STEREO Guidance & Control

Transcription

1 STEREO Guidance & Control J. Courtney Ray J. C. Ray 98/11/19 1

2 STEREO G&C Requirements Baseline System Software Some Analysis J. C. Ray 98/11/19 2

3 G&C Requirements - Drivers Spacecraft pointing - (3 σ) Roll Pitch/Yaw Knowledge: ± 20 arcsec ± 0.1 arcsec Control: ± 0.1 degree ± 20 arcsec Jitter: 30 arcsec RMS 1.5 arcsec (0.1 to TBD Hz) (with SCIP error signal, which is ± 0.1 arcsec ) Jitter is challenge Need high control bandwidth => High wheel torque Fast sampling rate Minimize disturbances Modern control techniques J. C. Ray 98/11/19 3

4 Interesting G&C Requirements Point LOS within 5 arcmin of sun for SCIP acquisition Requires good coalignment Nominal HGA pointing to 0.1 ; Maintain HGA pointing during thrusting; Complete autonomous thruster firings within 300 seconds Sets gimbal step frequency Need small impulse bit & small t In-flight HGA alignment cal? Momentum storage capacity > 4 days in operational mode Sizes wheel momentum Return from any attitude in < 12 minutes Thruster attitude control? May size wheel torque On-board orbit propagation for pointing HGA at Earth distance > 1e6 km Autonomous navigation? Solar pressure momentum bias within sun-angle limit Solar c.p. trim? J. C. Ray 98/11/19 4

5 STEREO Guidance & Control System Block Diagram DSADs IMU Attitude Interface Unit (AIU) Reaction Wheels Thrusters HGA Gimbal G&C 1553 Bus C&DH 1553 Bus to C&DH System Star Tracker Flight Computer LOS Error Signal G&C PS Inst C&DH Tel J. C. Ray 98/11/19 5

6 Baseline G&C Equipment Item Heritage Performance IMU NEAR HRG, 0.01 deg/hr ½ Star tracker TIMED 3 arc sec, 7.5 Mv stars Reaction Wheels NEAR Torque: Nm Momentum: 4 Nms Sun Sensors NEAR 0.5 deg quantization 0.25 deg accuracy AIU TIMED No Flight Computer TIMED Yes J. C. Ray 98/11/19 6

7 Inertial Measurement Unit (IMU) Supplier: Delco Electronics Gyros: Delco 130Y Hemispherical Resonator Gyros (HRG) Rate bias stability < deg/hr, over 16 hr ARW < 0.01 deg/hr 1/2 Redundancy: NEAR: redundant CPU, power; 4 gyros Cassini: single-string Projected P S (system function) = for mission life (4 gyro IMU) J. C. Ray 98/11/19 7

8 Star Tracker Supplier: Lockheed Martin Accuracy : 3 arcsec P/Y; 32 arcsec R (1σ) 7.5 Mv stars 8.8 square FOV Quaternion output Autonomous star ID within ~2 sec 5 Hz update, 1553 interface Flown on DS1, P59; to fly on TIMED, EO1, MAP, IMAGE,... J. C. Ray 98/11/19 8

9 Reaction Wheel Assembly (RWA) Supplier: Ithaco, Inc. (Type A) Characteristics: Brushless DC motor Bipolar tachometer Separate electronics, stacked to save weight & space Performance: Angular Momentum: 4 Nms (@ 5100 RPM) Torque: Nm Unbalance: static < 1.5 gm cm dynamic < 40 gm cm 2 Torque noise PSD: (Nm) 2 /Hz, 0.1 to 1 Hz Continuous operating life: > 4 years J. C. Ray 98/11/19 9

10 Sun Sensors (DSAD) Supplier: Adcole Digital Solar Attitude Detector (DSAD) system 5 detector heads, each measures 2-axis sun vector in ±64 FOV Accuracy: 0.5 deg quantization 0.25 deg bit transition-angle accuracy Flight proven, many times J. C. Ray 98/11/19 10

11 Guidance & Control Functional Block Diagram Thrusters T Spacecraft ω q Wheels T Dynamics DSADs IMU u th Thruster Control u wh Wheel Control ω q ω C q C SCIP q LOS Error Signal Safe Mode q G ωg ST Guidance & Navigation q q G q C s q G q ST ω Attitude Estimation J. C. Ray 98/11/19 11

12 G&C Software G&C software will be developed in Simulink Graphical tool for developing system models Runs in Matlab Used on TIMED Real-Time Workshop (RTW) used to automatically generate code: Testbed simulator FC AIU? System-level modules (e.g. Cmd, TLM, device handlers) may need to be hand-coded J. C. Ray 98/11/19 12

13 Simulink Model Top Level Title: /d0026/project/stereo/jcr/mdl1.eps Creator: MATLAB, The Mathworks, Inc. Preview: This EPS picture was not saved with a preview included in it. Comment: This EPS picture will print to a PostScript printer, but not to other types of printers. J. C. Ray 98/11/19 13

14 Simulink Model Dynamics Title: /d0026/project/stereo/jcr/mdl2.eps Creator: MATLAB, The Mathworks, Inc. Preview: This EPS picture was not saved with a preview included in it. Comment: This EPS picture will print to a PostScript printer, but not to other types of printers. J. C. Ray 98/11/19 14

15 Control Bandwidth (BW) Effects Response to Error Signal Noise 0.1 Response to Disturbances Frequency (Hz) J. C. Ray 98/11/ BW limited by: Wheel torque Sampling rate

16 RW Saturation Angle (deg) vs. Control Bandwidth 10 2 RW Tq = (NEAR) RW Tq = 0.14 (TRACE) RW Tq = 1.4 (MSX) Angle (deg) Saturation Angle: Pointing error at which RW reaches maximum torque, for the control gain producing given bandwith Bandwidth (Hz) J. C. Ray 98/11/19 16

17 Limit Cycling For Momentum Dumping H τ H θ + θ 0 θ 0 θ θ 0 θ = τ H 16 I J. C. Ray 98/11/19 17 H τ = α = RWA torque H is torque impulse bit, I = inertia For H = 0.02, α = 0.02, I=300 (SI units): θ = ~ 4 µrad, τ = 1

18 Momentum Bias Mode Possible for safe mode, or if y or z wheel fails - x wheel runs at large fraction of its max speed Other wheel(s) used to damp nutation Precession by thruster firings Degraded pointing accuracy - Stability dominated by nutation Accuracy limited by momentum precession Fuel for angular momentum precession: About 150 mgm/day for 1 deg/day precession (H=4 Nms, Isp=65 s) If x wheel fails - y & z RW control still possible 2-sided thruster limit cycle for x J. C. Ray 98/11/19 18

19 Redundancy Considerations Four Wheels Full capability if any one fails Enable wheel speed control to avoid troublesome frequencies Four Gyros Full capability if any one fails Lower noise if all four used Fine Sun Sensor In addition to, or in place of, coarse DSADs Enable mission pointing without LOS error signal J. C. Ray 98/11/19 19