Virtual Reality Scientific and Technological Challenges (1995) / Chapter Skim
Currently Skimming:
9 Telerobotics
9 Telerobotics
Pages 304-361
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 304... ...
Haptic interfaces that mechanically link a human to a telerobot nevertheless share similar issues in mechanical design and control, and the technology survey presented here includes haptic interface development. INTRODUCTION Telerobotic devices are typically developed for situations or environments that are too dangerous, uncomfortable, limiting, repetitive, or costly for humans to perform.
|
From page 305... ...
There is a continuum of human involvement, from direct control of every aspect of motion, to shared or traded control, to nearly complete robot autonomy. Any robot manipulator can be hooked up to a haptic interface and hence become a telerobot.
|
From page 306... ...
There are also unique problems in telerobotic control, having to do with the combination of master, slave, and human operator. Even if each individual component is stable in isolation, when hooked together they may be unstable.
|
From page 307... ...
The virtual environment is deemed an obvious and effective way to simulate and render hypothetical environments to pose "what would happen if" questions, run the experiment, and observe the consequences. Simulations are also an important component of predictive displays, which represent an important method of handling large time delays.
|
From page 308... ...
Examples include many industrial robots, such as the PUMA 560, as well as a number of commercial telerobots (Kraft, Shilling, Western Electric, ISE)
|
From page 309... ...
. Commercial examples include the Sarcos Dextrous Arm (Jacobsen et al., 1990a, 1990b, 1991)
|
From page 310... ...
. Almost all industrial robots have these special structures, but some for design convenience do not, such as the Robotics Research Arm, which has been used in teleoperation.
|
From page 311... ...
Yet gears bring serious drawbacks: substantial friction, backlash, and flexibility. The performance consequences are poor joint torque control, poor end-point force control, reduced accuracy, and slower response.
|
From page 312... ...
has designed a 6-axis magnetically levitated wrist, which can be used either as a hand controller or as a robot end effecter. Salcudean (Salcudean et al., 1992)
|
From page 313... ...
Sensors Sensor technologies for telemanipulators include sensors required to monitor the internal mechanical state of the arm (joint angle sensors and joint torque sensors) , the external contact state (wrist force/torque sensors and tactile sensors)
|
From page 314... ...
Optical encoders offer the highest resolution; for example, Canon produces an incremental laser rotary encoder with 24 bits of resolution. The Sarcos Dextrous Arm has 18 1/2 bit incremental encoders.
|
From page 315... ...
Accurate knowledge of joint torque is very important for precise control and, in the context of teleoperation and haptic interfaces, for force reflection. Despite this importance, very few manipulators actually have this capability.
|
From page 316... ...
Most frequently, a 4-beam arrangement in a Maltese cross configuration has been employed with strain gauges; commercial examples include the IR3 sensor and the Assurance Technology sensor. A significant problem is cross-axis interference, due to nonlinear beam bending (Flatau, 1973; Hirose and Yoneda, 1990~; this effect may produce errors up to 3 percent.
|
From page 317... ...
_ _,: _ _ , \ J REMOTE VEHICLES Remote vehicles, or mobile robots, encompass any basic transport vehicle that can be operated at a distance: indoor motorized carts, road vehicles, off-terrain vehicles, airborne or space vehicles, boats, and sub
|
From page 318... ...
This section highlights mobile robots exemplifying the current state of the art, major issues involved in the development of mobile robotic systems, and remaining research and development challenges. Systems The arguably perfect mobile robotic system would: (1)
|
From page 319... ...
. All major navigation and strategy decisions are made by a human operator using vehicle navigation, collision avoidance, and scene understanding sensors.
|
From page 320... ...
Data tethers, typically fiber optic cables, are much less bulky than power tethers. These cables can support very high-bandwidth, secure, nonjammable, non-line-of-sight telemetry to ranges of over 100 km without repeaters, and they do so without imparting significant drag to the remote vehicle, since they can be actively or passively payed out from it.
|
From page 321... ...
On-board automation is emphasized in order to remove the physical data tether yet still be capable of performing long-range missions. Class 3 systems have a telemetry connection to their control station, but it is a low-bandwidth, nonline-of-sight, connection incapable of supporting direct manual control by the human operator.
|
From page 322... ...
Collision avoidance sensors include a 9-element ultrasonic array and bumper-mounted collision detectors. Navigation is accomplished using a hybrid navigation scheme that combines compass/encoder-based dead reckoning and a wall-following/reindexing system.
|
From page 323... ...
Class 4 Systems: Nontethered, No Telemetry The final class of systems represents the perceived high ground of mobile robotics research and development. The premium on on-board automation is extremely high, and the remote vehicle carries out its mission without requiring human monitoring or intervention.
|
From page 324... ...
