ICAT '99
Applying an ‘encounter-type’ haptic display totelexistence
Takuya NOJIMA1), Masahiko INAMI2), Taro MAEDA1) and Susumu TACHI1)
1) School of Engineering, University of Tokyo
2) Center for Collaborative Research, The University of Tokyo
7-3-1 Hongo, Bunkyoku, Tokyo, JAPAN{tnojima, minami, maeda, tachi}@star.t.u-tokyo.ac.jp

Abstract
We have proposed an ‘encounter-type’ haptic display,which has many advantages over the ‘mounting-type’ indisplaying a realistic sense of touch. However, it wasdifficult to use this type of display in telexistence. In thispaper, we propose and discuss a method for applying
encounter-type displays to telexistence in the real world.
In addition, we report on prototype systems.
Keywords: Telexistence, Encounter-type, Haptic

display, Force display, Latency

1. Introduction

This type of haptic display generates a haptic sensationby changing its closed-loop rigidity according to theshape of the virtual object and the state of touch. ThePHANToM[1] is classified as this type, but it is wellknown that this type of display has many disadvantages.
For example,
1) It is difficult to display a real sense of collision with avirtual object. A sense of collision is considered to bea key sensation of touch.
2) Operators always have to always mount or holdactuators on their bodies, which restricts movement
and feels unnatural.

The ‘mounting-type’ haptic display, which always
mounts or holds the end-effector of a system on an

In this research, we propose applying an 'encounter-type'

operator’s body, is often used in telexistence systems.

haptic display to a telexistence system for solving theseproblems. In addition, we report another advantage ofusing the encounter-type haptic display.

2. The encounter-type haptic display
2.1. Characteristic of the encounter-type hapticdisplay
Some researches have already developed encounter-type
haptic displays. For example, K. Hirota et al. developed
Surface Display[2] and S. Tachi et al. developed Active
Environment

Display(AED,[3]);however,

theircreations were haptic displays only for the virtual world.
These displays could not use in telexistence in the realFig. 1 The strategy of encounter-type haptic display

world. The general strategy of this type of display is(Tangent plane approximation)

shown in Fig. 1. First, move the real object (display unit)
to approximate the surface around the contact point of

the virtual object. Second, the display unit waits for theHowever, in such a case, the back-derivability of theoperator touches. Third, the display unit is controlled

system will decrease, and the operator will sense anaccording to the operator movement and shape of theunnatural feeling especially when the operator does not

object, which enables the operator to experience a

touch anything. Although when using the encounter-type

realistic sense of touch. We call it “tangent planedisplay, we do not have to consider about this problem.

approximation”. Compared to mounting-type of display,

Because, in this type of display, the actuators are

the advantages of the encounter-type are summarized asseparated from the operator’s body.

follows.
•Sense of collision: When the user touches thevirtual object, the collision has actually occurred.
Hence, the operator experiences a real sense of

2.2. Applying the encounter-type haptic display
to telexistence
Considering the use of an encounter-type haptic display
in a telexistence system,collision.
•Vertices and edges: By using a real object, which•

The environment (or the object) on the slave’sside must be measured before contact.

has vertices and edges, as does a display unit, the•operator experience a realistic sensation of touching

The display unit has to be controlled to approximate thevertices and edges [3].

environment on the slave’s side before contact.

Toachieve an appropriate haptic sensation with an
If you do not touch anything, you do not feel anyload: The operator only has to mount or hold thepassive system to measure, point and orientation of
his/her finger. Hence, the operator is free from any
extra load when he/she is not touching anything.

encounter-type haptic display, the surface of the display
unit on the master’s side would have to be controlled toapproximate the object on the slave’s side before
contact.

Inotherwords,

information

about

theenvironment on the slave’s side (relative position andrelative orientation between the slave hand and slave
environment) would be necessary before contact. To
solve this problem, the following two methods areconsidered.
•Measure the slave environment around the contact
point locally for every contact.

•Construct an environment model in advance.

