Several factors, such as body size and shape, and the number of arms and their placement, will influence how well the Flight Telerobotic Servicer (FTS) is suited to its potential duties for the Space Station Program. In order to examine the implications of these configuration options, eight specific 2, 3, and 4 armed FTS configuration were simulated and used to perform a Space Station Orbital Replacement Unit (ORU) exchange. The strengths and weaknesses of each configuration were evaluated. Although most of the configurations examined were able to perform the exchange, several of the 3 and 4 arm configurations had operational advantages. The results obtained form these simulations are specific to the assumptions associated with the ORU exchange scenario examined. However, they do illustrate the general interrelationships and sensitivities which need to be understood

Howard, W. S.

Lewis, R. H.

Scott, R. D.

English

NASA Technical Reports Server

N89-10100 --
KINEMATIC STUDY OF FLIGHT TELEROBOTiC SERVICER
CONFIGURATION ISSUES
R.H. Lewis, R.D. Scott and W.S. Howard
RCA Astro Space Division
Princeton, NJ 08543-0800
Abstract
Several factors, such as body size and shape, and the number of arms
and their placement, will influence how well the Flight Telerobotic
Servicer (FTS) is suited to its potential duties for the Space Station
Program. In order to examine the implications of these configuration
options, eight specific 2, 3, and 4 armed FTS configurations were
simulated and used to perform a Space Station Orbital Replacement Unit
(ORU) exchange. The strengths and weaknesses of each configuration
were evaluated. Although most of the configurations examined were able
to perform the exchange, several of the 3 and 4 arm configurations had
operational advantages. The results obtained from these simulations are
specific to the assumptions associated with the ORU exchange scenario
examined. However, they do illustrate the general interrelationships and
sensitivities which need to be understood.
i '_/! _'_;""_
.-¢ j
The Flight Telerobotic Servicer (FTS) is intended to provide a
supplemental EVA capability on the Space Station without the extra risks
associated with actual manned EVA. It can be remotely controlled from
within the Station's pressurized volume as an IVA activity. A number of
tasks are under consideration as candidates for regular FTS assignments.
The exchange of Orbital Replacement Units (ORU)s is one such work
assigment. ORUs are standardized modules which contain replaceable
Station and instrument subsystem elements. These ORUs will be located
throughout the Space Station's trusswork on Station Interface Adapter
(SIA) pallets. Transportation of the FTS around the Space Station will be
provided by the Canadian Mobile Servicing Centre (MSC), which will carry
the FTS at the end of one of its remote manipulator arms. The MSC arm
also provides coarse positioning for the FTS over its local work area.
Before starting the FTS arm configuration kinematic study it was
necessary to make a number of initial assumptions. In all exchanges, one
arm is dedicated to providing stabilization by grasping the stability
fixtures, located on the MSC and SIA, and the remaining arms move and
operate the ORU. The exchange of a Work Package 4 electrical power
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https://ntrs.nasa.gov/search.jsp?R=19890000729 2020-03-20T06:04:23+00:00Z
syst,was baselined for all FTS configurations examined. These
ORLssumed to be 23"x25"x12". Two versions of these ORUs were
modr the two and three armed FTS configurations, the standard
attaevice was assumed to incorporate a combined handle and
attacechanism. This system, based on RCA designs, requires only
one ,with an appropriate end effector to both hold and operate the
connsconnection mechanism. For the four arm FTS
confi_ an alternate version of the ORU was employed. Its
attacechanism consisted of two bolts which required one arm
equip a special end effector to operate while a second arm
equil_ another end effector was used to hold the ORU handle.
Simusing this ORU required a minimum of three arms to
compe ORUs were assumed to be mounted in a three by three array
with _arances between adjacent ORUs. The ORU to be replaced in
all th_ions was the central ORU in the array. This represented
the mult exchange in terms of reach and clearance. The SIA was
locate of a standard, 16.4 foot (5 meters) on a side, truss cube. A
gener,vith one six degree of freedom, 35.3 foot long manipulator
arm _med. An illustration of this common trade study
enviro_ shown in Figure 1.
'ational reach of each FTS configuration is a function of the
manipngth, number of manipulators involved, and the size, shape
and lahe body used. Rather than model several different
manip.ms, only one design was used and all arms were identical.
This P_useful redundancy and flexibility during simulations.
Speciad effectors were assumed to be available, but their actual
desigrhot modeled in the simulations. A generic stub, 14 inches
long, n after the wrist joint represented this class of device. The
separaance between the shoulders of the FTS was an important
