erect posture, TLC’s of 0°, 20° and 40° were added to the matrix as seat

conditions. Table 12 shows the empty matrix for the BAC.

Table 12 - Simulation matrix for BAC study

 

TLC 0° 20° 40°
TRA

 

23°

 

28°

 

 

 

 

 

33°

 

Once the matrix of study conditions had been completed, a simulation
protocol was developed. In order to compare the LH simulation results to the
BAC results, the study conditions for the BAC and LH seat simulations had to be

as similar as possible. To help accomplish this, the 2-D JOHN model was

77

 

started in a 40° TLC posture for all simulations in the LH seat and BAC. This
was done to allow the seat geometry the opportunity to support a very erect
posture. Because of the forces acting on the 2-D JOHN model and because of
the nature of the mechanism controlling its movement, the model moved from an
erect to a slumped posture after it came into contact with the seat surfaces. If
the model was placed, for example, in a 20° TLC posture and placed into a seat
where the geometry of the seat would support a 30° TLC posture, the 2-D JOHN
model would not be able to conform and obtain the 30° TLC posture due to
gravity and the gear linkage controlling torso articulation. Therefore, the 2-D
JOHN model was started at an erect, 40° TLC posture in order to allow the
largest range of postural support by the seat model.

In addition, for all LH seat and BAC simulations, the starting TRA of the 2-
D JOHN model was modified to be the same as the torso angle promoted by the
seat. This was done for the same reason stated above. By starting the model
with the same recline angle as the seat frame and in an erect posture, the seat
model had the opportunity to support a very erect posture, not limited by the
starting condition of the 2-D JOHN model.

For the LH seat simulations, initial position of the 2—D JOHN model was
such that its HJC was 33 mm forward of and 6 mm above of the design H-pt
position. The 2-D JOHN model was placed there to simulate the actions of a
person sitting in the seat. The LH seat was designed with the SAE J826 tools,

and as discussed before, the SAE 3-D testing manikin represents a flat back, or

78

rather slumped posture. It is with this posture that the design H-pt location is
determined. In order to start the 2-D JOHN model in a 40° TLC, or erect,
posture it was reasoned that the initial position HJC of the 2-D JOHN model
would have to be farther ahead of the design H-pt. This was because when the
2-D JOHN model came into contact with the seat surface, the pelvis rolled
rearward on the ischial tuberosities and the model moved from an erect to a
slumped posture. As the model slumped, the HJC position moved rearward. If
the model had been started in an erect posture with the HJC at the design H-pt,

the response of the model would not have been realistic of a seated person.

 

This starting position of JOHN was determined by first placing the model i
at a 0° TLC with the HJC centered over the design H-pt location described on
the design drawing. The bottom of the pelvis was pinned to background at the
location of the ischial tuberosities. The pelvis was then rotated fonlvard 20° until
its position corresponded to a 40° TLC alignment. The location of the HJC was
recorded and the model started that horizontal (x) and vertical (y) distance away,
33 mm (x), 6 mm (y), from the design H-pt for every automotive model simulation
(Figure 46).

For the BAC simulations, the HJC of the 2-D JOHN model was positioned
with the ischial tuberosities of the pelvis 5 mm above the pivot on the pelvis
support (Figure 47). The design of the BAC is such that the pivot for the pelvis
support is approximately in the location of the ischial tuberosities of a seated

person.

79

 

 

 

TRA
TLC

A Recline Ber Angle

 

 

 

Figure 47: 2—D JOHN set in place over BAC

81

Before the HJC of the 2-D JOHN model was positioned, the TRA of the
model was adjusted to match the torso angle promoted by the seat for that
simulation. Then the simulation was started. Due to the effects of gravity and
the mass of the 2-D JOHN model, the 2-D JOHN model dropped into the seat,
and its final posture was determined by the location of the seat supports The
simulation was stopped when the 2-D JOHN model did not have a change in
TLC or TRA for 20 consecutive time-steps. When the simulation was stable, the
program was stopped and the posture recorded. For the Chrysler LH seat
simulations, the lumbar support and seat frame recline angle were adjusted to
another case in to matrix study conditions and the simulations run until all nine
cases described in the matrix had been modeled. In the BAC cases, the seat
support structures were modified and the simulation run again until all nine BAC
cases had be completed.

