Reserch On Active Electromagnetic Scanning Technique And Signal Processing Method

The performance of machines and parts are required more stringent with the rapiddevelopment of modern industrial technology, and their working environment is becomingmore complex and working conditions are more hostile. So the traditional electromagnetictesting technology can’t meet the request of NDT under the new situation. A new methodcalled active scanning electromagnetic NDT is put forward based on combining activemeasurement technology, electromagnetic detection technology and scanning technique inthis paper. Active electromagnetic scanning technique and signal processing method isresearched on mainly from the mathematic modeling of electromagnetic detection system,scheme research for active adjustment, time-space signal modeling, signal processingtechnology research for scanning technique, the design and research of experimental systemand so on.The active electromagnetic detection system is designed by using the method of lowfrequency transmission eddy current testing base on the electromagnetic detection theory andactive control thinking. The model for workpiece’s thickness measured is establishedaccording to the relationship which is established according to the electromagnetic theorybetween the workpiece’s thickness and output signal from the test coil. MATLAB program iswritten to simulate for the model. The control model is established by using the output voltageamplitude as feedback quantity base on the workpiece’s thickness measured mold. And thedetection device is adjusted by the Combination adjustment scheme what is not only the probeelectric parameter adjustment but also the test device mechanical structure adjustment. Andthen the feasibility of the implementation scheme is analyzed theoretically.The basic theory of the multisampling signal processing and the energy operatordemodulation approach were discussed in this paper. The time-space domain mathematicalmodel for the output signal of electromagnetic scanning detecting technology is establishedbase on the theory of digital signal processing and by means of Dirac delta function. Spectrumcharacteristics of the space-time signal, the influence to the output signal from scanning rateand its fluctuation, deceleration process and Start-stop process and the problems such asnonlinear modulation and non-uniform sampling of the detection signal are studied base onthe theory of the multisampling signal processing. Then the sampling frequency of the testingsystem and the sampling plan of time-space domain signal are determined. Velocityinformation reconstruction and defect airspace effective information recovery are studied bythe pretreatment method such as filtering and energy operator method.The experiment system of active electromagnetic scanning detecting that is mainly made of computer control system has been designed, the experiment of workpiece’s thicknessmeasured, active adjustment and scanning detecting are carried out. The results are as follows:The detection signal voltage amplitude of two workpiece which their thickness are1mm and2mm respectively are measured, and are compared with the results of theoretical calculation,the experimental error of two groups are within5%, the correctness of the model for theworkpiece’s thickness measured is verified; The system is adjusted by means of the activeadjustment scheme, the detection signal amplitude of before and after adjustment arecompareed, the signal amplitude of after adjustment increases obviously; the wire rope whichhas defect is detected by the speeds of15.7mm/s and31.4mm/s, the spectrum of the latterdetection signal is stretched about2times relative to the former. The results are consistentwith the simulation conclusion, and it provides strong support for the above theory.Putting forward and research on a new electromagnetic detection scheme according tothe detection on complex environment is a research focus in this paper, and it is preliminaryresearched on its problems such as the modeling and signal processing. This project providesa new strategy for the workpiece’s NDT working in complex environment, and settles thetheoretical foundation for the development of the new nondestructive detecting instrument.