Tactile Texture Display with Vibrotactile and Electrostatic
Friction Stimuli Mixed at Appropriate Ratio Presents Better
Roughness Textures
KEN ITO, SHOGO OKAMOTO, and YOJI YAMADA, Nagoya University, Japan
HIROYUKI KAJIMOTO, University of Electro-Communication, Japan
Vibrotactile and friction texture displays are good options for artificially presenting the roughness and frictional properties oftextures, respectively. These two types of displays are compatible with touch panels and exhibit complementary characteristics. We combine vibrotactile and electrostatic friction texture displays to improve the quality of virtual textures, consideringthat actual textured surfaces are composed of both properties. We investigate their composition ratios when displaying roughness textures. Grating roughness scales with one of the six surface wavelengths are generated under 11 display conditions,and in 9 of which, vibrotactile and friction stimuli are combined with different composition ratios. A forced-choice experiment regarding subjective realism indicates that a vibrotactile stimulus with a slight variable-friction stimulus is effective
for presenting quality textures for surface wavelengths greater than or equal to 1.0mm.
CCS Concepts: • Human-centered computing → Laboratory experiments; Empirical studies in HCI; • Hardware→ Haptic devices;
Additional Key Words and Phrases: Tactile texture display, vibrotactile display, electrostatic friction display, surface roughness
ACM Reference format:
Ken Ito, Shogo Okamoto, Yoji Yamada, and Hiroyuki Kajimoto. 2019. Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed at Appropriate Ratio Presents Better Roughness Textures. ACM Trans. Appl. Percept. 16, 4, Article
20 (September 2019), 15 pages.
https://doi.org/10.1145/3340961

1

INTRODUCTION

Tactile displays have received a considerable amount of attention with the widespread use of touch panel devices.
This study deals with the presentation of virtual textures using tactile displays installed on touch panels. Tactile
displays suitable for commercial touch panels are divided into vibrotactile-type (e.g., References [2, 11, 17, 38, 57])
and friction-type displays (e.g., References [4, 15, 35, 49, 58]). The former type of display mechanically deforms thefinger pad by driving the touch panel, and the latter type of display controls the panel friction. Vibrotactile stimuli
The work was supported by MEXT Kakenhi under Grants No. 15H05923 and No. 17H04697.
Authors’ addresses: K. Ito, S. Okamoto (corresponding author), and Y. Yamada, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan, 4648603; emails: itou.ken@a.mbox.nagoya-u.ac.jp, okamoto-shogo@nagoya-u.jp, yamada-yoji@mech.nagoya-u.ac.jp; H. Kajimoto, University
of Electro-Communication, 1-5-1 Nunogaoka, Chofu, Tokyo, Japan; email: kajimoto@hc.uec.ac.jp.
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https://doi.org/10.1145/3340961
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20

