ICAT 2002
December 4-6, Tokyo, JAPANElectro-Tactile Display
with Localized High-Speed Switching
Hiroaki Takahashi, Hiroyuki Kajimoto, Naoki Kawakami, Susumu Tachi
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 Japan{hiroaki, kaji, kawakami, tachi}@star.t.u-tokyo.ac.jp
Abstract
The electro-tactile display with localized high-speed
switching is a novel type of electro-tactile display basedon H-Bridge circuit that uses only one electrical source
at a time. By using this display electrical stimulations
can easily be presented and measured simultaneously. In
this paper, other new methods for electrical stimulation,
including scan-type ectrical stimulation and dipole
electrical stimulation, are introduced.
Key words: electro-tactile
switching, electrical stimulation

display,

pathway is directed by flipping the switches on and off.
By this flipping, electrodes can be in any of four states:“current source,” “current sink,” “open,” and “internalshort”(Fig. 2).

high-speed

1. Introduction
The electro-tactile display directly activates sensory
nerves through electrical current from surface electrodes.
Electrical stimulation is based on researches on theequivalent circuit of the nerve [1,2,3]. Compared withother tactile devices, the electro-tactile display can bemade very small and flexible because we can arrange
electrodes and the other instrument like a current source
separately, and the only requirement on the skin is a setof electrodes. Therefore, this display can easily be
mounted onto other haptic devices, and it effectively
indicates complex sensations. In previous electro-tactile
display systems, electrical currents from each electrodewere controlled independently, requiring one currentcontrol circuit module per electrode [4]. Hence such
systems were quite expensive, and fabrication withuniform accuracy was difficult.

Fig. 1 Basic concept of the electro-tactile display withhigh-speed switching
At the start, the electrode for the stimulation point is

A novel electro-tactile display with localized high-speed
switching is proposed here. The system is composed ofonly one current source and parallel H-Bridge switches,
which are commonly used in motor drivers and cochlear
implant[5].

2. Principle
2.1. Localized High-Speed SwitchingFig. 1 describes the basic concept of this display. The
system is mainly composed of H-Bridge circuits
arranged in parallel. Each electrode is connected to thebridge part of an H-Bridge circuit, and the current

Fig. 2 Four states of electrodedetermined, and the type of stimulation, anodic
stimulation or cathodic one, is selected. If the anodic
stimulation is chosen, the upper switch which leads tothe stimulation point is turned on, and then, lower
switches that are connected to the electrodes around thepoint are turned on one by one. In this process, only oneelectrode is retained as the cathodic one at any time. Thisis how quasi-concentric electrodes are made (the methodis called QCE), and the stimulation point is changed in amanner similar to video display.
The most important difference between the existing
stimulation method and QCE stimulation is the numberof current path at every moment of stimulation.
Generally, concentric electrodes are adopted for theelectrical stimulation; however, with such electrodes,
there are many current paths, and the stimulated space isenlarged as a result. At the same time, these existing
isotopic stimulations hardly take care of the variation ofsubcutaneous tissue.

assumed. This is why localized stimulation can be shownand high-resolution impedance map of the skin surface
can be obtained with dipole stimulation.

Fig. 4 Side paths to measure the voltage and the current

QCE stimulation simultaneously uses only twoelectrodes: anode and cathode. These dipoles arescanned around the stimulation point quickly. Hence, thestimulation is strongly localized, and the amplitude of
the stimulation to the path under the skin can be adjusted
in each instance.

2.2. Stimulation by Dipole
Dipole stimulation has many advantages over existing
methods. For example, about one third of the current isrequired for stimulation because of the localization of
the current paths. This reduces the load on the system
and makes it easy to build this display.

Fig. 5 Equivalent circuit of the skin2.4. Selected Stimulation
There are many receptors under the skin. Meissner’s
corpuscles and Merkel cells exist near the surface asshown in Fig. 6. Meissner’s corpuscles are assumed tobe concerned with low-frequency vibration, and Merkel
cells with pressure. It is likely all tactile sensations are
the result of the combination of activity of thesereceptors, which are called “tactile primary colors” aname taken from the primary colors in vision. Actually
the receptors were activated selectively in the previous
research [6]. Anodic stimulation showed low-frequency
vibration, Cathodic stimulation showed pressure because

