G. Scholl

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.

Quartz pressure sensor based on SAW reflective delay line

We have developed a wireless SAW (Surface Acoustic Wave) pressure sensor operating in the pressure range from 0 Pa to 250 kPa. In order to minimize the temperature sensitivity the pressure sensor is based on an all quartz package, which has been designed with the Finite Element Method. The package of the pressure sensor consists of a diaphragm and a cover, both made of conventional Y-cut quartz. A blind-hole was structured into the sensor cover. By attaching the cover and the diaphragm with an epoxy-adhesive, this blind-hole forms a closed cavity. The SAW element is a reflective delay line, working at approximately 434 MHz. The delay line consists of ten reflectors and extends over the whole diaphragm. The pressure is determined by evaluating the change of the carrier phase shifts of the reflected impulses at the reflectors. We show that it is possible to minimize the temperature sensitivity and to achieve good linearity by correct positioning of the SAW reflectors. The measurements of the SAW pressure sensor show a deviation from linearity of less than +/-0.7%. The temperature dependence is almost negligible in the range from -20/spl deg/C to 100/spl deg/C.

SAW devices for consumer communication applications

An overview of surface acoustic wave (SAW) filter techniques available for different applications is given. Techniques for TV IF applications are outlined, and typical structures are presented. This is followed by a discussion of applications for SAW resonators. Low-loss devices for mobile communication systems and pager applications are examined. Tapped delay lines (matched filters) and convolvers for code-division multiaccess (CDMA) systems are also covered. Although simulation procedures are not considered, for many devices the theoretical frequency response is presented along with the measurement curve.<<ETX>>

Review of models for low-loss filter design and applications

The most frequently used models for surface acoustic wave (SAW) devices are the impulse model, the equivalent circuit models, the Coupling-of-Modes model, and the matrix models. While the impulse-model is only a first order model the other models include second order effects, e.g. reflections, dispersion, and charge distribution effects. The influence of diffraction and refraction on the transfer function of a SAW filter can be described by the angular spectrum of straight-crested waves model. A survey of these different models will be given. The simulation of low-loss filters requires flexible analysis tools, which can cope with different geometries and substrates. Operating with a parameter set, which depends only on the substrate crystal and not on the specific geometry of the SAW filter, is advantageous. Due to the high insertion attenuation of conventional transversal filters the requirements on the accuracy of the analysis are focused on S21, whereas for low-loss filters all elements of the S-matrix are important. The comparison of simulations with a P-matrix model, which fulfills the above mentioned prerequisites, and measurements of different types of low-loss filters, e.g. SPUDT, DMS, and transverse-mode coupled resonator filters are presented

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.

Radio signals for SAW ID tags and sensors in strong electromagnetic interference

The applicability of wirelessly interrogated passive ID-tag type SAW sensors for measurements of the temperature of disc brakes of moving railway cars is shown. Here, a short radio channel allows relatively simple radio impulse processing by phase evaluation. Further, using elastic convolver measurement in large steel works, we discuss the feasibility of passive SAW sensors in a severely obstructed radio channel with interference. For such environments radio signals of the spread spectrum type and a receiver with the corresponding correlative SAW structure or a RAKE receiver are appropriate for remote readout

Radio accessible SAW sensors for non-contact measurement of torque and temperature

Surface acoustic wave (SAW) based sensors can easily be interrogated by low power radio signals. They therefore are ideally suited for non-contact measurement of torque and temperature on rotating shafts. It is advantageous to use lithium niobate as a substrate material because of its linear temperature response and its high electro-mechanical coupling efficiency. With this material, however, the sensitivities of temperature and torque measurement are strongly different: a small change in temperature of only a few Kelvin will cause a sensor signal of the same amount as the nominal torque of a typical shaft. It is experimentally demonstrated that, despite the strong temperature sensitivity of the sensors, exact measurements of both torque and temperature can be accomplished over a wide temperature range from 0/spl deg/C to more than 70/spl deg/C, while the resolution of torque measurement is well below 1%. Deviations from linearity are small, so that a third order polynomial in temperature and torque is sufficient to describe the sensor response characteristics.

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.

Surface Acoustic Wave Devices for Sensor Applications

Identification and sensor systems based on surface acoustic waves exhibit intriguing properties which have hitherto remained unexploited by semiconductor-based systems. They offer a long readout distance of up to more than 20 m with purely passive surface acoustic wave (SAW) devices. SAW devices operate with no battery or wiring, withstand extreme temperatures and work reliably and maintenance-free over many decades even in harsh industrial environments. Because they operate at frequencies in the GHz range, SAW identification and sensor systems are well protected from the electromagnetic interference that often occurs in the vicinity of industrial equipment such as motors and high-voltage lines. The fundamentals and design rules of numerous passive wireless SAW sensor and identification systems for industrial and domestic applications as well as relevant practical work will be presented.

Energy distribution in a quartz resonator

We present a theoretical analysis of the acoustical energy distribution in a surface acoustic wave (SAW) test resonator on the 34/spl deg/ cut of quartz. To calculate the energy density at each point for a known voltage, we chose a combination of three different methods: a P-Matrix algorithm for the longitudinal direction, a Stack-Matrix formalism across the aperture of the resonator and a Partial Wave Method normal to the substrate surface. We determined the distribution of energy in all three dimensions and located the peak stress at the metallization-vacuum interface.