Research On Detection And Imaging Technology Of Curve Surface Defects With Flexible Eddy Current Array Sensor

Flexible eddy current array(FECA)non-destructive testing technology has the advantages of conventional eddy current testing technology,as well as the advantages of array sensor single large scanning area,fast scanning speed,and the ability to adapt to surface defect detection,it has become a common research hotspot of sensor technology and non-destructive testing technology.In the field of nondestructive testing,the application of imaging technology can intuitively detect the location and size of defects,which not only improves the efficiency of inspectors,but also promotes the popularization of nondestructive testing technology.This paper studied the detection and imaging technology of FECA sensor,completed the detection and the imaging of natural defects on the surface of last stage blade of steam turbine by FECA sensor,In the research of surface defect detection technology of conductive body,traditional detection technology can only detect the presence or absence of defects,but it is difficult to visualize the shape,size and location of defects.The traditional eddy current array probe unit is large in size.Due to the limitation of the conventional winding process,the inductance of the detection coil is low and the sensitivity of the sensor is insufficient.In this paper,based on the principle of eddy current array detection,the parameters such as coil shape,outer diameter,number of turns and ferrite core are optimized by using finite element simulation.The mutual inductance effect between the probe elements is studied by using the simulated magnetic field density of the FECA probe coil and the turbine’s final stage blade at different frequencies.The characteristic curves of excitation frequency and inductance variation are derived and the range of reference excitation frequency is determined.Based on the simulation results,eight coil arrays of small size and high inductance detection probes are distributed on flexible substrates through FPCB technology.the FECA sensor is fabricated and a flexible eddy current array sensor detection system test bench is built.The detection system USES three-axis desktop robot arm to scan along the blade body direction and LC resonance method to detect circuit.Based on the detection principle of constant frequency and amplitude modulation,the voltage amplitude changes of eight detection coils are collected and converted into inductance signals,and the experimental data are exported to the computer.In this paper,the inductance change data containing defect information are processed by signal processing,and the median filter and compensation algorithm are used to successfully solve the problem of unevenness of probe caused by different lift height,material difference of probe and external interference is solved successfully.The two-dimensional and three-dimensional images of the defect are obtained in MATLAB to realize the defect of the flexible eddy current array sensor on the surface of the last stage blade of the steam turbine.Detection and imaging.Experiments show that the size,location and trend of natural surface defects of the last stage blades of steam turbines which have experienced pitting,erosion and corrosion can be correctly and intuitively reflected,which has a high guiding significance for the visualization of surface defects of conductive materials.