Mechanism of an Anthropomorphic 7-DOF Slave Arm for Telexistence
Riichiro Tadakuma, 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, kaji, kawakami, tachi}@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 otherpeople using gestures and remote dexterous
manipulation tasks. In this paper, we describe thespecifications of the experimental setup of the slavearm for demonstration of the feasibility of the mutualtelexistence concept.

1Introduction

We have been progressively moving towards theconclusion of a complete anthropomorphic sevendegrees of freedom (7-DOF) redundant arm for amaster-slave system to support “mutual telexistence,”
which is the concept of presenting the realistic senseof presence of a robot’s operator to a remote location.
This area of field robotics places special demands on
the system because it must be endowed with humanlike shape and movement, and is also safe for use ineveryday environments with ongoing human contact.
We describe here the mechanism design andfeasibility evaluation.

2Previous Work

Many anthropomorphic 7-DOF redundant arms ofhumanoid 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 the Japanese National
Project on Advanced Robot Technology [1]. Anotherexample was the MIA arm of Waseda University [2],designed to realize compliant motion.
Two of the full-size humanoid robot systems from
Honda, called P2 and P3, have anthropomorphic 7DOF arms [3]. H6 and ISAMU built by the Universityof Tokyo and Kawada Industries, Inc. also have 7DOF redundant arms [4]. However, these arms’ firstvertical yaw joints in their shoulders have a large
offset to enlarge the motion space, so they are a bit
different from normal anthropomorphic robotic arms.
Another example was built by SARCOS ResearchCorp. [5] The robot’s body was anchored at the waist

and most of its joints were actuated by hydraulicpumps using oil pressure, so it is infeasible to be usedin field robotics tasks such as moving around ineveryday settings.
The most recent example is NASA’s Robonaut [6].
It is a human-scale manipulator designed to fit within
the exterior volume of an astronaut's suit and to realize
thermal endurance to cope with eight hours of ExtraVehicular Activity (EVA).

3

System Details and Experimental Setup

3.1 Design of the arm
The arm was designed to be as light as possible inorder to move quickly and to be safe for human use.
By uniting the housing parts of harmonic drive gearsystem with other parts such as the rotational axes ofjoints, 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 humanarm. The distribution of the joints of the arm replicatesthe human arm’s structure in order to make it easy to
be operated by telexistence using kinaesthetic
sensation. This structure is also useful for interaction
with people without a sense of incongruity. A
complete view of the arm is shown in Figure 1.
In order to confirm the validity of our slave arm’sfunctions, we compared it with other anthropomorphic
7-DOF arms. For comparison, we chose designs
whose specification details were released in references,and the three axes of joints in their shoulders andwrists cross at one point, just like the structure of thehuman arm. The result is shown in Table 1 (humandata was taken from Refs. [7-9]). The degree values ofthis table are displayed considering the posture ofFigure 1 as a neutral point.
Through this comparison, we confirmed that ournew slave arm supports sufficient payload and speed
for mutual telexistence using gestures, and since it ismuch lighter than existent arms, the potential dangerof 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.

Table 1 Comparing designs of the new slave arm
Robot

Weight [kg]Mobility Shoulder
range
[deg]

J1
J2
J3Elbow

J4

WristJ5J6J7Payload [kg]Velocity [m/s]

New slavearm(Humanarm)
7.3-180/+180(-60/+180)
0/+180(0/+165)-180/+180(0/+100)
0/+135(0/+130)-180/+180(0/+180)-35/+35(-35/+35)-35/+35(-35/+35)
0.5
1.2

Mitsubishi
ExtremeCondition
Robot
34.2

Waseda
MIA arm-90/+180-50/+180-5/+165-35/+90-100/+100-90/+900/+135

0/+125-70/+220-90/+90-45/+85-90/+75-65/+65
52.8-32/+32
0.5
125

3.2 Back-drivability of the joints of the arm
In the design sequence of the slave arm
mechanism, a three-dimensional CAD system wasused to measure the distribution of the moment ofinertia and the position of the center of gravity of eacharm component, in order to calculate precisely thetorque needed to move each joint.
We also considered back-drivability of each joint,
which is a critical aspect for a humanoid robot that has
to interact with humans safely and to perform ineveryday environments. To estimate the backdrivability, dynamics, power and endurance of eachjoint of the slave arms, we made an experimental joint
with the same specifications as the shoulder of theslave arm. A complete view of the test joint is shownin Figure 2.
Timing pulley and belt
DC motor

Harmonic drive system

Figure 2 Experimental joint with harmonic drive
system and DC motor

Figure 1 Complete view of the 7-DOF arm

This joint has the same harmonic drive system andDC motor in each joint of the slave arm. The reduction
ratio of the harmonic drive is 50. Through a test
measuring the electric current of the motor and thepower output by the joint model, we confirmed thatthe model has enough back-drivability to ensure safety
for a communication robot and enough power to movethe arm in the same way as a human arm. The test
joint can provide 20.6 Nm of load moment and starts
back-drive at 3.46 Nm with no backlash.
Thus, the reduction ratio of the harmonic drive ofeach joint of the slave arm was set to 50 to maintainback-drivability. The torque needed to back-drive theharmonic drive of each joint is estimated with thespecifications table in the catalogue of the component
and the result of the joint model test described above.
Because of this back-drivability, we can measure thetorque of each joint of the slave arm by monitoring thecurrent that flows to the DC motor of the joint without
any additional force sensors. After the calculations andexperiments described above, we decided the power ofthe motor for each joint of the arm. The actuators forthe arm are shown in Table 2.

has a large offset from the output shaft of theharmonic drive system that drives the joint, and asmaller backlash and higher rigidity and backdrivability than a timing belt is required for amechanism after the output shaft of the harmonic drivesystem. This mechanism realizes all three axes of thewrist crossing at one point, just like a human wrist.

