Fibratus tactile sensor using reflection on an optical lever
Satoshi Saga∗
Tohoku University

Shinobu Kuroki†
The University of Tokyo

Naoki Kawakami‡
The University of TokyoSusumu Tachi§
The University of Tokyo

Abstract

2 Fibratus tactile sensor

Among several tactile sensors in use currently, none can evaluate
the sense of gentle touch. We have developed a fibratus tactile sensor that utilizes the property of reflection. This sensor enables toevaluate the sense of gentle touch. We propose a new interface device that employs this fibratus tactile sensor.

One tends to be kind when he strokes teddy bears or pets gently;
in the same manner, one feels the tenderness of another’s gentle
stroking. Therefore, the sensations detected by the fibratus tactile
sensor are capable of stimulating one’s intuition. If we use thesefibratus tactile feelings as interfaces, we can use the fibratus sensor
as an input device in computers and evaluate its intuitional sense.

Keywords: tactile sensor, fiber, image sensor, optical measurement, optical lever

1Introduction

In recent years, with the advancement in robotics, many tactile sensors have been developed in order to improve force sensation inrobots. Several tactile sensors are commercially available in themarket, for example, the 6-axis force/torque sensor that can measure the force at one point [BL AUTOTEC LTD ]. There exists another sensor can measure the distribution of the contact state or theforce distribution [Nitta Corporation ].
The disadvantage of these distribution-type force sensors is theirexcessive wirings. Each sensor unit is arranged in close proximity of the measurement surface in order to allow a small sensor tobe individually distributed, and the wiring that gathers informationfrom a unit is also individually wired. Therefore, a sensor itself
cannot prevent deterioration due to the stress of repeated measurements; further, the wiring assembly is complicated. Some opticalsensors [Yamada et al. 2002; Kamiyama et al. 2005] such as the
distribution-type optical tactile sensor have already been studied.
In these sensors, the sensing units and the corresponding wiringfrom the measurement surface can be eliminated by using a camera. However, as these sensors measure the motion of the markers
embedded in an elastic body, the sensor resolution, rather than thecamera resolution, is determined by the markers. Therefore, theresolution of the camera is not completely utilized.
We have examined a sensor that uses an optical lever [Saga et al.
2006]. In the optical lever technique, the displacement is magnified
by using reflection characteristics. We propose a sensor with a newsystem that exploits the optical lever and a flexible mirror surface
and measures the reflection image. Due to the reflection image we
can utilize the full-resolution of the camera. We propose a new typeof optical tactile sensor that can detect the surface deformation withhigh precision by using the principle of an optical lever. Transparent silicone rubber is used as the flexible mirror surface.
If we can evaluate touch sensation information with respect to fibratus salience, the acquired information can be very useful for interactive devices. We then implant fibratus salience into the flexible
mirror surface. From this salience, the end of the fibratus is considered to be the contact surface and the reflection characteristics areleft. This salience indicates that the sensor functions as a fibratus
tactile sensor.
∗ e-mail: saga@rm.is.tohoku.ac.jp† e-mail:shinobu kuroki@ipc.i.u-tokyo.ac.jp‡ e-mail: kawakami@star.t.u-tokyo.ac.jp§ e-mail:tachi@star.t.u-tokyo.ac.jp

Figure 1: Stroking a teddy gently& Stroked by mother gently: Left:
A girl is stroking her teddy gently. Right: A girl’s mother strokes
her daughter gently
Thus far, there exist some tangible interfaces that employ position
sensors such as buttons, dials, joysticks, and touch panels; forcesensors; and acceleration sensors. Many of these interfaces are theinput devices that derive information from only one point, and acquired values are linked to symbolic information. In recent years
some gaming interfaces have been using conventional sensors toobtain information on natural movements. However if we need to
employ interfaces, the development of sensors with innovative concepts becomes imperative.

Flexible fibers
Siliconerubber・hard・softImage
pattern

Reflective
surface( based on Snell ' s law )CameraFigure 2: Section of a sensor: Cross-section of the sensor. The
surface of the sensor contains some fibers
Our research focuses on distributed tactile sensors with soft tissues.
Utilizing the principle of the sensor we propose a fibratus tactile
sensor. By using this sensor, gentle touches can be evaluated and
new interfaces can be developed. For example, by stroking fibers
gently, one can create dynamic flows with one’s fingers, and intuitional senses may be used as inputs for computers. If softer fibers
are utilized, this sensor is capable of sensing even the blowing ofwind. In the near future by employing this sensor in a robot, it will
be possible to evaluate softer contact information than that detected

by existing tactile sensors, thereby causing resulting in an increasein the use of robots in daily life.
We have proposed a new optical tactile sensor that can detect thesurface deformation with high precision by using the principle ofan optical lever. Transparent silicone rubber is used as the flexible
mirror surface [Saga et al. 2006]. We implant fibratus salience, thehardness of which is a little greater than that of silicone rubber,
into the flexible mirror surface. From this salience, the end of thefibratus is considered to be the contact surface and the reflection
characteristics remain unchanged. This salience indicates that thesensor functions as a fibratus tactile sensor.

