Saw resonator temperature sensor

Saw resonator temperature sensor

Sensors and Actuators A, 30 (1992) 73 73-75 SAW resonator temperature sensor Frank Moller Ibnenau Insfrtute of Technology, Department of S&d State ...

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Sensors and Actuators A, 30 (1992)

73

73-75

SAW resonator temperature sensor Frank Moller Ibnenau Insfrtute of Technology, Department of S&d State Electromcs and Materrals, P 0

Box 327, D-0-63W

Ilmenau

(FRG)

Jens Kuhn Geraberger Thermometerwerke GmbH, Elgersburger Strasse 1, D-O-6306 Geraberg (FRG)

Abstract The temperature dependence of a SAW resonator IS used to determme temperature The high hneanty of the frequency shaft with temperature makes It possible to achieve a Hugh resolution and a high stablhty wth a simple oscdlator clrcult Prehmmary data for the resonator and the temperature-frequency charactenstlc are Even The operating pnnclple can also be used to measure the concentratron of chemical compounds or elements m hqulds or gases, pressure, humldlty of gases and many other propertles

The sensor described here consists of a resonator whose resonance frequency 1s determmed by the travelhng velocity of the acoustic wave and the geometry of the sensor metalhzatlon The resonance frequency IS influenced by temperature because of the alteration of matenal propertles like the elastic, plezoelectrrc and dlelectrlc constants [l] The sensor shows slmdar electric behavlour to that of a quartz resonator and can m general be used m the same basic oscdlator clrcults (Figs 1 and 2) An oscillator compnsmg the temperature sensor has a temperature-dependent frequency which can easily be converted to dlgtal data [2] Figure 3 presents the resonator frequency shift versus temperature In consequence of the high oscillator short-term stab&y, a resolution of about 1 mK or better is achevable The high lmeanty of the resultmg frequency shift makes It possible to measure the temperature v&h high precision over a wide temperature range [3] Prehmmary techmcal data for this resonator are gven m Table 1 Compared with a quartz thickness-mode resonator, one advantage of the descrtbed sensor (see Fig 4) is the better thermal contact with the heated medium because of the possible direct bracket mounting of the sensor element, unlike the free suspension of the tickness-mode resonator This enables a higher mechanical stab&y to be achieved, m addltlon to better measurement

dynamics The tigher sensor resonance frequency yields a better measurement resolution [3] At present, problems of sensor agmg and of the measurement range expansion are bemg mvestigated To ensure a low agmg rate, several metalhzmg systems, sensor packagmg and mountmg techniques have been exammed Some engmeers at ERMIC GmbH, Erfurt, are preparmg a mounting techmque which enables a stress-free sensor suspension to be achieved m addltlon to a good thermal contact with tigh mechanical stabdlty [4] A set of common packagmgs 1s shown m Fig 5 On the basis of the described sensor, some temperature probes have been constructed which reflect the sensor advantages and properties (see Figs 6 and 7) With one probe the sensor fun&on was tried out at a very low temperature (hqmd nitrogen) In tlus expenment the excellent sensor properties were verified Problems result especially from the pyroelectnclty of the substrate matenal and the feedback of the connection between the sensor and oscillator on the measurmg result The effect of pyroelectnclty may be cancelled by means of the oscillator circuit design or by specially contacting the sensor The feedback of probe technique on the sensor function may above all be numnuzed by using a special matenal and design for the connection between the sensor and osclllator arcmt [5] Elsewer Sequoia

I$ Rf

RI

CD

Cl Ll

Fig

I Eqmvalent circmt

Fig 4 Photograph

of the resonator

Fig 2 Oscillator c~rcmt

?kfp- -73 38ppmIK r P-099997

Fig 5 Various packagmgs

7171

-10

Tk4- -73 62ppm/K P --0 99997 * 0 10 20 30

Fig 3 Resonator

TABLE

10

50

for the temperature

sensor

60 T ,n ‘C

frequency vs temperature

Fig 6 Temperature

1 Prehmmary techmcal data

Substrate materlal Serial resonance frequency f; (25 “C) Parallel resonance frequency & (25 “C) Operatmg temperature Unloaded Q Loaded Q (50 ohm) Temperature coefficient of frequency Eqmvalent parameters (see Fig I)

LINbO, ZXL 38” II 98 MHz 78 II MHz -5o-+200°c 2000 1600 Tkf - 73 5 ppm/K R, = 47 ohm R,=25ohm c, = 20 83 fF L, = 200 /iH C,=63pF

Fig 7 Opened

probes with glass and metal heads

temperature

probe

wth

the odlator

cxcw1

75

common project Fig 8 Measurmg prmclple

Apphcatlon posslblhtles of this temperature sensor and the resultmg probes are m the fields of high-resolution laboratory thermometers with a high working range, as the probe of a precision temperature-control unit, m the production of high quality optlcal instruments, m vetermary medicine and m agriculture Usmg radio signal transnusslon, the sensor could also be installed at measuring places with difficult or no access Besides this, the operatmg prmcrple (Fig 8) also allows the concentration of chemical compounds or elements m gases or hqulds, pressure, the humidity of gases and many other properttes to be measured

Acknowledgement

This work 1s supported by the Bundesmmlstermm fur Forschung und Technologle

ERMIC GmbH, Unternehmensberelch Forschung und Entwlcklung, Abtellung Verfahrensentwlcklung/Labore, Rudolfstrasse 47, D-0-5023 Erfurt (FRG) Dip1 -1ng D Romhlld Tel (003761) 583440 Ilmenau Institute of Technology, Department of Solid State Electronics and Materials, P 0 Box 327, D-0-Ilmenau (FRG) Prof Dr SC techn W Buff Tel (0037672) 693124

References 1 P Petter, The temperature dependence of travelhng parameters of surface acoustic waves m hthmm mobate and posslbdltles of their usage m the temperature measurement techmque, Doctoral Theses, Ilmenau Institute of Technology, 1988 (m German) 2 D L Hammond and A BenJammson, The crystal resonator-a [email protected] transducer, IEEE Spec~rm, 6 (Apnl 1969) 53-58 3 J Kuhn, A contnbution to frequency-analog temperature measurement with one-port SAW-resonators, Doctoral Thms, llmenau Institute of Technology, 1989, p 112 (m German) 4 D Romhdd, personal commumcatlon 5 Sensors on the basis of surface acoustic waves, Research Reporf No 143 45, Ilmenau Instttute of Technology, 1990 (m German, unpubhshed)