The Zenith Star Experiment is designed to demonstrate and evaluate the performance of a laser in space to answer critical issues relevant to SDI. The spacecraft design, control system architecture, and vehicle control are discussed

Morine, L. A.

English

NASA Technical Reports Server

N90-21064
Zenith Star:
A Structural Control Challenge
L. A. Morine
Martin Marietta Aerospace Group
Denver, Colorado
3rd Annual NASA/DOD CSI Conference
San Diego, California
January 29 - February 2, 1989
PRECED:NG PAGE '_'p_._,_,,_"'_"NOT FILMED
17
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INTRODUCTION
The Zenith Star experiment (Figure i) is designed to demonstrate
and evaluate the performance of a laser in space to answer
critical issues relevant to SDI. This experiment is fully
compliant with the restrictive interpretation of the 1972 Anti-
ballistic Missile (ABM) Treaty. As such it does not directly
perform all of the functions of a defensive system nor to the
level required by an operational system. Its results however, do
provide a measure of the potential of the operational systems by
applying the appropriate scaling from the benchmarks achieved by
it in space.
[
i
....7 i
Figure 1: Zenith Star Experiment
18
The experiment (Figure 2) consists of a series of high power
evaluations of beam control and a series of low power evaluations
of the tracking and pointing functions of the system.
The high power experiments evaluate the beam control by direct
measurement of the far field beam performance with a high power
target board. Both space propagation and upper atmospheric
effects are measured.
The low power experiments evaluate the tracking and pointing
function performance while tracking a booster throughout its boost
phase flight. The Agile Control Performance is evaluatd by
performing structured characterization and large and small angle
repointing of the system against a star field, small test objects
(carried on board), and multiple boosters to exercise the system
under multiple conditions.
High Power
Characterization
.... _,_
AGILE PLATFORM PERFORMANCE
Repointing
al High Rates ,_,
j .... .--.
i "// StructuralII
t l _./ / " _ _ Chatacterizal_on
Capture, \"__c'_ _-_L ....
\_ '"\ Tracking and Pointing E.._
, \\ Fe,,F,e,0ao_ _ ----// Ropoo,,o0_\\\\ Maoa0emoo,*"°ck
t \\ Settling --
// ,-/" _vS"_Oual
/1 _estObiec,so,'_TestO_iects ..." ...._ ' Boost._,
Almospheric / /
Effects " " / /
Phenomenology Oalhering =
Figure 2: Zenith Star Objectives
19
ZENITH STAR SPACECRAFT
The basic hardward for the Zenith Star experiment is shown in
Figure 3. It consists of a chemical laser of the class of the
alpha program, a beam expander that utilizes the segmented LAMP
mirror for the primary optical element, an actuator for pointing
the beam expander, and an isolator for attenuating the laser noise
from the beam expander. The latter two are combined into one
subsystem called the actuator/isolator. The laser energy is
directed through the aft body to the beam expander by a series
of transfer optics and steering mirrors (beam control transfer
assembly) on the aft body. A capture track system (consisting of
a suite of sensors) is utilized to point the beam expander and
optical train for tracking a series of test objects. The
remainder of the equipment is a set of standard spacecraft
subsystems that allow it to be in orbit as a free flyer that is
commanded by ground operations personnel.
The system is delivered into orbit by two Titan IV launch vehicles.
The forward spacecraft is launched first and checked out completely.
Then the aft spacecraft-C0nsisting of the Alpha laser and
spacecraft support subsystems is launched into the same orbit as
the first, orbit phased, and remotely operated from the ground
for rendezvous and docking.
ACTUATOR/
h._OLATOR
{A/I)
ISOLATED
(IFBI
\
_ICATION
SYSTEM {PCS) ANTENNA
SEPARATION
PLANE
FV _LAR ARRAY
SOLAR
ARRAY
DEVICE
=
su Ys tcrrs)
Figure 3: Zenith Star Space Vehicles
20
CONTROL SYSTEM ARCHITECTURE
The control architecture for the space based laser is derived from
a series of stringent tracking and pointing requirements depicted
in Figure 4 and the resulting interactive implications lead to a
complex hierarchical control architecture. Tight accuracy and
jitter requirements combined with the need for rapid repointing of
the line-of-sight from one object to another necessitates
isolation and suppression of disturbances to the large beam
expander. The Zenith Star control system is designed to duplicate
this architecture so that the experiment results can be directly
related to the SBL performance.
The precision and jitter are analogous to hitting a basketball on
the Empire State Building in New York from Pike's Peak in
Colorado. It must accomplish this while tracking objects at
angular rates more than an order of magnitude higher than the
capability of the Hubble Space Telescope. To accomplish this, the
line-of-sight must be isolated from disturbances by as much as i00
million to one and yet be able to repoint from one object to
another in less than one second so that the system effectiveness
can be high.
Pikes
Hubble Space Telescope Can
Precision Point at 1/10 Degrees per
Second, as Fast as a Minute Hand
-_.9
SDI Must Precision Point at 5
