Study On The Measurement Method Of Remote Field Eddy Current Of Thickness

In the process of continuous casting production,on-line testing of billet solidification thickness is a technical difficulty in the field of continuous casting nondestructive testing.The commonly used detection methods are ultrasonic method,ray method,eddy current method and so on.The remote field eddy current non-destructive testing(NDT)can provide an effective method for solving this difficulty because of its wide range of detectable thickness.The principle of far-field eddy current testing is that when the excitation line has low frequency alternating current,the energy generated by the excitation coil is divided into direct coupling component and indirect coupling component,and the direct coupling component propagates forward along the axis.The indirect coupling component propagates forward through the plate.Direct coupling in the near-field region due to shielding The component decay is very fast and the indirect coupling component is very slow,which leads to the indirect coupling component of the magnetic field in the far-field region is stronger than the direct coupling component,which causes the secondary penetration phenomenon,thus forming the remote field eddy current effect.The induced voltage can be measured by the detection coil in the far field,and the thickness of the measured part can be obtained by studying the change of the induced voltage.Compared with the high-frequency eddy current,the far-field eddy current testing technology has a significant advantage over the high-frequency eddy current,but it is also affected by the lift-off effect.Therefore,according to the requirements of thickness detection of continuous casting billet shell,the following research work has been done in this paper:The solid phase and the liquid phase region of the casting blank in the continuous casting process are analyzed,and the cold simulation experiment measurement system is set up according to the different electromagnetic characteristics of different regions.The hardware platform of far field eddy current measurement mainly includes the following parts: far field eddy current sensor,signal conditioning circuit,test piece,data acquisition card,computer,etc.In a computer,a program is written using LabVIEW software.According to the requirement of thickness measurement of continuous casting billet shell,theremote field eddy current testing technology is studied,and the remote field eddy current sensor with shielded structure is designed,which produces the remote field effect in the billet structure.According to the different electromagnetic characteristics of metals in different temperature regions of continuous casting billet,the simulation model of the tested object is established by using Ansoft simulation software.Based on the simulation analysis of voltage amplitude,excitation frequency and sensor coil diameter,coil turn number,shielding structure,magnetic core structure and lift-away distance of remote field eddy current sine excitation signal,the image of detection results is studied.In response,the optimization parameters of the field eddy current sensor are given.By changing the thickness of the steel plate,a detection signal corresponding to different thicknesses is obtained,and the characteristic value of the signal waveform is combined and evaluated.The induction voltage signal of the remote field eddy current is analyzed,then the response of the signal characteristic value is compared to evaluate the effect of the signal in the thickness detection of the continuous casting slab,and the peak value and phase are used to evaluate the response of the detection signal.In addition,the thickness inversion formula can be obtained by analyzing the thickness of the measured metal in combination with the specific conditions.