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from the IEEE."The 2009 IEEE/RSJ International Conference onIntelligent Robots and Systems
October 11-15, 2009 St. Louis, USA

Virtual Active Touch II: Vibrotactile Representation of Friction
and a New Approach to Surface Shape Display
Sho Tsuchiya, Masashi Konyo, Hiroshi Yamada, Takahiro Yamauchi, Shogo Okamoto and Satoshi Tadokoro

Virtual active touchwith a pointer

Abstract— The tactile display for handheld devices requires
compact hardware and useful applications. To satisfy thesepoints, we have proposed the concept of ’Virtual Active Touch’that implements virtual exploration with a cursor on a screen
through a pointing-stick-type tactile interface. The objectiveof this study is to present three-dimensional shapes on a twodimensional screen without force feedback devices. It is possible
to present three-dimensional shapes such as a bump using alateral force [1]. In order to represent human perception ofgeometric surface shape, instead of the lateral force, we use thecutaneous sensation of friction that occurs when a human finger
strokes object surfaces. First, we confirmed that the Virtual
Active Touch interface could present cutaneous sense of friction.
Second, we evaluated the perception of surface height in thecontext of bumped shape induced by the friction display. The
experimental results agreed with our expectation that faster andlonger increases in friction sensation were perceived as higher
bumped shapes.

I. INTRODUCTION
Many handheld devices; such as mobile phones, PDAs,
and portable media players; have become popular in ourlife. Haptic interaction with them is expected to enhancetheir usability and provide strong impressive experiences. To
popularize such haptic interaction, more compact hardware
and more useful applications must be developed.
Recently, touch screens with vibratory tactile feedback
have been developed for handheld devices and several commercial mobile phones have employed them. For example,
Immersion Corp. has developed tactile feedback technology
with vibrations of touch screens [2]. However, actuationof the entire screen has limitation to produce richer tactile
information because large area and heavy mass of the screen
restrict controllability. On the other hand, Luk et al. [3] havedeveloped a handheld tactile display using a compact tactile
stimulator known as STRESS2 [4], which produced discrete
lateral skin stretches. The stimulator was attached on a slidertype controller located on the side of the handheld device.
The tactile feedback was generated in response to movementof the finger placed on the controller. The objectives of
their study were to deliver haptic icons as just symbolic
information.
We have proposed the concept of ’Virtual Active Touch’to overcome the size and weight limitations of a haptic
device [5]. This concept is implemented by applying atactile feedback function to a pointing-stick-type interface
as illustrated in Fig. 1. The pointing-stick is operated by thefingertip and generates tactile feedback. The cursor on the
S. Tsuchiya, M. Konyo, H. Yamada, T. Yamauchi, S. Okamotoand S. Tadokoro are with the Graduate School of Information Sciences, Tohoku University, 6-6-01 Aramaki Aza Aoba, Aoba-ku, Sendai,
980-8579, Japan {tsuchiya, konyo, yamada, yamauchi,

okamoto, tadokoro}@rm.is.tohoku.ac.jp

978-1-4244-3804-4/09/$25.00 ©2009 IEEE

Vibrator ona pointing stickFig. 1.

Concept of virtual active touch[5]screen can perform virtual exploration as a substitute for thefinger without actual hand movement. This approach makes
the tactile stimulator much smaller than the touch-screen typewhile providing richer tactile sensation because a smaller
stimulator can generate complex waveforms rapidly. Our
question was whether Virtual Active Touch could produce thesame tactile sensation as actual hand exploration. We havealready confirmed the performance of Virtual Active Touchfor perceiving roughness [5]. Psychophysical experiments
showed that Virtual Active Touch delivered perceived roughness identical to that provided by tactile interfaces involving
actual hand exploration. However, the suitability of Virtual
Active Touch for other sensations is not yet established.
In this second report, we verify the perception of friction
through Virtual Active Touch.
This study also proposes a new approach to inducing
human geometric shape perception using the proposed friction display. Conventional tactile displays have attempted toreproduce the tactile sensation itself and the tactile quality. In
contrast, our main target is the reproduction of the kinesthetic
sensation induced by the tactile sensation. In particular, we
aim to reproduce human perception of geometric surface
shape induced by the cutaneous sensation of friction. The
perception of geometric shape is regarded as kinesthetic
information [6]. Robles-De-La-Torre and Hayward [1] determined that lateral force between the skin and the surface
is the main cue in perceiving geometric shape through active
touch. If any tactile sensation can substitute for the lateralforce in shape perception, we can present three-dimensional
shapes on a two-dimensional screen without force feedback
devices. The shape display can be applied to representing
GUI icons such as a button and a slider, 3D-maps, movementof shapes, etc. Such technology would drastically expand
the range of tactile applications, compared with conventional
quality-oriented tactile displays.

