Research And Application Of Multi-layered Structure Thickness Measurement Using Eddy Current

Eddy Current Nondestructive Testing (ECNDT) is widely used to detect the thinness and defects of the sample, because of its characteristics of non-contact, high sensitivity and ease of detection. The principle of ECNDT is that: when a probe coil with the AC exciting source is put near a sample, eddy current will be produced in the sample which will conversely affect the impedance of the coil according to the Farad Theorem. Any un-uniformity in the sample will change the impedance of the coil and the output voltage. Monitoring the change will give us useful information about the sample.The paper begins from analyzing the principle of ECNDT, studies the feasibility of the clearance detection between two metal plates using eddy current, furthermore makes experiment research. The content of the dissertation is as follow:(1) An eddy current testing model of multi-layered metal structure was presented in the light of the work of Luquire and others. Using Maxwell electromagnetic equations, by solving the boundary-value problem of magnetic vector potential, analytical expression to impedance increment of the solenoid coil in eddy current testing of the medium in half space was given. Theoretical result was modeled in computer. Results showed the validity and feasibility of the model.(2) In the paper the structure of intelligent eddy current testing system was studied. The function frame of the system and its hardware circuit were introduced. To the defect of the system hard to equilibrate , a new circuit frame to make equilibrium was presented(3) Based on the analysis of interferences of the circuit, some relative means were taken to restrain the interferences. Furthermore in order to detect surface conveniently and to improve the testing accuracy some designs of the probe were introducer.(4) An approach involving the use of Fourier descriptors for the extraction of eddy current signal characteristics before their inputting into neural network for quantitative analysis was presented. The results showed that the output of neural network had high analysis accuracy.(5) Lastly the testing data was taken to curve fitting in least squares. The fitting results showed that: to the clearance detection the amplitude of output voltage would increase when the clearance between two metal plates increased. Otherwise ,the amplitude would decrease; to the thinness testing the amplitude would decrease when the thinness became thick. The experiment proved the correctness and feasibility of the eddy current testing model.