2020 IEEE 2nd Global Conference on Life Sciences and Technologies (LifeTech 2020)

Softness Presentation via Friction Force Control on
Electrostatic Tactile Panel Display
Giryeon Kim, Shogo Okamoto, Yasuhiro Akiyama, and Yoji Yamada
Abstract—This study proposes a softness presentation method
for when the ﬁnger slides on a hard touch panel. Herein,
an electrostatic tactile display was made to present a friction
stimulus, the frequency of which was changed by virtue of thenormal force and speed of the ﬁnger. Such a stimulus imitates
the stick-slip phenomenon that occurs between soft materials.
In the experiment where four types of stimulus conditions wereranked, one of which was a control condition with no active
friction stimulus, the new stimulus and previous method that
presented a constant low-frequency friction were subsequently
determined to be the softest among the four stimulus condition
types.

(a): Friction force: Normal force of the finger: Applied voltage: Electrostatic force
Insulator
Conductor- - - -(b)I. I NTRODUCTION
Substantial demand exists for the presentation of tactile
feedback on a touch panel. Many studies have introduced
methods for texture presentation; however, softness presentation on a hard touch panel is challenging because the panel
surface itself is hard. For achieving perception of softness,earlier studies focused on the pushing motion that can bereasonably employed to explore material softness. However,
because the touch panel is inherently hard and does notdeform by virtue of a ﬁnger’s force, pushing against it is
not particularly meaningful. Therefore, different approaches
should be studied when a hard touch panel is involved. To
date, only a few studies have attempted to present softness ona hard panel.
Some studies have presented softness by using lowfrequency vibrotactile stimuli [1]–[3]. In our previous study[4], we found that low-frequency friction stimuli present thesense of softness when the ﬁnger slides on a touch panel.
Ours was the ﬁrst study to present softness while the ﬁnger
slid along a hard panel. Furthermore, the rubbing motion isalso frequently used for exploring soft objects, in addition
to pushing. Arakawa and Okamoto demonstrated that surface
friction inﬂuenced the softness perceived when a ﬁnger was
made to slide on the surface [5]. Hence, softness presentation
using friction stimuli can be a practical solution in the case ofa ﬁnger sliding along a hard panel.
Friction between the ﬁnger and a soft object is affected by
not only the normal contact forces but also the ﬁnger speed [6],[7]. Therefore, effective softness perception can be achieved
upon employing dynamic change of the friction on the panel
according to the ﬁnger force and motion. In this study, weinvestigated the effects of a new method for controlling theAll authors are with the Department of Mechanical Systems Engineering,
Nagoya University, Nagoya, Japan. This study was in part supported by MEXT
Kakenhi (17H04697, 15H05923).

978-1-7281-7063-3/20/$31.00 ©2020 IEEE

Fig. 1. Structure of electrostatic tactile display. (a) Principle of electrostatic
tactile feedback. (b) Photograph of electrostatic tactile display. Participants
were instructed to grip the ground rode to improve the effectiveness ofelectrostatic feedback.

friction force between the touch panel and ﬁnger for presenting
softness and achieving perception of it.
II. E LECTROSTATIC TACTILE DISPLAY AND FRICTION
STIMULI

A. Equipment
The device manipulated the friction force between the ﬁnger
and touch panel by controlling the attractive electrostatic force
induced by the applied voltage between the ﬁnger and thetouch panel. A change in the friction force inﬂuenced thetangential deformation of the skin as the ﬁnger slid along thepanel.
A commercially available touch screen (SCT3250EX, 3M
Touch Systems, MN) was modiﬁed for presenting the electrostatic friction stimulus. A load cell (FSS015WNSX, TEConnectivity, Switzerland) was located under each corner of
the touch screen to calculate the load and the position of theﬁnger. The outputs of the load cells were sampled at 250 Hz.
The applied voltage was controlled at 2 kHz for smoothstimulus presentation. Further, the voltage was amplitudemodulated with the carrier frequency of 4 kHz. A similarequipment was also used in [8].

219

B. Friction stimuli
The four different stimuli were prepared for the experiment.
One of them was the test stimulus whereas the others werethe comparison stimuli. Stimulus A was the constant lowfrequency stimulus also employed in our previous study [4]and was determined using
x(t)
(1)
2λ
where A, x(t), and λ represent the amplitude of voltage, theposition of the ﬁnger, and the spatial frequency, respectively.
The A value was adjusted to lie within 96–132 V for individual
participants because the ﬁngertip conductivity depends on
the individuals. λ was 12 mm. The temporal frequency wasdetermined by the ratio of λ to the ﬁnger speed, and itapproximately ranged from 5 to 30 Hz according to the ﬁnger
movement.
According to earlier studies [1], [4], the frequency is an
important parameter for the presentation of softness. In thecase of the stick-slip phenomenon, the frequency of frictional
vibration changes according to the normal force and sliding
speed of the ﬁnger [7]. Therefore, we designed stimulus B suchthat its frequency depended on the normal force and ﬁnger
movement. The stimulus was determined using
Ve (t) = A sin 2π

