NASA

Technical

Paper

3013

May 1990

i

- 4

A General-Purpose

Balloon-Borne Pointing

System for Solar

Scientific Instruments

M. E. Polites
- __ L"

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NASA

Technical

Paper

3013

1990

National Aeronautics and

Space Administration

Office of Management

Scientific and Technical

Information Division

A General-Purpose

Balloon-Borne Pointing

System for Solar

Scientific Instruments

M. E. Polites

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama

TABLE OF CONTENTS

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

II. DESCRIPTION OF THE POINTING SYSTEM ...................................................

!II. SIMULATION RESULTS ............................................................................

IV. CONCLUSIONS ........................................................................................

REFERENCES ..................................................................................................

Page

1

I

2

3

17

111

PRECEDING PAGE BLAN_( HOT F!!,_,._ED P_G_,, II INTENTIONAI,¢_ BLANK
LIST OF ILLUSTRATIONS

Figure

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

!1.

12.

13.

Title Page

Control system block diagrams for the gimbal command mode ............................... 4

Control system block diagram for the Sun acquisition slew mode ............................ 5

Control law for the Sun point mode ............................................................... 6

Torque command steering laws for the Surl point mode ........................................ 7

Block diagram for the reaction wheel speed control system ................................... 8

Instrument attitude for three-axis control at lO-degrees Sun elevation angle ................ 9

Control torques for three-axis control at 10-degrees Sun elevation angle .................... 10

Instrument attitude for three-axis control at 80-degrees Sun elevation angle ................ 11

Control torques for three-axis control at 80-degrees Sun elevation angle .................... !2

Instrument attitude for two-axis control at 10-degrees Sun elevation angle ................. 13

Control torques tor two-axis control at IO-degrees Sun elevation angle ..................... 14

Instrument attitude for two-axis control at 80-degrees Sun elevation angle ................. 15

Control torques for two-axis control at 80-degrees Sun elevation angle ..................... 16

iv
TECHNICAL PAPER

A GENERAL-PURPOSE BALLOON-BORNE POINTING SYSTEM

FOR SOLAR SCIENTIFIC INSTRUMENTS

I. INTRODUCTION

NASA is placing new emphasis on high-altitude research balloons because of their low-cost and

quick-launch advantages over rockets, satellites, and the space shuttle [ 1]. Typically, research balloons

can get scientific payloads to float at altitudes of 130,000 ft for hours or even days. This is satisfactory

tbr a number of solar scientific instruments like the solar optical universal polarimeter (SOUP). The

SOUP has been developed by the Lockheed Missiles and Space Company in Palo Alto, California, and is

a candidate fbr balloon flight. It is an instrument that is approximately 6-ft long, 1.5 ft in diameter, and

weighs about 400 lb. To meet its scientific objectives, it nmst be pointed at the Sun, stable to within 0.25

arc-min rms in pitch and yaw and 2 arc-rain rms in roll.

This paper presents a general-purpose balloon-borne pointing system for accommodating solar

scientific instruments like the SOUP. It is configured for precise pointing, low cost, quick launch, and

lot use by a wide variety of solar instruments. It offers the option of three-axis control, pitch-yaw-roll, or

two-axis control, pitch-yaw, depending on the needs of the payload. This system is described in section

[I. Simulation results for it with the SOUP as the science instrument are given in section lII, for both the

three-axis and two-axis control options. Final comments are made in section IV.

II. DESCRIPTION OF THE POINTING SYSTEM

The general-purpose balloon-borne pointing system presented in this paper has most of its

hardware on the gondola, to provide a quick-launch capability. There is a reaction wheel on the gondola

for generating control torques in azimuth. There is a bearing and torque motor at the top of it that attaches

to the flight train. The torque motor torques against the flight train to put low-frequency torques on the

gondola in order to keep the reaction wheel speed from saturating. There are three silicon solar cells

mounted 120 degrees apart around the azimuth axis of the gondola that are used in the initial Sun acquisition slew. The control computer [br the pointing system is situated on the gondola. It should be a digital

device with the ability to readily change control law parameters when the payload is changed between

flights. Attached to the gondola is an elevation gimbai assembly with a torque motor, tachometer, and

resolver. If two-axis control is used, the solar scientific instrument mounts directly to the elevation

gimbal. If three-axis control is required, there is a cross-elevation gimbal assembly between the elevation

gimbal and the science instrument. Like the elevation gimbal assembly, it has a torque motor,

tachometer, and resolver. The cross-elevation gimbal assembly should be removed, if not needed by the

payload, to save weight. On the interface structure that the science instrument mounts to, there are three

single-axis rate gyros for measuring instrument pitch, yaw, and roll rates. There is also an inclinometer

