UNIVERSITY ROLE I N ASTRONAUT LIFE SUPPORT SYSTEMS: - .. GUIDANCE AND STABILIZATION I N
by Lonlzie C. Von Renner
Prepared by MAN-VEHICLE LABORATORY
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
Cambridge, Mass. 02 139
f o r
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION WASHINGTON, D. C. MARCH. 1972
https://ntrs.nasa.gov/search.jsp?R=19720011518 2018-09-08T17:20:14+00:00Z
1. Report No. 2. Government Accession No. NASA CR-1919
4. Title and Subtitle UIJrVERSITy ROLF 3N AS!lXOItAUT SUPPORT SYSl'EMS: EXTRAVMIW GUIDANCE AND S!CABILI!U'ITON I N SPACE
T
7. Author(s1
IxlNNJE C. VON RENNER
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The Bioenvironmental Systems Division, NASA Office of L i f e Sciences,
Office of Manned Space Flight, i s vi ta l ly interested in foster ing
academic research which w i l l advance the technological base of l i fe
support and crew equipment systems. Both exploratory and applied research
is needed to improve the state-of-the-& and evolve advanced concepts.
Many excellent ideas and operating 'systems have been brought t o f r u i t i o n
i n NASA laboratories and by associated contractors. However, involvement
of academic laboratories and graduate student study programs has not
been as great as was originally hoped. Perhaps, t h i s has been due t o
the fact that academic personnel were not aware of the c r i t i ca l problem
areas i n l i fe support and crew equipment systems, nor were they cognizant
of the fac t that NASA is interested in joining with colleges and univer-
sities t o develop new ideas to solve space flight problems.
This brochure on Extravehicular Guidance and Stab i l iza t ion in Space
is intended t o introduce you t o some of the existing technology involved
i n guidance and s tabi l izat ion of extravehicular maneuvering systems and
t o pinpoint areas where problems exist. We encourage you t o study this
brochure. If, in your research, you have already developed new ideas,
theories, and processes, which would be applicable to NASA's efforts, we
hope you w i l l feel incl ined to contact us to see where a joint research
e f for t can be ini t ia ted. Approved ef for t s w i l l be funded as grants with
funding sufficient to support the principal investigator and several
graduate students research assistants. Graduate students are encouraged
to par t ic ipa te as research assistants and t o conduct the graduate research
under such a cooperative effort.
Walton L. Jones, M.D. Deputy NASA Director for L i f e Sciences
iii
SUMMARY
The field of Extravehicular Activity is multi-disciplinary
to the point where one report cannot hope to cover in detail
the several broad areas of knowledge upon which this subject
is based. Therefore, this is a review primarily about two
problems: 1) the nature of and approach to the EVA mission
and 2) the guidance and stabilization required of such a
mission. The intent here is to reveal several broad areas
in which additional research is needed to provide an advance
to the state-of-the-art. Moreover, the research problems
cited were chosen from among many as those most amenable to
research and study in the university environment. As a rule,
they represent basic questions as yet unanswered, new
approaches as yet unproven. They are less hardware-oriented
than other current problems in the field. Some, perhaps,
are more free-wheeling than those currently receiving the
attention of private industry; others require descending
toward microthoughts aimed at improving existing procedures
or suggesting new ones. All share in common the need for
fresh and enthusiastic inquiry.
These areas are reiterated below.
V
Area
1.
The Nature of EVA 2 .
1.
2. EVA System Approach
1.
EVA Guidance and Stabilization
L .
3 .
4 .
5.
Problem
Standard definition of EVA tasks and timelines
Load transmission device
Non-anthropomorphic suit
Definition of Optimal System
Subjective effort model
Postural reflex model
Electromyographic control
Timing controller evaluation
Optimal manual control technique
ACKNOWLEDGEMENTS
The ancient canon "no man works in a vacuum" may prove
apocryphal in the space age; it is certainly heresy to the
subject of this review. Nonetheless, we earthbound remain
subject to such laws as is well exemplified in the prepara-
tion of this report.
The author is grateful to several people whose comments
and criticisms helped to mold the study's final form. They
include Messrs. Thomas L. Keller, Frank W. Parker, David F.
Thomas, Jr., Dr. David Richardson, and Major C. E. Whitsett, Jr.
Special thanks are due Mrs. Annette Markowitz who steered the
report through several typed drafts. Mrs. Henrietta Galiana
and Professor Laurence Young together supervised the project.
vii
TABLE OF CONTENTS
Chapter I page
INTRODUCTION 1
The Nature and Necessity of EVA 1
Environmental Constraints 4
Mechanics 4
Vacuum 6
Illumination 6
Radiation, Thermal and Micrometeoroid Protection 7
Recommendations for University Research 8
Chapter I1
APPROACHES TO AN EVA SYSTEM 9
Recommendations for University Research 11
Chapter I11
EXTRAVEHICULAR SUBSYSTEMS 13
Guidance and Stabilization 13
Requirements 14
Unstabilized (Open-Loop) System Designs 17
Self-Rotation Techniques 17
Hand-Held Maneuvering Unit 18
Foot Controllers x)
Stabilized (Closed-Loop) System Designs 29
The Astronaut Maneuveuvering Unit 29
Alternate Controllers 33
Shuttle 39
ix
Control Techniques
Control Alternatives
Comparative Studies
Manual Control
Recommendations for University Research
Guidance and Stabilization
REFERENCES
Figures
1.
2.
3 .
4 .
5.
6 .
7 .
8.
9.
10.
11.
12.
Work Platform
Rendezvous Geometry
Vector Diagram
Force and Control Moments Resulting from Rotation of Ankles
Platform for Balancing Reflex Expts.
Single Rotor Single Degree of Freedom CMG
Typical Control Moment Gyro Type Attitude Control and Stabilization System Utilizing Rate Gyro Feedback
Momentum Control System
Experimental Configuration, Artist Conception
Actual vs. Subjective Angular Velocity
Pseudo Rate Control Block Diagram
Block Diagram of an Automatic Discontinuous Attitude-Control System with Linear Rate and Position Feedback
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65
X
I . INTRODUCTION
On March 18, 1965, with the era of manned s p a c e f l i g h t
extending back less than fou r yea r s L t . Col. Alexsei
Leonov s t e p p e d o u t s i d e h i s s p a c e c r a f t t o become t h e f i r s t
man t o d i r e c t l y e n c o u n t e r s p a c e . The f i r s t " s p a c e w a l k "
w a s s h o r t , l a s t i n g o n l y t e n m i n u t e s , and a t a l l times t h e
cosmonaut w a s a t t a c h e d t o v i t a l l i f e - s u p p o r t s y s t e m s o f h i s
s p a c e c r a f t by an umbilical co rd . None the le s s t he f ea t
demonstrated t h a t men p r o p e r l y s u i t e d a n d s u p p l i e d w i t h l i f e -
g iv ing oxygen could survive exposure to the weight less vacuum
of space.
From t h a t day t o t h i s , r e s e a r c h and development i n t h e
f i e l d o f e x t r a v e h i c u l a r o p e r a t i o n s h a v e p r o c e e d e d i n t h i s
coun t ry and e l sewhere , t o fu l f i l l r equ i r emen t s fo r fu tu re
mis s ions i n wh ich man ' s ab i l i t y t o work o u t s i d e h i s s p a c e c r a f t
w i l l be e s s e n t i a l . The following review w i l l c i t e t h e scope
an2 t r e n d s o f t h i s work in t h e p a r t i c u l a r a r e a of Guidance and
S t a b i l i z a t i o n of an EVA a s t r o n a u t . O n t h e basis o f c u r r e n t work
s t a t u s , t h i s r e p o r t w i l l endeavor to l i s t per t inent p roblems
f o r u n i v e r s i t y s t u d y and research.
-. The "- Nature and Necessity o f Ex t r aveh icu la r Ac t iv i ty
E x t r a v e h i c u l a r a c t i v i t y (EVA) is de f ined as any a c t i v i t y
"~ - " .. . .~
by a space c r e w member conduc ted ou t s ide t he pa ren t spacec ra f t ( s )
i n s u p p o r t o f o p e r a t i o n a l m i s s i o n s o r s c i e n t i f i c an?. engineer ing
inves t iga t ions . Thus , EVA encompasses explorat ion of the lunar
t e r r a i n by a s t ronau t - exp lo re r s as w e l l as c c b i t a l " s p a c e
walks ." This s tudy w i l l concen t r a t e on the problems and hard-
ware associated with performing work i n z e r o o r low g r a v i t y
environments in which a p l ane t ' s a tmosphe re or s i i r face does
not p lay a part,. I t does not include an examinat ion of 'Inloon
r o v e r s " o r o t h e r p l a n e t a r y v e h i c l e s , n o r d o e s i t cons ide r
the vas t a r ray o f space " too ls" cur ren t ly under deve lopment .
