2015 IEEE International Conference on Systems, Man, and Cybernetics

Wearable ﬁnger pad sensor for tactile textures using
propagated deformation on a side of a ﬁnger:
Assessment of accuracy
Shunsuke SATO, Shogo OKAMOTO, Yoichiro MATSUURA, and Yoji YAMADA
Department of Mechanical Science and Engineering
Nagoya University, Japan

Abstract—Measuring the shear deformation of a finger pad
during active haptic exploration of materials is important foranalysis of textural sensations and development of tactile texture
displays. Thus far, there has been no general methods to measuresuch deformations for active touch. To create a sensor system, we
have been developing a new method for estimating the deformation of a finger pad based on the skin deformation propagated to
the radial side of a finger tip. In this study, in order to validate
the method, we compared the deformation or its acceleration of
finger pad estimated on the basis of the acceleration measured
at the radial side and those estimated by the shear force applied
to the finger pad while exploring a few types of materials.
The estimation errors for the deformations at 40–450 Hz were
smaller than human discrimination thresholds, indicating thatthe accuracy of our method is satisfactory compared with humanperceptual sensitivity.
Index Terms—Skin deformation, Skin impedance, Tactile sensor, Texture

Fig. 1.

meter [8]. Wiertlewski et al. measured the interaction force
between the ﬁxed ﬁnger pad and the material that slid on theﬁnger using a high precision force sensing system [9]. Nakatani
et al. estimated load on the ﬁnger pad based on the deformationof the radial ﬁnger skin by applying Poisson effect [10]. Tanaka
et al. measured the skin vibration on the lateral or palmar sides
of a ﬁnger using accelerometers or PVDF ﬁlm attached to theﬁnger tips [11], [12]. Okamoto et al. integrated differential
outputs of two accelerometers to estimate deformation in ﬁnger
pad’s when a handle held by the thumb and index ﬁnger was
pulled away [13]. These methods are not immediately available
for measuring the ﬁnger pad’s deformation under active touch.
For example, some of them can be used only for passive touch
where the ﬁnger and sensory apparatus are attached to eachother. Furthermore, many methods did not refer to ﬁnger pad
deformation but related information such as the vibration ofskin adjacent to the contact part. Therefore, there is no general
method, that can be used under active touch, for estimating
ﬁnger pad deformation thus far.
To solve these problems, we measured the accelerationof skin deformation at the radial side, and then estimatedthe acceleration of ﬁnger pad deformation [14]. Through thistechnique, a wearable sensor for estimating shear deformationof a ﬁnger pad that can be used even under active touch
can be developed because, in principle, it merely requires
an accelerometer attached to the radial side of the ﬁngertip.
We conﬁrmed that there is a high reproducibility of therelationships between the accelerations of the ﬁnger pad and its

I. I NTRODUCTION
Humans feel tactile sensations such as roughness, hardness,and friction by touching materials. The perceptual mechanisms
while sensing material textures have not been fully understood;
deformation of the skin when it comes in contact with materials
and related activities of cutaneous mechanoreceptors intimately
reﬂect texture perception. Therefore, it is important to measure
deformaitons in the ﬁnger pad or forces applied to the ﬁnger
pad to understand human perceptual mechanism and to develop
a technique for tactile texture display. For example, vibrotactile
techniques [1], [2], [3], [4], which are a major approach of
texture displays, present virtual textures by controlling thespectrum of vibratory stimuli. These stimuli are typically basedon the measurement of ﬁnger pad deformation.
However, it is difﬁcult to measure ﬁnger pad deformation
during active tracing of material surfaces. Thus far, direct
measurements have been limited to visual observation of the
ﬁnger pad through transparent materials [5], [6], which cannot
be used for general materials. Therefore, many researchers
have attempted to indirectly measure ﬁnger pad deformations
through alternative methods. For example, Bensmaı̈a et al.
measured the resulting ﬁngertip vibration while sliding onmaterials using a Hall–effect device and magnet attached at
the ﬁnger tip [7]. Manfredi et al. measured the deformationof skin near the contact surface using a laser Doppler velocity
978-1-4799-8697-2/15 $31.00 © 2015 IEEE
DOI 10.1109/SMC.2015.164

Finger pad deformation and its propagation to the radial side.

