Journal of Biomechanical
Science andEngineering

Vol. 7, No. 4, 2012

A Novel Braille Display Using the Vibration of
SMA Wires and the Evaluation of Braille
Presentations*Feng ZHAO**, Changan JIANG*** and Hideyuki SAWADA**** Faculty of Engineering, Kagawa University
2217-20, Hayashi-cho, Takamatsu-city, Kagawa, 761-0369, Japan
E-mail: sawada@eng.kagawa-u.ac.jp*** RIKEN-TRI Collaboration Center for Human-Interactive Robot Research,
2271-130 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, 463-0003, Japan

Abstract
In this study, a new Braille display is constructed based on the vibration of a ShapeMemory Alloy (SMA) wire. To present Braille information, a vibration actuator isused instead of conventional Braille dots. According to the temperature-dependent
characteristic shrinkage and the vest ration to the initial length of an SMA wire, avibration actuator which can be driven by several [Hz]-100 [Hz] pulse signals has
been developed. A method for presenting Braille information is proposed by theapplication of the developed actuator to a Braille display. In this research, multiple
actuators constructed by using metal pins (0.7 [mm] in diameter, 3 [mm] in length)
and SMA wires (50 [um] in diameter, 3 [mm] in length), are placed as to form
standard Braille. The actuators are vibrated by PWM signals with different
frequencies and appropriate timings, and the effectiveness of the proposed method
for Braille display is verified by experiments. From the experimental results, thehighest recognition rate of 100 [%] was achieved under the conditions of 50 [Hz]vibration frequency and 500 [ms] time delay. This means the vibration patterns
effectively stimulated the Meissner corpuscles, and helped the subjects properly
discriminate Braille characters presented by the developed display. Goodevaluations were received from the subjects who are visually-impaired.
Key words: SMA, Braille, Braille Display, Vibration, Vibration Actuator

1. Introduction*Received 28 May, 2012 (No. 12-0235)[DOI: 10.1299/jbse.7.416]

Copyright © 2012 by JSME

Humans communicate with each other using the five senses such as the vision, auditionand olfaction, together with the sensations through their bodies. Information transmitted
through non-verbal media directly affects emotions and feelings, and touch sensations
especially play an important role in the perception of an object and its surrounding
environment. For visually impaired people, auditory and tactile information is the primalcommunication media, and computerized devices and equipment have been introduced tohelp with their communication in daily life. Recently, accompanying the popularization of
personal computers, the digitization of information through E-book and E-mail has
developed rapidly. In the current information age, methods to make such types of
information easily and conveniently accessible to the visually impaired have become
important. The purpose of this study is to develop an energy-efficient and easily-portable
Braille display which can be mounted on any mobile electronic media.
The visually impaired usually get the information through Braille and speech

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Vol. 7, No. 4, 2012synthesizers (1). Since Braille is currently the most widely used communicative tool byvisually impaired people, electronic Braille displays with actively sliding surface, consistingof 20 - 40 characters per line as shown in Fig.1 (2), have been widely used. Such displays
have been developed by researchers and developers to replace traditional Braille printed on
the paper, and information in Braille can be easily accessed using these devices. For anormal Braille display, the user has to move their fingers on top of each dot in a Braille cellin order to read the Braille character. This method is called active touching.
The developed Braille displays are made by small solenoid electromagnetic vibrators(3), small motors (4), piezoelectric actuators (5), pneumatic actuators (6), Phase-Change
Microactuators (7), ultrasonic actuators (8) and electrostatic actuators (9). Small motors caneasily be adapted as vibration actuators, and have been applied to mobile phones (10), PDAs,
and Braille displays for visually impaired people in previous studies. By attaching an
eccentric mass, a vibrating force is produced, however, it is not easy to precisely control thevibration frequency and amplitude. Another disadvantage is that the electromagnetic device
is large, but the power consumption is greater than the energy provided by a mobile battery.
To solve these problems, piezoelectric actuators have been developed. An array of multiple
piezoelectric actuators can be used to present the Braille patterns, and the driving principle
is based on generated mechanical strain due to piezoelectric effect in such devices. The
simplicity of the structure does not require the coil windings, and enables to present plural
mechanical vibrations in a small space. However, the driving voltage of the piezoelectric
actuators need about several tens volts, and due to the influence of mechanical components,
it is difficult to apply them to mobile devices. In addition to that, due to the high power
consumption and large size, is difficult to combine existing Braille displays with portablemobile devices. The high cost is also one of the shortcomings.
Considering the above problems, we are developing a Braille display which has low
power consumption, small size, passively touched and can easily be combined with portablemobile devices. A Shape Memory Alloy (SMA) wire is employed to develop a vibration
actuator which is driven by a low voltage supply in this research. By using the vibration
actuators, a compact Braille display with 4 Braille cells has been constructed. Since the sizeof each Braille cell is just 2.0 [mm] x 4.5 [mm], the developed compact Braille display isportable. The cost of the developed compact Braille display is quite low, because each
vibration actuator is made using only a 3 [mm]-long SMA wire. Furthermore, through thevibration the users are able to read the Braille character without even moving their fingers
to feel each dot in a Braille cell. The user just has to put their fingertip on top of the Braillecell in order to read the presented character. For the purpose of verifying the effectivenessof the developed device, experiments were conducted with the support of the students from
Kagawa Prefectural School for the visually impaired.
In this research, it is assumed that the target of the Braille recognition rate would be
90 [%]. The target of the reading speed is still assumed to be 60 characters/minute which isless than 1/3 of the usual 200 characters/minute. The recognition rate of Braille attained inthis experiment is 100 [%] when the speed is 60 characters/minute. In previous studies, thevibration pin type tactile display (Braille display) Ino (11 ~ 13) required a recognition time of 2
seconds for each Braille cell and that the recognition rate is not as good (about 60 [%]) as
we achieved. Shimizu (14) found that the Braille recognition rate changes with the number ofvibrations (#1 ~ # 6) of Pins from 95 [%] to 0 [%].

