A test rig is being developed that will be used to demonstrate and evaluate approaches to limiting manipulator base reactions in microgravity environments. The demonstration will include a 4-degrees-of-freedom arm, control computing facilities, and a base reaction measurement system

Brush, Andrew S.

English

NASA Technical Reports Server

N88-23242
MICROGRAVITY MANIPULATOR DEMONSTRATION*
Andrew S. Brush
Sverdrup Technology, Inc.
(Lewis Research Center Group)
NASA Lewis Research Center
ABSTRACT
A test rig is being prepared that will be used to demonstrate and evaluate
approaches to limiting manipulator base reactions in microgravity environ-
ments. The demonstration will include a 4-degrees-of-freedom (DOF) arm, con-
trol computing facilities, and a base reaction measurement system.
*Work performed on-site for the Structural Dynamics Branch under contract
NAS3-24105 (task order 5232; monitor: Douglas A. Rohn).
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https://ntrs.nasa.gov/search.jsp?R=19880013858 2020-03-20T06:22:39+00:00Z
OVERVIEW
REACTION FOR SPECIFIED END-EFFECTOR PATH
In order to curtail the accelerations imparted to spacecraft by space experi-
mental activities, schemes have been developed for reducing the reactions at
the bases of robotic devices used for manipulation and motion on space plat-
forms. These call for the application of robotic joints combining smooth oper-
ation with zero backlash, to reduce vibration, and the use of path optimization
strategies to control base reactions due to gross motion. Although there is
theoretical and numerical evidence that these concepts are valid, a hardware
demonstration is desirable.
ARBITRARY PATH,
GEAR DRIVE
OPTIMIZED PATH,
ROLLER DRIVE
REACTION,
RCRT
TIME
CD-88-31729
1-218
MICROGRAVITYMANIPULATORDEMONSTRATOR
LABORATORYSETUP
The Microgravity Manipulator Demonstration will contain four main elements:
(i) A 4-DOFrobotic arm utilizing traction-driven joints. These joints are
expected to cause minimumvibration and have insignificant backlash.
(2) Data processing hardware based on VME bus architecture employing Motorola
68020 processors to implement arm control. In addition to providing
closed-loop position and velocity control, processing capability will be
available for path-optimization programs. Control software will be made
as modular as possible to facilitate parallel processing. The bus archi-
tecture will allow future expansion through addition of processor or data
acquisition boards.
(3) Optimization strategies that will select the best trajectory through
joint space for a required end effector path in order to have minimum
base reaction. These algorithms will be interchangeable modules in the
control software to allow easy switching of optimization methods.
(4) A base reaction measurement system that will allow quantitative determina-
tion of the differences in reaction levels between optimized and unopti-
mized trajectories.
This demonstration will contribute valuable test data for developers of path
optimization strategies, provide experience with programming techniques for
integrating position control and trajectory optimization, and leave a reaction
measurement test bed that will be useful to future microgravity robotics
research.
6-DOF FORCE/MOMENT TRANSDUCER 32-BIT COMPUTER SYSTEM
OPERATORINTERFACE
CD-88-31730
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POSTER PRESENTATION
OBJECTIVES
The Microgravity Manipulator Demonstration is being developed in order to
facilitate assessment of hardware, software, and theoretical approaches to pro-
viding a manipulation capability for space microgravity experimentation.
The demonstration will provide quantitative evaluation of strategies for limit-
ing manipulator base reactions and allow Lewis personnel to develop a knowl-
edge base of experience with robotic controls for reduction of base reactions.
Increased practical experience in this field is essential to the implementa-
tion of reaction-limited robotics in space.
The demonstration will also provide the opportunity to evaluate traction-
driven robot joints.
