This is a preprint of an article whose final and definitive form has been
published in ADVANCED ROBOTICS [2014], copyright Taylor & Francis andRobotics Society of Japan, is available online at:
http://www.tandfonline.com/Article DOI; 10.1080/01691864.2014.913502.

Toward Quality Texture Display:
Vibrotactile Stimuli to Modify Material Roughness Sensations
S. Asano, S. Okamoto, Y. Matsuura, and Y. Yamada,
Nagoya University, Nagoya, Japan
Address correspondence, Department of Mechanical Science and Engineering,
Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan

Abstract
For industrial purposes such as product design, texture displays should deliver a quality senseof touch to users. We have developed a vibrotactile texture display that uses real materials suchas fabric, wood, and leather to enable the presentation of quality textures to users. By applying
two types of vibrotactile stimuli to users’ ﬁnger pads through the materials, their ﬁne and macroroughness sensations can be selectively modiﬁed while maintaining their original perceptual characteristics. This approach is eﬀective for diﬀerent types of textures such as paper, wood, leather,and cloth unless they possess strong damping properties that may attenuate the vibratory stimuli
applied through them.

keywords: Tactile texture display, Vibration stimulus, Augmented reality

1

Introduction

The tactile sensation of materials is an important factor in the design and marketing of various products.
A good sensation imparts expensiveness to the product and increases its aﬃnity to users. We havedeveloped a tactile texture display to virtually present the tactile sensations of material surfaces suchas cloth or wood, and potentially help product designers engineer virtual materials and achieve thedesired tactile sensations. Most texture displays are designed to present either a broad range of textures
typically for virtual reality applications rather than speciﬁc materials with greater precision, or to achievemechanical simplicity of devices (cf. related study). Neither type of device aims at providing a tool for
product design.
The purpose of our research is to investigate the tactile texture display technology to identify anapproach for a device that can deliver the quality adequate for supporting product design. In thisstudy, we propose a vibrotactile texture display approach that involves applying vibratory stimuli of acertain range to real materials to slightly modify the sensation of their roughness without producing

1Figure 1: Vibrotactile texture display with real materials.

a strong sense of vibration. As shown in Fig. 1, in such a method, stimuli are applied to ﬁnger pads
as they scan the material surface. The application of real materials in texture displays enables thepresentation of more realistic material sensations than those done by previously reported techniques.
Moreover, the proposed method can potentially be used with various types of textures by just replacing
the materials. Nonetheless, because earlier vibrotactile texture displays used ﬂat or smooth contactors,
the eﬀectiveness of this approach needs to be conﬁrmed in a situation where the ﬁnger is exposed totwo stimuli sources, namely, the asperities of material surfaces and vibratory stimuli. The idea of using
real materials in texture displays was originally presented by the authors [1], wherein two types ofvibrotactile stimuli could selectively inﬂuence the ﬁne and coarse (or macro) roughness sensations ofmaterials. Experiment 1 of the present study is based on our previous report and demonstrates thatthe roughness sensation of materials can be controlled by vibrotactile stimuli. Experiment 2 conﬁrms
that the tactile sensation of materials is modiﬁed slightly and not completely; i.e., their perceptual
characteristics must be maintained upon the application of vibrotactile stimuli. Experiment 3 veriﬁes
whether the eﬀect of vibrotactile stimuli depends on the type of material. From these experiments, wehave constructed the foundation of the technology that combines vibrotactile stimuli with materials.
Figure 2 shows a possible application of texture displays in industrial design. It presents an imaginary
scenario of two industrial designers discussing textures of cloths used to cover a certain product wherein
one is holding and feeling the cloth material and attempting to deliver his preferences. Although aurally
communicating the feeling in such scenario is diﬃcult, modifying the texture oﬀers potential beneﬁt
as both designers can share their tactile experiences of the texture using a vibrotactile texture display.
Furthermore, although presenting a plethora of cloth samples with diﬀerent textures is also a practicalsolution, the studied technology enables users to interpolate sample textures and makes sharing tactile
experiences more eﬀective when face-to-face or telephonic communication is unfeasible.