Fig.1 Braille Display
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The rest of this paper is organized as follows. In Section 2, the structure of the vibration
actuator and the Braille display system configuration are introduced. The preliminary
experiment to determine the optimal parameters of pulse signal is described in Section 3. In
Section 4, the results and analysis of experiments on the practicability of the developed
Braille display are presented. Finally, the conclusion and our plans for future research aredescribed in Section 5.

2. Vibration Actuator and Braille Display System
2.1 Tactile Receptors
According to the previous studies, we know that the human fingertip has many tactile
receptors which could sense vibration, pressure, deformation and temperature, and thetactile receptors could sense many kinds of external stimuli (15). Tactile receptors are foundat the epidermis-dermis border down to the subcutaneous tissues. Tactile receptors can be
categorized according to specific morphologies and functions. The tactile receptors in theglabrous skin include four different types; Meissner corpuscle, Pacinian corpuscle, Merkel
disc, and Ruffini endings. The Meissner corpuscles and the Merkel disc have small
well-defined receptive fields with a diameter of a few millimeters. Conversely, the Pacinian
corpuscles and Ruffini endings form large and diffuse fields. The Meissner corpuscles andthe Pacinian corpuscles respond to mechanical vibration, while the Merkel disc and the
Ruffini endings respond to pressure or deformation of the skin. The Ruffini endings sense
stretching of the skin or bending of the fingernails as these stimuli compress the nerve
endings. Mechanical information sensed by the Ruffini endings contributes to ourperception of the shape of objects held in our hands. The Meissner corpuscles detectvibration stimuli in the range up to 10 ~ 150 [Hz], and the minimum threshold frequency is
40 [Hz]. The Pacinian corpuscles which have large receptive fields detect frequency
components greater than 100 ~ 300 [Hz], and the minimum threshold frequency is about
150 ~ 200 [Hz]. The Merkel discs respond to vertical displacement and detect vibrations
with the frequency of less than 2 [Hz]. The two-points discrimination threshold of the
Meissner and Pacinian corpuscles is between 1.3 [mm] and 2.4 [mm] (16). That is, if the
distance between two vibrating points is beyond the threshold, a human can feel two points.
Otherwise, a human just feel a smooth surface instead of two points.
Because of the characteristics of the various tactile receptions, in this study we focusedon the Meissner corpuscle, the Pacinian corpuscle and the Ruffini endings that respond tothe vibration stimuli to the fingertips.
2.2 Vibration Actuator
In order to produce a vibration stimulus which can be sensed by tactile receptors, wedeveloped a Braille display using filiformed shape memory alloys (SMA) (17). The
filiformed SMA wires (Toki Corp, BioMetal BMF50) respond to the specific temperatures,
T1 and T2, which are 68 [℃] and 73 [℃] respectively. By applying a weak current to the
SMA, heat is generated by the internal resistance, and the SMA shrinks up to 5 [%]lengthwise at the temperature T2. When the current stops and the temperature drops to T1, it
returns to the original length. Fig.2 shows an outline drawing that explains the relationshipbetween the temperature of the SMA wire and its length. The SMA wire is so thin that it
rapidly cools after the current stops, and returns to its original shape when the temperature
shifts from T2 to T1. This means that the shrinkage and the return to initial length of the
SMA wire can be controlled by the pulse current as shown in Fig.3. By applying a pulse
current to the SMA wire, the body temperature instantly rises due to the generated heatinside the body, and shrinks from its initial length. When the pulse current stops, the bodyinstantly cools down, and recovers its initial length. The process of shrinkage and return to418

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LengthPulse Signal
H [V]

L

0.95L
T1
T2
Temperature T
Fig.2 Temperature Characteristics
of SMA Wiret [ms]
L [ms]
W [ms]

H [V]: Amplitude
W [ms]: Pulse Width
L [ms]: Periodt [ms]: DurationtContraction and return
to initial length of SMA
wire of one unit
Fig.3 Pulse Signal for Driving SMA

