WebTelerobotic Web-based telerobotics

Web-based telerobotics

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.

Teleoperation 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 sophistication. For experimental telerobots see: + (Conway et al. 1990) * (Hirzinger et al. 1989)
Figure 20

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.
Figure 21

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.

Shared control

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:-