Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
SAW for sensing Basics: radiofrequency components
J.-M Friedt 1 , G. Martin2 , S. Ballandras2
Resonator interrogation
1
Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator
2
SENSeOR, Besan¸con, France (www.senseor.com)
FEMTO-ST/CNRS, Besan¸con, France (www.femto-st.fr)
slides and references available at http://jmfriedt.free.fr/
Delay line interrogation Acoustic device characterization
May 18, 2008
Conclusion
1 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation
Generating acoustic waves • Acoustic waves in solids: mechanical, thermal expansion, piezoelectric generation
• An RF voltage applied to an interdigitated transducer generates an acoustic wave
• Surface, bulk waves, shear/longitudinal/Rayleigh/guided (Love mode) • Delay line (single path) or resonator (reflectors define a cavity) material LiNbO3 LiTaO3 KNbO3 LiB4 O7 langasite Quartz
wave shear shear Rayleigh shear Rayleigh Rayleigh Rayleigh shear
(m/s) 4700 4100 2800 3500 3500 2900 3150 5100
TCF -90 ppm/K -36 ppm/K < 1 ppm/K ? -300 ppb/K2 -70 ppb/K2 -40 ppb/K2 -60 ppb/K2
comment ferro. & pyroelectric, TC > 1200o C 525 < TC < 700o C huge coupling, TC ' 430o C water soluble no Curie temperature, > 1000o C most used less coupled
Acoustic device characterization Conclusion
2 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation
Acoustic wave sensors • Interdigitated transducers (IDT) patterned (lithography) on piezo
substrate define wavelength • εpiezo εair ⇒ efficient electric field confinement in piezo substrate • Conversion from electric to acoustic wave • Wavelength in the micrometer to tens of micrometers range,
velocity ∈ [3000 − 10000] m/s ⇒ 2-3000 MHz depending on design
Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
Sensing principle: variation of velocity induces variation of propagation delay 3 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements
Acoustic wave sensors • Boundary conditions define velocity and insertion losses. • Thermal expansion/stress of the substrate change velocity c =
(E Young modulus, ρ density), complex in anisotropic materials
viscosity ... • ... select the dominant effect by selecting the appropriate design.
For example: selection of a temperature compensated orientation for temperature-independent sensor. 50
’TCF1’
o
propagation=0 propagation=10o propagation=20o propagation=30o o propagation=40
1st order temperature coef. TCF1 (ppm/K)
40
45 40 35 30 25 20 15 10 5 0 -5
35 30
Delay line interrogation
25
45 40 35 30 25 20 15 10 5 0 -5
20
Acoustic device characterization Conclusion
E ρ
• Intrinsically radiofrequency devices ⇒ no conversion from DC to RF • ⇒ sensitive to temperature, stress (pressure), gravimetric (mass),
45
Measurement principles, beyond the resonator
q
15
0 5
10
10 15 20
5
25
3.4 ppm/K
30
0 −5 0
35 40
θ=36 (AT)
5
10
15
20
25
30
35
40
45
45 50
40
30
20
10
0
θ (o)
Temperature coefficient of frequency (TCF) computation by M. Bruniaux (SENSeOR) 4 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor
Interrogating acoustic wave sensors • resonator = narrow band: look for resonant frequency (inverse
Fourier transform of pulse, or frequency sweep) ; or oscillator and measure output frequency • delay line = wide band: look for time delay of reflected signal or
phase ; or set frequency and monitor phase and insertion losses Radiofrequency emission ⇒ respect regulations. • 433 MHz ISM band is only 2 MHz wide → resonator • 2450 MHz ISM band is 80 MHz wide → resonator or delay line
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
5 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor
Interrogating acoustic wave sensors • resonator = narrow band: look for resonant frequency (inverse
Fourier transform of pulse, or frequency sweep) ; or oscillator and measure output frequency • delay line = wide band: look for time delay of reflected signal or
phase ; or set frequency and monitor phase and insertion losses Radiofrequency emission ⇒ respect regulations. • 433 MHz ISM band is only 2 MHz wide → resonator • 2450 MHz ISM band is 80 MHz wide → resonator or delay line
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
6 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator
Phase locked loop (PLL) Frequency multiplication: • a high frequency steerable oscillator (Voltage Controlled Oscillator:
VCO) is divided, • the resulting low frequency is compared (phase measurement) with
a stable low frequency oscillator (mixer or XOR + low pass filter), • and the control signal is defined by the phase difference between the
