Control of large space systems, modeling, and control difficulties are discussed. Ground based analysis of spaceflight data are presented to determine structural dynamics characteristics for the purpose of revising control laws, and to trim the surface contour. Systems identification for adaptive control and automatic surface control are also considered

Taylor, L. W., Jr.

English

NASA Technical Reports Server

CONFIGURATIONAL AND SYSTEM REQUIREMENTS
FOR CONTROL OF LARGE SPACE SYSTEMS
L. W. Taylor, Jr.
NASA Langley Research Center
Large Space Systems Technology-1981
Third Annual Technical Review
November 16-19, 1981
375
https://ntrs.nasa.gov/search.jsp?R=19820010428 2020-03-21T16:22:02+00:00Z
INTRODUCTION
An overview on the subject of control of large space systems is offered
in which anticipated modeling and control difficulties are discussed. Partic-
ular emphasis is given to issues that have received little attention in the
current literature on the control of large space structures. The status of
the control work that has been done to date on large space systems could be
pictured as being at "the back-of-the-envelope" stage in contrast to the de-
tailed analytical effort that will be required to ensure future operational
systems controlability.
\
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Figure 1
376
The discussion will be divided into two parts from the point of view of a
specialist in systems identification. The first part of the discussion will
deal with ground-based analysis of spaceflight data (I) to determine struc-
tural dynamics characteristics for the purpose of revising control laws, and
(2) to trim the surface contour. The second part of the discussion is con-
cerned with (i) systems identification for adaptive control and (2) auto-matic
surface control.
SYSTEMS IDENTIFICATION FOR
LARGE SPACE SYSTEMS
GROUND-BASED ANALYSIS OF SPACEFLIGHT DATA TO:
O
O
O
DETERMINE STRUCTURAL DYNAM ICS
REVISE CONTROL LAW
TRIM SURFACE CONTOUR
ON-LINE AND ON BOARD FOR:
O
O
ADAPTIVE CONTROL
AUTOMATIC SURFACE CONTROL
Figure 2
377
Onemight ask the questions "Whymust structural dynamicsbe determined?"
and "Whymight pointing control involve structural dynamics?" The answers to
these questions lie in the system requirements, structural dynamics character-
istics, and the disturbance level. An example is offered in which an
unacceptable 12-minutes settling time is required before a large flexible
antenna meets its pointing requirement of .03° following a disturbance due to
solar heating. For the example discussed, the effect of structural dynamics
is crucial in the design of the pointing control system.
WHY MUST STRUCTURAL DYNAMICS BE DETERMINED?
WHY MIGHT "POINTING CONTROL" INVOLVE STRUCTURAL DYNAMICS?
A POINTING ACCURACY OF .030 REQUIRES THAT THE SUPPORTING MAST
DEFLECTLESS THAN 1/10 OF AN INCH.
20m _-EEDj S"/_TR_'_
T _V_ST A STRUCTURALFREQUENCYOF.1HERTZANDDAMPING
-b_ _ _Y': .I ,,_. RATIO OF .005 RESULTS IN A TIME TO HALF AMPLITUDE
'_'" _ OF 221 SECONDS. LOG. 5
TI/2, = _ - 221 SECONDS
n
SHOULD A SUDDEN DEFLECTION OF THE MAST OF 1 INCH OCCUR DUE TO SOLAR
HEATING, THE POINTING ERROR WILL BE EXCESSIVE FOR 12 MINUTES.
Figure 3
378
In order to appreciate the importance of disturbances and noise, it is
useful to observe that, for linear systems, its quasi-static error (no command
input) is proportional to the amount of disturbance and noise. It behooves us
then, to give these matters more attention. Sources of disturbances include
solar (thermal and wind), on-board systems, and gravitational. Noise can come
from sensors, actuators, and computers.
'LINEAR' SYSTEM PERFORMANCE IS PROPORTIONAL TO THE
AMOUNT OF DISTURBANCE AND NOISE
_ SOLAR, THERMAL
_SOLAR, WIND
DISTURBANCES \_ON-BOARD SYSTEMS
"GRAV ITATI ONAL
SENSOR
ACTUATOR
