Development of a Whole-Sensitive Teleoperated Robot Arm using TorqueSensing TechniqueDzmitry Tsetserukou* Riichiro Tadakuma* Hiroyuki Kajimoto*† Naoki Kawakami*
Susumu Tachi*(*) The University of Tokyo, Department of Information Physics and Computing, Japan(*†) The University of Electro-Communications, Japan
E-mail: dima_teterukov@ipc.i.u-tokyo.ac.jp, r-tadakuma@aist.go.jp, kajimoto@hc.uec.ac.jp,
kawakami@ star.t.u-tokyo.ac.jp, tachi@ star.t.u-tokyo.ac.jp
Abstract
In this paper, we concentrate on the design of a
new whole-sensitive robot arm enabling torquemeasurement in each joint by means of developed
optical torque sensors. When the contact of arm withan object occurs, local impedance algorithm provides
active compliance of corresponding robot arm joint.
Thus, the whole structure of the manipulator can safely
interact with unstructured environment. The detailed
design procedure of the 4-DOF robot arm and opticaltorque sensors is described in the paper. The gravity
compensation algorithm was elaborated and verified
by implementation of the local impedance control of asimple two-link arm manipulator.

1. Introduction
In the recent years, robots that are able to perform
work in human daily environments, such as offices,
hospitals and homes, has been successfully developed[1]. This causes the necessity to support assurance ofsafe human-robot-environment interaction.
In teleoperation systems, the same problem arises
since a slave robot controlled by the human operator
mainly performs the manipulations in unknown andunstructured environment. Moreover, teleoperated
robot interacts with human in various cooperative tasks.
In these applications, the force sensing ability is a
crucial aspect of safe interaction in the environment.
The NASA/DAPRA Robonaut teleoperated robot has a
new tactile glove instrumented with 19 force sensors to
grasp control realization [2]. The operator moves therobot arm until the palm contacts the object.
However, the rest part of the robot arm still carries
dangerousness for the surrounding objects.

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Area of safeinteraction withenvironment

Collision with
object can occura) Slave robot armof Telesar II

Optical torquesensors
enabling wholesensitive armb) Proposed robot
arm

Figure 1. Concept of whole-sensitive robot
arm realization
The whole-sensitive arm manipulator covered witha sensitive skin capable of detecting nearby objects
was designed by V. J. Lumelsky et al. [3] to avoid
collisions in time-varying environment. The sensitive
skin is able to identify the point of collision rather thanthe external force value. S. Morinaga and K. Kosuge[4] proposed collision detection system without using
external sensors. The system recognizes collisions
based on the difference between the actual input torqueapplied to the joints and the reference input torquecalculated from manipulator dynamics. However,inaccuracy of the dynamic model parameters
deteriorates sensitivity threshold up to 17.4 N at the
elbow joint. The dexterous robot-human cooperative
work requires continuous measurement of force inrange of 0-50 N.
We have successfully developed master-slave robot
systems (TELESAR I and TELESAR II) based on thetelexistence concept [5]. The safe working area isprovided only around the wrist where the force sensor
is attached (Fig. 1a).
To realize friendly human-robot interaction, ourprimary idea is devoted to the design of wholesensitive slave robot arm (by using distributed torquesensors in each joint) and compliant motion realization
when contact occurs (Fig. 1b). The main issue here isthe cost of the commercially available compact
force/torque sensors providing redundant information
for the case of only one-axis moment measurement.
Besides, to incorporate the sensors with unique robot
design, the additional mounting plates have to beinstalled, making the joint structure bulky. The authors[6] implemented strain-gauge-based torque sensors tofacilitate torque-controlled lightweight robot. However,strain gauges have such inherent drawbacks as
considerable electrical noise level, easy breakableness,complicated calibration procedure, etc. Work
experience with strain-gauge-based 6-axis force/torquesensor showed inevitability of implementation of theadditional digital low-pass filter to get readable data
scarifying bandwidth of signal as small as 60 Hz.
In order to realize impedance control of each arm
joint, we developed new optical torque sensors having
high reliability, good accuracy, easy mounting
procedure,andlowprice.
The
4-DOF
anthropomorphic robot arm with torque measuring
ability in each joint has been designed for the slave
robot. To extract the value of contact force from thetorque sensor signal, we propose the procedure ofgravity force estimation.

during interaction with human is avoided. We kept thearm proportions the same as in average height human:
upper arm length L1 - 0.308 m; upper arm
circumference - 0.251 m (diameter 0.080 m); forearm
length L2 - 0.241 m; forearm circumference - 0.189 m(diameter 0.06 m). The 3D CAD model of developedarm and coordinate systems based on DenavitHartenberg convention are represented in Fig. 2.