2

Related studies

Tactile texture displays are potentially classiﬁed based on the following desired features: (a) applicability
to a broad range of textures and (b) simplicity. However, both categories of devices do not aim to present2Figure 2: Application image for communicating tactile sensation of textures.
textures with the quality adequate to aid product design. The proposed approach is unique in that it
focuses on neither feature but slightly modiﬁes the tactile sensations of real materials while maintaining
their perceptual characteristics.
The ﬁrst major category of devices focused on broad applicability, aiming to present a variety ofmaterials virtually using a single device rather than limited types of materials with high quality. For
example, the tactile interface, incorporating a vibrator and thermal transducer, developed by Caldwell
et al. presented the diﬀerences in the surface roughness, friction, and thermal conductivities of diﬀerent
materials [2]. A similar approach but with pin-array actuators was adopted by Bergamasco et al. [3]. Pinmatrix tactile texture displays controlled the height or pressure distributions of virtual surfaces to present
a variety of surface patterns [4, 5, 6]. Yamamoto et al. [7], Winﬁeld et al. [8], and Wiertlewski et al. [9]attempted to present a broad range of textures by producing either the tangential displacements of the
ﬁnger pads or the frictional forces between them and the texture displays. Vibrotactile texture displays
are representative approaches to create virtual material surfaces by applying vibratory stimulation toﬁnger pads at dynamically variable frequencies and amplitudes [10, 11, 12, 13, 14]. Although thesedevices have superior applicability to a broad range of textures, they are not designed for presenting
even limited types of textures with the quality adequate for aiding product design.
The second major category focused on using simple mechanical structures as devices for presenting
complex texture stimuli. For example, the display developed by Konyo et al. employed vibrotactile approach for presenting the sense of friction [15] whereas more complex mechanical structures are required
generally. Further, slippery or sticky surfaces can be presented by tangentially deforming the ﬁnger
pads [16, 17, 18, 19] without modifying the actual frictional properties of the tactors. With the aim ofpresenting virtual softness simplistically, Bicchi and Scilingo et al. [20, 21], Fujita et al. [22], and Kimura3

et al. [23] proposed displays that controlled the contact area between the human ﬁnger pad and thetactor. Electric tactile displays also have the potential to function as simple texture displays without
any mechanical structures [24, 25]. These devices focus more on device simplicity than the ﬁdelity ofthe physical interactions between real materials and ﬁnger pads.
Finally, Hollins et al. investigated the perception of vibrating roughness gratings [26] and suggested
that vibration increased their perceived roughness. However, they used constant vibrations irrelevant
to ﬁnger motions unlike our methods because their interest was not to control roughness sensations but
test roughness percepts under vibratory circumstances. Moreover, such constant vibrations imposed on
the gratings do not always increase the roughness percepts but may mask and curtail roughness [27].

3Vibrotactile stimuli to alter roughness sensations

In our study, we used two types of known vibrotactile methods to alter the ﬁne and macro roughness
sensations of materials. Fine roughness is experienced while touching surfaces with small, densely
arranged nubs or bumps [28], the sizes of which range from less than hundreds of micrometers to 1 mm.
Such roughness is observable typically in synthetic materials such as woven cloth or paper. In contrast,
macro (or coarse) roughness is experienced while touching surfaces with sparsely arranged, sharp nubs
or edges, a typical attribute of the surfaces of natural materials such as wood or stone [28]. These two
types of roughness are perceived by a diﬀerence in the sensory mechanism (e.g., [26, 29]).

3.1

Method for altering fine roughness using virtual wavy surface

A related study [30, 11] proposed a method to present a virtual wavy surface using vibrotactile stimuli,presenting tactile sensations such as touching a cloth or sandpaper with ﬁne roughness. Figure 3 shows
the schematic representation of the stimuli using this method of modifying the perceived ﬁne roughnessof materials. The displacement applied to the human ﬁnger pad is expressed as( x(t) )y1 (t) = A1 sin 2π
,
λ

(1)

where A1 , x(t), and λ are the amplitude, position of the human ﬁnger, and wavelength of the virtual
wave, respectively. The typical value of λ is set to 0.8 mm. In earlier vibrotactile texture displays, theuse of multiple surface wavelengths has eﬀectively improved perceptual quality [11]. However, in thismethod, real materials are composed of a large set of wave components and such multiplicity of artiﬁcial
waves is not required for improving perceptual quality. As vibratory stimuli are applied according toﬁnger movements, their frequency is directly proportional to the velocity of the movement, determined as|ẋ(t)| /λ. The perceived ﬁne roughness sensation increases with A1 , which is eﬀective at approximately
0.5–2.0 µm.

