A fast rotating differential accelerometer for fundamental physics and possible applications. Anna Nobili, for the GG collaboration

Transcription

1 A fast rotating differential accelerometer for fundamental physics and possible applications Anna Nobili, for the GG collaboration Strumentazione per la Geofsica, Pisa 16 Novembre 2007

2 Galileo Galilei-GG GG Goal: test the UFF (WEP) to m 250 kg total mass circular, equatorial orbit at 520 km passive satellite stabilization by 1-axis rotation at 2 Hz room temperature (capacitance read out) partial along track drag compensation with electric thrusters VEGA launch from Kourou ground operation from ASI station in Malindi, Kenia GG in the National Space Plan of Italian Space Agency (ASI) in 2006 Phase A-2 scientific & industrial Study approved by ASI in 2007 (to start soon) GGon the Ground-GGG laboratory prototype currently funded by INFN

3 GG: making high frequency signal modulation possible test cylinders weakly coupled to form a 2D differential accelerometer rotation around symmetry axis EP violation vector not affected by rotation/modulation (no forcing at the rotation frequency) rotation frequency can be much much higher than the coupling frequency of test masses (by 3 orders of magnitude in GG) and yet the violation signal is not attenuated!!!

4 Advantage of high frequency modulation Advantage of fast spin in a space test of the equivalence principle is apparent! Relative displacements of GG/GGG test cylinders are read by 2 co-rotating capacitance bridges. In GGG the analog output of each bridge is digitized 32 times per turn. Plot shows, as function of spin frequency, noise of readout electronics alone located inside a chamber at 35 ± 0.1 C. The noise of digital part was measured for several days, sampling at 32 times per spin period (up to 3 Hz). The noise of the analog part was measured with the spectrum analyzer. Curve shows the sum of the two (2007). At 2 Hz GG nominal spin, measured noise of m/ Hz close to the GG requirement (with a weak coupling frequency of the test masses of Hz in absence of weight)

5 Reduction of thermal noise at supercritical rotation Ultimately experiment will be limited by thermal noise a = 4K Tω mq th B n Hz (signal at orbital frequency: Hz ) (T/m): room temperature experiment, but large test masses (10 kg each) ok! ω 3 = 2 π( Hz) weak natural coupling frequency n Question: how can you have high Q at this low natural frequency? Answer: if the system spins faster than natural frequency, losses occur at spin frequency (not at natural frequency!), which means much smaller losses ( much higher Q) very good!

6 Q measurements Q measurements with GGG laboratory prototype (test masses of 10 kg each, as proposed for GG space experiment)

7 Sketch of GGG differential accelerometer 1 m size vacuum chamber 2 weakly coupled test cylinders 10 kg each In essence, a vertical beam balance with concentric test masses

8 GGG: experimental confirmation of theory (I) The normal modes Comandi et al., Rev. Sc. Instr. (2006)

9 GGG: experimental confirmation of theory (II) Self centering of test masses in supercritical regime (i.e. after resonance crossing)

10 GGG: experimental confirmation of theory (III) Self centering of test masses in supercritical regime: well defined equilibrium position of test masses determined by physical laws (unambiguous determination of the zero of the read out!) Nobili et al., IJMPD (Q2C 1) in press

11 Uniformity of accelerometer rotation at increasing spin frequency Hz Aug Hz Sept-Oct Hz In increasing the rotation speed of the rotor by a factor 9.5 its spin energy increases by almost 2 orders of magnitude, and yet the rotation noise has decreased by about 1 order of magnitude

12 Relative displacements of rotating test cylinders: raw data (July( 2005) ν = spin 014. Hz July 2005: several days of raw data as taken by the rotating capacitance bridges measuring the relative displacements of the test cylinders in the horizontal plane of the rotor. The black curves are the average of the same data over 5 periods of whirl; the center of the whirl orbit is the position of equilibrium of the test masses relative to each other which is affected by external forces, such as that of an Equivalence Principle violation

