IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS,

VOL. 11,

NO. 6,NOVEMBER/DECEMBER 2005

1

Development of Anthropomorphic Multi-D.O.F.
Master-Slave Arm for Mutual Telexistence
Riichiro Tadakuma, Yoshiaki Asahara, Hiroyuki Kajimoto,
Naoki Kawakami, and Susumu Tachi, Member, IEEE
Abstract—We developed a robotic arm for a master-slave system to support “mutual telexistence,” which realizes remote dexterous
manipulation tasks and close physical communication with other people using gestures. In this paper, we describe the specifications of
the experimental setup of the master-slave arm to demonstrate the feasibility of the mutual telexistence concept. We developed themaster arm of a telexistence robot for interpersonal communication. The last degree of the 7-degree-of-freedom slave arm is resolvedby placing a small orientation sensor on the operator’s arm. This master arm is made light and impedance control is applied in order togrant the operator as much freedom of movement as possible. For this development stage, we compared three control methods andconfirmed that the impedance control method is the most appropriate to this system.
Index Terms—Telexistence, virtual reality, force feedback, bilateral impedance control.

æ
1

INTRODUCTIONW

E developed a robotic arm for a master-slave system
to support “mutual telexistence,” which realizes
remote dexterous manipulation tasks and close physicalcommunication with other people using gestures. In thispaper, we describe the specifications of the experimental
setup of the master-slave arm to demonstrate the feasibility ofthe mutual telexistence concept. We developed the masterarm of a telexistence robot for interpersonal communication.
To facilitate the operator’s arm motions, the master arm wasdesigned as a 6-degree-of-freedom structure, thus avoiding
interference with the operator’s arm, particularly the elbow.
The last degree of the 7-degree-of-freedom slave arm is
resolved by placing a small orientation sensor on theoperator’s arm. This master arm is made light and impedancecontrol is applied in order to grant the operator as much
freedom of movement as possible.

2

TELESAR I FOR TELEXISTENCE

It has long been a desire of human beings to project
themselves into 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], [2], [3], [4]. The firsttelexistence master-slave system “TELESAR (Telexistence
Surrogate Anthropomophic Robot) I” is shown 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. The authors are with the Department of Physics and Computing (IPC),
Graduate School of Information Science and Technology (IST), Universityof Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 Japan.
E-mail: {tadakuma, asahara, kaji, kawakami, tachi}@star.t.u-tokyo.ac.jp.
Manuscript received 7 Dec. 2004; revised 25 Mar. 2005; accepted 24 May
2005; published online 8 Sept. 2005.
For information on obtaining reprints of this article, please send e-mail to:
tvcg@computer.org, and reference IEEECS Log Number TVCGSI-0235-1204.
1077-2626/05/$20.00 ß 2005 IEEEfocus on producing human-like, realistic movement for
TELESAR II.

3

THE MECHANISM OF THE MASTER-SLAVE ARM
OF TELESAR II

3.1 Slave Arm for Mutual Telexistence
We have been progressively developing a complete
anthropomorphic seven degrees of freedom (7-DOF) redundant arm for a master-slave system to support mutual
telexistence. This area of field robotics places special
demands on the system because it must be endowed withhuman-like shape and movement and is also safe for use ineveryday environments with on-going human contact.
3.2 Previous Work
Many anthropomorphic 7-DOF redundant arms of humanoid robots have already been designed and built to replicate
human skills. One of the first was the “Extreme Condition
Working Robot” of Mitsubishi Heavy Industries Ltd. for theJapanese National Project on Advanced Robot Technology[5]. Another example was the MIA arm of Waseda University[6], designed to realize compliant motion.
Two of the full-size humanoid robot systems from
Honda, called P2 and P3, have anthropomorphic 7-DOF
arms [7]. H6 and ISAMU, built by the University of Tokyoand Kawada Industries, Inc., also have 7-DOF redundantarms [8], [9], [10]. However, these arms’ first vertical yaw
joints in their shoulders have a large offset to enlarge themotion space, so they are a bit different from normal
anthropomorphic robotic arms.
Another example was built by SARCOS Research Corp.
[11]. The robot’s body was anchored at the waist and mostof its joints were actuated by hydraulic pumps using oilpressure, so it is infeasible for use in field robotics tasks
such as moving around in everyday settings.
The most recent example is NASA’s Robonaut [12], [13],[14]. It is a human-scale manipulator designed to fit within
the exterior volume of an astronaut’s suit and to realize
Published by the IEEE Computer Society

2IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS, VOL. 11,

NO. 6,NOVEMBER/DECEMBER 2005

TABLE 1Comparing Designs of the New Slave Arm

Fig. 1. The first telexistence master-slave system “TELESAT I.”

thermal endurance to cope with eight hours of ExtraVehicular Activity (EVA).