lbr measuring instrument roll orientation. The only hardware on the science instrument is a two-axis Sun

sensor lot pitch and yaw attitude of the instrument with respect to Sun center.
Thepointing systemhasthreemodesof operation.Theyarethegimbalcommandmode,theSun

acquisitionslew mode,and the Sunpoint mode.Oncethe balloonis at float altitudeandthe payload

launchclamphasbeenopenedto releasethescienceinstrument,thegimbal commandmodeis enabled,

andthe scienceinstrumentis commandedto a predeterminedelevationanglethatshouldbecloseto the

elevationangleof the Sunat thetime. If thecross-elevationgimbalassemblyis employed,zerocrosselevationgimbal is commanded.Thecontrol systemblock diagrams for the gimbal command mode are

shown in figure !. The second order transfer functions in the forward loops are shaping filters whose

coefficients can be uniquely specified for each payload, if necessary, to ensure system stability. Once the

science instrument reaches the commanded elevation angle, the Sun acquisition slew mode is also

enabled. This slews the gondola and science instrument toward the Sun in azimuth. A block diagram for

this mode is shown in figure 2. Once the Sun is in the field-of-view of the instrument-mounted Sun

sensor, the Sun point mode is enabled and the other modes are disabled by the control computer. The

science instrument is now controlled to point at the Sun. The control system for the Sun point mode has

two parts. One part is the control law shown in figure 3. It generates angular acceleration commands in

science instrument axes. These are input into the second part, which is the steering law that generates the

torque commands for the control actuators. A different steering law is used depending on whether threeaxis or two-axis control is employed, as shown in figure 4. In figure 3, the option exists of updating the

roll attitude using the inclinometer output in an attitude observer loop. This also compensates for roll rate

gyro drift. The ground should have the ability to enable or disable this update loop as it sees fit. Likewise, it should have the ability to uplink the biases Op(B) and Oy(B) that correct for misalignments between

the science instrument line-of-sight and the Sun sensor, determined in flight. Furthermore, the ground

should have the ability to uplink the pitch, yaw, and roll offset pointing commands Op<c_, Ov<c_, and Omc _,

respectively.

Finally, the reaction wheel speed control system shown in figure 5 is the last part of the pointing

system. It is used in the Sun acquisition slew mode and the Sun point mode to keep the reaction wheel

speed from saturating.

III. SIMULATION RESULTS

A computer simulation of the pointing system described in section II was developed by modifying

the one in reference 3. The solar scientific instrument was assumed to be the SOUP. It and the gondola

were modeled as rigid bodies whose mass characteristics are shown in table 1. The flight train is modeled

as a massless and extensionless cable with some torsional stiffness. The balloon can rotate in azimuth and

its motion is assumed to be unaffected by the dynamics of the gondola and the science instrument. Other

conditions for the simulation are listed in table 1. Four Sun point mode cases were simulated, two with

three-axis control and two with two-axis control. For each control configuration, Sun elevation angles of

10 degrees and 80 degrees were simulated. The simulation results are shown in figures 6 to 13. They

show good pointing system performance in all cases.

2
IV. CONCLUSIONS

In summary, this paper has presented a general-purpose balloon-borne pointing system for precise

pointing, low cost, and quick launch. It is designed to accommodate a wide variety of solar scientific

instruments. It has the option of three-axis control, pitch-yaw-roll, or two-axis control, pitch-yaw,

depending on the needs of the payload. Simulation results for both configurations were presented with

the SOUP as the scientific instrument. The results indicate good pointing performance for Sun elevation

angles ranging from 10 degrees to 80 degrees.

Table 1. Simulation conditions.

I.

II.

III.

IV.

Mass characteristics:

A. Science instrument:

I. Weight = 410 lb

2. Principal moments of inertia = 6,70,70 s-ft 2

B. Gondola:

I. Weight = 2,055 lb

2. Principal moments of inertia = 635,1392,1439 s-ft 2

Maximum control torques = 20 ft-lb in each axis

Control bandwidths = 1 hz

Roll attitude observer bandwidth = 0.03 hz

g. Input disturbances:

A. _+0.25 degrees pendulous oscillations with period

B. Sinusoidal balloon rotations in AZ with peak rates

period = 100s

= 17s

= +0.42 degrees/s and
8EL(c)

EL

TORQUE

MOTOR

EL

TACH

1

1

f

+ =1_+_+_°1 1_o_o.

I TACH XEL

SqSXEL I XEL I

IRESOLVERJ

Figure 1. Control system block diagrams for the gimbal command mode.
+

a2S2 +alS+ao I

S2 +blS+bo I

REACTION

"- WHEEL

* SIGN OF 0)cDETERMINED FROM

OUTPUTS OF SILICON SOLAR

CELLS

,I I COy

I

- I

I I ('OR J

IF 6EL(c ) < 45°, SWITCH UP

IF 6EL(c ) > 45°, SWITCH DOWN

I YAW

RATE

GYRO

ROLL I

RATE

GYRO

Figure 2. Control system block diagram for the Sun acquisition slew mode.

