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HMI Display Readability During Sinusoidal Vibration
HMI Display Readability During Sinusoidal Vibration
Abhilash Marthi Somashankar, Paul Weindorf
Visteon Corporation, Michigan, USA
James Krier, Wayne Nowicki(Former Visteon) Michigan, USA
Abstract Modern automotive cockpit design trends have increased the
number of displays and the locations and manner in how they are
packaged. One theme in particular is the packaging of the
displays in novel locations that may be marginal in terms of
dynamic stability during road load vibrations. Examples of this
include displays that adjust their position in the vehicle. The
image of the display may be partially or fully blurred during
vibration events which can produce a poor HMI experience and
potential safety issues.
This paper will present the results of a HMI study that evaluated
the readability of different sizes and contrast ratios of TFT color
display graphics via jury evaluation during varying vibration
acceleration and frequency levels in a controlled lab
environment. The result of this study was identification of
minimum natural frequencies and maximum acceleration levels
for the display mounting structure as a function of display
graphics size and contrast ratios. This information is intended to
be used by designers of the cockpit structures that package the
displays as well as guidance for the HMI design of the display
graphics
Author Keywords Display, Vibration, Readability, Angular Subtence
1. Introduction
The size and quantity of digital displays are increasing in the
vehicle as a general market trend. This trend as a general rule has
increased the mass of the displays. This increased mass coupled
with lower bracket stiffness can cause a reduction in the
mechanical natural frequency of these displays. Reduced natural
frequency increases the relative motion the observer sees in the
form of blurring the display characters when the vehicle is being
driven over rough roads. As displays replace traditional
mechanical switches, reduced viewability during vibration can
cause safety issues since increased viewing time can distract the
driver.
In addition to the mechanical aspects of structural stability, the
design of display character size and contrast ratio is included in
the paper for graphic screen design guidelines. The study varied
the contrast ratio and size of display characters at black on white
and white on black test screens to examine the impact on
viewability.
Existing knowledge of display HMI viewability [1] are static in
nature. They do not factor the vibration environment that needs
to be considered. As stated previously, for historical display
applications in clusters and centerstacks that achieve relatively
rigid mounting with higher natural frequencies, this is not an
issue.
This paper presents the results of a jury evaluation of a digital
display that was tested for readability at varying display character
parameters and vibration levels.
This paper will recommend design guidelines for natural
frequency and sinusoidal acceleration g-level as a function of
display character size, angular subtence and contrast ratio. The
information will be presented in the following outline.
1. General description of the test setup
2. Identification of the display type, detailed description
of the viewed characters, & illumination levels.
3. Jury information
4. Vibration test parameters
5. Data
6. Conclusions
2. Method An 8” diagonal Color TFT display was used in the study. It is a
typical size used in modern passenger vehicles found in the
centerstack location. The display utilized had the following
characteristics:
1000 cd/m2 (white)
800 x 480 resolution (117ppi)
Normally Black ASV type
Contrast Ratio = 2000 (typical)
Two test screens were designed for the study, see Figure 2-1 & 2-
2. Black letters on lighter backgrounds and white letters on darker
backgrounds were selected to study the effect of inverse contrast
ratios. 11 rows of text with increasing size were chosen to permit
discrimination at varying vibration levels. Contrast ratios were
also studied by selecting 4 columns with decreasing contrast
ratios by adjusting the background luminance.
Figure 2-1. Black letters on lighter background test screen.
Figure 2-2. White letters on darker background test screen
Calibri(Body) font was chosen, it is a common office type font
with relatively even font width. Text height was measured
optically on the display screen and reported in mm. The measured
heights of W, e & h are reported in Table 2-1. These letters
represent the range in height of the text characters in millimeters.
Table 2-1. Character font heights used for test screens
Background shading was chosen to decrease the contrast ratio of
the characters, Microsoft PowerPoint was used to create
transparent shading for the columns, and Table 2-2 identifies the
values used.