2.
Designofwhole-sensitive
anthropomorphic robot arm

Z2

In order to remove mechanical subsystems without
disassembling the main structure when the failures do
occur, we were using a modular approach while
designing anthropomorphic robot arm. Therefore, we
selected CSF-series gear head type of Harmonic Drive
instead of compact and lightweight component type.
The Harmonic drive offers such advantages as accurate
positioning, high torque capability, torsional stiffness,and high single stage ratios. Most of the robot parts
were made from aluminium alloys to obtain as muchlightweight structure as possible. The distribution of
the arm joints replicates the human arm structure inorder to make it easy to operate using kinesthetic
sensation during teleoperation. Developed teleoperated
robot arm has 4-DOF. Each joint is equipped with
optical torque sensor. The sizes and appearance of thearm were decided so that the sense of incongruity

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Distribution of the
optical torque sensors

+

Yaw

Z1
Y2

J2

Y1

Z0
Y0

X0 +
Roll

+

Pitch
J1

X1

X2

L1
J3

Z4

Y4

X4

Y3

J4

X3
Z3

Z5

L2

Y5

X5
Figure 2. 3D CAD arm model andcoordinate systems
The principal specifications and kinematicparameters of the developed arm are listed in Table 1
and Table 2, respectively.

Table 1. Principal specifications
Parameters
Shoulder: J1,
Mobility Pitch
range
Shoulder: J2, Roll
(Humanarm),
Shoulder: J3, Yaw[deg]
Elbow: J4, Pitch
Shoulder: J1, Pitch
Motor
Shoulder: J2, Roll
power,
Shoulder: J3, Yaw[W]
Elbow: J4, Pitch
Drive
Shoulder: J1,Pitch
rated
Shoulder : J2,RolltorqueShoulder: J1, Yaw[Nm],
Elbow: J4, Pitch(type)New slave arm-180 to 180(-60 to 180)
-180 to10(-165 to 0)-180 to 180(-60 to 180)
0 to 112 (0 to 130)90, Maxon RE 35
60, Maxon RE 35
26.6, Faulhaber 2657
26.6, Faulhaber 2657
7.8, (CSF-14-GH)
7.8, (CSF-11-2XH)
5, (CSF-11-2XH)
5, (CSF-11-2XH)

Table 2. Denavit-Hartenberg parameters of
the robot arm
i
1
2
3
4
5

αi-1 [deg]
90
-90
90
-90
0

a i-1 [m]
0
0
0
0
L2

di [m]00L100θi [deg]
θ1 - 90θ2 + 90
θ3 + 90
θ4 - 90
0

The motors are equipped with encoders having 512
pulses per revolution. To prevent influence of bending
moment on the torque value measurement, the simple
supported loaded shaft configuration was realized.
In addition to contact force, torque sensor
continuously measures the gravity and dynamic load.
Teleoperated slave robot arm usually works with small
speeds; and dynamics influence is negligible
comparing with gravity load. Besides, it cannot be
accurately calculated. To extract the value of contact
force from a sensor signal, we elaborated algorithm forcalculation of torque caused by contact with an object.