3.3 Mechanical Design of the Slave Arm
The arm we developed was designed to be as light as
possible in order to move quickly and to be safe for humanuse. By uniting the housing parts of the harmonic drive gearsystem with other parts, such as the rotational axes of joints,
we made the whole mechanism of the arm very light.
Moreover, by this design, we succeeded in making the arm
mechanism as slim as the outer human arm. The distribution of the joints of the arm replicates the human arm’s
structure in order to make it easy to operate by telexistence
using kinesthetic sensation. This structure is also useful forinteraction with people without a sense of incongruity. A
complete view of the arm is shown in Fig. 2.
In order to confirm the validity of our slave arm’s
functions, we compared it with other anthropomorphic
7-DOF arms. For comparison, we chose designs whose
specification details were released in references and thethree axes of joints in their shoulders and wrists cross at one
point, just like the structure of the human arm and our slavearm. The result is shown in Table 1 (human data was takenfrom [15], [16], [17]). The degree values of this table aredisplayed considering the posture of the left picture of Fig. 2
as a neutral point.
Through this comparison, we confirmed that our new
slave arm supports sufficient payload and speed for mutual
telexistence using gestures and, since it is much lighter thanexisting arms, the potential danger of injury due to malfunction is also greatly reduced. At the same time, our slave arm
has a larger mobility range in the joints above the elbow.

The reduction ratio of the harmonic drive of each joint of
the slave arm was set to 50 to maintain back-drivability. The
actuators for the arm are shown in Table 2.
The maximum force at the tip of the slave arm is 164 [N].
The maximum torque of each joint of the slave arm isshown in Table 3.

3.4 Control System of the Slave Arm
Fig. 3 shows the control system for the slave arm. The motor
driver controls the DC motors in the joints with a torquecontrol mode according to commands from the DA board.
The angular velocity and the posture of each joint aremeasured with an encoder attached to the motor. The
neutral point of each joint is defined by potentiometers or
photo-interrupters, whose signals are read by the AD board.
The slave arm’s control system is connected to the masterarm system through shared memory. The control system forthe master arm is the same as the slave arm system.
3.5 Master Arm for Mutual Telexistence
In order to realize a telexistence robot that produces aconvincing sense of presence, it is indispensable to capture
motion from an operator without restraining his or her freemovement and to correctly display useful force information
to the operator at the same time.
TABLE 2
Actuators of the Slave Arm

Fig. 2. The whole view of the slave arm of TELESAR II.

TADAKUMA ET AL.: DEVELOPMENT OF ANTHROPOMORPHIC MULTI-D.O.F. MASTER-SLAVE ARM FOR MUTUAL TELEXISTENCE

3

TABLE 3The Maximum Torque of Each Joint of the Slave Arm

To satisfy these criteria, we developed a master arm with
6 DOF that has an exoskeleton-type structure and assessed its
performance and efficacy through experiment. We describe
here the mechanism design and feasibility evaluation.

3.6 Mechanical Design of the Master Arm
A general anthropomorphic slave arm is built as a 7-DOF
mechanism that has the same structure as that of a person.
The master arm used as the operation system is also usually
given a 7-DOF structure, but it is difficult to realize freemotion of the operator because it tends to restrain theoperator’s elbow mechanically.
From the viewpoint of simplification of function andcontrol, we considered its high rigidity and the fact that theforce to be applied to the operator’s hand is along at most
six axes and we designed the master arm to specialize in theforce presentation function in these six axes and also madeits mechanism 6-DOF.
Because the master arm has the cantilever beam structure
as serial links, if the degree of freedom of the master arm increases, the length of the cantilever beam and the total
weight of the actuators also increase, driving down therigidity and stability of the master arm.
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 an acceleration sensor to measure the final DOF, which is critical toidentifying the posture of the operator’s whole arm.
Altogether, the master arm serves as a master system of
7 DOF for measurement of the arm’s posture and 6 DOF for

Fig. 3. Control system for the slave arm.

Fig. 4. The whole schematic of the master arm. (a) Plane figure of themaster arm. (b) Front view of the master arm.

force presentation. Since the posture sensor is very lightcompared with mechanical restraints on the operator’s
elbow, the sensor enables much freer movement of his or
her arm without any undesirable load on it.
We decided to use the exoskeleton structure because we
considered that it could be widely adapted for the movement of an operator with minimal size requirements, which
is an essential characteristic to correspond to a human’svarious actions in everyday life.
The whole schematic of the master arm is shown in Fig. 4.
The operator’s backbone is located near the center of the circle in Fig. 4. The 190 mm offset between the shoulder jointof the master arm and the elbow joint is the clearance foravoiding interference of the master arm and the operator’s
elbow and the turning radius of the gimbal mechanism at
the tip of the arm in Fig. 4 is the same length as the offset.
The whole view of the developed master arm describedin this paper is shown in Fig. 5.
A potentiometer and an encoder are installed in eachjoint and the operator’s initial posture is computed from theoutput signals from the potentiometers and, during movement, his or her arm’s joint angles and angular velocities arecomputed 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. 6, and a 6-axes force
sensor (MINI 4/20, made in BL AUTOTEC) is attached tothat point. Output signals from this sensor are used tomeasure the force that works between the wrist of themaster arm and the operator in the direction of the rotating