3boundary surface assumes the reflection characteristic of a mirror
surface. We design a sensor with a flexible mirror surface by using
the above-mentioned mirror surface reflection characteristic. By
arranging the image pattern, camera, and the transparent silicone
rubber as shown in figure 2, the light diffused from the image pattern is reflected from the silicone rubber boundary and captured by
the camera.

Principles

In order to realize sensing with regard to the movements of the fibratus salience, it becomes necessary to use some method detecting
small angular shifts in surfaces such as that shown in figure 3. Thissensor employs two silicone rubber layers and fibers. The outerlayer is harder than the inner layer. Owing to this difference inthe hardness, the fibers draw the harder silicone layer when they
are moved. Therefore the distribution of the angle on the silicone
rubber surface is changed by the movement of the fibers. In otherwords the movement of fibratus salience can be evaluated on thebasis of the distribution of the silicone rubber surface.

Figure 4: Snell’s law: Based on Snell’s law, total internal reflectionis achieved

nr sin φr

=φr

=φr

>

Subsequently, we utilize the reflection method proposed in [Sagaet al. 2006]. This reflection method employs an optical lever. Ourna sin φanaarcsin( sin φa )nrnaarcsin( )
nr(1)
(2)
(3)

In this paper we employ this method for detecting the movement ofthe fiber attached to the surface of the sensor.

3.2 Deformation of the Reflection Image

Siliconerubber・hard・soft
Figure 3: Reaction of a fiber: Pressing the fiber along the verticaldirection causes a symmetric shift of the silicone rubber. Pressing
the fiber along the horizontal direction cause an asymmetric shiftof the silicone rubber
aim is to develop a flexible mirror surface by using this optical lever.
The sensor detects the deformation of the surface by measuring thedisplacement via a reflection image from the mirror surface.

3.1

Reflection Condition

In this sensor, transparent silicone rubber is employed as the flexible mirror surface. With regard to the boundary between silicone
rubber and air (figure 4), the refractive indices are denoted by nr
and na and the refraction angles by φr and φa . The notations “r”
and “a” denote rubber and air, respectively. When the distribution of the refractive index at the boundary of silicone rubber andair satisfies equation 3, total internal reflection occurs; further, thisIn some optical tactile sensors, a camera is used as the optical sensor[Begej 1984; Kamiyama et al. 2005]; the sensor detects the deformation by tracing a marker. However, the resolution of this sensor
is limited because of the restriction due to the overlapping of markers. Further we propose a tactile sensor for the new system thatmakes use of a reflection image in order to utilize the resolution of
the camera to the maximum possible extent by combining an opticallever and a flexible mirror surface and fibratus salience.
If the contact object touches the fiber, the silicone rubber boundary
will be deformed (figure 3). Consequently, a deformation of thereflection surface occurs, thereby deforming the reflection imageof the image pattern. The sensor measures the deformation of thereflection surface by solving an inverse problem using this deformation. Therefore, the entire reflection image contains information.

3.3 Geometrical Optics
We denote the deformed depth of point Q1 on the reflection surface
q by d, the depth displacement between the adjacent points Q1 andQ2 by ∆d, the angle at each point on the surface by θ , and theangular displacement between the adjacent points Q1 and Q2 by∆θ . Here, we express ∆d by θ , and then express ∆d with known
values via ∆α . With some approximation we consequently obtain[Saga et al. 2006] the following:∆d=Z L0tan(θ +Θl)dlL(4)From equations 6, 9, and 11, we can express θ1 with the known
parameters and measured values.
By defining Q1 in equation 5 as the nth characteristic point anddefining ∆d as ∆dn from the continuation property of a boundary
surface, we can assume that∆dn ' ∆dn+1(12)

Based on this assumption, we can express equation 5 as follows:∆dn+1 =Figure 5: Geometrical optics: We express ∆d with known values
via ∆α∆lncos θnlog(||)∆θncos(θn + ∆θn )(13)Since we assume that d = 0 at l = 0 and l = L0 , we obtain ln and θnfrom equation 7. We obtain ∆θn from the given value of θn and calculate ∆dn+1 in equation 13. We obtain αn , dn , and the distributionof α and d by performing a step-by-step iteration of this calculation.

4 Implementation
The sensor developed in this study uses the additionpolymerization-type silicone rubber (KE109A, B and KE1052A,B [Shin-Etsu Chemical Co., Ltd. ]). We fabricate a triangularpole-shaped flask with transparent acryl, pour silicone rubber into
it and perform a lap reaction. Here, we use two types of silicone
rubbers. The surface of the sensor is made of a harder type ofsilicone rubber, KE109, and the sensor contents are made of asofter silicone rubber, KE1052. Then we put fibers to the surfaceof the sensor. The fibers are made of polystyrene.

