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L. Reindl, TU Clausthal, IEI, Page 1
Wireless Passive SAW Identification Marks and Sensors
2002 IEEE Int`l Frequency Control Symposium & PDA Exhibition New Orleans, LA, USA, 29-31 May 2002, with Tutorials on 1 June.
Leonhard M. Reindl Institute of Electrical Information Technology
Clausthal University of TechnologyLeibnizstrasse 28, 38678 Clausthal-Zellerfeld, Germany
L. Reindl, TU Clausthal, IEI, Page 2
The research and development project, which is presented in this tutorial, was carried out within the last years in the Corporate Technology Department of Siemens AG, Germany.The author would like to thank W.-E. Bulst and M. Guntersdorfer for continuously encouraging this work, and C.C.W. Ruppel, W. Ruile, G. Scholl, F. Schmidt, T. Ostertag, and V. Magori for their important contributions as well as J. Hornsteiner, C. Kordon, O. Sczesny, E. Riha, C.Seisenberger, M. Vossiek, and U. Wolff for their valuable contributions. Special thanks to W.Gawlik, U. Knauer, J. Zimmermann, S. Berek, B. Bienert, H. Zottel, T. Sofronijevi´c, and K.Riek for the manufacture of the SAW devices.
Very important contributions were developed at the Applied Electronics Laboratory of the University of Technology in Vienna, Austria by my teacher F. Seifert, by A. Pohl, R.Steindl, G. Ostermayer, and C. Hausleitner, and the students there, at the Institute for Communications and Information Engineering of the University of Linz, Austria, by R. Weigel, H. Scherr, T. Pankratz, and G. Schimetta, and at the Institute for Measurement Systems and Sensor Technology of the Technical University of Munich, Germany, by E. Schrüfer, and K. Pistor.
Thanks to A. Kirmayr from the University of Applied Sciences, Munich, for his work on the water content sensor and, last but not least, Mike J. Reindl for proof-
reading the text.
Acknowledgement
L. Reindl, TU Clausthal, IEI, Page 3
Outline
• Introduction: Classical SAW Sensors• SAW Radio Read Out• SAW Identification Tags• SAW Radio Readable Sensors• Application Examples• Conclusion
L. Reindl, TU Clausthal, IEI, Page 4
Surface Acoustic Waves (SAW’s)Elliptical particletrajectory Wave propagation at typ.
3000 m/s ≈10-5 vLight
Pene
tratio
nde
pth
≈1
λ
Rayleigh wave
At the same frequency, the acoustic wavelength is 10-5 times that ofelectromagnetic waves.
Einleitung
L. Reindl, TU Clausthal, IEI, Page 5
Seismologic surface acoustic wave
San Francisco, City Hall
April 17, 1906 April 18, 1906
L. Reindl, TU Clausthal, IEI, Page 6
History of SAW
1885 Lord Rayleigh characterizes Surface Acoustic Waves (earth quake)
1965 Invention of the Interdigital Transducer (White/Voltmer)
1970 First applications: pulse expansion and compression in radar systems
1985 SAW filters replace LC filter in TVs and VCRs1990 SAW filters allow for miniaturization of mobile
phones
L. Reindl, TU Clausthal, IEI, Page 7
Wave Excitation and Detection: IDTs
Interdigital Transducer (IDT) as• transmitter: converse piezoelectric effect ⇒ electric RF field generates SAW• receiver: piezoelectric effect ⇒ SAW generates electric RF fieldIn both cases maximum coupling strength for λSAW = vSAW / f = 2·p (~ 1…10 µm)
Top view Cross sectional view A-B
A B
p
Piezoelectric substrate
Electric field lines
Mechanicaldisplacement
L. Reindl, TU Clausthal, IEI, Page 8
SEM-photo of an interdigital transducer and two SAW pulses
Interdigital Transducer, IDT Surface Acoustic Wave, SAW
Bus Bar
L. Reindl, TU Clausthal, IEI, Page 9
Properties of some commonly used substrata materials
1) Cut = crystalline orientation of the substrate surface normal;2) Prop. = crystalline orientation of the wave propagation direction.
Orientation 1)
Cut Prop.
ST X37°rotY 90°rotX
Y Z41°rotY X128°rotY X
3
Material
Quartz
LiNbO3
LiTaO 36°rotY X leaky SH wave 4220X 112°rotY
Wave type vph
(m/s)
gen. RW 3158SH wave 5094pure RW 3488
leaky SH wave 4750gen. RW 3980
gen. RW 33016.6≈ 30
2k TCD
(%) (ppm/°C)
0.1 0≈ 0.1 04.1 94
15.8 695.5 75
0.88 181.35 20.9
Loss (db/µs)
433 MHz 2.45GHz
0.75 18.6- -0.25 5.8- -0.27 5.2
- -
3)3)
3) 3)
3) Depends on metallization.
L. Reindl, TU Clausthal, IEI, Page 10
Design of SAW filters
The design of SAW devices is based on • signal theory (e.g. Impulse response modelling) • network theory (e.g. P-Matrix-formalism, coupling of
modes, equivalent circuit,angular spectrum of strait-crested waves)
• field theory (e.g. FEM)
L. Reindl, TU Clausthal, IEI, Page 11
P-Matrix Formalism
bbi
aau
P P PP P PP P P
aau
1
2
1
2
11 12 13
21 22 23
31 32 33
1
2
=
=
P
Top view
u in
Pa1
b 1
a2
b 2
iin
withP33: transducer admittance [conductance],P13 and P23: stimulation elements [ econductanc ],P11 and P22: reflection elements [ ],P12 and P21: transmission elements [ ].