4

x(t )

Amplitude of A
vibrotactile stimulus

Fingerposition1

λVibrotactile Virtual surface
stimulus
wavelength

Figure 3: Vibrotactile stimulus to modify ﬁne roughness sensations.

3.2

Method for altering macro roughness using impulsive vibrotactile stimuli

Okamura et al. proposed a method using impulsive tactile stimuli [31] for presenting virtual roughness
similar to that perceived while scanning the surface of a material with sharp edges or coarse ridges on aplane. In this case, the perceived roughness is diﬀerent from the ﬁne roughness described in section 3.1.
Figure 4 shows the schematic representation of this method, which used a decaying sinusoidal wave toapply impulsive stimuli. The displacement to the human ﬁnger pad is expressed asy2 (t) = A2 exp(−at) · sin(2πf t),(2)

where A2 , a, and f are the amplitude, damping ratio, and damped natural frequency of the ﬁnger pad,respectively. The value of f was ﬁxed at 200 Hz based on a related study [32]. An increase in A2 results
in an increase in the magnitude of perceived roughness. As this method presumes relatively larger skindeformation, A2 was assigned a value in the range approximately 0.5–2.0 mm, signiﬁcantly greater thanA1 . Furthermore, we set a = 5.0 s−1 so that the sensations are perceptually similar to those of actualedges. However, in practice, such sensations do not strictly rely on f and a, and some deviations are
acceptable. Unlike the ﬁne roughness stimulus, this impulsive stimulus does not depend on the ﬁnger
movement, as it is presented at the moment at which the ﬁnger passes over the virtual projection. For
this study, we set the frequency of stimulation (optionally as the spatial density of the virtual edges) to
1 impulse/10 mm.

4

Vibrotactile texture display

4.1

Mechanical structure of vibrotactile display

The tactile texture display developed through our research presents vibrotactile stimuli using a voice coilmotor (X-1740, Aoyama Special Steel Co. Ltd., Tokyo, Japan, Maximum force: 2.42 N) as the actuator.
Figure 5 shows the schematic representation of its mechanical structure. The parts shown in dark graycolor (acrylic plate and coil) and the replaceable real material were linked and actuated along the y-axis,
with the moving range of the linked parts being 4 mm. The control value to the voice coil motor is5

Fingermovement

A2
Amplitude ofvibrotactile stimulus
Vibrotactile
stimulus
Figure 4: Vibrotactile stimulus to modify macro roughness sensations.

Y

Material
Acrylic plate
Restraint bar
Coil
MagnetAluminum base

X

Figure 5: Schematic representation of vibrotactile display based on voice coil motor.
frequency-shaped so that regardless of the frequency band, the target displacement is outputted [1].
This displacement, set for a load of 1 N (in the range of the typical load applied when a ﬁnger traces thematerial) varied with the load because the latter was not fed back to the control system. For example,
at 50 and 100 Hz, the displacements were, respectively, in the range of 0 to -0.6 dB and 0 to -0.5 dB
for loads of 0.5–2.0 N, with that at 1 N being 0 dB. Although this variation with load might have led
to individual diﬀerences in the results, the consistency of each participant’s answers was not possibly
corrupted given that he/she employed a consistent exploratory strategy during the experiment.

4.2

Control system

Figures 6 and 7 show the schematic representation of the experimental apparatus and block diagramof the display system, respectively. The position of index ﬁnger x(t) of the participant who traces thematerial mounted on the top of the acrylic plate of the display was captured using a camera (PlayStation
Eye, Sony Computer Entertainment, Tokyo, Japan, 320 × 240 pixel, 30 fps) that tracks the red marker
attached to the ﬁnger to determine its position. Both the spatial (0.625 mm) and temporal resolutions ofposition measurement were not adequately precise with respect to the criteria of texture rendering [33].
Hence, the ﬁnger position used for generating the tactile stimulus was fully computed by integrating theﬁnger speed at 5 kHz without any correction for slow sampling. In this method, the change in time series
is consecutive, and hence, the vibrotactile stimuli given by (1) was smooth but did not yield accurate6MicrocomputerFingerposition

CameraRed marker

Current
signalCurrent
driverVibrotactile
display

Figure 6: Schematic of experimental setup.

CameraFingermotionTactile display

FingermotionHumanfingerTactilestimulus

Current
Micro
VCM
computer D/AdriverControl signal Current input(Force command)

Figure 7: Block diagram of vibrotactile display.
position mapping of the real materials and virtual roughness stimuli. However, the present case did
not strictly require a spatial match between real and virtual textures. The latency of feedback fromthe camera was high at 33 ms plus a small computational overhead that was sustainable for real-time
operation. Therefore, the latency can be regarded as fairly smaller than its threshold for tactile texture
feedback, i.e., 40–60 ms [34].

