Jiri Erhart

Measurement of piezoelectric transformer vibrations by digital holography

A method for the measurements of the out-of-plane displacement on the surface of vibrating object is presented herein. This method is based on frequency-shifted time-averaged digital holographic interferometry, employing the principle of phase shifting. This approach allows for significant noise reduction, which results in high sensitivity of measurements. This method makes it possible to measure vibrations with amplitudes in the nanometer range over the whole measured surface. This method was applied to the visualization of the out-of-plane vibration modes of piezoelectric transformers. The amplitude and modal shapes were measured with a very high resolution. Furthermore, aspects influencing the measurement errors are discussed and the measurement results by holographic method were compared with the well-established single-point laser interferometry measurement method.

Analytical Modeling of Piezoelectric Transformers

Piezoelectric transformers (PTs) are used for transformation of alternating electric voltage. The best transformation ratio is achieved in the mechanical vibration resonance. A dual ring-dot disk PT design is presented in this paper. Simpler form of such ring-dot type design of PT was first proposed by Berlincourt. However, only experimental data were published previously. The analytical continuum model is presented in our work. The effects of dimensions, material properties and electrical load on the voltage gain as well as the efficiency are calculated. Experiment was performed on samples made of hard lead zirconate titanate ceramics (PZT, APC841 type). Studied PTs typically attain transformation ratio equal 20 without load and efficiency above 90% for matching impedance (the transformation ratio is degraded to around 1.5). The model is in good agreement with experimental data to the extent limited by approximations employed.

Interferometric Measurement of the Temperature Dependence of Piezoelectric Coefficients for PZN-8%PT Single Crystals

Piezoelectrically induced displacements were measured by a laser interferometric method in the temperature range 150 K–333 K for two domain-engineered 0.92Pb(Zn1/3Nb2/3)-0.08PbTiO3 single crystals poled in the [001]-direction. Assuming an effective tetragonal symmetry, we determined the piezoelectric d31 coefficient by measuring the displacement on (100) and (110) crystal faces. Coefficient d31 = −1280 pC/N was calculated for the [100]-direction at room temperature. Its value is reduced to −550 pC/N at 210 K. Piezoelectric coefficient d31 measured on a second sample in the [110]-direction exhibited a smaller value than the previous one, only about –555 pC/N at room temperature and –360 pC/N at 190 K. Different domain structures in both samples might be the possible reason for this difference. Temperature dependence of piezoelectric coefficient d33 was also investigated. The effect of aging is also discussed. The domain configuration in crystals was observed by optical microscopy.

Bar piezoelectric ceramic transformers

Bar-shaped piezoelectric ceramic transformers (PTs) working in the longitudinal vibration mode (k31 mode) were studied. Two types of the transformer were designed¿ one with the electrode divided into two segments of different length, and one with the electrodes divided into three symmetrical segments. Parameters of studied transformers such as efficiency, transformation ratio, and input and output impedances were measured. An analytical model was developed for PT parameter calculation for both two- and three-segment PTs. Neither type of bar PT exhibited very high efficiency (maximum 72% for three-segment PT design) at a relatively high transformation ratio (it is 4 for two-segment PT and 2 for three-segment PT at the fundamental resonance mode). The optimum resistive loads were 20 and 10 kΩ for two- and three-segment PT designs for the fundamental resonance, respectively, and about one order of magnitude smaller for the higher overtone (i.e., 2 kΩ and 500 Ω, respectively). The no-load transformation ratio was less than 27 (maximum for two-segment electrode PT design). The optimum input electrode aspect ratios (0.48 for three-segment PT and 0.63 for two-segment PT) were calculated numerically under no-load conditions.

Light-intensity-induced characterization of elastic constants and d33 piezoelectric coefficient of PLZT single fiber based transducers.

Enhanced functionality of electro-optic devices by implementing piezoelectric micro fibers into their construction is proposed. Lanthanum-modified lead zirconate titanate (PLZT) ceramics are known to exhibit high light transparency, desirable electro-optic properties and fast response. In this study PLZT fibers with a diameter of around 300 microns were produced by a thermoplastic processing method and their light-induced impedance and piezoelectric coefficient were investigated at relatively low light intensity (below 50 mW/cm2). The authors experimentally proved higher performance of light controlled microfiber transducers in comparison to their bulk form. The advantage of the high surface area to volume ratio is shown to be an excellent technique to design high quality light sensors by using fibrous materials. The UV absorption induced change in elastic constants of 3% and 4% for the piezoelectric coefficient d33.