from complicated calibration procedure, requireapplication of Digital signal processor for real-time
computation of measured force, and have an average
accuracy of 5% of Full Scale.
Our objective was to develop a torque sensor forthe specific application – teleoperated robot arm.
While elaboration of the optical torque sensor structure,
we followed the guidelines listed below:
1. Addition of the torque sensor to a robot joint
should cause a minimal modification in kinematics anddynamics. Therefore, lightweight sensor with small
width is preferable.
2. High Signal-to-noise ratio to ensure highresolution of torque measurement.
3. To avoid mechanical resonance, torsional
stiffness of the sensor should not considerably reduce
the natural frequency of the robot arm.
4. Mechanical structure should be machined froma single piece of metal in order to eliminate hysteresis.
5. Reliable, simple to manufacture, and low in cost.
The distinctive feature of our method is applicationof the ultra-small size photointerrupter (PI) RPI-121 asa sensitive element to measure relative motion of thesensor components. The linear section of thetransferring characteristic of PI can be used for
detection of the relative displacement of an object [9].
Compact overall dimensions of the sensor can berealized due to the small dimensions of PI (RPI-121:
3.6 mm × 2.6 mm × 3.3 mm) and its weight (0.05 g).
Furthermore, PI also has negligible influence from theelectromagnetic field and stray light. We developed
several sensor prototypes to find out the optimal
solution [10]. The optical torque sensor is set between
the harmonic drive and driven shaft of the joint (Fig.3).

Slider adjusted by screws with shield

3. New optical torque sensor design
Optical approach to torque measurement considers
measuring displacement between unmovable andflexible parts of the torque sensor by means of lightsource and photosensor. Comparing to electrical andelectromagnetic methods, it ensures better noise
immunity, easier sensing element installation
procedure, high robustness, and low cost
manufacturing. S. Hirose and K. Yoneda [7] proposed
to use a split type photodiode to detect thedisplacement of the light source (LED) caused by force
applied in two directions. In cooperation with Minebea
Co. OPFT series of the 6-axes optical force sensors
were manufactured [8]. These transducers are compact,
lightweight, and low in cost. However, they suffer

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PI

θ2
T

θ1

SpringDriven shaft adapter plate

Output shaft ofharmonic drive

Figure 3. Structure of the optical torque sensor

When a load is applied to the arm, the spring incorresponding joints is deflected. This causes theadapter plate rotation with small angle of twist, θ. The
shield displacement is detected by the degree ofinterruption of infrared light that falls on thephototransistor. Thus, the magnitude of output signalof the PI corresponds to the applied torque. The
relationship between the sensed torque, T, and theangle of twist is the following:
T = kθ = k (θ1 − θ 2 ) = k ∆x R ,force and contact force of ± 25 N at the tip of the arm.
Such materials as high yield-strength stainless steel 174PH, 13-8 VAR as well as AISI 4135 and 4130 steel
can be employed for manufacturing the spring member.
We decided to use AISI 4135 steel with tensile yieldstrength σyield of 785 MPa. The results of analysis
using FEM demonstrate tangential displacement in mm(Fig. 5a) and von Mises stress in MPa (Fig. 5b) of theloaded springs in elbow joint J4 under the torque TZ of
4.5 Nm.

(1)

where k – torsional stiffness of the spring member, ∆x– displacement of the shield in tangential direction, R –distance from the sensor center to the middle axis of
the shield plate in radial direction. The drawing of the
hub-spoke-shaped spring is presented in Fig. 4.
b54a) Tangential displacement

2

Y

R

Figure 5. Results of analysis using FEM

6

1

7
r

Z

X

D

l

b) von Mises stressT
3

The torsional stiffness is calculated from Eq. (1)using tangential displacement value (Fig. 5a). The
sensor was manufactured from one piece of steel using
wire electrical discharge machining (EDM) cutting.
Components and assembly of the torque sensor
incorporated in third/forth joints are presented in Fig. 6.

tFigure 4. Hub-spoke structure layout
The detector consists of the inner part 1 connectedby the flexure 3 with the outer part 2, fixed PI 5, andshield plate 4. For axial alignment of the spring with
output shaft of the harmonic drive, we provided for theround shape protrusion 6. The protrusion 7 is intended
for increasing the transferring torque rate. The
torsional stiffness of the sensor [11] is derived from:⎛ 1 3r 3r ⎞k = 4 NEI ⎜ + 2 + 3 ⎟ ,
l ⎠
⎝l l
2

(2)

Figure 6. Torque sensor of elbow joint
The test rig for calibration of the optical sensor has
been developed (Fig.7). Loading torque was created bymeans of attachment of reference weights to the leverarm.
Torque sensor

where N – number of spokes, E – modulus of elasticity,
I – moment of inertia of the spoke cross section, l – thespoke length, and r – inner radius of the sensor.
The value of I can be calculated as follows:
I = bt 3 /12 ,(3)

where b – beam width, t – beam thickness.
The sensing range of the sensors was decided basedon the value of torque in each joint caused by gravity

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Lever arm

WeightFigure 7. Calibration setup

The calibration was realized by changing the massof attached weights as well as lever length, andmeasuring the output signal from the PI. Calibration
results are presented in Fig. 8.

