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2012 IEEE RO-MAN: The 21st IEEE International Symposium on
Robot and Human Interactive Communication.
September 9-13, 2012. Paris, France.

Vibrotactile Display Approach that Modifies
Roughness Sensations of Real Textures
Shuhei Asano, Shogo Okamoto, Yoichiro Matsuura, Hikaru Nagano, and Yoji Yamada
Department of Mechanical Science and Engineering,
Graduate School of Engineering, Nagoya University, Nagoya, Japan
Abstract—In this study, we developed vibrotactile display
methods that can assist designers in product design. In orderto achieve realistic sensations required for such designing
purposes, we used real materials such as cloth, paper, wood,and leather and applied vibrotactile stimuli to modify theroughness sensations of these materials. This approach allowed
us to present textures of various virtual materials with a strong
sense of reality. We verified that our proposed methods could
selectively modify the fine and macro-roughness sensationsof real materials. The methods are expected to aid productdesigners in deciding tactile sensations suitable for theirproducts.
Index Terms—Vibrotactile texture display, Surface roughness
Fig. 1.

Schematic of vibrotactile texture display using real materials

I. I NTRODUCTION
Tactile sensation of materials is known as an importantfactor in the design of various products. Hence, we develop
a tactile texture display that aids designers in productdesign. The tactile texture display is a device that virtuallypresents the tactile sensations of material surfaces, suchas cloth or wood. Such devices potentially enable productdesigners to engineer virtual materials and achieve desired
tactile sensations. However, as described in sec. II, currently,
most texture displays employ either substitutional methods
that produce complex tactile stimuli using simple devices,or those capable of producing a broad range of virtual
textures using a single device. While the former leads tothe mechanical simplicity of devices, the latter, typicallyused in virtual reality applications, is designed to cover abroad range of textures rather than present precise specified
materials. However, since neither of these devices aims
realistic stimuli, designers cannot use them for designing
purposes.
Proposal (Vibrotactile texture display with real materials): We propose a vibrotactile texture approach withquality adequate to assist in product design. To this end,
we apply vibratory stimuli to real materials to slightly
modify the sensation of their roughness. As shown in
Fig. 1, in these methods, such stimuli are applied to finger
pads as they scan the material surface. The applicationof real materials in texture displays is a novel idea thatcan enable the methods to present more realistic material
sensations than those presented through previously reported
techniques. Moreover, the proposed methods can potentiallybe employed with various types of textures by replacing the978-1-4673-4605-4/12/$31.00 ©2012 IEEE

materials.
Applications: This texture display enables productdesigners to experience and decide a tactile feel suitable
for their products. For example, suppose that a designer
considers using wood as the packing material for a certain
product; then, using such texture displays that present a sensation of wood, the designer can experience various woodbased textures, which differ slightly in their roughness, after
which s/he can make an appropriate selection.
Objectives: Thus far, the potential of including realmaterials in combination with vibrotactile stimuli has notbeen studied. Therefore, to the best of our knowledge, thisis the first report establishing this potential. We evaluate
the proposed approach by investigating whether or not conventional vibrotactile techniques can modify the roughness
sensations of a variety of real materials, such as wood
(rough and hard), cotton (rough), leather (soft and uneven),and paper (smooth and hard). These techniques may be
unsuitable for some rough or soft materials, and theireffectiveness depends significantly on the material type.
Hence, we address this concern to establish the potential ofusing real materials in combination with vibrotactile stimuli.
II. R ELATED STUDIESTactile texture displays are classified on the basis of thefollowing desired features: simplicity or the applicabilityof the device to a broad range of textures. However,devices under both these categories do not aim to present
textures with quality sufficient to aid in product design. The
proposed approach is unique in that it focuses on neither of1001

these features. Instead, our approach slightly modifies thetactile sensations of real materials while maintaining theirperceptual characteristics.
The first category of devices focused on broad applicability, where a single device aims to present a variety of materials virtually, rather than to present limited
types of materials with high quality. For example, Caldwell et al. developed a tactile interface using a vibrator
and a thermal transducer and demonstrated that the interface transferred the differences in the surface roughness, friction, and thermal conductivities of different materials [1]. Pin-matrix tactile texture displays represented
by Ikei et al. [2] and Kyung et al. [3] were aimed atpresenting a variety of surface patterns with limited spatial
resolution. Yamamoto et al. [4], Winfield et al. [5], andWiertlewski et al. [6] attempted to present a broad rangeof textures through either the tangential displacements offinger pads or the frictional forces generated by the interactions between finger pads and the material surfaces.
Although, these single devices have superior applicability
to a broad range of textures, they are not designed forpresenting limited types of textures with quality sufficient
for aiding in product designs.
The second category focused on simple mechanicalstructures that could be applied as devices for complextexture stimuli. For example, Konyo et al. developed a friction display that used vibrotactile stimuli [7]. The display
presented vibration stimuli to the finger pads that mimicked
stick-slip phenomena observed while exploring frictionalsurfaces. With the aim of presenting virtual softness simplistically, Bicchi et al. [8] and Fujita et al. [9] proposed
displays that controlled the contact area between the humanfinger pad and a tactor. Further, slippery surfaces could
be presented even without modifying the actual frictional
properties of the tactors, by tangentially deforming thefinger pads [10], [11], [12]. Finally, electric tactile displays
have also been reported to have the potential to function
as simple texture displays without any mechanical structures [13]. These texture displays aimed to present textureswith mechanically simple devices. They focused more on
device simplicity than on the fidelity of physical interactions
between real materials and finger pads.
III. V IBROTACTILE STIMULI TO ALTER ROUGHNESSSENSATIONS

