Data entry task in mobile computing: effects of vibration, display colour, and user age
23
2.5. Measurement of vibration
For vibration measurement the evaluation procedure ISO 2631-1: 1997 was adopted. A tri-axial accelerometer Deltatron Type 4505, Bruel Kjaer was
mounted on the test vehicle seat to register the vibration level. This accelerometer has a detection range of 5 ms
2
to 7500 ms
2
and a frequency range of 1 Hz to 1000 Hz. This instrument can make simultaneous measurements in X, Y and Z
directions. The vibration level meter was calibrated in the X, Y and Z directions before measurement. To check the suitability of the basic evaluation method, the
crest factor was calculated for X, Y and Z directions. According to ISO 2631-1 1997, the crest factor is defined as the modulus of the ratio of the maximum instantaneous
peak value of the frequency-weighted acceleration signal to its root-mean-square rms value. The crest factor values for the X, Y and Z directions obtained were
within the limit prescribed by ISO 2631-1 1997. For vibration with crest factors
≤ 9, the basic evaluation method is normally sufficient. The accelerometer was connected
to a Whole Body Vibration front end Bruel Kjaer Type 2693 and this was connected to modular sound level meter Bruel Kjaer Type 2260 which was used
for both data collection and display; data were later downloaded to a PC for further analysis. Total equivalent vibration was calculated using ISO 2631-1 1997
recommendations. The vibration levels were measured with respect to the standard biodynamic coordinate system Figure 2. Equivalent vibration level means the
average power of the vibration measured in a specific period of time and was derived from the equivalent noise level of the sound level meter.
Total equivalent vibration = [1.4a
wx 2
+ 1.4a
wy 2
+ a
wz 2
]
12
1
Where: a
wx
, a
wy
and a
wz
are the weighted rms acceleration values in the X, Y and Z directions respectively, and the factor 1.4 is the ratio of the longitudinal to the transverse acceleration
limits for the frequency range in which humans are sensitive.
The total vibration varied from 0.30 ms
2
to 1.75 ms
2
Table 1. The vibration levels for the present study were set at 0, 0.85 or 1.65 ms
2
. Zero vibration occurred only when the vehicle was not moving. Measurements at this level were necessary
to determine the base line NCEPMWS value of the subjects.
Figure 2.
Whole body vibration biodynamic coordinate system for a seated subject
ZULQUERNAIN MALLICK AJE Vol.9 2009 19-32
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3. Results
Three experiments were conducted to study the effect of experience level on data entry task performance.
3.1. Experiment 1
In this experiment, the subjects of 48-62 years of age were tested for the data entry task performance under varying levels of vibration and textbackground color
combination Figure 3. Analysis of variance ANOVA Table 2 implied that the effects of vibration
and color combination of the text and background, and the interaction of these two factors, were statistically significant.
The significant interaction between the vehicular vibration level and the textbackground color combination necessitated an analysis of the simple main
effects; the result Table 3 indicated that level of vibration interacted significantly with textbackground color combination only at 0.85 ms
2
and 1.65 ms
2
for 48-62 year-old subjects in the real driving environment. However , the same was not true
in case of vibration level under the varying levels of textbackground color combination, i.e., black characters on white background and white characters on
black background. The mathematical relationship between NCEMWS for varying levels of vibration under the influence of particular textbackground color
combination was explored for subjects of age group 45-55 Years and the following models were obtained:
CE
B1
= 50.86 - 5.01V 2
CE
B2
= 68.72 - 8.24V 3
Where CE
B1
and CE
B2
represent characters entered per minute without spaces for textbackground color combinations B1black characters on white background and B2 white
characters on black background, and V is the value of vibration considered in this study 0, 0 .85 ms
2
and 1.65 ms
2
.
Table 2. Analysis of variance of vibration effects when operators 48-62 Years performed the
data entry task under varying levels of vibration and textbackground color combination.
Source of Variation SS
df MS
F
Between Subjects S 9
Within Subjects 50
A Vibration 1015.19
2 507.59
8.32 A x S Error I
1097.64 18
60.98 B Text background color
686.72 1
686.72 10.94
B x S Error II 564.75
9 62.75
A x B 660.20
2 330.10
6.02
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