Table Of ContentIF AC Conference Proceedings
ATHERTON: Multivariable Technological Systems
BANKS & PRITCHARD: Control of Distributed Parameter Systems
CICHOCKI & STRASZAK: Systems Analysis Applications to Complex Programs
CRONHJORT: Real Time Programming 1978
DUBUISSON: Information and Systems
GHONAIMY: Systems Approach for Development
HASEGAWA & INOUE: Urban, Regional and National Planning - Environmental Aspects
LEONHARD: Control in Power Electronics and Electrical Drives
NIEMI: A Link Between Science and Applications of Automatic Control
NOVAK: Software for Computer Control
OSHIMA: Information Control Problems in Manufacturing Technology
RIJNSDORP: Case Studies in Automation Related to Humanization of Work
SAWARAGI & AKASHI: Environmental Systems Planning, Design and Control
SINGH & TITLI: Control and Management of Integrated Industrial Complexes
SMEDEMA: Real Time Programming 1977
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AUTOMATIC CONTROL IN SPACE
Proceedings of the 8th IF A C Symposium,
Oxford, England, 2-6July 1979
Editor
C. W. MUNDAY
Honorary Secretary of the
Institute of Measurement and Control,
London, England
Published for the
INTERNATIONAL FEDERATION OF AUTOMATIC CONTROL
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First edition 1980
British Library Cataloguing in Publication Data
IFAC Symposium on Automatic Control in Space,
8th, Oxford, 1979
Automatic control in space.
1. Space vehicles - Automatic control -
Congresses
I. Title II. Munday, C W
III. International Federation of Automatic
Control
629.47'42 TL795 79-41745
ISBN 0-08-024449-1
These proceedings were reproduced by means of the
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The readers are therefore asked to excuse any deficiencies
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The Editor
Printed in Great Britain by
A. Wheat on & Co. Ltd., Exeter
Vlllth IFAC SYMPOSIUM ON AUTOMATIC CONTROL IN SPACE
Sponsored by
International Federation of Automatic Control (IFAC),
Organized by
The Institute of Measurement and Control on behalf of the
United Kingdom Automatic Control Council (UKAC) and
co-sponsored by the British Interplanetary Society and the
Royal Aeronautical Society
International Programme Committee
E. Gottzein (FRG) — Chairman, R.J.Jude (UK)
IFAC Space Committee P. Kant (The Netherlands)
C. W. Munday(UK) - Chairman, W. M. Lovell(UK)
National Organizing Committee J. L. Nevins(USA)
D. P. Atherton (Canada) M. J. Pelegrin (France)
P. M. Barham(UK) R. Pure (USSR)
J. L. Crowder(UK) J. Rodden (USA)
R. Dalziel (UK) J. Scull (USA)
D. B.DeBra(USA) N. N. Sheremetjevsky (USSR)
D. G. Ewan (UK) E. Slachmuylders (The Netherlands)
L. Flook(UK) K. C. Varghese(UK)
W. Haeussermann (USA) I. Vasiljev (USSR)
C.J.Harris (UK) H. D. Zago(FRG)
J. W. Hursh (USA) M.J. Yates(UK) — Secretary
National Organizing Committee
C. W. Munday (Chairman)
P. M. Barham
P.J. Conchie
J. L. Crowder
R. Dalziel
D. G. Ewart
L. Flook
K. F. Gill
C.J. Harris
R.J.Jude
W. M. Lovell
G. K. Steel
D. R. Towill
M.J. Yates (Secretary)
INTERNAL IMAGE MOTION COMPENSATION
SYSTEM FOR THE SHUTTLE INFRARED
TELESCOPE FACILITY
Kenneth R. Lorell*, Eric K. Parsons** and J. David Powell**
* Space Projects Division, Ames Research Center, NASA, Moffett Field,
California 94035, U.S.A.
** Department of Aeronautics and Astronautics, Stanford University, Stanford,
California 94305, U.S.A.
