Development of an Exoskeleton-type 6-DOF Master Arm for Telexistence
Riichiro Tadakuma, Yoshiaki Asahara, Ichiro Kawabuchi, Hiroyuki Kajimoto, Naoki Kawakami,and Susumu Tachi
Graduate School of Information Science and Technology, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
Phone: +81-3-5841-6917 FAX: +81-3-5841-8601
E-mail: {tadakuma, asahara, kawabuchi, kaji, kawakami, tachi}@star.t.u-tokyo.ac.jp
To solve this problem, we developed a master arm
with 6 DOF that has an exoskeleton-type structure witha small orientation sensor on the operator’s arm, andassessed its performance and efficacy throughexperiment. We describe here the mechanism design
and feasibility evaluation.

Abstract
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
last degree of the 7-degree-of-freedom slave arm is
resolved by placing a small orientation sensor on theoperator’s arm. Moreover, this master arm is madelight and impedance control is applied in order to grant
the operator as much freedom of movement as possible.
For this development stage, we compared three control
methods, and confirmed that the impedance control
method is the most appropriate to this system.

12

Mechanism design of the master arm

2.1 6-DOF arm
From the viewpoint of simplification of function
and control, high rigidity, small inertia, and the fact thatthe power to be applied to the operator’s hand is alongat most 6 axes, we designed the master arm tospecialize in the power presentation function in these 6axes, and also made its mechanism 6-DOF.
Since the measurable movement of the master arm
that follows the operator's hand has 6 DOF, we decided
to use a new lightweight posture sensor composed of
two acceleration sensors to measure the final DOF,
which is critical to identifying the posture of theoperator’s whole arm, including his or her elbow.
Altogether, the mechanism serves as a mastersystem with 7 DOF for measurement of the arm’sposture, and 6 DOF for power presentation. Since theposture sensor is very light compared with mechanicalrestraints on the operator’s elbow, the sensor enables
much freer movement of his or her arm without any
undesirable load on it.
Moreover, since the master arm has less than 7
DOF, it achieves power presentation with high rigidity
and stability.

Introduction

In order to realize a telexistence [1][2] robot that
produces a convincing sense of presence, it isindispensable to capture motion from an operator
without restraining his or her free movement, and tocorrectly display useful power information to theoperator at the same time.
The slave arm that we developed this time has 7DOF, because it aims at communication throughgestures [3]. It has the same structure as that of a person,with three DOF at both the shoulder and wrist, and asingle DOF at the elbow. For a slave arm with 7 DOF
like this one, existing master-slave systems have amaster arm used as the operation system, which also
has 7 DOF and has an exoskeleton-type structure tocapture the operator’s arm’s movement. However, it isdifficult to realize free motion of the operator because ittends to restrain the operator's elbow mechanically.
For the Humanoid Robotics Project (HRP) ofJapan [4], in order to mitigate an operator's feeling ofrestraint, a structure that detects the position of theoperator's elbow with optical sensors on the 7-DOF
exoskeleton-type master arm is adapted, and the mastersystem follows his or her elbow with force feedback
function. However, this mechanism introduces a serious
time delay to the motion of the slave arm’s elbow. Sucha time delay is severely problematic for transferring theoperator’s gesture.

2.2 Exoskeleton-type 6-DOF Master Arm
As described above, we should leave the motion of
the operator as unrestrained as possible in telexistence.
Therefore we adapted an exoskeleton-type mechanism
for the master arm.
Although the master arm does not necessarily needan exoskeleton-type structure to present forces along 6
axes, it has a mechanism similar to the human body ingeneral, and it always follows human motion. So, wedecided to use the structure because we considered thatit could be widely adapted for movement of an operator
with minimal size requirements, which is an essential1

A potentiometer and an encoder are installed ineach joint, and the operator's initial posture is computedfrom the output signals from the potentiometers, andduring movement his or her arm’s joint angles andangular velocities are computed from the output signals
from the encoders.
The three axes of the joints in the master arm’s
wrist cross at one point, as shown in Fig. 3, and a 6axes force sensor (MINI 4/20, made by BL
AUTOTEC) is attached to that point. Output signals
from this sensor are used to measure the power thatworks between the wrist of the master arm and theoperator in the direction of the rotating axis, and thetorque that works around the axis. The tip of the masterarm is currently just a simple grip, but in the near future,
we will attach the exoskeleton-type multi-fingered
master hand to the tip, and develop a bilateral system
including fingers.

