11

GP-B Attitude and Translation Control

John MesterStanford University

22

Space - reduced support force, "drag-free"

- roll about line of sight to star

Gyro - sphericity and mass unbalance

requirements: 10nm achieved

The GP-B Challenge

– Gyroscope (G) 107 times better than best 'modeled' inertial navigation gyros– Telescope (T) 103 times better than best prior star trackers– G – T

33

ATC Requirements

Pointing Requirement 110marcsec/√ Hz (Guide Star Valid) pitch and yaw

Roll Requirement 40 arcsec at roll rate

Acceleration Requirement Active Control of all 6 dof of Spacecraft

44

What is the ATC system?

First space vehicle to actively control all six degrees of freedom 3 Orientation 3 Translation

16* Cold gas proportional micro thrusters provide forces and torques Fueled by helium boil-off gas from cryogenic system * Two thrusters failed before science phase

Common-mode flow rate control maintains helium bath temperature

Pointing system controls the guide star tracking telescope and maintains roll phase Translation control system uses acceleration measurements from one of the science

gyroscope's suspension system to null out environmental forces Vehicle flies in a near-perfect gravitational orbit

55

Block Diagram

Rate Sensor

Orientation Sensors

Command Generator

Attitude Control

Translation

Control

Dewar Temp

Control

Thruster

Distribution

Magnetic

Torque

Rods

Thrusters

Vehicle

Dyanmics

Science Gyro

Suspension

System

Dewar Press

& Temp

Sensors

Magnetic Torquer

Distribution

+

-

++

Dewar

Thermodynamics

Attitude and

Translation

Control

Software

Sensors Actuators

GP-B

Spacecraft

+

-

PrimaryTelescope

Star Trackers

BackupMagnetometersCoarse Sun Sensors

Control Gyroscopes

66

ATC System Hardware

Sensors

• Primary Sensors• Attitude

• Telescope• Star Tracker• Rate Gyroscopes

• Translation - One of the fourscience gyro suspensionsystems

• Backup Attitude Sensors• Magnetometers• Coarse Sun Sensors

• Backup Translation Sensors -Three more science gyroscopes

Actuators

• Primary Attitude and TranslationControl Actuators - 16* ProportionalMicro Thrusters

• Backup Attitude Actuators - Magnetictorque rods

77

The Overall Space Vehicle

♠ 16 Helium gas thrusters, 0-10 mNea, for fine 6 DOF control.

♠ Roll star sensors for fine pointing.

♠ Magnetometers for coarseattitude determination.

♠ Tertiary sun sensors for verycoarse attitude determination.

♠ Magnetic torque rods for coarseorientation control.

♠ Mass trim to tune moments ofinertia.

♠ Dual transponders for TDRSSand ground stationcommunications.

♠ Stanford-modified GPS receiverfor precise positioning. Laserranging corner cube cross-checksGPS.

♠ Redundant spacecraftprocessors, transponders.

88

Control Gyroscopes

• Two gyro assemblies -one used, one backup

• Gyro mounting platformthermal distortion had abig impact on the controlsystem performance

ControlGyroscopes

99

Star Trackers

Star Trackers

• Two star trackers - oneprimary, one backup

• 8 deg field of view• Mounted at angles 10degrees apart - differentgroups of stars visible

1010

Magnetometers

• Measure vehicleorientation by comparingmagnetic field readingwith expected field

• Expected field dependson orbital position whichis propagated onboardand updated from ground

Magnetometers

1111

Coarse Sun Sensors

• Backup coarseorientation sensors

• Rough estimate ofvehicle orientation

• Only used if star trackersand magnetometers fail

Fwd SunSensors

Aft SunSensor

1212

Thrusters

• Vehicle has anominal mass flowrate of ~8 mg/s

• Maximum thrusterforce: 8mN

Thrusters

1313

Magnetic Torque Rods

• 3 Orthogonal torque rods• Two axis control dependingon magnetic field geometry• Orbit position propagatedonboard and updated withground processed GPS data• Thruster failure backup• Used during science toincrease fuel margin• Still used today to controlsatellite to ~5 deg

