GP-B Attitude and Translation Control. John Mester Stanford University

Transcription

1 GP-B Attitude and Translation Control John Mester Stanford University 1

2 The GP-B Challenge Gyroscope (G) 10 7 times better than best 'modeled' inertial navigation gyros Telescope (T) 10 3 times better than best prior star trackers G T <1 marc-s subtraction within pointing range Space Gyro Basis for 10 7 advance in gyro performance - reduced support force, "drag-free" - roll about line of sight to star - sphericity and mass unbalance requirements: 10nm achieved 2

3 ATC Requirements Pointing Requirement 110marcsec/ Hz (Guide Star Valid) pitch and yaw Roll Requirement 40 arcsec at roll rate Acceleration Requirement <10 11 g crass track average => Active Control of all 6 dof of Spacecraft 3

4 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 4

5 Block Diagram Command Generator Attitude and Translation Control Software + - Attitude Control Translation Control Dewar Temp Control + - Magnetic Torquer Distribution Thruster Distribution Orientation Sensors Primary Telescope Star Trackers Backup Magnetometers Coarse Sun Sensors Sensors Rate Sensor Control Gyroscopes Dewar Press & Temp Sensors Science Gyro Suspension System Magnetic Torque Rods Actuators Thrusters Vehicle Dyanmics + + GP-B Spacecraft Dewar Thermodynamics 5

6 ATC System Hardware Sensors Actuators Primary Sensors Attitude Telescope Star Tracker Rate Gyroscopes Translation - One of the four science gyro suspension systems Primary Attitude and Translation Control Actuators - 16* Proportional Micro Thrusters Backup Attitude Sensors Magnetometers Coarse Sun Sensors Backup Translation Sensors - Three more science gyroscopes Backup Attitude Actuators - Magnetic torque rods 6

7 The Overall Space Vehicle 16 Helium gas thrusters, 0-10 mn ea, for fine 6 DOF control. Roll star sensors for fine pointing. Magnetometers for coarse attitude determination. Tertiary sun sensors for very coarse attitude determination. Magnetic torque rods for coarse orientation control. Mass trim to tune moments of inertia. Dual transponders for TDRSS and ground station communications. Stanford-modified GPS receiver for precise positioning. Laser ranging corner cube cross-checks GPS. Redundant spacecraft processors, transponders. 7

8 Control Gyroscopes Control Gyroscopes Two gyro assemblies - one used, one backup Gyro mounting platform thermal distortion had a big impact on the control system performance 8

9 Star Trackers Star Trackers Two star trackers - one primary, one backup 8 deg field of view Mounted at angles 10 degrees apart - different groups of stars visible 9

10 Magnetometers Magnetometers Measure vehicle orientation by comparing magnetic field reading with expected field Expected field depends on orbital position which is propagated onboard and updated from ground 10

11 Coarse Sun Sensors Fwd Sun Sensors Backup coarse orientation sensors Rough estimate of vehicle orientation Only used if star trackers and magnetometers fail Aft Sun Sensor 11

12 Thrusters Vehicle has a nominal mass flow rate of ~8 mg/s Maximum thruster force: 8mN Thrusters 12

13 Magnetic Torque Rods 3 Orthogonal torque rods Two axis control depending on magnetic field geometry Orbit position propagated onboard and updated with ground processed GPS data Thruster failure backup Used during science to increase fuel margin Still used today to control satellite to ~5 deg 13

14 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) 14

15 Single Axis Pointing Error The attitude control goes through three different phases each orbit: 1) Guide Star Invalid Guide star is blocked by Earth 2) Guide Star Capture Guide star is re-centered in the telescope field of view 3) Guide Star Valid Nominal telescope pointing Pointing Angle (marcsec) 15,000 10,000 5, ,000 10,000 North-South Pointing vs Time (11/10/04) Vehicle in Solar Eclipse 2) Guide Star Capture 1) Guide Star Invalid 3) Guide Star Valid -15, Time Since Guide Star Invalid Start (min) 15

16 Target Plot Nominal guide star capture times range from 30 to 90 seconds The RMS vehicle pointing is controlled to less than 320 marcsec (1.55 microrad) Spacecraft Pointing (3/25/05) 3000 North-South Pointing (marcsec) RMS Pointing = 260 marcsec 3600 marcsec East-West Pointing (marc-sec) 16