Image understanding for mobile robotic applications, as exemplified by the CMU work, is currently the focus of intense research sponsored by the Advanced Research Projects Agency (1992~. Technologies and Directions Although all the major technology areas depicted in Figure 9-1 conhnue to be the focus of intense research and development, the most significant and relevant developments have been in the sensor, platform, actuator, high-level robotic control, and human-machine interface fields.
|
From page 325... ...
Recent developments in small, low-cost inertial linear accelerometers and angular rate sensors and the maturation of global positioning system (GPS) technology, however, are of particular import for mobile robot navigation and sensing.
|
From page 326... ...
with future systems slated to have positional accuracies on the order of 15 m SEP by using the military version of GPS and solid-state acceler ometers. Collision Avoidance and Scene Understanding Sensors and Systems Acoustical, optical, and electromagnetic sensors using proximity, triangulation, time of flight, phase modulation, frequency modulation, interferometry, swept focus, and return signal intensity ranging techniques have been employed on mobile robots for collision detection and scene understanding purposes (Everett et al., 1992~.
|
From page 327... ...
and Odetics exemplify the state of the art. The ERIM laser scanner, used by Carnegie Mellon University on its Navlab series of mobile robots, provides resolutions of 0.5 deg/pixel horizontal (80 deg field of view)
|
From page 328... ...
Legged platform research and development is particularly relevant to the design and control of figures and autonomous agents in virtual environments (see Badler et al., 1991~. High-Level Robotic Control A relatively new and controversial approach to high-level control for mobile robotics is reactive control.
|
From page 329... ...
Interfaces of the latter type have typically been designed in order to provide the human operator with some sense of remote presence or telepresence providing sensor feedback of sufficient richness and fidelity and controls of sufficient transparency that human operators feel as if they are present at the remote site. This approach is typically taken in order to engage the human's naturally evolved sensory, cognitive, and motor skills in the ways they are used in everyday tasks so as to minimize task completion times and the training required to operate the remote system (Pepper and Hightower, 1984~.
|
From page 330... ...
LOW-LEVEL CONTROL OF TELEOPERATORS Teleoperators are complex systems composed of the human operator, master manipulator (joystick) , communication channel, slave manipulator, and the environment (remote task)
|
From page 331... ...
Fortunately, most human movements are relatively smooth (Flash and Hogan, 1985~. The volume of space in which it is comfortable for the human operator to maintain hand position for extended periods is small compared
|
From page 332... ...
(1987) found that position control gave better completion times in simulated teleoperation, except for very slow simulated manipulators, for which rate control was slightly better.
|
From page 333... ...
The earliest systems used identical master and slave devices with decoupled controls of the individual joints in which joint torque for both master and slave was a function of position difference between them (Goertz and Thompson, 1954; Goertz, 1964~. As improved computing power became available in the 1970s, it became possible to use Cinematically different master and slave devices in which the master could be optimized for interfacing with the human operator, and the slave for its particular task (Bejczy and Salisbury, 1980, 1983~.
|
From page 334... ...
For practical reasons, many of these studies have been carried out using hardware designed for other purposes, with little capability for delicate force control. Often, for example, an industrial robot manipulator is used for the slave robot.
|
From page 335... ...
Delay A challenging issue arising in many applications is time delay between master and slave sites. This delay ranges from a few milliseconds in the case of computation delays to 10-100 ms delays induced by computer networks, to delays of seconds or more induced by multiple satellite communication links.
|
From page 336... ...
Therefore, the shortcoming of the passi~rity-transmission line theory is that performance of the system is not addressed. Robustness Recently, robust control concepts have been applied to teleoperator control, starting with earlier work in impedance control (Colgate and Hogan, 1988)
|
From page 337... ...
(1987) compared position control versus rate control, taking into account the joystick type (isotonic or isometric)
|
From page 338... ...
Decreasing bandwidth increased the gross motion and fine motion times about the same. Another issue is force reflection versus position control for teleoperators.
|
From page 339... ...
In a less strict sense, supervisory control means that one or more human operators are continually programming and receiving information from a computer that interconnects through artificial effecters and sensors to the controlled process or task environment, even though that computer does not itself close an automatic control loop. The strict and not-strict forms of supervisory control may appear the same to the supervisor, since he or she always sees and acts through the computer (analogous to a staffs and therefore may not know whether the computer is acting in an open-loop or a closed-loop manner in its fine behavior.
|
From page 340... ...
All the computers are controlled from a central control station in which the human supervisor cooperates with a computer to coordinate the control of the multiple automatic subsystems. The supervisor's functions are five, which in turn can be subdivided as shown in the upper part of the diagram by the upper case labels.
|
From page 341... ...
subdivided into elements, all of which must be attended to by the human supervisor, but any of which may be aided by a computer-based expert system or on-line decision aid. In addition, off-line and not shown is the supervisory function learn.
|
From page 342... ...
