Micro Displacement Measurement System Based On Capacitance Method

With the rapid development of science and technology,micro-displacement measurement technology has been widely used in many fields such as scientific research,biomedical,semiconductor manufacturing,aerospace and high-end equipment manufacturing,etc.The rapid development of electronics and integrated technology has created the advantages of capacitive micro-displacement measurement sensors with high measurement accuracy,great stability,simple structure,and non-contact measurement prominent,and their applications are in widely increase.Generally,the displacement range of capacitance sensing is on the order of micro / nanometer.Noise interference from space environment and other serious constraints restrict the further improvement of measurement accuracy,so high requirements are placed on the anti-interference ability of the measurement system.First of all,this paper analyzes the domestic and foreign research status of capacitive micro-displacement measurement,and designs the overall composition scheme of the micro-displacement measurement system based on the capacitive method.The mathematical model of the capacitive micro-displacement measurement system is established,and the influencing factors on the measurement accuracy of the system are analyzed from various aspects such as sensors,measurement conversion circuits and signal processing algorithms.Secondly,according to the analyzed influencing factors,the design scheme of each functional module of the detection circuit is further determined.In order to reduce the edge effect and noise interference of the capacitance sensor,an equipotential ring type unipolar plate capacitance probe is designed;In view of the small capacitance of the probe and the weak change with displacement,the capacitance measurement and conversion circuit is designed,which mainly includes a small capacitance Extraction circuits,differential amplifier circuits,and offset signal cancellation and secondary amplifier circuits.Thirdly,what expounded is the signal processing algorithm and software design of the micro-displacement measurement system.It mainly includes the use of a minimum system to generate high-frequency sinusoidal excitation signals,signal sampling and data transmission,data fitting,and signal recovery algorithms.The sampling of the noninteger period sampling and the non-integer period sine fitting are used to fit the known excitation frequency signal to effectively remove noise interference and improve the fitting accuracy.The algorithm uses linear fitting calculation method,and the calculation speed is faster.The upper computer realizes the function of controlling the lower computer STM32 through serial communication,including the changes of the excitation signal frequency,the changes of the sampling frequency,the real-time transmission and display of the sampling signals,the measurement signal processing and data storage,etc.Meanwhile,the micro-displacement measurement based on the capacitance method was completed on system automation and visual measurement experiment.Finally,the calibration experiment,repeatability experiment and stability experiment of the micro displacement measurement system are completed by using PZT calibration system.The least square fitting method is used to fit the experimental data,and the relationship between the amplitude of voltage signal and the displacement is obtained,and the basic vibration test experiment is carried out.The experimental results show that: the equivalent ring monopole capacitance probe can effectively avoid the influence of edge effect,the designed displacement measurement system has good stability and high repetition accuracy,within the range of 20 um,and the highest measurement resolution can reach 20 nm.