16th IEEE International Conference on Robot & Human Interactive Communication

MA1-2

August 26 ~ 29, 2007 / Jeju, Korea

Electrotactile Display for Integration with Kinesthetic Display
Katsunari Sato1, Hiroyuki Kajimoto2, Naoki Kawakami1 and Susumu Tachi1
1

Information Science and Technology , The University of Tokyo, Tokyo, Japan,
e-mail : Katsunari_Sato@ipc.i.u-tokyo.ac.jp, (kawakami, tachi)@star.t.u-tokyo.ac.jp
2
Department of Human Communication, The University of Electro-Communications, Tokyo, Japan,
e-mail:kajimoto@hc.uec.ac.jp

Abstract—The goal of this study is to develop a haptic interface for dexterous manipulation. To achieve this, we proposed electrotactile-kinesthetic integration. Our electrotactile
display presents natural touch sensations of objects. In addition, this display is so small that it is considered to not affect themoving range of fingers. The haptic interface for dexterous
manipulation is realized by mounting it on a kinesthetic display
having a wide workspace. The electrotactile display on the integrated haptic interface is used actively. Therefore, we actively evaluated the performances of the electrotactile display
on the single-fingered prototype system. The results revealed
the present performance of the system. Subsequently, it also
shows the possibility of further improving of the haptic interface by devising an electrotactile rendering method.

I. INTRODUCTION
When we handle objects using telepresence or virtual
reality systems, haptic feedback is necessary for dexterous
manipulation [11][12]. This haptic feedback is divided into
two types: kinesthetic (force) feedback and tactile (touch)
feedback. The former improves the stability of hand
movements [1][3] and the latter helps us perceive object
properties [12]. The sensations from both types of haptic
feedback should be available for dexterous manipulation.
For example, when we handle a pen, we can stably pinch itby our fingertips and feel a reflecting force from the kinesthetic sensation; the position of the pen can be determined by
the tactile sensations.
Thus far, several haptic interfaces have been developed[11][12].However, these are not suitable for dexterous manipulation because of inadequate tactile feedback. The tactile display on conventional interfaces presents only asymbolic “contact” sensation of an object. Therefore, wecannot feel the object on our fingertips. It is believed that
handling small objects such as pens is difficult without position information. Nowadays, there are some researches thatcan present the distributed touch sensation of an object[9][10][13]. Unfortunately, the systems proposed by theseresearches are too large for use in dexterous manipulation. A
large system limits the workspace of our fingers, i.e. themovement range of our finger to manipulate an object. This
limitation of workspace complicates manipulations such as
pinching.
Based on conventional studies, we set the goal of ourstudy to be the development of a haptic interface for dexterous manipulation. First, we summarize the requirements
for the touch feedback display intended for dexterous manipulations as follows:

978-1-4244-1635-6/07/$25.00

2007 IEEE.

1. It should present a highly realistic and intuitive touchsensation, i.e., it should present not only the “contact”
sensation but also the distributed touch sensation thathumans perceive.
2. It should be a compact body that does not invade theworkspace of our fingers. Such a body also affords
advantages in its implementation. It will simplify integration with the kinesthetic feedback display.
To fulfill these requirements, we used an electrotactile display [6] as the touch feedback display. We mounted thedisplay on a kinesthetic display having a wide workspace.
This integration provided a haptic interface that was suitablefor dexterous manipulations (Fig. 1).
In this paper, we propose electrotactile and kinesthetic
integration for dexterous manipulation. In section II, we
introduce the concept of electrotactile-kinesthetic integration. Subsequently, we construct a single-fingered prototype
and evaluate the basic performance of the electrotactile display in section III. In section IV, we discuss the results ofthe experiments and further improvements to our system.
II. CONCEPT OF SYSTEM
To achieve dexterous manipulation, we integrate theelectrotactile and kinesthetic displays. We introduce theirfeatures and their integration in this section.
A. Haptic integration
The electrotactile display that we have developed can
Haptic interface
for dexterous
manipulation

Electrotactiel
display

Robot hand

Kinesthetic
display
Slave side

Master side

Fig. 1 Conceptual diagram of electrotactile and kinesthetic integration for
dexterous manipulation. This is an example used for a master-slave system.

