LISA Pathfinder Coldgas Thrusters

Transcription

1 LISA Pathfinder Coldgas Thrusters Joseph Martino/Eric Plagnol - LPF collaboration Lisa Symposium September 2016 Zurich

2 Outline System Description External Disturbances and thruster noise In Flight dedicated experiment Conclusion

3 MicroNewton Coldgas Thrusters 2 micro-propulsion systems : Coldgas - Colloidal Description X-axis Y-axis thrusters ~10-500μN nominal mission Truster = Mass Flow Sensor connected to a piezo controlled valve 4 High Pressure N2 Gas Tanks Z axis

4 MicroNewton Coldgas Thrusters 2 micro-propulsion systems : Coldgas - Colloidal Description X-axis Y-axis thrusters ~10-500μN nominal mission Truster = Mass Flow Sensor connected to a piezo controlled valve 4 High Pressure N2 Gas Tanks Z axis 2% of the total Mass : 10kg 30% already used 3kg used during 9 months deltag driven emptying strategy

5 Drag Free Scheme 2 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1

6 Drag Free Scheme 2 1 o1 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1/ Read out TM1 position with the interferometer o1

7 Drag Free Scheme DFACS 2 1 o1 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1/ Read out TM1 position with the interferometer o1 2/ Feed the DFACS with the information

8 Drag Free Scheme DFACS 2 1 o1 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1/ Read out TM1 position with the interferometer o1 2/ Feed the DFACS with the information 3/ Actuate thrusters to nullout o1 with an authority set by the DFACS

9 Drag Free Scheme DFACS 2 1 o1 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1/ Read out TM1 position with the interferometer o1 2/ Feed the DFACS with the information 3/ Actuate thrusters to nullout o1 with an authority set by the DFACS The thrusters commanded Forces are a measurement of disturbances on the satellite

10 Drag Free Scheme DFACS 2 1 o1 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1/ Read out TM1 position with the interferometer o1 2/ Feed the DFACS with the information 3/ Actuate thrusters to nullout o1 with an authority set by the DFACS The thrusters commanded Forces are a measurement of disturbances on the satellite External Disturbances are reduced by the DFACS authority

11 Drag Free Scheme DFACS 2 1 o1 Attitude Control : sun orientation / earth communication Drag Free control (x y z and theta) : (i.e. : Satellite follow test Mass 1 on X) Goal : reduce external disturbances 1/ Read out TM1 position with the interferometer o1 2/ Feed the DFACS with the information 3/ Actuate thrusters to nullout o1 with an authority set by the DFACS See poster by Henry Inchauspe The thrusters commanded Forces are a measurement of disturbances on the satellite External Disturbances are reduced by the DFACS authority

12 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite

13 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite Commanded Force on Z Commanded thrusts = external disturbances

14 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite Commanded Force on Z 23.5uN Commanded thrusts = external disturbances

15 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite Commanded Force on Z 23.5uN Commanded thrusts = external disturbances => Constrain the mean Thrusts value at ~10μN per thrusters (thrusters geometry)

16 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite Commanded Force on Z Commanded Force on X and Y 23.5uN Commanded thrusts = external disturbances => Constrain the mean Thrusts value at ~10μN per thrusters (thrusters geometry)

17 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite Commanded Force on Z Commanded Force on X and Y AC contribution Events Thruster Noise Micro-meteorites Solar radiation pressure Magnetic Field Solar Wind Protons Daniel Hollington See poster by Ira Thorpe 23.5uN Commanded thrusts = external disturbances

18 External Disturbances DC contribution Solar Radiation Pressure (mainly Z-axis) Thermal IR of the satellite (mainly Z-axis) Self Gravity of the Satellite Commanded Force on Z Commanded Force on X and Y AC contribution Events Thruster Noise Micro-meteorites Solar radiation pressure Magnetic Field Solar Wind Protons Daniel Hollington See poster by Ira Thorpe 23.5uN Commanded thrusts = external disturbances