892

If we consider that a linear relationship holds betweenẍp and ẍs which were actually investigated later, a transfer
function is deﬁned by

Cradle
VCM
Accelerometer

G1 (s) =s2 Xs (s)L[ẍs (t)]=.
s2 Xp (s)L[ẍp (t)](1)

Using this transfer function, from a sample of the skin acceleration at the side of the ﬁnger ẍs1 , a corresponding accelerationof the ﬁnger pad ẍp1 can be estimated byLinear slider

s2 Xp1 (s) = s2 Xs1 (s)G1 (s).

(2)

III. E XPERIMENTAL APPARATUS AND METHODSElectronic scale

Two subjects participated in two types of experiments. In
the ﬁrst experiment, the transfer function of the ﬁngertip was
speciﬁed. In the second experiment, the accuracies of estimates
were compared between the two types of estimations: based on
the radial skin and interaction force to the ﬁnger pad. Finger
pad deformations were separately estimated from these two
types of measured values and will be compared in the later
section. During these two types of experiments, the subjects
maintained accelerometers attached to their ﬁngertips.

Fig. 2. Experimental apparatus for measuring accelerations at ﬁnger pad
and radial side. The solid contactor was attached to the ﬁnger pad using abonding tape. Hence, an accelerometer ﬁxed to the contactor largely measured
the acceleration at the ﬁnger pad.

Accelerometer

A. Specification of transfer function between the finger padand side

Material

Fig. 2 shows an experimental apparatus used for specifying
the transfer function between the ﬁnger pad and side. A
vibration generator was used for deforming the ﬁnger pad. The
vibrator drove a solid ﬁnger pad contactor which was attachedto the ﬁnger pad such that slippage did not occur between theﬁnger pad and the contactor. Hence, an accelerometer mounted
on the contactor recorded the acceleration of the ﬁnger pad
almost accurately. Furthermore, the ﬁnger PIP joint was ﬁxed
using a cradle to regulate the joint movement. The acceleration
at the side of the ﬁnger was measured using an accelerometerattached to the radial skin of the ﬁngertip. The measured
accelerations depend on the attached position. Therefore, thespeciﬁed transfer function also involves the effects of position.
Both accelerations were sampled at 8 kHz. The sinusoidal
vibration was within the range of 1–500 Hz. During a single
measurement cycle, 10 sweeps were repeated; ﬁnger pressing
forces were monitored and maintained at approximately 1 N.

Force sensor

Fig. 3. Experimental apparatus for measuring the shear force applied to theﬁnger pad and the acceleration at the radial side of a ﬁnger.

radial side for a wide frequency range. However, the accuracyof estimations was not examined in [14]. In this study, we
assessed the accuracy of the estimation by comparing theestimation based on the radial skin with that based on thetangential interaction force exerted to the ﬁnger pad during
active touch.
II. P RINCIPLE : E STIMATION OF THE FINGER PAD

B. Simultaneous measurements of shear interaction force andacceleration at the radial side

DEFORMATION BASED ON PROPAGATION AT THE FINGER ’ S
SIDEFor the purpose of validation, we simultaneously measured
the shear force applied to the ﬁnger pad and the acceleration
at the radial side of the ﬁnger tip during active exploration ofseveral types of materials.
As shown in Fig. 3, similar to the previous measurement,
the acceleration at the side of the ﬁnger side was measured
using the accelerometer. The shear force to the ﬁnger pad wasmeasured using a high precision load cell (9217A, Kistler)
installed on a metal plate on which the materials were placed.
The sampling frequency for the load cell and accelerometerwas 8 kHz. The test materials included cotton cloth, aluminum
plate, and two ABS-made grating scales with rectangle ridges