20:2 • K. Ito et al.
are suitable for controlling the sense of a surface roughness feature. Variable friction displays aim to represent
the friction distribution on a surface and frictional vibration. Both types of displays can evoke a sense of textured
surfaces such as grating scales [2, 38, 55, 58] and roughened surfaces [32]. A question is then naturally raised.
Does the integration of vibrotactile and variable friction stimuli improve virtual textures in terms of realism? For
the perception of fine roughness textures, skin vibration is the important cue [6, 20, 56], and both types of texture
displays can induce skin vibration. Hence, the two types of stimuli may present textures with similar realism,
and the combined stimuli do not substantially improve realism, because both types of stimuli can generate skinvibration. In contrast, it could be argued that the combination of multiple modalities is effective for resembling
realistic surface contact states.
The tactile perception of surfaces is expressed in a multidimensional space [5, 7, 28, 37]. Haptic displays that
control multiple modalities have been developed thus far to artificially present such perception [9, 11, 25, 30,
41, 60]. In this article, texture is referred to as surfaces featuring roughness and friction. Roughness and friction
perception are considered to be perceptually independent [3, 16, 19, 46, 61]; however, friction physically depends
on the surface roughness. Both macroscopic and microscopic surface roughnesses influence the friction forces
when a finger scans the surface [13, 47, 54]. For example, surface asperities influence frictional forces applied to
the finger pad when the finger slides over a surface [12, 14, 24]. Hence, the senses of roughness and friction always
arise when exploring textured surfaces. The roles of roughness and friction for texture perception and material
discrimination have not been fully understood partly because of the difficulties associated with independent
control or the separation of surface roughness and friction. In psychophysical experiments involving multiple
modalities, highly controlled stimuli are generally required; however, decoupling the surface roughness andfriction is demanding. Although lubrication is a method for weakening the adhesion between the finger pad andthe textured surface [1, 52], deformation friction and ploughing are rarely affected by lubrication. Moreover, theeffect of the viscosity of the lubricant must be considered when using lubrication.
The objective of the present study is to investigate whether a texture display combining vibrotactile andvariable-friction stimuli presents better texture stimuli. Thus far, no studies except for our previous ones [22, 23]
have attempted to combine the two types of stimuli for developing quality tactile textures; however, one study
reported a tactile display that could deliver both types of stimuli [41, 43] using vibrotactile and friction stimuli
separately for presenting fine textures and macroscopic surface profiles, respectively. The friction distribution
on a plane leads to the perception of macroscopic surface profiles such as bumps and holes [26, 39, 42]. The
combination of frictional and vibrotactile stimuli embodies a variety of functional haptic feedbacks and improvesuser experiences [31]. In addition, the perceptual interference between two types of stimuli was reported [41, 44]
where intense vibration masked the perception of electrostatic frictional vibration of detection levels. Anotherstudy synchronized electrostatic attraction and lateral vibration to generate a lateral force on a plane [34]. Thesestudies indicated that a joint technique of passive frictional stimuli and active vibrotactile stimuli broadens thepossibilities of tactile displays.
Our previous experiments [22, 23] indicated that the combination of two types of tactile stimulations is effective for improving the realism of virtual roughness textures. In the experiments, participants perceived virtual
grating scales under three conditions: textures presented by vibrotactile stimuli, electrostatic friction, and the
conjunction of vibrotactile and friction stimuli. While comparing the virtual and actual grating scales, the participants selected one of the three conditions in an alternative-forced-choice manner. The condition in which thetwo were combined was judged to be the most realistic by the majority of participants. In this study, we investigate the composition ratio of the two types of tactile stimulations for achieving better realism when the spatial
wavelengths of virtual grating scales vary between 0.5 and 3.0mm. In other words, we compared 11 conditions,
9 of which are conjunction conditions with different vibrotactile–electrostatic composition rates whereas ourprevious studies [22, 23] tested only one conjunction condition. The acquisition of a better composition ratio foreach roughness scale would contribute to the design of the hardware and rendering algorithms of tactile texture
displays.
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Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed • 20:3

Fig. 1. Principles of vibrotactile and electrostatic friction displays (modified from Reference [22]).

2

VIBROTACTILE AND ELECTROSTATIC FRICTION DISPLAYSBoth vibrotactile and electrostatic friction displays can be mounted on a touch panel. The characteristics foreach type of display are summarized below. Previous studies on tactile displays and products mostly use either
only vibrotactile-type or only friction-type displays. As aforementioned, a texture is composed of the surface
roughness and friction features. Hence, the combination of roughness-oriented vibrotactile stimuli and friction
stimuli is a reasonable way to represent texture stimuli.

2.1 Vibrotactile Display
Figure 1(a) shows the principles of a vibrotactile display. A touch panel is mechanically driven or vibrated by aset of high-fidelity actuators such as voice coil motors and piezoelectric devices. A finger pad on the touch panel
is then deformed to induce tactile sensations. Tactile stimuli are typically presented along the normal to thefinger pad. This display is a good option for depicting surface-roughness textures [2, 11, 29, 38, 57]; nonetheless,natural friction accrues between the flat panel and the finger pad when the finger slides on the panel. The
touch panel itself actively produces mechanical stimulation; hence, for presenting tactile stimuli, this display
does not necessarily need active finger movement. However, tactile stimuli synchronized with the movement
are considered to be a more natural form of tactile feedback.

2.2 Electrostatic Friction Display
Figure 1(b) shows the principles of an electrostatic friction display. This display controls the friction between afinger pad and a touch panel using the attractive force caused by the electric charge between the touch panel
and the finger pad. The friction changes by controlling the voltage applied between the finger pad and the panel,and thus the attractive force; then, the variation in the surface friction is simulated on the panel. This display is
generally a good option for depicting a texture dominated by friction [4, 35] and a macrosurface pattern [27, 39].
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20:4 • K. Ito et al.

