ISASE 2024

Response Surface of Softness Perceived
via Frictional Tactile Stimuli on Flat Touch-display
Giryeon KIM*, Shogo OKAMOTO** and Hisataka MARUYAMA** Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-shi, Aichi 464-8603, Japan
kim.giryeon.y2@s.mail.nagoya-u.ac.jp** Department of Computer Science, Tokyo Metropolitan University, 6-6 Asahigaoka, Hino-shi, Tokyo 191-0065, Japan

Abstract: An electrostatic frictional tactile display is used to present softness on a rigid flat-panel display. The display controlled thefriction force of a finger sliding on it. However, the effects of representative stimulus parameters, those are, the finger’s sliding velocity
and wavelength which determined the spatial change of friction on the panel, are unknown, whereas the low-frequency fluctuation ofsurface friction is known to be important for softness presentation. This study experimentally specified the response surface of subjective softness as a function of the stimulus wavelength and finger’s sliding velocity, involving ten participants. Consequently, the response surface was established, with a local maximum at the wavelength and sliding velocity of 10.55 mm and 198.6 mm/s, respectively.
This stimulus condition was considered effective for the softness presentation.
Keywords: Haptics, Softness, Tactile display, Electrostatic force, Friction force

1. INTRODUCTION

Slide
direction: Friction force: Normal force by the finger: Applied voltage: Electrostatic force

The presentation of softness on a rigid flat-panel display
is challenging because the surface touched by the fingers
is rigid. Controlling the contact area [1–5] or force [2] between the finger pad and the haptic display device according to the pressing motion of the finger is a general approach for presenting softness. However, these methods
are not suitable for flat and hard liquid crystal display panels because their surfaces do not deform. To overcome thisdifficulty, our research team proposed a method which
control frictional forces when a finger slides on hard flatpanel displays to present illusory softness [6, 7].
When a finger slides over an object’s surface, the friction force and its fluctuation are affected by the softnessof the object [8–11]. Further, when people explore soft objects, they adopt sliding motions as well as the pushingmotion [12–14]. Based on the relationship between friction and compliance, our research team controlled the friction force on a flat panel and presented softness perceptions [6, 7]. Electrostatic forces were used to control thefriction force and low-frequency fluctuations (approximately 10–20 Hz) of frictional stimuli were perceived assoft [6, 7]. However, the most effective frequency of thefluctuation is unknown. Furthermore, as described in Section 3.3., frequency depends on the sliding velocity of thefinger and spatial period of the stimuli. To optimize softness presentation, the effects of these parameters onInsulator
Conductor−−−−Figure 1: Principle of electrostatic friction force. Adapted from[15].

subjective softness need to be investigated. In this study,
as a preliminary step for optimization, the response surface of subjective softness as a function of the spatial period of frictional change and the sliding velocity of the finger was experimentally determined.
2. APPARATUS: ELECTROSTATIC FRICTIONALTACTILE DISPLAY
2.1 Principle: Electrostatic friction force
Figure 1 shows a schematic of the principle of an electrostatic tactile display presenting the frictional force using electrostatic adhesion. The friction force 𝐹 between afinger and the display is determined by the coefficient offriction 𝜇 between them and the finger’s normal force 𝐹 .
When a finger touches the surface of a conducted display,the electrostatic force 𝐹 attracts the finger skin to the display, leading to an increase in friction.

1

Table 1: Combinations of the wavelength and sliding velocity of the stimuli used in the experiment.

λ (mm)𝑣 (mm/s)λ (mm)𝑣 (mm/s)

5.00

7.93

7.93

15.00

15.00

150.0

79.3

220.7

50.0

150.0

15.00

22.07

22.07

25.00

250.0

79.3

220.7

150.0

Hino Campus, Tokyo Metropolitan University (H22-031).
3.3 Stimuli
The voltage level applied to the electrostatic tactile display was determined as follows:𝑥 𝑡
𝑉 𝑡
𝐴sin 2π22λwhere 𝑥 𝑡 is the horizontal position of the finger on thedisplay panel, and 𝜆 is the spatial wavelength of the stimulus. 𝐴 is the gain of the stimulus and was adjusted suchthat each participant felt the frictional changes satisfactorily.

Figure 2: Experimental apparatus.

The relationship between the electrostatic force and theapplied voltage is expressed as𝐹𝑘𝑉 𝑡 ,1where 𝑘 is an electrostatic coefficient that depends on the
dielectric constant, insulating film thickness, and contact
area [16]. Using this relationship, the frictional force isdynamically controlled.

Five different wavelengths λ and sliding velocities 𝑣
were selected in order to investigate their effects on theperceived softness based on the central composite design.
The ranges of λ and 𝑣 values were determined through apreliminary experiment involving the authors and theircolleagues, such that the most effective parameter set
would be included within the ranges. Following the central composite design, the λ values were either 5.0, 7.9,15.0, 22.1, or 25.0 mm, and the 𝑣 values were 50.0, 79.3,150.0, 220.7, or 250.0 mm/s. A total of nine combinationsof λ and 𝑣 were used for the experiment, as listed in Table
1. With these combinations, participants were expected toexperience temporal frequencies, that is, 𝑣/λ , ranging
from 3.3 to 30 Hz on their fingertip.