the initial length of the SMA wire synchronizes completely with the ON/OFF pulse current.
To control the magnitude of the vibration when presenting Braille information, theamplitude of pulse signals H and the duty ratio W/L should be determined based on thecalories exchanged. At the same time, the SMA temperature needs to be maintained
between T1 and T2 so as to initial length and contract most efficiently. A pulse-widthmodulated (PWM) rectangular wave signal with arbitrary frequency, amplitude andduty-ratio is generated in a PC, which is amplified to drive the actuators. The current
amplifier was specially designed for driving SMA actuators in variable frequencies andvariable voltage amplitude with current control. A small vibration can be generated bycontinuous initial length and shrinkage of the SMA wire while the continuous pulse current
flows through SMA wire. In this study, the diameter of the SMA wire is 50 [µm], and thelength is 3 [mm]. The length of the SMA shrinks about 1 ~ 2 [µm] by a current pulse withthe frequency of several tens of hertz. Since this vibration displacement is almost the sameas the size of the human tactile receptors such as the Meissner and Pacinian corpuscles, thephysical stimuli given by the vibration are perceived as various tactile sensations (18), andthe micro-vibration actuators have been applied to the research of the presentation ofvarious textures(19). However, the micro vibration for the purposes of this study is too small
to present Braille.
In order to amplify this micro vibration to make it usable as a Braille pin, we employ around-head pin, which is fixed on the SMA wire. Fig.4 illustrates the structure of thevibration actuator consisting of a 50 [µm] (diameter) x 3 [mm] (length) SMA wire and a 1.4[mm] (diameter) x 3 [mm] (length) round-head pin. The SMA wire generates continuoussynchronization with the state of the ON/OFF pulse current, causing the process of SMA
wire to shrink and return to its initial length. This vibration is transmitted to the round-headpin. With this structure, the round-head pin successfully amplifies the vibration of a SMA
wire, so that a user can recognize the Braille pins through the vibration, when he/she lightly
touches the vibration actuators with his/her fingertips as shown in Fig.5.
In this study, we confirmed that the mechanical vibrations were strong enough topresent tactile stimuli when a person touched the vibration actuator. Fig.6 shows the Brailledisplay employing the vibration actuators. Braille characters are presented by cells of six
raised dots arranged in a grid of two dots horizontally by three dots vertically. The dots arenumbered 1, 2, and 3 from the top of the left column and 4, 5, and 6 from the top of theright column. The presence or absence of dots gives the coding for the Braille. Japanese
Braille is conventionally regulated that the dot height is approximately 0.5 mm, thehorizontal and vertical spacing between dot centers within a Braille cell is approximately
2.0 mm, and the blank space between dots on adjacent cells is approximately 3.2 mmhorizontally. Vibration actuators are allocated in the same manner as a conventional Braillecharacter, two columns of three dots each. Furthermore, a rubber sheet with the thickness of
0.5 [mm] is set on the display surface to absorb vibration interference from other actuators.

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1.4 mmInsulator
Vibration
3 mmCoolingHeating

Pulse
Current-ON

SMA wire
Length: 3mm
Diameter: 0.05mm

Pulse
Current-OFF

Lead

Fig.4 Structure of Vibration Actuator
Tip of fingeri

3.2 mm

e

ka

ko

2

3

4

2.25
mm

1

2.0 mmVibration actuators
Fig.5 Figure Contact Site

Fig.6 Braille Display with 4 Cells

2.3 Configuration of Braille Display System
By using the designed vibration actuators, a Braille display system is constructed in thisstudy to transmit Braille information. The developed system is shown in Fig.7. The
generated pulse signal is transmitted to the Braille display through a DIO board. In order tomake the vibration of the actuators strong enough to be recognized, the transmitted signal
should be amplified by using a current amplifier. The amplifier is specially designed for
driving vibration actuators in variable frequencies and variable voltage amplitudes withcurrent control. The current amplifier has 24 channels, and it is possible to control eachchannel independently (one Braille cell has 6 Braille dots, and one dot corresponds to one
channel). Fig.8 shows the control interface for generating a pulse signal to a Braille display
for each channel by using a conventional computer.
In this study, the vibration frequency (see Fig.3) of each actuator is 2 [Hz] ~ 50 [Hz],the amplitude of voltage (see Fig.3) is 2 [V], and the ON-OFF duty ratio of pulse signal issuitably controlled in the range between 1:250 and 1:20. The above parameters can bevaried to modify the strength of vibrations according to each person’s ability to feel thevibrations. The maximum power consumption of each actuator is approximately 80 [mW],
when the actuator with its resistance 2.4 [Ω] is driven by a pulse signal with the amplitudeof voltage 2 [V], and the duty ratio 1:20. The battery of the mobile device is enough to drivethese actuators. When the shape memory alloy shrinks, the temperature becomes about 70[℃]. Since the length of the SMA is quite short and the diameter is also small, users do not
feel the heat conducted through the pin.

3. Selection of Parameters to Test Drive
In this study, the preliminary experiment was conducted by using pin-type vibration
actuators to determine the optimal parameters of the pulse signals. The Braille display with
4 characters used in the experiment is shown in Fig.6. The display is composed of 24
vibration actuators, which can be driven independently. The parameters to be determined in
the experiment are as follows (20).