As shown in Fig. 1, the shear deformation of the ﬁnger pad
induces skin deformation on the sides of a ﬁnger. Matsuura
et al. [14] measured accelerations of the shear deformationof a ﬁnger pad ẍp and radial skin ẍs . They observed a
reproducible consistency of the relationships between these
two types of accelerations for a wide frequency range. They
also attempted to establish a physical model of the propagationof skin deformations. However, the attempt was not successful
owing to the signiﬁcant individual difference of the shape ofeach ﬁngertip. In the present study, we employed an approach
independent from any available physical model.

893

Coherency

1

TABLE IM EAN ERRORS BETWEEN THE ACCELERATION ESTIMATED UNDER SHEAR

0.8

FORCES ON THE FINGER PAD AND THOSE ESTIMATED ON THE RADIAL
SIDE . GS STANDS FOR GRATING SCALE .

0.6
Participant
A
B

0.4
0.2
200
300
Frequency [Hz]

400

500

Fig. 4.
Coherency between accelerations of ﬁnger pad and radial side
(Subject A).
1

Gain

10

0

10

0

1

10

10
Frequency [Hz]

10

10

10

0

GS (10 mm)
m/s2
1.3×10−2
9.0×10−3−2−4
Fig. 6.

2

10

Cloth
m/s2
2.7×10−3
2.3×10−3

Cloth
Alminium
Grating scale (1 mm)
Grating scale (10 mm)

10

−1

10

GS (1 mm)m/s2
5.9×10−3
7.8×10−3



2

100

Magnitude of acceleration [m/s ]00Alminumm/s2
4.2×10−3
3.8×10−3

12

10Frequency [Hz]Accelerations of ﬁnger pad estimated by G1 (s) (Subject A).

IV. R ESULTS

Fig. 5. Gain between accelerations of ﬁnger pad and radial side (Subject A).

A. Gain between the finger pad and the radial skinof 1 mm in height and width (1 mm and 10 mm groove widths).
Each subject slid a ﬁnger on the material along the sensitive
direction of the load cell over a range of 50 mm with one
repetition per second for a duration of ten seconds.
1) Estimation of finger pad deformation using shear force
on the finger pad: Here, we describe how to calculate or
estimate ﬁnger pad deformation under the shear force applied
to it. Many earlier studies could at least in part successfullymodel a human ﬁnger pad as a one-d.o.f. spring and damper
system when a shear force is applied to it [9], [15]. This meansthat a linear relationship holds between the shear deformationon the ﬁnger pad and force. Under this approximation, theequation of motion for the shear deformation of the ﬁnger pad
is expressed bymx¨p (t) + cẋ(t) + kx (t) = f (t)Fig. 4 shows the coherency function between the accelerations on the ﬁnger pad and radial skin for subject A. The
coherency was satisfactorily high at 10–450 Hz, in which thesystem is considered linear. In addition, the coherency for
subject B exhibited a similar trend. Hence, we consider thatthe estimation based on the transfer function is valid for thisfrequency range. Fig. 5 shows the magnitude of G1 (s), which
is a gain function between the ﬁnger pad and radial skin, forsubject A. The gain increased up to 80 Hz and decreased at
greater frequencies with another peak around 320 Hz. The gainof subject B was not exactly same as that of subject A.
B. Estimated accelerations of finger pad deformation
Fig. 6 shows the accelerations for the ﬁnger pad that werecomputed by (1) using the accelerations measured at the sideof the ﬁnger. The accelerations for cloth and aluminum wererelatively small whereas those for the grating scales werelarger. This trend is in line with the degree of surface roughnessof these four types of materials.
Figs. 7–10 show the accelerations of ﬁnger pad deformation
estimated by (1) (based on radial skin) and (4) (based on shear
force) when subject A explored each material. Fig. 11 shows
the acceleration for the grating scale (1 mm) for subject B.
Ideally, the accelerations estimated using the two different
methods should match; unfortunately, this was not the case.
In terms of the acceleration estimated for the radial skin (redcurves), the peak commonly appeared at approximately 15 Hz
for all types of materials. Around this low frequency range, theaccelerations for the radial skin (red curves) were larger thanthose estimated by the shear forces (blue curves). In addition,(3)

