Wolf-Eckhart Bulst

Mechanical sensors based on surface acoustic waves

Abstract A survey is given of SAW sensor systems. Three categories of sensors are defined as transit-time, resonator and chirp-type devices and each category is subdivided into active and passive signal-processing systems. In these, the mechanical change of the SAW device is evaluated by difference measurements of the transit time, phase or resonance frequency, which are described quantitatively. To show the high resolution of active sensor systems, an electronic spirit level (based on SAW resonators) is described. New SAW chirp sensors allow the dispersive interdigital 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 interrogating radio impulse. After a SAW transit time, the sensor radiates back to the interrogator an impulse containing the sensors information. The time and frequency response of all sensor categories and the possibilities of temperature compensation are discussed.

State of the art in wireless sensing with surface acoustic waves

Surface acoustic wave (SAW) devices can be turned into novel identification and sensor elements (transponders) that do not need any power supply and may be interrogated wirelessly. Such a transponder picks up an electromagnetic request signal and stores it until all echoes caused by multipath propagation have died away. Then, a characteristic response signal is beamed back to the interrogator unit. In radio-link sensors, a physical or chemical quantity influences the propagation properties of the SAW and consequently changes the response pattern of the device. This contribution surveys the operating principle of such sensors and their state-of-the-art performance. The discussion is supported by illustrative examples such as temperature sensors and sensors for mechatronic applications.