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Subjects A and D, who were students in the Kagawa Prefectural School for the Visually
Impaired, took part in the experiments. Their profiles are shown in Table 1. Subject A is
male and is congenitally visually impaired, and Subject D is female who lost her sight at theage of 10. Ten Braille characters /a, i, u, e, o, ka, ki, ku, ke, ko/ were randomly presented tothe two subjects, and they gave the answers by telling which pins were vibrating. The
subjects read each Braille cell from left to right on the Braille display by using their index
fingertip with their preferred speed. Vibration duration (see Fig 3) of actuators was 50 ~ 900[ms].
The experiment was done in 2 steps:(1) Vibration duration experiment of the vibration actuator
We believed that the Meissner corpuscle responded well to vibration stimuli.
Therefore, in this experiment, we chose 50 [Hz] as the reference vibration
frequency for stimulation of the Meissner corpuscles and Ruffini endings.
(2) Vibration frequency experiment of the vibration actuator
We applied the experimental results obtained from the vibration duration
experiment to this vibration frequency of experiment. In this experiment,
considering the different responses to vibration frequency of the four kinds oftactile receptors, we chose 20 [Hz] and 30 [Hz] for the Meissner corpuscle, 50 [Hz]for the Meissner corpuscle and Ruffini endings (17), (20 ~ 23). These vibration
frequencies were used in the Japanese Braille recognition experiment.
Current Amplifier

Braille Display
Fig.7 Braille Presentation System
4 Braille Characters(ON/OFF of Vibration)

Parameter Setting Boxes
Fig.8 Dialog Interface for Parameter Settings421

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The results of the experiments are shown in Fig.9 and Fig.10. In Fig.9 the recognition
rate of each Japanese Braille cell with different vibration durations is shown. Highrecognition rates are obtained at 500, 700, 900 msec. By considering the recognition time,
500 msec interval is adopted in the following experiments, because the 500 [ms] vibration
duration gives the shortest reading time when compared to the other vibration durations
such as 700 [ms] and 900 [ms]. In Fig.10 the recognition rate of each Japanese Braillecharacter at different frequencies is shown. From the experimental results we can find thatthe driven frequencies of vibration actuator to 30 [Hz] and 50 [Hz] used for this study isfully possible, and it is the number of years that the subject had used Braille that cause
individual differences in the recognition results. Subject A who is congenitally visually
impaired could read Braille better. The recognition time of the Braille characters was around8 ~ 9 seconds for the 20 [Hz] and 30 [Hz], and less than 6 ~ 7 seconds for the 50 [Hz].
In the experimental results, when the number of vibrating pins in one cell was one or
two, the Braille letters were well recognized. However, as we had anticipated, therecognition rate decreased when the number of vibrating pins increased. There was aninterference of vibrations among the pins, which caused the difficulty in discriminating thedots. To solve these problems, we propose two driving modes as seen in the next section.
Table 1 Profile of Visually impaired People by 5 Subjects
Profiles Age

Subjects

A
B
C
D
E

males
females

14
16
21
15
15

Years of using Braille
12
10
9
5
4

Visually impaired Statuscongenitalcongenitalcongenital
lost the sight at the age of 10
lost the sight at the age of 10

Recognition Time [Sec]Recognition Rate [%]Recognition Rate [%]About 8 ~ 9About 8 ~ 9 About 6 ~ 7

100

100
90
80
70
60
50
40
30
50

100

200

300

500

700

900

90
80
70
60
50

Subjects
A

40

D

30
20
10
0
20 (Hz)30 (Hz)50 (Hz)Vibration Duration [ms]Vibration Frequency [Hz]Fig.9 Braille Recognition Rate bydifferent Vibration Durations

Fig.10 Braille Recognition Rate bydifferent Frequencies

#1

#4

#2

#5

#3

#6

(a) Three-stage mode

Time delay
VD [ms]Time delay
LD [ms]

#1

#4

#2

#5

#3

#6

(b) Six-stage mode

Fig.11 Vibration actuator Driven Mode

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4. Braille Display System and its Evaluation
In order to reduce interference from each vibratory stimulation of the actuators, weconsidered putting a time delay between the rising time of the vibration actuators to makethe actuators’ vibration rhythmic so that the vibrations can be more easily sensed. The
rhythmic vibration can be produced by controlling the parameters of the driving signal ofeach actuator. In this section, two Braille presentation experiments, one with a three-stage
mode and the other with a six-stage mode are described, and according to the obtained
experimental results, the comparison between these two modes is given. Subjects A ~ E whowere students in the Kagawa Prefectural School for the Visually impaired took part in theexperiments. All subject profiles are shown in Table 1. The Braille display presents 4 Braillecharacters simultaneously. During the experiments, the subjects used the same method toread Braille presented by our developed device as to read traditional Braille. The reading
method is to use both index fingers to read Braille character one by one from left to right.
When the subject reads the Braille character, they can extrapolate the meanings of thecharacter string. We managed to acquire the permission to conduct the experiment using
their pupils after explaining our intention to the person in charge of the Kagawa Prefectural
School for the Visually Impaired. In addition to that, we got the consent from eachindividual subject after giving them a proper explanation regarding the experiment. A
teacher from the school was present as an overseer during the experiments.
4.1 Two Different Driving Modes of Braille Display
The three-stage mode and the six-stage mode which add the time delays are used inexperiments on the Braille recognition systems.
In Fig.11 (a), six dots in one Braille character are divided into three groups and driven
with time delays (three-stage mode). Using this method, the actuators are driven in the orderof (#1, #4) → (#2, #5) → (#3, #6), where the numbers indicate the actuator's number. The
time delay between the two groups is presented by VD [ms] (Time-Delay of Vertical
Direction), which is added as a parameter between the groups.
Fig.11 (b) shows the other method that drives actuators one by one with a time delay(six-stage mode). Using this method, the actuators are driven in the order of #1 → #4 → #2
→ #5 → #3 → #6. The time delay between #1 → #4, #2 → #5, #3 → #6 actuators ispresented by LD [ms] (Time-Delay of Lateral Direction), and time delay between #4 → #2,#5 → #3 actuators is presented by VD [ms]. The vibration actuators generate rhythmic
vibrations which can be easily recognized by the tactile receptors in the fingers.
4.2 Experiments for Braille Presentation (Three-Stage Mode)In this experiment we can validate using the combination of the vibration frequency for
Braille recognitions. We could analyze the recognition rate and time delay to evaluate therecognition difficulty for the Braille vibrators. Since this experiment uses a three-stage
mode for improving the recognition rate of Braille, we believe that each stage can use a
different frequency to drive the vibrators in order to help the visually impaired distinguishdifferent tactile sensation from the different frequency vibrations. Considering thecharacteristic responses of tactile receptors to mechanical vibration frequency, 20 [Hz], 30[Hz] and 50 [Hz] were chosen to provide vibrating stimulation to the recognition of theposition of each vibrator easy.
The driving schematic diagram and the driving signal of Braille /ke/ are shown in
Fig.12, where Braille dots #1, #2, #4 and #6 were divided into three groups by threedifferent frequencies 20 [Hz], 50 [Hz], and 30 [Hz]. In order to help people easily
discriminate between each group, three frequencies were selected. Actuators #1 and #4
which were in the first group were driven simultaneously by the same frequency of 20 [Hz],and the time delay VD between the first group and second group was presented by 300 [ms].

