Surface Acoustic Wave Resonators as Passive Buried Sensors

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Wave Resonators as Passive Buried

Sensors Friedt &al.

Introductions Buried resonators Ground Penetrating Radar Delay line interrogation Measurement range estimate Conclusion

Surface Acoustic Wave Resonators as Passive Buried Sensors

J.-M Friedt¹, T. R´etornaz¹, G. Martin², T. Laroche², J.-P. Simonnet³, ´E. Carry², S. Ballandras²

1Senseor, France

2Time & Frequency Department, FEMTO-ST, Besan¸con, France

3Laboratoire de ChronoEnvironnement, Franche Comt´e University, Besan¸con, France

[email protected]

April 21, 2009

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Introductions

Buried resonators Ground Penetrating Radar Delay line interrogation Measurement range estimate Conclusion

Intoduction

Surface acoustic wave (SAW) devices meet ground penetrating RADAR (GPR):

• SAW devicesprovide transducers able to store and release energy provided as an electromagnetic wave⇒wireless

• Two major classes of transducers: narrowband resonators (frequency domain) and wideband delay lines (time domain)

• The velocity of the acoustic wave might be a function of a physical quantity (temperature, stress ...) depending on design⇒signature characteristic of the measured quantity

• GPRis a classical time-domain technique for probing dielectric interfaces in ground

• bistatic antenna configuration: a short (single) RF pulse is emitted and echos are monitored⇒scan to gather a map of the interfaces

The interrogation method of GPR is similar to that used to probe delay lines⇒use SAW device as GPR cooperative target

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Buried SAW resonators

• Narrowband SAW resonator designed as temperature sensors are buried at various depths ranging from 30 to 80 cm.

• Frequency sweep using a narrowband RADAR to identify the resonance frequency and hence the temperature

interrogation unit

waveguide (conducting wire) T sensor

clay 30 cm

• The emitted power is 10 dBm, receiver detection limit -70 dBm⇒ in this experiment, no direct signal was gathered when burying the SAW

• An electromagnetic waveguide is provided through a metallic wire thrown in the hole (no electrical connection on either side of the wire)

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Buried SAW resonators (2)

• These SAW devices were not individually calibrated⇒relative temperature observation

• Measurement during'1.25 year: SAW devices are robust to the environment

• No loss in RF signal quality over time

• The undergroud signal closely matches a running average over several weeks of surface temperatures.

0 50 100 150 200 250 300 350 400 450

−15

−10

−5 0 5 10 15 20 25 30

∝ T (o C)

temps (jours depuis 01/01/2008) σ(30 cm)

relative <30 cm>

σ(60 cm) relative <60 cm>

σ(80 cm) relative <80 cm>

7

Air temperatures fromhttp://www.meteo- franche- comte.fr/ 4 / 13

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GPR

• Measurement in agreement with the expected temperature trend but poor interrogation range because we wished to respect ISM band regulations (50% duty cycle ⇒mean power = 0.5 peak power).

• GPR is classically used in geophysics to probe dielectric interfaces several meters deep

⇒switch from resonator to delay line as GPR cooperative target _{5 / 13}

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SAW delay line

• Point-like reflectors appear as hyperbola

• The interface echo from the buried antenna will be followed by reflections within the delay line⇒the hyperbola associated with dielectic interface reflections will be followed by acoustic sensor echos.

• Challenge: GPR hardware must be adapted because a single RF pulse is insufficient to load energy in SAW delay line

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SAW delay line interrogator prototype (2)

We experimentally demonstrate that the best response of the delay line is achieved when the number of periods of the signal is equal to the number of interdigitated transducers

• Tradeoff between loading a delay line (multiple periods) and spatial resolution (single pulse)⇒development of a dedicated solution, using a 860 MHz SAW delay line (sensor from the Kongsberg Sentry temperature monitoring system)

• No ISM band regulations (wideband, here 30 MHz at 860 MHz), peak powermean power

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SAW delay line interrogation prototype

Experimental results using the Universal Software Radio Peripheral (USRP, http://www.ettus.com/) and GNURadio opensource software:

prototyping tools for developing RF interrogation units (opensource, modular)