Fig. 2. Overall structure of the tactile display for vibrotactile and electrostatic friction stimuli.

As shown in Figure 1(b), in our experimental setup, the voltage was applied to the conductive pad to control thecontact condition between the finger pad and the panel for the purposes of psychophysical experiments. Sucha conductive pad also allows us to produce large attractive forces and resultant friction forces with relatively
small applied voltages, whereas the sense of frictional variation is slightly different between conditions with andwithout the conductive pad.

3

APPARATUS: TACTILE TEXTURE DISPLAY USING VIBROTACTILE AND ELECTROSTATICFRICTION STIMULI

We used a tactile display that was developed in our previous studies [22, 23] and is shown in Figure 2. This
display simultaneously presents vibrotactile and electrostatic friction stimuli. We used actuators and amplifiers
that could deliver output stimuli far above the threshold levels for perception, because this was for a laboratory
prototype. Although the apparatus can present a visual image beneath the transparent panel, no textural images
were shown.
Vibrotactile stimuli were produced by four voice coil actuators (X-1741, Neomax Engineering Co. Ltd., Japan,
maximum output force: 6N) located at the four corners of the top panel. The four actuators were synchronously
driven by a current amplifier (ADS 50/5, Maxon Motor AG, Switzerland) and mechanical stimuli were produced
along the normal to the top panel. The mass of the top panel was 283g. The natural frequency of the normal
motion was 25Hz, around which mechanical resonance occurs; however, this range was rarely used in the experiments mentioned later.
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Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed • 20:5

Fig. 3. Grating scale (roughness scale) with a sinusoidal surface.

An electrostatic friction force was generated between the indium tin oxide (ITO) panel layer and the conductive
pad by applying a voltage (Ve ) between them (Figure 2(b)). An insulator (Kimotect PA8X, KIMOTO Co. Ltd., Japan,8μm) was fixed on the ITO plate. Ve was driven by a high-voltage amplifier (HJOPS-1B20, Matsusada PrecisionInc., Japan, maximum output: ±1 kV, response: 75kHz). A circular aluminum pad of 2 mm in thickness and 16 mm
in diameter was introduced to qualify the large frictional forces and regulate the contact condition on the panel
with no respect to individual finger size and its moisture condition. The pad directly transfers vibrotactile stimuli
from the panel to the finger pad because of its rigidity and does not change the experience of vibrotactile stimuli
when both vibrotactile and electrostatic stimuli exist. The surface friction between the panel and the conductive
pad was small, and the coefficient of friction was smaller than 0.1 when the normal load was 1N.
The position and velocity of a finger were detected by four load cells (FS2050-0000-1500-G, TE Connectivity,
Switzerland); each of these was affixed beneath each voice coil motor. The signal from each cell passed throughan analog low-pass-filter circuit to filter the effects of high-frequency noise and the mechanical vibration of
the system. The center of the force on the panel was calculated using the four load cells and considered to be
the finger position on the panel. The resultant positions involved static errors of which standard deviation was
1.0mm after low-pass filtering signal processing.
The two amplifiers and four load cells were connected to a data acquisition board (TNS-6812, Interface Corporation, Japan) controlled by a personal computer at 2kHz. Our display presented tactile stimuli based on thefinger motion, and with no finger movement, no tactile stimuli were provided.

4 EXPERIMENT
4.1 Actual Texture: Sinusoidal Grating Scale
We adopted a sinusoidal grating scale with periodic surface patterns (Figure 3) as reference stimuli during experiments. Trapezoidal or rectangular scales have local edges and are not effectively presented by our setup withthe flat touch panel. The scale was made of ABS resin defined by a spatial wavelength λ and had a wave depth ofh (Figure 3(a)). The effects of the combined texture stimuli depend on λ [22, 23]; hence, we considered six levelsof spatial wavelength (λ = 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0mm). h was 1.0mm when λ = 0.5–2.0 and 2.0mm when
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20:6 • K. Ito et al.
λ = 2.5 or 3.0mm. We determined the depths of the wave such that the finger pad did not reach the bottom of
the scale to avoid the effects of shallow inter-element spaces [50, 52], which our and extant texture rendering algorithms do not address. The surfaces of the grating scales were finely finished. As mentioned later, participants
could freely touch the actual texture as a reference during experiments.

