Dr. A.S.Rukhlenko

Hybrid Wireless SAW Sensor for Pressure and Temperature Measurement [email protected] www.intraSAW.com Neuchâtel, 2005

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Outline Introduction

1. Advantages of SAW Sensors 2. Hybrid SAW + MEMS Pressure Sensor 3. Tire Monitoring System 4. Hybrid Sensor Schematics 5. Transceiver Block Diagram 6. Hybrid Sensor Unit Prototype Construction 7. SAW Sensor Modeling 8. Temperature Compensation and Measurement 9. Hybrid Sensor Parameters Conclusions

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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)

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active

passive

Active (Battery Powered) Sensors 9 Limited lithium battery lifetime 9 Battery replacement inside the tire 9 Battery waste management problem 3

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Wireless Surface Acoustic Wave (SAW) Sensors Wireless interrogation (energy supply via the electromagnetic RF 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 4

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Hybrid SAW + MEMS Pressure Sensor Construction: Hybrid SAW sensor = SAW reflective delay line + high-Q capacitive micromachined 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 temperature 2) Compensation temperature in the pressure measurement 3) Wireless transmission of the measurand data (pressure, temperature) Simultaneous monitoring of the tire pressure and temperature becomes possible. 5

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Tire Monitoring System

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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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Hybrid SAW Sensor Schematics (Dual Track) DjR = jR2–jR1 jP L L

LC-matching circuit

capacitive micro-machined pressure sensor 7

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 interdigital transducer (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)

∼ ∼ ∼

Trigger

CAN

µC

A

XO

LO

10.7 MHz

423.22 MHz

I

QDM

A/D

LNA

Q

∼ ∼ ∼

LNA

XO – crystal oscillator LO – local oscillator LNA – low-noise amplifier QDM – quadrature demodulator

A/D – analog/digital converter µC – microcontroller CAN – controller area network A – antenna

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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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Micromachined Pressure Sensor

bond wires

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. 10

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 Qfactor as typically the electrodes based on the silicon technology are manufactured by doping the silicon that results in high serial resistance (2050 Ω).

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Hybrid Sensor Unit Prototype

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 dipole etched out of the copper layer on the 0.5 mm FR-4 substrate. Antenna gain is about –2.1 dB. 11

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 3 V I a1

a2

1

2 b1

b2 p

L=Np

1, 2 – acoustic ports, 3 – electric port Fig. 3. Mixed three-port representation of a SAW transducer 12

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Mixed Scattering Matrix of a SAW Transducer An ideal SAW transducer is a reciprocal and lossless three-port acoustoelectric network with two acoustic and one electric ports. Mixed scattering matrix of a SAW transducer

⎡ b1 ⎤ ⎡ m11 m12 ⎢ ⎥ ⎢ ⎢ b2 ⎥ = ⎢ m21 m22 ⎢ ⎥ ⎢ ⎢⎣ I ⎥⎦ ⎢⎢ m31 m32 ⎣

m13 ⎤⎥ m23 ⎥⎥ m33 ⎥⎥⎦

⎡ a1 ⎤ ⎢ ⎥ ⎢ a2 ⎥ ⎢ ⎥ ⎢⎣ V ⎥⎦

(1)

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

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Physical Meaning of Matrix Elements mii =bi /ai – reflection coefficient at the i-th acoustic port, i=1,2

mik= bi /ak– transmission coefficient from the k-th to i-th acoustic port, i, k=1,2, i≠k mi3= bi /V – acoustoelectric conversion function, i=1, 2 m3i= I /ai – electroacoustic conversion function, i=1,2 m33=I/V– transducer admittance Y(ω)=G(ω)+jB(ω)+jωC G(ω), B(ω) – radiation conductance and susceptance, respectively C – static capacitance Power conservation: 2 2

G (ω ) = Re{m33 (ω )} = m13 + m23 I

G(ω)

B(ω)

C

V

Y0(ω)

