2017 IEEE International Conference on Systems, Man, and Cybernetics (SMC)
Banff Center, Banff, Canada, October 5-8, 2017

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A Texture Display Using Vibrotactile andElectrostatic Friction Stimuli Surpasses One Basedon Either Type of Stimulus
Ken Ito∗ , Shogo Okamoto∗ , Hatem Elfekey∗ , Hiroyuki Kajimto† , and Yoji Yamada∗∗ Department of Mechanical Science and Engineering, Nagoya University, Nagoya, Japan† Department of Informatics, The University of Electro-Communications, Tokyo, Japan

Abstract—This study demonstrates that the simultaneous useof two types of representative principles for tactile texture
displays, vibrotactile and electrostatic friction stimulation surpasses either type of stimulation. We used a set of sinusoidal
grating scales with different surface spatial periods as simulated
textured surfaces for testing the quality of texture displays.
Experimental participants judged the best stimulus condition
among the vibrotactile, electrostatic friction, and the conjunctive
stimuli in terms of the reality perceived from the simulated
texture stimuli. The use of vibrotactile and electrostatic stimuli
in conjunction was judged to be the most realistic for the spatial
periods of 1 and 2 mm, whereas noticeable differences among
the three stimuli conditions were not reported for small spatial
periods of 0.2 and 0.5 mm. The simultaneous use is effective forsimulating the surfaces with middle-level asperity.

I. I NTRODUCTION
In the wake of the proliferation of touch panel interfaces
for personal use, vibrotactile feedback techniques have been
intensively studied to compensate for the lack of haptic sensation and to improve their usability. Thus far, many studies
have demonstrated that vibrotactile feedback for actions suchas button-pushing, dialing, sliding, and flicking [1], [2], [3]improves the usability of touch panel interfaces.
Unlike these successful vibrotactile feedback techniques for
operative motions, the feedback techniques for surface textures
still require technical progress. The main drawback of texture
feedback is believed to be the lack of perceptual dimensions
that the vibrotactile feedback can provide. In general, vibrotactile feedback excels at presenting roughness sensations [4],[5], [6], whereas some studies have devised supplementary
techniques for friction and softness sensations [7], [8], [9],[10]. Roughness, friction, softness, and temperature constitutethe perception of texture [11], and the vibrotactile roughness
feedback combined with friction and softness/hardness cues
can effectively present simulated materials [8], [12].
Ultimately, tactile texture displays should be able to simultaneously present roughness, friction, softness, and temperature
stimuli. However, it is demanding to equip flat, rigid, andtransparent touch panels with capabilities to present softness
and temperature stimuli. Therefore, by combining vibrotactile
and electrostatic friction stimuli that are suitable for touch
panels, we have been developing tactile texture display tocover roughness and friction sensations [13]. The principles ofeach type of stimuli are described in section II. Thus far, no
978-1-5386-1644-4/17/$31.00 ©2017 IEEE

TABLE IC OMPLEMENTARY CHARACTERISTICS OF VIBROTACTILE AND
ELECTROSTATIC TACTILE TEXTURE DISPLAYS .

Target texture
Energy type
Finger deformation

Vibrotactile
Surface roughness
ActiveNormal direction

Electrostatic
Variable friction
Passive
Shear direction

studies have attempted to combine the two types of stimuli for
quality tactile textures. One study reported a tactile display that
could deliver both types of stimulus [14] where the vibrotactile
and friction stimuli were used for separately presenting finetexture and macroscopic geometry, respectively.
In the present study, through experiments to evaluate subjective experiences, we demonstrate that the texture display using
vibrotactile and electrostatic friction stimuli in conjunction
presents better quality textures than texture displays using
either type of stimuli. We use grating scales with periodic
surface patterns as testbeds. In our former study [13] using
the grating scale with a spatial period of 3 mm, the combined
tactile stimuli were superior to either type of stimuli in termsof perceived reality. However, it is still unknown whether the
combined tactile stimuli are effective for finer textures forwhich the surface spatial period is lower than 3 mm. For
acquiring further general conclusions, in the present study,
we experimentally examine the effects of vibrotactile andelectrostatic friction displays for fine textured surfaces.
II. P RINCIPLES OF V IBRO - AND E LECTROSTATIC TACTILED ISPLAY
We focused on two types of texture presenting mechanisms
which are suitable for touch panels. These two have complementary characteristics and are summarized in Table I.
A. Vibrotactile display
Fig. 1(a) shows the mechanism of vibrotactile display. Thistactile display presents tactile stimuli by actively deforming
the finger pad in the normal direction by using actuatorssuch as voice coil motors or piezoelectric actuators. Thus, thevibrotactile display actively stimulates the finger pad and does
not require relative displacement in the tangential direction
between the finger pad and the panel.

