Surface acoustic wave (SAW) sensors

Basic operation

An acoustic wave sensor uses mechanical (acoustic) waves to sense multiple phenomena from the device's environment, 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:

1. An electromagnetic impulse signal is sent to the device via wired connection or wireless antenna
2. An input interdigital transducer (IDT) transduces the electromagnetic signal into a surface acoustic wave
3. The surface acoustic wave propagates along the delay line and is affected by its environment along the way
4. An output IDT transduces the acoustic impulse response wave back into an electromagnetic signal
5. 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 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.

Basic device components

The basic components of a SAW sensor are:

1. A piezoelectric substrate which generates electrical charges from mechanical force, and vice versa

2. At least one interdigital^A transducer (IDT) to convert electromagnetic waves to acoustic waves, and vice versa
3. 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

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 delay along a shorter delay line. The most common piezoelectric substrate materials are quartz, lithium niobate, and lithium tantalate.

IDTs and areas of propagation

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 (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.

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.

In bulk acoustic wave (BAW) sensors, the acoustic wave travels through the interior, or "bulk", of the piezoelectric substrate. Some sub-classifications are: thickness shear mode (TSM) resonators; shear-horizontal acoustic plate mode (SH-APM) sensors.

 In surface acoustic wave (SAW) sensors, the acoustic wave travels on the surface of the substrate. Some sub-classifications are: Rayleigh surface wave sensors (generally known as SAW sensors); shear-horizontal surface acoustic wave (SH-SAW) sensors, also known as surface transverse wave (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 

Figure 4: Propagation of a Love SAW with shear horizontal component.
Source: http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html

Rayleigh surface acoustic 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.

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:

• 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:

¹Vectron International, “Acoustic Wave Sensors”, (slide presentation and notes), http://www.sengenuity.com/tech_ref/AWS_WebVersion.pdf

²M. Hoummady 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

³J. Kirschner (2010), “Surface Acoustic Wave Sensors (SAWS): Design for Application”, Microelectromechanical Systems, December 6, 2010.

⁴M.F. Hribšek et al (2010), “Surface Acoustic Wave Sensors in Mechanical Engineering”, FME Transactions, Vol. 38, No. 1, 2010.

⁵A. Mamishev 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

⁶A. Pohl (2000), “A Review of Wireless SAW Sensors”, IEEE Transactions on Ultrasonics, Ferroelectronics, and Frequency Control, Vol. 47, No. 2, March 2000.

⁷V. Ferrari, R. Lucklum (2008), "Piezoelectric Transducers and Applications," 2nd edition, Springer Publishing.

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.