WP-14

16th IEEE International Conference on Robot & Human Interactive Communication
August 26-

29, 2007 / Jeju, Korea

Towards Safe Human-Robot Interaction: Joint Impedance Control
of a New Teleoperated Robot Arm
Dzmitry Tsetserukoul, Riichiro Tadakuma2, Hiroyuki Kajimoto3, Naoki Kawakami4, and Susumu Tachi5,
Member, IEEE

Dzmitry Tsetserukou, Riichiro Tadakuma, Naoki Kawakami, and Susumu Tachi, Department of Information Physics andComputing, University of Tokyo, Tokyo, Japan,
e-mail: dima teterukovgipc.i.u-tokyo.acjp, (tadakuma, kawakami, tachi)@ star.t.u-tokyo.acjp
3Hiroyuki Kajimoto, Department of Human Communications, University of Electro-Communications, Tokyo, Japan,
e-mail: kajimotoghc.uec.acjp

2'4'5

Abstract-The paper focuses on design and joint impedancecontrol of a new teleoperated robot arm enabling torquemeasurement in each joint by means of incorporation of devised optical torque sensors. When the contact of arm with anobject occurs, joint impedance algorithm provides active compliance of corresponding robot arm joint. Thus, the whole
structure of the manipulator can safely interact with unstructured environment. In the paper, we describe detailed design
procedure of the 4-DOF robot arm and optical torque sensors.
To extract force signal from measured data, the gravity compensation algorithm was elaborated and verified. The experimental results of joint impedance control show that proposed
strategy provides safe interaction of entire structure of robot
arm with human beings and ensures the collision avoidance.

I. INTRODUCTION

Since teleoperation system was successfully developedby Goertz [1], the vast spectrum of application fields, including rescue operations, surgery, and space exploration,
has been found. Typically, a slave robot controlled by theoperator mainly performs the manipulations in unknown andunstructured environment. Moreover, teleoperated robot
interacts with human in various cooperative tasks. In theseapplications, the force sensing ability is a crucial aspect ofsafe interaction in the environment. Recently, researchers
have made a big progress in the improvement of arm contact
detection ability. The NASA/DAPRA Robonaut teleoperated robot has a new tactile glove instrumented with 19 force
sensors to grasp control realization [2]. In the developed
system, the operator moves the robot arm until the palm
contacts the object. However, the insensitive part of therobot arm still carries dangerousness for the surrounding
objects. 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 intime-varying environment. The sensitive skin is able toidentify the point of collision rather than the external force
value. As this device integrates a huge amount of small
sensors incorporated in tiny rigid platform, and requires
complicated wiring and signal processing hardware, thesensitive skin has high cost and reliability issues. Besides,
there are some robot parts, such as elbow and shoulder joints,
which are extremely difficult to cover with skin sensors. S.
Morinaga and K. Kosuge [4] proposed collision detection
system without using external sensors. The system recog-

978-1 -4244-1 635-6/07/$25.00 i2007 IEEE.

Teleoperated slave robot

Operator cockpit

Fig. 1 Master-slave teleoperation systemnizes collisions based on the difference between the actual
torque applied to the joints and the reference torque calculated from manipulator dynamics, and then generates thecompliant motion in response to disturbance. However, inaccuracy and change of the dynamic model parameters deteriorate sensitivity threshold up to 17.4 N at the elbow joint.
The dexterous robot-human cooperative work requires continuous precise measurement of force in range of 0-50 N.
We have successfully developed master-slave robot systems (TELESAR I and TELESAR II) based on the telexistence concept [5], and have gradually improved performanceand stability of teleoperation. The operator cockpit has two
6-DOF exoskeleton-type structure arms with hands (Fig. 1).
The slave robot enables to perform dexterous operations bymeans of two 7-DOF arms and 8-DOF hands. The external
force is measured through the 6-axis torque/force sensor (BLAUTOTECH MINI 4/20) attached to the robot arm wrist.
The present teleoperated robot also provides safe working
area only around the wrist. To realize safe and friendly
human-robot interaction, our primary idea is devoted to thedesign of whole-sensitive slave arm (by using distributed
torque sensors in each joint) and compliant motion realization (when contact occurs). The main issue here is the cost ofcommercially available force/torque sensors providing redundant information for the case of only one-axis momentmeasurement. Moreover, to incorporate the sensors with theunique robot design, the additional mounting plates have to
be installed, making the joint structure bulky. In [6], authors860