divided frequency and the reference. fin
phase detector
VCO
fout = N × fin
divider 1/N
Delay line interrogation Acoustic device characterization Conclusion
⇒ we will work in the 0-100 MHz range, and multiply to reach the wanted ISM band ⇒ source noise is multiplied by N: ∆fout = N × ∆fin 7 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor
Direct Digital Synthesizer (DDS) A fixed quartz oscillator ... is multiplied to a clock frequency (internal PLL) ... 3 which increases a counter ... 4 whose values is converted through a look-up table to a sine output ... 5 converted to an analog output by a fast D/A converter Digital component, programmable output frequency Reference oscillator defines the output stability Complex output spectrum (fck , fck ± f wanted ...): low pass filter 1 2
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
From the Analog Devices AD9954 and AD9851 DDS datasheets 8 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components
Mixer and I/Q demodulator • Real and imaginary parts of the Fourier transform at the modulation
frequency ⇒ phase and magnitude • Analog Devices AD8302 includes auto-gain control from phase and
magnitude measurements
Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
60 dB dynamics, comparable with magnitude detector but provides demodulated output around reference signal
9 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization
General strategy • In order to comply with 433 MHz ISM regulations, use narrowband
sensors • In order to penetrate deep in dielectric substrates, avoid 2450 MHz
(+technological constraints) • Differential measurement (two resonances) to cancel correlated
noises and reference oscillator drift • Main issue: isolation between emission and reception stages (defines range) ⇒ generate a tunable frequency source, sweep ISM band and for each pulse, listen for response of resonator. If we are close to the resonance frequency, the energy loaded in the resonator empties to 1/e within Q/π periods.
Conclusion
10 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Frequency source
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator
• Fully software controlled strategy: program a frequency, send pulse,
listen, goto next frequency • Flexible approach allowing sub-band division of the ISM band • avoids synchronization between a (continuous) saw-tooth sweep of
VCO and listening period • two strategies: sweep DDS and lock PLL (multiplication), or mix
reference with variable frequency sources and band-pass wanted signal
Delay line interrogation Acoustic device characterization Conclusion
11 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Receiving
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor
Two strategies: wide band (power measurement) or narrow band (demodulated) detection • wide bandpass filter (listen to the whole ISM band) • amplify • magnitude detector
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation
We will always work at baseband, no mixing to reach IF since detectors up to 3 GHz provide the required sensitivity
Acoustic device characterization Conclusion
12 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
Radar mode, frequency sweep • a programmable frequency source (DDS) generates the frequency f
to be probed • RF switches emit the frequency for a duration τ • upon switching off the emission, listen for the magnitude of the
response of the resonator (exponential energy decay in Q/π periods, i.e. Q/(π × f ) s. Q = 8000 at f ' 434 MHz ⇒ 5.9 µs • repeat for all frequencies in the ISM band
⇒ 1 ISM band sweep requires ' 10 ms 14 V, 120 mA 13 V, 120 mA 12 V, 130 mA 11 V, 150 mA 10 V, 160 mA 9 V, 180 mA 8 V, 200 mA 7 V, 230 mA
attenuator
source 400 MHz
−1.5 to −32.5 dB
RS232
microcontroler ARM7 core
SPI
DDS 4
analog 1 Ms/s ADC
power detector
RF power measurement with 42 dB attenuator on antenna output 13 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt
Radar mode, digitized signal ⇒ poor selectivity since we listen at the energy in the whole ISM band: sensitive to other RF sources
Generation of acoustic waves
Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation
Improved resolution with averaging: sub-kHz resolution above 4 averages, usually 8 or 16 averages 433.53
frequencies (MHz)
Basics: radiofrequency components
433.525
433.52
433.515 0
σ (Hz)
∆f (MHz)
SAW for sensing
1000
2900
1080
1
2
2000
830
3000 4000 sample number (a.u.)
5000
590
520
480
8
10
12
6000
400
340
16
24
320
0.94
0.935
Acoustic device characterization 0.93
Conclusion
0
4 1000
2000
3000 4000 sample number (a.u.)