L. Reindl, TU Clausthal, IEI, Page 12
SAW Device ModellingBasic cells for the P-matrix model
Pa3b3
a4b4
iout
uoutuin
Pa1b1
a2b2
iin
charge density
electrode
P-matrices
... ...
... Pd o Pf o Pu o Pm o Pe o Pm o Pe o Pm...
Device Modelling
L. Reindl, TU Clausthal, IEI, Page 13
SAW Fabrication
Lift-off Technique
Cleaning Resist coating Development Metallization Lift-offExposure Structured Wafer
Etching Technique
Metallization Resist coating Exposure Development EtchingCleaning Structured Wafer
L. Reindl, TU Clausthal, IEI, Page 14
Fabrication - Electrode Widths
900 MHz: finger width = 1 µm
European ISM bands: 434 MHz, 869 MHz, 2.45GHz, 5,8GHz
100 MHz: finger width = 8 µm
250 MHz: finger width = 3 µm
human hair: 60µm thick
1800 MHz:finger width = 0.5 µm
L. Reindl, TU Clausthal, IEI, Page 15
Classical SAW Band pass Filter
AbsorberInput transducer(apodized)
Output transducer(uniform)
Bonding wire
Piezoelectric substratePiezoelectric substrate(quartz, LiNbO(quartz, LiNbO33, LiTaO, LiTaO33))
L. Reindl, TU Clausthal, IEI, Page 16
Block Diagram of a Typical Transceiver
Receiver (Rx)1st Intermediate frequency filter
LO
Modulator
Transmitter (Tx)
Duplexer
Power Amplifier
Low Noise Amplifier
Ant
enna
2nd Intermediate frequency filter
L. Reindl, TU Clausthal, IEI, Page 17
Sensitivity
( ) ( ) ( )
( ) S - S
∆ v
v1 ∆
L
L1
v v
LL =
τ τ
τ
vT
LT
ϑϑϑ
ϑϑ∂
∂ϑϑ∂
∂
ϑϑϑ
∆⋅≡∆⋅≡∆⋅=
⋅⋅−⋅⋅=
∆∆
−∆∆
∆∆
TCDS T
A delay line with centre-to-centre transducer spacing L exhibits a delay time τ of τ=L/v. Hence, a small variation ∆ in a measurand x, like a variation of the temperature ∆T results in change of the delay time change ∆τ of
L. Reindl, TU Clausthal, IEI, Page 18
Sensitivity
[ ]ySyfyyf T ∆⋅−⋅=∆+ τ1)()( 00
[ ]ySyyy T ∆⋅+⋅=∆+ τϕϕ 1)()( 00
[ ]ySyyy T ∆⋅+⋅=∆+ τττ 1)( )( 00
For SAW devices a change of velocity v or delay length Lresults in a change of the electrical measurable quantities
delay time τ,
corresponding phase ϕ,
and the centre frequency f:
The sensitivity Syx gives the change of the parameter x on the quantity y
Syx
xxy
= ⋅1 ∂
∂
L. Reindl, TU Clausthal, IEI, Page 19
Basic electronic circuitry for active SAW sensing
3 dBOscillator
Mixer
Low pass
SAW
Ref.
V~ cos( )ϕ − ϕSAW Ref.
Direct phase measurement
Oscillator
DetectorSAW
Pulsegenerator
fout
TriggerSAW
fout
"Sing-around" arrangement Oscillator
L. Reindl, TU Clausthal, IEI, Page 20
physical sensors Pressure DL 105 Quartz 3.8 ppm/kPaDL 90 AlN/Si 27 ppm/kPa
Selected SAW
from literature
Measurand Device Freq. Substrate Sensitivity
(MHz) Value Unit
Force DL 8.3 LiNbO 3 10.8 ppm/kNStrain R 140.2 Quartz 1.28 ppm/10 -6
DL 10.9 PZT 21 ppm/10 -6
Position (linear) DL 8.3 LiNbO 3 120.5 ppm/ µmPosition (angular) R 434 Quartz 2.86 ppm/mradAcceleration DL 251 Quartz 45 ppm/(m/s 2)
DL 10.9 PZT 8.7 ppm/(m/s 2)Rotation rate DL 10.9 PZT 25.7 ppm/s-2
Flow rate (gas) DL 73 LiNbO 3 204 ppm/(cm3/s)Flow rate (liquid) DL 68 LiNbO 3 105 ppm/(mm3/s)Liquid viscosity DL 30 LiNbO 3 2.7 ppm/cPLiquid density DL 6 ZnO /Si xNy 30000 ppm/(g/cm 3)Electric field DL 900 LiNbO 3 141 ppm/(V/ µm)(normal) R 85 Li 2B4O7 on
piezoceramic300 ppm/(V/ µm)
Electric field(transv.)