where xp (t), c, k, and f (t) are the shear deformation of the
ﬁnger pad, skin viscosity, skin elasticity, and the applied shear
force, respectively. In our experimental setup, m corresponds
to the effective mass of an aluminum plate.
The transfer function between f (t) and xp (t) is
G2 (s) =Xp (s)
1
=
.
2
F (s)
ms + cs + k(4)

For subject A, c and k were speciﬁed to be 0.21 N·s/m and
210 N/m, respectively. For subject B, these values were 0.53
N·s/m and 210 N/m, respectively. These values were close
to those reported in [9] and, [15]. Using G2 (s), the shear
deformation of the ﬁnger pad xp (t) is estimated by the shear
force f (t).

894

10

−4

10



1

2

Magnitude of acceleration [m/s ]

−4


1

10

−4

10



1

2

10
Frequency [Hz]



2

2

−2

10

10

−2

10

Fig. 9.
Estimated accelerations on ﬁnger pad for grating scale (1 mm)(Subject A).



0

10

10

Estimated on radial side
Estimated on shear force

10

Estimated on radial side
Estimated on shear force

010Frequency [Hz]Estimated accelerations on ﬁnger pad for cloth (Subject A).

Magnitude of acceleration [m/s ]Fig. 7.



2

−2

10

10

Fig. 8.

Magnitude of acceleration [m/s ]2

Magnitude of acceleration [m/s ]Estimated on radial side
Estimated on shear force

02

Estimated on radial side
Estimated on shear force

0

10

−2

10

−4

10

10
Frequency [Hz]

10

Estimated accelerations on ﬁnger pad for aluminum (Subject A).

1

2

10
Frequency [Hz]Fig. 10. Estimated accelerations on ﬁnger pad for grating scale (10 mm)(Subject A).



2

Magnitude of acceleration [m/s ]further discrepancies between the two methods appeared at afrequency higher than 100 Hz. We discuss the signiﬁcance of
these errors in the next section.
V. D ISCUSSION
Here, we discuss the signiﬁcance of the errors between thetwo methods. Remember that we do not know that both typesof estimation methods are correct. When the two types of
estimates are similar, we can infer that the measured values
are fairly reliable. However, when the errors are signiﬁcant,we can infer that either one or both types of measurements
are unreliable.
Furthermore we discuss whether or not the error is acceptable in terms of the discrimination threshold against vibrotactile stimuli. The discrimination threshold is a noticeable difference between two stimuli. In our case, this is the minimum
difference of vibratory amplitudes between two pure sinusoidal
vibratory stimuli that can be barely discerned. In other words,
the discrimination threshold is perceptual sensitivity to physical stimuli. According to literature [16], [17], the discrimination thresholds of the magnitudes of vibrotactile stimuli areapproximately 10–20% of the detection thresholds, which has
been speciﬁed in [18], for a wide range of frequencies.
Table II and Fig. 12 show the mean absolute errors of ﬁnger
pad deformation estimated by the radial acceleration and shear
forces. The table also lists the errors of accelerations. Meanvalues were calculated across the materials. The discriminationthresholds were calculated on the basis of [16], [17], and [18]and are shown in Table II and Fig. 12 Their general proﬁle is au-shape that reaches the bottom at approximately 200–300 Hz.
The mean errors between the two types of estimations were

