Preliminary demonstration of friction perception
generated by Laterally symmetric vibrotactile
stimuliAkihiro Imaizumi, Shogo Okamoto, and Yoji Yamada⋆

Abstract. Laterally asymmetric vibrotactile stimuli produce anisotropic
or direction-dependent friction force that function as a friction display
system [1]. In this study, we showed that laterally symmetric vibrotactile
stimuli increase friction perception by a similar principle. We measured
the frictional forces between a sliding finger pad and a contactor laterally vibrating at 3 or 5 H. We found that lateral vibrations led to zero
relative velocity between the finger pad and contactor, which resulted
in frequent sticking between the two bodies. Consequently, humans perceived stronger friction when scanning the vibrating contactor than when
scanning a stationary contactor.
Keywords: Texture display, Skin stretch, Friction, Vibrotactile

1

Introduction

Recently, we observed that perceived friction increased when human subjects
scanned a contactor that asymmetrically vibrated along the lateral direction offinger pad [1]. We measured the impulse of the frictional force applied to thefinger pad sliding on the contactor, and found that it was larger for sliding in
one direction than for the other owing to the anisotropic frictional condition produced by asymmetric vibration. This anisotropy was also verified in a perceptual
experiment. A possible explanation of this phenomenon is that the finger pad
sticks more frequently to the contactor when sliding in one direction, and thatthe sticking phase produces a frictional force larger than the sliding phase.
In principle, even symmetric vibration (e.g., a sinusoidal wave) could cause
more frequent sticking than a non-vibrating condition. In this study, we experimentally tested this possibility. We measured the frictional forces between thesliding finger pad and the contactor, when the latter vibrated at 0 (no vibration), 3 and 5 Hz. By comparing the frictional forces among these conditions, weconcluded that laterally symmetric vibrotactile stimuli could cause stick-and-slip
phenomena that led to increased friction and its perception. Although, some earlier studies [2–6] on the relationship between percepts for vibration and friction
have been conducted, no study appears to have increased friction perception bylaterally symmetric vibration.
⋆Department of Mechanical Science and Engineering, the Graduate School of Engineering, Nagoya University. This study was in part supported by MIC SCOPE(27-J-J6238).

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Imaizumi, Okamoto, & Yamada

Fig. 1. Lateral vibrotactile display based on a DC motor and equipped with a loadcell. The finger scans the contactor along the direction of vibration.

2

Experimental setup and Procedure

2.1

Lateral vibrotactile display

We constructed a vibrotactile display based on a DC motor (RE-40, Maxon)
with an encoder (Type L, Maxon), as shown in Fig. 1. The contactor, which wasmade of finely polished ABS plastic, was fastened to the motor through a loadcell (Model 1004, Tedea Huntleigh). We estimated the frictional force between
the finger pad and contactor by subtracting the inertial force of the contactor
from the force measured by the load cell.
2.2

Measuring frictional force

We measured the frictional forces while a voluntary participant slid his or her
finger along the contactor. Three contactor conditions were tested : vibrating
at 3 Hz, vibrating at 5 Hz, and stationary. Each participant repeatedly scanned
the contactor under these conditions in one direction for a sustained amount oftime. Fig. 2 shows two samples of measured frictional forces. For comparison
purposes, the frictional forces for the still contactor (solid curve) and those forthe vibrating contactor (dotted curves) are plotted on the same figure. Notethat the figures also show forces while the finger was not in contact with thecontactor. These non-contact phases are indicated by the periods with negligible
friction forces.
The frictional forces of the still contactor (solid curves) indicate that stickand-slip phenomena hardly occurred when the contactor was slid by the finger
because the fluctuations in friction were minuscule. Although some exceptions
were observed at the beginning of sliding motions, the measurements corroborated the notion that kinetic friction was dominant for the still plastic contactor.
In contrast, the frictional forces (dotted curves) between the vibrating contactor and finger pad significantly fluctuated. This suggests that the sticking andFriction force [N]Friction force [N]Title Suppressed Due to Excessive Length0.4A) 3 Hz

0.2

Still contactor(no vibration)

0Vibratingcontactor

0.43B) 5 Hz

0.204567Time [s]8910Fig. 2. Frictional forces when the finger scanned the contactor with and without vibration (A : 3 Hz, B : 5 Hz). During periods in which the frictional force was nearlyzero, the finger was not in contact with the contactor. Sticking was more frequently
observed between the vibrating contactor and finger than between the still contactorand finger. For visual clarity, components larger than 10 Hz have been filtered out.

slipping modes alternatively appeared. Given that the maximum static friction
is greater than the kinetic friction, the frictional forces observed with the vibrating contactor were largely greater than those observed with the still contactor.
Furthermore, such sticking between the finger pad and vibrating contactor wereperceived clearly by all those who experienced the laterally vibrating stimuli.

Finger velocity

Velocity000 000 000 000 000 000 000 000 000

000 000 000 000 000 000 000 000 000

Finger pad’s velocity
Contactor velocity
000 000 000 000 000 000 000 000 0000TimeStickbegins.

Friction

Frictional force

Time0
Stuck

Stuck

Stuck

Fig. 3. Velocity of the finger and vibrating contactor and the frictional force betweenthem4Imaizumi, Okamoto, & Yamada

3Discussion and Conclusion

Larger frictional forces were observed when scanning the vibratory contactor
than when scanning the still contactor. Fig. 3 shows a conjectural interpretationof this observation. In general, when the relative velocity between the two bodies
is non-zero, they are sliding. As described before, slippage was predominant when
the finger slid on the still contactor. On the other hand, when the finger slid on
the vibrating contactor, the sign of the relative velocities periodically switched.
This switch would have led to sticking of the two bodies and caused greater
friction. The friction increased, synchronizing with the occurrence of sticking orvibration because the maximum static friction is greater than the kinetic friction.
Our interpretation is supported by the fact that the frictional forces observed inFig. 2 showed frictional fluctuations whose frequencies were consistent with thevibratory frequencies of the contactor.
In summary, we speculate that when a finger slides on a laterally vibrating
contactor, sticking occurs between the two bodies and perception of this frictionis amplified. We will pursue the principles underlying this phenomenon in futurestudies.

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