Piezoelectric sensor is used for the measurement of pressure, acceleration and dynamic-forces such as oscillation, impact, or high speed compression or tension.
It contains piezoelectric ionic crystal materials such as Quartz.
On application of force or pressure these materials get stretched or compressed. During this process, the charge over the material changes and redistributes.
One face of the material becomes positively charged and the other negatively charged. The net charge q on the surface is proportional to the amount x by which the charges have been displaced. The displacement is proportion to force.
Therefore we can write,
q = kx = SF
where k is constant and S is a constant termed the charge sensitivity.