Robots operating in the microgravity environment of an orbiting laboratory should be capable of manipulating payloads such that the motion of the robot does not disturb adjacent experiments. The current results of a NASA Lewis Research Center technology program to develop smooth, reaction-compensated manipulation based on both mechanism technology and trajectory planning strategies are present. Experimental validation of methods to reduce robot base reactions through the use of redundant degrees of freedom is discussed. Merits of smooth operation roller-driven robot joints for microgravity manipulators are also reviewed

Lawrence, Charles

Miller, Jeffrey H.

Rohn, Douglas A.

English

NASA Technical Reports Server

NASA Technical Memorandum 103271 
Reaction-Compensation 
Technology for Microgravity 
Laboratory Robots 
( A SA - T ~- 1 0"1 L 7 1) C T I -' - C PE, I SAT I 0 
TECHNOl GY FUR IC GR V TY L B AT Y 
B TS (A A) 5 P C Cl 131 
Douglas A. Rohn and Charles Lawrence 
National Aeronautics and Space Administration 
Lewis Research Center 
Cleveland, Ohio 
and 
Jeffrey H. Miller 
Sverdrup Technology, Inc. 
Lewis Research Center Group 
Brook Park, Ohio 
Prepared for the 
i-SAIRAS '90 
90- 28062 
Unclds 
G3 ! j7 0~U2 5 
International Symposium on Artificial Intelligence, Robotics 
and Automation in Space 
Kobe, Japan, November 18-20, 1990 
NI\5/\ 
https://ntrs.nasa.gov/search.jsp?R=19900018746 2020-03-19T21:02:19+00:00Z
Technology development is required
to bring the disturbance regions below
the required level.
REACTION-COMPENSATION TECHNOLOGY
MULTI-DEGREE-OF-FREEDOM TEST BED
A multi-degree-of-freedom (DOF)
manipulator test bed, shown in figure
2, has been constructed to
experimentally evaluate and verify the
technologies developed under this
project. Brush [4] describes the
selection of the system's components,
shown schematically in figure 3. The
4-DOF robotic arm is driven by roller-
traction differentials which are
expected to cause insignificant
vibration and backlash. A computer is
employed for closed-loop arm control
and acquisition of joint performance
and reaction force and moment data.
The base reaction force/moment
transducer provides a digital output of
the six force and moment components.
ROLLER DRIVEN JOINTS
Roller, or traction, driven
actuators provide significant benefits
to servomechanism applications in space
by offering: zero backlash, high
torsional stiffness, low starting
friction, low torque ripple, potential
for nonlubricated operation (due to low
sliding), and over-torque protection
(ability to slip at predetermined
traction limits}. These
characteristics, as discussed by
Loewenthal, et. al. [5] are important
for smooth, controlled robot joints.
Under a NASA program to evaluate and
mature advanced telerobotic
technologies, Kuban and Williams [6]
designed a manipulator arm for a
Laboratory Telerobotic Manipulator
(LTM) which incorporated a 2-DOF
roller-driven joint. To simplify the
control system and provide the
necessary fineness of control, drive
system backlash had to be eliminated.
These requirements, plus the need to
consider operation in a vacuum were
factors contributing to the selection
of roller driven joints. The rollers
were hardened steel with ion-gold
plating to allow dry operation.
This joint design was incorporated
into the test bed at NASA-Lewis to
demonstrate the servomechanism
characteristics of roller-driven robot
joints, as well as to verify the
reaction-compensating trajectory
optimization strategies discussed in
the next section.
JOINT TRAJECTORY PLANNING
Dynamics and controls technologies
have been utilized to limit the
reactions transmitted by a robot
through its base to the orbiting
laboratory. Several methods for
momentum compensation were proposed by
Quinn and Lawrence [9]. The basis for
the reaction minimization strategy used
in this project is joint trajectory
planning through the use of redundant
DOFs. Manipulators used in space
applications may have kinematic
redundancy in order to facilitate the
performance of tasks. In certain
applications, the redundant degrees of
freedom may also be used to minimize
base reactions.
In deSilva, Chung, and Lawrence [8]
a method was developed for trajectory
design which employs kinematic
redundancy (extra degrees of freedom)
for base reaction minimization. The
method involves moving the extra
sections of the manipulator in a
direction inertially opposite to the
movement of the end-effector to
minimize the base reactions. The
procedure employs an optimization
