Wireless Surface Acoustic Wave Pressure and Temperature Sensor With Unique Identification Based on ${\rm LiNbO}_{3}$

Wireless sensor applications at elevated temperatures of around 200^°C and above call for robust technologies such as surface acoustic wave (SAW) sensor designs. A combined pressure/temperature sensor with identification has been developed based on LiNbO₃ 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.

I. Introduction

Pressure 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 a very small bandwidth where they are typically designed in the ISM band at 433.05–433.79 MHz [1].