We use two types of conventional vibrotactile methods to alter the fine and macro-roughness sensations ofmaterials. We define the fine-roughness sensation as theroughness experienced by touching surfaces with small nibs
or bumps that are densely arranged. The sizes of thesenibs/bumps range from less than hundreds of micrometers
to 1 mm. The roughness of such a fine surface is mediatedmainly by rapidly adapting mechanoreceptors [14], [15].
Fine roughness is observable typically in synthetic materials
such as woven cloth or paper. In contrast, we define macroFig. 2. Schematic representation of application of vibrotactile stimulus
for modifying fine-roughness sensations

Fig. 3. Schematic representation of application of vibrotactile stimulus
for modifying macro-roughness sensations

roughness as roughness resulting from sparsely arranged
sharp nibs or edges; it is a typical attribute of surfaces ofnatural materials such as wood or stone.
A. Method for altering fine roughness using virtual wavy
surface
Konyo et al. proposed a method for presenting vibrotactile roughness using a single vibratory source [16], [17]. In
this method, a virtual wavy surface with wavelength of λwas considered to present fine roughness. Fig. 2 shows aschematic representation of this method. The displacement
applied by the tactile display to the human finger pad isexpressed as x(t) ,y1 (t) = A1 sin 2π
λ(1)where A1 , λ, and x(t) are the amplitude and wavelengthof the virtual wave, and the position of the human finger,
respectively. We set λ = 0.8 mm for typical fine roughness.
In this method, vibratory stimuli are applied in response
to finger movements. Therefore, the frequency of vibratory
stimuli provided to the finger pads varies with the velocity
of movement of the human finger. This frequency is determined as |ẋ(t)|/λ. As the velocity of the finger movement
increases, the vibratory frequency of finger pad skin also
increases. Furthermore, the virtual roughness perceived byhumans increases with A 1 . Larger A1 produces stronger
roughness sensations. We employ this method for altering
fine roughness sensations.

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Fig. 4. Schematic representation of vibrotactile display based on voicecoil motor
Fig. 5.

B. Method for altering macro roughness using impulsive
vibrotactile stimuli

Schematic of experimental setup for vibrotactile display

Okamura et al. proposed a method for presenting virtualroughness using impulsive tactile stimuli [18]. The roughness presented by this method is similar to that by scanning
the sharp edges on a plane. This roughness sensation iscompletely different from the fine roughness describedabove. Fig. 3 shows the schematic representation of thismethod. We model the impulse as a decaying sinusoidal
wave. The cutaneous displacement applied by the tactile
display is expressed asy2 (t) = A2 exp(−at) · sin(2πf t),(2)

where A2 , a, and f are the maximum amplitude of theimpulse, damping ratio, and damped natural frequency ofhuman finger pad. The value of f was set at 200 Hz on
the basis a previous study [19]. An increase in A 2 results
in an increase in the magnitude of perceived roughness. We
set a = 5.0 s−1 so that the sensations become perceptuallysimilar to those of the actual edges. We employ this method
as the method for altering macro-roughness sensations.
According to this method, impulsive stimuli are provided
to the finger pads when they come across a virtual edge.
In this study, we set the spatial density of virtual edges as
1 impulse per 10 mm. This density can be set arbitrarily
unless it becomes too dense. When the density is extremely
high, the stimuli of individual impulses may overlap witheach other and the sensations perceived by this method will
no longer represent macro roughness.
IV. V IBROTACTILE TEXTURE DISPLAY
A. Structure and usage
1) Mechanical structure of vibrotactile display: Thevibrotactile texture display used in this study was based on avoice coil motor (X-1740, Aoyama Special Steel Co. Ltd.,Tokyo, Japan). The maximum continuous-output force ofthe motor was 2.42 N with a current of 0.96 A. Fig. 4 shows
the schematic of mechanical structure of our vibrotactile
texture display. The parts shown in dark gray and the realmaterial were rigidly linked and actuated along the y-axis.
They include the real material, acrylic plate, and coil. The
moving range of these parts was 4 mm. The real material

Fig. 6.