Theoretical Calculation of the Temperature Dependence for the Material Coefficients of the Domain-Engineered Ferroelectric Crystals

Domain-engineered ferroelectric samples exhibit multi-domain structures. Temperature dependence of the material coefficients is calculated from the Landau-Ginzburg-Devonshire theory. Single-domain material data are averaged using matrix method for twinned systems with neutral (110) and charged (110) domain wall structure in PbTiO 3 crystal. Temperature dependence for most of the material tensor components has been found less pronounced in twinned structures than for single crystal. Bigger differences between both twinned systems have been calculated in the direction perpendicular to the (110) domain wall.

Modeling of a ring rosen-type piezoelectric transformer by Hamilton's principle

This paper deals with the analytical modeling of a ring Rosen-type piezoelectric transformer. The developed model is based on a Hamiltonian approach, enabling to obtain main parameters and performance evaluation for the first radial vibratory modes. Methodology is detailed, and final results, both the input admittance and the electric potential distribution on the surface of the secondary part, are compared with numerical and experimental ones for discussion and validation.

Disc Piezoelectric Ceramic Transformers

In this contribution, we present our study on the discshaped and homogeneously poled piezoelectric ceramics transformers working in various planar-extensional vibration modes. Transformers are designed with electrodes divided into wedge, axisymmetrical ring-dot, moonie, Smile or Jin-Jiang segments. Electrodes are designed on one side of the transformer; opposite side is fully electroded. A set of transformers with different electrode pattern has been prepared from hard PZT ceramics (type APC841, diameter 20mm, thickness 0.8mm). Transformation ratio, efficiency, input and output impedance were measured for low-power signal. Transformer efficiency and transformation ratio were measured as a function of frequency and impedance load in the secondary circuit. Parameters were further measured for the fundamental and second resonance. Optimum impedance for the maximum efficiency has been found. Maximum efficiency and no-load transformation ratio can reach almost 100% and 52 for the fundamental resonance of ring-dot transformers and 98% and 67 for the second resonance of wedge 2 segment transformers. Maximum impedance was reached at optimum impedance, which is in the range from 500 Ω to 10k Ω depending on the electrode pattern and size. Fundamental vibration mode and its overtones were further studied using frequency modulated digital holographic interferometry. Normal displacement distribution on the transformer surface was visualized. The shape of vibration mode was influenced by the mechanical clamping of the transformer during its operation. Operating shapes were also simulated by the finite element method. Complementary information has been obtained by the infrared camera visualization of surface temperature profile at higher driving power.

A digital holographic method for the measurement of piezoelectric transformer vibrations

Method for the measurements of the out-of-plane displacement on the surface of vibrating object is presented. The method is based on the frequency-shifted time average digital holographic interferometry employing the principle of phase shifting. This approach allows for a significant noise reduction, which results in the high sensitivity of measurements. The method makes it possible to measure the vibrations with amplitudes in the nanometer range over the whole measured surface. The method was applied to the visualization of the out-of-plane vibration modes of piezoelectric transformers. The amplitude and modal shapes were measured with a very high resolution. Results of measurement were used to judge the influence of mounting on the electrical properties of piezoelectric transformers in operation.

PLZT-Based Light Controlled Piezoelectric Transformer

Novel functions of devices are sometimes realized by superimposing two different effects. The authors discovered light controlled Piezoelectric Transformers (PT), with light feedback on efficiency, taking advantage from sophistically coupled effects of piezoelectricity and photostriction. The PT is coupled with direct and converse piezoelectric effect, while the photostriction is associated with bulk photovoltaic effect. These “very smart” multifunctional transformers will be utilized as a promissing devices relevant to consumers market needs of adopting LCD screens backlight illumination to the surrounding light intensity. Application related measurements of this material, incorporated into the Piezoelectric Transformer structure revealed, that the efficiency and gain characteristics of PT device are addition result of starting material parameters and light intensity.