Evaluation of Transmission System for
Spatially Distributed Tactile Information
Katsunari Sato1 and Susumu Tachi2
1

Graduate School of Information Science and Technology, The University of Tokyo, Japansato@tachilab.org
2
Graduate School of Media Design, Keio University, Japantachi@tachilab.org

Abstract. Spatially distributed tactile information is essential for dexterous operation in telexistence. We propose a transmission system for spatially distributed tactile information using a finger-shaped GelForce and an electrotactile
display. Both devices are suitable for integration with multi-fingered teleoperation system because these devices are compact. In this study, we evaluate
the performance of a one-fingered transmission system for spatially distributed
tactile information by conducting recognition experiments. We discuss the efficiency and limitations of the proposed transmission system.
Keywords: finger-shaped force sensor, electrotactile display, spatially distributed tactile information, and telexistence.

1 Introduction
The transmission of haptic information in tele-operation systems is essential for interaction with remote objects. In particular, spatially distributed tactile information isessential for dexterous operation [1]. Thus far, several systems have been developed
for the transmission of spatially distributed tactile information [2] [3]. For example,
Methil et al. [3] have developed a tele-diagnostic system that can be used to detecttumors in remote patients. Unfortunately, because this system was developed for use
as a diagnostic tool for breast pathology, users are unable to grasp objects. The size ofthe tactile device limits the finger workspace, making it all the more difficult to fabricate a multi-fingered transmission system. Therefore, compact sensors and displays
are required to achieve dexterous grasping of remote objects in tele-operationsystems.
To achieve dexterous tele-operation, we have developed the following compact tactile devices: a finger-shaped force sensor named as finger-shaped GelForce [4] and anelectrotactile display [5]. Because these devices are small in size and light in weight,
they can be easily mounted on any multi-fingered tele-operation systems (Fig. 1).
Furthermore, the spatial resolution of these devices can be easily improved because of
their simple structures. In this study, we fabricated a one-fingered transmission system and evaluated the contact area recognition efficiencies of the system.
A.M.L. Kappers et al. (Eds.): EuroHaptics 2010, Part I, LNCS 6191, pp. 279–284, 2010.
© Springer-Verlag Berlin Heidelberg 2010

280

K. Sato and S. Tachi

Fig. 1. Transmission system for spatially distributed tactile information using finger-shaped
GelForce and electrotactile display

2 Transmission of Spatially Distributed Tactile Information
2.1 Finger-Shaped GelForceA finger-shaped GelForce sensor [4], which was developed in-house, was used toobtain spatially distributed tactile information. The finger-shaped GelForce consistsof a transparent elastic body, red and blue marker matrices, and a color chargecoupled device (CCD) camera that captures the movements of the markers. Thesemovements are used to calculate the distribution of the force vector (magnitude andangle) applied on the sensor surface. Thus, we easily realized a high-resolution sensor
that has a design, such as shape and impedance, similar to that of the human fingertip.
We use a human skin gel (Exseal Corp., Young’s modulus: 50.0 KPa) for the transparent elastic body of the finger-shaped GelForce to match the impedance of the human finger. We set 15 force sampling points at intervals of 2.5 mm. In this case, thespatial, magnitude, and temporal resolutions of the constructed finger-shaped
GelForce are 5.0 mm, 0.15 N, and 16 ms, respectively.
2.2 Electrotactile Display
The developed electrotactile display [5] produce spatially distributed tactile stimulus.
It comprises pin electrode matrices and directly activates nerve fibers under the skinby passing electric current through surface electrodes. The current flows from anelectrode to adjacent electrodes through the skin. By periodically changing the pin
used for stimulation, electrotactile stimuli can be presented at any point. Because theelectrotactile display does not require the use of any mechanical actuators, the displays can be easily attached to each fingertip.
In this study, we use an electrotactile display that has 15 electrodes. The diameterof each electrode and the distance between the centers of the electrodes are 1.25 mm
and 2.5 mm, respectively. A square wave having a pulse width and maximum pulse
amplitude of 20 µs and 5.0 mA, respectively, is used for stimulation. The frequencyof pulses applied at the electrode is 30 Hz. The line-width, two-line, and amplitude
discrimination thresholds are 2.5 mm, 7.5 mm, and 0.1 mA, respectively.
2.3 Methods for Information Transmission
In the proposed system, spatially distributed tactile information is transmitted fromthe finger-shaped GelForce to the electrotactile display (Fig. 2). The magnitude andEvaluation of Transmission System for Spatially Distributed Tactile Information