Block diagram of experimental setup of vibrotactile display

could be replaced. The remaining parts (light gray), i.e., themagnet, base, and restraint bars, were earthed. The restraintbars allowed the coil to move along only the y-axis.
2) Control signal flow of system: Figs. 5 and 6 show theschematic of the experimental setup and the block diagramof the display system, respectively. The user senses thematerial mounted on the top panel of the tactile display.
The position of the user’s hand, x(t), was captured usinga camera (Playstation Eye, Sony Computer Entertainment,
Tokyo, Japan). The camera tracked a red marker attached tothe user’s index finger to determine the finger position. In
our system, the spatial resolution of position measurement
was 0.625 mm. The refresh rate of the tracking system
was set to 30 Hz. A microcomputer determined the target
displacement given to the finger pad based on (1) and (2)and x(t) and the corresponding current values for the voicecoil motor. The motor was then driven by a current driver.
B. Compensation for frequency response characteristics ofvoice coil motor and finger pad
In order to ensure application of the set of target output
displacements to the finger pads, we compensated for thefrequency characteristics of human finger pad and thevoice coil motor system. Using a FFT analyzer (CF-7200,
Ono Sokki Co., Ltd., Yokohama, Japan), we measured thefrequency response with the input and output being theapplied electric voltage and the finger pad displacement,
respectively. The finger pad was placed on the acrylic panel
1003

Fig. 7. Frequency response characteristic of vibrotactile display with
output force of 0.425 N

Targetdisplacement

Voltage

G-1 (s)Force

Current

Current
driver

Voice coilmotor

Fig. 9. Frequency response characteristics of tactile display and finger
pad before and after application of compensation

the macro roughness, because the vibrotactile frequency
remains constant in this case.

Displacement
Finger

V. E XPERIMENT FOR ALTERING ROUGHNESSSENSATIONS OF REAL MATERIALS

G(s)Fig. 8. Block diagram of vibrotactile display system compensating for
nonlinear frequency responses of the finger pads and voice coil motor

linked to the voice coil. The finger displacements were measured using a laser displacement meter (CD5-30, OPTEX
FA Co. Ltd., Kyoto, Japan), where the laser was directed
toward the finger pad through a hole on the acrylic panel.
During measurement, the contact force between the finger
and the plate, determined using a scale, was maintained at
approximately 100 g. This force is comparable to that withwhich humans typically explore material surfaces.
The measured frequency response characteristics areshown in Fig. 7. We used the following second-ordertransfer function that well approximated the characteristicsof the high-frequency band (more than 5 Hz). The transfer
function was
6 × 104.
(3)
G(s) = 2
s + 800s + 106
As shown in the block diagram (Fig. 8), we applied theinverse of this function to compensate for the nonlinear
frequency responses of the finger pad and the voice coilmotor.
Fig. 9 shows the frequency characteristics before andafter the compensation. The dotted line indicates the characteristics before the compensation in which the voice coilmotor generated the constant force amplitude of 0.75 N
whereas the solid one indicates those after the compensation. The observed characteristics were not nearly flat atless than 5 Hz because the transfer function for the approximation did not match well in the low-frequency band.
However, this compensation is valid in our setup because
it does not include stimuli in such a low-frequency band.
We apply this compensation to the method for presenting
the fine roughness, wherein the vibrotactile frequency varies
according to the velocity of the human hand. We do not need
to apply this compensation to the method for presenting

We verified the effectiveness of our methods for presenting roughness sensation for real materials through experiments. We investigated whether the two methods forpresenting vibrotactile stimuli could selectively present thefine and macro-roughness sensations.
A. Participants and materials
Real materials with different textures were tested to verify the effectiveness of our roughness presentation methods.
As shown in Fig. 10, they were wood (rough and hard),
cotton cloth (rough and moderately soft), taurillon leather(uneven and soft), and tracing paper (smooth and hard).
Owing to the preliminary nature of this study, we decided
to invite four volunteers from the authors’ laboratory. (Notethat the authors did not participate in these experiments.)
They were blindfolded, wore headphones, and listened topink noises for shutting out the sounds generated by thevoice coil motor.
B. Stimuli
We investigated four levels for each of the fine andmacro-roughness stimuli. For the fine roughness stimuli,
determined using (1), the amplitudes of vibrotactile stimuli(A1 ) were set at 1.0, 1.3, 1.5, and 2.0 µm. For the macroroughness stimuli, determined using (2), the amplitudes(A2 ) were set at 0.5, 1.0, 1.5, and 1.8 mm. Each of thesestimuli was applied to each of the four types of materials.
Furthermore, the participants also tested the materials in theabsence of any vibratory stimuli. Hence, each material wastested following the application of nine different stimuli.
Thus, 36 stimuli (nine stimuli × four materials) wererandomly presented to each participant (one section). In all,
144 trials (four sections) were conducted for individuals.
a) Tasks: Triggered by a sound cue, the participants
explored the real materials mounted on the tactile display.
They reported their perceived fine and macro-roughness
for each stimulus using the magnitude estimation method,where the magnitude of perceived roughness, reported as1004