17 Proton Monitor Data Proton Monitor Hits 17

18 Telescope Detector Data Particle hits also affected the telescope detectors A plot of the number of hits in the telescope clearly shows the South Atlantic Anomaly region Telescope Detector Proton Hits 18

19 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 Angle (marc-sec) Vehicle X-Axis Attitude Error (5/9/05) Time Since Guide Star Valid Start (min) Percentage of Telescope Data Corrupted by Proton Activity 100 Percent Time Since Guide Star Valid Start (min) 19

20 Roll Rate Plot 400 Vehicle X-Axis Pointing Total RMS Pointing Roll Rate Component Mission-long pointing performance Shaded Regions - Roll Rate Notch Filter Turned ON Amplitude (marcsec) Orbit Number 20

21 Pitch/Yaw Attitude Dither Dither: Slow 30 marc-s calibration oscillations injected into pointing system { Vehicle pitch/yaw dither gyro output telescope output Scale factors matched for accurate subtraction Once telescope/gyroscope scale factor is known, vehicle motion can be subtracted out of the gyroscope signal 21

22 Roll Phase Control There are two star trackers onboard the GP-B satellite. Field-of-view: 8 o x8 o Boresight axes are 50 o and 60 o away from the satellite roll axis, out of phase by 180 o Two 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 ARP Close-loop control of the roll phase is implemented with 16 proportional cold gas thrusters by the Attitude and Translation Control (ATC) system. 22

23 Star Tracked Fields of View Red = Star Tracker B Field of View Blue = Star Tracker A Field of View 23

24 Roll Angle Plot Angle (arc-sec) Roll Phase Error (3/25/05) 40 arcsec The control gyroscopes and star trackers are used to control the spacecraft roll angle to an error less than 40 arcsec at a constant roll period of 77.5 sec Time Since Guide Star Valid Start (min) 24

25 Drag-free Satellite Technology Transit deployed on orbit 1959 George Pugh envisioned a tender satellite for first proposed test of General Relativity Ben Lange (Stanford) provided first detailed study of issues surrounding drag-free satellites Flight of a Disturbance Compensation System (DISCOS) on Transit I (experimental US Navy navigation satellite) 2004 Flight of Gravity Probe B Transit pre-launch 25

26 Drag Free Concept Control Spacecraft to follow an inertial sensor Reduce disturbances in measurement band Aerodynamic drag Magnetic torques Gravity Gradient torques Radiation Pressure Spacecraft Follows a purely Gravitational Orbit 26

27 Drag Free History 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 year DISCOS - Disturbance Compensation System - Stanford 3 axis translation control And Now Also GP-B 3 axis translation control 3 axis attitude control 27

28 DISCOS Performance Electrostatic Sensing of Proof Mass Pressurized gas On-Off Thrusters 3 Axis Translation Control Acceleration levels were below 5 x g averaged over 3 days limited by tracking data and earth gravity model 28

29 Propellant Source: Helium Boil-off 300K ambient Superfluid Helium 2400L Heat Leak 125mW Porous Plug Superfluid phase separator Wick 1.8 K Pyro vent valve Isolation Valves (TIV) - 16 Thrusters - 16 Superfluid phase separator porous plug permits gaseous Helium to leave main tank while keeping superfluid He inside. Mass Flow: 4-16 mg s 1 over plug temperatures of 1.6K to 2.0K without choke or breakthrough. Provides supply pressure of 660 to 2300 Pa (5 to 17.5 torr) to thrusters. 2.8K Each thruster is provided a dedicated isolation valve (TIV) to disable unit if a failure occurs. 29

30 Flight Proportional Thruster Design Thrust: 0 10 mn I SP : 130 sec Mdot: 6-7 mg s 1 Scale factor variation: 6% Bias variation: Noise: 25 µn Hz 1/2 0.2 mn 3.5mm dia Operates under choked flow conditions Pressure FB loop makes thrust independent of unit temperature Redundant voice coils From Dewar and manifold Supply: 5 to 17.5 torr Re ~10 Laminar flow! 30

31 Thruster Arrangement on Vehicle Guide Star m 1.19 m 2 SV Forward 7 Thruster Cluster Roll Axis 2.51 m 1.9 m 11 +Z +Y Space Vehicle Body Axes +X G1 SV Aft 23cm cm G m 1.19 m Thrusters are arranged in 4 clusters of 4 units each. Each cluster has thrust authority along 3 axis. Arrangement chosen to be robust to thruster failures. Provides 3DOF of attitude and 3DOF of translation control for the space vehicle. 31