Estimated on radial side
Estimated on shear force

0

10

−2

10

−4

10



10

1

2

10
Frequency [Hz]Fig. 11. Estimated accelerations on ﬁnger pad for grating scale (1 mm)(Subject B).

beneath the discrimination thresholds at high frequencies. For
example, the errors between the two types of estimations were
3.8 ×10−3 and 2.3 ×10−3 μm for 300 and 450 Hz, respectively, and thus smaller than the discrimination thresholds. In
contrast, at a few tens of Hertz, the error was comparable to thediscrimination threshold. The error for 30 Hz was 0.89 μm,
which was slightly larger than the threshold of 0.3 μm. For
15 Hz, the error between the two types of estimates was evenlarger (52 μm). The estimation errors at low frequencies are
signiﬁcant.
From our comparisons between two types of measurement
methods, the one using the radial skin is considered satisfactory
within the frequency range of 40–450 Hz.
VI. C ONCLUSION
The future goal of this study is to realize a wearable sensor
to estimate ﬁnger pad deformation while actively touchingmaterials. Thus far, such measurement has been restricted to a895

TABLE II
M EAN ERRORS BETWEEN THE FINGER PAD ’ S ACCELERATIONS AND[2] S. Okamoto and Y. Yamada, “Lossy data compression of vibrotactile
material-like textures,” IEEE Transactions on Haptics, vol. 6, no. 1, pp.
69–80, 2013.
[3] S. Okamoto, S. Ishikawa, H. Nagano, and Y. Yamada, “Spectrum-based
synthesis of vibrotactile stimuli: Active footstep display for crinkle offragile structures,” Virtual Reality, vol. 17, no. 3, pp. 181–191, 2013.
[4] S. Okamoto and Y. Yamada, “An objective index that substitutes for
subjective quality of vibrotactile material-like textures,” IEEE/RSJ International Conference on Intelligent Robots and Systems, pp. 3060–3067,
2011.
[5] M. J. Adams, S. A. Johnson, P. Lefèvre, V. Lévesque, V. Hayward,
T. André, and J.-L. Thonnard, “Finger pad friction and its role in grip
and touch,” Journal of Royal Society Interface, vol. 10, 20120467, 2013.
[6] M. Tada and T. Kanade, “An imaging system of incipient slip formodelling how human perceives slip of a ﬁngertip,” Proceedings ofIEEE International Conference on Engineering in Medicine and Biology
Society, vol. 1, pp. 2045–2048, 2004.
[7] S. Bensmaı̈a and M. Hollins, “Pacinian representations of ﬁne surface
texture,” Perception & Psychophysics, vol. 67, no. 5, pp. 842–854, 2005.
[8] L. R. Manfredi, H. P. Saal, K. J. Brown, M. C. Zielinski, J. F. Dammann,
V. S. Polashock, and S. J. Bensmaı̈a, “Natural scenes in tactile texture,”
Journal of Neurophysiology, vol. 111, no. 9, pp. 1792–1802, 2014.
[9] M. Wiertlewski, J. Lozada, and V. Hayward, “The spatial spectrum oftangential skin displacement can encode tactual texture,” IEEE Transactions on Robotics, vol. 27, no. 3, pp. 461–472, 2011.
[10] M. Nakatani, K. Shiojima, S. Kinoshita, T. Kawasoe, K. Koketsu, andJ. Wada, “Wearable contact force sensor system based on ﬁngerpad
deformation,” Proceedings of the 2011 IEEE World Haptics Conference,
pp. 323–328, 2011.
[11] Y. Tanaka, Y. Horita, A. Sano, and H. Fujimoto, “Tactile sensing
utilizing human tactile perception,” Proceedings of IEEE World Haptics
Conference, pp. 621–626, 2011.
[12] Y. Tanaka, D. P. Nguyen, T. Fukuda, and A. Sano, “Wearable skinvibration sensor using a PVDF ﬁlm,” Proceedings of IEEE World Haptics
Conference, pp. 146–151, 2015.
[13] S. Okamoto, M. Wiertlewski, and V. Hayward, “Anticipatory vibrotactile
cueing facilitates grip force adjustment,” Proceedings of IEEE World
Haptic Conference, pp. 525–530, 2013.
[14] Y. Matsuura, S. Okamoto, and Y. Yamada, “Estimation of ﬁnger pad
deformation based on skin deformation transferred to the radial side,”
Haptics: Neuroscience, Devices, Modeling, and Applications. Springer
Berlin Heidelberg, pp. 313–319, 2014.
[15] N. Nakazawa, R. Ikeura, and H. Inooka, “Characteristics of humanﬁngertips in the shearing direction,” Biological Cybernetics, vol. 82,
no. 3, pp. 207–214, 2000.
[16] J. C. Craig, “Difference threshold for intensity of tactile stimuli,”
Perception & Psychophysics, vol. 11, no. 2, pp. 150–152, 1972.
[17] G. Gescheider, S. Bolanowski, J. Zwislocki, K. Hall, and C. Mascia, “Theeffects of masking on the growth of vibrotactile sensation magnitude
and on the amplitude difference limen: A test of the equal sensation
magnitude-equal difference limen hypothesis,” The Journal of the Acoustical Society of America, vol. 96, no. 3, pp. 1479–1488, 1994.
[18] G. A. Gescheider, S. J. Bolanowski, and K. Hardick, “The frequency
selectivity of information-processing channels in the tactile sensory
system,” Somatosensory and Motor Research, vol. 18, no. 3, pp. 191–
201, 2001.

