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Feeling Softness on a Hard Touch Panel Using an
Electrostatic Tactile Texture Display
Ken Ito∗ , Shogo Okamoto∗ , Hatem Elfekey∗ , Hiroyuki Kajimoto† , and Yoji Yamada∗∗ Department of Mechanical systems Engineering, Nagoya University, Nagoya, Japan† Department of Human-Communication, The University of Electro-Communications, Tokyo, Japan(a)

Abstract—This paper presents a method to deliver soft tactile
textures on a hard touch panel, using an electrostatic tactile
feedback display, which produces high-frequency frictional textures with extremely low-frequency friction stimuli. Throughexperiments involving several participants, we found that lowfrequency signals are effective at transmitting soft sensations.
Although the underlying mechanism requires further study, thisnew perceptual effect may provide the touch panel industry withan attractive haptic advancement.

The movement of the finger
Load of the finger
Friction forceܸୣн н нннн

н

Attractive electrostatic force
Insulator
Conductor Ͳ Ͳ Ͳ Ͳ Ͳ Ͳ Ͳ Ͳ

I. I NTRODUCTION
The presentation of tactile feedback by a touch panel would
lead to an improvement in the operability of the touch panel
and the appearance of new content. The demand for theimplementation of a tactile feedback mechanism is increasing
and various principles are being studied. The presentation ofsoftness on a hard touch panel extends the tactile feedback
capabilities of tactile display; however, it is highly challenging
to present softness on a hard touch panel because of its
inherent hardness. Until now, some researches have been
conducted to present softness by using vibratory mechanicalstimuli. There have been studies that present softness to thefinger by using low-frequency vibrotactile stimuli [1], [2] andone that presents softness of virtual objects while holding theobjects [3]. However, no study has been conducted to present
the softness of virtual texture when the finger slides on thetouch panel. Hence, we aimed at presenting the softness ofvirtual texture on a touch panel based on the concept of lowfrequency frictional stimulus related to the movement of thefinger and the increase of friction when the finger slides on
the soft material.
The objective of this paper is to present fine textures andsoftness simultaneously using an electrostatic tactile display
which presents natural tactile feedback when the finger slides
on the touch panel. This paper is the first on an electrostatic
tactile display to present soft textures.
II. E LECTROSTATIC TACTILE DISPLAYFig. 1(a) shows the mechanism of an electrostatic tactile
display. It manipulates the friction between the finger pad
and touch panel by controlling the attractive electrostatic force
which is induced by the applied voltage between the finger pad
and the conductor. A change in the friction force influences thetangential deformation of the skin as the finger pad slides along
the panel. Fig. 1(b) shows our experimental setup [4]. The
frictional stimuli were produced by electrostatic forces inducedby a voltage (Ve ) between the conductive rubber pad and an
Indium Tin Oxide panel. The Indium Tin Oxide plate wascovered with an 8 µm an insulator (kimotect PA8X, KIMOTO

978-1-5090-4045-2/17/$31.00 ©2017 IEEE(b)
Conductive rubber pad

ITO panel

Liquid crystal display

Transparent insulator

Fig. 1. Structure of the electrostatic friction display. (a) Principle ofelectrostatic tactile display. (b) Photo of the tactile texture display.

Co. Ltd., Japan). The voltage Ve was driven by a highvoltage amplifier (HJOPS-1B20, Matsusada Precision Inc.,Japan). The conductive rubber pad was used for regulating
the frictional condition of the panel. Otherwise, the friction
operating between the finger and the panel would be adversely
affected by individual finger properties, such as the moisture
content of the finger pad.
A load cell (FS2050-0000-1500-G, TE Connectivity,Switzerland) was affixed beneath each corner. The center ofthe force on the panel was calculated at a 1 kHz sampling
frequency using the four load cells for tracking the fingerposition.
Our display delivered frictional tactile stimuli, based upon
the position and velocity of the finger through the pad. Without
finger movement, the display does not yield any tactile stimuli.
III. E XPERIMENTThe participants in the experiment compared two types offine texture stimuli presented by the touch panel. One wascombined with low frequency variation of friction, and theother was without. Using a forced-choice experimental option,
the participants were asked to choose the softer stimulus.