4

IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS, VOL. 11,

NO. 6,NOVEMBER/DECEMBER 2005

TABLE 4
Specifications of the Master Arm

Fig. 5. The master arm of TELESAR II.

axis and the torque that works around the axis. The tip ofthe master arm is currently just a simple grip, but, in thenear future, we will attach the exoskeleton-type multifingered master hand to the tip and develop a bilateralsystem including fingers.
Specifications of the master arm are shown in Table 4.
The degree values of this table are displayed considering
the posture of Fig. 4 as a neutral point.
The gear system of each joint of the master arm iscomposed of a combination of precise spur gears ofmodule 0.3, comparatively small size of the teeth, as shownin Fig. 7.
In the assembly process, the tooth outlines of the coupled
gears were precisely fitted to ensure high back-drivability
and even high reduction ratio of each joint, so the arm canbe used as a passive master arm.
We made the dynamic model of frictional resistance inthe gear system of all joints with identification experiments
and use that model for precise impedance control.
A gravity compensation system is realized by hanging the
tip of the master arm with wire, which enables the
manipulator to exhibit maximum performance. The tensionof the wire is 20.6 [N]. Because of this gravity compensation
system, the actuators of the master arm’s joints don’t have tocompensate for the gravitational torque applied to the masterarm. Therefore, the master arm is able to present forces to theoperator’s hand with small output torque and high accuracy.
The mechanism that suspends the arm tip component with awire is constituted as shown below. A passive link that has
two joints is attached above the master arm, as shown in Fig. 5.
The constant force spring passes inside the link, as shown in
Fig. 8. Constant tension works on the wire by passing it
through the pulley at the tip of the spring. The wire is attachedto the tip of the master arm. Since the joints of this link areparallel to the direction of gravity, the link can follow themaster arm smoothly with the wire that circulates through it,
while maintaining a horizontal posture.

Fig. 6. The three axes of the wrist cross at one point.

The maximum force at the tip of the master arm is 239 [N].
The maximum torque of each joint of the master arm isshown in Table 5.

4

CONTROL METHODS OF THE MASTER-SLAVEARM OF TELESAR II

4.1

Concept of Bilateral Impedance Control as Dual
Motion Transmission Method
We adapted bilateral impedance control for TELESAR II.
The basic principle of the impedance control type masterslave system is to give seemingly equivalent impedance toboth master and slave. Furthermore, by minimizing its
impedance, the master arm places only a slight load onto
the operator’s arm and he or she can perform a wide varietyof gestures and other body movement expressions without
being hindered.
Here, we explain the presently adopted Dual Motion
Transmission Method of the bilateral impedance control
that exchanges movement information between the masterand the slave [18]. In this control method, the master’s force
is equal to the slave’s and exact force presentation can be
performed in contact work. The conceptual image of Dual
Motion Transmission Method of the bilateral impedancecontrol is shown in Fig. 9. In this control method, both themaster arm and slave arm have force sensors at their wrists.
Specifically, control is performed based on the movement equations below:
For the master:€ m þ Bd X_ m þ Kd Xm  Ftm :Fo ¼ M d X

ð1Þ

For the slave:€ s þ Bd X_ s þ Kd Xs þ Fe :Fts þ Md X

ð2Þ

Here, ðMd ; Bd ; Kd Þ mean the target impedance parameters of the master arm and the slave arm, Fo means theFig. 7. The gear system of the joint of the master arm.

TADAKUMA ET AL.: DEVELOPMENT OF ANTHROPOMORPHIC MULTI-D.O.F. MASTER-SLAVE ARM FOR MUTUAL TELEXISTENCE

Fig. 8. The mechanism of the passive link for gravity compensation.

operational force to the operator, Ftm and Fts mean the innertorque of the master and the slave, Fe means the power thatworks from the object to the slave arm, Xm and Xs mean the
posture vectors of the master and the slave.
In the Dual Motion Transmission Method, the control
methods are as shown below.
For the master:€ s þ Bd X_ s þ Kd Xs :Ftm ¼ Md X

5

Fig. 9. The conceptual image of the Dual Motion Transmission Methodof the bilateral impedance control.

€ þ Bd ðX_  X_ d Þ þ Kd ðX  Xd Þ;
FEXT ¼ Md X
€ ¼ M 1 FEXT  M 1 Bd X_  X_ d
Xdd
 Md1 Kd ðX  Xd Þ:
4.

ð3Þ

ð6Þ
ð7Þ

Equation (8) shows the relationship between Cartesian space and the joint space.
_:
X_ ¼ J 

ð8Þ

For the slave:€ m þ Bd X_ m þ Kd Xm :Fts ¼ Md X

ð4Þ

5.

