ICAT 2002
December 4-6, Tokyo, JAPANA Vision-based Tactile Sensor (II)
Kazuto Kamiyama1), Terukazu Mizota2), Hiroyuki Kajimoto1),
Masahiko Inami1), Naoki Kawakami1), Susumu Tachi1)(1)The University of Tokyo, Bunkyo-ku Hongo 7-3-1, Tokyo, Japan,(2)CASTI, Chiyoda-ku Marunouchi 1-5-1, Tokyo, Japan{kazuto, mizota, kaji, minami, kawakami, Tachi}@star.t.u-tokyo.ac.jp
Abstract
Receiving tactile information from a slave-robot is anecessary component of telexistence with tactile display.
However, there are few tactile sensors that can measure
the distribution of three-dimensional force vectors on asurface of the sensor. For this reason, we developed asensor that provides three-dimensional force distribution
by detecting movement vectors in the transparent elastic
body with a video camera. In the previous report,
fundamental principle was explained. In this report, wedescribe about downsizing of the sensor and result ofevaluation experiment.
Key words: tactile sensor, telexistence,
force distribution, force vector

1. Introduction
The purpose of our research is the development of atactile sensor for telexistence of tactile sensation. When
handling an object accurately in environment oftelexistence, such as telesurgery, transmission of tactile
sensation is equally important as visual and auditory
sensation. To realize telexistence of tactile sensation, we
need both sensing device and displaying device. As adisplaying device, there has been extensive research on
haptic displays. As a sensing device, there also has been
research on tactile sensor[1]. However, the purpose ofthem is not telexistence originally. Therefore, wedeveloped a new tactile sensor specially designed fortactile display.

2. Necessary property and information
Our first step is to determine the property that the sensor
for haptic display should equip. The primary difference
from other sensations such as vision is the existence ofinteraction. When the finger contacts with an object,repulsive force causes the deformation of both finger
and object. If the sensor does not deform like the finger,acquired information becomes different. In other words,
same elasticity as finger is essential for the sensor body.
The second step is to determine what information thesensor should get. Several mechanoreceptors aredistributed within the finger. Merkel cell perceives
displacement, Meissner corpuscle and Pacinian
corpuscle receive low and high frequency vibration.

Combination of the activities of these receptors makes
various sensations. As both displacement and vibration
are generated by force applied on the surface of the skin,
if we can measure the surface force distribution, we canobtain sufficient information to calculate the receptor’sactivities. Force is vector and has direction. Besides
fingertip can measure distribution. Hence the sensor fordisplay needs to measure distribution of force vector.

3. Method of measurement
Here, we propose a new optical tactile sensor for
displaying tactile information, which can measure athree-dimensional vector distribution and have theproperty of elasticity. The concept of our proposed
tactile sensor is as follows. The sensor consists oftransparent elastic body and markers within the body.
When a stress is applied on the surface, we opticallymeasure the internal strain of the body as movement of
the markers. Finally force vector is calculated from thestrain using elastic theory.

3.1 Method of measuring strain
This proposed measuring method uses information ofstrain of an elastic body. There are many choices about
what kind of strain should be measured, and our current
approach is to measure the horizontal movement ofsmall markers in the elastic body, which are located N
by N at a specified depth. To gather sufficient
information for the reconstruction of the stress vectors,
we used two layers of markers that are located at
different depths (Fig.1). These layers could be
distinguished by the colors of the markers (red andblue). We set the x-y plane parallel to the sensor surface
and the z-axis extending vertically on the interior. By
measuring these markers from a positive z direction witha CCD camera, we could obtain two sets of twodimensional motion vectors at different depths, so theamount of information is increased and the distributionof stress vector can be readily obtained.

3.2 Method of calculating force distribution
We assume linearity of the elastic body, so the motion of
the markers is described as a simple superposition ofstress distributions[2][3].

Figure1: Model of sensing

Figure3: Distribution of force vector

4. Experiment
4.1 Sensor system
Figure 2 illustrates the developed sensor system. Part ofelastic body is a cylinder, which height is 10[mm] anddiameter is 35[mm]. Whole size of sensor is 40[mm] by40[mm] by 100[mm]. Red and blue spherical markers
are arranged in a depth of 3[mm] and 6[mm] from thesurface. Diameter of the markers is 0.6[mm]. Throughtransparent acrylic block with which the elastic body isfixed, the markers are captured by CCD camera.
Spherical markers inside,
Force
1.5mm interval

Figure4: Evaluation of linearity
35mm

5. Conclusion
10mm

Transparent
Elastic body

100mm

Transparent
Acrylic block

CCD

Figure2: Sensor system

Discussed at first section, our purpose is development oftactile sensor for telexistence of tactile sense. To
accomplish the purpose we developed the tactile sensor,
which has property of elasticity and can measure
distribution of three-dimensional force vector. In thisreport, linearity and accuracy of the sensor is evaluated.
From this result, we believe that the sensor works
effective for use of telexistence. As current and future
work, we are developing finger shaped tactile sensor andwill evaluate in detail.

Reference
4.2 Evaluation
Figure 3 illustrates calculated distribution of force
vector. This distribution is resulted by applied force to zdirection using 5mm cylinder. The length and directionof arrow represents magnitude and direction of force.
Additionally, Figure 4 is the result of linearity
evaluation. Applying forces to z-direction using thecylinder, we summated z components of distributed
forces and plotted them. This graph shows that thesensor has high linearity and accuracy of 20[gw].

[1] R.S.Fearing, "Tactile Sensing Mechanisms", The
International Journal of Robotics Research, Vol. 9, No.
3, pp 40-54, 1990.
[2] L.D.Landau,E.M.Lifshitz, "Theory of Elasticity",
BUTTERWORTHHEINEMANN, 1985
[3] K.Kamiyama, H.Kajimoto, M.Inami, N.Kawakami,
S.Tachi, "A Vision-based Tactile Sensor", International
Conference on Artificial Reality and Telexistence, 2001.