DL 1000 LiNbO 3 120 ppm/(V/ µm)
Voltage DL 900 LiNbO 3 0.93 ppm/VLiquid conductivity DL 51 LiTaO 3 13400 ppm/(S/m)Magnetic field DL 140 Fe-B/ Quartz 0.38 ppm/(A/m)Temperature DL 43 LiNbO 3 92.13 ppm/°CRadiation dose R 199 Quartz 0.48 ppm/(J/kg) 0.5
Thin film thickness DL 75 LiNbO 3 9.25 ppm/nm
From :G. Fischerauer, "Surface Acoustic Wave Devices," in: W. Göpel, J. Hesse, J. N. Zemel, H. Meixner, R. Jones (Eds.),Sensors. A Comprehensive Survey, Vol. 8. Weinheim: VCH, 1995(References: see there)
L. Reindl, TU Clausthal, IEI, Page 21
Outline
• Introduction: Classical SAW Sensors• SAW Radio Request• SAW Identification Tags• SAW Radio Requestable Sensors• Application Examples• Conclusion
L. Reindl, TU Clausthal, IEI, Page 22
Operating Principle of Wireless Identification or Sensor Systems
Response Signal
Request Signal
ReaderUnit
passive (SAW-)Transponder
L. Reindl, TU Clausthal, IEI, Page 23
Separation of the sensor response from the request signal
frequency division
time division
request signal
response signal
frequency
A request signal
environmentalechoes
response signal
time division
non linear sensors only
radio sensor
transmitter receiver
down link:
request
signal
up link:
response signal
space division
L. Reindl, TU Clausthal, IEI, Page 24
Operating Principle of Wireless SAW -Identification or Sensor Systems
sensor effect:delay time variation due to a changein length L or in the propagation velocity vadvantages:• wireless read out, read out distance ~ m• transponder is passive & maintenance free• free of ageing ("Quartz-stable")
ReaderUnit Request Signal
Response Signal
SAW Transponder
Interdigital transducer
ReflectorsTransponder
AntennaPiezoelectricPiezoelectricCrystalCrystal
[µs]
1 2
3
4
5
Amplitu
de
L. Reindl, TU Clausthal, IEI, Page 25
The time division of the RF response of a SAW transponder
Request Signal
Environmental Echoes Sensor Echoes
RF Response
Time
L. Reindl, TU Clausthal, IEI, Page 26
Design of Reader Units The reader units of wireless SAW identification or sensor systems applications resemble those used in traditional radar, and all designs used in radar technologies can be applied:Time domain sampling using pulse radar
+ suitable for measuring with a high dynamic resolution of fast changing or moving objects+ very simple duplexer by using a switching device – expensive due to the necessity of fast sampling and fast signal processing devices – low range due to a low duty cycle
Time domain sampling using a chirp radar+ same as a pulse radar, but with an increased range, because the TB-product improves the duty cycle
Restriction: Only medium processing gains are possible: B is restricted by the operating bandwidth of the SAW transponders, and T is restricted by the initial delay of the transponders
Frequency domain sampling using a network analyser structure+ low cost and low speed standard components+ lower demand on the signal processing devices+ high range due to maximum duty cycle– suitable only for measuring low speed changing or moving objects– only a circular device or two separated antennas as duplexer is possible– a high dynamic range of the receiver architectures is necessary
Frequency domain sampling using a FMCW design– same as a network analyser structure, but with a higher dynamic resolution due to the improved measuring speed
But: A high speed Fourier Transform is needed
L. Reindl, TU Clausthal, IEI, Page 27
Reader units utilizing time domain sampling
... assuming fast changing measurands:• the frequency band is transmitted in one burst • the duty cycle may be enhanced by using chip signals • need a fast sampling of the response signal
quadraturedemodulator
fast sampling
coherentoscillator
r(t)I(tn)
Q(tn)s(t)
fastswitching
passivesensor
High speed, but high costdue to fast components
L. Reindl, TU Clausthal, IEI, Page 28
Reader units utilizing frequency domain sampling
very low speed, low coststandard components
quadraturedemodulator
slow sampling
r(f) I(fm)s(f)
slowswitching
passivesensor
sweep-oscillator
synchronizedsweep-oscillator
Q(fm)
... assuming slowly changing measurands:• the frequency of the transmitted bursts is varied step by step Circuitry is like a
network analyser
L. Reindl, TU Clausthal, IEI, Page 29
Reader units utilizing a FMCW principle
Request signalReader Tag
2.45 GHz
Response signal
Pulsed read outSignal
t tamplitude
FMCW read out
Signal
t tfrequency
L. Reindl, TU Clausthal, IEI, Page 30
Modular structure of reader units in a FMCW radar system
SRAM Flash
DSP
UART I/OController AUX Input/Output
A/Dconverter
Prog.Synthesizer x 256
Filter
DSP Unit Baseband Unit RF Unit
2.45 GHzTX
RS 232Interface
LNA
LO
2.45 GHzRX
DSP Unit• FFT
• Communication
• System configuration
• FPGA programming
Baseband Unit• generates the frequency modulation
• A/D converter of the echo signal (time domain)
• controls Aux I/O
RF Unit• transmits (Tx) and receives (Rx)
in the 2.45 GHz ISM Band
• mixing of Tx / Rx
L. Reindl, TU Clausthal, IEI, Page 31
Extended Block diagram of a time domain sampling reader unit
Frequency Range 170 MHz - 2.77 GHz
RX-Bandwidth 1 MHz - 40 MHz
Dynamic Range 85 dB
Max. Output Power 40 dBm
Amplitude Resolution ±0.5 dB
Phase Resolution ± 1°
Noise Figure 5 dB
I
Q
2.7 GHz100 MHz -
70 MHz
70 MHz
RSSI
Q
I
70 MHz Log. Amp.Null
Burst70 MHz
Null
A
A
A
D
D
D
LO
RF IF
RF IF
LOLO
RF
LO
Clock
ReaderAntenna
L. Reindl, TU Clausthal, IEI, Page 32
Reader Unit operating at 2.45GHz, built up using standard ICs.
Open card chassis
RF printed circuit board
L. Reindl, TU Clausthal, IEI, Page 33
RF part of a Reader Unit operating at 434MHz
L. Reindl, TU Clausthal, IEI, Page 34
Current densitymap of the metallicspiral layers
Geometry with ground plane
Reflection Coefficient
frequency [MHz]
refle
ctio
n [d
B]
-6
-5
-4
-3
-2
-1
0
400 410 420 430 440 450 460 470 480
reflection [dB],measured
reflection [dB],simulated
Antenna in a steel package filledwith polymer (diameter = 20mm).
