THEORY AND APPLICATION OF PASSIVE SAW RADIO TRANSPONDERS AS SENSORS

Surface acoustic wave (SAW) radio transponders make it possible to rea d identification codes or measurement values from a remote location. T he decisive advantage of these SAW transponders lies in their passive operation (i.e., no power-supply), and in the possibility of wireless installation at particularly inaccessible locations. The passive SAW t ransponders are maintenance free. Identification marks respond to an i nterrogation signal with their nonchanging identification pattern. In wireless SAW sensors the physical or chemical properties to be detecte d change the propagation characteristics of the SAW. SAW radio transpo nders are advantageously placed on moving or rotating parts and in haz ardous environments such as contaminated or high voltage areas. They a lso can be used for contactless measurements in high vacuum process ch ambers, under concrete, extreme heat, or strong radioactive radiation, where the use of conventional sensors is complicated, dangerous, or e xpensive. In this paper we discuss the principles of wireless passive SAW transponders and present a radio frequency interrogation unit and several passive radio SAW sensors developed for noncontact measurement s of temperatures, pressures, torques, and currents.