2343

Touch panel
Displacement of
the panel

Touch panel

Displacement of
the panel

Actuator to movethe panel

Actuator to movethe panel(a) Vibrotactile display.
The movement of the finger
Load of the finger
The movement of the finger
Loadof thefingerFrictionforceнн н нн

Attractive electrostatic force

Insulator
Conductor
Friction force

нн
н

нн

Ͳнн нͲ н Ͳ

Ͳ

Ͳ

Ͳ

Ͳ

Ͳ

Attractive electrostatic force
Insulator(b) Electrostatic
tactile display.
ConductorͲ Ͳ ͲandͲ electrostaticͲ Ͳ Ͳ tactile
Ͳ displays.
Fig. 1. Principles of vibrotactile

In terms of textures, generally, this type of display is good atpresenting fine surface roughness [4], [5], [6], [15]. However,when it comes to the presentation of geometry or a macroscopically rough surface, the vibration feels unnatural. Although
the vibrotactile display can present roughness textures without
hand motion of the user [16], a stimulation method synchronized with the hand motion is more effective [4], [5], [6].
These general characteristics of vibrotactile texture display aresummarized in Table I.
B. Electrostatic tactile display
Fig. 1(b) shows the mechanism of electrostatic display. It
manipulates the friction between the finger pad and touch
panel. The friction force can be varied by controlling theattractive electrostatic force which is induced by the applied
voltage between the finger pad and conductor. The change inthe friction force influences the tangential deformation of theskin. The finger stroking-motion is necessary for the user toexperience the feedback from the display. According to this
aspect, the electrostatic friction display is a passive-type tactile
display.
In terms of textures, generally, this type of display is goodat presenting textures with low surface roughness but having
friction [17], [18] and geometry or a macroscopically roughsurface [13], [19]. The characteristics of electrostatic friction
display are summarized in Table I.
III. I NTEGRATED TEXTURE DISPLAY USING V IBROTACTILE
AND ELECTROSTATIC STIMULI

Fig. 2 shows the tactile display developed in this and ourprevious studies [13]. This display presents vibrotactile andelectrostatic stimuli simultaneously. As previously mentioned,

the conjunction of vibrotactile and friction stimuli is practically reasonable, provided that this technique is intended for
popular information terminals, such as touch panels. There
are various methods for the implementation of the two typesof stimuli. In the present study, because it is a laboratory
prototype, we used actuators and amplifiers that could deliver
output stimuli large enough when compared to the perceptual
threshold levels.
Vibrotactile stimuli were produced by four voice coil actuators (X-1741, Neomax Engineering Co. Ltd., Japan) locatedat the four corners of the top panel. The four actuators
were synchronously driven by a current amplifier (ADS 50/5,
Maxon Motor AG, Switzerland). Therefore, mechanical vibratory stimuli were produced along the normal to the top panel.
In Fig.2 (a), a voltage (Ve ) is applied between the Indium
Tin Oxide (ITO) panel layer and the conductor. Consequently,
an electric force is generated between the ITO layer and the
conductor. In doing so, an electrostatic stimulus was applied.
An insulator (Kimotect PA8X, KIMOTO Co. Ltd., Japan, 8µm) was fixed on the Indium Tin Oxide (ITO) plate. The
voltage Ve was driven by a high-voltage amplifier (HJOPS1B20, Matsusada Precision Inc., Japan).
A load cell (FS2050-0000-1500-G, TE Connectivity,
Switzerland) was affixed beneath each voice coil motor. The
center of the force on the panel was calculated at 1 kHz byusing the four load cells and regarded as the finger position on
the panel. Thus, the position and velocity of the finger weredetected by the load cells.
The two amplifiers and four load cells were connected toa data acquisition board (TNS-6812, Interface Corporation,
Japan) which was connected to a PC.
Our display presents tactile stimuli based on the positionand velocity of the finger through the pad. Without finger
movement, this display does not present tactile stimuli. The
pad with a conductor film was introduced to qualify thefrictional forces of the panel.
IV. E XPERIMENT: C OMPARISON OF REALITY PERCEIVEDBY TACTILE STIMULI

A. Design and justification
To evaluate the reality of simulated textures, a ranking task
was selected because it is suitable for proving that a stimulus
is more realistic than the other stimuli. Simple sinusoidal
grating scales were selected for the evaluation because thegrating scale has been used in a lot of research. It should be
noted that additional experiments are necessary for coming toconclusions about the other types of textures.
B. Ranking task
Participants experienced three types of stimuli (1. combination of vibrotactile and electrostatic, 2. vibrotactile only, and3. electrostatic only) through the pad and the grating scalewith a round tube shown in Fig. 3. They were instructed
to rank the three stimuli in the order of reality. Tie ranks
were permitted. If a participant ranked a stimulus as first, thestimulus was perceived to match the specimen the most. They2344

Vinyl chloride pad

TABLE II
λ AND h VALUES OF GRATING SCALE . λ AND h WERE SPACIAL WAVE

ܸ௘

Conductor

LENGTHS AND HEIGHTS FROM THE BOTTOM OF THE GRATING SCALES .