Folded spiral antenna for the 434 MHz band
L. Reindl, TU Clausthal, IEI, Page 35
ID System OIS-W
A company of theGroup
L. Reindl, TU Clausthal, IEI, Page 36
Estimation of the request distance
integration time ti [s]
radar equation
requ
est d
ista
nce
d [m
]
2.45 GHz, IL=40 dB
433 MHz, IL=35 dB
10-5 10-4 10-3 10-2 10-1 1 101
assumed values for estimationsPT: radiated power 14 dBmGT: transmitting antenna gain 0 dBGR: receiving antenna gain -3 dBi / 0 dBi
λ: wavelength 70 cm / 12 cmF: receiver noise figure 12 dB
S/N: signal to noise ratio for detection 20 dBIL: sensor insertion loss 35 dB / 40 dBT0: antenna noise temperature 300 Kti : integration time
1 101
100
10
1
0,1
dP G G t
k T FSN
IL
T T R i=2 2 4
0
4
λ
L. Reindl, TU Clausthal, IEI, Page 37
Multiple access of SAW radio sensorsWith only one sensor in the beam of the reader unit, everything is fine:
"...the temperature is 27 degree Celsius..."
27°C
L. Reindl, TU Clausthal, IEI, Page 38
More than one... ???
240°C
123°C27°C
"...the temperature is .......hm...???"
Example for a TDMA-sample
Request signal
Sum of responsesignals
L. Reindl, TU Clausthal, IEI, Page 39
Outline
• Introduction: Classical SAW Sensors• SAW Radio Request• SAW Identification Tags• SAW Radio Requestable Sensors• Application Examples• Conclusion
L. Reindl, TU Clausthal, IEI, Page 40
in
out
transponderantenna
SAW
transducer
Good design for quartz materials and other substrates with small dielectric and piezoelectric constants
Schematic layout of a SAW ID tag with several transducers wired together to a common bus bar
L. Reindl, TU Clausthal, IEI, Page 41
Schematic layout of a reflective SAW tag
in
out
transducer reflectors
SAW
Good design for LiNbO3 materials and substrates with a high dielectric and piezoelectric constants, half the chip size!
transponderantenna
L. Reindl, TU Clausthal, IEI, Page 42
SAW ID tag with every reflector arranged in a separate track
+No close multi reflections+No dependence of actual bit level on precursors - need reflectors with high reflectivity----- need a transducer with a huge aperture
L. Reindl, TU Clausthal, IEI, Page 43
SAW ID tag where all reflectors are arranged in the same acoustic track
+Small insertion loss-close multi reflection occur
L. Reindl, TU Clausthal, IEI, Page 44
Layout of typical transducers for SAW ID tags and radio requestable sensors
uniform transducer split finger transducer unidirectional transducer
pitch: λ/4 pitch: λ/4 and λ/8pitch: λ/8
L. Reindl, TU Clausthal, IEI, Page 45
Layout of reflectors
a) b) c) d)
uniform open
multistrip coupler effects may occur
shorted multistripcouplerhuge demandof wasted space per track
chevron-type
no multi reflections,but high loss
non-symmetrical reflectionsymmetrical acoustic reflection
L. Reindl, TU Clausthal, IEI, Page 46
Schematic of triple reflection resulting in a delayed spurious signal
N=1 2 3 4 5 6 7
mln i x
L. Reindl, TU Clausthal, IEI, Page 47
Loss of a 33 bit ID-tag as a function of the number of reflectors lined up in one track
Design parameter:• Amplitude weighting (ON/OFF)• 33 bits• dynamic (IL(ON)/IL(OFF)>20dB)• propagation loss between twocontiguous reflectors 0.38dB
• loss due to passing twice a reflector 0.75dB
Symmetrical reflector
Chevron type reflector
L. Reindl, TU Clausthal, IEI, Page 48
Layout, Photo and measurements of a mounted SAW ID tag comprising 33 reflectors in 4 tracks (f0 = 2.45 GHz) (Amplitude Coded)
Measurements: 8 "ON", 8 "OFF",8 "ON“, 8 "OFF", 1 "ON
L. Reindl, TU Clausthal, IEI, Page 49
Coding Schemes• Amplitude Coding (ON/OFF)
+ insensitive on small velocity variations+ temperature effects can be eliminated using a start and stop bit± problems with the uniformity can be avoided by using special OFF structures with
the same damping but no reflective properties than ON structures– high insertion loss due to the high amount of reflectors (and also due to the OFF
structures)• Phase Coding
+ 2PSK: lower bit error rate by using a 2-PSK coding scheme than by ON/OFF-amplitude coding with the same signal-to noise-ratio
+ 4-PSK and higher coding schemas are possible, which reduces the total amount of reflectors / symbols
– very sensitive to small velocity variations– temperature effects have to be cancelled very carefully
• Pulse Position Coding+ higher coding schemas are possible+ insensitive on small velocity variations and temperature variations+ small insertion loss due to the small amount of reflectors used
L. Reindl, TU Clausthal, IEI, Page 50
Layout and Photo of a mounted SAW ID tag comprised of 5 reflectors in one tracks (f0 = 2.45 GHz) using pulse position coding
2nd reflector
Possible coding positionof 1st reflector
1st reflector
AntennaPossible coding positionof 2nd reflector
Tag by
Pulse position coding schema
L. Reindl, TU Clausthal, IEI, Page 51
Outline• Introduction: Classical SAW Sensors• SAW Radio Request• SAW Identification Tags• SAW Radio Requestable Sensors
- (Reflective) Delay Lines- Resonators- (Reflective) Dispersive Delay Lines- Non-linear Sensors
• Application Examples• Conclusion
L. Reindl, TU Clausthal, IEI, Page 52
Photo of a mounted SAW radio readable temperature sensor and corresponding time domain response
Time domain response
Am
plitu
de [d
B]
Time [µs]
ϕ2−1ϕ3−1
ϕ4−1
bonding wiresreflectors
LiNbO3 chipLiNbO3 chipInterdigitalInterdigitalTransducerTransducer
phases in a polar chart
L. Reindl, TU Clausthal, IEI, Page 53
Evaluation of the phase difference
reference
... enhances the time resolution by a factor of 100 andyields to a relative resolution of 10-5 to 10-6.