Fig. 3 Methods of stimulation

2.3. Impedance Sensing
As the density of the electrodes increases, thestimulation must be more finely controlled. For example
an electrode located on a sweat gland could cause pain.
The condition of the skin must be known so that thestimulation can be adjusted to the structure under theskin. In this display, the tactile sense can be shown, andthe impedance of the skin can be calculated at the sametime. To make the only two side paths, one for voltage
and the other one for current, the voltage and current
between the anode and cathode in the dipole aremeasured as shown in Fig. 4. The values are applied tothe equivalent circuit (Fig. 5), where a symmetricproperty in the skin between the anode and cathode isFig. 6 Structure under the skin

anodic was superior when used in conjunction with
Meissner’s corpuscles, and Cathodic, with Merkel cells(Fig. 7).

withstand the high voltage that is required to stimulate
nerve fibers from electrodes on the skin surface. As aresult, the following power transistors are used: 2SJ130(Hitachi Semiconductor) for the upper switches, and2SK3113 (NEC Electronics) for the lower switches,
which may result in a quite large capacitance.

Fig. 7 Anodic and cathodic stimulation

3. System
The block diagram of the whole system is shown in
Fig.7. The system consists of five primal components:
the PC, current source, upper and lower switches,
impedance meter, and electrodes. Impedance metermeasures the voltage on the switches and the current
through it.

Fig. 9 Current source

Fig. 8 Block diagram of the system
The details follow.

3.1. Current Source
The current source is divided into two parts, a voltageto-current converter and a current mirror supplied by a380[V] source (Fig. 9). In the pilot study, it was affirmedthat this instrument can produce a 30[us] pulse widthwave. The current source is controlled with the digitalout put board (DO board) on the PC (PCI-2794,Interface).

3.2. Upper and lower switches
Upper and lower switches enable each electrode to takeone of the four states: “current source,” “current sink,”“open,” and “internal short.” All switches are also
controlled with the DO board on the PC (Fig. 10). The
board and switches are insulated with photo-couplers(HCPL-2601, Agilent Technologies) to protect the PC
from reverse. These switches have two requirements.
One is speed. The QCE stimulation must have afrequency that is faster than the time constant of the skinelectrical impedance. The other is that it must be able toFig. 10 Upper and lower switch circuit

3.3. Electrodes
For this display, seven stainless steel electrodes werearranged in closest packing. The distance between
adjacent electrodes was 2.5[mm], and the diameter of
each electrode was 1.0[mm]. Although stainless is highly
rust-resistant, after a long period of stimulation, a thinoxidized layer of rust was evident on the surface.
Polishing the surface constantly with a metal polishsolved this problem.

Fig. 11 Electrodes

4. Experiment4.1. Performance Evaluation
The purpose of the switching procedure was to makeconcentric circle electrodes virtually through the highspeed scanning around each individual stimulation point.
To realize this QCE mechanism, the switching speed
should be faster than the time constant of skin and nerve
fibers (a few hundred microseconds). Contrary to theserequirements, the system did not function properly. The
output waveform was distorted through the upper andlower switch circuit, which works as a low-path filter.
Switching procedures were devised to solve thisproblem. Extra switching procedures that make the“Internal short” state were added to the end of the eachstimulation, and any electrical charge in the display wascleared during the interval between switching. Fig. 12 is
the time chart. As a result, the stimulation waveform ispresented as gathered pulse waves in Fig.13. The form iscalled “burst wave”. In this experiments, the values ofeach parameter are adopted (the frequency is 100 [Hz],and the pulse width is 50 [us]). At each moment, there
was only one current path as shown in Fig. 14Fig. 14 Actual current through the electrodes(The number of the cathodes indicates the order of thescan.)4.2. Impedance Sensing

Fig. 12 Time chart example

In this display, the electrical voltage between electrodes
and the electrical current through the skin cannot bemeasured directly. Hence, at first, the equivalent circuitof the upper and lower switches must be obtained. An on
FET can be regarded as a resistance, and an off FET as acapacitance. As a result, the system can be considered tobe a H-Bridge circuit (Fig. 15). With this system, thevoltage and the current are measured, and the test
resistance is identified. In this identification, leastsquares method was adopted (Fig. 16,17). The resultwith an uncertainty of about fifty percent was obtained.
The equivalent circuit of the skin and the real skinsurface haven’t been tested yet. However, after fine
identification of this system, the skin impedance can beobtained, and the values of resistors and capacitance inthe equivalent circuit can be calculated.