32 Prime Drag-free Operation In prime drag-free, the ATC system flies the SV around the position of the proof 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 Translation Control Gravity Gradient feed forward Ext Distur - bances Resonance Modes + SV Translation PID Controller SV Ctrl Effort + + U 1 Ms 2 + R r Vehicle dynamics K2 K1 - Position Bridge 1 ms GSS Ctrl Gyro Suspension Effort Controller 2 + ( r! R) SV Position Gyro Position + R - + r ( r! R) Gyro Suspension Ext Disturbances Gyroscope dynamics Space Vehicle Dynamics 32

33 Backup Drag-free Operation In suspended (backup) drag-free, the ATC system nulls the gyro suspension control effort. Advantage: Gyro always suspended; 2) Accelerometer biases can be removed. Disadvantage: Suspension forces and torques reduced to preload minimums. Accelerometer bias compensation Translation Control + Gravity Gradient feed forward SV Translation PID Controller SV Ctrl Effort + + U Ext Distur - bances Resonance Modes 1 Ms 2 Vehicle dynamics + R r K2 K 1 - Gyro Suspension Controller GSS Ctrl Effort u Position Bridge + ( r! R) 1 ms 2 SV Position Gyro Position + R - + r ( r! R) Gyro Suspension Ext Disturbances Gyroscope dynamics Space Vehicle Dynamics 33

34 Example Drag-free Transitions Transition sequence: Position (nm) Science Gyro Position Drag free on Vehicle X Vehicle Y Vehicle Z 1. Backup drag-free 2. Prime drag-free 3. Non drag-free Force (nn) Science Gyro Control Effort Vehicle X 1. Prime drag-free mode Vehicle Y Vehicle Z not used on orbit due to unacceptable accelerometer biases. (20 nn on proof mass) Force (mn) Vehicle X Vehicle Y Vehicle Z Drag free off Space Vehicle Thrust Minutes Since 08/22/ :05 Note: data shown is early on-orbit test sequence; does not represent final performance. 34

35 Overall Drag-free Performance c e s. m, t r o -2 f f e l o r t n o C Gyro 1 - Space Vehicle and Gyro Ctrl Effort - Inertial space (2005/200, 14 days) Gravity Gradient Residual gyro acceleration Z Gyro CE Z SV CE 2!Orbit Frequency (Hz) Performance at 4x10-12 g level between 0.01 mhz and 10 mhz in inertial space. Suppression of gravity gradient acceleration by a factor of ~10,000. Meets needs of experiment to minimize support forces on gyroscope. 35

36 Dewar Dewar 36

37 Dewar control plots The ATC system maintains a constant dewar temperature of ~1.83 K by controlling the flow rate of helium boil-off from the dewar 37

38 Dewar Heat Pulse The temperature control behavior is observable during a heat pulse test (used to estimate the amount of helium remaining in the dewar) 38

39 Lesson: Accelerometer biases Bends Orbit Effective SV Z-axis thrust (mn) Accelerometer bias due to small patch charges generate drag-free force bias on vehicle. Bias identified via GPS and laser ranging; removed via parameter update. Necessitated backup drag-free operation to compensate for biases; no ability to compensate in prime drag-free. 39

40 Lesson: Dewar slosh mode coupling Dewar slosh mode resonance peak pumped by drag-free controller Managed via drag-free loop gain setting and crossover phase margin. Simple fluid wave model predicts period with high accuracy: Wave Velocity v = ad r d a =! 2 roll r T slosh = l 2 r v =! ad = T r roll d Periods: 100 to 200 sec (without the addition of SV roll period, 77.5 sec) 40

41 Slosh Mode Evolution Peak at 1.2x10-7 g 217 sec period 41

42 Conclusions 100marcsec/ Hz pointing Achieved 5marcsec at roll frequency Roll phase controlled to <40arcsec Drag-free performance on orbit established at the 4x10-12 g level. Reduced the gravity gradient accelerations on the proof mass by a factor of ~10,000. Proportional cold gas thrusters fueled from Helium boil-off worked well to control the space vehicle in 6DOF. Both prime and backup drag-free modes were demonstrated on orbit. Accelerometer biases were corrected in backup drag-free mode to keep the orbit constant. 6 Degree of Freedom Control Works 42