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ISA (ITALIAN SPRING ACCELEROMETER):

ISA (ITALIAN SPRING ACCELEROMETER): AN ACCELEROMETER TO MEASURE THE INERTIAL ACCELERATIONS ACTING ON THE MPO. Iafolla V . and the ISA Team Valerio.iafolla@ifsi-roma.inaf.it. ISA team. IFSI (Istituto di Fisica dello Spazio Interplanetario), INAF, Rome Italy AAS_I

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ISA (ITALIAN SPRING ACCELEROMETER):

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  1. ISA (ITALIAN SPRING ACCELEROMETER): AN ACCELEROMETER TO MEASURE THE INERTIAL ACCELERATIONS ACTING ON THE MPO. Iafolla V. and the ISA Team Valerio.iafolla@ifsi-roma.inaf.it

  2. ISA team • IFSI (Istituto di Fisica dello Spazio Interplanetario), INAF, Rome Italy • AAS_I • Harvard-Smithsonian Center for Astrophysics Cambridge MA, USA • Sternberg Astronomical Institute, University of Moscow Russia • Space Research Centre, Warsaw Poland

  3. Acceleration due to the planet gravity field gradients Centrifugal acceleration Non–Gravitational accelerations Angular acceleration Coriolis acceleration Z ISA proof–mass Inside the MPOframe Y X Acceleration acting on ISA installed on a satellite The acceleration on a point P (ISA proof–mass) close to the MPOCOM is: = Planet gravity; = MPO angular rate: = MPO angular acceleration; = MPO–proof–mass vector;

  4. the solar radiation acceleration: • Mercury albedo acceleration: Istituto di Fisica dello Spazio Interplanetario Istituto Nazionale Di Astrofisica Key rôle of ISA accelerometer The main NGP acting on the MPO in Mercury’s thermal environment are: • Direct solar radiation pressure; • Mercury albedo (indirect radiation pressure); • Mercury infrared radiation; The more important in terms of the magnitude of the disturbing acceleration on the MPO orbit are: V. Iafolla

  5. Istituto di Fisica dello Spazio Interplanetario Istituto Nazionale Di Astrofisica Key rôle of ISA accelerometer Mercury’s albedo radiation pressure An order of magnitude estimate: ĀMer 0.12 represents the average Bond albedo RMer  2439 Km represents Mercury’s equatorial radius a  3389 km represents the MPO semi–major axis In a simplified approach the albedo acceleration has a component along both the radial and solar direction. V. Iafolla

  6. Istituto di Fisica dello Spazio Interplanetario Istituto Nazionale Di Astrofisica Reference Frame The GAUSS co-moving frame: R  radial component; T  transversal component; W  out-of-plane component; V. Iafolla

  7. 1 Orbital period ½ Orbital period 1/3 Orbital period 108 m/s2 100 No modelling of the solar radiation Solar Radiation: Radial acceleration 3 orbital periods  7 hours The perturbing effect at the orbital period (2.3 h) is about 2 orders–of–magnitude larger than the accelerometer accuracy of  108 m/s2

  8. 108 m/s2 100 Solar Radiation: Transversal acceleration 3 orbital periods  7 hours … the most important in the MPO orbit reconstruction … The perturbing effect at the orbital period (2.3 h) is about 2 orders–of–magnitude larger than the accelerometer accuracy of  108 m/s2 No modelling of the solar radiation

  9. 108 m/s2 Albedo: Radial acceleration 3 orbital periods  7 hours 1 Orbital period 2 ? The perturbing effect at the orbital period (2.3 h) is only a factor 2 larger than the accelerometer accuracy of  108 m/s2 No modelling of the albedo radiation

  10. Albedo: Transversal acceleration 108 m/s2 The perturbing effect at the orbital period (2.3 h) is smaller than the accelerometer accuracy of  108 m/s2 No modelling of the albedo radiation

  11. RSE total noise

  12. ISA accuracy requirement inside the frequency band

  13. K mr ISA General Description ISA mechanical oscillator and its equivalence with a linear harmonic oscillator: Al 5056 The accelerometer works at frequencies lower than the resonance frequency of the mechanical oscillator, where the transfer function between the acceleration of the sensitive mass and its displacement is:

  14. ISA General Description   2fp • The displacements of a proof–mass due to a perturbing acceleration are detected by means of capacitive transducers in a bridge configuration, followed by a low noise amplifier; • The capacitive bridge (CB) is biased at high frequency ( fp = 10 kHz ) so that accelerations at frequencies fs produce their unbalance; • At the output of the CB the signals are seen as a modulation of the bias voltage at the two side–bands f = fp  fs in the frequency domain;

  15. ISA Electrical and mechanical Parameters

  16. ISA General Description ISA calibration Electromechanical actuator Superimposing an alternate voltage to the constant voltage V

  17. Geophysical Measurements at the Istituto Nazionale di Fisica Nucleare (INFN) Gran Sasso Laboratory Seismic Noise Teleseismic (free oscillation of Earth) Solid tide of Earth