Signal 1 Signal 2
ϕ1 n•2πϕtot is calculated with τtot
ϕ2
L. Reindl, TU Clausthal, IEI, Page 54
SAW Sensors using a Delay Line Configuration
[ ][ ]∆y )S(y)S(y∆
∆y )S(yτ)S(yτ∆τ
y,101y,20212
τy,101
τy,20212
ϕϕ ϕϕϕ −=
−=
−
−
In most sensing applications using a delay line a differential arrangement is applied and the change ∆ in the difference of
τ2-1 = τ2 - τ1or ϕ2-1 = ϕ2 - ϕ1
between two signals (#1) and (#2) is evaluated. For ∆ τ2−1 and ∆ ϕ2−1 we get:
L. Reindl, TU Clausthal, IEI, Page 55
SAW Sensors using a Delay Line ConfigurationIf the sensitivities Sτ
y,2 and Sτy,1 for the signals (#1) and (#2) are
equal, we get
ySff y ∆⋅⋅∆⋅=∆=∆ −−ττπτπϕ 2 2 1212
yS y ∆⋅⋅=∆ −−τττ 1212
2 • (number of acoustic wave-lengths between both reflectors),
typical several hundreds
The phase difference can be determined and measured very accurately. Thus the evaluation of the phase provides a high sensitivity.
L. Reindl, TU Clausthal, IEI, Page 56
Switch able SAW Tag operating at 2,45 GHz
acoustic track of label bit
acoustic track of alarm bit
SAWSAW--chipchipY,ZY,Z--LiNbOLiNbO33
}
}
housingantenna
alarm switch
I R
I: IDTR: Reflector
RI
L. Reindl, TU Clausthal, IEI, Page 57
Special reflective delay line: SAW device combined with an external classical sensor
YPPPYP sc +
⋅+=
33
213
,11112)(
ρ Z
RF RequestSignal
RFResponse
Antenna
Interdigital Transducer
Piezoelectric Crystal
Interdigital Transducer
Load Impedance(Switch)
Only one reflector is shown!
L. Reindl, TU Clausthal, IEI, Page 58
Layout of a SAW Device which can be combined with an external classical sensor
s 3ˆ µ= s 1ˆ µ= s 1ˆ µ=
Timing Housed SAW Chip
Reflector #2 is built up as transducer. The electrical port of the transducer can be loaded with the impedance of an external classical sensor. Therefore, the acoustical reflection of reflector #2 becomes a function of the applied complex load impedance, which is given by the external sensor element.
L. Reindl, TU Clausthal, IEI, Page 59
Typical Impulse Response
L. Reindl, TU Clausthal, IEI, Page 60
open4,1 pF
19 pF
46 nH
18 nH 60 nH
C
L
R
Measured acoustic reflectivity of a split finger IDT as a function of the applied electrical load
( )
load
scload
ZP
PPZP
12
33
213
1111
+
⋅+=
P Z Ploadsc
11 11 0( ) = ≈serial resonance:
0}1Im{ 33 =+loadZ
P
parallel resonance:
L. Reindl, TU Clausthal, IEI, Page 61
L. Reindl, TU Clausthal, IEI, Page 62
Binary SAW Sensors
time [µs]
s 11(t
) [dB
]s 1
1(t) [
dB]
open
shortedReed element- sector alignment indicators- radio accessible switches
- readout of classical sensors withvarying impedance
L. Reindl, TU Clausthal, IEI, Page 63
Schematic block diagram of a semi-active SAW tag using IDT reflectors
re-transmittedinformation
incoming RF-request signal
hybrid
antenna
SAW-device
RF-commutatornetwork
control
memory
powersupply (RF-rectifier or battery)
L. Reindl, TU Clausthal, IEI, Page 64
Circuitry for dynamic switching of one IDT reflector
G D
S
C
L
C
CR
R
L
L
IDTreflector on the SAW substrate
SAW-device
L. Reindl, TU Clausthal, IEI, Page 65
Photo of a mounted SAW chip in a hybrid commutatornetwork for switching 8 IDT reflectors
SAW ChipTransducer
switching circuitry for switching one IDT reflector
L. Reindl, TU Clausthal, IEI, Page 66
Measurements of the switching states of the switchable SAW ID-Tag shown in the last chart
all "ON"
all IDT reflectors "OFF" 3 "ON"
|S11
| [dB
]
|S11
| [dB
]
time [µs]
|S11
| [dB
]
time [µs]
time [µs]
L. Reindl, TU Clausthal, IEI, Page 67
Schematic drawing of a SAW resonator used as radio requestable sensor
Antenna Interdigital Transducer
Piezoelectric Crystal Reflector
RF RequestSignal
RF Response
Number of stored wavelengths nλ∼ Q loaded
L. Reindl, TU Clausthal, IEI, Page 68
Signal Processing: Evaluating the Resonant Frequency of Resonators
Frequency domain
time [µs]
S 11[d
B]
S 11[d
B]
not gated
gated
frequency [MHz]
time domain
L. Reindl, TU Clausthal, IEI, Page 69
The change in centre frequency f and phase ϕ is given by:
QOFW
≈10 000∆ϕ = ∆τ⋅ ⋅ ⋅2πQ S yy
∆ ∆f f S yy
= ⋅ ⋅0
τ-
Resonator Mathematics
Same sensitivity as a delay line, but significant reduction of chip size
L. Reindl, TU Clausthal, IEI, Page 70
High-Q Dielectric Resonators
2,06
2,07
2,08
2,09
2,1
2,11
2,12
2,13
300 350 400 450 500 550 600 650 700 750
temperature [°C]
reso
nanc
e fr
eque
ncy
[GH
z]
The resonator is stimulated by a RF signal, the decaying signal is detected after switching off the stimulus.e.g. for high temperature measurements
... utilizing the temperature shift of the resonance of a dielectric microwave
resonator.