characteristic to correspond to a human's variousactions in everyday life.
In our system, the number of degrees of freedom inthe master arm is less than that of a human arm, so it isimpossible for the master arm to follow an operator'swhole arm completely, and it introduces the possibilityof mutual interference between the human arm and themaster arm.
In order to be able to follow a human arm without
any interference with only 6 DOF, this master arm musthave sufficient clearance for movement of theoperator's elbow. The whole schematic of the masterarm is shown in Fig. 1. The operator's backbone islocated near the center of the circle in the figure. The
190 mm offset between the shoulder joint of the masterarm and the elbow joint is the clearance for avoiding
interference of the master arm and the operator's elbow,
and the offset absorbs the turning radius of the gimbal
mechanism at the tip of the arm in Fig. 1.

Fig. 3 The three axes of the wrist cross at one point
The system compensates for gravity by hanging thetip of the master arm with wire, which enables themanipulator to exhibit maximum performance.
Specifications of the master arm are shown in
Table 1. The degree values of this table are displayedconsidering the posture of Fig. 1 as a neutral point.

Fig. 1 The whole schematic of the master arm

2.3 Structure of the master arm
The perspective view of the master arm developed
this time is shown in Fig. 2.

JT1JT2
JT3
JT4JT5JT6

Mobility range ofeach joint-75〜60 [deg]-30〜100 [deg]-140〜0 [deg]-180〜180 [deg]-50〜230 [deg]-180〜180 [deg]Output of
the motor
150 [W]
150 [W]
36 [W]
12 [W]
12 [W]
12 [W]

Reduction ratio of
the gear system
125
625/6
625/6
525/4
525/4
525/4

Table 1 Specifications of the master arm
The gear system of each joint of the master arm iscomposed of a combination of gears of module 0.3 withgrease, as shown in Fig. 4. This ensures high backdrivability, so the arm can be used as a passive masterarm.
The mechanism in which an arm tip component ishung with a wire is constituted as shown below. The
passive link with two joints is attached above themaster arm as shown in Fig. 2. The constant force
spring passes inside the link as shown in Fig. 5.
Constant tension works on the wire by passing it

Fig. 2 The whole view of the master arm

2through the pulley at the tip of the spring. The wire isattached to the tip of the master arm. Since the joints ofthis link are parallel to the direction of the gravity, thelink can follow the master arm smoothly with the wirethat circulates through it, while maintaining ahorizontal posture.

means the operational force to the operator, F1 and F2
mean the inner torque of the master and the slave, Femeans the power that works from the object to the slavearm, X m and X s mean the posture vectors of themaster and the slave, and C1 and C2 mean the control
methods of the master-slave as proposed here.
In the presently adopted Dual Motion Transmission
Method, the control methods are as shown below.
C1 = M d X&& s + Bd X& s + K d X s
C = M X&& + B X& + K X
2systemwith

3.1 Control methods of master-slave system
The previous version of the Telexistence Surrogate
Anthropomorphic Robot (TELESAR) [1] adapted
unilateral control, but we adapted bilateral control for
TELESAR II because of the improvements intechnology such as computational power.
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, byminimizing 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.
Specifically, control is performed based on themovement equation and the control equations below.
For the master:F0 = M d X&&m + Bd X& m + K d X m − F1
F1 = C1
For the slave:

