Some specific properties of piezoceramic pickups

Abstract

An equivalent circuit of a pickup with a piezoceramic transducer, which takes its electric properties into account more fully, is proposed. Experimental d a~ are presented.Pickups of mechanical quantities based on polarized ferroelectrie ceramics form the main part of the assembly of modern piezoelectric means of measurement. Their fundamental properties are described fairly completely in the elassical theory of piezoelectric transducers [1], which is used by practically all workers in this field. However, with the increasing requirements imposed on measurement accuracy, it has become necessary to supplement it in each special case. We are talking here of the effect of conversion nonlinearity, since this is unimportant in the majority of the pickups that are used.A ferroelectrie ceramic is a material in which the electrical polarization processes are more complex than in an ordinary dielectric. This is characterized, in particular, by the fact that its permittivity e is represented by the sum of two terms [2]where e** is the permittivity due to fast polarization processes, g and 0 are parameters of the material, where 0 has the dimensions of time, and a, is the angular frequency. The resistive component of the currents through a ferroelectric material, situated in a field, is determined by these parameters, and we can therefore assume that the permittivity is a complex quantity. Its real part is described by Eq. (1), and its imaginary part is given by 4~o~o(2) Ect-1+r~202-According to these formula.~, the electric circuit of a ferroelectric ceramic element must have the form shown in the dashed frame in Fig. 1 (the resistance of the insulation is ignored --it is very high). Here 0 = re'. The circuit of a "classical" piezoelectric transducer under the same conditions is simply the capacitance C. The extent to which both circuits differ is mainly determined by the ratio C'IC. According to experimental data obtained on samples of TsTS ceramic in the audiofrequency range it has a value of the order of 0.01-0.03. The conductive component of the admittance is small (the loss tangent is of the order of 0.01) and can be ignored. The same applies to the relative reduction in the capacitance with frequency-over this range.This lack of constancy of the capacitance should affect the property of a piclo_,p with a ceramic piezoelectric transducer. In fact, according to the data'of companies well known for the care they take in developing apparatus (Bruhl and Kjar and Endevco), the charge conversion factor of such pickups over this frequency band, where neither the finite resistance of the load nor the resonance drop has an effect, falls with frequency by 2-3 % per decade. At the same time, the voltage conversion factor under these conditions remains constant.On the basis of existing experimental data, we can assume that for ferroelectric ceramic piezoelectric materials the piezoelectric moddus d depends on the frequency in the same way as the permittivity. This is quite logical since in both cases it is polarization of the mat...