described the implementation of strain-gauge-based torquesensors aimed at facilitation of torque-controlled robot.
However, strain gauges have such inherent drawbacks as
fragility and complicated calibration procedure. The
low-pass filter is required to get readable data from thesesensors because of their high sensitivity to electrical noise.
The torque measurement through the elasticity of the harmonic drive flexsplines allows keeping the same stiffness of
the robot [7]. This method requires the strain gauges to beinstalled on the flexsplines, and suffers from nonlinear behavior of the drive output shaft. In order to facilitate realization of impedance control of each arm joint, we have developed new optical torque sensors having high reliability,
good accuracy, easy mounting procedure, and low price.
The first part of the paper is addressed to the descriptionof the development of whole-sensitive robot arm and new
optical torque sensors. In the second part, we discuss theprocedure of gravity force estimation, the position-based
impedance control algorithm, and experimental results.

Torque sensor distribution in
Shoulder Pitch and Roll Joints

_

x\l

I

I

s SensorsIin Yaw- and Elbow, Jointszo

Yaw

II. DESIGN OF THE NEW ANTHROPOMORPHIC ROBOTARM

In teleoperation and service application of robots, human-robot interaction is considered as crucial factor for arobot design, and imposes strict requirements on its behavior
and control. From the safety point of view, to minimize injures in case of collision, most of the robot parts were madefrom aluminium alloys to obtain lightweight structure, andthe robot links were made with round shape reducing impact
force. The distribution ofthe arm joints replicates the humanarm structure in order to make it easy to operate using kin-esthetic sensation during teleoperation.
In order to remove mechanical subsystems without disassembling the main structure when the failures do occur, wehave been using a modular approach while designing anthropomorphic robot arm. Therefore, we selected CSF-series
gear head type of Harmonic Drive instead of compact andlightweight component one. The Harmonic Drive offers
such advantages as accurate positioning, high torque capability, torsional stiffness, and high single stage ratios. Developed teleoperated robot arm has 4-DOF. Each joint isequipped with optical torque sensor, directly connected tothe output shaft of Harmonic Drive. The sizes and appearance of the arm were chosen so that the sense of incongruity
during interaction with human is avoided. We kept the arm
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 the developed arm and coordinate systems basedon Denavit-Hartenberg convention are represented in Fig. 2.
The motors are equipped with magnetic encoders having
512 pulses per revolution. To prevent influence of bending
moment and axial force on the torque value measurement,
the simple supported loaded shaft configuration was implemented using couple of bearings.
The principal specifications of the developed arm aregiven in Table I.

Y2
J3

Z-4

Y41

Y3

X4/~~~J
z3

Y51

z5
x5

Fig. 2 3D CAD arm model and coordinate systems
TABLE I
PRINCIPAL SPECIFICATIONSParameters
New robot arm-180 to 180 (-60 to 180)
JI, Pitch
Mobility range
J2, Roll-180 tolO (-165 to 0)(New robot arm/ Shoulder
Human arm)
-180 to 180 (-60 to 180)
J3, Yaw[deg]
O to 1 12 (O to 130)
Elbow
J4, Pitch
JI, Pitch
90, Maxon RE 35
ShoulderMotor power
J2, Roll
60, Maxon RE 35[W], motor type26.6, Faulhaber 2657
J3, Yaw
Elbow
26.6, Faulhaber 2657
J4, Pitch
JI, Pitch
7.8, (CSF-14-GH)
Harmonic drive
ShoulderJ2, Roll
7.8, (CSF-14-GH)
rated torqueJ3, Yaw
5.0, (CSF-11-2XH)[Nm], (type)Elbow
J4, Pitch
5.0, (CSF-1 1-2XH)

