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OCR for page 313
D.~. Introduction / Background
One important aspect of creating an ergonomic workstation in transit buses is
choosing an appropriate seat. The most ideal seat would be one that adjusts to all the
ranges needed in the workstation design, attenuate as much vibration as possible, and is
comfortable for the operator. There are several methods to measure vibration exposure to
a seated person, and thus indicate what seat is the "best", some of which are described in
(Gilmore, 1995~.
Further, the following methods will be used to determine which seat is acceptable
for the workstation prototype and guidelines, all taken from (Griffin, 19901. There are
two approaches, time domain analysis and frequency analysis. In time analysis, one of
the most common measures used is the root-mean-square (r.m.s.) value:
R M S = [N ia2'(i)]
where N is the total number of data points and aw is the weighted acceleration value. The
r.m.s. value is an average measure of the peak values, and therefore is not subject to one
or two extreme values. However, in motions where shocks occur, the r.m.s. value may
not be an appropriate measure. To quantify this, some definitions are necessary. The
peak value is defined as the maximum deviation from the r.m.s. value in a time series.
Also, the crest factor is the ratio of the peak value over the r.m.s. acceleration. If the crest
factor is greater than six, then the r.m.s. value is not a good representation of the vibration
D-]
OCR for page 314
levels. Therefore, another measure, the root-mean-quad (r.m.q.), accounts for more of the
higher acceleration levels and is a better method for high crest factor motions.
RMQ=[NiaiV(i)]
Further, a method that measures the cumulative exposure of vibration is through the
vibration dose value (V.D.V.) which is defined as:
V D V = [N ~ a4,~i)1
where Ts is the duration of the motion being analyzed. Finally, the last time analysis
method used will be the seat effective amplitude transmissibility (SEAT, pronounced
'see-at') value, which is a ratio of the VDV of the seat over the VDV of the floor. The
SEAT value gives another indication of what vibration is passed from floor to human. A
value of 1 00% indicates similar comfort to sitting on the floor, and values less than 1 00%
indicate an improvement over the floor.
in the frequency domain approach, the methods used will be the power spectral
density (PSD) and the transfer function or transmissibility. The PSD indicates the
dominant frequencies of the r.m.s. value of the data, and the transfer function reveals how
much acceleration is being passed on and at what frequencies. The seated human has
natural frequencies around 4 Hz and ~ Hz (Griffin, 1990), and the bus has natural
frequencies at 1-2 Hz and 10-12 Hz (Boileau and Boutlin, 19901. Therefore, it is
important to investigate how the seats attenuate the vibration at the critical frequencies.
D-2
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D.2. Problem Statement
The objective testing of the seats is made up of two distinct phases. Phase ~
includes the static evaluation of the seats. This includes evaluating the seat features
(armrests, height adjust, etc.), and experimentally finding various seat parameters (i.e.
cushion stiffness, suspension damping, etc.~. Phase I] involves the dynamic testing in a
bus on the PTT test tracks.
The goal of the dynamic testing of the transit bus seats is to relatively compare the
ability of the seats to attenuate vibration. From this comparison, the seat that best isolates
the vibration to the bus operator will be used in the bus operator workstation prototype.
There are several different factors which effect how these seats operate during a typical
transit route, and the test should be devised to accurately account for all of these
variables. In this experiment, the same bus will be used for all of the trials since this will
provide a control for the experiment. This bus is the Chevy bus used in previous studies
by PT] (Figure D-.. (Note: Unless otherwise noted, all figures are located in Sub-
appendix Dl). Also, a human ride simulator will be used as a control for human
physiology (Wambold, 19861. This simulator represents a 50th percentile male and a
good correlation with actual human subjects, yet is not susceptible to psychological
influences such as personal mood. The two natural frequencies of the simulator are 4.25
Hz and 7.5 Hz.