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In this study, a friction display method using vibratory
stimulation as we have proposed [7] is applied in supportof the above concept. This display method provokes thesensation of friction using tactile vibrations instead of lateralforce. The vibratory stimulation is generated in responseto stick-slip transitions between the skin and the surface
depending on hand exploration speed and normal force. We
expect that cutaneous friction sensation can replace lateralforce in shape perception.
The objective is to verify the possibility of tactile-induced
shape perception using the friction display through Virtual
Active Touch. We confirm the potential of our new approachto presenting three-dimensional surface shapes via tactile
sensation. To use the friction display through Virtual Active
Touch, we confirm that the friction displaying method [7]can be applied in Virtual Active Touch.
In this paper, we first give an overview of the proposed
friction display method and local modifications. Next, we
describe two types of experimental apparatus to evaluate
the friction display through Virtual Active Touch. Finally,
we evaluate tactile-induced shape perception by changing
friction sensations.
II. FRICTION DISPLAY BY VIBRATORY STIMULI
We use a vibrotactile display method that presents friction
sensations to users [7]. The method focuses on stick-slip
contact of finger skin with an object. The method delivers
friction sensations by controlling activity of FA II type tactile
receptors at the moment of stick-to-slip transition using highfrequency vibrations more than 200 Hz, which are selected to
fit the frequency response characteristics of the FA II type
receptor. We use the same stick-slip friction model in theprevious paper and improve a waveform of vibratory stimuli
to produce more natural friction sensation.
A. Stick-slip Friction Model
The stick-slip phenomena of the contact between finger
skin and object surface were simulated by a 1-DOF vibration
model with Coulomb friction, shown in Fig. 2. The model
approximates the dynamic characteristics in the shear direction of a finger pad surface in contact with a flat object.
The model involves stiffness k, mass m and viscosity c. The
finger contacts a flat object with a normal force N , and lateral
friction force F . The velocity of the object is V , and ẋ(t) is
the finger velocity. The contact state of the finger (stick orslip) was calculated based on this model. Motion equation
when stick-to-slip transitions occur is expressed asmẍ + cẋ + k(x − x0 ) = µs N,(1)where x0 is the position of the object when the spring isin its natural length and µ s is the static friction coefficient.
Since the finger pad is sticking with the flat object in its
sticking phase, both the finger pad and object move at aconstant velocity V and ẍ is equal to zero. The details of
the simulation are described in the literature [7].
Stick-slip phenomena depend on the stiffness and thedifference ∆µ between the static friction coefficient µ s andthe kinetic friction coefficient µ k . For example, stick-slip
phenomena are less likely to occur with smaller ∆µ. In

Finger pad

x(t)

k

m
N

F

∆µ

c
Flat object

0Starting pointof stickFig. 2.

