hybrid saw pressure and temperature sensor - intrasaw - home

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Dr. A.S.Rukhlenko

[email protected]

Neuchâtel, 2005

www.intraSAW.com

Hybrid Wireless SAW Sensor forPressure and Temperature

Measurement

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Introduction

1. Advantages of SAW Sensors2. Hybrid SAW + MEMS Pressure Sensor3. Tire Monitoring System4. Hybrid Sensor Schematics5. Transceiver Block Diagram6. Hybrid Sensor Unit Prototype Construction7. SAW Sensor Modeling8. Temperature Compensation and Measurement9. Hybrid Sensor Parameters

Conclusions

Outline

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Basic Wireless Monitoring Techniques

Radio transmission with an active sensor unit

RF carrier signal power supply (AC/DC conversion)

Inductive coupling (short-distance)

RF signal reflection (passive transponder)

Active (Battery Powered) Sensors

Limited lithium battery lifetime

Battery replacement inside the tire

Battery waste management problem

active

passive

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Wireless Surface Acoustic Wave (SAW) Sensors

Wireless interrogation (energy supply via the electromagneticRF field of the transceiver unit)

Large readout distance (2-3 m, ~mW)

Temperature stability

No battery required

No aging

Low mass and size

Low cost

Batch (group-type) mass production

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Hybrid SAW + MEMS Pressure Sensor

Construction:

Hybrid SAW sensor = SAW reflective delay line + high-Q capacitivemicromachined pressure sensor as the electrical load.

Micromachined Capacitive Pressure Sensor Functions:

1) Measure pressure (direct function)2) Load electrically (capacitive high-Q load) the SAW sensor

SAW Sensor Functions:

1) Measuring the temperature2) Compensation temperature in the pressure measurement3) Wireless transmission of the measurand data (pressure, temperature)

Simultaneous monitoring of the tire pressure and temperature becomespossible.

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Tire Monitoring System

G. Schimetta, et al. Wireless pressure and temperature measurement using a SAW hybrid sensor. 2000 IEEE Ultrasonics Symposium Proc., Vol. 1, p. 445 – 448.

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LC-matching circuit

capacitive micro-machined pressure sensor

Hybrid SAW Sensor Schematics (Dual Track)

jP

DjR = jR2–jR1

L L

G.Schimetta, et al. Optimized design and fabrication of a wireless pressure and temperature sensor unit based on SAW transponder technology. Microwave Symposium Digest, 2001 IEEE MTT-S Int., vol. 1, 20-25 May, p. 355 – 358.

The measurement cycle is initiated by a RF burst signal emitted from the wheel arch antenna of the central transceiver unit. This signal is received by the antenna of a SAW transponder unit mounted on the rim. The interdigitaltransducer (IDT) connected to the antenna transforms the received signal into a surface acoustic wave (SAW). All of the three acoustic reflectors are placed within the acoustic paths of the SAW transponder. The first and third reflector are used as reference, whereas the second one is electrically connected to (impedance loaded by) a pressure sensor. In the IDT the reflected acoustic waves which contain the sensor information are reconverted into an electromagnetic pulse train to be retransmitted back to the central transceiver unit, where the received signal is amplified, down converted and analyzed.

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Equivalent Hybrid Sensor Circuit

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Transceiver Block Diagram (433.92 MHz)

∼∼∼

∼∼∼I

XO10.7 MHz

LO423.22 MHz

LNA

LNA

µC

CAN

QDMA/DQ

XO – crystal oscillatorLO – local oscillatorLNA – low-noise amplifierQDM – quadrature demodulator

A/D – analog/digital converter

A – antennaCAN – controller area networkµC – microcontroller

A

Trigger

G. Schimetta, et al. A wireless pressure-measurement system using a SAW hybrid sensor. IEEE Trans. Microwave Theory and Techniq., vol. 48, No. 12, 2000, pp. 2730-2735.

This is virtually a pulse radar scheme.

The transmitted burst signal is created by switching an IF continuous-wave signal, where the switch is triggered by a microcontroller. The generated 10.7 MHz burst signal is filtered and mixed with the 423.22 MHz signal. The resulting amplified burst meets the specifications of the 433.92 MHz industrial-scientific-medical (ISM) channel. After having transmitted the interrogation burst signal, the transceiver is switched into receiving mode. The incoming sensor signal is amplified by a low-noise amplifier (LNA), down converted to the intermediate frequency and filtered. Finally, it is demodulated in the quadrature demodulator unit. The digitized I and Q signals are processed by a microcontroller connected to the controller area network (CAN) interface which is responsible for the calculation of the sensor data and providing it to automotive safety and stability systems.

