Sviluppo di sensori per test ad 1-g di strumenti dedicati alla

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 25-26 Marzo 2009
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 =αg
α
Effects of (small) local accelerations ( ∝ mi ) and of terrain
tilts α = a / g ( ∝ m g ) are conceptually indistinguishable
because of the Equivalence Principle ( m g / mi = +1)
l
-a
g
The instruments needed to sense (and compensate for) these
effects are accelerometers/tiltmeters
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)
Tilt control as an auxiliary tool to allow lab tests of high sensitive 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!
Tilt control as an auxiliary tool to allow lab tests of high sensitive 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 10-10 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!
Tilt signal vs temperature time variations
Direz. Sensibile X
Correlation between tilt signal and
temperature variations is apparent!
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)
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 T2. Improving
sensitivity requires to increase pendulum length …
unpractical…
T = 2π
L
g
∆α =
1
T2
L = 1m , T = 2 s
∆x
L
L ∝T2
∆α ∝
∆x = 1µ m ⇔ ∆α = 10−6 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”)
Tdp = 100 s Leq = (50) 2 L2 s = 2500m
Longer oscillation
period obtained by
Donato Bramanti with
his first prototype
∆x = 1µ m ⇔ ∆α = 10−10 rad
…with capacitance sensors it is possible to
measure displacements smaller than µm…
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
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)
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 = 4 pF d =1mm
∆C min
∆x
∆xmin =
d 0.06 µ m ⇒ ∆α min = min 2.5 ⋅10−11 rad
2C
Leq
(with T=100s; Leq=2500 m and ∆C=5x10-4 pF)
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