Figure 6: Geometrical optics: This is a part of figure 5=Lcos θlog(||)Θcos(θ + Θ)

α1 = β1 − 2θ1
m=p

w=p(5)(6)cos(α1 − γ )cos(α1 − α0 )(7)cos(β1 − γ 0 )l 02 + d 2cos(β1 − β0 )(8)l2 + d2

The sensor can measure m, w, β1 , and β2 while L0 , α0 and β0 areknown. The distribution of θ and d are the unknown quantities.
The sensor can construct ∆d if ∆θ and ∆l are known from equation 5; further, it can reconstruct θ and d if ∆θ and ∆d are known.
However, because there are few equations, the unknown values inequations 7 and 8 cannot be solved if θ equals θ + ∆θ and l equals
l + ∆l. Here, we define∆α = α1 − α0

Depending on the difference in hardness the fibers draw the hardersilicone layer when the fibers are moved. Thus the angle distribution of the fibers on the silicone surface changes due to the movements. After the lap reaction, we perform exfoliation using an acrylboard and prepare the reflection surface. One of the remaining acryl
surfaces functions as the pattern surface and the other one functions
as the captured surface.
In this case, we use a lattice-patterned paper as the pattern surface.
In this manner, we prepare a sensor with a flexible mirror surface(figure 7). We install a camera to adequately capture the flexible
mirror surface (figure 8).

Fibratus tactile sensor

60mm(9)From equation 7, we get
m2 cos2 ∆α − (l 2 + d 2 )(cos2 (α0 + ∆α )) = 0(10)When ∆α is expressed as γ = 0 and ∆α ' 0 from the second Taylor
series expansion, then ∆α can be expressed as follows:
22−(dv + l ) cos α0 sin α0 u 2 2 2 2 u (d + l ) cos α0 sin2 α0 u ±t −(m2 − (d 2 + l 2 ) cos2 α0 )2×((−m2 ) + (d 2 + l 2 )(cos2 α0 − sin α0 ))∆α =(11)(−m2 ) + (d 2 + l 2 )(cos2 α0 − sin2 α0 )Image patternFigure 7: Silicone rubber with a lattice pattern: the rubber istransparent and the pattern can be seen. The sensor is almost cube
shaped with a side length of 60 [mm]Here we confirm whether or not the fiber movement induces a deformation of the surface shape. Figure 9 shows the image before
fiber movement, and figure 9 shows the image after fiber movement.

Fibratus tactile sensor

USB Camera

Image patternFigure 8: Overview of the sensor: The optic axis of the camera islocated perpendicular to the captured surface

Figure 9: Before deformation: White objects on the right side of
the image are the end of the implanted fibers

These two figures show that the fibers can induce a deformation in
the surface shape. We can reconstruct the surface shape by using
these images and equation 13. According to the surface shape thefibers movements can be estimated.

5User Experience

We have already developed a simple reflection sensor. This system requires one computer. By using this system the concept ofour sensor can be experienced and comprehended. Additionally wewill create two fibratus-type sensor systems. Each of these systems
also requires with one computer. We plan to activate these sensor systems by using LCDs and show interactive demonstrations sothat attendees can comprehend the performance of our sensor. Furthermore, two or three people can simultaneously touch the sensor.
Further we plan to display a description video of the sensor on alarge screen by means of a projector.

6Figure 10: After deformation: Acquired image is deformed by thetouch toward the fibers

For example, (1) multi sensing, contact-less sensing, interactive devices; (2) sensing by means of active patterns; and (3) thin device
and rapid sensing can be realized by using this sensor. Our future
work involves the extensive use of these sensing devices and utilizing each of their properties [Saga et al. 2006].

Conclusions and Future Work
References

In this paper, we have proposed a tactile sensor that employs theprinciple of an optical lever and a flexible reflection surface. We
have designed a tactile sensor that takes advantage of the reflection
image whose deformation can be detected with high precision using an optical lever; the sensor also takes sufficient advantage ofthe resolution of a camera by using transparent silicone rubber as
the flexible mirror surface. Further we have developed a prototype
made of silicone rubber with fibers and have confirmed that thefibers can induce shape deformation of the surface and a change inthe reflection image can be captured.
The proposed fibratus tactile sensor employs the full-reflection
based on the distribution of the refractive index. Thus variable soft
fibratus salience and silicone rubber can be used as the sensor material. Sensing by means of dense distribution of soft fibratus andsensing by a sparse distribution of hard fibratus can be realized. According to this property, the sensor can not only be a conventional
tactile sensor but also a more sensitive tactile sensor that can evensense the blowing of wind. Thus, the sensor can not only realize
the possibility of a novel interface but also makes the robots to be
spread in daily life.
In addition the simplicity of the sensor facilitates not only the use offibratus sensors but also its use in a variety of variable applications.

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