Study On Influencing Factors Of Accuracy For Eddy Current Displacement Measurement

Precision machining of the fuel tank grid plays an important role on the safety and economy of rocket launch. The mirror milling method developed in recent years has become the effective way to solve this problem by virtue of its high efficiency, versatility and high precision. In order to improve the precision of milling, the central axis of both processing terminal and support end should be along the normal direction of thin-walled parts. The normal direction would be measured by arrayed eddy current sensors through the real-time displacement measurement. The precision of normal direction measurement is a prerequisite to ensure the accuracy and stability of mirror milling. Moreover, the accuracy of eddy current displacement measurement is the basis for the normal direction measurement. Thus, in order to ensure the accuracy and realize the compensation for measuring error, the influence of tilt angle and coupling interference on the eddy current displacement measurement accuracy will be studied in this paper.The basic working principle of eddy current testing is introduced and the coil equivalent impedance is analyzed by means of the transformer equivalent circuit. Through the electromagnetic field simulation software ANSYS Maxwell, the modeling and simulation of eddy current sensor is completed. Moreover, the effects of parameters such as coil dimensions, excitation frequency and measured material on the performance of sensor are also explored. It is discovered that the sensor will have a higher measurement sensitivity if the height of coil is small or the coil inner radius is large; the eddy current density will increase with excitation frequency while the eddy current depth decreases with the frequency; the variation law of coil equivalent inductance is opposite for the ferromagnetic and non-ferromagnetic material.The theoretical analysis models for tilt angle and coupling interference are established. Based on the tilt angle model, the distribution and density of induced eddy current under different tilt angles are analyzed. Through the analysis of relationship between lift-off, tilt angle and coil equivalent impedance, it is found that the impact of lift-off and tilt angle on the coil equivalent impedance is opposite. In other words, the tilt angel leads to the smaller measured value and the error increases with tilt angle according to the Gaussian function. And, the distribution and density of eddy current under different spacing are also analyzed based on the coupling model. Through the analysis of the relationship between coil mutual inductance, sensor spacing and lift-off, it can be discovered that both the sensor spacing and lift-off have an impact on the strength of coupling interference. And then, the variation laws of coil mutual inductance under different excitation frequencies have also been investigated.The experimental research on eddy current displacement measurement is carried out and the measuring error is compensated. The platforms for tilt angle and coupling interference are set up. By comparison between different groups of tilt angle experiments, it is verified that the sensor's measured value is smaller due to the effect of tilt angle and the average measuring error increases with the tilt angle according to the Gaussian function. Furthermore, by the comparison between different groups of coupling experiments, it is also testified that the percentage of reduction of the measuring range will decrease with the sensor spacing according to the exponential function. Besides, the piece-wise effect caused by lift-off on the coupling interference is also discovered.