Effects of Patch Potentials on the GP-B Performance
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Effects of Patch Potentials on the GP-B Performance Results and Lessons Learned Sasha Buchman, John Turneaure HEPL June 24 2009 & GP-B March 24 2009 Page 1 GP-B Performance 6606 Geodetic effect Roll averaged 1,000 to 2,500 marcs/yr MISALIGNMENT 1000 Apparent linear drift LINEAR DRIFTS about 500 marcs/yr Repeat events RESONANCE to 200 marcs 100 39.0 Frame dragging effect 10 marcsec/yr 1 1.00 Original GP-B requirement Drift rate (deg/hr) rate Drift 0.50 Tighter req. 1995 DESIGN 0.21 Single gyro expectation 0.12 4 Gyro expectation 0.1 0.08 Readout alone (4 gyros) 0.01 1 marcsec/yr = 3.2 × 10-11 deg/hr Page 2 Goals and Outline ¾ Identify and understand “anomalous effects” ¾ Identify single cause of effects if possible ¾ Establish physical base for data analysis ¾ Experimental Observations ¾ Ground Measurements ¾ Classification of Effects on GP-B ¾ Discussion of Effects ¾ Remaining Work ¾ Lessons Learned Page 3 The Relativity Mission Concept 1 GM α GI ⎡3R ⎤ dγ 3 Ω d ⎛ ⎞ ⎛ 1 ⎞ ⎛ ⎞ Ω =⎜γ +⎟ 2× 3()R v +⎜γ +1 +⎟ 2 3⎢ ⋅ω 2 ()e ⋅R − e ω⎥ = ⎜ ⎟ ⎝ 2c⎠ R ⎝ 4⎠c 2 R ⎣ R ⎦ γ 2⎝ Ω ⎠geodet ic γ , α - 1 Frame Dragging 1 dγ ⎛ marcs⎞ Geodetic Effect 2= 3. × − 104⎜ ⎟ Space-time curvature Rotating matter drags space-time γPage 4 ⎝ yr ⎠ de Sitter (1916) Pugh and Schiff (1959, 1960) Experimental Observations Coupling of rotor-fixed frame to the Gyro Suspension System (GSS) 3 Modulation at low spin ωSL and polhode ωP frequency in the ~ 1.5 Hz GSS band -7 1 ¾ ωSL=1.3 Hz mod. control effort: 30% of ~1.5×10 N ¾ ωSL=1.3Hz mod. position at : 40 nmpp; ~0.1% gap -8 2 ¾ ωP mod. z (telescope axis) bias at 80 Hz spin : 10 N -8 3 ¾ ωP mod. control effort at 80 Hz spin: ~10 N “Triangle” = dipole + ‘all’ harmonics 2 1 Page 5 Possible Causes of Enhanced Coupling Rotor-fixed Mechanisms Explain “30% rotor bumps” (103 of expected) 1. Rotor geometry a. Mass unbalance: ~10nm measured (3×10-4 gap) @ Smaller than observed coupling by ~ 103 b. Surface waviness: ~10nm measured (3×10-4 gap) @ Smaller than observed coupling by ~ 103 2. Trapped flux interacting with magnetic fields Three independent calculations @ Smaller than observed coupling by > 103 3. Non uniform potential of rotor surface @ Coupling consistent with ~50 mV – 100 mV patch effect modulating Ve ≈ 200 mV suspension voltages ¾ Variation of electric potential over the surface ¾ It can arise due to the polycrystalline structure Gyroscope ¾ It can be affected by presence of contaminants d ¾ Modeled as dipole layer ¾ Patch fields present on rotor and housing walls Housing ¾ Cause forces and torques between surfaces PageData 6 explained by patch potentials of ~50-100 mV on rotor and housing Ground Patch Effect