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The high sensitivity Italian Spring Accelerometer (ISA) for fundamental physics in space
Adv. Space Res. Vol. 25, No. 6, pp. 1241-1244,200O 0 2000 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-I 177/00 $20.00 + 0.00 PII: SO273-1177(99)00993-X
Pergamon www.elsevier.nl/locate/asr
THE HIGH SENSITIVITY ITALIAN SPRING ACCELEROMETER (ISA) FOR FUNDAMENTAL PHYSICS IN SPACE V. Iafolla*, A. Mandiello*,
and S. Nozzoli*
*Istituto di Fisica dell0 Spazio Interplanetario CNR, Via de1 Foss0 de1 Cavaliere 00133 Rome, Italy
ABSTRACT Many space experiments in fundamental physics require accelerometers of high sensitivity. We present here the “Italian Spring Accelerometer” (ISA), operating at low frequencies (lOa + 1 Hz), whose sensitivity is expected to be better than 1O-l2 g /
&
An ISA prototype has been completed and used for ground tests.
We describe ISA and report the results obtained from the experimental measurements. We have conceived a differential accelerometer based on the same physical principles as ISA which, if operated at low temperatures, Equivalence
could reach a sensitivity Principle
better than lo-l4 g / &
in short-time free fall conditions.
and be of interest for testing the Weak
0 2000 COSPAR. Published by Elsevier Science Ltd.
THE ISA ACCELEROMETER A wide range of experiments in fundamental physics, in space as well as on the ground, require to detect the effects of extremely small forces acting on the proof mass of a harmonic oscillator (Blaser et al. 1996; Chan et al. 1987; Nobili et al. 1996; Worden et al. 1997). For the effects of these small forces to be as large as possible, the resonance frequency of the oscillator has ri%ma plater Of c2ontra capacitors to be as low as possible; ideally zero (for a free, \\ unconstrained proof mass). ISA is a three-axis instrument made by three mechanical oscillators whose sensitive axes are orthogonal to each other. Figure 1 shows one such mechanical unit with the sensitive axis in the z direction. The proof mass (m=0.2 kg) is constrained by two torsional arms to make rotation possible about their symmetry axis (axis r in Figurel). The sensitive axis is perpendicular to the face of the proof mass (axis z). The natural resonant frequency is 0,=2~.3.5 radhec. It can be lowered by adding a negative electrostatic coupling by means of an electric field across the control capacitors. The central part is manufactured from a single plate of 5056 AZ alloy.
2
Fig. 1. One of the 3 mechanical On the proof mass described
units of the ISA accelerometer here accelerations
of 1O-l2 g I fi
(that with sensitive axis in the z direction). produce a rotation whose corresponding
is x,,, = am,”I w,’ = 1O-” m I & , which is detected by means of a capacitive push-pull transducer inserted in a capacitance bridge and followed by a low-noise amplifier. How the read out works is shown in Figure 2. The system is in essence a capacitance bridge, which would be unbalanced by any force mean displacement
1241
V. Iafolla er
1242
al.
= 2OkHz and the output signal is seen like an
acting on the proof mass. The bridge is driven at f, = Rl2n unbalancing
of it, from points A and B.
Fig. 2. Simple scheme of the ISA read out system (K, sensitive axis; the other letters are mentioned As a result frequency, frequency. factor: the
represents
the torsional
elastic constant,
z the
in the table 1).
of the fact that the signal is transformed from low to high frequency, the amplifier works at high where its l/f noise is lower. After amplification the signal is demodulated and acquired at low In order to obtain the desired sensitivity, the mechanical oscillator must have a high quality higher the quality factor, the smaller its brownian noise.
SYSTEM CALIBRATION One major difficulty with high sensitive accelerometers like ISA, operating calibrate it, since it is very hard to reduce seismic noise at low frequencies. Separate Measurements
at low frequencies
is how to
of the Relevant Parameters.