• DEMONSTRATE (PROOF OF CONCEPT) PATH OPTIMIZATION
FOR LIMITING REACTIONS DUE TO MANIPULATOR MOTION
• DEVELOP AND EVALUATE CONTROL PROGRAMS FOR
MICROGRAVITY MANIPULATORS
• EVALUATE TRACTION DRIVE JOINT TECHNOLOGY
CD-88-31731
1-220
APPROACH
The test rig must be able to provide all capabilities required to meet the
objectives stated previously. This will require
(I) A multi-DOF robot arm
(2) Software implementing optimization strategies to reduce base reactions
(3) A data processing facility capable of providing full control of the arm,
managing data acquisition, and running optimization programs under test
(4) A modular approach to control software that will allow easy substitution
of different strategies for accomplishing the objectives of robot control
(5) A means of measuring the reactions at the base of the robot in order to
evaluate the success of the reaction-limiting scheme
• MULTI-DOF MANIPULATOR ARM UTILIZING 2-DOF TRACTION JOINTS
• STRATEGY TO OPTIMIZE JOINT TRAJECTORIES TO MINIMIZE BASE REACTIONS
• HIGH-SPEED, 32-BIT, MULTIPROCESSOR CONTROL HARDWARE
• MODULAR CONTROL SOFTWARE
• MEASUREMENT OF BASE REACTIONS
CD-88-31734
1-221
LABORATORYSETUP
The Microgravity Manipulator Demonstration will contain four main elements:
(i) A &-DOFrobotic arm using traction-driven joints. These joints are
expected to cause minimumvibration and have insignificant backlash.
(2) Data processing hardware based on VME bus architecture employing Motorola
68020 processors to implement arm control. In addition to providing
closed-loop position and velocity control, processing capability will be
available for path-optimization programs. Control software will be made
as modular as possible to facilitate parallel processing. The bus archi-
tecture will allow future expansion through addition of processor or data
acquisition boards.
(3) Optimization strategies that will select the best trajectory through
joint space for a required end effector path in order to have minimum
base reaction. These algorithms will be interchangeable modules in the
control software to allow easy switching of optimization methods.
(4) A base reaction measurement system that will allow quantitative determina-
tion of the differences in reaction levels between optimized and unopti-
mized trajectories.
6-DOF FORCE/MOMENT TRANSDUCER 32-BIT COMPUTER SYSTEM
OPERATORINTERFACE
CD-88-31730
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TESTPLAN
Oncethe control system is satisfactorily debugged, strategies for minimiza-
tion of base reactions will be evaluated by executing point-to-point motions
of the end effector using standard and optimized trajectories in the joint
space. The measuredbase reactions will be comparedto determine the extent
to which dynamic forces are mitigated.
REACTION
MAGNITUDE,
RCRT
/- UNOPTIMIZED
/- OPTIMIZATION STRATEGY "A"
/
/ /-OPTIMIZATION STRATEGY "B"
/ /
__// /_"_fw .--_
' /
TIME
CD-88-31733
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BASEREACTIONMEASUREMENT- PERSPECTIVE
Until researchers becameinterested in experiments that take advantage of the
low acceleration levels available in on-orbit spacecraft, there was no particu-
lar concern over the possible effects of small accelerations due to forces
imparted by experiments or associated manipulators. Designers of the EURECA
space platform discovered that they wanted to know the characteristics of the
acceleration caused by each experiment that would fly on EURECA,so that they
could asses the effects on other experiments. They were not able to develop a
suitable 6-DOFforce-moment transducer, however, so they recorded vibration
levels at the attachment points of each experiment and calculated the forces
using their knowledge of the dynamics of the experiment frame and the support-
ing structure. This was not an accurate method because of the estimation
involved in determining the dynamic characteristics of the structures.
Some6-DOFtransducers have been developed that are apparently suitable for dy-
namic measurementof robot base reactions to the desired accuracy. The direct
measurementof reactions is preferable and will be used in the demonstration.
• SMALL SPACECRAFT ACCELERATIONS NOT A MAJOR CONCERN UNTIL THE ADVENT
OF laG EXPERIMENTS
• REDUCING SOURCES OF FORCE IMPROVES laG ENVIRONMENT
• HOW CAN WE MEASURE SMALL DYNAMIC FORCES?