The time differentiation of (8) gives the angular€ , as shown below.
acceleration 

From (1)-(4), the relationship between Fo and Fe isderived as shown below:€ þ J_ _;€ ¼ J
X

ð9Þ€m  X€ s Þ þ Bd ðX_ m  X_ s Þ þ Kd ðXm  Xs Þ:
Fo ¼ Fe ¼ Md ðX
ð5Þ_:€ ¼ J 1 X
€  J 1 J_ ð10Þ

The block diagram of the adopted impedance control
system is shown in Fig. 10. The sequence of this block
diagram is as follows:
1.

2.

3.

With the encoder attached to each joint of the arm,
_ are
the joint angle  and joint angular velocity 
measured and, with the 6-axes force sensor attachedto the wrist, the external force FEXT applied to thewrist is measured.
From the measured joint angle , the position andorientation of the wrist X in Cartesian space iscalculated with forward kinematics LðÞ and X_ is_ withalso calculated from the equation of X_ ¼ J Jacobian matrix J.
Equation (6) shows the relationship of impedance.
€ is calculated
From this equation, the acceleration X
as shown in (7).

TABLE 5The Maximum Torque of Each Joint of the Master Arm

6.

Resolve inverse dynamics by the Newton-Euler
method with the parameters of the arm’s motion€;_ ; ; FEXT Þ, and the output of thisand force, (calculation, torque , is to be used as the torque foreach actuator of the arm.

_ ; GðÞThe actual dynamics of the arm, M ðÞ; V ; in the third block is calculated from the parameters ofinertia of the arm. These parameters were calculated by thethree-dimensional CAD system used to design the masterand slave arms. In the case of the master arm, X is Xm and,in the case of the slave arm, X is Xs . For both the master andslave arms, (Xd ; X_ d ) denotes information coming from the
opposing arm.

4.2

Modified Bilateral Impedance Control for 7 DOF
Anthropomorphic Slave Arm
In the bilateral impedance control between the 6 DOF
master arm and our new anthropomorphic 7 DOF slavearm, the Jacobian matrix is rank deficient (6  7) and theinverse Jacobian matrix in the second block of the
impedance control diagram of Fig. 10 cannot be calculated.
So, we opted to use the posture sensor that can measure theposition and orientation of the operator’s elbow. Thisposture sensor is composed of an acceleration sensor thatmeasures its own posture with the direction of thegravitational acceleration. We attached the sensor to theoperator’s upper arm and, with the information from thissensor, we measure the angle of one joint of the operator’sshoulder and control the same joint of the anthropomorphic

6

IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS, VOL. 11,

NO. 6,NOVEMBER/DECEMBER 2005

Fig. 10. The block diagram of the adopted impedance control.

7 DOF slave arm in PD control and control the other six
joints in impedance control. In this case, the Jacobian matrix
has full rank (6  6) and its inverse in the second block of
the diagram may now be calculated. The concept of thismodified bilateral impedance control is shown in Fig. 11.
In this way, we can control the anthropomorphic 7 DOF
slave arm with a redundant degree of freedom in bilateral
impedance control.
In this measurement method, the master side has 7 DOF for
position and orientation, while it has 6 DOF for force
feedback, as shown in Fig. 11. As the operator’s arm can be
regarded as a redundant 7 DOF manipulator, conventional
methods to solve inverse kinematics of the redundant arm
can be applied to measure the operator’s posture. One
popular method is to define a swivel angle of an arc of acircle, which the elbow traces, lying on a plane whose normal
is parallel to the wrist-to-shoulder axis. Given the wrist
position, orientation, and elbow swivel angle, the algorithm
can compute the joint angles analytically. Note that the wrist
position and orientation of the operator is identical to that of
the master manipulator. In order to get the remaining
information (swivel angle), we used the acceleration sensor(ADXL202E, Analog Devices, Inc.) attached to the operator’s
upper arm. The sensor has a high enough frequency response(1kHz), while its small size and light weight allow theoperator to move freely. We used an axis of the sensor
corresponding to the change of the swivel angle, as shown in
Fig. 12. The method to solve inverse kinematics of the slavearm is as follows: The origin ð0; 0; 0Þ is coincident with theshoulder position and ðL1 ; L2 ; L3 Þ denotes the length of theupper arm, lower arm, and the distance from the goal position
to the shoulder of the slave arm. First, the goal position P andorientation R (rotation matrix) of the slave arm’s wrist are
determined by forward kinematics of master arm. The joint
angle of the elbow 4 is then determined geometrically, as
shown in Fig. 13.
In Fig. 13, n is a unit vector from the shoulder to thewrist. u and v are unit vectors forming a local coordinate
system for the plane containing the circle traced by theelbow. u is determined arbitrarily to correspond to  ¼ 0.
Then, v is set as v ¼ n  u. Similarly to 4 ,  is determined
and the center of the circle c traced by the elbow and its
radius r can be computed as

Fig. 11. Concept of modified bilateral impedance control for the masterslave system with different degrees of freedom.

