Surface acoustic wave
A surface acoustic wave , short AOW ( engl. SAW for surface acoustic wave ) is a structure-borne sound -wave, the planar on a surface, thus only in two dimensions, spreads.
Basics and classification
Sound waves occur in solid media as longitudinal and transverse waves ; Only longitudinal waves can exist in liquids and gases because the shear modulus required for the transmission of transverse waves is missing there.
However, waves that have both a transverse and a longitudinal component can still propagate on the surface of liquids and solids; a surface point describes an elliptical movement when such a wave passes through . Such waves can be generated on the surface of solids and have a very high propagation speed due to the existing shear module. The sound wave propagates in a planar manner without penetrating deep into the material. The penetration depth is practically limited to one wavelength.
Waves in water (surface) is not to elastic waves, but gravity waves , in which the restoring force of the force of gravity is. In addition, there is a mass transport that cannot be neglected.
Applications
Frequency filter
Acoustic surface waves are used in particular in SAW filters , which consist of piezocrystals and electrode structures applied to them .
With the help of these electrodes, electrical signals can be converted into sound waves that propagate on the substrate surface. For example, frequencies can be selected through the shape of the electrodes or other shape parameters . At the filter output, the surface acoustic wave is converted back into electrical signals; the electrode structures required for this are basically the same as those used for generation.
These filters are used for frequency selection and for the implementation of defined transmission curves in transmitters , radio receivers , television sets and in oscillators .
A SAW filter is used in high-frequency oscillators and takes on the tasks of a quartz oscillator . Its maximum overtone frequency is well below 300 MHz. The picture shows the circuit of a Colpitts oscillator . Together with a transistor and two capacitors, a surface acoustic wave filter stabilizes the transmission frequency at 433 MHz.
RFID and wireless sensors
SAW structures with a connected antenna are passive components and return part of the radiated radio signal as an echo via the same antenna after the signal has passed through the SAW structure and has been reflected there by two or more structures. Due to the different locations of these reflector structures for each component, echoes that can be assigned to the component are created. The time interval between the echoes of a component depends on the position of the reflector structures in relation to one another and on the speed of sound of the substrate. Depending on the substrate and its crystal orientation, the speed of sound depends on the temperature and mechanical stresses.
RFID tags
Surface wave structures (SAW) can be used to produce digital identification marks ( SAW tags ) (compare also RFID ). For this purpose, a sound transducer is applied to a suitable substrate, which can receive electromagnetic signals via an antenna and convert them directly into surface waves. These are then reflected back by several reflectors attached to the substrate at individual intervals and emitted to the outside via the same transducer and antenna. The pulse sequence generated in this way can now be read out with a suitable reading device.
Applications are, for example, the control of the flow of goods or the contactless identification of medical implants, suture materials, surgical equipment, surgical instruments or blood reserves during and after sterilization. Compared to barcode labeling, SAW tags have the advantage that they can be better protected against dirt and abrasion. Compared to electronic coding, SAW tags offer the option of geometric coding, which is particularly robust.
Sensors
SAW sensors use the dependence of the surface wave speed on the mechanical tension (deformation), the mass impact (deposits on the surface) or the temperature (temperature coefficient of the speed of sound). They withstand high temperatures of up to around 400 ° C and have a high resistance to gamma rays - they withstand 500 kGy (kilo gray ) without any loss of performance; the upper limit is currently unknown.
Such SAW sensors can be suitable wherever, for certain reasons, the points to be measured are difficult to access or cabling is impossible. Rotors, couplings or shafts can be wirelessly monitored for their temperature or position in different operating states.
SAW sensors for pressure measurement can measure pressure and vibrations at various locations in the development of internal combustion engines . With their individual micro-structuring, the reliable identification and the subsequent clear assignment of the calibration data to the respective sensor succeed.
SAW sensors can measure the temperature of the conductors of high-voltage overhead lines and thus improve the load monitoring of high-voltage lines. The passive sensors, which are placed around the conductor, are queried over short distances of up to about 30 m by radio with regard to their echo-response sequence, which changes with the temperature. With a transmitter / receiver device similar to a radar , several sensors can be read out at the same time if they have individual structural dimensions. Measurement tolerances of up to 0.2 K are possible.
By using this information, less or more energy can be transported via the monitored lines, depending on the existing environmental conditions. The first installations are already running. The data transmission over the long distances along the line takes place, for example, with GPRS .
Other high-voltage components can also be temperature-monitored in this way, for example varistors (surge arresters).
