Surface acoustic wave sensor
Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.
In some devices, a mechanical absorber or reflector is added between the IDTs and the edges of the substrate to prevent interference patterns or reduce insertion losses, respectively.
The structure of the basic surface acoustic wave sensor allows for the phenomena of pressure, strain, torque, temperature, and mass to be sensed.
The mechanisms for this are discussed below: The phenomena of pressure, strain, torque, temperature, and mass can be sensed by the basic device, consisting of two IDTs separated by some distance on the surface of a piezoelectric substrate.
A surface acoustic wave temperature sensor can be fashioned from a piezoelectric substrate with a relatively high coefficient of thermal expansion in the direction of the length of the device.
Due to the ability of Surface Acoustic Wave sensors to operate within electromagnetically noisy environments and in close proximity to magnets it has been found that they can be embedded into electric motors in order to improve control by providing active torque and temperature measurement of the machine rotor shaft.
The velocity v of a wave traveling through a solid is proportional to the square root of product of the Young's modulus E and the density
The inherent functionality of a surface acoustic wave sensor can be extended by the deposition of a thin film of material across the delay line which is sensitive to the physical phenomena of interest.
If a physical phenomenon causes a change in length or mass in the deposited thin film, the surface acoustic wave will be affected by the mechanisms mentioned above.
Some extended functionality examples are listed below: Chemical vapor sensors use the application of a thin film polymer across the delay line which selectively absorbs the gas or gases of interest.
When exposed to ultraviolet radiation, zinc oxide generates charge carriers which interact with the electric fields produced in the piezoelectric substrate by the traveling surface acoustic wave.
Ferromagnetic materials (such as iron, nickel, and cobalt) change their physical dimensions in the presence of an applied magnetic field, a property called magnetostriction.
The resulting strain (i.e., the deformation of the surface of the substrate) produces measurable changes in the phase velocity, phase-shift, and time-delay of the acoustic wave signal, providing information about the magnetic field.