Fig. 12. Swivel angle, an acceleration sensor attached to an operator’s
upper arm, and its axis a.

cos  ¼

L21 þ L23  L22;
2L1 L3c ¼ L1 cos n; r ¼ L1 sin :

ð11Þ
ð12Þ

If we set u as the direction such that the output of theacceleration sensor a is equal to the gravitational
acceleration g,
u¼

ðnz0  n ÞTpﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃx ;n2x þ n2z

ð13ÞTADAKUMA ET AL.: DEVELOPMENT OF ANTHROPOMORPHIC MULTI-D.O.F. MASTER-SLAVE ARM FOR MUTUAL TELEXISTENCE

7

Fig. 13. Geometric relationship between shoulder, elbow, and wrist.

Fig. 14. Configuration of the human arm and the slave arm.

the swivel angle  is obtained as

In this way, the equation that shows the relationshipbetween Cartesian space and the joint space changes fromasin  ¼ :g

ð14ÞThen, the elbow position EðÞ is given byEðÞ ¼ c þ rðcos u þ sin vÞ:



x_ ; y_ ; z_ ; _ ; _ ; _

T


T
¼ J _1 ; _2 ; _3 ; _4 ; _5 ; _6 ; _7

ð16Þ

to
ð15Þ

From the relation between this elbow position EðÞ andwrist position P, three joints angles of the shoulder, 1  3 ,are determined. Next, from the orientation R of the wrist,
three joint angles of the wrist, 5  7 , are determined.
As described above, we can solve inverse kinematics ofthe redundant slave arm that satisfies the wrist’s positionand orientation obtained by the master arm and elbow
position computed from an acceleration sensor’s output.
With this method, the slave arm can replicate the operator’s
motion and posture even though the master arm has adifferent kinematic configuration from the slave arm andthe operator’s arm.
With this method of inverse kinematics using the elbow
posture sensor, we can get all the joint angles of theoperator’s arm (1  7 in Fig. 14). Among these joint
angles, we decided to use 3 , meaning the operator’s arm’s
roll joint angle at the shoulder to control the slave arm’s rolljoint at its shoulder with PD control, and to control the othersix joints of the slave arm with impedance control. We use
the Newton-Euler method of inverse dynamics to calculate
the torque for each joint of the slave arm in the third blockof the block diagram of the impedance control (Fig. 10) bysetting the angular acceleration €k to zero in order to control
the one joint with PD control. So, we choose the joint whose
angular acceleration, on average, is the smallest of all jointsof the human wrist and shoulder that are related to theredundant degree of freedom’s posture to make the interference of PD control with impedance control as small as
possible. To find this type of joint, we analyzed the angularaccelerations of all joints of a human arm in several gestureswith an optical motion capture system, OPTO-TRAK. Aftersome experimentation and comparison, we found theangular acceleration of the roll joint of human shoulder isthe smallest on average. So, we decided to control thisjoint of the slave arm with PD control. In this control
method, the Jacobian matrix becomes a full rank matrix(6  6) and we can calculate its inverse matrix for thesecond block of the block diagram of impedance control.

T
x_ ; y_ ; z_ ; _ ; _ ; _ ¼ J _1 ; _2 ;T
_4 ; _5 ; _6 ; _7ð17Þin impedance control. In this case, €3 ¼ 0. x_ ; y_ ; z_ ; _ ; _ ; _mean the time differentiations of the position and orientation of the operator’s hand. This control method is used inbilateral impedance control between the 6 DOF master arm
and the anthropomorphic 7 DOF slave arm in the last partof Section 4.
Another merit of bilateral impedance concerns operation
safety. For contact operations of the slave arm with anotherperson, the operator can feel the force from the person witha high sense of reality, so the potential danger of giving him
or her superfluous force can be avoided.
If we control the master-slave system with a Dual Motion
Transmission Method, in a state where the slave arm is
moved without applying external force from an object in theouter environment, since the master’s force and the slave’s
are not equal, the operator receives the force by the productof target impedance and the posture error. However,making target impedance as small as possible can minimize
undesirable load, unlike other control methods.

4.3 Verification of Bilateral Impedance Control
In order to verify the optimality of this system, we conductedsimple comparison experiments with symmetry and force
feedback, 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, asshown in Fig. 15.
Specifically, control is performed based on the movement equation and the control equations shown below:
For the master:€ m þ Bm X_ m þ Ftm ;F0 ¼ Mm X

ð18Þ



Ftm ¼ Kp ðXm  Xs Þ þ Kv X_ m  X_ s :

ð19Þ

8

IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS, VOL. 11,

Fig. 15. The conceptual image of Symmetry type control.

NOVEMBER/DECEMBER 2005

Fig. 16. The conceptual image of Force-feedback type control.