In the case that we construct an environment model ofthe slave side in advance, it is known that we could use
this method if the transmit latency is very big. But itlacks of flexibility in the variance of the environment. In
addition, the dynamics of the object on the slave’s side
are basically unknown, which presents another major
problem if the model is to be constructed. On the otherhand, when using the latter method, measuring theFig. 2 The state before contact

environment every contact time, we could not use it if
the transmit latency is too big, but we could get

Displaying a haptic sensation with a mounting-type
haptic display should increase rigidity and torque of theactuators to display a sense of collision and edges.

flexibility against the variance of the environment, andwe could measure the dynamics of the object. In thisresearch, we choose the latter one to make a prototype

Cm[sec] for control latency of the display system on thesystem. If the problem about pre-measurement is solved,

master’s side, v(t)[m/sec] for the speed of the operator’s

another advantage is gained.

arm/finger and d[m] for the distance of the pre-•measurement, that is, the maximum value of the distanceCorrectness against latency: The system could
display haptic sensation correctly in spite of theexistence of transmit latency or control latency.

between the finger of the slave arm and the slave
environment during the pre-measurement state(Fig. 3).
The τ is for the time when starts pre-measurement. If it ismounting-type system, d in the condition would be zero,so the condition may not be completed.

Fig. 4 Implementation of the system
Fig. 3 About transmit latency

3. System design

To use an encounter-type haptic display in a telexistence

When displaying haptic sensation in a telexistencesystem, the information about the environment on thesystem, the information about the slave arm andslave’s side must be obtained before contact, which

environment on the slave’s side should be displayedmeans that the slave’s side environment must be pre-through visual and haptic sensation to the operator on themeasured. On the other hand, the pre-measurement

master’s side. However if the difference between visualenables us to give haptic sensation correctly (Fig. 3).

and haptic sensation about position and orientation of theWhen we consider 1-dimensional situation, we can get

arm became larger, the operator would feel an unnatural

the next inequality (1) as a condition for appropriate

sensation (Fig. 2). Therefore, some consideration must

haptic presentation, where L[sec] for transmit latency,

be given to these problems. Here, we consider about theaccuracy of position and the gradient of the line on thed≥τ + L +Cm

∫ v(t )dt (1)τfingertip. Some researches have examined abouthuman’s position recognition of the fingertip[4] andcurve

recognition[5][6].

Whenconsideringthepresentation of haptic sensation with the encounter-typeprobe-like system to pre-measure the environment on thehaptic display, first, it will be the problem that the gap

slave’s side. In this case, the information that we need is

between the position and orientation of the surface of thethe relative position and orientation between the pen anddisplay unit around the contact point and the same of thethe tangent plane around the contact point. The overview

shape of the object which presenting. We could considerof the system is shown in Fig. 4. The system on the slavethat if the gap is under the threshold, the haptic display

side is shown in Fig. 5. We used a linear servo actuator

could make haptic sensation appropriately (Fig. 2). The

for the movement in the horizontal plane. Its resolution

positioning accuracy of the human fingertip without

about the position is 0.5[µm]. For pre-measurement, wevisual feedback is about 1[cm][4]. The resolution about

used a probe-like system shown in Fig. 6. The probe

the angle of the line of the fingertip without visual

makes contact with the environment on the slave’s side

feedback is about 4-6[deg][5]. These are the assumptions

before the pen contacts. We use the gradient between

used as conditions of the system.

two probes as the gradient of the pen against theenvironment. The probes are kept parallel with the pen
by using a parallel link system. The encoder in the probe
is 3000[pulse/round] unit. In this system, the accuracy ofthe position of the pen of the slave’s side is 1[mm], andthe accuracy of the orientation of the probe on theslave’s side is 0.12[deg](Table 1). They satisfy thecondition we refer to. Here, if we assume that L=20[ms],Cm=40[ms] and v=20-60[cm/sec], the d would be about
1.2-3.6[cm].
Table 1 System specifications

Fig. 5 The system of the slave’s side

Degree of Freedom
Range
Accuracy
The range measured bythe probe
Resolution of the probe
The distance of premeasurement

2-DOF in the horizontal plane
50[cm] for each DOF
1[mm]21.60[deg]〜157.02[deg]Fig. 6 Left: The sensor on the slave’s side (normal)
Right: The sensor on the slave’s side (just before contact)

4.Implementation of the system
As a slave system, we made a 2-DOF system because it
was easy to make. It moves in a horizontal plane. The
operator holds a pen-like device to measure his/hermovement. For accuracy of the measurement, we use

Fig. 7 The AED

0.12[deg]
3.7[cm]In this situation, the distance of pre-measurement

fusion of visual and haptic in the curvature

3.7[cm] is almost sufficient. As a master system, we useperception,”

AED ([3],Fig. 7). The AED has 6-DOF, but we restrictSociety of Japan 4th Annual Conference, pp.181-

its DOF according to the DOF of the slave system.

184, 1999.

5. Conclusion
In this research, we have proposed to use an encountertype haptic display to telexistence and examined theconditions to be considered. We have made a prototype
system according to the conditions. In the future, we willconduct research regarding direct haptic presentation tothe human finger without using a pen device.

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