2019 IEEE 8th Global Conference on Consumer Electronics (GCCE)

Shear Stress Sensor for Soft Material with
Built-in Piezoelectric Polymer Films
Fengyu Li, Yasuhiro Akiyama, Xianglong Wan, Yoji Yamada, and Shogo Okamoto
Graduate School of Engineering
Nagoya University
Nagoya, Japan

Abstract—Currently, it becomes more important to anticipate
the intention of the user who touches the product. However, incase of the soft material, the internal deformation and stress
distribution, which is the information of the way of touch, arehard to be estimated nor observed using current interactive
shear force sensors except for limited situations. In this study,
we developed a sensor system, which is embedded in the soft
material and measures shear deformation and stress distribution
inside of soft material. The validity of this sensor system, which
consists of the array of piezoelectric sensors of Polyvinylidene
fluoride (PVDF), was tested by measuring the stress distributionof an artificial human skin.
Index Terms—Embedded sensor, Soft material, Shear stressmeasurement, Piezoelectric sensor

A. Measuring principle using PVDF film
PVDF, which was manufactured in the form of micro film,
was applied as sensing element. The PVDF films embeddedin the soft material is deformed like a beam along with thedeformation of the substrate material. The electric charges qi
induced by piezoelectric property vary along with different
local stress [6]. Based on the idea that the sensor films arearranged inside of soft substrate with different embedded
lengths as illustrated in of Fig. 1 (b), the three-dimensional
distribution of stress can be obtained.
Substrate material

PVDF films

I. I NTRODUCTION
Most of current shear force sensors are set at the contact
surface or under the contact area. Thus, they cannot measure
the condition inside of the material. Especially for the soft
material whose structure is sometimes anisotropic or multilayer, the measurement of the internal deformation and stress
distribution is difficult to embed current shear force sensors.
Furthermore, it is also difficult to estimate the internal stress
distribution of such complex structure.
However, for instance, the observation of the distribution ofshear force inside of soft tissue is demanded in the field ofcontact safety [1], [2] such as the anticipation of bedsores [3],[4] and friction blister when using a device which touches thehuman skin [5].
Therefore, in this study, a shear deformation and stresssensor system with piezoelectric polymer films embedded
in soft material was developed. The designing principle andprocess will be firstly introduced, and the actual measurementof the shear force distribution in different underneath depth
will be followed. It is also expected to be applied to theorganic robotics, such as artificial skin of humanoid robot
or prosthesis, or simulated organ for surgical practice andpathology.
II. S TRUCTURE OF SENSING SYSTEM
The basic structure of shear stress measuring system usedin this study is shown in Fig. 1 (a). The sensing elements ofPVDF are fixed perpendicularly to the surface and embeddedin the soft substrate material.

Testbed and support(a)‫ݍ‬ଵ ‫ݍ‬ଶ ‫ݍ‬ଷ ‫ݍ ڮ‬௜(b)Fig. 1. Shear force measuring device

PVDF is usually attached to flexible plate such as PET sheet,
whose rigidity decides the PVDF’s deformation behavior.
Thus, the performances of 3 kinds of sensor elements with50, 25, and 16 µm thickness of PET sheets were compared.
As a result, the thinnest one, whose flexibility was highest,
was selected because of the highest sensitivity.
B. Substrate material and testbed
In this paper, a kind of artificial skin (HITOHADA GEL,EXSEAL Co., Ltd., Japan) with ASKER-C hardness of 0
was utilized as the substrate material. Using this substrateand PVDF film, a testbed inlaying clamps matching films’
thickness was manufactured as shown in Fig. 1 (a). All thesensors are supported vertically without being clamped thesensing area, and then electric wiring from the pins for signaltransmission is done below the testbed.

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978-1-7281-3575-5/19/$31.00 ©2019 IEEEIII. S HEAR STRESS DISTRIBUTION MEASUREMENTIN DIFFERENT UNDERNEATH DEPTH

Manipulator

To test whether this system can observe the deformation andstress distribution of substrate, an experiments were conducted.
At the same time, the effect of the distance of adjacent PVDF
elements in horizontal direction was also evaluated.

Acrylic plate

V+Sensor signal
V-Charge amplifier circuit board

B. Experimental result
The clear and highly sensitive sinusoidal output signal could
be observed. Thus, the peak to peak value of the output signal,
which was obtained as the sinusoidal wave, was used as therepresentative parameter for comparison among conditions.
Firstly, the signal differed obviously among embedded lengths
as shown in Figs. 4 and 5. The shape of graphs became
approximately an exponential form, which reflects the mechanical characteristics of soft material. This trend verifies thefeasibility for deep directional force distribution measurement
as mentioned below.

pk-pk charges [pC]Interval :15mm8 mm
6 mm

A4 mm

ComputerFig. 3. Experimental system

50010 mm

Strain info

Plastic testbed

The artificial skin material was manufactured in a thicknessof 12 mm. 25 PVDF films are arranged inside it. They weredivided into 5 groups with different intervals from wide tonarrow in 10, 8, 6, 4, and 2 mm respectively. In each group,5 PVDF films are aligned and differ in the embedded lengthsof 10, 8, 6, 4, and 2 mm respectively from left to right. The
arrangement is schematically illustrated in Fig. 2.
The dynamic simple shear deformation, whose direction wasorthogonal to the PVDF film, was applied to the surface of theexperimental device. The dynamic simple shear deformation
was exerted in sinusoidal pattern with amplitude of 2.5 mm
and frequency range of 0.5 to 5 Hz based on the previous
study, which measured the interaction force and deformationof skin surface when wearing an physical assistant robot [7].
This surface deformation was applied with a manipulator(MOTOMAN-MH5F, Yaskawa Electric Co., Ltd., Japan) as
shown in Fig. 3. The part which contacted to the substratematerial surface was designed as a square acrylics board insize of 80 mm×80 mm at the central position, and could cover
all the elements. It should be noted that the contact part did
not slip on the substrate.

