The development of multi-axis hand controllers for use in telemanipulator systems is described. Experience in the control of the SRMS (shuttle remote manipulator system) arm is reviewed together with subsequent tests involving a number of simulators and configurations, including use as a side-arm flight control for helicopters. The factors affecting operator acceptability are reviewed

Kruk, Ron

Mckinnon, G. M.

English

NASA Technical Reports Server

MULTI-AXIS
N90-22943
CONTROL OF TELEMANIPULATORS
G.M. McKinnon 1 and Ron Kink 2
CAE Electronics Ltd.
Montreal, Canada
ABSTRACT
This paper describes the development of multi-axis hand controllers for use in telemanipula-
tor systems. Experience in the control of the SRMS arm is reviewed together with subsequent
tests involving a number of simulators and configurations, including use as a side-ann flight con-
trol for helicopters. The factors affecting operator acceptability are reviewed.
INTRODUCTION
The success of in-orbit operations depends on the use of autonomous and semiautonomous
devices to perform construction, maintenance and operational tasks. While there are merits to both
fully autonomous and man-in-the-loop (or teleoperated) systems, as well as for pure extravehicular
activity (EVA), it is clear that for many tasks, at least in early stages of development, teleoperated
systems will be required.
This paper reviews some experience gained in the design of the human-machine interface for
teleoperated systems in space. A number of alternative approaches have been proposed and evalu-
ated over the course of the work described, and some basic design principles have evolved which
may appear mundane or obvious after the fact, but which nevertheless are critical and often
ignored.
One key design objective in the implementation of human-machine interfaces for space is that
of standardization. Astronauts should naturally and comfortably interpret their input motions in
terms of motions of the manipulator or task. This "transparency" is achieved by careful design to
ensure that task coordinates and views are always presented in a clear, unambiguous and logical
way, and by ensuring that standardized input devices are used in standardized modes. If conven-
tions are established and systematic modes of control are respected, training time is reduced and
effectiveness and performance are improved. The end objective in the design of displays and con-
trois for telemanipulators is to establish a "remote presence" for the operator.
THE SRMS SYSTEM
A number of manual control input devices have been used in space over the years. For the
most part these devices were designed as flight controls for the various satellites and modules
1 Director, R&D. (Dr. McKinnon died in spring of 1989.)
2 Group Leader, Human Factors.
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https://ntrs.nasa.gov/search.jsp?R=19900013627 2020-03-19T22:07:31+00:00Z
whichhaveflown. Thefirst truly roboticcontroldevicewasthatusedon theSRMSor
CANADARM systemof theSpaceShuttle.Thecontrolinterfacein thiscaseconsistedof two
three-degree-of-freedomdevicesusedin conjunctionwithadisplaysandcontrolspanel,CCTV
visualfeedbackfrom cargobayandarm-mountedcameras,augmentedbylimiteddirectviewing.
A TranslationalHandControl(THC) allowedtheastronautto controltheendpointof thearmin
thethreerectilineardegreesof freedomwith theleft hand,andaRotationalHandControl (RHC)
wasusedin thefight handto controlrotationaldegreesof freedom.
TheTHC wasdesignedspecificallyfor theSRMSapplicationbyCAE Electronics,while the
RHC wasamodifiedversionof the Shuttle flight control produced by Honeywell. The geometry
and overall configuration of the RHC was thus predetermined and was not matched to the task.
The device does not have the single centre of rotation which is considered by the authors to be an
advantage in generalized manipulator control. The RHC differed from the flight control version in
several ways:
• The forces and travels were modified to reflect task requirements.
Auxiliary switches and functions were changed to comply to task requirements. In fact
all auxiliary switches were located on the RHC -COARSE/VERNIER, RATE HOLD
and CAPTURE RELEASE.
A switch guard was added to CAPTURE/RELEASE to prevent inadvertent release of a
payload.
• Redundant electronics were eliminated in view of the reduced level of criticality.
The THC differed from the RHC in that it incorporated rate-dependent damping through the
use of eddy current dampers driven by planatary gears. A hand index ring was added to the THC
after initial evaluations of prototype units. The ring provided a reference for position and led to the
use of the device as a fingertip control, whereas the RHC with its larger hand grip was clearly a
hand control. Force levels and gradients on the THC were low, and the rate dependent damping
enhanced the smooth feel of the device. The x and y inputs of the THC were not true translations,
but an effort was made to optimize a linkage in the available space to reduce the curvature due to a
displaced pivot point.