3

present intuitive tactile sensations. It comprises a pin electrode matrix. It directly activates nerve fibers within the skinsurface by an electrical current from the surface electrodes(Fig. 2). The electrical currents flow from the electrode to
adjacent electrodes through the skin. It can selectively
stimulate each type of receptor and produce vibratory andpressure sensations with an arbitrary frequency. Some applications of this device include the Smart Touch [5] andForehead Retina system [7]. They allow us to perceive theposition, edge, and field sensation of an object. In addition,
the electrode plate of this display is small and lightweight.
Therefore, it is considered to not affect the workspace.
Further, we can easily mount it on all types of kinesthetic
displays.
The kinesthetic display presents the reaction and friction
force of object surfaces. Nowadays, several types of kinesthetic displays are used [1]. In this study, we consider asmall-sized display that has multiple degrees of freedom(DOFs) such as PHANToM (SensAble Tec.) and CyberGrasp (Immersion Tec.). Some of them provide a wide
workspace and sufficient kinesthetic feedback to our hand.
When a user touches objects in a remote or virtual environment using our integrated system, he/she can perceive the
distributed touch sensation and reaction force of objects.
From these sensations, the user can easily recognize theobject position, posture, and shape, i.e., he/she can easily
recognize what object he/she is touching. For example, fromthe rounded kinesthetic sensation and the tactile sensation of
concave-convex surfaces, we can recognize that we aretouching a gear (Fig. 3). We believe that this haptic information will also help the user manipulate objects dexterously.

17 mm

12 mm

Electrode

Electrotactile display

Kinesthetic
sensation

Touch sensationof an object

Fig. 3 Presentation of touch sensation by electrotactile andkinesthetic integration.

B. Electrotactile rendering
The electrotactile display presents distributed touchsensations. This implies that the electrostimulus is provided
by the electrodes at the position corresponding to the contact
position of a finger pad and an object. For example, when thefinger pad contacts the face of the cube, all electrodes send acurrent to the finger. When the center of the finger pad
touches the edge of the cube, the electrodes located in a line
send the current.
In this tactile rendering method, it is necessary to becareful when deciding the strength of the electrostimulus ateach electrode. We assume the case when we touch an object
by our finger pad. The fingertip is elastic and thus it has thecharacteristic of a spatial band-path filter. Therefore, stressconcentrates at the edge of a contact surface. It emphasizes
the edge automatically. However, when we perceive
touching sensation of an object using the electrotactile display, the sensation of this phenomenon cannot be produced.
To present the stress concentration on the fingertip byusing the electrotactile display, we have to change thestimulus strength for every electrode responding to thecontact surface. Therefore, we consider that the electrotactile
display should present a stimulus by gradual strengthening
it; it should have an adequate strength resolution for theelectrical stimulus.

1 mm

III. EXPERIMENT

Nerve
fibersWhen we manipulate an object using the abovementioned
haptic interface, the electrotactile display will dynamicallymake contact with our finger pad. Therefore, we evaluate thebasic performance of the electrotactile display when using it
actively. We examine the space resolution of the electrotactile display by distance and width discrimination. Subsequently, we evaluate the strength resolution of the electricalstimulus by strength discrimination.

Mechanoreceptor

Electricalcurrent

Tactilesensation

Inside the skinFig. 2 (Above) Picture of electrotactile display (Electrode plate).
(Below) Method of electrical stimulus (Reconstructed from [6]).