Abstract. The Shuttle Infrared Telescope Facility (SIRTF) is being designed as a 1-m, cryo-
genically cooled telescope capable of a thirty-fold improvement over currently available infrared
instruments. The SIRTF, mounted in the Orbiter bay on the Instrument Pointing System (IPS),
requires that the image at the focal plane be stabilized to better than 0.1 arcsec with an absolute
accuracy of 1 arcsec in order to attain this goal. Current estimates of IPS performance for both
stability and accuracy indicate that additional stabilization will be necessary to meet the SIRTF
requirements. An Image Motion Compensation (IMC) system, utilizing a Charge Coupled Device
(CCD) star tracker located at the focal plane and a steerable mirror in the SIRTF optical path,
has been designed to work in conjunction with the IPS. The CCD tracks multiple stars simulta-
neously and through the use of a high-performance microcomputer generates three-axis attitude
errors and gyro drift estimates to correct the IPS gyros. A high-bandwidth feed-forward loop,
driven directly from the IPS gyro package, controls the steering mirror in order to correct distur-
bances not compensated for by the IPS control system. The SIRTF IMC system is described in
detail and performance is analyzed with respect to system noise inputs, environmental distur-
bances, and error sources such as bending and feed-forward scale factor. It is concluded that the
SIRTF accuracy and stability requirements can be met with this design.
Keywords. Attitude control; control system analysis; discrete systems; feed forward; micro-
processors; tracking systems.
INTRODUCTION at the focal plane. The entire internal structure of the
telescope operates at a temperature of 20° K and below.
The Shuttle Infrared Telescope Facility (SIRTF) is being This low temperature is made possible by a concentric
designed as a 1.2-m, cryogenically cooled, infrared tele- shell design in which the inner optical portion of the
scope to be flown as a Shuttle-attached payload in the telescope is connected to the outer structural cylinder by
mid-1980's (Witteborn and Young, 1976). The SIRTF, thin, thermally resistive members, thereby minimizing
mounted in the Orbiter bay on the European Instrument heat transfer losses (McCarthy, 1976). Unfortunately, the
Pointing System (IPS), will make possible previously mechanical coupling between the SIRTF mounting
unobtainable photometric and spectrographic observa- fixture and the optical system is relatively light and flexi-
tions over a wide range of IR wavelengths (Fazio, 1978). ble, making precise knowledge of the optical axis
uncertain.
In order to maximize the effectiveness of SIRTF as a
scientific instrument, it is necessary that the optical axis A second difficulty peculiar to infrared astronomy, and
be pointed with an absolute accuracy of 1 arcsec and that the source of a major shortcoming of previous IR tele-
the image at the focal plane remain stable to 0.1 arcsec scope pointing systems, is the lack of a visible component
(Young, Witteborn, and Kiya, 1978). The signal-to-noise associated with many infrared objects. These objects are
ratio during the observation of faint sources is further therefore not visible to the astronomer, nor can the
enhanced by chopping the incoming beam with an oscil- pointing system use them for augmentation of the basic
lating mirror located in the optical train. The modulated gyrostabilization loops as is often done in optical astron-
image has the same accuracy and stability requirements omy. Tracking a nearby visible object is of course possi-
as the un chopped one. Meeting these stringent pointing ble, but multiple star trackers are then required to
and stability requirements is a challenging task even provide three-axis error information. Even when multiple
under ideal circumstances. However, infrared astronomy star trackers are used to provide the off-axis tracking
and the SIRTF in particular place additional demands on function, the problems of target identification and
the design of the pointing system. limited interaction with the control system still remain.
Primary among the factors that further complicate the The SIRTF will be mounted on the IPS in the Orbiter
analysis and design of the SIRTF controller are the tele- bay. The most recent simulations of IPS operation have
scope's cryogenically cooled optics which must not inter- indicated that while quiescent performance is in the 1- to
fere thermally with the extremely sensitive IR detectors 2-arcsec range, disturbance errors may be as much as
1
C.A.C.S.B—
2 K. R. Lorell, E. K. Parsons and J. D. Powell
3 arcsec. Thus, although the IPS can provide positioning algorithm in order to effectively enhance the CCD reso-
for target acquisition and a stable base to isolate the lution (Salomon and Goss, 1976), and generates a point-
SIRTF from large amplitude Shuttle disturbances, it does ing error signal for each of the three axes based on the
not provide the necessary precision or stability for the relationship between the desired and the current posi-
SIRTF. Additionally, the image dissector tube star tions of the tracked stars in the CCD field of view
trackers utilized by the IPS have no star field display (Murphy, Lorell, and Swift, 1975).
capability.
