IEEE SENSORS JOURNAL, VOL. 13, NO. 5, MAY 2013 1801 Wireless Surface Acoustic Wave Pressure and Temperature Sensor With Unique Identification Based on LiNbO 3 Alfred Binder, Gudrun Bruckner, Norbert Schobernig, and Daniel Schmitt Abstract— Wireless sensor applications at elevated tempera- tures of around 200 °C and above call for robust technologies such as surface acoustic wave (SAW) sensor designs. A com- bined pressure/temperature sensor with identification has been developed based on LiNbO 3 substrate for condition monitoring applications in the baking industry. Due to the given temperature specification and to avoid adhesives, the design of the sensor was conceived as a classical beam arrangement supported by three balls. This paper presents an overview on the design process of the sensor beginning with the mechanical simulation of the sensor and the derivation of the SAW delay line to measure pressure, temperature, and to realize an ID functionality. The main result is a pressure sensitivity of 3.7 rad/bar with a temperature drift of 0.013 rad/°C. The sensitivity of the pressure sensor is also temperature dependent. This compound temperature drift can be compensated with the inherent temperature information of sensor. Index Terms—Lithium niobate, surface acoustic wave (SAW) pressure and temperature sensor, wireless pressure sensor. I. I NTRODUCTION P RESSURE sensor developments using SAW principles can first be distinguished in terms of substrate materials used. The majority of developments have been done on quartz substrates partially driven by the quest for a passive, wireless tire pressure monitoring system [1]–[8]. The advantage of using quartz is the availability of temperature compensated cuts and a good sensitivity to strain. Disadvantages are its bandwidth limitations and poor coupling coefficient [9]. Lithium Niobate (LN) is well suited for higher frequencies and offers good piezoelectric coupling. Interestingly the motivation for the LN sensors investigated by the research groups [10] and [11] lie also in the development of wireless tire pressure sensors. A second way to distinguish SAW pressure sensors is whether they are SAW resonators or delay lines. The sensor related frequency shift of resonators can be evaluated within Manuscript received November 9, 2012; revised January 11, 2013 and January 14, 2013; accepted January 15, 2013. Date of publication January 18, 2013; date of current version April 2, 2013. This work was supported by the COMET–Competence Centers for Excellent Technologies Programme by BMVIT, BMWFJ and the Federal Provinces of Carinthia and Styria. The associate editor coordinating the review of this paper and approving it for publication was Dr. Stefan J. Rupitsch. A. Binder, G. Bruckner, and N. Schobernig are with Carinthian Tech Research AG, Villach 9524, Austria (e-mail: alfred.binder@ctr.at; gudrun.bruckner@ctr.at; norbert.schobernig@ctr.at). D. Schmitt is with Fraunhofer-Institut für Biomedizinische Technik, IBMT, St. Ingbert 66386 Germany (e-mail: daniel.schmitt@ibmt.fraunhofer.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2013.2241052 TABLE I TARGET SENSOR SPECIFICATIONS Operation temperature +20 to +200 °C +300 °C short term Temperature accuracy < 1% full Scale Applied pressure 0 bar to 5 bar Pressure accuracy < 5% full Scale Number of ID to 256 a very small bandwidth where they are typically designed in the ISM band at 433.05–433.79 MHz [1]. Delay lines principally need a higher bandwidth of at least 20 MHz to be evaluated. In consequence only the ISM band at 2400–2480 MHz can be used to comply with radio regulations. Another advantage of the higher bandwidth of 80 MHz is that the resolution of the round trip delay is high as it is inversely proportional to bandwidth. While temperature compensation for quartz is inherent for certain crystal cuts, differential evaluation means have to be considered when temperature sensitive substrates as for instance LN are used. The work presented in this paper summarizes the devel- opment of a combined pressure and temperature sensor with unique sensor identification. The target sensor specifications are listed in the Table I. One will notice that the given number of unique ID’s is with 256 quite low compared to the state of the art. This comes from the initial target application where no higher numbers are needed per installation. In the general case the number of unique ID’s can be chosen much higher. While combined pressure, temperature and ID sensors have been investigated on 440 MHz devices [12] the novel approach of the present work lies in the realization of 2.4 GHz structures and a mechanical assembly completely free of adhesives. The need for distinguishable sensors gave rise to the chosen substrate LN where reasonable ID functionality can be realized. Another reason for this choice is the combination of a temperature and pressure sensor in a single SAW delay line. The mechanical setup and stress simulations will be dis- cussed in Section II. Section III explains the SAW design and the chosen temperature compensation technique for the pressure evaluation. The experimental results are discussed in Section IV and the outlook V concludes this paper. II. MECHANICAL SENSOR SETUP The elevated operating temperature of up to +200 °C and +300 °C short term exposure lead to the conclusion, that a 1530-437X/$31.00 © 2013 IEEE