The Principle of Piezoelectric Sensing

The piezoelectric transduction is based on the conversion of mechanical vibrations into electrical signals and vice versa. The piezoelectric effect involves the generation of electric charge by a material when subjected to a mechanical deformation. The redistribution of electrical charges in the piezoelectric material causes variations of the electric field inside the material. The reverse piezoelectric effect is defined by the appearance of a mechanical deformation as reaction to the externally applied voltages.

While structures like diodes or bipolar transistors show piezoelectric properties, silicon (which is normally used in these devices) is not a piezoelectric material; thus, layers of piezoelectric materials are deposited to obtain this functionality in integrated devices. Piezoelectric materials that are typically used in sensors are zinc oxide (ZnO), aluminium nitride (AlN), PZT, just to name a few. They are deposited by RF sputtering, evaporation, etc.

Mechanical waves are usually produced in the piezoelectric material by using sinusoidal voltage excitation. This gives place to mechanical resonance at high enough frequencies. Mechanical resonances originate resonance peaks in the measured admittance between the electrodes of the capacitor.

Piezoelectric coefficients are temperature dependent. The temperature variation in sensors has to be compensated.

Related: Types of Sensors used in Measurement and Process Control

Application of Piezoelectric Sensing

• Acoustic sensing often employs piezoelectric transducers. They can be used in viscous liquids. This is used to make microbalances and other types of sensors. In bulk acoustic wave (BAW), the mechanical wave propagates through the material. Resonances are found at frequencies at which the material thickness is an integer number of half wavelengths. BAW resonators often work at moderate frequencies permitting the use of low-cost electronics. These devices find use in gravimetric sensing.

Piezoelectric devices are frequently applied to sensors like gas or biosensors and to RF devices like filters.

Also read: Variable Capacitance Sensors