5Experiments for altering roughness sensations of materials

We evaluated ways to modify the perceived roughness through three kinds of experiments. Throughexperiment 1, we validated that the ﬁne and macro roughness sensations could be selectively modiﬁed.
Through experiment 2, we conﬁrmed that it is possible to slightly modify the tactile sensation of thematerial using vibrotactile stimuli while retaining its original perceptual characters. Through experiment3, we checked whether the detection threshold of the vibrotactile stimuli depends on the material andtested their eﬀects on diﬀerent materials. The experiments, including the procedure, methods, andparticipants, obtained approval in the Nagoya University Department of Engineering Ethic Sectional
Meeting.

7

2 mm

2 mm

Cotton

Wood
2 mm

2 mm

Taurillon leather

Tracing paper

Figure 8: Optical microscopy images of materials to which vibratory stimuli were applied

Figure 9: Reference materials. Left: reference for ﬁne roughness. Right: reference for macro roughness.
Top: top view. Bottom: cross-sectional view.

5.1

Materials and participants

Materials with diﬀerent textures were tested to verify the eﬀectiveness of our roughness presentation
methods. As shown in Fig. 8, we used four types of material surfaces: cotton cloth (rough, moderately
soft, and 1.03 mm thick), taurillon leather (uneven, soft, and 2.06 mm thick), wood (rough, hard, and4.88 mm thick), and tracing paper (smooth and 0.196 mm thick). The participants (12 students of the
Nagoya University chosen through a public advertisement) were blindfolded so that their perceptionsof roughness were not biased by the appearances of the material surfaces. They also wore headphones
and listened to pink noises for canceling out the sounds generated by the motor and mounted the redmarker on their index ﬁnger for their ﬁnger position to be measured.

8

5.2

Experiment 1: Selective modification of fine and macro roughness sensations

5.2.1

Stimuli

We investigated four levels each for ﬁne and macro roughness stimuli. For determining the two types ofstimuli using (1) and (2), respectively, the values of A1 and A2 were set to 0.75, 1.0, 1.25, and 1.5 µm
and 0.7, 1.0, 1.3, and 1.6 mm, respectively, across the levels. The participants tested the materials foreach type of stimuli applied to each of the four types of materials, as well as in the absence of any
vibratory stimuli. Thus, each material was tested for the application of nine diﬀerent stimuli, and 36
stimuli (9 stimuli × 4 materials) were randomly presented to each participant (one set). Totally, 144
trials (four sets) were conducted across all individuals.
5.2.2

Tasks

Triggered by a sound cue, the participants explored the surface of the material mounted on the tactile
display. Their exploratory motions were not controlled but monitored during the experiment for any
substantial irregularity. They reported their perceived ﬁne and macro roughness for each stimulus using
the magnitude estimation method, in which they adopted a number according to the perceived roughness
that compared with a reference value. As shown in Fig. 9, we provided ﬁne artiﬁcial lawn grass andembossed paper as the reference for ﬁne and macro roughness, respectively. Both reference values wereset at 50 and provided to the participants once in every 18 trials (half set).
5.2.3

Results

Figures 10 and 11 show the geometric averages of the roughness perceived by the participants upon
application of ﬁne and macro roughness stimuli, respectively. As seen in Fig. 10, an increase in theamplitude of ﬁne roughness stimuli resulted in an increase in the perceived ﬁne roughness (Fig. 10 top)
but not in the perceived macro roughness (Fig. 10 bottom). The application of macro roughness stimuli
also increased only the perception of macro roughness (Fig. 11). The diﬀerences between the perceived
roughness values were tested using a two-way ANOVA, with the vibrotactile stimuli (or the lack of it)
and the type of material as the factors for analysis.
First, we applied ﬁne roughness stimuli with A1 = 1.25 µm and compared the perceived ﬁne roughness of materials with and without stimuli. We concluded that the application of stimuli signiﬁcantly
increased the perceived ﬁne roughness (F0 (1, 280) = 18.12, p = 2.8 × 10−5 , comparison 1) but the perceived macro roughness was not aﬀected (F0 (1, 280) = 1.01, p = 0.315, comparison 2). Furthermore, we
observed the same results for A1 = 1.5 µm; i.e., the application of ﬁne roughness stimuli only aﬀected
the perceived ﬁne roughness.
Next, we applied macro roughness stimuli with A2 = 1.3 mm and compared the perceived ﬁneroughness of materials with and without stimuli. The result indicated that the perceived ﬁne roughness