D-3
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D.3. Equipment
50 Ib. weights
P.T.T. Chevy Bus
B.S.T. portable 486-66 Mhz computer with Labview software
Dytran amplifier board, capability of up to six channels
5 Dytran 3 ~ 27A accelerometers and cables
Seven seats from various manufacturers
Air tank capable of at least ~ 00 psi
Ride quality simulator
Durability and economy tracks at the P.T.~. Bus Testing Facility
D.4. Procedure
In the Phase ~ testing, various seat parameters and general features of the seats are
measured. The seat parameters include suspension mass, suspension damping ratio,
cushion stiffness, and cushion mass. (The 'suspension' is defined as the part of the seat
from the seat riser to the bottom of the cushion.) Natural frequency and stiffness of the
seat suspension can be found using the data from Phase I! (See Section D.5.) In order to
find cushion stiffness, a simple experiment is conducted. The cushion is removed from
the seat, placed on a rigid surface, and the ride quality simulator base is placed on the
seat. Fifty pound weights are placed on the base, and the displacement of the base is
D-4
OCR for page 317
measured. Using linear regression, the slope of the force / displacement line is the
stiffness of the cushion, assuming linearity. Next, to find suspension damping ratio, the
seat is set up in its normal operating conditions (air hookup, etc.~. The ride dummy is
placed on the seat, and the seat is given an initial displacement and released. One can
then look at the acceleration response, calculate the maximum overshoot, and use that to
calculate the damping ratio (a). Cushion mass is found by using a standard spring force
scale, and suspension mass is obtained from manufacturer data. Finally, all the general
construction features of each seat are observed, such as height adjust, fore-aft adjust, etc.
In the Phase T! testing, the experiment must minimize the effect of outside
influences on the evaluation of the seat. Consequently, each of the seats tested during
this experiment must be subjected to the same conditions (road surfaces, speeds, driver
position, etch. There are seven seats for the testing, numbered I-7 (Table D4.] below).
At this time, this table should not and will not be released to any seat manufacturers or
anyone who isn't affiliated with this project. However, the project investigators do intend
to publish these results at a later date. Figure D-.2 - D-~.8 include photographs and
general kinematic schematics of each seat and suspension linkage. Note that all seats are
standard air suspension systems, except Seat #2, which is an active control air suspension
(continuously varies air pressure to pneumatic spring) , and Seat #5, which is a height
adjustable rigid support system.
D-S
OCR for page 318
Table D4. I: Key for Seat Numbering
TABLE INTENTIONALLY REMOVED.
For each experimental trial, the seat is placed on an aluminum mount with a
universal bolt configuration for all seven seats at the center of gravity of the Chevy bus.
(See Figure D-.9 for schematics of the mount). It was found that the natural frequency
of a 40 ft New Flyer bus chassis was very close to the natural frequency of the Chevy
chassis (Belfiore, ~ 992~. Therefore, accelerations passed to the seat base at the center of
gravity of the Chevy bus are similar to that of a 40 foot transit bus. As illustrated in
Figure D- ~ . ~ 0, a total of five accelerometers are utilized, two for the seat and three for the
ride simulator. One accelerometer (#!~ is placed on the top surface of the mount near the
base of the seat. Another accelerometer (#2) is placed on the rigid support underneath
the seat pan cushion and as close as possible to the location where the operator would sit
as recommended in ISO 2361. The other accelerometers are placed on the two masses
suspended in the human ride simulator (#4 on lower mass, #5 on upper mass) and on the
rigid frame of the simulator near the molded base (#3~. Figure DO .l ~ shows a photo of
D-6
OCR for page 319
the ride quality simulator in a seat. Also, for those seats which have an adjustable
stiffness in the air spring, the seat will be adjusted to a midride setting. This will be
midway between the minimum and maximum heights when the air in the spring (spring
stiffness) is varied. Finally, all seats will be placed at an air pressure of ~ 00 psi, which is
in the operating range for all the seats.