dmdm

Another merit of bilateral impedance concernsoperation safety. For contact operations of the slave
arm with another person, the operator can feel thepower from the person with a high sense of reality, so
the potential danger of giving him or her superfluous
power can be avoided.
If we control the master-slave system with a Dual
Motion Transmission Method [5] that exchanges
movement information between the master and theslave, the master’s power is equal to the slave’s, andexact power presentation can be performed in contact
work.
In a state where the slave arm is moved without
applying external power, since the master’s power andthe slave’s are not equal, the operator receives power
equivalent to the product of target impedance and theposture error. However, making target impedance assmall as possible can minimize undesirable load, unlike
other control methods.
In order to verify the optimality of this system, we
conducted simple comparison experiments withsymmetry and force-feedback, which are generalcontrol method types.
The symmetry type is a control method in which
the master and the slave transmit their posture
information to each other, and apply a suitable gain
according to the difference of their postures to produce
a torque output.
Specifically, control is performed based on themovement equation and the control equations shown
below:For the master:F0 = M m X&&m + Bm X& m + F1
(8)
F1 = K v (X& m − X& s ) + K p ( X m − X s )(9)Fig. 5 The constant force spring in the link

Themaster-slave
impedance controlm

F0 = Fe = M d (X&& m − X&& s ) + Bd (X& m − X& s ) + K d ( X m − X S ) (7)Fig. 4 The gear system of the joint of the masterarm

3d(5)
(6)

(1)
(2)

For the slave:

F2 = M d X&& s + Bd X& s + K d X s + Fe(3)(4)
F2 = C2
Here, (M d , Bd , K d ) mean the target impedance

F2 = M s X&& s + Bs X& s + Fe
F2 = K v (X& m − X& s ) + K p ( X m − X s )parameters of the master arm and the slave arm, F0

3(10)
(11)

Here, (M m , Bm ) mean the master arm’s impedance
parameters, andIn all three kinds of comparison experiments, theimpedance of the arm in the impedance control typewas set as M = 4 [kg], B = 63.2 [kgf/(m/s)], and K =250 [kgf/m].
First, we describe an experiment that confirmed theability for the master arm to follow the slave arm. The
accuracies of following in the symmetry type, force
feedback type and impedance control type are shown in
Fig. 6-8.
As indicated in these three graphs, the symmetry
type, force feedback type and the impedance control
type have almost the same accuracies of following.
Next, we describe the experiment that considered
the power that works on the operator. The powers fromthe master arm that works on the operator in thesymmetry type, the force feedback type and theimpedance control type are shown in Fig. 9-11.
For the symmetry type, if the operator provides
acceleration, a large operational force is needed
because of the inertia of both the master arm and theslave arm.
In the force feedback type, because the master iscontrolled by the power difference between the masterand the slave, the power that works on the operator iscomparatively close to the value of the slave that does
not receive any power.
In the impedance control type, the operational
force is relatively small because of the reduction ininertia.
Almost the same power is needed in the force
feedback type and the impedance control type. This isconsidered to be because of the identical structures ofthe master and the slave in this experimental system.
Finally, we conducted an experiment in whichconstant force works on the master, and the slave
pushes the wall. The results of the measurement of thepower in the experiments are shown in Fig. 12-14.
In the symmetry type and the impedance control
type, the slave presents almost the same power as themaster, but in the force feedback type, the slave pushes
the wall with greater power than the power given by theoperator to the master. It is potentially very dangerous
for a power greater than the operator’s to be presented
by the slave robot when the slave is interacting withother humans.
The comparison results of the three control
methods are summarized in Table 2. As shown here,
the characteristics of the impedance control are superior
to other two control methods for all three functions.

(M s , Bs ) mean the slave arm’simpedance parameters. K v and K p are the feedback
gains of the velocity and the position.
The power presented to the operator is as shownbelow.
(12)F0 = M m X&& m + Bm X& m + M s X&& s + Bs X& s + Fe
The force-feedback type is a control method inwhich the master transmits posture information to theslave, and the slave transmits power information to themaster. The master receives the torque command value
from the product of the suitable gain and the differenceof the master’s power and the slave’s, and the slave
receives the torque command value from the product ofthe gain and the difference between the master’s
posture and the slave’s.
Specifically, control is performed based on themovement equation and the control equation shown
below.
For the master:(13)
F0 = M m X&& m + Bm X& m + F1
(14)
F1 = K f (Fe − F0 )For the slave:
F2 = M s X&& s + Bs X& s + Fe
F2 = K v (X& m − X& s ) + K p ( X m − X s )

(15)
(16)

