What is the Principle of surface acoustic wave sensor?

Answer 1

The resonant frequency of a SAW device is directly proportional to the velocity of the wave as it travels across the surface. The velocity is mostly determined by the material upon which the SAW propagates (such as quartz, lithium niobate, or lithium tantalate, to name a few common substrates). However, the velocity is also somewhat sensitive to a number of other parameters, including the stiffness, mass, and electrical conductivity of the substrate material or any other materials with which the wave comes into contact.

SAW sensors can, in principle, be used to measure all kinds of things, including temperature, acceleration, electric field strength, etc. However, their most common use is for chemical sensors (usually gas sensors). Most SAW chemical sensors work by virtue of mass sensitivity. The surface of a SAW resonator is coated with a thin layer of material (usually some sort of polymer) which chemically absorbs the gas of interest, kind of like a sponge soaking up water. A sponge, however, will soak up all kinds of different things - water, milk, juice, whiskey - pretty much any liquid. The polymer films on SAW gas sensors, on the other hand, are usually designed to absorb very, very specific chemicals. When the sensor is exposed to the chemical, gas molecules are adsorbed onto the polymer film, where they effectively increase the mass of the film. This tiny increase in mass causes a corresponding decrease in the velocity of the SAW, which in turn causes a decrease in the resonant frequency of the device. Since frequency can be measured with extremely high precision, gases can often be detected at extremely low concentrations, easily down in the parts per billion (ppb) or even parts per trillion (ppt) range.