5000
26 6000
14 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Radar mode, pulse duration
J.-M Friedt Generation of acoustic waves
Tradeoff between resolution (the longer the pulse, the narrower the bandwidth) and sweep time
SAW for sensing
Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation
500 moyennes sur 10 balayages 450
400
350 σf (Hz)
Basics: radiofrequency components
300
250 compteur=40 émission 24 µs
200
150
100 10
20
30
40 50 durée émission (compteur)
60
70
80
Acoustic device characterization Conclusion
15 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Radar mode : results
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
With these parameters: 7.7 ms/sweep × number of averages • flexible frequency emission does not necessarily require equally
spaced frequencies (zoom ...) • accumulate sweeps until the pre-defined number of averages is
reached (⇒ known noise level on the measurement) 16 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Radar mode, wideband interrogation
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator
• Generate a short radiofrequency burst: in the 433 MHz ISM band,
the burst must be tuned to comply with regulations while spreading energy on a wide enough frequency range (here '40 kHz) • Sample returned signal • Inverse Fourier Transform (DFT) ⇒ identify resonance frequency • iterative process converges quickly towards the resonance frequency
⇒ heavy requirements on signal processing hardware (fast sampling rate and Fourier transform)
Delay line interrogation Acoustic device characterization
This solution is used by Transense http://www.transense.co.uk/technologies/technical publications/
Conclusion
17 / 52
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components
CW mode • Continuous radiofrequency emission: avoids wideband signal
associated with chopping • Impedance variation measurement as transmitted power through
coupler, or I/Q demodulator ⇒ close to the principle of RFID (magnetic coupling of antennas) but lower dynamics and sensitivity Coldfire 5282
Resonator interrogation Example: temperature sensor
ethernet
GPIO
Acoustic device characterization Conclusion
AD9851
tunable radio frequency source
Delay line interrogation
ADC
4
Signal processing improvements Measurement principles, beyond the resonator
control and acquisition microcontroler
30 MHz TTL
MAX274
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
reference oscillator 54.233+/−0.2 MHz
synthetiser
mesurement PLL
VCO phase shift
f/8 reference frequency multiplication
I/Q demod.
low−pass filter 10 kHz 20 dB low−pass filter 10 kHz 20 dB phase magnitude
AD8302
Vdc
18 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components
Radar mode v.s. CW mode • No demodulation ⇒ poor signal to noise ratio associated with the
wide band reception filter • but cancelling direct signal in a CW configuration is difficult (30 dB
at best)
Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
Blue: VCO polarization, i.e. frequency ; red: phase measurement From left to right: ∆ϕ = 0, 90, 180o on the reference arm
⇒ signal shape change with distance can be compensated for by ∆ϕ in reference arm 19 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
IQ Mobil: modulated 2.4 GHz carrier
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator
• A 2.4 GHz carrier (compatible with ISM regulations) is modulated
at the interrogation frequency (' 10 MHz) • A non-linear element on the passive receiver removes the carrier and
probes the resonator • The decay of the resonator is monitored as an antenna impedance
variation which modulates the continuously emitted 2.4 GHz carrier. ⇒ lower range due to high carrier frequency and need of a nonlinear element on the receiver
Delay line interrogation Acoustic device characterization
http://www.iqmobil.com/index.php?m=IQLECTURES&language=en
Conclusion
20 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor
Local reference stability issue Differential measurement: the uncertainty on the local oscillator is seen on the frequency difference If a differential (two resonator setup) is not feasible • Assume we want a sensor working in -20 to 120o C range. • Assume we wish to comply with 433-ISM regulation (1.5 MHz
bandwidth) • Assume we have a referenced (2 resonance) temperature sensor
⇒ 750 kHz/140 K=5.4 kHz/K i.e. 12 ppm/K.
Signal processing improvements Measurement principles, beyond the resonator
Due to fabrication dispersion, we actually use 6 ppm/K
Delay line interrogation
For 0.1 K accuracy, we must provide a long term local oscillator stability better than 0.5 ppm over the whole temperature range.