9

Figure 10: Perceived ﬁne and macro roughness

Figure 11: Perceived ﬁne and macro roughness

values upon application of ﬁne roughness vibra-

values upon application of macro roughness vi-

tory stimuli to real materials. ** indicates a sig-

bratory stimuli to real materials. *** indicatesniﬁcance level of 0.01.

a signiﬁcance level of 0.001.

was not aﬀected (F0 (1, 280) = 0.39, p = 0.53, comparison 3) but the application of macro roughness
stimuli increased the macro roughness sensation (F0 (1, 280) = 17.85, p = 3.2 × 10−5 , comparison 4).
Again, the same observations were made with A2 = 1.6 mm.
The above results indicate that both methods for the presentation of roughness stimuli could selectively modify the ﬁne and macro roughness sensations, respectively.
The gradient of the curve for perceived roughness indicates the sensitivity of roughness perception
with vibration. To understand whether the eﬀects of vibrotactile stimuli depended on the type ofmaterial, we calculated the gradients of the perceived ﬁne and macro roughness in the range of A1 =0.75–1.5 µm and A2 = 0.7–1.6 mm, respectively, for individual participants and investigated theirdiﬀerences across materials. The averages and standard deviations for cotton, leather, wood, and paper,
respectively, were 9.9±3.8, 14.7±6.3, 13.8±7.5, and 13.2±5.7 µm−1 and 10.6±3.5, 8.8±2.6, 11.3±7.0, and11.5 ± 5.0 mm−1 for the perceived ﬁne and macro roughness, respectively, after removing the maximum
and minimum values for each material. A one-way ANOVA revealed no diﬀerences among materials forboth ﬁne (F0 (3, 36) = 1.22, p = 0.32, 1 − β = 0.77) and macro (F0 (3, 36) = 0.66, p = 0.58, 1 − β = 0.77)
roughness sensations, with possible diﬀerences equal to the variances within groups of 5.7 µm−1 and4.8 mm−1 , respectively. These analyses suggested that the diﬀerence in the eﬀects of vibrotactile stimuli
on the diﬀerent types of materials is nonsigniﬁcant for both ﬁne and macro roughness sensations.

10

5.3

Experiment 2: Similarity analysis of textures modified by vibrotactile
stimuli

As described before, the present approach aims to slightly modify the roughness sensations of realmaterials while sustaining their unique textures, and hence, we do not expect that the applicationof vibrotactile stimuli to totally change their textures and cause them to be perceived as diﬀerent
materials. To verify the slight modiﬁcation of textures, this experiment investigated the degree ofperceptual dissimilarity between textures, judged on the basis of comprehensive tactile percepts including
roughness, friction, softness, and vibration sensations. Subsequently, we constructed a multi dimensional
space where the textures were located based on their perceptual characteristics. This analysis enabled us
to capture the human material percepts whereas only roughness percepts were discussed in experiment
1.
5.3.1

Stimuli

For the ﬁne and macro roughness stimuli determined using (1) and (2), respectively, the values of A1
and A2 were set at 1.25 µm and 1.3 mm, respectively. In addition, there was one case of presenting
the material without vibration stimulus. Each of these stimuli was applied to each of the four types ofmaterials and hence, 12 stimuli were applied in experiment 2. We presented paired stimuli from these
12 conditions to participants at random.
5.3.2

Tasks

Similar to experiment 1, triggered by a sound cue, the participants explored the surface of materials
mounted on the tactile display and reported their subjective non-similar degree of perception, that is
also known as perceptual distance or dissimilarity, across the paired stimuli using a numerical value.
We instructed the participants to report zero in the case of same tactile perceptions under paired
stimuli conditions. However, there were no participants who perceived the same stimuli conditions.
As in experiment 1, participants’ exploration was not speciﬁcally controlled but monitored through anexperimenter.
5.3.3