1414

Attitude Control

• Science Mode– Pitch/Yaw Pointing

• Telescope• Control gyroscopes

– Roll Control• Star tracker• Control gyroscopes

• Coarse Control– Pitch/Yaw Pointing

• Star tracker• Control gyroscopes• (Magnetometers)• (Coarse sun sensors)

– Roll Control• Star Tracker• Control Gyroscopes• (Magnetometers)• (Coarse sun sensors)

1515

Single Axis Pointing Error

0 10 20 30 40 50 60 70 80 90 100-15,000

10,000

5,000

0

5,000

10,000

15,000

North-South Pointing vs Time (11/10/04)

Time Since Guide Star Invalid Start (min)

Poin

ting

Angl

e (m

arcs

ec) Vehicle in Solar Eclipse

2) Guide Star Capture

1) Guide Star Invalid 3) Guide Star Valid

The attitude control goes through three different phases each orbit:1) Guide Star Invalid – Guide star is blocked by Earth2) Guide Star Capture – Guide star is re-centered in the telescope field ofview3) Guide Star Valid – Nominal telescope pointing

1616

Target Plot

-4000 -2000 0 2000 4000

-3000

-2000

-1000

0

1000

2000

3000

Spacecraft Pointing (3/25/05)

East-West Pointing (marc-sec)

Nor

th-S

outh

Poi

ntin

g (m

arcs

ec)

3600 marcsec

Nominal guide star capture times range from 30 to 90 seconds The RMS vehicle pointing is controlled to less than 320 marcsec (1.55microrad)

RMS Pointing = 260 marcsec

1717

Proton Monitor Data

ProtonMonitor Hits

1818

Telescope Detector Data

• Particle hits also affected the telescope detectors• A plot of the number of hits in the telescope clearly shows the South AtlanticAnomaly region

TelescopeDetector

Proton Hits

1919

SAA Data Plot

GP-B flies through the SAA at least three out of every fifteen orbits losing,at times, up to 80% of the telescope data to proton corruption

0 10 20 30 40 50 60-1000

-500

0

500

1000Vehicle X-Axis Attitude Error (5/9/05)

Time Since Guide Star Valid Start (min)

Ang

le (m

arc-

sec)

0 10 20 30 40 50 600

20

40

60

80

100Percentage of Telescope Data Corrupted by Proton Activity

Time Since Guide Star Valid Start (min)

Per

cent

2020

Roll Rate Plot

2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500

0

50

100

150

200

250

300

350

400Vehicle X-Axis Pointing

Orbit Number

Ampl

itude

(mar

csec

)

Total RMS PointingRoll Rate Component

• Mission-longpointingperformance

• Shaded Regions- Roll Rate NotchFilter Turned ON

2121

gyro output Scale factorsmatched foraccuratesubtraction

telescope output

Dither: Slow 30 marc-s calibration oscillations injected into pointing system

{Vehicle pitch/yaw dither

Once telescope/gyroscope scale factor is known, vehicle motion can be subtracted outof the gyroscope signal

Pitch/Yaw Attitude Dither

2222

Roll Phase Control

There are two star trackers onboard the GP-B satellite.• Field-of-view: 8ox8o• Boresight axes are 50o and 60o away from the satellite roll axis,

out of phase by 180oTwo Attitude Reference Platforms (ARPs) are mounted on the

graphite ring around the dewar of the satellite.One control gyroscope package and one star tracker are mounted on

each ARPClose-loop control of the roll phase is implemented with 16

proportional cold gas thrusters by the Attitude and TranslationControl (ATC) system.

2323

Blue = Star Tracker A Field of View

Red = Star Tracker B Field of View

Star Tracked Fields of View

2424

Roll Angle Plot

0 10 20 30 40 50 60-80

-60

-40

-20

0

20

40

60

80Roll Phase Error (3/25/05)

Time Since Guide Star Valid Start (min)

Angl

e (a

rc-s

ec)

40 arcsec

• The controlgyroscopes andstar trackers areused to control thespacecraft roll angleto an error less than40 arcsec at aconstant roll periodof 77.5 sec

2525

Drag-free Satellite Technology

• 1959 – George Pugh envisioned a “tender satellite” for first proposedtest of General Relativity.