4.2 Virtual Roughness Texture
Normal (vibrotactile) and friction forces were output by the tactile texture display to present the virtual grating
scales. We adopted rendering methods for each type of force from previous studies [2, 55, 60]. The two types ofstimuli were synchronized in a spatiotemporal manner, because they referred to the same temporal and spatial
bases (i.e., t and x in the equations that follow).
4.2.1 Vibrotactile Stimulus. We determined the driving force by voice coil motors as follows to simulate thedisplacement of a sinusoidal grating scale or the skin vibration when the finger pad slides on it:
x (t ),(1)
F v (t ) = A sin 2π
λwhere x (t ) and A denote the position of the finger pad on the panel and the maximum output force, respectively.
This formula determines the vibrotactile stimulus corresponding to finger movement.
4.2.2 Electrostatic Friction Stimulus. When a finger slides on a bump on the surface, the friction force is afunction of the gradient of the surface displacement [14, 42]. In other words, the phase of the frictional force
and surface displacement on a sinusoidal grating scale is π /2 rad. Hence, the applied voltage for the electrostatic
friction stimulation is determined as follows:x (t ),(2)
Ve (t ) = B cos 2π2λwhere B is the maximum voltage. The friction force between the finger and the panel is determined as follows
by the law of electrostatic force and Coulomb’s law of friction:

F e (t ) = μ W + kVe2 (t )
(3)




kB 2
x (t )=μ W +
1 + cos 2π
,
(4)
2
λ
where μ, W , and k denote the coefficient of friction, the normal load of the finger, and a constant of the electrostatic friction force, respectively. k mainly depends on the contact area of the conductive pad and the relative
dielectric constant of the insulator and its thickness. The first term in the braces is related to the finger load, while
the second term is related to the electrostatic attractive force. We confirmed that the friction force followed thisquadratic formula by using the DC voltages by using the method described in Section 4.2.3.1
4.2.3 Stimuli with Different Composition Ratios of Vibrotactile and Friction Stimuli. We focused on the composition ratio of the vibrotactile and electrostatic friction stimuli in this experiment. In other words, we investigatedwhat combination of two stimuli will present a highly realistic texture. For this purpose, two wide-ranging andcontinuous variables, which are the magnitudes of the vibrotactile and friction forces, have to be investigated.
However, classical psychophysical methods are not available in a straightforward manner for such purposes. For
instance, the two-alternative forced-choice task that is one of the most mathematically grounded approaches
produces a large number of combinations to be tested when it is applied to a two-variable problem. Further, the

1 It is likely that the quadratic relationship between the friction force and applied voltage also holds even for AC voltages. We could notinstall the precise force sensing element on the apparatus and measure the dynamic frictional force; however, the peak acceleration values
measured on the conductive pad quadratically changed as a function of B.
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Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed • 20:7

Fig. 4. Combination ratios of vibrotactile and friction stimuli. At level 10, the strength is tuned such that the stimulus is felt
realistic when either of the vibrotactile or electrostatic friction stimuli is presented. The stimulus is just perceivable at level
1 but not present at level 0. The strength is linearly determined in terms of levels 2–9.

