Surface acoustic wave
A surface acoustic wave SAW briefly (English for SAW surface acoustic wave) is a body of sound - wave planar on a surface that is only in two dimensions, spreads.
Basics and Classification
Sound waves enter into solid media in a longitudinal and transverse waves; in liquids and gases can exist only longitudinal waves because there is missing the required for forwarding of transverse shear modulus.
On the surface of liquids and solids, however, can still propagate waves that have both a transverse and a longitudinal component; a surface point describes the shaft passage of such a wave has an elliptical motion. Such waves can be generated on the surface of solid bodies and due to the existing translator module a very high propagation speed. The sound wave spreading out into a planar from without penetrating deeply into the material. The penetration depth is practically limited to one wavelength.
For water waves (surface ) is not elastic waves, but gravity waves, in which the restoring force is gravity. In addition, a non-negligible mass transport occurs.
Applications
Frequency filter
Surface acoustic waves are in particular used in SAW filters, which consist of piezo crystals and thereon electrode structures.
Electrical signals can be converted with the help of these electrodes into sound waves propagating on the substrate surface. By the shape of the electrodes or other shape parameters, for example, frequencies can be selected. At the filter output, the surface acoustic wave is converted back into electrical signals; the necessary electrode patterns identical in principle to those used for the production.
Apply this filter for frequency selection and the realization of defined transmission curves in transmitters, radio receivers, TV sets and in oscillators.
A SAW filter is used in high frequency oscillators and assumes the responsibilities of a quartz crystal. Meanwhile overtone peak frequency is significantly below 300 MHz. The picture shows the circuit of a Colpitts oscillator. Together with a transistor and two capacitors to stabilize a surface acoustic wave filter, the transmission frequency of 433 MHz.
RFID and wireless sensors
SAW structures with a connected antenna are passive components and give a portion of the incident radio signal back as an echo of the same antenna, after the signal has passed through the SAW structure and was reflected there at two or more structures. By individually for each component of other places this reflector structures are created to associate the component echoes. The time spacing of the echoes of a component depends on the position of the reflector structures to each other and the speed of sound of the substrate. The speed of sound depends on the temperature and mechanical stresses depending on the substrate and the crystal orientation.
RFID tags
By means of surface acoustic wave structures (SAW ) can be digital identification marks ( SAW tags) (see also RFID) produce. For this, a transducer is applied to a suitable substrate, capable of receiving electromagnetic signals via the antenna and convert directly to surface waves. These are then reflected from a plurality of intervals in the individual reflectors mounted on the substrate and is discharged through the same transducer and the same antenna to the outside again. The pulse sequence produced in this way can now be read by a suitable reader.
Applications include the control of flows of goods or contactless identification of medical implants, sutures, surgical equipment, surgical instruments or blood products during and after sterilization. Compared to bar code labels have SAW tags the advantage that they can be better protected against dirt and abrasion.
Sensors
SAW sensors utilize the dependency of the surface wave velocity of the mechanical stress ( deformation ) of the Massenbeaufschlagung ( deposits on the surface ) or the temperature ( temperature coefficient of sound velocity ). They withstand high temperatures up to about 400 ° C and have a high resistance to gamma rays - they are 500 kGy (kilo Gray) without loss of performance; the upper load limit is currently still unknown.
Wherever the points to be measured are difficult to access for some reason or wiring is impossible, the application of such SAW sensors may be suitable. Rotors, shafts, or coupling can be monitored wirelessly in different operating states of their temperature or position.
SAW sensors for measuring the pressure can measure the pressure and vibration at various locations in the development of internal combustion engines. With their individual micro - structuring secure identification and subsequent unambiguous assignment of the calibration data for each sensor succeed.
SAW sensors can measure the temperature of the conductors of high-voltage overhead lines, thus improving the load monitoring of high voltage power lines. The passive to the rope ladder down sensors are doing is queried over short distances up to about 30 m above with respect to their radio with temperature shifting echo response sequence. With a transceiver similar to a radar several sensors can also be read when they have individual structural dimensions. Measurement tolerances to 0.2 K are possible. By using this information you can, depending on the existing environmental conditions, carry less or more energy over the monitored lines. First installations are already underway. The data transmission over long distances along the line occurs, for example with GPRS. Other areas of high voltage components can thus be temperature controlled, such as varistors ( surge arrester).
Mass transport
The Australian nanotechnology company Nanotechnology Victoria has developed an inhalation device that is based on the surface acoustic wave allow the intrapulmonary administration of high molecular weight therapeutics (proteins such as insulin and erythropoietin ).
Research activities
Among other things, the Coburg University is also working on sensors based on surface acoustic waves. This showed a sensor for measurement of fluid properties, for example.
On the applicability of surface waves for pumping liquid media (acoustic streaming) is currently being researched.
Also the use of the linear movement and positioning of superposed flat bodies is examined.
The Carinthian Tech Research (CTR ) has the use of SAW sensors performed following feasibility studies or are in preparation: remotely queried Temperature measurement in ovens. Distance measurement and positioning machines in underground and above daily mining, non-contact temperature measurement in microwave furnaces.