• 1964 – Ben Lange (Stanford) provided first detailed study of issuessurrounding drag-free satellites.

• 1972 – Flight of a Disturbance Compensation System (DISCOS) onTransit I (experimental US Navy navigation satellite)

• 2004 – Flight of Gravity Probe BTransitpre-launch

Transit deployed on orbit

2626

Drag Free Concept

Control Spacecraft to follow an inertial sensorReduce disturbances in measurement band

Aerodynamic dragMagnetic torquesGravity Gradient torquesRadiation Pressure

Spacecraft Follows a purely Gravitational Orbit

2727

Drag-Free Satellites have flown successfully

TRIAD I : Johns Hopkins Applied Physics Laboratory Navy Transit Navigation System

Launched September 2, 1972 Polar Orbit at 750 km Mission Lifetime over one yearDISCOS - Disturbance Compensation System - Stanford 3 axis translation control

And Now Also GP-B 3 axis translation control3 axis attitude control

Drag Free History

2828

• Electrostatic Sensing of Proof Mass

• Pressurized gas “On-Off” Thrusters

• 3 Axis Translation Control

• Acceleration levels were below 5 x 10–12 g

averaged over 3 days

– limited by tracking data and earth

gravity model

DISCOS Performance

2929

Propellant Source: Helium Boil-off

Isola

tio

n V

alv

es (

TIV

) -

16

Th

rust

ers

-1

6

Pyro

vent

valve

Su

perf

luid

Heliu

m

240

0L

Heat

Leak

125mW

Poro

us P

lug –

Su

perf

luid

ph

ase s

epa

rato

rW

i ck

• Superfluid phase separator“porous plug” permitsgaseous Helium to leavemain tank while keepingsuperfluid He inside.

• Mass Flow: 4 - 16 mg·s−1 overplug temperatures of 1.6K to2.0K without choke orbreakthrough.

• Provides supply pressure of660 to 2300 Pa (5 to 17.5 torr)to thrusters.

• Each thruster is provided adedicated isolation valve(TIV) to disable unit if afailure occurs.

2.8K

1.8 K

300Kambient

3030

Flight Proportional Thruster Design

From Dewar and manifoldSupply: 5 to 17.5 torr

Re ~10 – Laminar flow!

Thrust: 0 – 10 mNISP: 130 secMdot: 6-7 mg·s−1

Scale factor variation: 6%

Bias variation: 0.2 mN

Noise: 25 µN·Hz−1/2

Operates under choked flowconditions

Pressure FB loop makesthrust independent of unittemperature

3.5mm dia

Redundant voice coils

3131

Thruster Arrangement on Vehicle

• Thrusters arearranged in 4 clustersof 4 units each.

• Each cluster hasthrust authority along3 axis.

• Arrangement chosento be robust tothruster failures.

• Provides 3DOF ofattitude and 3DOF oftranslation control forthe space vehicle.

+X

+Y

+Z

7

8

14

1

11

12

2

16

3

9

1310

6

5

15

4

1.19 m

1.19 m

1.19 m

1.19 m

2.51 m

1.9 m

Space Vehicle

Body Axes

Roll Axis

Thruster

Cluster

Guide

Star

SV

Forward

SV Aft

G1

G4

23cm

8.8

cm

3232

“Prime” Drag-free Operation

• In prime drag-free, the ATC system flies the SV around the position of theproof mass– Suspension control is turned “off” (up to a given safety radius)– Advantage: Suspension forces/torques minimized (somewhat)– Disadvantages: 1) unsuspended spinning “bomb”, 2) cannot correct for

accelerometer biases

( )r R!��

R� r

�SV Translation

PID Controller

Resonance

Modes

+ +

21Ms

+

Position

Bridge

Vehicle dynamics

Gyro Suspension

Controller 21

ms

Gyroscope dynamics

+

+

Gravity Gradient feed forward

GSS Ctrl

Effort

K1K2

Gyro

Position

SV

Position

+

-

-

Gyro Suspension

Translation Control

Space Vehicle Dynamics

Ext

Distur -

bances

+

Ext

Disturbances

SV Ctrl

Effort

U

R�

r�

( )r R!��

3333

“Backup” Drag-free Operation

• In suspended (backup) drag-free, the ATC system nulls the gyro suspensioncontrol effort.– Advantage: Gyro “always” suspended; 2) Accelerometer biases can be

removed.– Disadvantage: Suspension forces and torques reduced to preload

minimums.