19 Thrusters noise Hypothesis : Main external disturbance = Thruster Noise Preliminary TM position readout noise Presence of Lines (also in the laser pump current ) We fit a 2 components noise model white noise + 1/f

20 Thrusters noise Hypothesis : Main external disturbance = Thruster Noise Preliminary TM position readout noise Presence of Lines (also in the laser pump current ) We fit a 2 components noise model white noise + 1/f LPF in Flights measurements = 0.13μN/sqrt(Hz) down to 0.2mHz

21 Thrusters noise Hypothesis : Main external disturbance = Thruster Noise Preliminary TM position readout noise Presence of Lines (also in the laser pump current ) We fit a 2 components noise model white noise + 1/f LPF in Flights measurements = 0.13μN/sqrt(Hz) down to 0.2mHz On Ground Gaia measurements = 0.1μN/sqrt(Hz) limited at 2mHz by the test bench

22 Thrusters noise Hypothesis : Main external disturbance = Thruster Noise Preliminary TM position readout noise Presence of Lines (also in the laser pump current ) We fit a 2 components noise model white noise + 1/f LPF in Flights measurements = 0.13μN/sqrt(Hz) down to 0.2mHz On Ground Gaia measurements = 0.1μN/sqrt(Hz) limited at 2mHz by the test bench Upper Limit

23 Thrusters noise Hypothesis : Main external disturbance = Thruster Noise Preliminary TM position readout noise Presence of Lines (also in the laser pump current ) We fit a 2 components noise model white noise + 1/f LPF in Flights measurements = 0.13μN/sqrt(Hz) down to 0.2mHz On Ground Gaia measurements = 0.1μN/sqrt(Hz) limited at 2mHz by the test bench Upper Limit A part of the 1/f could be due to the attitude control

24 Thrusters noise long term Thruster 1 to 6 white noise evolution Thrusters white noise is decreasing Thrusters 1 and 2 are less noisy. Noise is correlated 0.5 to 0.8 Preliminary Nominal Mission : March - June

25 Thrusters noise long term Thruster 1 to 6 white noise evolution Thrusters white noise is decreasing Thrusters 1 and 2 are less noisy. Noise is correlated 0.5 to 0.8 Preliminary Nominal Mission : March - June This time evolution could indicate that we measure another external noise source. Temperature related?

26 Thrusters noise long term Thruster 1 to 6 white noise evolution Thrusters white noise is decreasing Thrusters 1 and 2 are less noisy. Noise is correlated 0.5 to 0.8 Preliminary Nominal Mission : March - June This time evolution could indicate that we measure another external noise source. Temperature related? Or a common thrusters noise source drifting with time. They share the same electronic/coldgas feed line.

27 Thrusters Impact on differential acceleration Thruster Noise contribute to Jittering of the Spacecraft Preliminary

28 Thrusters Impact on differential acceleration Thruster Noise contribute to Jittering of the Spacecraft Preliminary Misalignment between TM and Optical Bench + motion of the SC See poster by Gudrun Wanner

29 Thrusters Impact on differential acceleration Thruster Noise contribute to Jittering of the Spacecraft Preliminary Misalignment between TM and Optical Bench + motion of the SC Projection of Stifness coupling for LISA with LPF controller On LPF this noise is coherent for TM1 and TM2 See poster by Gudrun Wanner

30 Thrusters Impact on differential acceleration Thruster Noise contribute to Jittering of the Spacecraft Preliminary Misalignment between TM and Optical Bench + motion of the SC Projection of Stifness coupling for LISA with LPF controller On LPF this noise is coherent for TM1 and TM2 See poster by Gudrun Wanner Quasi DC drift due to Propellant depletion See Poster by Valerio Ferroni

31 Thrusters dedicated investigation 6 injected forces on each thruster between 20mHz -30mHz 2 Inertial Sensors 1 Common analysis architecture with DRS/colloidal

