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Wold solar challenge: Long distance driver fatigue in lightweight
vehicles – Objective evaluation and monitoring of seat
discomfort
Executive Summary
The purpose of this study is to investigate an objective measurement of seat discomfort for
long distance drivers in lightweight vehicles, by comparing subjective rating with the
co
elation between full body vi
ation and pressure distribution on a seated body. Pressure
distribution will be mapped out using tactile sensor system, which will allow the collection of
data for the duration of testing. This will be evaluated against perceived discomfort, which
will be acquired through scoring by test subjects, as well as the vi
ation transmitted onto the
driver, recorded via an accelerometer. Literature has indicated that similar studies have been
undertaken, but the vi
ational aspect has been left out in many cases or when included other
aspects have been investigated. This proposal includes the theoretical background and the
experimental setup to be able to reach the sub aims and goals. These aims and goals include
the evaluation on how pressure changes with excessive vi
ation present, determining a
suitable a subjective scoring method, test procedure and possible discrepancies between the
data sets. This has been amalgamated into a project plan where a tactical approach has been
made to deliver the completed project within 14 weeks. The potential findings of this research
will allow for impartial seat evaluation, which can be used to be able to find a seat with
optimal comfort to weight ratio, or for the monitoring of seated behaviour.
1. Introduction
The University of Technology Sydney is aiming to compete in the world solar challenge
competition in 2019. The competition requires a vehicle running on solar energy as its only
power source, requiring the vehicle to be extremely lightweight. This will cause excessive
vi
ations, which are transmitted straight onto the driver. High magnitudes of vi
ation have
een seen to cause fatigue at a quick rate due to the mental and physical strain, interfering
with the performance of the driver. Whilst lower magnitudes of vi
ation can be annoying
ut not as prominent, the exposure under a long period of time does eventually cause
discomfort and physical fatigue (Mansfield 2012).
The competition covers a drive of 300 kilometres, where a driver will be seated for hours
under harsh conditions. To be able to find a seat to fit the application in the solar car there is
a need to evaluate if and how discomfort of a driver is affected when exposed to vi
ation.
The proposed project aims to investigate the subjective rating of discomfort and pressure
distribution when high frequency vi
ation is present. The ideal goal is to find an objective
method to evaluate seat discomfort, independent from driver perception, physical
characteristics and behaviour. This way the method can be used to evaluate an optimal seat
for the solar car, where weight of the seat is restricted.
Coments: Was penalised - should be more concise.The outline should
e inlcude in the Introduction.
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Literature indicates that similar research has been undertaken previously, though the studies
have not shown if the co
elation of perceived discomfort and pressure changes when
excessive vi
ation is present. A lightweight vehicle such as the solar car can be expected to
e exposed to frequencies of about 10-20 Hz. These types of frequencies were not presented
in any of the investigated research. The seat is also considered to be relatively rigid due to
safety requirements of the competition combined with the weight constraint, meaning the
transmissibility from the vehicle body onto the driver will be high.
2. Literature Review
The sensation of maintaining a seated position for an extensive period of time, is often
efe
ed to as seat discomfort (Balasu
amanian & Jagannath XXXXXXXXXXSeat discomfort will
occur regardless to vi
ation being present or not, however earlier onset to discomfort has
een found when vi
ation is introduced (Mansfield, 2012; Mansfield, Sammonds & Nguyen
2015). Although an early onset to discomfort with vi
ation present has been established,
seat performance has been recommended to be evaluated after a period of at least 2 hours
for accuracy (Sammonds, Fray & Mansfield XXXXXXXXXXThis could be seen in a study by Mandsfield,
Sammonds & Nguyen XXXXXXXXXXwhere the short-term seat comfort did not show a significant
difference when evaluating seat foam composition, whilst the long-term comfort test was
emarkably improved.
Due to the non-linear behaviour of the human body and the seat when exposed to vi
ation
it has been found challenging to evaluate the combined effect of the two (El Falou et al. 2003;
Kim, Fard & Kato 2016).
The seat effective amplitude transmissibility (SEAT) was found to be significantly greater than
measured subjective discomfort for vi
ations at low frequencies (Basri & Griffin XXXXXXXXXXThis
would suggest that there is a difference between perceived discomfort compared to objective
calculations. However, the study only measured a single frequency axis and vi
ation in
vehicles is co
ectly measured via the x-axis, y-axis, z-axis and the rotational motion yaw, pitch
and roll (Mansfield 2012).
In a study looking at pressure distribution for discomfort and postural changes, it was
suggested that there is a co
elation between subjective discomfort and pressure distribution.
