A shuttle-attached manipulator is currently proposed as the payload-handling device for the space shuttle. Basic requirements for the manipulator involve length, force, compliance, and control. Approaches for studying control methods are presented and simulation methods are discussed. Basic details about the two earthbased manipulators selected for simulation experiments are related to the test methods. Preliminary data from one test are shown as an example of the direction of the testing. A computer-generated simulation is explained, and the relationship of the three simulations to the design problems is discussed

Burroughs, E. G.

English

NASA Technical Reports Server

MANIPULATOR TECHNOLOGY FOR THE SPACE SHUTTLE
By E. Glenn Burroughs*
ABSTRACT
A shuttle-attached manipulator is currently proposed as the payload-handling de-
vice for the space shuttle. Basic requirements for the manipulator involve length,
force, compliance, and control. In this report, approaches for studying control methods
are presented and simulation methods are discussed. Basic details about the two earth-
based manipulators selected for simulation experiments are related to the test methods.
Preliminary data from one test are shown as an example of the direction of the testing.
A computer-generated simulation is explained, and the relationship of the three simula-
tions to the design problems is discussed.
INTRODUCTION
The orbital delivery system for space flight in the future will be the space shuttle.
The current design for the shuttle orbiter vehicle specifies a payload bay having dimen-
sions of 4.57 by 18.29 meters (15 by 60 feet) and having provisions for handling pay-
loads. The payloads will have a wide variety of sizes, weights, and shapes. Examples
of the handling functions to be performed are deployment, capture and retrieval, and
stowage of the payloads. The various payloads may be handled either by individual
manipulative devices, each designed to perform simple functions, or by a general-
purpose manipulator designed to span the range of handling functions. In this report,
a preliminary design for a shuttle-attached manipulator (SAM), which is a general-
purpose manipulator, is described.
No manipulators have been used in space, except for such simple devices as the
Surveyor surface-sampler scoop. However, the technology of earth-based manipulators
provides an adequate base from which to design the SAM. Significant aspects of the
SAM design deviate from earth-based manipulator designs; thus, a logical program to
extend the technological base toward the design of the SAM is necessary. The salient
parts of this program and the reasoning used in the extension of the technology are
discussed in this report.
*NASA Manned Spacecraft Center, Houston, Tex.
267
https://ntrs.nasa.gov/search.jsp?R=19730010162 2020-03-17T08:20:32+00:00Z
DESIGN REQUIREMENTS
A conceptual drawing of a SAM and its installation in a shuttle is shown in fig-
ure 1. The shuttle payloads identified in current mission models can be as small as
a few cubic feet and weigh as little as several hundred pounds or can be as large as
4.57 meters (15 feet) in diameter and 18.29 meters (60 feet) in length and weigh as much
as 29 478 kilograms (65 000 pounds). The desirability of grasping a heavy module near
its center of gravity and the need to grab a small satellite located toward the aft end of
the payload bay establish a minimum length of approximately 9.14 meters (30 feet) for
the SAM. Apparently, there is no requirement for the length to exceed 18.29 meters
(60 feet). Current plans are for a SAM to be 9.14 to 12.19 meters (30 to 40 feet) in
length because, with a SAM of this length, 90 percent of all the payloads in the traffic
model located anywhere in the payload bay can be reached, and 100 percent of the pay-
loads can be reached if the smaller satellites are not located far aft. The shorter
lengths are lighter and easier to control. The degrees of freedom of the SAM are dis-
tributed so that they correspond anthropomorphically to a shoulder, elbow, and wrist.
Tip Force
The minimum tip force required for the SAM is primarily a function of the arm
length, the payload weight, and the ability of the operator and his control system to
control acceleration and velocity safely. The minimum tip force of 44.48 newtons
(10 pounds), used with a maximum length arm, is required to provide sufficient energy
to complete many operations within a reasonable time. The energy also is required to
stop the heavy payloads at residual closing velocities. Arms shorter than the maximum
length require larger force because the available stopping and starting distances de-
crease, and the accelerations must be increased to supply equivalent energy. The max-
imum useful force is more difficult to establish. Because the heaviest payload imposes
design constraints on minimum force, the selection of a force levei only marginally
greater than the minimum will result in a low-weight system. An upper limit of
444.82 newtons (100 pounds) was established, based on combinations of the heaviest
payloads and the shortest arms.
Compliance
Compliance in the SAM is defined as the amount of tip deflection per unit of tip
force. Compliance is caused by the flexure of the arm structure, the elastic deflection
and tolerances within the joints, and the electronic response of the servocontrols. As
in other orbiter systems, the weight of the SAM must be minimized. The values of
stiffness (the reciprocal of compliance) postulated in preliminary designs are suffi-
ciently high that the design of the arm structure is controlled by stiffness instead of by
