. 

NASA 

Tech n ica I 

Memorandum 

GROUND FACILITY FOR LARGE SPACE STRUCTURES 

DYNAMICS AND CONTROL VERIFICATION 

By Henry Waites 

Systems Dynamics Laboratory 

Science and Engineering Directorate 

November 1986 

5561 G&OUIC PACILITP POE LA36E Ii87-18bOO 

YEBIPZCAIIGI (BAS&) 15 p CSCL 22B 

Unclas 

G3/18 43797 

National Aeronautlcs and 

Space Ad mi n 1st rat ion 

George C. Marshall Space Flight Center 
TABLE OF CONTENTS 

. 

Page 

INTRODUCTION ............................................................... 1 

INITIAL LSS CONTROL GOALS ................................................ 2 

DYNAMIC KODELING .......................................................... 3 

LSS CONTROL SYNTHESIS .................................................... 6 

FUTURE LSS ACTIVITIES ..................................................... 9 

iii 
Figure 

1. 

2. 

4. 

5. 

6. 

7. 

8. 

LIST OF ILLUSTRATIONS 

Title Page 

Multimission spacecraft ............................................... 1 

Methodology for LSS control verification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 

Dynamic modeling. . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . 4 

Closed tree topology options .... . ... ..... .... .... . ... .... . . .. ... ... ... 

LSS control for configuration 1 ....................................... 

LSS control for configuration 2 .............. ......................... 

Advanced Solar Observatory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

AS0 ground verification ................. ............................. 

5 

7 

8 

9 

10 

iv 
TECHNICAL MEMORANDUM 

GROUND FACILITY FOR LARGE SPACE STRUCTURES DYNAMICS 

AND CONTROL VERIFICATION 

INTRODUCTION 

With the increase in complexity of the spacecraft (Fig. l), the experiments have 

become more ambitious and multifaceted. On one such LSS, the stationkeeping of the 

spacecraft would be disturbances to any fine pointing system(s) or some low gravity 

material’s processing or both. The vibrational modes of the structure are excited by 

the disturbances which in turn interact with the experiments in a deleterious way. 

Either active vibrational control or disturbance isolation control or both are required 

to isolate or reduce the structural interactions with the experiments so as to improve 

payload system performance. 

Due to a more active use of space for Earth sciences, solar physics, astrophysics, material sciences, and defense, spacecraft structures and requirements have 

become more complex and stringent. To meet the desired objectives of these more 

complex space projects, the MSFC started a program to deal with such issues as the 

dynamic modeling, control development and synthesis, dynamics verification, and the 

hardware flight systems for these space structures, most of which were very large. 

Since these large spacecraft issues cover a wide range of technical disciplines, a 

team development was necessary in the areas of control, structures, optics, sensors 

Figure 1. Multimission spacecraft. 
and effectors, thermal propulsion, and materials. 

now an integral part of the MSFC Large Space Structure (LSS) control verification. 

A block diagram of this methodology is shown in Figure 2. 

The multidiscipline methodology is 

I 

6. 

7. 

8. 

Establishment of a Proof of Concept (POC) ground demonstration test. 

Development of real time test procedure for control optimization. 

Data reduction to feret out pertinent model parameters. 

1 

1 DYNAMIC MODELING I 

CONTROL SYNTHESIS 

VERIFICATION 

HARDNARE FLIGHT SYSTEM 

Figure 2. Methodology for LSS control verification. 

INITIAL LSS CONTROL GOALS 

The goals for the LSS control verification team were to automate as many of 

To that end, the initial 1982 

these technical disciplines as possible and to integrate where possible, these discipline tools into a user friendly analysis methodology. 

objectives of the LSS Control Verification Team were as follows : 

1. Develop control system design technique based upon closed-loop pole 

Investigate control system design techniques to isolate disturbance forces 

Develop centralized controller for Large Space Structure Orbital Experiplacement to assure desired performance. 

2. 

utilizing computer software. 

3. 

ment (LASSOE) test article. 

4. 

isolation. 

Conduct trade study of centralized, decentralized, and software disturbance 

5. Integrate control concepts to determine actual sensor and effector need for 

the LASSOE test article. 
9. Verification of modeling techniques. 