λ [mm]
0.2
0.5
1.0
2.0

Insulator‫ݔ‬
ITO panel

‫ݕ‬

‫ݖ‬

Voice-coil actuator
Load cellh [mm]
0.2
0.5
2.0
4.0

ܸ௩

present the spatial deformation on the finger pad. The shape
was a sinusoidal wave which was determined by λ and hshown in Fig. 3. Table II presents four combinations of λ andh. Four grating scales of each combination were prepared.
A similar experiment was performed under the condition of
λ = 3 mm in our previous research [13]. Thus, we selectedfour spatial wave lengths λ , which were each smaller than
3 mm. The values of h were large enough so that the roundtube did not reach the bottom.

(a) Schematic view of the structure.

E. Roughness texture stimuli
We used the vibrotactile and electrostatic roughness texture(b) Photograph of the display. A picture of a grating scale isstimuli of the grating scale. The presentation algorithm fordisplaced beneath the transparent top panel.
each stimulus was the same as that used in numerous earlier
Fig. 2. Overall structure of the tactile display for vibrotactile and electrostatic
studies.
friction stimuli
1) Simulated vibrotactile texture: We used the presentation
algorithm that had been already reported [6], [8], [20].
Movement of the finger
When the finger pad experiences the sinusoidal grating scale,deformation of the finger pad occurs. This deformation is aRound tube
sinusoidal wave of the same amplitude and frequency as theߣ
10 mmexperienced grating scale. Thus, the driving force Fv in thenormal direction for vibrotactile stimulus was determined by݄x(t)Fv (t) = A sin 2π(1)
λFig. 3. Sinusoidal grating scale. Participants experienced the grating scalewith the round tube.

could change the stimuli when they wanted and experience
the three stimuli and specimen as many times as they wanted.
They were instructed to move their finger at a normal speed(approximately 15 cm/s). The same tasks were repeated undereach condition for four wave lengths.
C. Participants
Six volunteers (all students, non-experts, right handed, withan average age of 23.3) participated in this experiment. They
were not informed about the objective of the research. They
were ascertained that they can perceive stimuli or not before
the experiment.
D. Sinusoidal grating scale (actual texture)
Fig. 3 shows an image of the sinusoidal grating scale, which
was the actual target texture. Participants experienced it using
the round tube shown in Fig. 3 because our display could not

where x(t) and A are the position of the finger pad on the panel
and maximum force respectively. This force causes the finger
pad to deform in coordination with the finger motion, which
is known to be effective in presenting surface roughness [6],[8], [20].
2) Texture stimulus by electrostatic stimuli: When the finger
moves up the surface of the grating scale, the friction force
becomes higher. When the finger moves down the surface ofthe grating scale, the friction force becomes lower. Hence,the phases of the stimuli for surface displacement and friction
were separated by π /2 [21], [22]. Thus, the applied voltage
for electrostatic was determined by
Ve (t) = B cos 2π

x(t)
2λ

(2)

where B is the maximum value of the voltage. From the lawof the static electricity attraction force and friction force, thefriction force between the finger pad and panel was determined
by

2345

were significant differences between the combination stimulus
and vibrotactile stimulus under the conditions of wave length
λ = 1.0 and 2.0 mm. As an overall trend, the smaller thewave length of the grating scale, the smaller is the difference
between each stimulus. The results for each λ value aredescribed below.

TABLE IIIM AXIMUM DRIVING FORCE Fv AND ELECTROSTATIC FRICTION FORCE Fe

λ [mm]
0.2
0.5
1.0
2.0

Combination
Fv [N]Fe [N]
1.69
0.70
1.78
0.84
1.90
0.88
2.01
0.99

Vib. only
Fv [N]
2.01
2.19
2.25
2.35

Elec. only
Fe [N]
0.80
0.91
0.95
1.08

A. λ = 0.2 mm
For the stimuli with λ 0.2 = mm, every stimulus was ranked
first at least once. There were no significant differences.