Output voltage U, V

4.0

Calibration data
Best fit line

3.5
3.0
2.5
2.0

In this section, we consider the problem ofcomputing the joint torques corresponding to thegravity forces acting on links with knowledge ofkinematics and mass distribution. It is assumed that,
due to small operation speed, the angular accelerations
equal zero. The Newton-Euler dynamics formulation
was adopted. In order to simplify the calculation
procedure, the effect of gravity loading is included by
setting linear acceleration of reference frame 0ϑ0 = G ,where G is the gravity vector. First, link linear
accelerations i +1ϑ&C of the center of mass (COM) of

1.5
1.0

4. Gravity compensation algorithm and
impedance control implementation

i +1

-4

-2

0

2

4

Torque T, Nm

each link are iteratively computed for i = 0 to i = 4 (4).
Then, gravitational forces i+1Fi+1 (5) acting at the
COM of the first and second link (i = 2 and i = 4) arecomputed:

Figure 8. Output vs. loading torqueϑ&0 = gZˆ0 ; i +1ϑ&C =i +1 Ri iϑ&i
i +1
F = m i +1ϑ& ,(4)

0In order to determine sensitivity and non-linearityof the optical torque sensor, we used the best fit line
approach. Calculated sensitivity of the transducer
attached to the joint J4 equals 0.317 V/Nm. Nonlinearity of 2.5 % of Full Scale was calculated using
maximum deviated value from the best fit line.
Specifications of the developed optical torque sensors
and analogous 6-axis T/F sensor are listed in Table 3.
Table 3. Technical specifications
SensorSensing
range,[± Nm]
Torsional
stiffness,[Nm/rad]
FactorofsafetyOuter
diameter[mm]
Thickness,[mm]
Mass, [g]

First
joint

Secondjoint

Third/
fourthjoint

ATI Mini
45 SI-29010 [12]

12.5

10.5

4.5

10.0

i +1

i +1

2 730

950

35 000

1.76

1.7

1.82

5.4

i

42

37

45

7.0

6.0

5.0

15.7

68.4

49.8

34.2

92.0

The main advantages of the ATI sensors [12] arehigh torsional stiffness and large factor of safety.
Nevertheless, such features as compactness, easy
manufacturing process, low cost, and simple
calibration procedure make the developed sensors
preferable for torque measurement in robot arm joints.

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ni =

i
i +1

i +1

fi = i +1i R i +1 fi +1 + i Fi

R ni +1 + PCi × Fi + Pi +1 ×
τ = i nT i Zˆ ,
i

i

i

i

i

i

(6)
i
i +1

R

i +1

fi +1

(7)
(8)

where i PC – vector locating the COM for the link i,
i

Pi +1 – vector locating the origin of the coordinate
system i+1 in the coordinate system i.
The joint torques were calculated using theapproach cited above. The experiment with simple 2link arm (Fig. 9a) was conducted in order to measure
the joint torque caused by gravity forces and estimate
the measured error by comparison with reference
model. The system consists of: DC motors with speed
reducers (gear ratios of 2548) to guarantee very low
backdriving of joints; manufactured torque sensors(sensing range is ±0.8 Nm); and rigid links. The plotof measured (while rotating) and calculated values of
torque acting on the first joint is shown in Fig. 9b.
i

45

(5)

Ci+1

where mi+1 – mass of the link i+1, i+1Ri – matrix ofrotation between successive links calculated using
Denavit-Hartenberg notation (Table 2).
While inward iterations, we calculate force ifi (6)and torques ini (7) acting in the coordinate system of
each joint. In the static case, the joint torques causedby gravity forces are derived by taking Z component of
the torque applied to the link (8).