The SRMS system has proven to be operable but not optimal. With training, astronauts can
become proficient in performing required tasks. In general, however, the tasks must be carefully
programmed and significant training and practice is required before an astronaut feels comfortable
with the system. Even with training, the skill of the astronaut is still a limiting factor on system
capability. Tasks requiring coordinated or dextrous motions are difficult to achieve.
While there is no hard data to compare alternatives, the shortcomings of the SRMS design in
part can be attributed to the limitations of the RHC and THC described above, but mainly to the
unfortunate location of the two hand controls and lack of direct correspondence between the axes
of the controls and those of the visual displays.
The SRMS system incorporated no force-reflective feedback aside from indications of motor
parameters from each joint. Positional feedback of the end point is strictly visual--either direct
viewing or through CCTV. The axes of the presented display depend on the view selected: direct,
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cargobayor arm-mountedcamera.Controlis in theresolvedratemode. In thecaseof a
large-scalearmsuchastheSRMS,amasterslaveor indexedpositionmodeisnot suitablebecause
of scalingproblems.
Figure1 showsasimulationof theSRMSDisplaysandControlsSystemin SIMFAC. The
RHCis locatedto thelowerrightof theD&C panelandabreadboardmodelof theTHC to the
upperleft. TheCCTVdisplaysareto therightandthedirectviewingportsareoverheadand
immediatelyabovetheD&C panel.
MULTI-AXIS STUDY
Following the design of the SRMS system, the authors conducted a study of multi-axis con-
trois (1). The purpose of the study was to determine the feasibility of controlling six degrees of
freedom with a single hand control. According to the guidelines laid down for the study, mode
changes were to be avoided so that coordinated control was required simultaneously in all axes.
No specific application was defined; however, the controller was to be usable either to fly a space-
craft or to "fly" the end point of a manipulator.
The study included a review of the literature, observation of available multi-axis controllers,
and discussions with experts. Although a prototype device was not required by the contract, one
was assembled. Interestingly, the consensus of opinion at the time amongst the knowledgeable
community was that coordinated control in six axes was desirable, but probably not feasible.
A number of six-degree-of-freedom controls were reviewed. The most notable were devices
with force feedback operated in the indexed position mode. A prototype laboratory version was
developed by R. Skidmore at Martin Marietta and evaluated in various dynamic and graphic simu-
lations. A similar design and evaluation was done at Jet Propulsion Laboratories by A. Bejczy (2).
These devices were both unsuitable in design for implementation in a mature control system, but
permitted laboratory evaluation of force characteristics, displacements, and interactions with visual
feedback. Another approach was developed by D. Whitney at the Draper Laboratory. This was
elegantly designed from the mechanical viewpoint, but difficult to use due to the absence of tactile
feedback.
This study uncovered no mature or workable concept for a six-degree-of-freedom controller
and a lot of skepticism amongst practitioners as to the feasibility of implementing more than four
degrees of freedom. A more recent study of hand controls was done by Brooks and Bejczy (3).
DEVELOPMENT PROCESS
At the conclusion of the study, in spite of the climate of skepticism, the authors felt that there
was no reason why a well-coordinated, six-degree-of-freedom controller could not be designed.
Experiments with a variable-geometry test rig demonstrated that the only way to avoid inherent
cross-coupling between axes, achieve the ability to make discrete inputs where required, and still
have a direct correlation between control inputs and resulting action was to center all axes at a
single point positioned at the geometric center of the cupped hand. In this way, control of the end
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pointrelatedto handmotions.Alignmentof controlleraxesin a logicalway to theaxesof visual
displayswasalsoconsideredessential.
Oneinitial concernwastheissueof isometric (purely force) versus displacement control. An
isometric controller is rugged and easily constructed from a mechanical standpoint. Unfortunately,
the concept leads to overcontrol, particularly in stressful situations, because of the lack of proprio-
ceptive indication of input commands. In some situations operators tend to saturate the controller
to the extent that they quickly suffer fatigue. While there may be tasks in which isometric control
is adequate and acceptable, in general the addition of displacement with suitable breakout gradients
and hard-stop positions improves performance. For this reason, most manual controls designed
on the isometric principle have been modified to include compliance.
