PEIZOELECTRIC SENSING AND ACTUATION || MODULE 2 || KTU MEMS || MEMS KTU || LECTURE 15

Hi all today, we are discussing piezoelectric sensors and actuators So what are piezoelectric materials? How it can be used for both sensing and actuation purpose? Piezoelectric materials are used for both sensing and actuation application So piezoelectric materials are certain materials that can generate electric charge or voltage when it is under mechanical stress. This is called direct effect of piezoelectricity and that same material would be able to produce mechanical deformation or force when an electric field is applied to it. It is called inverse effect of piezoelectricity Piezoelectric crystals have a mass of minute crystallities Direction of polarization of neighboring crystal domains is 90° and 180° No overall polarization or piezoelectric effect is exhibited in the material due to random distribution of domains Polarization is the process of exposing a crystal to a strong electric field at a higher temperature Domains are aligned according to the field and lengthened according to the material and lengthened according to the field When the field is removed, the dipoles are locked in the approximate alignment and the crystal undergoes a permanent polarization and deformation To depolarize a piezoelectric element, it is exposed to a strong electric field in the opposite polarity Its field strength for depolarization depends on the material grade, the time the material is subjected to depolarization field, and temperature The field strength of depolarization depends on the material grade and the depolarization field In the case of static field, the threshold is between 200 and 500 V per mm In the case of alternating field, the threshold is between 200 and 500 V per mm Mechanical depolarization occurs when mechanical stress on piezoelectric element becomes high enough to disturb the orientation of domains and hence destroy the alignment of the dipoles Mechanical depolarization occurs when mechanical stress on piezoelectric element becomes high enough to disturb the orientation of domains Next is Curie point or Curie temperature The critical temperature at which if a piezoelectric element is heated, the crystal vibration may be strong that domains become disordered and the element becomes completely depolarized The operating temperature is between 0 degree and Curie point Next is mathematical description of piezoelectric effect Piezoelectric effects are strongly orientation dependent The direction of positive polarization is Z axis Normal stress component XY and Z are represented as 1,2,3 And its shear stress component is represented as 4,5,6 respectively You can see in this figure 1,2,3 is normal stress component 4,5,6 is shear stress component In piezoelectric crystal, the relation between electrical polarization and applied mechanical stress is d is equal to dT plus epsilon E d is piezoelectric coefficient matrix Epsilon is electrical permittivity E is electric field D is electrical polarization Electrical polarization is contributed by two parts Electrical biasing and mechanical loading Electrical field is absent E is equal to 0 Electrical polarization d is equal to dT The equation d is equal to dT plus epsilon E We can write it in full matrix form T1 through T3 is normal stress Axis 1,2,3 is normal stress T4 through T6 is shear stress We have already explained it using the figure Unit of piezoelectric constant is the unit of electrical displacement divided by unit of stress After substituting the values, we will get unit of piezoelectric constant is Coulomb per Newton Units of various parameters are given in the figure After substituting the values, we will get unit of piezoelectric constant is Coulomb per Newton We can describe the inverse piezoelectric effect in the same way The total strain is related to both applied electric field and any mechanical stress according to the equation S is equal to S T plus dE In the case of piezoelectric material, there is an electric field and electrical signals corresponding to mechanical deformation The equation S is equal to S T plus dE S is the compelence matrix If there were no mechanical stress present, the equation S is equal to D into E Dij component is related to strain and applied field Electromechanical coupling coefficient is K K is the measure of how much energy is transferred from electrical to mechanical or vice versa