Acoustic device characterization Conclusion
One conceivable solution (if applicable): lock local oscillator on GPS. 21 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
GPS reference
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor
Motorola Oncore VP
Thales A12
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
Novatel Superstar2 200 ns on a 1 s 1PPS signal ⇒ 0.2 ppm relative stability (< 0.5 ppm) on the long term since the GPS signal will not be affected by thermal drift, stress etc ... (aging and drift monitored and compensated for by the ground segment of GPS) 22 / 52
Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator
• Embedded sensor monitoring ⇒ the reference oscillator is subject to
large temperature variations (TTL oscillator) 8
x 10
f[Hz]=0.0432*T3−3.4975*T2−107.35*T+cst
15
4.34
10
2 kHz
5
4.34
200
400
600
800
1000
1200
1400
1600
1800
temps (minutes)
1
4.34
60
0.5
0
4.3399
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
4.3399 4
x 10 20 (T/12/4096*1.5)/0.0035−291
4.3399
15
Delay line interrogation
20 MHz resonator, multiplied to 400 MHz (PLL) + mixer at 34 MHz
20
f (Hz)
SAW for sensing
stability in the 10−9 − 10−10 range under stable environmental condition
mesures hobo (oC)
Generation of acoustic waves
• A reference oscillator might display a relative short term ( DAC (u.a.)
80
100
120
PC (recording)
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
• Evolution of the
frequency as a function of DC tuning voltage (open loop). • 1 kHz tuning around
4 MHz (250 ppm) 25 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Local reference stability: time analysis
J.-M Friedt Generation of acoustic waves SAW for sensing Basics: radiofrequency components Resonator interrogation Example: temperature sensor Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization
Open loop: -1 ppm/K around 25o C, temperature fluctuations are visible on the frequency
Closed loop: temperature fluctuations are visible on the feedback control
Conclusion
26 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control
Local reference stability issue (5)
J.-M Friedt −5
10
MSP430, open HP53131 open MSP430, closed HP53131, closed
Generation of acoustic waves −6
10
SAW for sensing
στ
Basics: radiofrequency components
3 s/year
−7
10
1 day
Resonator interrogation −8
10
Example: temperature sensor
−9
10
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization
0
10
frequency deviation ∆f • 5 × 10
−8
1
10
2
10
3
10 τ (s)
4
10
5
10
6
10
relative frequency deviation ∆f /f
= 0.05 ppm is consistent with 50 ns relative stability
• Such a stability is enough for our sensing applications • Global reference for all interrogation units, even widely spaced apart
Conclusion
27 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves SAW for sensing
Temperature sensor design • Dual resonator SAW sensor to subtract correlated noise sources
(stress, environmental effect of antenna) • Two resonators on a same substrate, different orientations for
different temperature drift coefficients
Basics: radiofrequency components Resonator interrogation Example: temperature sensor
micro processor
tunable frequency source power detector
Signal processing improvements Measurement principles, beyond the resonator Delay line interrogation Acoustic device characterization Conclusion
Interrogation unit: principle of radar: 1 switch on radiofrequency source at f 2 wait τ seconds until resonator is loaded (τ ≥ Q/f ) 3 switch off emission and listen for resonator discharge 4 repeat for f → f + fstep 5 after sweeping ISM band, search max=resonance frequency 28 / 52
Wireless and Mobile Acoustic Sensor Interrogation for (Bio)Chemical Sensing and Industrial Control J.-M Friedt Generation of acoustic waves
Temperature sensor on a wheel Example of a temperature measurement on a wheel rotating at 3000 RPM ⇒ 0.2 to 1o C relative temperature measurement
SAW for sensing Rotation, fit 3 measurements, 12.4 ms/10 sweeps
Basics: radiofrequency components
f (MHz)
433.4
Resonator interrogation
433.35 433.3 433.25 433.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2 4
0
Measurement principles, beyond the resonator
Acoustic device characterization Conclusion
0.2
0.4
0.6
0.8
1
time (a.u.)
1.2
1.4
1.6
1.8
2
10 8 6 4 2 0
averages (a.u.)
∆f (MHz)
Signal processing improvements
Delay line interrogation
x 10
signal loss (wheel stopped)
Example: temperature sensor
4
x 10
• Absolute temperature requires preliminary calibration • Interrogation time
