Friction Sensation Produced by
Laterally Asymmetric Vibrotactile Stimulus
Akihiro Imaizumi, Shogo Okamoto, and Yoji Yamada⋆

Abstract. Vibrotactile texture stimuli have commonly been used to produce sensations of roughness. The extension of such stimuli to other textural modalities
enhances their applicability. We found that laterally asymmetric vibrotactile stimuli cause a sensation of friction rather than vibration. When a vibrotactile contactor moves in one direction, it sticks to the finger pad and induces lateral skinstretch. In contrast, when the contactor moves in the other direction, it slips because of its quick motion and induces little skin stretch. As a result, humans experience frictional sensations in scanning vibrating contactors with their fingertips.
We examined participants’ subjective responses and measured interactive forces
between the finger pad and the contactor. Both perceptual and physical experiments corroborated the hypothesis of the production of a sensation of friction.
Laterally asymmetric vibrotactile stimuli increased stretching of the finger padskin and increased the sensation of friction.
Keywords: Texture display, Skin stretch, Friction, Vibrotactile

1Introduction

Tactile texture displays have been extensively studied as a user interface technologythat complements visual and auditory displays. Texture sensations consist of multiple
modalities, including roughness, softness, friction, and thermal sensations. A texture
display that addresses more modalities is considered to be more valuable. In the field ofvibrotactile texture displays, which have been studied extensively because of their easeof use and commercial availability, many research groups have developed display techniques for roughness sensations. In addition, researchers have studied that the accurate
representation of vibratory signals that are generated by rubbing materials impart theirtextures (e.g., [1]). If friction sensations can be conveyed in addition to these sensations,broader types of textures can be represented by vibrotactile stimuli.
The objective of this study was to examine how to increase friction sensations using
vibrotactile stimuli. Although many studies on frictional vibration between fingers andtextures have been conducted [2–4], there has been little research on the production of
the sensation of friction using vibrotactile stimuli. This is partly because a mechanismwith an unlimited motion range is necessary to present friction sensations when the contactor of a tactile display is traced by a finger in one direction and a friction force works
continuously in the opposite direction. Hence, spherical contactors that rotate infinitely
in one direction [5] and skin stretchers that do not cause slippage between a finger pad⋆ Department of Mechanical Science and Engineering, Nagoya University. Nagoya, Japan. This

study was in part supported by KAKENHI Shitsukan (25135717).

2Imaizumi, Okamoto, & Yamada

Fig. 1. Laterally asymmetricvibration

Fig. 2. Friction produced by rubbing asymmetrically vibrating contactor with fingertip

and a stimulator [6, 7] have been adopted in practice. In addition, the squeeze effect [8,9] and static electric friction [10] have been used to increase or decrease friction. A
unique vibrotactile method was developed by Konyo et al., to convey friction sensations using vibratory stimuli normal to a finger pad and hence simulate high-frequency
stick-slip vibration [11]. However, vibrotactile approaches to induce lateral skin stretching for conveying friction sensations have yet to be reported (see also the last paragraphin Section 2).
As described in Section 2, we asymmetrically vibrated a tactile contactor or stimulator in a direction tangential to a finger pad to induce the sensation of friction between
the finger pad and the contactor. The effectiveness of this type of asymmetric vibrotactile stimulus in producing the sensation of friction was evaluated using a psychologicaltest and friction force measurement.

2

Assumed principle: Friction sensations produced by laterallyasymmetric vibrotactile stimuli

Humans perceive friction from the shear deformation of the finger pad [6]: a larger shear
deformation evokes a sensation of greater friction. We separated the frictional conditions of a finger pad moving across a contactor into two phases: sticking and slipping
phases. Static and kinetic friction forces act during the sticking and slipping phases,respectively. Naturally, the finger pad stretches more in the sticking phase. We can also
induce friction sensations by manipulating the finger pad stretch. In this section, we
describe how laterally asymmetric vibration may preferentially cause stick or slippage.
As Fig. 1 shows, we assume that the contactor vibrates asymmetrically, moving
more quickly to the right, and more slowly to the left, whereas the maximum displacements are the same in both directions. When the fingertip moves rightward across thecontactor, and the contactor moves quickly, the relative velocity between the fingertip
and the contactor is sufficiently large for them to slip and hardly stick (Fig. 2, Case A).
In contrast, when the contactor moves slowly leftward, sticking frequently occurs because the relative velocity between the fingertip and the contactor is sufficiently small

Friction display by Asymmetric Lateral Vibrotactile Stimuli

3to cause sticking (Case B). Hence, humans feel more friction in Case B and less friction
in Case A. As a result, a sensation of friction is induced rather than a sensation of vibration. We call these combinations of fingertip movements and asymmetric contactor
motions “sticking conditions.”
When the fingertip moves leftward, the shear deformation is moderate. When thecontactor moves quickly, the finger pad deformation is small because the slipping phases
is dominant owing to the largely relative velocity between the two bodies (Case C).
When the contactor and the fingertip move in the same direction at similar velocities,
the skin stretch is minimal because of the small relative displacement between the fingertip and the contactor (Case D). As a result, in these two cases, humans perceive weakfriction sensations. We call these conditions “slipping condition.”
Also, in cases A and D, the direction of finger deformation may become the sameas the finger motion, which never happens in scanning still surfaces. Under such cases,
the sense of friction is weakened [12]. Hence, we assume that the phenomenon reported
in this article cannot be fully explained by physics but also by perceptual effects.
The overall idea is somewhat similar to the method developed by Chubb et al. [9].
They combined a tangential vibration and ultrasonic vibration to selectively produce
shear forces in one direction. Although they used ultrasonic vibrations to reduce frictions when the contactor moves in one direction, we use fast lateral displacement of thecontactor to reduce frictions. As a result, our method takes advantage of asymmetriclateral vibrations whereas the earlier study used symmetric ones. Asymmetric vibratory cues were also used for a different purpose in the field of haptics. They provide
directional force sensations to a hand in which a vibratory source is grabbed [13, 14].