strategy for identifying the joint
motion solution set which minimizes the
resulting base reactions.
Chung and Desa [9] assessed the
effect of various weighting functions
on the base forces and moments. From
these results it was determined that a
suitable weighting matrix could be
constructed by using average values of
base moments and forces. This
weighting function can also be tailored
to minimize a partial set of reaction
components, such as only the forces or
only the moments.
Quinn, et.al [10] also incorporated
the strategy into a general computer
program to simulate and control
manipulators with any number of links,
joints, and degrees of redundancy. It
was found that it is possible to design
manipulators through the proper
selection of redundancy which will be
capable of operating with minimal base
reactions. As shown in figure 3, an
arbitrary planar manipulator, with two
redundant joints, can even exhibit zero
net base reaction. In most cases, it
is not possible to completely eliminate
/ _IttII_IIIQtii&Jilli 2 PRECEDING PAGE BLANK NOT FILMED
base reactions. However,use of these
techniques can be employedto lower
robotic disturbances to below desired
laboratory levels.
Further, Chung and Desa [11] showed
that it is possible to eliminate the
base reaction function peaks in figure
3 through use of an integral approach.
Rather than attempting to minimize the
reaction function at every time step,
as done in the previously discussed
strategy, the integral method seeks to
minimize the area under the reaction
function curve.
REACTION COMPENSATION EXPERIMENTS
Implementation of the above control
strategies on the multi-DOF test bed
begins with generating a set of joint
angles as a function of time for the
desired robot-end motion from the
output of the optimization code. This
set-point file is downloaded to the
control computer. The manipulator is
then commanded through the motion in
robotic-control, position-feedback
mode, while six-axis reaction and joint
angle data are acquired. Static,
gravity-induced moment loads are
removed from the data by subtracting a
non-linear function of joint angles
based on known physical dimensions and
the measured joint angles.
Since the test bed has only a 4-DOF
arm, the possible end-effector
positions and orientations as well as
available redundancies and
corresponding joint trajectories are
limited. Thus, the theoretically
possible improvement from arbitrary to
optimized may be relatively small.
However the data is expected to verify
the trajectory design strategy, which
is applicable to robots with any number
of DOFs.
Currently efforts are underway to
complete the laboratory measurements
and compare the data to dynamic model
predictions for arbitrary and optimized
moves. Other tasks will include
measurement of end-effector
acceleration and vibration to help
address the problem of precise motion,
and to evaluate the roller-driven joint
concept.
SUMMARY
The goal of the microgravity
robotics technology program at NASA
Lewis Research Center is to develop
reaction-control technology for use in
robots for microgravity laboratories.
Roller drive design, analysis, and
experimentation are underway to provide
smooth robot drive systems under a
variety of environmental and dynamic
conditions. Optimization schemes have
been developed which can control
reactions in a redundant-joint robot.
These and future results will help
prevent excessive disturbances to the
on-orbit microgravity environment of
future space laboratories.
REFERENCES
I. Rohn, D. A., Lawrence, C, and Brush,
A.S.: "Microgravity Robotics
Technology Program," ISA 88-1642,
1988.
2. Harman, P. E. and Rohn, D. A.: "The
Impact of an IVA Robot on the Space
Station Microgravity Environment,"
Proceedings, NASA/Army Space and
Military Applications of Robotics
Conference, Huntsville, AL, June 21-
23, 1988.
3. Dodd, W. R., Badgley, M. B., and
Konkel, C. R.: "User Needs,
Benefits, and Integration of Robotic
Systems in a Space Station
Laboratory," NASA CR-185150,
October 1989.
4. Brush, A. S.: "Microgravity
Manipulator Demonstration," LST'88,
NASA CP-3OO3-VOL I, May 1988, pp.
217-227
5. Loewenthal, S. H., Rohn, D. A., and
Steinetz, B. M.: "Application of
Traction Drives as Servo
Mechanisms," 19th Aerospace
Mechanisms Symposium, NASA CP-2371,
May 1985.
6. Kuban, D. P. and Williams, D. M.:
"Traction-Drive, Seven-Degree-of-
Freedom Telerobot Arm: A Concept
for Manipulation in Space," 21st
Aerospace Mechanisms Symposium, NASA
CP-2470, May 1987.