For the slave:€ s þ Bs X_ s þ Fe ;Fts ¼ Ms X

ð20Þ



Fts ¼ Kp ðXm  Xs Þ þ Kv X_ m  X_ s :ð21ÞHere, ðMm ; Bm Þ mean the master arm’s impedance
parameters and ðMs ; Bs Þ mean the slave arm’s impedance
parameters. Kv and Kp are the feedback gains of thevelocity and the position.
The force presented to the operator is as shown below:€ m þ Bm X_ m þ Ms X
€ s þ Bs X_ s þ Fe :F0 ¼ Mm X

NO. 6,

ð22Þ

The operator has to cope with the inertial and velocityrelated forces from both the master arm and the slave armin the symmetry type.
The force feedback type is a control method in which themaster transmits posture information to the slave and theslave transmits force information to the master, as shown in
Fig. 16. In this control method, both the master arm andslave arm have force sensors at their wrists. The masterreceives the torque command value from the product of thesuitable gain and the difference of the master’s force and theslave’s and the slave receives the torque command value
from the product of the gain and the difference between themaster’s posture and the slave’s.
Specifically, control is performed based on the movement equation and the control equation shown below:
For the master:€ m þ Bm X_ m þ Ftm ;F0 ¼ Mm X

ð23Þ

Ftm ¼ Kf ðFe  F0 Þ:

ð24Þ

€ s þ Bs X_ s þ Fe ;Fts ¼ Ms X

ð25Þ



Fts ¼ Kp ðXm  Xs Þ þ Kv X_ m  X_ s :ð26Þ

For the slave:In the following text, the word “master” refers to theright arm of the master manipulator system and the word“slave” refers to its left arm.
For the first fundamental experiment, only the yaw-axesof the shoulders of the right and left master arms weremoved. Master-slave control was performed such that,when the right arm is moved to the left, the left arm also
moves to the left, in the same direction.
We investigated the following two concerns in thiscomparison experiment:
1.

When the right arm (virtual master arm) is moved,
how well does the slave arm follow?
2. What is the presented force when force is applied by
the operator?
In all of the comparison experiments, the impedance of
the arm with the impedance control type was set as
M ¼ 6 ½kg, B ¼ 2 ½kgf=ðm=sÞ, and K ¼ 2 ½kgf=m.
First, we describe the experiment that considered theforce that works on the operator. The forces from the masterarm that work on the operator in the symmetry type, theforce feedback type, and the impedance control type areshown in the following three graphs (Figs. 17, 18, and 19).
In the symmetry type, if the operator provides acceleration, a large operational force is needed because of theinertia of both the master arm and the slave arm.
In the force feedback type, because the master iscontrolled by the force difference between the master andthe slave, as shown in (24), the force that works on theoperator is comparatively close to the value of the slave that
does not receive any force. The reason is the slave does notreceive any force from the outer environment, Fe ¼ 0 and
Ftm ¼ Kf Fo . This torque output Ftm reduces Fo and also
decreases the difference between Fe ð¼ 0Þ and Fo .
In the impedance control type, the operational force isrelatively small because of the reduction in inertia.

Here, Kf is the feedback gain of the force. The force
presented to the operator is as shown below:
F0 ¼


1 
€ m þ Bm X_ m þ Kf Fe :Mm X
1 þ Kf
1 þ Kf

ð27Þ

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 the mechanicaldifference between the master and the slave and to compare
the control methods themselves.

Fig. 17. Force on the operator in symmetry type.

TADAKUMA ET AL.: DEVELOPMENT OF ANTHROPOMORPHIC MULTI-D.O.F. MASTER-SLAVE ARM FOR MUTUAL TELEXISTENCE

9

Fig. 18. Force on the operator in the force feedback type.

Fig. 19. Force on the operator in the impedance control type.

Almost the same force is needed in the force feedback
type and the impedance control type. This is considered tobe because of the identical structures of the master and theslave in this experimental system.
In addition, we conducted several experiments using
only the yaw-axes of the shoulders of the right and left
master arms, and classified the three control methods asshown in Table 6.
However, these results can be predicted with the control
equations of the three types of control methods described in
Section 4, so these experiments simply confirm each control
theory. So, we conducted the second experiment using all
six joints of each arm to verify the optimality of this systemwith bilateral impedance control. In this case, the result of
the experiment is not easily predictable.
In the second experiment, we activated all six axes of themaster arm and the virtual slave arm. The slave arm traced
the wall and we measured the operational force and the
opposing force from the wall applied to the slave arm.
Results of this experiment are shown in Fig. 20.
For the symmetry type, the operational force is large
because the operation itself requires large force. In the force
feedback type, the virtual slave arm pushes the wall withsuperfluous force and the operator can’t feel it. In the
TABLE 6Classification of Three Control Methods

Fig. 20. Results of the second experiment of comparison. (a) Symmetry
type. (b) Force feedback type. (c) Impedance control type.

impedance control type, tracing with stable force wasrealized. So, almost the same tendency of three control
methods shown in Table 6 as the results of the experimentswith one joint is also shown in this experiment with six joints.
In the third experiment, we checked the influence ofdifferent inertia in comparison with the symmetry and
impedance control types. We attached a weight of 0.5kg to
the forearm of the virtual slave arm to increase its inertia andmeasured the operational force. The results of this experiment are shown in Fig. 21. Compared with the increase of theoperational force in the symmetry type (from 7.4 N to 9.3 N),
the increase of operational force in the impedance control
type is very small (from 3.7 N to 3.9 N). The inertia of theweight on the forearm of the slave arm is included in the
dynamic model of the slave arm in the impedance control
type and that is the reason why the operational force does notincrease so much in impedance control.
As shown in the experiments described above, thecharacteristics of the impedance control are superior tothe other two control methods. So, we decided to adapt
impedance control for the master-slave system with theactual anthropomorphic slave arm.