Fig. 12. Perceived fine and macro-roughness values upon application offine-roughness vibratory stimuli to real materials

Fig. 13. Perceived fine and macro-roughness values upon application ofmacro-roughness vibratory stimuli to real materials

a number, was compared with a reference value. For fineroughness, the reference material was fine artificial lawn
grass, as shown in Fig. 11, that is typically used fordioramas or miniatures. This reference value was set at
50. Similarly, the roughness of an embossed paper thatis also shown in Fig. 11 was used as a reference macroroughness. This value was also set at 50. These referenceswere provided to participants after every 18 trials (halfsection).

Fig. 10. Optical microscopy images of real materials to which vibratory
stimuli were applied in experiments

Fig. 11. Reference materials. Left: reference for fine roughness. Right:
reference for macro roughness. Up: top view. Bottom: cross-sectional view.

C. Results
Figs. 12 and 13 show the geometric averages of theroughness perceived by the participants upon application offine and macro-roughness stimuli, respectively. As shownin Fig. 12, an increase in the amplitudes of the fineroughness vibratory stimuli increased the perception of fineroughness. However, such an increase was not observed in
the perceived macro-roughness. In contrast, an increase in
the amplitudes of macro-roughness stimuli increased theperception of only the macro-roughness (Fig. 13).
The statistical relevance of the differences between
the perceived roughness values was tested using two-way
ANOVA. The vibrotactile stimuli or the lack of it and thetype of material were used as factors for the analysis.
First, we tested the effects of application of fineroughness vibrotactile stimuli. We compared the perceived
fine roughness of the materials in absence of the vibrotactile
stimuli and that perceived upon application of stimuli with
A1 = 1.5 µm (Comparison 1). From the analysis, we concluded that the application of fine-roughness stimuli significantly increased the perceived fine roughness (F 0 (1, 88) =10.23, p = 1.92 × 10−3 ). In contrast, the perceived macroroughness was not affected (F 0 (1, 88) = 1.69, p = 0.20,

1005Comparison 2). Furthermore, we observed similar resultsfor A1 = 2.0 µm (data not included).
Next, we tested the effects of the application of macroroughness vibrotactile stimuli. We compared the perceived
fine-roughness (Comparison 3) and macro-roughness (Comparison 4) measured upon the application of the macroroughness vibrotactile stimuli with amplitude of A 2 =1.5 mm with that in the absence of the stimuli. While themacro-roughness vibrotactile stimuli did not significantly
affect the perceived fine-roughness (F 0 (1, 88) = 0.78, p =0.38, Comparison 3), it did significantly affect the perceived
macro-roughness (F 0 (1, 88) = 20.38, p = 1.96 × 10−5 ,Comparison 4). Again, similar observations were made with
A2 = 1.8 mm.
The above analysis results indicate that the two methods
for the presentation of roughness stimuli could selectively
modify the perception of fine roughness and macro roughness.

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VI. C ONCLUSION
To the end of realizing tactile texture displays with
sufficient quality, we devised a novel tactile display thatuses real materials. We applied vibrotactile stimuli to realmaterials such as wood, cloth, leather, and paper, andmodified their roughness sensations. To the best of ourknowledge, this is the first attempt of using real materials
in combination with vibrotactile texture stimuli. Hence,
we testified the applicability of conventional roughness
presentation methods to real materials through experiments.
As a result, we confirmed that the perceived fine andmacro-roughness could be selectively modified. Various
vibrotactile texture methods, other than those described in
the present study, should also be verified in the future
for establishing techniques that assist product designers inselecting tactile sensations for their products.
Finally, one may concern the visuo-tactile cross-modal
effects on our approach. For example, the users can feela sheet of paper very rough with the vibratory stimuli
applied whereas it looks as smooth as the ordinal paper.
Such discrepancy between visual and tactile sensations
may poorly reflect on the achievement of our approach.
We suppose that small discrepancy between two modes is
acceptable, however the limitation of the approach remains
to be clarified.
ACKNOWLEDGMENT
We thank Prof. Susumu Hara, Nagoya University, forproviding valuable advices. This study was supported inpart by MEXT KAKENHI (23135514).
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