...
SAW sensors may also include an additional filtering or packaging element to sense many types of phenomena indirectly. The sensor in this case is not directly sensing the phenomenon, but instead sensing the response of the filtering or packaging element to its presence.
The piezoelectric
...
effect
The IDTs and propagation area of a SAW sensor is built on a piezoelectric substrate, which 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 long delay along a relatively 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 and vice versa.
...
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.
...
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 34: Propagation of a Rayleigh SAW with shear vertical component.
Source: http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html
Figure 45: Propagation of a Love SAW with shear horizontal component.
Source: http://www.tjhsst.edu/~jlafever/wanimate/Wave_Properties2.html
...
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.
Sensor characteristics
Transfer functions
Due to the application-specific nature of IDTs, substrate materials, device configurations, and other considerations, transfer functions for SAW sensors are also often application-specific and based on experimental data.
However, transfer functions for the two-port delay line configuration may be generalized. One such transfer function used for rapid simulation tools is as follows:
where where f is the frequency, k is the piezoelectric coupling coefficient, τ is the delay between IDTs in wavelengths, CS is the capacitance for an IDT digit pair per unit length, NP is the number of IDT digit pairs, and X is defined as:
A more simplified transfer function for a two-port delay line can be written as:
where H_1and H_2 are individual IDT transfer functions, and τ is the delay of the electric signal due to the delay line.
While the magnitude of the delay line transfer function depends on the characteristics of the individual IDTs, the phase of the response is only dependent on the delay and signal and synchronous frequencies:
Sensitivity
SAW sensor sensitivities are also dependant on their wavetype, configuration, components, materials, and applications. Some typical sensitivities are listed below:
| Physical quantity | Linear coefficient |
|---|---|
| Temperature | up to 100 ppm/K |
| Pressure, stress | 2 ppm/kPa |
| Force | 10 ppm/kN |
| Mass loading | 30 ppm/μg·cm2 |
| Voltage | 1 ppm/V |
| Electric field | 30 ppm/V·μm−1 |
Figure 6: Linear coefficients for physical effects on SAW sensors6.
SAW sensors are often valued for their high degree of sensitivity due to the concentration of energy at the device’s surface, where the external environment can have a greater effect. However, this is oftentimes a design challenge. For example, whereas surface acoustic waves with shear vertical components are very sensitive to changes in gaseous environments, they can undergo severe damping in liquid environments. Furthermore, in environments with large temperature fluctuations, a SAW sensor’s piezoelectric substrate can be affected by these fluctuations, often necessitating an additional “reference” configuration to control for such effects.
Interface electronic circuits
Excitation
SAW sensors are fundamentally passive, in that they need no additional energy other than an excitation impulse to operate. However, the sensor’s response must be processed to evaluate synchronous frequency, amplitude, and/or phase shift before any useful data is recovered from the sensor system.
Signal conditioning
Input impedance varies depending on the sensor package, but is typically ~50 ohms for easy impedance matching with test equipment.
As SAW devices often use a low-power signal for excitation input, and response signal power is derived from the input signal, response signals typically require amplification for processing. However, wireless signal transmission from the SAW is possible without amplification up to short distances.
Highly sensitive SAW sensor systems may need to quantitatively determine very small signal changes. High-precision methods for phase detection and frequency response analysis are often needed for such applications, requiring digital signal processing techniques like the fast Fourier transform, zero-crossing, and sine-wave fitting.
Noise
Noise can distort SAW outputs due to many unwanted second-order effects, including:
- Electromagnetic feedthrough between IDTs, causing amplitude and phase ripple
- “Triple-transit interference” associated with SAW reflections, causing ripple effects
- Mass-loading by IDT digits, causing SAW velocity changes
- Unwanted bulk wave emissions accompanying SAW emissions, causing passband distortion
- Finite source and load impedances, causing frequency-dependent voltages across IDTs
- IDT diffraction similar to optical systems, causing changes in transition band and shape
- Harmonic frequencies generated by the input IDT (may be desirable or undesirable, depending on application)
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:
...
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
10C.K. Campbell (1989), “Applications of Surface Acoustic and Shallow Bulk Acoustic Wave Devices,” Proceedings of the IEEE, Vol. 77, Issue 10, Oct 1989.
11W.C. Wilson, G.M. Atkinson (2007), “Rapid SAW Sensor Development Tools,” NASA Langley Research Center.