method of adjustment is usually to manipulate one parameter. Hence, we adopted an m-alternative forced-choicetask [10, 33, 36, 53] that is an expansion of the two-alternative forced-choice task. We prepared for a set of m
possible composition ratios, and participants selected the one that felt most realistic. This may not be a sophisticated method compared with classical ones especially in terms of statistical tests; however, our objective would
be accomplished by this approach.
First, we tuned gains A and B for each spatial wavelength of the grating scale such that each stimulus wasmost realistic when considering only vibrotactile or electrostatic friction. These values of A and B were regarded
as level 10 for each type of stimulus. The smallest stimulus felt through our tactile texture display was level 1.
These settings were attained through consensus by five experienced people, including those conducting theexperiments. Each of them determined the values for levels 1 and 10 by the method of adjustment. Their averagevalues were then used as the final values. Levels 2–9 were determined by dividing the values of the output force
between levels 1 and 10. In this way, 11 levels of stimuli were prepared for both vibrotactile and electrostatic
friction stimuli. Among the 11 levels, level 0 denoted no stimuli. The stimuli sets for all 11 combinations ofvibrotactile and electrostatic friction stimuli were prepared as follows for each spatial wavelength of the grating
scale: 0:10, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9:1, and 10:0. Figure 4 shows an image of some of these combinations.
The perceived magnitudes experienced from these 11 stimuli were comparable. In contrast, combinations such as
10:10 and 9:9 induced subjective magnitudes greater than the above 11 stimuli set. Similarly, combinations suchas 1:1 and 2:2 were perceived as weak. Hence, stimuli that felt stronger or weaker than the best tuned stimuli(i.e., 10:0 and 0:10) were excluded from the test stimuli.
In our previous reports [22, 23], the number of stimulus conditions was 3. In the present study, we prepared
11 conditions to investigate the effects of the composition ratios in detail, but other numbers of levels can also be
used. We linearly split the stimuli on the basis of physical quantity, i.e., force. Another option is separation basedon perceptual indices, such as a discrimination limen. In the latter case, the stimulus quantities are nonlinearly
divided, while we used linearly separated stimuli.
Table 1 lists the maximum values of the vibrotactile force F v and friction force F e by electrostatic friction
stimuli. The vibrotactile forces were calculated on the basis of nominal torque constant of the actuator andapplied currents. The friction force was measured with DC voltages and a weight of 100 g (W  1 N) placed on
the aluminum pad pulled with a force gauge (DS2-20N, Imada, Japan, nominal resolution: 0.01 N) at 1cm/s. The
values of F v and F e in Table 1 differ depending on the value of λ. A larger output force was needed for large valuesof λ to perceive the texture stimulus. F v was used to drive the panel as well; therefore, the actual vibrotactile
force on the finger depends on the inertial resistance or acceleration of the panel.
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20:8 • K. Ito et al.
Table 1. Maximum Vibrotactile Force F v and Electrostatic Friction Force F e Measured with W = 1 N(a) λ = 0.5 mm
Vb:Fr
10:0
9:1
..
.

F v (N)
2.1
1.9
..
.

F e (N)
0.094..
.

(b) λ = 1.0 mm
Vb:Fr
10:0
9:1
..
.

5:5
..
.

1.26
..
.

0.33
..
.

1:9
0:10
(c) λ = 1.5 mm
Vb:Fr
10:0
9:1
..
.

0.59
-

0.63
0.70

F v (N)
2.7
2.5
..
.

5:5
..
.
1:9
0:10
(e) λ = 2.5 mm
Vb:Fr
10:0
9:1
..
.

F v (N)
2.5
2.3
..
.

F e (N)
0.21
..
.

5:5
..
.

1.7
..
.

0.49
..
.

1.1
-

0.77
0.84

F e (N)
0.35
..
.

1:9
0:10
(d) λ = 2.0 mm
Vb:Fr
10:0
9:1
..
.

F v (N)
2.9
2.74
..
.

F e (N)
0.38
..
.

1.8
..
.

0.63
..
.

5:5
..
.

2.1
..
.

0.7
..
.

1.1
-

0.91
0.98

1.46
-

1.02
1.1

F v (N)
4.2
4.0
..
.

F e (N)
0.55
..
.

1:9
0:10
(f) λ = 3.0 mm
Vb:Fr
10:0
9:1
..
.

F v (N)
5.0
4.8
..
.

F e (N)
0.62
..
.

5:5
..
.

3.4
..
.

0.84
..
.

5:5
..
.

4.2
..
.

0.97
..
.

1:9
0:10

2.8
-

1.1
1.19

1:9
0:10

3.6
-

1.3
1.4

Part of the vibrotactile force is used to drive the top panel (weight: 283g) of the display. Vb. and Fr. indicate vibrotactile andfrictional stimuli, respectively. The applied peak-to-peak voltages were 385, 350, 330, 300, 260, and 200V for λ = 3.0, 2.5, 2.0, 1.5,
1.0, and 0.5mm, respectively, for the 0:10 combination.

4.3 Participants
Fifteen university students (mean age: 23, 12 men, 3 women) participated in the test with written informed
consent. They were not informed of the objective of the research.