After the 18 simulations described above had been completed and the
data analyzed, another five simulations were run in the automotive seat model.
These simulations were run to study the effect of the upper thorax support on the
final posture of the model. The seat configuration used for these simulations
was the one that produced the greatest amount of lumbar curvature in the
automotive seat The postural effect of the proximity of the upper thorax support
on the 2-D JOHN model was studied by moving the upper thorax support
rearward, perpendicular to the seat frame, in 10 mm intervals, until the 2-D

JOHN model was no longer in contact with the upper thorax support. The thorax

82

support was moved, the simulation was run, and the TLC and TRA of the 2-D

JOHN model were recorded when the model had stabilized. Then the upper

thorax support was moved and the cycle repeated.

1.; RESULTS AND DISCUSSION

3.2.1 Chgsler LH results

The results for the Chrysler LH automotive seat simulations can be seen

in Table 13. Figures 48 - 56 show the final posture of the 2-D JOHN model for

each simulation.

Table 13 - Simulation results for automotive seat study

 

 

 

 

 

Lumbar
Prominence 0 mm 12.5 mm 25 mm
Seat frame
recline angle
23., TLC = -10° TLC = -10° TLC = -10°
TRA = 25° TRA = 24° TRA = 21°
28° TLC = -2° TLC = -8° TLC = 3°
TRA = 30° TRA = 29° TRA = 29°
33° TLC = -1° TLC = 5° TLC = 14°
TRA = 35° TRA = 38° TRA = 38°

 

 

 

 

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LH simulation results

 

 

 

 

 

 

 

Mid M
38 a
or
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g 0
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0 +28°frame angle
23 " IMid +33° frame an Ie
Max
18 [P f T r 1 r Jr r r I T .
-15 -10 -5 0 5 10 15 20 25 30 35 40 45 ‘_'

TLC '-'.

 

 

 

Figure 57: Graph plotting TRA vs. TLC at constant frame angle for LH
simulations 5;.

 

The results from the automotive seat study, shown in Table 13, are
plotted in Figure 57. The graph plots TRA vs. TLC for the different levels of
lumbar prominence at constant frame angles. For viewing purposes, 0 mm of
lumbar prominence is represented on the graph as 0, 12.5 mm is shown as Mid,
and 25 mm of prominence is represented by Max. The results show that for the
seat frame angles of 28° and 33°, the TLC of the 2-D JOHN model increased as
the amount of lumbar prominence increased. At each of those seat frame
angles, the 2-D JOHN model TLC was the greatest when the seat had the
maximum amount of lumbar prominence. In addition, as the seat frame angle
increased from 28° to 33°, the TLC also increased, represented by the rightward

shift in the 33° frame angle line versus the 28° frame angle line in Figure 57.

93

The increase in TLC occurs because the center of mass of the thorax is allowed
to move rearward due to the interaction with the increased lumbar prominence
and more reclined seat. The TLC was greatest in the simulations with the
maximum amount of lumbar prominence because the rearward shift of the thorax
COM, due to the increased recline angle, was enhanced because of the early
contact between lumbar segment and the lumbar support. This contact created
an arching effect over the lumbar support and allowed the thorax to continue to
rotate rearward, enabling the 2-D JOHN model to be supported with a higher

TLC.

 

However, in the three simulations with 23° seat frame recline angle l:
(Figures 48-50), the geometry of the model and seat were such that the
equivalent COM of the thorax was forward of the intersection point of the two
gears controlling the lumbar motion. As the 2-D JOHN model comes into contact
with the seat cushion surface, the pelvis begins to rotate rearward about the
ischial tuberosities, and the thorax rotates forward due to the coupled motion of
the two segments. Because there were no supports immediately behind the
pelvis or bottom of the thorax to stop the rotation of those segments relative to
each other, it was impossible for the model to maintain an erect posture. At a
seat frame recline angle of 23° and with only a lumbar support and not pelvic
and thoracic supports, the geometry on the seat and model are such that the
thorax COM cannot remain rearward far enough to maintain an erect posture at

all. The 23° seat frame angle plot in Figure 57 shows that the seat set up was

94

unable to support any change in the TLC of the 2-D JOHN model, represented
by the vertical line, and that the TLC supported was the most slumped posture
possible, -10° TLC.