10. Application of the design and control methodology to larger structures with 

various performance constraints. 

11. Shape control Scope of Work development. 

The expected outputs of the LSS Control Verification Team were as follows: 

1. An automated control design technique for space vehicles. 1 

2. Demonstration of utility of decentralized controllers. 

3. Development of a disturbance isolation control system. 

4. Establishment of POC ground demonstration test approach for Large Space 

Structures. 

5. Real time testing procedures for space systems. 

6. Development and utilization of parameter estimation methods. 

7. Improvement of modeling techniques. 

8. Shape control RFP. 

As can be seen from these objectives and outputs, they are parochial relative 

to controls, sensors and effectors, and structures, but with the program evolution 

came an awareness that a multi-discipline methodology was necessary for LSS control 

verification. 

DYNAMIC MODELING 

One of the more time consuming areas of LSS control verification is the develop. 

ment of the structural model. In most space projects the data trickles in a substructure at a time until the whole spacecraft is defined and that is usually for a fixed 

substructure orientation. Kost of the LSS have several flexible substructures that 

are changing their orientation which implies that a multitude of structural models are 

required to effect an LSS control verification. This scenario is not only time consuming, but it also contains many possible sources of error. 

To eliminate the aforementioned problem (time and errors) and to work within 

the constraints of the system (substructure determinations and different orientations) 

a user friendly computer analysis tool was developed to automatically effect nonlinear 

modeling and simulation of complex flexible structure (Fig. 3). 

features of the computer analysis tool are as follows: 

The significant 

1. Modular concept allows for rapid reconfiguration. 

2. Models large angle rotations and angular rates for any module. 

3. Allows for chain, tree, and ring topologies of flexible bodies. 

3 
4 

v) 

w 

a 

A 

h 

n 

m 
4. A variety of control modules are available. 

5. Allows equality and inequality constraints between any two or more elements 

of the substructures. 

The ring topology of flexible bodies is the latest feature that has been added to 

this computer analysis tool. The ring joints for each substructure has either equality 

constraint or inequality constraints (Fig. 4). 

kinematic conditions and the inequality constraints are set conditions such as : 

The equality constraints consist of 

1. Hard stops. 

2. Coulomb damper. 

3. Velocity squared damper. 

4. Solid damper. 

5. Displacement squared springs. 

0 CONTOPS 

- EQUALITY CONSTRAINTS (KINEMATICS) 

- INEQUALITY CONSTRAINTS, 

oo HARD STOPS, 

oo COULOMB DAMPER (SIGN (6) 1 

oo VELOCITY SQUARED DAMPER (Q/d/), 

00 SOLID DAMPER (Q SIGN b>, 

oo DISPLACEMENT SQUARED SPRINGS ( Q/Q/) a 

0 CONTOPS ENtIANCEMENTS 

- GG MODEL, 

- ATMOSPHER 1 C MODEL, 

- MAGNETIC MODEL, 

- MODAL SELECTION METHOD, 

- CMG MODELS, 

Figure 4. Closed tree topology options. 

5 
The ring topology with the joint constraints can effect the dynamic model and 

simulation of many LSS, but future enhancements are needed to upgrade the disturbance models, selection of critical structural modes, and the modeling of effectors with 

momentum. The enhancements for the nonlinear modeling and simulation program will 

be a gravity gradient model, an atmospheric model, a magnetic model, modal selection 

methods and momentum effector models. With these additions, very complex structures 

can be modeled and simulation, using various control options, with relative ease and in 

a small time period. 

reasonable times to model LSS, were achieved with the development and use of the 

computer analysis tool. 

The system model objectives, which were relative ease and 

LSS CONTROL SYNTHESIS 

Another facet of the total plan is the control synthesis which uses the model 

generated by the user friendly computer tool, CONTOPS. Essentially, there are two 

control synthesis techniques and they are pole placement methods and quadratic 

minimum techniques. Initially, the synthesis tack was in the direction of pole placement techniques because of the parallels to the well understood frequency techniques. 

