sensors S. Ballandras 1, J.-M Friedt 2 slides and references available at March 17, 2008

Transcription

1 Surface acoustic S. Ballandras 1, 2 1 FEMTO-ST/CNRS, Besançon, France 2 SENSeOR, Besançon, France slides and references available at March 17, 28 1 / 17

2 Generating 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 wave (m/s) TCF comment LiNbO 3 shear 47-9 ppm/k ferro. & pyroelectric, T C > 12 o C LiTaO 3 shear ppm/k 525 < T C < 7 o C KNbO 3 Rayleigh 28 < 1 ppm/k huge coupling, T C 43 o C shear 35? LiB 4 O 7 Rayleigh 35-3 ppb/k 2 water soluble langasite Rayleigh 29-7 ppb/k 2 no Curie, > 1 o C Quartz Rayleigh ppb/k 2 most used shear 51-6 ppb/k 2 less coupled 2 / 17

3 Acoustic wave Interdigitated transducers (IDT) patterned (lithography) on piezo substrate define wavelength ε piezo ε air efficient electric field confinement in piezo substrate Conversion from electric to acoustic wave Wavelength in the micrometer to tens of micrometers range, velocity [3 1] m/s 2-1 MHz depending on design Sensing principle: variation of velocity induces variation of propagation delay 3 / 17

4 Acoustic wave Boundary conditions define velocity and insertion losses. E Thermal expansion/stress of the substrate change velocity c = ρ (E Young modulus, ρ density), complex in anisotropic materials Intrinsically radiofrequency devices no conversion from DC to RF sensitive to, stress (pressure), gravimetric (mass), viscosity select the dominant effect by selecting the appropriate design. For example: selection of a compensated orientation for -independent. 1st order coef. TCF 1 (ppm/k) 5 propagation= o propagation=1 o 45 propagation=2 o 4 propagation=3 o propagation=4 o ppm/k θ=36 (AT) TCF θ ( o ) TCF computation by M. Bruniaux 4 / 17

5 Interrogating acoustic wave 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 245 MHz ISM band is 8 MHz wide resonator or delay line 5 / 17

6 Interrogating acoustic wave 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 245 MHz ISM band is 8 MHz wide resonator or delay line 6 / 17

7 Temperature design Dual resonator SAW to subtract correlated noise sources (stress, environmental effect of antenna) Two resonators on a same substrate, different orientations for different drift coefficients micro processor tunable frequency source power detector 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 + f step 5 after sweeping ISM band, search max=resonance frequency 7 / 17

8 Surface acoustic Temperature on a wheel Example of a measurement on a wheel rotating at 3 RPM.2 to 1o C relative measurement Rotation, fit 3 measurements, 12.4 ms/1 sweeps f (MHz) f (MHz) time (a.u.) averages (a.u.) x 1 signal loss (wheel stopped) 4 x 1 Absolute requires preliminary calibration Interrogation time <1 ms Range '2 m in free space, 3 cm in soil, a few cm in living body 8 / 17

9 Burried no battery and wireless monitoring compatible with long-term monitoring of building complement to current Ground Penetrating Radar (GPR) methods ( cooperative target ) range of 3 cm demonstrated in wet soil, should be possible beyond considering the range of GPR Data provided by L. Chommeloux (SENSeOR) 9 / 17

10 Burried (2) Evolution of the 3 and 5 cm deep, in wet soil T ( o C) std 3 cm moy 3 cm std 6 cm moy 6 cm time (days since 1/1/28) ε soil 9 2 λ soil / cm 1 / 17

11 Temperature and pressure Packaging challenge: include a reference cavity at known pressure Temperature is needed to compensate reference pressure change Three resonators: one reference, one, one pressure Freq. Variations with (P,T) ; P =.. 5 bars 434,8 434,6 434,4 F1,F2,F3 (MHz) 434, ,8 433,6 433,4 433, Temperature T ( C) Data provided by L. Chommeloux (SENSeOR) 11 / 17

12 Wired v.s wireless Wireless sensing opens new applications for passive, but measurement precision=25 Hz (sweep time+signal processing), Closed loop (oscillator) still most sensitive solution: cm 2 iron beam loaded with 2 g weights, 2 oscillators g, 35 Hz 192 g, 33 Hz oscillator1 oscillator MHz f (MHz) g time (s) Design and realization of the oscillators: G. Martin 12 / 17

13 Delay line measurement Increasing f reduces Q and response time is Q/π periods Delay line needs long enough pulse to generate acoustic wave, but short enough to define each reflection Classically used for identification (SAW-tags): here we want to add measurement Time domain analysis, but information can also be extracted from inverse Fourier transform of wideband frequency sweep (network analyzer) 25 2 maximum position (parabola fit, pixel) freezing freezing heating curve number (~1 curve/second) 13 / 17

14 Need for simulation tools Fundamental parameters: evolution of piezoelectric substrate properties as a function physical parameter IDT width and thickness reflection coefficient of delay line mirror = Q of resonator design parameters for accurate prediction of working frequency mixed matrix model for acoustic propagation characteristics BUT requires tabulated parameters: quartz is the best characterized piezoelectric substrate. I s g s d = Y jβ jβ jβ jr exp( jϕ) t exp( jϕ) jβ t exp( jϕ) jr exp( jϕ) V e g e d 14 / 17

15 3D modelling Harmonic analysis (frequency sweep) + biperiodic structure + Green function radiation p = 1 µm, LiNbO 3 YX, Rayleigh wave (2D analysis, 3D fields) p = 2 µm, w = 5λ, quartz YX, Rayleigh wave (3D analysis) 15 / 17

16 Modelling a delay line LiNbO3 128 Rayleigh Reflectivity a/p h/lambda Reflection coefficient v.s h/λ and a/p ap_5_h_.4um_1pc.data u 1:2 ap_5_h_.8um_2pc.data u 1:2 ap_5_h_1.2um_3pc.data u 1:2 ap_5_h_1.6um_4pc.data u 1:2 ap_5_h_2um_5pc.data u 1: apidt_.2.data u 1:2 apidt_.3.data u 1:2 apidt_.4.data u 1:2 apidt_.5.data u 1:2 apidt_.6.data u 1:2 apidt_.7.data u 1:2 apidt_.8.data u 1:2 apidt_.9.data u 1: Al thickness: h/λ a/p IDT Simulations performed by M. Bruniaux 16 / 17

17 Conclusion Use of surface acoustic wave for passive, RF, wireless Methods for charaterizing SAW resonator and delay lines of physical quantities (, pressure, stress, torque) in moving or hostile environments Accurate prediction of behavior requires modelling tools / 17