x(t)V

1-DOF vibration model of stick-slip motion

order to observe the phenomenon with smaller ∆µ, the model
requires smaller V and higher N .
The parameters (k, m, c) in the friction model affectthe frequency of the stick-to-slip transition. We set theseparameters so that the frequency of the model was close tothat of a real finger pad in contact with a flat acrylic board,which was measured by filming the contact with a high-speed
camera. The selected parameters values were as follows:k = 2.0 [N/mm], m = 1.0 × 10−6 [kg], and c = 1.0 × 10 −5[Ns/mm].
B. Control of Vibratory Stimuli
Fig. 3 shows the approach to controlling vibratory stimuli
for simulating friction sensations adopted in this report. The
finger pad is given vibratory stimuli according to the timingof the stick-to-slip transitions calculated with the stick-slip
friction model. We improved a waveform to have a gradualincrease of vibratory amplitude at the moment of stick-toslip transitions, while the previous report had a step increase
which cause ragged surface sensation.
The intensity of FA II type receptors’ activities is controlled by modulating the amplitude of the applied voltage.
The maximum amplitude A of the voltage applied to avibrator is proportional to elastic energy due to the spring.
From the equation (1), the elastic energy at the moment ofstick-to-slip transition is expressed as
A = B(µs N − cẋ),(2)where B is constant. The amplitude increases as the static
friction coefficient µ s or the normal force N in the friction
model increase. The vibration frequency is constant at 300
Hz. After switch from the stick state to the slip state,
the applied amplitude increases linearly for 5 ms and thendecreases for 30 ms in a quadratic form.
Fig. 4 shows waves of applied voltage (the normal force
N = 0.5 or 2.0). Both the amplitude of the applied voltage
and the period of the stick-to-slip transition are influenced
by N .
III. EXPERIMENTAL APPARATUS
A. Two Types of Tactile Interface
In this research, in order to examine whether or not
friction sensations can be presented using Virtual Active
Touch, a pointing stick that can deliver tactile sensations has
been developed. This pointing stick is an interface which3185

Stick-slip transition
Stick

Slip

Stick

Slip Stick

Direction ofvibration

Slipz

VibratoryA

77 [mm]Appliedvoltage

t

x

z
y

x
Force sensor

Fig. 3.

Control of vibratory stimuli

Amplitude [V]10550 [mm]Amplitude voltage
Envelope curve

Fig. 5.

0
−5
−10
0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Direction
Direction ofofvibration
vibration

Time [s]a) Normal force = 0.5 N
10Amplitude [V]Pointing-stick-type interface

5

y

x

−5

0

z

x

0

−10

Vibratorzz yy

Amplitude voltage
Envelope curve

77 [mm]xLinear slider

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Linear encorderTime [s]b) Normal force = 2.0 N

49 [mm]Fig. 4. Waveforms of the vibratory stimulation corresponding to normal
force changes.
Fig. 6.

manipulates a cursor on a computer screen by a force input.
This interface is named the Pointing-Stick-Type (PS-type)interface. Note that the objective of this study is to confirm
the validity of Virtual Active Touch. Therefore, we gave
more importance to the performance of the experimental
equipment than to miniaturization. We use a vibrator thathas sufficiently large output force (800 N) and a sufficiently
precise force sensor to perform the pointing operation. However, in our past study [7], a thin-model piezoelectric vibrator
was enough to generate stimuli for practical purposes.
Next, in order to compare friction sensation by PS-type toactual touch movement, a Linear-slider-type interface (LStype) that allows a user to move his finger in a straightline has been developed. The LS-type had the same kindof vibrator and the same approach to control of vibratory
stimuli as the PS-type. Therefore the effect of input motion
could be determined.
B. Pointing-Stick-Type Interface
The PS-type is shown in Fig. 5. When an operator applies
a tangential force with his fingertip to the x-axis, the cursor
on a computer screen moves according to the applied force.
In addition, according to the speed of the cursor and theforce along the z-axis, vibratory stimuli are presented to thefinger of the operator.
A contact shoe was installed on top of the vibrator toprovide an adequate contact area between the fingertip andthe vibrator. The shoe was circular with a diameter of 20mm and a thickness of 10 mm. A six-axial force sensor (BLAUTOTECH, MINI2-10) was installed beneath the vibrator

