Sviluppo di sensori per test ad 1-g di strumenti dedicati alla fisica fondamentale nello spazio

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1 Sviluppo di sensori per test ad 1-g di strumenti dedicati alla fisica fondamentale nello spazio Raffaello Pegna, Dipartimento di Fisica E. Fermi & INFN, Pisa ASI Workshop Studi di Osservazione dell Universo Roma Marzo 2009

2 The motivation Many high sensitive experiments in Fundamental Physics need to be performed in space All such experiments require the measurement of very small forces, by means of sophisticated payload instruments It is crucial for these instruments to be tested in the lab at 1-g before launch but the lab is dominated by tilt/local acceleration noise: microsismic tilt/acceleration noise, diurnal temperature induced tilt/acceleration effects a a = α g α l ( m ) Effects of (small) local accelerations i and of terrain tilts α = a / g ( m g ) are conceptually indistinguishable because of the Equivalence Principle ( mg / mi = + 1) g -a The instruments needed to sense (and compensate for) these effects are accelerometers/tiltmeters

3 ISA (Italian Spring Accelerometer): an instrument for space and lab ISA is designed for space and lab. In the noisy lab environment ISA proves its performance by relying on rejection (..best in the frequency range of lower seismic noise)

4 Tilt control as an auxiliary tool to allow lab tests of high sensitive sitive instruments (I) Tilts sensed with commercial spirit level tiltmeter and compensated with PZT sensors (in closed loop) at INFN lab in Pisa to provide a very quiet environment at very low frequency by significantly reducing tilt effects at diurnal frequency The challenge of diurnal ground noise!

5 Tilt control as an auxiliary tool to allow lab tests of high sensitive sitive instruments (II) Tilt control loops can be made very effective, even by means of small, cheap commercial tiltmeters and PZTs: a horizontal surface can be maintained horizontal within a few rad at diurnal frequency for very long timespans but, is this horizontal surface the true zero? I.e., is the surface we are maintaining as horizontal perpendicular to the direction of the true local vertical? if the tiltmeter signal is affected by temperature variations, such temperature dependent components of the signal will be re-introduced as spurious tilts by the PZT actuators in the close tilt control loop!

6 Tilt signal vs temperature time variations Correlation between tilt signal and temperature variations is apparent! Direz. Sensibile X Direz. Sensibile Y These measurements have been performed with a small ad-hoc version of ISA tiltmeter manufactured in order to replace in the tilt control loop the spirit level tiltmeter (visible on top of ISA tiltmter box)

7 Development of a high sensitive double pendulum tiltmeter: : the principle Simple pendulum: sensitivity to tilts α of a simple pendulum of length L and oscillation period T is inversely proportional to T 2. Improving sensitivity requires to increase pendulum length unpractical L x 2 T = 2π α = L T g L 1 α 2 T 6 L= 1 m, T = 2s x = 1µ m α = 10 rad Double pendulum: simple + inverted, aligned: the mid point of the thin cantilever connecting the 2 test masses is ideallly a zero force point the oscillation period can be much longer (corresponding to a pendulum of much longer equivalent length ) Longer oscillation period obtained by Donato Bramanti with his first prototype 2 dp eq 2s T = 100 s L = (50) L = 2500m 10 x = 1µ m α = 10 rad with capacitance sensors it is possible to measure displacements smaller than µm

8 Development of high sensitive double pendulum tiltmeter: : construction details (I) Top knife suspension (in widia) of the (top) simple pendulum (x direction) Bottom knife suspension (in widia) of the (bottom) inverted pendulum

9 Development of high sensitive double pendulum tiltmeter: : construction details (II) Double pendulum skeleton showing the thin cantilever connecting the 2 test masses (in blue) As the top platform tilts, the top test mass falls back to the vertical (stable) while the bottom one falls off (unstable). For very small tilts (tilt control loop) the mid point of the cantilever connecting them is almost a zero force point ( very long period, very high sensitivity to tilts)

10 Development of high sensitive double pendulum tiltmeter: : the read-out The displacements caused by tilts are sensed by 2 small capacitance sensors arranged to form a capacitance bridge (4pF, 1mm gap) The electronics is based on the Analog Device AD7745 capacitance-to.digital converter (24 bit) which is very adequate for small capacitances: tests show that capacitances up to 4pF can be measured to a few tenths of femtofarad C = 4pF d = 1mm Cmin xmin 11 xmin = d 0.06µ m αmin = rad 2C L eq (with T=100s; Leq=2500 m and C=5x10-4 pf)

11 Development of high sensitive double pendulum tiltmeter: : expected temperature effects The double pendulum skeleton is symmetric w.r.t its sensitive point The structure of the tiltmeter is also symmetric w.r.t its sensitive point We expect the effects of temperature variations with time to be minimized..but symmetry does not help against spatial temperature gradients between the two sides of the structure at its top and bottom plates, giving rise to a banana shape deformation best results with very good thermal uniformity