测试与检测技术基础----24.ppt

Fundamentals of Measurement Technology(10)
Prof. Wang Boxiong
A piezoelectric transducer is an active transducer or self-generating transducer. It utilizes the piezoelectric effect of some materials, which generate electrical changes on some of their surfaces when acted by external forces. So piezoelectric transducers are often used to measure pressure, stress, and acceleration, etc., and have wide applications in engineering.
Piezoelectric transducers
Certain materials can generate an electrical charge when subjected to mechanical strain or, conversely, can change dimensions when subjected to voltage. Pierre and Jacques Curie are credited with its discovery in 1880.
The materials that exhibit a significant and useful piezoelectric effect fall into three main groups:
natural (quartz, Rochelle salt) and synthetic (lithium sulphate, ammonium dihydrogen phosphate) crystals
polarized ferroelectric ceramics (barium titanate, etc.)
certain polymer films.
Piezoelectric effect
Piezoelectric effect
Fig. Axis numbering system for piezoelectric constants
Piezoelectric effect
In Fig.
subscript 1 corresponds to x-axis
subscript 2 to y-axis
subscript 3 to z-axis.
It is evident that the piezoelectric constants are different when the directions of force application and the generated deformations are different.
Piezoelectric effect
Quartz crystal is monly used material. A quartz crystal has a shape of hexahedral structure with three mutually perpendicular axes as its crystal axes
The longitudinal axis Z-Z is known as optical axis
The axis X-X going through the edge of the hexahedron and being perpendicular to the optical axis is called electrical axis
The axis Y-Y perpendicular to both the axis X-X and the axis Z-Z is called mechanical axis
Piezoelectric effect
The piezoelectric effect generated by the force applied in the electrical axis X-X is called longitudinal piezoelectric effect
The piezoelectric effect gene