Piezoelectric Material Parameters Testing System Based On Digital Quadrature Transformation

Nowadays, piezoelectric materials have been widely applied in various fields, such piezoelectric transducers based on positive piezoelectric effect and piezoelectric drives based on inverse piezoelectric effect. These growing applications require accurate measurements of piezoelectric parameters including the resonant frequency, impedance, mechanical quality factor and electromechanical coupling coefficient, etc.Traditional piezoelectric parameters testing method is as the follow steps: At first, add the sample into the testing circuits, and adjust the frequency of sinusoidal signal in testing circuit which is the excitation signal, then measure the voltage of components in the circuit, and finally calculates the piezoelectric parameters of the samples. In recent years, piezoelectric testing technology has been improved from traditional methods such as transmission line method and IRE standards method to the new testing method such as vector resistance method. The improvements of testing accuracy require more rigorous testing environment. Recently, almost each kind of piezoelectric parameters testing system required low-distortion sinusoidal signal for the excitation signals, but because of the high-frequency low-distortion sinusoidal signal is very difficult to obtain, the accuracy and testing scope of many piezoelectric parameters testing was limited.To solve the above problems, we developed a new structure of piezoelectric parameters testing system, in which the digital quadrature sampling method was introduced. We have studied two aspects: piezoelectric parameters testing method and theory of digital quadrature. The core of this system is DSP chip of TMS320F2812. The signal source of the system was composed of the crystal oscillator and digital counter 82C54, and we designed automatic range selection circuit to improve the testing environment. Then we sampled the voltages of the reference resistors and the testing sample by high-precision AD chip of AD7654. And we used the method of digital quadrature to calculate the plural-impedance of the piezoelectric sample in different frequency, and finally we can calculate the other piezoelectric parameters.Testing results showed that this method could accurately measure the resonant frequency, mechanical quality factor and resonance frequency (testing errors within 1%), and other piezoelectric electrical parameters could also be tested relatively precise (testing errors within 8.5%). Additionally, our system could accurately measure the impedance parameters in a range of frequencies and the testing accuracy had been further improved higher than previous test systems.