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bond wires

Micromachined Pressure Sensor

A new capacitive differential pressure sensor featuring metallized electrodes with a series resistance of Rs = 3 Ω was developed. It consists of three layers of structured borosilicate glass forming a hermetically sealed cavity. A pressure sensor prototype has the dimensions 5 x 5.7 x 1mm.

G. Schimetta, et al. Wireless pressure and temperature measurement using a SAW hybrid sensor 2000 IEEE Ultrasonics Symposium Proc., Volume 1, p. 445 – 448.

Surface and bulk micromachined capacitive pressure sensors have low Q-factor as typically the electrodes based on the silicon technology are manufactured by doping the silicon that results in high serial resistance (20-50 Ω).

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The patch antenna with the integrated sensor board is mounted on the rim with a stress ribbon. The antenna is the capacitively shortened λ/2 dipoleetched out of the copper layer on the 0.5 mm FR-4 substrate.Antenna gain is about –2.1 dB.

Hybrid Sensor Unit Prototype

G. Schimetta, et al. Wireless pressure and temperature measurement using a SAW hybrid sensor 2000 IEEE Ultrasonics Symposium Proc., Volume 1, p. 445 – 448.

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SAW Sensor Modeling

Fig. 3. Mixed three-port representation of a SAW transducer

p

1, 2 – acoustic ports, 3 – electric port

a1

b1

a2

b2

1 2

3VI

L=Np

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Mixed scattering matrix of a SAW transducer

1 11 12 13 1

2 21 22 23 2

31 32 33

b m m m ab = m m m aI m m m V

⎡ ⎤⎡ ⎤ ⎡ ⎤⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎢ ⎥⎢ ⎥ ⎢ ⎥⎣ ⎦ ⎣ ⎦⎢ ⎥⎣ ⎦where

a1, a2 - incident waves at the acoustic ports 1,2 b1, b2 - reflected waves at the acoustic portsI - terminal current at the electric port 3V - voltage applied to the transducer bus-bars

(1)

An ideal SAW transducer is a reciprocal and lossless three-portacoustoelectric network with two acoustic and one electric ports.

Mixed Scattering Matrix of a SAW Transducer

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mii =bi /ai – reflection coefficient at the i-th acoustic port, i=1,2mik= bi /ak– transmission coefficient from the k-th to i-th

acoustic port, i, k=1,2, i≠kmi3= bi /V – acoustoelectric conversion function, i=1, 2m3i= I /ai – electroacoustic conversion function, i=1,2m33=I/V– transducer admittance Y(ω)=G(ω)+jB(ω)+jωCG(ω), B(ω) – radiation conductance and susceptance, respectivelyC – static capacitancePower conservation:

Physical Meaning of Matrix Elements

CG(ω) B(ω)

Fig. 4. SAW transducer equivalent scheme

2 233 13 23( ) Re ( )G m m mω ω= = +

Y0(ω)V

I

0I Y V= −

According to the power conservation law, all the electrical power delivered to the transducer is radiated acoustically in both directions.

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SAW Transducer Wave Scattering Matrix

2 0 1313 31 13 3211 12

0 0 02 0 2323 31 23 32

21 220 0 0

0 31 0 32 00 0 0

Y mm m m mm m

Y Y Y Y Y Y

Y mm m m mm m

Y Y Y Y Y Y

Y m Y m Y YY Y Y Y Y Y

⎡ ⎤⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎢ ⎥⎣ ⎦

− −+ + +

= − −+ + +

−− −

+ + +

S

where Y0 =1/Z0 - characteristic admittance (source/load) at the electric port.

Wave scattering matrix of a SAW transducer

(4)

Assumption: m11=m22=0 mechanical (mass-electrical loading) reflectionsare negligible.

Validity: f0≠v/2p where f0 – central frequency, v – SAW velocity, p – IDTperiod.

Short-circuit: Y0=• → s11=0Open-circuit: Y0=0 → s11=-1Matched: Y=Y0* → s11=-1/2Impedance loaded: Y0>>Y → s11=-m13m31Z0, Z0 – matched load impedance

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Temperature Measurement1. Monitoring the temperature inside the tire is desirable.2. The phase shift caused by the thermal variation is superimposed

on the phase shift due to the variable impedance load.

The temperature can be determined by measuring the time delayt =L/v between the two reference reflectors.

Time delay method provides worser accuracy than the phasemeasurement. However, this accuracy is sufficient for monitoringpurpose.