stress. Consequently, it would be desirable to allow more compliance (less stiffness),
use higher stress levels, and reduce weight. Just as too little compliance increases
weight, excess compliance decreases safety and lessens positioning accuracy. The
compliance value of 0.06 cm/N (0.1 in/lb) was used in the preliminary designs because
it would produce conservatively high weight and better-than-adequate positioning. Re-
cent experiments that will be described have produced data indicating that more compli-
ance is tolerable.
268
Currently, electric actuators are planned for use in the SAM. Conditioned elec-
tric power will be available on the orbiter, whereas hydraulic power sources probably
will not be active during payload-handling periods. Electric motors also will be more
easily space qualified than hydraulics because of contamination restrictions.
Control System
The effect of man in the servocontrol loop is difficult to assess, particularly be-
cause partial computer control probably will be used. In preliminary designs, two
desirable modes of computer operation have been identified. In the first mode, active
when man is in primary control, system status is monitored by computer, and collision
avoidance is ensured by the computation of the position of the arm with respect to all
fixed objects. Automatic collision avoidance is particularly desirable with respect to
the position of the elbow. Requiring the operator to be constantly aware of the elbow
and wrist, points that are at least 4.57 rr/eters (15 feet) apart, reduces his capability
to operate the secondary television and information systems while also operating the
arm. In the second computer mode, the computer operates the arm and the man is in
monitor status. This mode will be used only in situations in which all bodies are at
known locations. The reason for using full computer control is to make precise opera-
tion of a controller for a long timespan unnecessary. Examples of operations in which
the second mode is used are deploying the arm from the stowed position, stowing the
arm, and translating a payload between positions near (but not in contact with) other
bodies. These operations do not need precise position control from a safety standpoint
but do require careful force control to minimize operation times. The operator can
prepare the peripheral systems for the next operation to be completed while the com-
puter is in control.
The type of hand controller to be used by the operator for his real-time control is
being studied. A variety of types of controllers probably would be suitable. Examples
of candidate controllers are hand controllers, joysticks, buttons, and master-slave
devices. The c_rrent work is to determine which controller offers the best combination
of safety, reliability, utility, and weight penalty.
SIMULATION METHODS
The design requirements for reach and force were developed by mea.nsof a sim-
ple analysis of the payload-handling functions and dynamics. The determination of
tolerable levels of compliance 'and optimal computer and controller mechanization is
not susceptible tO analysis because man adds an active part to the control equations.
Consequently, empirical methods are being used, and the obvious approach is to build
and test a SAM. The force levels previously mentioned were in a zero-g environment,
but a limited study has indicated that simulating zero-g performance in a one-g field
for a SAM is not practical early in the design process.
A review of manipulator technology reveals'that there is no manipulator system
currently using a combination of reach, force, compliance, and complex control that-
even approximates the SAM. The remaining approach is to find systems that approxi-
mate some characteristics of the SAM and to use appropriate testing to extrapolate
their performance. Thus, the performance of the SAM is approximated by a series of
269
simulations using current systems with the man in operational control. This approach
is relatively economical and allows the performance of the SAM to be demonstrated by
experiment instead of by analysis very early in the design process.
Manipulative Systems
A wide variety of manipulative systems are available for experiment. The sys-
tems range from such large but nonadvanced devices as derricks to extremely complex,
anthropomorphic systems used in materials handling. The force level of the SAM ap-
pears to offer no difficult problems, and the length per se is just a design detail. The
experimentation must facilitate information acquisition regarding compliance and con-
trol in a remotely controlled, dextrous system. The materials-handling devices that
have great dexterity embody the most complicated and versatile control systems. Two
manipulators, an E-2 and a CAM 1400, were good choices for study because of perform-
ance capability and availability.
The E-2 (fig. 2) is an anthropomorphic, bilateral, six-degree-of-freedom, master-
slave, electric manipulator. The motions and distribution of the degrees of freedom of
the E-2 arm are anthropomorphic. The controller (master) is a duplicate of the working
arm (slave); operations are performed by moving the handle of the master through the
motions and with the forces desired of the slave. Bilateral refers to the fact that force
feedback generated at the master arm is equal to the working force of the slave arm. A
useful test is possible with the E-2, even though the slave arm has approximately one-
tenth the effective reach of the SAM. The purpose of the test is to demonstrate the
ability of a man to control high-inertia dynamics by the application of small forces and
to quantify his response to different compliances. The massive body to be controlled