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The #2 actuator, which was in the second group was driven by the frequency of 50 [Hz],and the time delay VD between the second group and the third group was presented by 300[ms]. Actuator #6 which was in the third group was driven by the frequency of 30 [Hz]. The
vibration duration t [ms] of each actuator was set to 500 [ms]. In the above example, weused 20 – 50 – 30 (300) to indicate its parameter settings.
In this experiment, we chose 16 conditions (see Table 2) which were combined withdifferent VD and vibrating frequency to present Braille to the subjects. In addition, theamplitude of voltage was set to 2 [V], the width of the pulse was set to 1 [ms]. 18 Japanese
Katakana characters which are shown in Fig.13 were regarded as the test content. Fivevisually impaired subjects (A ~ E, two males, and three females) took part in the experimentand gave their results for the selected characters under 16 different conditions.

500 [ms]

CH 1

The number ofVibration actuator

CH 4
20 [Hz]CH 2

50 [Hz]CH 5

30 [Hz]

VD

#1

#4

#2
VD

CH 3

#6

CH 6
300[ms]300[ms]500 [ms]

Fig.12 Vibration Actuator Driving of Three-stage mode /ke/
a

i

u

e

o

ka

ki

ku

ke

ko

hi

te

ti

n

.

fu

mi

re

●○ ●○ ●● ●● ○● ●○ ●○ ●● ●● ○●○○ ●○ ○○ ●○ ●○ ○○ ●○ ○○ ●○ ●○○○ ○○ ○○ ○○ ○○ ○● ○● ○● ○● ○●●○ ●● ●○ ○○ ○○ ●● ●○ ●●●○ ●● ●● ○● ●● ○○ ●● ●●●● ●○ ●○ ●● ○● ●● ●● ○○

14
25
36

Fig.13 Japanese Braille
Table 2 Combination of Vibration Frequency and Time Delay
Vibration
duration t [ms]Experimental
selection* Vibration
Frequency
VF [Hz]** Time Delay of VD
[ms]

500
1

2

3

4

20-30-50

20-50-30

50-30-50

50-50-50

0 100 300 500 0 100 300 500 0 100 300 500 0 100 300 500

VD = 0 [ms]VD

VD = 100 [ms]VD

VD = 300 [ms]VD

VD = 500 [ms]VD

* VF: Vibration Frequency** Time – Delay of VD: Time – Delay of Vertical Direction

