This chapter commences with an examination of machine control schemes
and their suitability for web telerobotics. As there were no web
telerobots prior to this project, conventional telerobotics is examined
to determine control schemes, problems that have arisen, solutions
proposed and the form that operator work stations have taken. This is
followed by a look at the growth of the internet, an examination of
telerobots that have used the internet but not the Web and finally a
discussion of some of the other web telerobots that have been built.
and machine control schemes
Teleoperation has been used with a variety of control schemes ranging
from pure manual control to sophisticated supervisory systems. In
Figure 20 the range of machine control schemes are classified in
increasing order of sophistication from manual through to autonomous
control. Examples are provided for each control scheme as well as the
implications of each for teleoperation. Autonomous control is the ideal
control scheme but it often requires too great a development effort.
Supervisory control falls between the extremes of manual and autonomous
control and provides a mechanism for implementing the reduced
instruction set robotics described in section 2.7. The following
sections describe each control scheme in detail.
Classification of machine control schemes in increasing order of
For experimental telerobots see: + (Conway et al. 1990) * (Hirzinger et
Manual closed loop control
Manual closed loop control is the method normally used for operating a
machine tool or device. The operator can be local or remote. For the
case of a remote operator the essential components of manual closed loop
control are shown in Figure 21.
The essential components of a manual closed loop system.
In this scheme, the human operator forms part of the control loop.
Manual closed loop control is the earliest form of teleoperation and the
most studied. In its simplest form, communication is through mechanical
linkages and feedback is direct viewing plus force feedback through the
linkages. Such equipment has long been used in the nuclear industry but
is only suitable when the operator is in the immediate vicinity of the
work. Such a device is not always thought of as being teleoperated. To
enable an operator to be further from a task, electrical or digital
communications can be used.
A system of the type in Figure 21 will become unstable if the
communications delay is large. Sheridan explains the problem thus:
"Driving the controlled process sufficiently to achieve negative
feedback requires a loop gain greater than unity in the frequency range
of interest. However if the loop gain is greater than unity at such a
frequency that half a cycle is equal to the time delay, this will result
in positive feedback rather than negative. This means that energy at
this frequency is continually added to the loop and the amplitude of the
signal traversing the loop grows without bound. For short time delays,
instability is avoided because frequencies at which good tracking is
required are lower than those at which loop time delay equals one half
cycle, and the dynamics in the open loop attenuate the loop gain to less
than unity by the time the critical frequency (at which one half cycle
is short enough to equal the time delay) is reached (Sheridan 1993)".
With visual feedback, a human will achieve stability with a closed loop
system by adopting an open loop control strategy when necessary. That
is, the human operator will move as large an amount as they consider
reasonable without risking an error and wait for a response. This will
considerably slow the time it takes to complete a task in a predictable
way. The time taken to complete a two degree of freedom telemanipulation
task investigated by Ferrel (1965) increased by an order of magnitude
with a 3.2 second communication delay.
Many experimenters such as (Hannaford and Wood 1991) have shown that
when the operator forms part of the loop, as shown in Figure 21, much
improved performance can be achieved with force feedback. Force
reflecting systems cannot avoid instability using the move and wait
strategy. However, it is possible to achieve stability under any time
delay by controlling the communication link so that it emulates the
passive transmission line of an electrical distribution network.
Anderson and Spong (1988) were the first to recognise this and Neimeyer
and Slotine (1991) have developed an energy based formulation where the
total power into the network is given by:-