4A. Material
We constructed the single-fingered prototype system of
the electrotactile and kinesthetic integration for the experiment. Figures 4 and 5 show the overview and configurationof the system, respectively. This prototype comprises theelectrotactile and kinesthetic displays, a laptop PC, and an
H8 microcomputer (Akizuki, Coop.). The kinesthetic display is controlled by the PC and the electrotactile display iscontrolled by the PC and H8 microcomputer.
In this prototype, we used the PHANToM Omni
(SensAble Tec.) as a kinesthetic display. The PHANToM
has six DOFs and presents triaxial force feedback at 1 kHz.
It allows us for a wide workspace and presents sufficient
force for one finger. We mounted the electrotactile display
on the end-effector of the PHANToM.
The electrotactile display we used comprises a 3 × 5
electrode matrix (Fig. 2). The size of the electrode plate is 12
× 17 × 1 mm. This small size does not affect the workspaceof the PHANToM. The surrounding electrode is used as anelectrical ground. The diameter of each electrode is 1.25 mm
and the distance between the centers of the electrodes is 2.5
mm. The pulse width is 40 µs and the pulse frequency is 60
Hz. The maximum strength of the stimulus is 5.0 mA and thestrength can be controlled by the user.
The position data of the user’s index finger is captured by
the PHANToM and translated to the PC. Then, the positionof the cursor in the virtual environment is updated. Based on
this cursor position, the reflection force and the electric
current at the electrode pin are calculated. The reflection
force is calculated from the spring-damper model. Current ispassed through the electrodes based on the position of thecontact field between the cursor and the virtual object. In thisprototype, the strengths of the electrostimulus are the sameat all electrodes, i.e., the electrotactile rendering method that
we mentioned in section II was not implemented.
B. Methods
The participants were volunteers in their 20’s and comprised two males and two females. Before beginning theexperiments, they were explained to the participants. The
experiment was conducted using the system that is explained
in section A. The participants placed the tip of their index
finger on the electrotactile display and moved theend-effector of the PHANToM. They can control the cursor
in the virtual environment using their fingertips. The fingertip was fixed on the end-effector by rubber bands.
There were three experimental conditions: 2-line, width,and strength conditions. In each condition, there was a floor,
a cursor, and two lines (standard and comparison lines) in
the virtual environment. In the dist 2-line condition, theparticipants touched the lines simultaneously and reported
whether they were identical or not. In the width and strength
conditions, the participants alternately touched the standardline and the comparison line. Subsequently, for the widthcondition, they determined whether the comparison line waswider than the standard line. In the strength condition, theparticipants evaluated whether the strength of the electricalstimulus in the comparison line was stronger than that in the

Kinesthetic display(PHANToM Omni)

PC (Laptop)
Cursor

Virtual environment

Electrotactile display
on end-effector

Microcomputer(H8)

Rubber bands
Fig. 4 (Above) Overview of the single-fingered prototype.
(Below) Electrotactile display on end-effector of PHANToM.

PC (Laptop)
・Update virtual world・Caluculate reflecting force
・Set electrical stimulation

Microcomputer
(H8)
60 Hz

・Control electrotactile display

1 kHz
Kinesthetic Display(PHANToM Omni)
・Get finger position
・Feedback reflection force

Electrotactile
Display・Electrostimulus

Fig. 5 Configuration diagram of prototype system.

standard line.
When we manipulate objects that are in contact with ourfinger, there are two types of relative movements between
the finger and the object—static and dynamic. Therefore, we
specified two modes of touching the lines—pushing andsliding (Fig. 6). In the pushing mode, the participants canonly push and remain stationary on the line (down the cursor
to the floor along the Y-axis). In the sliding mode, the participants push and slide their finger vertically along the lines(along the Z-axis).
In the experiment, the participants were able to view thefloor, the landmarks for determining the location to touch,and the cursor that they controlled. In addition, they wereable to feel the reflecting force from the floor and the electrotactile sensations from the lines. However, they were notable to view the lines and feel the reflecting forces from them.
Therefore, they were not able to report on the visual andkinesthetic sensations.

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Down thecursor

Z

Y
X
Lines
Pushing

Slide thecursorZ

D. Results

YX

Sliding

Landmarks
Fig. 6 Experiment 1: Two modes of touching lines.
(Note that participants were not able to view lines during experiments.)

