Proceedings of the 1999 lEEE/RSJ
International Conference on
Intelligent Robots and Systems
Bilateral ‘Ibleoperation under
Time-Varying Communication Delay
Yasuyoshi YOKOKOHJI, Takashi IMAIDA, and Tsuneo YOSHIKAWA
Department of Mechanical Engineering, Graduate School of Engineering
Kyoto University, Kyoto 606-8501, Japan
{yokoko j i I imaida24 I yoshi}Qmech. kyoto-u. ac . jp
Abstract
Several approaches based on wave variables have al-
ready been proposed for bilateral teleoperation control
under time-varying delay. However, these approaches
are too conservative for keeping the system passiv-
ity or for estimating the amount of virtually constant
time delay. These conservative approaches result in
too much degradation of maneuverability from the con-
stant time delay situations.
In this paper, we propose a new control scheme
that is also based on wave variables, but minimizes
the performance degradation due to the fluctuation of
the communication time delay. The proposed method
is simple and easy to implement. We conducted some
simulation studies with a one DOF system. Simula-
tion results show the validity of the proposed scheme.
1 Introduction
Master-slave manipulators are often used for tele-
operation in hazardous environment. Control laws
for master-slave manipulators are either unilateral
(without force reflection) or bilateral (force-reflecting).
Force feedback in bilateral control improves the per-
formance of teleoperation in most cases.
When the slave site is located far from the mas-
ter site, communication delay between the two sites
is not negligible. For example, communication delay
between a ground station and a space robot in a satel-
lite orbit is 3-6 seconds[6]. When we use a conven-
tional bilateral control law, such as symmetric type
or force reflection type, even a small communication
delay may easily destabilize the system. A stable con-
trol method under the communication time delay was
developed by Anderson and Spong[l], and studied fur-
ther by Niemeyer and Slotine[4]. These methods are
based on wave variables instead of the conventional
power variables, i.e., velocity and force.
Recently “networked robotics”, i.e., telerobotics
over the computer network, is getting considerable at-
tentions. Any communication method over the com-
puter network has transmission delay and the delay
time fluctuates in most cases. This time-varying delay
brings some new problems when we apply the wave-
variablebased control scheme that supposes a con-
stant time delay. When the delay is constant, the
waveform of the transmitted data is not distorted. If
the delay fluctuates, however, the waveform is dis-
0-7803-5 184-3/99/$10.00 0 1999 IEEE
torted after pass through the communication line.
Consequently, position drift between the master and
the slave appears and the system passivity is no longer
maintained.
In this paper, we first show the behavior of a con-
ventional bilateral control scheme under a constant
time delay and briefly explain the wave variables in
section 2. We then point out some problems under
time-varying delay in section 3. We propose a new
wave-variablebased scheme in section 4, which is the
main contribution of this paper. Simulation results
are shown in section 5. We summarize our result and
discuss the future work in section 6.
2 Existing Bilateral Control Scheme
2.1 Destabilization due to the communi-
cation time delay
With an ideal bilateral control scheme, the operator
can execute a task as though he were present at the r e
mote site[8]. Therefore, a bilateral control scheme can
provide better maneuverability than a unilateral one.
Symmetric type and force reflection type are well used
in practice[7]. However, even a small communication
delay may destabilize the system with such bilateral
control methods. Fig.1 illustrates a simulation result
with a one DOF master-slave manipulator when 70
millisecond time delay is inserted in the communica-
tion link. Although the master arm behaves stably
without delay, it is destabilized by the time delay.
2.2 Bilateral control based on the wave
The destabilizing problem caused by conventional
bilateral schemes was studied by Anderson and
Spong[l]. They proposed a bilateral control law that
maintains stability under the communication delay us-
ing the scattering transformation theory.
Niemeyer and Slotine[4] studied further on this in-
stability problem. They introduced the notion of
“Wave Variable”. They showed that instead of trans-
mitting force or position information directly, trans-
mitting wave variables, which are transformed from
forces and velocities at the master and slave sides,
makes the system stable even under some communi-
cation time delay. They also suggested a symmetric
under Constant Delay
variables
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