E x t r a v e h i c u l a r a c t i v i t y w i l l be G s e f u l i n i n s u r i n g t h e
s a f e t y of a p r i m a r y v e h i c l e s y s t e m a n 6 i n a s s i s t i n g i n t h e
accomplishment of future missions. The r o l e o f o r b i t a l EVA
as a back-up f o r i n t r a - v e h i c u l a r t r a n s f e r i n the Apollo Program
has been descr ibed by Bond, e t a l . (4). The o r b i t a l m i s s i o n s
which appear l ikely for implementat ion during the 1969-1980
period have been reviewed ( 3 4 , 51, 6 7 , 7 9 ) . They may be
l i s t e d uizrl.er e ight broad Headings.
Table 1: EVA Orb i t a l b l i s s ions 1969-1980
1. Environmental Survey
2. Biomedical Experiments
3 . Earth Sensing
4 . Orb i t a l Opera t ions Development
5 . Astronomical Sensing
6 . Orb i t a l Assembly
7. S a t e l l i t e I n s p e c t i o n and Repair
8. O r b i t a l :,aunch Vehicle Assembly
2
Some assignments , such as the as t ronomica l sens ing
mission, w i l l r e q u i r e a h igh degree o f as t ronaut mobi l i ty and
manual d e x t e r i t y i n p e r f o r m i n g t a s k s s u c h as changing the l ens
on an o r b i t i n g t e l e s c o p e ( 7 3 ) . Others , such as t h e o r b i t a l
launch vehicle assembly, w i l l r e q u i r e EVA t o checkout and
c o r r e c t any malfunctions of a.presumed automatic assembly
procedure ( 6 ) . I n o t h e r cases i t is d i f f i c u l t t o e x t r a p o l a t e
prec ise in format ion f rom a descr ibed miss ion of the EVA suppor t
r equ i r ed . A t p r e s e n t , n o s t a n d a r d e x i s t s as t o an accurate
fo rma t fo r desc r ib ing t he des i r ed EVA. I n a l l of t h e l i s t e d
ca t egor i e s , man ' s con t r ibu t ion can be analyzed as f a l l i n g
w i t h i n three d i s t i n c t domains: observat ion (sensory) , d e c i s i o n
(mental) , a n d a c t i v i t y s u c h as cons t ruc t ion , r epa i r , modu la r
rep lacement , ad jus tments , f lu id connec t ions (phys ica l ) ( 6 5 ) .
From t h e m i s s i o n s l i s t ed i n T a b l e 1, t y p i c a l s u p p o r t
a c t i v i t i e s c a n be def ined .
Table 2 : EVA S u p p o r t A c t i v i t i e s ( 4 5 )
Maintenance
Assembly
Resupply - packages
Resupply - f l u i d s
Rescue
P e r s o n n e l t r a n s f e r
These a c t i v i t i e s , i n t u r n , s u g g e s t t h a t c e r t a i n a t t r i b u t e s
w i l l be required of an EVA a s t ronau t . O the r s , wh i l e r equ i r ed
f o r some miss ions , may be omi t ted for o thers . These are l i s t e d
i n T a b l e 3 . Taken as a g roup , t hese de f ine t he s cope o f EVA
as it i s cu r ren t ly env i s ioned .
3
Table 3: EVA Requirements ( 3 4 )
Life support extension Tool bin (optional)
Communications Work aids
Mobility - dexterity Manipulators (optional) Power source (optional) Umbilical capability
Fixed work base Rescue provisions
Environmental Constraints
Two factors determine the needs of the extravehicular
astronaut. These are the type of work which he must perform
and the constraints imposed by the environment. Having studied
the mission requirements, the environment in which they will be
executed is here considered.
Four conditions which most seriously affect an EVA mission
in orbital space are:
1. Mechanics
2. Vacuum
3 . Illumination
4 . Radiation, Thermal and Micrometeoroid Protection
These will be considered individually.
TMechanics
The mechanics of motion in a zero-gravity environment
pose several problems, among which are: untethered object
scattering, peculiar inter-object navigation requirements,
and changing relative bearings between objects. Costick has
determined in simulation that a two pound force applied for
4
one-tenth of one second by an as t ronaut aga ins t h i s space-
c r a f t w i l l . c a u s e him t o d r i f t beyond a r m r each w i th in
t h i r t y s e c o n d s ( 9 ) . The force o f an o rd inary sneeze is
s u f f i c i e n t t o tumble t h e u n r e s t r a i n e d i n d i v i d u a l a t a rate
of one- f i f th o f a revolut ion per minute provided, of course. ,
he i s not wearing a he lmet wi th facepla te . The e f f e c t s o f
weight lessness on a human being from a dynamical point of
view have been studied (80). Medica l e f fec ts o f the weight -
less env i ronmen t have a l so r ece ived cons ide rab le a t t en t ion .
(See, f o r exa.mple, 3 , 49 , 54. )
Trout and H i l l h a v e o b s e r v e d t h a t t h e l a c k o f t r a c t i o n
encountered in space accounts for much of the degrada t ion
i n EVA performance as experienced by a s t r o n a u t s i n t h e
Gemini Program ( 5 3 , 55 , 7 6 ) . The metabol ic cos t o f do ing
work i n z e r o g r a v i t y i s much g r e a t e r t h a n t h a t of doing the
same work on e a r t h o r e v e n on t h e moon, a s t h e r e s u l t s o f
Apollos 11 and 1 2 i n d i c a t e d . To see t h i s c o n s i d e r t h e
fo l lowing ana lys i s (84 1 . L e t :
E = e f f i c i e n c y
LQm = metabol ic cos t o f work
Qw = amount of energy used in pe r fo rming u se fu l work on
e a r t h
Q w c = ene rgy spen t i n coun te r - ac t ive fo rce
Qwr = energy t o r e s t o r e body t o a prework pos i t ion ( to rque)
Qs = energy s tored as body h e a t
Q, = n e t h e a t loss
5
Then
cQm * E = (2, ( e a r t h )
am = Q, + Qwc + Q,, + 9, + Qn (sDace)
Thus , t he i nhe ren t absence o f t r ac t ion i n zero-g causes
e n e r g y p a r a m e t e r s t o rise a n d e f f i c i e n c y t o be lowered.
-Vacuum
The vacuum o f s p a c e a f f e c t s a n EV4 a s t r o n a u t i n t h e
fol . lowing ways ( 4 2 ) :
1. Forces man i n t o a space s u i t o r o t h e r a i r t i g h t e n c l o s u r e
( 4 7 ) .
2. Poses spec ia l p roblems for maneuver ing f rom one exc losure
t o a n o t h e r ( a i r l o c k d e s i g n ) .
3 . R e q u i r e s r a d i a t i v e h e a t d i s s i p a t i o n .
4 . Necessitates communication by wire, c o n t a c t , o r e l e c t r o -
magne t i c r ad ia t ion .
The s o u r c e s o f i l l u m i n a t i o n i n s p a c e d i f f e r i n e x t r e m e s .
When v iewed f rom outs ide the ear th ' s a tmosphere the sun i s
almost twice as b r i g h t as f rom the ea r th ' s su r f ace (7x10 8
millilamberts vs. 4 . 4 ~ 1 0 ~ mL) . The e a r t h ' s day s i d e i s a
b r i l l i a n t s u r f a c e w i t h a luminance as g r e a t as 9 . 4 ~ 1 0 ~ mL.
When compared t o a f u l l moon viewed from ear th (8x102 mL) o r
w h i t e p a p e r i n good r e a d i n g l i g h t ( 2 ~ 1 0 ~ mL) , t h e i n t e n s i t i e s
of sDace i l l u m i n a t i o n are apparent .
b
The b r i l l i a n c e h a s two o p t i c a l e f f e c t s . The f i r s t is
on Trisual adapta t ion ; the eye mus t be sh ie lded and g iven time
t o a d a p t b e f o r e it w i l l respond t o low i n t e n s i t i e s s u c h a s
stars. I f obse rv ing an ob jec t i l l umina ted on one s ide by
t h e s u n , t h e o t h e r s i d e c a n n o t b e s e e n , e v e n i f i l l u m i n a t e d
by t h e moon o r by t h e e a r t h . Fi l ters do no t he lp t h i s p rob lem.
