...
The electromagnetic response is then analyzed to compare its frequency, phase, and amplitude to some reference. Based on this comparison, certain properties of the device's environment may be deduced, such as temperature, strain, pressure, force, and mass.
SAW sensors may also include a filtering element as a first step to sensing, for example, a particular chemical or biological compound. The 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.
Basic device components
The basic components of a SAW sensor are:
A piezoelectric substrate which generates electrical charges from mechanical force, and vice versa
- At least one interdigitalA transducer interdigital transducer (IDT) to convert electromagnetic waves to acoustic waves, and vice versa
- An area of propagation, in some cases conceived as a delay line (see below), 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
Piezoelectric substrate
SAW sensors may also include a filtering element as a first step to sensing, for example, a particular chemical or biological compound. The 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.
The piezoelectric substrate
The IDTs and propagation area of a SAW sensor is built on a piezoelectric substrate, which The device uses the piezoelectric effect to respond to mechanical forces by generating a voltage, and vice versa. This voltage is proportional to the amount of force applied to the device as well as the type of force applied (i.e. tension and compression produce opposite polarities). Furthermore, this effect is reciprocal, so the device 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 device's 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 long delay along a shorter delay linerelatively short area of propagation. The most common piezoelectric substrate materials are quartz, lithium niobate, and lithium tantalate.
Interdigital transducers (IDTs)
An interdigital transducer consists of a series of comb-like conductive structures with an interleaving pattern that resembles fingers, or "digits" (see Figure 3). Using the piezoelectric effect, the IDTs
...
convert the electromagnetic current of the impulse signal into acoustic waves which propagate in both directions of each "digit," creating constructive and destructive interference among the waves.
Figure 3: A generic IDT with pitch p (left); Frequency response of a generic IDT (right). Adapted from source9.
Where constructive interference and wave amplitude is maximized, the wave is said to be at the synchronous frequency, or fs (also termed the characteristic frequency). The period length, or pitch, between an IDT's fingers affects this frequency respective to the velocity of the waves along the propagation area. This relationship is expressed as:
This relationship shows that changes in either an IDT's pitch or in the velocity of the SAW can be identified by determining the synchronous frequency of the device and comparing it to some reference.
Device configurations
Typically, SAW devices use either a one-port resonator or a two-port delay line configuration. The two-port delay line configuration (pictured above) consists of one input IDT, one output IDT, and an area of propagation in between called the delay line.
The one-port resonator configuration includes only one IDT element for transducing both impulse and response signals. In these devices, the area of propagation leads to a reflector element, which reflects the acoustic wave back into the same IDT that produced it. The slow propagation speed of the mechanical wave (mentioned above) allows sufficient time for the electromagnetic impulse to be completely transduced (or dissipated) before the reflected acoustic response is captured by the single IDT.
Acoustic wave 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 wave or a bulk acoustic wave device.
...
SH-SAW sensors employing acoustic waves without a shear vertical component (e.g. surface transverse waves) are better suited for operation in liquid environments since their shear horizontal components do not lose much energy into liquids external to the device.
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:
...
Acoustic wave devices have been in commercial use for over 70 years, and their most common use is in the telecommunications industry as filters for signal processing applications. Recently, however, interest in acoustic wave devices for sensing applications has risen greatly due to their low cost, reliability, sensitivity, flexibility to measure many phenomena, and mature technology.
References and further reading
...
1Vectron International, “Acoustic Wave Sensors”, (slide presentation and notes), http://www.sengenuity.com/tech_ref/AWS_WebVersion.pdf
...
8Drafts, Bill (2001) “Acoustic Wave Technology Sensors”, IEEE Transactions on Microwave Theory and Techniques, Vol. 49, No. 4, April 2001, http://www2.nkfust.edu.tw/~jcyu/Paper/Acoustic%20wave%20technology%20sensors.pdf
Footnotes
AThe interdigital transducers do not involve A/D conversion or digital data. Here, the term "digital" refers to the resemblance of IDTs to human fingers.9M.I.Rocha Gaso et al (2013), "Love Wave Biosensors: A Review," from "State of the Art in Biosensors - General Aspects," Intech. http://www.intechopen.com/books/state-of-the-art-in-biosensors-general-aspects/love-wave-biosensors-a-review