Results

The degrees of dissimilarity reported were normalized within each participant and then averaged acrossall participants. Thereafter, we arranged the 12 stimuli in a Euclidean plane using the Torgerson’s multidimensional scaling method of multivariate analysis to evaluate stimulation in multidimensional space
using the degree of dissimilarity as the distance on the plane, in this case. The coordinates of stimulation
were decided such as to preserve the distance relationship reported by participants. Figure 12 shows
the result of experiment 2, in which the two-dimensional system describes the variance of data withthe contribution ratio being 0.88. The images of the stimuli on the (a) material only (white image),(b) material with the application of ﬁne roughness stimulation (gray image), and (c) material with the11Figure 12: Perceptual locations of materials based on their dissimilarity.

application of macro roughness stimulation (black image) for each material are represented, with theimages of the same material being closely located. The stimuli applied to each material being similar
perceptually, we thus veriﬁed that the tactile sensation of the material was modiﬁed not totally but
slightly using vibrotactile stimuli.

5.4

Experiment 3: Independence of roughness stimuli on materials

In experiment 1, the nonsigniﬁcant dependence of vibrotactile stimuli on the type of material was
veriﬁed using the gradients of perceived roughness upon the application of the stimuli, which speciﬁed theperceptual characters of supra threshold stimuli. In contrast, from experiment 3, we veriﬁed whether thedetection thresholds of vibrotactile stimuli (the minimum amplitude of stimulus required for inﬂuencing
texture perception) depended on the type of material.
5.4.1

Stimuli

We used the method of limits that gradually raised or lowered the amplitude of stimulation to measure
its threshold. The amplitude steps for the application of ﬁne and macro stimulations were set at 0.15 µm
and 0.15 mm, respectively. From the result of experiment 1, the least eﬀect of ﬁne roughness stimuli
was found on cotton cloth material and the most was on taurillon leather, whereas the diﬀerences in theeﬀects of macro roughness stimuli were minuscule. Thus, we compared the thresholds of vibrotactile
stimuli for cotton cloth and taurillon leather by considering four trials (2 materials × 2 applications ofroughness stimuli) as one set, and conducted the experiment with two sets per person.
5.4.2

Tasks

Participants were exposed to materials with paired stimulations (a pair of materials with and without
vibrotactile roughness stimuli) and were asked to report whether they felt the same or diﬀerent roughness
sensations. The amplitude of application of roughness stimulus was raised or lowered when participants

12Figure 13: Perceptual thresholds of vibrotactile ﬁne and macro roughness stimuli. Left: Fine roughness
stimuli. Right: Macro roughness stimuli.
regarded similar or diﬀerent sensations, respectively. For example, after the point when participants
reported same roughness sensations (when the amplitudes of vibrotactile stimuli were adequately small),the amplitudes were continuously raised until they felt a diﬀerence in the perceived roughness, after
which the direction of stimulation was changed from upwards to downwards (or vice versa). This pointof change is called the transition point, and the trial was concluded with ten achievements of this point.
Similar to the previous two experiments, participants’ exploration was not speciﬁcally restricted.
5.4.3

Results

We calculated the perceptual thresholds of ﬁne and macro roughness stimuli for cotton cloth and taurillon
leather as the average of the amplitudes at the transition points obtained in the individual trials.
We obtained 24 thresholds from 12 participants for each combination of material and modiﬁcation ofroughness sensations. We removed the two maximum and minimum values for each combination andapplied the remaining thresholds to statistics. The thresholds of cotton cloth and taurillon leather were0.86 ± 0.24 and 0.81 ± 0.26 µm, respectively, for ﬁne roughness stimuli (Fig. 13 left) and 0.91 ± 0.25
and 1.07 ± 0.37 mm, respectively, for macro roughness stimuli (Fig. 13 right). From the result of the
ANOVA, a meaningful diﬀerence was not recognized between the thresholds of the two materials upon
application of ﬁne (F0 (1, 39) = 0.4, p = 0.53, 1 − β = 0.56) and macro (F0 (1, 39) = 3.08, p = 0.087,
1 − β = 0.56) roughness stimuli, with possible diﬀerences equal to the standard deviations, of 0.25 µm
and 0.31 mm, respectively. The results suggested that the eﬀect of vibrotactile stimulation did not
signiﬁcantly depend on the material. The threshold may rise about the material that clearly damps
vibration (e.g., sponge). However, we do not need to be particular about the eﬀects of material types
used in this study.

