Study On A New Probe Sensor For Measuring The Eddy Current Magnetic Field Directly

The eddy current testing technology is based on Faraday principle of electromagnetic induction,to judge the distribution of defects through the change of eddy current field in the conductor produced by a probe sensor.The performance of the probe sensor determines the accuracy of eddy current testing.The traditional impedance analysis method is used to judge whether there is a defect in the conductor by the impedance variation of the probe coil,but it cannot describe the size,shape and position of the defect directly,and the excitation signal has a great influence on the impedance of the coil.Starting from the principle of eddy current testing,this paper designed a new probe sensor for measuring the eddy magnetic field directly,eliminated the influence of excitation signal,and the distribution of eddy current field is directly determined by the change of induced electromotive force in the coil.Finite element simulation and experiment are used to study the influence of the shape,size and distribution of defects on the induction electromotive force of the coil,verified the feasibility of the probe sensor for measuring the eddy current magnetic field directly,and provided theoretical guidance for the development of new eddy current testing instrument.The main research contents and results are as follows:(1)Based on the related literature and current research at home and abroad,the finite element method is used as the numerical analysis method of eddy current testing,and two-dimensional axisymmetric model has been used in the finite element simulation study of the eddy current testing.But as the defects in the conductor is not distributed regularly,and the probe detection moving above the conductor,the very idealized two-dimensional axisymmetric model cannot accurately describe the influence of defects on the distribution of eddy current field in the conductor.Therefore,this paper uses a three dimensional finite element model to refine mesh generation and eliminate the interference of irregular defects,boundary conditions,etc.(2)The mathematical model,boundary condition of the engineering electromagnetic field and the skin effect of eddy current are introduced.According to the working principle of eddy current testing,a new probe is designed to directly measure the eddy current magnetic field,which contains three coils,including two excitation coils reverse winded by the same enameled wire and a detection coil with more turns.Eliminated the influence of excitation signal,the induction electromotive force in the detection coil is completely generated by the eddy current magnetic field in the conductor.(3)With the basic theory of finite element method and the tool of finite element software ANSYS Maxwell,the parametric method is used to determine the specific structure of the probe coils,eliminated the effect of excitation signal on induction electromotive force of the detection coil.The modeling of ANSYS Maxwell software includes the establishment of geometric model,the management of material properties,the boundary conditions and the excitation source,the subdivision of mesh,calculation and post-processing.(4)The change of eddy current field in the conductor is not only affected by the distribution of defects,but also the frequency of excitation signal,the lift-off height of probe sensor,the material and thickness of tested conductor.By using the finite element method to simulate the effect of parameters on the strength of eddy current field within the conductor,finding there's no eddy current in the center position below the probe coil.And if the probe coil center is at the origin,as the eddy current field radiated around from the radial center,it achieved maximum value at the radius distance,until reduced to zero at infinity.(5)In order to simulate the effect of defects on the induction electromotive force of the detection coil and the difference between the induced electromotive force when there is a defect and no defect,appropriate system parameters should be selected,including the excitation signal of 1 kHz,the probe lift-off of 1.2 mm,the aluminum plate of the tested conductor selection.The simulation results show that no matter the defects are in the conductor center or the edge,the change rules of detection coil's induction potential difference remain consistent,and the rules are similar to the distribution of eddy current field in the conductor.The maximum value is reached when the probe radial moves to the outside radius of the coils,but the difference of the induced electromotive force is reduced to zero faster than the eddy current magnetic field.(6)Experimental platform is built to analyze the influence of different shape,size and distribution of defects on the induction electromotive force of the coils.The platform consists of four parts,including the probe sensor,tested conductor,signal excitation module and signal detection module.Experiment shows that the experimental results are in agreement with the simulation results when there are defects in the conductor,no matter where the defects are,the difference of induced electromotive force achieves maximum when the probe moved to its diameter distance,and it is zero at the top and infinity of the defects.And when the diameter and depth of the defects increase,the difference of induced electromotive force becomes larger.