The s e c o n d o D t i c a l e f f e c t i s t h e p o t e n t i a l i n j u r y o f
r e t i n a l b u r n i f t h e e y e ' s f i e l d of v i ew inc ludes t he un f i l t e r ed
s u n f o r more than a f e w seconds.
-Radiation, Thermal, and Micrometeoroid Protect ion
The ex is tence o f rad ia t ion in space has been rev iewed ( 5 2 ) .
Cosmic r a y s , t h e Van Allen B e l t s , and t h e r a d i a t i o n spewed
from s o l a r f l a r e s a l l p o s e h a z a r d s t o t h e EVA a s t ronau t . The i r
e f f e c t s on t h e hurnan being have been e x t e n s i v e l y t reated i n
o t h e r r e p o r t s ( 2 0 , 5 2 ) . P r o t e c t i v e s h i e l d i n g i s r e q u i r e d f o r
an a s t ronau t du r ing EVA. Such s h i e l d i n g must be so designed
a s t o p r o v i d e a comfortable ranue of temperatures .
7
Recommendations for Universi ty Resear 'ch
- Review major EVA miss ions and t imel ines wi th goa l in
each case of reducing required EVA f o r s u c c e s s f u l O p e r a t i o n .
- Determine a s t anda rd se t o f s p e c i f i c a t i o n s t o b e Tmposed
by NASA upon c o n t r a c t o r s f o r u s e i n d e f i n i n g EVA missions
r e q u i r e d i n s u p p o r t of p r o j e c t s .
- Dete rmine an accep tab le fo rma t t o u se fo r e s t ab l i sh ing
EVA t imel ines for each miss ion based on t a s k s t o be done,
d i s t a n c e s t r a v e r s e d , e tc .
- Design a sys t em, u s ing eng inee r ing app l i ca t ions of s t a t i c s ,
k i n e m a t i c s , and dynamics, of load t ransmission devices that w i l l
improve the e f f ic iency of zero g r a v i t y work t o p e r m i t m e t a b o l i c
r a t e s comparab le t o t hose expe r i enced on e a r t h .
8
11. APPROACHES TO AN EVA SYSTEM . .. . .
An ex t r aveh icu la r sys t em i s a small s e l f - c o n t a i n e d u n i t
o p e r a t i n g i n s p a c e or on a p l a n e t f o r a s h o r t p e r i o d of t i m e ,
u s u a l l y m e a s u r e d i n h o u r s , a t a l i m i t e d d i s t a n c e f r o m a p a r e n t
s p a c e c r a f t . Several t y p e s of EVA systems have been proposed
and they can be r o u g h l y c l a s s i f i e d a c c o r d i n g t o s i z e , complex-
i t y , and endurance.
The f i r s t s u c h c l a s s i f i c a t i o n , least complex and providing
s h o r t e s t a v a i l a b l e w o r k i n g t i m e is t h e s p a c e s u i t w i t h b a c k -
pack l i f e s u p p o r t s y s t e m . T h i s d e s i g n i s c u r r e n t l y b e i n g u s e d
in the Apol lo Program. Opera t iona l t i m e i s determined by,
among o t h e r t h i n g s , s u i t d i s c o m f o r t ( f o u r t o e i g h t h o u r s )
(31, 3 4 ) .
The second conf igu ra t ion i s that of the non-anthropomorphic
s p a c e s u i t , t h a t i s , one wh ich encapsu le s t he en t i r e a s t ronau t
i n a f o r m - f i t t i n s s h e l l . Th i s concept has not been demonstrated,
b u t t h e o r e t i c a l l v i t o f f e r s s e v e r a l a d v a n t a g e s o v e r c o n v e n t i o n a l ,
f o r m - f i t t i n g s u i t s : l i m i t e d comfort time i s g r e a t l y i n c r e a s e d ,
s e l f - f eed ing and waste d i s p o s a l becomes p r a c t i c a l , a n d self-
r e p a i r i s made p o s s i b l e .
A space su i t wi th backpack maneuver ing un i t and l i f e
suppor t sys t em i s a t h i r d s y s t e m c u r r e n t l y r e c e i v i n g much
a t t e n t i o n . @ p e r a t i o n a l time i s determined by l i f e s u p p o r t
and p ropu l s ion r equ i r emen t s ( t h ree t o fou r hour s ) .
A s p a c e s u i t w i t h work p la t form incorpora t ing maneuver ing
and l i f e s u p p o r t c a p a b i l i t y i s y e t a n o t h e r c l a s s i f i c a t i o n .
9
Tools could be car r ied a long , and the p la t form would have a
var ie ty o f p lug- in modules for u se i n p a r t i c u l a r m i s s i o n s .
A capac i ty fo r r emote re t r ieval would be highly desirable .
An a r t i s t ' s concept ion of t h e p l a t f o r m a p p e a r s i n F igure 1.
The as t ronaut would per form in a s p a c e s u i t e x p o s e d t o t h e
e n v i r o n m e n t a n d c o u l d e x i t f r o m t h e f r o n t t o o p e r a t e a t
umbi l i ca l distances. A t o t h e r times, he would be res t r ic ted
i n t h e p l a t f o r m by a b e l t which would permit him t o work
wi th l i t t l e l e g e x e r t i o n . The primary advantaqes of such a
sys tem inc lude : increased propuls ion capac i ty , ( i . e . range)
increased too l s torage a rea , added mechanica l a ids , improved
g u i d a n c e a n d s t a b i l i z a t i o n , a b i l i t y t o a n c h o r a t work s i tes ,
work i l lumina t ion , heavy du ty power su?ply, TV and te lemetry
channels , umbi l ica l power and communications for remote
o p e r a t i o n s , and t h e a b i l i t y f o r r e m o t e r e c a l l .
The l a s t c o n c e p t c o n s i d e r e d h e r e i s th3.t of a s h u t t l e o r
work boa t wi th p ressur ized compar tment enc los ing an as t ronaut
with back-up spacer-uit . The m o s t s o p h i s t i c a t e d o f a l l d e s i g n s
mentioned, i t has the chief advantaqe of extending EVA oper-
a t i n g t i m e t o s eve ra l days and i nc reas ing work ing d i s t ances
from t h e main s p a c e c r a f t t o s e v e r a l miles ( 3 4 ) .
10
Recommendations for Univers ' i ty Research
- Determine a p r a c t i c a l o p e r a t i o n a l time f o r u s e w i t h a
non-anthropomorphic space s u i t and determine the need, i f any,
f o r EVA o p e r a t i o n s o f t h i s d u r a t i o n .
- On the bas i s o f the above s tudy , de te rmine the requi re -
ments t o s u p p o r t a n a s t r o n a u t f o r t h i s p e r i o d , n o t t o e x c e e d
e i g h t h o u r s a s a p r a c t i c a l work l i m i t .
- Evaluate designs which would sat isfy the above require-
ments f o r a non-anthropomorphic sui t .
- Determine the EVA system(s) of maximum e f f i c i e n c y i n
meet ing the to ta l requi rement o f EVA miss ions , inc luding
rescue. Consider : 1) the op t imal usage for each sys tem
t h u s f a r ? r e s e n t e d , 2 ) compar ison wi th to ta l requi rements
and inteqrat ion of system. concepts where possible , 3 ) i d e n t i -
f i c a t i o n and s o l u t i o n t o t h e p a r t i c u l a r p r o b l e m s a s s o c i a t e d
with each system such as a i r l o c k s , r e s c u e , a s t r o n a u t i n t e r f a c e ,
use of manipulators , 4 ) a study of modular concepts for
v e r s a t i l i t y i n t a s k p e r f o r m a n c e t h r o u g h e a s y i n t r o d u c t i o n a n d
removal of di f ferent subsystems, 5 ) c l o s e c o o r d i n a t i o n w i t h
spec i f i c mi s s ions cu r ren t ly p l anned by NASA f o r t h e rest of
t he cen tu ry .
11
111. EXTRAVl3HICULAR SUBSYSTEMS "
An EVA system resembles a small manned spacecraft with a
limited duty cycle. Its major difference from a parent space-
craft in addition to size is the absence of any re-entry
capability. Extreme reliability over a long period is required,
however, for each of several subsystems. These include Life
Support, Power, Communication, Propulsion, and Guidance and
Stabilization. While each of these systems is worthy of
extensive study, the remainder of this report will be
confined to a review of problem areas in Guidance and
Stabilization.