Separate measurements of the various parameters which contribute to determine the sensitivity of the ISA instrument give the first indication of the its sensitivity. Table 1 lists these parameters, their measured values, those which are within reach, and the ones which could be obtained at low temperatures. Table 1. Electromechanical parameters of the ISA accelerometer Actual value Possible value amin
T fP
c, and c, c, and c, C,and
C,
Sensitivity
g / JHZ
Proof mass
(kg)
4’10-‘4
Cryogenic
3.3*10-‘2 0.22
10
5.7 * 1o-‘5 10
Frequency of resonance
(Hz)
3.5
1
1
Polarisation
(kHz)
10
10
10
300
300
300
External fixed capacities (pF)
300
300
300
Control capacitors
300
300
300
AD743iAD
OPA128
OPAl28
3*lo-9
15*10+
15*10-9
frequency
Sensing capacitors
(pF)
(pF)
Electronic device Voltage noise of amplifier ( V / JHZ
)
Current noise of amplifier
)
Temperature
(A / &
noise of amplifier (K)
Electromechanical
transducer factor
Total quality factor Brownian noise
(m/set’)’
Electronic noise
(m I set’)’ /Hz
I Hz
7*10-‘5
o.l*lo-‘5
0.1* lo-‘5
0.76
0.06
0.06
2.8*lo-2 5.7*10’
7.6*10”
1.9*10-’
6.3*104
IO5
2.9 * IO-=
l.6*10-25 4.9 * lo-26
1.4 1o-27
8.4 * 1o-*2
l.9*lo-27
1243
The Italian Spring Accelerometer
In the first column we report, for each parameter, the value measured with the current ISA prototype; the measurements are obtained with a driving frequency around 10 kHz and the noise amplifier is not matched. The minimum acceleration detectable is given by the formula:
The first term represents the total brownian noise or thermal noise, acting on the proof mass; it accounts for the loss of the harmonic oscillator and the loss of the capacitive transducers. The second term is the amplifier noise contribution. (Z,, is the noise amplifier impedance). Thermal noise depending on the loss of the oscillator acts on the proof mass directly at low frequency, while the transducer capacity loss and the amplifier noise give major contribution to noise around the polarisation frequency (1 OkHz) directly at the exit; this noise is then translated to low frequency (back-action). Thermal noise and electronic noise around the polarisation frequency can be regarded to be flat. With the measured values listed in the first coloumn of Table 1 we get a minimum value for the acceleration noise of amin= 3.3 * lo-‘* g/G . From the table it is possible to note that in no matching conditions an electronic device with a very low noise voltage (Texas Instruments AD743/AD) is used. The next column in the Table shows improvements which appear to be possible. In this case the amplifier, using a low noise temperature device (Analogue Device: OPA128), matches the transducer impedance. By increasing the mass of the oscillator and lowering the operating pressure, a quality factor close to lo5 appears to be within reach. The sensitivity evaluated in matched conditions is given by the following formula:
and with the measured values of column 2 in Table 1 the acceleration sensitivity is a,,,, = 4 * 1O-l4g / &, Laboratorv Measurements. Using a single sensitive element like a horizontal tiltmeter (gravity acceleration parallel to n axis of Figure 1) it is possible to perform measurements of acceleration in the horizontal plane, or variations of the angle of the mounting plane (inclinometer). The angular variations expressed in radiants are equal to the acceleration expressed in g. Figure 3 shows the horizontal seismic noise measured in the underground INFN laboratory at Gran Sasso, L’Aquila (Fuligni et al. 1997). It shows a minimum in the frequency range of lo-* -lo-‘Hz of about 10m9g / 6. This value is in point of fact an upper limit for the sensitivity of the instrument.
A
0.016500 urms
Fig. 3. Seismic noise measured at the Gran Sasso. A gravitational calibration has also carried out. Figure 4 shows the spectral density of the accelerometer output signal. The accelerometer is suspended on a pendulum for high frequency insulation and
0
Fig. 4. Gravitational Calibration
1.56 Hz
1244
V. Iafolla et al.
gravitationally the gravitational
excited by a system of two masses rotating at 0.16Hz. The arrow at 0.32Hz, corresponds signal of 10m9g . All the other peaks are structural frequency of the suspension
A DIFFERENTIAL
to
system.