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The average recognition rates and dispersion value under the different conditions areshown in Fig.14. From this figure, it can be observed there are large variations in thesubjects’ Braille recognition rates. One reason for the big variation in the subjects’
recognition rates was the differences in the period of time they had been using Braille.
Some subjects have more experience than others. Therefore, the margin of error of theBraille recognition rates occurred.
An analysis of the vibration characteristics of tactile mechanical receptors shows thatthe recognition rate of Braille obtained when using the same vibration frequency to drive
the vibrators on three stages is better than the results obtained when using different
vibration frequencies on three stages. According to Miyaoka’s research on recognition
thresholds for vibratory stimulation (16), the holding vibration stimulation with the same
vibration frequency can make the recognition threshold of tactile mechanical receptors
stable. With the stable recognition threshold the subjects could feel regular vibration
stimulation, and could distinguish whether the vibrators were vibrating or not. So therecognition rate for Braille can be improved. This was also proven in our experimental
result for the 50 – 50 – 50 [Hz] combination when compared with other frequency
combinations as illustrated in Fig.14. In addition, the pin-type vibration actuator used in thisstudy was driven by a 50 [Hz] vibration stimulation which could produce a continuous
deformation of the skin. That means the vibration on the skin produces a tension, whichcaused the Ruffini endings to become stimulated to reach the active state. So, the subject
could improve the positioning accuracy of each vibrator (15) to recognize the Braille better.
On the other hand, when using several frequency combinations, the recognition
threshold of the tactile mechanical receptors is decided based on the highest vibration
frequency. That means the subjects could not easily recognize the vibration whose
frequency is lower than the threshold. In fact they found it difficult to distinguish whether
the vibrators are vibrating or not. The more different the frequencies used, the worse
recognition rate for Braille became. Furthermore, since the capacity field area of the
Ruffini endings is larger than that of the other tactile receptors, and the location of Ruffini
endings is deeper than the others, Ruffini endings can not position each vibrating actuator
well although skin deformation can be produced by different vibration frequencies. From
our experimental results under the conditions of 20 - 30 - 50 (0), 20 - 50 - 30 (0), 50 - 30 50 (0) and 50 - 50 - 50 (0), we noticed that the increase of the use of 50 [Hz] vibrations therecognition rate of increases.
With reference to time delay, we discovered the effect from four different time delays
on the Braille recognition rate. According to Kikuchi’s analysis (24) on time and spatial
characteristics of point vibration stimulation, we found that the recognition rate of Braille isdifferent when using different time delays. If the time delay is too short (vibration duration
is 500 [ms]), the subjects cannot distinguish the vibration of the vibrators well because of
the interaction between the vibrations. From the experimental results obtained using 100[ms] time delay, we can see the recognition rate of Braille, under the conditions of 20 – 30 –
50 (100), 20 – 50 – 30 (100), 50 – 30 – 50 (100) and 50 – 50 – 50 (100) are not good.
If the time delay is too long, when the next vibrator begins to vibrates, the subjects will
forget the position of the last vibrating vibrator, which causes difficulties in distinguishing
the Braille. This is determined by the human’s physiology, psychology, and nervous
system. So we set the time delay not longer than the maximum vibration duration 500 [ms].
From the experimental results, we could see that the recognition rate of Braille became
higher along with the increase in the time delay. We speculated that the increase in the timedelay could reduce the interaction between the vibrators, and enhanced the subjects’ ability
to recognize the vibrations produced by each vibrator, thereby the recognition rate of Brailleimproved. Table 3 shows the relationship of the Braille reading times, for each VF and VD.
We can see although the VD time increased, the total recognition time of Braille is425

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Vol. 7, No. 4, 2012shortened. In other words, by adding the appropriate time delay, Braille identification
becomes easier, and the recognition time becomes shorter. With a vibration frequency of 50– 50 – 50 [Hz], the average reading speed was about 15 characters/minute.
There are some exceptions to the results shown in Fig.14, which are 20 – 30 – 50(500), 20 – 50 – 30 (300), 50 – 30 – 50 (0), and 50 – 50 – 50 (100). Two reasons are
considered to the occurrence of the exceptions. The first reason is that there are individual
differences in tactile sensation. Even though we used the same parameters, each subject’s
tactile sensations are different. The second reason is, that in Japan the percentage of peopleusing Braille amongst the visually impaired is low (about 20%), and we were unable to findmany subjects for the experiment in our region. In addition, among the subjects thevariability in the number of years using Braille is large (from 4 years to 14 years).
110

Recognition Rate [%]

100
90
80
70
60
50
40
30
20
10
0
0 100 300 500 0 100 300 500 0 100 300 500 0 100 300 500
20-30-50

20-50-30

50-30-50

50-50-50

VD
VFTime Delay VD [ms] and Vibration Frequency VF [Hz]Fig. 14 Average Recognition Rate of Five Subjects
Table 3 Braille Recognition Time by VF and VD
500
t [ms]
20-30-50
20-50-30
50-30-50
50-50-50
VF [Hz]VD [ms] 0 100 300 500 0 100 300 500 0 100 300 500 0 100 300 500*PT [s] 0.5 0.7 1.1 1.5 0.5 0.7 1.1 1.5 0.5 0.7 1.1 1.5 0.5 0.7 1.1 1.5**TRT [s] 7 6.6 5.8 5.2 7.1 6.7 5.5 4.9 6.8 6.1 5.1 4.5 6 5.1 4.2 3.8***RT [s] 6.5 5.9 4.7 3.7 6.6 6 4.4 3.4 6.3 5.4 4
3 5.5 4.4 3.1 2.3* PT: Maximum Presentation Time of each Braille cell (t + 2VD)** TRT: Total Recognition Time (RT + PT)*** RT: Recognition Time Only

4.3 Experiments for Braille Presentation (Six-Stage Mode)In the above experiment, we used the three-stage mode to drive the vibration actuators,
and analyzed the effect of the vibration frequency and time delay on the recognition rate ofBraille based on the vibration characteristics of the tactile senses. The above experimental
results showed that using the same vibration frequency on vibrators and rationalizing thetime delay can improve the subjects’ recognition rate. In this section, we used a six-stage
mode to drive the vibrators and conducted a more detailed division of the vibration
frequency and time delay. The parameters are shown in Table 4. We used the parameterswith good experimental results from the three-stage mode and continued using thesevibration frequencies (VF) in this experiment. The experiment content and the subjects are
the same as those used in the three-stage mode. The randomly selected 180 patterns were
presented to the subjects in one trial. Based on the parameters of VD time delay, we added426