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Answer ratio: Softer

(a)

(b)

Ύ
0.8
0.6
0.4
0.2
0
Without low- With lowfreq. stimuli freq. stimuliAnswer ratio: Softer2017 IEEE 6th Global Conference on Consumer Electronics (GCCE 2017)

1.0
0.8
0.6
0.4
0.2010 12 14 16 18

The spatial wave lengthof low-freq. stimuli [mm]Fig. 2. Answer ratios of “feeling softer”. (a) Mean and Standard Deviation
values of answer ratios at which the stimulus was felt softer than the other.
∗ indicates p < 0.05. (b) By each low-frequency component.

A. Stimuli
Based on previous research that reported on the perceptionof softness as being related to low-frequency mechanicalstimulus [2], [3], the presented stimuli were determined by
the equation as follows:
Ve (t) = A sin 2πx(t)x(t)+ B sin 2π2λh2λl(1)

where A and B were maximum values of each voltage andx(t) was the position of the finger on the panel. λh and λl are
the spatial wave length of fine textures and long spatial wave
length for presentation of softness, respectively. They wereselected randomly from determined ranges. λh was a range of
0.2–1.0 mm by 0.2 mm, and λl was a range of 10–18 mm by
2.0 mm. We prepared five parameters for λh and λl . The firstpart of (1) is related to fine texture, and the second is related
to the low-frequency stimulus for the perception of softness.
The frictional force applied to the pad was determined by thefollowing equation:{}(2)
Fe (t) = µ W + kVe2 (t)where µ , W , and k are the coefficients of friction, the load of
the finger, and the electrostatic force constant, respectively.
B. Alternative forced-choice task
Participants experienced two kinds of texture stimuli: thosecombined with low-frequency variations of friction (A =110 V, B = 0 V) and those without (A = 25 − 100 V, B =200 V). Because the magnitude of the perceived friction(related to A and B) while sliding the finger pad could affectthe participant’s responses, we tuned the gains of A and B suchthat the magnitude of the stimuli were felt close to each other.
They chose the stimulus that felt softer, and the one that felt
rougher. The task of choosing the rougher texture constituted,
which was a fake task. Each participant compared 20 pairs ofstimuli. They were instructed to move the pad at a comfortablespeed (about 15 cm/s). They could freely switch between thetwo types of stimuli as frequently as they wanted.
C. Participants
Five volunteers (non experts, average age 23.4) participatedin this experiment. They were not informed of the research
objective.

978-1-5090-4045-2/17/$31.00 ©2017 IEEE

D. Results
The experimental results are shown in Fig. 2; (a) shows
the answer ratio of softness and (b) shows the answer ratio
per low frequency applied to each λl . Fig. 2(a) shows thatthe stimuli including the low-frequency frictional change by
λl were judged as softer by the participants (t-test, p < 0.05).
These results indicate that low-frequency stimuli influenced
the perception of softness. Fig. 2(b) shows that the values of λl
did not affect the answer ratios since there were no significant
differences in the answer ratios among the spatial wave lengthsof low-frequency stimuli.
IV. D ISCUSSION
Our experimental results showed that the virtual texture
was perceived softer by adding the stimulus of spatial wave
length of 10–18 mm when the display presented the finetexture of spatial wave length of 0.2–1.0 mm. Because there
were no significant differences among the values of lowfrequency spatial wave lengths, softness was perceived when
the low-frequency wave length was sufficiently longer thanthat of the fine texture stimulus. The causes of this perception
demand more research, but, our hypothesis is as follows.
The increase of friction from the softness would be related
to this experiment. The adhesion and hysteresis friction isnegative power of the Young’s modulus of objects [5]: thesofter the material, the larger the friction. Additionally, thefriction depends on the load and the velocity of the finger.
Normally, the movement of the finger is low frequency. Hence,
the frictional force from the softness is also low frequency.
The low-frequency friction might have led the participants toimagine the softness of material.
V. C ONCLUSION
With this research, we experimentally presented soft-feelingtextures on a hard touch panel with an electrostatic tactile display by adding low-frequency frictional stimuli. Thus far, such
results have not been formally reported. This approach would
extend the possibilities of software-driven tactile displays.
ACKNOWLEDGMENT
This study was in part supported by MEXT Kakenhi(15H05923).
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