Degrees per Second, as Fast
as a Second Hand
- Two Orders of Magnitude Faster
than the State-of-the-Art HST
Precision Pointing '_
Is Critical To
System Effectiveness
¢, Basketball
__ Empire State
Building
Retarget Time Is Critical to System Cost
/
Retargeting in Less
Than One Second
Number of
Engagements
80--
60 _ Desired40 Technology
2o
i ! 1 !
12345
Average Retarget Time
SDI Payloads Must
Be Isolated from
the Noisy
Spacecraft
Spacecraft Bus : Payload Must Be
Is Noisy World Held Quiet
I
Isolation 10,000:1
Structural Suppression 100:1
Precision Steering 100:1
Total Isolation 100,000,000 : 1
Figure 4: Space Based Laser Control Requirements
21
While the structure is made as stiff as possible, there is
sufficient deformation (Figure 5) of the beam expander structure
and optical geometry resulting in line-of-sight disturbances to
require isolation of aft body noise from the beam expander. There
are other self-induced beam expander disturbances such as fluid
flow and rapid repointing that require structural disturbance
suppression on the beam expander itself.
Either technique can be readily handled without two body
interaction, but when combined an actuator/isolator is required
between the bodies. This actuator/isolator must provide six
degrees of freedom operation which introduces other control
issues, such as translation and beam walk control, that further
complicate the controls problem.-
I
I
l
• LOS
Desired _ _ Errors
Line-of-Sight
, I Distorted
_ Line-of-Sight
Torque
LOS
Errors
Target
Position
Time _I II
Figure 5 Beam Expander Optical Distortions
22
In order to ease the burden of pointing the line-of-sight of
the system, a precision pointing set of controllers is introduced
to provide beam expander off-axis pointing and stabilization so
that the structure control can be relaxed within a small field of
view and as shown in Figure 6. So long as the line-of-sight
disturbance is within the range of the precision pointing
controller authority the beam expander controller requirements are
eased. In other words, the settling time is satisfied when the
beam expander line-of-sight is within this band.
Laser
Target
Position
Steering Tracker
Alignment --v Mirrors _ \
Sensor
Aft Body ---, Actuator/Isolator !
Inertial Reference
Increased Fine
Pointing Field of
Line Regard Allows
of Earlier/Longer
Sight Lase Periods
_t_
" T
Pointing
Tolerance
Figure 6 Precision Pointing Control System
23
VEHICLE CONTROL
The formulation of the control architecture for the beam expander
can be described as follows in the next series of figures in
Figure 7.
An easy method of isolation (Figure 7a) of the aft body
disturbances from the beam expander is to provide a gap between
the forward and aft bodies, control the beam expander to point
to the test object from on-board sensor data by external torques
(such as control moment gyros), and control the aft body to follow
this motion by external forces and torques to maintain the desired
gap within some tolerance. This is ideal isolation since there is
no actuation between the bodies to force alignment of the two
bodies, hence there is no transfer of disturbances from one body
to the other. _=_ ........ _ :_......
Since each body tends to rotate about its own center of mass there
will be large translational displacements (Figure 7b) at the
optical interface between the beam expander and aft body. Also
since the beam expander disturbances are to be minimized the aft
body must be translated as well as rotated by external forces and
torques to maintain the proper separation.- @h_s is not practical
for a highly agile control system because of the large heavy aft
body and the fact that the gap must be small, on the order of
centimeters. Consequently an actuator between these bodies is
required.
L
This actuator i_ntroduces a coupling path from the aft body to the
beam expander which then requires @n isolato[ b@tween the bodies
(Figure 7c). This actuation and isolation must be combined into
one subsystem because of this interaction This subsystem is
called the actuat0r/isolator and it must minimize this coupling
while producing the desired pointing forces and torques. This
function is nontrivial even for the baseline magnetic isolator
because of nonl_near magnetic forces and eddy currents which
must be cancelled,
L
Self-induced disturbances on the beam expander arising from fluid
flow and rapid pointing must be dissipated through damping in the
structure, transferred to noncritical structural motion (non-
critical modes_ o[ transferred off the beam expander to the aft
body (Figure 7d) . The incorp6rition of the actuator/isolator
allows this energy to be transferred to the aft body which can
then remain isolated. Hence the beam expander line of sight can
be stabilized while still tracking objects.
The pointing of the beam expander causes severe disturbances. In
order to move the line-of-sight from one object to another (rapid
repointing) it is desirable to make maximum use of the available
torque from the actuator/isolator. In fact, the optimal
repointing for a rigid body is a bang-bang command. This,
however, causes severe disturbances to the line-of-sight.
24
Aft Body
2_
Figure 7a
Beam
Gap Expanded
All Body I
2_
_Z
2_
Figure 7b
