SURFACE ACOUSTIC WAVE (SAW)

RADIO TRANSPONDERSBy:

Utsav Gami

1171110175

WHAT IS A TRANSPONDER ?

A communications satellite's transponder, is the series of interconnected units which form a communications channel between the receiving and the transmitting antennas. It is mainly used in satellite communication to transfer the received signals.

In air navigation or radio frequency identification, a transponder is a device that emits an identifying signal in response to an interrogating received signal.

In optical fibre communications, a transponder is the element that sends and receives the optical signal from a fibre. A transponder is typically characterized by its data rate and the maximum distance the signal can travel.

WHAT IS SURFACE ACOUSTIC WAVE ?

A surface acoustic wave (SAW) is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.

SAWs were first explained in 1885 by Lord Rayleigh.

Rayleigh waves are a type of surface acoustic wave that travel on solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects. They are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized Rayleigh waves.

Experimental image of surface acoustic waves on a crystal of Tellurium Oxide

WHAT IS A SAW TRANSPONDER ?

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Surface acoustic wave (SAW) radio transponders make it possible to read identification codes or measurement values from a remote location. A sensor system based on surface acoustic waves (SAW) consists of a reader with transmitting and receiving antenna(s), a SAW transponder that may for example be designed as a reflective delay line and a transponder antenna. The measurement signals are contained in the SAW transponder’s high-frequency response signal, which the reader records and evaluates. Depending on the design of the piezoelectric crystal in the SAW transponder, its natural sensitivity for example to temperature can be utilised.

Schematic drawing of a wireless transponder system based on a passive SAW transponder.

Surface acoustic wave (SAW) devices are based upon the piezoelectric effect and on the surface related dispersion of elastic (= acoustic) waves at low speed. If an (ionic) crystal is elastically deformed in a certain direction, surface charges occur, giving rise to electrical voltages in the crystal (application: piezo lighter). Conversely, the application of a surface charge to a crystal leads to an elastic deformation in the crystal grid (application: piezo buzzer). Surface acoustic wave devices are operated at microwave frequencies, normally in the ISM range 2.45 GHz.

SAW PRESSURE SENSOR

WORKING

SAW sensors can be built with a SAW delay line element connected to an antenna. The SAW delay line consists of a substrate, an inter-digital transducer (IDT), and a reflector.

1. The transceiver sends RF interrogation signal which is received by the antenna of the SAW sensor.

2. The IDT which is connected to the antenna, transforms the received signal which is an electrical RF voltage applied between the two opposing electrode combs into a SAW.

3. The SAW propagates on the piezoelectric crystal and is partially reflected by reflectors placed in the acoustic path.

4. The reflected waves are reconverted into an electromagnetic pulse train by the IDT and are retransmitted to the radar unit.

5. The high frequency electromagnetic signal is amplified and down converted to the baseband frequency in the RF module of the radar unit.

6. Then the sensor signals are analysed with a digital signal processor.

7. Finally the measurement results can be transferred to a personal computer for post processing and data storage.

SAW PRESSURE SENSOR OPERATION :

The SAW propagates on a quartz diaphragm, bending under hydrostatic pressure. To bend the diaphragm in a defined manner, there has to be a constant reference-pressure at the other side of the diaphragm. This is realized by a hermetically closed cavity with the reference pressure inside. Therefore with a sand-blast unit a blind-hole was structured into a quartz cover plate, which is of the same substrate material as the diaphragm. A monolithically packaged SAW radio transponder and pressure sensor are developed for the application to a TPMS. The device contains the wireless transponder, which converts analog signal into digital one without any auxiliary electronic circuits and transmits the converted data wirelessly. The realization of the mechanical A/D conversion is possible since the SAW radio transponder is connected to the touch-mode capacitive pressure sensor.

TIRE-PRESSURE MONITORING SYSTEM

A tire pressure monitoring system (TPMS) is an electronic system designed to monitor the air pressure inside the pneumatic tires on various types of vehicles. TPMS report real-time tire-pressure information to the driver of the vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light. TPMS can be divided into two different types — direct (dTPMS) and indirect (iTPMS)

Targets

Avoiding traffic accidents due to under-inflated tires by early recognition of the malfunction of tires

Reducing rolling resistance thus increasing overall fuel efficiency

IT’S SENSOR USES THE SAW DEVICE AS A DIAPHRAGM BETWEEN THE SIDE OF THE SENSOR

SUBJECTED TO TIRE PRESSURE AND A SEALED REFERENCE CHAMBER. THE ENERGY NEEDED IS PROVIDED

FROM THE SIGNAL OF THE RECEIVER COMPONENT. THE TRIPLE SAW PRESSURE DEVICE PROVIDES

TEMPERATURE COMPENSATED PRESSURE MEASUREMENT FROM A SINGLE QUARTZ DIE OPERATING IN A

SIMPLE BENDING MODE. FIGURE BELOW SHOWS HOW THE SAW SENSOR IS USED IN TPMS.

APPLICATIONSTemperature measurement on high-voltage power

lines is important additional information for optimum capacity utilisation of the grid. In this case SAW sensors are mounted on the electricity lines. The special benefit of SAW sensors in this application is that they operate completely passively and do not therefore require any maintenance (e.g. battery replacement).

SAW sensors are also used to monitor the rotor temperature in high-speed electric motors. The sensor itself is located at the point of interest inside the rotor and is connected to the antenna on the front of the rotor. The reader is housed in the stator.

Torque Sensors.Current Sensors.Fixed Coded SAW ID Tags.

In metal working SAW sensors are used for example to measure temperature in refractory components. The measurements enable conclusions to be drawn regarding the state of the components, thus contributing to safety and reliability.

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.

TPMS

ADVANTAGESThe decisive advantage of these SAW transponders lies in their passive

operation (i.e. no power-supply).The possibility of wireless installation at particularly inaccessible

locations.The passive SAW transponders are maintenance free.SAW radio transponders are advantageously placed on moving or

rotating parts and in hazardous environments such as contaminated or high voltage areas.

They can also 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.

High performance.Small size.Exact reproducibility.

REFERENCESIEEE paper on “Theory and application of passive SAW radio

transponders as sensors”. Wikipedia.www.gorferay.comhttp://www.ctr.at/en/r-d-technologies/wireless-sensors/tempera

ture-sensors.html

THANK YOU…