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Put simply, acoustic wave sensors send and receive acoustic waves "within" the device to promote an effect from the device's environment on these waves. The device operation itself is fairly simple:
- An electromagnetic impulse signal is sent to the device via wired connection or wireless antenna
- An input interdigital transducer (IDT) transduces an electric the electromagnetic signal into an acoustic wave
- The acoustic wave propagates along the delay line and is affected by the environment along the way
- An output IDT transduces the propagated acoustic impulse response wave back into an electric signal electromagnetic signal
- The electromagnetic response signal is transmitted for processing
The electric signals can be compared electromagnetic response is then analyzed to determine what changes the acoustic wave underwent during its propagation, i.e. how the . These changes in frequency, phase, or amplitude of the sent signal differs from the received signal. These changes and amplitude can in turn be used to determine the properties of the environment through which the acoustic wave traveled.
Acoustic wave sensors may also include a filtering element as a first step to sensing, for example, a particular chemical or biological compound. The acoustic wave sensor in this case is not directly sensing the compound, but instead sensing the response of the filtering element to the presence of the compound.
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A piezoelectric substrate which generates electrical charges from mechanical force, and vice versa
- Interdigital transducers (IDTs) to generate and receive acoustic wavesAt least one interdigital1 transducer (IDT) to convert electromagnetic waves to acoustic waves, and vice versa
- An area of propagation, oftentimes conceived as a delay line, through which the acoustic wave propagates
Figure 2: Diagram of a surface acoustic wave sensor using a delay line. Source: http://en.wikipedia.org/wiki/File:Surface_Acoustic_Wave_Sensor_Interdigitated_Transducer_Diagram.png
Instead of multiple IDTs (as shown in the figures above), many acoustic wave sensors include only one IDT element for transducing both impulse and response signals. These devices use a reflector element to reflect the acoustic wave back into the same IDT that produced it. The slow propagation speed of the mechanical wave (compared to the electromagnetic impulse) along the delay line allows sufficient time for a short electromagnetic impulse input to dissipate before the reflected response is captured by the single IDT.
Piezoelectric substrate
A piezoelectric substance is a crystalline mineral which responds to a mechanical force by generating a voltage. This voltage is proportional to the amount of force applied as well as the type of force applied (i.e. tension and compression produce opposite polarities). Furthermore, this effect is reciprocal, so a piezoelectric substance will also respond to an electric field by generating a mechanical response that is proportional to the field's strength and polarity.
The material of the piezoelectric substrate determines the velocity of the acoustic wave, which is in the range of 1500-4800 m/s. This is 105 times slower than the electromagnetic wave velocity, allowing for a longer delay along a shorter delay line. The most common piezoelectric substrate materials are quartz, lithium niobate, lithium tantalate, zinc oxide, and bismuth germanium oxide.
Interdigital transducers
An IDT consists of a series of electrodes...
Area of propagation
Resonator vs delay line...
Common sensor types:
Acoustic wave sensors are generally classified based on the propagation mode of the acoustic wave. Some common wave types and sensors are:
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SAW devices are particular among this group since surface acoustic waves include a vertical shear component, which greatly affects the are more sensitive to velocity and amplitude of the wave along changes due to the environment on the surface of the delay line. This results in higher sensitivity among SAW devices than shear-horizontal wave sensorsto environmental stimuli such as humidity, radiation, and viscosity.
Figure 3: Propagation of a Rayleigh SAW surface acoustic wave with shear vertical component vs a Love SAW .
Source: http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html
Figure 4: Propagation of a Love surface acoustic wave with shear horizontal component.
Consider the state of the environment above the waves, e.g. if medium were air or water and their relative damping effects on the waves. Source: http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html
Generally, "SAW" sensors propagate Rayleigh surface acoustic waves. Rayleigh waves include a vertical shear component which further increases sensitivity to the device's external environment. However, this vertical shear component also undergoes severe damping when placed in a liquid medium, rendering Rayleigh SAW devices best suited for only gas and vacuum environments.
TSM, SH-APM, and SH-SAW devices are better suited for operation in liquid environments since their shear horizontal waves do not lose much energy into liquids.Other acoustic wave sensors exist for other types of waves, e.g. flexural plate waves, Love waves, surface skimming bulk waves, and Lamb waves. (See http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html for some visualizations of different wave propagation modes).
Applications:
Acoustic wave sensors are very versatile in that they may be used alone or as part of a filtered sensor to measure many phenomena, including:
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Pohl, Alfred (2000), “A Review of Wireless SAW Sensors”, IEEE Transactions on Ultrasonics, Ferroelectronics, and Frequency Control, Vol. 47, No. 2, March 2000.
Footnotes
1The interdigital transducers do not involve A/D conversion or digital data. Here, the term "digital" refers to the resemblance of IDTs to human fingers.