Initial designs by the authors were based on the use of force transducers to generate input
signals. The controls were designed to allow for the inclusion of compliance and adjustable force
characteristics, although the device could also be configured for isometric operation in all axes. It
was quickly established that some compliance was advantageous. Since there was always signifi-
cant displacement, the force transducers were replaced by position transducers, thus permitting the
use of rugged, compact, noncontact, optical position sensors and eliminating the tendency to gen-
erate noise signals due to vibration or shock. In addition, a purely position system made it easier
to eliminate cross-coupling between axes when pure motions in a single axis were required.
An intermediate step of isometric translational axes and displacement in rotation, a so-called
"point and push" approach, was unsuccessful because of the problems described above in the iso-
metric axes.
In the final analysis, a prototype design was constructed which included significant dis-
placement in all six axes. The prototype unit is shown in figure 2.
PROTOTYPE DESIGN
The design concept was to ensure that all six axes pass through a single point. The mechani-
cal components and transducers for the rotational axes were mounted within a ball. The ball in turn
was mounted on a stick which was free to translate in three mutually orthogonal axes. All axes had
appropriate breakout forces, gradients and stop-force characteristics generated by passive compo-
nents. The output of the device was a position signal sensed by optical transducers. No additional
rate-dependent damping was included. While rate-dependent damping does enhance the "feel" of
the controller, the additional mechanical complexity is probably not justified.
The relationship between breakout forces and gradients is task-dependent. In general, the
breakouts should be sufficient that pure inputs can be generated easily in a single axis; however,
breakouts do have a negative impact on controllability for small coordinated movements in multiple
axes simultaneously.
Various handgrip shapes were investigated, but with the emergence of the coincident axis
concept as previously described, there was a fundamental need to provide a face perpendicular to
the direction of commanded motion. The other prime requirement was a shape which ensured the
correct positioning of the hand relative to the geometric center of the system. The natural solution
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wasasphere.As developmentof themechanismandsensingsystemsprogressed,theball size
wasreducedto its presentconfiguration.Thisapproximatesto thesizeof abaseball,andhas
shownto becomfortablefor bare-handed,gloved,andpressure-suitedoperation.
Severalderivativesof thebasicdesignevolvedfor specialapplications.A bang-bangdevice
wasconfiguredfor testson theMMU simulator.A four-axis(threerotationsonaverticalpurely
rate-dependentdampedlinearaxis)modelwasevaluatedfor flight controlin helicopters.In some
configurationsa protuberancewasaddedto provideatactilecuefor orientation.Auxiliary
switcheswereaddedon thisprotuberance.
TEST AND EVALUATION
To date a number of tests have been carded out. It is difficult to compare data between tests
since different tasks and performance metrics were used. In general, though, subjective ratings
and measures of performance were consistent and some basic design principles were established.
Tests performed were
Johnson Space Flight Center
Initial tests were performed using the controller to control computer graphic representations
of docking tasks.
Subsequent tests were also made using the full-scale mockup of the SRMS ann (MDF).
Comparisons were made between the conventional SRMS (two three-degree-of-freedom con-
trollers) configuration and the single six-axis device. NASA human factors personnel, technicians
and astronauts participated in the tests.
Martin Marietta
The controller was evaluated with computer graphics representations of docking maneuvers.
Astronaut evaluations of a bang-bang configuration were done on the MMU simulator. Tests
were performed for operation in pressurized space suits, as shown in figure 3.
Marshall Space Flight Center
A six-axis controller was used to control a six-axis ann as shown in figure 4. The system
has been operated over the past 2 years with a variety of operators and tests.
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Grumman
Tests were carded out using two six-degree-of-freedom controllers to control two six-degree-
of-freedom dextrous manipulators as shown in figure 5. Comparisons were done with master/
slave control in the same environment.
Tests were carded out using the six-degree-of-freedom controller with the LASS simulator
for various "cherry picker" tasks.
National Aeronautical Establishment
Four-axis versions of the design were installed and flown in a variable-stability helicopter as
shown in figure 6. Evaluations were performed by numerous military and civilian pilots, including
test pilots from major airframe manufacturers. Cooper-Harper ratings were recorded for a variety
of maneuvers at various levels of control augmentation. Results were comparable to conventional
controls. For the most part flight tests were performed by highly experienced pilots.
It should be noted that, in the case of the four-axis version, the use of a relatively conven-
tional handgfip superimposed on the ball was possible while respecting the principle of a single
centre. The addition of another translation axis with a similar handgrip would introduce cross
coupling.
