This paper is focused on self resonant FDS. They are also called auto resonant or resonant sensors and the main advantages of resonant sensors over other kind of sensors are their stability, high resolution and quasi-digital output, among others [11].
As shown in Fig. 1, resonant sensors require a counter to measure frequency and, despite the fact that there many types of resonant sensors, this paper is focused on surface acoustic wave (SAW) sensors. These consist of two interdigitated electrode patterns (see Fig. 2), screen printed and fired onto an alumina substrate of thickness 630 µm [12].
The piezoelectric thick film (thickness 40µm) is deposited over the electrodes. The width and gap between the electrodes is 200 µm. The piezoceramic material is polarized between the ‘fingers’ of the electrode pattern.
One of the interdigitated transducers (IDTs) acts as an input device. A sinusoidal voltage is applied to the electrodes and, because of the piezoelectric effect; an elastic wave is generated within the film and travels towards the other IDT.
At the receiver, the acoustic wave is translated back into an electrical signal as a result of the direct piezoelectric effect. The wave velocity, v, is determinate by the acoustic properties of the medium. The wavelength, λ, is equal to the centre- to centre-distance between adjacent pairs of fingers. The typical frequency of the SAW devices is around 5 MHz. The information bearer in such sensors is primarily the time delay of the SAW or the central frequency of the SAW device [13].
Fig.1 Schematic representation for resonant sensors
Fig.2. A thick-film elastic wave sensor (SAW)