Guidance and Stabilization
The attitude control subsystem of the EVA integrated
system is essential for, without the means of controlling one's
attitude and position with respect to a work site, little can
be accomplished.' Attitude control for target viewing and
for aiming tranlational thrusters can also serve as an .
inertial reference system. The specification of this subsystem
involves first listing of requirements placed on it by the pro-
posed mission. Next, one of two basic systems must be chosen.
The first, referred to as unstabilized or open-loop, relies
for its successful performance on the astronaut who has complete
control over such aspects as timing and vectoring of thrusters,
a
for example. The second system, called stabilized or
closed-loop, describes a condition whereby attitude-hold,
translation, and attitude changes can in some way be programmed
into an automatic control system which then effects the
assignment and retrieves some feedback information for
reference. Within these two categories one must choose the
precise controller which will be used to activate the system.
Finally, a controller technique or strategy must be chosen
such as to optimize the "feel" of the system for the EVA
astronaut. Each of these steps are reviewed below.
Requirements
Any attitude control system (ACS) must have a 360°
command capability in each axis so that the astronaut can
orient himself in any direction. Experiments in the simulation
of various maneuvers have shown that angular velocities should
be available at levels from 5 to 3 0 deg/sec and angular ac-
celerations between 10 and 15 deg/sec2. Possible disorientation
due to vestibular effects determine the upper extreme ( 2 2 ) .
Translational acceleration between - 2 and . 5 ft/sec2 is
desirable.
Presuming some attitude error in manual or open-loop
control, and the existence of an error dead-band limit cycle in
the closed-loop configuration, a limit must be set as to the
size of such errors. The determining factor is found in the
accuracy required for translational maneuvers. Several tech-
14
niques for translation and rendezvous with a target are
available, but all require the astronaut to monitor his
approach and make necessary corrections.
Consider the following guidance scheme which neglects
second order orbital effects ( 1 3 ) .
1) The EVA astronaut establishes the initial conditions
and measures range, range rate, and cross-range velocities,
with the help of sensors and perhaps a small computer.
2% The astronaut, using his knowledge of these conditions,
thrusts toward the target to attain a predetermined range
rate (or relative velocity) v.
31- Due to thrust misalignments and resi.dua1 cross-range
velocity there will be some error angle E between the line-of-
sight and the relative velocity. This angle is measured in
the plane of the target, interceptor, and relative velocity.
This plane, shown in Figures 2 and 3, may change after each
corrective thrust.
4 ) . Immediately after thrust, the astronaut will note the
bearing of the target; when the bearing changes by angle, 1,
(the threshold) , the Law of Sines gives sin (1+E) = sin 1/ (k/R).
Since the astronaut knows R and V and can measure time from
start until the bearing changes, tl, he can find k as follows:
k = vtl. Using this value he can find 1+E from the expression
for its sine function above.
5). The vector diagram of Figure 3 shows that Av = vtan(l+E) . Since for angles up to 200 milliradians tan x differs from sin x
by less than five percent, and since all the angles are in this
range, then Av 2 vlR/k. 6) In order to complete the solution, steps 2 ) through 5)
are iterated, with R replaced by ( R - k), as many times as necessary.
7) When the astronaut.sees that he is approaching the
target, he will apply braking thrust.
The consensus among investigators is that the astronaut
will be able to judge distance accurately at fifty to two
hundred feet, but that during retro-thrust he will be too busy
to make normal corrections to the guidance system for the last
fifty feet. It is also assumed that he can adjust for errors
of up to two feet by arm reach. A two foot error subtends an
angle of forty milliradians at fifty feet. This, then, is the
upper limit for the threshold. A safety factor of two can be
introduced, and the nominal threshold is determined at twenty
milliradians (k 1.2O) (13). This represents the accuracy which
must be available in attitude control, neglecting insignificant
errors in state-of-the-art optical sighting devices.
Other guidance and rendezvous techniques have been
described elsewhere (60). Tests performed in a six-degree of
freedom simulator indicate that flight maneuvering within two
hundred feet of the spacecraft should be accomplished with
ease (87) .
16
Unstabilized .~ (Open-L-stem ~ Designs
- Self-Rotation Techniques
The simplest possible means of attitude control and
translation in space is that of self-locomotion, without the
assistance of any propulsive or stabilizing device. A
knowledge of the inertial characteristics of the human being
is essential in planning for this system, as well as for a l l
subsequent systems. Anthropometric data must be gathered
before dynamic response characteristics can be determined for
a body in weightless free-fall. While any analytical repre-
sentation of biomechanical properties is approximate, several
studies have proved useful as a basis for estimating dynamic
response. DuBois developed a semi-analytic technique based
on computer analysis of data in a double axis compound
pendulum test of subjects (14). Whitsett developed a
distributed mass model based on the U S A F "mean man" which
Drissel extended to a suited astronaut (13, 80). Five
postural variations were defined. The effect of a homogeneous,
weight-distributed one hundred-ninety pound backpack was also
determined. Math models of the human body were developed ( 4 3 , 2 4 ) .
On the basis of such data, research was conducted at
Stanford University to determine the change in orientation of
the human body in weightlessness resulting from specified
motions of the limbs relative to the body. A computer program
assisted the evaluation (37, 62). The mobility restraints
inherent in currently used soft space suits places a severe
I
limitation on such a system and effectively challenges the
overall practicality of the technique. Nonetheless, analytical
treatment of the human body as a non-rigid mass has become
important. Experiments and actual flight data in the Lunar
Module have demonstrated that torques induced by limb motions
are sufficient to cause difficulty in maintaining automatic
stabilization.
Limited maneuvering by judicious handling of an
astronaut tether ha5 received some attention (1). Motion is
restricted to certain transfers and poses several significant
problems in accuracy and even astronaut safety.
Manual locomotion techniques may be practical where the
tasks are confined to a spacecraft surface such as
inspection and maintenance.
- Hand-Held Maneuvering Unit
Among the simplest propulsive devices used in conjunction
with body motions for orbital locomotion is the Hand-Held
Maneuvering Unit (HHMU). Like most unstabilized control
schemes, the HHMU offers a relatively light means for achieving
short distance translations and for carrying small payloads.
First flown aboard Gemini IV and further evaluated on later
Gemini flights, this variable-thrust device has received
extensive review (53, 54). A summary of Gemini X HHMU char-
acteristics appears in Table 4 .
In determining the dynamics of an astronaut using the
HHMU, it was suggested that an astronaut operating at the end
of a tether would be less likely to encounter large rotations
in any axis other than that of the tether itself. The explained
reason is that the rotational energy causing wrap-up must be
converted to translational kinetic energy for wrap-up to continue,
which would eventually null the rotation.
Table 4 ( 5 4 )
GEMINI X HAND HELD MANEUVERING UNIT CHARACTERISTICS
Propellant, gas . . . . . . . . . . . . . . . . . . . Nitrogen Thrust, tractor or pusher, lb . . . . . . . . . . . . 0 to 2
Total impulse, lb-sec . . . . . . . . . . . . . . . . 6 67 Total available velocity increment, ft/sec . . . . . 84 Trigger preload, lb . . . . . . . . . . . . . . . . . 5
Specific impulse, sec . . . . . . . . . . . . . . . . 63
Trigger force at maximum thrust, lb . . . . . . . . . 8 Storage tank pressure, psi . . . . . . . . . . . . . 5000
Weight of usable propellant, lb (in spacecraft) . . . 10.75
Regulated pressure, psi . . . . . . . . . . . . . . . 125 f 5 Nozzle area ratio . . . . . . . . . . . . . . . . . . 51: 1
HHMUweight, l b . . . . . . . . . . . . . . . . . . . 3 Gross weight of extravehicular pilot, lb . . . . . . 260
Finally, the control logic employed by astronauts was
simply to aim the thrusters constantly toward the target,
positioning them with respect to the body as required to null
out any disturbing torques. Another logic preferred by some
pilots employed six decoupled control positions for six degrees-
of-freedom. Pure rotations were not possible but performance
was improved ( 8 7) . Research has continued to fabricate a hand-held hydrazine
thruster which decomposes this fuel into nitrogen and hydrogen
gas (32).
- Foot Controllers
It should be clear that in addition to its relatively
short lifetime, a major disadvantage of the HHMU lies in the
requirement that one hand and arm be used exclusively for
effecting such control. Other more desirable controllers can
be suggested and will be discussed in greater detail later.