( )r R!��

R� r

�

SV Translation

PID Controller

Resonance

Modes

+ +

21

Ms+

Position

Bridge

Vehicle dynamics

Gyro

Suspension

Controller2

1ms

Gyroscope dynamics

+

+

Gravity Gradient feed

forward

GSS Ctrl

Effort

K 1K2

Gyro

Position

SV

Position

+

-

-

Gyro Suspension

Translation Control

Space Vehicle Dynamics

Ext

Distur -

bances

+

Ext

Disturbances

SV Ctrl

Effort

R�

r�

u�

U�

( )r R!��

Accelerometer

bias

compensation

3434

Example Drag-free Transitions

Transition sequence:

1. Backup drag-free

2. Prime drag-free

3. Non drag-free.

1. Prime drag-free modenot used on orbit due tounacceptableaccelerometer biases.(20 nN on proof mass)

• Note: data shown isearly on-orbit testsequence; does notrepresent finalperformance.

0 20 40 60 80 100 120

-10

0

10

Science Gyro PositionPo

sitio

n (n

m)

0 20 40 60 80 100 120-100

-50

0

50

100Science Gyro Control Effort

Forc

e (n

N)

0 20 40 60 80 100 120-5

0

5Space Vehicle Thrust

Minutes Since 08/22/2005 18:05

Forc

e (m

N)

Vehicle XVehicle YVehicle Z

Vehicle XVehicle YVehicle Z

Vehicle XVehicle YVehicle Z

Drag free off

Drag free on

3535

Overall Drag-free Performance

• Performance at4x10-12 g levelbetween 0.01 mHzand 10 mHz in inertialspace.

• Suppression ofgravity gradientacceleration by afactor of ~10,000.

• Meets needs ofexperiment tominimize supportforces on gyroscope.

10-4

10-3

10-2

10-1

100

10-11

10-10

10-9

10-8

10-7

10-6

Frequency (Hz)

Control effort, m.sec

-2

Gyro 1 - Space Vehicle and Gyro Ctrl Effort - Inertial space (2005/200, 14 days)

Z Gyro CE

Z SV CE

2!OrbitGravity Gradient

Residual gyroacceleration

3636

Dewar

Dewar

3737

Dewar control plots

• The ATC systemmaintains a constantdewar temperature of~1.83 K by controllingthe flow rate of heliumboil-off from the dewar

3838

Dewar Heat Pulse

• The temperaturecontrol behavior isobservable during aheat pulse test (usedto estimate theamount of heliumremaining in thedewar)

3939

Lesson: Accelerometer biases Bends OrbitEf

fect

ive

SV Z

-axi

s th

rust

(mN

)

• Accelerometer biasdue to small patchcharges generatedrag-free force biason vehicle.

• Bias identified viaGPS and laserranging; removed viaparameter update.

• Necessitated backupdrag-free operation tocompensate forbiases; no ability tocompensate in primedrag-free.

4040

Lesson: Dewar slosh mode coupling

• Dewar slosh mode resonance peakpumped by drag-free controller

• Managed via drag-free loop gain settingand crossover phase margin.

• Simple fluid wave model predictsperiod with high accuracy:

2slosh

roll

l rT

v ad

rT

d

!= =

=

Periods:100 to 200 sec

(without the addition of SVroll period, 77.5 sec)

v ad=

2

rolla r!=

d

r

Wave

Velocity

4141

Slosh Mode Evolution

Peak at 1.2x10-7 g 217 sec period

4242

Conclusions

• 100marcsec/√ Hz pointing Achieved– 5marcsec at roll frequency

• Roll phase controlled to