The T/4 mile durability track at P.T.~. is used for the rough road surface
simulation, and the ~ mile fuel economy track is used for the smooth road simulation. A
trial on the durability track consists of one lap counterclockwise around the track (Lane ~
and Lane 6, elements ~ through 14) at the normal posted speeds (See Figure D-~.12 - D-
~ . ~ 8 for durability track elements and speeds), all starting at a common point, marked in
Figure D- ~ . ~ 2. On the economy track, a trial consists of one lap counterclockwise around
the track at thirty-five mph, all starting from a common point (Figure D-~.191. The
testing consists of three trials on the durability track and three trials on the economy
track. Each of the seven seats are tested with the ride quality simulator on each seat for all
trials. To help insure repeatability in the experiment, the same person drives the bus for
all trials. Further, the seats are tested in as small a time frame as possible in order to
eliminate the effect of temperature variability on the road surface and to a certain extent,
tire pressure.
After the data is collected, it is run through two filters. The first filter is a low
pass Chebychev Type ~ filter at 30 Hz to filter out the high frequency data. Because of
the human resonances at 4.25 Hz and 7.5 Hz, only frequencies up to 20 Hz are important
for this study. The second filter is a frequency weighting filter as presented in ISO 263 I.
D-7
OCR for page 320
From the resulting acceleration data, several methods of analysis are used,
including both time series and frequency domain approaches. The time series approach
will include measures of r.m.s., peak value, crest factor, r.m.q., vibration dose value
(VDV) , and SEAT%. For frequency domain analysis, transmissibilities for the
accelerometer locations relative to the floor and power spectral densities of each
accelerometer location are investigated. This allows a comparison of the seats to
determine what seat isolates the most vibration from the body at the various locations and
at the frequencies 4.25 Hz and 7.5 Hz, indicating the "best" seat to use in the bus operator
workstation prototype. The following table (Table D4.~) summarizes the procedure for
the experiment, with a listing of accelerometer locations on which each method will be
applied.
D-8
OCR for page 321
Table D4.2: Summary of Data Analysis
Seat Model:
Trial: (1, 2, 3)
Durability Track Economy Track
Measurements l (Accel. Channel (Accel. Channel
l l Application) I Application)
1~1~ - acre! vs Lime 1; 2; 3; 4; 5 1; 2; 3; 4; 5
RMS 1 1;2;3;4;5 1 1;2,3;4;5
i Peak Value 1 1;2;3;4;5 1 1;2;3;4;5
Cmsr ~r I: 2 ·: 4: ~1; 2; 3; 4; 5
j RMQ 1 1;2;3;4;5 1 1;2;3;4;5
i VDV 1 1;2;3;4;5 1 1;2;3;4;5
S~19o I,' 1~3
PSD* ~1 1; 2; 3; 4; 5 1 1; 2; 3; 4; 5
Transfer Function* l 1,2; 1,3; 1,4; 1,5 | 1,2; 1,3; 1,4; 1,5
* For the durability track' the frequency analysis procedures can only be applied to
one specific element of the track at a time. See Section D.5.
l
D-9
OCR for page 322
D.5. Results / Discussion
The results from Phase T are shown below in Table D5.l. Various features such
as tore-aft adjustment, seatbelts, armrests, etc. are listed, as well as seat parameters such
as cushion stiffness, suspension damping, etc. Note that these criteria are important, but
doesn't automatically rule out a certain seat, because the potential exists for retrofitting a
seat to meet the required adjustment (i.e. fore-aft adjust, etc.).
Table D5.1: Seat Comparison
T 1 T 2 ~
Seat
3 1 4
y
y
N
-
-
N
N
Auto
1~.3
~ 1 7 C
Feature
5 1 6
N
y
Lap
y
3
y
-
9.52
11.4
36.29
7
-
N
Lap
y
Armrest |
Headrest Adjustable r
Seatbelt
Seat Back Tilt
Lumbar Air Chambers
_
Seat Back Side Air Chamber
Pan Side Air Chamber _
Height Adjust (cm) _
Fore-Aft Adjust (cm) _
N
y
Lap
y
2
y
N
12.4
24.i
38.56
1.59
37.1
0.741
8.48
_
2.36 _
7
N
-
Lap
-
y
-
Lap
-
N
y
N
y
N
N
2 1 2
2*
y
N
, _
l _
| N _
12.7
24.0
40.82
N
N
6.99
15.2
38.10
9.21
30.3
38.56
8.32
13.~
Suspension Mass (kg) _
Number of Shocks _
Cushion Mass (kg) _
_
Cushion Stiffness (kN/m) _
Susp.Damping Ratio (if) _
Suspension Stiffness (kN/m)
Natural Frequency (Hz)
1
2
1
1
1
3.15
20.9
0.440
3.18
.49
8.9
2
1.80
20.1
0.434
7.37
_
2.20
8.9
1.36
~ .