The concept of foot controllers and foot-located thrusters
is one such alternative; it will be discussed here because of
the open-loop nature of most such systems which have been
proposed. The preliminary research and development in this
area has been accomplished primarily by three groups:
1) Langley Research Center, 2 ) Grumman Aircraft Engineering
Corporation, and 3 ) General Dynamics/Convair. These three
approaches will be treated below in turn.
The basis of all theory in foot controllers rests in the
supposition that the human balancing reflex can be adapted
for use under zero-gravity conditions. This view was first
propounded by Charles Zimmerman of NASA in the early 1 9 5 0 ' s .
H i s central thesis was that the learned pattern of reflexes
used by a person in standing is essentially the same as that
required to balance a force-vector supported platform and,
hence, should be directly applicable to the control of hover-
ing vehicles (91). His demonstration of this concept caused
much interest and subsequent research among aeronautical
engineers.
The word "reflex" is, perhaps, inaccurate as some
physiologists believe the balancing ability to be the learned
coordination of many simple reflexes. A person in a normal
stance remains balanced by making continual fine adjustments
of foot and leg muscles in response to various sensory inputs.
For example, if a person is forcibly tilted forward or
backward he instinctively varies the amount that he pushes
with his toes to remain balanced. If he stands on a rug
which is pulled gently forward, he is tilted gently backward:
he instinctively responds with an appropriate decreased toe
pressure. Conversely, when a person reduces his toe pressure
he expects to be tilted forward. Suppose now, that the person
can control the direction of this rug: pushing his toes down
accelerates the rug forward: reducing his toe pressure ac-
celerates the rug backward. In essentially this manner can
one control, in theory, a jet thrust device attached to the
feet. See Figure 4 .
On the basis of early successful flights of man standing
on a jet-supported platform, tests were made to determine the
flight qualities of a man standing on a platform supported by
a teetering rotor ( 3 0 ) . Controllability was improved when
the two fans were counter-rotated to negate angular momentum
vectors and when the rotor assembly was spring-restrained.
Additional study has continued at Langley Research Center to
determine the impingement forces, moments, and centers of
pressure on a flat plate produced by small jets operating in
a vacuum (35). Work has also proceeded on the development of
an automatic balancing system for such platforms ( 2 5 ) . Balance
is achieved by measuring the difference in total impulse
21
exerted by opposite torquers used for attitude control in
each axis during some time interval of limit cycle operation.
The system then moves a small weight by an appropriate
distance to compensate for the measured imbalance.
In projects of a similar nature, Bell Aerosystems Company
has modified their "flying felt" hardware to provide various
body-controlled fixed-thruster vehicles for application in
low-gravity environments (2). This work, and that of rocket
lift devices by Aerojet-General Corporation are well document-
ed ( 6 $ ) .
A project which grew directly out of Zimmerman's early
work was that of the "jet shoe" for use in extra-vehicular
motion. The somewhat instinctive movement of feet and legs
used by skin divers to maneuver in a medium that simulates to
some extent the "free fall" condition of space suggested the
concept of placing jets on the shoe soles of an EVA astronaut.
This was done at Langley Research Center (75). A switch was
mounted in the toe of the shoe so that a downward flexing of
the toes closed the switch and turned on the jet. Each jet
shoe was independently controlled, and the nozzle of each was
tilted forward thirty degrees from the perpendicular to make
possible equal pitching moments in both directions. Tests
to determine the feasibility of such a concept were performed
on five facilities including two air-bearing facilities. In
two of the later simulations, six degrees-of-freedom were
achieved. Pilot control, allowing for simulator artifacts,
22
demonstrated a potential for such a device. The chief ad-
vantages are simplicity and freedom of hands.
Several disadvantages are inherent to this system, how-
ever. No pure rotation is ever possible because a force
couple does not exist. Pure translation is difficult to achieve
because it requires a thrust vector passing directly through
the center of gravity. All pure translations must be in a
head-first direction. Retro-thrusts require first a rotation
to reverse one's attitude.
An extensive research program continues in the adaptation
of the original flying platform to lunar rovers (29). Grumman
Aircraft Engineering Corporation followed up the work of
Zimmerman described earlier with preliminary experiments
designed to provide more information about the human balancing
reflex. Keller and O'Hagan reported development of a hydrau-
lically-driven carriage which permits a pilot to make limited
(ten feet) excursions in response to small tilting motions of
a control platform on which he stands ( 3 8 ) . Provision is made
for adjustment of platform height relative to its pivot point
and for simulation of parameters such as spring, damping, and
moment of inertia. A system block diagram appears in Figure 4
Early experiments located optimal positions for system gain
(carriage acceleration per unit platform tilt), platform
height, and a preference for acceleration rather than velocity
as a response to platform tilt. A second series of experiments
dealt with other parameters. The addition of platform inertia,
for example, was found to be highly detrimental, contributing
to overshoot and difficult low frequency oscillation.
The basic goal toward which Grununan researchers had
worked was the adaptation of successful one-g platform control
of five degrees-of-freedom to a zero-g environment. The first
simulated zero-g test of the system, however, revealed that
little control ''feel" was experienced by the subjects and,
therefore, control was difficult. This was attributed to
1) the low gains (angular acceleration per degree of ankle
deflection) which were required in order to prevent bizarre
motions, and 2 ) the very brief periods of thruster "on-time"
required to produce rotation. These weaknesses exist as well
in the "jet shoe" concept discussed above, and strongly suggests
that such a system would be restricted to accomplishing trans-
lational thrusts without precise controllability. The use
of a single thruster to control five degrees-of-freedom in
a weightless environment does not now appear practical.
New tests were prepared and carried out on a three
degree-of-freedom air-bearing platform (one rotational, two
translational degrees in the horizontal plane) (40). A
subject was positioned on his side in the "zero-g scooter"
and tests in uncoupled pitch rotation were conducted using ankle
defection as a con.troller. Translaton artifacts due to un-
balanced force vectors were cancelled by the addition of thrust-
ers to form a pure couple. Results were optimistic and sub-
sequent tests were performed on fore-aft
24
translations (bending the torso forward to cause forward
motion) and up-down translations (legs stretched for upward
motion, semi-squat for downward). A l l three degrees-of-
freedom were next tested simultaneously, and the results were
highly encouraging. Yaw control was tested on a separate
stationary platform capable of rotation only. Body twist was
found to be a "natural" control in yaw. Roll motion con-
trolled by differential foot lifting was also tested.
The question of whether proportional acceleration control
or on-off control is best in such tests in unanswered. On-off
control is a simple mechanism. The control strategy assumed
by subjects using on-off control has a tendency toward se-
quential attendance to the various degrees-of-freedom as opposed
to the more smooth, simultaneous operation characterized by
proportional control. Also, simultaneous operation i.n three
degrees-of-freedom in a plane does not necessarily mean that
spacial operation will be feasible. Finally, the question of
space suit interference has yet to be satisfactorily addressed.
Test results imply that the body deflections required for
motion are quite small, but full-dress simulations are clearly
called for. Despite these questions, the concept of separate
uncoupled control of several degrees-of-freedom looks promising.
The desirability of locating thrusters adjacent to the
corresponding "control elements" of the human body is evident
from the results of such tests: in this position they function
also as a means of providing proprioceptive feedback ( 4 0 ) .
A somewhat related approach was taken by investigators
at General Dynamics/Convair. Wrench and Greensite proposed
a system using twin two degree-of-freedom control moment
gyros ( 8 5 ) . Shoe-mounted units, controlled by muscle action
about the ankles, was envisioned. The principle of control
moment gyros and their possible use in an EVA control system
has received detailed scrutiny (7, 19, 55, 5 7 ) . The simplest
such system is a gimballed wheel rotating at constant speed.
Newton's second law states that the total external moment, T,
acting on a system is equal to'the time rate of change of
angular momentum with respect to inertial space:
T = Ia = dL/dt; L = Iw
where L = L - - system Lastronaut + L~~~
See Figure 6 .
To predict physical behavior of a simple single degree-
of-freedom wheel consider the case where external torques are
zero, T = 0. Then L = constant. Hence, rotation of the gyro
through some small angle 6 as shown in the figure causes a
component of gyro momentum to appear in the +x direction. To
first order approximation the 2 component remains unchanged.
If overall momentum is to be conserved, then the astronaut must
assume some momentum in the -x direction. If this corresponds
to a principal body axis, then he will rotate purely in that
axis. The gimballed angle controls the change in momentum
which, in turn, is proportional to the time integral of the
created torque. S o the torque is proportional to gimbal angle
rate. In the absence of external torques, the astronaut
could create a small rotation rate by moving the gimbal through
some small angle, and then could stop the motion by restoring
the gimbal to its former position. A block diagram of a CMG
attitude control system appears in Figure 7 .
