SAW day

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SAW tags are defined as a transponder technology that uses the SAW effect. Weak microwave signals are converted into ultrasonic signals that run over the surface of a piezoelectric crystal and are reflected at markings on the surface.

history

The first technical investigations on surface acoustic wave (SAW) elements took place in 1965 in the American Bell Telephone Labs. Until the late 1980s, this new technology was strongly supported by its main customer, the US military.

functionality

The SAW tag reflects the query signal as soon as an object with a code carrier reaches the reading area. The reader receives the reflected response signal and uses it to calculate the code. The use of this echo principle enables the passive, battery-free energy supply of the SAW tag. Basically, the functionality of the code query corresponds to the functionality of a distance radar device.

The SAW tags currently available are all fixed-code data carriers. Since the code is determined during production, the term “code carrier” is generally used in this text instead of “data carrier”. Analogous to the inductive code carriers, a “single chip” structure can also be found here. Instead of a coil and a silicon chip, a flat antenna is used, which is connected to a SAW element. The combination of antenna print and SAW element is accommodated in a specific housing, depending on the application.

A SAW element basically works as follows: The antenna converts the incident microwave into electrical excitation, which in turn is directed to the SAW element via a line. In the SAW element, the transducer converts the electrical excitation into a surface acoustic wave (SAW). This SAW propagates along the substrate surface and is reflected in itself by a mirror. The same transducer converts the reflected SAW back into electrical excitation, which is radiated via the antenna. The coding takes place serially with a geometrically embossed pattern.

properties

SAW tags differ significantly from normal RFID systems:

  • Temperature resistance: The high resistance of the SAW tags to high temperatures (up to 250 ° C, 320 ° C for 40 seconds) results from the fact that, on the one hand, no battery and, on the other hand, no electronics are used for communication and code storage. Siemens has shown that this is possible in principle up to 1000 ° C. Long-term SAW day measurements based on lithium niobate crystals showed temperature stability over 250 hours at 350 ° C and over 3000 hours at 300 ° C.
  • No battery: The lack of a battery allows high continuous temperatures of up to 180 ° C. Fastening the code carrier is usually associated with handling costs, especially in a robust industrial environment.
  • Large reading distance: A major advantage over inductive systems is the relatively large reading distance, which can be 1 to 2 meters even in very difficult surroundings. The more different codes are required, the smaller the response signal of the SAW tag and thus the maximum achievable reading distance.
  • Fast identification: The SAW principle allows the identification of fast objects (500 km / h) that are only briefly in the antenna field, as no active communication has to be established.
  • Mechanical and chemical robustness: The high mechanical strength of the SAW tags results from the robust “single chip” structure. The SAW element is packed in a very resilient steel or ceramic housing.

Individual evidence

  1. ↑ René Fachberger, Gudrun Bruckner, Jochen Bardong Carinthian Tech Research (CTR), Leonhard Reindl (Uni Freiburg IMTEK): High temperature RFID system using passive SAW transponders . In: Proceedings European Microwave Association , Vol. 4 (2007), pp. 288-1194 (Special Issue on RFID Systems).
  2. ↑ Klaus-Peter Dziggel (Siemens AG), Christoph Olszak (TU Dortmund): Detecting and identifying vehicles by surface wave techniques. In: Reinhard Jünemann u. a. (Ed.): Identification technologies. A guide through practice and research; Conference proceedings for SMAID '97 . UZV, Frankfurt / M. 1997, ISBN 3-930007-96-7 , pp. 217-222.
  3. ↑ Roland Stierlin (Baumer IDent AG): Heat- resistant ID tags for use in rough industrial environments . In: Kalus Krämer (Ed.): Basis of company processes. Identification technology . UZV, Frankfurt / M. 1999, ISBN 3-930007-97-5 , pp. 161-175.