YawOutput shaft ofthe harmonicdrive system

Steel belt andpulley

Table 2 Actuators of the slave armShoulder
Motor
Elbow Motor
Wrist Motor

J1, J2J3J4J5, J6, J7

Reduction Ratio

150 [W]
90 [W]
90 [W]
17.5 [W]

PitchRoll

50

The drive mechanism used for the pitch axis of
the wrist is shown in Figure 3. The output shaft of theDC motor is input into the harmonic drive gear systemthrough the timing belt and pulley. Because the output
shaft of the harmonic drive system with a highreduction ratio is connected to the joint directly, theelasticity of the timing belt can be neglected. The
same mechanism is adopted in all joints of the arm.
Harmonic drive system
DC MotorFigure 4 Mechanism of the wrist joints
3.4 Mechanism of the Pitch axis of the shoulder
Since the Pitch axis of the shoulder is a cantileverstructure, we adopted angular contact ball bearings
with pressurization at both ends of the rotational axis,
in order to receive the thrust forces that work on thebase of the Pitch axis as shown in Figure 5.
These thrust forces work at the base of theshoulder by decomposing the bending moment fromthe weight of the mechanism below the shoulder into apair of forces, as shown in Figure 6.
This support mechanism realized the stabilized
rotation of the Pitch axis of the shoulder.
Pressure

Timing pulley and belt

Figure 3 Drive mechanism used forpitch axis of the wrist
3.3 Mechanism of the wrist joints
A steel belt and pulley were used for the lastrotational axis of the Roll joint of the wrist mechanismof the arm as shown in Figure 4, because the Roll joint

Angular contact bearings

Figure 5 Position of angular contact bearings

Thrust6Acknowledgments

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

7ThrustBending moment

Figure 6 Forces on the Pitch axis of the shoulder

4Control System

Figure 7 shows an overview of the control system
for the arm. HPtec µSB-4A is adopted as a motor
driver for each wrist actuator. µSB-10A is adopted forthe actuator above the elbow joint, because 90W and150W motors require more current than the motors inthe wrist joints.
CPU Board (Pentium 4 - 2.4GHz)

6 axes force sensor
AD Board (PCI-3177C, 64ch)
Potentiometer/Photo Interrupter

PCI
Back Plane

DA Board (PCI-3346A, 32ch)

Encoder Counter Board(PCI-6205C, 32ch)

Motor Driver

DC Motor
EncoderReferences[1] T. Oomichi, M. Higuchi, and K. Oonishi:“Design method of the Dexterous Fingered
Manipulator,” Journal of the Robotics Society ofJapan, Vol. 16 No. 4, pp. 508-517, 1998.
[2] T. Morita, N. Tomita, T. Ueda, and S. Sugano:“Development of Force-Controlled Robot Arm
Using Mechanical Impedance Adjuster,” Journalof the Robotics Society of Japan, Vol. 16 No. 7,pp. 1001-1006, 1998.
[3] M. Hirose, T. Takenaka, H. Gomi, and N.
Ozawa: “HUMANOID ROBOT,” Journal of theRobotics Society of Japan, Vol. 15 No. 7, pp.
983-985, 1997.
[4] K. Nishiwaki, T. Sugihara, S. Kagami, F.
Kanehiro, M. Inaba, and H. Inoue, “Design andDevelopment of research platform for perceptionaction integration in humanoid robot: H6,” Proc.
IEEE/RSJ Int. Conf. On Intelligent Robots andSystems, Takamatsu, pp. 1559-1564, 2000.
[5] 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: “Usinghumanoid robots to study human behavior,”
IEEE Intell. Syst. Appl. 15 (4), pp. 46-56, 2000.

Figure 7 Control system for the arm

5Conclusions

This paper described the mechanical design anddevelopment of the new anthropomorphic 7-DOF
slave arm for Telexistence. It has enough degrees offreedom, joint angle range and torque to satisfydynamic gestures for daily conversation and remotedexterous manipulation tasks. Further, the backdrivability of each joint ensures safe operation forhuman contact. In the near future, the robot will moveand match its mechanical impedance according to the
dynamic model of its operator’s body. With thissystem, the operator will be able to move the robot in
the remote place very smoothly, and “mutualtelexistence” with a high sense of presence will
become possible for both the operator and the remoteobservers.

[6] 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 Journal, August 2000.
[7] D. A. Winter: “Biomechanics and Motor Controlof Human Movement,” 2nd edn. Wiley, New
York, NY, 1990.
[8] M. Rosheim: Robot Evolution: The Developmentof Anthrobotics, Wiley, New York, NY, 1994.
[9] V. M. Zatsiorsky: “Kinematics of HumanMotion,” Human Kinetics, Urbana, IL, 1998.