with 26.69 newtons (6 pounds) of available force is a 181.44-kilogram (400 pound) weight
suspended from a 6.71-meter (22 foot) pendulum (fig. 2). The weight swings in the
plane containing the slave arm and is released from a position 0.61 meters (2 feet) from
rest on the far side from the arm. The slave arm captures the weight at the point of
closest approach and brings it to a rest as soon as possible. A potentiometer used as
a position transducer: measures the lateral displacement of the weight from rest for
recording on an oscillograph. In figure 3, the data record of the position transducer
for one test run of one operator is shown. Different compliances are introduced by
making adjustments to the servoamplifiers between runs. The operator reacts essen-
tially the same to servocompliance as to structural compliance of the SAM. Four dif-
ferent compliance values were used. Each of the nine operators made practice runs
before making test runs. The data point for each combination of compliance and opera-
tor is the average of several consistent runs. In figure 4, the overall average for all
operators at each compliance is shown. The significance of the curve is that it shows
that operators can adapt to almost any compliance of interest to the SAM design and
that only small performance changes result.
The CAM 1400 is an anthropomorphic, bilateral, five-degree-of-freedom,
master-slave, electrohydraulic manipulator (fig. 5). The CAM is not bilateral master-
slave in all five degrees of freedom. The three degrees of freedom, two at the shoulder
and one at the elbow, that translate the end point of the arm are controlled by a master
arm that kinematically resembles the slave and that has 22.24 newtons (5 pounds) of
force feedback. The remaining degrees of freedom, which position the wrist motions,
are controlled by one constant-rate switch and one position potentiometer. A more
270
significant difference between the E-2 and the CAM is that the E-2 has an effective
reach of 0.91 meter (3 feet) and the CAM has an effective reach of 6.71 meters (22 feet).
The CAM will be used in conjunction with a 5°49- by 10.67-meter (18 by 35 foot) air-
bearing surface, air-bearing freebodies, and television systems to simulate all major
handling functions of the SAM. Examples of tests are tracking and grasping a moving
small satellite, payload stowage and deployment, and payload inspection. In tests pres-
ently being conducted, the CAM moves a 3175.1-kilogram (7000 pound) weight mounted
on air bearings through a maze in order to evaluate positioning capability and the effects
of compliance changes. Compliance will be varied as it was in the E-2 tests, and per-
formance data will be taken. Other tests will be used to evaluate television systems
and interactions between direct vision and television. The combination of buttons,
potentiometers, and master-slave control methods also will give early indication of
operator preference of control types.
Computer-Generated Scene
At the NASA Manned Spacecraft Center, a computer-generated scene, which is
a method of simulation not incorporating an actual manipulator, is being used to re-
solve problems involving control methods. The scene is composed of points, lines,
and colors. A typical scene, shown in figure 6, consists of the cargo bay area of the
orbiter vehicle, the SAM, and a large payload module. The computer program can
handle rigid-body dynamics for the bodies and is capable of imposing such restrictions
as rate limiting. The scene is displayed on a television screen. Force feedback is not
available currently. A variety of controllers can be used because only programing
changes are needed to interface the physical controller with the simulation. Controllers
covering the range from a button box to a master arm without feedback will be used.
CONCLUDING REMARKS
The technology base necessary for the design of the SAM is being derived from
simulations using earth-based manipulator equipment and computer simulation. These
simulations are being used to give operators the subjective feel of a space manipulator
and to generate design data. Two hardware simulations, using the E-2 and the CAM,
and the computer simulation form an experiment program that covers the characteris-
tics of the SAM that need definition early. The computer scene generator will provide
control-system definition, and the E-2 experiments have provided needed data on com-
pliance and man-control interaction. The CAM experiments will allow exploration of
compliance, man-control interactions, vision-control interactions, and even such pro-
cedures and limited hardware work as evaluation of arm terminal devices. By the use
of this program early in the SAM design, the gap between the technology of earth-based
manipulators and the technology needed for the SAM can be bridged efficiently.
271
ELBOW
(2 DEGREES
OF FREEDOM
WRIST
(3 DEGREES
OF FREEDOM
ISION
A AN
LIGHTS
-SHOULDER
(2 DEGREES
OF FREEDOM)
MODULEj
ISION
CAMERA
AND
LIGHTS
"TERMINAL
_DEVICES
PAYLOAD
GRASPIN
POINTS
ARM
STOWAGE
CLAMPS
Figure 1. - The SAM installation.
Figure 2. - The pendulum test setup.
272
INCREASING
TIME
+0.61 m (+2 FT)
LATERAL
WEIGHT DISPLACEMENT
-0.61 m (-2 FT)
REST
RELEASE
CAPTURE
Figure 3. - Sample test data.
STOP
6.0
TIME REQUIRED TO BRING
PENDULUM TO REST, SEC
5.0
AVERAGE VALUE
ALL SUBJECTS
FOR
4.0
!
0
I I
0.6 (1.0) 1.2 (2.0)
ELBOW COMPLIA_NCE, cm/N (IN/LB)
Figure 4. - The effect of compliance on performance.
273
Figure 5. - The CAM 1400.
Figure 6. - The computer-generated scene.
274


Manipulator technology for the space shuttle

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server