ceramic substrate
dielectric resonator
coupling patch antenna
L. Reindl, TU Clausthal, IEI, Page 71
Special Radio Readable Resonator: Pulling of the SAW Resonator with an External Sensor
Antenna Interdigital Transducer
Piezoelectric Crystal Reflector
RF InterrogationSignal
RF Response
Z
L. Reindl, TU Clausthal, IEI, Page 72
Dispersive Delay Line - Principle
Antenna
Interdigital Transducer Reflector
Piezoelectric Crystal
RF InterrogationSignal
RF Response
L. Reindl, TU Clausthal, IEI, Page 73
Dispersive Delay Line - Mathematics
sensitivity of a delay line
( ) ( )
ySfB
T
yfBTySy
y
y
∆
∆∆±∆=∆∆
ττ
ττ
00
0
τ 01= m
ySS ylinedelay ∆⋅⋅=∆ −τϕ 12
may be 10 - 100 times of thesensitivity of a delay line
L. Reindl, TU Clausthal, IEI, Page 74
Dispersive Delay Line - Effectsynchronous frequency y
τττ
τ τ ∆⋅⋅⋅
⋅±=∆ yS
BfT1
delay timeτ0
∆ f
∆τ
τ0(y+∆y)
f0 y+∆y
B
T
L. Reindl, TU Clausthal, IEI, Page 75
Dispersive Delay Line - Measurement
Gro
up d
elay
tim
e τ 2
−1[n
s]
Temperature [°C]
up chirp
not chirped =linear phased
down chirp7000
7500
0 50 100
8000
L. Reindl, TU Clausthal, IEI, Page 76
Outline• Introduction: Classical SAW Sensors• SAW Radio Request• SAW Identification Tags• SAW Radio Requestable Sensors• Application Examples
– ID Tags– Temperature Sensors– Mechatronic Sensors– Current Sensors– Water Sensors
• Conclusion
L. Reindl, TU Clausthal, IEI, Page 77
Application of fixed coded or writeable tags
host
slave1 slave 2 slave 3 slave 4
material flow
host
4
material flow
321
data flow
Schematic of a material accompanying control system:write able tags are needed
Schematic of a master-slave control system:fixed coded tags are sufficient
L. Reindl, TU Clausthal, IEI, Page 78
Application of SAW ID-Tags a Norway Toll System
L. Reindl, TU Clausthal, IEI, Page 79
OFW ID System SOFIS installed on the Munich Subway System
SAW ID-Tag mounted on each subway carSAW ID-Tag mounted on each subway car
Tag housing antenna of the 2.45 GHzinterrogation unit
L. Reindl, TU Clausthal, IEI, Page 80
SAW Identifikation Systemsfor Manufacturing/ Logistics Management
long readout distance, - highly flexible assembly set-up,high temperature stability - only one single ID system for entire
production process
SAW ID-Tags Reader UnitAntenna
Tag
L. Reindl, TU Clausthal, IEI, Page 81
SAW Temperature Sensors∆ ∆ϕ π τ= 2 0f∆ ∆
∆∆∆
∆ •∆τ
τ= −
=1 1
1llT v
vT
T TTCD
ampl
itude
[dB
]
∆t = 0.8µs∆t = 0.8µs
∆t = 2.27 µs∆t = 2.27 µs
temperature variation [°C]ph
ase
varia
tion
[rad
/ 2 π
] variation of the phasessensor response in time domain
delay time [µs]
∆t = 0.8 µs∆t = 2.27 µs
∆tmax=2.27µs accuracy: 0.02 °C, but small range of unambiguity∆tmin=0.03µs range: 450 °C
L. Reindl, TU Clausthal, IEI, Page 82
phas
e di
ffere
nce
reflectors 1->11reflectors 1-> 2
9 π
Phase difference between 3 selected reflectors on LiNbO3-YZ-Cut versus temperature
6 π
3 π
12 π
15 π
18 π
-20 0 20 40 60 80 00 120 140temperature [°C]
L. Reindl, TU Clausthal, IEI, Page 83
60,00
70,00
80,00
Time
Tem
pera
ture
°C
Brake (with attached SAW temperature transponder, not seen)
Brake temperature of a train entering a station
90,00
100,00
reader antenna
L. Reindl, TU Clausthal, IEI, Page 84
0 25 50 75 10020
40
60
80
100
120
140
rotationoperation under load
reference measurement
roto
r tem
pera
ture
time [min]
Measurement of the rotor temperature in a 11 kW asynchronous motor
readerunit
antenna ofthe reader unit
eddy-currentbrake
attached SAW temperature transponder
rotor
Sketch of the set-up
measurement curve
L. Reindl, TU Clausthal, IEI, Page 85
Online Temperature Monitoring System for High-Voltage Surge Arresters
SAW Temperature Sensor
Antenna Radar Unit
RF-Frontend
MO-resistors
SAW-sensor withslot antenna housing
Ø 70 mm
70 m
m
Sensor chip
Aluminum housing
Slot
360 kV surge arresters equipped witha SAW temperature monitoring system
360 kV surge arresters equipped witha SAW temperature monitoring system
SAW temperature sensorin a stack of MO varistors
SAW temperature sensorin a stack of MO varistors
L. Reindl, TU Clausthal, IEI, Page 86
Field Test Results
the main featuresthe main features • aging: long term temperature increase• event counter: sudden temperature rises• energy monitoring: due to a simple correlation
between temperature and absorbed energy
-8
0
8
16
24
32
0 95 190 285 380Time [d]
T [°C
]
Upper unit
Ambient temperature
Lower unit
L. Reindl, TU Clausthal, IEI, Page 87
Signal attenuation versus temperature on LiNbO3-YZ
70
30
40
50atte
nuat
ion
[dB
]
•Up to 200ºC standard assembly, interconnect and package techniques can be used.