~ {. 1
0.59:
0.89
2.60
_ 5.1 _
3.15
3.15
19.6
2.27
18.0
~1
8.38
2.36
11.4
_ 30.4 _
_ 0.50: _
_ 8.60
_ _
2.62
8.9
Cushion Thickness (cm)
* Non-air lumbar chambers
D-10
8.9
OCR for page 323
Table D5. I: Seat Comparison (cont.)
Seat | Cushion Bottom l
1 No rigid bottom, steel plate on seat
suspension
2 No rigid bottom, steel plate on seat
suspension
3 No rigid bottom, steel plate on seat
suspension
4 Metal plate
Steel skeleton around edge
3 belts along bottom
Steel skeleton around edge
3 belts along bottom l
Plywood
Misc.
-
Electronically controlled
Pan can extend forward
Extra 5.08 cm of fore-aft when at full
height
Pneumatic control of fore-aft lever
Height adjusts from weight
Pan rear & fore raise/lower
Pan fore raise (~2.54 cm)
Rigid Suspension
Pan fore raise (~2.54 cm)
For Phase TI of the testing, several methods were used, as stated above. The time
analysis methods include rms, rmq, vdv, and SEAT values. Figure D-.20 - D-.24 show
line graphs of the values obtained for each seat trial per location (floor, frame, etc.) for
the I" random chuck holes element (Figure D-. of the durability track. Since each
element is designed to invoke a specific response, using the entire durability track trial is
not appropriate. The chuck holes element was used because it most closely simulates
rough road conditions and results in nearly vertical motion of the seat and ride simulator.
The other elements invoke more roll and pitch motion in the system, which complicates
the results. Next, the economy track line graph results are shown in Figure D-~.25 - D-
~ .29. These two sets of data show at a certain location, such as dummy frame (essentially
the vibration felt by the seated person), what acceleration levels are present for each seat.
Dot!
OCR for page 384
10'
10°
lo-l
1ol
loo
lo 1
SuspensionIFloor
it,
1o1
10°
10-1
[: ummy Lower MassIFloor
0 ~ 10 15 0 ~10 15
Dummy FramelFloor
.
I . ~_\
'O ~
Frequency (Hey
1o1
10°
Dummy Upper MassIFloor
1~
~~\
1
10-' ~. I
10 16 0 ~ 10
Frequency (Hz)
Figure D-~.47: Seat 2 Average Transm~ssibilities, Economy
D- 72
OCR for page 385
Floor
0.04
O. 02
w
o
0.04
0.01
I
0.005
O.
Dummy Lower Mass
'O ~ 10
Suspension
. . 0.02
~ ~: ~ odor ~
15 0
~ 10 15
Dummy Upper Bless
. . 0.04
;4002k~;3 002~;~
0 ~ 10 15
Dummy Frame
,~ :
O ~
Frequency (Hz)
O ~
10 15
Frequency (Hz)
10 15
Figure D-~.48: Seat 3 Average PSD Functions, Economy
D - 73
OCR for page 386
10
10°
lo-l
10'
100
lo l
SuspensionIFloor
.
1 ~
· - ~
~,:~
. .
O ~
Dummy FramefFloor
10
100
10-'
10 15 0
Dummy Lower h1assIFloor
it,
~ 10 15
Dummy Upper MassIFloor
1 ~ 10'1 -
1~ 10°~
1.0 ~
0 ~ 10 15 0 ~10
Frequency (Hz)
Frequency (Hz)
Figure D-1.49: Seat 3 Average Transmissibilities, Economy
D - 74
OCR for page 387
Floor
0.04
at, 0. 02
~ 0.01
0.02
0.02
~ 0.01
. .