Table 2-2. Microsoft PowerPoint transparency shading settings used to generate background contrast values
The luminance and contrast ratios of the test screens were
measured using a Lumicam 1300 video photometer per Table 2-
3 & 2-4. To reduce measurement sampling noise, special test
screens that permitted a substantial area of measurement for the
Lumicam were used instead of attempting to measure only the
relatively small text area. These special test screens were
comprised of creating black or white squares to represent the text
using the same background shading color per Figure 2-3. Figure
2-4 shows the display with the special test screens and the
Lumicam. For the actual measurements, a dark hood was used to
prevent any ambient light from influencing the camera
measurements.
Figure 2-3. Lumicam special test screens to measure luminance
Font Type
Font Size
(Point) "W" "e" "h"
Row 1 Calibri (Body) 18 1.7 1.2 1.8
Row 2 Calibri (Body) 20 2 1.7 2.1
Row 3 Calibri (Body) 24 2.4 1.8 2.5
Row 4 Calibri (Body) 28 3 2 3.1
Row 5 Calibri (Body) 32 3.2 2.4 3.4
Row 6 Calibri (Body) 36 3.7 2.9 3.8
Row 7 Calibri (Body) 40 4.3 3.3 4.5
Row 8 Calibri (Body) 44 4.8 3.5 5
Row 9 Calibri (Body) 48 5 3.9 5.4
Row 10 Calibri (Body) 54 5.8 4.8 6.2
Row 11 Calibri (Body) 60 6.5 5.9 7
Measured Display Letter Height
(mm)- "We hold"
Contrast Values, Microsoft Powerpoint
Column A Column B Column C Column D
White
Background
35% Transparancy
Black Background
"Fill"
15% Transparancy
Black Background
"Fill"
5% Transparancy
Black Background
"Fill"
Column A Column B Column C Column D
Black Background
35% Transparancy
Black Background
"Fill"
15% Transparancy
Black Background
"Fill"
5% Transparancy
Black Background
"Fill"
Black Letters, Lighter Background
White Letters, Darker Background
Figure 2-4. Display and Lumicam prior to luminance measurements
Table 2-3: Luminance measurements of Figure 1 black text on lighter background
Table 2-4: Luminance measurements of Figure 2 white text on darker background
Some variation in the measurements was recorded, this was
attributed to variation in the backlight behind the display glass.
A sinusoidal vibration test was designed to cover varying
acceleration levels and frequencies. Sinusoidal was chosen over
random because although random is a more realistic test condition
in the automotive environment, the details of a random vibration
test’s power spectral density were expected to be more
complicated to cover the various packaging locations in the
vehicle compared to a simple sinusoidal acceleration vs frequency
test.
Some preliminary tests were run to set acceleration (“g’s”) and
frequency (Hz) ranges that would minimize jury evaluation test
time. Table 2-5 & Table 2-6 are the values that were chosen for
the test with calculated maximum displacement and maximum
velocity listed, respectively.
Table 2-5: Acceleration & frequency versus peak to peak displacement
Table 2-6: Acceleration & frequency versus maximum velocity
The most difficult viewing conditions would be expected to occur
at higher G level and lower frequency since the displacements and
velocities are at their greatest values that would cause increased
blurring.
The display was mounted to an electromagnetic vibration table
using a fixture that approximated viewing the display normal to
the viewer per Figure 2-6. A viewing distance of 27” (686mm)
was chosen to represent a taller driver that would represent a
worst case viewing distance typical for a centerstack or
instrument cluster display location.
Figure 2-6. Test viewing condition
A jury of 12 people were used for the study. Age and genders are
listed per Table 2-7.