C. Settings
The settings of each experimental condition are summarized in Tab. 1. In each condition, we recorded the participants’ answers. We consistently used a constant method to
examine a discrimination threshold on this experiment. The
number of the parameter value was six. Each value waspresented 20 times in one experiment.
In 2-line discrimination, the widths of the lines were 2.5mm and the distances to their centers were varied according
to the values 2.5, 4.0, 5.5, 7.0, 8.5, and 10.0 mm.
In width discrimination, the distance between the centersof two lines was fixed at 40.0 mm and the width of thestandard line is 7.5 mm. The widths of the comparison lines
were 2.5, 4.5, 6.5, 8.5, 10.5, and 12.5 mm.
Tab. 1 Experiment 1: Settings of experiment in each condition.
Con: condition, Dis: distance between centers of lines, Wid: width ofline,
Str: strength of stimulus, Std: standard line, Cmp: comparison line
Con
2-lineLine

Dis (mm)Wid (mm)Std2.5, 4.0, 5.5,
7.0, 8.5, 10.0

2.5

Cmp
Std

Wid

Std
Str

7.5

Cmp

Cmp

40.0

In strength discrimination, the distance between thecenters of two lines was 40.0 mm. The widths of the lines are
15.0 mm. The standard stimulus strength was decided by theparticipants. The comparison stimulus differed from thestandard stimulus according to the values –0.25, –0.15,–0.05, +0.05, +0.15, and +0.25 mA.

2.5, 4.5, 6.5, 8.5,
10.5, 12.5
15.0

Str (mA)

Set byParticipants

std ± 0.05,
0.15, 0.25

The experimental results in each setting are shown in
Figures. 7, 8, and 9. Each “circle” and “rhomboid” represents the average response ratio in the pushing and sliding
modes, respectively. The straight and broken lines represent
the fitting curve in each mode. The error bars represent thestandard deviation.
In Fig. 7, the horizontal axis represents the distance between the centers of lines and the vertical axis represents the
response ratio of “identical.” From this figure, we can estimate that the threshold of 2-line discrimination (75% discrimination) is around 10.0 mm in the pushing mode. In thesliding mode, it is around 9.2 mm.
In Fig. 8, the horizontal axis represents the width of thecomparison line and the vertical axis represents the response
ratio of “wide.” From this figure, we can estimate the lower
and upper thresholds of width discrimination (25 and 75%
discrimination) to the 7.5 mm width line. In the pushingmode, they are approximately 6.0 and 10.5 mm respectively.
In the sliding mode, they are approximately 5.5 and 9.5 mm.
In Fig. 9, the horizontal axis represents the stimulus
strength of the comparison line and the vertical axis represents the response ratio of “strong.” From this figure, we canestimate the lower and upper thresholds of strength discrimination (25 and 75% discrimination) to the strength ofthe participants set. In the pushing mode, they are approximately –0.15 and +0.08 mA, respectively. In the sliding
mode, they are approximately –0.10 and +0.05 mA.
Note that the strengths of the electrical stimuli set by theparticipants were approximately 2.0 mA. Based on the participant’s feedbacks, they could comfortably feel electricalstimuli of around 1.25 to 2.75 mA.
IV. DISCUSSION
A. Physiological viewpoint
In the results of the 2-line discrimination, the threshold isapproximately 9.5 mm. This value is large as compared tothe two-point discrimination threshold of the index finger,
which is approximately 2.0 mm [8]. On the electrotactile
display, the electrical current flows from the electrode only
to the adjacent electrodes. Therefore, the discriminationthreshold should be around 5.0 to 7.5 mm. However, underpractical conditions, the electrical current leaks to the surrounding electrodes. This leakage current causes a wide areaof contact sensation. Therefore, we believe that it willcomplicate the identification of whether the lines are identical or not.
We believe that this space resolution will improve by
increasing the density of the electrodes. However, this will6

to be sufficient from this theoretical value. Therefore, theabovementioned leakage current does not affect the width
discrimination.
In all experiments, the results reveal the tendency of theslide mode to be more sensitive than the pushing mode. Thisagrees well with the knowledge about active touch [4]. The
ability of tactile perception improves by moving the finger
pad to the surface of an object. However, the difference issmall as compared to the conventional results of active
touching. This reveals that the tendency of the electrotactile
stimulus to reduce the effect of active touching.