Material transport
The Australian nanotechnology company Nanotechnology Victoria has developed an inhalation device that is supposed to enable the intrapulmonary administration of high-molecular therapeutics ( proteins such as insulin and erythropoietin ) on the basis of surface acoustic waves .
Flow measurement
Surface acoustic waves can also be used to measure the flow of liquids. For the SAW technology, wave propagation is used as in seismic activities: starting from an initial center of the excitation, a wave front propagates on the surface of a solid material. So-called interdigital transducers are excited by an electrical impulse and generate SAWs that propagate in a similar way to earthquake waves, but in this case directed and on the one hand on the pipe surface and one to several times running through the medium. The interdigital converters work as transmitters and receivers. If it is active as a transmitter, the two interdigital transducers furthest away from it act as receivers. The SAW spreads on the measuring tube surface, but also decouples into the liquid. The decoupling angle depends on the liquid or the propagation speed of the wave resulting from it. Once on the other side of the measuring tube, part of the wave couples in again and runs on the tube surface to the next interdigital transducer. Another part is decoupled again and moves back to the other side of the measuring tube, where the same effect occurs again and the transducer on this side of the tube receives the wave. So the excitation of each transducer leads to a sequence of received signals to two others. Two transducers transmit in the direction of flow, the other two against the direction of flow. The time difference between the propagation time in the forward and backward directions of the flowing liquid is proportional to the flow rate.
Temperature measurement
Temperatures can also be measured using surface waves. The advantage of the technology is that this type of temperature measurement is wireless and passive. This means that neither a data cable, power cable nor a battery is needed at the measuring point. This results in a highly flexible use of the technology at up to 300 degrees Celsius. Temperature measurement is used in ovens for real-time evaluation of the food being cooked in the oven.
Research activities
I.a. the Coburg also working on sensors on the basis of surface acoustic waves. This resulted in z. B. a sensor for measuring fluid properties.
Research is currently underway into the usability of surface waves for pumping liquid media ( acoustic streaming ).
The use for linear movement or positioning of flat bodies on top is also being investigated.
The Carinthian Tech Research (CTR) has carried out the following feasibility studies for the use of SAW sensors or is in progress: Remote temperature measurement in ovens. Distance measurement and positioning of machines in underground and surface mining, non-contact temperature measurement in microwave ovens.
See also
literature
- R. Hauser, R. Fachberger, G. Bruckner: Tagging of metallic objects in harsh environments . In: Sensors and measuring systems 2006: Lectures at the 13th ITG / GMA symposium from March 13th to 14th, 2006 in Freiburg / Breisgau . VDE, 2006, ISBN 3-8007-2939-3 , p. 619-624 .
- R. Fachberger, G. Bruckner, R. Hauser, L. Reindl: Wireless SAW based high-temperature measurement systems . In: International Frequency Control Symposium and Exposition, 2006 IEEE . 2006, p. 358-367 , doi : 10.1109 / FREQ.2006.275412 .
Individual evidence
- ↑ Measuring system for cylinder pressure measurement on internal combustion engines , AT 7781 U2 (2005)
- ↑ Teminova Renata: Use of passive surface wave sensors that can be queried by radio in electrical energy technology ; Dissertation, University of Darmstadt 2007.
- ↑ CTR: Measure high voltage lines - monitor the temperature during operation
- ↑ RITHERM / SAW TECHNOLOGY. Doble Lemke company, archived from the original on March 7, 2011 ; accessed on June 15, 2016 .
- ↑ azonano.com: “NanoVentures Australia Reaches Important Milestone in Development of Pulmonary Drug Delivery Technology” from May 31, 2009
- ↑ Product from Bürkert Fluid Control Systems , elektroniknet.de: Flow measurement for the first time using the SAW method from June 2, 2014
- ↑ tip300 - wireless temperature sensor. In: pro-micron. Retrieved November 26, 2019 (German).
- ↑ Wireless temperature sensor. In: All-Electronics.de. April 30, 2015, accessed November 26, 2019 (German).
- ↑ Monique Robinson: MIELE ENHANCES DUAL FUEL AND GAS RANGES FOR ULTIMATE CULINARY EXPERIENCE. Miele, accessed on November 26, 2019 (English).
- ↑ Using surface acoustic waves for actuation. (No longer available online.) Senter an agency of the Ministry of Economic Affairs Netherlands, Drebbel Institute of Mechatronics (University of Twente), Philips Center for Industrial Technology, March 2004, archived from the original on June 15, 2016 ; accessed on June 15, 2016 .