Investigations SEM images of gyro (Nb) & electrode (Ti) coatings ¾ Pre-launch investigation ¾Contact potential differences ~ 0.1V - 1V ¾Patches mitigated/eliminated by grain size 0.1 μm << 32 μm rotor-electrode gap ¾Kelvin probe measurements on flat samples ¾ Post-launch ground investigations dipole + harmonics1 39 40 2 3 38 0.65 4 ¾Work function profile by UV photoemission 37 0.6 5 36 0.55 6 35 0.5 7 ¾Detailed analytical modeling 34 0.45 8 0.4 33 0.35 9 32 0.3 10 ¾Kelvin probe measurements 0.25 31 0.2 11 30 12 Kelvin probe scans Kelvin probe 29 13 28 14 27 15 26 16 25 17 24 18 23 22 20 19 21 Work function polar plot, UV photoemission ST-7, LISA, LPF, LIGO find: Page 7 Patch potentials 30 -100 mV Causes of Patch Potential Variations Gyroscope ¾ The rotor coating process can lead to variations in the patch potential ¾ Coating is the result of many layers ¾ Each layer covers about 2/3 of the rotor surface ¾ Coating is axi-symmetric, but varies with angle to deposition source ¾ Thickness variations ¾ Impurity variations ¾ Crystal structure variations ¾ Contaminants Housing ¾ All suspension electrodes coated with same axi-symmetric process ¾ Small variation from center to edge ¾ 6 separate depositions ¾ Ground plane coating ¾ Substantial variation expected due to coating process / angle ¾ 2 separate depositions ¾ Thickness variations ¾ Impurity variations ¾ Crystal structure variations ¾ Contaminants Page 8 ‘Eight’ Patch Potential Effects on GP-B ¾ Coupling to GSS Î Vpp 50-100 mV ¾ z axis force ¾ At zero frequency ¾ At polhode harmonics (mentioned) ¾ Torques ¾ Misalignment Î following talks ¾ Resonance Î following talks ¾ Dissipation mechanisms ¾ Spin-down ¾ Polhode damping ¾ Charge measurement bias Page 9 Patch Potential in Spherical Harmonics Two Approximations: A. Dipole only B. Harmonic expansion ¾VR: rotor potential, VH: housing potential ¾ Potentials are real 1 ∞ l ′′ ′′ V(,)Rθ φ = ∑∑RVY l,,, m(,) lθ m φ m * (Y1 , 0 0 , 0l=−0 m ) = l VR,, l− = m ( −1) VR,, l m 1 ∞ l V = −1 m V * ′′′′ ′′′′ H,, l− m ()H,, l m VH(,)θ φ = ∑∑HVY l,,, m(,) lθ m φ (Y1 , 0 0 , 0l=−0 m ) = l Statistical Model ¾ Spherical harmonics terms are uncorrelated ¾ Assume a power spectrum characterized by the parameter r ¾ Assume that VR,l,m and VH,l,m are both Gaussian and have the same σ * * * VV, 1RR , 0 , 1 , 0 * VV, 1HH , 0 , 1 , 0 RVV l,,,, m R l= m r HVV l,,,, m H l= m r Page 10 l l z-axis Force Effects at f = 0 Axial thrust towards guide star (z axis) for the GP-B mission ¾ Average at f = 0 0.6 Day #310 z Bias Calibration Day #212 z Bias Calibration ¾ Varying with VR Gyros Discharged 0.4 0.2 N) μ 0 -0.2 Day #432 Day Day z Bias Calibration #157 #180 Gyro 1 Drag-Free -0.4 Gyro 1 Drag-Free to End of Mission GP-B orbit Semi-Major axis -0.6 Day #360 z Bias