The pole placement control synthesis technique was divided into several control categories and they were centralized control, centralized control with disturbance isolation, 

distributed sensor control, distribution sensor control with disturbance isolation, 

distributed control, distributed control with disturbance isolation, decentralized 

control, and decentralized control with disturbance isolation. All of these categories 

will eventually be analyzed in the Ground Facility for Large Space Structure Control 

Verification (GF /LSSCV) . 

With the constraints of time and hardware, two preliminary control techniques 

were demonstrated in the GF/LSSCV. The first test configuration for the centralized 

control technique was comprised of the Base Excitation Table (BET), the Advanced 

Gimbal System (AGS), the Voyager Magnetometer Boom (VlLzB), the tip inertial reference unit, and the cruciform. This configuration had 15 modes below 2.5 Hz of which 

the fundamental mode was 0.5 Hz and these modes had damping of 1.5 percent or less. 

With this test article, a preliminary centralized control design was effected. The open 

loop rate gyro responses to an Orbiter thruster-like disturbance at the BET are shown 

in the first column time plots. 

stimuli are shown in the second column time plots. The comparison of the two columns 

of time responses show the effectiveness of the centralized control on this generic 

Large Space Structure (Fig. 5). 

The closed loop rate gyro responses to the same 

The Structures/Controls Test Facility Configuration 2 is comprised of the following elements : 

1. Shake table. 

2. Three axis base accelerometers. 

3. Three axis base rate gyros. 

4. Three axis tip rate gyros. 

5. Three axis tip accelerometers. 

6. Bidirectional thrusters. 

6 
Figure 5. LSS control for configuration 1. 

7. Optical detectors. 

8. Reflectors. 

9. Laser. 

10. Two axis gimbal system. 

11. N2 gas bottles. 

12. Two planar sets of Linear Momentum Exchange Devices with accelerometers. 

Items 1 through 5, 7 through 10, and 12 are presently in-hand for configuration 2 while items 6 and 11 are on order. With items 1 through 12, several control/ 

structures techniques will be demonstrated, which fall into the control categories of 

either decentralized or distributed control on configuration 2. The control/ structures 

techniques to be demonstrated are active image motion compensation for a long flexible 

focal length, vibrational suppression using linear cold gas thrusters, evolutionary 

control by using the Voyager Magnetometer Boom in various stages of deployment, and 

simultaneous closed loop parameter estimation /control. 

vide much insight into the structurallcontrol interaction of LSS but much work still 

remains (Fig. 6). 

These demonstrations will proOne area of interest relative to control hardware is an unobtrusive sensors and 

effectors. Currently most control hardware concepts are distribution lump mass 

elements and these hardware configurations are such that they alone change the 

7 
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K 

2s 

2 0 

L 0 c- 0 Y a X 

W 

k 

0 

ccc 

tn 

tn 

d 

8 
structural character of the payload. 

assuage this type of problem for LSS control. 

LSS vibrational suppression is piezo-electric polymer. 

used as either a sensor or an effector or both. 

trusive sensor is another viable alternative as is using remote sensing techniques 

such as optical reflectors. 

multi-discipline technology in which more work remains. 

objectives for the MSFC Ground Facility for LSS control verification. 

The unobtrusive sensors and effectors would 

One material that could be used for 

The piezo material could be 

The use of fiber optics as an unobThe implementation of unobtrusive sensor and efforts is a 

This is one of the future 

FUTURE LSS ACTIVITIES 

The structural interaction with spacecraft experiments is a most complex issue 

for evaluating the performance of a spacecraft. In particular , one control challenge 

is the Kultiple Payload Pointing Nount (IKPPM) situation on a flexible support structure. Little work has been effected to analyze the problems which are endemic to the 

NPPM experiment. The Advanced Solar Observatory (ASO) is an example of such a 

situation in which at least two pointing mounts will be operating independently of one 

another while secured to a flexible structure. This is depicted in Figure 7. 

To address the AGPPM problem, MSFC plans to erect, in its LSS ground facility , an experiment situation similar to the ASO. 

construction of the air bearing table, which will allow translation in a plane and rotation perpendicular to that plane, and the experiment mount with the Pinhole/Occulter 

Facility (POF). 

mounting plate on which will be located an inertial reference unit and the SAFE-I boom. 