Thus, a precise measure is available from the CCD of the
A number of areas are, therefore, apparent in which meet- current orientation of the telescope optical axis, irrespec-
ing the pointing and stability requirements, as well as tive of thermal distortion, mechanical bending, or
facilitating efficient operational use of an infrared tele- external star tracker alignment. The error signal derived
scope, may be difficult or impossible. These areas are as from this measurement is processed by a Kaiman filter
follows : in the SIRTF microcomputer to correct the gyro outputs
both in pointing offset and drift. The time constant for
1. The thermal distortion due to temperature differen- these error corrections is naturally quite long, of the
tials between the inner and outer telescope sections order of tens of seconds (or hundreds of seconds when
2. The light mechanical coupling between the inner and tracking M+14 stars).
v
outer telescope shells
3. The difficulty of providing off-axis pointing using The problem of short-term, high-bandwidth stability is
current star tracker systems solved by controlling the image with a gimbaled mirror
4. The lack of a star field display for the operator which also functions as the second folding flat in the
5. The inability of the IPS to meet the SIRTF funda- double-folded Gregorian optical system of the SIRTF.
mental pointing and stability requirements Using this mirror as an image steering device is a natural
application since it already utilizes a high-performance,
The present work deals with the development of an two-axis servo system in order to spatially chop the
image motion compensation (IMC) system, internal to infrared science image.
the SIRTF, which provides the stability and control to
meet the SIRTF's pointing requirements. This system Any motion of the IPS-SIRTF sensed by the gyros, but
uses a charge coupled device (CCD) array located at the not corrected by the IPS controller, will be compensated
SIRTF focal plane combined with outputs from the IPS for by moving the steering mirror. The IMC system effec-
gyros to drive a mirror in the telescope optical train. tively increases the system bandwidth by acting on dis-
Operation of the CCD and of the IMC system is con- turbances to which the IPS cannot respond. The steering
trolled by a small, high-performance microcomputer mirror is driven directly in a feed-forward mode by the
whose extremely fast cycle time permits tracking multi- IPS gyro outputs. This feed-forward loop provides two
ple stars and calculating error commands for all three important advantages: (1) the bandwidth is limited only
axes. by the bandwidth of the gyros themselves, not the closed-
loop system bandwidth of the IPS-SIRTF; and (2) the
The basic concepts developed in this paper are based in possibility of SIRTF structural resonance exciting a con-
part on the Video Inertial Pointing (VIP) system devel- trol system instability is eliminated because there is no
oped from 1974 to 1977 at Ames Research Center direct closed loop between the sensor and actuator. Feed-
(Lorell, Murphy, and Swift, 1976; Powell and Throck- forward loops of course are sensitive to errors in the scale
morton, 1975). The VIP system also utilized a CCD to factor. The effects of scale-factor error have been investi-
track multiple stars and combined the star tracker infor- gated and will be discussed in the next section.
mation with gyro outputs. However, the concept of steer-
ing a mirror inside the telescope is a major difference Figures 1 and 2 are schematic and block diagrams, respec-
between VIP and the SIRTF/IMC. In part, the steering tively, of the IMC system. Note the direct interfaces
mirror concept evolved in response to the stringent
SIRTF pointing requirements and because a movable
mirror was already part of the SIRTF optical design. STEERING MIRROR
SERVO AND CHOP
MOTION DRIVE
SYSTEM DESCRIPTION
The SIRTF/IMC System operates by combining the out-
puts of the IPS gyros, which provide high-bandwidth,
short-term stability and an 800 X 800 element CCD array
at the telescope focal plane tracking multiple stars to pro- COOLED INNER SHELL
vide low-bandwidth, low-noise stability. The image on the _L
IPS CONTROL
focal plane is stabilized both by moving a two-axis steer-
SYSTEM
able mirror located in the SIRTF optical system and by COMPENSATION
inputs to the IPS gyros. TO CONTROL
CCD ELECTRONICS
- CONSOLE
AND MICROCOMPUTER
AND CRT
The readout and control of the CCD are performed by a DISPLAY
small, high-performance microcomputer built into the
SIRTF on-board electronics. The computer accepts raw Fig. 1. Schematic diagram of the SIRTF
video data from the CCD, performs an interpolation IMC system.
