Thickness Measurement Of Metallic Coating From Pulsed Eddy-current Signals

A novel electrom agnetic nondestructive testing method adopting the technology of pulsed eddy current (PEC) has been develo ped in recent decade s. As a kind of testing method in the tim e domain, PEC testing method has its own advantages, for example, it is non-intrusive with high spatia l resolution. At the same time it doesn’t need coupling agent and special treatm ent before detection which m akes the information processing m ore easily. Theref ore, it gains wide application in m etal thickness measurement and non destructive testing (NDT).PEC testing m ethod is based on the pr inciple of electrom agnetic induction. According to Faraday’ s law of induction, when the m etal specimen is closed to the coil that carries pulse signal, eddy m agnetic field which can influence the original field in return will be p roduced in the specimen. The signal detected in the receiver which indirectly contains some useful information of the specimen (such as thickness, lift distance and defeat size) is changed too. Relevant information of the specimen can be achieved by analyzing the change of received signal with certain algorithm.This paper is based on th e research status of PEC testing method in China and abroad and the main work contains: study the application of PEC technology in metal thickness m easurement and NDT, design ha rdware pl atform, co mpile soft ware program, simulate H differential sensor with different software and verify the stability of the system through experiment.The thesis could be divided into four main parts:Firstly, a combination of PEC testing m ethod and H differential sensor has been designed for the first tim e, which effectively improveds the sensitivity of the sens or and reduced the im pact of background signal. T he results derived from experim ents showed that this design had a high accuracy.Secondly, hardware circuits which c ontain baseboard, exciting circuit and acquisition cir cuit ha ve been d esigned. F ilter circuits with dif ferent cut-o ff frequencies which were controlled by the host com puter were chosen. In the experiment, the optim al filter circuit could be selec ted by com parison of the data obtained from different filter circuits.Thirdly, finite-elem ent analysis was done by ANSOFT wh ere tim e dom ain simulation was adopted . Different from traditional simulation of frequency dom ain, during the sim ulation, step signal was set as stimulus and co rrespondence between thickness of the specim en and charac teristic signal was quantif ied. Meanwhile equivalent circuit m odel was deduced and qualitative analysis of relevance between the thicknes s of the specim en and the ch aracteristic signal was obtained by the simulation software of ORCAD/PSpice.Last, the experim ent was conducted on the basis of the above work. By measuring the different thickness of aluminum foil, the results showed that the system could accurately measured the th ickness of aluminum foil with sim ple construction and high stability.