ISA ACCELEROMETER
An evolution of this accelerometer is a differential prototype designed for equivalence principle experiments, in free fall from stratospheric altitude (Iafolla et al. 1997; Lorenzini et al. 1994). Figure 5 shows its exploded prospect view, where Al, A2, Bl and B2 are the fixed faces of the transducers. Two sensing masses of different material are connected by two couple of torsional arms to a rigid frame, r and s in the Figure 5 represents the torques axis. In absence of Equivalence Principle violation the gravitational force produces the same torque on both masses. The experiment should be performed in vacuum, in free fall from stratospheric altitude and by rotating the instrument at about 1 Hz for high frequency modulation of the expected signal. The experiment in cryogenic conditions is expected to reach sensitivity
better than one part in 1014.
CONCLUSIONS ISA is a sensitive accelerometer, also by comparison with similar instruments under development world-wide. In addition, it is well suitable for calibration in the laboratory and for long term ground testing (it involves low mechanical suspensions and has no floating masses). Some further improvements are well feasible. Even a low temperature version of ISA, involving the use of a SQUID, does not appear to be a major challenge. Fig. 5. Schematic design of a differential accelerometer.
ISA
REFERENCES Blaser J. P., J. Cornelisse, Publication
A.M. Cruise. T. Damour,
F. Hecler et al., STEP M3 Phase A Report, ESA
SC1 (96) 5 March 1996.
Chan H.A., M.V. Moody, and H.J.Paik, Physical Review D 35, 3572-3597 (1987). Fuligni F., and V. Iafolla; Nuovo Cimento C 20,619-628 Iafolla
V., E. Lorenzini,
Gravitational Lorenzini
V. Milyukov,
(I 997).
and S. Nozzoli,
Gravitation
and Cosmology
(J. of Russian
Society) 3, 15 1 (1997).
E.C., 1.1. Shapiro, F. Fuligni, V. Iafolla, M.L. Cosmo et al., Nuovo Cimento B 109, 1195 (1994).
Nobili A.M “GALILEO GALILEI” (GG) Phase A Report, AS1 (Agenzia Spaziale Italiana) (1998), htttp://tycho.dm.unipi.itfnobili/ggweb/phaseA Worden Jr P.W., R.Torii, and C.W.F. Everitt, Advances in Space Research 20, (1997) (in press).
Pergamon www.elsevier.nl/locate/asr
THE HIGH SENSITIVITY ITALIAN SPRING ACCELEROMETER (ISA) FOR FUNDAMENTAL PHYSICS IN SPACE V. Iafolla*, A. Mandiello*,
and S. Nozzoli*
*Istituto di Fisica dell0 Spazio Interplanetario CNR, Via de1 Foss0 de1 Cavaliere 00133 Rome, Italy
ABSTRACT Many space experiments in fundamental physics require accelerometers of high sensitivity. We present here the “Italian Spring Accelerometer” (ISA), operating at low frequencies (lOa + 1 Hz), whose sensitivity is expected to be better than 1O-l2 g /
&
An ISA prototype has been completed and used for ground tests.
We describe ISA and report the results obtained from the experimental measurements. We have conceived a differential accelerometer based on the same physical principles as ISA which, if operated at low temperatures, Equivalence
could reach a sensitivity Principle
better than lo-l4 g / &
in short-time free fall conditions.
and be of interest for testing the Weak
0 2000 COSPAR. Published by Elsevier Science Ltd.
THE ISA ACCELEROMETER A wide range of experiments in fundamental physics, in space as well as on the ground, require to detect the effects of extremely small forces acting on the proof mass of a harmonic oscillator (Blaser et al. 1996; Chan et al. 1987; Nobili et al. 1996; Worden et al. 1997). For the effects of these small forces to be as large as possible, the resonance frequency of the oscillator has ri%ma plater Of c2ontra capacitors to be as low as possible; ideally zero (for a free, \\ unconstrained proof mass). ISA is a three-axis instrument made by three mechanical oscillators whose sensitive axes are orthogonal to each other. Figure 1 shows one such mechanical unit with the sensitive axis in the z direction. The proof mass (m=0.2 kg) is constrained by two torsional arms to make rotation possible about their symmetry axis (axis r in Figurel). The sensitive axis is perpendicular to the face of the proof mass (axis z). The natural resonant frequency is 0,=2~.3.5 radhec. It can be lowered by adding a negative electrostatic coupling by means of an electric field across the control capacitors. The central part is manufactured from a single plate of 5056 AZ alloy.