I = −Y0V

Fig. 4. SAW transducer equivalent scheme 14

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 Wave scattering matrix of a SAW transducer ⎡ m m ⎢ m − 13 31 ⎢ 11 Y + Y 0 ⎢ ⎢ m m S = ⎢⎢ m − 23 31 21 Y + Y 0 ⎢ ⎢ Y m ⎢ 0 31 ⎢ − Y +Y ⎢⎣ 0

m m m − 13 32 12 Y + Y 0 m m m − 23 32 22 Y + Y 0 −

Y m 0 32 Y +Y 0

2 Y m ⎤ 0 13 ⎥ Y +Y ⎥ 0 ⎥

2 Y m ⎥⎥ 0 23 Y +Y ⎥ 0 ⎥ Y −Y 0 Y +Y 0

(4)

⎥ ⎥ ⎥ ⎥⎦

where Y0 =1/Z0 - characteristic admittance (source/load) at the electric port. Assumption: m11=m22=0 mechanical (mass-electrical loading) reflections are negligible. Validity: f0≠v/2p where f0 – central frequency, v – SAW velocity, p – IDTperiod. Short-circuit: Y0=• → s11=0 Open-circuit: Y0=0 → s11=-1 Matched: Y=Y0* → s11=-1/2 Impedance loaded: Y0>>Y → s11=-m13m31Z0, Z0 – matched load impedance 15

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Temperature Measurement 1. 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 delay t =L/v between the two reference reflectors. Time delay method provides worser accuracy than the phase measurement. However, this accuracy is sufficient for monitoring purpose. ∆τ = (α l − α v ) ∆T = ατ ∆T , τ = L / v, ∆ϕ = 2π f 0 ∆τ (5) τ where at = al- av - temperature coefficient of delay (TCD).

1 dL - temperature coefficient of expansion (TCE). L dT 1 dv αv = - temperature coefficient of velocity (TCV). v dT

αl =

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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=160ºC Substrate material: YZ LiNbO3, at =94ppm/ºC. Central frequency: f0=432 MHz (ISM) Time delay: 4, 7, 10 µs.

∆τ

τ

= ατ ∆T = 94 × 160 × 10−6 ≈ 0.015

(6)

The phase shift caused by the thermal variation:

∆ϕ = 2π f 0 ∆τ = 2π × 434 × 4 × 0.015 ≈ 163.6 rads

(7)

Temperature compensation must be done!

∆ϕ = ϕ P − ϕT = ϕ R + ϕ R2 ϕ R − ϕ R1 ∆ϕ R = ϕP − 1 = ϕ P − ϕ R1 − 2 = ∆ϕ PR − 2 2 2 17

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Pressure Sensor Capacitance

Fig. 5. Measured sensor capacitance versus pressure 18

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.

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Measurement Results

Fig. 5. Reflection magnitude |s11| and phase ∆ϕs11 versus tire pressure 19

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.

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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 dB 6. Pressure range 100-400 kPa 7. Pressure accuracy ±15 kPa 8. Temperature range -30ºC to +130ºC 9. Temperature accuracy ±10ºC 10. Interrogation cycle 20 ηs

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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. 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.

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Conclusions The principles and design of the pressure and temperature measurement (monitoring) system based on a hybrid of the reflective surface acoustic wave (SAW) delay line (SAW transponder) with the high-Q micromachined capacitive pressure sensor are presented. The hybrid sensor unit integrated with antenna does not require power supply (electrical battery) and serves for simultaneous measurement of the pressure and temperature. With a new approach to matching the capacitive sensor impedance to the SAW transponder impedance both a high signal-to-noise ratio and a wide signal dynamic range can be achieved. The prototype tire pressure sensor (Siemens AG, Germany) is discussed. 21

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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 requestable conventionanal 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 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. 8. G. Schimetta, et al. Wireless pressure and temperature measurement using a SAW hybrid sensor. 2000 IEEE Ultrasonics Symposium 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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The End End

Thanks for your attention. Questions?

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