On further observation, the amount of TLC the 2-D JOHN model exhibits
in the Chrysler LH model seems to be dependent on the lumbar prominence and
seat frame recline angle. As can be seen in Figure 57, changing the lumbar
prominence does not produce a very large change in TLC at a specific recline
angle. A long line would indicate a wide range of TLC values supported at a

constant seat frame angle, but the constant seat frame angle lines are not very

 

long. Overall, however, by adjusting the seat frame recline angle, in addition to
varying the lumbar prominence, the seat does support a range of TLC values,
though none of these values supported by the Chrysler LH seat model are above
15° TLC, leaving quite a range of more erect TLC values unsupported.
3.2.2 BAC results

The simulation matrix results for the BAC simulations are listed in Table

14. Figures 61 -69 describe the final posture of the 2-D JOHN model for each

 

 

 

 

 

simulation.
Table 14 - Simulation results for BAC study
TLC o O O
TRA 0 20 40
23° TLC = -1° TLC = 20° TLC = 39°
TRA = 22° TRA = 23° TRA = 23°
28° TLC = -2° TLC = 21° TLC = 40°
TRA = 27° TRA = 28° TRA = 28°
33° TLC = -2° TLC = 21 ° TLC = 39°
TRA = 32° TRA = 33° TRA = 33°

 

 

 

 

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BAC simulation results ,
+34° Recline bar angle
+29° Recline bar angle
38 __ —e—3Q° Recline bar angle
0 20 40
g 33 -- g-t i
20 4o
'- 28 -~ 0 4a: x
x.___
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F 20 40
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-15 -10 -5 0 5 10 15 20 25 30 35 40 45
TLC

 

 

 

 

Figure 67: Graph plotting TRA vs. TLC at constant recline bar angle for BAC
simulations

The TLC values of the 2-D JOHN model in the BAC match those
corresponding to the placement of the seating supports within 2° or less for
every simulation. As can be seen by the length of the graph lines in Figure 67,
which plots the TRA values versus the TLC values for each level of constant
recline bar angle, the BAC can support a wide range of lumbar curvature (TLC),
from 0° to 40°. In addition, the TLC change is not a function of the recline angle
(TRA) of the BAC, meaning that the model can move the full range of TLC for
any constant TRA value, unlike in the Chrysler LH seat model. This is shown in
Figure 67 by the fact that the plotted line have very little slope, if any. To

contrast the BAC results to the Chrysler LH results, recall that the TRA vs. TLC

105

plot for a constant seat frame angle of 23° in the Chrysler LH seat model was
vertical, indicating no change of TLC possible at that seat frame angle, whereas
for all three values of recline bar in the BAC simulations the TRA vs. TLC plotted
lines are flat, showing that the BAC was able to support the 2-D JOHN model in
a wide range of postures at any Torso Recline Angle.

The most striking result from the BAC simulations is the control that the
pelvis support has over the TLC of the 2-D JOHN model. In all nine simulations
the pelvis comes to an immediate stop when it contacts the pelvis support. The

TRA and thorax position continue to adjust slightly, but TLC change stops soon

 

after the pelvis stops. The different amounts of TLC possible in the BAC are I:
directly dependent on the starting distance between the back of the 2-D JOHN

model pelvis, in an erect 40° TLC posture, and the pelvis support. When this

distance is very small, as in the case of all three simulations to support 40° TLC,

the pelvic support constrains any rotation of the pelvis, thereby keeping TLC at

40°. When the distance is the largest, as with the cases for 0° TLC, the pelvis

will rotate rearward, resulting in a decrease in TLC, until it comes into contact

with the pelvis support. At that time the rotation of the pelvis support stops, and

the TLC does not change any further.

106

2.3 Upper thorax support location results

The results of the simulations run to study the effect of the upper thorax
support in the automotive seat on the final posture of the 2-D JOHN model are

shown in Table 15 and can be seen in Figures 6669.