DEFORMATIONS ESTIMATED BY SHEAR FORCES AND THOSE ESTIMATEDBY RADIAL SIDE ’ S ACCELERATIONS .

Acc. err. m/s2
Disp. err. μm
Discrim. th. μm

15 Hz
0.46
52
0.4

30 Hz
0.00320.89
0.25

100 Hz0.00276.8×10−3
0.04

300 Hz0.014
3.8×10−3
0.013

450 Hz0.018
2.3×10−3
0.022

Discrim. th. indicates the discrimination thresholds [16], [17] towardvibratory stimuli to ﬁnger pad.

Displacement (dB re 1.0 μm peak)

30

Detection threshold
Difference threshold
Displacement error

20
10
0
-10
-20
-30
-40
-50
-60

1

10

100

1000

Frequency [Hz]Fig. 12. Detection threshold (red) [18] and discrimination threshold (blue)
against the magnitudes of vibrotactile stimuli. Estimation error of ﬁnger pad
deformation between the two types of measurement methods is shown as a
black curve. Estimation accuracy is considered valid at the frequency range
for which the black curve is beneath the blue one.

camera-based approach using transparent materials. To achieve
this goal, we use the propagation of skin deformation to theradial side in our novel approach. A linear relationship holds
for a wide range of frequencies between the deformations of
the ﬁnger pad and the radial skin. This enables us to estimate
the deformation of one from the other. Speciﬁcally, the purposeof the present study was to validate the estimation method
in terms of the estimation accuracy. For this purpose, we
compared ﬁnger pad deformations estimated by our methodwith those estimated under shear force applied to the ﬁnger pad
while four types of actual materials were explored. The estimation errors between these two methods in the frequency range
40–450 Hz were smaller than the discrimination thresholds ofvibrotactile stimuli indicating that the errors are perceptuallyminor within this region. However, the estimation errors of thetwo methods were signiﬁcant at lower frequencies for whichfurther research remains to be pursued.
ACKNOWLEDGMENT
This study was in part supported by KAKENHI Shitsukan
25135717 and 15H05923.
R EFERENCES[1] Y. Matsuura, S. Okamoto, H. Nagano, and Y. Yamada, “Multidimensional matching of tactile sensations of materials and vibrotactile spectra,” Information and Media Technologies, vol. 9, no. 4, pp. 505–516,
2014.

896