The first phase of this plan will be the 

The POF will consist of a 3-axis gimbal system with its payload 

Figure 7. Advanced Solar Observatory. 

9 
The SAFE-I boom will have an end plate similar in mass characteristics to the tungsten 

end plate of the POF. 

possesses similar structural characteristics of the POF . After "tuning" this structure, 

a dynamics and control verification will be effected so that any possible "surprises" 

can be studied and eliminated before the POF flight. 

The total structure configuration will be "tuned?? so that it 

A natural extension to this facility would be the addition of a horizontal structural member with two more pointing mounts and their associated payloads. The two 

additional payloads which will be models of an optical telescope and an infrared device, 

along with the POF will simulate the AS0 or the MPPM problem (Fig. 8). This facility 

should provide a sufficient challenge to the present control verification technology 

until a system akin to this configuration is flown. L 

Figure 8. AS0 ground verification. 

10 
APPROVAL 

GROUND FACILITY FOR LARGE SPACE STRUCTURES DYNAMICS 

AND CONTROL VERIFICATION 

By Henry Waites 

The information in this report has been reviewed for technical content. 

of any information concerning Department of Defense or nuclear energy activities or 

programs has been made by the MSFC Security Classification Officer. 

in its entirety, has been determined to be unclassified. 

Review 

This report, 

G. F. McDONOUGH 

Director, Systems Dynamics Laboratory 

*U.S. GOVERNMENT PRINTING OFFICE 1986--730-067/60018 

11 
TECHNICAL REPORT STANDARD TITLE PAGE 

1. REPORT NO. (2. GOVERNMNT ACCESSION NO. 13. RECIPIENT'S CATALOG NO. 

Ground Facility for Large Space Structures Dynamics and Control Verification 

NASA TM -86558 I I 

1. TITLE AND SUBTITLE 15. REPORT DATE 

November 1986 I 

6. PERFORMING ORGANIZATlON CODE 

7. AUTHOR(S) 

Henrv Waites 

3. PERFORMING ORGANIZATION NAME AND ADDRESS 

George C. Marshall Space Flight Center 

Marshall Space Flight Center, Alabama 35812 

2. SPONSORING AGENCY NAME AND ADDRESS 

8. PERFORMING ORGANIZATION REPOR r 

10. WORK UNIT NO. 

11. CONTRACT OR GRANT NO. 

13. TYPE OF REPORi & PERIOD COVEREC 

Technical Memorandum 

National Aeronautics and Space Administration 

Washington, D .C . 20546 

Unclassified 

- 

14. SPONSORING AGENCY CODE 

Unclassified 14 NTIS 

I 

15. SUPPLEMENTARY NOTES 

Prepared by Systems Dynamics Laboratory, Science and Engineering Directorate. 

16, ABSTRACT 

NASA Marshall Space Flight Center has developed a facility in which closed loop 

control of Large Space Structures (LSS) can be demonstrated and verified. 

main objective of the facility is to verify LSS control system techniques so that 

on-orbit performance can be ensured. 

or payload which is connected to a 3-axis angular pointing mount assembly that 

provides control torque commands. 

to a base excitation system which will simulate disturbances most likely to occur for 

Orbiter and DOD payloads. The control computer contains the calibration software, 

the reference systems, the alignment procedures, the telemetry software, and the 

control algorithms. The total system is suspended in such a fashion that the LSS 

test article has the characteristics common to all LSS. 

The 

The facility Consists of an LSS test article 

The angular pointing mount assembly is attached 

17. KEY WORDS 

Large Space Structures 

Con t r o 1 

Dynamics Verification 

19. SECURITY CLASSIF. (of thl. repart) 120. SECURITY CLA. 

18. Dl STRl BUTlON STATEMENT 

Unclassified - Unlimited 

;IF. (of thh page) 21. NO. OF PAGES 22. PRICE 

WFC - Form 3292 (hY 1969) 

For .ale by National Technical Information Service, Springfield. Virginia 221 61