This was observed by comparing subjective ratings of discomfort did pressure change, which
was increased over time (Na, Lim, Choi & Chung XXXXXXXXXXHowever, the study did not expose
subjects to vi
ation and the pressure changes only accounted for pressure variation rather
than pressure mean and peak values. The pressure variations were concluded to be due to
postural changes (Na, Lim, Choi & Chung XXXXXXXXXXSimilar findings were made by Kyung &
Nussbaum XXXXXXXXXXwhere different pressure variables where investigated and compared to
subjective ratings. The testing period was only 20 minutes and there was no vi
ation exerted
on the test subjects.
Le et al XXXXXXXXXXalso tried to quantify objective data using pressure distribution and perceived
discomfort. It was found that shorter and taller test subjects reported the discomfort
differently. This was concluded to be because of the ways they are seated differently to be
able to fit. This could co
espondingly be seen on the pressure mapping as they distributed
the pressure differently (Le et al XXXXXXXXXXThe study allowed for a testing time of 2 hours but
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did not expose the subjects to vi
ations. Due to unclear presentation of data, there was no
evident co
elation between the pressure distribution and perceived discomfort for the
individuals.
When vi
ation was introduced during a research using pressure mapping, it was found that
the overall pressure increased. The pressure peaks could be located where vi
ation caused
esonance in the seat. The subjects exposed to vi
ation on a ridged seat experienced higher
pressure than when on a cushioned seat (Wu, Rakheja and Boileau, XXXXXXXXXXThe research did
not mention exposure duration, or how the pressure changed over time, nor how discomfort
was perceived by subjects.
3. Research Objective
The objective of this study is to investigate an impartial measurement of seat discomfort for
long distance drivers in lightweight vehicles, by comparing subjective rating with the pressure
distribution on a seated body when excessive vi
ation is present.
The study aims to evaluate how and at what rate the pressure distribution changes when
vi
ation is present. The ideal goal is to find an objective method to evaluate seat discomfort,
independent from driver perception, physical characteristics and behaviour. An example of
an application the method could be used for is to find optimal comfort for a seat with
estricted properties such as weight limitations. To complete this research the project has
een divided into sub aims as following:
Determine suitable subjective scoring in consideration to the pressure mapping.
Determine suitable test procedure, including: test time, how and how often subjective
scoring will be recorded and if the experimental setup is applicable for the data
acquisition.
Evaluate possible discrepancies between data, postural, external factors, human
e
ors.
The proposed project intends to answer the following research questions:
1. Does the pressure distribution areas co
espond with the areas of perceived
discomfort when vi
ation is present?
2. Does the vi
ation affect the pressure distribution?
4. Theoretical Content
Full Body Vi
ation
Per the standard ISO 2631-1:1997 “Mechanical vi
ation and shock- Evaluation of human
exposure to whole-body vi
ation”, the vi
ation coordinate system is defined as 6DOF
system for vehicles. The horizontal and vertical motions refe
ed to via the x-axis, y-axis, z-
axis and the rotational motion yaw, pitch and roll (Mansfield XXXXXXXXXXSince seats often are
slightly inclined and the coordinate system needs to be shifted co
espondingly.
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The vi
ation magnitude can be calculated from the acceleration at the point an
accelerometer is placed. The signal can digitally be processed to model the response to
different frequencies. Mathematically the vi
ation, measured in acceleration root-mean-
square (m/s2 r.m.s.), can be expressed from the frequency-weighted acceleration ? ?,
measurement duration (T) and the frequency-weighted acceleration at time t ( ? ?(?)).
? ? = √
1
?
∫ ??
2 (?)??
?
0
Equation 1. Frequency-weighted m/s2 r.m.s. (Mansfield 2012)
To measure the vi
ation exerted on the driver by the seat, the seat transmissibility needs to
e obtained by comparing the vehicle vi
ation and the vi
ation of the seat surface. The
dynamic response of transmissibility (?(?)) can be expressed via the ratio between the seat
and vehicle at any frequency.
?(?) =
?????(?)
???ℎ????(?)
Equation 2. Transmissibility (Mansfield 2012)
Since the transmissibility is a function of frequency, performance of a seat is not constant as
oad conditions vary. An alternative method to evaluate the performance is by using the seat
effective amplitude transmissibility (SEAT), which is expressed by:
????% =
????????? ?? ????
????????? ?? ?????
∗ 100
Equation 3. Seat Effective Amplitude Transmissibility (Mansfield 2012)
For the vi
ations at the different points the frequency-weighted acceleration can be used or
the vi
ation dose value (VDV). The VDV takes varying vi
ation conditions into consideration
y combining the exposures into a single dose value. VDV can be calculated from the testing
duration (T) and the frequency-weighted acceleration at time t ( ? ?(?)):
??? = √∫ ??
4 (?)??
?
0
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Equation 4. Vi
ation Dose Value (Mansfield 2012)
SEAT values under 100% indicate that the seat is reducing the vi
ation, whilst