Internal image motion compensation system 3
RIGID Figure 3 illustrates the physical relationship between the
TELESCOPE quantities in the model, and Fig. 4 is the Laplace trans-
\ » 1 è 1 Θ Θ-^Μ form block diagram of the continuous system. Note that
) JTS s
the effect of scale-factor error in the feed-forward loop
<
Λ and an estimate of the first structural bending mode have
been included.
GYRO
IMAGE POSITION IN FOCAL PLANE: 0T = Θ + 20B - KSCALE (0g -
Ιφ^
Kp +KwS + K|/i L OPTIMAL STATE
ESTIMATOR / 0MX M
Fig. 2. Block diagram of the SIRTF IMC system.
between the SIRTF microcomputer, the IPS three-axis
gyro package, and the steering mirror. The IMC is also
coupled to the SIRTF control console in order to provide
the operator with a video star field display, control of the
telescope optical axis, and CCD operational parameters
(e.g., threshold, integration time, and number of tracked
stars).
Performance Analysis of the IMC System
A number of system- and environment-related factors
were involved in the design and performance analysis of
Fig. 3. SIRTF pointing system geometry with bending
the IMC system. The fundamental problem was one of
mode. / = gyro rate integration, C = IPS compensation,
synthesizing a system that would be insensitive to a vari-
ety of noise and disturbance inputs, yet could operate Gm = Ty-axis gimbal, Gy 7^-axis gyro, M mirror.
with the limitations imposed by tracking Af +14 stars.
v
Once a design concept utilizing a combination of digital
and continuous loops had been established, the analysis o(S)
proceeded along two distinct paths. First, the inspection
m r
of the response of the continuous portion of the system T T
to Shuttle disturbance torques, feed-forward scale-factor
error, and gyro rate noise; and second, optimization of JT1S 9(S) s1 0(S)■.(vS8 ► 0T(S)
the discrete portion of the system in order to minimize
errors from CCD noise, long-term gyro drift, and thermal
distortion. Modeling of the system was done only for the
single-axis case; however, accurate numerical data to the
con(S)- C(S)
greatest extent possible were obtained for all relevant
IPS and Shuttle parameters to make the results realistic
and useful.
JTS2 + VBS+KB OSCPATILCEA LFA CTOR
(MIRROR)
BENDING 0„(S)
Continuous Loop DYNAMICS
-ÖC(S)
The governing equations of motion for the continuous — +Kp + KvS
portion of the system are: B(S)' VBS + KB
IPS SERVO '
-JjB = JTèB + VßÖß + Κβθβ + MTrTa0 KSCALE
FEEDFORWARD 2
SCALE FACTOR
Vßeß = -KßOß^K(e-e) + K[m + 0-e
I g c p c
Fig. 4. Block diagram of the continuous portion
of the IMC system.
θ = ω θ-θ
8 η+ τ €
The response of the system to a step command in angle
θτ~θ -2θ + θ
Β Μ is seen in Fig. 5 and illustrates the bandwidth of the IPS-
SIRTF. The mirror deviation 0^f *s clearly seen respond-
ing to the transient with the result that virtually all of
scalè
%=-r-(Vöc) (1)
4 K. R. Lorell, E. K. Parsons and J. D. Powell
ORBITER + PAYLOAD:
MASS 9.2 x 104 kg
MOMENT OF INERTIA
ABOUT Ys AXIS 8.9 x 106 kg-m2
Fig. 7. Shuttle-SIRTF geometry.
TIME, sec
.20
Fig. 5. IMC system response to position
.15
step command.