NOISE _'_'-COMPUTATI ONAL
_QUANTI ZATI ON
Figure 4
379
Because of the relative ease with which linear systems can be analyzed,
nonlinearities are often neglected. Unfortunately, the nonlinearities can
greatly affect system response, particularly at very large and very small
amplitudes of motion. Nonlinear behavior generally comes from sensors, actu-
ators and inertial coupling. It can be expected that system nonlinearities
will result in limit cycles in pointing, in orbit maintenance, and probably at
some structural modal frequencies.
SENSOR AND ACTUATOR NONLINEARITIES WILL RESULT IN
LIMIT CYCLES IN POINTING, ORBIT MAINTENANCE, AND PROBABLY
AT SOME STRUCTURALMODAL FREQUENCIES
.ACTUATOR AND SENSOR SATURATION
_ACTUATOR HYSTERESI S
NONLINEARITIES "x_SENSOR DEADBAND
_INTENTIONAL CONTROL CHARACTERI STICS
Figure 5
380
To close the first part of the discussion, we consider the computational
aspects of ground-based analysis of spaceflight data. Transient analysis
techniques will be required, either in the frequency or time domains.
Attempts to determine the steady-state response to a sine-wave input, for
example, are easily frustrated by lightly damped modes with little frequency
separation. The instrumentation requirements are outlined and seen to be
ordinary. Although the ground-based computer requirements are expected to be
quite modest, the analysis techniques should first be tested using ground test
data.
THE COMPUTATIONAL TASKS OF GROUND-BASED
ANALYSIS OF SPACEFLIGHT DATA ARE ONLY
MODERATELY DIFFICULT
STRUCTURAL DYNAMICS MODELING
O
O
O
O
O
O
O
O
TRANSIENT ANALYSIS TO BE USED
NO SPECIAL INSTRUMENTATION IS REQUIRED
SPECIAL CONTROL INPUTS REQUI RED
PERHAPS I0 MODES CAN BE MODELED
NEED SAMPLE RATE OF 5 TO 10/SECOND
NEED DATA LENGTH OF ABOUT 30 SECONDS
GROUND-BASED COMPUTER REQUIREMENTS ARE INSIGNIFICANT
SHOULD TEST ANALYSIS TECHNIQUE WITH GROUND TEST DATA
Figure 6
381
Turning to the problem of adjusting the surface contour of a large space
system, we can expect that a static analysis will be sufficient, though exten-
sive special instrumentation will be required. Repeated measurements serve to
improve the accuracy in determining the surface contour and can detect sur-
face motion. Analysis procedures should be tested using ground test data.
.SURFACECONTOUR ADJUSTMENT
O
O
O
O
O
O
O
STATIC ANALYSIS TO BE USED
EXTENSIVE INSTRUMENTATION REQUIRED
CONTROL INPUTS, DISTURBANCES NEED TO BE MINIMIZED
PERHAPS 240 SURFACE MEASUREMENT LOCATIONS
PERHAPS 4 SETS WILL BE NEEDED
MUST INVERT 240 x 240 MATRIX
SHOULD TEST ANALYSIS TECHNIQUE WITH GROUND TEST DATA
Figure 7
382
Examination of the distribution of modal frequencies reveals the diffi-
culty in controlling large space systems at frequencies exceeding the first
structural mode. A control bandwidth reaching a frequency ratio of 3.0 might
become a typical limit. Difficulties arise in modeling, also the modes
become packed. The particular dynamic characteristics must be examined, how-
ever, to make a meaningful assessment.
A SYSTEM HAVING A LARGE NUMBER OF CLOSELY SPACED MODES
IS DIFFICULT TO MODEL
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383
It should be noted that the limits to the effectiveness of control
systems depends on the degree to which the control law can be adjusted. The
bandwidth of a fixed control law will probably be limited to about the first
structural mode of a large space system. If adjustment of the control law is
possible after analyzing spaceflight data, then it should be possible to
increase the control bandwidth, perhaps by a factor of two. Adaptive control
would allow even tighter control by continually adjusting control in response
changing conditions.
"THERE ARE LIMITS TO THE EFFECTIVENESS OF CONTROL SYSTEMS"
3.O
WBANDWlDTH
WSTRUCTURE
2,0
/0
0