4.4 Procedure: An 11-alternative Forced-choice Task
We investigated the composition ratio of the vibrotactile and frictional stimulation perceived to be realistic foreach value of λ. The participants selected the most realistic stimulation among 11 types of stimuli for each value of
λ in comparison with the actual grating scale that they were allowed to experience freely. The 11 types of stimuli
were randomly assigned to keys on a keyboard and could be switched by participants. From observations andinterviews, their common strategies can be summarized as follows. First, participants experienced all stimuli
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Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed • 20:9
and remembered a few stimuli that could be their answers. They were then carefully compared before a final
judgment was made. They could experience virtual and actual stimuli as many times as desired. Typically, one
trial required 2–3min, but the time for each trial was not limited.
We did not intend to strictly control the finger speed on the panel. The finger speed was calculated as thederivative of its position on the panel and shown on the personal computer for control. We specified the finger
speed to be 10–20cm/s, which was monitored by the experimenters during the task, to avoid extremely high or
low speeds2 . Each participant performed 18 trials (i.e., three trials for each of the six values of λ). The order ofthese trials was randomized. No visual images of the grating scales were presented on the texture display panel.
Auditory cues were masked by wearing headphones playing pink noise.

4.5 Results
Figures 5(a)–5(f) show the ratios that the participants selected for the most realistic stimuli for every value of λ.
Figure 5(g) depicts the average over all values of λ.
We tested whether specific composition ratios were preferred over the others for each value of λ. All ratios
should be equally perceived by the participants if no specific ratios were better than the others in terms of being
more realistic. Hence, we conducted a goodness-of-fit test with the null hypothesis that all composition ratios
are equally perceived by the participants (i.e., uniform probability distribution). The null hypothesis was not
rejected when λ was 0.5mm but was rejected for λ = 1.0–3.0mm. Additional details are discussed as follows.
For λ = 0.5, the most selected composition ratio was (vibrotactile:friction) = (6:4). Nonetheless, we did not findany statistically significant nonuniformity in the probability distribution (χ 2 (10) = 16.8, p > 0.05).
For λ = 1.0, the most selected composition ratio was (vibrotactile:friction) = (8:2) followed by (0:10) and (10:0).
The responses were statistically nonuniform (χ 2 (10) = 19.3, p < 0.05).
For λ = 1.5, the most selected composition ratio was (vibrotactile:friction) = (8:2). After that, ratios (10:0) and(7:3) were equally selected. The responses were statistically nonuniform (χ 2 (10) = 25.2, p < 0.01).
For λ = 2.0, the most selected composition ratio was (vibrotactile:friction) = (8:2). The 11 composition ratios
were significantly nonuniform (χ 2 (10) = 29.6, p < 0.01).
For λ = 2.5, the most selected composition ratio was (vibrotactile:friction) = (8:2) followed by (7:3). Stimulus
conditions weighting vibrotactile stimuli were preferred to friction-weighted conditions. The 11 composition
ratios were significantly nonuniform (χ 2 (10) = 24.7, p < 0.01).
For λ = 3.0, the most selected composition ratio was (vibrotactile:friction) = (8:2). Similar to λ = 2.5, thevibrotactile-weighted stimuli were mainly selected by participants. The 11 composition ratios were not significantly uniform (χ 2 (10) = 24.2, p < 0.01).
Figure 5(g) shows the averaged selection of each composition ratio for all spatial wavelengths. Across all valuesof λ, the most selected composition ratio was (vibrotactile:friction) = (8:2), where the selection ratio of 0.22 wasgreater than the chance level (z 0 = 7.5, p < 0.001). The selection of composition ratios 10:0 and 0:10 followedthat of 8:2, which might have been because these two types of stimuli were carefully tuned by experienced
people, whereas the stimulus strengths of the other ratios were automatically determined. In total, the ratios(vibrotactile:friction) = 8:2, 10:0, and 0:10 were selected 59, 39, and 33 times by the participants, respectively.
Post-hoc tests suggested that 8:2 was selected more than 10:0 (z = 2.02, p < 0.05) and 0:10 (z = 2.62, p < 0.01)3 .
The results were divided depending on the spatial wavelengths of the grating scales. The participants werelikely to select 8:2 or a similar ratio when λ was large, whereas such trends were not strong when λ = 0.5mm.
We discuss the possible explanations of these observed responses later.

