MECHANICAL SENSORS BASED ON SURFACE ACOUSTIC-WAVES

A survey is given of SAW sensor systems. Three categories of sensors a re defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal-processing syste ms. In these, the mechanical change of the SAW device is evaluated by difference measurements of the transit time, phase or resonance freque ncy, which are described quantitatively. To show the high resolution o f active sensor systems, an electronic spirit level (based on SAW reso nators) is described. New SAW chirp sensors allow the dispersive inter digital transducer or reflector geometry to be adjusted to the sensor system resolution and measurement range. In passive systems the sensor device is connected to an antenna. The SAW is excited by an interroga ting radio impulse. After a SAW transit time, the sensor radiates back to the interrogator an impulse containing the sensor's information. T he time and frequency response of all sensor categories and the possib ilities of temperature compensation are discussed.