ISMCR2004

Development of Multi-D.O.F. Master-Slave Arm with Bilateral ImpedanceControl for Telexistence
Riichiro Tadakuma, Kiyohiro Sogen, Hiroyuki Kajimoto, Naoki Kawakami, and Susumu Tachi
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
Graduate School of Information Science and Technology, The University of Tokyo
Phone: +81-3-5841-6917 FAX: +81-3-5841-8601
E-mail tadakuma@star.t.u-tokyo.ac.jp
Abstract
We developed the robotic arm for a master-slave
system to support “mutual telexistence,” which realizes
close physical communication with other people using
gestures and remote dexterous manipulation tasks. In thispaper, we describe the specifications of the experimental
setup of the master-slave arm for demonstration of thefeasibility of the mutual telexistence concept. We
developed the master arm of a telexistence robot forinterpersonal communication. To facilitate smoothgestures of the operator, this arm has 6-degree-offreedom structures to free the operator's elbow. The lastdegree of the 7-degree-of-freedom slave arm is resolvedby placing a small orientation sensor on the operator’sarm. Moreover, this master arm is made light andimpedance control is applied in order to grant theoperator as much freedom of movement as possible.

1. Introduction
It has long been a desire of human beings to project
themselves in a remote environment, i.e., to have asensation of presence in a remote place.
The concept of projecting ourselves by using robots iscalled telexistence (tele-existence) [1]. Telexistence is anadvanced type of teleoperation system that enables anoperator to perform remote manipulation tasks
dexterously with the feeling that he or she exists in theremote environment where the robot is working in. Thefirst telexistence master-slave system “Telesar(Telexistence Surrogate Anthropomophic Robot) I” isshown in Fig.1.
The second prototype of a telexistence master-slave
system for remote manipulation experiments is being
designed and developed with improved technology. The
robot built for this system is called “Telesar II”. We focus
on producing human-like, realistic movement for TelesarII.

Fig. 1: The first telexistence master-slave system “TelesarI”

2. The mechanism of the master-slave arm ofTelesar IIThe slave arm of Telesar II is a 7 D.O.F. light weightarm [2]. The anthropomorphic slave arm is shown inFig.2.
The master arm used in experiments of telexistence
manipulation is shown in Fig.3 [3]. This master arm hasforce feedback.

Fig. 2: The whole view of the slave arm of Telesar II

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that works from the object to the slave arm, X m and X sFig. 3: The master arm of Telesar II

3. Control methods of master-slave system
Telesar I adapted unilateral control, but we adaptedbilateral control for Telesar II because of theimprovements in technology such as computationalpower.
Many types of bilateral systems are currently underdevelopment, but in this study we control the system
based on the impedance control.
The basic principle of the impedance control typemaster-slave system is to give seemingly equivalent
impedance to both master and slave. Furthermore, by
minimizing its impedance, the master arm places only aslight load onto the operator’s arm, and he or she canperform a wide variety of gestures and other body
movement expressions without becoming hindered.
Another merit of bilateral impedance concerns
operation safety. For contact operations of the slave armwith another person, the operator can feel the power fromthe person with a high sense of reality, so the potential
danger of giving him or her superfluous power can beavoided.
If we control the master-slave system with a Dual
Motion Transmission Method [4] that exchanges
movement information between the master and the slave,the master’s power is equal to the slave’s, and exact
power presentation can be performed in contact work.
In this control method, the control equations are asshown below.
F0 = Fe = M d (X&& m − X&& s ) + Bd (X& m − X& s ) + K d ( X m − X S )Here,(M d , Bd , K d ) mean(1)

the target impedance

mean the posture vectors of the master and the slave.
In a state where the slave arm is moved without
applying external power, since the master’s power and theslave’s are not equal, the operator receives the power by
the product of target impedance and the posture error.
However, making target impedance as small as possible
can minimize undesirable load, unlike other control
methods.
In order to verify the optimality of this bilateral
impedance control system, we conducted simple
comparison experiments with symmetry and forcefeedback, which are general control method types.
The symmetry type is a control method in which themaster and the slave transmit their posture information toeach other, and apply a suitable gain according to thedifference of their postures to produce a torque output.
The force-feedback type is a control method in which
the master transmits posture information to the slave, andthe slave transmits power information to the master. The
master receives the torque command value from theproduct of the suitable gain and the difference of themaster’s power and the slave’s, and the slave receives thetorque command value from the product of the gain andthe difference between the master’s posture and theslave’s.
For the fundamental experiment, we comparedsymmetry, force feedback, and impedance control types.
At first, we used the left arm of the master system as thevirtual slave arm, and used the right arm of the mastersystem as the master arm to eliminate mechanicaldifference between the master and the slave, and compare
the control methods themselves.
In this experiment, we activated all 6 axes of the masterarm and the virtual slave arm. Results of this experimentare shown in Fig.4.
In symmetry type, the operational force is large
because the operation itself needs large power. In force
feedback type, the virtual slave arm pushes the wall withsuperfluous power, and the operator can’t feel it. In
impedance control type, tracing with stable power wasrealized.
In the second experiment, we checked the influence ofdifferent inertia with comparison of symmetry andimpedance control type. We attached a weight to thevirtual slave arm to increase its inertia, and measured theoperational force. The result of this experiment is shownin Fig.5. Compared with the increase of the operational
force in symmetry type (from 7.4N to 9.3N), the increaseof the operational force in impedance control type is verysmall (from 3.7N to 3.9N).

