Surface acoustic wave delay lines as passive buried sensors
J.-M Friedt1, T. R´etornaz1, S. Alzuaga2, T. Baron2, G. Martin2, S. Ballandras1,2, M. Griselin3, J.-P Simonnet4
1SENSeOR SAS,2FEMTO-ST,3Th´eMA,4Chronoenvironnement, Franche-Comt´e University/CNRS, Besan¸con, France
Wireless, passive (battery-less) sensors interrogated by Ground Penetrating RADAR
Objective: complementing widely available geophysical characterization instruments (GPR) with sensing capability :
Ground Penetrating RADAR (GPR):
• bistatic antenna configuration, pulse emitter
• stroboscopic (equivalent time sampling) receiver
• ⇒low power, low cost setup for baseband measurement (magnitudeand phase informations)
• ⇒typical center frequencyfc ∈[100−1000] MHz, sam- pling frequency'10×fc
• sampling duration'3µs (225 m in ice)
• peak power 2 kW, interrogation raange≥150 m in ice
• BUTsensitive to dielectric/conductivity changes
distance to surface (m)
time (s), radar speed ∼ 15 k m /h
20
40
60
80
100
120
140
0 50 100 150 200 250 300 350 400
emitter receiver
Rock
Ice raster scan
R=
√
εr(ice)−√
εr(rock)
√εr(ice)+√
εr(rock)
Reflection coefficient from Fresnel eq.
Ice-rock interface (East Loven Glacier, Spitsbergen, Norway, 79oN)
Surface Acoustic Wave (SAW) delay line:
• electromechanical sensor based on piezoelectric substrates (LiNbO3(YXl)/128o for its high coupling)
• sensing based on mechanical wave velocity measurement
• time delay between reflection∼physical quantity
• intrinsic radiofrequencysensor (no DC conversion)
• linear device (no rectifier threshold voltage)
• acoustic velocity'10−5×electromagnetic velocity
⇒3µs delay =9 mm path length = 4.5 mm long sensor
• BUT large antenna (λEM/2'1 m at 100 MHz)
50 100 150
−20
−15
−10
−5 0
frequency (MHz)
|S11|
0 0.5 1 1.5 2 2.5
−80
−60
−40
−20
time (us)
|S11|
emitted RADAR pulse spectrum
delay line S11
RADAR echo time domain analysis
Delay line dimensions
4000
2400 3100 3700 780
SAW GPR
emitter
GPR
receiver
RF pulse
SAW reflections
Comparison of the GPR and SAW delay line spectra (100 MHz)
trace number
time (s)
1. echo position v.s time
100 200 300 400 500
1.3
1.4
1.5 1.6
1.7 x 10−6
0 10 20 30 40 50
0 1000 2000 3000 4000
index
|FFT|
2. echo peak power position trace 200 trace 500
0 100 200 300 400 500
−4
−3
−2
−1 0 1 2 3
trace number
φ(FFT)
3. phase of FFT at peak power position 1.3 us−1.5 us 1.6 us−1.8 us
0 100 200 300
0 20 40 60 80
time (s)
temperature (degC)
4. scale to convert phase to temperature Pt100 probe φ(FFT)*33+90
50 cm 1 meter
temperature measurement
time delay dependent with sensor distance
Sensor located at 1 m & 50 cm from receiving antenna
⇒Practicalmeasurement strategydemonstration:
1. probe delay line sensor with GPR,
2. identify delay response through Fourier transform (FFT), 3. measure absolute FFT phase,
4. phase difference insensitive to GPR-sensor distance, 5. representative of physical quantity (preliminary calibration),
here temperature Rangedestimate :
1. point-like target⇒Free Space Propagation Loss asd−4 2. dSAW = dinterf ace×10(ILinterf ace−ILSAW)/40, with typical
SAW insertion loss (IL) at -35 dB, and ice-rock interface -19 dB (εrock = 5, εice= 3.1, Fresnel eq.)
Conclusion and perspectives
• Interrogation range: demonstrated at 5 m, signal to noise ratio⇒40 m?
• only requires post-processing, no need to modify GPR hardware
• mandatory differential measurement(poor local oscillator resolution + distance de- pendence)
• temperature with ∼K accuracy
• applicable totemperature probe,pressuresensor, straingage
• application to acousticresonators ? (narrowband, low loss transducer)
500 cm
45 cm 1 m
0
1
2
3
4
time (us)
distance (a.u.)
-2 0 2 4 6 8 10
5 10 15 20 25 30 35 40
phase (rad)
trace number echo 1 echo 2 echo 3
1 1.5 2 2.5 3 3.5 4
5 10 15 20 25 30 35 40
phase (rad)
trace number echo 1-echo 2 echo 1-echo 3
echo2 echo1
echo3
echo4
Reference: Friedt & al,Surface acoustic wave devices as passive buried sensors, J. Appl. Phys109(3), 034905 (2011)