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the LD time delay (time delay of lateral direction, for example, between #1 and #4, between#2 and #5 and between #3 and #6), and made the vibration time of each vibrator in each
Braille cell completely independent. That is, there is never more than one vibrators
vibrating at the same time. In Fig.15, the presentation of the Japanese Braille /ke/ is shownas an example, where the parameter is set to 50 – 50 – 50 (300) (300). In this figure, weused steps to describe the vibration process of the vibrators. The CH1 vibrator was vibrated
for 500 [ms] at 50 [Hz] (i). After waiting for 300 [ms] LD time delay (ii) from the end of
CH1 vibration, the CH4 vibrator was vibrated for 500 [ms] at 50 [Hz] (iii). Then a 300 [ms]VD time delay (iv) is added before the vibrator of CH2 begins to vibrate (v). The vibratorsof CH5 and CH3 did not need to vibrate, but the interval should include all the time from(vi) to (x). Finally, the CH6 vibrator was vibrated for 500 [ms] at 50 [Hz] (xi).
Fig.16 shows that by using parameters which combine the vibration frequency and thetime delay, the average recognition rate and the dispersion value of Braille improved after
adding the LD time delay. This shows that based on VD time delay the LD time delay can
improve subjects’ recognition rate of Braille. Furthermore from the figure it is observablethat the dispersion values of 50-50-50 (500) (300), 50-50-50 (300) (500), 50-50-50 (500)(500) variability show the decreasing influence of individual differences. Therefore, we canknow the versatility of parameter settings for the Braille display and can help the subjects
with less experience achieve high recognition rates. Table 5 shows the relationship between
the Braille reading times and each VF, VD and LD. We can see that although the VD and LD
time increased, the total recognition time of Braille shortened. In Table 5 the recognition
time is negative. That means the average of the actual total recognition time is shorter than
the standard Braille characters presentation time which was set as the longest time among
Table 4 Combination of Vibration Frequency and Time Delay
Vibration durationt [ms]
Experimental
selection* Vibration
Frequency
VF [Hz]** Time - Delayof VD [ms]*** Time – Delayof LD [ms]

500
1

2

50-30-50

50-50-50

300

500

0

0

300
0

100

500

300

500

0

100

300

500

* VF: Vibration Frequency** Time – Delay of VD: Time – Delay of Vertical Direction*** Time – Delay of LD: Time – Delay of Lateral Direction(i) t [ms]The number ofVibration
actuator

CH 1
CH 4

50 [Hz]50 [Hz]50 [Hz]LD (iii)(ii)
VD (v)
(iv)

CH 2

#1
LD
(vi) (vii)

CH 5

#2

VD
(viii) (ix)

CH 3

#4

LD (xi)
(x)

#6

CH 6
300
[ms]

300
[ms]

300
[ms]

300
[ms]

300 500[ms] [ms]Fig.15 Vibration actuator Driving of Six-stage mode /ke/427

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Recognition Rate [%]18 Braille character presentation time. Although 50 – 50 – 50 (500) (500) took a longer timethan the other cases, the recognition rate of Braille is better.
For this experiment, we made sure that these 4 Braille characters have no semantic
association with each other (exp. /mi/, /ku/, /hi/, /u/; /te/, /e/, /re/, /o/). In this case, thereading speed is 14 characters/minute. Here, we should mention that in real life Braillecharacters usually have a semantic association with next one (exp. /fu/, /ku/, /o/, /ka/; /ko/,/u/, /e/, /n/). Therefore, readers can guess the meaning according to semantic association.
The reading speed when semantic associations were allowed, was observed to be faster thanwhen there was no semantic association with the characters. It illustrates the authors pointof view that using the same vibration frequency on the vibrators and rationalizing the timedelay can improve the recognition rate.
Furthermore, we found that individual difference can have a significant impact on therecognition rate. Subject A who is a congenital visually impaired, began to learn Braillefrom the age of two. He has about 12 years experience of reading Braille. Subject D whose
visual acuity declined from the age of 8 began to learn Braille as a 10 – year – old. Her
experience in reading Braille is just 5 years. From the results in Table 6, subjects A (a) madefewer mistakes (marked in gray) than subjects D (b). This shows that the length of years
reading Braille can have an impact on the Braille recognition rate.
From the above experimental results we can conclude that after adding the LD timedelay, the recognition rate of Braille is improved and more accurate than that of thethree-stage mode. However, the reading time also becomes longer than the reading time of
the three-stage mode. In order to improve the practicality of our system, we will shorten thesubjects’ reading time and keep the higher recognition rates in our future research.

120
110
100
90
80
70
60
50
40
30
20
10
0

LD
VD
VF
Time Delay VD and LD [ms] and Vibration Frequency VF [Hz]Fig. 16 Average Recognition Rate of five Subjects

Table 5 Braille Recognition Time by VF, VD and LD
500
t [ms]
50-50-50
VF [Hz] 50-30-50300
500
VD [ms] 300 500
LD [ms] 000 100 300 500 0 100 300 500*PT [s] 3.6 4 3.6 3.9 4.5 5.1 4 4.3 4.9 5.5**TRT [s] 6.8 6.2 6.5 6.0 5.2 4.8 5.5 5 4.5 4.2***RT [s] 3.2 2.2 2.9 2.1 0.7 -0.3 1.5 0.7 -0.4 -1.3
* PT: Maximum Presentation Timeof each Braille cell (6t + 2VD + 3LD)** TRT: Total Recognition Time (RT + PT)*** RT: Recognition Time Only
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Table 6 Recognition Results of Each Character by two Subjects A and D(a) Subject A
BrailleData