¢j1
eP(B)

ep(c)

+

a2S2+ alS + ao

b2S2* blS * bo

STEERING LAW

0)p PITCH

RATE

]

SUN

9y SENSOR

03y YAW

RATE

ey(c)

+

eR(C)

+

÷

ey(B)

÷ ÷

a2S2+ alS + ao

b2S2+ blS + bo

STEERING LAW

a2S2+alS+aol aR(C) } TO

b2S2+ blS + bo I STEERING LAW

÷

03R ROLL

RATE

INCLINOMETER

Figure 3. Control law for the Sun point mode.
FOR PITCH-YAW-ROLL CONTROL (i.e., WITH CROSS-ELEVATION GIMBAL):

.4

TAZRW(C)

TEL TM(C)

_TxEL TM(C_

/&_-_ o o0 IEL 0

C_ELIxE,) o IxE, L(-SSE,

R(C)"_

P(C)_"

Y(C).J

• FOR PITCH-YAW CONTROL (i.e., WITHOUT CROSS-ELEVATION GIMBAL):

TAZRW(C)

"_ TEL TM(C) _" =

TXEL TM(C_

o o

/ /

o o o kt_<clJ

NOTE: S6EL AND CSEL ARE ELEVATION RESOLVER OUTPUTS

Figure 4. Torque command steering laws for the Sun point mode.
O0

+

AZ

TORQUE

MOTOR

AZ

RW

TACH

Figure 5. Block diagram for the reaction wheel speed control system.
PITCH

IN

RAD

YAW

IN

RAD

ROLL

IN

RAD

XEL

IN

RAD

k

1:8|ti,

-I I

0 10 _0

0 • 0008 -,

O•0007 _

I:ISll:

|:|II_:

qp.oqpo2-

o.e

-0.O00t ....

0 10

30 40 50 60 "70 80 g0

• U " ' "

;'O 3e 40 50 60 "70 80 90

L00

00

f • I jn

l

1 / .... .

l ....

0 10 20 30 40 50 60 70 80 g0 100

0.03 ........

o.e2 A _ I f"_ •

-o.el

-,..

.., ...., _ r,\

___

I,,,\

xv

.... / , ,,\ _/

,

-qll.e3 ......

0 l0 20 30 40 50 GO 70 80 90 le0

TIME IN SEC

Figure 6. Instrument attitude for three-axis control at lO-degrees Sun elevation angle.
AZ

TORQUE

IN

FT-LBS

EL

TORQUE

IN

FT-LBS

XEL

TORQUE

IN

FT-LBS

a_

t5

le

G

o

-5

-1o

-Is

-2e

1.5

i

1,0

0.5

O.e

-$.5

-I .0

-I .5

-2.5

-3.0

-3.5

5.e

i ,

i i

.J

(

0 10 26 30

2.5

O.O

-2.5

-S. O

-7.!;

-le.0 +

-12.5

-15.0 .-

-17.5

-2O.O ,.

0 1Q 20 30 4e • . . i . .

$$ 60 "70 80 90 104

TIME IN SEC

Figure 7. Control torques for three-axis control at 10-degrees Sun elevation angle.
PITCH

IN

RAD

YAW

IN

RAD

ROLL

IN

RAD

XEL

IN

RAD

II. 0

e.llll

o.q

O.I

e

(

LOll

L .... m •

O.Olle4

)

lO0

0.0008

O.iOOll

-t.Ollll_

-0.0004

-O.O00G

-O.OllOB k

O 10 20 30 40 50

I II 8:881i

-lillX: / -_-t!tt i

e

60 "tO 80 ill loll

• " ' t \\! t

\ t \

\ s \ / _, t

\v I _.c,..,. 1

ill 20 30 40 50

_ 1 /

\ #

% l

\ 1

\ 1

\ /

6e 7e

\ /

• i

\ l

\ /

\ /

80 90 leo

TIME IN SEC

Figure 8. Instrument attitude for three-axis control at 80-degrees Sun elevation angle.
AZ

TORQUE

IN

FT-LBS

EL

TORQUE

IN

FT-LBS

XEL

TORQUE

IN

FT-LBS

........ ,

-I.S

-;!.e

-;l.S

-3.e

-3.S

e

1.5-

10 ;_11 3e 4e 5e 6e 7ll 8e 90 1tie

1.e-' L e.S.' I

e.e-

-e.5-_

-1 .e-

-1.5-

-il. II.

-a.5: ........