{}µ W + kVe2 (t)
(3)
{
kB2
x(t) }= µ W+(1 + cos 2π
)
(4)
2
λ
where µ , W , and k are the coefficient of friction, loadof the finger, and constant involving the electrostatic force,
respectively. The first member of (3), which is W, expresses
the load of the finger, and the second member expresses theelectricity attraction force; k mainly depends on the area ofthe electrode, the relative dielectric constant of the insulator,
and thickness of the insulator.
3) Texture simulated by vibrotactile and electrostatic friction stimuli: This type of texture stimulus was designed basedon the combination of vibrotactile and electrostatic stimuli.
4) Perceived magnitude of simulated textures: The gain parameters A and B in (1) and (2), respectively, partly determine
the magnitude of perceived textures. Hence, they should be
carefully adjusted. The direct measurement of normal andshear forces when a finger slided on a grating scale was oneof options to adjust gains, however, it is very sensitive anddepends on the environment of the measurement. Thus, theexperimenters determined these parameters manually such thatthe simulated textures felt close to the actual grating scale foreach λ value. As a result, the three types of perceived strengthsof texture stimuli were the same under the condition of eachspatial wave length λ .
The maximum generative forces Fe and Fv are listed inTable III. The ranges for Fe were evaluated under the onset ofa pressing force W of 1 N. The intensity of the combinationof the two stimuli was lower than the intensity of each single
stimulus as a result of tuning by experimenters.
Fe (t) =V. R ESULTS
Fig. 4 shows the experimental results. Each graph shows
the ranks assigned to each stimulus condition by individual
participants under each condition of the four spatial wave
lengths.
For an overall analysis, we conducted ANOVA on thenumber of times that each simulated texture was ranked first.
The analysis conducted was a two-way ANOVA involving thetype of simulated stimulus and λ value as the two factors. The
type of stimulus was found to be significant in influencing themost favored stimulus (F0 (2, 6) = 5.14, p < 0.01).
For each λ value, we used the Wilcoxon rank sum test
to evaluate the priority of the combination stimulus. ThereB. λ = 0.5 mm
For the stimuli with λ 0.5 = mm, every stimulus was ranked
first at least once, the same as λ = 0.2 mm. There were no
significant differences. The combination stimulus was ranked
first the most.
C. λ = 1.0 mm
For the stimuli with λ 1.0 = mm, there was a significant
difference between the combination stimulus and vibrotactile
stimulus. However, there was no significant difference between
the combination stimulus and vibrotactile stimulus. The vibrotactile stimulus was not ranked first and the ranks that wereassigned to the electrostatic friction stimulus were divided intotwo.
D. λ = 2.0 mm
For the stimuli with λ 2.0 = mm, there was a significant
difference between the combination stimulus and vibrotactile
stimulus. However, there was no significant difference between
the combination stimulus and electrostatic friction stimulus.
VI. D ISCUSSION
Although we had expected the conjunction of two typesof stimuli to be more effective across wide λ values than theindividual stimuli, in our experiments, the superiority of tactile
stimuli depended on the fineness of the surface roughness.
When the surface was fine with λ values being 0.2 – 0.5 mm,
the superiority was opaque. In contrast, for a coarser surface
roughness with λ values greater than 1 mm, the stimuli
combining the vibrotactile and electrostatic friction stimuli
exhibited a better performance than either the vibrotactile
or electrostatic friction stimuli. The reasons explaining such
results are discussed below.
A. Fine roughness textures are well presented by either vibrotactile or electrostatic friction displays
For fine surface roughness with λ < 1 mm, the roughness
perception is largely mediated by skin vibration caused inscanning the surface [15]. Both the vibrotactile and electrostatic friction stimuli are good at presenting vibratory stimulus.
One speculation is that the skin vibration derived from surface
displacement and that derived from friction are perceptuallyclose for fine roughness. Consequently, two types of tactile
stimuli are equally capable of presenting fine roughness textures.

2346

;ĂͿߣ
сϬ͘Ϯ
;ĂͿߣ
сϬ͘Ϯ
;ĂͿߣ сϬ͘Ϯ;ĂͿߣ(a) λ = 0.21stmmсϬ͘Ϯ

Electrostatic friction
may be effective.

Vibrotactile orelectrostatic friction.

Rank
Rank
Rank
Rank

1st
1st
2nd
2nd
1st
2nd
3rd3rd2nd
3rd

ߣ [mm]
0.2 0.5

1

2

3

4

5

Combination is effective.

+ Elec.
Vib.
+ Elec. Vib.Vib. Elec.Elec.
3rd Vib.
Vib. + Elec.