281

distribution of the force vectors are related to the amplitude and position of the electrotactile stimuli. The electrotactile display can only indicate the magnitude and distribution of the force vectors, not their orientation. The magnitude of the force vector|fi| is calculated as follows:fi =f ix + f iy + f iz
222(1)

where i denotes the number of force sampling points. When the magnitude of force
at a point is less than half its value at adjacent sampling points, we set the magnitude of force at that point to 0.0. This elimination technique seems to virtually improve the spatial resolution of the finger-shaped GelForce from 5.0 mm to 2.5 mm[4]. The amplitude of the electrotactile stimulus at a pin electrode, |Ai|, is given as
follows:
Ai = α f i + β(2)where i, α and β denote the number of pin electrodes, gain, and bias, respectively.
Each value of α and β is pre-examined by a user.

3 Experiment
We evaluate the transmission system for spatially distributed tactile information fromthe view-point of contact area recognition efficiency.
3.1 Material and MethodMaterial. We employed PHANToM Omni (SensAble, Technologies) to touch theremote object in the transmission system for spatially distributed tactile information.
We mounted the electrotactile display and the finger-shaped GelForce on the endeffectors of the master PHANToM and slave PHANToM, respectively (Fig. 3). The
participants had to place the tip of their index finger on the electrotactile display andmove the end-effector of the master PHANToM to control the finger-shaped
GelForceon the slave PHANToM. In this experiment, we used the simplest symmetricposition servo type control algorithm for robotic master-slave control, which can berepresented by the following equations:Fig. 2. Transmission of spatially distributed tactile information. Contact with (a) edged objectand (b) flat object. The lengths of the arrows represent the magnitude of the measured force,
whereas the size of the sphere represents the amplitude of the electrostimulus.

282

K. Sato and S. Tachi

Fig. 3. Experimental setup: one-fingered transmission system for spatially distributed tactile
information employing PHANToM master-slave system
f s = (x m − x s )K p

fm = − fs(3)where fs and fm denote the forces of the slave and master systems, respectively; xs andxm, the positions of the slave and master systems, respectively; Kp, the gain. We limited the participant’s finger movement to a single direction (along the y-axis, as indicated in Fig. 3). Participants were instructed to push the object, but not to stroke thesurface of the object or to change the contact position.
Method. Participants conducted three recognition experiments: position, width andangle. In the position experiment, the participants touched the edge of a circular cone at
an arbitrary position (Fig. 4a). The circular cone is made from plastic and its diameterand height are 20.0 mm and 10.0 mm, respectively. The contact position of the cone
was selected randomly from five positions, namely, the center of the sampling point, 2.5mm away from the center and toward to left, toward the right, forward, and backward.
Subsequently, participants reported the position in which they made contact with thecone. In the angle experiment, the participants touched at rectangular prism that wasoriented at 0°, 45°, 90°, or 135° (Fig. 4b). The rectangular prisms are made from plastic,and they have a width and length of 2.5 mm and 20.0 mm, respectively. The angle oforientation of the prism was selected randomly. Then, the participants reported theorientation of the prism that they touched. In the width experiment, the participants
touched closely-arranged rectangular prisms (Fig. 4c) using the transmission system.
The number of prisms was randomly selected as 1, 2, 3, and 4, and therefore, their total
widths were 2.5 mm, 5.0 mm, 7.5 mm, and 10.0 mm, respectively. Then, participants
reported on the number of prisms they had touched.