study _er. Given identical arms, a greater shoulder separation
distanq allow the FTS a greater reach.
rJ_..e.d.Y__'em e nts
T_rs influence the size and shape of the FTS in a direct way.
The recnt that the FTS must be IVA servicable implies that the
entire _t have overall dimensions which are compatible with the
Space _hatches and passageways. The other factor is the
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Figure 1. Common Trade Study Work Environment
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requirement that the FTS is not responsible for its own transport.
Because the MSC transport system positions the FTS in its proper work
space, the FTS need not be capable of simultaneously reaching the five
meter distance from one truss node to the next. (Physical contact with
the truss members under normal conditions is not allowed). Elimination
of the need for a five meter reach promotes compatibility with the hatch
size requirement. All of the FTS configurations examined will fit through
a Space Station hatch as it is currently known.
Arm Confiouration Descriotions
Every FTS configuration examined used identical six degree of
freedom manipulator arms as a common component. When necessary a
seventh degree of freedom could be added. Although the arms were less
agile than a human arm, they were of anthropomorphic design. The
shoulder provided pitch and yaw, the elbow provided pitch, and the wrist
provided pitch, roll, and yaw. The shoulder and elbow position the wrist,
and the wrist orients the end effector relative to the work area. The arm
is made up of a 22 inch upper arm and a 22 inch forearm.
The bodies of the simulated FTS configurations fell into three basic
categories based on the number of arms used. Two, three, and four arm
FTS configurations were modeled. Within each of these three categories
the major variable was the separation distance between adjacent
shoulders. The following eight configurations were modeled:
A two arm bar shaped robot with a 48 inch shoulder separation
A three arm equilateral triangle shaped robot with 24 inch
shoulders
A three arm equilateral triangle shaped robot with 48 inch
shoulders
A four arm square shaped robot with 24 inch shoulders
A four arm square shaped robot with 36 inch shoulders
A four arm square shaped robot with 48 inch shoulders
A four arm rectangular shaped robot with both 24 and 48 inch
shoulders
A four arm kite shaped robot with both 24 and 48 inch
shoulders
These configurations are illustrated in Figures 2 and 3.
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48 Inch Two Arm FTS
24 Inch Three Arm FTS 48 Inch Three Arm FTS
Figure 2. Two and Three Arm FTS Simulation Configurations
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24 Inch Four Arm FTS 36 Inch Four Arm FTS
48 Inch Four Arm FTS 24/48 Inch Four Arm FTS
Kite Four Arm FTS
Figure 3. Four Arm FTS Simulation Configurations
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Confiourational Trades. Two Arm FTS ORU Exchanoe Simulation
A two arm FTS with a shoulder separation of 48 inches was used in
this simulation. The ORU exchange simulation started with the FTS being
moved inside the truss structure by the MSC arm. In order to pass through
the trusswork the FTS orients its arms to minimize its chance of
collision. Once inside the trusswork the MSC arm positions the FTS in
front of the SIA containing the ORUs. The FTS then grasps a support point
on the SIA with one of its arms to stabilize itself relative to the work
site. The other arm is used to remove the ORU identified for replacement.
Once the ORU has been removed and is clear of the SIA, the FTS releases
the support point on the SIA and assumes a posture which minimizes its
chance of collision. The MSC arm then transports the FTS and ORU back to
the base of the MSC. Once positioned properly, the FTS grasps a support
point on the MSC base with its free arm, attaches the ORU to the MSC, and
picks up the new replacement ORU. The MSC then transports the FTS and
replacement ORU back inside the truss structure. The FTS grasps a
support point on the SIA, and places the new ORU in the spot previously
occupied by the removed ORU. The FTS is then transported back to the
MSC base.
The grasping of the support points on both the SIA and MSC by the
FTS is a crucial step in the ORU exchange. In a real exchange, the support
point permits the FTS to identify its relative position. The FTS
calculates its position in space precisely, since the arm joint angles,
location of the work site, and the support point position are known.
Automated routines can then be inititated. The support point also allows
the FTS to carry the loads associated with connecting and disconnecting
the ORU against itself rather than the MSC arm. In the simulation, the
locations of the support points were examined to verify their usefulness.
Figure 4 shows the two arm FTS removing an ORU. The MSC and other
details have been removed for clarity.
The results of this simulation demonstrate that it is possible to
exchange an ORU with a two arm FTS. However, two arms are the absolute