32 Thrusters dedicated investigation 6 injected forces on each thruster between 20mHz -30mHz 2 Induce large motion of the spacecraft : Stiffness + electrostatic forces negligible Inertial Sensors 1 Common analysis architecture with DRS/colloidal

33 Thrusters dedicated investigation 6 injected forces on each thruster between 20mHz -30mHz Inertial Sensors 2 1 Induce large motion of the spacecraft : Stiffness + electrostatic forces negligible Reconstruct the motion of the SC with the inertial Sensors of TM1 Common analysis architecture with DRS/colloidal

34 Thrusters dedicated investigation 6 injected forces on each thruster between 20mHz -30mHz Inertial Sensors 2 1 Induce large motion of the spacecraft : Stiffness + electrostatic forces negligible Reconstruct the motion of the SC with the inertial Sensors of TM1 Fits a model using the commanded forces with Gains Delays and Center Of Mass combining all the Degrees of Freedom at every frequencies Common analysis architecture with DRS/colloidal

35 Thrusters dedicated investigation 6 injected forces on each thruster between 20mHz -30mHz Inertial Sensors 2 1 Induce large motion of the spacecraft : Stiffness + electrostatic forces negligible Reconstruct the motion of the SC with the inertial Sensors of TM1 Fits a model using the commanded forces with Gains Delays and Center Of Mass combining all the Degrees of Freedom at every frequencies Common analysis architecture with DRS/colloidal See Poster by Jacob Slusky

36 Thrusters dedicated investigation Motion on X Motion on X seen by the Inertial Sensors Preliminary Motions due to the thrusters at 6 different frequencies

37 Thrusters dedicated investigation Motion on X We subtract the thrusters commanded forces model Preliminary Residual after subtraction = external disturbances + Systematics

38 Thrusters dedicated investigation Motion on X We compare the same model during a noise run Preliminary external disturbances during a noise run external disturbances + Systematics

39 Thrusters dedicated investigation Motion on Theta Motion on Theta seen by the Inertial Sensors Preliminary external disturbances during a a noise run run external disturbances + Systematics +

40 Thrusters dedicated investigation Motion on Theta Motion on Theta seen by the Inertial Sensors Preliminary Systematics external disturbances during a a noise run run external disturbances + Systematics +

41 Thrusters dedicated investigation results Main results Limits Next Gains = 0.91 and 1 - Thruster 4 off by 8.3% Center Of Mass seems offset by ~ 4 cm in Z Limited by measurements systematics - Set the estimation errors to a few percent. Model is not complete - Moment Of Inertia - Cross Sensing - thrusters position Consolidate some geometrical parameters like housing position This experiments = Calibration of the thrusters against IS Repeat the experiment during Acceleration Mode (= TM follow SC) to calibrate against Electrostatic Forces/Torques.

42 Conclusion Thruster Noise measured at 0.13μN/sqrt(Hz) - noise is flat down to 0.2mHz We extent the frequency range characterization of more than an order of magnitude compare to the on-ground measurements Set an upper limit on the 1/f and the white noise part. This white noise measurements is 30 % higher than the on-ground measurements (0.1μN/sqrt(Hz)) But the thruster noise is decreasing with time -> could be another external disturbance Thrusters Gains =1 within percent accuracy except thruster4 ~ 8.3% off

43 Conclusion Thruster Noise measured at 0.13μN/sqrt(Hz) - noise is flat down to 0.2mHz We extent the frequency range characterization of more than an order of magnitude compare to the on-ground measurements Set an upper limit on the 1/f and the white noise part. This white noise measurements is 30 % higher than the on-ground measurements (0.1μN/sqrt(Hz)) But the thruster noise is decreasing with time -> could be another external disturbance Thrusters Gains =1 within percent accuracy except thruster4 ~ 8.3% off Cold gas micronewton are good for LISA => with the same controller performances