(The number of the lower switches indicates the orderof the scan.)Fig. 13 Burst wave
Fig. 15 Equivalent circuit of the switchesFig. 16 Amplitude fitting

Fig. 17 Phase lag fitting

4.3. Differences in the Switching Methods
The objective of this experiment is to determine the best
way to make a QCE stimulation. Six scanning methods
were tested, three for anodic and the other three forcathodic current pathways. The methods, called “rotation
scan,” “interlace rotation scan,” and “diagonal line scan”
shown in Fig. 18, were psychophysically tested andcompared with existing stimulation methods, in which
all surrounding electrodes were connected to the ground.
Subjects adjusted pulse height for each trial andcomment on the quality of the stimulation and spatialextensity.
Compared with existing types, anodic and cathodic QCE
stimulations are broader and have time instability. In
addition, with the cathodic methods, it was moredifficult to recreate concentric stimulation than with theanodic ones. For example, cathodic methods can easily
cause a pain because the current required to show
stimulation is close to the current which causes pain, andthe current that is a little bit smaller than sufficient value
can show only quite unclear point stimulation.
Moreover, these difficulties hardly depend on scanning
methods. Among these scan methods, “rotation scan”
method is indicated to be closest to existing concentric
electrode method.

Fig. 18 Scanning methods(The number of each display indicates the pulse order inthe burst wave.)

4.4. Dipole Stimulation
The stimulation with only one dipole electrode pair was
researched to determine the reason for the advantage thatthe anodic stimulation had over the cathodic one.
Although the dipole stimulation was not thoroughly
examined, the stimulation was very important. Eachcharacteristic of QCE stimulations depends mainly onthat of the dipole stimulation because all scan-type
stimulations consist of dipole stimulation at each
moment. A pilot study showed that this stimulation has aline segment or two points that have different intensity.
Nevertheless, dipole stimulation has some line-shapebroadness. Whether the dipole stimulation can show
certain direction or not, even if it has some deviation
about the strength, were researched.
Two adjacent points were chosen randomly, one for an
anode and the other for a cathode. Subjects indicated thesegment that was shown in the display. The burst
waveform was used in a comparison with the scan-type
stimulation, and the frequency was 50[Hz]. The pulse
width was 50[us].
The results are shown in Tab. 1. About the direction of
the segment, if the answer was not the accurate segmentitself, but parallel with the true, it was considered correct
as shown in Fig. 19. The phrase “including anode”
means that the segment that the subjects answered
contained the anode of the true stimulation, as shown in
Fig. 20. The same applies to the phrase “includecathode.”
Tab. 1. Results of direction test
The direction is correct“Including anode”“Including cathode”

48.7%
90.0%
50.0%

5. Results
The objective of this paper, that was to prove theadvantages of a noble electro-tactile display withlocalized high-speed switching, was accomplished. In
fact, compared with existing display, this one requires
less voltage. The unified current path can show localized
sensation and observe fine impedance of the skin.
More research will be required to determine somefundamental characteristics, such as the length of an
ideal electrode interval and the method to acquire a moreaccurate current waveform. Moreover, to make thisdisplay work effectively, the uses of an impedance
sensing system will need to be studied, and the best
scan-type stimulation for QCE stimulation will need tobe determined.

References1.

Fig.19 Direction of the segment
2.

3.

4.

5.

Fig. 20 “Including anode” and “including cathode”
Almost all anodic electrodes are cognized exactly.
Hence the accuracy of direction mainly depends on thepercentage of cathodic electrodes answered correctly.
These results fit those of 4.3.

6.

Donald R.McNeal, “Analysis of a Model for Excitationof Myelinated Nerve,” IEEE TRANS. ON
BIOMEDICAL ENGINEERING，VOL.BME-23, No. 4,
pp. 329-337, 1976.
Frank Rattay, “Analysis of Models for Extracellular
Fiber Stimulation，” IEEE TRANS. ON BIOMEDICALENGINEERING，Vol.36, No.7, pp. 676-682, 1989.
J.T.Rubinstein, F.A.Spelman, “Analytical Theory for
Extracellular Electrical Stimulation of Nerve with Forcal
Electrodes Ⅰ.Passive Unmyelinated Axon,”
Biophys.J.，vol.54，pp. 975-981, DES, 1988.
Hiroyuki Kajimoto，Naoki Kawakami, Taro Maeda,Susumu Tachi, “Electrocutaneous Display with Receptor
Selective Stimulations,” IEICE TRANS.，vol.j84-D-Ⅱ，
pp. 120-128, 2001.
Hugh McDermott, “An Adovanced Multiple Channel
Cochlear Implant,” IEEE TRANS. ON BIOMEDICALENGINEERING，Vol.36, No.7, pp. 789-797, 1989.
Hiroyuki Kajimoto, Naoki Kawakami, Taro Maeda,
Susumu Tachi, “Electrocutaneous Display with Receptor
Selective Stimulation(II) -Skin Impedance Based Control,” Proceedings of the Virtual Reality Society of Japan theFifth Annual Conference，pp. 307-310, SEP, 2000.