Basic operation
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 input interdigital transducer (IDT) transduces an electric 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 wave back into an electric signal for processing
The electric signals can be compared to determine what changes the wave underwent during its propagation, i.e. how the frequency, phase, or amplitude of the sent signal differs from the received signal. These changes 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.
Basic device components
The basic components of an acoustic wave sensor are:
A piezoelectric substrate which generates electrical charges from mechanical force, and vice versa
- Interdigital transducers (IDTs) to generate and receive acoustic waves
- An area of propagation, oftentimes conceived as a delay line, through which the acoustic wave propagates
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
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:
- Bulk acoustic wave (BAW): wave travels through the piezoelectric substrate
- Thickness shear mode resonator (TSM)
- Shear-horizontal acoustic plate mode sensor (SH-APM)
- Surface acoustic wave (SAW): wave travels on the surface of the substrate
- Rayleigh surface waves sensor (generally known as a SAW sensor)
- Shear-horizontal surface acoustic wave sensor (SH-SAW), also known as the surface transverse wave sensor (STW)
SAW devices are particular among this group since surface acoustic waves include a vertical shear component, which greatly affects the velocity and amplitude of the wave along the delay line. This results in higher sensitivity among SAW devices than shear-horizontal wave sensors.
Propagation of a Rayleigh SAW with shear vertical component vs a Love SAW 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
However, this vertical shear component also undergoes severe damping when placed in a liquid medium, rendering 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:
- mass
- temperature
- pressure
- stress, strain, and torque
- acceleration
- friction
- humidity and dewpoint
- UV radiation
- magnetic fields
- viscosity
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:
Drafts, 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
Vectron International, “Acoustic Wave Sensors”, (slide presentation and notes), http://www.sengenuity.com/tech_ref/AWS_WebVersion.pdf
Hoummady, Moussa et al (1997), “Acoustic wave sensors: design, sensing mechanisms and applications”, Smart Materials and Structures, Vol. 6, No. 6, December 1997, http://www.uta.edu/rfmems/BMC/0720/0902_backup/Background/sm7601.pdf
Kirschner, Jared (2010), “Surface Acoustic Wave Sensors (SAWS): Design for Application”, Microelectromechanical Systems, December 6, 2010.
Hribšek, Marija F. et al (2010), “Surface Acoustic Wave Sensors in Mechanical Engineering”, FME Transactions, Vol. 38, No. 1, 2010.
Mamishev, Alexander et al (2004), “Interdigital Sensors and Transducers”, Proceedings of the IEEE, Vol. 92, No. 5, May 2004, http://www.rle.mit.edu/cehv/documents/81-Proc.IEEE.pdf
Pohl, Alfred (2000), “A Review of Wireless SAW Sensors”, IEEE Transactions on Ultrasonics, Ferroelectronics, and Frequency Control, Vol. 47, No. 2, March 2000.