0( ) , / , 2l v T T L v fττ α α α τ ϕ π ττ∆

= − ∆ = ∆ = ∆ = ∆ (5)

where at = al- av - temperature coefficient of delay (TCD).1

ldL

L dTα = - temperature coefficient of expansion (TCE).

1v

dvv dT

α = - temperature coefficient of velocity (TCV).

W.D.Suh, et al. Design optimization and experimental verification of wireless IDT based micro temperature sensor. Smart Mater. Struct., v.9, 2000, pp. 890-897.

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Temperature Compensation

Temperature range: from -30ºC to +130ºC → DT=160ºCSubstrate material: YZ LiNbO3, at =94ppm/ºC.Central frequency: f0=432 MHz (ISM)Time delay: 4, 7, 10 µs.

02 2 434 4 0.015 163.6f radsϕ π τ π∆ = ∆ = × × × ≈

(6)

The phase shift caused by the thermal variation:

694 160 10 0.015Tττ ατ

−∆= ∆ = × × ≈

(7)

Temperature compensation must be done!

1 2 2 1

12 2 2

P T

R R R R RP P R PR

ϕ ϕ ϕϕ ϕ ϕ ϕ ϕϕ ϕ ϕ ϕ

∆ = − =+ − ∆

= − = − − = ∆ −

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Pressure Sensor Capacitance

Fig. 5. Measured sensor capacitance versus pressure


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Measurement Results

Fig. 5. Reflection magnitude |s11| and phase ∆ϕs11 versus tire pressure


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Prototype Hybrid SAW Sensor Parameters

1. Phase modulation range is about 110º (pressure range 100-400 kPa).2. Amplitude modulation is about 8 dB (ambiguous and obsolete for this

case).3. The pressure resolution is not constant (non-linear dependence).4. Maximum sensitivity can be controlled by tuning the matching circuit.5. Signal-to-noise ratio 20 dB6. Pressure range 100-400 kPa7. Pressure accuracy ±15 kPa8. Temperature range -30ºC to +130ºC 9. Temperature accuracy ±10ºC10. Interrogation cycle 20 ηs



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ConclusionsThe principles and design of the pressure and temperaturemeasurement (monitoring) system based on a hybrid of thereflective surface acoustic wave (SAW) delay line (SAWtransponder) with the high-Q micromachined capacitive pressuresensor are presented.

The hybrid sensor unit integrated with antenna does not requirepower supply (electrical battery) and serves for simultaneousmeasurement of the pressure and temperature.

With a new approach to matching the capacitive sensor impedanceto the SAW transponder impedance both a high signal-to-noise ratioand a wide signal dynamic range can be achieved.

The prototype tire pressure sensor (Siemens AG, Germany) is discussed.

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References

1. R. Steindl, et al. Impedance loaded SAW sensors offer a wide range of measurement opportunities. IEEE Trans. Microwave Theory and Techn., v.47, No. 12, 1999, pp. 2625-2629.

2. A. Pohl, et. Al. Monitoring the tire pressure at cars using passive SAW sensors. 1997 IEEE Ultrasonics Symp. Proc., p. 471-474.

3. R. Steindl, et al. SAW delay lines for wirelessly requestableconventionanal sensors. 1998 IEEE Ultrasonics Symp. Proc., p. 351-354.

4. G. Schimetta, et al. A wireless pressure-measurement system using a SAW hybrid sensor. IEEE Trans. Microwave Theory and Techniq., vol. 48, No. 12, 2000, pp. 2730-2735.

5. H. Scherr, et al. Quartz pressure sensor based on SAW reflective delay lines. 1996 IEEE Ultrason. Symp. Proc., pp. 347-350.

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References (Cont’d)

7. G.Schimetta, et al. Optimized design and fabrication of a wirelesspressure and temperature sensor unit based on SAW transpondertechnology. Microwave Symposium Digest, 2001 IEEE MTT-S Int.,vol. 1, 20-25 May, p. 355 – 358.

8. G. Schimetta, et al. Wireless pressure and temperature measurement using a SAW hybrid sensor. 2000 IEEE UltrasonicsSymposium Proc., Volume 1, p. 445 – 448.

9. A. Pohl, F. Seifert. Wirelessly interrogable surface acoustic wave sensors for vehicular applications. IEEE Trans. Instrumentation and Measurement, Vol. 6, No 4, 1997, p. 1031 – 1038.

10. W.D.Suh, et al. Design optimization and experimental verification of wireless IDT based micro temperature sensor. Smart Mater. Struct., v.9, 2000, pp. 890-897.

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hybrid saw pressure and temperature sensor - intrasaw - home

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