2 This condition was also employed when determining the values of A and B mentioned in Section 4.2.3.
3 Ideally, we should conduct multiple pair-wise tests among the conditions. However, this overly lowers the value of α for each pair-wise

test. Regarding this point, our m-alternative forced-choice design remains to be improved.
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20:10 • K. Ito et al.

Fig. 5. Experimental results. The horizontal axes denote the composition ratios of the vibrotactile and electrostatic friction
stimuli. The vertical axes denote the frequency selected as the most realistic stimulus. Fr. and Vb. represent friction andvibrotactile stimuli, respectively. The error bars in (g) are the 95% confidence intervals across panels (a)–(f). Asterisks *, **,and *** indicate statistically nonuniform ratios at p < 0.05, 0.01, and 0.001, respectively.

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Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed •20:11

Fig. 6. Results of the follow-up experiment showing the average frequency of each stimulus condition selected as the most
realistic stimulus. Error bars are the 95% confidence intervals.

4.6 Follow-up Experiment
In the above experiment, the composition ratio of 8:2 was significantly selected more than the vibrotactile- andelectrostatic-only conditions, i.e., 10:0 and 0:10. However, one question is whether there exist conditions better
than 8:2 when the sum of the two stimulus levels is not restricted to be 10. As a follow-up experiment, we
compared five types of stimulus conditions near 8:2.
4.6.1 Stimuli and Tasks. Five stimulus conditions were prepared for each value of λ = 1.0, 1.5, 2.0, 2.5, and3.0mm. The stimulus conditions were (vibrotactile:friction) = (8:2), (7:2), (8:1), (8:3), and (9:2). Participants selected
one of the five conditions in terms of realism. Each value of λ was tested three times in total. Hence, 15 trials(three repetitions for each of the five values of λ) were conducted for each individual. Other conditions including
sound masking, comparison with actual grating scales, and random stimulus assignment were the same as thetask in Section 4.4. Ten university students who also participated in the experiments of Section 4.4 participatedin this follow-up experiment.
4.6.2 Results. The selection ratios averaged across the five values of λ are shown in Figure 6. We tested theuniformity of these average selection ratios, because no distinct trends were found across the five values of λ, anduniformity was not rejected (χ 2 (4) = 8.53, p > 0.05). Even post-hoc pair-wise tests found a significant difference
only between the combination ratios for 7:2 and 8:2 (z 0 = 2.63, p < 0.05). These results suggest that stimulus
conditions similar to that of (8:2) were equally selected, and they were not distinguished in terms of realism.

5

DISCUSSION

This experiment was intended to present the texture of a sinusoidal grating scale made of ABS resin. Note thatthe results may not be fully applied for other virtual textures or materials.

5.1 Effectiveness of Either Vibrotactile or Electrostatic Friction Texture Display
for Fine Roughness Texture
For small spatial wavelengths (λ = 0.5mm), the virtual textures of the grating scales presented by each composition ratio were of the same quality in terms of their realism according to the uniformity test of frequencies. In
other words, no specific composition ratio was considered to be more effective than the others.
These results may be explained by the perceptual mechanism of fine roughness textures. The exploration of
actual and virtual fine textures causes high-frequency skin vibration. Whether a quality difference in perception
exists between tangential and normal skin deformations at a high frequency is debatable, although they leadto different perceptual magnitudes or thresholds [8]. Assuming that the direction of the force that causes skinvibration is not significant for the perception of fine roughness and that the frequency and magnitude of skinvibration are rather important, all composition ratios were judged to be of equal quality. In other words, when
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20:12 • K. Ito et al.
the criterion was realism, the participants did not distinguish the vibratory texture presented by the normal
vibrotactile and tangential friction forces.
Nonetheless, for λ = 0.5, the ratio of 6:4 appeared to be more frequently selected than the other ratios, as shownin Figure 5(a). The composition ratios above the chance level were 4:6, 5:5, 6:4, and 8:2. At least, the combined
stimulus did not deteriorate the presentation of fine roughness textures. In terms of even smaller values of λ, oursetup may not be able to precisely deliver the stimulus because of the limited frequency responses of the display
system.

