Ulrich Wolff

Theory and application of passive SAW radio transponders as sensors

Surface acoustic wave (SAW) radio transponders make it possible to read identification codes or measurement values from a remote location. The 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 transponders are maintenance free. Identification marks respond to an interrogation signal with their nonchanging identification pattern. In wireless SAW sensors the physical or chemical properties to be detected change the propagation characteristics of the SAW. SAW radio transponders are advantageously placed on moving or rotating parts and in hazardous environments such as contaminated or high voltage areas. They also can be used for contactless measurements in high vacuum process chambers, under concrete, extreme heat, or strong radioactive radiation, where the use of conventional sensors is complicated, dangerous, or expensive. 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 measurements of temperatures, pressures, torques, and currents.

Wireless passive SAW sensor systems for industrial and domestic applications

The authors discussed the principle of wireless sensing with passive SAW devices and investigated binary SAW sensors and other SAW radio sensors for the non-contact measurement of temperatures, pressures and torques. An RF identification system for the Munich subway, a SAW sensor system for the diagnosis and control of electric motors and a temperature monitoring system for high voltage surge arresters are the applications which were presented. Finally a low-cost radar unit for domestic applications was described.

SAW sensors for harsh environments

Surface acoustic wave (SAW) sensors are rugged components made on highly stable substrate materials. In addi- tion, by their operating principle they lend themselves to wireless readout by radio signals. For these reasons, they are a first choice for sensing in harsh environments. A review is given on SAW device design, instrumentation of sensor systems, and on the physical interactions underlying sensing mechanisms. Recent progress in chemical as well as physical sensing toward various applications, e.g., in pollution control, biomedicine, and industry, is highlighted. Index Terms—Chemosensors, coatings, detection limit, high temperature, instrumentation, resolution, selectivity, sensing mechanisms, sensitivity, sensor materials, surface acoustic waves, torque.

SAW-based radio sensor systems for short-range applications

Wireless autonomous surface acoustic wave (SAW) sensors offer high flexibility for modern sensor systems. Because no battery or wiring is required for power-supply and communication tasks, they can be advantageously employed for nearly all kinds of short-range identification and measurement applications, where the use of conventional sensors, e.g., on moving or rotating parts or in industrial process chambers. Here, the basics of SAW-based radio sensor systems are reviewed and different examples out of a manifold of possible applications are given. Wireless SAW identification and sensor systems operate stable and maintenance free over many years even in harsh industrial environments. With a fast readout of only a few microseconds, a readout-distance of up to several meters, and a high sensor stability, even at temperatures above 200 /spl deg/C, these highly flexible SAW-based radio systems are ideally suited for a multitude of measurement tasks in industrial, automotive, transportation, and domestic applications.

Borderline applications of QCM-devices: synthetic antibodies for analytes in both nm- and μm-dimensions

Abstract Mass-sensitive devices are able to monitor both degradation processes of complex mixtures, such as automotive oils, and microorganisms by synthetic antibodies allowing detection of nm and μm particles. Pure ceramic materials (TiO 2 ) were synthesised by a sol–gel process, e.g. from titanium(IV) alkoxides (Ti(OR) 4 ) and imprinted by long chain carbonic acids. The sensor effect is based on the re-inclusion of oil oxidation products, e.g. carbonic acids. Surface imprinting with biological structures, such as microorganisms, yields pits for their adhesion and strongly enhanced mass-sensitivity of the sensor device. This effect depends very sensitively on both geometrical fitting and chemical interaction between the structured sensor layer and the analyte. Surprisingly, the specific interaction results in Sauerbrey and non-Sauerbrey sensor behaviours.

QCM and SAW transducers allow analyte detection from nanometer- to micrometer-dimensions using imprinting techniques

Mass-sensitive devices like the quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices show a major advantage as a transducer principle as every analyte can be detected due to its mass. In order to transfer QCMs and SAWS into chemical sensors a layer has to be applied in which the desired analyte is preferentially incorporated. Such coatings can vary from molecular hollows like calix[n]arenes to monolayers and molecular imprinted polymers (MIP). MIPs are produced by polymerization of carefully selected monomers around a template, the desired analyte. Such monomers can carry functional groups which interact with the analyte. When the template is removed by evaporation or washed out, it leaves behind specially adapted hollows in respect to size and interactions in which the analyte can be re-included. With these sensitive layers it was possible to achieve selectivities for poly aromatic hydrocarbons which are comparable to that of natural antibodies. Bulk imprinted MIPs allow the synthesis of,highly packed artificial, receptor sites for small organic molecules. The high amount of sites within the coating of a QCM/SAW allows detection limits down to the ppb range. Due to diffusion limitations the imprinting technique has to be adapted to the size of the analyte. The technique is not limited to single compounds, complex mixtures can also be used as templates. In this way it was possible to determine motor oil degradation. Even whole cells can act as imprinting media.