Seismic Noise & Vibration Isolation Systems. AIGO Summer Workshop School of Physics, UWA Feb Mar. 2, 2010

Transcription

1 Seismic Noise & Vibration Isolation Systems AIGO Summer Workshop School of Physics, UWA Feb Mar. 2, 2010

2 Seismic noise Ground noise: X =α/f 2 ( m/ Hz) α: 10-6 ~ f = 10 Hz, x = m GW detector requirement: Hz Vibration isolation essential Ground displacement noise m/sqrt(hz) Quiet site 10-9 /f Frequency Hz

3 Vibration isolation basic ideas Mechanical low pass filter Below resonant f << f 0 x = x 0 follow excitation At resonant f = f 0 x > x 0 motion amplified Above resonant f >> f 0 x > x 0, vibration isolation

4 Vibration Isolators horizontal and vertical components Vertical Spring/Mass system y0 + y Horizontal Pendulum x0 x * Transfer function Input output TF output TF = = input x x 0

5 Transfer Function single stage X/X 0 TF= (f 0 /f) Frequency Hz

6 Some numbers 1 Hz isolator, TF@10Hz~(1/10) 2 Hz, seismic x 0 =10-11 m x=x 0 TF=10-14 m Not good enough Multiple stages: TF=TF1 TF2 TF3 TF high frequency Input TF1 TF2 TF3 output

7 Multi-stage isolator k 1 l 1 m 1 Vertical isolator spr ing/mass system k 2 m 2 k i m i x i + m i m 2 l i m 1 Integrated 3-D isolator l 2 θ i Horizontal isolator pe ndulums k n l n x i m n m n Roll off slope (f 0 /f )2( f 1 /f) 2( f 2 /f) 2.(f n /f )2 If f 0 =f 1 =f 2 =.., then slope (f 0 /f )2n

8 Transfer function multi-stage 5 stage 1 Hz pendulum Single 1Hz pendulum slope x/x TF=(1/10) 2 5 X=x0 TF=10-21 m Wonderful Frequency Hz

9 Some facts Coupled system: the total transfer function is not simply multiplication At high frequency, TF=TF1xTF2xTF3 Near resonant frequencies, the peaks spilt Corner frequency highest resonant peak Spring mass system with identical stages: fc~2f 0 Pendulum system with identical stages: fc~2f 0 N

10 At low frequencies. Vibration isolation mechanical low pass filter High resonant peaks at low frequency amplitude Q-factor of the isolation components Hard for interferometer cavity locking control Needs damping to reduce the peaks height Or.

11 Pre-isolation stages Have an Ultra-low frequency (ULF) pre-isolation stage in front of the isolation chain with f 0 ~0.1 Hz. Then the peaks of the chain will fall on the slope of the this preisolator. Trnasfer function frequency

12 A single ultra ultra low f stage? 50 0 Trnasfer function Not as simple as that! frequency

13 Internal frequencies Any structure will have internal resonances. Chladni figures are the results of the internal resonates of the plate

14 Example: cantilever Mode 1 Mode f Mode 2 Mode 5

15 Pre-isolation stage working range Low frequency pre-isolation is usually large structures with low internal f Usually the first internal frequency is roughly 10~100 times of f 0. It roll off does not goes on forever Needs to combined with low f isolation chain

16 Remarks Due to the cross coupling between the vertical and horizontal direction ~10-3 (nothing is perfect), it is important to have good vertical isolation system. It is relatively easy to create horizontal low frequency isolation system pendulums Vertical low frequency isolation system is not so straight forward (coil spring, cantilever ) Needs to support heavy load spring creeping Low frequency larger structure lower internal modes

17 Pre- isolation First stage of seismic noise reduction Very low frequency Two approaches Active, stiff system Passive, soft system

18 Stiff system Basic Idea: Sensing the seismic motion, then move the stiff system accordingly to hold it still seismometer Stiff system Require very sensitive commercial seismometer Large forces on rigid system LIGO system is stiff system

19 Requirements for Active systems Complicated servo control Needs very good seismometer to sense the motion of m. Noise in seismometer would be taken as displacement signal and fedback Remark: the passive isolation chain is actually a very good seismometer Usually the a isolation system is a combination of both but more active-like or more passive-like

20 LIGO isolation system Active isolation platform + quadruple pendulum hydraulic external pre-isolator (HEPI) active isolation platform quadruple pendulum

21 Basic idea: Soft system Spring-mass system (or equivalent) Transfer function falls as (f 0 /f) 2 at f 0 >>f i, seismic will be reduced Soft system is a very good seismometer itself Small control force mainly for alignment and damping VIRGO, TAMA, ACIGA adopted the soft system

22 Pre-isolation Linkages Anti-springs Example: Inverted pendulum (wobbly table) Gravity acted as anti-spring

23 Inverse Pendulum 1 st horizontal ULF Pre-isolation ~100mHz

24 Lacoste Linkage Vertical ULF preisolation stage ~100mHz

25 Zero length spring ka 2 =bw f=0

26 Robert linkage 2 nd ULF horizontal Pre-isolation stage ~100mHz Key feature Two stages allow super-spring implementation

27 Robert s Linkage Suspension point P moving in a shallow potential well

28 Key feature Passive critical damping Self-Damped pendulums (3 Stages) ~

29 Self Damped Pendulum thin fibre pendulum link simple wire pivot 2-d gimbal pivot Eddy current viscous coupling magnets copper to next stage Eddy current damped rocker vertical Euler springs Couple pendulum motion with very low frequency rocking motion use eddy current damping between these two motions

30 Vertical Euler Springs (3 Stages) Euler spring with anti-spring is near critically damped Tuned near pendulum frequency

31 Euler Spring mass load motion l clamped end Low stored energy means low mass high internal mode frequency superior to blade springs

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33 UWA Vibration Isolation and Suspension System Inverse Pendulum (Horizontal) 0.05 Hz Roberts Linkage (Horizontal) 0.05 Hz LaCoste (Vertical) 0.5Hz 4 horizontal Self-damped pendulum stages (~0.6Hz) 4 Vertical Euler Springs stage (~0.6Hz) Test mass

34 Newtonian Noise fundamental limit Gravity Gradient Noise Local gravitational field fluctuation: masses (people, vehicles, kangaroos) moving ground density change due to land wave.. (go underground) Environment is coupled to test masses by a equivalent spring with typical frequency G f grav 2 π ρ Hz