III. DEVELOPMENT OF THE NEW OPTICAL TORQUESENSORIn our previous work, we described the techniques of thetorque measurement, their advantages and shortcomings,
and pointed out the motivations behind using an optical approach [8]. While designing a new optical torque sensor, wefollowed the guidelines listed below:
1. Addition ofthe torque sensor to a robot joint shouldcause a minimal modification in kinematics and dynamics.

861

2. Sensitive element should have high signal-to-noise
ratio to ensure high resolution of torque measurement.
3. Mechanical structure should be machined from asingle piece of metal in order to eliminate hysteresis.
4. Simple to manufacture, low in cost, and robust.
Hirose and Yoneda [9] proposed to use a split type photodiode to detect the displacement ofthe light source (LED).
However, the output signal ofthis sensor is highly nonlinear.
The novelty of our method is application of the ultra-small
size photointerrupter (PI) RPI-121 as sensitive element [10].
We use linear section of the PI transferring characteristic tomeasure the shield displacement. The dimensions of the
RPI- 121 (3.6 mm x 2.6 mm x 3.3 mm) and its weight of 0.05
g allow realization of compact sensor design. Furthermore,
PI also has small influence from the electromagnetic fieldand stray light. The sensor is set between the driving shaft ofthe harmonic transmission and driven shaft of the joint(Fig.3).

Fig. 4 Torque sensor of shoulder pitchjoint

The sensor calibration results are presented in Fig. 5.
A 1)
4.U-

,|
-* Calibration data
~ Best fit line

3.5-1
3.0-

a)

Ct

2.5-

0

2.00

Slider adjusted by screws with shield

1.5I n-

PI

-4

-2

0

Torque T, Nm

2

4

Fig. 5 Output vs. loading torque

To determine sensitivity and nonlinearity of the sensor,
we used the best fit line approach. Calculated sensitivity
equals to 0.317 V/Nm, and nonlinearity is 2.5 00 of Full
Scale. Specifications of several developed sensors andanalogous 6-axis T/F sensor are listed in Table II.
T

01

SpringDriven shaft adapter plate

Output shaft ofharmonic drive

Fig. 3 Construction of the optical torque sensor

When the load is applied to the arm, the spring in corresponding joints is deflected. This causes the adapter plate
rotation with small angle of twist, 0. The shield displacement
is detected by the degree of interruption of infrared light that
falls on the phototransistor. Thus, the magnitude of theoutput signal ofthe PI corresponds to the applied torque. The
relationship between the sensed torque, T, and the angle oftwist is the following:
T =ko =k(01-02)'(1)

where k is the torsional stiffness of the spring member.
Since angle of twist is small, it can be calculated from thedisplacement of the shield in tangential direction, Ax:
T = k Ax/R,(2)

where R is the distance from the sensor center to the middle
axis of the shield plate in radial direction.
The sensors were manufactured from one piece of steel
using wire electrical discharge machining cutting (Fig. 4).

TABLE IITECHNICAL SPECIFICATION
Third joint
Firt t Second

joint

Fourth

joint

12.5

ATI Mini
45SI-29010

10.5

4.5

10.0

3 230

2 730

950

35 000

1.76
45.0

1.7

1.82

5.4

42.0

37.0

45.0

6.0
50.81

5.0
34.95

15.7
92.0

SensorFirstjoint

Sensing range,[± Nm]
Torsional stiffness, [Nm/rad]Factor of safety
Outer diameter,[mm]

Thickness, [mm]Sensor mass, [g]