Vib.

Elec.

Vib. + Elec.
;ďͿߣ
сϬ͘ϱ
;ďͿߣ
сϬ͘ϱ
(b) λ = 0.5;ďͿߣmmсϬ͘ϱ

Vib.

Elec.

Fig. 5. Type of tactile stimuli that deliver natural texture feelings. For λsmaller than 1 mm, either of vibrotactile or electrostatic friction stimuli issatisfactory. For λ of 1–3 mm, the combined stimulus is superior. For λ >3 mm, electrostatic friction stimuli may be better.

;ďͿߣ сϬ͘ϱ

Rank
Rank Rank
Rank

1st1st
1st
2nd2nd2nd
1st
3rd3rd
3rd
2nd

3rd

cues such as the friction field on the surface and might haveenhanced the perceived reality of textures in our experiments.
Fig. 5 shows the summary of the present and our paststudies [13]. For grating scales with λ = 0.2 − 0.5 mm, either
vibrotactile or electrostatic friction display is effective. For
those with λ = 1 − 3 mm, the display combining the two typesof stimuli is recommended. Regarding much lower or higher λvalues, further studies are necessary; however, our prediction
is as follows. For greater λ values, both types of stimuli
are not effective because the surfaces have macroscopically
distributed information, for which texture displays are nolonger effective. Nonetheless, electrostatic friction stimuli may
be still better [18] than vibrotactile stimuli. For λ values lower
than 0.2 mm, the methods used in the present study may
not be effective because the resultant temporal frequencies ofthe stimuli easily overcome the rendering capability of thecomputer.

Vib. + Elec. Vib.
Elec.
Vib.++Elec.
Elec. Vib.
Vib. Elec.
Elec.
Vib.

(c) λ = ;ĐͿߣ
1.0 mmсϭ͘ϬVib. + Elec.
;ĐͿߣ сϭ͘Ϭ
сϭ͘Ϭ
Ύ

Vib.

Elec.

ΎΎ

Rank
Rank Rank
Rank

1st;ĐͿߣ сϭ͘Ϭ
1st
1st
2nd
2nd
2nd
1st
3rd
3rd
3rd

2nd

3rd

Ύ

Vib.
+ Elec.
Vib.
+ Elec. Vib.
Vib. Elec.
Elec.
Vib. + Elec.
Vib.
Elec.

(d) λ = 2.0mmсϮ͘Ϭ;ĚͿߣ;ĚͿߣ сϮ͘Ϭ+ Elec.
;ĚͿߣVib.
сϮ͘Ϭ Ύ Ύ

Vib.

Elec.

Ύ

Rank
Rank Rank
Rank

1st
;ĚͿߣ
1st сϮ͘Ϭ
1stΎ
2nd2nd2nd
3rd1st3rd
3rd+ Elec.
2nd Vib.
Vib.
+ Elec. Vib.
Vib. Elec.
Elec.
Fig. 4. Experimental results.
Frequenciesof ranks foreach stimulus. ∗
Vib.
+
Elec.
Vib.
Elec.
3rd
indicates p < 0.05.

Vib. + Elec.

Vib.

Elec.

B. Texture displays using vibrotactile and electrostatic friction
stimuli in conjunction are superior to those based on either
type of stimulus
When λ = 1.0 and 2.0 mm, the stimulus combining the two
types of stimuli was judged better than the individual vibrotactile stimuli. In our previous report [13], the combined stimuli
were better than single types of stimulus when λ = 3.0 mm.
Collectively, the combination of the two types of stimuli
is effective when λ = 1 − 3 mm. The perception of coarse
surfaces with such λ values is mediated by spatially distributed
cues, rather than skin vibration [11]. An intuitive explanationof these results is as follows. Only with the vibrotactile
stimulation, is the friction associated with surface roughness
missing, whereas the information on surface displacement
is delivered. In contrast, the electrostatic friction stimulation
lacks the information on surface displacement. Actual textures
involve both surface displacement and friction. The combination of friction and surface displacement presented spatial

VII. C ONCLUSION
To improve the quality of textures presented by a tactile
display, we combined two complementary types of texture
presenting methods, vibrotactile and electrostatic displays. We
prepared grating scales with four different wave lengths andpresented virtual texture of those wave lengths. As the resultof a ranking task, there were no significant differences inthe small wave-length conditions, but there was a significant
difference between combination and vibrotactile stimuli inthe long wave length conditions. Therefore, we proposed aneffective stimulus presenting method under each wave length.
ACKNOWLEDGMENT
This study was in part supported by MEXT Kakenhi(15H05923) and SCOPE (142106003).

2347

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