minimum number necessary to complete the exchange. The dimensions of
this particular FTS configuration were compatible with those of the work
site. It would be advantageous to have a third arm on the FTS to carry the
replacement ORU along when the MSC arm positions the FTS within the
truss structure. This would eliminate the need for an extra trip through
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Figure 4. Orthogonal Views of a Two Arm FTS Removing an ORU
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the trusswork. The simulation also demonstrated the need for close
coordination between the MSC operator and the FTS operator. There were
many instances where a lack of coordination between the two operators
could result in damage. The movement through the truss and the approach
to the SIA and MSC base are of particular concern.
Confiquration Trades. Three Arm FTS ORU Exchanae Simulation
Two versions of a three arm FTS were simulated. Both versions had
equilateral triangle bodies which differed only in size. One used 24 inch
shoulder separations, and the other used 48 inch separations.The ORU
exchange scenario used for these two simulations was similar to that
used previously in the two arm case. Once again, the scenario begins with
the MSC arm transporting the FTS. The first stop this time is the MSC
base. The FTS uses one arm to grasp a support point and then uses another
arm to attach to the replacement ORU. The ©RU is disconnected from the
MSC and lifted away, the other arm then releases the support point. The
MSC arm then transports the FTS and ©RU into the truss structure to a
location in front of the SIA. In order to do this, both FTS configurations
had to assume postures which reduced their collision cross section. This
posture is shown in Figure 5. The FTS then uses one arm to grasp the
support point on the SIA. While keeping the replacement ORU safely out of
the way, the FTS uses its third arm to remove the target ORU. Please
refer to Figure 6. The target ORU is moved out of the way and the
replacement ORU is moved and attached to the SIA. The FTS releases the
SIA support point and the MSC arm transports it back through the
trusswork to the MSC base. The FTS grasps a support point and connects
the target ORU to the MSC base. The exchange is then complete.
The three arm FTS configurations both have distinct operational
advantages over the two arm FTS. An exchange requires significantly less
use of the MSC arm and is therefore safer and faster. The 24 and 48 inch
shoulders were both capable of performing the ORU exchange. However,
the 24 inch shoulder represents the smallest feasible size given the work
site dimensions and the length of the FTS arm. With this smaller shoulder
separation, the FTS had to stretch its arms to full length to accomplish
the exchange. The 48 inch shoulder separation allowed the FTS improved
reach under less constrained conditions. The three arm FTS could also
emulate the two arm FTS, if necessary. The three arm FTS is capable of
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Figure 5. Three Arm, 48 Inch Shoulder, FTS Prior to Insertion
into the Truss Cube.
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IFigure 6. A Three Arm FTS, with a 48 Inch Shoulder Separation,
Exchanging an ORU
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holding and connecting or disconnecting an ORU which required two arms
to operate. This use was not investigated for the three arm
configurations, but was baselined for the four arm simulations.
Configuration Trades. Four Arm FTS ORU Exchange Simulation
Five variations of a four arm FTS were simulated, including three
square FTS configurations with 24, 36, and 48 inch shoulder separations.
A rectangular FTS with two 24 inch and two 48 inch shoulders, and a kite
shaped FTS with two adjacent 24 inch shoulders and two adjacent 48 inch
shoulders were also investigated.
The ORU exchange scenario simulated for the four arm configurations
used ORUs which require two separate arms. One FTS arm holds the ORU
and a second operates the connection and disconnection mechanism.
Instead of having a specially designed single handle which can be used to
both grasp and release/attach the ORU, the new ORU has a handle used
only to hold the ORU, and two tie down bolts used to release or attach the
ORU. The size of the QRU remains the same.
The scenario used for the simulation started with the MSC arm
transporting the FTS to a point near the MSC base. The ITS then uses one
arm to grasp a support point, and another to grasp the ORU handle. Using
the remaining two arms, the FTS disengages the ORU from the MSC base.
These arms are then moved out of the way and the third arm releases the
MSC support point. The MSC arm then transports the ITS and ORU to the
SlA inside the truss structure. The FTS uses one arm to grasp the SlA
support point, and uses the other two arms to grasp the ORU handle and
disengage the target ORU. The target ORU is then moved to a point out of
the way by one arm. The replacement ORU is then moved into position and
the free arm is used to connect it to the SlA. The ITS then releases its