2
Fig. 1. One of the 3 mechanical On the proof mass described
units of the ISA accelerometer here accelerations
of 1O-l2 g I fi
(that with sensitive axis in the z direction). produce a rotation whose corresponding
is x,,, = am,”I w,’ = 1O-” m I & , which is detected by means of a capacitive push-pull transducer inserted in a capacitance bridge and followed by a low-noise amplifier. How the read out works is shown in Figure 2. The system is in essence a capacitance bridge, which would be unbalanced by any force mean displacement
1241
V. Iafolla er
1242
al.
= 2OkHz and the output signal is seen like an
acting on the proof mass. The bridge is driven at f, = Rl2n unbalancing
of it, from points A and B.
Fig. 2. Simple scheme of the ISA read out system (K, sensitive axis; the other letters are mentioned As a result frequency, frequency. factor: the
represents
the torsional
elastic constant,
z the
in the table 1).
of the fact that the signal is transformed from low to high frequency, the amplifier works at high where its l/f noise is lower. After amplification the signal is demodulated and acquired at low In order to obtain the desired sensitivity, the mechanical oscillator must have a high quality higher the quality factor, the smaller its brownian noise.
SYSTEM CALIBRATION One major difficulty with high sensitive accelerometers like ISA, operating calibrate it, since it is very hard to reduce seismic noise at low frequencies. Separate Measurements
at low frequencies
is how to
of the Relevant Parameters.
Separate measurements of the various parameters which contribute to determine the sensitivity of the ISA instrument give the first indication of the its sensitivity. Table 1 lists these parameters, their measured values, those which are within reach, and the ones which could be obtained at low temperatures. Table 1. Electromechanical parameters of the ISA accelerometer Actual value Possible value amin
T fP
c, and c, c, and c, C,and
C,
Sensitivity
g / JHZ
Proof mass
(kg)
4’10-‘4
Cryogenic
3.3*10-‘2 0.22
10
5.7 * 1o-‘5 10
Frequency of resonance
(Hz)
3.5
1
1
Polarisation
(kHz)
10
10
10
300
300
300
External fixed capacities (pF)
300
300
300
Control capacitors
300
300
300
AD743iAD
OPA128
OPAl28
3*lo-9
15*10+
15*10-9
frequency
Sensing capacitors
(pF)
(pF)
Electronic device Voltage noise of amplifier ( V / JHZ
)
Current noise of amplifier
)
Temperature
(A / &
noise of amplifier (K)
Electromechanical
transducer factor
Total quality factor Brownian noise
(m/set’)’
Electronic noise
(m I set’)’ /Hz
I Hz
7*10-‘5
o.l*lo-‘5
0.1* lo-‘5
0.76
0.06
0.06
2.8*lo-2 5.7*10’
7.6*10”
1.9*10-’
6.3*104
IO5
2.9 * IO-=
l.6*10-25 4.9 * lo-26
1.4 1o-27
8.4 * 1o-*2
l.9*lo-27
1243
The Italian Spring Accelerometer
In the first column we report, for each parameter, the value measured with the current ISA prototype; the measurements are obtained with a driving frequency around 10 kHz and the noise amplifier is not matched. The minimum acceleration detectable is given by the formula:
The first term represents the total brownian noise or thermal noise, acting on the proof mass; it accounts for the loss of the harmonic oscillator and the loss of the capacitive transducers. The second term is the amplifier noise contribution. (Z,, is the noise amplifier impedance). Thermal noise depending on the loss of the oscillator acts on the proof mass directly at low frequency, while the transducer capacity loss and the amplifier noise give major contribution to noise around the polarisation frequency (1 OkHz) directly at the exit; this noise is then translated to low frequency (back-action). Thermal noise and electronic noise around the polarisation frequency can be regarded to be flat. With the measured values listed in the first coloumn of Table 1 we get a minimum value for the acceleration noise of amin= 3.3 * lo-‘* g/G . From the table it is possible to note that in no matching conditions an electronic device with a very low noise voltage (Texas Instruments AD743/AD) is used. The next column in the Table shows improvements which appear to be possible. In this case the amplifier, using a low noise temperature device (Analogue Device: OPA128), matches the transducer impedance. By increasing the mass of the oscillator and lowering the operating pressure, a quality factor close to lo5 appears to be within reach. The sensitivity evaluated in matched conditions is given by the following formula:
and with the measured values of column 2 in Table 1 the acceleration sensitivity is a,,,, = 4 * 1O-l4g / &, Laboratorv Measurements. Using a single sensitive element like a horizontal tiltmeter (gravity acceleration parallel to n axis of Figure 1) it is possible to perform measurements of acceleration in the horizontal plane, or variations of the angle of the mounting plane (inclinometer). The angular variations expressed in radiants are equal to the acceleration expressed in g. Figure 3 shows the horizontal seismic noise measured in the underground INFN laboratory at Gran Sasso, L’Aquila (Fuligni et al. 1997). It shows a minimum in the frequency range of lo-* -lo-‘Hz of about 10m9g / 6. This value is in point of fact an upper limit for the sensitivity of the instrument.
