Submitted to Ninth Annual Symposium on
HAPTIC INTERFACES FOR VIRTUAL ENVIRONMENT
AND TELEOPERATOR SYSTEMS
on Feb. 2000

Virtual Three Legged Race using Networked Locomotion Interfaces
Haruo Noma†, Fumitaka Nakaizumi††, Hiroaki Yano††, Tsutomu Miyasato†, Hiroo Iwata††
ATR-Media Integration &Communications Research Lab.†, Univ. of Tsukuba††
The trial virtual three-legged race system
The trial virtual three-legged race system consisted of two locomotion interfaces, GaitMaster[2] and ATLAS[3,4]. GaitMaster wasdeveloped at Tsukuba university in Tsukuba Science City, and ATLAS was develop at ATR in Kansai Science City. Although they
are 600 km apart, we connected them via a digital network (Fig.1).
A typical locomotion interface has two basic functions: a walking motion detection unit and a motion canceling unit. The motion
detection unit traces a user’s walking motion and analyzes themotion. Using the result, the motion canceling unit keeps the user
in a location. GaitMaster is equipped with movable foot-plate. A
user’s foot and the foot-plate are connected by a small mechanicallink. The link measures the user’s walking motion on the plate, andthe GaitMaster controller drives the two foot plates according to
the user’s foot motion to simulate a virtual surface. On the otherhand, ATLAS is a treadmill based locomotion interface. It measures a user’s walking motion with a video camera and analyzes
the user’s walking speed and position on a belt. Then, it adjusts thebelt speed of the treadmill to keep the user on the belt.
In the trial virtual three-legged race system, we exchanged theresults of the motion detection units of GaitMaster and ATLAS,and put them into the motion canceling units alternately. As shownin Fig. 1, the left foot motion data of the user of GaitMaster and theright foot motion data of the user of ATLAS were exchanged in thetrail system. If they did not walk in exact timing, their steps werestuck in different ways. GaitMaster restricted tied foot motion of
the user, and ATLAS did not drove its belt as user’s motion. Going

Introduction
Our focus is on communications while walking together. When
we walk together with another person, we sometimes find that oursteps are synchronized. In addition, if we stop suddenly or quicken
our pace while walking together, we change the subject with themotion as a trigger. In a group tour, moreover, although each member may intend to behave independently, the whole party moves
together with no specific communications or rules among the members. In these cases, we suppose that people can perceive theirpartner’s motions and react to these motions unconsciously. Consciously, they do other complicated tasks such as talk, observe something and so on.
We expect such unconscious and nonverbal interaction to be akey point in enhancing the reality and existence of a remotely located partner in the next-generation telecommunication system. We
call such a communication method Tel-E-Merge[1]. We have been
investigating Tel-E-Merge in the exchange of users’ motions. In
this paper, we propose a virtual three-legged race as one exampleof the system use.
In the virtual three-legged race, two remotely located users are
virtually couples at one leg, and need to synchronize their walking
motion to walk around a virtual world together, like in a real threelegged race. In other words, they need to harmonize their motions
to go forward. We expect that both of them can independently recognize their partner’s motions under the virtual restrictions. Experimental results showed that the degree of motion synchronization increased in the trial system.
Locomotion interface
controller
Walking motion
detecting unit