parameters of the master arm and the slave arm, F0 means
the operational force to the operator, Fe means the power

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Symmetry type

Symmetry type

Force feedback type

Impedance control typeFig. 5: Result of the second experiment of comparison

Impedance control typeFig. 4: Result of the first experiment of comparison

In the third experiment, we used the actual slave arm ofTelesar II to confirm the validity of bilateral impedancecontrol for master-slave system with different mechanicalstructure. The overall view of the experimental setup is
shown in Fig.6. In this experiment, the slave arm moved
in free space, and we checked the accuracy of following
ability in three axes of the universal coordinate. The
results of this experiments is shown in Fig.7. As shown inthese three graphs, sufficient accuracy for gestures andcommunication was confirmed in this experiment.
In the fourth experiment, we checked the accuracy offorce presentation of this system. The slave arm traced thewall, and we measured the operational force and theopposing force from the wall applied to the slave arm.
The result of this experiment is shown in Fig.8. Small
operational force is realized, forces on the master arm andthe slave arm are comparatively close, and the stable trace
is also realized. Namely, impedance control type is also
effective in this system.

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0.45

distance(m )

0.4

0.35

0.3

0.25

0.2
05

Fig. 6: The overall view of the experimental setup using
the actual slave arm of Telesar IIzzzzzzzzz

Reproduction of the operator’s work
Precise force feedback
Stable contact
Gesture is enabled
Smooth Operation
Sufficient accuracy of following ability
Safe for humans
Precise force feedback
Stable contact operation (for shaking hands, etc.)

15

X-axis
-0.25
02

4

6

8

10

1214

10

1214

16

-0.3
distance(m )Through all the experiments shown above, the featuresthat must be realized in good master-slave system fortelexistence were actually realized in impedance control
type, so the validity of impedance control type wasconfirmed. The features were as follows:10
tim e(s)

-0.35

-0.4

-0.45

-0.5
tim e(s)Y-axis-0.05
04. Head mechanism

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Now we are developing the head mechanism withvision system of Telesar II for more efficient experimentsof telexistence with vision feedback. The overall view ofthe head mechanism is shown in Fig.9, and its allcomponents designed in the virtual environment ofPro/Engineer, the three dimensional CAD system, areshown in Fig.10. It has the screw mechanism to adjust thedistance between two cameras to the distance between
two eyes of the operator.
In the presentation, we will show the additional
experiments using vision feedback of the headmechanism of Telesar II to demonstrate the feasibility ofthe mutual telexistence concept.

distance(m )

-0.1
-0.15
-0.2
-0.25
-0.3
-0.35
tim e(s)Z-axisMasterSlave

Fig. 7: Results of experiments to check the accuracy offollowing ability

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ISMCR2004M asterSlave

5. Conclusions(M aster)-(Slave)108
6force(N)420-202468

10121416-4

This paper described the mechanical design anddevelopment of the new master-slave arm for mutualtelexistence. The validity of the bilateral impedancecontrol was confirmed in fundamental experiments. With
this system, the operator will be able to move the slave
robot in a remote location very smoothly with a highsense of presence.

-6-8

6. Acknowledgments-10tim e(s)Fig. 8: The result of the experiment using the slave arm

This work has been supported by the project of Japan
Science and Technology Agency (JST), Core Research for
Evolutional Science and Technology (CREST).

7. References[1] S. Tachi, H. Arai, and T. Maeda: “Tele-existence
master slave system for remote manipulation,” Proc. Int’lWorkshop on Intelligent Robots and Systems (IROS’90),
pp. 343-348, 1990.
[2] R. Tadakuma, I. Kawabuchi, H. Kajimoto, N.
Kawakami, and S. Tachi: “Mechanism of an
Anthropomorphic 7-DOF Slave Arm for Telexistence,”
Proceedings of 13th International Symposium on
Measurement & Control in Robotics (ISMCR’03),
Madrid, Spain, pp. 79-82, 2003.

Fig. 9: The overall view of the head mechanism

CCD camera[3] Riichiro Tadakuma, Yoshiaki Asahara, Ichiro
Kawabuchi, Hiroyuki Kajimoto, Naoki Kawakami, andSusumu Tachi: “Development of an Exoskeleton-type 6DOF Master Arm for Telexistence,” Proceedings of 35th
International Symposium on Robotics (ISR2004), Paris,France, TU33-3, 2004.
[4] S. Tachi, T. Sakaki: “Impedance Controlled MasterSlave Manipulation System Part I: Basic Concept andApplication to the System with Time Delay,” Journal ofRobotics Society of Japan, Vol. 8, No. 3, pp.241-252,
1990.

DC motorFig. 10: The all components of the slave head

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