13

6

Discussion

In most previous studies, the contactors in vibrotactile texture displays had ﬂat surfaces. In contrast, inthe present study, the contactors were materials, and their surface asperities caused skin deformations
including vibratory signals when being explored by ﬁnger pads. The results of experiment 1, in which
the two types of vibrotactile stimuli successfully modiﬁed the roughness percepts of materials, encourage
the use of vibrotactile display even under the condition of mixing both material- and vibrotactile-derived
skin deformations, and oﬀer scope for further research and development.
Despite functioning well in a series of experiments, the limitation of the proposed method is that thevibrations in the materials attached to the display are apparent when the amplitudes of the vibrotactile
stimuli are signiﬁcantly large. Hence, as described before, our approach is unsuitable for modifying
the sensations of materials to a greater extent. Although the magnitude of the vibrotactile stimulus
used in the experiments was small enough for users to feel the natural material textures, they could
discern vibrations when the stimuli were even moderately increased, and this could have hindered theperceptual quality or realism of textural stimuli. The deﬁnition and measurement of naturalness arecomplex, and there seems to be no general method for specifying such a concept. For example, one of themost reliable and frequently used evaluations to measure the quality of tactile displays is classiﬁcation,in which assessors classify randomly presented stimuli to the type of material that feels most similar
to the simulated texture (e.g., [9, 11, 25]). The ratios of correct classiﬁcations become the criteria of
their quality; i.e., the higher the better. However, because this is applicable for the comparison of realand simulated textures, it is not relevant for the present study. Among such diﬃculty, analysis using
multidimensional perceptual spaces is regarded the most eﬀective and fair approach among known test
methods (e.g., [13, 35, 36]), oﬀering the beneﬁt of testing the similarity between textures from multiple
perceptual aspects or comprehensive human percepts. Hence, it is diﬃcult to supplement the test resultsby explicitly designating perceptual criteria such as roughness or hardness. Although experiment 2 may
not fully advocate the quality of vibrotactile textures, it is regarded to be one of the best measures toverify the eﬀectiveness of our approach.
The introspective reports of the experimental participants led to the suggestion that the perceived
naturalness of the stimuli relies on the combination of material types and virtual wavelength of vibrotactile ﬁne roughness stimuli, considering which we should be cognizant of the dependence of our methodon the type of material, although experiments 1 and 3 revealed no diﬀerences in the eﬀects of vibratory
stimuli on the type of material.
Our methods modify the roughness of materials, which is only a part of the overall characteristics
of tactile textures. In related studies, vibrotactile display methods for other characteristics such as
hardness or softness [11, 30, 37, 38], and friction [15] are discussed. Although our method causes anincrease in the perceived roughness of materials, it can be reduced as desired by using our method withultrasonic wave vibration [8, 39]. The technical applicability of our proposed method can improve when
it is successfully combined with these methods.

14

7

Conclusion

In summary, we have devised a tactile texture display using real materials to present quality tactile
sensations. This approach enables us to modify the roughness sensations of material surfaces and experience realistic textures. We have conﬁrmed that the perceived ﬁne and macro roughness sensationsof materials could be selectively modiﬁed (experiment 1) while maintaining their tactile characteristics
using slight (experiment 2) instead of total modiﬁcation. Furthermore, our approach, which combines
vibrotactile stimuli and real materials, has been found eﬀective regardless of the type of material (suchas cotton cloth or taurillon leather (experiments 1 and 3)). Although these experiments have provided
the base for vibrotactile texture displays using real materials for presenting quality textures, some topics
remain to be studied in the future as the known experimental designs seem inadequate to assert thatthe textural quality of materials are sustainable upon the application of strong vibrotactile roughness
stimuli. Moreover, the two types of vibrotactile stimuli used in this study may not be the best and may
be improved or other eﬀective vibratory proﬁles may be used. One potentially successful method toimprove the naturalness of sensations is using good approximation of physics. For example, it is known
that vibrotactile signals decay with increase in their frequencies [40] whereas in our ﬁne roughness presentation, the amplitude of vibrotactile stimuli was maintained constant across the entire frequency
range. Fortunately, because many researchers are actively working on the development of vibrotactile
texture stimuli, the combination of real materials and vibrotactile texture stimuli has profound application potential in the future. Such display techniques are expected to facilitate the texture design ofcommercial products. Our method enables designers to engineer material surfaces using texture display
by allowing them to adjust textures by carefully selecting the surface ﬁnishing processes or kinds oftextile ﬁbers based on their experience and sharing tactile feelings.

Acknowledgment
This study was in part supported by KAKENHI Shitsukan (23135514 and 25135717).

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