hold on the SIA support point and is transported by the MSC arm to the
base of the MSC. Once there, the ITS grasps the MSC support point and one
arm moves the ORU into place, where the other two arms connect it to the
MSC. Figures 7 and 8 illustrate two steps in this scenario for the kite
shaped FTS and the 24 inch square FTS.
The tasks simulated in this scenario were considerably more
challenging than those used in the previous simulations. The ability of
each of the five configurations to reach both the SIA support point with
one arm and the proper ORU on the SIA with the remaining three arms was
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Kite Shaped Four Arm FTS Removing an ORU from the
.=MSC
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Figure 8. A 48 Inch Square Four Arm FTS Attempting to
Exchange an ORU on the SIA
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est,luare Fable to accomplish this.
s v,.oits arpt reach far enough. The
aremable lsh the same tasks for the
]sooulder arms of the FTS too far
w tlether the ORU. The rectangular
3o _)te theven though its 48 inch
dlo_hold tl_oint and another to hold
_ndlcould ram the support point end
:o tl_ince tl'm carried the replacement
Id r Only our arm FTS
ns,_are FT(ite shaped FTS, were able
Jlly ORU cs it was intended.
inwas w(the exchange scenario, its
_eitl too shle shaped FTS represents
ted3cenarits arms is located
f fr(ee. Thble to reach easily
,, Oport p_lA, and still have three
Iog_ the cd exchange. This example
)s trly undich tasks the FTS must
)re ptimiz(d overall design.
Ehe,apparne four arm FTS
Tlfour at" in close proximity
,ithignificaype of situation, six
re_e not _cient. Seven degrees of
vidry flexbid interarm interference.
._d,gree oYas incorporated at the
his)ility rer intervention to
E
_uown thdesign can benefit from a
;tanded u also shown that seven
reessary )eration of more than one
trio. The also pointed out that the
hethe opa FTS need to work in close
to, safet_pecially true during the
)u._cture _1approaches to the MSC
A.
All FTS configurations examined used one arm for stabilization at
the local work site. Thus, two arms are the minimum number necessary
for the baseline ORU exchange using the RCA ORU design. The two and
three arm FTS configurations used the RCA ORU design with the combined
handle and attachment mechanism. This was the only type of ORU that the
two arm FTS could use. A generic ORU with separate handle and
attachment mechanism, which requires at least two arms to hold and
work, was also simulated. This design was used in all of the four arm
FTS configuration simulations. The two arm FTS configuration with 48
inch shoulder separation was compatible with the exchange scenario and
the local work sites' dimensions and layouts.
A third arm produces an operational advantage over the two arm FTS
in that it can be used to carry the replacement ORU during the first
passage through the Station truss and thereby eliminate an extra trip
back to the MSC base. A four arm configuration also has this operational
advantage. The use of an extra TV camera at the end of a free arm would
also provide a remote operator with a useful alternate viewpoint when
the work space becomes visually congested.
Both the 24 and 48 inch shoulder separation, equilateral triangle, 3
arm FTS configurations were compatible with the exchange scenario and
local work environments. However, the 48 inch shoulder separation had
better reach characteristics than the 24 inch design.
The 24 inch shoulder square, 48 inch shoulder square, and rectangular
(24 and 48 inch shoulders) FTS designs were not compatible with the
baseline work environment and could not accomplish the ORU exchange.
The 36 inch shoulder square and 24/48 inch shoulder kite FTS
configurations were compatible with the baseline work environment and
were able to perform the ORU exchange. Of these two, the 24/48 kite FTS
represented the most appropriate design for the baseline generic ORU
exchange scenario.
This simulation exercise illustrates how the tasks and work
environment associated with one specific ORU exchange scenario
influenced the success of each FTS configuration examined. The actual
FTS will be expected to be able to accomplish a minimum number of tasks
in a minimum number of work environments. Simulations of each of these
situations and scenarios will be needed before a serious FTS design can
be produced. The final FTS design will, in all probability, bethe best
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compromise achieved between the task optimized designs used in these
simulations. The ability of the FTS to perform future tasks, which are
currently unrecognized, will depend on how well these new tasks and
work environments can be understood and modeled and on how much they
differ from those used in designing the FTS originally.
Acknowledaements
The simulation exercise was funded by the NASA Goddard Space Flight
Center as part of contract NAS 5-29400.
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J


Kinematic study of flight telerobotic servicer configuration issues

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