The proposed shoe employed two identical wheels, as
shown in Figure 8 , to eliminate cross-coupling with large
angles. The two degree-of-freedom mounting in each shoe was
figured so that one shoe would control pitch and yaw rotations,
the second pitch and roll. Four wheels would, thus, be
available in pitch where maneuverability is desirable and the
astronaut's inertia is high. Three operational modes were
envisioned. If the torque shaft were locked to the platform,
the platform could be rotated freely in space without exhibit-
ing gyroscopic torques. A spring restrained shaft would pro-
vide angular rate sensing which would be used to provide
precessional feedback forces applied tactilely to the foot.
The torque shaft would also be powered by the astronaut's foot
motions: toes down or up, right or left, and sidewise rota-
tion. See Figure 9.
Two shortcomings characterize this system despite ex-
tensive theoretical development. Maneuvers are limited to
sequential rotations if cross-coupling is to be avoided at
large gimbal angles. Momentum saturation is always a limiting
factor in such systems. While the CMG can handle cyclic
torques for extended periods, an EVA astronaut is likely to
encounter constant external torques. These, in turn, produce
a constant gimbal angle rate and cause the CMG to reach maxi-
mum capacity. Some active source is required to desaturate the
CMG, implying an additional weight penalty. Reaction control
jets might possibly perform this function by 1) torquing the
system at times dependent on gimbal angle or rate, or 2) using
fixed-time pulses initiated by gimbal angle. This, of course,
requires the introduction of thrusters into the attitude con-
trol system and makes questionable the advantage of CMG's.
Slow steady-state rotation rates and the present uncertainty
of the degree to which an astronaut could "naturally" or
otherwise control such a system make its use in the near
future appear unlikely.
The previously discussed studies of a passive system for
extravehicular attitude control have demonstrated high
potentiad for future use, but also some limitations. Among
the latter is the fundamental stability problem encountered
when a system is designed in the "pusher mode", with thruster
behind the astronaut's center of gravity. In addition, passive
systems rely on the capacity of the human vestibular system
to accurately sense angular rate, since one can expect that
frequently visual inputs will be minimal. The semicircular
canals have dynamics similar to that of an overdamped, second
order torsion pendulum (90). A subject's subjective angular
velocity as a function of time is shown in Figure 10. Notice
that were an astronaut to encounter a constant velocity spin
and subsequently succeed in stopping it, he would feel that he
had begun rotating in the opposite direction. Corrective
28
action based on this false information could cause him to re-
enter. the original spin.
Stabilized (Closed-Lo.op). System Designs
- The Astronaut Maneuvering Unit
The limitations of a passive EVA maneuvering system
emphasize the potential of active systems in providing a means
for long range (less than one mile) translations. The term
Astronaut Maneuvering Unit (AMU) has generally been associated
with the attitude-stabilized backpack designed for this
purpose. The AMU provides life support, propulsion, communi-
cations, and automatic attitude stabilization; and it permits
the astronaut to operate as an independent small maneuverable
spacecraft system. An orthogonal arrangement of fixed
thrusters located symmetrically abaut the astronaut/AMU center
of gravity assures precise control. Combined with automatic
stabilization, it provides great versatility.
The Modular Maneuvering Unit (MMU) and the AMU built by
LTV Astronautics Division for the Gemini Program have been
described extensively (43, 5 0 , 53, 54 , 5 6 ) . The operational
requirements and unit characteristics are listed in Tables 5
and 6.
Table 5 : AMU Operational Requirements ( 5 6 )
1) Extravehicular mission duration - 4 hours
2) Multiple mission capability - 6 missions 3 ) Maneuvering range - 1000 feet 4 ) Payload weight - astronaut + 168 lbs.
5 ) Total velocity change - 285 fps
6) MMU redundancy philosophy
7) Stabilization and control
auto stabilization - 3 axes rate command orientation - 3 axes automatic/manual mode
The propulsion system chosen was one of hydrogen peroxide
with a total impulse of 3500 pounds of thrust each. The flight
control system provided manual and automatic three-axis atti-
tude control and stabilization and manual translation in two
axes. Two redundant systems were available. Control commands
were made manually through the control handles located on the
controller arms. The left-hand controller provided translation
commands; the right-hand controller provided attitude control.
The left-hand controller included a mode selection switch
which was used to choose between automatic and manual control.
In the manual control mode, translational inputs resulted in
accelerations of .35 ft/sec2 for the duration of the input.
Pure translation was difficult because of the offset of
the center of gravity from the center of thrust. In the auto-
matic (stabilized) mode, pure translations could be obtained
30
Table 6: Gemini IX-A AMU Characteristics (53)
Propellant ................ 90 percent hydrogen peroxide Total thrust (fore-and-aft or up.and.down), lb ........................ 4.6
Pitch moment, in.-lb ..................... 63.5 Roll moment. in.-lb ...................... 44.2 Yaw moment. in.-lb ....................... 47.7 Specific impulse, sec .................... 169 Total impulse. lb x sec .................. 3100 Total available h V , ft/sec ............... 250 Controller characteristics: Breakout:
Fore-and-aft. lb ................... 4.5 Up-and-down. lb .................... 4.5 Pitch. lb .......................... 4.0 Roll. lb ........................... 4.0 Yaw ................................ Small
Fore-and-aft. lb ................... 9.75 Up-and-down. lb .................... 9.75 Pitch. lb .......................... 10.5 Roll. lb ........................... 10.5 . Yaw. in.-lb ........................ 13.0
Fore-and-aft ....................... 6 Up-and-down ........................ 6 Pitch .............................. 6 Roll ............................... 6 Yaw ................................ 4.5
Pitch .................................. 59 Roll ................................... 50 Yaw .................................... 3.2
Maximum force :
Maximum deflection. deg:
Attitude-limit cycle periods. sec:
Attitude deadband, deg ................... (3 axeslk2.4 Maximum control rates. deg/sec:
Pitch .................................. 18 Roll ................................... 27 Yaw .................................... 18
Maximum nitrogen tank pressure, psi ...... 3500 Regulated hydrogen peroxide pressure, psi .......................... 455
Nozzle-area ratio ........................ 40:1 Weight of propellant. lb ................. 24 Weight of Astronaut Maneuvering
Unit. lb ............................... 168 Weight of extravehicular pilot. lb ....... 407
from translation inputs, but the acceleration level was ap-
proximately halved due to attitude control requirements on
thrusters. The translation controller would produce constant
linear acceleration (interrupted to make automatic attitude
corrections) for as long as it was activated. A priority was
incorporated on the jet-select logic for the forward and aft-
firing thrusters which gave yaw first priority, pitch second,
and translation third. The AMU did not include lateral
thrusters. Pulse width modulation was utilized, with thruster
on-time directly proportional to the inpht signal. Above a
certain error level (called saturation) thruster firing was
continuous; below this level thruster firing became inter-
mittent with thruster ON pulses becoming shorter and further
apart as the error decreased until below a certain level
(threshold) the thrusters were off continuously. The resultant
motion was an average angular acceleration proportional to the
error in the region between the saturation and threshold levels.
When a rotation was commanded, the error was the difference
between the preset fixed rate for the commanded rotation and
the actual rate.
Rotational inputs in the manual mode resulted in angular
accelerations of 11, 13, and 25 degrees/sec2 in roll, pitch,
and yaw respectively for the duration of the input. Pure
rotation was not possible in the manual mode but could be
attained in the automatic mode. Accelerations would occur on
command at the levels specified until an angular rate of 18
degrees/sec in pitch and yaw, or 2 6 degrees/sec in roll was
achieved. Angular acceleration would then stop, and a continued
input to the attitude controller would result in this rate
being maintained. Releasing the controller head would allow
it to return to a neutral position and deceleration would begin.
When rotation ceased, the system would go into an attitude-hold
mode, maintaining attitude within 22.4 degrees/sec in each axis.
When the sum of the angular displacement and the angular
velocity about a given axis exceeded 22 degrees (deg/sec) an
attitude correction would be made about that axis. Thus,
automatic attitude corrections were made dependent not only
on the angular displacement but also on the rate of angular
displacement. A system of rate and integrating gyros sensed
these parameters about all three axes. In the absence of
external torques, the period of limit cycle operation within
this deadband was greater than twenty seconds ( 4 3 , 5 4 ) .
