Sebastian Bösersboeser@ifh.de

Development of glaciophones and acoustic transmitters for ice

1st International ARENA Workshop

Zeuthen

May 2005

Acoustic sensors and transmitters – 2 sboeser@ifh.de

Overview

Motivation Thermoacoustic model Target material properties

Sensors Principle and design Calibration

Piezoceramics Sensors

Transmitters Transmitter design HV signal generators

Acoustic sensors and transmitters – 3 sboeser@ifh.de

Thermoacoustic modell

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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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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

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Sensors

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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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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 sboeser@ifh.de

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