/ \ 0.10
.10
the error in θγ is removed. The response in θβ is negli- IM T
.05
gible as indicated on the greatly expanded scale. «SCALE = °·95
0
A Bode plot of system response to rate inputs to the gyro VBI
such as rate noise (ω ) or rate commands (ß) is shown
η c
in Fig. 6. The complementary nature of 0 and θ„ is quite -.05
evident and demonstrates that the resulting response of
.10
θγ remains nearly uniform within the limitations of the
feed-forward scale-factor error and the bending response. -.15
-.20 I I
Responses to Shuttle disturbances have been analyzed 5
TIME, sec
using astronaut wall push-offs and Shuttle reaction-
control-system limit cycling as the primary input sources.
Fig. 8. IMC system response to astronaut wall push-off.
Figure 7 defines the geometry of the Shuttle with the
IPS/SIRTF mounted in the bay, and Figs. 8 and 9 illus-
trate image position response to these inputs. Note in
J I I I I I I I I I l I I I
5 10
TIME, sec
Fig. 6. Bode plot of IMC system parameters Fig. 9. IMC system response to Shuttle RCS
with respect to rate input. limit cycling.
Internal image motion compensation system 5
Fig. 8 that the IPS gimbal angle exceeds 1.5 arcsec error for tracking a Λί+14 star and using the Ferranti 125
ν
in response to the astronaut pushoff. The limit-cycle gyro.
disturbance, which produces a 1.8 mm/sec2 acceleration
lasting 600 msec at the IPS pivot point every 10.6 sec, is
seen to cause an image excursion in excess of 0.1 arcsec; CCD
the excursions last about 1 sec. The magnitude of this SHUTTLE aD0I(SST) URBANCE» OjiS) TFRUANNCSTFIOENR
error is directly related to ^scaie, and based on our a0(S) 7Γ/2Τ
assumption of a realistic value of 5% scale-factor mis-
match, it may be necessary to inhibit the RCS during GYRO RcoAnT(SE) NOISE 0T(S) U
critical viewing periods in order to meet the 0.1 sec CJ„(S)
requirement. RANDOM WALK
DRIFT
ω(Ζ) —
DRIVING RANDOM (Z-1)2 ' ' "
Digital Drift Filter __ SEQUENCE | Öm(Z)
GYRO NOISE MODEL AVERAGE
OVER
The digital control loop provides attitude and attitude INTERVAL
rate commands to the IPS gyros at discrete intervals. The A
updates are outputs of a Kaiman filter designed to obtain
maximum-likelihood estimates of image angular position
based on noisy star position measurements. The synthe- 0C(S) OjiS)
sis of the filter requires balancing the improvement in 0C(S)
estimation accuracy with decreasing sample period
against the gain in CCD measurement accuracy as inte- 0C(S)
gration time is lengthened. ÖC(S)
A second consideration is related to the time delay asso- I ZERO ORDER HOLDS
ciated with the CCD integration period. It is essential that _ |
the filter model this delay if instabilities are to be avoided -ω0(Ζ)
for the longer integration periods. The filter design is,
therefore, sensitive to the brightness of the tracked stars; öm(z)
o (z)
that is, dimmer stars require longer CCD integration m
times. Thus, the optimal filter for tracking Mv+7 stars -ω0(Ζ)
will have a different gain matrix from one designed to 0m(Z)
track Λ/+14 stars. The flexibility provided by the system
ν
digital computer, however, permits the use of optimum DISCRETE ESTIMATER
filter gains regardless of the required integration period.
The discrete portion of the IMC system was analyzed Fig. 10. Block diagram of the discrete portion
using the loop shown in Fig. 10. Shuttle disturbances, of the IMC system.