European Space Agency
A model of the controller has been ordered by ESA for evaluation use in the European Space
Program.
DISCUSSION
Tests to date have demonstrated that six-axis control using a single hand is not only feasible
but, providing certain design guidelines are respected, preferable to approaches in which axes are
distributed amongst separate controllers. Statements to the effect that six degrees of freedom is too
much for one hand ignore the fact that the humans have the ability to make complex multi-axis
movements with one hand using only "end point" conscious control. The coordinate transforma-
tions required are mastered at an early age and the inverse kinematics are resolved with no con-
scious effort. To operate a system using two separate three-axis controllers requires a conscious
effort on the part of the operator, thereby increasing his or her work load. The operator requires
considerable training and practice with a 2 x 3 axis system before achieving the same level of con-
trol as is immediately possible with the single six-axis device. NASA experience has shown that
the weeks of training necessary for the former become less than 30 sec for the latter. While the
guidelines have been verified only in specific environments for specific tasks, the authors feel con-
fident in making the following statements:
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1. A proportionaldisplacementcontrollerwill provideimprovedperformanceandin many
casesmorerelaxedcontrolthananisometricdevice. Performancewith isometricdevicesvaries
morebetweenindividual subjectsthanthatwithdisplacementcontrol.
2. Forcegradientsandcharacteristicshouldbecorrelatedto thetaskbeingperformed.
Theremaybeajustificationfor standardizingforcecharacteristicsandcontrollerconfigurationsfor
all space-relatedequipmento ensurecommonalityandto reducetrainingrequirements.
3. An obviousandconsistentorientationbetweencontrolleraxesandthoseof visual feed-
backdisplaysis essential.This is anareawherestandardizationbetweentasksandsystemsis a
keyelement.A single controller design would be suitable for all applications, provided that basic
axis orientation and control mode standards are maintained.
4. The use of force-reflecting feedback has not been evaluated by the authors, although a
program is under way to investigate some unique and novel approaches. In general, direct force
feedback is useful only in a system with high mechanical fidelity. In the presence of abrupt non-
linearities such as stiction or backlash and particularly transport lag force, feedback can in fact be
denimental in excess of 100 msec.
5. For some tasks with some manipulators, a master/slave system can provide equal or
superior performance to that of a manual control in resolved-rate mode. Resolved rate is, how-
ever, universally applicable and can provide a standardized approach for virtually all manipulator or
flight-control tasks.
6. In tasks in which lag exceeds 1 sec, it may be assumed that real-time interactive control in
the strict sense is not feasible. Providing physical relationships are stable or static, a reconstructive
mode using generated graphics for a "prehearsal" of manipulator movement may be used, stored in
memory, then activated. When lags are 100 msec or less, resolved-rate control may be used to
directly control the end-effector (position control is inadequate when any substantial excursion may
be required). The lag regime between 100 msec and 1 sec causes difficulty because there is a ten-
dency to compensate for delay or system instability (e.g., arm-flexing modes) with more complex
drive and "prediction" algorithms. Our experience thus far is that the simplest control algorithm
which permits stable response generally provides the best performance.
In conclusion, tests have shown that six-degree-of-freedom controllers can be used naturally and
effectively to control tasks requiring dexterity and coordination.
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REFERENCES
. Lippay, A.L., McKinnon, G.M., King, M.L., Multi-axis Hand Controllers, a State of the Art
Report, Proceedings of Seventeenth Annual Conference on Manual Control, Los Angeles,
June 1981.
. Bejczy, A. K., Handlykon, M., Experimental Results with a Six Degree of Freedom Force
Reflecting Hand Controller, Proceedings of Seventeenth Annual Conference on Manual
Control; Los Angeles, June 1981.
3. Brooks, T. L., Bejczy, A. K., Hand Controls for Teleoperation, VPL Publication 85-11, Cali-
fornia Institute of Technology, Pasedena, March 1985.
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ORIGINAL PAGE
Figure 1.- SIMFAC.
Figure 2.- Six-degree-of-freedom prototype.
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ORIGINAL PAGE
BLACK A_D WHITE P_HOIOGRAP.H
Figure 3.- MMU tests at Martin Marietta.
Figure 4.- Control of robot at Marshall Space Flight Center.
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O_IGINAE PAGE
Figure 5.- Simultaneous control of two arms.
Figure 6.- Four-degree-of-freedom controller installed in helicopter.
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Multi-axis control of telemanipulators

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