•Up to 350ºC aluminium can be used for electrodes material
•LINBO3 can not be used for temperatures higher than 400ºC for short time and300ºC for long time operation.
Aluminium electrodesAluminium electrodesform dropletsform droplets
-100° 0° 100° 200° 300° 400° 500°
temperature [°C]
L. Reindl, TU Clausthal, IEI, Page 88
Delay line for testing the high temperature features of Langasit (La3Ga5SiO14)
SMD-Ceramic Housing
Platinum Bonding Wires
SAW SAW SubstratSubstratLaLa33GaGa55SiOSiO1414
InterdigitalTransducer (IDT) madewith Platinum (50 nm) on Titanium (4 nm)
Chip fixing with Platinum wires
L. Reindl, TU Clausthal, IEI, Page 89
SMD-Ceramic Housing
IDT IDT
IDTIDT
Chip made fromX,Y-Langasit (La3Ga5SiO14)
Chip fixing with Platinum wires
Platinum Bonding Wires
IDTs made with 50 nm Platinum on 4 nm Titanium
L. Reindl, TU Clausthal, IEI, Page 90
ss [d
B] 25 °C1000 °C
-40SAW Sensors with Platinum Electrodes
Inse
tion
lo
frequency [MHz]92 96 100 104 108
-80
-60
-20High Temperature
on Langasit(La3Ga5SiO14)
Test chip at room temperature Test chip at 1000°C
L. Reindl, TU Clausthal, IEI, Page 91
Increase in Insertion Loss of a Delay Line on X,Y-Langasit (La3Ga5SiO14) with Pt-electrodes with Increasing Temperature
90 92 94 96 98 100 102 104 106 108
Inse
rtio
n L
oss [
dB]
Frequency [MHz]
-60
-40
-30
-80
-90
-20
-50
-70
25 …955 …1000 …1060 °C
L. Reindl, TU Clausthal, IEI, Page 92
SMD - packaging with W/Ni/Au-metallization before heating
L. Reindl, TU Clausthal, IEI, Page 93
detail after 1 hour at 500 °C …
ca. 80 µm
...after 1 hour at 500°C
0,8mm
...after 1 hour at 700°C
...after 1 hour at 800°C
detail after 1 hour at 700 °C …
... and after heating at higher temperatures
L. Reindl, TU Clausthal, IEI, Page 94
Wirelessly Readable Passive Sensors for Force and Mechanical Displacement
force
displacementIDT reflectors
... if a mounting is chosen, which letsforces act on the SAW chip to bend it, we get a wirelessly readable passive
sensor for force or mechanicaldisplacement.
... due to the bending of the substrate, both the surface’s length and theelasticity constants are changed.
0200400600800
10001200140016001800
0 20 40 60 80 100
120
140
160
displacements [µm]
phas
e sh
ift [
degr
ees]
The dynamic range of monitoringthe forces with SAW can be up to several tenths of kHz
L. Reindl, TU Clausthal, IEI, Page 95
SAW Torque SensorAn applied torque induces astretching of one and a correspondingcompressing of the other sensor.
The torque is calculatedfrom the phase differences.
The dynamic range of monitoringthe torque with SAW can be up to several tenths of kHz
strain in ‰
phas
e di
ffere
nce
in d
egre
e
0°
50°
100°
150°
200°
150°
0 0,05 0,10 0,15 0,20
SAWsensors
rotatingshaft
rotating trans-ponderantennas
fixed reader antennas
measurement set-up
L. Reindl, TU Clausthal, IEI, Page 96
SAW Rotary Torque SensorElectronicinterface
SAWRF-couple
Rotationalstresses
www.sensors.co.uk
Surface strain due to torque acting on a shaftSurface strain due to torque acting on
a shaft
F F
www.sensors.co.uk
L. Reindl, TU Clausthal, IEI, Page 97
SAW pressure sensor
diaphragm
adhesive
cover-plateclosed cavity withreference-pressure
antenna
1,001,201,401,601,802,002,202,402,602,803,00
14:30
:5514
:30:59
14:31
:0414
:31:09
14:31
:1314
:31:18
14:31
:2214
:31:27
14:31
:3214
:31:36
14:31
:41
pressure[Bar]
time
Two Track Railway Crossing
Adjacent Water Channel
A SAW pressure sensor type, which uses a direct bending of the SAW chip, results in a resolution of about 1% of full range.
Wafer processingof SAW pressure sensors
L. Reindl, TU Clausthal, IEI, Page 98
tire pressure sensor, presented by G. Schimetta
matchingcircuit
capacitive micro-machined pressure sensor
sensor mountingThe patch antenna with the integrated sensor board is mounted on the rim with a stress ribbon.
bond wires
L. Reindl, TU Clausthal, IEI, Page 99
SAW sensorelementthread element
The intelligenttire
The friction coefficient between a car tire and the road surface, which is a key parameter when stabilising a vehicle in critical situations, can be measured by evaluating the mechanical strain in the tire surface contacting the road. This can be done by monitoring the deforming of the tread elements.