Fig. 4. Conditions of object for each position, width, and angle recognition experiment

Evaluation of Transmission System for Spatially Distributed Tactile Information

283

One male and two female participants in the age-group of 23-26 volunteered forthe experiments. In each discrimination experiment, each experimental condition wasperformed ten times. Throughout the experiment, there was no visual or audio feedback for the participants.
3.2 Result
The experimental results are shown in Fig. 5. In the position experiments, the averaged correct answer ratio was 0.75. Using ANOVA, we determined that there is nosignificant difference in the results obtained for the different contact positions on thecircular cone (F(4,14)=1.33, p=0.34). In the angle experiments, the averaged correct
answer ratio was 0.74. There is no significant difference in the results obtained fordifferent orientation of the rectangular prism (F(3,11)=2.37, p=0.17). Finally, in thewidth experiments, the averaged correct answer ratio was 0.53. There is a significant
difference in the results obtained by changing the widths of the rectangular prisms(F(3,11)=6.51, p<0.05). Furthermore, the results of multiple comparisons demonstratethat there are significant differences in the results when the widths of the prisms are
2.5 mm and 7.5 mm (t(11)=4.11, p<0.05).

Fig. 5. Experimental results. The graphs on the left, at the center, and on the right represent theresults of the position, angle, and width experiments, respectively. The horizontal axis represents the experimental condition, whereas the vertical axis represents the ratio of correct answers. The bars represent the averages of all participants. The error bar represents standarddeviation.

3.3 Discussion
In this evaluation, we employed a finger-shaped GelForce and an electrotactile display. These devices have similar spatial resolutions as those employed in previousstudies. From the spatial resolutions of these devices, we expected that participants
would correctly discriminate the 2.5 mm differences more than 75% of the time. The
results of the experiment involving the position and angle demonstrate that the proposed system is capable of effectively determining the contact position and orientation of the object. Therefore, we expect that an improvement in the spatial resolutionsof our tactile devices would result in the improvement in the efficiency of the transmission system.
However, it appeared that participants were unable to discriminate the 2.5 mm difference in the width experiment. A previous study [5] showed that the participants candiscriminate this difference using the electrotactile display. Therefore, we assume that

284

K. Sato and S. Tachi

the measurement error of the finger-shaped GelForce causes a decrease in the accuracy of the transmission system. It is believed that the measurement error can be decreased by increasing the resolution of the CCD camera [4].

4 Conclusion
In this study, we fabricated a one-fingered transmission system for spatially distributed tactile information using a finger-shaped GelForce and an electrotactile display.
The results of the contact area recognition experiment showed that our system is efficient for transmission for spatially distributed tactile information.
However, the transmission of other haptic information such as compliance or texture is also essential for tele-operation systems. Therefore, in the future, we would
aim to conduct a comprehensive evaluation of our system as a haptic transmission
system. Then, we will design a multi-fingered transmission system and evaluate its
efficiency from the viewpoint of achieving dexterous grasping.

Acknowledgement
This research was partly supported by CREST and Grant-in-Aid for JSPS Fellows(20·10009). The development of the electrotactile display was partly supported by Dr.
Hiroyuki Kajimoto (The University of Electro-Communications).

References
1. Lederman, S.J., Klatzky, R.L.: Sensing and Displaying Spatially Distributed Fingertip
Forces in Haptic Interfaces for Teleoperator and Virtual Environment Systems. Presence 8(1), 86–103 (1999)
2. Khatchatourov, A., Castet, j., Florens, J.-L., Luciani, A., Lenay, C.: Integrating tactile andforce feedback for highly dynamic tasks: Technological, experimental and epistemologicalaspects. Interacting with Computers 21, 26–37 (2009)
3. Methil, N.S., Shen, Y., Zhu, D., Pomeroy, C.A., Mukherjee, R., Xi, N., Mutka, M.: Development of supermedia Interface for Telediagnostics of Breast Pathology. In: Proceedings ofIEEE ICRA 2006, pp. 3911–3916 (2006)
4. Sato, K., Kamiyama, K., Kawakami, N., Tachi, S.: Finger-shaped GelForce: Sensor for
Measuring Surface Traction Fields for Robotic Hand. IEEE Transaction on Haptics 3(1),37–47 (2010)
5. Sato, K., Kajimoto, H., Kawakami, N., Tachi, S.: Electrotactile Display for Integration with
Kinesthetic Display. In: Proceedings of 16th IEEE RO-MAN 2007, pp. 3–8 (2007)