A review of wireless SAW sensors

Wireless measurement systems with passive surface acoustic wave (SAW) senso rs offer new and exciting perspectives for remote monitoring and control of moving parts, even in harsh environments. This review paper gives a compre hensive survey of the present state of the measurement systems and should h elp a designer to find the parameters required to achieve a specified accur acy or uncertainty of measurement. Delay lines and resonators have been used, and two principles have been emp loyed: SAW one-port devices that are directly affected by the measurand and SAW two-port devices that are electrically loaded by a conventional sensor and, therefore, indirectly affected by the measurand. For radio frequency (RF) interrogation, time domain sampling (TDS) and frequency domain samplin g (FDS) have been investigated theoretically and experimentally; the method s of measurement are described. For an evaluation of the effects caused by the radio interrogation, we discuss the errors caused by noise, interferenc e, bandwidth, manufacturing, and hardware tuning. The system parameters, di stance range, and measurement uncertainty are given numerically for actual applications. Combinations of SAW sensors and special signal processing tec hniques to enhance accuracy, dynamic range, read out distance, and measurem ent repetition rate (measurement bandwidth) are presented. In conclusion, a n overview of SAW sensor applications is given.