sebastian böser [email protected] development of glaciophones and acoustic transmitters for ice 1 st...
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Sebastian Bö[email protected]
Development of glaciophones and acoustic transmitters for ice
1st International ARENA Workshop
Zeuthen
May 2005
Acoustic sensors and transmitters – 2 [email protected]
Overview
Motivation Thermoacoustic model Target material properties
Sensors Principle and design Calibration
Piezoceramics Sensors
Transmitters Transmitter design HV signal generators
Acoustic sensors and transmitters – 4 [email protected]
Signal amplitudes
Wasser (20℃) Ice (-50℃)Density
ρ 1 0.92Energy deposition
L ≈ 5 m d ≈ 10 cm
Velocity of soundvs 1480 3900
Peak frequencyfpeak 7.4 19.5
Expansion coefficientα 200 · 10-6 150 · 10-6
Heat capacityCp 0.999 0.5
Peak pressure amplitude
Pmax 0.22 · 10-3 2.2 · 10-3
3cmg
sm
kHz
1K
Kkcal
Acoustic sensors and transmitters – 5 [email protected]
Sensor design
Requirements: sensitive to mPa pressures all-φ sensitivity / radial symmetry (directional information)
Environmental: deployment in
hot-water drilled holes Water tight temperature: -30℃ to -55℃ Refreezing:
pressures up to 200 bar
Electrical: very small signals
high gain shielded against EM noise
Piezoelectric ceramics: well understood cheap
Housings: thick walls or solid (cast out)
Amplifiers: custom build
Simplicity vs. Suitability
Acoustic sensors and transmitters – 6 [email protected]
Piezoelectric ceramics
material: lead zirkonium titanate
(PXE5 = PZT) pervoskit structure polycrystalline
poling: heat above Tcurie ≈ 300 ˚C
cool in strong E-Field (E ≈ 2 MV/m)
reorientation of
polarization domains
sensitivity: d33 ≈ 500pC/N
typical signal:
0.1 V @ 1 mPa
T > Tcurie T < Tcurie
shapes: tubes plates cylinders
resonances: mode frequency
Acoustic sensors and transmitters – 7 [email protected]
Sensor design: schematic
signal: U ∝ Δl ∝ ma
mass/spring load
amplifier: three stages ( +80 dB ) low noise ( ≈ 8V )
housing: high pressure thickness impedance matching
resonances
Z (25 kHz)
ice 3.59*106 15.6 cm
brass 28.5*106 13.6 cm
PXE5 14.2*106 7.4 cm
piezoceramics
housing
amplifier
(brass) head
Acoustic sensors and transmitters – 9 [email protected]
Lab measurements
Medium: ice water
Linearity: all sensors nicely linear absolute values
calibration
Self noise: power supply temperature
Temperature: increasing with lower temp
not understood
Pressure: no results (yet)
Frequency response: need larger volume than in lab
calibration
Excitation: piezoceramics laser proton beam
Acoustic sensors and transmitters – 10 [email protected]
Calibration of piezoceramicsstability:
stable with temperature, time, … manufacturing variations
problem: input impedance of voltmeter
decharge = R•C ≈ 3 s
charge integration
Acoustic sensors and transmitters – 11 [email protected]
Calibration of sensors
Problem interesting frequency ≈ 20 kHz
λwater = 7.5 cm λice = 20 cm
“Ringing” signal reflections distort signal need container with xcont » λ
Setup at HSVA water tank 12m × 3m × 70m deep section 12m × 5m × 10m
Sensors Reference Hydrophone
Sensortech SA03163.3±0.3 dB re 1 V/µPa ( 5 to 65 kHz) Glass Ball, Iron Ball
Transmitter piezoceramic in epoxy
arbitrary signal generator
Acoustic sensors and transmitters – 12 [email protected]
Sensitivity: Method
Method transmit same signal to
reference sensor to calibrate
compare response relative calibration
Transmitted signals gated burst
precisely measuresingle frequency limited by
system relaxation time reflections
pulse in one shot measure full spectrum limited by
noise level
Acoustic sensors and transmitters – 13 [email protected]
Sensitivity: Gated burst
Time window start: after initial excitation stop: before 1st reflection
Fit
A(t) = A0sin(2πf·t + φ) + bt +c free phase and amplitude fixed frequency linear offset term
very good χ2
But: low-f and DC background
large error for small signals
probably overerstimated
Acoustic sensors and transmitters – 14 [email protected]
Sensitivity: pulse method
Transmitted signal
P ∞ ∂2Uin/ ∂t2 “soft” step function
Received signal
Fourier transform compare spectral components
Errors and noise
A(t) = Σf s(f)ei (2πft + φs) + n(f)ei (2πft + φn)
coherent signal: φs(f) = const
random noise: φs(f) = random
Noise spectrum from
average fourier transform
fourier transform average
define signal dominated freq. ranges
Acoustic sensors and transmitters – 15 [email protected]
Comparison of methods
Results high sensitivity and S/N
Glass ball: factor ≈ 20 Iron ball: factor ≈ 50
very good agreement strongly structured
many different resonance modes only valid for water
Acoustic sensors and transmitters – 16 [email protected]
Equivalent noise level
Method fourier transform
scaling, frequency range
inverse transform
Problem noise recording from water tank lab self noise higher due to EM coupling
Equivalent Noise Level [mPa]
Frequency range [kHz]
5 - 120 5 - 65
Hydrophone 50.1± 0.7 40.3 ± 8.3
Glass Ball 17.1 ± 1.7 15.9 ± 1.7
Iron Ball 6.6 ± 0.6 4.7 ± 0.7
Acoustic sensors and transmitters – 17 [email protected]
How to do it for ice ?
Theoretical use formula for transmission
Problem temperature dependance
resonance modes amplifier gain× bandwidth
solid state vs. liquid
Practical use large ice volume (glacier, pole) use small ice block with changing boundary conditions
(e.g. air, water) determine reflections from comparison
Acoustic sensors and transmitters – 18 [email protected]
Transmitters
Large absorption length
Need high power transmitter
Piezoceramics can be driven with kV signals easy to handle cheap well understood
Ring-shaped piezoceramic azimuthal symmetry larger signals than cylinders more expensive
Acoustic sensors and transmitters – 19 [email protected]
Ring vs. cylinder
Linearity tested from 100 mV to 300 V
perfect linearity
Frequency response three resonance modes
width, thickness and diameter
wide resonance at lower frequencies
Testing frequency sweep
dominated by reflections
resonance modes of container
white noise signal
reflections not in phase
resonance modes of transmitter
Acoustic sensors and transmitters – 20 [email protected]
HV signal generation
Problem build a HV generator for
arbitrary signals
Imax = 2πf Ctot Umax
Cring = 16 nF f = 100 kHz Umax = 1kV k33 = 0.34
Imax = 16 A, P ≈ 5.4 kW too large
Solution large capacity at low duty cycles
100 cycle burst 1ms 16 W large inductivity
discharge via capacitance shortcut after N cycles
Acoustic sensors and transmitters – 21 [email protected]
Summary
Developed sensors are cheap and sensitive
Developed transmitters are powerful
Problem: HV signal generation
Properties of both need to be better understood
Testing in ice limited by limited volume and freezing time
With only two years R&D,glaciophones are already quite successful