7. Quinn, R. D. and Lawrence, C.:
"Robots for Manipulation in a
Microgravity Environment," Sixth
VPI&SU/AIAA Symposium on Dynamics
and Control of Large Structures,
June 61987.
8. DeSilva, C. W., Chung, C. L., and
Lawrence, C.: "Trajectory Design
for Robotic Manipulators in Space
Applications," submitted to
International Journal of Robotics
Research.
.10.
11.
Chung, C. L. and Desa, S.: "Base
Reaction Optimization of Redundant
Manipu]ators For Space
App]ications," ASME Mechanisms
Conference, Orlando, FL, Sept. 25-
28, 1988.
Quinn, R.D., £hen, J. L. and
Lawrence, C: "Redundant
Manipu]ators for Momentum
Compensation in a Micro-Gravity
Environment," AIAA Guidance and
Control Conference, Minneapolis,
MN, August I988.
Chung, C. L. and Desa, S.: "A
Global Approach for Using
Kinematic Redundancy to Minimize
Base Reactions of Manipulators
Used in Microgravity
Environments," IEEE Robotics and
Automation Conference, 1988.
r_ _ ArbitraryA
= .... Opti- II
Four-DOF _ :o_ ,mized /I
manipulator _ Yaw _. _ ._ / [
Yaw _-._ _._ fr_ rr f,..._ A "
,wo__._--_ Pitch L , Time r- _u
axis _--._r__.._r.._._._/_ Path optimization strategies
joints I IFIL> "_ "" R '
"_ I lit I Pitch oDot, Control algorithms
__Da_ a%qauisition_ i_
force/moment 32-Bit computer system
transducer operator interface
Figure 2.-- Microgravity manipulation demonstration
test bed [4].
[] Gravity gradient [_ Rms operation
[] Air drag D Misc. machinery
[] Reboost I_1 Lab. robotics
[] Crew, attitude control, etc.
101 _ Smooth
Reaction motion
force effects
10001-- ^ effects _ L ,
space . E - _- _- q
# .4 - s!ation.
requ rement --7 E - - _ii.;;i !
8 10-5 _] ,' -: ii:_i!;i!i!i!iii:i!ilili!
-7 iiiiiiiiiiii',iiiil;iil10 iiiiiiiiii!iiiiii!ii_i_!
10 10_5 10_ 4 10_ 3 10_ 2 10-1 100 101 102
Frequency, hz
Figure 1.-- Potential disturbances to the Space
Station Freedom's low gravity environment.
_- 80
.o
e--
"- 64
r-
E
o
E 48
t-
3e
t"
._o
m
0
Base joint fixed,
1 redundant DOF
Base joint free,
2 redundant DOF
-- Four joint, planar manipulator;
arbitrary straight-line motion of j
end-effector in X-Y plane. /
/
-- For two joints fixed, /
no redundancy, /
A fun_ion_ch hign_
r I I
.3 .6 .9 1.2
Time
Figure 3. -- Typical optimization of base re-
actions for Four-Joint Planar Manipulator
over arbitrary trajectory, with one and two
redundancies. Base reaction function is
defined as a weighted sum of the square._
of reaction forces and moments [10].
.5
National Aeronautics and
Space Administration
1. Report No. 2. Government Accession No.
NASA TM-103271
4. Title and Subtitle
Reaction-Compensation Technology for Microgravity Laboratory Robots
Report Documentation Page
7. Author(s)
Douglas A. Rohn, Charles Lawrence, and Jeffrey H. Miller
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135-3191
3. Recipient's Catalog No.
5. Report Date
6. Performing Organization Code
8. Performing Organization Report No.
E-5713
10. Work Unit No.
694-03-03
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546-0001
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the i-SAIRAS '90, International Symposium on Artificial Intelligence, Robotics and Automation in
Space, Kobe, Japan, November 18-20, 1990. Douglas A. Rohn and Charles Lawrence, NASA Lewis Research
Center. Jeffrey H. Miller, Sverdrup Technology, Inc., Lewis Research Center Group, 2001 Aerospace Parkway,
Brook Park, Ohio 44142.
16. Abstract
Robots operating in the microgravity environment of an orbiting laboratory should be capable of manipulating
payloads such that the motion of the robot does not disturb adjacent experiments. The current results of a NASA
Lewis Research Center technology program to develop smooth, reaction-compensated manipulation based on both
mechanism technology and trajectory planning strategies are presented. Experimental validation of methods to
reduce robot base reactions through the use of redundant degrees of freedom is discussed. Merits of smooth
operation roller-driven robot joints for microgravity manipulators are also reviewed.
17. Key Words (Suggested by Author(s))
Robotics
Robot control
Microgravity
Microgravity robotics
18. Distribution Statement
Unclassified- Unlimited
Subject Category 37
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of pages
Unclassified Unclassified 5
NASAFORMlS2Soc'r 86 *For sale by the National Technical Information Service, Springfield, Virginia 22161
22. Price*
A02


Reaction-compensation technology for microgravity laboratory robots

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server