Basic operation
Put simply, an An acoustic wave sensor sends and receives acoustic waves "within" the device to promote an effect uses mechanical (acoustic) waves to sense multiple phenomena from the device's environment on these waves, which are registered as changes in the wave's phase, amplitude, and/or frequency relative to some reference. For surface acoustic wave (SAW) sensors, 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 the electromagnetic signal into an a surface acoustic wave
- The surface acoustic wave propagates along the delay line and is affected by the its environment along the way
- An output IDT transduces the acoustic impulse response wave back into an electromagnetic signal
- The electromagnetic response signal is transmitted for processing
Figure 1: Overview of surface acoustic wave sensor operation. Graphics edited from source7.
The electromagnetic response is then analyzed to determine what changes the acoustic wave underwent during its propagation. These changes in frequency, phase, and amplitude can in turn be used to determine the properties of the environment through which the acoustic wave traveled .
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when compared to some reference.
SAW 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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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.
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Acoustic wave
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types:
SAW sensors are only a subset of acoustic wave sensor devices. Acoustic wave sensors are generally classified based on the propagation mode of the acoustic wave employed, firstly ordered as either a surface acoustic waves wave or a bulk acoustic waves. Some common acoustic wave types and sensors are:
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wave device.
In bulk acoustic wave (BAW)
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sensors, the acoustic wave travels through the
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interior, or "bulk", of the piezoelectric substrate. Some sub-classifications are: thickness shear mode (TSM) resonators; shear-horizontal acoustic plate mode
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(SH-APM) sensors.
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In surface acoustic wave (SAW)
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sensors, the acoustic wave travels on the surface of the substrate
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. Some sub-classifications are: Rayleigh surface
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wave sensors (generally known as
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SAW
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sensors); shear-horizontal surface acoustic wave
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(SH-SAW) sensors, also known as
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surface transverse wave
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(STW) sensors.
SAW devices are typically more sensitive to velocity and amplitude changes due to the propagation of the wave on the surface of the delay line, where it can be affected more easily by the external environment. This results in higher sensitivity to environmental stimuli such as humidity, radiation, and viscosity.
Figure 3: Propagation of a Rayleigh SAW with shear vertical component.
Source: http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html
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