Table 15 - Simulation results for upper thorax support location study

 

 

 

Distance of 0 mm 10 mm 20 mm 30 mm
shoulder
support
TLC 14° 18° 22° 25°

 

 

 

 

 

 

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Upper thorax location simulation results

 

 

 

 

 

 

 

 

30 mm
Mid Max
0
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g '40—.
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0 +23° frame angle
23 __ IMid +28° frame angle
Max +33 frame angle
18 t : i i 4. i 'r : i e .
-15 -10 -5 0 5 10 15 20 25 30 35 40 45
TLC

Figure 72: Upper thorax location results with LH seat results

The results from the upper thorax simulations are plotted as an addition to
the LH results and are show in Figure 72. As mentioned previously, the
simulations for this study were run with the Chrysler LH seat geometry
generating the largest TLC value, a 33° seat frame recline angle and with
maximum lumbar prominence. Moving the thorax support rearward 30 mm, just
over 1 inch, allowed the final posture of the model to be 11° greater. This can
be seen in Figure 72 by observing the extra length added to the constant 33°
frame angle line. Each 10 mm increment resulted in about a 4° change in
lumbar curvature. As can also be seen in Figure 72, the TRA of the 2-D JOHN
model changed only 2° as the upper thorax support was moved rearward. In this

case, allowing the upper thorax of the 2-D JOHN model to rotate rearward

112

provided a large increase in TLC with minimal change in TRA, which is like the
results from the BAC simulations. This shows that if the final posture of the
model is supported, or restricted, by the upper thorax support, then allowing the
upper thorax support to move rearward can have a dramatic effect on the final

posture of the model.

a; OBSERVATIONS

From the 2-D JOHN model simulation results, it can be seen that the rear
pelvis support has the greatest influence on the final posture of the model. This
can be seen most clearly in the BAC simulations, where as the distance between
the rear pelvis support and the starting position of the pelvis decreases, there is
a dramatic increase in the TLC supported. When the back of the pelvis comes
into contact with the rear pelvis support in either the automotive or BAC
simulations, there is an immediate constraint of the thorax to pelvis rotation, or
change in TLC. The closer this support is to the back of the pelvis when the 2-D
JOHN model is in a 40° TLC posture, the greater the final TLC will be.

Of second most importance to the final position of the 2-D JOHN model is
the proximity of the upper thorax support. If the upper thorax support is
positioned too far forward, the 2-D JOHN model will come in contact with the
upper thorax support before the rear pelvis support and an erect posture is not
possible. Due to the nature of the mechanism controlling the coupled movement

of the thorax, lumbar and pelvic segments, the contact with the upper thorax

113

 

support stops the rearward translation of the torso of the model, the TRA, and
forces any motion to occur to be rotation of the thorax. Because the thorax is
coupled to the pelvis this causes a rapid decrease in TLC, resulting in a more
slumped posture. One of the reasons the 2-D JOHN model could not obtain a
very erect posture in the LH seat model was due to the interference with the
close upper thorax support.

From the simulation results, it was observed that the human torso can be
thought of as two structures and a linkage, the structures being the thorax and
pelvis, and the linkage the lumbar segment. Controlling the posture of the torso
by influencing the position of the structures, like the BAC, is much more effective
than trying to control the posture of the torso by pushing on the lumbar linkage,
as is the case in many current automotive seats. It seems clear from the
simulation results that in order to control the posture of seated occupants it is
necessary to replace the current method of a single lumbar support with a more
influential support structure. This new structure would involve a rear pelvis
support, lower thorax support and ability to let the upper part of the shoulders
m'ove rearward. If a seat model can support a simplified mechanistic model of
the human body that has no muscle forces acting on it in an erect posture, then

it can surely support a relaxed human body in a real seat.

114

 

[gar

 

CONCLUSION

The objectives of this thesis were to develop two kinetic computer models
that simulate the human body for automotive seating design. The first computer
model represented the current SAE 3-D testing manikin, defined in the SAE J826 Eb
Standard. The second model was a representation of the human body model

developed at Michigan State University called JOHN. The JOHN model is an

 

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improvement over the SAE J826 models because JOHN has the ability to
represent a change of timber curvature. The 2-D SAE computer model was
used to simulate experimental data collected with the SAE 3-D testing manikin.
The 2-D JOHN model was used in a seat study comparing a current production
automotive seat designed with the SAE tools and an articulating prototype seat.

The results from the 2-D SAE computer model show that it is possible to
reproduce data collected with the SAE 3-D testing manikin with good accuracy.
This method of predicting the penetration of the SAE 3-D testing manikin into an
automotive seat cushion may prove valuable to automotive seat designers.