  18. Differential Accelerometer: Mechanical Arrangement 08/06/2006 Emiliano Fiorenza

  19. Pick Up System 08/06/2006 Emiliano Fiorenza

  20. Rejection: Electrical scheme 08/06/2006 Emiliano Fiorenza

  21. Seismic Noise Rejection

  22. Vibrational random noise on board the MPO inside the frequency band Micro-vibration random noise on board the MPO outside the frequency band

  23. ISA Microvibration noise ISA Block Diagram

  24. Z B Z Rotation axis ISAcom YB COM Y X B X ISA Positioning (XYZ) represents the MPO frame with origin in COM; (xyz) define the MPO position with respect to Mercury instantaneous orbital plane; X–axis, along the radial direction (Mercury–MPO radius vector); Z–axis, along the out–of–plane direction (perpendicular to the orbital plane); Y–axis, along the transversal direction in the along–track direction ( );

  25. comISA COM Z–sensitive axis X–sensitive axis Rotation axis Y–sensitive axis ISA Positioning This result suggest for the best configuration of the accelerometer a location with the three sensitive masses aligned along the rotation axis of the MPO, and with the com of the mass with sensitive axis along the rotation axis coincident with the com of the accelerometer as well as with the MPO one:

  26. ISA mechanical configuration

  27. ISA Positioning Angular rate and angular acceleration The angular rate and acceleration along the MPO axes are: where and are the nominal values: Satellite mean motion f = true anomaly where M represents the MPO mean anomaly around Mercury, and e  0.162 the MPO eccentricity;

  28. Solution for the gravitational (tide) and apparent accelerations: general case

  29. ISA Positioning In order to determine the constraints in the possible displacements along the three sensitive axes, we need to compare the previous formula with the accelerometer accuracy:

  30. Case A: com≡ COM Case B: com +20 cm ≡ COM

  31.  ±2 °C  ±12.5 °C  4 °C/Hz ISA thermal system overview One of the main characteristics for the accelerometer to be considered in the BepiColombo mission to Mercury is its thermal stability, i.e., the immunity of the accelerometer to temperature variations: over one orbital period (2.3 h) of the MPO; over one sidereal period (44 days) of Mercury; Random noise; In the actual version of the ISA accelerometer the thermal stability is: That is a temperature change of 1 degree produces a voltage output equivalent to:

  32. ISA thermal system overview GR=0,0067 COVER_BOX ENVIROMENT a=e=0.9 GR=0,0005 OVEN BOX ACCELEROMETER PAKAGES GL=0,3 QI=0.078W GR=0,008 CONTROL ELECTRONIC THERMAL INSULATOR GL=5 GL=0,003 QI = 3.7 W GR=0,0028 GL=0,8 PCB QI=0.88W OVEN_PCB GL=0.03 BOTTOM_BOX GL=3 GR=0,05 GL=0,5 GR=0,014 SPACECRAFT ISA Thermal Mathematical model

  33. ISA Error Budget: Pseudo Sinusoidal Contributions "ISA accelerometer onboard the Mercury Planetary Orbiter: error budget“ Celestial Mech Dyn Astr DOI 10.1007/s10569-006-9059-0 http://dx.doi.org/10.1007/s10569-006-9059-0

  34. ISA Error Budget: Random Contributions

  35. ISA PAYLOAD OVERVIEW ISA Detector Assy ISA Control Electronics

  36. ISA DETECTOR ASSY OVERVIEW Accelerometer Package

  37. ISA Simulator (simulink) ISA simulator diagram Mechanical Signal ISA Mechanical system Mechanical noise Output ISA Thermal system Thermal noise Input signal Electro-mechanical system Thermal system with active control Mechanical oscillator

  38. ISA Signal and noise in the radial direction ISA displacement: [4cm+/-5mm; 4cm+/-7mm; 20cm+/-15mm]

  39. Integration

  40. References Bertotti, B., Iess, L., Tortora, P., A test of general relativity using radio links with the Cassini spacecraft, Lett. to Nature 425, 2003; Fuligni, F., Iafolla, V., 1997. Measurement of small forces in the physics of gravitation and geophysics. Il Nuovo Cimento 20 C (5), 637–642; Fuligni, F., Iafolla, V., Milyukov, V., Nozzoli, S., 1997. Experimental gravitation and geophysics. Il Nuovo Cimento 20 C (5), 637–642; Iafolla, V., Nozzoli, S., Mandiello, A., 1998. High sensitive accelerometer for fundamental physics in space. Second Joint Meeting of the International Gravity Commission and the International Geoid Commission, Trieste, 7–12, September; Iafolla, V., Nozzoli, S., 2001. Italian spring accelerometer (ISA) a high sensitive accelerometer for ‘’BepiColombo‘’ ESA CORNERSTONE. Plan. Space Science, 49, 1609–1617; Iafolla, V., Lucchesi, D.M., Nozzoli, S., 2004. On the ISA accelerometer positioning inside the Mercury Planetary Orbiter. Plan. and Space Scien., in press. Milani, A, Vokrouhlicky, D., Villani, D., Bonanno, C., Rossi, A., Testing general relativity with the Bepicolombo radio science experiment, Phs. Rev. D 66, 2002; Milani A, Rossi, A., Villani, D., The BepiColombo Radio Science Simulations, Version 2, 11 April (2003);

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