7.0

70.15

The main advantages of ATI sensors [1I1] are high torsional stiffness and large factor of safety. Nevertheless, suchfeatures as compactness, easy manufacturing, low cost andsimple calibration procedure make developed sensors preferable for torque measurement in robot arm joints.
In addition to contact force, torque sensor continuously
measures the gravity load. To extract the value ofthe contact
force from sensor signal, we elaborated the gravity
compensation algorithm.
IV. GRAVITY COMPENSATION ALGORITHM AND
JOINT IMPEDANCE CONTROL

A. Gravity Compensation
In this section, we consider the problem of computing thejoint torques corresponding to the gravity forces acting on862

links with knowledge of kinematics and mass distribution. It
is assumed that due to small operation speed the angularaccelerations 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 Ov0 = G, where G is
the gravity vector [12]. First, link linear accelerations i+b C1of the center of mass (COM) of each link are iteratively
computed (3). Then, gravitational forces '+'Fi+1 acting at the
COM of the first and second link are computed from (4):O0=gZ=9; i+l) Ci1

i+lF

0.010-

0.005-.0 00 4I49---

-T

.0

f7

.0

'i+ ,P~ii

(3)

t~

(4)

0

where mi+l is the mass of the link i+1,' +Ri is the matrix ofrotation between successive links.
While inward iterations, we calculate force j (5) andtorques 'ni (6) acting in the coordinate system of each joint.
In the static case, the joint torques caused by gravity forces
are derived by taking Z component of the torque applied to
the link (7).

i+'. R'.+l +l + Fi=

0.015-

4c

i+1 '6=m

The plot of error the torque value measurement is shownin Fig. 7. The maximum error value of 0.015 Nm is considered as threshold magnitude when contact occurs.

(5)ni = i+' Ri+1ni+l +'Pc x 'Fi + 'Pi+, x i+'.R' '+f+l(6)

ci = 'niT .,(7)

where ipc is the vector locating the COM for the link i, ipi+lis the vector locating the origin of the coordinate system i+ 1
in the coordinate system i.
The joint torques were calculated using the approachcited above. For example, for the second joint we have:x2 = LCM2g(C3C4(C1C3S2 +S1S3)+S3(ClC4s2s3 -C3C4S1 -ClC2S4)) ++Ln1gc9C2s2 +L2M2g (s3 (C1s2s3 C3S1) + C3 (C1C3S2 + S1S3 )), (8)-where Lc, Lc are the distances defined using 3D CAD
model to the COM of the first and second link; cl, C2, C3, C4,
S1, S2, S3, and S4 are abbreviations for cos(OI), COS(02), COS(03),COS(04), sin(0O), sin(02), sin(03), and sin(04), respectively.
The plot of measured (while rotating) and calculated
values of torque acting on the fourth joint is shown in Fig.6.
0.40
0.35X
0.30

.0
20

40

60

80

100

120

Joint angle 04, (deg)

140

160

180

Fig. 7 Error of torque measurement

Thus, using gravity compensation algorithm we can easily determine the contact occurrence with an environment.
B. Joint Impedance Control

The dynamic equation of an n-DOF manipulator in joint
space coordinates (during interaction with environment) is
given by:
M(O)O + C(O, 0) + If (0) + g(O) = I + IEXT

(9)where 0, 0, 0 are the joint angle, the joint angular velocity,and the joint angle acceleration, respectively; M(0) E R"' isthe symmetric positive definite inertia matrix; C(
Q0, ) Rn isthe vector of Coriolis and centrifugal torques; Tf (0) E R' isthe vector of actuator joint friction forces; g(0) E Rn is thevector of gravitational torques; X E R' is the vector of actuator joint torques; TEx e Rn is the vector of external disturbance joint torques.
People can perform dexterous contact tasks in daily activities while regulating own dynamics according totime-varying environment. To achieve such skillful behavior,
the robot has to change its dynamic characteristics depending on time-varying interacting forces. The most efficientmethod of controlling the interaction between a manipulator
and an environment is impedance control [13], [14]. Thisapproach enables to regulate response properties ofthe robot
to external forces through modifying the mechanical impedance parameters. The graphical representation of joint
impedance control is given in Fig. 8.
FJEXT.~0.25-I