- Alternate Controllers
While the AMU described above fulfilled the requirements
of the scheduled Gemini IX-A EVA, it relied upon the use of
both hands for successful control. In this respect, it was
less practical for use in performing space work than some other
systems already described. In addition to the desire to
minimize use of the hands in controller operation, it is also
necessary to preserve body and limb dexterity, to the degree
that is possible given an inflated soft space suit. Controller
"arm rests" on either side of the AMU pilot limited arm
motion; they could also interfere in a cramped work site
condition. The need exists for the development of alternate
33
means of control. Such means, each with its own control
logic, should be traded off against the hand controller concept
which is identical in technique to that of a spacecraft and
therefore requires less training. In addition to the re-
quirements already mentioned, a system cannot either obstruct
visibility or interfere with normal voice communication.
W. E. Drissel, et. al. conducted a survey of controller
concepts for use in an AMU (13). Hand, oral, eye, and body
controllers were studied and compared for such traits as
command capability, accessibility, accuracy, "naturalness",
and reliability. Hand controllers have already been discussed.
Oral controllers offer a variety of alternatives because of the
number of elements available for use: the lips, tongue, the
teeth, breath, speech, whistling, etc. Only breath, tone, and
voice controllers appear acceptable to pilots and these are
now considered.
Breath controllers possess attractive features: complete
hand freedom, simple mechanization, and the on-off, incremental,
or, (in the case of a set of actual mouthpieces), proportional
commands. The disadvantages include an increased burden on
the environmental control system in terms of both oxygen supply
and water removing capability, the space limitations inside a
helmet, and the artificiality of the control code. This last
factor may not be important, except that under conditions of
great stress regression occurs, during which highly artificial
relationships are temporarily forgotten. Similar disadvantages
attend the use of a singing or humming tone for control
purposes. 34
The operational simplicity of a voice (speech) controller
is at once attractive; ten natural language words could con-
ceivably suffice. This approach was chosen for a detailed
feasibility study (13). Thirty-six commands were determined
as fulfilling requirements; several of these utilized repeated
words of the vocabulary to restrict the number of actual words
to ten. This vocabulary is listed in Table 7 . Two rules
were established:
- In attitude, continuous utterances command continuous
rotation. Silence commands constant attitude.
- In translation, continuous utterances command continuous
acceleration; silence commands constant speed (a special case
of which is zero speed relative to the astronaut's parent
vehicle). An output logic was developed and the system was
simulated using a computer to control an oscilloscope pattern.
Voice commands were relayed to a human controller who would
enter the commands into the computer in normal fashion. Re-
sults were encouraging. Subsequent development has led to the
definition of a completely mechanized system which is currently
under assembly ( 2 7 ) . The primary disadvantage of this system
lies in the need for a computer program to perform tasks in
voice-recognition and logic.
The use of the eye in attitude control was investigated
from three approaches: sensing the deflection of a beam of
light reflected off some portion of the eye; sensing the
vector position of the front-to-back potential of the eye;
and sensing the action potentials of the eye muscles.
35
Specific problems of the first approach are the excessive
equipment required and the precise initial alignment which must
be maintained. The second approach utilizes the fact that the
eye behaves like a small battery, being electrically positive
at the front (cornea) and negative at the back (fundus).
Table 13: Suggested Vocabulary for Speech Controller (13)
Word Primary Function
Roll
Pitch
Yaw
X
Y
Z
Plus
Minus
stop
Cage
Denotes roll rotation
Denotes pitch rotation
Denotes yaw rotation
Denotes translation along X axis
Denotes translation along Y axis
Denotes translation along 2 axis
Denotes positive direction of motion
Denotes negative direction of motion
Removes all commands from ACS
Places ACS in synchronous mode
Pairs of electrodes placed around the eye will sense its rota-
tion in terms of altered potentials. The need for frequent
calibration and the variations made possible by galvanic skin
response and other extraneous signals make this approach
questionable in value. Similar arguments are used against
sensing eye muscle action potentials. The primary advantage
36
of using the eye is that it acts as an optical self-correcting
error'-detecting device. It is not, however, a controller in
the strict sense but merely an aiming device that could
provide steering information to the AMU. The ACS still needs
command inputs to remove or insert the eye in the control
loop, to provide an "execute" signal at the proper time, and to
command a fast or slow speed of execution. More research in
this area could produce encouraging results, however.
The use of the body as a controller has already been
considered somewhat with an earlier discussion of leg and
foot control. In addition to the difficulties cited earlier,
such a mechanism could entail a loss of worker mobility at the
work site. Torso controllers appear to be impractical due to
the strong mechanical intercoupling between the AMU and the
torso. This leaves on ly the head as a possibility.
The head may be used for control in two ways: with and
without a visual sighting mechanism. When used in conjunction
with sight, this concept is similar to eye controllers. Pickoffs
sensing the relative position of the helmet to the AMU would
provide signals to the ACS translating the astronaut to the
observed target. As with the eye controller, auxiliary controls
would be required.
The second method of using head position is to instrument
the helmet so that signals are generated by nods, turns, and
tilts of the head. These could be made to command pitch, yaw,
and roll, respectively. Speed level, execution, and lockout
controls would be required. Possible interference with visual
37
functions is a disadvantage to be considered. The visual
capability of a suited astronaut is shown in Table 8 and the
neck mobility in three axes in Table 9.
Table 8 : Range of Vision (Suited) ( 7 8 )
Horizontal plane 120°1eft, 120Oright
Vertical plane 105Odown, 9Ooup
Table 9 : Maximum Neck Mobility Requirements ( 7 8 )
Flexion (forward-backward) 1200 zero torque
Flexion (lef t-right) 3 Oo zero torque
Rotation (Abduction-Adduction) 140° zero torque
Still another possible use of the body lies in the field
of myoelectrics. Most processes in living organisms are
accomplished by electrical changes. Actions of voluntary
muscles in operating the limbs exhibit such changes. The measure-
ment of these biocurrents is called electromyography, and is
being used now primarily in the operation of prosthetic devices
for amputees. Raw electromyographic signals are character-
istically spiked, having amplitudes in the low milli-volt range.
A practical, reliable technique must be found for sensing,
amplifying, filtering, conditioning, and decoding these voluntary
signals for command and control purposes. Such signals would
undoubtedly be useful for command and control where the human
operator finds normal manual control impractical due to restraint
or other use of the limbs.
- Shuttle
The stabilized shuttle or work boat offers great
advantages in terms of protection, range of operation, and, with
mechanical manipulators, greater ability to perform work. The
significant disadvantages include heavy weight (900-1600 lbs.),
high cost, and interface complexity which, in turn, could de-
grade reliability (82). Maneuvering and attitude control
requirements could be expected to be similar to that of a
larger spacecraft.
Control Techniuues
Once a controller has been chosen, it is necessary to
determine the technique by which attitude will be maintained.
If the system is open-loop, a signal will be generated direct-
ly by the astronaut: if a closed-loop or stable system, a
signal will automatically be generated when certain chosen
parameters exceed pre-set limits. Haeusserman has reviewed in
detail the types of sensors available for this purpose in
automatic systems (23). The treatment of these signals must be
determined to optimize a chosen set of parameters. The follow-
ing paragraphs will make note of the systems which have been
suggested both for automatic and manual control.
- Control Alternatives
Several traditional control techniques are available for
use (13, 34). Simple acceleration command applies the signal
directly to the reaction thrusters which apply a torque pro-
portional to signal strength. The resulting rotational ac-
39
celeration is likewise proportional from the formula
T = Ia.
Angular rate command or rate feedback is the traditional tech-
nique used in airplane autopilots but is subject to a high
power requirement. Another technique is known as the orbit
mode which uses fixed pulse widths of rocket thrust correspond-
ing to a set of predetermined angle errors. This method pro-
vides for low residual rates and low propellant consumption at
the cost of complex circuitry and large errors in the presence
of disturbing torques. Derived rate control employs a lead
network to feed a rate plus attitude signal to jets. Disad-
vantages include the noise sensitivity which is produced by
the.lead network and the possibiltiy of saturation. Pseudo-
rate feedback lags a signal proportional to angular acceleration
in a pseudo-integration process and sums this signal with that
of attitude. The result is a pseudo-rate control signal which
is less noise sensitive than rate feedback and capable of re-
ducing limit cycle rates to a low figure with small impulses.
A block diagram of this system appears in Figure 11.