Here, K f is the feedback gain of the force.
The power presented to the operator is as shown
below.
F0 =Kf1(M m X&& m + Bm X& m ) +Fe
1+ K f1+ K f(17)

For each experimental trial, we used the left arm of
the master manipulator system as the slave arm because
the real slave arm was still under development. In thefollowing text, the word “master” refers to the right armof the master manipulator system, and the word “slave”
refers to its left arm.
In this fundamental experiment, only the yaw-axesof the shoulders of the right and left master arms weremoved. Master-slave control was performed such thatwhen the right arm is moved to the left, the left arm
also moves to the left, in the same direction.
We investigated the following three concerns inthis comparison experiment:
1. When the right arm (virtual master arm) is moved,
how well does the slave arm follow?
2. What does the power that the operator gives
become in the former experiment?
3. When constant power is applied at the master arm,
how strong does the slave arm push the wall?

3.2 Slave arm for master-slave system
In the near future, the efficacy of the impedancecontrol will be assessed with an experiment using all 6
joints of the master arm, and the master-slave control
with the slave arm that has different dynamic featuresfrom the master arm. For this experiment, the 7-DOF
robotic slave arm with the same distribution of thedegrees of freedom as the human arm and a different
mechanical structure from the master arm is being

4

developed at this laboratory. The whole view of theslave arm is shown in Fig. 15.

4

impedance control was confirmed in a fundamental
experiment using one joint of the master arm. In thenear future, the superior performance of the bilateral
impedance control in the master-slave system shall beconfirmed through experiment using all 6 joints of themaster arm. With this system, the operator will be able
to move the slave robot in a remote location verysmoothly with a high sense of presence.

Conclusions

This paper described the mechanical design anddevelopment of the new exoskeleton-type 6-DOF
master arm for telexistence. The validity of the0.3

1010

8

6

700

5

10

15

20-0.120-2024

6810121416

1820

force [N]40.1
force [N]distance [m]0.2time [s]0-100Fig. 9 Power on theoperator in symmetrytype

0.3

71520-0.1

20-20246810121416

18

20

force [N]0time [s]41-8-0.3

52-6

MasterSlave

63-4-0.2

Master
Slave

840.1

20

96

force [N]distance [m]

0.2

15

108

10

10
time[s]Fig. 12 Force on the armin symmetry type105

5time [s]Fig. 6 Accuracy offollowing in symmetry type04

1-8-0.3

52-6

MasterSlave

63-4-0.2

MasterSlave

9800-10510time [s]152025time [s]Fig. 7 Accuracy of following
in force feedback typeFig. 10 Power on theoperator in force
feedback typeFig. 13 Force on thearm in force feedback
type

0.3
10100.270510

15

20-0.1

20-2

02

4

6810

12

1416-0.3

time [s]Fig. 8 Accuracy of followingof Impedance control type

Accuracy offollowing
Operationalforce
Power
presentation

6542-6MasterSlave

203-4-0.2

18force [N]40force [N]distance [m]860.1MasterSlave

981-80-10

time [s]Fig. 11 Power on theoperator in impedancecontrol type

Symmetry
TypeModerate

Force feedback
eedback
type
Moderate

Impedance control
type
Moderate

BadGoodGoodGoodBadGood

05time10 [s]15Fig. 14 Force on the arm
in impedance control
type

Table 2 Classification of three control methods

Fig. 15 The whole view of the slave arm

520

5Acknowledgments

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

6References[1] S. Tachi, H. Arai, and T. Maeda: “Tele-existence
master slave system for remote manipulation,” Proc.
Int’l Workshop on Intelligent Robots and Systems(IROS’90), pp. 343-348, 1990.
[2] S. Tachi and K. Yasuda: “Evaluation Experimentsof a Teleexistence Manipulation System,” Presence,Vol. 3, No. 1, Winter 1994, pp. 35-44.
[3] 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.
[4] S. Tachi, K. Komoriya, K. Sawada, T. Nishiyama, T.
Itoko, M. Kobayashi, and K. Inoue: “Telexistence
cockpit for humanoid robot control,” Advanced
Robotics, Vol. 17, No. 3, pp. 199-217, 2003.
[5] 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.

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