Research On Magnetic Induction Thickness Measurement Method Based On Dynamic Magnetic Field

The functional coating is widely used in engineering manufacturing,transportation,aerospace and other fields,so that the product has specific functions such as anticorrosion,rust prevention,wear resistance and so on.Thickness is one of the key indicators affecting the coating quality,and the demand for high-precision measurement of this indicator widely exists in the production and maintenance process of products.As one of the main methods of coating thickness measurement,magnetic induction thickness measurement is an important means to detect the thickness of non ferromagnetic coating on ferromagnetic substrate.At present,the mainstream magnetic induction thickness measurement method is mainly based on static magnetic field,and does not consider the influence of different magnetization processes of the matrix material on the measurement error during the measurement process,resulting in unstable measurement results and poor repeatability.To solve the above problems,a dynamic magnetic induction thickness measurement method based on low frequency magnetic field is proposed in this paper.In this method,the low-frequency dynamic magnetic field is used as the excitation signal to make the matrix material undergo a complete closed-loop process of magnetization and demagnetization in the measurement process.The saturation field characteristic quantity in multiple change periods is selected to eliminate the influence of different magnetization processes on the measurement results,and the accuracy and repeatability of the thickness measurement method based on magnetic induction are improved.The main work of this paper is as follows:(1)The principle and mathematical derivation process of three mainstream magnetic induction thickness measurement methods are analyzed,and the characteristics and errors of each method are analyzed.The results show that the current mainstream magnetic induction thickness measurement methods are based on static magnetic field,ignoring the influence of magnetization process.By analyzing and deriving the influence of hysteresis loop on the measurement results,this paper proposes a magnetic induction thickness measurement method based on low-frequency dynamic magnetic field.Through the theoretical analysis of the relationship between the three characteristic quantities in the hysteresis loop and the spatial magnetic field signal under the dynamic magnetic field,the mathematical mapping model between the characteristic quantities and the coating thickness is established,which establishes the theoretical basis for the method proposed in this paper.(2)Taking cobalt steel,low carbon steel,nickel steel and other materials with different permeability as the matrix,the magnetization process under the condition of dynamic magnetic field is simulated.The correctness of this method is verified by the nonlinear fitting and error analysis of the simulation data.At the same time,the excitation amplitude,frequency and other key parameters are simulated and analyzed,and the final characteristic quantity is selected,which provides the guidance of specific parameters for the construction of experimental platform and experimental verification.(3)Based on the nmlevel micro-displacement platform and high-precision magnetic sensor,an experimental verification platform is built.Thickness measurements based on static and dynamic magnetic fields were performed on four different material substrates(DT4,DT4 A,DT4C,DT4E).At 0 μm to 7000 μm range,the maximum measurement error based on static magnetic field detection method is-46.10 μm,measurement accuracy ±(1 μm+6.81 %H),maximum standard deviation is 18.23 μm,and maximum measurement error based on dynamic magnetic field is-31.27 μm,32.17% higher than static magnetic field detection method;The measurement accuracy is ±(1 μm+3.56 %H),47.72% higher than static magnetic field detection method;Maximum standard deviation is 12.80 μm,29.79% higher than static magnetic field detection method.This method has been verified to improve the measurement accuracy and repetition compared with the mainstream magnetic induction thickness measurement methods,and has reached the purpose and technical indicators of this study.