f (t)
x(t)
2Fn (t)f (t) = fl + (fh − fl )Fn,maxVe (t) = A sin 2π(2)(3)

where fh , fl , Fn , and Fn,max are the maximum and minimum
frequencies, normal force, and predetermined maximum normal force, respectively. Fn was bounded to Fn,max . fh and fl
were set to 12 Hz and 2.5 Hz, respectively. These parameters
were determined by the authors, and their optimization was not
the objective of the present study. As a result, the frequencyof the stimuli ranged from 5 to 40 Hz according to Fn andmovement of the ﬁnger.
Stimulus C did not present any electrostatic friction. Thiscondition was prepared to be a control condition.
Stimulus D presented a high frequency roughness texture.
This stimulus was included to test the suggestibility of experimental participants.
III. E XPERIMENT
A. Participants
Eight males and one female Japanese university students
in their 20s participated in experiment. They were all righthanded, and none of them was aware of the objectives of thestudy.
B. Tasks
The four types of stimulus were presented to the participantsin a randomized order. The participants were then asked torank the provided stimuli according to the order of softness
perceived from them. Two trials were conducted for eachindividual over a span of 10 to 15 min. The participants slid

Fig. 2. Box plots of ranks among nine participants. The blue x marks indicate
the average rank of each stimulus. ∗∗ indicates the signiﬁcance level of 0.01
found by pair-wised Friedman tests.

their right index ﬁngers along the panel, and they could switch
the stimuli freely using a key board. The participants wiped
their ﬁngers with a cloth before the experiments. They weremade to listen to pink noise through headphones to mask thefrictional sounds generated.
IV. R ESULTS
Fig. 2 shows the result of the experiments. The horizontal
axis indicates the type of stimulus, and the vertical axis
indicates the rank of perceived softness. The ranks assigned
to the four types of stimuli were signiﬁcantly different (χ2 =39.28, p < 0.01, Friedman test). The post-hoc tests (pairwised Friedman test without p value correction) indicated thedifferences between all the stimulus pairs except for stimuliA and B.
Stimuli A and B were ranked higher than the other twostimuli, and there was no signiﬁcant difference between thetwo. Therefore, the new stimulus, the frequency of which waschanged according to the normal force and ﬁnger speed, wasas effective in presenting softness as the previously proposed
stimulus A, the frequency of which was constant.
V. C ONCLUSION
This study investigated a new method for controlling thefriction force for presenting softness on a hard touch panel.
The electrostatic friction stimulus, the frequency of whichdepended on normal force and ﬁnger speed, was found to be
as effective as the constant low-frequency stimulus used in [4].
Therefore, Although further studies are required to elucidate
the principles and optimize the stimulus, this new method will
expand the possibilities for softness presentation on hard touch
panels.
R EFERENCES[1] L. B. Porquis, M. Konyo, and S. Tadokoro, “Representation of softness
sensation using vibrotactile stimuli under amplitude control,” in 2011
IEEE International Conference on Robotics and Automation, 2011, pp.
1380–1385.
[2] T. Yamauchi, S. Okamoto, M. Konyo, Y. Hidaka, T. Maeno, and S. Tadokoro, “Real-time remote transmission of multiple tactile properties
through master-slave robot system,” in 2010 IEEE International Conference on Robotics and Automation, 2010, pp. 1753–1760.

220

[3] Y. Visell and S. Okamoto, “Vibrotactile sensation and softness perception,” Multisensory softness, vol. Section 1, pp. 31–48, 2014.
[4] K. Ito, S. Okamoto, H. Elfekey, H. Kajimoto, and Y. Yamada, “Feelingsoftness on a hard touch panel using an electrostatic tactile texture
display,” in 2017 IEEE 6th Global Conference on Consumer Electronics(GCCE), 2017, pp. 282–283.
[5] N. Arakawa and S. Okamoto, “Less frictional skin-like materials feel
softer in a tribologically inconsistent manner,” in Proceedings of IEEE
World Haptics Conference, 2019.
[6] K. Inoue, S. Okamoto, Y. Akiyama, and Y. Yamada, “Effect of material
hardness on friction between a bare ﬁnger and dry and lubricated artiﬁcialskin,” IEEE Transactions on Haptics, 2020.
[7] K. Nakano and S. Maegawa, “Stick-slip in sliding systems with tangential
contact compliance,” Tribology International, vol. 42, no. 11-12, pp.
1771–1780, 2009.
[8] K. Ito, S. Okamoto, Y. Yamada, and H. Kajimoto, “Tactile texture display
with vibrotactile and electrostatic friction stimuli mixed at appropriate
ratio presents better roughness textures,” ACM Transactions on Applied
Perception, vol. 16, issue 4, no. 20, 2019.

221