Basic device components:
- Piezoelectric substrate which uses the piezoelectric effect to generate electrical charges from mechanical force, and vice versa
- Two (2) interdigital transducers (IDT), one acting as a generator and the other as a receiver
- Delay line through which the acoustic wave propagates
Diagram of a surface acoustic wave sensor interdigitated transducer.Source: http://en.wikipedia.org/wiki/File:Surface_Acoustic_Wave_Sensor_Interdigitated_Transducer_Diagram.png
Basic operation:
- Sensor transduces an electric signal into an acoustic wave. The transducer itself is “flexible,” in that it is made to be sensitive to changes in pressure, strain, temperature, etc. so that the generation
- The acoustic wave is propagated, at which time it is affected by its environment
- Sensor transduces the acoustic wave back into an electric signal
- The signals are compared to determine what changes the wave underwent during its propagation. These changes can then be used to determine the properties of the environment through which the acoustic wave propagated
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
- 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.
However, this vertical shear component also undergoes severe damping when placed in a liquid medium, rendering SAW devices best suited for only gaseous 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 devices of course exist to sense 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 pretty gifs 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, torque, acceleration, friction, humidity, UV radiation, magnetic fields, and viscosity.
They are frequently used for materials sensing and as RF filters for signal processing applications.
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.