measurand grows into the change of electrostatic charge or tension, arising up in some materials at their mechanical tension. Tension usually appears under the action of forces of compression, tension or bend, which are a measurand and affect pickoff either directly or by means of some mechanical connection.
To perceive the change of charge or tension, to piezoelectric material connect two metallic plates, which form the plates of condenser the capacity of which is determined in a kind actually
C = Q/V
where Q is a charge; V is tension (voltage).
As piezoelectric material, applied in the construction of such transducer, used: 1) natural crystals, such, as a quartz or rochele salt; 2) synthetic crystals, for example sulfate of lithium; 3) the polarized ferromagnetic ceramics, for example титанат of barium.
Piezoelectric sensors based on the use piezoelectric effect. The essence of it is that on the faces of certain crystals when they are compressed or stretched appear charges. As materials for sensors use quartz, barium titanate. There are direct and inverse piezoelectric effect.
The direct piezoelectric effect consists in the fact that under the influence of mechanical stresses on the faces of the crystals appear electric charges. When removing efforts the crystal returns in not electrified condition.
The reverse piezoelectric effect is that when you make a piezoelectric crystal in an electric field there is a change in the geometric dimensions of the crystal (compression or tension).
Direct piezoelectric effect is used for the measurement of fast dynamic processes. Great use got the gauges, vibrators, flow meters, acceleration etc.
Inverse piezoelectric effect has found application in ultrasonic generators. Quantitatively, the piezoelectric effect is estimated by the relationship:
q = k ⋅ F
where k is the piezoelectric constant (piezoelectric module); F is the applied force.
As a material for sensors most commonly used quartz because it has high piezoelectric and insulating properties, low temperature dependence and high resistance.