Locomotion interface
controller
Walking motion
detecting unit

ISDN 64kbps

B's leftfoot motion

A's right
foot motion

Motioncanceling unit
controller

User AA's leftfoot motion

B's right footmotion

RemoteuserMotion
canceling unit
controller

User B
LocaluserMotion
sensorMotionsensor
Virtualrestriction

Locomotion
Interface GaitMaster

Locomotion
Interface ATLAS

Figure 1 Overview of the virtual three legged race systemforward together without stumbling, the users had to synchronize
their walking steps like in a real situation.
The controller of ATLAS and GaitMaser were carried out at 20Hz
using a 64Kbps digital network. We also used one more channel
for a TV-phone system. Frame rate of the system is 15Hz with atime delay of a few frames. In particular, a voice channel, that hadno time delay, played an important role in synchronizing the stepsof the novice subjects at their first trial.
Experiment
We conducted an experiment to examine the degree of synchronization of the users’ walking motions in the trial system. Fourpairs of subjects were asked to walk together as fast as they could.
To estimate the degree of synchronization, we measured two timelags in the motion of the bound feet for each step. t1 was the time
lag in starting the swing phase, and t2 was the time lag in finishing
the swing phase. We asked the subjects to walk for 15 minutes andrecorded the time lags of all of the step. They were defined basedon the motion of the ATLAS user; in short, when the subject onGaitMaster started his/her motion later than the subject on ATLAS,
the time lag was a plus value. Furthermore, all of the subjects were
novices at system use, and so the subject on ATLAS was allowed
to call out to his/her partner on GaitMaster to synchronize theirsteps through the TV-phone.
Figure. 2 shows results from the four pairs. We divided each trail
into three periods, i.e., the first, middle, and final periods. One period lasted five minutes. A column in the figure represents the average time lag for one period and the error bar represents the standard errors.
In the first period, t1 and t2 vary widely, but they converge as thetrial goes on. The converged value t1, i.e., the time lag in starting
the swing phase, deeply depends on the pair. On the other hand, t2,

Time lag t1(sec)

0-5min.
5-10min.
10-15min.

i.e., time lag in finishing the swing phase, converges into about 0.4
sec for all pairs. From these results, we focused on the standarderrors of t1 and t2, which represented the degree of convergence ofthe time lags. As a result of statistic analysis, there was a significant difference in the standard errors in the measured periods(P(F=9.50)=0.01) and between the time lags (P(F=6.14)=0.048).
There was no difference in the pairs of subjects (P(F=0.98)=0.46).
All of this meant that the subjects could synchronize their motions step by step, but it seemed that how the walking motions wereharmonized depended on each pair’s strategy. Furthermore, it wasmore difficult to synchronize motions finishing the swing phasethan motions starting the swing phase. In fact, observing the trials,
the subjects failed at motion synchronization even if they confirmed
their steps by calling out to each other during the first period. In thefinal period, however, they could step successively by calling out.
These results lead to the conclusion that the subjects could becomeaware of their partner’s motions through the locomotion interfaces
and adjust their motions by referencing their partner’s motion.
Conclusion
In this report, we proposed a virtual three-legged race system as
one instance of a new communications method using body motion.
The experimental results using the trial system showed that thesubjects could synchronize their walking motions step by step as
expected.
As future work, we need to discuss how subjects can recognize
and react to their partner’s motions through the motion synchronizing process in the trial system.
We expect to apply the virtual three-legged race system as tool
for rehabilitation, especially therapy by walking. Using the system, a therapist can train a patient to walk by synchronizing his/hermotion. In short, the therapist can diagnosis and lead the patient by
her/his own body motion simultaneously. Also, their motions canbe digitized as shown in the trial system. Accordingly, the therapist
can diagnosis the patient from a remote location over a network.

Time lag t2(sec)

Reference

AB

C

D

Subjects

Figure 2 Time lag of starting and finishing swing
phase between dound feet:[1] Miyasato, Nakatsu,"User Interface Technologies for a Virtual Communication Space", Proc. of CVVRHC'98(IEEE and ATR Workshop on
Computer Vision for Virtual Reality Based Human Communications),
pp.105-110, (1998)[2] Iwata, "Locomotion Interface for Virtual Environments", Proceedingsof 9th International Symposium of Robotics Research (ISRR’99) pp.220226, (1999).
[3] Noma, Miyasato, "A New Approach for Canceling Turning Motion inthe Locomotion Interface, ATLAS", Proc. of ASME-DSC-Vol.67, pp.405406, (1999).
[4] Noma, Miyasato, "Design for Locomotion Interface in a Large Scale
Virtual Environment, ATLAS: ATR Locomotion Interface for Active SelfMotion", Proc. of ASME-DSC-Vol.64, pp.111-118, (1998).