3

Experimental setup

3.1

Tangential vibrotactile display

We used a vibrotactile display based on a DC motor (RE-40, Maxon) with an encoder(Type L, Maxon), as shown in Fig. 3. The contactor was made of finely polished ABS
plastic that was fastened to the motor through a strain-gauge load cell (Model 1004,Tedea Huntleigh). This load cell measured the inertial force of the contactor and thefrictional force between the contactor and a finger pad. We estimated the friction force
by subtracting the inertial force, which is the product of the inertia of the contactor andits acceleration, from the measured force values.
3.2

Laterally asymmetric vibrotactile stimuli

Fig. 4 shows an example of the displacement and velocity of the asymmetric vibrotactile stimuli and the estimated friction force between the finger pad and the contactor.
The displacement of the contactor was similar to a saw-tooth profile with the contactor
moving in one direction with a peak speed of approximately 90 mm/s and in the otherdirection with a peak speed of 40 mm/s. The peak-to-peak displacement of the contactor was approximately 3.5 mm, and its frequency was set at 5 Hz. The motion of thecontactor was feedback-controlled to maintain asymmetric motion even during contact

4Imaizumi, Okamoto, & Yamada

Fig. 3. Lateral vibrotactile display

Fig. 4. Saw-toothed vibrotactile stimulus and estimated frictional forces between finger pad andcontactor

with the finger pad. We determined the feedback gains using the linear quadratic integral design method. It should be noted that asymmetric motions are not necessarilyensured with large disturbance forces.

4 Experiment: Perceptual and physical measurement of friction
under laterally asymmetric vibrations
4.1

Methods

We tested the differences in the friction and frictional sensations between the stickingand slipping conditions in two types of experiments.
Five blindfolded and auditory-masked volunteers participated in paired comparisontasks. They were unaware of the objectives of the experiment. They scanned the contactor leftward and rightward for 10 s and identified the direction that corresponded to a

Friction display by Asymmetric Lateral Vibrotactile Stimuli

5stronger sensation of friction in a forced-choice manner. Ten trials were conducted foreach volunteer, with the direction of asymmetric vibration randomly changed between
trials.
After these tests were completed, we measured the frictional forces between theparticipants’ finger pads and the contactor as each of the participants touched the contactor, using the load cell installed on the contactor. The participants slid their fingertips
across the contactor in each of the two directions for 10 s.
4.2

Result: Differences in perceived friction between the two conditions

Table 1 shows the ratios at which the participants felt that the sticking condition produced more friction than the slipping condition. Overall, the sticking condition wasjudged to produce more friction than the slipping condition (t0 (4) = 4.81, p = 0.0085,
two-tailed paired t-test), except for participant 1. As described later, this participant experienced exceptionally substantial frictional forces for both conditions, potentially due
to the large forces in the normal direction with which he pressed the contactor.
4.3

Result: Frictional force

Fig. 5 shows the estimated frictional forces measured during two trials for participant 2.
The left and right figures correspond to the sticking and slipping conditions, respectively. During the masked periods, the frictional forces were small for a sustained periodof time, which means that the participant’s finger was not in contact with the contactor.
The data for these periods were not used in the calculation of the statistics reported
later.
Both the magnitude and duration of friction that the participants experienced may
affect the participants’ perception of friction; therefore, we calculated the impulse offriction forces (integral of friction over time) as a point of reference. Table 2 shows
the impulses calculated for a single trial, typically eight strokes for 10 s, under thetwo conditions. These impulses were larger under the sticking condition than under theslipping condition, except for participant 1, for whom unusually large frictional forceswere measured. We excluded the data for participant 1 from the statistics because of
their inconsistency with the results for the other participants. Although conclusions
drawn from the results should be viewed with caution because of the small number ofparticipants, the trend observed was statistically valid (t0 (3) = 7.98, p = 0.0021, twotailed paired t-test).

5Conclusions

In this study, we examined the friction sensation caused by vibrotactile stimuli. We assumed that asymmetric vibrotactile stimuli cause anisotropic shear deformation of thefinger pad and influence the perception of friction, although such mechanism should bediscussed further. We controlled the motion of the contactor to ensure a saw-toothedlike displacement profile that produced an asymmetric vibrotactile stimulus. The participants felt stronger friction when scanning the contactor in one direction than in the6Imaizumi, Okamoto, & Yamada

Table 1. Answer ratios in forced-choice taskof friction between sticking and slipping conditions.

Table 2. Impulse of friction. Averages andstandard deviations were calculated for participants 2–5.

Participant Answer ratioNo.
(stick > slip)
10.6
20.8
31.0
40.9
51.0
Ave. ± S.D. 0.86 ± 0.17

Participant
Impulse [N·s]No.
StickSlip
1
2.14
2.41
2
0.44
0.31
30.40
0.23
40.30
0.20
50.98
0.83
Ave. ± S.D. 0.53 ± 0.31 0.39 ± 0.30

Fig. 5. Sample of friction forces

other direction. In addition, the impulses of the friction forces between the participants’finger pads and the contactor were different between these two directions. This approach to producing the sensation of friction using vibrotactile stimuli broadens theapplicability of vibrotactile texture displays.

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