Jd(i+ 1)00.20T ET(i+l )0.15*:0.10-

ff

H° 0~
00 X
0

A

;Reference torque

0.0

H0.00

1)

Measured torque

20

40

60

80

100

120

Joint angle 04, (deg)

/TET,

140

Fig. 6 Measured and referenced torques

160

180
Fig. 8 Concept of the local impedance control

863

The desired impedance properties of the i-th joint ofmanipulator can be expressed as:
JdiAOi + Ddii + KdiAi IxTi; A0i

Oi Odi'(10)where Jdi, Dd1, Kd, are the desired inertia, damping, andstiffness of the i-th joint, respectively; TEXTi is the torqueapplied to the i-th joint and caused by external and gravity
forces; AOi is the difference between the current position O,iand desired one Odi. The state-space presentation of theequation of local impedance control is written as follows:
Avj

0K
1 dj o
d-i/

01Jj

i()

(11)

or:

AOj =A 0 j + B-cFTi ()(12)

where the state variable is defined as vi = A01; A, B are thematrices. After integration of (12), the discrete time presentation of the impedance equation is expressed as:

LAO kl1 ALAOk 1+d
j+ BdTEXT(k)AO. ljL=Ad

In many studies, the model-based impedance control isadopted [15]. However, due to parametric uncertainties of
the robot dynamics model, it is difficult to obtain the complete dynamics description. Furthermore, because of
nonlinearity of dynamics, the identification and estimation
methods could not be easily used. To improve the service
task effectiveness, we decided to implement position-based
impedance control (Fig. 9). In this case, compliant trajectory
generated by the impedance controller is tracking by PD
control loop. Thus, inherent dynamics of the robot does notaffect the performance of target impedance model. To verify
the theory and to evaluate the feasibility and performance ofthe proposed impedance controller, the experiment was
conducted with newly developed teleoperated robot arm(Fig. 10). During the experiment, the force was periodicallyapplied to the tip of the second link in opposite directions.
The experimental results for the elbow joint - applied torque,angle generated by impedance controller, and joint angle in
the function of calculation cycle - are presented in Fig. 11,
Fig. 12, and Fig. 13, respectively.

(13)

To achieve the fastest possible non-oscillatory responseon the external force, we assigned the eigenvalues XI and X2of the matrix A as real and equal X,=X2=X, X>0. By using

Cayley-Hamilton method for matrix exponential determination, we have:= e)1
1 1(4J
AdA
DdT

(14)

Id) 1-)IT--DdTlJd ]

Bd= (Ad I)A-'B

eXT (1

2XT) -1
-,

Kd

L

( Dd

1(2Jd ))2

(15)

TeXT

where T is the sampling time; I is the identity matrix.

Fig. 10 Developed whole-sensitive robot arm

Fig. 9 Block diagram of position-based impedance control864

this arm with joint torque measuring ability, the new opticaltorque sensors characterized by good accuracy, high signal-to-noise ratio, and compact sizes were designed andmanufactured. The effectiveness of the position-based impedance control for providing safe human-robot interaction
was experimentally illustrated on a new whole-sensitive
robot arm. Our next goal is realization of the intelligent
variable impedance control with regard to human-robot
cooperation tasks.

_o
.-

Q--

Q:L

¢:L

REFERENCES

Time t, (sec)[1] R. C. Goertz, "Fundamentals of general purpose remote manipulators," Nucleonics, vol. 10, no. 11, 1952, pp. 36-42.

Fig. 11 Applied external torque[2] M.A. Diftler, C. J. Culbert, R. 0. Ambrose, Jr. R. Platt, and W. J.
Bluethmann, "Evolution of the NASA/DARPA Robonaut control
system," in Proc. of the 2003 IEEE Int. Conf on Robotics andAutomation, 2003, Taipei, pp. 2543-2548.