Table 2-7: Jury age and gender
The test procedure for the jurors was to examine the white letters
with darker background and black letters with lighter background
test screens without vibration and identify the smallest text from
row D that was readable. No time limit was used to determine
this value. The purpose of this static test was to act as a “eyetest”
to discriminate the juror’s vision for very difficult low contrast
Black Text, Lighter Background
Column A Column B Column C Column D
Black Text (Cd/m2) 2.7 2.1 1.6 1.4
Background (Cd/m2) 615.8 69.7 11.4 2.1
Contrast Ratio (Background/Text) 228.1 33.2 7.1 1.5
White Text, Darker Background
Column A Column B Column C Column D
White Text (Cd/m2) 602.9 67.4 9.9 1.8
Background (Cd/m2) 1.5 1.1 1 1
Contrast Ratio (Text/Background) 401.9 61.3 9.9 1.8
Peak to Peak Displacement (Millimeters): G's vs Hz
G Level 10 20 30 40 50
0.25 1.24 0.31 0.14 0.08 0.05
0.5 2.48 0.62 0.28 0.16 0.10
0.75 3.73 0.93 0.41 0.23 0.15
1 4.97 1.24 0.55 0.31 0.20
Frequency (Hz)
Max Velocity (Millimeters/Second): G's vs Hz
G Level 10 20 30 40 50
0.25 39.02 19.51 13.01 9.75 7.80
0.5 78.04 39.02 26.01 19.51 15.61
0.75 117.06 58.53 39.02 29.26 23.41
1 156.08 78.04 52.03 39.02 31.22
Frequency (Hz)
Juror Age Gender
1 23 Male
2 25 Male
3 25 Male
4 38 Female
5 48 Female
6 52 Female
7 55 Male
8 59 Male
9 59 Male
10 61 Male
11 61 Male
12 65 Male
ratio conditions.
The next task was to have the jurors identify the smallest text row
number that could be read during vibration at each of the different
G and Hz levels. The criteria was readability at a glance to
simulate safe driving conditions in the car. If the juror couldn’t
decide upon two different text row sizes, the larger text value was
chosen. During the test, the jurors were instructed to either close
their eyes between readings or glance away from the display.
3. Result Figure 3-1 is a graph of the static “Eyetest” used to discriminate
the relative eyesight of each of the jurors. The results represent
the smallest text height that could be read without vibration of the
lowest contrast column D of the test screens. The chart is shown
by age order. White & black text screen results are shown along
with a 2nd degree polynomial curve fit.
Figure 3-1: Static “Eyetest” of Column D
A histogram of the distribution of the jurors for both test screens
is presented in Figure 3-2 and Figure 3-3
Figure 3-2: Eyetest histogram of black text, lighter background, column D
Figure 3-3: Eyetest histogram of white text, darker background, column D
The definition of subtended angle is shown below in Figure 3-4
for reference and the equation (1) used for it [2] is shown below
the figure. Subtended angle was selected for reporting of results
instead of character height to produce results that are independent
of viewing distance and character height.
Figure 3-4: Subtended Angle definition used to report results in terms of angular subtence
Angular Subtence in degrees = 2 arctan (𝑡𝑎𝑟𝑔𝑒𝑡 ℎ𝑒𝑖𝑔ℎ𝑡
2 𝑥 𝑣𝑖𝑒𝑤𝑖𝑛𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒)
Angular Subtence in minutes of arc = 60 x 2 arctan
(𝑡𝑎𝑟𝑔𝑒𝑡 ℎ𝑒𝑖𝑔ℎ𝑡
2 𝑥 𝑣𝑖𝑒𝑤𝑖𝑛𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒)
= (3438 𝑥 𝑡𝑎𝑟𝑔𝑒𝑡 ℎ𝑒𝑖𝑔ℎ𝑡
𝑣𝑖𝑒𝑤𝑖𝑛𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒) (1)
Figure 3-5 thru 3-10 are summaries of the average Angular
subtense that was legible during the test vibration G levels and
frequencies. A separate chart is shown for each column
representing a unique contrast ratio for both the black letters on
lighter background of Figure 2-1 and the white letters on darker
background of Figure 2-2.
Figure 3-5: Column A Legible vibration response
Figure 3-6: Column B legible vibration response
Figure 3-7: Column C legible vibration response
Figure 3-8: Column A legible vibration response
Figure 3-9: Column B legible vibration response
Figure 3-10: Column C legible vibration response
Table 3-1 thru 3-6 is the same average legibility data as Figure 3-
5 thru 3-10 but in table form.