Response ratio of "identical"

1
push
slide

0.75

push
slide

0.5

0.25

0
2.5

4

5.5

7

8.5

10

Distance between lines (mm)Fig. 7 Experiment 1: Result of 2-line discrimination.

1
Response ratio of "wide"

push
slide

0.75

push
slide

0.5

0.25

0
2.5

4.5
6.5
8.5
10.5
Width of compared line (mm)12.5

Fig. 8 Experiment 1: Result of width discrimination.

Response ratio of "strong"

1
push

0.75

slide
push
slide

B. Viewpoint from application
The electrotactile display has sufficient space resolution
in width. Therefore, it is considered that several contact fieldsizes can be presented. For example, we can perceive thedifference in the touch sensation of a pen from a bat. Unfortunately, 2-line resolution is not sufficient and thus theperception of a fine slot is difficult. Therefore, we require
further improvements in the electrotactile display in order topresent fine features of an object surface.
At the strength discrimination, the upper and lower
thresholds are approximately 0.12 and 0.06 mA, respectively.
These thresholds are considered to be small as compared to
the range of the strength of the electrical stimuli that theparticipants could feel comfortably (1.5 mA). Therefore, we
believe that the electrotactile display has a high strength
resolution. Based on this result, it is possible to implement
the electrotactile rendering mentioned in section II. We believe that this implementation will improve not only theneutrality of the stimulus but also the space resolution.
Based on the results, the effect of the touching modes on
the resolution is small. This is convenient. When manipulating an object, we sometimes cannot move the finger after
the first contact. In this case, we have to recognize the size ofthe contact field. Therefore, we require a high space resolution at the static mode of touching, in a manner similar to the
dynamic mode. The result shows that this requirement canbe satisfied by using the electrotactile display.

0.5

V. CONCLUSION
0.25
0
-0.25

-0.15

-0.05

0.05

0.15

0.25

Difference of electrostimulus strength (mA)
Fig. 9 Experiment 1: Result of strength discrimination.

cause an increase in the electrical current which in turn willcause an increase in the leakage current. We have to examine
this trade-off relationship more closely and set the optimum
density of the electrodes.
The width discrimination threshold for the 7.5 mm line isapproximately 2.0 mm. Based on the distance of the centerof the electrodes, the width discrimination threshold is considered to range from 0.0–2.5 mm. The result is considered

In this paper, we proposed electrotactile-kinesthetic integration for haptic interfaces. We believe that this integration will enable us to manipulate objects dexterously. We
constructed a single-fingered prototype. Subsequently, we
dynamically evaluated the basic performance of the electrotactile display. The results revealed the present performance of the electrotactile display. Subsequently, theyalso showed the possibility of further improving the electrotactile display by devising an electrotactile rendering
method.
In the future, we will enhance the system to enable users
to perceive a more intuitive haptic sensation than that perceived currently. First, we will implement the electrotactile
rendering method introduced in section II. Second, we willdevelop the electrodes in order to realize a higher spaceresolution. To achieve this, we will examine the effect of the7electrical current and electrode density on the haptic perception. Finally, we will construct a multifingered haptic
interface for dexterous manipulation.
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[2]

[3]
[4]
[5]

[6]

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[7]

H. Kajimoto, Y. Kanno and S. Tachi, “Forehead Electro-tactile Display for Vision Substitution,” in Proceedings of EuroHaptics 2006,
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[8] E.R. Kandel, J.H. Schwartz and T.M. Jessell, “Principles of Neural
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[13] C.R. Wagner, D.P. Perrin, R.L. Feller, R.D. Howe, O. Clatz, H.
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