Bilateral Teleoperation under Time-Varying Delay using Wave Variables
Massimo Satler, Carlo Alberto Avizzano, Antonio Frisoli, Paolo Tripicchio and Massimo Bergamasco
Abstract—Any teleoperation system involving two distant
devices is affected by communication delay due to the physical
gap between the devices. Several approaches based on wave
variables have already been proposed to deal with time-varying
delay. However, these approaches are too conservative resulting
in high degradation from the constant time delay case. In
this paper, we propose a new control scheme for bilateral
teleoperation under time-varying communication delay entirely
developed in the wave variables domain. The proposed method
minimizes the performance degradation from the constant
time delay case. Experimental results show the validity of the
proposed scheme.
I. INTRODUCTION
Telerobotics is the field of robotics that deals with distant,
yet coupled robots. Teleoperation systems have been origi-
nally used in hazardous environment (Goertz [9]). Control
laws for such systems are either unilateral (i.e. without
force feedback) or bilateral. For an overview on the control
challenges and a survey on current approaches see [18], [11]
and [4], [1] for a quantitative comparison of different control
architectures.
A Bilateral teleoperation system consists in a double infor-
mation flow that allows the user to interact with the remote
environment and, at the same time, provides him with a force
feedback. It has been proved that adding force feedback to
a teleoperation system emphasizes the sense of telepresence
improving the user’s ability to perform complex tasks [27].
Position-Position (PP) and Position-Force (PF) schemes are
examples of conventional bilateral control law which are well
used in practice. Although bilateral teleoperation can provide
better performances than unilateral one, in combination with
even a small communication delay stability problems may
arise.
Several results using H1 optimal control appeared in the
mid 1990s [15], [12], [29]. Leung [16], introduced a compen-
sator for delayed teleoperators that achieved stability for a
prescribed time delay margin while optimizing performance
specifications. Sano et al. [26], [25], suggested the use of
gain scheduled H1 controller using measured time delay.
Sliding-mode approach has also been used: one degree of
freedom teleoperated system without delay [5], and time-
varying delayed communication by Park et al. [24], [23] and
Cho et al. [6]. Anyway these kinds of approaches assume
that the slave environments and the human users are known
linear time-invariant systems.
This work was supported by PERCRO Laboratory
M. Satler, C. A. Avizzano, A. Frisoli, P. Tripicchio and M.
Bergamasco are with PERCeptual RObotics Laboratory, Sant’Anna
Superior School of Advanced studies, Pisa m.satler@sssup.it,
carlo@sssup.it, a.frisoli@sssup.it,
p.tripicchio@sssup.it, bergamasco@sssup.it.
Fig. 1. CREATE system installation
In the late 1980s and early 1990s, network theory came
into play through impedance representation, hybrid repre-
sentation and passivity-based control. The passivity-based
approach paved the way for stable time-delayed teleoper-
ation. Anderson and Spong [2], [3], introduced the notion
of scattering variables which were well-known in transmis-
sion line theory. A conceptually similar formulation to the
scattering variables appeared subsequently in Niemeyer and
Slotine [21], the so called wave variables. These approaches
provided the basis for the modifications of classical two-
channel teleoperation architecture such as PP and PF scheme,
in order to deal with communication delays. In essence, wave
variables method introduces a new telemanipulator controller
design approach in which the stability of the overall system
will be dependent on both the local controllers and the
interacting environment. Dependence on the communication
channel latencies is no longer present.
Recently, computer networks, as e.g. the Internet, are
becoming very attractive since they allow widespread and
economic installation of teleoperation systems [13], [20]
and [28]. Kosuge pointed out the problem of time-varying
delay over a computer network, especially on the Internet,
and proposed a virtual time delay method. All data are
temporarily stored into a buffer and later re-extracted with
a fixed sample time. The method seems too conservative
since the system behaves like under a constant time delay
that is estimated at the worst case. Niemeyer and Slotine
also proposed a model which guarantees both no drifting
and passivity. Their approach keeps the system passivity
rigorously and thus the performance, especially in blackout
stage, seems to degrade too much. Brady and Tarn used a
forward time observer developed for supervisory control over
the Internet.
Recently, interesting results were obtained by Lozano [17],
The 2009 IEEE/RSJ International Conference on
Intelligent Robots and Systems
October 11-15, 2009 St. Louis, USA
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