Table 3-1: Column A legible vibration angular subtense values
Table 3-2: Column B legible vibration angular subtense values
Table 3-3: Column C legible vibration angular subtense values
Table 3-4: Column A legible vibration angular subtense values
Table 3-5: Column B legible vibration angular subtense
Table 3-6: Column C legible vibration angular subtense values
4. Discussion The test provided data for relative viewability of different text
sizes and contrast ratios at varying vibration G and frequency
levels. In order to extract conclusions from the data, a number of
assumptions need to being made.
Assumptions:
-The jury’s eyesight is representative of the population as a
whole.
-Average jury text size values are being reported for a given test.
Several of the jurors appeared to have worse eyesight than the rest
of the group. This should bias the results to a somewhat
Dark Letters, Lighter Background
Hz
1/2G 3/4G 1 G
50 7.04 7.29 7.46
40 7.46 7.33 7.67
30 7.71 8.46 9.88
20 8.92 11.08 12.55
10 10.83 13.22 16.95
Column A - Averages
Dark Letters, Lighter Background
Hz 0.5G 0.75G 1.0G
50 7.50 7.50 7.58
40 7.54 7.79 8.00
30 8.21 8.67 9.96
20 9.38 11.08 12.55
10 11.34 13.51 17.88
Column B - Averages
Dark Letters, Lighter Background
Hz 0.5G 0.75G 1.0G
50 7.79 8.08 8.29
40 8.25 8.25 9.04
30 9.04 9.63 10.38
20 10.21 11.01 12.55
10 12.13 14.61 18.99
Column C - Averages
White Letters, Darker Background
Hz
1/2G 3/4G 1 G
50 7.58 7.29 7.54
40 7.79 8.00 7.92
30 8.21 8.88 9.00
20 10.08 11.50 12.76
10 12.79 14.69 18.03
Column A - Averages
White Letters, Darker Background
Hz 0.5G 0.75G 1.0G
50 7.92 7.38 7.63
40 8.17 8.08 8.17
30 8.83 9.38 9.29
20 10.08 11.09 12.54
10 13.35 15.07 18.24
Column B - Averages
White Letters, Darker Background
Hz 0.5G 0.75G 1.0G
50 8.67 7.63 8.67
40 9.46 8.17 9.46
30 10.33 9.29 10.33
20 12.88 12.54 12.88
10 19.66 18.24 19.66
Column C - Averages
conservative value.
-The sinusoidal vibration test generates a fixed observer and
moving display producing a given amount of relative motion.
Actual vehicle vibration will introduce motion of both the
observer and the display which could either increase or decrease
the amount of relative motion depending on if the driver and
display are moving in phase or out of phase with each other.
-Only linear components of vibration in one degree of freedom
are considered.
5. Summary and Conclusion The test results show that:
- As the frequency is decreased at a given G level, a
larger character angular subtense (i.e. larger Character
Height) is required to maintain visibility.
- As the G level is increased at lower frequencies (10-
30Hz), a larger character angular subtense (i.e. larger
Character Height) is required to maintain visibility.
From the results, it can be concluded that the target display
module frequency range should be greater than 25Hz at a given
G level. Below this frequency, the angular subtense increases
above a normal font size.
6. References [1] INTERNATIONAL STANDARD ISO/FDIS 15008 SAE,
Road vehicles — Ergonomic aspects of transport
information and control systems — Specifications and
compliance procedures for in-vehicle visual presentation
ISO/FDIS Standard 15008:2002(E)
[2] INTERNATIONAL STANDARD ISO 13406-2,
Ergonomic requirements for work with visual displays
based on flat panels -- Part 2: Ergonomic requirements for
flat panel displays
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About Visteon Visteon is a global company that designs, engineers and manufactures innovative cockpit electronics products and connected car solutions for most of the world’s major vehicle manufacturers. Visteon is a leading provider of instrument clusters, head-up displays, information displays, infotainment, audio systems, telematics and SmartCore™ cockpit domain controllers. Visteon also supplies embedded multimedia and smartphone connectivity software solutions to the global automotive industry. Headquartered in Van Buren Township, Michigan, Visteon has approximately 10,000 employees at more than 40 facilities in 18 countries. Visteon had sales of $3.16 billion in 2016. Learn more at www.visteon.com.
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