A study conducted by Honeywell, Inc. for an AMU resulted
in selection of a combination of orbit mode control and pseudo-
rate control, the latter being initiated when the error con-
tinued beyond a certain range deemed wholly within the capability
of orbit mode (13) . - Comparative Studies
The analytic techniques through which a comparison of
approaches can be made is based in nonlinear feedback control
theory. Nonlinear effects of hysteresis, dead zone, and thrust
time delays on limit cycle characteristics were studied using
the phase plane by Brown ( 5 ) . The additional effects of pulse
frequency and pulse width modulation for a fixed thrust level
have been investigated (11). The effects of external torques
must also be considered in developing a strategy for minimizing
fuel consumption assuming position and rate sensing. Design
charts for determining system parameters of an automatic dis-
continuous attitude-control system with linear rate and
position feedback have been constructed ( 7 4 ) . Figure 12 is a
block diagram of such a system.
Other studies have considered the practical design aspects
given the requirements of high torque capability and very
efficient operation when no disturbing torques are present (21).
Finally, Ergin et al. have prepared a four volume report
on the problem of nonlinear attitude control (17). Pulse
modulation is here viewed as being essentially a compromise
between proportional control, which is dominated by the limit
cycling of the system about a reference command, and simple
"bang-bang" control, which, if provided with sufficient thrust
to satisfy torquing requirements, would then suffer high
propellant usage rate when in a "steady state" mode of limit
cycle operation (17).
Ergin also suggests the use of timing devices in lieu of
rate sensing equipment to improve performance while increasing
system simplicity. One such method measures the "on-time"
during each firing of the jets and uses this information to
41
reduce the "on-time" of the next firing by some factor K.
Ultimately, under quiescent conditions, the minimum jet "on-
time", will be reached and will determine the steady-state
limit cycle characteristics and the fuel consumption rate. tmin
Another example of torque compensation by means of a
timing device is that of a "'staircase Torque generator". Here
"rate" information is obtained by measuring the time between
zero-crossing and torque-swikching. When the attitude error
reaches the dead-space value of the switch, an impulse is
imparted to reverse the direction of travel. The magnitude of
this impulse will be inversely proportional to the elapsed
time between zero-crossing and the dead-space value. Such systems
are capable of very low limit cycle rates. Further comparative
simulations seem warranted.
- Manual Control
Selection of an optimum control technique for an automatic
attitude control system depends on which parameters are to be
maximized or minimized and to what values. For a manual, open-
loop control system, selection must rely heavily on how each
candidate system "feels" to the pilot and how his performance
compares. The response of a manual system is very dependent,
as well, on the minimum impulse that a pilot can activate.
Recent studies of remote manipulators have been made to de-
termine an optimal system ( 3 3 ) . Crawford and Kama conducted
four groups of experiments designed to produce an ordered
list of preferred manipulator control techniques based upon
performance of a human operator at a given task (10). The
42
results demonstrated a preference for proportional rate systems
over a fixed rate on-off controller. A comparison between
position control and rate control was difficult to assess, and
more research was deemed necessary which would consider such
factors as operator fatigue.
Control of a remote manipulator is different from control
of one's own attitude and work must be done in this area to
determine the optimal manual system. Simulations conducted
by Emerson demonstrated that angular acceleration command would
maintain adequate attitude control if center of gravity
thruster misalignments were small and accelerations were not
large (16). A rate command system is sufficient according to
these studies unless, at great ranges from the target, drift
rates in the ACS are misinterpreted as real translational
velocities of the target and "corrected" accordingly. The in-
corporation of an attitude-hold feature such as afforded by
inertia-exchange devices or a system of reaction jets was found
by simulation to be an effective means of overcoming the need
for precise rate control. Further experiments involving human
operators are called for, featuring a visual simulation of
variable range.
Comparative studies in manual control of several EVA
maneuvering units are being planned for the Orbiting Workshop
Program (OWS) of the Apollo Applications Program. The object
of Skylab Experiment 17-509 Astronaut Maneuvering Equipment is
to investigate the utility of several maneuvering unit control
concepts in the relative safety of a pressurized workshop area.
43
Among the control units currently planned to receive this
evaluation are.the M U , the HHMU, and a modified foot-controller
unit (80). Whitsett and other project planners will give
particular attention to system performance and man's response
to the overall handling qualities in the weightless regime.
One secondary advantage will be the feedback information
provided on the fidelity of current ground-based simulation
techniques (48) .
Recommendations for University Research -
- Guidance and Stabilization
- Develop effective models for station-keeping dynamics,
including consideration for gravity gradient perturbations.
This is an area that has received no attention comparable to
that given the dynamics of terminal rendezvous.
- Continue analysis of the human body as a non-rigid
mass for the purpose of establishing such control system para-
meters as displacement and rate deadband adjustments.
- Perform experimental study with goal of identifying a
postural control model for use in foot and waist-controlled
attitude control systems ( 3 9 ) .
- Consider the use of control moment gyros (CMG's) for
use in both stabilization or attitude sensing and actual
attitude control or motion. Explore mechanical or other means
of torquing with the goal of minimizing the additional weight
required.
44
- Design and perform experiments to determine precisely
pilot subjective "effort" as a function of degree of ankle
deflection, waist-banding, differential foot-lifting, torso
twist, etc. as well as rotation rates in these modes. Clearly,
subjective effort increases from zero to infinity at some
finite deflection and rotation rate.
- Conduct computer-simulated tests followed by zero-g
tests with subjects to determine feasibility of eliminating
yaw rotation as a required capability for an attitude control
sys tem.
- Study proportional versus on-off control with respect
to "naturalness" to the human reflex system and smoothness
of operation.
- Work to improve existing hand controllers. One approach
might include a single controller which can command both rota-
tion and translation.
- Study the possible effects of variable spacecraft space
suit atmospheres, background noise, and change of personnel
in operation of a voice recognition system for use in attitude
control.
-Design and evaluate a tone controller on a simple task
capable of control by incremental inputs. Measure oxygen
uptake. Determine the effect of sound initiation and the
feasibility of using portamento. Determine the nature of
any side reference that may be required, and evaluate its
effect on the pilot and the overall system. Evaluate this
45
technique in various simulated "emergency" modes.
- Survey several approaches to eye position measurement
for possible controller adaptation, given the restrictions which
such applications impose on any measuring technique.
- Design a low-volume system for control using astronaut
helmet position pick-offs in conjunction with a visual sight-
ing device for "aiming". Conduct tests.
- Perform experiments to determine subjective "effort"
as a function of head nods, turn, and tilts, their displacement
and rates of turn.
- Design and conduct experiments to determine feasibility
of electromyographic control using head movements. Determine
optimal control technique and evaluate technique in terms
of astronaut comfort, reliability, and helmet interface.
- Perform computer simulations to compare timing devices
with traditional rate sensing based on optimization of several
parameters.
- Conduct control tasks with subjects, comparing position
and rate control, considering factors such as nature of task,
shape of hand controller, and subject fatigue.
- Test acceleration command under simulated conditions
of short and long range (greater than two hundred feet)
rendezvous to determine the degree of ambiguity at greater
distances between rotation rate and target translation. Test
incorporated rate control with attitude-hold feature to optimize
time lines and fuel consumption.
46
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54
I "
F i g u r e 1: Work Platform ( 3 4 )
55
Figure 2 : Rendezvous. Geometry (13)
V
Figure 3 : Vector Diagram (13)
(a) Translation only no rotationa I con t yo I
Negative control moment
(b) Rotationa I contro i
F i g u r e 5: P la t form for Balanc ing Reflex Expts . (38)
58
I
X
Figure 6: Single Rotor Single Degree of Freedom CMG
I .
c
Body At t i tud -e
I ' I f - I Momer '
Exch,
I I I I
"
I n t e g r a t o r N u l l e d When Command Siqnal Given
F igure 7 : Typica l Cont ro l Moment Gyro Type A t t i t u d e C o n t r o l a n d S t a b i l i z a t i o n Sys t em Ut i l i z ing Rate Gyro Feedback ( 6 4 )
TORQUE SHAFT
Figure 8 : Momentum Control System. ( 85
r
1 . :
a c t u a l angul .ar v e l o c i t y of a s t r o n a u t .
5 1 0 15 20
Time ( sec )
angular v e l o c i t y as p e r - ccfved b y a s t r o n a u t
-.-.””- -. I _” ”.-
appa ren t r e v e r s a l of d i r e c t i o n
E.:.., ATTITUDE
%%
JET CONTRO
F igu re 11: Pseudo Rate Control Block Diagram (13)
64
I Rate feedback
L A t t i t u d e f eedbocl: -
Figure 1 2 - Block diagram of an automatic discont inuous a t t i tude-control system with l inear ra te and posi t ion feedback. (74)
