The Measurement Of The Electromechanical Coupling Coefficient Of The Giant Magnetostrictive Material

As a new type of functional materials,the giant magnetostrictive material plays a positive role in promoting the development of related industries. As an important characteristic parameter, if the electromechanical coupling coefficient of the giant magnetostrictive material can be measured accurately, it will be useful for the designers to making an in-depth study of the material and to designing a variety of devices with it more effectively. As the giant magnetostrictive material is usually made the shape of rod to use its axial magnetostrictive effect, so in this thesis, the main work is to show how to measure the axial electromechanical coupling coefficient k33.This thesis describes the development of the electromechanical coupling coeffi-cient measuring instrument in three parts.The first part is about the measuring principle of the electromechanical coupling coefficient. According to the physical definition, there are three ways to measure the electromechanical coupling coefficient: the three-parameter method, the resonance method and the admittance diagram method. In this thesis, I choose the resonance method. In order to use the calculating formula of the resonance method, we need to know two parameters: the resonance frequency and anti-resonance frequency. They can be determined by measuring the frequency-impedance curve of the enwinding-sample coil. In the instrument, according to the measuring circuit design, the current through the coil is a constant value. So the resonance frequency and anti-resonance frequency are determined by measuring the frequency-voltage curve. Moreover, in different bias magnetic field, the material's resonance frequency and anti-resonance frequency are different, so we should also measure the bias magnetic intensity. The second part is about the hardware structure of the electromechanical coupling coefficient measuring instrument. In the instrument, the single-chip microcomputer AT89C52 is the controlling core and circuit design mainly includes the following sections: the magnetic intensity measuring circuit, the frequency-voltage curve measuring circuit, the data acquisition system, the optoelectronic isolator, the data storage, the serial communication interface with the computer, the keyboard circuit, the display unit and etc.The bias magnetic intensity measuring circuit is mainly composed of the Hall probe, the constant current source, the inequipotential offset circuit and the signal amplification circuit. In the instrument, the constant current source which provides the constant current for the Hall probe is converted from the constant voltage source by the operational amplifier. Because the existence of the inequipotential voltage in the Hall probe, the bridge compensation circuit is used to realize the amplitude compensation of the Hall probe by adjusting the variable resistor, which make the output voltage of the Hall probe to be zero when it isn't in the magnetic field. Last, there are some gain-programmable amplification circuits to achieve the conversion of the different measurement range.The frequency-voltage curve measuring circuit is mainly composed of the oscillation circuit and the phase-locking amplifier. The programmable waveform generator AD9833 is chosen as the oscillation source, which provide incentives signal for the sample and provide reference signal for the phase-locking amplifier. The phase-locking amplifier section includes: the zero-drift amplifier AD8556 used to pick-up the coil voltage, the analog switch 4052 used to design the phase-sensitive detection circuit, and the integrated operational amplifier OP07 uesd to design low-pass filter circuit.The analog-to-digital conversion chip MAX110, as the core of the data acquisition system, converts the input analog signal to digital signal that can be read by the single chip AT89C52. In order to improve the instrument's SNR by suppressing the sharp pulse of digital circuit and other noise interfering in the analog circuit, TPL521 is selected in the low-frequency control lines and 6N137 is selected in the high-frequency control lines to isolate the analog circuit from the digital circuit related with AT89C52. In order to achieve the automation of measurement, the instrument uses X24C45 as the data storage. Controlled by the program, the setting-state parameters can be automatically stored when the instrument is shutting down and loaded after start-up, that will facilitate the user to choose whether or not to use the same settings as the previous to measure the sample. In order to achieve the serial communication between the host machine and the computer, the low power chip MAX232 is used to complete the voltage conversion between TTL and RS232 in the instrument. The keyboard and display compose the auxiliary part, which can complete some basic functions when the instrument is not connected with the computer.The third part is about the software design of the instrument. It mainly includes the host machine's monitoring program using assembly language and the computer application software using VB language.The host machine's monitoring program mainly achieves the automatic control. It includes the initialization of all programmable chips in the circuit, A/D conversion, reading and writing X24C45, the keyboard interrupt, the serial port interrupt, the data transmission, display, correlative algorithm and etc.The computer application software mainly makes use of the controls provided by Measurement Studio, and achieves the integration of the automatic control and data acquisition and analysis. Software functions mainly include setting the host machine operating parameters, data acquisition, data processing, serial communication, plotting, accessing the test results document and etc.