2.2 Hardware
Figure 2 shows the experimental setup and electrostatic
frictional tactile display used in the experiment. A loadcell (FSS015WNSX, Honeywell International Inc., USA)was placed at each corner of the panel to calculate finger
movement. Signals from the load cells were sampled at250 Hz.
A capacitive touch panel (SCT3250EX, 3M Touch Systems, USA) was modified to present the electrostatic force.
The voltage signal to the panel was amplitude-modulatedwith a carrier frequency of 2 kHz and then conditioned using an amplifier (HJOPS-1B20, Matsusada Precision Inc.,
Japan). The maximum applied voltage was adjusted forindividual participants and ranged from 200 to 300 V. The
ground of the panel was attached to the middle phalanx of
the index finger using a surgical tape to fully present theelectrostatic friction force.

3.4 Procedures
The participants slid the index finger of their dominant
hand along a 200 mm straight line on the surface of theelectrostatic tactile display, as shown in Figure 2. The participants controlled their own slide speed with the help of
a guide monitor on which a black square moved along a200 mm horizontal line in a reciprocal manner. The blacksquare moved at five constant velocities (50.0, 79.3, 150.0,220.7, and 250.0 mm/s), same as 𝑣 mentioned in Section
3.3. The participants were instructed to keep their finger
motion synchronized with the black square displayed on
the guide monitor.
Stimuli of five different wavelengths (5.0, 7.9, 15.0,22.1, and 25.0 mm) were presented to the participants asthey slid their fingers on the display. The combinations ofλ and 𝑣 were selected from Table 1 in a randomized order.

3. EXPERIMENTAL METHODS
3.1 Participants
Ten paid university students (seven males and three females in their 20s) participated in the experiment after
providing written informed consent.
3.2 Ethical statement
This study was approved by Institutional Review Board,2where 𝑎 , 𝑎 , 𝑎 , 𝑎 , 𝑎 , and 𝑎of their respective terms.

are the coefficients

4. RESULTS
The combination of λ and 𝑣 with the highest mean softness score was λ 7.93 mm and 𝑣
220.7 mm/s. Theapproximated response surface is shown in Figure 3, andthe constants are listed in Table 2. The adjusted coefficientof determination for the response surface was 𝑅
0.81.
The local maximum value of the response surface was at
λ 10.55 mm and 𝑣
198.6 mm/s.

Figure 3: Response surface approximated from the experimental results. The dot plotted on the surface is thelocal maximum point on the surface (λ 10.55mm, 𝑣
198.6 mm/s). Mean value of each of thenine stimuli conditions were used for the computation.

5. DISSCUSSIONS

Table 2: Constants of response surface.

𝑎𝑎𝑎𝑎𝑎𝑎

6.48
10

5.26
10

1.38
10

3.54
10

4.91
100.282

The response surface exhibited the highest score of softness at λ 10.55 mm and 𝑣 198.6 mm/s, and the score
decreased as the distance from this point increased, as
shown in Figure 3. A verification experiment is required
to determine whether this stimulus combination is felt
softer than the other combinations.
According to the response surface, as the 𝑣 values
change, the λ values for achieving the highest softness
values also change. For instance, for 𝑣
50.0, 150.0 and
250.0 mm/s, the response surface exhibited maximums at
λ 4.35, 8.52, and 12.70 mm, respectively. The frequency of friction can be simplified as 𝑓 𝑣/λ, and thecorresponding frequencies are 11.5, 17.6, and 19.7 Hz, respectively. At the maximum softness score, that is, λ10.55 mm and 𝑣 198.6 mm/s, the frequency is 18.5 Hz.
Hence, the effective frequency of frictional stimuli forsoftness presentation may lie between 10 and 20 Hz,aligning with earlier studies [6, 7].
As a limitation of the experimental protocol, the conceptof softness was not precisely defined, and the participants
could have had different images of softness. Cavdan et al.
[17] suggested four types of different softness perceived
by humans. This may have resulted in a greater deviationin participants’ answers.

Each combination of λ and 𝑣 was presented twice for eachindividual participant. Hence, 18 trials (9 stimulus combinations 2 repetitions) were performed for each. Aftereach trial, the participants scored the stimulus using a 10graded scale ranging from 0 (as hard as a glass panel) to 9(soft) to indicate how soft the surface felt. The participants
were asked, “How soft was the surface?” A score of 0 wasdefined as the hardness of the glass touch panel without
an electrostatic stimulus applied. If the surface felt softer
than that at a score of 0, the participants scored it 1 orgreater. The participants experienced the stimulus repeatedly in each trial until they were confident in the score.
Prior to the main experiment, participants completed apractice session. They experienced each stimulus combination once during the practice session. The experiment
took at most 30 min, including instruction and practice. Abreak was taken if required. A pink noise was played
through a headset during the experiment.
3.5 Analysis
The scores for the nine stimulus combinations were normalized within participants by dividing the raw scores by
the highest score. Using the mean of the nine normalized
scores among the participants, a response surface wascomputed with λ and 𝑣 being the explanatory variables
and the softness scores (𝑧) being the objective variable byusing the least-squares method. A total of nine mean values were used to determine the response surface. Themodel of the response surface is as follows:𝑧𝑎 𝜆𝑎 𝜆𝑣𝑎 𝑣𝑎 𝜆𝑎 𝑣

𝑎6. CONCLUSION
In this study, to investigate the effects of stimulus parameters on the softness experienced when sliding a finger
on an electrostatic friction panel, the response surface ofthe subjective softness as a function of the surface wavelength and finger’s sliding speed was experimentally specified. The response surface exhibited the local maximumat λ 10.55 mm and 𝑣 198.6 mm/s, which generated africtional fluctuation of 18.5 Hz. Future studies should33

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ACKNOWLEDGMENT
This study was in part supported by MEXT Kakenhi(23H04360 and 22KJ1587).
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