Displacement
Figure 7c
Figure 7 Vehicle Control Architecture Evolution
Figure 7d
Beam Expander
Actuator/Isolator
,K Fluid
Disturbances
Repointing
Disturbances
25
The severity is dependent on the relationship of the angle to be
repointed (frequency of the bang-bang torques) and the structural
frequencies. Figure 8 shows the effects of a single structural
frequency of 4 Hz and 8 Hz separately as a function of repointing
angle. The time to hand over is the time that the line-of-sight
error takes to settle to within the field of reguard of the beam
expander where the fine off-axis steering takes over. The rigid
body response is included since it represents the lower bound of
maneuver time for the system.
> 4
O
c
T
2
_
A
/
4Hzf
\
.7Hz
/ L-
7, I
2 4
I 1 I 1 1 1 I
0 6 8 10 12 14 16 18
Repointing Angle
Figure 8 Effects of Structural Frequencies on Repointing Time To Precision Track
Handover
26
!
When all structural modes are considered the picture is not quite
so easily displayed because the relative effects on the line-
of-sight are intermixed. An envelope of these effects is
indicated in Figure 9 where the lower bound is limited by the
rigid body response and the upper bound depends on advanced
structural controllability.
The regions of interest for structural control are the torque-
limited and rate-limited regions. The algorithm-limited region is
the area of responsiveness of the precision off-axis controlsys-
tem for scene interpretation and control.
Repointing
Time
10
0.1
,_ Torque ___.3
Limited "1 //
-- i
' ;/X Rigid Body
AIg°rithm I _/_/ Limit
__Limited fi /S Rate
1 [ Settling Time
RegionI I
,11 I iI I,l 1
.I I 10 100
Repointing Angle
Figure 9 Repointing Characteristics
27
Figure i0 shows the improvement in repointing time that can be
made by a simple modulation. The technique is based on the
relationship of the repointingangle and the knowledge of the struc-
tural frequencies of the beam expander. By properly commanding or
modulating the torque commands, disturbances can be minimized as
shown in the figure for one technique called modulated bang-bang
control.
This technique concentrates on modal avoidance and cancellation
and its effectiveness. There are other techniques that have been
investigated by several members of the community that should also
be evaluated in space. These include both other modal avoidance,
modal suppression, and modal displacement.
14-
10
E
6
Modulated Bang-Bang
.,......._.._ Rigid :13:0clyOptimal
_
I I I | L
10 20 30 40 50
Retargeting Angle, mrad
Figure 10 Multiple Mode Controller Characteristics
=- =
28
SUMMARY
The space based laser control tasks are indeed challenging because
of the variety of requirements that demand different types of
controllers, all competing simultaneously. The architecture
derived for the SBL resulted in a hierachical control formulation
that demands advanced control techniques. Each portion of the
architecture has interaction with the others which demands careful
orchestration of the control commands to fulfill the control
requirements.
The Zenith Star duplicates the SBL functions and provides
performance levels close enough to the SBL performance to provide
valid scaling for evaluation of the SBL expectations.
One dominant crucial control function is the beam expander
controller. It must place and stabilize the beam expander line-
of-sight within a few hundred microradians of the object tracked
in a very short time. The accuracies involved require careful
control of the structural deformations even with structural
resonances on the order of 20 Hz.
There is no precedence for this type of structure control since
this is the first opportunity to control a structure of this
nature in space. Experiments such as structural identification
and modal surveys are also planned for in the experiment
objectives. Utilization of other techniques for controlling the
structure, such as distributed actuator structural control, are not
currently available on Zenith Star but may be available in the
future depending in the interest within the community and the risk
to the other Zenith Star objectives.
In either case there is ample opportunity for industry
participation during the Zenith Star mission operations. This
can be accomplished by submitting ideas for structural control
techniques to SDIO for consideration. If approved, these
experiment ideas will be integrated into the experiment objectives
and the implementation incorporated into the mission planning.
Space Based Laser Control ComDlex & Challenginq
• Stressing Pointing & Tracking
• Repointing in=Short Time Requires New Control Thinking
• Control Large Optical Structures Requires Interactive Control Strategies
Zenith Star ¢hpllenqeF Rival the $BL Control Difficulties
• Pointing & Tracking is Severe
• Repointing & Structural Control is Scaleable to SBL
• Results from Beam Expander Control is First Attempt in Space
Zenith Star Offer_ Opportunity
• Demonstrate & Validate Wide Variety of Structural Control Issues
• Industry Wide Participation in Large Structure Experiments in Space
Figure II Summary 29
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Zenith Star: A structural control challenge

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