5.2 Effectiveness of Combined Vibrotactile and Electrostatic Stimuli
for Roughness Scales with Large λ Values
The probability of selection for each stimulus was not uniform for grating scales with large spatial wavelengths (λ ≥ 1.0mm) in the first experiment. The ratio of 8:2 was mostly selected for every spatial wavelength.
Stimuli with similar ratios such as 8:3 and 9:2 were equally selected in the follow-up experiment. In other words,
the combination of vibrotactile and slight variable-friction stimuli is effective for presenting texture stimuli preferred in terms of realism, which is consistent with our earlier studies [22, 23], where combined stimuli were
judged to be better for wavelengths of 1.0 and 2.0mm than vibrotactile or electrostatic stimuli but not for wavelengths smaller than 1.0 mm. The vibrotactile and electrostatic stimuli in [22, 23] correspond to 10:0 and 0:10
conditions in the present study, respectively.
For grating scales that are considered perceptually roughness-dominant, the vibrotactile stimuli that havebeen used for rendering roughness stimuli would be reasonable. However, the composition ratio of 8:2, which
has a slight friction variation, would be selected with a high probability, because a variation in the friction force
accrues when the finger pad slides on a real grating scale. This result implies that the deformation of the finger
pad’s skin along the normal direction mainly determined by surface asperities and the tangential deformation
caused by friction may play different roles in texture perception [18, 40]. One-dimensional quantities pertaining
to skin deformation were measured in many studies related to the normal deformation [6, 56], tangential deformation [13, 45, 48, 57], or whole finger vibration [51], where skin deformation and texture perception werediscussed. The information related to deformation or vibration in different dimensions is closely coupled, because
they propagate in the fingertip. Hence, even unidimensional information may be satisfactory to be statisticallylinked with texture perception. However, the normal and tangential directions are different in terms of the mechanical characteristics comprising the mechanical impedance and vibration transfer properties [8, 59]. Normal
and tangential skin surface stimuli have different influences on the mechanical receptors deep inside the skin,suggesting that combined normal and tangential deformations are different from either type of deformation interms of texture perception.

5.3 Further Aspects and Limitations of the Study
Part of our research motivation, which was combining the effects of complementary texture displays, was clarified herein; however, some aspects remain to be studied. Although we used vibrotactile stimuli normal to thefinger pad, it is intriguing to imagine the perceptual effect of the combination of tangential vibrotactile stimuli(e.g., References [21, 57]) and frictional stimuli. Another aspect is the generalization of the findings. Apparently,
the preferred ratios differ among materials. In the case of finely polished ABS plastic used in this study, the ratioof 8:2 was selected; however, other ratios may be selected for sticky or slippery materials. We need a design
guideline, because it is costly to repeat experiments similar to the one conducted in the present study for various
types of materials. Note that inappropriate composition ratios deteriorate the realism of textures, because mostcombined stimuli were less preferred than vibrotactile- and electrostatic-only stimuli in our experiments.
Although we introduced a conductive pad to control the frictional conditions, this pad is unlikely to be used forcommercial applications. Hence, for the practical applications with bare fingers, for example, feedback control of
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Tactile Texture Display with Vibrotactile and Electrostatic Friction Stimuli Mixed •20:13

the frictional force is demanded to ensure that users are subjected to set stimulus forces. Further, the conditionsof slippage may differ between when a fingertip slides on a flat panel and when a conductive pad slides on a flat
panel, which may influence the realism of frictional textures.

6

CONCLUSION

We presented virtual roughness textures with a spatial wavelength ranging from 0.5 to 3.0mm. A tactile texture
display that could present vibrotactile and electrostatic friction stimuli was used. In the experiment, the participants selected the stimulus that they perceived to be most realistic among the 11 types of combined vibrotactile
and electrostatic stimuli. The two extreme stimuli among them were the vibrotactile- and friction-only stimuli,
whereas the other nine stimuli were the combination of the two types of stimuli. The result was different depending on the spatial wavelength. We did not find a significant difference in the response for 11 types of stimuli
for a relatively small wavelength (λ = 0.5mm). In the range of relatively large spatial wavelengths (λ ≥ 1.0mm),
vibrotactile stimuli with slight electrostatic friction stimuli were effective for presenting virtual grating scales.
These findings will contribute to the determination of the specifications of high-quality texture displays.

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Received January 2019; revised May 2019; accepted May 2019

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