Research On Sensing Technology Of Eddy Current Displacement

Micro-displacement measurement plays an important role in semiconductor industry,precision machining,material science,automatic control and other fields.There are some sensors can measure micro-displacement,such as optical sensors,capacitive sensors,eddy current sensors and so on.Eddy current displacement sensors are widely used in aviation,aerospace,electric power,petroleum,chemical industry,metallurgy and other fields due to their advantages of large measuring range,high resolution,high sensitivity,fast response,simple structure,low cost and so on.Eddy current displacement sensors not only can achieve non-contact measurement,but also are not affected by oil,dust,humidity and other media.Therefore,they have more competitive advantages in measuring high-speed movement such as shafts,rotors and so on in harsh environment.Although the technology of eddy current displacement sensor has been developed for more than 40 years,there are still various problems to be overcome,such as the measurement accuracy of nanometer and even sub nanometer level.Therefore,it is vital to study the high-performance eddy current displacement sensors.In this thesis,a simple and effective physical model of the eddy current displacement sensor was established,and the working principle of eddy current displacement sensor was described.The COMSOL finite element method was used to analyze the relationship between the impedance variation of detector coil and displacement.According to the influence of the target board of different metal materials,different thickness,the signal source of different excitation frequency and the detector coil of different installation position on the detector coil impedance,the design parameters were determined after iterative optimization.The AC bridge circuit was used to demodulate and output the impedance variation of detector coil caused by the change of displacement.The millivolt level small signal of the coil impedance was decoupled by the phase sensitive detection principle.The voltage value of the impedance was output after filtering and amplifying,and the relationship between the voltage and the displacement was obtained.Multisim software was used to simulate the signal sources with different excitation frequencies of 1MHz,500 k Hz and 200 k Hz.The simulation results showed that the nonlinear errors were all less than ±2.5%,especially the nonlinear error of 500 k Hz excitation frequency was less than ±2.0%.Due to the higher excitation frequency of the signal source,the higher sensitivity of the eddy current displacement sensor was,but the greater its demand for the signal processing circuit was.By improving the traditional impedance separation method,the measurement sensitivity was improved,and the linearity was increased by about 1 times at different excitation frequencies.Especially the excitation frequency of 500 k Hz,the nonlinear error could be reduced from the original ±2.0% to ±0.4%.The influencing factors of temperature drift of eddy current displacement sensor were analyzed,which were drift of metal conductivity,drift of signal demodulation circuit,thermal expansion and contraction of detector coil and target plate,then the corresponding measures to suppress temperature drift were given.The simulation results showed that the eddy current displacement sensor designed in this thesis had a resolution of 10 nm and a response bandwidth of 10 k Hz(-3d B)in the 200?m range at room temperature.By improving the signal demodulation circuit,the measurement sensitivity was increased by nearly 50% and the nonlinear error was reduced to 20% at 500 k Hz excitation frequency.