i

3 230

i +1

6. References[1] K. Kaneko, F. Kanehiro, S. Kajita, et al. “Humanoid
Robot HRP 2”, Proc. of the 2004 IEEE Int. Conf. on
Robotics and Automation, New Orleans, 2004, pp. 10831090.
a)F1=m1gF2=m2gb)

Figure 9. Experimental setup andmeasurement results
The error of the torque value measurement wasconsidered as a threshold magnitude when contact
occurs.
The local impedance control was realized for 2link arm to ensure safety in interaction using gravity
compensation approach. Results of response are shownin Fig. 10. We used the following equation describing
desired dynamics of robot joint i in joint space:τ EXTi = J di ∆θ&&i + Ddi ∆θ&i + K di ∆θi ; ∆θ i = θ ci − θ di , (9)where τEXTi – torqueapplied to joint iand caused byexternal and gravity
Experimental results offorces, Jdi – desired
response on external
inertia,
Ddi
–
force
desired damping,
Kdi–
desiredstiffness, θci –Figure 10. Response graphoutput joint angleof the impedance model, θdi – desired joint angle.

5. Future work
The developed robot arm has been alreadyassembled. In near future, we will start to makeexperiments. Implementation of described approach
can significantly contribute to safety improvement of
human-robot interaction. Proposed torque sensor
eventually diminishes stiffness of the arm andintroduces angular error. However, it does notdeteriorate the performance of human-robot interaction.
The robot arm with torque sensing ability can also be
applied as a haptic device. The user can manipulate itin impedance control mode to perform action withremote objects.

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[2] M.A. Diftler, C. J. Culbert, R. O. Ambrose, Jr. R. Platt,and W. J. Bluethmann, “Evolution of the NASA/DARPA
Robonaut Control System”, Proc. of the 2003 IEEE Int. Conf.
on Robotics and Automation, Taipei, 2003, pp. 2543-2548.
[3] V. J. Lumelsky and E. Cheung, “Real-time Collision
Avoidance in Teleoperated Whole-sensitive Robot Arm
Manipulators”, IEEE Trans. on Systems, Man, andCybernetics, vol. 23, no. 1, 1993, pp.194-203.
[4] S. Morinaga, and K. Kosuge, “Compliant Motion Controlof Manipulator`s Redundant DOF based on Model-based
Collision Detection System”, Proc. of the 2004 IEEE Int.
Conf. on Robotics and Automation, New Orleans, 2004, pp.
5212-5217.
[5] R. Tadakuma, Y. Asahara, H. Kajimoto, N. Kawakami,
and S. Tachi, “Development of Anthropomorphic MultiD.O.F. Master-slave Arm for Mutual Telexistence”, IEEE
Trans. on Visualization and Computer Graphics, vol. 11, no.
6, November 2005, pp. 626-636.
[6] G. Hirzinger, A. Albu-Schaffer, m. Hahnle, I. Schaefer,and N. Sporer, “On a New Generation of Torque ControlledLight-Weight Robots”, Proc. of the 2006 IEEE Int. Conf. on
Robotics and Automation, Seoul, 2004, pp. 3356-3363.
[7] S. Hirose and K. Yoneda, “Robotic Sensors with
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Industrial Robotics, 1989, pp. 271-278.
[8] A. Kitamura, S. Adachi, and S. Tanabe, “Optical
Displacement Sensor and External Force Detecting Device”,
U. S. Patent 7 057 154 B2, Jun 6, 2006.
[9] “Photointerrupter Design Guide”.
ROHM, pp. 6-7, 2005.

Product catalog of[10] D. Tsetserukou, R. Tadakuma, H. Kajimoto, and S.
Tachi, “Optical Torque Sensors for Implementation of Local
Impedance Control of the Arm of Humanoid Robot,” Proc.
of the 2006 IEEE Int. Conf. on Robotics and Automation,
Orlando, 2006, pp. 1674-1679.
[11] D. Vischer and O. Khatib, “Design and Development ofHigh-performance Torque Controlled Joints,” IEEE Trans.
on Robotic and Automation, vol. 11, no. 4, August 1995, pp.
537-544.
[12] “Multi-axis F/T Sensor,” ATI Industrial Automation,[Online], Available: http://www.ati-ia.com