The results from the 2-D JOHN model show that a prototype articulating
seat called the Biomechanically Articulating Chair (BAC) can support the 2-D
JOHN model in a greater variety of postures than a current automotive seat.

These results show that the current method of supporting the human torso with a

115

 

lumbar support may not be a effective as supporting the torso with a combination

of pelvic and thoracic support.

Kinetic modeling of both the 2-D JOHN model and the SAE J826 tools will
be very useful for designers and manufacturers of seating products. There are
no current computer models that show lumbar articulation and none that allow
kinetic evaluation. Allowing the seat designers to see the results of their work
quickly will be of great benefit to them because it will help to lower prototyping
costs and shorten design time. The 2-D JOHN model can be used to better
understand the interaction between the body and the seat by observing the
influence of different supporting conditions on the posture of the model.

From the results and conclusions of this research the following future
work is recommended;

1. A study to compare the 2—D JOHN model results obtained from the
simulations in the automotive seat model to postural data collected from
human subjects seated in the same automotive seat. This study would not
only compare the model results to individual data but to the average posture

' of the seated subjects. It would be useful for determining the reliability of the
2-D JOHN model to predict general human seated posture.

2. Research that would combine the spring—damper support modeling of the
SAE 2-D drafting template with the more advanced 2-D JOHN model postural
measures. This research would also investigate modeling of the seat back

cushion as a spring-damper support system.

116

 

3. Develop and construct models representing the 5% female and 95% male
and perform simulations to study the effect of body size on the deflection into

the seat and final posture of the model.

117

 

BIBLIOGRAPHY

1. SAE Standard J826, “Devices for the Use in Defining and Measuring Vehicle
Seating Accommodation”, Body Engin. Comm, Soc. Auto Eng, Approv. Nov.
1962.

2. Roe, RW. “Occupant Packaging”, Automotive Ergonomics, eds., B. Peacock,
W. Karwowski, Taylor and Francis Ltd., 1993.

3. Hubbard, R.P., W.A. Haas, R.L. Boughner, R.A. Canole, N.J. Bush, “New
Biomechanical Models for Automobile Seat Design”, paper no. 930110, lntemat.
Cong, Soc. Auto. Eng, 1993.

4. Haas, W.A., “Geometric Model and Spinal Motions of the Average Male in
Seated Postures”, Thesis of the Master of Science Deg, Dept. Of Metal,
Mechanics, and Mat. Sci, Michigan State Univ., 1989.

5. Schneider, L.W., D.H. Robbins, M.A. Pflug, and RC. Snyder, “Development
of Anthropometrically Based Design Specifications for an Advanced Adult
Anthropometrically Family” Vol. 1, Fin. Rep. No. DOT-HS-806—715, US. Dept. of
Trans, NHTSA, 1985.

6. Robbins, D.H., “Anthropometric Specifications for Mid-sized Male Dummy”,
Vol. 2, Fin. Rep. No. DOT-HS-806-716, US. Dept. of Trans. NHTSA, 1985.

7. Boughner, R.L., “A Model of Average Adult Male Human Skeletal and Leg
Muscle Geometry and Hamstring Length for Automotive Seat Designers”, Thesis
for Master of Science Deg, Dept. of Metal, Mechanics and Mat. Scl, Michigan
State Univ., 1991.

8. Bush, N.J., “Two-dimensional Drafting Template and Three dimensional
Computer Model Representing the Average Adult Male in Automotive Seated
Pastures", Thesis for Master of Science Deg, Dept of Metal, Mechanics and
Met. Sci, Michigan State Univ, 1992.

9. Hubbard, R.P., C. Beckett, C. Gedraitis, “Method for Determining the

Stiffness of an Automotive Seat Cushion”, lntemal Document for SAE sponsored
ASPECT program, 1996.

118

10. Beer, F P, E.R. Johnston Jr., “Vector Mechanics for Engineers: Statics and
Dynamics”, 5th ed., McGraw-Hill Pub. 00., pp. 176-77, 1988.

11. Hubbard R.P., “Seat with Biomechanical Articulation” US. Patent Appl. #
08/259,662, filed June 14, 1994.

 

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