ADAPTIVE CONTROL
I
!
I
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J L ADJUSTABLE
LARGE
SPACE
SYSTEMS
I
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/ k FIXED
I
I
/,0
WBANDWl DTH
WACTUATION/SENS IN(;
Figure 9
384
If the size of large space systems becomesboundedby control limita-
tions, stiffer configurations will have an advantage. Their relatively higher
structural frequencies enable control bandwidth requirements to be met for
larger sized configurations. The hoop/column antenna, for example, is about
four times as stiff as the offset wrap-rib configuration of the samedia-
meter. For multi-frequency applications, this advantage narrows to a factor of
two because of the lack of blocking for the offset configuration.
"IFTHE SIZE OF LARGE SPACE SYSTEMS BECOMI'_BOUNDEDBY CONTROL LIMITATIONS,
STIFFERCONFIGURATIONS WILL HAVE AN ADVANTAGE"
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Figure I0
385
On-line systems identification for adaptive control of large space
systems requires additional development, although simple forms of adaptive
control would require less work. Practicalities are expected to limit the
number of structural modes that can be modeled, on-line, to about four.
Intentional disturbances will be required to produce response signals which
can be separated from noise and unmodeled disturbances. The computational
load is not insignificant, but lies within a practical range.
ON-LINE SYSTEMS IDENTIFICATION FOR ADAPTIVE CONTROL
REQUIRES ADDITIONAL DEVELOPMENT
O
O
O
O
O
O
O
O
SIMPLE FORMS OF ADAPTIVE CONTROLREQUIRE LESS DEVELOPMENT
MAXIMUM LIKELIHOOD ESTIMATION WOULD BE USED
NO SPECIAL INSTRUMENTATION IS REQUIRED
SOMEWHAT OBTRUSIVE CONTROL INPUTS WOULD BE REQUIRED
PERHAPS 4 STRUCTURAL MODES CAN BE MODELED, ON-LINE
NEED SAMPLE RATE OF 10/SECOND
UPDATE AFTER 30 SECONDS MIGHT BE POSSIBLE
REQUIRES COMPUTATIONAL RATE ON THE ORDER OF 2. 2 M OPS
SENSITIVITY EQUATIONS - MX2_ NC _ N = 142. I00" 300
INFORMATION MATRIX -- MZ2_ NC2 _ N = 20 • 1002" 300
INVERSE -- NC2/3 = 1003/3
Figure I I
386
Automatic surface control of a large space structure is straightforward
but is dependent on the accuracy and rapidity of surface measurements. The
larger number of sensors and actuators involved in surface contour control
will require fault management. The need for active surface control has not
yet been established, but will dependon surface accuracy specifications,
structural damping, and disturbance levels. Surface control at a particular
point would need surface deflection information over a limited region.
AUTOMATIC SURFACECONTROL IS STRAIGHTFORWARD
O
O
O
O
O
O
BANDPASS OF SURFACE CONTROL IS PACED BY RATE OF MEASURING
SURFACE DEFLECTION
MUST ACCOUNT FOR FAILED SENSORS AND ACTUATORS
PERHAPS 240 SURFACEMEASUREMENTLOCATIONS
SIMPLE, CONSTANT GAIN, FEEDBACK CONTROLADEQUATE
COMPUTATIONAL REQUIREMENTSARE MODEST"." .50K OPS
CONTROL-"-"_ _ 9.240. 1 = 2160
FILTER ; 20" 240. 10 = 48,000
REDUNDANCY--_, ?
NEED FOR AUTOMATIC SURFACE CONTROL DEPENDS ON:
RF REQUIREMENTS
SURFACE RESPONSE TO DISTURBANCES, CONTROL,
DEGREEOF STRUCTURAL DAMPING
J J
= _,m NZ,m
ui,J "J-
Figure 12
387
CONCLUDING REMARKS
II STRUCTURAL DYNAMICS MUST BE CONSIDERED IN DESIGNING EVEN
POINTING CONTROL FOR LARGE SPACE SYSTEMS.
O
I
DISTURBANCES, NOISE, AND NONLINEARITIES DRIVE THE CONTROL
SYSTEM DESIGN.
ADJUSTMENTS OF CONTROL GAINS AND SURFACE CONTOUR USING
GROUND-BASED ANALYSIS ARE ONLY MODERATELY DIFFICULT.
e ADAPTIVE CONTROL PROMISES IMPROVED CONTROL BUT REQUIRES
ADDITI ONAL DEVELOPMENT.
II AUTOMATIC SURFACE CONTROL IS STRAIGHTFORWARD.
I CONTROL AND ANALYSIS TECHNIQUES NEEDTO BE TESTED USING
GROUND AND TEST-FLIGHT DATA.
388


Configurational and system requirements for control of large space systems

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