13 Relative displacements of rotating test cylinders: raw data (September( 2006) ν = spin 014. Hz September 2006: same as in previous plot (see previous caption). The raw data have improved from 100 µm to 20 µm peak-to-peak relative displacements. More importantly, the average over 5 whirl periods (black curves), i.e. the position of relative equilibrium of the test cylinders, is now much more stable in time. NOTE: The advantage of high frequency modulation (provided by rotation) will become apparent only after demodulation, i.e. after transformation to the non rotating horizontal plane of the laboratory

14 Relative displacements of rotating test cylinders: raw data (September( 2007) ν = spin 133. Hz

15 Whirl control of all 3 natural frequencies of the system A rotation speed of 1.33 Hz is supercritical for all 3 natural frequencies of the GGG system. They therefore develop whirl motions which are damped by small capacitance sesnros/actuator ( whirl control ) natural diff. freq Hz natural common mode freqs. ν = spin 133. Hz 2 ν spin

16 Temperature compensated active tilt control PC (LabView) RS232 PICO 104 T tilt PT100 Sensors Trotor PID with auto-tuning NI DAQ AO To Piezo Actuators AI From Tilt meters (x,y) Sensor signal Conditioning Low pass digital filters Temperature correction T(tilt), T(rotor) Drift Correction DC off-set (Data Storage for post processing)

17 Relative displacements in the non rotating frame (PSD)

18 Where do we stand? From the best experiment run (red curve; from 5 data sets lasting 3.5 days each) the result is Hz (signal freq.in space): x 10 m 12 (target of space experiment is in 1 week integration time) m a ms η Hz (signal freq on ground): space 8 x 10 m a ms η ground ground

19 ISA tiltmeter built (Iafolla et al. ) specifically for GGG (I) Direz. Sensibile X Direz. Sensibile Y I due sensori sono alloggiati all interno della scatola in modo da formare due direzioni sensibili ortogonali tra loro. All interno della scatola è alloggiata anche una scheda elettronica di acquisizione che fornisce 3 segnali analogici (X, Y e temperatura) attraverso un connettore a 9 pin 12 cm x12 cm x13.7cm

20 ISA tilt measurements at GGG lab (as( function of time) temporary mounting on 3 legs (in separate room, not on rotor; it can be easily improved) enclosed by small chamber with temperature stabilized (heating only) to about 0.2 C analog signal digitized and acquired with GGG 24-bit card (to check acquisition software)

21 ISA tilt measurements at GGG lab (PSD) temporary mounting on 3 legs (in separate room, not on rotor; it can be easily improved) enclosed by small chamber with temperature stabilized (heating only) to about 0.2 C analog signal digitized and acquired with GGG 24-bit card (to check acquisition software)

22 ISA tilt measurements at IFSI lab (PSD) performance at 10-5 Hz is 2 orders of magnitude worse than in the GGG lab (previous slide): it is clearly correlated to diurnal temperature variation (green curve), which in these measurements was not stabilized

23 Our expertise, available for possible applications high sensitive capacitance measurements of displacements/accelerations at ultra-low frequency noise reduction by high frequency signal modulation through supercritical rotation of sensor+read-out supercritical rotation of lab size weakly suspended masses suitable for small force measurements high Q (low dissipation) even at low frequency

24 Recent Papers Dynamical response of the GGG rotor to test the Equivalence Principle: theory, simulation and experiment. Part I: The normal modes, Comandi et al., Rev. Sc. Instr /1-15 (2006) Dynamical response of the GGG rotor to test the Equivalence Principle: theory, simulation and experiment Part II: The rejection of common mode forces, Comandi et al. Rev. Sc. Instr /1-10 (2006) Experimental validation of a high accuracy test of the Equivalence Principle with small satellite Galileo Galilei- GG, Nobili et al., Int. J. Mod. Phys. D 2007 in press Limitations to testing the Equivalence Principle with Satellite Laser Ranging, Nobili et al., Gen. Rel. & Grav., 2007 in press In preparation, based on measurements performed in 2006: Self centring of non rigid rotors for high accuracy tests of the Equivalence Principle, Comandi et al., GG/GGG Website: