Acoustic Sensors

This works on the principle that any change in the environment concentration is reflected as a change in mass of the sensing interface which is then measured as a variation in vibration frequency. Sensors working by this principle are alternately termed "surface acoustic wave (SAW) resonators."

15.2.3.1. Piezoelectric Effect

The piezoelectric effect is due to the fact that certain crystals contain positively and negatively charged ions, which separate when the crystals are subjected to stress. Vibration of a crystal involves its passing from a deformed configuration through its equilibrium configuration to an opposite deformed configuration and then back through the equilibrium configuration to the original deformed configuration.

When the vibrating crystal is piezoelectric, this cycle of oscillating deformity produces an oscillating electrical field; the frequency of the electrical oscillation is identical to the vibrational frequency of the crystal. At the same time, placing a piezoelectric crystal in an oscillating electrical field causes it to vibrate at its frequency. This transfer of energy from the electric field to the crystal is highly inefficient except when the frequency of the oscillating electrical field is the same as the resonant frequency of the crystal.

The crystals most commonly used are 5, 9, and 10 MHz quartz of 10-16-mm disks, squares, or rectangles that are approximately 0.15-mm thick. The metal electrodes are 3000-10,000-A thick and 3-8 mm in diameter and can be made of gold, silver, aluminum, or nickel.

The quartz wafer is sandwiched between two electrodes bonded to the wafer surface. These electrodes are used to induce an oscillating electrical field perpendicular to the surface of the water. The electrical field then produces a mechanical oscillation, a standing wave, in the bulk of the quartz wafer as described in Eq. (2). Mechanical oscillation of the crystal is maximum where the electrode pads overlap and diminishes rapidly in areas where the oscillating electrodes do not overlap.

where

A fo = Change in the resonant frequency fo = Initial (applied) frequency at zero mass A = Surface area of the resonator Xq = Shear modulus of the quartz crystal pq = Density of the resonator S = Proportionality constant.

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