Calibration Gyros Discharged 1.3 Effective Z-axis thrust ( Effective -0.8 Day #151 - Sept 18, 2004, 140 190 240 290 340 390 440 490 Gyro 3 polhode resonates 1.2 with orbit period Time (days from Day #1; Apr. 20, 2004) 1.1 Day #318 - March 4, 2005, Drag-free off for 5 days 1.0 Semi-Major Axis ( km + 7,017 km ) km 7,017 + km ( Axis Semi-Major 0.9 0 100 200 300 400 500 600 Page 11 Time (days from Day #1; Apr. 20, 2004) Polhode Modulation of z-axis Force plitude of Control Effort at PomAAxis)Harmonics :#1 (a lhode Gyro 10-7 83 s,001=), T33-131 #1y04 (da-31 20g 29uA Mar 7- #321y9 2005 (da1 s= 3,12), T23-3 10-8 1 s), T= 3,1226-424 #4y05 (daJun 19-21 20 10-9 10-10 Control Effort (N) 10-11 100 101 102 Polhode Harmonic Number Polhode Modulation ¾ Patch potential model gives the force for the pth harmonic of polhode ¾ Using the observations and the statistical model 2 l lmax ⎡ ⎤ ⎛ a ⎞ 4ε 0 VR,,,0 l d 0()− 0 γ −H,, V0 l l −1 l+1 Fz, p = ⎜ ⎟ cos(pω P t× ) ⎢ V()R,,,1 l d 0p 0()−γ l +R V,,,1 l 0p 0−() dγ ⋅()1 + ⎥ l d 2 ∑ − + ⎝ ⎠ ()Y1,(,)0 0 l=20 ⎣⎢ +l 1 ⋅ 2() l − 1 + 1 ⎦⎥ Page 12 z-axis Forces Due to Rotor Charge z- xis force: zero frequency averagea Vd 0 = 72 mV ¾ Model: Vd0 rotor - oushing dipole component ¾ Force FZ = 9 nN; VR = 10 mV 8π ε r 2 F = 0 VV 3 d 2 R0 d z- axis force: variation with rotor charging Vd 0 = 81 mV ¾ Model: Vd0 rotor - oushing dipole component dV dV Δ =F 8 nN; ΔV =R t Δ10mV; = R 0= . 12mV/day Z R dt dt 8π ε r 2 ΔF =0 ⋅VV Δ' ⋅ Z 3 d 2 R d0 Data explained by patch potentials of ~50 - 100 mV on rotor and housing Page 13 Misalignment Torque ¾ Torque model for given l and m = 0 C d C N= −VVt [d l ()β =] lt l()1 + ) V V β l 3 R, , l 0 H , , ld 0β 0 , 0 6 R, , l 0 H , , l 0 ¾ Statistical model 2 Magnitudeateof Drift R v s. AngleMsaliig of nmetn ~1800 4 Ct ⎛l() l+1 ⎞ N = VV ⎜ ⎟ β Gy 1or RH1,,,, 0 1∑ 0 r 3.5 6 l=1 ⎝ l ⎠ Gy 2or 3 Gy 3or Gy 4or Equivalent dipole field as a function r 2.5 2 1.5 (as/day) Rate Drift )darcsec/yate (a RtfirD 1 0.5 0 0 20. 0.4 0.6 0.8 1 enmglignAlais Mfoe et (dnegre)s Data explained by patch potentials ~50-100 mV on rotor and housing Page 14 Roll – Polhode Resonance Torque ¾ Torque model C d C l( + l1) N= −Vt V()()l d γ []d l β = − t V Vl () d γ l 3R, , l 1 H , , l 1 0 m ,P dβ 0 , 1 β =0 6 R, , l 1 H , , l 1 0 m ,P ¾ Statistical Model 2 C ~1800 ⎛l() l+1 dl ()γ ⎞ N= −VVt ⎜ 0,m P ⎟ RH1,,,, 0 1∑ 0 ⎜ r ⎟ 6 l= m ⎝ l ⎠ Equivalent dipole field as a function r rcseca 343 393 443 ) 2004. 20,rp; A #1yDa romf syad(Time Data explained by patch potentials ~30-150 mV on rotor and housing Page 15 Spin-Down Measurements Gyro 1 dfs/dt vs. Time Gyro 2 dfs/dt vs. Time ‐0.15 ‐0.12 ‐0.13 ‐0.16 ‐0.14 /dt (nHz/sec) ‐0.17 /dt (nHz/sec) s s ‐0.15 df df ‐0.18 ‐0.16 163 254 345 436 163 254 345 436 Time (days from Day #1; Apr.