Linear-slider-type interface

to measure the tangential force along the x-axis and thenormal force along the z-axis applied to the vibrator.
We employed a linear transformation between the applied
force f (t) and the cursor velocity v(t). The transformation
equation is expressed asv(t) = αf (t),(3)

where α is the gain. v(t) corresponds to V in Fig. 2. The
computation frequency for calculating the cursor velocity
was 1 kHz. The gain α used in this paper is 316 [mm/Ns]on the basis of a preliminary experiment. In the preliminary
experiment, a participant stroked a virtual object at constantcursor velocity and the constant normal force. The participant
adjusted the gain so that the vibrotactile sensations presented
by the PS-type were similar to the vibrotactile sensations
presented by the LS-type. Three male participants aged
between 20 and 30 performed the tasks. We used the meanof the gains adopted by participants.
C. Linear-Slider-Type Interface
The LS-type is shown in Fig. 6. When the operator places
his finger on the vibrator and moves his hand along thelinear guide, the cursor on the computer screen moves andtactile stimuli are presented to the operator’s finger on thebasis of the cursor velocity and the force along the z-axis.
The vibration actuator was installed on top of the linear
slider, and was moved along the x-axis with touch movement.
The linear guide could slide along the x-axis over a range

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of approximately 200 mm. The position of the vibrator
on the slider was measured by an optical encoder with aspatial resolution of 0.4 µm. The force along the z-axis
was measured by a force sensor (VISHAY, 1004) installed
below the linear slider. There was a contact shoe as in the
PS-type. The velocity of the cursor controlled by the LStype was equal to the actual hand velocity measured by theencoder. The computation frequency required for calculating
the cursor velocity was 1 kHz.
D. Vibrator

Amount of displacement [μm]The vibratory actuators used for the presentation of friction sensations were piezo stack actuators (NEC/TOKIN,
AHB800C801FPOLF). The relationship between the applied
voltage and the displacement of the vibrator was linear, as
shown in Fig. 7. The figure shows the voltage-displacement
plot of the vibrator when it was actuated with a frequencyof 300 Hz, the frequency used in experiments.
10
9
8
7
6
5
4
3
2
1
0
0

Fig. 7.

2

4

6
8
Voltage [V]

10

12

14

Amplitude-frequency characteristics on 300[Hz]E. Visual Stimuli
The image used as a visual stimulus in the experiments
should be of a familiar object such that the every experimental subject can imagine its actual dimensions. Otherwise, thesubjects would not consider the tactile stimuli to be natural.
In our experiments, we used the image of a brick as shownin Fig. 8. Its dimensions as observed on a computer screen
were 120 mm × 60 mm. The cursor manipulated by the

Cursor

60[mm]

120[mm]Virtual object

Fig. 8.

interface was a 6-mm-square. When the cursor slid on the
brick, vibratory stimuli were displayed through the tactileinterfaces.
IV. EVALUATION OF FRICTION SENSATION
THROUGH VIRTUAL ACTIVE TOUCH
We estimated whether or not a friction sensation waspresented by Virtual Active Touch. We compared the friction
sensation presented by the PS-type to that of the LS-type.
A. Task and Procedure
The experiments were performed with both the PS-type
and the LS-type apparatus. The sensory magnitudes of theperceived friction were evaluated as two parameters (thestatic friction coefficient µ s and the difference between thetwo friction coefficients ∆µ ) in the stick-slip friction model
were changed. Magnitude estimation method was applied toevaluate the two types of interface. Participants were asked toassign an arbitrary number to quantify the subjective strengthof the friction sensation after stroking a virtual object on acomputer screen within 30 seconds. Ten values of µ s wereselected from 0.375 to 0.6 at intervals of 0.025. Three valuesof ∆µ = 0.1, 0.2, 0.3 were applied. Thus, one set of friction
conditions contained 10 × 3 combinations. A participant
evaluated one set (30 trials) on both the PS-type and the LStype with a short break. The ordering of the parameter value
combinations was random. In this experiment, the normal
force N had a constant value of 2 N, the average force along
the z-axis when human stroke objects. The participants wore
headphones through which pink noise was played to mask thesounds generated by the vibrator. Five men and one woman
aged from 20 to 40 performed the tasks.
B. Experimental Result
The magnitudes of perceived friction were calculated
as the geometric average of the normalized ratings of allparticipants. Fig. 9 and Fig. 10 show the normalized results
for the two types of apparatus.
According to Fig. 9, the LS-type successfully increased
the perceived friction as the static friction coefficient µ sincreased. This result agrees with a related study [7]. Therefore, the LS-type presents the same friction sensation as in
the related study.
According to Fig. 10, the PS-type successfully increased
the perceived friction as the static friction coefficient µ sincreased as well as the LS-type did. Therefore, the PStype presents friction sensation. However, the increases ofperceived friction from the PS-type were smaller than fromthe LS-type. There is a possibility that the difference between
these two tendencies was influenced by some factors, suchas a weight of vibrator. But, it appears certain that the gain
α of the transformation from the applied force to the cursor
velocity influences perceived texture in the first report [5].
Correlation coefficients between the PS-type and the LStype on for ∆µ series were calculated. Correlation coefficients for ∆µ = 0.3, 0.2, 0.1 were r = 0.595, 0.869, 0.616.
In particular, the correlation coefficient for ∆µ = 0.2 washigher than those for ∆µ = 0.3, 0.1.
As a result, although the sensitivity with perceived friction
increased as the friction coefficient increased in the PStype was different from that of LS-type, the ability of

Visual stimuli

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1.6

Normalized magnitudesof perceived friction

1.4

N

1.2

(a) Perceived bumped slope

0.8
0.6

ObjectΔμ=0.3Δμ=0.2Δμ=0.1

0.4
0.2
0
0.35

Cursor
0.4

Fig. 9.

0.45
0.5
0.55
Static friction cofficient μs

0.6

0.65

(b) Visual information on the screen

Perceived friction for LS-type

Normal load

1.6
1.4
Normalized magnitudesof perceived friction

HeightFriction

1

1.2

N2
k
1

N1

1

L
L0

0.8

L1

L2

L3 Position(c) Normal load function

0.6
Δμ=0.30.4

Δμ=0.20.2

Δμ=0.1

0
0.35

0.4

Fig. 10.

0.45
0.5
0.55
Static friction coefficient μs

0.6

Fig. 11. Tactile height illusion via virtually controlling normal finger-force

0.65

Perceived friction for PS-type

Virtual Active Touch to present friction sensations using thesuggested method for synthesizing vibratory stimulation was
confirmed.
V. EVALUATION OF SURFACE SHAPESPERCEPTION BY FRICTION SENSATION
A. Purpose
We examined the possibility of presenting geometricshapes by changing the friction sensation as an example ofkinesthesia induction by tactile perception. We confirmed
whether geometric shapes could be perceived by changing
friction during touch movement as described in the Introduction.
As an example of geometric shapes, we aimed to present
the shapes of sloped edges of bumps. It appears that the shapeof the slope is perceived as an increasing virtual lateral force
as the friction sensation gradually increases during touch
movement. It also appears that a higher slope is perceived
when either the rate or the span of the increase in friction sensation is larger. The reason is that the lateral force increases
faster when the slope is sharper. Therefore, we examined
whether different heights were perceived depending on therate or span of increasing friction sensation.
B. Task and Procedure
To control the friction sensation, the normal force wasincreased artificially. The method we have suggested forsynthesizing a friction sensation is based on Coulomb friction
model. Therefore, as the normal force increases, a lateralforce (fixing strength) increases. Moreover the period of the
stick-slip transition is influenced. The virtual normal force
was increased from N 1 to N2 at a rate of increase k over aspan L when a user stroked a virtual object on a computer
screen as shown in Fig. 11. The normal force was N 1 = 0.5
on the left side of the virtual object, and was changed to N 2
depending on L and k in the middle of the virtual object.
The normal force N (x) at the position x was expressed as
(L0 ≤ x < L1 )
 N1k(x − L1 ) + N1 (L1 ≤ x < L2 )N (x) =
(4)
 N
(L2 ≤ x < L3 ).
2
The perceived height of a slope was influenced by thechanging normal force. In this experiment, after the normal
force reached N 2 , the normal force did not change.
Six values of the increasing span L = 5, 10, 15, 20, 25, 30[mm] and three values of the rate of increase k =0.05, 0.1, 0.15 [N/mm] were used. Thus, one set of conditions contained 6 × 3 combinations.
A magnitude estimation method was used to evaluate
the perceived height of the slope. Participants were asked
to subjectively assign an arbitrary number to quantify theperceived height within 30 seconds after the moment ofreaching the largest normal force N 2 at the end of the span.
Each participant evaluated one set (18 trials). The orderof the stimulation combinations was random. Participants
experienced maximum L and minimum L for each k before
the experiments. They wore headphones through which pinknoise was played to cover the sounds generated by thevibrator. Five men and one woman aged from 20 to 40
performed the tasks.

3188

Magnitudes of perceived height2

hand movement because no movement of the finger affectthe kinesthetic perception.

k=0.05k=0.1
k=0.15

1.5

1

0.5

0
0

5

10

15
20
Span L[mm]25

Fig. 12.

Magnitude of perceived height

30

35

C. Experimental Result
The magnitudes of perceived height were calculated as thegeometric average of the normalized results from all participants. Fig. 12 shows that the perceived height increased
as L increased and as k increased. However, the perceived
height only increased slightly above L = 15 with k = 0.15;
apparently the perception of shape by friction sensation was
limited by a ceiling.
D. Discussion
These results indicated that the heights of slopes could
be perceived from an increase in friction sensation. Higher
perceived slopes when the rate or span of the increase infriction sensation was higher corresponded to the anticipatedresult. Therefore, it would appear that the friction sensation
presented in this experiment operated similarly to the lateralforce in the method of presenting geometric shapes proposedby Robles-De-La-Torre et al.
From interviews with participants, we confirmed that they
perceived both a bump when their hand moved from left toright and also a dent when their hand moved from right to left
in Fig. 11. It appeared that the heights perceived depended
on both the bump and the dent, because participants moved
their hands from side to side in experiments.
The reason that perceived height only increased slightly
above L = 15 with k = 0.15 was a problem in the methodof synthesizing the friction sensation. A continuous sense of
stroke was not perceived, because the period of the stick-toslip transition of the friction model became too long when
the normal force was too high. More continuous slopes might
be presented by restricting increase of the period of the stickto-slip transition when the normal force is too high.
In this study, participants needed to image the shapesby themselves, because the visual stimulus was constantand the slope was not represented visually. We expect that
it would be easy to present geometric shapes by tactile
sensation combined with visual representations of the shapes
in practical systems. Virtual Active Touch has an advantage
to add visual effects on the screen because the virtual finger
does not hide the screen. In addition, Virtual Active Touch
would be easier to image the shapes represented by thetactile stimulation than tactile interfaces including actual

VI. CONCLUSION
Our target was the reproduction of the kinesthetic sensation induced by the tactile sensation. In particular, we aimed
to reproduce human perception of geometric surface shapeinduced by the cutaneous sensation of friction. This approach
will drastically expand the range of tactile applications,compared with conventional quality-oriented tactile displays.
To use the friction display through Virtual Active Touch,we confirmed that the proposed friction method can beapplied in Virtual Active Touch, which is the concept that
implements virtual exploration through a pointing-stick-type
tactile interface.
First, we compared the perceived friction sensation fromthe pointing-stick-type tactile interface with the linear-slidertype tactile interface, which allows actual hand movement.
Psychophysical experiments showed that the proposed friction display method could deliver perceived friction corresponding to the friction coefficient as well as the linearslider-type with slight differences in sensitivity to changes
in friction coefficient.
Second, we evaluated the perception of surface heightin the context of bumped shape perception induced by thefriction display. The experimental results agreed with our
expectation that larger rates and longer spans of increasing
friction sensation were perceived as higher bumped shape.
These results strongly suggest that the friction sensation
presented here has the same effect as lateral force in the
shape perception reported by Robles-De-La-Torre et al [1].
VII. ACKNOWLEDGMENTS
This research was partially supported by the Ministryof Education, Science, Sports and Culture, Grant-in-Aid
for Scientific Research (B) #18760179, Grant-in-Aid forYoung Scientists (A) #21686024, and by the Ministry ofInternal Affairs and Communications, Grant for the Strategic
Information and Communications R&D Promotion Program#082102006.

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