VF (Hz)VD and LD(ms)50-30-50(300)(0)50-30-50(500)(0)50-50-50(300)(0)50-50-50(500)(0)50-50-50(300)(100)50-50-50(500)(100)50-50-50(300)(300)
50-50-50(500)(300)
50-50-50(300)(500)50-50-50(500)(500)

/u/
14

/e/
124

/ki/
126

/ku/ /ke/
/ti/
/te/
/hi/
/fu/ /mi/
146 1246 1235 12345 1236 1346 12356

14

124

126

146 126

1356

1235

136

1346 1236

14

124

126

146 1246 1235

1345

13

1346 1236

14

124

126

146 1246

135

134

1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356

14

124

126

146 1246 1235 12345 1236 1346 12356(b) Subject D

BrailleData
VF (Hz)VD and LD(ms)/u/ /e/14 124/ki/
126/ku/ /ke/
146 1246

/ti/
1235

/te/
/hi/
/fu/ /mi/12345 1236 1346 12356

50-30-50
1 24
16
46 146 1356 1235 136 136 1236
(300)(0)50-30-50
14 24 126 16 1613
13
13
136 136(500)(0)50-50-50
145 124 1236 16 1236 135
134
13 1346 12356(300)(0)50-50-50
14 123 126 46 126 1356 1235 136 136 1236
(500)(0)50-50-50
1 12
16 146 126
135
134
13
13 12356(300)(100)50-50-50
14 124 126 146 1246 1235 12345 126 1346 12356(500)(100)50-50-50
14 124 126 146 1246 12345 12345 123 1346 12356(300)(300)
50-50-50
14 124 126 146 1246 1235 12345 123 1346 12356(500)(300)
50-50-50
14 124 126 146 1246 1235 12345 1356 1346 12356(300)(500)50-50-50
14 124 126 146 1246 1235 12345 1236 1346 12356(500)(500)

4.4 Analysis of Experiments
In this study, a vibration actuator using SMA was developed and applied to a new
Braille display for the visually impaired. In order to verify the effectiveness of thedeveloped Braille display, experiments on Braille presentation with a three-stage mode anda six-stage mode were conducted. The conditions including vibration frequency, vibration
duration and time delays on both the vertical and lateral directions affected recognition
results.
We found that when the vibration frequency was 50 – 50 – 50 [Hz] the recognition rate
was higher than those with other vibration frequencies. The reason for the good recognition
rate is that Meissner corpuscle and Ruffini endings could easily sense the vibration stimulus

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Vol. 7, No. 4, 2012at 50 [Hz] (21 ~ 23). The vibration generated by the vibrators could produce a continuous
deformation of the skin which stimulated the Ruffini endings. The subjects could improve
the positioning accuracy of each vibrator, and as a result could recognize the Braille better.
Moreover, by changing the time delay from 0 [ms] to 500 [ms], the recognition rateincreased. From the results shown in Fig.14 (Three-stage mode), Fig.16 (Six-stage mode),
Table 3 and Table 4, it can be shown that the recognition rate increased after adding timedelays between the vibration actuators. The reason is that the Ruffini endings are locateddeep in the skin, and slowly adapt to change (16) (25). Therefore, that is to say since the timeand spatial resolution of the Ruffini endings for vibration stimulation is poor, it leads to a
low recognition rate of Braille with a 0 [ms] time delay.
While more than one vibration actuators can produce stimulus at the same time, thenerve impulse from the tactile receptors simultaneously arrive at the brain through the nerve
fibers. It is difficult for the brain to classify the stimulation sources. This is the reason why
the recognition rate of the six-stage mode is higher than that of the three-stage mode. So, toreduce the interference of each vibration actuators, adding time delays on the vertical andlateral direction is necessary and regarded as important parameters in improving therecognition rate. The longer the time delays are, the higher the recognition rate is. However,the speed of reading Braille will still be slow. In the future work, shortening the reading
time while keeping a high recognition rate is our target.

5. Conclusion and Future Work
A compact Braille display which is driven by a PWM current was developed using
Shape Memory Alloy wires. The merits of the developed Braille display are low power
consumption, high-speed response, clear presentations of vibration stimuli, and can be fitted
to mobile phone or other devices which would allow visually impaired people to always
carry it. In this study, the vibration actuator was used to construct the Braille display. Fromthe experimental results, the recognition rates were the highest under the conditions of 50[Hz] vibration frequency. This means the vibration patterns effectively stimulated the
Meissner corpuscles, and helped the subjects discriminate the Braille characters more easily
and accurately. In addition, the use of the same vibration frequency gave a better
performance. By considering the time delay, which was the most important parameter forthe recognition rate, the highest performance was obtained when using a 500 [ms] timedelay with each vibration actuator. The results of this experiment show that the recognition
rate of Braille characters was higher compared with the results for recognition rates inprevious studies. The effectiveness of using the developed Braille display to present
Japanese Katakana characters for the visually-impaired was verified by using the above
experimental results. To make the Braille display more practical and keep the highrecognition rate, the recognition time should be further considered for the instantrecognition. In the near future, we will try to study the optimal parameters to shorten therecognition time so that users can read smoothly.

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