10IEEE TRANSACTIONS ON VISUALIZATION AND COMPUTER GRAPHICS, VOL. 11,

NO. 6,NOVEMBER/DECEMBER 2005

TABLE 7
Maximum Overshoot of the Slave Arm’s Wristin Universal Coordinate

result of this experiment is shown in Fig. 23. Small
operational force is realized, forces on the master arm
and the slave arm are comparatively close and a stabletrace is also realized. Namely, impedance control type isalso effective in this system.
Through all the experiments described above, thefeatures that must be realized in a good master-slave
system for telexistence were actually realized using theimpedance control type, so the validity of the impedancecontrol type was confirmed. The features were as follows:

Fig. 21. Results of the third experiment of comparison. (a) Symmetry
type. (b) Impedance control type.

. reproduction of the operator’s work,. precise force feedback,. stable contact,. gestures are enabled,. smooth operation,. sufficient accuracy of following ability,. safe for humans,. precise force feedback, and. stable contact operation (for shaking hands, etc.).
In the final experiment, we used the master-slave headsystem for vision feedback. The robot head is composed of
two CCD cameras and two DC motors. It has a screw
mechanism to adjust the distance between the two cameras
to match the distance between the two eyes of the operator.
Fig. 24 shows the system diagram of the robot head.

Fig. 22. The overall view of the experimental setup.

In the fourth experiment, we used the actual slave arm of
TELESAR II to confirm the validity of bilateral impedancecontrol for a master-slave arm with a different mechanicalstructure, such as degrees of freedom and inertia.
The overall view of the experimental setup is shown in
Fig. 22. In the fourth experiment, the slave arm moved infree space and we checked the accuracy of its following
ability in three axes of the universal coordinate. The resultsof this experiment are shown in Table 7. This table shows
the maximum overshoot in the direction of each axis of theuniversal coordinate. As shown in this table, sufficientaccuracy for gestures and communication was confirmed inthis experiment.
In the fifth experiment, we checked the accuracy offorce presentation of this system. The slave arm traced thewall, and we measured the operational force and the
opposing force from the wall applied to the slave arm. The

Fig. 23. The results of the experiment using the slave arm.

Fig. 24. The system diagram of the robot head.

TADAKUMA ET AL.: DEVELOPMENT OF ANTHROPOMORPHIC MULTI-D.O.F. MASTER-SLAVE ARM FOR MUTUAL TELEXISTENCE

CCD cameras are connected to up scan converters andvision information in NTSC format is converted to
VGA format for the head-mounted display (HMD). The
HMD is connected to an ADL1 that measures the orientation of the operator’s head with forward kinematics.

5

CONCLUSION

This paper described the mechanical design and control
methods of a new anthropomorphic multi-DOF masterslave arm for mutual telexistence. The validity of thebilateral impedance control was confirmed in experiments
using the master arm and the slave arm. With this system,
the operator will be able to move the slave robot in a remotelocation very smoothly with a high sense of presence.

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

REFERENCES
[1]
[2]
[3]

[4]
[5]
[6]
[7]
[8]

[9]

[10]
[11]

[12]

[13]
[14]