gyro rate noise, and random-walk drift are inputs to the
system. Performance is evaluated by plotting image posi- Figure 11 is an indication of filter performance as a func-
tion on the CCD vs time. tion of integration time for tracking an Af+14 star
y
(worst-case noise input). Note the sensitivity of the error
The transfer functions for the Shuttle disturbance, rate to not only CCD integration period, but also gyro quality
noise, and estimator outputs, as well. Small filter gains are necessary because the
tracker noise is significantly higher than the gyro drift
6j{S) ej{S) 6 (S) Θ (Ξ)
T Τ
A(S) ω (Ξ) ejfs) 6 (S)
0 η C
The low filter gains result in rather poor transient
are those defined by Eqs. (1) and in Fig. 4. The drift response, as shown in Fig. 12 which represents the closed-
filter has the structure shown below, loop response of the complete system to initial offsets in
CCD star position and gyro drift (Θ j and ω^). Perfor-
■»/>] l oo κ~ M X' mance improves as a direct function of tracked-star
brightness with the filter time constant decreasing by a
ωΒ 0 1 0 K2 Ù>D K2 factor of 2.5 for a change of one visual magnitude. It is
emd 0 0 0 K3 emd K, rrii also possible to select gains that minimize transient
l?wi_ 0 0 1 K2 om KA roenscpeo nthsee etirmroer, htahse nd escwreiatcshe dt ob etlhoew n oanis ea-cocpetpimtaball eg tahinre sshet-
z+i
old. This technique, which takes advantage of the flexi-
bility inherent in an on-board digital processor, was used
where
successfully on the VIP System (Lee, 1978).
r*~
+ 0.1420
\K + 8.375 X 10"4
2
=
r3 -0.1336
[tf_ 0 (2)
4
K. R. Lorell, E. K. Parsons and J. D. Powell
SYMBOL TABLE
INTEGRATED RANDOM WALK
GYRO MODEL RATE NOISE DRIFT RATE (At 1 hr )
(arcsec rms) (arcsec/sec rms) English
A. HAMILTON STANDARD 0.11 0.01
RI-1139 lateral acceleration of pivot (m/sec)
«o
B. BENDIX 64-PM 0.11 0.0005
Jrp telescope moment of inertia about pivot
C. FERRANTI 125 0.017 0.0005
= 28175kg-m2
Kp IPS proportional feedback
= 5.7X 105 Nm/rad
IPS rate feedback
*v
= 1.29 X 105 Nm/rad/sec
Kr IPS integral feedback
= 3.42X 105 Nm/rad-sec
J I I I I MM J I I I I Mil Kr bending mode stiffness
1 10 100 = 2.818 X 108 Nm/rad
STAR TRACKER INTEGRATION/SAMPLE TIME, sec
K scale feed-forward scale factor = 0.95
Fig. 11. Estimate error vs star tracker integration time
for three candidate IPS gyros. Filter designed for
K, K, K, Κ4 Kaiman filter gains
1 2 3
M +14star.
v
telescope mass = 2140 kg
S Laplace variable
T sample time
bending mode damping
FILTER FOR Mv + 14 STAR = 2.818 X 104 Nm/rad/sec
shuttle-fixed coordinate frame
XS> SY'ZS
Z Z-transform variable
telescope center of mass offset = 2.5 m
200 300
TIME, sec
Fig. 12. IMC system response to initial offset of Greek letters
1 arcsec and drift rate of 0.1 arcsec/sec.
0„ interpolated star tracker measurement
m
(rad)
CONCLUSIONS Θm d delayed value of dm (rad)
The performance of an image motion compensation θ,θ IPS gimbal angle and angle rate (rad),
system for the Shuttle Infrared Telescope Facility, which (rad/sec)
uses a focal plane CCD star tracker and a steerable
mirror, has been analyzed. The IMC system is necessary ΊΌ pointing error estimate
because stringent pointing requirements for SIRTF
cannot be met by the Shuttle-mounted pointing system. rotational bending error of secondary
The SIRTF-IMC system acts to increase the effective mirror (rad)
pointing system bandwidth and thereby desensitizes the
SIRTF to Shuttle disturbance sources. Use of the CCD gyro output (rad)
tracker at the SIRTF focal plane not only provides gyro
offset and drift correction, but facilitates astronomer- °c>ec commanded pointing angle and rate (rad),
pointing system interaction by providing a star field (rad/sec)
display for the SIRTF operator. Results of this analysis
show that 0.1-sec pointing stability can be achieved using commanded mirror angle (rad)
*M
the SIRTF-IMC, even with scale-factor errors of as much
as 5% and in the presence of large amplitude IPS gimbal image angular position on focal plane
excursions. (rad)