• ACCELEROMETERS--REQUIRE PRECISE KNOWLEDGE OF DYNAMIC OF TEST ARTICLE
AND SUPPORTING STRUCTURE
• DIRECT MEASUREMENT OF FORCE USING 6DOF TRANSDUCER
CD-88-31732
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SIX-DOFTRANSDUCERCONSTRUCTION
The construction of a 6-DOFtransducer involves constructing a structure with
predictable stiffness to forces applied at the attachment point and then
instrumenting it to determine the strain or displacement in enough (six mini-
mum)directions to fully determine the load.
Strut-type transducers attempt to resolve all forces and momentsinto purely
axial loads in struts supporting an attachment flange (load application point).
Onepopular type of robust transducer uses eight flexures to measure the hori-
zontal and vertical forces in perpendicular planes. This type maybe stiff
and strong but will have more crosstalk than a strut-type transducer.
Any shape of flexure maybe used, as long as at least six orthogonal compo-
nents of displacement can be measuredwith sufficient resolution. The previ-
ous types of fixtures are successful because they contain areas of high enough
strains to allow use of electrical resistance strain gages.
High-precision displacement measurementtechniques may present opportunities
to use unconventional flexure designs.
STRUT TYPE TRANSDUCER MULTIPLE FLEXURE
TRANSDUCER
CD-88-31735
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BASEREACTIONMEASUREMENT- DATAREDUCTION
Measurementof forces arising from the motions of the manipulator and its pay-
load are complicated in an earth-bound laboratory by the presence of static
loads due to gravity. In the case of a manipulator, the static momentsin the
plane of the floor (Mx and My) are determined by a nonlinear function of thejoint angles. This static componentof the measured load must be subtracted
from the total measured reaction:
Rt(t,@) = Rd(t) + Rs(e)
where Rs(e) must be known for all possible points in the joint space.
The interpolation error caused by using a lookup table with a reasonable
numberof points (<I0 000) will be excessive, so we need to develop an equa-
tion for the Rs(e). This equation is a function of e, having as parameters
the massdistribution of the arm segments. Wewill eliminate having to esti-
mate the massdistributions by fitting an equation of the proper form to exper-
imental data.
The present demonstration does not include real-time feedback of reaction, but
this maybe possible if processing resources are sufficient.
• STATIC BASE FORCES DUE TO GRAVITY MUST BE IGNORED
• GRAVITY MOMENT IS A NONLINEAR FUNCTION OF JOINT ANGLES
• NEED TO KNOW GRAVITY MOMENT AT ALL POINTS IN JOINT SPACE
• LOOKUP TABLE IS IMPRACTICAL
• PARAMETERS IN EQUATIONS CANNOT BE EXACTLY DETERMINED ANALYTICALLY
• ANSWER: USE PARAMETER IDENTIFICATION TECHNIQUES
• EQUATIONS ARE COMPUTATION INTENSIVE...
REAL-TIME REACTION FEEDBACK MAY REQUIRE ADDITIONAL PROCESSOR
CD-88-31736
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CONCLUSIONSANDBENEFITS
This demonstration will provide proof of concepts that will reduce robot base
reactions by optimizing joint trajectory. Additionally, it will contribute
experience with controls and hardware for microgravity manipulation to a knowl-
edge base that will benefit designers of hardware for space experiments.
The force measurementsystem and computer system will be available after the
4-DOFdemonstration for research using other types of manipulators.
• KNOWLEDGE BASE WILL BENEFIT DESIGNERS WHO REQUIRE
REACTION LIMITATION
• FORCE MEASUREMENT SYSTEM AND CONTROL DATA FACILITY
WILL BE AVAILABLE TO TEST FUTURE MANIPULATORS AND
ACTUATORS
CD-88-31737
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