S. Tachi, H. Arai, and T. Maeda, “Development of an Anthropomorphic Tele-Existence Slave Robot,” Proc. Int’l Conf. Advanced
Mechatronics (ICAM), pp. 385-390, 1989.
S. Tachi, H. Arai, and T. Maeda, “Tele-Existence Master Slave
System for Remote Manipulation,” Proc. Int’l Workshop Intelligent
Robots and Systems (IROS ’90), pp. 343-348, 1990.
S. Tachi, H. Arai, T. Maeda, E. Oyama, N. Tsunemoto, and Y.
Inoue, “Tele-Existence Experimental System for Remote Operation with a Sensation of Presence,” Proc. Int’l Symp. Advanced Robot
Technology (SART ’91), pp. 451-458, Mar. 1991.
S. Tachi and K. Yasuda, “Evaluation Experiments of a Teleexistence Manipulation System,” Presence, vol. 3, no. 1, pp. 35-44,
1994.
T. Oomichi, M. Higuchi, and K. Oonishi, “Design Method of the
Dexterous Fingered Manipulator,” J. Robotics Soc. Japan, vol. 16,
no. 4, pp. 508-517, 1998.
T. Morita, N. Tomita, T. Ueda, and S. Sugano, “Development ofForce-Controlled Robot Arm Using Mechanical Impedance
Adjuster,” J. Robotics Soc. Japan, vol. 16, no. 7, pp. 1001-1006, 1998.
M. Hirose, T. Takenaka, H. Gomi, and N. Ozawa, “HUMANOIDROBOT,” J. Robotics Soc. Japan, vol. 15, no. 7, pp. 983-985, 1997.
K. Nishiwaki, T. Sugihara, S. Kagami, F. Kanehiro, M. Inaba, and
H. Inoue, “Design and Development of Research Platform for
Perception-Action Integration in Humanoid Robot: H6,” Proc.
IEEE/RSJ Int’l Conf. Intelligent Robots and Systems, pp. 1559-1564,
2000.
S. Kagami, K. Nishiwaki, T. Sugihara, J.J. Kuffner, M. Inaba, and
H. Inoue, “Design and Implementation of Software Research
Platform for Humanoid Robotics: H6,” Proc. Int’l Conf. Robotics andAutomation (ICRA ’01), pp. 2431-2436, 2001.
S. Kagami, K. Nishiwaki, J.J. Kuffner, K. Okada, M. Inaba, and H.
Inoue, “Low-Level Autonomy of Remote Operated Humanoid
Robot H6 & H7,” Proc. 10th Int’l Symp. Robotics Research, 2001.
C.G. Atkeson, J.G. Hale, F. Pollick, M. Riley, S. Kotosaka, S. Schaal,
T. Shibata, G. Tevatia, A. Ude, S. Vijayakumar, and M. Kawato,“Using Humanoid Robots to Study Human Behavior,” IEEE
Intelligent Systems, vol. 15, no. 4, pp. 46-56, July/Aug. 2000.
R.O. Ambrose, H. Aldridge, R.S. Askew, R. Burridge, W.
Bluethman, M.A. Diftler, C. Lovchik, D. Magruder, and F.
Rehnmark, “ROBONAUT: NASA’s Space Humanoid,” IEEE
Intelligent Systems, vol. 15, no. 4, July/Aug. 2000.
M.A. Diftler and R.O. Ambrose, “ROBONAUT, A Robotic
Astronaut Assistant,” Proc. Int’l Symp. Artificial Intelligence,
Robotics, and Automation in Space, June 2001.
W. Bluethmann, R. Ambrose, M. Diftler, S. Askew, E. Huber, M.
Goza, F. Rehnmark, C. Lovchik, and D. Magruder, “Robonaut, aRobot Designed to Work with Humans in Space,” Autonomous
Robots, vol. 14, pp. 179-197, 2003.

11[15] D.A. Winter, Biomechanics and Motor Control of Human Movement,
second ed. New York: Wiley, 1990.
[16] M. Rosheim, Robot Evolution: The Development of Anthrobotics. New
York: Wiley, 1994.
[17] V.M. Zatsiorsky, “Kinematics of Human Motion,” Human Kinetics,
Urbana, Ill., 1998.
[18] S. Tachi and T. Sakaki, “Impedance Controlled Master Slave
Manipulation System Part I: Basic Concept and Application to the
System with Time Delay,” J. Robotics Soc. Japan, vol. 8, no. 3,
pp. 241-252, 1990.
Riichiro Tadakuma received the BEng degreewith First Class Honors in mechano-aerospaceengineering from the Tokyo Institute of Technology in 2000 and the MEng degree in mechanicaland aerospace engineering from the Tokyo
Institute of Technology in 2002. He received
the PhD degree in advanced interdisciplinary
studies from the University of Tokyo in 2005. He
is a research fellow of the University of Tokyo
with a fellowship from the Japan Science andTechnology Agency. He was a research fellow of the Japan Society for
the Promotion of Science from 2003 to 2005. He was awarded the
Hatakeyama Award from the Japanese Society of Mechanical Engineers (JSME) in 2000. His research interests includes robotics,
telexistence, and virtual reality.
Yoshiaki Asahara received the BSci degreefrom the Department of Earth and Planetary
Science at the University of Tokyo in 2002 andthe MEng degrees in information physics andcomputing from the University of Tokyo in 2004.
He has worked at Toyota Motor Corporation,
Aichi, Japan, as an engineer.

Hiroyuki Kajimoto received the MS and PhD
degrees in mathematical engineering and information physics from the University of Tokyo.
He is a research associate in the Department ofInformation Physics and Computing at the
University of Tokyo. His research interests
include tactile displays, tactile sensors, andvirtual reality.

Naoki Kawakami received the PhD degree inadvanced interdisciplinary studies from the
University of Tokyo. He is a lecturer in the
Department of Information Physics and Computing at the University of Tokyo. His researchinterest is virtual reality.

Susumu Tachi received the PhD degree inmathematical engineering and information physics from the University of Tokyo. He is a
professor in the Department of Information
Physics and Computing at the University ofTokyo. He is a founding director of the Robotics
Society of Japan (RSJ), a fellow of the Society of
Instrument and Control Engineers (SICE), and is
the founding president of the Virtual Reality
Society of Japan (VRSJ). His research interests
include virtual reality, telexistence, augmented reality, and robotics. He
is a member of the IEEE.