O __
0 ~ 10 15
Suspension
0.04
0.02
0.1
0.05
Dummy Lower Mass
,~,
10 15
Dummy Upper Class
few
0 ~ 10 15 0
Dummy Frame
-1 ' ' 1
~ 10 15
Frequency (H.)
~ 10 15
Frequency (Hz)
Figure D-1.50: Seat 4 Average PSD Functions Economy
D- 75
OCR for page 388
1 o 1
10°
10
sol
10°
1 o -1
SuspensionIFloor
10'
10°
-1
Dummy Lower h~lassIFloor
~w
~v~x
0 ~ 10 15 0 ~10 15
Durnnny FramefFloor
O ~
Frequelloy (Hz)
1ol
10°
10 15 0
[: ummy Upper MassIFloor
;
An:
-11
~ 10 15
Frequency (Hz)
Figure D-1.5 1: Seat 4 Average Transmissibilities, Economy
D- 76
OCR for page 389
0.02
~ 0.01
o
I 0.02
0.01
o
0.02
;4 0.01
i
Floor
'a
O ~
:: ~ ~
. _
O ~
Dummy Frame
. .,
Dummy Lower Mass
0.04
10 15 0
Suspension
002 J: A
o
~ 10 15
Dummy Upper Mass
0.1,
0.05
o
10 15 0 ~
O ~,~
0 ~ 10 15
Frequerley (Hz)
At
10
Frequency (Hz)
Figure D-1.52: Seat 5 Average PSD Functions, Economy
15
D- 77
OCR for page 390
Dum~ny Lower hdassfFloor
10'
100~
10 ' _
O
S;uspensionfFloor
Jo
10°
10-1
10 15 0 ~
lol
10°
10.1
Dummy FramelFloor
, fit -.
1
0 ~ 10 15
Frequency (Hz)
Figure D-1.53
10'
.:
I,
10 15
Dummy Upper MassfFloor
10°
1Q '
'
o
~ 10 15
Frequency (Hz)
Seat 5 Average Transm~ssibilities, Economy
D - 78
OCR for page 391
Floor
~ 0.02
I
~ 0.01
0.02
~ 0.01
o
10
Suspension
it.
O ~
Dummy Frame
10 15
0 005 ~. ~
O ~
Dummy Lower Mass
0.04
15 -O
0.05
10 15
Frequency (H~)
. .
0.02 /~`
~ 10 15
Dummy Upper Mass
~ 10 15
Frequency (Hz)
Figure D-.54: Seat 6 Average PSD Functions, Economy
D - 79
OCR for page 392
10'
104
10
10'
10°
10
SuspensionfFloor
. .
'my
14
,
O ~
Dummy FramelFlcor
· 10'
· .
0 ~ 10
Frequency (Hz)
10'
10°
10-1
Dummy Lower 191assIFloor
' '' '1
~~N
10 15 0 ~ 10 15
Dummy Upper MassIFloor
10°
10 '
16 0 ~ 10
Frequency (Hz)
Figure D-1.55: Seat 6 Average Transmissibilities, Economy
D - 80
.
OCR for page 393
Floor
0.05
I
W
o
0.04
;4 0. 02
0.02
~ 0.01
o
Dummy Lower Bless
art
0 ~ 10
Suspension
:1
l
o
Dummy Frame
~ ~ - ~
O ~
0.04
002
15 0
0.05
Lo
_:
-
10 15
Dummy Upper Mass
1 {~
o
10 15 ~0 ~
Frequency (Hz)
10 15
Frequency (Hz)
10 15
Figure D-:.56: Seat 7 Average PSD Functions, Economy
D- 81
OCR for page 394
10
104
10-1
1ol
10°
SuspensionIFloor
~ f
O ~
10 15
Dummy FramelFloor
J
r
10'
10°
10
10'
100
_ .
10-1
~ 0 ~ 10 15 ~ 0
Frequency (Hz)
Dummy Lower hlassIFloor
-
~ 10 15
Dummy Upper MassIFloor
it.
.:
. _ L
~ 10
Frequency (Hz)
Figure D-~.57: Seat 7 Average Transm~ssibilities, Economy
D - 82
15
Representative terms from entire chapter:
upper mass