USRP prototyping toolset

resonatorSAW

antenna configuration monostatic

communicationUSB

transceiver433 MHz transceiver860 MHz

FPGA 6V DC

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SAW delay line interrogation prototype (3)

0 200 400 600 800 1000 1200 1400

10 20 30 40 50 60 70 80

|IQ|

time (15 ns/point)

-2 -1 0 1 2 3 4

10 20 30 40 50 60 70 80

φIQ

time (15 ns/point)

1 2 3 4

800 1000 1200 1400 1600 1800

0 200 400 600 800 1000 1200

time (1.5 ns/point)

time (repetition rate)

1st peak 2nd peak 3rd peak 4th peak

<4th>10

• Magnitude and phase of the reflections of the delay line: the phase is defined but does not vary, probably because the received signal is too weak (insufficient receiver amplification)

• Signal processing: polynomial fit of the reflection magnitude and identification of the position of the maximum referenced to the emission pulse.

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SAW delay line temperature measurement

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000

1000 1500 2000 2500

time (15 s/point)

delay (a.u.) reflection1

reflection2 reflection3 reflection4 temperature(degC)x10+800

−20 0 20 40 60 80 100 120 140 160

1000 1500 2000 2500

temperature (degC)

delay (a.u.)

• Evolution of the duration between excitation pulse and reflections (4 reflections) as a function of temperature.

• These data are extracted from the magnitude only (' ±3^oC), since the receiver gain was insufficient to define the phase.

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Interrogation range estimate

How to estimate the interrogation range of SAW devices considering the detection range of GPR used on ice-rock interface ?

Two approaches: planar wave approach (Fresnel equation, reflection coefficient from permittivity change) or FDTD modelling.

1 εice'3.1,εrock '5⇒reflected powerR=^√_ε

ice−√ ε_rock

√ε_ice+√ ε_rock

2

, here -19 dB

2 Free Space Propagation Loss=20 log₁₀d+ 20 log₁₀f + 32.44 (f in MHz andd in km): at 100 MHz, FSPL(2×100 m)'58 dB and FSPL(2×15 m)'42 dB.

3 2D-FDTD simulation considering a hole (air,ε= 1⇒R'-8 dB) in rock.

Since typical SAW insertion losses are in the -35 dB range, the interrogation range will be reduced from 100 m to 15 mdue to lower reflection coefficient of SAW delay line.

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Sensor signature/2D-FDTD simulations

ε1 ε2

Tx Rx

emission

SAW antenna interface1

SAW signature

(b)(a) (d)(c) (e)

−100

−80

−60

−40

−20 0

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Electric field modulus (dB)

Time(µs) (a) infinite radius, 5 m deep, source in ground (b) infinite radius, 5 m deep, source in air (c) 30 cm radius, 5 m deep, source in ground (d) 30 cm radius, 5 m deep, source in air

(e) 30 cm radius, 10 m deep, source in ground 30 cm & 1 m diameter, 5 m deep in soil (εr= 25)

Expected signal measurement technique: the sensor appears as multiple hyperbola, the summit of the 1st one being the sensor position and the delay to the next ones

being related to the physical quantity. _{12 / 13}

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Conclusion and perspectives

1 We have demonstrated that SAW devices provide rugged sensors compatible with underground measurements.

2 We have illustrated the detection range of GPR on ice/rock interface and verified that distances up to 100 m can be reached

3 We have demonstrated the use of a prototyping tool – USRP – for testing RADAR interrogation stategies of delay lines

4 We have estimated the interrogation range of delay lines using GPR.

We still have to

1 demonstrate the wireless signal detection of buried 860 MHz delay lines using USRP

2 develop a SAW delay line working efficiently at 100 MHz, a frequency providing good compromise between antenna size and interrogation range

3 finalize 3D-FDTD simulation to assess the interrogation range

Acknowledgement: GPR measurements were performed as part of the Hydro-Sensor-FLOWS ANR Projet, under the supervision of M. Griselin and C.

Marlin.

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