The computer graphic model provides depth cues in the form of lines from the underside of the object to the floor as described above. Another algorithm checks for collisions with the environment by any part of the teleoperator or objects carried by it (again, as best known)
|
From page 343... ...
intervening and Learning If the automation fails, if the programmed actions end, or for other reasons, occasionally the human supervisor must intervene to reprogram or take over manual control. Criteria for doing this and which takeover mode is best tend to be context dependent.
|
From page 344... ...
Although the dynamics of the computer model used for prediction and planning must be synchronized to the dynamics of the actual task, the use of that model by the human operator need not be synchronized. In other words, an easy maneuver in free space may require no attention by the operator, and as that easy maneuver is occurring he or she may wish to focus attention on a more complex maneuver coming up later, even running the prediction of the complex maneuver in slow time or repeating it.
|
From page 345... ...
For very simple tasks, one might expect direct control to be quicker because instruction of a machine, as with that of another person, requires some
|
From page 346... ...
Given the diverse components of a VE or teleoperation system, a decentralized computing architecture with an individual computer attending to just one component, such as a haptic interface, seems more appropriate than a monolithic supercomputer. Software and operating systems have to facilitate the programming and interaction of such networks of computers, walking a delicate line between efficiency and features.
|
From page 347... ...
In the simplest case, the control design may be approached by considering each joint of a complex robot (or haptic interface) as a single-input/single-output system, with joint torque as input and joint position as output.
|
From page 348... ...
The calculation of these trajectories will in turn require an additional computational demand, which depends on the number and the nature of the constraints that need to be enforced. As a general rule, each control system at a level of a control hierarchy must handle subsystems, which grow more complex as we move up the control hierarchy, but if the design is correct, their complexities will be hidden by the lower levels.
|
From page 349... ...
The conclusion is: if latency can be avoided, it must be even if it means simpler control algorithms. For low-level control, latency requirements for manipulators and haptic interfaces are identical to the sampling period, which is of the order of a few tens to hundreds of microseconds.
|
From page 350... ...
Processors such as Transputers and the Texas Instruments TMS320C40 (C40) DSP chips are not meant for general-purpose computing platforms, but for fast numerical computation and I/O.
|
From page 351... ...
A common bus is the most traditional approach; industry standards with a roughly equivalent performance of 40 to 80 Mbytes/s include VME Bus, MultiBus II, FutureBus, EISA, NuBus, SBUS, and PCI Bus. Each computing unit is equipped with fast local memory, and a global memory bank is made available to all computing units (Bejczy, 1987; Chen et al., 1986; Clark, 1989; Narasimbam et al., 1988~.
|
From page 352... ...
Although considerably slower, it is also possible for processors to communicate across networks. Researchers have become interested in running robotic experiments or virtual environment setups across a computer network with nodes located in different cities or even in different continents.
|
From page 353... ...
An example of this category is HELIOS by Distributed Software Ltd., which offers an eight-level priority realtime scheduler, virtual message routing, X and Microsoft Windows graphic support, as well as SUN and PC host interfaces. Clearly, processing nodes with microsecond interrupt latency requirements can and must be stripped of the inherently slow high-level functionality.
|
From page 354... ...
Another reason is the inability of robots to handle unstructured environments, which obviates the possibility of even partially automating a task the reason for having telerobots in the first place. In the control of telerobots, the issues of haptic interfaces, computergenerated environments, and real-time systems are important.
|
From page 355... ...
Exactly how to formulate a controller that addresses both stability and performance, under various time delays, needs further work. Force reflection under delays is more problematic than position control.
|
From page 356... ...
determine whether to use position control or force reflection as a function of delay and tasks; (2) determine how to configure the slave manipulator's force controller as a function of a priori knowledge of the task; and (3)
|
From page 357... ...
Examples are locomotion on different surfaces, the behavior of the operator's hand tissues in the interaction with the haptic interface, and the behavior of the remote manipulator with such compliant environmental substances as human tissues (in surgical applications) and rock or soils (in mining applications)
|
From page 358... ...
could be fast and flexible, as there could exist numerous communication links between the two. Furthermore, the boundary could be laid dynamically, depending on the requirements of each system.
|
From page 359... ...
Robust Proximity Sensors As a step toward supervisory control, small adjustments in grasping or an approach to a surface should be performed with 1ncal .sen.~in~ The 1 - ~ -c 1 · . ~ nte~gence requ~recl Is much less than for the general case of autonomous control.
|
From page 360... ...
As mentioned earlier in this chapter, robotics also has this goal. As an example, it is not fully understood when to apply rate control versus position control, or how to include force feedback into rate control.
|
From page 361... ...
With more sophisticated abilities to interact with the environment and to complete such tasks as the generic peg-in-hole problem, the need for force reflection will diminish. A step toward such autonomous control capabilities would be a higher-level transfer of skills between the operator and the telerobot.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.