-3.e-

-3.'S,

-4.e. . ,.... i ..... .. . . ,

le 211 3e 4e 5e 6e ?e 8e 9e lee

TIME IN SEC

Figure 9. Control torques for three-axis control at 80-degrees Sun elevation angle.
PITCH

IN

RAD

YAW

IN

RAD

ROLL

IN

RAD

LO 20 30 40 50 60 '70 BO 9O

.,i

i

tOO

. ,,,

TIME IN SEC

Figure 10. Instrument attitude for two-axis control at lO-degrees Sun elevation angle.

¢,_
AZ

TORQUE

IN

FT-LBS

EL

TORQUE

IN

FT-LBS

5 _

Q-

-S-

-IO -

-1S -

-20 _ _

0

L,$,

Q.S_

Q,e. i

-Q.S.

-l.o2

-! .S-

-t!.e.'

-11,6-

-3.e --

-3.S ,.

o

i, i

IQ alp 3O

10 p-e 30

TIME IN SEC

Figure 11. Control torques for two-axis control at lO-degrees Sun elevation angle.
PITCH

IN

RAD

YAW

IN

RAD

ROLL

IN

RAD

@.eg45

Q.0940 0.003S I

0.092S0.0020-

8.0_ISo.oe10-

o.eeose.eeee-

-o.000_-

0

o.oQs -

0.004 _ I

0.003 -

0.0(12 1

0.0tll 1

0.000 1_

V

--@.001

@

0.03-

0.02

0.01

0.00

-0.0L

- O. 02

-0.03

10 20 30 40 50 60 70 80 90 100

lO 20 30 40 50 60 70 80 90 100

/

V

O IO 20 30 40 50 GO

f

/

/

70 80 90 I01

TIME IN SEC

Figure 12. Instrument attitude for two-axis control at 80-degrees Sun elevation angle.

i...a

¢,a-l
AZ

TORQUE

IN

FT-LBS

EL

TORQUE

IN

FT-LBS

;:tll

,._II

till

.t Ill

_,.tlII"vv_

_,_flil

@

I.S

1°@

@°S

@o@" _

-CoS

-1 .@,

-t .S

-Z.@

-I!.S

-3°@

-3.S

^At

_VV

V"

10 20 3@ 40

10 _0 3O 4O

A

AAA/V_

• g *

.^AA_

V_A/vv_',,,MA^ ^_

"vvv

.,, . ...... . . - . ,,, j

S@ G@ 70 80 @0 10@

• ,, , ,

gO GO 70 80 go 1@@

TIME IN SEC

Figure 13. Control torques for two-axis control at 80-degrees Sun elevation angle.
REFERENCES

I ,

,

.

Nordwall, B.D.: "U.S. Places New Emphasis on the High-Altitude Balloons." Aviation Week &

Space Technology, McGraw-Hill, New York, NY, October 23, 1989, pp. 34--36.

Davis, J.M.: "SOUP Requirements." Internal memorandum ES52, NASA Marshall Space Flight

Center, Huntsville, AL, April 24, 1989.

Lightsey, W.D., and Polites, M.E.: "Modeling, Designing, and Simulating a Pointing Control

System for Balloon-Borne Solar Experiments." NASA TM-100337, NASA Marshall Space Flight

Center, Huntsville, AL, July 1988.

17

Report Documentation Page r LL1,,, ,, /k,i,,, q,it, ir ,ll

. P,,,, ,, I" ,I

I. Report No.

NASA TP-30i3

2. Government Accession No.

4. Title and Subtitle

A General-Purpose Balloon-Borne Pointing System

for Solar Scientific Instruments

7. Author(sl

M.E. Polites

9. Performing Organization Name and Address

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama 35812

12. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546

3. Recipient's Catalog No

5. Report Date

May 1990

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No.

M-628

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Paper

14. Sponsoring Agency Code

15. Supplementary Notes

Prepared by Structures and Dynamics Laboratory, Science and Engineering Directorate.

16 Abslrilcl

This paper describes a general-purpose balloon-borne pointing system for accommodating a

wide variety of solar scientific instruments. It is designed for precise pointing, low cost, and quick

launch. It offers the option of three-axis control, pitch-yaw-roll, or two-axis control, pitch-yaw,

depending on the needs of the solar instrument. Simulation results are presented that indicate good

pointing capability at Sun elevation angles ranging from 10 degrees to 80 degrees.

17. Key Words (Suggested by Author(s))

Balloon-borne

Pointing control system

Solar scientific instruments

18. Distribution Statement

Unclassified - Unlimited

Subject Category: 31

19 Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No. nf pages

24

22. Price

A03

NASA FORM 1626 OCT 86

For sale by the National Technical Information Service, Springfield, VA 22161-2171

NASA-Langley, 1990

_

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