A
0.016500 urms
Fig. 3. Seismic noise measured at the Gran Sasso. A gravitational calibration has also carried out. Figure 4 shows the spectral density of the accelerometer output signal. The accelerometer is suspended on a pendulum for high frequency insulation and
0
Fig. 4. Gravitational Calibration
1.56 Hz
1244
V. Iafolla et al.
gravitationally the gravitational
excited by a system of two masses rotating at 0.16Hz. The arrow at 0.32Hz, corresponds signal of 10m9g . All the other peaks are structural frequency of the suspension
A DIFFERENTIAL
to
system.
ISA ACCELEROMETER
An evolution of this accelerometer is a differential prototype designed for equivalence principle experiments, in free fall from stratospheric altitude (Iafolla et al. 1997; Lorenzini et al. 1994). Figure 5 shows its exploded prospect view, where Al, A2, Bl and B2 are the fixed faces of the transducers. Two sensing masses of different material are connected by two couple of torsional arms to a rigid frame, r and s in the Figure 5 represents the torques axis. In absence of Equivalence Principle violation the gravitational force produces the same torque on both masses. The experiment should be performed in vacuum, in free fall from stratospheric altitude and by rotating the instrument at about 1 Hz for high frequency modulation of the expected signal. The experiment in cryogenic conditions is expected to reach sensitivity
better than one part in 1014.
CONCLUSIONS ISA is a sensitive accelerometer, also by comparison with similar instruments under development world-wide. In addition, it is well suitable for calibration in the laboratory and for long term ground testing (it involves low mechanical suspensions and has no floating masses). Some further improvements are well feasible. Even a low temperature version of ISA, involving the use of a SQUID, does not appear to be a major challenge. Fig. 5. Schematic design of a differential accelerometer.
ISA
REFERENCES Blaser J. P., J. Cornelisse, Publication
A.M. Cruise. T. Damour,
F. Hecler et al., STEP M3 Phase A Report, ESA
SC1 (96) 5 March 1996.
Chan H.A., M.V. Moody, and H.J.Paik, Physical Review D 35, 3572-3597 (1987). Fuligni F., and V. Iafolla; Nuovo Cimento C 20,619-628 Iafolla
V., E. Lorenzini,
Gravitational Lorenzini
V. Milyukov,
(I 997).
and S. Nozzoli,
Gravitation
and Cosmology
(J. of Russian
Society) 3, 15 1 (1997).
E.C., 1.1. Shapiro, F. Fuligni, V. Iafolla, M.L. Cosmo et al., Nuovo Cimento B 109, 1195 (1994).
Nobili A.M “GALILEO GALILEI” (GG) Phase A Report, AS1 (Agenzia Spaziale Italiana) (1998), htttp://tycho.dm.unipi.itfnobili/ggweb/phaseA Worden Jr P.W., R.Torii, and C.W.F. Everitt, Advances in Space Research 20, (1997) (in press).