Fundamentals of Measurement Technology (10) Prof Wang Boxiong
Fundamentals of Measurement Technology (10) Prof. Wang Boxiong
4. 8 Piezoelectric transducers 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
• 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. 4.8 Piezoelectric transducers
4.8.1 Piezoelectric effect u 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 U 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
❑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. 4.8.1 Piezoelectric effect
4.8.1 Piezoelectric effect We use polarization intensity vector to express the piezoelectric effect of a material P=P+p+p (4.78) Where x, y and z form a coordinate system relating to crystal axes Fig. 4.55 Axis numbering system for piezoelectric constants
We use polarization intensity vector to express the piezoelectric effect of a material: P = Pxx + Pyy + Pz z (4.78) Where x , y and z form a coordinate system relating to crystal axes 4.8.1 Piezoelectric effect Fig. 4.55 Axis numbering system for piezoelectric constants
4.8.I Piezoelectric effect Writing the polarization intensity P in the form of axial stress and shear stress t gIves P=d10m+d120m+d13-+d14t+dl15t+d 16 d20x+d2y+d2+d24n+d2tx+d262y(4.79) P2=d310x+d320m+d3=+d341+d 35 7+d 36 Where dm. n is the piezoelectric constant with its subscript m as the axis direction perpendicular to the surface on which electrical charges are generated and as the axis direction in which stress is applied
Writing the polarization intensity P in the form of axial stress and shear stress gives = + + + + + = + + + + + = + + + + + z z xx yy z z yz z x xy yy xx yy z z yz z x xy xx xx yy z z yz z x xy P d d d d d d P d d d d d d P d d d d d d 3 1 3 2 3 3 3 4 3 5 3 6 2 1 2 2 2 3 2 4 2 5 2 6 1 1 1 2 1 3 1 4 1 5 1 6 (4.79) Where m n d , is the piezoelectric constant with its subscript m as the axis direction perpendicular to the surface on which electrical charges are generated, and n as the axis direction in which stress is applied. 4.8.1 Piezoelectric effect
4.8. 1 Piezoelectric effect In Fig. 4.55 subscript i corresponds to x-axis subscript 2 to y-axis subscript 3 to z-axis uIt is evident that the piezoelectric constants are different when the directions of force application and the generated deformations are different
❑ In Fig. 4.55 • 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. 4.8.1 Piezoelectric effect
4.8.1 Piezoelectric effect Quartz crystal is a commonly 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
• Quartz crystal is a commonly 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 XX and the axis Z-Z is called mechanical axis 4.8.1 Piezoelectric effect
4.8. 1 Piezoelectric effect applied in the electrical axis x-x is called ce The piezoelectric effect generated by the longitudinal piezoelectric effect The piezoelectric effect generated by the force applied in the mechanical axis Y-Y is called transverse piezoelectric effect Ua force applied in the optical axis z-z doesnt generate any piezoelectric effect
➢ The piezoelectric effect generated by the force applied in the electrical axis X-X is called longitudinal piezoelectric effect ➢ The piezoelectric effect generated by the force applied in the mechanical axis Y-Y is called transverse piezoelectric effect ❑A force applied in the optical axis Z-Z doesn’t generate any piezoelectric effect. 4.8.1 Piezoelectric effect
4.8. 1 Piezoelectric effect Z y (a) Fig 4.56 Quartz crystal (a)quartz crystal shape(b) the coordinate system for crystal axes(c)a cut of the crystal
4.8.1 Piezoelectric effect Fig. 4.56 Quartz crystal (a) quartz crystal shape (b) the coordinate system for crystal axes (c) a cut of the crystal
4.8.1 Piezoelectric effect There are three major piezoelectric effects, namely the longitudinal the transverse and the shear effect Q ++ +++ L== Longitudinal Traverse ear effect effect Fig. 4.57 Acting directions of piezoelectric effects
• There are three major piezoelectric effects, namely the longitudinal, the transverse and the shear effect. 4.8.1 Piezoelectric effect Fig. 4.57 Acting directions of piezoelectric effects