2.7-

-

u

1.8-<

0.9

0

0.0

u

\

[3] V. J. Lumelsky and E. Cheung, "Real-time collision avoidance inteleoperated whole-sensitive robot arm manipulators," IEEE Trans. on
Systems, Man, and Cybernetics, vol. 23, no. 1, January, 1993, pp.
194-203.

I-~ -0.9

[4]

0

-1.8
r_o

-.

o

4

8

12

Time t, (sec)

16[5] R. Tadakuma, Y. Asahara, H. Kajimoto, N. Kawakami, and S. Tachi,"Development of anthropomorphic multi-D.O.F. master-slave arm for
mutual telexistence," IEEE Trans. on Visualization and Computer
Graphics, vol. 11, no. 6, November, 2005, pp. 626-636.

Fig. 12 Output angle of impedance model-3-[6] G. Hirzinger, A. Albu-Schaffer, M. Hahnle, I. Schaefer, and N. Sporer,"On a new generation of torque controlled light-weight robots," inProc. of the 2004 IEEE Int. Conf on Robotics andAutomation, Seoul,
2004, pp. 3356-3363.

-4-5-

11-

.h

[7] M. Hashimoto, Y. Kiyosawa, and R. P. Paul, "A torque sensing technique for robots with harmonic drives," IEEE Trans. on Robotics andAutomation, vol. 9, no. 1, February, 1993, pp. 108-116.

-6-

a)

7-7

4

-8

o9

S. Morinaga and K. Kosuge, "Compliant motion control of manipulator's redundant DOF based on model-based collision detection
system," in Proc. of the 2004 IEEE Int. Conf on Robotics andAutomation, New Orleans, LA, 2004, pp. 5212-5217.

4Time t, (sec)

812

11 6

Fig. 13 Measuredjoint angle

The experimental results show the successful realizationof joint impedance control. While contacting with human,the robot arm generates compliant motion in opposite direction to the external force vector. Thus, safety in interaction with environment is secured. As we assigned the critically damped response of impedance model to disturbanceforce, output angle (A0k,) has ascending-descending exponential trajectory. The value of the joint impedance caneasily be adjusted according to the task by selection of appropriate parameters of desired impedance model.
V. CONCLUSION AND FUTURE WORK

New whole-sensitive teleoperated robot arm was devel-

oped to provide human-like capabilities of contact task
performing in a broad variety of environments. To facilitate[8] D. Tsetserukou, R. Tadakuma, H. Kajimoto, and S. Tachi, "Opticaltorque sensors for implementation of local impedance control of thearm of humanoid robot," in Proc. of the 2006 IEEE Int. Conf on
Robotics andAutomation, Orlando, FL, 2004, pp. 1674-1679.

[9] S. Hirose and K. Yoneda, "Development of optical six-axial force
sensor and its signal calibration considering nonlinear interference," inProc. of the 1990 IEEE Int. Conf on Robotics and Automation, Cincinnati, OH, 1990, pp. 1674-1679.
[10] Photointerrupter Design Guide, Product catalog of ROHM, 2005, pp.
6-7.

[11] ATI, Mlulti-axis FIT sensor, ATI Industrial Automation, [Online],
Available: http:/w[12] J. Craig, Introduction to Robotics: Mechanics and Control, Addison-Wesley, New York: 1989, ch. 4.
[13] N. Hogan, "Impedance control: an approach to manipulation," ASME
J. of Dynamic Systems, Measurement and Control, 1985, pp. 1-23.
[14] N. Hogan, "Stable execution of contact tasks using impedance control," in Proc. of the 1987 IEEE Int. Conf on Robotics and Automation, Raleigh, NC, 1987, pp. 1047-1054.
[15] R. Carelli and R. Kelly, "Adaptive impedance/force controller for
robot manipulators," IEEE Trans. Automat. Contr., vol. 36, no. 8,
August, 1991, pp. 967-972.

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