Manipulator control

A. Experiment Setup

A

PVDF films

Artificial skin

2 mmInterval 10mm
Interval 8mm
Interval 6mm
Interval 4mm
Interval 2mm

400300200100

Artificial skin 100×100×12 mm302468

1012Embedded length of PVDF films [mm](a) Top view

Artificial skin0Acrylic plate
PVDF films
Plastic testbed

Embedded length : 10 8 6 4 2 mm(b) A-A sectional front viewFig. 2. PVDF films’ arrangement

Fig. 4. Signals differ with embedded length among different intervals in 2Hz

Considering the effect of interval between sensing elements,according to Fig. 4, any bad effect was not observed even incase of interval of 2 mm compared to the condition of 10mm interval. Instead, Fig. 4 suggested that the group locatedcloser to central area generated larger charges. Furthermore,
the output signals of the pairs symmetrically located sidewaysof 6 mm, the intervals of 10 and 2 mm, 8 and 4 mm, matchedeach other.

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soft material. These results suggest the feasibility of proposedsensor system.

pk-pk charges [pC]

400
0.5Hz
1Hz
1.25Hz2Hz
2.5Hz
3.125Hz
5Hz

300

200

ACKNOWLEDGMENT
This paper was supported by JSPS KAKENHI Grant Number 16H03134.
R EFERENCES

1000024681012Embedded length of PVDF films [mm]Fig. 5. Signals differ with embedded length among different frequencies of
2 mm interval

Then, the result of signal variation among different frequencies is shown in Fig. 5. The decrease of signal in the deeper
section of high frequency condition suggested the attenuationof stress wave propagation from surface through the softmaterial [8]. This trend coincides with the transmissibility
characteristics of viscoelastic soft material.

[1] T. Jacobs and G. S. Virk, “ISO 13482-the new safety standard forpersonal care robots,” ISR/Robotik 2014; 41st International Symposium
on Robotics, pp. 1–6, 2014.
[2] Y. Akiyama, Y. Yamada, and S. Okamoto, “Interaction forces beneath
cuffs of physical assistant robots and their motion-based estimation,”
Advanced Robotics, vol. 29, no. 20, pp. 1315–1329, 2015.
[3] J. E. Grey, K. G. Harding, and S. Enoch, “Pressure ulcers,” Bmj, vol.
332, no. 7539, pp. 472–475, 2006.
[4] R. M. Allman, “Pressure ulcers among the elderly,” New England Journalof Medicine, vol. 320, no. 13, pp. 850–853, 1989.
[5] X. Mao, Y. Yamada, Y. Akiyama, S. Okamoto, and K. Yoshida, “Safetyverification method for preventing friction blisters during utilization ofphysical assistant robots,” Advanced Robotics, vol. 31, no. 13, pp. 680–694, 2017.
[6] J. Sirohi and I. Chopra, “Fundamental understanding of piezoelectricstrain sensors,” Journal of intelligent material systems and structures,vol. 11, no. 4, pp. 246–257, 2000.
[7] Y. Akiyama, Y. Yamada, K. Ito, S. Oda, S. Okamoto, and S. Hara, “Testmethod for contact safety assessment of a wearable robot-analysis ofload caused by a misalignment of the knee joint,” in 2012 IEEE ROMAN: The 21st IEEE International Symposium on Robot and Human
Interactive Communication. IEEE, 2012, pp. 539–544.
[8] R. S. Lakes, Viscoelastic solids. CRC press, 1998, vol. 9.

IV. D ISCUSSION
According to Figs. 4 and 5, the signal of sensors, whoseembedded length was 10 mm, became lower than that of 8mm. The top of PVDF film of these sensors was distant fromthe surface only 2 mm. It seems that this sensor system perhaps
cannot measure the force when it was embedded closer to thesurface because of the complex deformation of the film.
The tendency that the centered group outputted larger signals in Fig. 4 probably reflected the stress distribution in theperpendicular direction under the contacting area.
Furthermore, because of the individuality of sensors, which
probably reflects the installation error of PVDF film occurredin the embed process, calibration is necessary for precise
measurement of force distribution.
The arrangement of PVDF sensing elements should bedecided considering the practical application. For example, thespatial resolution of measurement can be improved by more
intensive arrangement of sensing element. Though this paper
concentrated on the shear force sensitivity, the normal stresswould be also applied at the same time. Thus, the research
about the identification of normal and shear components fromoutput signal will also be expected.
V. C ONCLUSION
In this study, a sensor system which measured the distribution of shear stress and deformation using built-in PVDF
films was developed. By using group of PVDF films, whoseembedded length differed, force distribution in the depth
direction was successfully observed. The frequency responseof deformation transmissibility can reflect the viscoelasticity of684