Schematic drawing of an experimental SAW bending beam
force acceptorcover for SAW chip
Intelligent tire due to a sensor in the tire / road contact area
L. Reindl, TU Clausthal, IEI, Page 100
SAW sensor for tire friction control
mounted SAW sensor
Radiography of a tire with integrated
SAW sensor
SAW sensor integrated into a standard tire
L. Reindl, TU Clausthal, IEI, Page 101
SAW sensor for tire friction control
bristle model
propulsion braking
Free rolling
entering running outprofile element
-8
-6
-4
-2
0
2
4
6
8
0 100 200 300 400 500 600 700 800 900
Def
orm
atio
n of
pro
file
elem
ent
-8
-6
-4
-2
0
2
4
6
8
0 100 200 300 400 500 600 700 800 900
time / 25 µs
dry road surface
Def
orm
atio
n of
pro
file
elem
ent
wet road surface
The deformation of a profile element gives information of the friction coefficient betweentire and road
time / 25 µs
L. Reindl, TU Clausthal, IEI, Page 102
A radio requestable SAW accelerometer can be attained if seismic mass is added
SAW accelerometer configurations:
Seismic massSAW chip Acceleration
Flexuredcantilever beam
Tensioned cantilever beamCircular diaphragm
L. Reindl, TU Clausthal, IEI, Page 103
Wireless Measurement of Deceleration
Passive SAW sensor fixed to a dDart arrow, invading the target-2
-1
0
1
2
3
4
5
6
1
dece
lera
tion
time
arrow invadingthe wooden back plane
arriving thesurface made offoam PE
invading
SAW sensor
viscousdamping
seismic mass
SAW accelerometer using a seismic mass and a flexuredSAW cantilever beam
L. Reindl, TU Clausthal, IEI, Page 104
SAW Current Sensors
Impu
lse
resp
onse
[dB
]
Time [µs]
antennamagnetoresistort1 t2 BB
SAW delay linemagnetoresistor
Diff
eren
ce in
Am
plitu
de [d
B]
Current [A]
resolution ~ 5% of full scale
L. Reindl, TU Clausthal, IEI, Page 105
Sketch of a tire wear sensor using a GMI wire and magnetize able particles
magnetizeable particles
steel belt GMI wire
SAW
treadelements
inside layer withtextil cord
h
antenna
B0
L. Reindl, TU Clausthal, IEI, Page 106
Water Content Sensor: Scenario
soil
RF requestsignal
requestunit
RF responsesensor
L. Reindl, TU Clausthal, IEI, Page 107
Schematic of the SAW Water Content Sensor
L2L1L1
SAW-Element
Rod Socket
Sensing Electrodes (Rods)
CircuitBoard
Matching Element
Antenna6cm 1.5cm
.5cm
One reflector is electrically loaded by the impedance of the sensing electrodes
L. Reindl, TU Clausthal, IEI, Page 108
Schematic of the Water Content Sensor Electrodes (Rods) and Corresponding Matching Circuitry
Overall sensor circuitry
sandy soil
antenna
transducer CL
electrodes
L
L. Reindl, TU Clausthal, IEI, Page 109
Electrical Reflection Coefficient of the Sensor Electrodes (including Matching Circuitry) versus Water Content
ε‘~30ε‘~55< ε‘< 30
water content
Change of the permitivity ε‘ of sandy soil with increasing water content
Dry Soil
FreeWater
WaterSaturatedSoil
IncreasingWaterContent
0
4'c
fl ⋅⋅⋅−=∆
πεϕ
L. Reindl, TU Clausthal, IEI, Page 110
Two Measurements
Moist Soil (21 %)Dry Soil (7 %)
L. Reindl, TU Clausthal, IEI, Page 111
Amplitude and Phase Differences of the Echo Signals of Reflector #2 and #1
Water content Θ
100º
200º
300º
Diff
eren
ce o
f the
am
plitu
des o
f th
e si
gnal
s at 3
and
4 µ
s 0 dB
-10 dB
-20 dB
-30 dB
-40 dB0% 5% 10% 15% 20%
Phas
e-di
ffer
ence
L. Reindl, TU Clausthal, IEI, Page 112
Outline
• Introduction: Classical SAW Sensors• SAW Radio Request• SAW Identification Tags• SAW Radio Requestable Sensors• Application Examples• Conclusion
L. Reindl, TU Clausthal, IEI, Page 113
The potential of remotely read SAW sensorsMonitoring of physical and chemical quantities in inaccessible or hazardous zones (heat, cold, moving parts, high voltage, radiation, vacuum, poison, behind concrete, danger of explosion).
Examples:• Identification marks (cars, persons, ...)• Temperature of moving parts (drives, turbine blades, rotating anodes) or in
vivo• Torque of a rotating shaft• Force, pressure, light, corpuscular radiation, contamination, current, voltage,
humidity, …• Burning off in high-power switches• Chemical concentration in closed containers or in waste water• Numbered sensors (identification function), "read-me flag”, positioning sensors• Hybrid devices comprising variable-impedance elements and SAW devices
L. Reindl, TU Clausthal, IEI, Page 114
Resolution of SAW Passive Wireless Remote Sensingmeasurand physical effect resolutionidentitification analysis of signal 32 Bittemperature variation of SAW velocity 0.1 Kmechatronic measurands (pressure,torque, acceleration, tire-roadfriction)
variation of elastic constants 1% of fullscale
impedance sensors variation of amplitude and phaseof reflected signal
5% of fullscale
distance signal delay 20cmrelative position continuous measurement of
Doppler phase2cm
angular positioning measurement of Doppler phase 3 degrees
L. Reindl, TU Clausthal, IEI, Page 115
Conclusion
• The generation and the physical properties of SAWs,• the operating of a SAW identification system,• the design of SAW ID tags and radio sensors• applications of SAW ID tags• and SAW radio readable sensors
has been presented.
Recommended